IAEA-TECDOC-315

PROTEROZOIC UNCONFORMITYAND

STRATABOUND URANIUM DEPOSITSREPORT OF THE WORKING GROUP ON URANIUM GEOLOGY

ORGANIZED BY THEINTERNATIONAL ATOMIC ENERGY AGENCY

EDITED BYJOHN FERGUSON

A TECHNICAL DOCUMENT ISSUED BY THEINTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1984

PROTEROZOIC UNCONFORMITY AND STRATABOUND URANIUM DEPOSITSIAEA, VIENNA, 1984IAEA-TECDOC-315

Printed by the IAEA in AustriaOctober 1984

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FOREWORD

The great surge of interest and activity in exploration for uraniumdeposits over the last decade has added significantly to our knowledge ofuranium geology and the nature of uranium deposits. Much of the informationthat has been developed by government and industry programmes has not beenwidely available and in many cases has not had the benefit of systematicgathering, organisation and publication. With the current cut-back inuranium exploration and research efforts there is a real danger that muchof the knowledge gained will be lost and, with the anticipated resurgenceof activities, will again have to be developed, with a consequent loss oftime, money and effort. In an effort to gather together the most importantinformation on the types of uranium deposits, a series of reports is beingprepared, each covering a specific type of deposit. These reports are aproduct of the Agency's Working Group on Uranium Geology. This group, whichhas been active since 1970, has gathered and exchanged information on keyquestions of uranium geology and coordinated investigations on importantgeological questions. The Working Group has been carrying on several projectsto prepare comprehensive reports on major types of uranium deposits. Thesereports are planned for completion and release in 1984. Findings of the workwill be presented at the International Geologic Congress in Moscow in August1984.

The topics of the Working Group on Uranium Geology projects and thenames of the project leaders are:

Sedimentary Basins and Sandstone Type Deposits -Warren Finch

Uranium Deposits in Proterozoic Quartz-Pebble Conglomerates -Desmond Pretorius

Vein Type Uranium Deposits -Helmut FuchsProterozoic Unconformity and Stratabound Uranium Deposits -John Ferguson

Surficial Deposits -Dennis Toens.

The success of the projects is due to the dedication and efforts ofthe project leaders and their organisations, and the active participationand contribution of world experts on the types of deposits involved. TheAgency wishes to extend its thanks to all involved in the projects fortheir efforts. The reports constitute an important addition to theliterature on uranium geology and as such are expected to have a warmreception by the member states of the Agency and the uranium community,world-wide.

I.A.E.A., Vienna, May 1984

EDITORIAL NOTE

The authors who have contributed to this project are all actively engagedin research work in areas relating to their respective contributions. Thisadvantage has allowed for updated syntheses which frequently includes work thathas a restricted distribution and additionally incorporates new data. It ishoped that this report will help serve as an aid for exploration for Proterozoicunconformity and stratabound uranium deposits.

I am indebted to Dell Stafford for typing most of this manuscript and tothe BMR Drafting Office for the production of numberous line drawings and maps,in particular, I would like to thank Ivon Chertok, Ingo Hartig, Natasha Kozinand Diana Pillinger.

John Ferguson

CONTENTS

STRATIFORM AND STRATA-BOUND TYPES

Proterozoic strata-bound uranium deposits of Zambia and Zaire .................................... 7L. Meneghel

Uranium in Early Proterozoic Aillik Group, Labrador .................................................... 35S.S. Gandhi

The Olympic Dam copper-uranium-gold deposit, Roxby Downs, South Australia ............ 69D.E. Roberts, G.R.T. Hudson

Strata-bound uranium deposits in the Dripping Spring Quartzite, Gila County,Arizona, USA: A model for their formation ................................................................ 95C.J. Nutt

The Lianshanguan uranium deposit, Northeast China: Some petrological andgeochemical constraints on genesis ................................................................................ 115Zhong Jiarong, Guo Zhitian

UNCONFORMITY-RELATED TYPES

Pine Creek Geosyncline, N.T. ............................................................................................ 135G.R. Ewers, J. Ferguson, R.S. Needham, T.H. Donnelly

Uranium mineralization at Turee Creek, Western Australia ................................................ 207G.N. Lustig, G.R. Ewers, C.R. Williams, J. Ferguson

Unconformity-related uranium deposits in the Athabasca basin region, Saskatchewan ...... 217V. Ruzicka

Uranium geology, Beaverlodge Area ................................................................................ 269D.M. Ward

Geology and discovery of Proterozoic uranium deposits, central district of Keewatin,Northwest Territories, Canada .................................................................................... 285A.R. Miller, J.D. Blackwell, L. Curtis, W. Hilger, R.H. McMillan, E. Nutter

The RioPreto uranium occurrences, Goiâs, Brazil ............................................................ 313P.M. de Figueiredo Filho

Proterozoic unconformity and strata-bound uranium deposits: Epilogue ........................ 325/. Ferguson

PROTEROZOIC STRATA-BOUNDURANIUM DEPOSITSOF ZAMBIA AND ZAIRE

L. MENEGHELAGIP Nucleare,Milan, Italy

Abstract

The Katanga System, host to uranium and copper mineraliz-ation, is several thousands of metres thick ana rests unconform-ably on an older complex of crystalline rocks and meta-sedimentsand is locally covered by Karoo sandstones or Kalahari sands.The deposition of the Katanga System took place during the LateProterozoic in a wide complex basin extending from Shabaprovince in Zaire through a large part of Zambia and into easternAngola. The sediments were affected by different grades ofmetamorphism, tectonic events, and by thermal events associatedwith post-tectonic metamorphism.

At the base of Katanga system there are 84 known copperdeposits and 42 uranium occurrences. It is suggested that allthe known uranium and copper occurrences are of an essentiallysyngenetic sedimentary origin. The mineralisation is found inthe Lower Roan Formation near the base of the Katanga Systemoccurring in rocks produced in similar environmental conditionsand thus being stratigraphie controlled, however, their arealdistribution is localised producing a regional metal zonation.Many of the uranium occurrences have a typical vein aspect.These transgressive relationships are not inconsistent with asyngenetic origin as evidenced by the vein morphology.

Exploration activities and genetic studies are ongoingthroughout this metallogenic province.

INTRODUCTION

The Copperbelt areas of Zaire (Shaba Province) and Zambiawhich host the uranium deposits are located on the southern flankof the Congo Craton which is one of the major structural blocksof the African continent south of Sahara.

00 TABLE 1. Correlation of the Katanga System of the Domes, Zambian, and Shaban Copperbelt Areas

Domes area

McGregor, 1964; Edwards, 1974; Arthur, 1974;Klinck, in press; and Appleton, in press

Stratigraphie

West Lungaformation

Luigishiformation

Wamikumbiformation

Wushingwiformation

units Rock type

Limestone and dolomite(Tillite)

Carbonaceous shale

Schists

Dolomite and magnesite

Dolomitic schists

Talc-speculariticquartzite

Quartzite (marker)

Mica schistsBiotite schists

Local conglomerate

Zambian Copperbelt area

Mendelsohn, 1961; Binda and Mulgrew, 1974

Stratigraphie units Rock type

ShaleTillite

Kundelungu Shalegroup Dolomite and shale

Tillite

Carbonaceous shaleMwashya group

Argillite

Dolomite

(Roan unit 1)

Roan group Argillite, shaledolotnitic schists(Roan units 2,3, and 4)Quartzite (Roan unit 5)

A r g i l l i t e , micaceous quartzite,impure dolomite (Roan unit 6)

(Quartzite, conglomerate)(Roan unit 7)

Shaban Copperbelt area

Francois, 1974; Cahen, 1974; Cailteux, 1977

Stratigraphie type Rock type

Sandstone and shaleTillite

Kundelungu Sandstone and shalesupergroup Dolomite

TilliteGrandConglomerateand Mwashyasupergroup Black shale

(Roan unit 4.2)

Dolomite

(Roan unit 4.1)

Roan supergroup Dolomitic sandstoneDolomiteShale (Roan units 3(?)and 2.3)Siliceous dolomite(Roan units 2.1 and 2.2)

Dolomitic sandstone(Roan unit 1)

Basement complex: granites, schists, gneisses, etc.

Uranium mineralisation was first discovered in the ShabaProvince of Zaire in 1915 and sometime later in the ZambianCopperbelt, the discoveries in the North-Western Zambian DomesArea are fairly recent.

For sometime these three districts (Fig. 1) have beenregarded as indépendant of one another forming three separatedepositional basins, as suggested by the fact that there aremarked differences between the three areas. A more carefulexamination of the geological environments demonstrates howeverthat in each of these areas the lithologies merge into eachother indicating that they form a single depositional domain.

The three areas are considered to be a part of a single geo-logical unit. The mineralised areas are thought to have acommon origin and to be linked to the same important metallogen-etic epoch; later tectonic overprinting has masked theirpetrogenetic links. This modelling served as the basis for therecent discovery of several uranium showings and deposits in theDomes Area of Zambia by Agip S.p.A.

While a considerable amount of data and information iscurrently available on the Copperbelt area of Zambia and Zaire,the studies on the Domes Area and on the basin's general palaeo-geography and evolution are still scanty and fragmentary. Morespecific information on the Domes Area geology is included inthis paper; as far as the other areas are concerned the inter-ested reader is referred to the available literature and partic-ularly to Benham et al. (1976); Binda and Mulgrew (1974);Cahen (1974); Derricks and Oosterbosh (1958); Derricks and Vaes(1956); Demesmaeker et al. (1962); Francois (1974); Garlick(1972); Oosterbosh (1962); Thoreau et du Trieu de Terdonck (1933)

REGIONAL GEOLOGICAL SETTING

The Katanga System, at the base of which all the copper,cobalt, uranium and nickel mineralisations are found (Table 2),unconformably rests on an older complex (gneisses, mica schists,quartzites, phyllites and granite) and is locally covered by Karoosandstones or Kalahari sands.

The reconstruction of the history of the Katanga System basinis quite difficult not only because of its large size and its

051 SAND

I '-'. KATANGA SYSTEM

BASAMENT COMPLE

Fig 1. Distribution of the Katanga System(after Francois, 1962)

original complexity but also because its domains underwent diff-erent tectono-metamorphic evolutions. The lack of adequateregional mapping makes for further difficulties. It followsthat virtually unknown areas separate other areas for which arich harvest of data is available.

The deposition of the mainly marine Katanga System tookplace in the period 840 m.y. and 1,300 m.y. (Cahen, 1970) in awide complex basin extending from the Shaba province in Zairethrough a large part of Zambia and eastern Angola. The south-western deposits are covered by more recent formations (Fig. 1) ,The full sequence is several thousands of metres thick: at thebase are found the copper deposits of the Copperbelt areas ofZambia and Shaba.

10

ê•5

11

In the Zambian Copperbelt area the sequence is subdivided,from bottom to top, into the Roan, Mwashya and Kundelungu Groups(Tables 1 and 2). The Roan Group is subdivided into a pre-dominantly clastic, Lower Roan sub-Group, up to 1,000 m thick,and a predominantly dolomitic Upper Roan sub-Group, 500 to 800 mthick (Binda and Mulgrew, 1974). In Shaba the sequence issubdivided into the Roan, Mwashya and Kundelungu Supergroups,the Roan Supergroup being subdivided into four groups (Cahen,1974). In the Domes Area the sequence is subdivided from bottomto top into Wushingui, Wamikumbi, Luigishi, and West LungaFormations.

There appears to be a general increase in metamorphic gradein the Katanga System from east to west and from north to south.In Shaba there is little evidence of metamorphism, and in theZambian Copperbelt the metamorphic grade normally does not exceedgreenschist faciès (Mendelsohn, 1961). In the Solwezi areamineral assemblages indicate metamorphism of the greenschistfaciès, with local development of amphibolite faciès (Arthurs,1974). Lower Roan schists contain quartz, muscovite, and tracesof phlogopite and iron oxides, with or without chlorite, biotite,or epidote. Kyanite occurs as individual crystals and needles,or in coarse aggregates at the unconformity between the basementand rocks of the Lower Roan Group. In the Kabompo Dome area,the metamorphic grade reached amphibolite faciès in the basementand in the Wushingwi and the Wamikumbi Formations (Appleton, inprep.; Edwards, 1974). Assemblages characteristic of theWushingwi Formation (Lower Roan) are kyanite + quartz, quartz +talc + phlogopite + chlorite, and kyanite + quartz + talc +phlogopite + chlorite. Chlorite is indicative of post-tectonicretrogressive metamorphism.

Shaban Copperbelt areaThe arcuate Shaban Copperbelt covers an area of approximately

300 km. Uranium, copper, cobalt and nickel are found together,mainly along the southern margin of the arc, particularly atKalongwe, henda, Kasompi, Swambo and Shinkolobwe (Fig. 2).Copper-cobalt mineralisation, sometimes accompanied by uranium,is found in the central part of the arc at Husonoi, Kamoto,Mashumba, Kalumbwe, Mwinga, Mashitu, Kambove, Luishya and many

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Table 2. Distribution of Copper Deposits and Uranium Occurrences in the Katanga System

Shaban Copperbelt

Stratigraphie Number of Cu Number of Uunits deposits occurrences

K.s.3K.s.2.2K.s.2.1K.s.l. 3. 2K.s.l. 3.1K.s.l. 2. 2K.s.l. 2.1K.s.1.1K.i.1.4K.i.1.3K.i.1.2.2 2K.i. 1.2.1K.i.1.1R.4.2R.4.1 2

R.37.R.2.3

R.2.1.R.2.2 64 22R.I? 45?

Total 66 22

Zambian Copperbelt Domes area Total

Stratigraphie Number of Cu Number of U Stratigraphie Number of Cu Number of U Number of Cu Number of Uunits deposits occurrences units deposits occurrences deposits occurrences

K.u. tilliteK.I. shale K.I. shale

K.I K.Ilimestone limestone

K.I. tillite K.I. tilliteMwashya MwashyaUpper Roan Upper Roan(R.u. 1) 1? 2 1?R.u.2R.I. 4, R.I. 3R.I. 6, R.I. 5 17 5 Lower Roan 3 15 84 42R.I. 7Basement Basement 1? 1?

17 5 3 15 86 42

Roan subdivision from Binda et al. (1974)^Correlation between Roan of Shaban and Zambian Copperbelt established by Cailteux (1977)One deposit of hydrothermal origin and one of supergene enrichment (Francois, 1974)The four deposits are of supergene enrichment (Francois, 1974)

Abbreviations: K = Kundelungu; R « Roan; s = superior; i = inferior; u « upper; 1 = lower

other localities. In the northern part of the arc only tracesof copper and cobalt, not accompanied by uranium, have been found.

The mineralised dolomitic rocks of the Lower Roan Group, inShaba are found as separate masses enclosed in breccia. Incentral Shaba, the rocks are only slightly deformed, but farthersouth they are complexly folded and faulted. Because the contactbetween the Katanga System and the basement is not exposed, itis difficult to reconstruct the paleoenvironment. Francois(1974) suggested that transgression and terrigenous sedimentationinitially took place on a continental platform in an oxidizingenvironment with the formation of a Roan 1 unit. The Roan 2, 3and 4 units were then deposited under euxinic conditions,accompanied by local volcanic activity, especially at the Roan 4level, and with the concomitant development of copper and uraniummineralisation at the Roan 2 level. Finally, the basin wasfilled with Kundelungu sediments in a variety of environmentalconditions.

Mineralisation occurs in the basal 30 to 40 m of the Roan 2unit of the Lower Roan Group (Table 2). The host rock isgenerally a siliceous dolomite, the main minerals being dolomite,quartz, magnesite, sericite, and chlorite, with apatite, tourmal-ine, and monazite as accessory phases. Original clayey sedimentswere weakly metamorphosed, with the formation of authigenicsericite ana chlorite (Francois, 1974). The usual ore mineralassemblages are chalcocite-bornite-carrollite or chalcopyrite-carrollite. The common uranium minerals are thorium-freeuraninite and secondary uranium minerals. The highest uraniumconcentrations are found in fracture zones or near the watertable. All the disseminated black uranium minerals are localizedat the base of the zone of copper-cobalt mineralisation (Cahenet al, 1971).

Recent isotopic age determination indicate that Roansedimentation began more than 1,000 m.y. ago (Cahen, 1974).Three subsequent orogenic phases were dated at 840 + 40 to710 m.y., 670 + 20 m.y., and 620 + 20 m.y. (Cahen, 1970). Agedeterminations (Cahen et al., 1971) on the uranium minerals fromdifferent localities give the following results: ^706 m.y.,670 + 20 m.y., 620 + 10 m.y., 582 + 15 m.y., 555 + 10 m.y. and520 + 20 m.y. (Fig. 3). Modification of the protore has been

14

brought about, with local conversion into rich ore, by tectonic,volcanic and thermal events, and alteration phenomena.

The uranium-copper-cobalt-nickel association is found in adifferent isopic zone from that of the main copper-cobaltassociation (Oosterbosch, 1962), the mineralisation has a typicalvein aspect with epigenetic characteristics. The origin of themineralisation, particularly its structural control, is acontroversial one. A metallogenetic epoch different from thecopper-cobalt association has been postulated (Oosterbosch, 1962).A magmatic origin has been proposed and the deposits at Shinko-lobwe have been classified as hydrothermal (Thoreau et al., 1933;Derricks and Vaes, 1956; Derricks & Oosterbosch, 1958;Dahlkamp, 1978. Ngongo-Kashisha (1975) suggests for the twoassociations a unique metallogenetic process and Francois (1974)suggests a similar origin. Cahen et al. (1971) consider thatthe uranium deposits are due to the remobilisation of pretectonicmineralisation.

Zambian Copperbelt areaIn the Zambian Copperbelt, which covers an area 112 km long

and 56 km wide, uranium mineralisation is found at Nkana,Mushoshi, Luanshya, and Chibuluma in the basal part of theKatanga system. Mineralisation occurs as disseminations orfracture fillings stratigraphically below the copper ores, as atChibuluma, or in rocks of shallow water origin adjacent to thecupriferous zones, as at Nkana (Mendelsohn, 1961). The hostrocks are generally argillite, fine-grained carbonaceous quartz-ite, micaceous quartzite, and shale; they represent the earliestsediments deposited under reducing conditions, as in the Shabanarea. The common accessory minerals are rutile, apatite, zircon,monazite and pyrite. The ore-bearing formation was affected byup to three phases of deformation during the Lufilian Orogenyand by the same thermal and volcanic events and alterationprocesses as the sediments of the Shaban area.

The uranium minerals are uraninite and its alterationproducts. Age determinations show that there are several gener-ations of uranium mineralisation: 520 ± m.y., 468 +; 15 m.y.,365 + 40 m.y., and 235 + 30 m.y. (Cahen et al., 1971). The firstgeneration included copper, cobalt, iron and uranium, but sub-

15

sequently the proportions of the other metals decreased untilonly uranium remained. No uranium mineralisation older than 520m.y. has been found. The dissemination and the vein-typeuranium mineralisation have clear epigenetic characteristics(Cahen et al., 1961). The uranium mineralisation contrasts withthe copper mineralisation, for which a syndepositional origin canbe regarded as established.

Domes AreaThe Domes Area (Fig. 2) in the northwestern province of Zambia

extends for about 300 km from west to east and includes theKabompo, Mwombezhi, Solwezi and Luswichi Domes. Rocks of pre-Karoo age in the area belong to the basement complex and theKatanga System.

Gneisses, schists and granites of the basement complex occurin the cores of the Kabompo, Mwombezhi and Solwezi Domes. Inthe Kabompo Dome, the basement rocks are divided into (a) gneisses,including leucogneiss, scapolite gneiss, and biotite-hornblendegneiss and (b) psammitic and pelitic rocks affected by two phasesof migmatisation (Klinck, in press). Appleton (in press)similarly describes an older, partly migmatised gneiss complexand a younger formation of schists, both feldspathised andunfeldspathised. A gneiss group found in the Solwezi andMwombezhi Domes comprises biotite gneiss, schistose gneiss, amph-ibolite, and hornblendite. A later granite group, separatedfrom the gneiss group by a migmatite zone, is composed of margin-ally foliated granite (Arthurs, 1974). The high radioactivebackground over large areas suggests that the basement complexcould have been the original source of the uranium found in thebasal Katangan rocks.

The lower units of the Katanga System crop out peripherallyaround the domes, whereas the higher stratigraphie units are foundfarther out in the surrounding country.

In the Kabompo Dome and adjacent areas, the Wushingwi Form-ation unconformably overlies the basement complex. The formationis composed of a sequence of talc-muscovite-kyanite quartzitesand schists, with intraformational breccias and conglomerates(Appleton, in prep.). A basal conglomerate occurs locally. At

16

some localities, notably in the Solwezi Dome area, the base ofthe formation is not clearly defined, the assumed contact beinga zone of kyanite schist (Arthurs, 1974). The Wushingwi Form-ation gives rise to prominent quartzitic ridges.

The overlying Wamikumbi Schist Formation comprises mainlybiotite schists, variously with garnet, kyanite, actinolite,carbonate minerals, and scapolite. Quartzites, marbles,graphitic phyllites, banded ferruginous quartzites and schistosetillites are also included. In the Kansanshi borehole NK6 inthe Solwezi area, the formation is represented by dolomite andmuscovite-chlorite-talc schist, passing into micaceous anddolomitic quartzite (Arthurs, 1974). Unfoliated basic meta-volcanic rocks occur in the Wamikumbi Formation to the south andto the north of the Kabompo Dome (Appleton in press: Klinck, inpress) as lenticular masses up to 3 km in length.

In western part of the Domes Area, the Luigishi Formationappears to overlie conformably the Wamikumbi Formation (Edwards,1974), but in other areas, especially to the north of theKabompo dome, it represents a more arenaceous faciès of theWamikumbi formation (Appleton, in prep.). It includes calcar-eous psammitic schists and psammites, variously containingbiotite, scapolite, actinolite and garnet. The West LungaFormation which follows is a thick sequence of folded peliticmetasediments with units of marble and tillites (Edwards, 1974).The tillites are considered to be predominantly glacial inorigin, although in part they were probably modified by massflow processes. The youngest Katangan unit, the KanyidmaFormation, crops out in the northwest. It is composed of cal-citic marbles (Appleton, in prep.).

In the Wushingwi Formation the development of cross-beddingand intraformational breccias in a sedimentary sequence includ-ing carbonate (some probable bioherms) and coarse elastics, tog-ether with apatite and graphite, may indicate a shallow water,near-shore, reducing marine environment of deposition. Near-shore, possibly lagoonal, conditions are considered to haveprevailed at the beginning of the deposition of the WamikumbiFormation, followed by a transition to marine sedimentation. Theintercalation of carbonate rocks may imply deposition in shallowwater. Edwards (1974), considered the rocks of the Luigishi

17

Formation to have been originally sandy marls but was unable tofind identifiable sedimentary structures. Shallow waterdeposition, possibly in a shelf environment, is suggested byKlinck (in press) for the West Lunga Formation.

Resulting from the absence of age determinations, thecorrelation of the Katanga sediments of the Domes Area can onlybe based on litho-stratigraphic features and on tectonic andmorphological considerations. The Wushingwi, Wamikumbi Schist,Luigishi, and West Lunga Formations are included in the KatangaSystem because they lie unconformably on the basement complexand are lithologically similar to the Katanga rocks of theCopperbelt. Although the rocks of the Wushingwi Formation arerelatively highly deformed and metamorphosed, they can becorrelated with similar arenaceous units around the Mwombezhiand Solwezi Domes, and with arenaceous Lower Roan rocks of theCopperbelt. If the tillites are indeed of glacial origin, theWest Lunga Formation may be correlated with Kindelungu of theCopperbelt (Edwards, 1974).

The Katanga metasediments contain unfoliated gabbro, meta-gabbro, metadolerite, and granitic rocks. Gabbroic rocks, includ-ing olivine gabbro, commonly form sills and plugs in the Luigishiand Wamikumbi Formations, and there are minor occurrences in theWest Lunga Formation. Rocks of granitic composition includeadamellite, quartz monzonite, tonalité, and diorite. Hornblende-epidote adamellites in Katangan metasediments younger than theWushingwi Formation in the northwest of the Domes Area mayrepresent the post-Lufilian Hook granite (Appleton, in press).

The Domes Area lies within the southern portion of the Lufi-lian arc and is composed of Katangan rocks subjected to threephases of deformation and metamorphism during the LufilianOrogeny (840-620 m.y.). The mode of formation of the domes isuncertain, but it is generally believed that they were formedlate in the Lufilian Orogeny, possibly by the interference offolds of the second and third phases (Klinck, in press).

Three major low-grade stratiform copper deposits are knownin the Lower Roan of the Mwombezhi Dome. The ore-bearinghorizon is a slightly carbonaceous shale. Deposition of thehost rock ana distribution of the sulfides are closely related tothe palaeotopography. Mineralisation seems to be of syn-

18

sedimentary origin related to reducing conditions (Benham et al.,1976).

Uranium mineralisation occurs in the Lower Roan Group,which crops out for 600 km around the domes. Uranium mineralis-ation is confined to two horizons, one above and one below aquartzite marker horizon. At Kalaba and Katontu around theKabompo Dome, uranium arsenates and vanadates, carnotite, gold,copper sulfides, traces of nickel, cobalt, lead, chromium andmolybdenum occur in a brecciated talc schist just above thequartzite horizon. To the east of the West Lunga River, 15 kmfrom the western edge of the Kabompo Dome, uranium and coppermineralisation occurs in a carbonaceous shale of the KatanganSystem, and metatorbernite has been identified in siltstone. Twoboreholes, located on a radioactive anomaly, intersected leacheduranium mineralisation at a 40 m depth (Edwards, 1974). Uran-inite, brannerite, metatorbernite, monazite, molybdenite, anagold occur in quartz-carbonate veins at the Kansanshi copper mine(C. Legg, in Arthurs, 1974).

Below the quartzite marker horizon, uranium mineralisationoccurs as uraninite, disseminated or in small veins, and as yellowsecondary minerals. Five of the 15 known occurrences have beeninvestigated by drilling and encouraging results have beenobtained.

In the Kawanga area (Kabompo Dome), uranium occurs mainlyas traces of autunite coating mica schist cropping out in thearea of two small radiometric anomalies. Drilling intersecteduranium mineralisation in mica schist below the quartzite markerhorizon. Discontinuous mineralisation occurs at one or morelevels the thickness of which can be several metres and has a U000j ocontent up to 1 percent. One hole intersected mineralisationof such a grade over a thickness of about 25 m at a depth of morethan 200 m. The mineralisation consists of thorium-free uranin-ite disseminated in schist or filling fractures several centi-metres wide, and of secondary uranium minerals coating fracturesurfaces. Kyanite schist from the mineralised zone carriesquartz, epidote, kyanite, and muscovite and has granoblastic andpoikiloblastic textures. Quartz shows a mosaic texture andepidote is present as irregularly shaped grains or as prismaticcrystals. Kyanite occurs as individual crystals or in

19

aggregates. Accessory minerals include iron oxides, apatite,monazite, and rutile. The uranium mineral is autunite. Adark biotite schist, in which the schistosity is marked by flakesof biotite and muscovite and equidimensional grains of quartz,occurs below the mineralised zone. Tourmaline, apatite, zirconand opaque minerals are also present. The rock overlying themineralised zone is a talc schist, containing unorientated platesof talc and crystals of rhombic pyroxene and kyanite. Accessoryminerals include monazite and scapolite.

During the course of exploration for copper in the MwombezhiDome area, Mwinilunga Mines Limited found occurrences of uraniumon the surface and subsurface in the Lumwana area (McGregor,1964; Benham et al., 1976). The uranium mineralisation is foundin mica schist, as in the Kawanga area. In the Solwezi Domearea, numerous small occurrences of uranium mineralisation havebeen found, and drilling has been carried out at Dumbwa,Kapijimpanga, Kimale, and Mitukuluku. The mineralisation issimilar to that already described. Secondary uranium mineralsare found coating flakes of mica, talc, or chlorite alongfractures and in veinlets containing quartz, talc, muscovite,apatite, and kyanite. The uranium minerals include autunite,meta-autunite, sabugalite,phosphoranite, vandendrlesscheite, andgummite. At depth, the mineralisation consists of uraninite,disseminated or forming scattered veinlets, and secondary uraniumminerals, as at Kawanga. The best mineralisation intersected indrill holes occurs at the Mitukuluku showing, where 1.4 percentU000 was found over a thickness of 9 m (C. Legg, in Arthurs, 1974)3 oThe host rock is quartz-mica-kyanite-talc-epidote-apatite-chloriteschist, underlying the marker quartzite.

The absolute ages of six samples from the Kawanga, Kimale,and Dumbwa areas, were determined at the Institute of GeologicalSciences, Geochemical Division, London. The mean value for the207/206 (U) ages is 536 + 12 m.y.

The uranium mineralisation of the Domes Area is of thesame age or is younger than that of the Shaban Copperbelt and isof the same age or older than that of the Zambian Copperbelt(Fig. 3). In the Domes Area and in the Zambian Copperbelt themineralisation lies close to the basement where reactivation ofgranite gave rise to thermal events after such influences had

20

ceased elsewhere. In the Domes Area and in the Zambian Copper-belt, any uranium mineralisation ages older than 520 m.y. couldhave been suppressed by these later thermal events, during whichuranium and other minerals were remobilized. In the ShabanCopperbelt, older uranium mineralisation was isolated in up-faulted blocks of the Lower Roan and only a very few mineralsparticularly sensitive to temperature were thermally affected.

S2O582

Kamoto . .. Kolwtzi

52O

'AKalongw« • Swambo

Z A I I* E

555• Kambova

. «hmliolobwo

/IwmilMRfla

;V 706

_ ' Kiw.ng .536 520. Kamanihi

S36.l«n,al.

I;**\ fLUBUMBASHI t

.. -..x7 '">\^iiu

. Dumbw»536

M BLuanthya*

365

\

\ •"Li

SO 16O 2OO Km

Fig 3 Isotopic ages of Uranium occurrences in theKatanga system (Modified after Cahenet al,l97l )

THE DEPOSITS' GEOLOGICAL SETTING

Shaban Copperbelt areaIn the Shaban Copperbelt area Francois (1974) classifies

22 uranium occurrences as 7 proven deposits, 2 indicated deposits,3 inferred deposits, and 10 showings. The most notable of thesedeposits is Shinkolobwe Mine (Derricks and Vaes, 1956; Derricksand Oosterbosh, 1958), the only one exploited. Although noofficial data have been published, the amount of uranium exploitedcould have been over 30,000 tonnes (U-.Oa) at an average gradebetween 0.4 and 0.5%. The mine was opened in 1921 and closed in1960 as a result of a fire.

The uranium was found as uraninite or as secondary minerals(over 20 species) in the lower part of the R2 units at the baseof the copper mineralisation. Uranium preferentially concen-trated in the mylonite zones but originally could have had a

21

lensoid or stratigraphie dissemination similar to but more irreg-ular and patchy than that of the copper mineralisation. Further-more since the two elements have different geochemical behaviourcharacteristics they tend to fractionate. Protore could haveundergone a long series of later remobilizations, the mostimportant of which could be linked to the faulting of the minesalloctonous tectonic unit enclosed into an impermeable talc-schistbreccia. Within this unit the uranium could have been free tomove and concentrate in small fractures. Such remobilisationwould have been responsible for the production of economicuranium grades. The host rock is a slab of Roan Group involvedin a thrust zone and enclosed into a tectonic breccia; it restsover younger Kundelungu Strata. The mineralisation is foundalong open fractures within the dolomite. Mineralisation stopsat the boundary with the enclosing breccia; the larger faultsare always barren. After development of mineralisation inthe slab only minor displacements followed. Uranium mineralsare found both in veins (from a few decimetres up to a metrethick) and disseminated in the host rock. Uranium was accompan-ied by Cu, Co, Ni and traces of Mo, W and rare earth elements.Some native gold occurred as encrustations on uraninite. Goldgrades from 10 to 60 grams per tonne. Some platinum and pallad-ium is also present .

The following parageneses have been recognised - first phase:magnesite; 2nd phase: uraninite; 3rd phase: pyrite, selenium,molybdenum, monazite, chlorite; 4th phase: Co-Ni sulphides (andtectonization); 5th phase: chalcopyrite. Absolute agedeterminations on uranium minerals at Shinkolobwe yielded age>706 m.y., 670 +_ 20 m.y. and 620 _+ 10 m.y. Fluid inclusionsfound in the Cu-Co and U-Ni-Co-Cu mineralisation within unit R2show identical characteristics suggesting that they were formedduring the same metallogenetic event.

Zambian Copperbelt areaIn the Zambian Copperbelt at least five uranium occurrences

are known. The Nkana mineralisation is the only one that hasbeen so far exploited with a total production of 110 tonnes U000,o oUranium ore occurred in the transition zone between a copper

22

_C Nt «IT 0 • {i » «ota "&»«oto

I A'r**i. i C A i e I

Fig. 4.- Diagram illustrating the topographiccontrol of Lower Roan ore depositionfrom the Nkana area.

23

orebody and barren dolomites, this barren gap reflecting a buried'basement' hill (Fig. 4).

Uranium mineralisation was present in complex argilliticsediments (the Cherty Ore) and in alternating sandy and argill-aceous dolomites (the Banded Ore). The dominant primary uraniummineral is uraninite, occurring as fine disseminations and blebs,with a little brannerite and coffinite. Near surface oxidationproduced uranophane, beta-uranotile and gummite. Numerouscalcite, quartz, feldspar and anhydrite veins cut the deposit andwere mineralised with brannerite and uraninite and traces ofbornite and chalcopyrite.

Mineralisation has been found both as veins and disseminat-ions. When the veins intersect a disseminated mineralisedhorizon the strata-bound ore grade decreases suggesting that theveins uranium came from the disseminations themselves. Quartz-feldspar veins that do not intersect the mineralised beds arealways barren. Field evidence demonstrates that the entiremineralisation postdated the Lufilian Orogeny. Absolute agedeterminations at Nkana yielded ages of 520 +_ 20 m.y.,468 + 15 m.y. and 235 + 30 m.y. The first generation (520 m.y.)showed the following mineral association: uraninite, melanite,bornite, chalcopyrite, digenite, calcite, chalcocite, quartz,biotite, albite, chlorite. The 468 m.y. old one showed uraninite,carbonate, pyrite, finally the third one (235 m.y.) showedbotryoidal uraninite only..

Domes AreaOver 15 uranium occurrences are known around the basement

domes of North-Western Zambia. So far none of them has beenexploited and exploration is still going on. The best known ofthem, namely Kawanga has proved reserves of a few thousand tonnesU000 at an average grade of about 0.4%.J o

URANIUM ORE GENESIS AND CONTROLS

The protore and its relation to copper mineralisationStratigraphie correlations have been established between

the Zambian and Shaban Copperbelts (Cailteux, 1976), but correl-ation of these areas with the Domes Area is uncertain. The

24

basins of deposition were for the most part of limited extent,especially for the lower parts of the Katanga System, so thatthere are many local faciès changes. The basal sediments inthe Zambian Copperbelt and Domes Areas are predominantlyarenaceous, whereas those in the Shaban area are carbonates, butall are associated with copper, cobalt, and uranium mineralis-ation. The sediments deposited later were more uniform incomposition and of greater lateral extent and are, thereforemore readily correlatable. The sediments of the Katanga Systemwere deposited in the period 1,300 to 620 m.y., those of theRoan Group probably before 1,000 m.y. (Cahen, 1970).

There is evidence of stratigraphie control, since theuranium occurrences and main radioactive anomalies of the Shabanand Zambian Copperbelts and the Domes Area appear to berestricted to a particular stratigraphie interval at the base ofor adjacent to the copper-bearing horizons or to occur indifferent isopic zones. The known copper, cobalt, and uraniumdeposits all occur in an interval of sediments 100 to 150 mthick (30-40 m in the Shaban area) at the base of an 8,000 mthick sequence of sediments. In the Shaban area, all the blackuranium oxides and more than 70 percent of the uranium occurrencesare localised at the base of the zone of copper mineralisation.Secondary uranium minerals are found along fault zones and otherfractures in decreasing amounts away from the mineralisedhorizon (Cahen et al. 1971).

Environmental controls are similar in the three areas forboth the copper and the uranium mineralisation, as indicated bythe presence of bioherms in the vicinity of the mineralised zones.It seems likely that the bioherms provided the required condit-ions to cause initial syngenetic precipitation of copper, cobaltand uranium in shallow marginal marine waters. Subsequentenrichment could have taken place during diagenesis. The zoneof mineralisation therefore corresponds to reducing conditionsin the depositional environment as attested by the presence ofcarbonaceous matter in most host rocks. Where oxidizedminerals, such as hematite, rutile and ilmenite, are prominent,uranium mineralisation is weak or absent; there seems to be anirreconcilability between uranium mineralisation and the presenceof some oxides. The host lithologies, therefore, can be said

25

to vary quite considerably but not the chemistry of the deposit-lonal environment which was the vital factor.

A mineral zoning is evident in the Zambian Copperbelt(Garlick 1972) , at Lumwana in the Domes Area (McGregor, 1964) ,and in the Shaban area (Oosterbosch, 1962); from the footwallupward or from the shoreline seaward the sulfide minerals occurin zones represented by chalcocite, bornite, chalcopyrite, andpyrite. In the Shaban area particularly, uranium occurs belowthe copper mineralisation (Cahen, et al. 1971). In the ZambianCopperbelt and Domes Area, lithologie consideration suggestseveral cycles of transgression and regression, with the resultthat the uranium mineralisation occurs at various horizonswithin the zone of copper mineralisation and not exclusively atthe base. An explanation of this distribution could be thatinitially precipitated uranium was again taken into solution asconditions changed from reducing during transgression to oxid-ising during regression.

Regional mineral zoning in the Shaban area has been studiedby Ngongo-Kashisha (1975) who shows that in the southern part ofthe 300 km arc, uranium, copper, cobalt, nickel, monazite, andmagnetite are predominant. In the central part of the arc,there is an increase in the amount of copper and cobalt and adecrease in the amount of uranium, nickel and magnesite, withmonazite present only in traces. Only traces of copper andcobalt are found in the northern part of the area. On a region-al scale in the Domes Area, the higher grades of copperoccurrence, such as Kimale, are almost free from uranium andcopper is absent from richest of the uranium occurrences includ-ing Kawanga and Mitukutulu; however both of these elements arefound in deposits of intermediate grade, such as the Kapijimpangaoccurrence. A similar relationship is apparent in the ZambianCopperbelt, where uranium of good grade occurs in a weak copperzone at Nkana, and in the Shaban Copperbelt area (Oosterbosch,1962). According to syngenetic concepts, the mineral zoningappears to be related to the changing chemical and physicalconditions within the zone of sedimentation as the depth of waterincreased or decreased with the advance or retreat of the shore-line (Garlick, 1972) .

26

Convincing evidence is available for the syngenetic sedim-entary origin of copper mineralisation; it seems that theevident stratigraphie, palaeotopographic and environmentalcontrols require a similar interpretation for the uranium miner-alisation. These conclusions are corroborated by the identicalfluid-inclusion characteristics of the copper-cobalt mineral-isation and uranium-copper-cobalt-nickel mineralisation (Ngongo-Kashisha, 1975), the fluid inclusions would be expected to bedifferent if the uranium association belonged to an independentmagmatic epoch as previously stated by Derricks et al. (1956,1958), as postulated by Oosterbosch (1962) and as generallyaccepted in the geological literature, at least as far as theShinkolobwe, Swambo and Kalongwe deposits are concerned.

Protore-changing processesMany of the uranium occurrences have a typical vein aspect

and all the isotopic ages of the minerals are post-sedimentary.These facts are not inconsistent with the previous conclusionsif the processes which affected the protore are examined.These processes include diagenesis, metamorphism, tectonic events,supergene enrichment and thermal events associated with post-tectonic metamorphism; they modified the protore and convertedit into ore with vein morphology.

In the Shaban area, the effects of metamorphism were slight.In the Domes and Zambian Copperbelt areas, the rocks were re-crystallised and to a certain extent the economic minerals weresegregated into veins which are bounded by barren zones.Generally it seems that, metamorphism only resulted in minoruranium migration over short distances.

A major role in the protore-changing processes was playedby tectonic events. The following generations of uranium miner-alisation are known: >706 m.y. (Shinkolobwe); 670 + 20 m.y.(Shinkolobwe and Swambo); 620 + 10 m.y. (Shinkolobwe, Kalongwe,and Luishya); 555 + 10 m.y. (Kambove West); 536 + 12 (Kimale,Kawanga and Dumbwa) ; 520 _+ 20 m.y. (Nkana, Kansanshi, Mushoshiand Kamoto) ; 468 +_ 15 m.y. (Nkana) ; 356 + 40 m.y. (Luanshya) ;and 235 +_ 30 m.y. (Nkana). In the Shaban area, the range ofmineralisation is from 706 to 520 m.y., whereas in the otherareas the range is from 520 to 235 m.y. The earlier dates apply

27

Table 3 - Paragene t ic of Minera ls Association( m o d i f i e d a f t e r Cahen et a l . , 1971)

Age of minerals ParagenesisMain syngenetic

metal

Ni Co Cu Fe U

670 m . y . ( S w a m b o )

670 and 620 m.y.(Shinkolobwe)

620 m.y. (Kalongwe)

582 m.y. (Kamoto P)

555 m.y.(Kambove)

536 m.y. (Dumbwa,Kimale, Kawanga)

520 m.y. (Nkana)

520 m.y. (Kansanshi)

520 m.y. (Musoshi)

520 m.y. (Kamoto P)

468 m.y. (Nkana)

365 m.y. (Luanshya)

(a)Uranmite;pyrite,mona-zite,chlonte ;vaesite,siegenite;chalcopyrite

(a)Magnesite;uraninite; py-rite .molybdenite.rnona-zite;vaesite,cattierite,siegenite;chalcopynte.

(a)Uraninite; pyrite, chlo-rite;carrolite;bornite,chalcopyrite

(a)Uraninite;chlorite;car-rolite;bornite.chalco-pyrite

(a)Uraninite;chlorite;car-rolite;bornite,chalco-pyrite

(b)Uraninite,muscovite, apa-t ite.molybdenite,chlor-ite

(b)Uraninite,me Ionite, bor-nite,chalcopyrite, dige-nite.caleite,chalcocite,quartz.biotite,albite,chlorite;molybdenite

(b)Brannerite,chalcopyrite,calcite,pyrite,rutilebornite.molybdenite

(b)Albite.uraninite,pyrite,carbonate,quartz,moly-bdenite

(b)Uraninite.carbonate.pyrite

(b)Uraninite.carbonate,pyrite

(b)Pitchblende,calcite,dolomite

235 m.y. (Nkana) (b)Pitchblende

(a) Paragenetic order; (b) paragenetic association; P = Principal

28

to the Shaban area where older generations were isolated inup-faulted blocks and preserved from the successive thermaleffects following the tectonism. In the Zambian Copperbelt andDomes Areas, where the original mineralised zones remained closeto the basement, the uranium minerals were exposed to thesucceeding thermal effects and repeated mobilisation and redepos-ition took place; consequently, more uranium generations exist.According to Cahen et al. (1971) the uranium generations of 706,670 + 20, and 620 +_ 10 m.y. are connected with the first, secondand third phases, respectively, of the Lufilian Orogeny. Allthree generations of uranium mineralisation are found atShinlolobwe. The 670 m.y. generation represents an epigeneticreworking of the 706 m.y. mineralisation, and the 620 m.y.generation is a reworking of the 670 m.y. mineralisation. The520 + 20 m.y. generation has been related to a large thermalevent associated with post-tectonic metamorphism (Belliere, 1961)which affected the three areas but had its greatest effect inthe southeast. This major thermal event has been confirmed bypotassium-argon age determinations and other geologicalconsiderations. Other lesser thermal influences up to 235 m.y.ago caused further redistribution of uranium mineralisation.Isotopic ages and their references to geologic events can bequestionable, but metamorphic events, three main tectonic phases,one major thermal event, and supergene alteration are well knownand documented facts in the Katanga System geology.

The paragenetic association of ore minerals confirms thehypothesis that the number of elements decreases after eachremobilisation. All of the main ore metals, nickel, cobalt,copper, iron and uranium are present at the 670 m.y. stage. At620 m.y. nickel has disappeared and cobalt, copper and irondisappear successively at the 520 m.y., 468 m.y., and 365 m.y.stages leaving only the highly mobile uranium at the final 235 m.ystage (Table 3). In the paragenetic order uranium is normallythe first metal to appear. The gradual impoverishment of theparagenetic associations after each remobilisation, withoutintroduction of new metals, supports the hypothesis of oneoriginal mineralisation.

Supergene alteration affected the mineralisation to a greateror lesser extent. In the Shaban area the uranium minerals

29

occurring in up-faulted blocks were leached out and reconcentratednear the water table. Francois (1974) states that intenseweathering often cancels form, original composition, and originof the mineralised bodies. The Shinkolobwe deposit is onesuch example. In the Zambian Copperbelt and Domes Areas,uranium enrichment occurs in structures near the water table orin fracture fillings.

The newly described uranium occurrences of the Domes Areasupport the view of Cahen et al., (1971) that the present uraniumconcentrations are the results of remobilisation of the originalmineralisation, reflecting well-documented geological events withno endogenetic processes being involved.

As far as the uranium origin is concerned, no specificinvestigation has been carried out. Nevertheless it can beassumed, as it has been done for the copper, that it came fromthe weathering of the basement although a possible contributionfrom submarine volcanism cannot be completely ruled out. It ishowever believed that whatever the source of uranium could havebeen, the protore formation has been syngenetic for both metals,uranium and copper.

Analogy with other mineralisationThe uranium occurrences of the Katanga System show certain

similarities with occurrences elsewhere in the world. Referenceis here made to the protore and not to the subsequent remobilis-ation events. A rough comparison can be attempted with depositsof the Northern Territory (Australia) and Saskatchewan (Canada).In the three regions the uranium bearing sediments were depos-ited at the base of a marginal marine sequence in a basinbordered by granites, gneisses, migmatites, and other metamorphicrocks. The presence of bioherms in Australian deposits(Needham & Stuart-Smith, 1976) and in the Zambian-Zaireanoccurrences indicates a euphotic zone, i.e. a shallow water orlagoonal environment. The presence of carbonaceous matter,adjacent to or intercalated with the mineralisation, suggests areducing marine environment. The Olympic Dam deposit on theRoxby Downs Pastoral Station, South Australia, appears to have simil-arities to the deposits associated with the Katanga System (seeRoberts & Hudson, this volume). The original mineralisation

30

subsequently underwent different geological histories, not onlyin the different regions but also in different places of the sameregion, leading to the present type of the mineralisation.

CONCLUSIONSUranium and copper mineralisation is present at the base of

Katanga System. The mineralisation is considered to be closelyrelated to a marine transgression over an ancient crystallinebasement. An originally stratiform mineralisation was changedby later tectonic, metamorphic and thermal events, giving rise tothe relatively high-grade vein-type occurrences now seen whichrepresent epigenetic enrichment of the syngenetic protore.

In the Shaban Copperbelt area, the stratiform nature of theuranium mineralisation, closely associated with the southernfaciès of the Lower Roan Group, provides a target for explorationalong 300 km of the mineralised arc. The probability of findingsub-outcropping mineralisation is minimal, but new technology -such as alpha-detection could be tested in the search for hiddenconcentrations of ore. Although large deposits of high-gradeuranium mineralisation are unlikely to be found, low-grade occurr-ences could provide valuable by-product uranium for the copperindustry. In the Zambian Copperbelt and Domes Areas, where theuranium isopic zones of the Lower Roan Group are not welllocalised, it may be necessary to elucidate the palaeogeographyand palaeoenvironment of the known uranium occurrences.

Although the geology of the copper deposits has beenthoroughly investigated, less attention has been given to thepeculiar conditions of uranium mineralisation. It has beenshown at Shinkolobwe, Nkana, Kawanga and Mitukutulu that richconcentrations of copper and uranium are mutually exclusive, sothat exploration techniques for copper detection are differentfrom those required for the location of uranium.

It is difficult to evaluate the possible uranium reservesof the Katanga System; at the moment exploration is still goingon in all the three areas involving international companies.Tentatively the total amount of U000 concentration present at

J Othe base of the Katanga System could be estimated between50,000 and 100,000 tonnes.

31

REFERENCESAPPLETON, J.D. in press: The geology of the Kabompo Gorge area.

Exploration of degree sheet 1225 NW quarter: Zambia Geol.Survey, Kept. 40.

ARTHURS, J.W., 1974: The geology of the Solwezi area. Explor-ation of degree sheets 1226 NW quarter, and 1126, part ofSW quarter: Zambia Geol. Survey, Rept. 36_, 48.

BELLIERE, J., 1961: Manifestations métamorphiques dans la regiond'Elisabethville: Pub. Etat d'Elisabethville, Katanga,July 1, 175-179.

BENHAM, D.G., GREIG, D.D. and NINK, B.W., 1976: Copper occurr-ences of the Mombezhi area northwestern Zambia: Leon. Geol.,J71, 433-442.

BINDA, P.L. and MULGREW, T.R., 1974: Stratigraphy of copperoccurrences in the Zambian Copperbelt, in Bartholome, P.,éd., Gisements stratiformes et provinces cuprifères:Liège, Soc. Geol. Belgique, 180-201.

CAHEN, L., 1970: Etat actual de la Geochronologle du Katangien:Mus. Royal Afrique Centre, Annales, Ser. in 80, Sei. Geol.6J> 7-14.

_________ 1974: Geological background to the copper-bearing strataof southern Shaba (Zaire) in Bartholome, P., ed. Gisementsstratiformes et provinces cuprifères: Liège, Soc. Geol.Belgique, 57-77.

CAHLN, L., PASTLELS, P., LEDENT, D.r BOURGUILLOT, R., VAN WAMBEKE,L. and EBLRHARDT, P., 1961: Recherches sur l'âge absolu desminéralisations uraniferes du Katanga et de Rodesie du Nord:Mus. Royal Afrique Centre Annales, 41, 1-54.

CAHEN, L., FRANÇOIS, A. and LEDENT, D., 1971: Sur l'âge desuraninites de Kambove ouest et de Kamoto principal et révisiondes connaissances relatives aux minéralisations uraniferesdu Katanga et au Copperbelt de Zambia: Soc. Geol. Belgique,Annales, 94, 185-198.

CAILTEUX, S., 1976: Corrélation stratigraphique des sédimentsde âge Roan du Shaba et de Zambie. Soc. Geol. Belgique,Annales, 99, 31-45.

32

DAHLKAMP, F.J., 1978: Classification of uranium deposits:Mineralium Deposita, 13, 83-104.

DERRICKS, J.J. and OOSTERBOSCH, R., 1958: The Swambo and Kalongwedeposits compared to Shinkolobwe: Contribution to the studyof Katanga uranium in United Nations Internat. Conf. on thepeaceful uses of atomic energy, 2nd, Geneva, Proc., 2.Survey raw material resources: New York, United Nations,663-695.

DERRICKS, J.J. and VAES, J.F., 1956: The Shinkolobwe uraniumdeposit. Current status of our geologic and metallogenicknowledge in Internat. Conf. on the peaceful uses of atomicenergy, 1st, Geneva, Proc., _6. Geology of uranium and thorium:New York, United Nations, 94-128.

DEMESMAEKER, G., FRANCOIS, A. and OOSTERBOSCH, R., 1962: La tec-tonique des gisements cuprifères stratiformes du Katanga, inLombard, J., and Nicolini, R., eds., Gisements stratiformesde cuivre en Afrique, Paris, Assoc. des Services Gëbl.,47-115.

EDWARDS, R.A., 1974: The geology of the West Lunga River area.Explanation of degree sheet 1224, NW quarter: Zambia, Geol.Survey Kept. 43, 86.

FRANCOIS, A., 1974: Stratigraphie tectonique et minéralisationsdans l'arc cuprieferes du Shaba, in Bartholomé', P., éd.Gisements stratiformes et provinces cuprifères: Liège,Centenaire Soc. Gé"ol. Belgique, 79-101.

GARLICK, W.G., 1972: Sedimentary environment of Zambian copperdeposition: Geologie Mijnbouw, 51, 277-298.

KLINCK, B.A. in press: The geology of the Kabompo Dome area.Exploration of degree sheet 1224, NE quarter: Zambia, Geol.Survey Kept. 44.

MCGREGOR, J.A., 1964: The Lumwana copper prospect in Zambia:Unpub disser., Grahamstown, South Africa, Rhodes Univ., 171.

MENDELSOHN, F., 1961: The geology of the northern RhodesianCopperbelt: London, Macdonald and Co., 523.

NEEDHAM, R.S., and STUART-SMITH, P.G., 1976: The Cahill Format-ion-host to uranium deposits in the Alligator River uraniumfield. Australia: BMRJ, 1, 321-333.

33

NGONGO-KASHISHA, 1975: Sur la similitude entre les gisementsuranifères (type Shinkolobwe) et les gisements cuprifères(type Kamoto) au Shaba Zaïre: Soc. Gë*ol. Belgique,Annales, 98, 449-462.

OOSTLRBOSCII, R., 1962: Les minéralisations dans le système deRoan au Katanga, in Lombard, J. and Nicolini, R., eds.,Gisements stratiformes de cuivre en Afrique: Paris, Assoc,des Services Géol., 71-136.

THOREAU, J., et du TRILU DE TERDONCK, R., 1933: Le gîte d'ura-nium de Shinkolobwe-Kasolo (Katanga): Extrait des Mem.publiés par l'Inst. Colonial Belge, J_, 18-64.

34

URANIUM IN EARLY PROTEROZOICAILLIK GROUP, LABRADOR

S.S. GANDHIGeological Survey of Canada,Ottawa, Ontario,Canada

Abstract

Uranium occurrences are widely distributed in tuffaceous-argillaceous beds and rhyolitic volcanic rocks of the Aillik Group ofEarly Proterozoic age. They include partially developed Kitts andMichelin deposits, containing 1560 and 7366 t U respectively.Uranium occurs mainly as disseminated fine pitchblende grains inlenticular concordant zones, and rarely as coarse pitchblendeaggregates and thin veinlets.

The occurrences in the argillaceous-tuffaceous beds, includingthe Kitts deposit, are within a steeply dipping stratigraphie zoneless than 50 m thick and over 20 km long. They are interpreted as1syngenetic', with a source of uranium in an active felsic volcanicfield in adjacent terrain. The occurrences in the rhyolitic rocks,including the Michelin deposit, are in elongate structural-stratigraphie belts or zones, and are characterized by a pronouncedenrichment of soda and depletion of potash associated withmineralization. They are interpreted as 'epigenetic', relatedclosely in time to the felsic volcanism that formed the host rocks1767±4 Ma ago accoring to a Rb/Sr isochron date.

The mineralized zones were affected to a varying degree byHudsonian deformation and metamorphism. Some of the pitchblendesyielded nearly concordant dates close to 1750 Ma. Others from thesouth near the 'Grenville Front1 indicate disturbance of isotopicequilibrium at the time of Grenville orogeny approximately 1100 Maago. This is corroborated by the isotopic composition of lead(Pb: 206/204: 532.45; 207/204:97.163; 208/204:36.726) in galena fromthe Michelin deposit. The overwhelmingly dominant uranogeniccomponent of this lead has a Pb207/Pb206 ratio of 0.1585 consistentwith its extraction at 1180 Ma from 1767 Ma old pitchblende.

35

INTRODUCTION

The uranium district of central Labrador coast has been exploredintermittently since the first discovery of a pitchblende vein therein 1954. Exploration to date has located a number of occurrences ofuranium and molybdenum, including a few promising deposits andprospects although none have been productive as yet.

The first comprehensive study of the geological setting andgenetic aspects of the uranium occurrences in the district waspublished by the writer in 1978. Since then additional explorationwork has been carried out, and several geological studies have beendone, notably by Clark (1979), Evans (1980), Kontak (1980), White andMartin (1980), Bailey (1979), Gower et al. (1982) and Ryan andKay (1982). These have added valuable data and new perspectives.Many problems however remain unresolved in this geologically complexand economically promising area. Divergent opinions have beenexpressed on several important aspects of geology of the area assummarized by Gower et al. (ibid). An attempt is made here toemphasize those aspects that are relevant to the genesis of theuranium deposits.

REGIONAL GEOLOGY

Early Proterozoic Aillik Group was deposited on an Archeanbanded gneiss complex (Figs. 1 and 2) , and was deformed into east-northeast to north trending folds, metamorphosed in the range ofupper greenschist to lower amphibolite faciès, and intruded bygranites, during Hudsonian cycle approximately 1750 to 1550 Ma ago(Gandhi et al., 1969, Gandhi, 1978). To the south, these rocks arebounded by the 'Grenville Front', which is the northern limit of anextensive area affected strongly by deformation, metamorphism andintrusions approximately 1000 to 1200 Ma ago (Stevenson, 1970).Position and nature of the 'Grenville Front' are not well known inthis region because of scarcity of outcrops due to extensive glacialdrift cover.

Aillik GroupThe Aillik Group is dominantly a bimodal sequence of basaltic

and rhyolitic volcanic rocks interbedded with clastic sediments(Fig. 3). Rhyolitic volcanic rocks predominate in the upper part ofthe sequence. Faciès variations over short distances are common, andstratigraphie relations are obscured by varying degree of deformationand intrusions. Thickness of the group is estimated as in the order

36

J | j Hadrynianl u supracrusi

C1~T i Grenvtlle gnet.I I J gramfe

(a/s

eiss

^_- -j3 Neoheliktan'----"] supracrusfa/s

Elsonianintrusions

P* *I __ ^__

E - ~] Paleohelikian___ _J supracrustals

»pheb/ansupracrustalsgneiss granite

n Archean gneiss_L J granite

56"

Figure L Location map of the study area and generalgeological features of the surrounding region of theeastern Canadian Shield (from Gandhi, 1978).

of 7600 m (Gandhi et al., 1969) or 8500 m (Clark, 1973J . It isinformally divided into the lower and upper Aillik groups.

The lower Aillik group is exposed only in the northwestern partof the study area along the Kitts-Post Hill belt near Kaipokok Bay(Figs. 2 and 3). On the southeast side of the bay, it isapproximately 2700 m thick, generally dips steeply to the east-southeast, and is overlain by the upper Aillik group which is up to900 m thick in this belt (Marten, 1977). Contacts of the sequencewith the Archean basement to the northwest are faulted and sheared.On the southeastern side, the basement is thrust over the sequence.The thrust zone is obscured in part by intrusion of granodioritegneiss and in part by extensive drift cover.

37

Figure 2. General geological map showing the Aillik Group and uranium occurrences in thegroup, Kaipokok Bay-Big River area, Labrador (from Gandhi, 1978, with minor modifications).Uranium occurrences:

10: Knife, 19:11: Nash East, 20:12: Nash Main, 21:13: Nash West, 22:14: Witch, 23:15: 'J' & 'B1, 24:16: Anna Lake North, 25:17: Michelin, 26:18: Rainbow,

1:2:3:4:5:6:7:8:9:

Long Island,Anderson Ridge,Kitts 'A' zone,Kitts 'B' and 'C' zones,Kiwi,South,Punch Lake,Gear,Inda,

Burnt Lake,Emben,Winter Lake,Pitch Lake,Round Pond-FaHs Lake,John Micheiin,Sum'!,Cape Makkovikmolybdenite deposit

The basal unit of the lower Aillik group is metasedimentary, andis comprised predominantly of micaceous schists and gneisses derivedfrom siltstones and sandstones, with pyritic and graphitic horizons.It is approximately 800 m thick. A basaltic unit capping Post Hill,and extending to the southwest across the Kaipokok Bay, is underlainand overlain by some of the metasedimentary rocks, and is up to1000 m thick (Fig. 3). The metasedimentary rocks from Kitts to Post

38

UPPERAILLIK

GROUP

LOWERAILLIKGROUP

KITTS-POSTHILL BELT

WHITE BEARMOUNTAIN-WALKER

LAKE AREA

Quartz feldspar porphyry,extrusive and intrusive

Rhyolite flows, tuffs,agglomerate, ignimbrite;interbedded volcaniclasticsediments

Conglomerate, agglomerate,sandstone, sittstone, bandedquartzite; mostly felsicvolcaniclastic sediments;minor shale and limestone

Basaltic flows, massive andpillowed; mafic tuffs

Sittstone, sandstone, shale;some pyritic and graphiticbeds

Figure 3. Generalized lithostratigraphy of the Aillik Group in three partsof the study area and tentative correlations.

Hill are overlain by well-exposed massive and pillowed basalt flowsand mafic tuffs, approximately 900 m in thickness. The basalts rangefrom marginally subalkaline (tholeiitic) to mildly alkaline, withsoda predominant over potash (Evans, 1980; White and Martin, 1980).

The thick basaltic unit is overlain by a relatively thinstratigraphie zone or unit, less than 50 m thick, and referred tohere as 'argillaceous-tuffaceous unit1, which is an important host ofuranium occurrences. It includes chert and argillite to thenortheast, and laminated tuffs and tuffaceous siltstones to thesouthwest, as shown in Figure 4. It is described further under 'HostRocks' of uranium occurrences. It grades into overlying felsictuffaceous siltstones and banded quartzite, in the southwestern partof the belt, and into interbedded mafic tuffs, siliceous andcalcareous beds in the middle part of the belt, and is overlain by

39

NE

-^V

V

V

V

V

V V V V

' : ^^ r i ^^v

v v v v v v v v

J~*~^ V"*"-*1 1- ' -^"v^ V

V V V V

v v v v

V V V V

v vV

V

V

V

V

V

V V

V

V V V

V V V

V

\f V

Anderson Kilts A B and South Gear Inda Knife East Nash NashRidge C zones showing showing zones zone Nash Mam west

Showing rone zone zone

Boundary ofargillaceous-tutfaceous unit

Felsic tuffs,siltstones, sandstone,banded quartzite

Mafic tuffs, siliceous andcalcareous beds

Intertedded siltstonesand laminated tuffs

Laminated tuffs,varicoloured

Argillite, pyrrhotrte-nchlenses

Cherty quartzite, localmagnetite layers

Basaltic flows, massiveand pillowed

Stratiform zone ofdisseminated uranium

Figure 4. Relationship of stratigraphy and uranium occurrences in the Kitts-Post Hill belt,Labrador (modified after Gandhi, 1978).

pillowed basalt to the north. The laminated tuffs and tuffaceoussiltstones contain abundant felsic layers that are interpreted asderived from rhyolitic tuffs. Influx of this felsic material marksthe beginning of felsic volcanism of the upper Aillik group, hencethe argillaceous-tuf f aceous unit is interpreted here as the basalunit of the upper Aillik group. Stratigraphically equivalent rocksare not exposed in the remainder of the area of the Aillik Group tothe east and south of the belt.

The argillaceous-tuffaceous unit is overlain by an assemblage ofinterbedded sandstone, siltstone, waterlain felsic tuffs, finegrained banded quartzite and calcareous beds, with an aggregatethickness of approximately 300 m. They dip steeply to the east-southeast. To the southeast, a quartz feldspar porphyry dykestriking parallel to the beds and upto 300 m wide, separates the bedsfrom a conglomerate-agglomerate unit which is approximately 260 mthick. East of this unit are a mafic volcanic lens, and rhyoliticflows and tuffs that are southern continuation of the thick andextensive rhyolitic unit near Makkovik Bay (Gandhi et al., 1969;Gandhi, 1978) . They may in part be interbedded with the sedimentaryassemblage and the conglomerate-agglomerate unit. The quartzfeldspar porphyry is related to the felsic volcanic activity of theupper Aillik group.

40

Appearance of the sedimentary assemblage deposited in shallowwater, with contributions from the felsic volcanic activity, in thesequence of Kitts-Post Hill belt indicates a significant change inthe depositional environments and provenance of the basincharacterized by extensive pillow basalts and deeper water sedimentsof the lower Aillik group in the earlier stages (Gandhi, 1978,p. 1497).

Rocks of the upper Aillik group are extensive in the Aillik-Makkovik area and the White Bear Mountain-Walker Lake area, to thenortheast and south of the Kitts-Post Hill belt respectively.Generalized lithostratigraphic sequence in the two areas is shown inFigure 3, but there are no marker beds to make definitivecorrelations from one area to the other, and between these areas andthe Kitts-Post Hill belt. A few tentative correlations however aresuggested as shown in the figure.

The oldest rocks in the two areas, exposed in the cores of majoranticlines (Fig. 2), are interbedded sandstones, silstones, bandedquartzites with some magnetite-rich beds and intraformationalconglomerates, totalling over 1000 m in thickness. At a fewlocalities shales, calcareous beds, and a chert-magnetite-amphiboleiron formation, are present (Gandhi, 1978). Some of the beds in theunit show small scale cross-bedding and graded bedding. Theconglomerates contain abundant rhyolitic clasts along with otherlithologies, and at places grade into, or are interbedded with,rhyolitic agglomerate.

Basaltic volcanic rocks occur at the upper boundary of thesedimentary unit, and also in rhyolitic sequence above it. They aremore abundant in the Aillik-Makkovik area than in the White BearMountain-Walker Lake area. They are relatively thin, with a maximumthickness of 150 m, compared to the basaltic units in the lowerAillik group, and are stratigraphically higher than them.

Rhyolitic volcanic and volcaniclastic rocks, and geneticallyrelated thick sill-like bodies of quartz feldspar porphyry, in partextrusive and in part intrusive, extends over most of the Aillik-Makkovik and White Bear Mountain-Walker Lake areas, and attain anaggregate thickness of over 3500 m in both the areas (Figs. 2 and 3).The rhyolitic rocks are varied in character, and include feldsparporpnyritic, subporphyritic and nonporphyritic flows and tuffs, finegrained massive and flow-banded rhyolites, fine to coarsepyroclastics, air-fall tuffs and ash-flow tuffs or ignimbrites, andlaharic flows. Hence the volcanic activity was partly if notlargely, subaerial. Some of the tuffs are reworked by surface

41

waters. Some lenses and beds of calcareous and siliciclasticsediments are also interbedded with the rhyolitic rocks.

The quartz feldspar porphyry contains coarse subhedral toeuhedral crystals of feldspar and blue quartz set in a fine grainedquartzo-feldspathic matrix, and is deformed to a varying degree. Inthe White Bear Mountain-Walker Lake area, the porphyritic unit isapproximately 1500 m thick, and is generally massive to weaklyfoliated, but strong foliation and lineation are developed at manyplaces, and obscure contacts with the rhyolitic rocks. In someoutcrops, the porphyritic rock apparently grades into the rhyoliticrocks that include ignimbrites (Stevenson, 1970; Watson-White, 1976;Gandhi, 1978; Bailey, 1979). In addition to the major unit, thereare dykes and irregular small bodies of relatively little deformedquartz feldspar porphyry in the area, and these may represent youngerintrusions. In the Aillik-Makkovik area, a quartz feldsparporphyritic gneiss unit is extensive and has an estimated thicknessof 3050 m (Gandhi et al., 1969). This may however be an over-estimate because the Aillik Group as a whole in this area is tightlyfolded, but the folding is not apparent in this essentially uniformunit. Its gneissic texture is interpreted to be due torecrystallization of quartz feldspar porphyry, with development ofquartz and hornblende streaks and growth and deformation ofporphyroblasts of feldspar (Clark, 1971, 1973; Gandhi, 1978; Whiteand Martin, 1980). It has sharp and generally concordant boundarieswith other rhyolitic rocks of the area. It may be a sill or anextrusive unit.

An assemblage of conglomerate, siltstones, banded quartzites andrhyolitic rocks exposed at Big Bight on the east side of the Aillik-Makkovik area may be the youngest part of the Aillik Group in thestudy area.

The rhyolitic rocks of the upper Aillik group are marginallysubalkaline, and show effects of alkali metasomatism as discussedlater (Watson-White, 1976; Gandhi, 1978; Evans, 1980; White andMartin, 1980; Gower et al., 1982). Rb/Sr isotopic agedeterminations on the rhyolitic rocks reported by Watson-White (1976), Gandhi (1978) and Kontak (1980) are 1676 ± 16 Ma(4 point isochron, recalculated), 1659 Ma (errorchron, unreliable)and 1767 ± 4 Ma (5 point isochron) respectively. These are regardedas minimum ages.

42

IntrusionsRocks intrusive into the Aillik Group include pre-kinematic

amphibolitic dykes, small gabbro bodies, and quartz feldspar porphyrydykes and irregular bodies; large synkinematic intrusions of gneissicgranodiorite to monzonite, referred to as Long Island gneiss, andlenses of granite gneiss; late kinematic to post-kinematic graniteplutons, the gabbro-diorite differentiated intrusion of Adlavik Bay;and younger diorite-syenite, diabase and lamprophyre dykes. Thequartz feldspar porphyry intrusions and some of the granitic plutonsare believed to be epizonal intrusions related to the rhyoliticvolcanics of the upper Aillik group.

The gneissic granodiorite of Long Island yielded a K-Ar date of1832 ± 58 Ma (Gandhi et al., 1969) which is anomalous with respect tothe Rb/Sr isotopic dates on the rhyolites intruded by it. This maybe due to excess argon inherited from basement rocks and/or from thenumerous mafic inclusions of unknown origin. Other K-Ar, Ar-Ar, andRb-Sr dates on rocks of the Aillik Group, metamorphosed dykes,Adlavik gabbro and granitic intrusions range from 1750 to 1500 Ma(Gandhi et al., 1969, Gower et al., 1982). They suggest a longperiod of Hudsonian intrusive activity during the late Aphebian toearly Paleohelikian time.

A large gabbro sheet-like body trending east-northeast near thesouthern boundary of the study area (Fig. 2) is one of the HelikianMichael gabbro intrusions which are extensive in the Grenvilleprovince (Stevenson, 1970; Gower et al., 1982). Other younger dykesin the study area range in age from 1000 to 400 Ma (Gandhi et al.,1969; Gandhi, 1978; Gower et al., 1982).

StructureIn the Kitts-Post Hill belt, the unconformity between the

Archean basement and the Aillik Group was obliterated by intensetectonic sliding. Furthermore, along the east side of this belt, thebasement is thrust over the group. Marten (1977) interpreted sixstages of Hudsonian deformation: two early stages of thrust-slicingand tectonic interleaving in the belt, followed by three foldingevents in the Aillik Group, one major and two minor ones, and a latestage of faulting. On the west side of Kaipokok Bay, rocks of thelower Aillik group display similar polyphase deformation, accordingto Ryan and Kay (1982) who, in addition, suggested the possibility ofpost-Archean migmatization in the basement prior to the tectonicsliding.

43

In the Aillik-Makkovik area, Gandhi et al. (1969) recognizedtwo broad, open anticlinal structures with synkinematic granitegneiss domes at the core, and adjacent tightly folded zones withlocal shearing and mylonitization. More detailed structural studiesby Clark (1971, 1973, 1979) have shown four stages of deformation, ofwhich the second was the most important one. It resulted in thenortherly trending upright folds, a subvertical schistosity andsubhorizontal mineral lineation in the plane of schistosity.

In the White Bear Mountain-Walker Lake area, sedimentary unitsalong the north side are tightly folded with axial planes nearlyvertical or overturned to the northwest (Fig. 2), but in therhyolitic rocks to the south the folding is obscure because of lackof marker units. Folding on a small scale, however, is observed atmany places. The degree of development of foliation in the rhyolitesvaries considerably across the regional east-northeast trend (Gandhi,1978) . Foliation commonly dips 50 to 60 degrees to the south-southeast, subparallel to recognizable lithologie boundaries in therhyolites. En echelon shear zones and mylonitized zones are present.Bailey (1979) has interpreted a number of faults subparallel to theregional trend but relative movements along them are not known.

TectonicsThe Aillik Group is affected by the Hudsonian structural,

metamorphic and intrusive events that resemble an orogenic cycle, butits lithological characters, in particular the abundant rhyoliticvolcanics, are interpreted by Watson-White (1976), Gandhi (1978), andWhite and Martin (1980) to indicate a nonorogenic, continental riftenvironment of deposition rather than a classical géosynclinalaccumulation. The rhyolitic rocks are marginally subalkaline and atleast some of them are subaerial. They predominate over the basaltsin a bimodal sequence of volcanics which lacks andesitic rocks. Theabsence of andésites is an important consideration in interpretationof the tectonic setting.

Basalts of the lower Aillik group are considered a part of thebimodal volcanic sequence of the Aillik Group as a whole by White andMartin (1980), whereas Clark (1979) and Wardle and Bailey (1981)proposed a fundamentally different tectonic setting for the lowerAillik group from that of the upper Aillik group, and proposed amajor angular unconformity separating the two groups. The lowerAillik group, according to these authors, was deposited in a shelfenvironment transitional southwards into deeper water environment,

44

similar to that of Labrador Trough. Clark (ibid) suggested thatdeposition of the lower Aillik group was followed by collision of twocratons, erosion of the group in the Aillik-Makkovik area,development of transcurrent faults, major felsic volcanism andplutonism and continued deformation. Wardle and Bailey (ibid) on theother hand favoured a rift environment for the upper Aillik group,but regarded the apparent synorogenic timing of its felsic volcanismas problematic.

The possibility of a major unconformity between the lower andupper Aillik groups was earlier considered by Gandhi (1976) but fieldevidence for it was not found. Wardle and Bailey (ibid) inferred anangular unconformity from what they considered as relatively moreintense polyphase deformation and higher grade of metamorphism in thelower Aillik group in comparison to simple structural style and lowergrade of metamorphism in the upper Aillik group. The differences,however, are not as pronounced as they suggest, and can be attributedto the original lithological differences and to inhomogeneousdeformation and metamorphism during a single 'orogenic' type of cycleas proposed by Gandhi et al. (1969) and Marten (1977). Wardle andBailey (ibid) also referred to a polymict conglomerate unit in theKitts-Post Hill belt, described by Marten (ibid) as the base of theupper Aillik group. According to Gandhi (1970, 1978) andEvans (1980) however, the conglomerate is structurally andstratigraphically above the east dipping siltstone-banded quartziteunit, which contains abundant waterlain felsic tuffs and conformablyoverlies the uranium-bearing argillaceous-tuffaceous unit. Theconglomerate is thus an intraformational unit in the upper Aillikgroup rather than a basal conglomerate marking a major unconformity.Southern part of this 260 m thick and over 9 km long unit isdescribed as metamorphosed rhyolitic agglomerate by Burbidge (1977).Evans (ibid) interpreted the unit as a submarine lahar. In any case,similar agglomeratic and conglomeratic intraformational units alsooccur elsewhere in the upper Aillik group.

Some structural readjustment of Hudsonian faults and shears inthe proximity of the 'Grenville Front' is likely to have occurredduring the Grenvillian events (Gandhi, 1978). For example, anAr/Ar date of 1004 ± 5 Ma on biotite in a sample from a shear zoneobtained by Archibald and Farrar as reported in Gower et al. (1982,p. 46), indicates Grenvillian effects. Gower et al. (ibid) inferredone major east-west fault extending from the head of Kaipokok Bayalong the Adlavik Brook to the mouth of the Big River, and a dextraldisplacement of 18 km along it, and regarded it as Grenvillian in

45

age. Bailey (1979) has interpreted the broad mylonitized zones andnearly vertical L fabrics in the White Bear Mountain-Walker Lake areaas Grenvillian developments along Hudsonian normal faults. Shearedand mylonitized zones however occur as far north as Cape Makkovikwhere Grenvillian effects are likely to be negligible, and aretherefore regarded here as Hudsonian structures. Presence ofmuscovite in some of the rocks south of the Adlavik Brook andalteration of the post-Hudsonian east-west trending gabbro sheet areattributed to the Grenvillian metamorphism by Gower et al. (1982).

URANIUM OCCURENCES

Over 70 uranium occurrences* or groups of occurrences,containing pitchblende with negligible thorium, are known in rocks ofthe Aillik Group in the study area (Fig. 2), and some of these areeconomically promising prospects. Several pegmatitic uranium andthorium occurrences are found in the Archean basement complex and ingranites intrusive into the Aillik Group. In addition, there are anumber of molybdenite, pyrite and fluorite occurrences in the AillikGroup, but only a few of them are closely associated with the uraniumoccurences in the group. These minerals however are common in thepegmatites associated with the intrusive granites.

DistributionMost of the uranium occurrences in the Aillik Group are located

in four elongate stratigraphie-structural belts or zones, namely theKitts-Post Hill belt, Cape Makkovik-Monkey Hill belt, Falls Lake-Shoal Lake-Bernard Lake belt, and the White Bear Mountain-Walker Lakebelt (Fig. 2) as described by Gandhi (1978). Those in the Kitts-PostHill belt are hosted by a stratigraphie zone of argillaceous andtuffaceous beds, less than 50 m thick and traceable for over 20 kmalong the strike. In the other three belts, the host rocks arerhyolitic flows and tuffs.

Shape and SizeMost of the uranium occurrences are lenses or groups of echelon

lenses of disseminated uranium minerals, subparallel to themoderately to steeply dipping host beds or lithologie units, and to

* Uranium concentration of approximately 0.01 per cent over 10 mstrike length, or greater, detected radiometrically and/or byanalyses of representative samples.

46

Table 1: Approximate tonnage and average grade of important uranium andmolybdenum deposits in the Kaipokok Bay-Big River area,Labrador

Deposit orprospect (no.)

Kitts (3,4)Inda (9)Nash Main (12)Gear (8)Michelin (17)Rainbow (18)Burnt Lake (19)Sunil (25)

Tonnes347,000496,000216,00033,000

7,366,00090,000146,000344,000

Average grade(per cent U)0.4490.1190.1870.1020.1000.1270.0670.024 +

Total U(tonnes)

156059040434

73661449882

Cape Makkovik (26) 2,000,0000.102% MoS20.250% MoS2+locally upto 0.004% U

Data from Gower et al., 1982, p. 61, and A.A. Burgoyne, Brinco MiningLimited, Vancouver, British Columbia, personal communications, 1984.Numbers in bracket: Location number in Fig. 2.Uranium resources in the Kitts, Nash and Michelin deposits areregarded as 'Estimated Additional Resoruces1 in high price categoryin terms of NBA/IAEA classiciation. Estimates for the Cape Makkovikmolybdenum deposit include 30 per cent dilution by barren dykes.

the regional structural trend. The lenses are a few tens to a fewhundreds of metres long in both the strike and dip directions, and upto a few metres thick. Elsewhere uranium minerals are disseminatedirregularly and/or are confined to veins of variable orientation.High-grade veins and aggregates do occur, but they are rare.

Exploration to-date has outlined a small high-grade deposit,namely the Kitts deposit, and a larger, lower grade deposit, namelythe Michelin deposit, and in addition a few prospects, as listed inTable 1. There are a number of other occurrences but they areinsufficiently explored to arrive at their grade and tonnage, andmost of them appear to be smaller or of lower grade than the oneslisted in the table.

Host RocksA distinctive argillaceous-tuffaceous unit along the Kitts-Post

Hill belt and the rhyolitic rocks east of this belt are the hosts ofuranium occurrences in the Aillik Group (Figs. 2 and 4). Recognition

47

of the regional stratigraphie control of uranium mineralization inthe Kitts-Post Hill belt was the most important guide in thediscovery and exploration of prospects along the belt (Gandhi, 1970,1977) , and should have an impact on further exploration.Stratigraphie control in the rhyolitic sequence is less evident, duemainly to the lack of marker units, but three major stratigraphie-structural zones or belts have been recognized as favourable ones,namely the White Bear Mountain-Walker Lake belt, the Cape Makkovik-Monkey Hill belt, and the Falls Lake-Shoal Lake-Bernard Lake belt(Gandhi, 1970, 1978). Exploration efforts in the past wereconcentrated along these belts.

Argillaceous-tuffaceous unit (Kitts-Post Hill Belt): This unit isless than 50 m thick and is continuous along the strike for 20 kmfrom the Kitts deposit to the Nash West showing (Figs. 2 and 4).Lithological variations are common along and across the strike(Fig. 4). To the north, argillite beds host the Kitts A, B, and Czones, and other occurrences including the Kiwi, Anderson Ridge,South and Gear showings (Fig. 2). The beds are grey to dark grey anda few millimetres to several centimetres thick. They are tightlyfolded, contorted, intruded by gabbro, sheared and metamorphosed atthe Kitts deposit (Gandhi, 1978). They include some thin beds andlenses rich in graphite (1 to 2.5 per cent) and pyrrhotite (up to10 per cent, with minor pyrite and chalcopyrite) that are the mainhosts of pitchblende. Magnetite is present in varying amounts, andis locally abundant and forms distinct layers as at the Gear showing.Calcite occurs mainly as veinlets, and amounts up to 7 per cent atKitts deposits. Pink variety of calcite is common at uraniumoccurrences. Pelitic beds commonly have porphyroblasts of garnet,and andalusite or chiastolite in a matrix of hornblende, sodicplagioclase, quartz, and some pyroxene and chlorite. Sillimanitetufts are developed in a buff white bed in contact with a gabbrointrusion near the Kitts deposit and are interpreted to have formedby contact metamorphism (Gandhi, 1977, 1978). Siltstones,metamorphosed to biotite-albite-quartz assemblages, and mafictuffaceous beds and lenses are interbedded with the argillaceousbeds. The siltstones (referred to as 'greywacke1 by Evans, 1980)contain little or no graphite, but contain cummingtonite-anthophyllite ± actinolite and minor magensium chlorite.Available chemical analyses of the argillites (11 reported byGandhi, 1977, and 9 by Evans, 1980) and of the greywackes(14 reported by Evans, ibid) show close affinities to basaltic rocks.

48

The argillite beds overlie and are locally interbedded withchert which at places contains magnetite layers. Relatively morecompetent chert has been brecciated and disrupted into isolatedlenses in incompetent argillite during deformation. It is regardedas inorganic, volcanogenic in origin (Gandhi, 1978) .

The chert and argillite apparently pinch out to the south of theInda showing (Fig. 4). Faciès changes are most marked at thisshowing, and mafic tuffs, interbedded mafic and felsic tuffs,argillaceous and calcareous beds are present. Pitchblende occurs ina main stratiform lens and three associated lenses at slightly higherstratigraphie levels. The main host rocks are fine grained, well-laminated tuffaceous beds, with green mafic and grey and pink felsiclaminae. They are overlain by calcareous mafic tuffaceous beds,which also host some pitchblende. Pitchblende is closely associatedwith aegirine-augite in the laminated tuffs and with phlogopite incalcareous mafic tuffs (Evans, 1980). The latter also containgarnets which are vanadiferous. In the northern part of the Indashowing, some low-grade zones have disseminated pitchblende in sodicplagioclase-magnetite-sphene rich layers containing quartz,hornblende ± garnet, according to Evans (ibid).

Spotty and lean mineralized zones at the Knife and Nash Eastshowings are in the rocks similar to that of the lower mineralizedzone at the Inda showing. The Nash Main and Nash West deposits are ina distinctive laminated subunit that overlies pillow basalt and mafictuff. The subunit is fine grained, tuffaceous with green, dark grey,buff and pink laminae. The minerals present are pyroxene (salite),garnet (grossular-andradite, vanadiferous), epidote, ferroanpargasite, albite, microcline, magnetite, calcite and minor sphene(Evans, 1980). Red colouration due to hematite is developed in therocks where uranium content is relatively high. The laminae arecontorted with folds plunging commonly to the south. Uraniumconcentration is commonly greater at fold hinges than along thelimbs.

Host rocks at the Punch Lake showings (Fig. 2, No. 7), 3 kmsouth of the Kitts deposit, where uranium occurrences are vein-typeand disseminated-type, include mafic and laminated tuffs similar tothe Nash West zone, according to Harder (1981).

The Witch showing, located approximately 5 km southwest of theNash West showing (Fig. 2, No. 14) is in grey to pink coloured,felsic tuffaceous beds. These beds may be stratigraphically close tothe laminated tuffaceous subunit of the Nash West and Nash Maindeposits.

49

The J & B prospect is located approximately 12 km southwest ofthe Witch showing (Fig. 2, No. 15), on the west shore of KaipokokBay, and is in sheared rhyolitic tuffs and associated mafic flows orsills, of the upper part of the Aillik Group (Ryan and Kay, 1982).The rocks may be stratigraphically close to those at the Witchshowing. Uranium-bearing zones, according to Lever andDavidson (1978), are stratiform.

In addition to the stratiform occurrences mentioned above,several uranium-bearing veins and fracture-fillings occur along theKitts-Post Hill belt e.g. at the southwest corner of Long Island andone kilometre to the west of Punch Lake. These are hosted byquartzite-siltstone-argillite, basalts and paraschists.

Metapelitic host rocks at the Anna Lake North occurrence, 8 kmnorth of Walker Lake (Fig. 2, No. 16), are in granite gneiss-graniteterrain (Willy, 1981). They appear to be similar to theargillaceous-tuffaceous unit, but they are too remote and isolatedfrom the unit to make a reliable correlation.

Rhyolitic rocks of the upper Aillik group: Rhyolitic flows, tuffs,and fragmental rocks are hosts to numerous uranium occurrences eastand south of the Kitts-Post Hill belt. The rocks vary considerablyin texture, but chemically they fall within a rather smallcompositional range. Some units are magnetic due to an abundance ofaccessory magnetite.

Textural varieties distinguished in the field are massive andflow banded rhyolites, coarse feldspar porphyritic and subporphyriticrhyolite flows or tuffs, laminated and thinly bedded tuffs,fragmental rocks with rhyolitic lapilli, bombs and blocks, andtuffaceous sedimentary rocks. At the Michelin deposit, severalmineralized lenses are subparallel to a sequence of interbeddedcoarse feldspar porphyritic and subporphyritic rhyolitic tuffaceousbeds and fine grained, thin felsic beds and lenses, although indetail the mineralized lenses transect the lithological boundaries atlow angles (Gandhi, 1978). Extensive quartz feldspar porphyry andporphyritic gneiss, which may in part be intrusive and in partignimbritic, are hosts to several occurrences, most of which areclose to the contacts with other rhyolitic rocks. The contacts aresharp, and distinctive as the rhyolites are generally fine grainedand lack the coarse quartz crystals that are abundant in the porphyryand prophyritic gneiss. Most of the occurrences along the CapeMakkovik-Monkey Hill belt are along such contacts either between themajor units or of lenses of rhyolites in the porphyritic gneiss.

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The rhyolitic rocks are deformed to a varying degree, fromrelatively little deformed to schistose and highly streaked quartzo-feldspathic gneisses, and mylonitized or granulated varieties. Theyare buff white, pale pink and light grey in colour, and have adistinctive red colouration, due to finely disseminated hematite, atand around high-grade uranium concentrations.

Some of the host tuffs are more mafic than rhyolitic tuffse.g. at the Rainbow prospect, 2 km south of the Michelin deposit(Fig. 2, No. 18), the host bed is chloritic and calcareous. Itoverlies a mafic tuffaceous bed, and is overlain by aphaniticrhyolite. The host rock resembles some of the host rocks at the Indaand Gear showings in the Kitts-Post Hill belt. Furthermore, layers,lenses and aggregates rich in mafic silicates are preferentiallymineralized in some of the occurrences e.g. at the Burnt Lakeshowings (Fig. 2, No. 19), stratiform mineralized zones occur inrelatively more mafic layers containing aegirine-augite and magnesio-riobeckite (Bailey, 1979; Kontak, 1980).

A number of chemical analyses of unmineralized and mineralizedrhyolitic rocks have been reported by Barua (1969) , Watson-White (1976), Minatidis (1976), Gandhi (1978), Bailey (1979),Evans (1980), Kontak (1980), White and Martin (1980) andGower et al. (1982) . Gower et al. (ibid) listed 125 analyses of10 groups of unmineralized rocks, and all of them except for a groupof 4 dacitic rocks, are within a range of 70 to 78 per cent silica,3 to 5 per cent soda and 4 to 6 per cent potash. No significantchemical differences are apparent among the groups of porphyriticrhyolite, nonporphyritic rhyolite, rhyolite tuff and quartz feldsparporphyry.

The mineralized rocks are texturally identical to theunmineralized equivalent rocks, except for the finely disseminatedpitchblende and hematite amounting to a fraction of a per cent in themineralized rock. There is however a significant increase in sodaand corresponding depletion in potash and to a lesser extent ofsilica, reflected in albitization of feldspar (Gandhi, 1978; Whiteand Martin, 1980; Evans, 1980). At other localities, potashenrichment and corresponding soda depletion are observed in theunmineralized rocks (Watson-White, 1976; White and Martin, 1980).Alkali metasomatism is widespread in the upper Aillik group. Hencesome of the rhyolitic rocks have alkaline or mildly peralkalineaffinités, although in general the rocks are high K., calc-alkaline incharacter.

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MineralogyPitchblende with negligible thorium, is virtually the only

uranium-bearing phase in most of the occurrences. Coffinite andsoddyite are associated with pitchblende in a few occurrences butthese minerals are quantitatively insignificant compared topitchblende. Yellow secondary uranium minerals are developed in thesurface exposures, particularly at high-grade spots.

In the Kitts-Post Hill belt, pitchblende occurs mostly asdisseminated grains less than 10 microns in diameter along thinlaminae and beds and as small string-like aggregates along beddingplanes in the argillaceous-tuffaceous host rocks (Hughson,1958 a, b, c; Gandhi, 1977, 1978; Evans, 1980). It also occurs inhigh-grade concentrations in fold hinges, veins and shears, as in theKitts deposit. These are apparently due to local remobilization andconcentration, during deformation, of the originally disseminatedmineral grains. In addition, there are late stage calcite veinswhich at places carry pitchblende.

The most important silicate mineral associated with pitchblendeis hornblende. It contains abundant inclusions of opaque mineralsincluding pitchblende. It is normally green in thin sections, buthas brownish red colouration in highly radioactive spots. Texturesobserved at the Kitts, Gear, Inda and Nash deposits show that thehornblende is a post-mineralization metamorphic mineral(Marten, 1977; Gandhi, 1978; Evans, 1980). Other mafic silicatesthat are also associated with pitchblende and at places carryinclusions of it, are pyroxene, biotite, and chlorite. The latter iscommon along shear zones. Electron microprobe analyses done byEvans (1980) show iron-rich character of these minerals. He alsofound clinozoisite, pumpellyite and prehnite which apparentlyrepresent retrograde metamorphism. Quartz, albite and sphene areother silicates commonly present.

Sulphides, graphite and magnetite are present in varying amountsin the occurrences along the Kitts-Post Hill belt, but there is noapparent systematic spatial relationship between these minerals andpitchblende. Pyrrhotite, pyrite, chalcopyrite, and traces ofmolybdenite and arsenopyrite, are common in the argillite. Thesulphides occur as disseminated grains, layers, lenses and also asveinlets. Hematite is not seen in argillite, but is common intuffaceous beds rich in felsic components.

Analyses of mineralized samples and bulk ore samples show onlytraces of precious metals, thorium, yttrium and other rare earths,

52

and generally low concentrations of base metals, molybdenum andvanadium (Hughson, 1958, a, b, c; Gandhi, 1978; Evans, 1980).Fluorite is rare or absent in the uranium occurrences of the Kitts-Post Hill belt, in contrast to some of the occurrences in rhyoliticrocks elsewhere in the Aillik Group.

In the rhyolitic host rocks, uranium occurs as disseminatedpitchblende or uraninite grains less than 10 microns in diameter,commonly as inclusions in mafic silicates and also at grainboundaries and in streaky aggregates with hematite and maficminerals. Uraniferous grains and aggregates are concentrated alongfoliation and shear planes.

At the Michelin deposit, the main host and closely associatedminerals are metamict sphene and aegirine augite, which formaggregates with alkali amphibole, ilmeno-magnetite partiallyoxidized to hematite, zircon, calcite, and apatite, and scarcephlogopite, andradite fluorite and pyrite, in quartz-albite rock.Variation in abundance of the mafic minerals imparts distinctivebanding to some of the coarse feldspar porphyritic units. The bandsare parallel to the lithologie boundaries, and range in widths up toseveral centimetres. In the nonporphyritic hosts rocks, the bandsare light grey, reddish brown and dark grey (Gandhi, 1976).

Compositions of pitchblende or uraninite, shpene, aegirine,alkali amphibole and albite were determined by energy dispersivespectra and x-ray diffraction (Gandhi, 1978; Evans, 1980, White andMartin, 1980). Feldspar in both phenocrystic and groundmass phasesof strongly mineralized rocks is nearly pure albite, with less than1 per cent anorthite and orthoclase molecules (Gandhi, ibid, andEvans, ibid), which is indicative of soda metasomatism. Thephenomenon is also evident from the chemical data. Analyses of57 mineralized rocks reported by Gandhi (ibid), Evans (ibid) andWhite and Martin (ibid) show NaaO content of 7 to 12 per cent and K2Ûless than 0.5 per cent, whereas the unmineralized rhyolitic rocks incomparison contain 3 to 5 per cent NaaO and 4 to 6 per cent K2O.There is also a small decrease in silica content in the mineralizedrocks compared to the unmineralized rocks. White and Martin (ibid)suggest that relatively mildly metasomatized rocks evolved byNa-for-K exchange, and more intensely metasomatized rocks evolved by(Na ± Al)-metasomatism, involving silica depletion, rather than bysimple ion exchange. Evans (ibid) recognized increasing intensity ofmetasomatism towards the core of ore zones in the drill sections hestudied, and outlined inner, outer and transional metasomatic zonesthat envelope the ore zones. Some K20-enrichment is observed at themargins of the metasomatized zones.

53

Available data show little difference in contents of Th, rareearth elements, Mo and F between the mineralized and unmineralizedrocks. Zr, Pb, C02 and Cl show an increase, and Th, Cu and S show adecrease with desilication of rocks according to White andMartin (1980). Data presented by them, and by Gandhi (1978) andEvans (1980) , show that the strongly mineralized rocks commonlycontain 1000 to 1500 ppm Zr in contrast to 300 to 700 ppm Zr in theunmineralized rocks. A slighlty higher C02 content in mineralizedrocks (0.9 to 1.3 per cent) compared to unmineralized rocks(0.60 to 0.63 per cent) is noted in a suite of 146 samples from46 drill holes, analysed by Brinex Limited in search for guides tomineralization.

Mineralogy of most of the occurrences in rhyolites is similar tothat at the Michelin deposit, with some variations in relativeproporitons of minerals and intensity of albitization. Thus theshowings at Burnt Lake (Fig. 2, No. 19) are in coarse feldspar quartzporphyritic rhyolite, chemically similar to those at the Michelindeposit, and are associated with nonporphyritic rhyolite, tuffs, andlenses of lapilli tuffs and agglomerate (Gandhi, 1978).Mineralogical studies by Kontak (1980), including electronmicroprobe analyses, show that uraninite or pitchblende ispreferentially associated with aegirine-augite, acmite and magnesio-riebeckite, that occur as disseminated grains, aggregates and layersor bands up to 0.5 cm thick. The pyroxenes predominate overamphibole. Other minerals present are biotite, zircon, apatite,hematite, magnetite, sphalerite, chalcopyrite, coveJlite, bornite,chalcocite, fluorite and galena. The feldspar is nearly pure albite.Kontak (ibid) regarded the mineralization as stratiform. This isalso seen in the Emben South showing, 5 km east of Burnt Lake(Fig. 2, No. 20), where mineralization is restricted to a 0.5 to1.5 m thick unit containing up to 10 per cent aegirine and 80 percent albite, and notable amounts of riebeckite, magnetite, sphene,apatite, pyrite and some quartz, calcite and fluorite. The unit istightly folded within an area 150 x 150 m with two antiforms andsynforms plunging moderately to the southwest (Minatidis, 1976;Gandhi, 1976; Willy, 1982). A few small occurrences of pyrite,molybdenite and fluorite are found in rhyolitic rocks close to theeast shore of Walker Lake.

In the Cape Makkovik-Monkey Hill belt, disseminated molybdenite,pyrite and fluorite occur with disseminated pitchblende (uraninite)at the Sunil showing (Fig. 2, No. 25) and in the Cape Makkovikmolybdenum deposit (Fig. 2, No. 26) which contains only traces of

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pitchblende in some places. The deposits are stratiform; the Sunilshowing is 350 m long and up to 5 m thick, and the Cape Makkovikdeposit is 2300 m long and up to 25 m thick. They dip 60 to70 degrees to the east and are drilled to a depth of 100 to 150 m.They contain approximately 5 per cent pyrite and subordinate amountsof molybdenite (Table 1). The sulphides in the deposits occurcommonly as discrete streaks several millimetres long, parallel tothe foliation or schistocity of host rocks, and locally as veins(King, 1963; Gandhi et al., 1969; Barua, 1969; Gandhi, 1978).Pyrite also occurs as euhedral crystals scattered through the hostrock. Pitchblende occurs as streaks at the Sunil showing(Barua, 1969). It is commonly not associated intimately withmolybdenite-pyrite streaks. The host rhyolitic rock at both depositsis strongly enriched in soda and depleted in potash as at theMichelin deposit, and the feldspar in it is albite (Morse, 1961;Gill, 1966; Gandhi, 1967; Barua, ibid). Fluorite occurs as purpleand colourless crystals closely associated with the sulphides. Otherminerals present are quartz, hornblende, magnetite, zircon, apatite,actinolite, sphene, garnet, acmite or aegirine, epidote and calcite.Traces of chalcopyrite, sphalerite, galena, pyrrhotite and of silverare present in the Sunil showing.

In the vicinity of Makkovik, weak radioactivity occurs in skarn-like aggregates of pyroxene, garnet, magnetite and hematite, sphene(titanite) , epidote, pyrite and scheelite in lenses of limestone thatoccur in the predominantly rhyolitic sequence (Beavan, 1958) .

Occurrences in the Shoal Lake-Falls Lake-Round Pond zone alsocontain disseminations of fine grained pitchblende in rhyoliticflows, tuffs and relatively more mafic tuffaceous units. Host rocksare enriched in soda and depleted in potash (Gandhi, 1978). Tracesof silver and selenium are noted in a few occurrences (Gandhi, ibid).The main occurrence at Winter Lake (Fig. 2, No. 21), approximately12 km southeast of Monkey Hill peak, is at the folded contact ofrhyolitic tuffaceous rock and what appears to be a mafic dyke.Pitchblende disseminations and veins occur at the crests of minorfolds, along the contact for 100 m strike length. An associatednarrow sulphide-calcite vein at one end of the showing containssphalerite, covellite and stromeyerite (Beavan, 1958). A number ofveins carrying chalcopyrite, pyrite, pyrrhotite, molybdenite andfluorite occur near Round Pond (Fig. 2, No. 23) in conglomerateunderlying rhyolitic volcanic rocks containing lean disseminatedpitchblende mineralization (Gandhi, 1969). Veins of pitchblende aremore common in the Round Pond - Falls Lake area (Fig. 2, No. 23) than

55

elsewhere in the Aillik Group. The vein at Pitch Lake (Fig. 2,No. 22), 5 km to the south-southeast of Monkey Hill peak, which wasthe first uranium occurrence discovered in the area in 1954, containsmassive and botryoidal pitchblende, gummite, hematite and fergusonite(Beavan, 1958). It occurs in a large amphibolite (mafic lava) lensintruded by granite.

Pitchblende and Galena Age DeterminationsU-Pb isotopic ratios of pitchblende samples are plotted on the

concordia diagram (Fig. 5).Nine samples from the Kitts deposit yielded nearly concordant

results. One of these, as reported by Gandhi (1978) , gave anessentially concordant date of 1730 Ma from the Pb207/Pb206 ratio.Eight other samples analyzed by Kontak (1980) yielded resultsindicating a very small range close to a concordant date of 1770 Ma.Two of the samples represented disseminated-type mineralizationwhereas others represented vein-type mineralization. The dataindicate a single crystallization event approximately 1750 Ma ago,with little loss of radiogenic daughters since.

Samples from the Inda and Gear showings, on the other hand,yielded discordant results representing uranium loss or lead gain.The precise mechanism that can cause this is not known.

0 6 4

sQ_

o Kins depositA Gear showing• Inda showing0 Michelin depositA John Michelin showingD Emben showingx Pitch Lake showing• Burnt Lake showing

0 1

Figure 5. Concordia plot of pitchblendes fromuranium occurrences in the Aillik Group, KaipokokBay - Big River area, Labrador (after Gandhi, 1978,with additional data points from Kontak, 1980).

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Evans (1980) estimated ages of pitchblendes in the Kitts, Indaand Nash West deposits from elemental U/Pb ratios obtained bymicroprobe analyses of pitchblende grains. His calculated ages are:1785 ± 115 Ma for Kitts (24 grains) , 1844 ± 209 Ma for Inda(16 grains), and 1795 ± 159 Ma for Nash West (four sites in one largegrain).

Among the uranium occurrences in rhyolitic rocks of the upperAillik group, the John Michelin showing with disseminatedpitchblende, located 2 km southwest of Makkovik (Fig. 2, No. 24),yielded a nearly concordant date of 1745 Ma from Pb207/Pb206 ratio(Gandhi, 1978). This is very close to the dates obtained for theKitts deposit. Uranium mineralization predates at least the laststages of Hudsonian deformation and metamorphism (Gandhi, 1978) ,hence these dates represent the time of these events. Together witha Rb/Sr isochron date of 1767±4 Ma for the rhyoitic rocks 4 kmnorthwest of the Michelin deposit (Kontak, 1980) , these dates imply arather short time mineral for formation of the host rocks, uraniummineralization and Hudsonian deformation and metamorphism.

Samples from the Michelin deposit and the Emben showing gaverelatively much younger Pb207/Pb206 dates, of 1244 and 1364 Marespectively. Both the samples however are from high-grade 'vein-type' concentrations exposed in trenches, and probably representremobilization of uranium from originally disseminated-type, lowergrade, more extensive mineralization (Gandhi, 1978). Two samplesfrom the Burnt Lake showings gave discordant results; theirPb207/Pb206 dates are 1770 and 1680 Ma (Kontak, 1980). The olderdate is from a drill core sample and the other one is from a trenchsample. On the concordia diagram these two samples, and the onesfrom the Michelin deposit, and from the Emben and John Michelinshowings, plot close to a line which intercepts the concordia atapproximately 1800 Ma and 1100 Ma. This indicates that thedisseminated-type uranium mineralization as represented by theJohn Michelin sample is close to 1800 Ma in age, and in case of theBurnt Lake, Michelin and Emben samples, it was affected by an episodethat disturbed its isotopic equilibrium approximately 1100 Ma ago.This is supported by a date of 1017 Ma obtained by Evans (1980) forpitchblende grains in a foliated mafic dyke in the Michelin deposit,using the electron microprobe technique. It is in good agreementwith an Ar"*°/Ar39 date of 1004 ± 5 Ma mentioned earlier on biotite ina sample from a regional shear zone in the area (Gower et al., 1982,p. 46). A K-Ar date on another foliated, mafic dyke, approximately25 cm thick, in the Michelin deposit, reported by Gandhi (1978), issomewhat younger at 919 ± 26 Ma.

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The episode of disturbance of isotopic equilibrium and/or localredistribution of uranium leading to high-grade concentrations,approximately 1100 Ma ago coincides with the time of Grenvilleorogenic cycle. It is therefore attributed to the effects ofGrenvillian structural and metamorphic events on the rocks in thesouthern part of the study area near the 'Grenville Front'. Maficdykes are present near the Michelin and Emben sample localities, andGandhi (1978, p. 1515-1516) suggested that local redistribution ofuranium there may be due to the intrusion of the dykes. The dykeshowever are more than 10 m from the localities and their contactmetamorphic effects, as in case of other dykes in the study area, arerestricted to within a few centimetres from the contacts, beyondwhich there is no visible effect on the country rocks including themineralized rhyolitic rocks. Furthermore, ages of these dykes arenot known. Their effects on the samples thus remain speculative. Incase they were emplaced prior to or during Grenville time, theirinfluence on isotopic equilibrium of the samples, if any, would bedifficult to isolate from the effects of Grenvillian structural andmetamorphic events.

The pitchblende vein at Pitch Lake yielded a Pb207/Pb206 date of934 Ma (Beavan, 1958; Gandhi, 1978). The vein dips gently to thesouth and is exposed on a north-facing cliff along a fault trendingeast-northeast. It is possible that the relatively young date is aresult of loss of radiogenic lead due to movements along the fault ordue to weathering.

A recent isotopic study of galena from the Michelin depositstrongly supports the interpretation from the pitchblende data, ofdisturbance of isotopic equilibrium and local redistribution ofuranium in occurrences in the southern part of the study area, duringGrenville time. The sample is from a rare, lenticular aggregate ofcoarse grained calcite, amphibole and galena, approximately 10 cm indiameter and upto 1.5 cm thick, subparallel to regional foliation, inmineralized coarse feldspar porphyritic rhyolite. It is located1.5 m south of a coarse feldspar porphyritic dioritic dyke which isapproximately 20 cm thick and trends east-southeast. It is part ofthe mineralized rhyolite sample SSG-157-'75 described byGandhi (1978, p. 1508-1509; from south wall of drift, 241 m east ofcrosscut). The isotopic analysis was performed by GeospecConsultants Limited (Edmonton, Alberta, Canada), and yielded ratiosas follows: Pb2°6/Pb2°": 532.45, Pb2°7/Pb2°":97.163 andPb2 ° 8/Pb20 "* : 36.726. Because of the overwhelming dominance ofuranogenic component in this lead, choice of any reasonable

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Proterozoic common lead will yield essentially the same Pb207/Pb206

ratio (0.1585) for the radiogenic component.The ratio puts contraints on the maximum and minimum time of

formation of galena and of the source pitchblende. The calculatedmaximum age of the source pitchblende, assuming a continuousaccumulation of the radiogenic lead in galena until the present time,is 2440 Ma. The calculated maximum age of galena, and hence minimumage of the source pitchblende, assuming that all the radiogeniccomponent was generated and accumulated within a very short timeinterval, is 1490 Ma. Further constraints are imposed by the knowngeology, the pitchblende data and the Rb/Sr isochron date of1767±4 Ma for the rhyolitic host rocks (Kontak, 1980). Calculationsusing 1767 Ma as the age of the host rocks as well as of the initialuranium mineralization, which is supported by their geologicalrelations and the pitchblende data, yield 1180 Ma as the time ofgalena formation. Other calculated pairs of dates for formation ofthe source pitchblende and of the galena respectively are: 1750 Maand 1201 Ma, and 1800 Ma and 1138 Ma. The younger dates in thesepairs are in approximate time range of 1200 to 1000 Ma generallyaccepted for Grenvillian orogeny. They are thus in general agreementwith the time of episodic loss of radiogenic lead from pitchblendeindicated by the concordia plot.

GenesisThe main features of uranium mineralization in the Aillik Group

are:

(i) Most of the pitchblende is finely disseminated,(ii) High-grade veinlets and fracture fillings that occur at

and close to the zones of disseminated pitchblende appearto be younger than, and most probably resulted from localremobilization and concentration of originallydisseminated pitchblende.

(iii) The disseminated mineralization predated at least the lastphase of Hudsonian deformation and metamorphism(Gandhi, 1978).

(iv) The age of pitchblende in the disseminated zones isapproximately 1750 Ma, and corresponds closely with theoldest Rb/Sr isochron date of 1767 ± 4 Ma obtained for theupper Aillik group rhyolites (Kontak, 1980).

(v) Disseminated pitchblende is in stratiform layers andlenses that occur within relatively thin stratigraphie

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zones; in an argillaceous-tuffaceous unit in thé Kitts-Post Hill belt, and in dominantly felsic tuffaceous unitsin the rhyolitic sequence in the upper Aillik group,

(vi) The favourable stratigraphie zones are characterized byrapid lithofacies variations,

(vii) Rhyolitic host rocks are strongly enriched in soda anddepleted in potash,

(viii) Uranium is preferentially associated with layers andlenses rich in graphite, pyrite, magnetite and hornblendein the argillite, and with sodic pyroxene, sodic amphiboleand sphene in the rhyolitic rocks,

(ix) Disseminated molybdenite, pyrite and fluorite occur insome of the disseminated pitchblende zones in the northbut are scarce or absent elsewhere,

(x) Thorium and rare earths do not increase with uranium inthe mineralized zones,

(xi) Finely disseminated hematite is common in the mineralizedzones, except for those in the argillite.

(xii) Isotopic equilibrium in occurrences to the south near the'Grenville Front1 was affected by Grenvillian eventsapproximately 1150 Ma ago, which caused localredistribution of uranium and formation of rare galenarich in uranogenic component.

These features, in particular the close spatial and temporalrelationships of the rhyolitic rocks and uranium mineralization,invite linking genetically the mineralization to the felsic volcanicactivity that was extensive at the time of the upper Aillik group.

A genetic model proposed by Gandhi (1978) and modified slightlyhere, invokes (a) syngenetic deposition of uranium derived fromfelsic volcanics by meteoric waters, in euxinic basins like the onewhere host beds of the Kitts-Post Hill belt were deposited, and(b) epigenetic mineralization in the rhyolitic rocks elsewhere byfluids generated at depth in the magma that gave rise to therhyolites. The thick rhyolitic sequence is believed to be subarealin part at least, from the presence of units interpreted as air-falltufts and agglomerates, ignimbrites and lahars.

(a) Influx of felsic tuffaceous material at the time ofdeposition of the argillaceous-tuffaceous host unit in the Kitts-PostHill belt (Figs. 3 and 4), marks the beginning of the felsicvolcanism that produced a source of uranium-enriched sediments aswell as of uranium in solution in meteoric waters. Strongly reducing

60

environment prevailed in part of the basin where argillite wasdeposited as indicated by sulphides and graphite in it, and favouredprecipitation of uranium from the meteoric waters that entered thebasin during sedimentation and diagenesis. Reducing conditionsadequate to precipitate uranium in the same fashion, extended mostprobably where interbedded mafic and felsic tuffaceous, andcalcareous sediments of the host unit were deposited.Such conditions may also have prevailed in some lakes during thesubaerial volcanism, and may have led to concentration of uranium inlacustrine sediments (Sherborne et al., 1979; Curtis, 1981). Mafictuffaceous-calcareous host beds of restricted extent at the Rainbowzone may represent such an environment.

(b) Uranium mineralization in the rhyolitic rocks on the otherhand was brought about by reaction of mineralizing magmatic fluidswith the host rocks (Gandhi, 1978). The fluids were enriched insodium, uranium, and volatiles in particular CO2, F and Cl. Suchfluids were probably generated throughout the felsic volcanic cycle,but may have culminated during the waning stages of volcanism. Theymigrated through permeable avenues in the sequence, viz. non-weldedtuffs and pyroclastic rocks, devitrified lavas, zones of frequentinterlensing of various lithological units and of shears and faultsdue to incipient structural disturbances prior to major deformation.They reacted with the wall rocks, and brought about soda metasomatismand uranium mineralization in favourable zones. Uranium wasprecipitated by reactive titanium- and iron-bearing minerals, namelysphene, pyroxene and hornblende. The precipitation was most probablyaided by boiling off of volatiles. Original distribution of thereactive minerals in the host rocks as disseminations and alongcertain layers and lenses, is largely responsible for the observeddistribution of pitchblende. Molybdenite-pyrite-fluoritemineralization may be synchronous with the uranium mineralization ormay represent a separate stage of evolving magmatic fluids. Itsincreasing abundance to the north suggests a regional metallogeniczoning.

Studies on Tertiary rhyolitic ash and lava flows in the westernUnited States and experimental work show that heated, mildly alkalineoxygenated meteoric waters play an important role in leaching ofuranium from devitrifying rhyolitic source rocks and precipitating itat favourable sites to form deposits (Rosholt et al., 1971;Zielinski, 1978, 1979, 1981; Zielinski et al., 1977). In case of theupper Aillik group, differing views have been expressed regardingrelative importance of meteoric waters and juvenile fluids from the

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volcano-plutonic complex, in bringing about the observed uraniummineralization. Gandhi (1978) regarded it unlikely that mineraliza-tion in the rhyolitic rocks was brought about entirely by themeteoric waters without any magmatic input, in view of the intensesoda metasomatism which resembles fenitization, and the mineralogy,texture and geological setting of the mineralized zones. Lowtemperature hydrous minerals are notably absent, and it seemsunlikely that evidence of their presence was completely obliteratedby later metamorphism. Kontak (1980) favoured a combined role ofmeteoric waters that leached uranium from glass shards and felsictuffs, and the volatile-rich fluids emanating from vents. White andMartin (1980) visualized convection cells of mildly alkaline aqueousfluids of meteoric origin near intrusive plugs and around inferredsubjacent plutons related to volcanism. Gower et al. (1982) alsopostulated similar convection cells, but regarded the upper Aillikgroup as resting unconformably over deformed lower Aillik group atthe time of mineralization. Evans (1980) on the other hand invokedonly the low temperature, neutral to weakly alkaline, oxidizinggoundwaters of meteoric and/or of connate marine origin, for leachingof uranium as well as zirconium from the felsic volcanic source rocksunder diagenetic to zeolite faciès pressure-temperature conditions.The metals, according to him, were transported and also precipitatedat low temperature (zeolite faciès), and redox changes and adsorptionon Fe-Mn-Ti hydroxides played an important role in the precipitation.In contrast, White and Martin (ibid) emphasized a high temperature ofprecipitation of uranium, above the upper limits of the greenschistfaciès, implied by desilication of the Na-metasomatized host rocks(now albitites).

Post-mineralization deformation and metamorphism make itdifficult to evaluate the influence of structure, in particularshearing, in the transport and deposition of uranium. Marten (1977)and Evans (1980) postulated a premineralization deformation in theKitts-Post Hill belt, that produced permeable shear zones in the hostunit, and derivation of uranium from the overlying felsic tuffaceousbeds by solutions that eventually deposited it in the shear zones.It seems improbable, however, that the postulated favourable shearzones were restricted to the incompetent argillaceous and tuffaceoushost beds and were parallel to the bedding, and that such shear zoneswere not developed in the underlying and overlying units of theAillik Group. Marten (ibid) furthermore visualized the mineralizingsolutions to be of metamorphic origin, rather than of magmatic ormeteoric origin or a mixture of the two. Textures of the mineralized

62

zones however show that the growth of metamorphic minerals likehornblende effectively protected the pre-existing uranium in the hostrock. This feature is inconsistent with the hypothesis ofmetamorphic remobilization of uranium from the source rocks.

CONCLUSIONS

Uranium in the Aillik Group is genetically related to the felsicvolcanic rocks that are thick and extensive, and predominant overbasalts in the upper part of the group. The bimodal volcanicsequence, with interbedded clastic sedimentary rocks, was depositedin a continental rift environment. Circulation of mineralizingfluids of either magmatic or meteoric or mixed origin, through therhyolitic rocks, some of which are subaerial, either introduced orredistributed uranium in permeable zones at the time of volcanism orsoon after it, but prior to major Hudsonian deformation andmetamorphism. Some uranium was transported by meteoric waters to aneuxinic basin at the beginning of the felsic volcanism where it wasdeposited syngenetically or during diagenesis in argillaceous bedsand in interbedded mafic and felsic tuffaceous and calcareoussediments.

Isotopic data support the genetic hypothesis, and indicate thetime of initial uranium mineralization in 1800 to 1750 Ma range.They also reveal a disturbance of isotopic equilibrium in theoccurrences to the south near the "Grenville Front1 approximately1150 Ma ago.

AcknowledgementsBrinco Mining Limited kindly permitted inclusion of unpublished

information presented here.Thanks are due to R.T. Bell and V. Ruzicka of the Geological

Survey of Canada, Ottawa, for critical review of the paper andvaluable comments. Discussions with R.T. Bell, S.M. Roscoe andR.I. Thorpe of the Geological Survey of Canada resulted in aconsiderable improvement of the manuscript. The opinions expressedhere, however, are writer's own.

REFERENCES

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63

Barua, M.C., 1969: Geology of uranium-molybdenum-bear ing rocks ofthe Aillik-Makkovik Bay area, Labrador; UnpublishedM.Sc. thesis, Queen's University, Kingston, Ontario, 76 p.

Beavan, A.P., 1958: The Labrador uranium area; Proceedings of theGeological Association of Canada, v. 10, p. 137-145.

Burbidge, G.H., 1977: A metamorphosed rhyolite breccia in the AillikGroup near Kaipokok Bay, Labrador; Unpublished B.Sc. (Rons.)thesis, Queen's University, Kingston, Ontario, 14 p.

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_____________ 1973: A reinterpretation of the stratigraphy anddeformation of the Aillik Group, Makkovik, Labrador;Unpublished Ph.D. thesis, Memorial University, St. John's,Newfoundland, 346 p.

_____________ 1979: Proterozoic deformation and igneous intrusionsin part of the Makkovik Subprovince, Labrador; PrecambrainResearch, v. 10, p. 95-114.

Curtis, L., 1981: Uranium in volcanic and volcaniclastic rocks —examples from Canada, Australia and Italy; American Associationof Petroleum Geologists, Studies in Geology, No. 13, p. 55-62.

Evans, D., 1980: Geology and petrochemistry of the Kitts andMichelin uranium deposits and related prospects, CentralMineral Belt, Labrador; Unpublished Ph.D. thesis, Queen'sUniversity, Kingston, Ontario, 312 p.

Gandhi, S.S., 1967: A pétrographie study of 1966 drill core fromCape Makkovik area, Labrador; British Newfoundland ExplorationLimited, Unpublished document G-67011, 26 p.

____________ 1968: Report on exploration in Aillik-Makkovik area,Labrador, during 1967: Brinex/Cominco joint venture;British Newfoundland Exploration Limited, Unpublisheddocument G-68009, 45 p.

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______________ 1970: Geology and uranium occurrences of theKaipokok Bay — Big River area, Labrador; British NewfoundlandExploration Limited, Unpublished document G-70004, 33 p.

64

Gandhi, S.S., 1976: Geology, isotopic ages and origin of uraniumoccurrences of Kaipokok Bay — Big River area, Labrador;British Newfoundland Exploration Limited, . Unpublisheddocument G-76001, 25 p.

_____________ 1977: The relationship of stratigraphy and uraniummineralization in the Kitts area, Labrador;British Newfoundland Exploration Limited, Unpublisheddocument G-76013, 73 p.

____________ 1978: Geological setting and genetic aspects ofuranium occurrences in the Kaipokok Bay - Big River area,Labrador; Economic Geology, v. 73, p. 1492-1522.

Gandhi, S.S., Grasty, R.L., and Grieve, R.A.F., 1969: The geologyand geochronology of the Makkovik Bay area, Labrador; CanadianJournal of Earth Sciences, v. 6, p. 1019-1034.

Gill, F.D., 1966: Petrography of molybdenite-bearing gneisses,Makkovik area, Labrador (abstract); Canadian Mining Journal,v. 89, p. 100-101.

Gower, C.F., Flanagan, M.J., Kerr, A., and Bailey, D.G., 1982:Geology of the Kaipokok Bay - Big River area, Central MineralBelt, Labrador; Newfoundland Department of Mines and Energy,Mineral Development Division, Report 82-7, 77 p.

Harder, D.G., 1981: Detailed exploration at Punch Lake SouthProspect, Area A, Labrador; Brinco Mining Limited, Unpublisheddocument G-81009, 28 p.

Hughson, M.R., 1958a: Mineralogical report on a uranium ore sample('K-2') from British Newfoundland Exploration Limited; Kittsproject, Makkovik area, Labrador, Newfoundland; Canada Dept.Mines Tech. Surveys, Mines Branch, Ottawa, Report IR-58-54,9 p.

____________ 1958b: Mineralogical report on a uranium ore sample{'K-4') from British Newfoundland Exploration Limited; Kittsproject, Makkovik area, Labrador, Newfoundland; Canada Dept.Mines Tech. Surveys, Mines Branch, Ottawa, Report IR-58-94,6 p.

____________ 1958c: Mineralogical report on a uranium ore sample('K-5') from British Newfoundland Exploration Limited; Kittsproject, Makkovik area, Labrador, Newfoundland: Canada Dept.Mines Tech. Surveys, Mines Branch, Ottawa, Report IR-58-162,5 p.

65

King, A.F., 1963: Geology of the Cape Makkovik peninsula, Aillik,Labrador. Unpublished M.Sc. thesis, Memorial University,St. John's, Newfoundland, 114 p.

Kontak, D.J., 1980: Geology geochronology and uranium mineralizationin Central Mineral Belt of Labrador, Canada; UnpublishedM.Sc. thesis, University of Alberta, Edmonton, Alberta, 378 p.

Lever, T. and Davidson, G., 1978: Reconnaissance geology andprospecting of the Albert Lake area, including detailed geologyof the J & B prospect, Area B, Labrador; Brinco Mining Limited,Unpublished document G-78025, 13 p.

Marten, B.E., 1977: The relationship between the Aillik Group andthe Hopedale gneiss, Kaipokok Bay, Labrador; UnpublishedPh.D. thesis, Memorial University, St. John's, Newfoundland,389 p.

Minatidis, D.G., 1976: A comparative study of trace elementgeochemistry and mineralogy of some uranium deposits ofLabrador, and evaluation of some uranium exploration techniquesin glacial terrain; Unpublished M.Sc. thesis, MemorialUniversity, St. John's, Newfoundland, 216 p.

Morse, S.A., 1961: Pétrographie notes on the Aillik molybdeniteshowing, Labrador (NTS 130/3); British NewfoundlandExploration Limited, Unpublished document G-61003, 4 p.

Rosholt, J.N., Prijana and Noble, D.C., 1971: Mobility of uraniumand thorium in glassy and crystallized silicic volcanic rocks;Economic Geology, v. 66, p. 1061-1069.

Ryan, A.B. and Kay, A., 1982: Basement-cover relationships andplutonic rocks in the Makkovik sub-province, north ofPostville, coastal Labrador (13J/13, 130/4); in CurrentResearch for 1981, Edited by C.F. O'Driscoll and R.V. Gibbons,Newfoundland Department of Mines and Energy, MineralDevelopment Division, Report 82-1, p. 109-121.

Sherborne, J.E. Jr., Buckovic, W.A., Dewitt, D.B., Hellinger, T.S.,and Pavlak, S.J., 1979: Major uranium discovery involcaniclastic sediments, Basin and Range Province, Yavapaicounty, Arizona; American Association of Petroleum GeologistsBulletin, v. 63, p. 621-646.

Stevenson, I.M., 1970: Rigolet and Groswater Bay map area,Newfoundland (Labrador); Geological Survey of Canada,Paper 69-48, 24 p.

66

Wardle, R.J., and Bailey, D.G., 1981: Early Proterozoic sequences inLabrador; in. Proterozoic Basins in Canada, P.H.A. Campbell,éditer; Geological Survey of Canada, Paper 81-10, p. 331-359.

Watson-White, M.V., 1976: A petrological study of acid volcanicrocks in part of the Aillik Series, Labrador; UnpublishedM.Sc. Thesis, McGill University, Montreal, Quebec, 92 p.

White, M.V.W., and Martin, R.F., 1980: The metasomatic changes thataccompany uranium mineralization in the nonorogenic rhyolitesof the Upper Aillik Group, Labrador; Canadian Mineralogist,v. 18, p. 459-479.

Willy, A.J., 1981: Initial diamond drilling on the Anna Lake NorthProspect, Summer 1981, Extended Licence No. 1524 BX, Area B,Labrador; Brinco Mining Limited, Unpublished document G-81008,61 p.

Willy, A.J., 1982: Detailed exploration on the Main and South Zones,Emben (Matthew Ben) Showings, Area A, Labrador; Brinco MiningLimited, Unpublished document G-82003, 44 p.

Zielinski, R.A., 1978: Uranium abundance and distribution inassociated glassy and crystalline rhyolites of the westernUnited States; Geological Society of America, Bulletin, v. 89,p. 409-414.

____________ 1979: Uranium mobility during interaction ofrhyolitic obsidian, perlite and felsite with alkaline carbonatesolutions: T = 120 C, P = 210 kg/cm2; Chemical Geology,v. 27, p. 47-63.

____________ 1981: Experimental leaching of volcanic glass:implications for evaluation of glassy volcanic rocks as sourcesof uranium; American Association of Petroleum Geologists,Studies in Geology, No. 13, p. 1-12.

Zielinski R.A., Lipman, P.W. and Millard, H.T. Jr., 1977: Minorelement abundances in obsidian, perlite and felsite of calc-alkaline rhyolites; American Mineralogist, v. 62, p. 426-437.

67

THE OLYMPIC DAMCOPPER-URANIUM-GOLD DEPOSIT,ROXBY DOWNS, SOUTH AUSTRALIA

D.E. ROBERTS, G.R.T. HUDSONRoxby Management Services Proprietary Ltd,Unley, SA,Australia

Abstract

The Olympic Dam copper-uranium-gold deposit in South Australia was discoveredin 1975. It has an areal extent exceeding 20km^ with vertical thicknesses ofmineralization up to 350 metres. The deposit is estimated to contain in excess of2,000 million tonnes of mineralized material with an average grade of 1.6 percentcopper, 0.06 percent uranium oxides, and 0.6gm per tonne gold.

The unmetamorphosed coarse clastic Proterozoic sediments hosting themineralization are probably no older than 1580 Ma and appear to fill a graben ingranitic rocks beneath 350m of unmineralized, flat lying Adelaidean to Cambriansediments.

Disseminated strata-bound bornite-chalcopyrite-pyrite is the dominant type ofsulfide mineralization with lesser amounts of younger transgressivechalcocite-bornite. Uraninite, with lesser coffinite and brannerite occurs with thesulphide mineralization in a variety of textural forms and rare earths mineralsbastnaesite and florencite occur in, and adjacent to sulphide mineralized zones.Gold and silver are a minor but significant component of the mineralization.

INTRODUCTION

The Olympic Dam copper-uranium-gold deposit constitutes a major resource ofcopper and uranium with an areal extent exceeding 20km^ and vertical thicknessof mineralization measured in tens to hundreds of metres (Roberts and Hudson,1983). The deposit contains at least 2,000 million tonnes of mineralized materialwith an average grade of 1.6 percent copper, 0.06 percent uranium oxide (U3Og),and 0.6 g/tonne gold. Higher grade mineralization has been estimated at 450million tonnes of 2.5% Cu, 0.08% UgOg and 0.6gm/tonne Au. In additionsignificant concentrations of rare earths and silver occur. The deposit is locatedon Roxby Downs pastoral station, 650 kilometres north-northwest of Adelaide and25 kilometres west of the opal mining town of Andamooka (Fig. 1).

69

Pi P E A K E ANDC JOENISON RANGES

Lower lo Middle Pro le ro io ic \ *~

Fig. 1. Geologic map of the southern part of South Australia showing the mainlithologie <3c tectonic features and the location of the Olympic Dam deposit.

Exploration for copper by Western Mining Corporation Limited which resulted inthe discovery of the deposit was focused on South Australia in 1972 followingresearch during the period 1969 to 1971 which related the formation of strata-bound copper deposits to solutions that had aquired copper during oxidativealteration of basalts (Haynes, 1972). In the Stuart Shelf region, regional gravityand magnetic data resulting from surveys by the Commonwealth Bureau ofMineral Resources and the South Auscralian Department of Mines and Energysuggested the possibility of basalts at depth and the interpretation of gravity andmagnetic anomalies, and lineament mapping resulted in the selection of specifictarget areas, including the Olympic Dam area (Fig. 2). In 1974 a commitment wasmade to drill stratigraphie holes on two of the selected targets to test for thepresence of favourable host rocks in the Upper Proterozoic cover sequence, andsource rocks in the basement.

The first hole, RDI in the Olympic Dam area, was completed in July 1975 afterintersecting 38 metres of 1.05 percent copper in the basement at a depth of 353metres. This encouraging, although uneconomic, mineralization led to the drilling

70

Fig. 2. Positive gravity and magnetic anomalies associated with the Olympic Damdeposit. Magnetic contours in nanoteslas; gravity in milliGals; H = centre ofanomaly; RDI = discovery hole.

of a pattern of holes in the area. The first two, RD3 and RD4 were barren, RD5and RD8 intersected additional mineralization containing 1 percent copper, RD6and RD7 were barren, and RD9 low grade. RD10 intersected 170 metres of 2.1percent copper and over 0.05 percent uranium oxide and provided the firstindication of significant uranium mineralization and the great potential of thearea that has been confirmed by subsequent drilling.

The gravity anomaly which was attributed to a shallow basement of basaltic rockis now known to result from the dense hematite-rich breccias associated with themineralization. The magnetic anomaly is still largely unexplained and modellingindicates it results from a large magnetic body beyond the current depth ofdrilling. Host rocks for the type of strata-bound copper deposit predicted in theoriginal concept do not occur in the area and mineralization is confined to rocksbeneath the Upper Proterozoic sedimentary sequences.Detailed assessment of the prospect commenced in 1979 as a joint venturebetween Roxby Mining Corporation, a wholly owned subsidiary of Western MiningCorporation Limited, BP Australia Limited and BP Petroleuni Development

71

TABLE 1

TYPICAL Cu-U MINERALIZED DRILL HOLE INTERSECTIONS

Drill Hole

RDI

RD10

RD19

RD20

RD24

Interval(m)

353 - 391

348 - 518

422 - 436684 - 758

369 - 580

343 - 649811 - 1003

1003 - 1128

Thickness(m)

38

170

1474

211

306192125

Cu(%)

1.05

2.12

2.003.04

2.10

1.000.601.10

U,0n/ i* " \(kg/tonne)

0.10

0.59

tr0.45

0.80

0.200.160.33

Au(gm/tonne)

-

-

15.4

-

-

RD39 368 - 414 146 3.25 0.68

Location of drill holes indicated in Figure 3.

o <0.5% Cu OR 20m%• 20 - 100m%• 100 - 200m%

200 - 500m%

500m%

WHENAN SHAFT

200 000 N

î- . \J W1 km» . _ . . —————j

Fig. 3. Location of diamond drill holes showing greater than 0.5 percent copperintersections. See Table 1 for intersection in numbered drill holes.

72

Limited. Intensive vertical diamond drilling over an area in excess of 25 squarekilometres has resulted in a success rate of over 80 percent mineralized drill holes(Fig. 3, Table 1). The Whenan Exploration Shaft has been completed to 500m andunderground exploration and bulk sampling is in progress. All observations,interpretations and speculations presented in this paper are on data from diamonddrilling.

REGIONAL GEOLOGICAL SETTING

The Olympic Dam deposit occurs within the Stuart 'Shelf Region (Fig. 1), an areaof flat-lying Adelaidean and Cambrian sediments separated from the AdelaideGeosyncline to the east by a major zone of north-south faulting termed theTorrens Hinge Zone by Thomson (1969), (Preiss et al., 1980). To the west andsouthwest, the sediment cover of the Stuart Shelf laps onto exposed rocks of theGawler Craton (Rutland et al., 1980). The basement to the Stuart Shelf isconsidered to consist of Gawler domain rocks and it is within this basement thatthe Olympic Dam deposit occurs. No correlation between the Olympic Damsuccession and specific units of the tectonically composite Gawler domain can bemade at this stage although regional stratigraphie data suggest that the OlympicDam sequence is undeformed Middle Proterozoic sequence. The deposit straddlesa west-northwest trending photolineament corridor at the intersection of a majornorth-northwest trending gravity lineament (O'Driscoll and Keenihan, 1980;O'Driscoll, 1982).

Early Proterozoic metasediments and reworked Archaean gneisses of the GawlerCraton (Fig. 1) were strongly deformed and intruded by granite as a result of theKimban Orogeny (1820 to about 1580 Ma; Webb et. al., 1982). Voluminoussubaerial felsic volcanics (The Gawler Range Volcanics) are relatively flat lyingand form a large basin-like feature in the approximate centre of the Gawlerdomain, surrounded by post tectonic granites (Fig. 1). The volcanics andassociated granites lack penetrative deformation and have ages between about1525 and 1450 Ma (Webb et. al., 1982).

The Adelaide Geosyncline to the east contains a 10km thick sequence of UpperProterozoic and Cambrian sediments and minor basic volcanics. The BedaVolcanics which occur in the south eastern part of the Stuart Shelf and in theGeosyncline have been dated at 1076 Ma (Webb and Coates, 1980). The rocks ofthe Geosyncline display a marked 500 Ma deformation and granite intrusion event,the Delamerian Orogeny but those of the Stuart Shelf are unaffected by the 500Ma event.

73

In the Olympic Dam area about 350 metres of cover sediments are separated fromthe underlying basement rocks by a major unconformity. A number of prospectscontaining sub-economic copper or copper-uranium mineralization have beenidentified outside the Olympic Dam area by drilling of geophysical anomalies. Thestyle of mineralization in some of these is very broadly similar to that underdiscussion.

GEOLOGY OF THE OLYMPIC DAM DEPOSIT

Stratigraphy

The host sequence for the Olympic Dam mineralization is un metamorphosedmedium to coarse-grained, potassium feldspar-rich, alkali feldspar granitesurrounding sedimentary breccias or rudites (Roberts and Hudson, 1983). Doleriteintrudes the granite and breccias, but not the cover sediments.

Breccias have been classified into two broad groups on the basis of clast type andmatrix content: (1) Monomict: one clast type present. Matrix content usually lessthan 10 percent; and (2) Polymict: two or more clast types in various proportionsand matrix content 10 to 80 percent, usually greater than 30 percent. Thepolymict group is further divided into three sub-groups: (1) one clast typepredominant (greater than 80 percent of clasts), (2) two clast types predominant(about 40 percent each), and (3) three or more clast types with nonepredominant. The two clast and three or more clast groups contain most of thesignificant mineralization.

A large number of different clast types occur in the polymict breccias includingaltered basement granite; felsic, intermediate and mafic volcanics; various typesof hematite-rich rocks including banded iron formation and hematitic siltstone;arenite; carbonate; fluorite; barite and sulfides (Fig. 7). Reworked clasts ofbreccias are common, particularly towards the top of the sequence. Clast sizesrange up to 12m but most are in the range 1 to 3 cm.

Eight different types of matrix are defined on the basis of composition andcolour. Crystalline black and grey hematite, granular brown hematite and earthyred hematite are the main matrix types but sericite, chlorite and siderite-richtypes are also observed. Arkosic matrices are also common in some parts of thedeposit. The grain size of the matrix material is 0.05mm to 2mm and it isgenerally a fine-grained equivalent of the clasts present in the breccia. Silty andclayey matrices are uncommon.

The sediments have been divided into two main lithostratigraphic units, theOlympic Dam Formation and the Greenfield Formation. Both formations have

74

Unconformity

I A | Dolerite

GREENFIELD FORMATIONMEMBERS

[w] Volcanic

[Tj Hematite Breccia

[sj Lower Silicified

OLYMPIC DAM FORMATIONMEMBERS

D Upper GraniteBreccia

[UT| Brooks

[D| Whenan

| 7 [ Black Hematite

D Lower GraniteBreccia

[T] Alkali Granite

Fig. 4. Diagrammatic composite stratigraphie section of pre-Adelaidean units.Lithologies are summarized in Table 2. Approximate vertical extent is 1,500 to2,000m.

/ v v v v v v v vV V V

V

GREENFIELD FORMATION

ALKALI GRANITE

mLU

OLYMPIC OAM FORMATION

Upp*r Grinlt« Br«cel« Member

I 1 Brooh«-Wh»nan-BI»ck Hcmallt* M«mb«r>

Sldvrlt« rich zon«

Fig. 5. Generalized geologic plan of the Olympic Dam deposit at the -450m level.

75

TABLE 2

OLYMPIC DAM STRATIGRAPHY

Unit

Andamooka Limestone

Wilpena Group

Arcoona Quartzite

Corraberra Sandstone

Tregolana Shale

Thickness(max.)

60m

200m

Lithology

Massive, locally bedded limestone.

White & red massive & cross beddedsandstone.

20m Red, massive & cross bedded sandstone.

200m Thinly laminated red &. green shales.

- - UNCONFORMITY - - - - - - - - - - - - - - - -

Greenfield Formation

Volcanic Member 250m

Hematite Breccia Member 500m

Lower Silicified Member 275m

Olympic Dam Formation

Upper Granite BrecciaMember

Brooks Member

1000m

300m

Whenan Member 350m

Black Hematite Member 150m

Lower Granite Breccia 800m +Member

Interbedded altered felsic volcanics,volcanic breccias, conglomerates & bandediron formation.

Hematite rich breccias with thin interbedsof polymict breccias & hematitesiltstones. Veined by silica and barite.

Strongly hematized & sihcified polymictbreccias dominated by quartz, hematite &altered volcanics.

Granite breccia & weakly polymict granitebreccias, matrix-poor. Sencite & hematitealtered. Chalcocite-bormte mineralizedzones.

Polymict breccias with a variety of clasttypes - sencitized felsic volcanic clastsdiagnostic. Discrete felsic volcanic lenses.Variable matrix of brown and blackhematite. Reworked clasts of underlyingmembers. Variably mineralized with bornite(chalcocite), & chalcopynte. Sidenteclasts locally abundant.

Polymict breccias, red & black hematiteclasts & matrices. Cyclic zones ofmatrix-rich & matrix-poor breccias.Bedded & graded units common. Stronglymineralized with bormte-chalcopynte(pyrite).

Black hematite dominated breccias both asclast & matrix. Specular hematite,chlonte & siderite in matrix. Porous,often vuggy. Pyrite (chalcopynte)mineralized.

Granite breccia with thin polymict breccialenses often containing distinctive chlontealtered volcanic clasts. Chlonte alteredcontact with above member & granophyreclasts.

76

WEST EAST

GREENFIELD FORMATIONMEMBERS

COVER [VJ Volcanic

DOLERITE [A] Hematite Breccia

\~] Lower Silicified

•~~~ Unconformity

Fault

OLYMPIC DAM FORMATIONMEMBERS

O Upper Granite Breccia

(V] Brooks

E3 Whenan

0 Black Hematite

FT] Lower Granite Breccia

Fig. 6. East-west cross section of the Olympic Dam deposit at 200,OOON (A-A1 inFig. 5).

several members. Contacts between the formations and members withinformations are often disconformable. Drilling has not penetrated the base of theOlympic Dam Formation.

A composite stratigraphie section of the basement units is illustrated in Figure 4,their broad distribution in plan at the -450m level, and in section at 200,OOON isshown in Figs. 5 and 6. Table 2 summarises their lithology.

Olympic Dam Formation

The Olympic Dam Formation is in excess of 1000m thick and comprises fivemembers as indicated in Table 2 (Roberts and Hudson, 1983).

Lower Granite Breccia Member: This member consists of predominantlymatrix-poor breccias containing basement granite clasts with rare, thinlenses of pyrite and hematite-rich polymict breccias. The breccias showvarying degrees of alteration to sericite, hematite and chlorite. Chloritealteration occurs at both the upper and lower contacts of the polymictbreccia lenses and is particularly well developed below the Black HematiteMember. In some areas microgranite dykes intrude this member but do notextend into the overlying members.

77

c

E

Fig.7. Photographs (A-D) of cut sections of drill core and photomicrographs (Eand F) of stratabound mineralization. A. Polymict breccia of granite (G),hematite (H), fluorite (black), and chalcopyrite (cp) with disseminatedchalcopyrite and fluorite in the matrix. Cross-cutting fluorite vein. Note grossbedding; top = right. B. Thin graded-bedded units in Whenan Member. Notegrading of chalcopyrite (white), micro-faulting and load cast around granite clast(G), top = right. C. Clastic and disseminated chalcopyrite (white) in matrix-richbreccia. D. Disseminated and clastic bornite (bn) in red-brown hematite matrix-rich breccia with altered granite clast (G). E. Chalcopyrite (cp) with pyrite (py),intergrown with hematite laths. F. Bornite (bn) with fine-grained annular zones ofhematite, and chalcopyrite (cp) in hematite with quartz clasts and a zonedintergrowth of uraninite-bornite (u).

78

Black Hematite Member: This is the basal member of the main mineralizedzone. It contains beds of massive unbrecciated hematite as well as brecciaswith black crystalline hematite dominant both as clasts and in matrices.Spéculante, fluorite, siderite and chlorite are present in the matrix and thebreccias contain thin vuggy and porous zones. Pyrite-rich mineralization ischaracteristic of the breccias in this member.

Whenan Member: This member consists of a series of thin (up to 15m thick)beds of polymict matrix-rich breccias and matrix-poor granite-rich brecciasaggregating to a total thickness in excess of 200m in many areas. Thebreccias are intercalated with thin bedded silty lenses and arenitic sedimentbands. The hematite in the lower beds is dominantly black and that in thebeds towards the top of the member is reddish brown. Sedimentary featuressuch as graded bedding (Fig. 7B) and upwards-fining beds up to 3 metresthick occur, and series of upwards-fining beds are not uncommon. Loadcasts, scours and, less commonly, cross-bedding are present. This memberhosts most of the chalcopyrite-bornite mineralization in the deposit. Mostof the sulfides occur in the matrix-rich beds.

Brooks Member: This member is not always developed and granite clasts aremore abundant than in the underlying Whenan Member. The matrix contentis variable and is predominantly brown hematite. Characteristic clasts inthis member are sericitized felsic volcanics and reworked Whenan Memberand Black Hematite Member clasts. Siderite-rich clasts are also common insome areas. Mineralization is commonly restricted to the matrix-rich zonesand is irregularly developed. Bornite and chalcopyrite are the dominantsulfides with minor chalcocite in some areas. Discrete lenses of pale greensericitized volcanics are common.

Upper Granite Breccia Member: Although lithologically similar to the LowerGranite Breccia Member, this member is not chloritic, but is usually sericite,hematite and silica altered. Mineralization is restricted to thin (less than10m thick) lenses, and veins composed of hematite, fluorite, chalcocite andbornite. Thin lenses and dikes of strongly sericitized felsic volcanics arecommon.

Distribution of Members: The five members of the Olympic Dam Formationare not always intersected in drill holes and they display complex facièsrelationships in many areas. In the northwestern part of the deposit thethree middle members are extensively interbedded with the Upper GraniteBreccia Member although features of the Black Hematite Member, WhenanMember and Brooks Member are observed. On the northeast flank of the

79

main mineralized zone (Fig. 5) siderite is a major matrix and clastcomponent in the three members and their definition is less clear,particularly the Black Hematite Member. Elsewhere, the Black HematiteMember and Whenan Member may be separated by barren granite-breccialenses.

Greenfield Formation

The Greenfield Formation is up to 700m thick and comprises the very hematite-rich upper units of the stratigraphie sequence preserved in downfaulted blocks, ithas both conformable and disconformable contacts with the Olympic DamFormation (Fig. 5). The unit has been subdivided into the Silicified Member, theHematite Breccia Member and the Volcanic Member. The Silicified Memberconsists of polymict breccias containing clasts of hematite, quartz and volcanicsin hematite-rich matrices. The breccias are strongly Silicified and veined by silicaand barite. The members of the Greenfield Formation are poorly mineralized.

Distribution of Members: The Greenfield Formation is restricted in itsdistribution and within it, the Silicified Member and the Volcanic Memberare also restricted. The Silicified Member is developed along the easternpart, and the Volcanic Member appears to be restricted to the northern andeastern parts of the Greenfield Formation block (Fig. 5).

Structure

Locally the sequence hosting the Olympic Dam mineralization is confined by afault-bounded trough informally called the Olympic Dam Graben. The graben hasa probable northwest trend with a length in excess of 7km and a maximum widthof 5km. The base of the sedimentary sequence in the graben has not beenintersected in drilling and the sequence therefore has a minimum thickness of1km. The main structural elements of the graben are shown in Fig. 8.

The graben is arched about a northeast-trending axis. Limited evidence alsosuggests local arching within the graben about north-west axes with thedevelopment of small localised domes and basins in some areas (N. Oreskes and W.Brown, pers. comm., 1982). Dip-slip and strike-slip faults occur parallel to boththe graben and main arch axis and fault displacement is concentrated on thenortheast margin of the graben. Faults roughly parallel to the graben axis aredisplaced by those at a high angle to the axis. None of the faults dislocate thecover sediments. The arching, faulting and erosion prior to the deposition of thecover sediments exposed lower members of the Olympic Dam Formation on the

80

\ \ DOLERITE

GREENFIELD FORMATION

LIMIT BROOKSBLACK HEMATITE

WHENANMEMBERS

Fig. 8. Plan showing main structural elements of the Olympic Dam graben.

unconformity in the vicinity of the crest of the arch (east of Whenan Shaft).Upper members of the Olympic Dam Formation are thicker to the southeast andnorthwest. Remnants of the previously more extensive Greenfield Formation havebeen preserved in downfaulted blocks in the crest of the arch.

The Olympic Dam Formation dips to the southwest at 15° to 20° in thesoutheastern half of the graben. Limited data suggest a flatter north or northeastdip in the northwest half. Local steepening and reversal of dips are observed nearfaults, synsedimentary slump breccias and local domes. Widespread slumpstructures are present in thinly bedded units.

Intense shearing has not been observed but intervals of closely jointed andfractured drillcore, and zones of very intense alteration, could be indicative offaults. There are marked variations over short lateral distances in the depth todistinctive units within the stratigraphie sequence. Thick lithological unitsdisappear and upper stratigraphie units can be adjacent to lower stratigraphieunits indicating vertical displacement. Extensive lateral (strike-slip) displacementalso occurs in the graben-fill sequence with maximum displacement parallel to thegraben long-axis. In many areas contemporaneous (growth) faults dislocate thelower members of the Olympic Dam Formation but do not affect the Brooks

81

Member and Upper Granite Breccia Member. Evidence of widespread penetrativedeformation and intense folding has not been observed.

Alteration

All rocks in the Olympic Dam Graben are altered to varying degrees. Thedominant alteration minerals are hematite, sericite, chlorite, silica andcarbonate. There are two types of alteration - a weak pervasive type affecting allrocks and a distinct, more intensive type related to mineralization.

1. Pervasive alteration. Hematite, sericite and chlorite alteration affects allrock types with dolerite least affected. Localized areas of silica andcarbonate alteration occur. This style of alteration is most apparent inbreccias with a high granite component where feldspars and ferromagnesianminerals are replaced.

2. Intense alteration. Intense hematite and chlorite alteration are associatedwith mineralization in the lower parts of the Olympic Dam Formation.Sericite and silica alteration are predominant in the upper members of thisformation.

The broad vertical zonation of intense alteration can be used as a general guide toposition in the stratigraphie sequence when distinctive units are lacking.

MINERALIZATION

Copper, uranium, rare earths, gold and silver mineralization occur in the OlympicDam deposit. The bulk of the potential ore grade mineralization occurs in theOlympic Dam Formation. There is normally a direct relationship between theamount of mineralization and the amount of matrix in the host breccias.

Copper Mineralization

Two distinct types of copper mineralization are defined on the basis of sulphideassemblage, rock association and stratigraphie position (Roberts and Hudson,1983). The older mineralization is strata-bound and the younger is largelytransgressive. Significant uranium, rare earths and gold mineralization isassociated with both types. The two types are normally mutually exclusive butsome areas of overlap occur.

Bornite (chaleocite)-chalcopyrite-pyrite strata-bound mineralization: Thestrata-bound type occurs in the three hematite matrix-rich members of the

82

1000m

Greenfield Formation Block

Transgress ive Chalcocite-Bornite

Strata-Bound Bornite-Chalcopyrite-Pyr ite

Fig. 9. Plan of the distribution of sulfide mineralization types.

Olympic Dam Formation throughout the graben (Fig. 9), but reaches itsmaximum thickness (up to 350m thick) in the north east crest zone of themain structural arch. Most of the Sulfides occur in the matrix as uniformdisseminations (0.1-2mm) that make up 5 to 20 percent of the rock but someare present as coarse clastic fragments up to 5cm in size (Fig. 7).Replacement and micro-cavity fill textures are common and in some finelybedded thin layers stratiform sulfides are present. Rock clasts containingdisseminated sulfides, and sulfide rims on clasts are abundant in some areas.

The dominant gangue minerals in the strata-bound type are hematite, quartz,sericite and fluorite in high grade copper zones.

Vertical zoning of sulfides is broadly consiste.it, grading from a relatively sulfur-rich copper-poor assemblage (pyrite(chalcopyrite)) at the base of the BlackHematite Member, to a relatively copper-rich sulfur-poor assemblage (bornite-chalcopyrite) at the top of the Whenan Member, and in the Brooks Member.Supergene processes are not considered to be a major factor in sulphide zonation(Roberts and Hudson, 1983).

Minor ore minerals in the strata-bound mineralization are carrollite, cobaltite,native silver, silver telluride and gold.

83

Fig. lu. Photograph (A) and photomicrograph (B) of transgressive mineralization.A. Disseminated and iensoid chalcocite-Dornite (white,) with fluorite (olack) inhematite matrix, ß. Complex vermiform intergrowth of chalcocite (white) andDornite (gray).

Transgressive chalcocite-bornite mineralization: Cross cutting veins andirregular conformable lenses of uranium-bearing chalcocite-bornitemineralization are found in the Brooks Member and Upper Granite BrecciaMember of the Olympic Dam Formation (Fig. 10). This youngermineralization is confined to linear zones parallel to the long axis of thegraben with the main zone being at least 6km long and up to 0.7km wide(Fig. 9). Smaller zones are developed to the northeast and southwest of themain trend. The thickest intersections of mineralization of this type are inexcess of 300m. The thickness of mineralized zones is not necessarilydependent on the host unit thickness or lithology and there is no consistent

84

relationship between the concentration of sulfides and matrix content as inthe strata-bound mineralization.

The grain size of the sulfides varies from sub-micron inclusions in hematite tomassive clasts up to 10 centimetres across, but most are less than 0.5mm. In alltextural types the chalcocite and bornite are complexly intergrown (Fig. 10).

The dominant gangue constituents are hematite, fluorite and quartz with lessersericite and chlorite.

Minor ore mineral constituents are native gold and silver, digenite, covellite,cobaltite, a copper-nickel-cobalt arsenate and native copper.

Uranium Mineralization

Three uranium mineral species have been identified. The pitchblende variety ofuraninite (130%) is the most abundant. Coffinite (U(SiO4)1_x(OH)4x) is subordinateto uraninite while brannerite { (U,Ca,Ce)(Ti,Fe)2C>6) accounts for only a minorproportion of the uranium mineralization. The grain size of the uranium mineralsvaries from sub-micron to several millimetres but most of the grains are less than50 microns.

Uraninite and coffinite are intimately associated and occur throughout themineralized zones with sulfides, sericite, hematite, fluorite and to a lesser extent,chlorite. Their form varies markedly and includes globular aggregates, micro-veinlets penetrating grain boundaries, fracture coatings, grain and crystal rimsand oriented intergrowths with sulfides (T.R. Peachey pers. comm., 1981, Grey,1978), (Fig. 11). A close association with bornite occurs in areas of uraniummineral veining.

Brannerite is restricted in its distribution and usually occurs in veins in the BrooksMember of the Olympic Dam Formation. It is closely associated with rutile(anatase), sericite and hematite and usually occurs as patchy aggregates ofirregular grains within sericite or as grain composites with hematite. Thebrannerite grains are always surrounded by small (1 to 10 microns) grains oftitanium oxide (rutile, anatase). Zircon crystals and grains are also present in thehalo of oxide grains.

Other Mineralization

Rare earths minerals bastnaesite (Ce,La)CO3(F,OH) and florencite(CeAl3(PO4)2(OH)g) and free gold are associated with both types of copper-uranium mineralization.

85

D

Fig. 11. Uranium mineralization, photomicrographs (A-D) and scanning electronmicrographs (E, F) of typical mineral textures. A. Coarse-grained uraninite (darkgray) with coffinite (light gray) replacing and surrounding bornite (white).B. Uraninite veinlets (gray) with bornite (white) in quartz. C. Concentricallyzoned intergrowth of chalcopyrite (white), bornite (gray) and uraninite (dark gray)in quartz with hematite and chalcopyrite. D. Uraninite-coffinite (u) surroundinga silicate clast with hematite (white). E. Very fine-grained uraninite (white)rimming bornite. F. Uraninite (white) in bornite; note crystallographic control ofveinlets (E & F after Grey, 1978).

86

TABLE 4

Cu %

U3°8AuAgLa

Ce

Pb

Zn

Ni

Co

Mo

As

Sb

Bi

Te

HgSn

Cd

Pt

Pd

Y

F %

BaO

Fe %

SiO2 %CO 2 %

P2°5 %

MnO %S%

IMPOSITION OF MINERA

1

1.15

4400.41.0

13002000

901303001355085844

0.08421

0.050.01110

1.377250

37.7029.60

0.970.200.102.03

Strata-boundbn-cp-py

2

1.72

6400.52.0

17002700

8580

290120

4865

68

100.05

301

0.050.05135

1.503450

30.4034.501.900.500.102.00

32.553352.47.0

180026503060280101101304410

0.05451

0.050.0535

0.90930017.4049.100.200.800.010.66

Transgress! vecc-bn

42.804400.154.02002200608547036604410

0.05201

0.050.0590

0.90352021.5044.500.080.150.011.13

Concentrations in ppm unless stated.

bn-bornite, cc-chalcocite, cp-chalcopyrite, py-pyrite

1. and 2. - Composite bulk samples from 10 (5 in each) drill hole intersections nearthe Whenan Shaft.

3. - Composite bulk sample from 6 drill hole intersections south of the WhenanShaft.

4. - Composite bulk sample from 7 drill hole intersections in the northwest part ofthe deposit.

All analyses by Comlabs Pty. Ltd., Adelaide, South Australia.

87

Lower Granite*BrecciaMember

Fig. 12. Down-hole plot from drill hole RD55 of Cu, U3O8, La, Ce, F and Fe instrata-bound mineralization showing relationship of these elements to lithology.

LITHOLOGY

Lower

Silicified

Member

Upper

Granite

Breccia

Member

wt% ppm

2 4 6 810 oo

toOO

ooo

La

PpmCOOOO

Ce

ppm wt%

Fe

wt%

Fig. 13. Down-hole plot from drill hole RD47 of Cu, U3Og, La, Ce, F and Fe intransgressive mineralization.

88

Geochemistry

The general composition of the two main mineralization types are indicated inTable 3 and the concentrations of the main ore elements copper, uranium,lanthanum, cerium, fluorine and iron and their relationships to lithology in thestrata-bound mineralization are illustrated in Fig. 12. A general positivecorrelation is evident for these elements in matrix-rich breccia units. Thecorrelation is not as well defined on a detailed scale but positive correlations existbetween uranium and the light rare earths lanthanum and cerium. A less definedpositive correlation exists between these elements and fluorine.

Ore element relations in the transgressive mineralization are shown in Fig. 13.

A number of general trends are apparent from simple element plots. These are:

Copper, uranium, iron, lanthanum, cerium and fluorine are concentrated inmatrix-rich polymict breccia units.Iron increases towards the base of strata-bound mineralized zones.Copper distribution is more uniform towards the base of mineralized zones.Uranium and lanthanum plus cerium can occur in relatively highconcentrations without associated high copper concentrations but thereverse is rarely observed.Cobalt concentration is elevated in the lower, more pyrite-rich, parts ofstrata-bound mineralized zones.Barium content tends to be antipathetic to copper, uranium, lanthanum,cerium.Copper and uranium contents are low in siderite-rich zones which are usuallylower in the sequence except in the northeast part of the main mineralizedarea.Lead and zinc contents are very low throughout the copper mineralized area.Silver content tends to be higher in bornite rich mineralized zones but itsdistribution is erratic within these zones.

DISCUSSION

The Olympic Dam deposit is an example of a very unusual type of sediment-hostedmineralization and no analagous deposits appear to have been documented. As thedelineation of the deposit is at an early stage and data are incomplete, thefollowing discussion is a preliminary attempt at synthesizing some of the featuresof the lithology and mineralization.

89

Formation of the Deposit

The initial activity leading to the formation of the Olympic Dam deposit appearsto have been the development of a thick pile of sedimentary breccias (the OlympicDam Formation) in a northwest trending trough in response to major faulting. Theessential features of the pile are a series of matrix-rich polymict breccia units(Black Hematite Member, Whenan Member and Brooks Member) which containstrata-bound copper, uranium, rare earths and gold mineralization and areunderlain and overlain by barren granite breccias.

The breccias were probably deposited in an arid subaerial environment by debrisflows, mudflows, lahars and rock avalanches along active fault scarps.Depositional environments and coarse-grained sediments with similar lithologicalcharacteristics to those in the Olympic Dam deposit have been described in theliterature (see Roberts and Hudson, 1983) but none of these examples ismineralized.

The first mineralizing event occurred during sedimentation, with deposition ofstrata-bound hematite, sulfides, uranium and rare earths minerals, gold and silver,fluorite, siderite and barite. This event could have been related to geothermalactivity resulting from volcanism but has some similarities with stratiform copperdeposits (Roberts and Hudson, 1983). The later epigenetic chalcocite-bornite andgold mineralization with associated uranium and rare earths was introduced infavourable structural and lithological zones during the waning stages of the earliermineralizing event (Fig. 14). Fluorite, barite, siderite and hematite veins werealso formed at this time.

The mineralized part of the sequence is overlain by distinctive hematite-richbreccias, banded iron formation, volcanic breccias and conglomerates, felsic lavasand tuffs which indicate that volcanism occurred during deposition of at least partof the breccia sequence. The thinly bedded iron formations were probablydeposited in a playa lake environment with iron possibly being introduced from afumarolic source.

Rapid faciès changes in the basin of deposition, contemporaneous faulting andlater faulting produced a complicated geometry in the mineralized zones.Dolerite dykes intruded the sequence which was then eroded and later covered bystable shelf sediments. The absence of penetrative deformation andmetamorphism in the deposit indicates an age younger than the 1580 Ma and olderdeformation events of the Gawler domain to the south and older than the doleritedikes which do not intrude the Adelaidean cover sequence.

90

VeinsBornite

Chalcopyrite Pyrite

ChalcociteBornite

Lenses in Matr ixRich Breccias

Sericite Quartz Al terat ion

ChalcociteBornite Chalcopyrite (Pyrite)

Bornite (Chalcocite)Bornite-Chalcopyrite

Chalcopyrite (Pyrite)

Chalcopyrite-Pyri te

Pyrite (Chalcopyrite)

Siderite Pyrite(Chalcopyrite)

Chlorlte Alteration

Fig. 14. Diagrammatic representation of the two types of mineralization in theOlympic Dam deposit.

The unusual association of copper, iron, uranium, rare earths and gold, the overalloxidized host environment, the sulfide zoning and alteration related tomineralization, pose a number of interesting conceptual problems. The highpotassium, rare earths, barium and fluorine contents are suggestive of alkaline-igneous activity in the region but the alkaline igneous rocks have not been foundat Olympic Dam. The copper, iron and gold probably originated from a maficsource. Some of the hematite may also have resulted from the in-situ oxidation ofsiderite, magnetite and chlorite.

The Hematite Association

The strata-bound mineralization occurs within hematite-rich rocks. Much of thehematite appears to have been deposited from solution, and textures indicatesimultaneous, rhythmic and sequential deposition of sulfides and hematite. Theassociation of hematite with sulfides, the sulfide-hematite textures, theoccurrence of barite adjacent to mineralized zones, the antipathetic relationbetween barite and sulfides, and the relative absence of sulfides in siderite-bearing rocks indicate that the sulfide precipitation could have been caused by thesimultaneous oxidation of ferrous iron and reduction of sulfate. The ferrous ironcould have been introduced by hot springs along the active faults into the sulfate-rich waters of playa lakes in the Olympic Dam graben (D.W. Haynes, pers. comm.,1981).

91

An analogous process of sulfate reduction and ferrous iron oxidation has beendescribed by Mottl et al. (1979) and Seyfried and Dibble (1980). The well-developed chlorite alteration zone below the mineralization may be the result ofinitial percolation of ferrous iron-rich electrolyte into the underlying porousbreccia.

The higher level, transgressive, chalcocite-bornite-gold mineralization appears tobe epigenetic and later than the strata-bound mineralization. The commonassociation of sulfides and directly precipitated quartz and the well-developedalteration halo around sulfide veins suggests that the temperature of depositionwas probably higher than in the strata-bound type.

Uranium and Rare Earths

The uranium and rare earth mineral textures indicate that they were depositedboth during and after the main sulfide mineralizing event. The composition andclose association of uranium and rare earth minerals with sulfides, fluorite andhematite indicate that fluorine, iron, carbonate, phosphate and sulfur were closelyrelated to the transport and deposition of uranium and rare earths. Theassociation of uranium-fluorine is a common one in many volcanic-related uraniumdeposits in continental environments (Curtis, 1981) and supports themineralization-volcanism hypothesis. The very close association of uranium anddirectly precipitated hematite in what appears to be a low-temperature strata-bound deposit is very unusual (Nash et al., 1981). However, the intimateassociation of the tetravalent uranium species and hematite could indicate thatthe ferrous iron oxidation and reduction of hexavalent uranium (present as a

sulfate in the playa lakes) was a possible precipitating process for some of theuranium (D.W. Haynes, pers. comm., 1982).

Conclusions

The most unusual features of the Olympic Dam deposit are:(1) the association of high concentrations of iron, copper and gold with high

concentrations of uranium, fluorine and rare earths;(2) the evidence of simultaneous rhythmic and sequential deposition of sulphides

and hematite;(3) the deposition during sedimentation in a very high energy environment; and(4) the size of the deposit.

The present uncertainty of interpretations will diminish with further drilling,underground development and research, and until this is accomplished, attempts toexplain the genesis of the deposit will continue to be speculative.

92

Acknowledge™ ent

This paper is abridged from that published in Economic Geology, Vol. 78, No.5.,August 1983 and we thank the Editor for permission to reproduce parts of it here.

We are grateful to the management of Roxby Management Services Pty. Ltd.,Western Mining Corporation Limited and British Petroleum Limited for permissionto publish this paper. A number of the staff of RMS and WMC have contributed tothe data presented and we thank them all.

Mrs. J. Hall typed the manuscript, Mr. J. Collins and Mr. C. Furstner prepared thefigures.

REFERENCES

Curtis, L., 1981: Uranium in volcanic and volcaniclastic rocks - examples fromCanada, Australia and Italy, in Goodaell, P.C., Waters, A.C., eds., Uraniumin Volcanic and Volcaniclastic Rocks; Am. Assoc. Petroleum GeologistsStudies in Geol. 13, p.37-54.

Grey, I.E., 1978: An X.R.D. and S.E.M. study of uranium and rare-earth bearingminerals in W.M.C.'s Olympic Dam copper deposit. Unpubl. C.S.I.R.O.Mineral Chem. Communication, June 1978.

Haynes, D.W., 1972: Geochemistry of altered basalts and associated copperdeposits: Unpub. PhD. thesis, Aust. Nat. Univ., Canberra, A.C.T., 244 p.

Mottl, M.J., Holland, H.D., and Corr, R.F., 1979: Chemical exchange duringhydrothermal alteration of basalt by seawater - ÏÏ. Experimental results forFe, Mn, and sulfur species: Geochem. et Cosmochim. Acta, 43, 869-884.

Nash, J.T., Granger, H.C., and Adams, S.S., 1981, Geology and concepts ofgenesis of important types of uranium deposits. Econ. Geol. 75th Anniv.Vol., 63-116.

O'DriscoU, E.S.T., and Keenihan, S.L., 1980: The Toowoomba-CharlevilleLineament in southern Queensland: Aust. Petrol. Explor. Assoc. Jour., 20,16-24.

O'Driscoll, E.S.T., 1982: Patterns of discovery - the challenge for innovativethinking: 1981 P.E.S.A. Distinguished Lecture. Petrol. Expl. Soc. Aust.

Preiss, W.V., Rutland, R.W.R., and Murrell, B., 1980: The Precambrian of SouthAustralia - the Stuart Shelf and Adelaide Geosyncline, in_ Hunter, D.R., ed.,Developments in Precambrian geology 2, Precambrian of the SouthernHemisphere; Amsterdam, Elsevier Scientific Publishing Co., 327-360.

Roberts, D.E., and Hudson, G.R.T., 1983: The Olympic Dam copper-uranium-golddeposit, Roxby Downs, South Australia: Econ. Geol., 78, 799-822.

Rutland, R.W.R., Parker, A.J., and Pitt, G.M., 1980: The Precambrian of SouthAustralia - the Gawler Province, in_ Hunter, D.R., ed., Developments inPrecambrian Geology 2, Precambrian of the Southern Hemisphere;Amsterdam, Elsevier Scientific Publishing Co., 309-327.

93

Seyfried, W.E. and Dibble, W.E., 1980: Seawater-peridotite interaction at 300°Cand 500 bars : implications for the origin of oceanic serpentinites: Geochem.et Cosmochim. Acta, 44, 309-321.

Thomson, B.P., 1969: Precambrian basement cover - the Adelaide System, in_Parkin, L.W., ed., Handbook of South Australian geology: South Aust. Geol.Surv. Govt. Printer, Adelaide, 49-83.

Webb, A.W., and Coats, R.P., 1980: A reassessment of the age of the BedaVolcanics on the Stuart Shelf, South Australia, South Aust. Dept. Mines andEnergy Kept. Bk. No. 80/6.

Webb, A.W., Thomson, B.P., Blissett, A.H., Daly, S.J., Flint, R.B., and Parker A.J.,1982: Geochronology of the Gawler Craton, South Australia: South Aust.Dept. Mines and Energy Kept. Bk. No. 82/86.

94

STRATA-BOUND URANIUM DEPOSITSIN THE DRIPPING SPRING QUARTZITE,GILA COUNTY, ARIZONA, USAA model for their formation

C.J. NUTT,U.S. Geological Survey,Denver Federal Center,Denver, Colorado,United States of America

Abstract

Uranium deposits in the Proterozoic Dripping Spring Quartziteare stratabound in diagenetica1ly altered, potassium-rich andcarbonaceous volcanogenic siltstones. The deposits are localizednear Proterozoic diabase intrusions and along monoclines that de-form the essentially flat-lying sequence. Uraninite and coffiniteare both in vertical veins and in sub-horizontal to horizontalveins along stylolites and bedding planes, and are disseminated infine-grained, potassium-feldspar-rich layers.

The uranium deposits in the Dripping Spring Quartzite arethought to be diagenet ic/sedimentary concentrations that were laterremobilized during diabase emplacement. Uranium, which may havebeen released from the volcanogenic sediments during diagenesis,could have been transported and concentrated in carbonaceoussiltstones at the time of diagenesis and stylolite formation.Circulating fluids associated with diabase emplacement remobilizedand concentrated uranium in carbonaceous siltstones close todiabase.

INTRODUCTIONUranium deposits in the Proterozoic Dripping Spring Quartzite

have been known since the 1950's, when more than 100 occurrenceswere identified in Gila County, Arizona, mostly in the Sierra Ancharegion (Fig. 1). Renewed interest in these deposits during the1970's resulted in additional exploration and drilling. Thedeposits and prospects are of particular interest because theyare the only known examples in the United States of Proterozoicstrata-bound uranium deposits in essentially flat-lying, onlylocally metamorphosed, sedimentary rocks. Granger and Raup

95

(1969a,b), who studied the deposits in detail during the period ofmining in the 1950's, proposed that the uranium was derived fromdiabase intrusions, but Williams (1957) and Nut t (1981) suggested

that the deposits wereremobilized sedimentaryconcentrations. This paper,an extension of the ideas ofNutt (1981), is in part basedon the study of core drilledby Wyoming Mineral Corporationin the Workman Creek area andon data supplied by Uranerz,U.S.A., Inc.

REGIONAL GEOLOGYThe Dripping Spring

Quartzite is part of theProterozoic Apache Group ineast-central and southeastern

Figure 1. Location map of the study area Arizona (Fig. 2). The Apache,in Gila County, Arizona, U.S.A., a neariy horizontal sequence ofshowing uranium deposits (•) andmonoclines. Modified from Granger and clastic sedimentary, carbonate,Raup (1969a,b). and volcanogenic rocks,

unconformably overlies abasement of folded and locally metamorphosed volcanic andsedimentary rocks, the Alder Group and Mazatzal Quartzite, thatwere intruded by granitic rocks (Fig. 2). The zircon U-Pb ages ofthe basement rocks range from about 1705 m.y. for the Alder Group(Ludwig, 1974; oral comm., 1983) to 1430 ± 30 m.y. for graniticintrusions (Silver, 1968; Ludwig and Silver, 1977). TheProterozoic Troy Quartzite lies unconformably above the ApacheGroup. Granger and Raup (1964, 1969a), Shride (1967), andBergquist and others (1981) have mapped and described the rocks inthe Sierra Ancha region. Smith (1969, 1970), Smith and Silver(1975), and Nehru and Prinz (1970) studied diabase s i l l s in theSierra Ancha. Wilson (1939) and An ders on and Wirt h (1981) examinedregional relations of the basement rocks, and Anders on and W i r t h(1981) have suggested regional tectonic settings for theProterozoic rocks.

96

uöNO

ot_CL

T3TJ

Troy IQuartziteN

+-< <A \ at \

MescalLimestone1

DrippingSpring

Quartzite

The uranium deposits in the Dripping Spring Quartzite, as wellas outcrops of the Apache Group, are predominantly in Gila County(Fig. 1), and par ticularily in the Sierra Ancha region, althoughthe Dripping Spring Quartzite is present elsewhere in southeastern

Arizona. The Sierra Ancharegion is in the southernpart of the Colorado PlateauProvince, in a zonetransitional to the Basin andRange Province to the south.As typical throughout theColorado Plateau, the strataare nearly horizontal exceptwhere folded by monoclines(Fig. 1). Faults and jointscut the Apache Group.

During the 1950's 21,360 tof ore with an average gradeof 0.24 percent U^Og wereproduced from 14 minesin the Dripping SpringQuartzite, 80 percent ofthe production coming fromthe Workman Creek and RedBluff properties (Otton andothers, 1981). An additional

PioneerFormation'

Arkos«. pebbly••ndsion«. quartz!te

UnconformityUNNAMED BASALT

UPPER PARTArgillite, dolomite

UNNAMED BASALTLOWER PART

Dolomite

Unconformity

UPPER PARTSiltstone

(In part volcano-genie), arkosicsandstone

LOWER PARTArkosic and feld-spathic sandstone,quartzite

Alder Group,MazatzalQuartzite,and 1430±30m.y.-oldand oldergranites

BARNES-CONGLOMERATEMEMBERTuffaceous siltstone,

arkose, feldspathicsandstone

SCANLANCONGLOMERATE.MEMBER

Unconformity

Granite, meta-sedimentary andmeta-volcanic rocks

Figure 2. Stratigraphie section of theApache Group. Modified from Grangerand Raup (1964).

grade of 0.05 percent

1,170 t averaged 0.07percent. Recent drillingat Workman Creek has outlinedan orebody with an estimated4,410 t U30g indicated inplace resources at a cut-off

and an average grade of 0.11 percent.

A p a c h e G r o u p

The Apache G r o u p , cons i s t i ng of the P ionee r F o r m a t i o n ,D r i p p i n g Spr ing Q u a r t z i t e , and Mesca l L imes tone ( F i g . 2 ) , over l ies

1430 ± 30 m . y . - o l d g r a n i t i c p l u t o n s ( S i l v e r , 1968; L u d w i g and

S i lve r , 1 9 7 7 ) and is i n t r u d e d by 1100-1150 m . y . - o l d d i a b a s e

( S i l v e r , 1960; Smi th and S i l v e r , 1 9 7 5 ) . In the S ie r ra A n c h a , the

97

1440 m.y.-old Ruin Granite (Ludwig and Silver, 1977) underlies theApache Group. The Pioneer, the oldest unit in the Apache Group,consists of a hasal conglomerate, the Scanlan Conglomerate Member,overlain by tuffaceous siltstone containing d e v i t r i f i e d glassshards (Gastil, 1954), arkose, and feldspathic sandstone.Geologists from Uranerz, U.S.A., Inc., report that in the SierraAncha a we 11-deve1 oped regolith separates the underlying RuinGranite from the Scanlan Conglomerate Member. The basal conglom-erate, the Barnes Conglomerate Member, of the Dripping SpringQuartzite unconformably overlies the Pioneer Formation and isoverlain by fine-grained sandstone and siltstone of the DrippingSpring Quartzite. The Mescal Limestone unconformably overlies theDripping Spring Quartzite and consists of a lower member of chertydolomite with halite molds and an upper member of cherty, stromat-olitic dolomite, argillite, and, locally, basalt. Basalt flows capthe Mescal. A s tratigraphica1ly continuous, 4.5- to 1 2.0-m-thickcarbonate breccia underlying the carbonate sequence has beeninterpreted by Shride (1967) to be the result of dissolution ofevaporites and subsequent collapse of higher portions of thesection.

The Troy Quartzite, although not part of the Apache Group, isProterozoic in age and was probably deposited in the same basin asthe Apache Group. Fine- to medium-grained arkose, pebbly sand-stone, and quartzite make up the unit, which lies unconformablyabove the Mescal.

Diabase intrudes nearly all sections of the Apache Group andthe Troy Quartzite. The diabase occurs predominantly as sill-likesheets 10-350 m thick, but contacts are locally discordant. Thelarge Sierra Ancha sill, formed by multiple intrusions of a high-alumina magma, has a central layer of feldspathic olivine-richdiabase and upper and lower layers of olivine diabase (Smith,1970). The diabase is extensively deuterically altered along thecontacts.

Granophyre, coarsely recrystallized rocks, and hornfels occuralong the contact separating diabase and the Dripping SpringQuartzite. The granophyres formed by melting and remobilizat ion ofsedimentary rocks (Smith, 1969; Smith and Silver, 1975). Miaro-litic cavities are common in the granophyres.

98

Dripping Spring QuartziteThe Dripping Spring Quartzite - actually misnamed as it is

largely composed of potassium-feldspar-rich rocks - is divided intolower and upper parts (Fig. 2) by Bergquist and others (1981).Granger and Raup (1964) defined additional units during their studyof the Dripping Spring Quartzite and the uranium deposits, but theregional mapping units are sufficient for this discussion.

The lower part, 85-110 m thick, consists of fine- to medium-grained arkosic and feldspathic sandstone, underlain by the basalBarnes Conglomerate Member. Chemical analyses from Smith (1969)show that two arkosic sandstones collected in the Dragoon Mountains(southeast of the Sierra Ancha) have ^0 contents of 6.2 and 8.3percent, which are higher than the normal values for arkosic andfeldspathic sandstones. Minor quantities of fine-grained potassiumfeldspar in the matrix are seen in thin sections from some samplesof the lower part.

The upper part, the ore-bearing sequence, is 75-110 m thickand consists of siltstone and arkosic sandstone. Rare impurecarbonate rocks are preserved as pyroxene-rich layers in contactmetamorphosed siltstone. Carbonaceous siltstones, locallyinterlayered with arkosic sandstone, occur in the lower section ofthe upper part and host the uranium deposits. Arkosic sandstoneand siltstone (not carbonaceous) compose the upper section of theupper part.

Stylolites are common in the upper part, where as many as 30stylolites occur in a one centimeter-thick layer. Stylolitescommonly separate coarse- and fine-grained strata and are concen-trated within fine-grained layers. Residuum, which accumulates asthe more soluble material is lost to intrastrata] fluids, is alongthe stylolite surfaces and consists predominantly of sphene, TiU2 ,carbonaceous matter, and pyrite.

Mineralogy and chemistry of siltstones in the upper partThe siltstones in the upper part are composed predominantly of

fine-grained potassium feldspar that is generally <5um in diameter(Granger and Raup, 1964). Granger and Raup (1964) identified thefeldspar as monoclinic, except in hornfelsic rocks where thefeldspar was microcline (triclinic). G. A. Desborough (oral comm.,1983) confirmed by X-ray diffraction the presence of both mono-clinic feldspar and microcline, but also identified microcline in

99

rocks that do not appear hornfelsic in hand specimen. Potassiumfeldspar composes from approximately 50 to more than 90 percent ofthe siltstone in the ore-bearing sequence of the upper part (visualestimate from thin sections). The chemical composition of theserocks reflects this high feldspar content with correspondingly highK^O values of 8 to 13.8 percent (Table 1). Medium-grained quartzand feldspar grains, commonly with irregular boundaries, arescattered through the potassium-feldspar-rich layers.

Siltstones s trat igraphically higher in the upper part havepotassium feldspar content visually estimated from thin sections tobe commonly 50 to 60 percent, but X-ray diffraction work has alsoidentified samples containing as much as a 95 percent potassiumfeldspar (G.A. Desborough, oral comm., 1983). Smith (1969) notedthat these carbon-poor rocks in the upper section of the upper partwere also K„0-rich.

The significance of the presence of fine-grained potassiumfeldspar and high K^O values.

High K^O content and large amounts of fine-grained potassiumfeldspar in siltstones suggest that the rocks are authigenicallyaltered volcanogenic rocks (Kastner and Siever, 1979). Thepresence of authigenic feldspar in altered tuffs has been welldocumented by Sheppard and Gude (1968, 1969, 1973). The feldsparthat forms during diagenesis of marine strata is monoclinic (G. A.Desborough, oral comm., 1983). Although diagenesis has destroyedprimary textures, the high l^O content and the abundance of fine-grained potassium feldspar in the upper part of the Dripping SpringQuartzite indicate that this unit has a large volcanic component.The rocks composed of about 95 percent feldspar may bediagenet ically altered tuffs, and those with lesser amounts offeldspar are probably reworked volcaniclastic rocks interlayeredwith detrital quartz and feldspar.

The high K20 and low Na2U contents of these rocks cannot beeasily explained by isochemical alteration of volcanic rocks. Sa-line pore water trapped or evolved during diagenesis of theDripping Spring Quartzite or fluids from the overlying evaporite-bearing Mescal Limestone are possible sources of potassium. Sodiummay have been lost to solutions during formation of potassiumfeldspar from zeolites, which form as an intermediate product

100

Table 1—Chemical analyses of »auples fron ehe Dripping Spring Quantité

SlOjAl20jFe2°3FeOMgOCaONa20K20H20+H20"TlOjP2°5MnOco2Total ZLOI

Organic C IU (ppm)Pb (ppm)Cu (ppm)Mo (ppm)V (ppn)Zn (ppm)

1-1721

62.215.22.21.30.461.40.2112.50.470.101.20.170.040.0197.5*

0.0529940

100050*

13

|AS

2-119256.814.12.81 .72.73.10.2

11.91.20.791.20.090.080.32

97.0*

0. 144210500930569554

4-38363.214.83.6*

0.420.410.68

11.50.890.411.30.09

<0.010.01

97.3*

0.636.73

<2090

<109714

5-95*43.610.811.12.85.20.970.078.02.62.50.950.150. 100.1189.. 02.71.15

11,2001020

41 ,000<IO8276

5-126557.814.84.21.61.80.820.11

12.11.30.860.940. 160.030.0796.60.60.27

269020064036806

7-71*61.316.22.11 .20.620.680.1013.80.620.240.770.170.010.1297.90.310.13

611903091466

7-S8759.014.74.01.71.30.200.11

12.31.10.820.720.080.040.0296.00.680.25

84208302400

43519

DS78-5862.916.40.280.080.120.180. 16

13.30.820.400.920.180.020.0295.83.12.92

16.44879

4922

DS78-20A2962.015.21.74.92.30. 180.238.82.70.250.660. 130. 100.02

99.2*

0.035.18

544145<270

152Rapid rock analyses of major oxides measured at U.S. Geological Survey laboratories by N.

Skinner, D. Ko bills, and ?.. Kami in. Method described under "single solution" In U.S.Geological Bulletin 1401.

U determined by delayed neutron activation at U.S. Geological Survey laboratories by K.Htllard, Jr., H. Coughlin, B. Vaughn, H. Schneider, W. Stang, S. Lasater, B. Keaten, C.Bliss, and C. McFee.

Cu, Pb» Mo, V, and Zn determined by optical spectroscopy at U.S. Geological labratories by P.Briggs, G. Riddle, V. Shaw, and B. Hatfteld

C determined by combustion» LECO

I . Upper pa r t » hornfelsic volcanogenlc siltstone. Ko microscopically visible uraniumminerals, but radloluxograph shows radioactivity concentrated In finely layered strata.Sample from Workman Creek core.

2. Upper part, finely laminated hornfelsic volcanogenlc slltstone with visible uraninite andcoffinite. Sample from Workman Creek core.

3. Upper part, hornfelsic carbonaceous volcanogenlc si Its tone. Sample from Workman Creekcore.

4. Upper part, hornfelsic carbonaceous volcanogenic stIt stone with coffinite both disseminatedand In veins. Sample f rom Workman Creek core *

5. Upper part, hornfelsic voIcanogenlc siltstone with uranlferous chlorite veins andcoffinite. Sample from Workman Creek core*

6. Upper part, hornfelsic volcanogenlc siltstone* Uranium minerals are not visiblemicroscopically, but radloluxographs show disseminated radioactivity. Sample from WorkmanCreek core.

7. Upper part, hornfelsic volcanogenlc slltstone with coffinite In veins and disseminated.Sample from Workman Creek ore.

8. Upper part, altered(T) carbonaceous volcanogenlc slltstone. Collected by J. K* Otton, fromSE V* sec. 11, T5N, R14E, McFadden Peak Quadrangle, Arizona (1:62,500).

9. Upper part, altered(7) slltstone with volcanogenlc component and fine-grained sandstone.Collected by J. K. Otton, from near Buckaroo Flats, T7N, R13E, Copper Mtn. Quadrange,Arizona (1:24,000).

between volcanic glass and feldspar: NaAlSi2Ûg'H20 4- SÎU2 +K4" -> KAlSi308 + H20 + Na+ (Surdatn, 1977).

The carbonaceous siltstone in the lower section of the upperpart has the highest K^O values, the most authigenic potassiumfeldspar, and presumably the greatest volcanic component, but above

101

average K^O values and the presence of fine-grained potassiumfeldspar elsewhere in the Dripping Spring Quartzite indicate thatmuch of the sequence may have some volcanic component. Detailedstratigraphie sampling is needed to show the abundance and distrib-ution of potassium-feldspar-rich rocks. Interestingly, the argil-lite in the Mescal Limestone also has high K^O content, suggestingthat this part of the Mescal also has a volcanic component.

The effect of diagenesis and me t amorphism on the Dripping SpringQuart zi t e

Diagenesis has profoundly altered the Dripping SpringQuartzite. Volcanogenic siltstones now have unusually high K^Ocontents and are composed of fine-grained potassium feldspar, andvolcanic textures and vo1 caniclas tic fragments have been largelydestroyed. Detrital quartz and feldspar grains and overgrowths ofthe same are irregular in outline because of extensive dissolution,especially where the grains are in contact with authigenic potas-s ium feldspar.

Widespread stylolite occurrence suggests extensive dissolutionand fluid circulation. Because dissolution that results in stylo-lite formation may cause thinning of the stratigraphie section - asmuch as 50 percent volume loss (Glover, 1969) - the presence ofnumerous stylolites in the Dripping Spring Quartzite suggests thatan appreciable amount of the section has been removed. Dissolvedsediments may include evaporites, a possible source of K^O, andcarbonate rocks, which are now only locally observed as calc-silicate layers in hornfelsic rocks.

Diabase intrusions are found throughout the thinly bedded ore-bearing upper part. Although macroscopically visible effects ofthe diabase on the Dripping Spring Quartzite are restricted togranophyre and re crys t al1ized rocks within about 30 m of thediabase (Granger and Raup, 1964) and metasomatism was confined torocks near the diabase (Smith, 1969; Smith and Silver, 1975), thewidespread presence of sphene and what Granger and Raup (1964)called spotted rocks (rocks with spherically shaped altered spots)suggests that diabase intrusions extensively altered the DrippingSpring Quartzite. The occurrence of microcline, instead of mono-clinic feldspar, outside the immediate vicinity of the diabasesuggests that a large volume of rock was affected by low-gradethermal metamorphism, probably caused by extensive fluid movement

102

and heating of the Dripping Spring Quartzite during diabaseemplacement; the extent of metamorphism probably depended on thepermeability of the rocks. Detailed sampling of stratigraphiesections and X-ray identification of the types of feldspar in therocks are needed to determine the actual extent of metamorphism inthe Dripping Spring Quartzite.

Smith and Silver (1975) suggested that the presence ofmiarolitic cavities in the granophyres and the extensive alterationof diabase near contacts indicate that water from the country rockscirculated through the diabase.

Depositional Environment of the Apache GroupThe Apache Group was predominantly deposited in a shallow

water environment in a stable cratonic setting (Anderson and Wirth,1981). The occurrence of carbonaceous siltstones in the DrippingSpring Quartzite is indicative of deposition of these sediments ina restricted environment. Carbonate rocks, evaporite molds, andstromatolites in the overlying Mescal Limestone suggest a supra-tidal to intertidal setting. The presence of relict glass shardsin the Pioneer Formation, high I^O contents and authigenicpotassium feldspar in the Dripping Spring Quartzite and MescalLimestone, and basalt in the Mescal Limestone is evidence ofvolcanic activity at the time of deposition of the Apache Group.The products of this intermittent volcanic activity were mostvoluminous during deposition of the Pioneer, the upper part of theDripping Spring, and the upper part of the Mescal and followingdeposition of the Mescal.

GEOLOGIC SETTING OF URANIUM DEPOSITSThe uranium deposits in the Sierra Ancha region are strata-

bound in the Dripping Spring Quartzite (Fig. 3). The deposits arein the carbonaceous volcanogenic siltstone sequence in the upperpart, are near diabase sills, and are predominantly restricted tostrata above and below a quartzite stratigraphie marker bed. Themost productive areas in the Sierra Ancha were near Workman Creekand Red Bluff (Fig. 3). Granger and Raup (1969a) noted uraniumenrichment in a carbonaceous shale in the Mescal Limestone; thisuranium occurrence is essentially the same as those in the DrippingSpring Quartzite where uranium is locally enriched in carbonaceouszones in siltstones. Uranium occurrences in the Sierra Ancha were

103

1 1 1 00'T E R T I A R Y

Troy Quartzite andMesca l L imestoneand Pioneer Formationof Apache Group

Apache Group,undi f ferent ia ted ,including DrippingSpring Quartzite

MIDDLE P R O T E R O 2 O I C

Dripping SpringQuartz i te

- Monocline, showingtrace and plunge:dashed whereapproximate ly located

Fault

Uranium deposit

3 3 4 5 ' —

12 km

2 mij

Figure 3. Uranium deposit localities and geologic map of the Dripping SpringQuartzite and diabase in the area of the Sierra Ancha and Cherry Creekmonoclines (Cherry Creek monocline is coincident with fau l t s ) . Geology isfrom Bergquist and others (1981), Granger and Raup (1964), and Wilson andothers (1959). Deposit locations are from H. C. Granger (wri t ten comm.,1983).

a l s o f o u n d b y g e o l o g i s t s o f U r a n e r z , U . S . A . , I n c . i n u r a n i f e r o u s

q u a r t z - h e m a t i t e - a p a t i t e m a t r i x f i l l i n g f r a c t u r e s a n d shear zones i n

t h e g r a n i t i c r e g o l i t h a t a n d n e a r t h e u n c o n f o r m i t y s e p a r a t i n g t h e

R u i n G r a n i t e a n d t h e A p a c h e G r o u p .

A l t h o u g h t h e d e p o s i t s a r e s t r a t a - b o u n d , u r a n i u m c o n c e n t r a t i o n

was a p p a r e n t l y i n f l u e n c e d by bo th s t r u c t u r e and the p r e s e n c e o f

104

diabase. Granger and Raup (1969a) were impressed by the proximityof diabase to uranium deposits, and an airborne magnetic surveyconducted by Uranerz, U.S.A., Inc. confirms the association ofdiabase and uranium anomalies (oral comm., 1983). It should benoted, however, that in the Sierra Ancha diabase sills are sowidespread in the Dripping Spring Quartzite that most outcrops ofDripping Spring Quartzite are near diabase or, possibly, near areaswhere the diabase has been eroded. Many of the deposits arelocalized along the trend of the monoclines (Fig. 1), most commonlyon the downdip side. Uranerz geologists have correlated thenumerous deposits of the Workman Creek area with the intersectionof an anomalous magnetic zone and complex faults associated withthe Sierra Ancha monocline. The Red Bluff deposit is situated neardiabase intruded along a fault and deposits are also clustered nearthe Cherry Creek monocline and associated faults (Fig. 3).Williams (1957) states that the ore is localized along north-northeast and north-northwest fractures that formed prior to oreformation. The uraniferous quartz-hematite-apatite fracture andbreccia matrix in the regolith at the unconformity is also concen-trated along the trend of the monoclines, near the uranium depositsin the Dripping Spring Quartzite.

Uranium in the Dripping Spring Quartzite is both disseminatedand in veins. The mines worked in the 1950's were developedprimarily in high-grade vertical veins. Core from the WorkmanCreek area, however, shows that uranium is also strata-bound onboth the outcrop and thin-section scale and is concentrated inhorizontal and subhorizontal veins, commonly along bedding planesand stylolites (Fig. 4). Even in coarse-grained recrystallizedrocks uranium is concentrated along remnant stylolites. Dissemi-nated uranium is in fine-grained, potassium — feldspar-rich layers;uranium is conspicuously absent from coarse-grained clastic andpyroxene-rich layers (Fig. 4).

Uranium ore occurs as uraninite and uranium silicatemineral(s). Uranium silicate was identified by the presence ofuranium and silicon in qualitative analyses on the electronmicroprobe energy dispersive X-ray analysis system, and powdercamera X-ray work confirmed the presence of coffinite mixed withother unidentified fine-grained minerals. Secondary uraniumminerals are also present. Radioluxographs of fine-grained samplesthat contain no visible uranium minerals show the presence of

105

s t y I o I i te s

\ s t y l o l i t e - r i c h_ _ _ , z o n e

c o a r s e - g r a i n e d ,lens

Figure 4. Radioluxographs of thin-section samples from the Dripping SpringQuartzite. Light areas have high radioactivity. a. Radioactivity isdisseminated and concentrated along stylolites. Radioluxograph exposuretime, 46.5 hours. b. Radioactivity is noticably absent in coarse-grainedlens within fine-grained, stylolite-rich section. Radioluxographexposure time, 7.5 hours.

uranium, either in submicros copic grains, on grain boundaries, orincorporated in minerals such as sphene. Sphene-fi lled stylolitesare commonly very radioactive.

Uraninite (called uraninite 1 in Granger and Raup, 1969a) isin veins and disseminated in layers that contain as much as 40percent uraninite. Grains are euhedral and subhedral and < 20 v m insize. An unidentified uranium-silicate mineral commonly forms thinrims around uraninite, and coffinite is concentrated along theedges of some uraninite-rich layers. Uraninite is associated withsphene, chalcopyri te, pyrite, galena, molybdenite, pyrrhotite,graphite, and, in a few places, ilmenite. All these minerals,except graphite, may contain inclusions of uraninite; uraniniteinclusions are also in recrys tallized feldspar matrix and phlogopite

Coffinite (called uraninite 2 in Granger and Raup, 1969a)distribution is similar to that of uraninite. Disseminatedcoffinite is yellow brown in transmitted light, <25ym in size, andeuhedral to subhedral. Coffinite in veins locally has a mottledgray-brown color in reflected light and differing ability to trans-mit light; this variability in color and reflectivity is probablydue to the presence of fine-grained minerals mixed with thecoffinite. Fine-grained galena cubes are invariably scatteredthrough coffinite and aid in identification of the coffinite.Coffinite is associated with chlorite, chalcopyrite , pyrrhotite,pyrite, molybdenite, and galena.

Veins of coffinite contain inclusions of uraninite that makeup major to minor parts of the veins. These uraninite inclusions,

106

the thin rims of uranium silicate on uraninite, and the coffiniteconcentrated along the edges of uraninite-rich layers suggest thatcoffinite formed after uraninite.

Alteration, which occurs only locally in the Dripping SpringQuartzite deposits, is primary composed of chlorite, iron sulfides,iron oxides, and unidentified, fine-grained, clear to brown, highlybiréfringent minerals and is most common in coffinite-rich rocks.Coffinite, chlorite, pyrite, and chalcopyrite occur together inpods that are scattered throughout uraniferous rocks. Near oreminerals and veins, feldspar grains have a cloudy, brown appearanceprobably caused by partial alteration to clays. Radioactivejarosite veins fill cross-cutting fractures.

The major element chemistry of uranium-bearing and barrensiltstone samples is quite similar (Table 1). Both have anoma-lously high K^O values, indicating potassium was not introducedduring ore-forming events. Magnesium and Fe2Uß values are highestin chlori tically altered rocks, the latter in part due to thepresence of fine iron sulfides and oxides mixed with the chlorite.

Other metals are also appreciably enriched in the uraniumdeposits. In 42 samples of high 1^0 siltstone from the WorkmanCreek prospect, molybdenum values ranged from less than 10 to 297ppm and averaged 59 ppm (values of less than 10 - the lower limitof detection - were assigned a value of 5 ppm); in these samesamples copper values ranged from 27 to 41,000 ppm and averaged1,998 ppm. Within the prospect, copper and uranium correlatepositively; the correlation coefficient for the 42 samples is0.7108. In contrast, within the deposit molybdenum has no corre-lation with either copper or uranium; the correlation coefficientfor molybdenum and copper is -0.0857, and the molybdenum - uraniumcorrelation coefficient is -0.0652.

Anomalous silver (0.7-70 ppm), cobalt, nickel, zirconium, tin,and lanthanum were reported in vertical veins and wall rocks(Uranerz, U.S.A., Inc., written comm., 1983), although it is notclear that these elements were carried by the same fluids thatcarried the uranium. Silver values are commonly greater in thewall rocks than in the veins. Vanadium is not enriched in the orezones and zinc values are not high in the core from Workman Creek,although zinc is enriched in veins sampled by Uranerz. Lead valuesaverage 121 ppm in the Workman Creek core, with the range of valuesfrom less than 20 to 1,020 ppm.

107

The age of high-grade veinlets of uranium minerals were deter-mined to be about 1050 m. y. (Granger and Raup, 1969a), approxi-mately the same age as the diabase sills. A more extensive projectof age determinations of uraninite- and coffinite-rich samplesmight clarify the relationship between diabase intrusions anduranium deposition.

SOURCE, TRANSPORT, AND DEPOSITION OF URANIUMThere are at least two possible sources for uranium in the

Sierra Ancha region. The volcanogenic component within theDripping Spring Quartzite could have been a nearby source ofuranium, and the underlying Ruin Granite constitutes a possibledistal source. The Ruin Granite typically has 2.5 to 8 ppm uranium(Anderson and Wirth, 1981) and the uraniferous quartz-hematite-apatite fracture and breccia matrix in the regolith has as much as1350 ppm uranium. Uranium derived from the granite or regolithcould have moved up structures into the Dripping Spring Quartziteand been reduced in carbonaceous zones. However, because of theextensive diagenesis, dissolution, and thermal metamorphism thathave affected and apparently moved fluids through the DrippingSpring Quartzite, the simpler and favored model derives uraniumfrom altered volcanogenic rocks.

The diabase, thought by Granger and Raup (1969a) and Neuerburgand Granger (1960) to be the source of uranium, was probably mostimportant as a heat source in moving fluids and uranium within thesedimentary rocks. Only local metasomatism of the siltstoneoccurred along the edges of the diabase, and there is no evidencefor development in the diabase of either an early or late stagevolatile fluid carrying incompatible elements. On the contrary,the extensive deuteric alteration and the miarolitic cavities inthe granophyres indicate that fluids moved from the country rockinto the margins of the diabase. No large scale loss of uraniumalong the margins of the diabase has been documented.

The uranium in the quartz-hematite-apatite fracture andbreccia matrix in the granitic regolith may have been derived fromeither the Dripping Spring Quartzite or the granitic basement.Because the uranium occurrences in the granite along the uncon-formity are in the vicinity of the Dripping Spring Quartzitedeposits, these apatite-rich concentrations may be exploration

108

guides to undiscovered uranium deposits in the Dripping SpringQuartzi te.

Uranium derived from the Dripping Spring Quartzite could havebeen liberated from volcanogenic siltstones during the diageneticalteration that formed the potassium feldspar. Analyses of theuranium content of volcanic zircons from the Dripping SpringQuartzite could be used to determine if the volcanic rocks wereenriched in uranium. Uranium, released to solutions during diagen-esis and stylolite formation, could have been deposited where itencountered a reducing environment associated with carbonaceousmatter. Granger and Raup (1969a) describe small, low-grade strata-bound concentrations of uranium in the Dripping Spring Quartzitethat may be protore that developed during the extensive diagen-esis. Inclusions of uraninite in metamorphic minerals suggest thatsedimentary or diagenetic uranium concentrations were later meta-morphosed .

Regardless of the validity of the postulated development of asedimentary protore, the spatial association of diabase and uraniumdeposits and the presence of vertical uranium veins suggest thatthe diabase was involved in remobilization of uranium and formationof the deposits as they now occur. The presence of fine-grainedmicrocline, indicative of thermal metamorphism, in the alteredvolcanogenic siltstones suggests extensive fluid circulation inpermeable rocks in the upper part of the Dripping Spring Quartzite,perhaps during diabase intrusion. Fluid drawn toward the diabasemay have traveled along bedding planes, stylolites, and throughporous rock, carrying uranium originally liberated during diagen-esis and depositing uranium in reducing environments associatedwith carbon-rich sedimentary rocks. Concentration of uranium alongstylolites, bedding planes, vertical structures, and in fine-grained potassium-feldspar-rich layers was probably dependent onthe permeability of these rocks and structures. Cemented clasticlayers and rare carbonate strata, which contain little uranium now,were impermeable to the uranium-bearing fluids. The distributionof the deposits near a quartzite stratigraphie marker bed suggeststhat these strata were an impermeable barrier to either diageneticor hydrothermal uranium-bearing fluids, or, alternatively, thaturanium-bearing fluids preferentially traveled through this unit.Areas disrupted by monoclines and associated fracturing probably

109

were most susceptible to hot circulating fluids, and, therefore,were the areas most likely to have concentration of uranium.

Chloritic alteration remobilized uranium locally, but appar-ently had little effect on the geometry of ore bodies. The timingof the chloritic alteration event is unknown.

The association of uranium, copper, and molybdenum in thedeposits may be indicative of deposition of copper and molybdenumfrom the same circulating fluid that carried uranium or may be aresult of a separate period(s) of fluid movement along the samestructures.

DISCUSSIONThe elements proposed as critical to the formation of the

deposits in the Dripping Spring Quartzite are: volcanogenicsediments, which may have been the source of uranium; extensivediagenetic alteration and dissolution during stylolite formationthat could have liberated and transported uranium; carbonaceousmatter and (or) sulfur that could have established a reducingenvironment for deposition of uranium; and diabase intrusions thatremobilized and concentrated uranium. Alteration only locallyaffected the uranium distribution.

There are intriguing similarities between the deposits in theSierra Ancha region and the large Australian and Canadianunconformity-type deposits: Proterozoic age; strata-boundcharacter of the deposits; shallow water, carbonaceous host rocks;an unconformity separating the ore-bearing sequence from a graniticor gneissic basement; development of a regolith at an unconformity;uranium enrichment in the regolith near the unconformity; apatite-rich concentrations near deposits; and diabase intrusions.Striking, and perhaps critical, differences between the DrippingSpring Quartzite and unconformity-type deposits are the lack ofmetamorphism and intense alteration in the Dripping SpringQuartzite and the differences in ages: the Dripping SpringQuartzite and basement rocks are Middle Proterozoic; the largerunconformity-type deposits are in Lower and Middle Proterozoicrocks overlying Archean basement. Regardless of their relationshipto unconformity-type deposits, the uranium deposits in the DrippingSpring Quartzite may be considered an intermediate stage betweenstrata-bound sedimentary or diagenetic concentrations and the

110

remobilized, high-grade deposits in metamorphosed and highlyaltered rocks.

ACKNOWLEDGEMENTSI thank H. C. Granger for sharing information from his

detailed study of the Dripping Spring Quartzite, for valuablediscussions, and for assistance with figure 3.

I am grateful to Wyoming Mineral Corporation for allowing meto sample and study core and to Uranerz, U.S.A., Inc. for allowingaccess to its data.

REFERENCES CITEDAnderson, Phillip, and Wirth, K. R., 1981: Uranium potential in

Precambrian conglomerates of the Central Arizona Arch. Rep.U.S. Depart. Energy GJBX-33(81), 121 p.

Bergquist, J. R. , Shride, A. F., and Wrucke , C. T., 1981: Geologicmap of the Sierra Ancha Wilderness and Salome study area, GilaCounty, Arizona. Misc. Field Studies Map U.S. geol. Surv. ME1162-A, 1 plate, scale: 1:48,000.

Gastil, R.G. 1954: Late Precambrian volcanism in southeasternArizona. Amer. J. Sei., 252 , 436-440.

Glover, J.E., 1969: Observations on stylolites in WesternAustralian rocks. J. royal Soc. W. Aust., 52, 12-17.

Granger, H. C., and Raup, R. B., Jr., 1964: Stratigraphy of theDripping Spring Quartzite, southeastern Arizona. Bull. U.S.geol . Surv. JLJL§-8> 119 p.

Granger, H. C., and Raup, R. B., Jr., 1969a: Geology of uraniumdeposits in the Dripping Spring Quartzite, Gila County,Arizona. Prof. Pap. U.S. geol. Surv. 595, 108 p.

Granger, H. C., and Raup, R. B., Jr., 1969b: Detailed descriptionsof uranium deposits in the Dripping Spring Quartzite, GilaCounty, Arizona. Open-File Rep. U.S. geol. Surv., 145 p.

Kastner, Miriam, and Siever, Raymond, 1979: Low temperaturefeldspars in sedimentary rocks. Amer. J. Sei. , 279, 435-479.

Ludwig, K., 1974: Precambrian geology of the central MazatzalMountains, Arizona. Ph.D thesis, California Institute ofTechnology. 218 p.

Ludwig, K., and Silver, L. T., 1977: Lead isotope inhomogeneity inPrecambrian igneous feldspars. Geochim. et cosmoch. Acta, 41,1457-1471.

Ill

Nehru, C. E., and Prinz, Martin, 1970: Petrologic study of theSierra Ancha sill complex, Arizona. Bull, geol. Soc. Amer., 81,1733-1766.

Neuerburg, G. J., and Granger, H. C., 1960: A geochemical test ofdiabase as an ore source for thé uranium deposits of theDripping Spring district, Arizona. Neues Jb. Miner. Abh., 94,759-797.

Nutt, C. J., 1981: A model of uranium mineralization in theDripping Spring Quartzite, Gila County, Arizona. Open-File Rep.U.S. geol. Surv. 8J[-524 , 49 p.

Otton, J. K., Light, T. D., Shride, A. F., Bergquist, J. R., andWrucke, C. T., Theobald, P.K., Duval , J.S., and Wilson, D.M.,1981: Map showing mineral resource potential of the SierraAncha Wilderness, Salome Study Area, and vicinity, Gila County,Arizona. Misc. Field Studies Map U.S. geol. Surv. ME-1162-H, 1plate, 6 p, scale: 1:48,000.

Shapiro, Leonard, 1975: Rapid analysis of silicate, carbonate, andphosphate rocks--revised edition. Bull. U.S. geol. Surv. 1401,76 p.

Sheppard, R. A. and Gude, A. J., 3d, 1968: Distribution andgenesis of authigenic silicate minerals in tuffs of PleistoceneLake Tecopa , Inyo County, California. Prof. Pap. U.S. geol.Surv. 597, 3 8 p.

Sheppard, R. A. and Gude, A. J., 1969: Diagenesis of tuffs in theBarstow Formation, Mud Hills, San Bernardino County,California. Prof. Pap. U.S. geol. Surv. 634, 35 p.

Sheppard, R. A. and Gude, A. J., 1973: Zeolites and associatedauthigenic silicate minerals in tuffaceous rocks of the BigSandy Formation, Mohave County, Arizona. Prof. Pap. U.S. geol.Surv . 830, 36 p.

Shride, A. F., 1967: Younger Precambrian geology in southernArizona. Prof. Pap. U.S. geol. Surv. 566, 89 p.

Silver, L. T., 1960: Age determinations on Precambrian diabasedifferentiates in the Sierra Ancha, Gila County, Arizona[abs.]. Bull. geol . Soc . Amer. , 71, 1973-1974.

Silver, L. T., 1968: Precambrian batholiths of Arizona. Spec. Pap.geol. Soc. Amer., 121, 558-559.

Smith, Douglas, 1969: Mineralogy and petrology of an olivinediabase sill complex and associated unusually potassic

112

granophyres, Sierra Ancha, central Arizona. Ph.D thesis ,California Institute of Technology, 314 p.

Smith, Douglas 1970: Mineralogy and petrology of the diabasicrocks in a differentiated olivine diabase sill complex, SierraAncha, Arizona. Contrib. Mineral. Petrol., 27, 95-113.

Smith, Douglas, and Silver, L. T., 1975: Potassic granophyreassociated with Precambrian diabase, Sierra Ancha, centralArizona. Bull, geol. Soc. Amer., 86 , 503-513.

Surdam, R. C., 1977: Zeolites in closed hydrologie systems: inMumpton, F. A., (Ed.), Mineralogy and geology of naturalzeolites. Mineral. Soc. Amer, short course notes, 4, 65-91.

Williams, F. J., 1957: Structural control of uranium deposits,Sierra Ancha region, Gila County, Arizona. U.S. Atomic EnergyCommission RME-3152, 121 p.

Wilson, E. D., 1939: Pre-Cambrian Mazatzal revolution in centralArizona. Bull, geol. Soc. Amer., 50, 1113-1164.

Wilson, E. D., Moore, R.T., and Peirce, H.W., 1959: Geologic mapof Gila County Arizona. Arizona Bureau of Mines, scale:1 :375,000.

113

THE LIANSHANGUAN URANIUM DEPOSIT,NORTHEAST CHINASome petrological and geochemicalconstraints on genesis

ZHONG JIARONG, GUO ZHITIANBeijing Research Institute of Uranium Geology,Beijing, China

Abstract

The Lianshanguan uranium deposits are located in the LianoningProvince of northeastern China. The deposits occur in a transit-ional zone lying between an Archaean craton and Lower Proterozoicmiogeosynclinal sequences. Three types of uranium mineralisationare present in the area; syngenetic mineralisation occurs in thebasal psammitic units of the Lower Proterozoic rocks which, duringmetamorphism in the period 1900 to 1800 Ma, was partly mobilisedand concentrated in the meta-sediments. Late-orogenic graniteemplacement in the Lower Proterozoic saw the development of uranium-bearing alkali solutions that initially produced metasomatic whitemigmatite in fracture zones marginal to the granites. Ore gradesare relatively rich in these alkali hypo-meso metasomatic uraniumdeposits. With precipitation of potassium and sodium from themetasomatising solutions uranium became further enriched in theevolved fluids. Precipitation of uranium from these fluidsproduced meso-epithermal mineralisation of the fissure fillingtype. All three types of uranium mineralisation have been over-printed by tectonic movements associated with activation of theplatform cover in the period 90 to 220 Ma.

INTRODUCTIONLianshanguan uranium deposit is located 120 km south of

Shenyang, eastern Liaoning province, at 40°59'N and 123°30'E (Fig.l)The geological setting of the region is similar to other

uranium-producing regions occurring in Precambrian platforms.Ground radiometry has revealed that some uranium occurrences occurnear the unconformity surface separating Lower Proterozoicsequences and Archaean granite. In 1978, a uranium deposit wasfound having surface exposure and extensions verified by drilling.

115

The preliminary results of the earlier work was published by Qin Fei& Hu Shaokang (1980) .

In this paper we present the results of x^ork recently carriedout which includes regional setting, petrology and geochemistry.This data has enabled further constraints to be placed on metall-ogenic modelling for the province.

REGIONAL GEOLOGICAL SETTINGThe Lianshanguan uranium deposit is located in the transit-

ional zone between an Archaean craton of the Northern ChinaPlatform and a Lower Proterozoic Miogeosyncline sequence (LiaoheGroup) which occurs on the southern edge of the Lianshanguan DomeAnticline.

Three tectono-stratigraphic units are present in the area,namely the Archaean crystalline basement, a Lower Proterozoic meta-sedimentary sequence and an Upper Proterozoic-Phanerozoicsedimentary platform cover (Fig. 1).

Fig. 1 Generalized geologic map of Lianshanguan regionshoxving the location of the uranium deposit1 . Upper Proterozoic-Phanerozoic sedimentary coverLoxver Proterozoic Units

2. Metavolcanic rocks 3. Schist4. Quartzite

Archaean5. Metamorphic strata 6. K-granite7. White migmatite (alkaline metasomatite)8. Fault9. Uranium deposit

116

Crystalline basementCrystalline basement is composed of Archaean K-granite and

xenoliths of Anshan Group which are distributed in the core ofanticline. The dominant rock is monzogranite with lesser grano-diorite. U-Pb concordia on the granitic zircon indicates an ageof about 2530 Ma. The late Lower Proterozoic regional metamorphismlocally produced gneissic textures in the granites and metasomaticeffects were responsible for the introduction of K and Na. Theolder Anshan Group rocks now comprise amphibolite, chlorite-actino-lite schist, granulite, banded magnetite quartzite and mica-quartz-schist. Originally these rocks are thought to have been basiccalc-alkaline volcanic rocks and argillo-arenaceous iron-richsediments.

Resulting from the late Archaean intrusive event, the metamor-phic units mostly occur as screens and xenoliths in the graniticrocks. The crystalline rocks and inclusions have anomalously highU-contents thus making them a potential provenance source for theuranium deposits.

Metasediment seriesThe Lower Proterozoic Liaohe Group, formed in miogeosyncline

conditions, is widely developed in Lianshanguan area. It isdistributed along the margin of the Archaean craton, and unconform-ably overlies it. In the Lianshanguan area, the Liaohe Groupsurrounds an arch anticline. The lower part of the Liaohe Groupcomprises metamorphosed sandstone, conglomerate, graphite-bearingbiotite schist, and marble intercalated with acid volcanics.Thick-bedded carbonates intercalated with schist comprise themiddle portion. The upper Liaohe Group schist and slate are inter-calated with quartzite. The sediments produce a whole rock Pb-Pbisochron age of about 2185 Ma suggesting an age of deposition.

Platform type sedimentary coverThe sedimentary cover is mainly distributed in the northern

part of Lianshanguan area (Fig. 1). Upper Proterozoic rockscomprise slightly metamorphic feldspathic-quartzite-sandstone, marland shale; these are unconformably overlain by the Cambrian-Ordovician sediments with no angular discordance. The Palaeozoicrocks consist of sandstone, shale and limestone and are frequentlyin faulted contact with Archaean-Lower Proterozoic sequences.

117

EVOLUTION OF THE AREAIn the Lianshanguan area the Archaean greenstone-gneiss

terrane underwent multiple metamorphism and complex deformation,with fold and fault structures well developed. Following onthese tectonic events the Archaean craton underwent erosion andpeneplanation. Then crust rifted in Lower Proterozoic times anda new geosyncline was developed. Initial sedimentation saw theaccumulation of terrigenous course elastics, in which the mainuranium-bearing unit occurs. The Lianshanguan area is situated inthe marginal parts of the geosyncline. With increasing sediment-ation marine incursion occurred in the deepening basin with peliticand carbonate sediments being deposited.

Sedimentation terminated with a major tectonic event dated at1900 to 1800 Ma. Metamorphism reached amphibolite faciès andadditional E-W trend linear folds were developed. Reactivationof the basement granites resulted in emplacement into the overlyingformations. Owing to the difference in physico-mechanic propert-ies between the crystalline rock and sedimentary strata frequentintense compressions took place along the interface of these twounits. Mica-quartz schist with marked schistosity occupy thecentral parts of the compressed zones. The mobile elements, suchas K and Na, were activated and carried in solutions to bring aboutmetasomatism within these structural zones, thereby producing thewhite migmatite zone ranging in width from tens of metres to inexcess of 100 m. This alkali metasomatism has controlled thelocation of economic uranium mineralisation in the area.

The Lianshanguan area is flanked by the Hercynian orogenicbelt to the north and by the western pacific plate in the east.Movements within these tectonic belts has resulted in the platformrocks of the Late Paleozoic-Mesozoic era, being faulted andintruded. The platform activatior has resulted in movement ofradiogenic lead producing galena in the environs of theLianshanguan uranium deposit.

GEOLOGY OF THE URANIUM DEPOSITSThe Lianshanguan uranium deposit occurs in quartzite and schist

at the base of Lower Proterozoic and additionally occurs in thewhite migmatite and altered fracture zones developed in the granitewithin the unconformable structural zone. The ore-bearing

118

horizons belong to Liaohe Group, Langzishan Formation, the totalthickness of which is 300-630 m. One representative section is asfollows :

Overlying beds : dolomitic marble, metamorphic volcanic rockLangzishan Formation :Upper member : 9) Graphite mica schist

8) Tremolite marble intercalated withmica-schist, granulite (21 m thick)

Middle member: 7) Garnet mica schist intercalatedmica-quartzite (40 m thick)

6) Feldspathic quartzite intercalatedwith garnet mica schist (22 m thick)

5) Graphite-staurolite-garnet mica-schist (99 m thick)

Lower member

Basement

4) Garnet hornblende schist, garnetbiotite schist (10 m thick)

3) Mica quartzose schist, staurolite-garnet-mica-schist intercalatedwith mica-quartzite (ore-bearing bed)(20 m thick)

2) Quartzite, feldspathic-quartzite,metamorphosed conglomerate intercalatedwith mica-quartzite (ore-bearing bed)(30 m thick)

1) Metamorphosed granitic sand-conglomerate(Ancient weathering mantle) (1-4 m thick).Red granite

From the section it is apparent that the Langzishan Formationis composed of a suite of terrigenous detrital rocks, belonging tothe littoral and neritic faciès. The dark grey quartzite andmica-quartz schist in the second and third beds are the primaryuranium-bearing units.

The main economic uranium mineralization is distributed alongthe metasomatite-structural zone between the Granite Complex andLower Proterozoic metamorphic rocks (Fig. 2). The mineralizedzone is concordant with the bedding, trending MW-SE and dipping tothe S at angles varying from 30° to 60°. The uranium ore-bodiesare located in the brecciated, jointed and deformed zone within thewhite migmatite, as well as in quartzite or mica-quartzite schist.

119

Fig. 2. Geologic map of theLianshanguan uranium deposit1 . Quartzite2. Schist3. Marble4. Metavolcanic rocks5. White migmatite6. K-granite7. Fault8. Uranium ore bodies

Table 1. Uranium mineralization characteristics

types of uranium metamorphosedmineralization

hypo-mesothermal meso-epithermal

mineralizationcharacters

sedimentary metasomatic filling

host rocks

form of orebodiestexture &structure

dark grey quartz- white migmatite altered graniteite mica quartz-ite biotite granu-litelayer, layer-like lens, vein-like veinlet

impregnated,cemented

economic uranium uraniniteminerals

cemented breccia,ooliticspherulitic,concentricmeta-pitchblende pitchblende

associatedminerals

v/all rockalteration

temperature ofmineralizationU-Pb dating

pyrite, galena,graphite, biotite,garnet

unaltered

450°-550°C

2,100 m.y.

alkali feldspar,chlorite,muscovite,galena, pyritealbitization,chloritizationsericitization

280°-250°C

1,900 m.y.

calcite,flourite quartzgalena, pyrite,hematitefluaritization,carbonatization,silicification,hematitization170°-230°C

1,830 m.y.

Types of uranium mineralizationUranium mineralization occurs in three situations: sediment-

ary metamorphosed; hypo-meso hydrothermal metasomatic; and meso-epithermal filling. They differ from one another in terms oftheir host rocks, mineral association, texture and structure of

120

uranium ore, wall rock alteration, form of ore bodies and occurr-ence (Table 1). The major and trace elements of the host rocks inthe U-deposits are shown in table 2 .

(i) Metamorphosed sedimentary mineralizationThe uranium mineralization of the metamorphosed sedimentary

type is distributed within the structural-metasomatic zone

Table 2. Major & trace elements of host rocks in theLianshanguan U-deposit

host rock

mainelement(%)Si02Ti02A12°3Fe2°3FeOMnOMgOCaONa20

P2°5SCNa20

traceelements

(ppm)CuZrGaCrNiVPbUThY

quartzite

94.030.062.380.180.590.0100.200.230.150.02001.53

6272420151415127.445

urano-ferousquartzite

93.590.061.260.290.720.0100.240.070.310.040.660.020.23

804371020302507303417

granite

75.210.11

13.240.810.910.030.190.483.055.440.20

-0.56

1645131210108.48.74538

alteredgranite

75.680.08

13.240.810.520.030.170.603.584.690.13

-0.76

whitemigmatite

75.160.1314.00.690.850.030.170.524.383.300.17

-1.33

31481412121325163735

ore-bearingwhitemigmatite

76. ]70.14

12.560.560.900.040.281 .084.112.360.09

-1.74

8483111419142407533845

121

NE24°

Fig. 3. Cross section of theLiashanguan uranium deposit1. Mica schist2. Amphibole schist3. Interbedded zone of

quartzite and schist4. Quartzite5. White migmatite6. K-granite7. Metamorphosed sediment-

ary uranium ore body8. Metasomatic hydrothermal

uranium ore body9. Shear zone10. Fault

Fig. 4. Microuraninite (grey)with pyrite 'white) distributein the groundmass

(Fig. 3). The host rocks are dark-grey quartzite, mica-quartzschist and biotite granulite. Ore bodies are stratified orstrataform with only moderate thickness. Generally, the ore gradeis less tha 0.1% U.

There is no clearly defined boundary between uranium mineral-ized zones and wall rock. The composition of ore is rather simple,the main economic uranium minerals are microuraninite, occurring inthe voids of detrital mineral grains or in form of disseminationsincluded in biotite, garnet, feldspar and other minerals (Fig. 4).

The chemical composition of ore-bearing and barren rocksindicates a slight enrichment of AI, K, Fe, C, S, in the mineral-ised rocks. A positive correlation exists between U and the totalcontent of S and C. This correlation suggests that uranium owesits concentration to reducing and absorbent conditions. Addition-ally ferrous iron and pelitic material may have aided the concen-

122

tration process. Trace elements of both ore and wall rock zonesare almost identical except for Pb which was probably produced byradioactive decay of U.

(ii) Alkali hypo-meso metasomatic mineralizationThis type of uranium mineralization is strictly controlled by

the structural-metasomatic zone. The ore bodies occur in thestructural breccia and border fracture zone of the white migmatite,as lenses or lenticles. Ore grade is relatively rich, locallyreaching 5% U. The main economic uranium mineral is meta-pitchblende occurring in colloidal form. Pentagonal and hexagonalcrystal-grains are present and can be seen under high magnification(Fig. 5). The paragenetic mineral sequence is albite, K-feldspar,chlorite, muscovite, minor amount of pyrite, galena and pyrrhotite.

The original rocks comprising the white migmatite were mainlyquartzite, feldspar-quartzite and granite. During the metasomaticprocess abundant Na and K was introduced to form sodic plagioclaseand K-feldspar. Relative to the granite the white migmatite hasan increased Na^O/K^O ratio (0.56 to 0.76) and with quartzite, theratio has increased from 0.29 to 1.33 (Table 2). It shows thatthe alkalic metasomatising solution was enriched in sodium. TheNa^O/K^O ratio is much higher in the ore-bearing metasomatic rock,averaging 1.74. This indicates that there is a close relationshipbetween uranium mineralization and Na-metasomatism. Apart fromthe higher Pb content in the ore, the remaining trace elements showlittle variation within mineralized and non-mineralized zones.

(iii)Mesoepithermal filling type mineralizationThis type of uranium mineralization occurs mainly within

fractures of altered granite. The ore bodies form mono-veins andstockwork or reticulated-veins occur on a small scale.

The main economic uranium minerals of this type are pitch-blende having spherulitic or oolitic texture. The parageneticsequence of the opaque minerals are pyrite, galena, sphalerite andchalcopyrite; the gangue minerals are mainly composed of calcite,fluorite, quartz and sericite. The pitchblende is in close assoc-iation with sulphides, dark-grey or light red calcite and dark-violet fluorite.

123

Fig. 5. Meta-uraninite withpentagonal and hexagonal form

5 0 0 -

2 100-

O.

acn

50-

10-

5-

38950 ppm

2220 ppm (u)

310 ppm (Si)x370 ppm 41)

X 680 ppm

La Ce Nd SmEu Gd Dy Er Yb

Fig. 6. Chondrite-normalized REE patterns of differentgenetic type uranium ore

1. Schist2. Metamorphosed sedimentary uranium ore3. Hydrothermal metasomatic uranium ore4. Hydrothermal filling uranium ore

GEOCHEMISTRYRare-earth elements

Rare-earth elements of ore and host rocks are shown in table3 and figure 6. The chondrite-normalized patterns of rare-earthelements of the ore-bearing quartzite, schist in LangzishanFormation and Basement Granite are quite similar. The La/Yb ratio

124

Table 3. Rare-earth elements of the three types ofuranium mineralization (ppm)

mineralizationtypes

metamorphosedsedimentary

hypo-mesothermal meso-epithermalmetasomatic filling

sample no.

LaCePrMdSmEuGdDyErYb

EREELREEÏÏÏÏEELa7b"EuEu*U

292

60.4109.114.637.69.73.18.54.54.13.2

254.811.4

18.9

1.13370

177

19.423.43.111.44.51.33.82.61.91.6

68.55.8

9.3

0.94740

242

24.15610.126.77.11.83.43.43.13.0

138.79.6

8

1680

317

51.688.717.151.831.56.5

28.615.411.48.7

3113.8

5.9

0.662220

121

190.440768.7

227.110622.267.330.941.119.8

1180.56.3

9.6

0.7638950

245

136.5236.133.3101.1243.3155.94.23.4

56318.5

40.1

0.5310

is between 8 and 15 indicating enrichment in the light rare-earth elements. The reason for the Eu-anomaly is unclear. Thereare some differences between the REE patterns of ore-bearing rocksand normal quartzite and schist. The ore zones have lower REEconcentrations. Usually with uranium increase there is a reduct-ion of REE content probably because uranium is concentrated in thefine grained detrital rocks with only minor amounts of heavyminerals.

The REE pattern within ores of the hydrothermal-metasomatictype are similar. The higher the uranium content, the moreabundant are the REE. The fractionation of light rare-earthelements is poor, the La/Yb ratio is between 4 and 10, and dropsas the uranium content rises.

Although there is a low uranium content in the ore of thehydrothermal filling type of mineralization the REE content is

125

relatively high with the La/Yb ratio reaching 40. It would appearthat the low temperatures associated with this type of mineral-ization results in light REE enrichment.

Sulfur IsotopesThe sulfur isotopes of pyrite, both in the ore and sundry

wall rocks, indicate that the uranium mineralization has charact-eristics of both sedimentary metamorphism and metamorphosed hydro-thermal alteration (Fig. 7).

os34 femeanT

+ 20 + 15 +10 +5 0 -5 -10Fig. 7. Sulfur isotope composition of thepyrite in host rocks and uranium ore1. Pitchblende vein2. White migmatite3. Uranium-bearing quartzite and schist4. Quartzite and schist

Q I6S of pyrite, present in quartzite or schist, varies from-8.4% to +16.8%; the average value is +6.1%. The fact that theo /

6 S range is similar to sulfate present in palaeo-marine waterssuggests a similar origin. The pyrite associated with uranium

o /minerals in the ore-bearing quartzite has an average ô S of +6.3%This is about the same as the wall rock values found in the quartz-ite host suggesting a sedimentary origin for this type of uraniummineralization.

o /The 6S of pyrite in the white migmatite and vein fillingtype uranium mineralization all have positive values. Thisindicates that the sulfur does not come from a deep source, but isprobably derived from the reconstituted country rocks; theassociated uranium is also probably derived from the wall rockshaving been mineralized after hydrothermal metasomatism.

U-Pb Radiometrie isotopesU-Pb isotope dating shows that the Lianshanguan uranium

deposit is a product of multistage mineralization. Three diff-

126

erent metallo-genetic ages of uranium mineralization can be dated.The age of metamorphosed sedimentary uranium mineralization is

+ 1292114 -no? ^a ' ^e alkali-0 nietasomatic stage is dated at-r 7Q +'3'}1891 _7^ Ma; and the hydrothermal filling stage at 1829 _£Q Ma.

The last two ages approach the model U-Pb age of the three stagesof the white migmatite (1917 + 77 Ma). Additionally the threetypes of uranium mineralization have all been overprinted bytectonic movements associated with activation of the platform duringthe period 90 to 220 Ma.

FLUID INCLUSIONSThe formation temperature of the vein pitchblende has been

measured by the decrepitation method, and by the homogenizationtemperature of fluid inclusions in fluorite, calcite and quartzassociated with the pitchblende. The decrepitation and homogeniz-ation temperatures of these minerals could be divided into twogroups. The first is 280°-350°C found in the hypo-mesohydrothermalrninerogenesis, this represents the formation temperature of alkalicmetasomatic U-mineralization. The mineral association of which ismeta-pitchblende, alkali feldspar, quartz and small amounts ofsulfides. The second is 160°-230°C occurring in the meso-epithermal minerogenesis, this represents the formation temperatureof hydrothermal filling uranium mineralization. The main mineralassociations here are pitchblende, calcite fluorite and microcry-stalline quartz.

METALLOGENETIC CONTROLSAs stated above, the formation of Lianshanguan uranium deposit

underwent complex processes, it is controlled by sedimentation,regional metamorphism, structural movement and metasomatism. Themajor control factors are different for the various mineralizationtypes; these controls can be generalized as follows:

Uranium sourceThe U abundance and U-extracting rate of the main hosts in

this region are arranged in tables 4 and 5 and in figure 8. Itshows that the U contents of these rocks are all higher thanaverage crustal abundances. The U abundance of quartzite and thatof the white migmatite are especially high, they also have highU-extracting rates. This suggests that there is a high degree of

127

Quartzi te

Samples

Whi te mi gmat 11 e

70 Samples

Fig. 8. histogram of theuranium content of the main hostrocks.

10 20 30 40 0 10 20 30 40 ppm <u>

Table 4. Uranium content of the main host rocks

li thology

grani teschistquar tzi tewhitemigma ti te

number ofsamples

77bl12670

average contentot uranium^ ppm)

661014

.25

.65

.20

.36

mean squaredeviation( 6X)

4.5.8.8.

50607284

Table 5. Uranium extracting rate of main host rocks

li thology

grani teschistquartzi tewhitemigma ti te

uranium^ppm)

9.2-1416

21.0-8819-130

extractingrate(%)

6.014.123.12b.4

extractingcondi t ion

in normal tempera-ture and 1% Na-CO-solution, the ratioof sample to solut-ion is 1:10

mobilized uranium in these rocks. As the basement granite andore-bearing hosts in this region also have high U contents it isapparent that the area was anomalously eririchea in uranium. Thebasement granite, undergoing a long period of weathering anderosion, provided the uranium source for the formation of syn-genetic U-bearing horizons in the basal members of the LowerProterozoic. Economic uranium grades were produced during meta-morphism and metasomatism.

128

SedimentationMost of the uranium occurrences in the region are localised in

terrigenous detrital rocks of the Langzishan Formation which hasstratabound characteristics. This uranium-bearing formation wasdeposited on the margins of the Archaean craton under miogeosynclineconditions during Lower Proterozoic times. The LangzishanFormation unconf orinably overlies the basement granite complex,belonging to the lower part of a transgressive cycle. ThisFormation, although widely distributed, only has uranium concentra-ted where the lithologie lithofacies and palaeogeographic condit-ions are favorable.

The spatial distribution of the original horizons, rich inuranium, is controlled by the base trough near shoreline conditions.In the late Archaean, the basement granitic complex was exposed tothe surface and underwent peneplanation. On the peneplain surfacelocal rivers scoured troughs which controlled the thickness andlithofacies of the U-bearing horizons. In general, theLianshanguan deposit is located in a NS striking basal trough withbetter uranium values mainly distributed in the centre and easternslope of the trough (Fig. 9).

120°

Fig. 9. Longitudinal section of the Lianshanguanuranium deposit ^symbols as in figure 3)

The interbeds of sandstone and argillite in the base troughare the favourable lithofacies for formation of horizons rich inuranium. The meta-sedirnentary uranium mineralization in theLianshanguan deposit is present mainly in black-grey quartzite andmica-quartz schist developed in the basal part of LangzishanFormation. Where the ratio of quartzite thickness to the totalthickness of the ore-bearing zone is less than 35%, uranium

129

grades improve. Where this ratio is higher than 50% mineral-ization is poor. All this suggests that the sedimentary litho-facies exerted a controlling role in the distribution of uraniummineralization. The change of sedimentary facies is stronglycontrolled by the paleogeographic environment. In the upliftedparts the thick-bedded macroclastic sediments occur in an oxidizingenvironment, which was unfavorable for uranium precipitation andconcentration. In the distal facies of the basal trough argillicand sandy sedimentary beds were produced. This zone lies in atransitional redox environment, that was favorable for uraniumprecipitation and concentration. Also these interbedded sand-stones and argillites produced permeable and impermeable layers,which was advantageous to the circulating of U-bearing ground waterresulting in further uranium precipitation.

Those lithologies rich in reducing mediums resulted in localuranium concentration. The U-bearing hosts of the Lianshanguandeposit are rich in clays, carbonaceous matter and pyrite andpossess a dark colour. These rocks have a sulfur content of 0.05-0.98%, 0.05-0.74% in organic carbon; a positive correlation ispresent between U and the total amount of S and C. This suggeststhat U-bearing horizons were formed under weak reducing conditions.Regional metamorphism

The ore-bearing rocks in the region have undergone a low tomedium grade of regional metamorphism. The metamorphic grade wasfavorable for uranium concentration, with the lower grade meta-morphic rocks being a favorable place for the production of meta-sedimentary uranium mineralization.

The main metamorphic mineral associations of the ore-bearingstrata are staurolite-almandine-biotite-muscovite-quartz andmuscovite-biotite-plagioclase-quartz, which belong to the green-schist facies or almandine-staurolite subfacies of the amphibolitefacies. The Mg/Mg+Fe+Mn distribution coefficient (Fig. 10) of thebiotite-garnet mineral pair in the rocks suggests a temperature offormation of about 500°-550°C at a pressure of 5-10 kb. Hydro-thermal solutions associated with this metamorphism reconcentratedthe syngenetic uranium to form micro-grained uraninite, which isheld in the metamorphic silicate minerals, or distributed alongmineral grain boundaries and cracks. As a result of metamorphismthe meta-sediinentary uranium deposits were formed from originalsyngenetic concentrations.

130

Fig. 10. Relationship betweendistribution coefficient ofMg/Mg+Fe+Fn for the garnet-biotite pair 0 - 2 0 - 4 0 - 6 0 - 8

Biotite Mg/Mg + Fe *• Mn

1-0

Tectonic conditionTectonism was an important controlling factor of the distribu-

tion of uranium in the Lianshanguan uranium deposits. Tectonicmovements associated with late Lower Proterozoic times resulted instructural deformation characterized by plastic flow. In the courseof plastic fold deformation, tectonic emplacement of the reactivatedbasement granite took place along the folding axis to form theLianshanguan arch anticline. At a late-stage of granite emplace-ment compressive fractures were formed along the contact zonebetween the granite and meta-sedirnentary rocks and also parallel tofolding axes. These faults possess characteristics of multistageactivation, which were welded or filled by white rnigmatite in theearly stage and subjected to brittle faulting in the later stagethat gave rise to fractures and crush zones. These fractures pro-vide the passageway for uranium-enriched fluid migration. Thesezones are the favorable sites for the hydrothermal rich ore bodies.The major economic orebodies of the deposit are located in strikefaults and interstratified fracture zones (.Fig. 3). The main ore-zones are brecciated and developed in areas where the dip changesfrom gentle to steep; the crushed zones of the elongated belt ofwhite migmatite formed along interstratified fracture zones.Other structure forms such as schistosity, gneissosity and rnicro-cracks are also favorable sites for uranium concentration.

Migma-metasomatism and alkalic hydrothermalismThe hydrothermal uranium mineralization in this region is

spatially closely related to the alkalic metasomatism that produc-ed the white migmatite. The main expressions are:

131

a) Hydrothermal uranium mineralization occurring in theinner structural breccia zones or border fracture zonesof the white migmatite. Pitchblende is especially inclose association with the late fine-grained albite.The correlation between U and Na is positive.

b) The paragenetic association of minerals and traceelements of ore-bearing and non ore-bearing whitemigmatites is essentially similar. The compositionof uranium ore is rather simple, apart from the highercontent of U, Th, Pb and REE no other metallogenicelements display enrichment.

c) The uranium metallogenic age is similar to the age ofthe white migmatite. U-Pb dating of white migmatiteis 1,917 Ma, and the age of alkalic hydrothermal meta-somatic uranium mineralization is 1,891 Ma.

d) The formational temperature of pitchblende is between250 -350°C, belonging to the range of hypo-mesotherrnalhydro thermal ism.

The above information suggests that the hydrothermal uraniummineralization has characteristics of succeeding to and evolvingfrom the migma-metasomatic solution in respects of time, space,composition and temperature. The metallogenic hydrothermal solut-ion was derived from the alkalic migmatite solution in its post-migmatitic metasoiuatic stage. The controlling role of migma-metasomatism alkalic hydrothermalism in uranium mineralization canbe summarized as follows:

a) The migmatization in this region was generated duringthe regional metamorphism in the late Lower Proterozoicera. In the late stages of regional metamorphismreactivation of the basement granitic complex resultedin mobilisation of elements such as K, Na and U. Thismetasomatism occurs on both sides of the contact zoneand forms the primary alkalic U-bearing fluid faciès(migmatitic solution).

The migration of this alkalic U-bearing fluid throughthe fracture zones resulted in wall rock metasomatismand extraction of uranium. The U-bearing primary alkalicsolution became concentrated in local parts of the white

132

migmatite (e.g. uranium content and extracting rate ofwhite migmatite in Lianshanguan deposit are all higherthan those of other rocks). These solutions becamefurther enriched in uranium to form the late alkalicsolutions, thus acting as a rich uranium source for thehydrothermal uranium mineralization.

b) The Na-bearing alkalic hydrothermal solution formed inthe post-rnigma-metasomatic stage, uranium could becarried in the form of a carbonate uranyl complex.The higher the pH of the solution, the more stablethis uranyl complex became. A favorable depositionalsite could have been produced by decreasing temperatureand pressure, changing redox conditions, reducing of pH(caused by separating alkalic metal from solution in theprocess of alkalic metasomatism). The uranyl complexcould be uncoupled under these circumstances withreduction taking place producing pitchblende.

c) Migmatitic or hydrothermal alkalic metasomatism of thearea is mainly represented by the replacement ofpotassium by sodium. Because the ion radius of sodiumis smaller than that of potassium, the equal-ionreplacement of K by Na resulted in an increase in voidspace within the minerals resulting in higher porosityof the rocks, and a permeable spongy texture. It issuggested that inhomogeneity resulting from meta-somatism led to a reduction in the mechanical strengthof the rocks allowing them to be easily fractured andthereby forming favorable structural traps for uraniummineralization.

THE SUGGESTED GENETIC MODELPetrogenetic modelling of the uranium mineralization at the

Lianshanguan deposit can be summarised as follows:After the late Archaean orogeny, the uranium enriched

metaraorphic rocks and granite complex underwent weathering anderosion. Labile uranium was leached out from these rocks andcarried by runoff into the offshore basal trough. The uraniumwas fixed by absorption on clays and reduced by carbonaceousmatter, pyrite and changing redox conditions to form low-grade

133

syngenetic uranium concentrations in the basal part of the LowerProterozoic psammitic sequence. Subsequently the rocks were meta-morphosed during the late Lower Proterozoic to greenschist amphi-bolite faciès. Part of the syngenetic uranium was mobilised dur-ing metamorphism and concentrated to form the meta-sedimentarytype of mineralization.

Late-orogenic granite emplacement took place during the reg-ional and metamorphic event. This event produced a series ofcompressive faults and interstratified fracture zones parallelingthe granite contact. During this tectonisra uranium-bearing alkalimigmatitic solutions were produced. These metasomatising solut-ions migrated along fracture zones and replacement of the wallrocks took place to form an elongated belt of white migmatite. Inthis process, alkali solution extracted uranium from the surround-ing rocks; with precipitation of some K and Na the uranium becameenriched. These solutions further evolved into the uranium-richpost-migmatitic alkalic hydrothermal solutions. In these hydro-thermal solutions uranium was transported as the carbonated uranylcomplex. Under suitable physiochemical conditions uranium wasprecipitated in fracture systems producing the alkalic hydrothermalmetasomatic type of mineralisation.

After formation of the alkalic metasomatic uranium mineral-ization, the residual hydrothermal solution migrated to lowerpressure regimes. With falling temperature, and separating outof the alkalic metals, the solutions evolved became weakly acidand formed meso-epithermal mineralization of the fissure fillingtype. These deposits are characterised by the mineral associationsulfide-pitchblende or fluorite-pitchblende found in tension cracksfar removed from the original thermal source.

ACKNOWLEDGEMENTSThe authors would like to thank the Liaoning Exploration Team

of Uranium Geology for making information available to us.

REFERENCESQIN FEI & HU SHAOKANG., 1980: Present explortion status of the

Lianshanguan uranium deposit, northeast China: in Ferguson,John and A.B. Goleby (Eds) "Uranium in the Pine CreekGeosyncline" (655-662) Proc. of IAEA 760p.

134

PINE CREEK GEOSYNCLINE, N.T.

G.R. EWERS, J. FERGUSON, R.S. NEEDHAMBureau of Mineral Resources,Canberra City, ACT

T.H. DONNELLYBaas Becking Geo-Biological Laboratory,Canberra City, ACT

Australia

Abstract

The Pine Creek Geosyncline comprises about 14 km of chrono-stratigraphic mainly pelitic and psammitic Early Proterozoicsediments with interlayered tuff units, resting on granitic lateArchaean complexes exposed as small domes. Sedimentation tookplace in one basin, and most stratigraphie units are representedthroughout the basin. The sediments were regionally deformed andmetamorphosed at 1800 Ma. Tightly folded greenschist facièsstrata in the centre grade into isoclinally deformed amphibolitefaciès metamorphics in the west and northeast, granulites arepresent in the extreme northeast. Pre and post-orogenic contin-ental tholeiites, and post-orogenic granite diapirs intrude theEarly Proterozoic metasediments, and the granites are surroundedby hornfels zones up to 10 km wide in the greenschist facièsterrane. Cover rocks of Carpentarian (Middle Proterozoic) andyounger ages rest on all these rocks unconformably and conceal theoriginal basin margins.

The Early Proterozoic metasediments are mainly pelites whichare commonly carbonaceous, lesser psammites and carbonates, andminor rudites. Volcanic rocks make up about 10 percent of thetotal sequence. The environment of deposition ranges fromshallow-marine to supratidal and fluviatile for most of thesequence, and to flysch in the topmost part.

The Early Proterozoic strata are overlain with marked angul-arity by mostly subhorizontal fluviatile sandstone and basalt ofMiddle to Late Proterozoic age. A veneer of Mesozoic and Cainoz-oic sediments conceals much of the Early Proterozoic sequence,particularly in coastal areas.

135

The uranium deposits post-date the -1800 Ma regional metamor-phic event; isotopic dating of uraninite and galena in the orebodies indicates ages of mineralisation at -1600 Ma, -900 lia and-500 Ma. The ore bodies have a number of features in common;they are stratabound, located within breccia zones, are of ashallow depth, contained in rocks that underwent low-temperatureretrogressive rnetamorphism and metasomatism, and occur immediatelybelow the Early/Middle Proterozoic unconformity. It is suggestedthat these ore bodies were produced by downward percolatingmeteoric waters transporting uranyl complexes which due to reduc-ing conditions in the breccia zones precipitated uranium oxicte.

As continental crustal development in post-2,200 lia timesappears mainly to follow uniformitarlan lines, the only variablewhich could explain the concentration of vein-type uranium in the2,200-1,700 lia period appears to be a steadily evolving atmosphere.It is suggested that during these times the hydrosphere wassufficiently oxidising for uranyl transport, but that rapidlyreducing conditions were met short distances into the lithosphère.Reduction resulted in precipitation of uranium as DO- from meteoricwater into suitable structural traps, which were largely developedduring periods of prolonged erosion. The structural traps mayalso have been active during the early sedimentation of the MiddleProterozoic cover rocks. Rapid development of impermeable coverrocks preserved the uranium deposits.

INTRODUCTIONThe Pine Creek Geosyncline is a roughly triangular area of2about 66 000 km south and east of Darwin, which contains Early

Proterozoic metasediraents, dolente and granite interspersed withminor granitoid Archaean basement domes. It is surrounded byyounger sedimentary basins from Middle Proterozoic to liesozoic inage, and is mantled in many parts by Cainozoic deposits. Theeconomic uranium occurrences define 3 fields, the Rum Jungle andAlligator Rivers fields each centred on inliers of Archaean base-ment, and the South Alligator Valley field in an area of extensivedip-slip faulting (Fig. 1).

Of the present known reserves of vein-type uranium in thewestern world, approximately 90% are confined to rocks of Proteroz-

136

GEOLOGY OF THE PINE CREEK GEOSYNCLINECRETACEOUS

PERMIAN

CAMBRIANORDOV'CIAN

} Sandstone stltslone

~~! Stltstone sandstone minor limestonej conglomerate

| L/mestone sandstone stitstone basaltI conglomerate

FITZMAURICE GROUP |

AUVERGNE GROUP

BULUTA GROUP

TOLMER GROUP

MOUNT RIGG GROUP

KATHERINE RIVERCROUP

Sandstone snate siltstone dolomiteconglomerate

Siltstone shale sandstone minordolomite

Siltstone minor dolomite

Sanc-alt-^ dolomite siltstone

Sandstone conglomérats limestone

Sandstone minor limestone siltstoieand conglomerateInterbedded intermediate to basicvolcanics

OENPELLI DOLER'TE | fdo | OMne dolente and differentiates

EDITH RIVERVOLCANICS

HERMIT CREEKMETAMORPHICS

MYRA FALLSMETAMORPHICS

LITCHFIELD COMPLEX

NIMBUWAH COMPLEX

NOURLANGIE SCHIST

ZAMU DOLERITE

FINNISS RIVERGROUP

SOUTH ALLIGATORGROUP

MOUNT PARTRIDGEGROUP

FORMATION

I Acid and minor basic volcanicsI pyroclastics sandstone

Granite adametlite granodionteminor syenite

I Schist gneiss amphiöolite migmatite

i Pelitic schist lit par lit gneiss

'•$, Granite granodionte pegmatite1 migmatile

I Migmatite granite granodionte

I Quartz schist

1 Dolente and differentiates| amphlbollte in east

Siltstone greywacke sandstone acid tobasic lavas pyroc/astics

Carbonaceous and ferruginous shale withchart bands carbonate tuff andésite bil

I Sandstone siltstone shale quartzite anVoseconglomerate dolomite scnlsl and gneiss m eastScÜlsl 9"8'ss Quartzite carbonaceous with

^j carnanate 3naoaic silicate gneiss in lower part.,..,„„.,. „„„ , I ; 1 Calcareous and carbonaceous shale sandstoneNAMOONA GROUP i Pn limestone, dolomite erkose conglomerate

\ i schist ana marble in east

—3—*" Anticline showing plungi

—t—*• Syndine showing plunge

—&— Overturned synclinp

sc Prevailing strike and dtp of strata

Prevailing strike and dip of foliation

Significant uranium mine or deposit

Uranium prospect or minor mine

MANTON GROUP

KAKADU GROUP

NANAMBU COMPLEX

RUM JUNGLECOMPLEX

WATERHOUSE COMPLEX

leastMagnesite dolomite conglomerate

Leucogneiss quartzite schist

Leucograntte migmattte gneiss granite schist

Gneiss granite schist metssedtments

Grange gn&iss amphtboliie migmalitedolomite metasetftments

oie age. If this skewness is real, a unique set of conditions isrequired to account for it.

The development of uranium in quartz-pebble conglomerates,confined as they are to the period 2,800-2,200 Ma (Robertson,1974), has been ascribed to changing plutonic activity in theperiod 3,000-2,700 Ma (Moreau, 1974) as well as to the prevailingatmospheric conditions (Simpson and Bowles, in press). Develop-ment of quartz-pebble conglomerate uranium deposits thus coincideswith major tectonic changes corresponding to the first developmentof stabilised cratons and ensuing development of intracratonicsedimentary basins (Glikson, 1979; Sutton, 1979). This tecton-ism, coupled with the first development of algal colonies (Nagyet al., 1976) created a favourable set of conditions for theformation of quartz-pebble conglomerate uranium deposits (Pretor-ius, 1975).

If, however, we look at the post-2,200 Ma period, which alsocorresponds to the first extensive development of red beds (Cloud,1976) , there does not appear to be any unique geological and/orchemical conditions to account for the original concentration ofvein-type uranium deposits within the 2,200-1,700 Ma time span.

Our studies of vein-type uranium deposits in the Proterozoichave been confined mostly to those within the Lower Proterozoicsequence of the Pine Creek Geosyncline. Using this area as ourtype-model, we have attempted to account for the extensive devel-opment of vein-type uranium deposits originally formed within thetime span 2,200-1,700 Ma.

REGIONAL GEOLOGYUp to 14 km of Early Proterozoic metasediments with inter-

bedded volcanics and mafic sills are preserved in a layercakesequence which generally thins westwards (Fig. 1; Table 1). Aregional metamorphic event between 1870 and 1800 m.y. metamorpho-sed the strata to mainly greenschist faciès; however, amphiboliteand minor granulite faciès dominates in the northeast, in theAlligator Rivers region. Proven Archaean rocks are restrictedto mainly granite-gneiss of the Rum Jungle, Waterhouse andNanambu Complexes, which form mantled gneiss domes near thepresently exposed western and eastern margins of the inlier.

138

TABLE 1. SUMMARY OF ARCHAEAN TO MIDDLE PROTEROZOIC STRATIGRAPHY OF THE PINECREEK GEOSYNCLINE

wSaöSt* oW Uä§s0-

Unit

MINOR DOLERITE

MUDGINBERHI PKONOLITEMUNMARLARY PHONOLITE

TOLMER GROUPKATHERINE RIVER GROUP

OENPELLI DOLERITE

EDITH RIVER GROUP

Lithology

quartz dolerite dykes and small plug-likebodiesphonolite dykes

sandstone, dolomite, siltstonesandstone, conglomerate, minor greywacke,siltstone. Interbedded basalt-andesitevolcanics and pyroclasticslayered tholeiitic dolente lopolithsignimbrite, microgranite, rhyolite, minor

Thick-ness(m)

1

10001200

<2501200

Age(Ma)1200 +

1316 +

1648 +

1688 +1808 +

35

50

29 (basalt)

138 (ignimbrite)

(J X

2«l g Z POST-OROGENIC: < o GRANITE EMPLACEMENT) in' T1 EL SHERANA GROUP

basalt and cherty sediments; basalsandstone, arkosebiotite granite, adamellite, syenite,granodiorite (numerous plutons)rhyolite, greywacke, siltstone, sandstone,basalt

1730 - 1800

< 2 o -Hs"! 'a "

MYRA FALLS METAMORPH ICSt NOURLANGIE SCHISTNIMBUWAH COMPLEX

ZAMU DOLERITE

FINNISS RIVER GROUP(flysch)

layered schist, gneiss (metamorphosed andpartly migmatised Early Proterozoic sediments)granitoid nugraatite, granite, gneiss, schist(anatexis of Early Proterozoic granite)layered tholeiitic dolerite sills and minor <2500dykessiltstone, slate, shale, greywacke, arkose 1500-quartzite, schist, minor interbedded 5000volcanics

1800

1803 - 1870

1914 + 170

SOUTH ALLIGATOR GROUP(shallow marinechemical, volcanic)

MOUNT PARTRIDGE GROUP(fluviatile, near-

pyritic black shale and siltstone, chert- <5000banded and nodulated hematitic siltstone andblack shale, algal carbonate, banded ironformation, jaapilite, tuff, greywacke neartopsandstone, siltstone, arkose, shale, conglom- <SOOOerate, quartzite, carbonaceous siltstone d

1884 + 3 (dacite)

i-l (NO 1N O

EARL

Y PR

OTE

RC-1

87

shore chemical, supra-tidal)CAHILL FORMATION(supratidal, fluviatile)NAMOONA GROUP(shallow marine,chemical, detrital,supratidal)KAKADU GROUP(fluviatile)

NANAMBU COMPLEX

shale, dolomite, magnesite; minor interbeddedvolcanicsquartz schist, pelitic and partly carbon- 3000aceous near base with lenses roagnesitepyritic carbonaceous shale and siltstone <3500calcareous in places, calcareous sandstone,tuff, agglomerate; arkose, sandstone andmassive dolomite in west.sandstone, arkose, siltstone, conglomerate, -1000quartzite, schist, gneissgranite, augen gneiss, leucogneiss, minorquartzite and schist (includes accreted Early

1800 (gneiss)-2500 (granite)

Sj S RUM JUNGLE COMPLEXO in WATERHOUSE COMPLEXS3

Proterozoic metamorphics)coarse, medium, and porphyritic adamellite,biotite-muscovite granite, migmatite, gneiss,schist, pegmatite, meta-diorite, banded ironformation

2500

The higher grade metasediments grade northeastwards into themigmatitic Myra Falls Metamorphics, where in places sedimentary1 resistors' indicate the sedimentary parentage and allow correl-ation with the un-migmatised terrane. The intensity of foldingalso increases generally towards the northeast - tight to isocli-nal folds with steep limbs in the central parts progress throughoverturned tight to isoclinal folds to polyphase isoclinal foldsin the Alligator Rivers region. In the extreme northeast pre-

139

orogenic granite was partly migmatised along with the surroundingmetasediments.

The metamorphics are unconformably overlain by valley-fillfelsic volcanics and pyroclastics and these are intruded by post-orogenic granites and dolerite. All these rocks are overlainwith marked regional unconformity by Middle and Late Proterozoicplateau sandstone and volcanics.

Basement ComplexesArchaean ages are established for the Rum Jungle and Water-

house Complexes in the western part of the Geosyncline, and forparts of the Nanambu Complex in the east (Page, 1976; Page et al.,1980; Richards et al., 1977). Walpole et al., (1968) interpretedthe Litchfield Complex and Hermit Creek Metamorphics, in theextreme west, as wholly or partly Archaean; little isotopic workhas been done on these rocks, but Berkman (1980) reports muscovite-biotite schist in the Litchfield Complex as a probable correlativeof the Burrell Creek Formation (topmost unit in the géosynclinalpile), and Needham et al., (1980) suggest that granites in theComplex have some features in common with the migmatitic granites.Page (1980) indicates that the Rb-Sr systematics of the LitchfieldComplex are similar to those of the 1870-1800 Ma Nimbuwah Complex.

Rum Jungle and Waterhouse ComplexesThe Rum Jungle and Waterhouse Complexes are roughly oval

9granite-gneiss domes up to 400 km in the west of the Geosyncline.They are the dominant rock units in the Rum Jungle district, withanomalous concentrations of uranium (12 and 13 ppm respectively,Ferguson & Winer, 1980) . Each Complex contains several graniteand gneissic granite phases with veins and dykes of pegmatite andamphibolite and tourmaline veins. Metasediments, banded ironformation, volcanics and metabasics in the Complexes are thoughtto pre-date the granites. On field relationships the leucocraticgranite of the Rum Jungle Complex is considered to be the youngestphase; this rock type gives an age of about 2400 Ma (Richards &Rhodes, 1967; Page et al., 1980). Rhodes (1965) compared theRum Jungle Complex to a mantled gneiss dome; Stephansson andJohnson (1976) suggested that both Complexes were domed by solid-state diapric intrusion of granite during the 1870-1800 Ma orogeny.

140

Nanambu ComplexRocks of this Complex are poorly exposed over an area of

about 45 km by 80 km between the South and East Alligator Rivers inthe northeast of the Geosyncline. Needham & Stuart-Smith (1980)defined three groups of rocks in the Complex: Archaean two-micagranite; porphyroblastic foliated biotite granite and augen gneisswhich is Archaean granite metamorphosed by the 1870-1800 Ma event;and pegmatoid leucocratic gneiss, migmatite and schist which isEarly Proterozoic arkosic material accreted to the granitic Complexat 1870-1800 Ma. Field distinction of the 3 phases is not alwaysclear. The age relationships of the phases were indicated byisotopic dating techniques (Page et al., 1980). On the southernand western margins of the Complex the leucocratic gneiss maycontain resisters of Early Proterozoic quartzite, and the leuco-cratic gneiss grades out into meta-arkose of the Early Proterozoicmetasedimentary sequence.

The Nanambu Complex is a likely source of uranium for theAlligator Rivers area uranium deposits, as the two-mica Archaeangranite gneiss is known to contain accessory uraninite (McAndrew& Finlay, 1980). Mean uranium content is 9 ppm U (Ferguson &Winer, 1980).

Géosynclinal sedimentationThe stratigraphy of the Early Proterozoic sediments and

interbedded volcanic rocks is summarised in table 1, and diagramm-atic relationships are shown in figure 3.

At least 14 km of sediments and volcanics were deposited inparts of the basin. The margins of the basin are concealed byyounger strata, so the size of the basin is unknown. The basinis probably continuous under cover with other Early Proterozoicsequences in northern Australia such as the Arnhem, Tennant Creek,Granites-Tanami and Arunta Blocks (Fig. 2). Sedimentation tookplace between 2400 Ma (the age of the youngest basement rocks)and 1870 Ma (the age of pre-metamorphic granodiorites) in thenortheast (Page et al., 1980). Volcanics interbedded with thesediments (in the South Alligator Group) yield an age of about1890 Ma (R.W. Page, pers. comm. May 1983). The sediments aremainly pelites, which are commonly carbonaceous and/or pyritic and/or calcareous at depth. Other lithologies present are sandstone,crystalline carbonate, calcarenite, tuff and conglomerate.

141

Money Shoal / Arafura Basin

r^NANAMBU COMPLEX/

Bonaparte Gulf RUM JUNGLE COMPLEBasin LITCHFIELD COMPLE

FITZMAURi££_MOBIlE ZONE/\COEN INLIER

Browse Basin

21 «Off

Carpentaria BasinBATTEN FAULT ZONE

\NICHOLSON £W/« • / '!•• LAWN HILLPLATFORM

TENNANT CREEKINLIER• HALLS CREEK

INLIER .. . X

THE GRANITES-• . ' TANAMI INLIER

GeorginaBasin i •

lDAVENPORT

INLIER• I

Broken RiverEmbayment, ARUNTA INLIER

NGALIA BASIN.

• AMADEUS BASIN

129°00- MUSGRAVE BLOCK

OROGENIC PROVINCES/ f

/ /

my/

Tasman

Central Australian

North Australian

West Australian

PLATFORM COVERS

* •• • *.*

Trans-Australian

Central Australian

North Australian

Fig. 2 Principal tectonic elements, northern Australia(Plumb et al., 1981).

The psammitic units are mainly fluviatile. The provenanceof the Namoona and Kakadu Group psammites was probably the adjacentbasement complexes, whilst grainsize distribution suggests anorthern provenance for the Mount Partridge Group, and a westernsource for the Finniss River Group (Stuart-Smith et al., 1980).Widespread algal carbonate and evaporite pseudomorphs indicatemainly very shallow to supratidal environments for the carbonates(Crick & Muir, 1980). The pelites were probably deposited inneritic conditions, and the greywackes may represent tectonic deep-ening of the basin heralding the 1870-1800 Ma event.

Sedimentation took place in an intracratonic basin coveringoat least 100 000 km . The earliest known Early Proterozoic

sediments, the psammites of the Narnoona and Kadadu Groups, rest

142

unconformably on Archaean basement and exposure is generally con-fined to areas adjacent to exposed basement. These basal sedi-ments may extend over much of the basin at depth. Those of theNamoona Group surround the Rum Jungle and Waterhouse Complexes,andcontain arkosic rudite, psammite, conglomerate, and minor shale.The Kakadu Group, which is best developed adjacent to the NanambuComplex, comprises mainly meta-arkose and paragneiss, which inplaces grade into leucogneiss of the Complex.

These psammites are overlain by carbonaceous and calcareouspelites and psammites of the Masson Formation, which extends fromwest of the Rum Jungle Complex almost to the South Alligator River;further east it is probably equivalent to the lower member of theCahill Formation, a partly calcareous and carbonaceous sequence ofmicaceous quartzo-feldspathic schist, with lenses of massivecarbonate near its base (Needham & Stuart-Smith, 1976). TheCahill Formation is host to uranium and other mineralisation in theAlligator Rivers area. Volcanic breccia and basalt of the StagCreek Volcanics are conformable on the Masson Formation in thecentral part of the Geosyncline. Elsewhere, constituting theMount Partridge Group, a wedge-shaped assemblage of sandstone,siltstone and conglomerate of the Mundogie Sandstone and CraterFormation, overlies the Masson Formation. It represents anextensive fluvial fan which grew from the north, although coarserclasts in the Rum Jungle area indicate rejuvenation of basementinliers in the west. Stromatolitic carbonate of the Celia andCoomalie Dolomites indicate that Archaean basement formedislands in the Rum Jungle area until at least mid-Mount PartridgeGroup time.

The alluvial fan deposits fine upwards into banded partlycarbonaceous siltstone of the Wildman Siltstone. Its equivalentin the Rum Jungle area, the Whites Formation, hosts U-Cu-Pb-Zndeposits at or near the contact with the Coomalie Dolomite below.Minor mafic volcanic units lie near the top of the WildmanSiltstone up to 100 km east of Rum Jungle.

A distinctive assemblage of iron-rich sediments, carbonate,and tuff of the South Alligator Group rests, in places unconform-ably, over the older groups. The lowest member of this group, theKoolpin Formation, is mostly pyritic carbonaceous shale whichcommonly contains chert-bands and nodules, algal carbonate, and

143

banded iron formation. These rocks are generally altered near-surface to chert-banded hematite siltstone and massive siliceousrocks. Chert-banded hematitic siltstone also crops out as inter-beds in the overlying massive tuff and argillite of the GerowieTuff. Rocks similar to the ferruginous siltstone of the KoolpinFormation also crop out in the Alligator Rivers Uranium Field,and are critical to correlation in the northeastern part of theGeosyncline.

Conformity of the ash-fall rocks of the Gerowie Tuff verifiesthe chronostratigraphic nature of the sedimentary pile. TheGerowie Tuff is overlain by Kapalga Formation rocks, which are atransitional suite between Koolpin-type rocks (less iron-rich andless chert than in the Koolpin Formation) and greywacke of theFinniss River Group, with interbeds of Gerowie Tuff-type tuff andargillite.

Uranium + gold deposits of the South Alligator Valley areaare hosted by pyritic highly carbonaceous shale of the KoolpinFormation, and commonly lie on the faulted contact with pyro-clastic sandstone of post-orogenic, late Early Proterozoic age.

The uppermost part of the Early Proterozoic succession isrepresented largely by a monotonous sequence of siltstone, slate,shale, greywacke, and minor arkose, quartzite, conglomerate, andschist, of the Burrell Creek Formation of the Finniss River Group.In the west, where greywacke is mostly volcanolithic, these rocksgrade laterally and upwards to quartz sandstone and minor conglom-erate of the Chilling Sandstone (Walpole et al., 1968). TheBurrell Creek Formation commonly displays turbiditic sedimentaryfeatures, including graded beds, scour marks, flute casts andflame structures.

Transitional igneous activityAt or near the end of sedimentation, the Zamu Complex, a suite

of continental tholeiites with a composite thickness of about1.5 km, intruded the sediments mainly as sills. The sills werefolded and metamorphosed with the host rocks by the 1870-1800 Maregional event (Ferguson & Needham, 1978). In the medium to highgrade metamorphic domain and in the contact aureoles of the post-orogenic granites the rocks are amphibolite, but elsewhere theoriginal textures and mineralogy are partly to wholly preserved;in places hornfelsic zones in the adjacent sediments are apparent.

144

The rocks range from quartz dolerite with felsic differentiates,to dacite, and are commonest in the South Alligator Group.

The close of Early Proterozoic sedimentation is probablyindicated by intrusion of granodiorite and tonalité of the NimbuwahComplex, in the extreme northeast of the Geosyncline, at about1870 Ma (Page et al., 198Q). This event may also have triggeredoff a long period of deformation and metamorphism which culminatedat about 1800 Ma, the age which is ubiquitously recorded by K-Arisotopic systems in the metamorphics and many of the basement rocksof the region. The tonalités and granodiorites of the NimbuwahComplex underwent partial migmatisation in this period, as did theEarly Proterozoic sediments in the same area. Resisters ofquartzite, carbonaceous schist, calc-silicate gneiss and marble,indicate that some of these rocks were originally of the KakaduGroup or Cahill Formation; those metamorphic rocks to which nosedimentary parentage can be objectively determined are ascribed tothe Nourlangie Schist, or the partly metamorphically differentiatedMyra Falls Metamorphics (Needham, 1982) .

The metamorphics were intruded by numerous post-orogenicgranite plutons, which have an age range of about 1800-1740 Ma(Page et al., 1980). Several plutons coalesced to form the CullenGranite batholith in the centre of the Geosyncline. This is the2largest granitic body (3600 km ), and extends under cover rocks ofthe Daly River Basin (Tucker et al., 1980). Diapiric intrusion of'solid state1 granite below the Rum Jungle and Waterhouse Complexeshas been described as a mechanism for rejuvenation of theseComplexes by uplift (Stephansson & Johnson, 1976).

Felsic and minor mafic volcanics and volcaniclastics of thelate Early Proterozoic El Sherana Group and Edith River Volcanics(-1760 Ma; Leggo in Walpole et al., 1968) are probably comagmaticwith the diapiric granites. A significant period of erosionbetween the 1870-1800 Ma metamorphic and deformation event, andextrusion of volcanic rocks in these two groups, is indicated bytheir valley-fill nature.

The final igneous event prior to Middle Proterozoic platformsedimentation was intrusion at 1699 Ma (Page et al., 1980) of aseries of continental tholeiitic lopoliths - the Oenpelli Dolerite -at about 1-2 km depth (Stuart-Smith & Ferguson, 1978) . Theseintrusions consist of porphyritic olivine dolerite, quartz dolerite

145

and granophyre sheets, in places over 250 m thick, and minor dykes.The lopoliths form long, arcuate basement ridges to platformsedimentation, indicating another substantial period of erosion.

The pattern of sedimentation followed by orogenesis and aperiod of post-orogenic igneous activity seen in the Pine Creek Geo-syncline is consistent with the model of evolution of Proterozoicmobile belts described by Glikson (1976), and applied to the PCG byStuart-Smith et al., (1980).

Cover rocksMiddle Proterozoic plateau-forming sandstone and minor inter-

bedded, mainly intermediate, volcanics of the Kombolgie Formationrest with marked unconformity on older rocks, covering the easternextent of the Early Proterozoic metasediments and their gneissicequivalents which are exposed in a few inliers within the sandstoneplateau. Minor dolerite intrusions of Late Proterozoic age cutthe sandstone in places; some additional similar intrusions may berepresented by long linear magnetic patterns coincident with majorfractures in the sandstone. These intrusions, and Late Proterozoicphonolite dykes which intrude the Nanambu and Nimbuwah Complexes,represent the final known igneous events in the region (Pageet al., 1980) .

Late Proterozoic sediments are exposed at the margins ofextensive Cretaceous tablelands in the southwest of the Pine CreekGeosyncline. The age of the sandstone-dominant Tolmer Group whichcrops out in the same area is obscure, as no relationships areexposed between it and rocks of proven Late Proterozoic age; itappears basal to the Late Proterozoic sequence. The northern andsouthern margins of the Geosyncline are concealed by Mesozoic andPalaeozoic strata.

A palaeo-weathering profile is common below the KombolgieFormation, and a rare regolith is developed in some places. Aregolith is common below the Cretaceous, and lateritisation tookplace during the Tertiary. Hays (1965) identified three Cainoz-oic land surfaces which are commonly superimposed, and extensivedeep weathering profiles (up to 60 m) are developed.

The proximity of Kombolgie Formation sandstone to the uraniumdeposits of the Alligator Rivers area and South Alligator Valley,and of the Depot Creek Sandstone Member of the Tolmer Group to theuranium deposits of the Rum Jungle area, has focused attention on

146

the sandstone as a possible source of uranium or as a vital part ofuranium ore genesis. The Kombolgie Formation is not however richin uranium and airborne radiometric anomalies in the Formation arerestricted to latérites developed downslope from interbeddedintermediate to mafic volcanic members (Needham et al., 1973).Heavier elements may be concentrated in purple beds which arecommon in some areas near the base of the unit. The lithologiesare mainly coarse, clean, moderately well-sorted subangular tosub-rounded buff to white quartz sandstone, with pebbly horizonsand conglomerate beds more common near the base. Clasts aremainly moderately to well-rounded quartzite and vein quartz;clasts of Early Proterozoic rocks are rare. There are very raresiltstone beds up to 5 cm thick. The sandstone is commonly devoidof a matrix except nearer the base where a white clay matrix isoften present. The rock ranges from loose and friable, to wellindurated, to silicified on stable exposed surfaces and alongjoints.

The unconformity plane has a morphology similar to that ofthe undulating sand plains which cover much of the Geosynclinetoday. The unconformity undulates gently with an amplitude ofabout 20 m although there are scattered basement hills and ridgesup to 250 m. The Oenpelli Dolerite forms large arcuate basementridges up to 100 m high and the post-orogenic granites in placesform plateaux about 100 m above the general level of the unconform-ity. The unconformity plane is displaced by numerous faults whichmainly throw the sandstone only a few tens of metres. However, athrow of at least 200 m has been substantiated for the Koongarrareverse fault (-Foy & Pedersen, 1975) . Extensive faulting ofblocks of Kombolgie Formation sandstone is recorded in or near theRanger 1 and Jabiluka 1 ore bodies (Eupene et al., 1975); Hegge,1977). There does not appear to be any broad relationshipbetween the elevation of the unconformity and the distribution ofmineralisation. The contact at the base of the Depot CreekSandstone Member is regionally anomalously radioactive, but theunconformity of the base of the Kombolgie Formation has no radio-metric expression. However, metamorphics and granites below theKombolgie Formation are commonly altered, in places to more than50 m below the unconformity. Granite and gneiss at the top ofthis palaeo-weathering-profile are commonly altered to a blotchypale-green and purple quartz-clay rock. In places this profile

147

may be more radioactive than its unweathered parent, but noanalyses are available of the amount and distribution of uraniumwithin this profile.

DISTRIBUTION OF URANIUM OCCURRENCESSixty-seven uranium occurrences (Table 2) with visible

uranium minerals, or significant uranium prospects, are scatteredmainly in the east in a broadly Nt-trending belt which terminatesabruptly at a linear edge along the South Alligator Valley uraniumfield, and along a NVv trend running from Rum Jungle to Katherine,(Fig. 4). The pattern ^artly reflects areas of most intenseexploration for uranium, i.e. around the deposits of the threemajor fields, and along the Stuart Highway (the latter by hand-held geiger counters clutched by 'weekend prospectors' in the1950's; Annabell, 1971) , but some alignments are indicated whichseem to control the broad distribution of the deposits. Thisapparent control is strongest in the South Alligator Valley area,where most of the deposits are close to a major NW strike-slipfault (Crick et al., 1980).

In an analysis on the distribution of metallic mines andprospects in the Pine Creek Geosyncline, Needham (in press) indic-ates a strong relationship between the distribution of uraniumoccurrences and the Cahill, Whites, and Koolpin Formations(Fig. 5a; 26, 13 and 22% of occurrences respectively.

The dominance of favoured stratigraphie units in determininguranium distribution is even clearer when uranium is expressed astonnes U.,0R (Fig« 5b). There is also a strong positive relation-ship between distribution of uranium occurrences and proximity toArchaean masses (Fig. 6), although no occurrences of significanceare known within them. A low-order relationship is evident withlineaments (as both lineament density and lineament intersections),and alignments represented by the distribution of uraniumdeposits could indicate a fundamental fracture control to thedistribution. No relationship is evident with the distributionof the post-orogenic granites, even though most of the uraniumprospects in the south are hosted by them. The relationshipbetween uranium distribution and the present-day position of thecover-rock unconformity may be more apparent than real, and moreproperly related to the thinner Mesozoic and Cainozoic cover over

148

TABLE 2. URANIUM MUTES AND PROSPECTS OF THE PINE CREEK GEOSYNCLINE (aaended from Needhaa, 1981)

PC

Black Rock

Tadpole

Babarlek

Oumgarn

Oorrunghur

Caramal

Béatrice

Arrara

Jabiluka 1

Jabiluka 2

Jabiluka 3

Ranger 5Ranger 66

Ranger 4

7J

«* TT 'S<V -P

B £ ë

£ |S ~-£°v o ï^ «

5 fl 1 is!S I f l f e a1 U prospect

2 V prospect

3 U 12,000(stockpiled.being treated)

4 u" prospect

5 TJ prospect

6 0* prospect

7 ö prospect

8 0 prospect

9 U prospect

10 Ü (3,484)

11 U, Au (203,800)(8.1 Au)

12 U prospect

13 U proapect

14 a testingincomplete

15 U prospect

16 U, Au prospect

eT* K

• \ï S

- +i JZ

i ! ?ë £ édisseminated irregular, disseminated uraniniteoffset by faulting

irregular to planar, thin uraninite in pegmatite

1.98* steep tabular to shallow massive and disseminatedcigar pitchblende, near-surface

oxidised halo

tabular 60 secondary near surface

tabular 60 secondary near surface

secondary near surface

arcuate band ?0° sooty pitchblende inbreccia veins

irregular to planar steep minute dissemination andvein-filling films ofsooty pitchblende,secondaries in weatheredzone

tabular shallow secondary, at base ofpre-Kombolgie weatheringprofile

0.25e* folded layers and leases disseminated & Tein filling0-70 , in 4 main strat- pitchblende, A selvages toiform ore-bearing horizons chlorite A alteration zones.

Minor coffinite andbrannerite

0.39* » "15, WT Au

similar to Jabiluka 1 A 2

b«st reported irregular breccia pipe sooty & colloform pitch-drill inter- blende. Minor secondariessections 13 m •0.36*, 8 m 00.76*, 28 m 00.38*

'medium to low irregular to tabulargrade1 small body

'viable ore body disseminated pitchblende,likely1 near-surface secondaries

ta

°

«H

chlorite * mica quartzschist

quartz mica schist +quart zi te

chlonte mica quartsschist

quartz mica schist andgraphite schist

quartz mica schist andgraphite schist

quartz mica schist

hematitic chlorite quartsschist

sericite hematite micachlorite quartz schist

chlorite quartz sericiteschist

quartz sericite chlonte+ graphite schist

it

similar to Jabiluka 1 & 2

chlontised breccia &pegmatoid, quartz seric-ite chlorite schist;nearby carbonate

chloritic pegmatoidbreccia in dolomite

chlorite + graphite schist,hematitic near surface

§Îtu

3:

NimbuwahComplex

M

Myra PallsMetamorphic 3

CahillFormation

H

It

Myra PallsMetamorphics

NimbuwahComplex

KoolpinFormation?

CahillFormation

H

H

n

it

m

»

i

o.

1£

retrogradedm.amphibolite

n.amphibolite

retrogradedm.amphibolite

*

m.amphibolite

«

retrogradedo.amphibolite

«

»

retrogradedm.amphibolite

H

N

m.amphibolite

retrogradedm.amphibolite

«

retrogradedm.amphibolite

r-<

F.

a

unconformity, îfault,chlonte alteration

pegmatite

shear zone ?unconformity,îchlorite alteration

stratabound, quartzstockwork breccia

unconformity, ?stratabound(relict), chlorite alter-ation?

shear zone breccias,unconformity

pre-Kombolgie weatheringzone, ?stratabound, ?fault

stratiform, unconformity,chlonte alteration,brecciation

n

stratabound?

chlorite alteration &brecciation at NanambuCompleVCahill Fmcontact

ti

stratabound, chloritealteration, unconformity

g

^£

SmetaMorphic hydrothermal

related*

hydrothermal

metamorphic hydrothermalune . related , ? stratabound(relict)

stratabound, metamorphichydrothermal, + epigenetic,uncf. related?

stratabound?, metamorphichydrothermal + epigenetic,uncf. related?*

metamorphic hydrothermal -fepigenetic, uncf. related?

stratabound, uncf. related,?epigenetic

stratabound, metamorphichydrothermal & epigenetic,uncf. related?

it

stratabound & metamorphichydrothermal + epigenetic,uncf. related?

H

stratabound + metamorphiohydrothermal •»• epigenetic,uncf. related?

17 B (727) pitchblende in fractures &breccia zones, minor thuc-olite, near-surface second-aries

chlorite schist, chloriticbreccia

chlorite alt°ration andbrecciation at NanambuCompleJt/Cahill Pm contact,unconformity

Table 2 cont'd

00

G4i

t- i-ii ï î

Es û: X

»ustato» 1 18 U

Ranger 1 No. 4 Anomaly 19 U

No. 3 Orebody 20 U

No. 1 Orebody 21 0

No. 2 Anooaly 22 U

No. 9 tnoialy 23 U

No. 5 taooaly 2« V

Koongarra 25 U

Tive Sisters 26 0

Graveside 27 U

AnouQy 2J 28 U

Teagues 29 Ut Au

Bockhole 30 U, Au

O'Dwyers 31 U, Au

Sterrets 32 D, Au

Airstrip 33 D

Sandstone 34 U

t,aj

*

o '^3 ~* §

1 1 S 3

(confidential)

prospect

(50,000)

(50,000)

prospect

prospect

prospect

(12,300)

prospect

prospect

prospect

152Au includedunder ElSherana

prospect

prospect

°°"•3 >,

1 !best reporteddrill intersect-ions 1.5 m 0 0.48S6,5 m 0 0.2156'thin, low grademi neral i sat i on '

0.36 shallow-dipping tabularto lenaoid body withconformable base, andblind, down-dip lenses

0.386 inverse cone withdown-dip conformablelenses

' significantprimary & second-ary mineralisat-ion1

'medium gradesecondary miner-alisation1 (nodeeper drilling)

0.34656 coalesced stratiformlenses form elongatewedge, dip 55

tabular steep lodes

0.99*

linear near-surface

1.156 several irregular tabularsteep bodies along reversefault

-

"

near-surface x4radioactivity

300 ppm U 0 ,4850 ppm TK,( 14 ppm U 0 atdepth) 3 8

w 1t. « ~ oo -i

« .n e .e t

| ^ ï 1 1 3

s 1 J 2 I ndisseminated pitchblende in Lower C thill schists Cahill retrograded chlorite alteration and btratabound +• metamorphic

CompleVCahill Fm uncf. related?^.ontact, unconfcroity

tt n

n »

f i lmy pitchblende mainly gneiss, s i l ic i f ied tarb- & Nanambu chlon te alteration, sil- hydrothermal + epigeieticin chlorite rock and chlor- bonate, massive chlorite, Complex icifi.cation, unconformity - probably carbonate s Ijt-ite schist, and carbonac- graphite schist C<»1 " ion. Uncf. i elated'ecus schist, particularly r catit. i

H n

n ti

disseminated pitchblende, quartz chlorite * graph- " " stratabound to stratiform stritabouid + oetaoorphicsecondary tail near ite schists, garnet chloride alteration, brecc- hydrothermal + epigeneti ,surface quartz muscovite chlorite iation, ?faulting, imcf. related'

schist 'unconformity

no visible uranium miner- carbonaceous phyllite Nou'-langie " carbon, unconformityals Schist

Hit«, nearby aeta-dolerite

'yellow-grepn micacpois weathered tuffac«>ous Stag Creek " faulting, ?stratdbound syngenetic/supergeneuranyl phosphate' shale & basalt Volcanics

patchy secondaries along carbonaceous siltstone, Koolpin Fm & l.greenschisL faulting, carbonaceous eoigeneticfaulted sst/sltst contact sandstone Coronation Sst shaK , nearby acid& in fractures volcanics, unconformity

patchy pitchblende, near- H tt tt n itsurface secondaries

»H H K " H H

no visible minéralisation " Koolpin n it itFormation

" andésite, sandstone Plun Tree unmetamorphosed stratabound supergonelenses Volcanics

El Sherana West

El Sherana

Koolpin

Scinto 6

Palette

Skull

Saddle Ridge

Scinto 5

Coronation Hill

Sleisbeck

ABC

Edith River

Tennyson s

Höre 4 O'Connors

Yenbeme

Yenbeme

Fergus son River

Fleur de Lys

George Creek

35

%

37

36

39

40

41

42

43

44

45

46

47

48

49

50

515253

5455

56

57

58

H,Au,Ag

D, Au

U

U

D, Au

V

0

U

U, iu

B

U

U

V

uu0

uD

W, Mo, Bi,n

v

u

H, Cu

D

185 0.8*.007 Ag

226 0.5*.33 Au

3 0.12*

3 0.15*

124 2.5*Au incl. underEl Sheraufl

3 0.5*

78 0.2*

22 0.4*

75 0.3*

3 0.46*

prospect 0.4$

prospect

prospect

prospect

prospect 0.1-0.2$

prospect at surface

prospect

prospect

prospect

0.125 Mo160 wo

prospect

prospect

0,15

1 0.2»

irregular to carrot-shapedbodies aligned alongstrike

pipe inclined 45 ,cuts across shale/sandstone contact

irregular

steep tabular

irregular

several irregular oreshoots

irregular near-surfacein breccia

shallow irregular

patchy bodies in N orNW shears

"it

n

"H

«

steep tabular reefs,parallel to aplite dykes.Also disaem. in joints& mica-rich seams

patchy bodies in N orNW shears

patchy bodies in N orNW shears

patches in conformableshear zones 4 joints 4along bedding planes

pods & stringers in weaksteep N shear

patches, veins 4 dissemin-ations of pitchblende,mainly in fractures. Nearsurface secondaries.Minor Au, Pb

sooty 4 massive pitch-blende in fractures

secondary mineralisation

vein 4 massive nodularpitchblende, veins nativegold, minor secondaries

pitchblende nodules inshears

secondaries, mainly meta-torbermte, minor sootypitchblende below orebody

secondaries, minor pitch-blende

sooty pitchblende, dissem-

gold

pitchblende, secondaries,with phosphate and minorCu, Hi, Co, As

autunite, phosphuranylite

disseminated torbermte &meta-autunite + apatite &hematite in quartz veins

•it

it

"

"

"

greisenised quartz aplitedykes containing wolfram4 minor molybdenite andtorbermte in quartz veins.Fe, Cu, Bi sulphides atdepth

disseminated torbermte &meta-autunite + apatite 4hematite in quartz veins

disseminated torbermte &meta-autunite + apatite &hematite in quartz veins

uranini te+pyri te+chal-copyrite in shears. U+Cp

patchy pitchblende, pyrite,chalcopyrite in joints 4

at surface

cherty ferruginous silt-stone , carbonaceoussiltstone, sandstone

carbonaceous shale

sheared and jointedvolcanics

carbonaceous shale,sandstone

carbonaceous shale

bleached carbonaceousshale, rhyolite 4 tuff

bleached white & redferruginous shale

volcanolithic and carbon-

chZoritic carbonaceousschist, quart zi te, mud-atone

interbedded tuff andamygdaloidal basalt

ailicified greisenisedgranite

n

n

it

"

n

"

aplite dykes in granite

silicified greisenisedgranite

silicified grftisenisedgranite

siltstone, slate

it

Koolpin FB 4 l.greenschistCoronation Sst

Koolpin 1 . greenschi stFormation

Coronation unmetaaorphosedSandstone

Koolpin Fm & l.greenschistCoronation Sst

Koolpin l.greenschistFormation

Koolpin Fm & n

Coronation Sst

Koolpin M

FormationCoronation unmetamorphosedSandstone

Kapalga l.greenschistFormation

Kombolgie unmetamorphosedFormation

Cullen Granite "

1 H

H It

ff ff

N ff

H N

H H

K ff

.

Koolpin l.greenschistFormation

Burrell Creek *Formation

carbonaceous shale, near-by acid volcanics, Tfault-ing, unconformity

?fauiting, unconformity

n

H

Tfaulting, unconformity

carbonaceous shale, nearbyacid volcanics, fault,unconformity

n

carbonaceous shale, brecc-lation, unconformity

?breccia

host volcanics, ?nearbydeep fracturing

hydrothermal alterationzones 4 quartz veining ingranite

H

H

It

N

n

H

altered aplites near roofof granite

n

hydrothermal alterationzones 4 quartz veining ingranite

hydrothermal alteration zones4 quartz veining in granite

altered shears and joints

•

epigenetic

epigenetic

epigenetic supergeneT

epigenetic

epigenetic

*

supergene?

"

?epi gene tic

epi geneti c/ syngene ti ct

N

tt

H

It

tt

"

»

M

-I

II

II

hydrothermal ; possiblee xhal at i ve/epigeneti c

•

Uïto

Table 2 cont'd

1Adelaide River

White 's

Dyson1 s

Mount Fitch

Mount Burton

Dolente Ridge

Area 55

Rua Jungle Creek

Rum Jungle Creek South

c*x

£

cc

c£ "o -c

K X

59 D

60 U.Cu.Pb,Co, Ni

61 U

62 D, Cu

63 ü, Cu

64 Ü.Pb.Cu,Zn

65 U,Pb,Zn,Cu

66 U

67 D

S

l '

'i«!

S 1 1tfc S o n

45

10691.8 4g, 861 Co,15 000 Cu,4100 Pb

534

(1500)(1885 Cu)

12.6100 Cu

prospect

prospect

prospect

2612

°°

;

•êo

0.5*

0.16*25.7 g/T 4g2.2* Cu0.6* Pb

0.34*

.042*290,000 T «0.65* Cu

0.17*1.3* Cu

0.4*

ETso

steep N shear zone,irregular ore zonepitches 345°

roughly confomable steep17-27 m wide NE shearzone

conformable NE she^r 8 mwide, steep

2 U bodies, E 110x30x20 mdeep, N l60x110x100 m deep,3 Cu lenses in -J tan con-formable enriched zone

small conformable lens

irregular pods in shears& plunging fold axes

horizontal cigar 60 » x245 m, 30 m depth

t-

S1

1

pitchblende, pyrite, chal-copyrite in irregularquartz veins, & dissen. inhost rocksuraninite , torbernite ,chalcopyrite, bornite +galena as disseminations+ veins

vein * dissem. uraninite,saleeite

fine ?thucolite in clayeygraphite schist bands,veinlets in magnesite.Near surface Cu-enri chmentin clays

dissem. torbemite, pitch-blende , malachite , chalco-cite

fine sooty pitchblendecoatings, minor veins &stockworks

S

I-1

-

sheared graphitic seric-itic, chlontic & pyriticshale

sheared graphitic shalehematitic dolomite

chlorite aericite griph-ite schist, magnesite

pyritic black slate,minor quart zi te

pyritic black shale,dolomite

chlontic A graphiticshale

chlontic A graphiticpyritic shale

c

1•23

-

WhitesFormation

Whites Fœ &CoomalieDolomite

Whites FB &CooaalieDolomite

WhitesFormation

N

Whites Fm &CoomalieDel OBI te

WhitesFormation

«

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Fig 3. Diagrammatic stratigraphy of the PineCreek Geosyncline

Early Proterozoic bedrock near the cliff-like edge of the Middle-Late Proterozoic plateau sandstones. The thinner cover hasallowed more effective use of conventional uranium explorationtechnology in these areas in contrast to the poorly and ineffect-ually explored areas marked by thicker cover, particularly nearthe ocast.

153

Fiy 4. Distribution of significant uraniuwoccurrences in the tine Creek Geosyncline.(Numbers cross-refer to Table 2)

South Alligatoj valleyUianium Field

50 km

Middle Proterozoic toMesozoic cover units

Tolmer GroupKombolgie Formation

Oenpelli DolenteEdith River VolcanicsPost otogenic granite

Nimbuwah ComplexLitchfield Complex

Hermit Creek MetamorphicsNourlangie Schist

Myra Falls Metamorphics

Zamu DoleriteMeeway VolcanicsBerinka VolcanicsDorothy Volcanics

Chilling SandstoneBurrell Creek Formation

Kapalga and Mount Bonnie Fm.Gerowie Tuff

Koolpin FormationWildman Siltstone

Mundogie SandstoneStag Creek Volcanics j

Cahill FormationMasson Formation

Kakadu GroupCoomalie Dolomite

Crater FormationCelia Dolomite

Beestons Formation

Nanambu ComplexRum Jungle ComplexWaterhouse Complex A -

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No. of mines and prospects2000 4000 6000

Tonnes U308

8000 10000

Fig 5. Distribution of uraniumA) number of occurrences, andB) tonnes U.,0Q production and reserves in Pine Creek rock units

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Distance from Archaean granite Distance from post-orogenic granite

Fig 6. Proximity of Pine Creek uranium occurrencesfrom Archaean and Proterozoic granite

GEOLOGICAL SETTING OF DEPOSITSThe clustering of uranium occurrences naturally defines 3

major uranium fields, and a scatter of prospects aligned NWbetween Darwin and Katherine (Fig. 4). Within each field theuranium deposits share many common features and lie in the samehost formation, but the host formation is different in each field,

Descriptions of individual uranium depsoits and some pros-pects in the Pine Creek Geosyncline have been given elsewhere inliterature (e.g. Ferguson and Goleby, 1980). Table 2 summarisesthe main features of these occurrences in terms of their grade,tonnage, geometry, host formation and lithologies, and orecharacteristics, and also outlines possible controls on mineral-isation. In this section the three main uranium fields are

156

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Koolpin Formation

MOUNT PARTRIDGE GROUP

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Mundogie Sandstone

Cahill Formation

KAKADU GROUP

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Munmarlary Quartzite

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Fig 7. Geology of the Alligator Rivers uranium Field

discussed, with the emphasis on similarities and differencesbetween deposits rather than detailed deposit descriptions.

Deposits in the Alligator Rivers Uranium Field (ARUF) are themost recent (post-1970 A.D.) and spectacular U discoveries in thePine Creek Geosyncline (PCG), and are either in the early stagesof mining or yet to be mined. Some of these deposits have beenthe subject of a wide variety of detailed geochemical and petro-

157

logical studies and as development proceeds further studies canbe expected to enhance our understanding of them. By contrast,the deposits in the Rum Jungle Uranium Field (RJUF) and SouthAlligator Valley Uranium Field (SAVUF) were discovered in the1950's, and mine and milling operations ceased in the 1960's.They did not receive the same attention and consequently cannotbe documented as thoroughly as the Alligator Rivers deposit.

ALLIGATOR RIVERS URANIUM FIELD (ARUF)2The Alligator Rivers Uranium Field covers about 22 500 km in

the northeast of the PCG and contains the Jabiluka, Ranger 1,Koongarra and Nabarlek deposits (Fig. 7). The area is distinctfrom other parts of the geosyncline in that metamorphic grade(medium to high-grade), and degree of deformation (four periodsof deformation and isoclinal folding), are markedly greater(Ferguson, 1980; Needham & Stuart-Smith, 1980). The uraniumdiscoveries were made in 1970 and 1971. Subsequent formation ofa National Park in the area has precluded effective exploration ofthe region since the discovery years.

The area between the South and East Alligator Rivers isdominated by the Nanambu Complex, which contains granitoids thatrecord Archaean (up to 2500 Ma) and -1800 Ma erogenic events, andaccreted Early Proterozoic feldspathic and quartzitic gneiss andschist. The gneiss is, in places (mainly south of the complex),gradational outwards into Mount Basedow Gneiss, the lowestrecognisable part of the Early Proterozoic metasedimentarysequence. The Munmarlary Quartzite is a quartz-rich equivalentof the gneiss on the western flank of the complex. The overlyingCahill Formation onlaps in places to rest directly on Archaeangranite of the Nanambu Complex. This formation contains all theuranium deposits and most of the uranium prospects of the area,and is the prime guide to uranium exploration. Needham andStuart-Smith (1976) have divided the Cahill Formation into twomembers: a lower member, which is host to the uranium mineralis-ation and is characterised by the presence of carbonaceous unitsand carbonate rocks, and includes an interlayered sequence of micaschist, chloritised feldspathic quartzite, quartz schist, para-amphibolite and calc-silicate rock; an upper more psammitic memberwhich contains quartz schist, feldspathic schist, feldspathicquartzite, and conglomerate. The Cahill Formation is overlain,

158

in places unconformably, by the Nourlangie Schist, a monotonoussequence of mainly quartz mica schist. Ferruginous siltstonewith finely crystalline quartz bands exposed in the north may bemetamorphosed equivalents of the Koolpin Formation.

East of the East Alligator River the Early Proterozoic meta-sediments grade into metamorphically differentiated schist andgneiss of the Myra Falls Metamorphics. Some distinctivelithologies can be mapped out as 'resisters1 which can be correl-ated with the Kakadu Group and Cahill Formation. 'In the extremeeast, pre-orogenic granite intruded at -1870 Ma was metamorphosedand partly differentiated along with the metasediments to formthe migmatitic Nimbuwah Complex; its composition ranges fromgranitic to granodioritic in the north, to tonalitic in placessouth of Nabarlek.

The orogenic event probably began at the time of intrusion ofthe pre-deformation granite, and culminated at about 1800 Ma.Pre-orogenic tholeiitic dolerite sills (Zamu Dolerite) are pres-erved throughout the area as strongly foliated amphibolite.Post-orogenic (1730-1780 Ma) granite - trondhjemite forms smallstocks east and south of Nabarlek, and post-orogenic (-1690 Ma)'tholeiitic dolerite intruded as extensive lopoliths throughout theregion east of the Nanambu Complex.

The Middle Proterozoic Kombolgie Formation forms a sub-horizontal sheet of sandstone and interbedded mainly andesiticvolcanics about 200-1000 m thick, generally marked by a sheercliff rising abruptly above the lower undulating terrain formedby the older rocks. A major erosional period is represented bythe unconformable contact, as at least 1-2 km of supracrustalswere removed to expose the Oenpelli Dolerite lopoliths. Smallpockets of tuffaceous sediments along the contact are probabledistal equivalents of the mainly felsic late Early Proterozoicvolcanism centred in the South Alligator Valley Uranium Field.Minor phonolite and dolerite dykes were emplaced between about1370-1200 Ma ago, concentrated in the Nanambu and NimbuwahComplexes. Mesozoic and Cainozoic marine to terrestrial depositsform a northward-thickening veneer (up to 80 m thick) in thenorthwest, and colluvial sand and latérite masks much of the EarlyProterozoic rocks throughout the region.

159

Jabiluka, Ranger, Koongarra and most of the other prospectsare stratabound within the Early Proterozoic Cahill Formation.Nabarlek is hosted by the Myra Falls Metamorphics, but this unitis thought to be a correlative of the Cahill Formation (Needham,1982). The carbonate rocks near the Jabiluka, Ranger 1 andKoongarra deposits were derived from evaporites (Crick & Muir,1980), indicating a supratidal depositional environment. CahillFormation carbonate west of the Nanambu Complex, where nosignificant uranium occurrences are known, was deposited underopen marine conditions (Needham, 1982); this and the strataboundnature of the deposits suggest that supratidal deposition was apossible prerequisite for uranium concentration in the lower CahillFormation.

The major deposits (geological plans and cross sections aregiven in Figures 8-14) are in close proximity to a sharply definedunconformity with overlying Middle Proterozoic Kombolgie Formationsandstone; an unconformity which approximates closely with thepresent day land surface. The Kombolgie Formation generallyforms a prominent escarpment in close proximity to the deposits;although the Jabiluka 2 deposit is concealed by up to 220 m ofKombolgie Formation sandstone. The deposits have a strongspatial association with granitoid/metasedimentary contacts. TheArchaean Nanambu Complex is known to exist 1.5 km south ofJabiluka; it forms the footwall to the Ranger orebodies; and atKoongarra, it crops out 6.5 km NNW of the deposit. At Nabarlek,the late Early Proterozoic Nabarlek Granite crops out 16 km to theeast of the deposit and has been intersected at a vertical depthof 470 m below the orebody.

Host rocks to the major deposits are predominantly quartzmuscovite chlorite schists (Figs 8-14) which are commonly charac-terised by segregation banding (in places accompanied bycrenulation) where any one of the three minerals may be dominantin individual bands. The sequence at Nabarlek differs in thatquartz tends to be minor or absent, except toward the southern endof the orebody and in metasediments below the Oenpelli Dolerite.In all deposits, quartz tends to be tabular, have wavy extinction,and to show horse-tail fractures passing continuously across adjac-ent grains and containing minute inclusions. In strongly deformedschists, the quartz has developed brittle fracture and pressure

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Kombolgie Formation sandstoneoverlying Cahill Formation

Topographic contours (metres above sea fevef I

Fig 8. Solid geology of Jabiluka 1 and 2 uranium-golddeposits (adapted from revised unpublished databy courtesy of Pancontinental Mining Ltd)

SOUTH NORTHJABILUKA 1 Kombolgie Formation sandstone

Cahill Formation schists

SOUTH NORTHJABILUKA 2

UGSHWSMMSFWS

LMS1LSS1IMS 2LSS2

Upper Graphit» SarlasHinging Wall SanasMam Mm« SanasFootwall SanasLowar Mma Sanas 1Lowar Schist Sanas 1Lowar Mma Sana* 2Lowar Schist Sanas 2

Uranium ore

WEST EAST

Fig 9. Generalised long-section and cross-sections ofJabiluka 1 and 2 uranium-gold deposits (adaptedfrom revised, unpublished data by courtesy ofPancontinental Mining Ltd)

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Surface expressionof orebody

Fig 10.Generalised geological plan ofthe Ranger 1 orebodies andprospects (after Eupene & others1975; Hegge & others 1980)

*No 9 ANOMALY

solution features, which has resulted in partial recrystallisation.Usually muscovite and less commonly biotite and chlorite show twoperiods of growth. Early distorted flakes define the schistosity,ana later undeformed flakes are superposed on the foliation. Finesericitic mica is generally present, and may occur as an alterationproduct of minor feldspar. At Nabarlek, sericite rich patches setin a chlorite matrix give rise to minor spotted schists, andelongated grains of leucoxene +_ sphene, which are parallel to theschistosity, give some rocks a flecked appearance. Porphyroblastsof almandine garnet are locally abundant in all deposits. Thinquartz veins, which are both discordant ana concordant to theschistosity and appear to predate and postdate the regional meta-

162

-200m

Zone of surface oxidation

Chlonte rock

Footwall shear zone

Dolente

Pegmatite, pegmatoid

Hanging wall schist

Upper mine schist

Lower mine chert

Chlontic carbonate

Lenticle schist

Recrystallised carbonate

Footwall schistand gneiss

Cahill Formation

—A— Base of weathered zone

&%% Approximate ore outline

N B. Faults and most breccia zones not shown

Cross-section of the Ranger 1 No. 1 orebody(after Eupene & others, 1975; Hegge & others,1980)

163

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CARRENT ARIAN

Kombolgie Formation

immun Escarpment

v—r Koongarra reverse fault

— — Cross fault

Main primary ore zone

— — Outline of secondary ore zone

Fig 12. Plan of the Koongarra deposit (after Foy &:edersen, 1975; Hegge & others, 1980)

NW

CARRENTARIAN Kombolgie Formation

Sandstone conglomerate

EARLY PROTEROZOIC Cahill FormationQuartz ch/onte schist

50m

f^HJ Graphitic quartz chlome schist

Siliceous bands

"| fault breccia

Outline of primary and secondary ore zones

—n — Base of weathered zone

Fig 13. Cross-section of the Koongarra No. 1 orebodyalong line A-A in Figure 7 (after Foy &Pedersen, 1975; Hegge & others, 1980)

SW NE

Quartz-m/ca schist, siliceous chlorite schist

Ch/onte-mica schist, micaceous chlorite schist

Chlorite schist

Chlorite rock ± hematite ± senate

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—A— Base of weathered zone

•••••••• Pit outline

^^^ Ore zone

Fig 14. Cross-section at the southern end of theNabarlek orebody. Modified from minesection 9553; published by courtesy ofQueensland Mines Ltd

morphism, are common at Jabiluka, Ranger and Koongarra, and rarerat Nabarlek.

In the vicinity of all of the deposits, a pervasive chloritedominated alteration halo surrounds the mineralisation. AtJabiluka, this halo extends into the overlying Kombolgie Formationsandstone cover sequence (Gustafson and Curtis, 1983) and is knownto extend laterally more than 200 m from ore (Binns et al., 1980).The chlorite ranges from cryptocrystalline to flaky, and generallyoccurs as a partial or complete replacement of biotite andmuscovite. At Jabiluka, it completely pseudomorphs garnet andamphibole, and in tourmaline-bearing granitoid pegmatite dykes,alkali feldspar has also been extensively chloritised. At Ranger1, the chloritisation is more restricted and less intense; theHangingwall Sequence contains fresh garnet and K-feldspar inpelitic schist, and unaltered hornblende and plagioclase occur ininterbedded amphibolite. Minor K-feldspar, plagioclase, andhornblende persist in the Upper Mine Sequence. Chloritisation is

165

most intense in the Ranger ore zones, especially the Lower MineSequence, where feldspars in granitic pegmatites have altered tochlorite and dolente has been completely chloritised. AtKoongarra, garnet is either completely chloritised or surrounded byan outer layer of chlorite elongated parallel to the schistosity.The garnet ma^ contain helical trails of opaque inclusions. Foy& Pedersen (1975) have reported feldspar in several drillholes andhave also noted the presence of partially or completely chloritisedhornblende. Chloritisation is pervasive at Nabarlek in meta-sediments above and below the Oenpelli Dolente; it also affectsthe margins of the dolente and the underlying Nabarle* Granite,which has been intersected below the orebody at a aepth of 470 m.Chlorite forms pseudomorphs after biotite, actinolite, muscovite,and appears to have replaced garnet.

With the exception of Nabarlek, the uranium deposits are allassociated with massive bedded dolomite and/or magnesite. Thisassociation is characterised by the disappearance, or markedthinning of carbonate sequences within the ore zones, and a corres-ponding increase in the degree of disruption and brecciation in theorebodies. Chert replacement of the carbonates is a commonfeature (Lwers and Ferguson, 1980), and Eupene (1979) has post-ulated that this disruption in the ore zones has been caused by thesilicification of carbonate resulting in volume reduction andsubsequent collapse. Ferguson et al. (1980) have suggested thatsolution of massive carbonate led to the formation of collapsedolines, and that these acted as structural traps and sites for ore-formation. At Nabarlek, the absence of massive carbonate in thevicinity of the orebody indicates that collapse dolines are notrelevant to the structural setting of this deposit (Ewers et al.,1983) .

Accessory minerals found in the wall rocks of some or all ofthe deposits include tourmaline, apatite, sphene, leucoxene, zircon,epidote-group minerals, rutile, gypsum and opaques. Euhedralprisms of pleochroic green tourmaline, sphene, apatite and epidote-group minerals are common and tend to be concentrated in muscovite-rich and chlorite-rich zones. Rutile and gypsum are generallyrare, though sagenitic growths of rutile are common in flakychlorite in wall rocks at Nabarlek. The opaques include graphite,hematite and minor sulphide (mainly pyrite). Graphite occurs

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throughout the sequence at Jabiluka, whereas it is confined to thebase of the Upper Mine Schists at Ranger, and is restricted to aschist overlying the primary mineralisation at Koongarra. Graphitewas reported in handspecimen only from schist adjacent to theNabarlek orebody (Lees and Tammekand, 1979); however, its presencehas not been confirmed in a more recent study (Ewers et al., 1983).Hematite is a common accessory mineral, particualrly in wall rocksat Nabarlek, where it occurs as fine disseminations or elongategrains which parallel the schistosity.

The two most striking features in all of the deposits are thetendency for mineralisation to occur within zones of brecciation,and for it to be intimately associated with chlorite. Reasonsfor this brecciation and the association of ore with chlorite havebeen considered in a variety of ore genesis models and will bedealt with later in this chapter. However, it is worth notingthat, unlike the other deposits, the Nabarlek orebody occurs inwhat is interpreted as a shear zone (Anthony, 1975) and is grosslydiscordant with the enclosing metasediments. The breccia frag-ments are typically chlorite- and/or sericite-bearing meta-arenites,intensely chloritised material, and chert which is commonly areplacement of carbonate. At Jabiluka, Ranger and Koongarra,fragments may contain graphite or hematite. At Nabarlek, theoriginal composition of the fragments (and also the matrix) isdifficult to ascertain because of the intensity of chloritisationand a later sericitisation in the ore zone (Ewers et al., 1983);however, it is reasonable to assume that the fragments are from theenclosing host rocks. In all deposits, the fragments are usuallyangular and rotated. Textures and lithology generally indicatethat the brecciation developed after the regional metamorphism(ca. 1800 Ma) and that there was a high percentage of voids betweenthe fragments. The breccias are cemented by chlorite _+ quartz +_hematite + uraninite, and at Jabiluka, Ranger and Koongarra,graphite and minor sulphides can occur in the matrix. Graphitehas not been identified in the Nabarlek ore-zone rocks; however,recent isotopic work on minor carbonate associated with the miner-alisation suggests that organic material was originally present(Ewers et al., 1983). At Ranger, vugs and fractures whichremained open at the time the breccias were initially cemented,have subsequently been infilled with euhedral quartz and chlorite,and can also contain hematite, sulphides, and carbonates. There

167

is also evidence that later deformation in the Ranger deposit hascaused some breccias to be rebrecciated before being cemented asecond time with chlorite.

Massive chlorite +_ sericite + hematite rocks and intenselyaltered schists are common in the Nabarlek ore zone and highlight asignificant difference between ore zone rocks at Nabarlek and thosein the other deposits. At Nabarlek, hematite and sericite aremajor constituents and are spatially related to mineralisation eventhough their development is minor and seemingly unrelated to miner-alisation in the other deposits. Recent pétrographie work hasindicated that the ore zone at Nabarlek has undergone progressiveand pervasive sericitisation (probably about 920 Ma), resulting inthe widespread replacement and breakdown of chlorite, the formationof hematite, and the solution and redeposition of uraninite (Ewerset al., 1983). To date, these events have not been recognised inthe other deposits.

Multiple generations of chlorite have been observed in alldeposits; the grainsize, colour, pleochroism, and interferencecolours of these different generations are wide ranging, and theirrelationships to one another very complex. In the ore zones, thereappears to be no correlation between mineralisation and the compos-ition or texture of a particular variety of chlorite. Chloritecompositions in both the host rocks and ore zone overlap and arehighly variable with respect to Al, Fe, Mg, and to a lesser extentSi; although, in general, Mg-rich and Al-rich varieties predom-inate .

The primary ore mineral assemblage in each deposit is dominatedby uraninite, though minor coffinite and brannerite occur in theJabiluka and Nabarlek deposits. Eupene et al. (1975) have alsoreported minor brannerite at Ranger. Thucolite is known fromJabiluka (Binns et al., 1980) and has been reported from the Ranger1 No. 3 orebody (G.H. Taylor in Eupene, 1979). The closeproximity of all deposits to the present land surface and theabsence of a sufficient thickness of cover rocks in most instanceshas exposed the primary mineralisation to the effects of weathering.Uraninite has been oxidised and replaced by an array of secondaryuranium minerals, to form a wide low grade envelope, which isparticularly well developed over the Koongarra and Nabarlekdeposits.

168

The uraninite is invariably associated with chlorite,particularly where it fills interstices between breccia fragments.In many instances, subsequent sericitisation in the Nabarlek orezone has resulted in the wholesale replacement of chlorite and thepreservation of residual massive uraninite; however, theseresiduals commonly contain uraninite-chlorite intergrowths.

The uraninite exists in a variety of forms - disseminateduraninite occurs as cubes 10-100 urn across, but these may coalesceto form clusters, strings, and massive uraninite. Colloformtextures with associated concentric banding, and radial fracturesare common in massive varieties, and can give way to reticulatedand lacework patterns where silicate inclusions are abundant.Uraninite also forms thin veins coating foliation planes and thesemay become braided where the uraninite becomes interstitial toflaky micas. Fine stringers of uraninite sometimes fill fractureswhich are discordant to the foliation and appear to postdate theearliest mineralisation. Complex intergrowths in which chloriteexerts its platy habit against uraninite are also common. Raremassive spherules of uraninite up to 0.7 mm in diameter are foundat Jabiluka. At Ranger, the disseminated form is particularlycommon, and "atoll-type" textures occur where only the rims ofuraninite cubes take a polish; Ewers and Ferguson (1980) havereported the progressive replacement of these cubes from the coreoutwards by chlorite, thereby establishing that, at different timesuranium has been remobilised. In polished sections, at highmagnification and with the aid of oil immersion lenses, the alter-ation of uraninite to darker varieties is visible and is ascribedto the partial oxidation of U to U . This alteration occursalong grain boundaries, in fractures, and as isolated patches inmassive uraninite. The extent of the alteration is very pronoun-ced in residual uraninite found in sericitised ore-zone rocks atNabarlek.

Sulphide occurs in minor amounts in all deposits, though itshould be added that there is no correlation between the presenceof sulphur and the concentration of uranium (Ferguson and Winer1980; Binns et al., 1980). Galena is most abundant; itsrelationship to the uraninite and its Pb isotopic compositionindicate a radiogenic origin (Hills and Richards, 1976). Itusually occurs as disseminated euhedral grains (generally <5 ym

169

across), fracture fillings in uraninite, and as stringers andirregular grains in massive chlorite. Pyrite and chalcopyrite arepresent and can be replaced by galena. Traces of chalcocite andcovellite have been found in all deposits, and bornite, digenite,arsenopyrite, cobaltite, and marcasite have been identified atNabarlek. Hegge (1977) has reported the rare occurrence oftellurides (altaite, tellurbismuth, and melonite) at Jabiluka.Weathering at Nabarlek has led to the breakdown of copper sulphidesand the minor supergene enrichment of native copper and cuprite.Gold has been noted in all deposits except Nabarlek, and occurs ineconomic quantities in the Jabiluka 2 orebody.

Graphite is found in the ore zones at Jabiluka, Ranger, andKoongarra; however, there appears to be no correlation between thepresence of carbon and concentration or uranium, and there is noclear evidence to suggest that the removal of carbon (as saymethane and carbon dioxide) produced a widespread reducing environ-ment in which uranium deposition took place. Vein and vug carbon-ates occur in all deposits, but they are a rare feature atNabarlek.

While the field relationships and age dating have provided areasonably coherent picture of the sequence of events on a regionalscale (Page et al., 1980) , the relationship and timing of the mainmineralising event in each deposit and subsequent events in theore zones to the regional framework is less certain. It seems tobe generally accepted, on the basis of field and laboratory work,that mineralisation took place later than the 1800 Ma regionalmetamorphism, although the evidence indicates that remobilisationhas occurred in the ore zones, probably many times. Uraninite andgalena ages indicate two main periods of possible mineralisation ormobilisation, namely -1600 Ma and -900 Ma (Hills & Richards, 1976;J.H. Hills, R.W. Page, B.L. Gulson & G.H. Riley, pers. comm.).Rb-Sr ages on rocks from the alteration zones associated withuranium mineralisation at Nabarlek and Jabiluka give an averagemodel age of about 1600 Ma, additionally a younger event <920 Ma,is recorded at Nabarlek in both ore and the sericitised ore-zonerocks (Page et al., 1980). K-Ar muscovite ages from altered andunaltered Cahill Formation rocks at Jabiluka are consistent atabout 1800 Ma - an indication that the alteration associated withthis ore zone took place below the threshold retention temperatureat about 35Û°C (Page et al., 1980).

170

Stable isotopesStable isotope studies of bedded sulfides (dominantly pyrite)

in graphitic rocks from the Cahill Formation, adjacent to the majoruranium ore deposits (Jabiluka 1 & 2. Koongarra and Ranger 1),

34hâve 6 S values in the same range as mantle sulfur (Table 3).This indicates that mantle sulfur was the sulfur source for theformation of these sulfides and that Archaean-type conditions (i.e.anaerobic world conditions) prevailed during sedimentation (cf.Skyring and Donnelly, 1982). The sulfides formed from hydrothermalsolutions probably derived by volcanogenic processes.

34The ô S values of vein or vug sulfides from the uraniumdeposits mentioned above have, relative to the bedded sulphides, awide range from -6 to +7%» (Table 3). The presence of S-enriched346 S values, outside the standard deviation limits of mantle sulfur,

indicates sulfate was now present (Rye and Ohmoto, 1974) at the34time of uranium ore formation. The variable 6 S values are best

explained as metal sulfide precipitation in organic-rich zones fromH2S which was produced by anaerobic sulfate reducing bacteria.

Bedded carbonates (mainly dolomite) from samples adjacent tothe ore zones have 6 C values around 0%«, in accord with normal

-10 1 Qmarine carbonate <5 C values (Keith and Weber, 1964). Their 60values, however, range from +13 to +19%»and are significantly belowthe range reported for early Proterozoic marine carbonates (Veizerand Hoefs, 1976; +15 to + 25%J, indicating some groundwater re-crystallisation.

13 18The 6 C and ô O values of vein or vug carbonate from theore zones are variable ranging from -20 to 0%»and +7 to + 20%»,respectively (Table 3). The <5 C values indicate carbonateformation, at least in part, from organically derived CO9, while

18the 6 0 values show to a greater degree the effect of groundwaterrecrystallisation. Vein or vug carbonates show two reasonable

T O "1 Ocorrelation lines between 6 C and 6 O values for the two mostextensively sampled deposits (Jabiluka 1 and 2 and Koongarra).These correlations support the argument of in situ reactions inwhich oxidising groundwaters have recrystallised carbonate andoxidised organic matter. Under the anaerobic conditions withinthe breccia zones sulfate reducers were active. A greater ground-water reaction is indicated at Koongarra and this is supported bythe signif(Table 3).

171

the significantly altered ore zone organic matter ô C values

Table 3: Isotopic compositions of minerals associated with, andfrom strata adjacent to, major uranium deposits from theAlligator Rivers Area (Ayres and Eadington, 1975;Donnelly and Ferguson, 1980; Ewers et al., 1983)

Location and mineral form

1. Alligator Rivers AreaBedded sulfides in barren Cahill Fmadjacent to:Jabiluka 1 & 2)Koongarra )Ranger 1 )

Vein or vug sulfides in breccia ore zonesCahill FmJabiluka 1 & 2)Koongarra )Ranger 1 )

Bedded carbonates barren Cahill FmVein or vug carbonate in breccia orezones Cahill FmJabiluka 1 & 2)Koongarra )Ranger 1 )

Sedimentary organic matter in brecciaore zonesJabiluka 1 & 2Koongarra

Ore zone carbonate Nabarlek

2. South Alligator Rivers AreaBedded sulfide in barren Koolpin FmSulfides associated with uranium ore

Mantle Sulfur

Early Proterozoic marine carbonates

Sedimentary organic carbon

Isot6 S(CDT)*

+2 +_ 1

-6 to +14

-1.6-6 to +12

+2 +_ 2

opic Composi613C(PDB)*

av. 0

-20 to 0

-30 to -24-22 to -15

-20 to -15

0 +_ 4-25 _+ 5

tions %6180(SMOW)*

+13 to +19

+7 to +20

+25 to +29

+15 to +25

'•International reference standards:CDT - Canyon Diablo TroilitePDB - Peedee BelemiteSHOW - Standard Mean Ocean Water

172

Table 4: Isotopic compositions of minerals from (1) three basemetal deposits in the Rum Jungle area, and (2) barrenorganic-rich sedimentary rocks of the Pine CreekGeosyncline (Donnelly and Roberts, 1976)

Location and mineral form<S34S(CDT)

Isotopic Compositions %.0 I 3 C(PDB) (5180(SMOW)

1. Woodcutters Pb/Zn sulfide depositin Whites Fm

2. Browns Pb/Zn sulfide deposit inWhites Fm

3. Mount Bonnie Pb/Zn/Cu sulfidedeposit in the Mount Bonnie Fm

Massive carbonate

Coomalie Dolomite (av. of 2 samples)

Celia Dolomite (av. of 2 samples)

Disseminated and vein sulfides fromcarbonaceous-rich sedimentary rocksfrom various strata

Kapalga Fm

Koolpin Fm

Wildman Siltstone

Coomalie Dolomite

-5.4 to +9.4

+0.2 to +6.0av. +2.0

av. +12.4

av. + 2.6

av. +14.8

av. + 5.0

- 3.5graphite av.-15.7

+5.5 to -6.9graphite-19 to -30av.-11.5 cc.av.- 8.8 do.graphite -22.3

+0.9

+5.2

+10 to + 1 4 . 7

+9 .7 to +13.4

av.+ 1 1.6 ccav.+ 8.8 do

+ 15.8

+ 15.0

Donnelly and Ferguson (198Q) showed, using sulfur isotopegeothermometry, that bedded or ore zone coexisting sulfides recordthe same temperature (T ~270°C). A similar temperature wasobtained from one sulfide pair by Binns et al. (1980). It wassuggested in both studies that the temperature event was post-ore,and it was suggested that the event may have been the -1688 Ma(Page, et al., 1980) emplacement of the Oenpelli Dolerite.

In contrast to the stable isotope studies of the Jabiluka,Ranger and Koongarra deposits, which indicate low temperature orezone reactions, Ewers et al. (1983) have noted that isotopiccompositions of the Nabarlek ore zone carbonate (Table 3) indicatehigh temperature reactions. A low fluid/rock ratio produced180-enriched carbonates. Low fluid/rock ratios were also found in

18a 6 0 study of fluid reactions involving the Oenpelli Dolerite

173

at Nabarlek (Ypma and Fuzikawa, 1980). The Nabarlek 13C-depletedcarbonates (to -20%0) indicate incorporation of organically derivedCO.-, in their formation.

A wide variety of geochemical studies on deposits of the ARUFhave been undertaken; however, they usually relate to a particularorebody and do not permit comparisons between the deposits. Torthe Jabiluka orebodies, Ferguson and Winer (1980) noted that wherewholerock U values exceeded 40 ppm, U correlated with Cu, W, As,Nb, Mo, Pb, Sc and Co and where whole-rock U values were below 30ppm no correlations existed. Binns et al. (1980) have concludedthat altered schists in the broad alteration halo surrounding theJabiluka orebodies have undergone the following changes compared totheir unaltered amphibolite faciès equivalents: enrichment in Liand Mg, and to a lesser extent Scr V, Ga, and Nb; and depletion inNa, Ca, Sr and Ba, and to a lesser extent Cr, Mn, Fe, P, Zn and Pb.In the immediate vicinity of ore (<1 m), K and Rb were alsodepleted. They also concluded that uraninite at Jabiluka wassignificantly enriched in Ça, Se, Y, radiogenic Pb, rare earthelements (RLE), Cu, As, Mo, Ag, and Sn. G.A. Cowan (reported inFerguson et al., 1980) established that uraninite from Jabilukaalso concentrates Li, B, Bi, Sb, Ga, Co and Ti. The concentrationof REE (heavy REE in particular) has also been reported for ore-zone samples from Jabiluka, Ranger and Koongarra (McLennan andTaylor, 1980). Mercury enrichment has been observed in primaryores from Jabiluka, Nabarlek and Ranger; the highest concentrat-ions being in gold-rich samples from the Jabiluka 2 orebody (Ryall,1981; Ryall and Binns, 1980). Marked increases have been notedin the U/Th ratios in the ore zones of all deposits when comparedto U/Th ratios in intrusives and metasediments throughout the geo-syncline (Ferguson and Winer, 1980; Ewers et al., 1983); thishas been interpreted as indicating that U rather than Th wastransported under oxidising conditions in the hexavalent state tosites of deposition (Ferguson et al., 1980). Ewers and Ferguson

2+ 3 +(1980) reported that the Fe /Fe ratio of chlorite appeared todecrease with increasing uranium content in the whole-rock sample,and suggested that redox reactions involving Fe in the chloriteand uranium in solution (transported as uranyl complexes) couldlead to uranium deposition.

A study of the groundwater geochemistry near the Jabilukadeposit has indicated that uranium concentrations in these waters

174

are low and not useful as a prospecting tool (Deutscher et al.,1980). At Koongarra, the distribution of secondary uraniumminerals, which form a dispersion fan in the weathered zone abovethe primary mineralisation, has been interpreted in terms oflateral groundwater movement (Snelling, 1980). Radiometrie dis-equilibrium between uranium and its daughter isotopes has also beenshown to result from weathering and the circulation of groundwaters(Dickson and Snelling, 1980). Shirvington (1980) has suggested

234 238that the measurement of U/ U activity ratios in soil samplesderived from weathered schists associated with uranium mineral-isation may be a useful exploration tool.

Fluid inclusion studies have been confined to the Jabilukaand Nabarlek deposits (Ypma and Fuzikawa, 1980) and are restrictedto fluid inclusions occurring in quartz and carbonates, mainly inlate stage veins. Although uraninite can be associated withsuch veins and related to the fluids from which they formed, thesignificance of these fluids in terms of the earlier main miner-alising events must be considered to be suspect. It has alreadybeen noted that the mineralisation in all deposits is primarilyassociated with chlorite, a mineral not amenable to fluidinclusion studies. At Nabarlek, quartz is a minor constituentin the ore zone and is unrelated to the primary mineralisingevent; it is absent from most rocks, although minor amounts occurin breccia fragments, some schists, and in late veins. Ypmaand Fuzikawa (1980) have observed two types of fluid inclusionsin quartz and calcite associated with mineralisation in theNabarlek and Jabiluka deposits: type 1 inclusions contain salinebrine dominated by CaCl,,, MgCl~, and NaCl, and have homogenisationtemperatures between 100° and 160°C; type 2 inclusions arecharacterised by low salinities and CO,, in the vapour phase.

In summary, there are major features common to all depositsin the ARUF to which various authors (Eupene, 1979; Hegge et al.,1980; Ewers and Ferguson, 1980; Needham and Stuart-Smith, 1980)have drawn attention. These are:

1) a spatial relationship of the deposits to the Early/Middle Proterozoic unconformity which approximates tothe present day land surface,

175

2) the proximity of deposits to the Kombolgie Formationescarpment, and to the contact between Early Proterozoicmetasediments and granitoid complexes,

3) the stratabound nature of deposits within the EarlyProterozoic Cahill Formation,

4) with the exception of Nabarlek, the presence of massivebedded carbonates outside ore zones and their dis-appearance within the ore zones,

5) brecciation within the ore zones,6) an intimate association between primary uranium mineral-

isation (predominantly uraninite) and chlorite,7) an intense and pervasive chlorite dominated alteration

halo in host rocks surrounding the mineralisation.

RUM JUNGLE URANIUM FIELD (RJUF)Uranium was first discovered in northern Australia at Rum

Jungle in 1949, and mining and milling continued until 1970.

The Rum Jungle and Waterhouse Complexes are basement highsto the Early Proterozoic sediments (Fig. 15). Both containschist, gneiss, granite gneiss, and several varieties of granite.Rhodes (1965) divided the Rum Jungle Complex into six units, whichin order of decreasing relative age are schist and gneiss; granitegneiss; metadiorite; coarse granite; large feldspar granite;and leucocratic granite. Veins and dykes of pegmatite and amphi-bolite, and quartz-tourmaline veins are also present. Johnson(1974) recognised the same units in the Waterhouse Complex, andalso noted the presence of banded iron formation in both complexes.Richards et al. (1966) determined a Pb-U age measured on zirconsof 2550 Ma for the Rum Jungle Complex, and Page (1976) reported a2400 Ma Rb-Sr isochron for the Waterhouse Complex. Richards andRhodes (1967) obtained a Rb-Sr whole rock age of about 2400 Ma forthe leucocratic granite of the Rum Jungle Complex, which provedthat all units of the complex are Archaean. Rhodes (1965) compar-ed the Rum Jungle Complex to a mantled gneiss dome; Stephanssonand Johnson (1976) suggest that the Rum Jungle and WaterhouseComplexes were domed by intrusion of late Early Proterozoicgranite at depth.

176

The Namoona Group represents a basal rudite, arenite carbon-ate assemblage (the Beestons Formation and Celia Dolomite) whichmake up a conformable sequence encircling both granitic complexes,except north of the Rum Jungle Complex, and west of the WaterhouseComplex, where they pinch out.

Coarse pebbly sandstone and conglomerate of the CraterFormation represents the unconformable base of the MountPartridge Group, which oversteps onto the complexes in places.The basement complexes formed islands at least until CraterFormation time, as it commonly contains clasts of banded ironformation derived from the Rum Jungle Complex. The CoomalieDolomite above the Crater Formation is similar lithologically tothe Celia Dolomite; both contain common clasts after gypsum andhalite indicating a supratidal environment of deposition, and inplaces extensive algal mats of Conophyton sp. are developed in theCelia Dolomite. Both carbonate units are pervasively altered tomagnesite, and a later alteration event is evidenced by abundantdevelopment of talc.

Outside the Rum Jungle area most of the Mount Partridge Groupis represented by interbanded grey siltstone and black carbonac-eous siltstone of the Wildman Siltstone, but at Rum Jungle this islargely represented by shallow-water faciès variants; the CoomalieDolomite is overlain by calcareous carbonaceous shale of theWhites Formation, and it is at or near this contact that alluranium and base-metal mineralisation is found: the Acacia GapQuartzite Member of the Wildman Siltstone represents a higher-energy phase in the shallow marine to intertidal conditions whichpersisted in the area almost to the end of Mount Partridge Grouptime.

The South Alligator Group rocks thin in the Rum Jungle areaand are not known further west, where Burrell Creek Formationgreywacke and siltstone form an extensive monotonous terrain.Except for some uranium prospects of no economic importance(mainly in the Koolpin Formation east of the Waterhouse Complex)these units are essentially unmineralised in the Rum Jungle area.

During the -1800 Ma orogeny the depositional dips around theArchaean basement highs were accentuated, and the Early Proter-ozoic sequence folded along axes trending mostly north to north-

177

east. Dips are commonly about 60°, and the sediments regionallymetamorphosed to low grade. The Giants Reef Fault, a dextralwrench fault, trends northeasterly and displaces the Rum JungleComplex and Early Proterozoic units by about five kilometres,west block north. The subsequently produced re-entrant of EarlyProterozoic metasediments into the area of Rum Jungle Complex isknown as 'The Embayment1, where most uranium and base metaldeposits of the area are located.

The Early Proterozoic sequence is overlain unconformably bythe Middle or Upper Proterozoic Depot Creek Sandstone Member.The uranium deposits at Rum Jungle, Adelaide River and GeorgeCreek are all near the present eastern limit of outcrop of thesandstone and are at, or immediately below, the unconformitysurface. Like the Kombolgie Formation, the Depot Creek SandstoneMember overlies the older rocks and is mostly quartz sandstonewith minor pebble conglomerate beds, but there are no interbeddedvolcanics.

The RJUF contains numerous prospects and significant uraniumdeposits at Dyson's, White's, Rum Jungle Creek South (RJCS), andMt Fitch (Table 2; Figs 15 & 16). Over 4300 tonnes U000 have-J Obeen mined and treated from the first three deposits, and Mt Fitchhas reserves of 1500 tonnes U000. Unlike deposits in the ARUF,j othese deposits contain considerably smaller tonnages of U_0 0, may3 obe associated with significant base-metal mineralisation(particularly Cu), and occur in a low-grade regional metamorphicterrain. Geological cross sections for the Dyson's, White's,and RJCS deposits are given in Figure 16 (and can be referred toin relation to the following text).

The deposits all show a strong stratigraphie and lithologicalcontrol; they occur within carbonaceous, pyritic, and chlorite-rich shale and schist of the Early Proterozoic Whites Formation,which in turn overlies or is in faulted contact with massivecarbonate belonging to the Coomalie Dolomite (Berkman, 1968;Fraser, 1975; Berkman and Fraser, 198Û). Unlike the otherdeposits, the RJCS orebody is not localised at a carbonate contact,although dolomite has been intersected in drillholes within 70 mof the orebody. The orebodies are between 30 m and 100 m belowthe present land surface; a surface which coincides closely with

178

I-O<oc-J Q.

ONOCCUJ

OITQ-

CAINOZOIC

TOLMER GROUP

Depot Creek Sandstone Member

Zamu Dolente i

FINNISS RIVER GROUP

Burrell Creek Formation

SOUTH ALLIGATOR GROUP

Mount Bonnie Formation

Gerowie Tuff

Koolptn Formation

MOUNT PARTRIDGE GROUP

Wildman Siltstone

Whites Formation

Coomalie Dolomite

Crater Formation

t3°00' -

M ANTON GROUPCelia Dolomite

Beestons Formation

r er1 si RUM JUNGLE andWATERHOUSE COMPLEXES

© Uranium deposit

• Uranium prospect 10 km

uroo'

Fig 15. Geology of the Rum Jungle Uranium Field

an unconformity separating the Early Proterozoic rocks from over-lying sandstone and hematite-quartz-breccia of probable LateProterozoic age. The orebodies are typically associated withzones of shearing or brecciation, and as a consequence of theirproximity to the surface have been weathered to give a variety ofsecondary uranium minerals which are predominantly phosphatic.

The uranium mineralisation in the Rum Jungle deposits variesin its mineralogy and association with other metals. At RJCS,the mineralisation is dominantly uraninite (base-metal sulphidesare virtually absent) occurring as fine sooty coatings on shearsand joints in pyritic and dolomitic shale ana schist; some veinsand stockworks are developed on a minor scale at the contact withunderlying graphitic shale (Berkman, 1968). Several near surfacepods of saleeite in the weathered zone were also mined. AtDyson's, saleeite is the dominant ore mineral and occurs through-out the orebody, particularly on cleavage planes, joints, andfractures in graphitic shales. Sklodowskite and autunite are

179

LATE PROTEROZOICDepot Creek Sandstone Member

Whites Formation

Coomalie Dolomite

Uranium mineralisation

Fault

Present ground surface

Fig 16. Diagrammatic sections of U deposits atDyson's (A), Rum Jungle Creek South (B)and White's (C). (Modified afterBerkman, 1968 and Fraser, 1980).

minor, and uraninite occurs in graphitic and strongly pyriticshales below the base of oxidation at 25 m. Base metals areagain absent. The Mt Burton deposit occurs in graphitic shalesinterbedded with grey pyritic quartzite and consists of torberniteat the surface grading to uraninite at depth. Malachite,chalcocite and native copper are present in the oxidised zone.Mineralisation in the White's orebody is the most complex ana hasbeen described by Spratt (1965) as consisting of two orebodies, acopper-uranium orebody overlain by a base-metal orebody in shearedgraphitic, sericitic, chloritic and pyritic shales. The copper-uranium orebody mainly contains uraninite, secondary uraniumminerals, chalcopyrite, and bornite. The base metal orebody hasbeen subdivided into three zones: a copper-cobalt zone (mainly

180

chalcopyrite, bornite, chalcocite, linnaeite and carollite), acobalt-nickel zone (mainly fine grained linnaeite, carollite,bravoite and gersdorffite), ana a cobalt-lead zone (mainly galenawith minor carollite and sphalerite). The Mt Fitch copper-uranium deposits are hosted by chloritic, sericitic and graphiticschists, with the main uranium orebody containing fine graineduraninite (Craven, 1983) and confined to a major breccia zonein magnesite. The copper mineralisation is mostly secondary(malachite, native copper, and chalcocite) and confined to resid-ual clays overlying the magnesite (Berkman and Fraser, 1980).

Studies of the petrology, geochemistry and geochronology ofthe deposits in the RJUF are very minor. Roberts (1960) deducedfrom mineragraphic studies of ore samples from White's deposit,that uraninite and pyrite mineralisation preceded a period ofshearing which was later followed by the introduction of Cu, Co,and Pb sulphides. Richards (1963) obtained a 207Pb/206Pb age of1015 Ma on a uraninite sample from White's, but concluded fromRoberts' work that the uraninite was invariably altered andtherefore probably older. No fluid inclusion studies have beenundertaken on any of the deposits.

Stable isotopesTo date only limited stable isotopic studies have been

carried out in the Rum Jungle area. The following discussionis a summary of a preliminary study (Donnelly and Roberts, 1976)and more recent studies by Donnelly and Crick (in prep.) andDonnelly et al. (in prep.).

Sulfides in Barren Organic Rich Sedimentary Rocks: Sulfidesfrom barren sedimentary rocks (pyrite and pyrrhotite) from variousstratigraphie units of the Rum Jungle area have a range of34positive ô S values (Table 4), which show two distinct distribut-ions. These data are interpreted as indicating two generationsof sulfide: one formed during sedimentation, the other wasintroduced from hydrothermal fluids after consolidation of the

34sediments. The 6 S values of the first generation sulfide aresuggested to be similar to that found for bedded sulfides in theCahill Formation, indicating formation from sulfur of mantle

34origin. The more positive ô S values of the introduced sulfidesindicates their formation from high temperature (>250°C) sulfate

181

(u) Uranium deposit

• Uranium prospect or smafl deposit(< WO tonnes U308)

(_)0

Ol-OL- ZLU LU

ouoc ce

ïïgoQ?

OOceLU

o0_

_la:<LU

ce

S5ce.LL§a.

>-K

KU

<S)DOz

zoC/i2

a:K

CAINOZOIC

EOCENE

CRETACEOUS

Kombolgie Formation

Oenpelh Dolente

EDITH RIVER GROUP

Plum Tree Creek Volcanic:

Kurrundie Sandstone

Malone Creek Granite

EL SHERANA GROUP

Big Sunday Formation

Pul Pul Rhyolite

Coronation Sandstone

Zamu Dolente

-r$1

in"•'-"'•"'.+ ++A A A

A A A

ÜI•

FINNISS RIVER GROUP___Burrell Creek Formation

SOUTH ALLIGATOR GROUP

Mount Bonnie and Kapalga Formations

Shovel Billabong Andésite

Gerowie Tuff

Koolpm Formation

MOUNT PARTRIDGE GROUPMundogie Sandstone

NAMOONAGROUPStag Creek Volcamcs

Masson Foimation

Fig 17. Geology of the South Alligator Valley Uranium Field

182

reduction. Although there is only limited isotopic data onl fimassive carbonates of the area the fact that their 60 values do

not exceed ~+15% , similar to the values found for the base metalvein carbonate (Table 4), is significant. Within the Alligator

l RRivers area bedded carbonates have 6 0 values which range up to~+20%, the average value for early Proterozoic marine carbonate.These data suggest that in the Rum Jungle Area significant amountsof carbonates have been recrystallised, or formed, from the hydro-thermal fluids; a conclusion in agreement with the extent ofintroduction of the second generation pyrite or pyrrhotite.

No stable isotopes have been carried out on minerals assoc-iated with uranium mineralisation on the RJUF. Three base metaldeposits have however been investigated and the stable isotope

34values presented in Table 4. The significantly positive ô Svalues found for ore sulfides indicates the presence of sulfateand it is suggested that these deposits are the result of thepervasive hydrothermal fluids moving through the ore zones as wellas the country rocks.

SOUTH ALLIGATOR VALLEY URANIUM FIELD (SAVUF)The upper reaches of the South Alligator River flow along a

northwest-trending belt of tightly folded Early Proterozoic géo-synclinal metasediments about 60 km long by 10 km wide. Thesestrata are deeply eroded and form strike ridges up to 200 m highin places, the tops of which are commonly capped by late EarlyProterozoic to Middle Proterozoic felsic volcanics and sandstone.This area is known as the South Alligator Valley. Basins of thefelsic volcanics and sandstone flank this northwest-trending belt,which in its narrowest part contains twelve of the thirteenuranium deposits of the South Alligator Valley Uranium Field.Sleisbeck, the thirteenth deposit, lies 32 km to the southeast(Fig. 17). The deposits were discovered between 1952 and 1954,and mine and milling operations ceased in 1965.

The Early Proterozoic géosynclinal sequence generally youngseastwards, but forms a complex syncline in the central part wheremuch of the western limb of the structure is concealed by youngerrocks. The Masson Formation is the oldest unit and containsmainly carbonaceous shale, siltstone, carbonate, calcarenite andsandstone. The pelitic rocks, particularly the carbonaceous

183

ones, are strongly iron-stained to brick red shale at the surface,and form low undulating ridges. The calcarenite (porous sandstoneat surface) and sandstone form continuous ridges which rarelyexceed 60 m in height. The Stag Creek Volcanics is a sequence ofpoorly exposed altered basalt breccia, flows, tuff, and dark greentuffaceous shale conformably above the Masson Formation, and isabout 1000 m thick. It is exposed along the southern flank of aprominent and continuous ridge commonly 100 m high formed by theunconformably overlying Mundogie Sandstone, which contains feld-spathic quartzite and conglomerate.

Rocks of the South Alligator Group dominate the géosynclinalsequence in the valley and rest unconformably on older rocks,although owing to the strong folding clear unconformable contactsare seen only in fold hinges north of the area. The basal unitis the Koolpin Formation, interbedded dolomite, siltstone andcarbonaceous shale. Its base is marked by a massive chert-bandedferruginous siltstone (carbonaceous shale with carbonate bands atdepth), or by massive dolomite with algal structures. Stronglyferruginised or silicified beds form continuous ridges up to 40 mhigh but other strata rarely crop out. Tuff and argillite ofthe Gerowie Tuff are interbeaded with the upper part of the KoolpinFormation, and thicken upwards as interbeds of Koolpin Formationrocks become progressively thinner. They are probably relatedgenetically to the Shovel Billabong Andésite, a flow of varioliticandésite and microdiorite 100-300 m thick near the base of theGerowie Tuff. The Mount Bonnie and Kapalga Formations, separatedby the South Alligator Fault, form the uppermost part of the groupand are similar assemblages of chert-banded ferruginous siltstoneand shale, with greywacke, but only the Mount Bonnie Formationcontains tuff. The fault may have been active during deposition,so that upward movement on the east side may have led to removalby erosion of any tuffaceous sediments deposited there duringSouth Alligator Group time.

The Fisher Creek Siltstone is the youngest unit in the géo-synclinal sequence, and contains a monotonous sequence of silt-stone, feldspathic sandstone, phyllite, greywacke and arkose.

The Zamu Dolente forms extensive sills mainly in the KoolpinFormation. The sills are folded with the géosynclinal sediments

184

and comprise a tholeiitic differentiated suite of mostly quartzdolerite.

During the 1800 Ma orogenic event the géosynclinal sequencewas metamorphosed to low-grade (most sedimentary textures arepreserved; pelitic rocks have a well developed slaty cleavage andcontain sericite, chlorite and rare epidote, and psammitic rocksare commonly fractured and veined by quartz) and deformed intooverturned tight to isoclinal folds with subhorizontal axes.

Following orogenesis, two suites of dominantly felsicvolcanics and related volcanoclastics accumulated in a graben-likestructure roughly coextensive with the South Alligator Valley.Each is valley-fill in character and strongly unconformable at thebase, and they are separated in time by intrusion of the MaloneCreek Granite. The older El Sherana Group contains basal coarsesandstone of the Coronation Sandstone, massive rhyolite,ignimbrite and minor tuff of the Pul Pul Rhyolite, and interbeddedgreywacke and siltstone of the Toilis Formation which are horn-felsed by the granite. These rocks are tightly folded incontrast to the generally gently folded rocks of the younger EdithRiver Group, which includes basal polymictic conglomerate andsandstone of the Kurrundie Sandstone overlain by an extensivesheet of ignimbrite with minor basalt of the Plum Tree CreekVolcanics. These rocks unconformably overlie granite of similarage to the Malone Creek Granite outside this area.

Earlier workers (Walpole et al., 1968) described volcanicvents in the Pul Pul Rhyolite, including one which hosts a uraniumdeposit, but more detailed examination indicates that exposure ofintrusive centres in the area are confined to small syenite plugsalong the South Alligator Fault near the Malone Creek Granite.

The relative age of the Edith River Group volcanism and ofintrusion of the Oenpelli Dolerite lopoliths at -1690 Ma is as yetnot known, but the dolerite intrusion probably represents the lastevent in the period of 'Transitional Igneous Activity' at the endof the Early Proterozoic. The dolerite and both volcanic suitesform a very rugged terrain over which sandstone of the MiddleProterozoic Kombolgie Formation was deposited with marked uncon-formity .

185

MIDDLE PROTEROZOICKATHERINE RIVER GROUP

"J Kombolgie Formation-. .~\

EARLY PROTEROZOICEL SHERANA GROUPPul Pul Rhyolite

Coronation Sandstone

SOUTH ALLIGATOR GROUPKoolpin Formation

Uranium mineralisation

Fault

Fig 18. South Alligator uranium deposits. A. Generalisedcross section of Palette (after Shepherd, 1967;and Crick et al., 1980). B. Composite crosssection of Rockhole (after Hare, 1961; Ayres andEadington, 1975; and Crick et al., 1980).C. Longitudinal projection of El Sherana and LlSherana West (after Foy, 1975 and Crick et al., 1980)

Faulting took place in the area in the episodes of géosyncl-inal sedimentation, igneous activity, and platform sedimentation,formed a graben as a depository for the volcanic and volcani-clastic rocks, and probably provided foci for extrusive pipes.Géosynclinal sequence rocks have been thrown against rocks of thevolcanic suites and platform cover. Thickened sequences ofMesozoic and Tertiary rocks indicate that faults in the area haveremained active until relatively recent geological time.

Deposits in the SAVUF field differ from those in the ARUF andRJUF by their association with Early Proterozoic sedimentary rocks

186

higher in the sequence, and in some deposits, an association withacid volcanic rocks; however they are similar in that they occurnear an Early Proterozoic/Middle Proterozoic unconformity, andlike the Rum Jungle deposits, occur in a low-grade metamorphicterrain. Generalised sections through some of the more signif-icant deposits in the SAVUF are illustrated in Figure 18.

An idealised orebody in the South Alligator Valley has thefollowing main features:

1) it occurs below the Early Proterozoic/MiddleProterozoic unconformity, usually in fractured chertyferruginous and, at times carbonaceous, siltstones ofthe Early Proterozoic Koolpin Formation and adjacentsandstone lenses of the Coronation Sandstone,juxtaposed by faulting,

2) the mineralisation does not occur more than 100 m belowthe unconformity, and does not extend more than a fewmetres above it (for example refer to cross-sections ofthe Palette and El Sherana deposits). About 70 percentof the production from the South Alligator Valleydeposits has come from the Koolpin Formation and theremainder from the Coronation Sandstone,

3} mineralisation is localised by major to minor faults,shears, and fractures,

4) uraninite is either massive or occurs as veins andsmall lenses, and is associated with minor galena,chalcopyrite, pyrite, and native gold. Rutherfordine,niccolite, gersdorffite, clausthalite and coloraditehave also been recorded,

5) in oxidised ore zones, phosphate-rich secondary uraniumminerals predominate and include gummite, metatorbernite,autunite, phosphuranylite, uranophane, and soddyite.

Two of the more significant deposits exhibit some divergencefrom this idealised orebody; the Coronation Hill deposit occursin a breccia of unknown origin containing fragments of altered ElSherana Group bnd blocks of carbonaceous shale; and the Saddle

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Ridge deposit consists almost entirely of secondary uraniumminerals and is associated with tuff and rhyolite of the ElSherana Group.

Hills and Richards (1972) and Cooper (1973) have reinterpretedU and Pb isotope measurements obtained by Greenhalgh and Jeffrey(1959) on uranium ore specimens from the SAVUF and have indicatedan age of 815 to 710 Ma, with a possible remobilisation or furtherphase of mineralisation at approximately 500 Ma. This is inagreement with the two generations of uranium mineralisationreported by Threadgold (1960) for ores from Rockhole, El Sherana,and Palette.

Apart from a geochemical and pétrographie study by Ayres andEadington (1975) of carbonaceous shales and acid volcanic rocksassociated with the uranium mineralisation, there are no furthersubstantive studies of this type reported for deposits in theSAVUF. Ayres and Eadington (1975) concluded that U correlateswith Cu, V, and Ga, but not with carbon, even though there is aclose association between uraninite ana carbonaceous shales.

Stable isotopes34Although only a limited 6 S study of barren and ore zone

sulfides was carried out there are features in common with similarstudies of the major uranium deposits. That is, sulfide in

34barren carbonaceous shale has a 6 S value compatible with deriv-ation from a mantle sulfur source, while introduced ore zone

34sulfides have a 5 S range which the authors interpret as indicat-ing sulfide formation as the result of the actions of sulfatereducing bacteria. The origin of these deposits, the authorssuggest, was the result of low temperature fluid transport ofoxidised uranium and reduction and precipitation when the fluidreached reduced zones.

CONTROLS OF ORE FORMATIONIn the development of any genetic models applying to the

uranium deposits of the Pine Creek Geosyncline (PCG) cognisanceof a number of features common to these deposits would have tobe taken into account. These features include:

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Stratabound nature of the ore zones.

Association of ore bodies with zones of brecciation orshearing.

The regional association of most of the mineralisationwith carbonate rocks and an antipathetic relationshipwith carbonate in the ore zones.

Shallow depth of the ore deposits.

The intense and pervasive chloritisation associatedwith the ore zones, wall rocks and cover rocks.

Absence of mineralisation in the Middle Proterozoic coverrocks despite the sometimes abundant presence ofchloritised zones.

The spatial relationship of the deposits to the Early/Middle Proterozoic unconformity which approximates tothe present-day land surface.

High U/Th ratios in mineralised zones.

The wide spread of apparent ages of mineralisation.

Proximity of the deposits to the overlying MiddleProterozoic cover rocks.

The proposed supergene genetic model for uranium mineral-isation which we develop here is not universally accepted andthe reader is referred to Binns et al., (1980) and Gustafson &Curtis (1983) for alternative models. The proposed model isbased primarily on the deposits in the ARUF which are the mostextensively studied. As there are a number of features commonto the ore zones within the three uranium fields of the PCG itwould seem likely that they share a common origin. Developmentof breccia zones took place after regional metamorphism. Thebreccia ore zones associated with carbonate-rich sequences wereprobably produced during peneplanation of the Early Proterozoicstrata and before deposition of Middle Proterozoic cover rockstook place. Solution of these carbonate rocks appears to haveproduced collapse with the subsequent development of an

Î89

insoluble residue of breccia fragments. At Nabarlek and inother deposits from the RJUF and SAVUF, brecciation appears tobe tectonically controlled by shear and fracture systems. Thesebreccia zones probably pre-dated the development of the MiddleProterozoic cover rocks. Besides the development of brecciafragments in the solution cavities, other insoluble residuesprobably included micas, chlorite, clays, quartz and graphite.Textures and lithologies clearly indicate that the breccias dev-eloped after the regional metamorphism (Ewers & Ferguson, 1980)and that there was a high percentage of voids between the frag-ments at times reaching 50 percent by volume. The depth ofsolution cavity development at Jabiluka, which is thought toextend to 600 m below the Middle Proterozoic cover, was probablyfacilitated by the presence in the area of a palaeotopographichigh of at least 250 m (G.A. McArthur, pers. comm.).

The low grade alteration within the ore zones suggests thatthe main mineralising event was low temperature and took placeafter the development of the brecciation and fracturing. Thepalaeoclimate during the peneplanation of the Early Proterozoicsurface with its abundant development of breccia zones wasprobably arid and oxidising as indicated by the red-bed character-istics of the Middle Proterozoic cover rocks (Gustafson & Curtis,1983). Lateritic soils were probably developed and clays fromthese soils could also have contributed to the insoluble materialpresent in the breccia zones. It is also possible that decayingmatter, now represented and identified as thucolite in somedeposits was washed into these traps. The suggested role ofbacterial sulphate reduction and the presence of biogenic carbon-ates within the breccia zones further supports the presence oforganic matter (Donnelly & Ferguson, 1980). The introducedmaterial could have been enriched in uranium, and have beencapable of further enrichment through absorption.

Most of the Archaean and Early Proterozoic rocks within thePCG are enriched in uranium relative to world abundances forequivalent rock types (Ferguson & Winer, 1980). The Archaeangranitoids within the PCG are thought to be representative of theprovenance area for the Early Proterozoic sediments. Thesegranitoids and the later Early Proterozoic high level granitoidsand their extrusives are enriched in uranium by 2 to 6 times

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world average abundances for granitoids (Ferguson et al., 1980).The uranium in the granitoids is present in the accessoryminerals uraninite, monazite, zircon, uranothorite, xenotine andapatite (McAndrew & Finlay, 1980). The accessory uraninite inthese granitoids ensured the presence of labile uranium in theprovenance rocks. The Early Proterozoic metasedimentary sequenceis also a potential source of uranium; however, uranium contentsin unmineralised areas of the PCG average no more than 6 ppm andrarely exceed 14 ppm for any given formation or rock type (Ewers,1982). The syngenetic concentration of uranium was aninsufficient concentrating mechanism to produce ore deposits asall of the uranium deposits post-date the regional metamorphicevent. There is no apparent concentration of uranium during themain regional metamorphic event (Ferguson & Winer, 1980). Theabundant late-tectonic I-type granitoids and extrusive equivalentswhich developed during the -1800 Ma regional metamorphic eventhave slightly higher uranium contents than the Archaeangranitoids.

The nature of the ore bodies suggest that there was adownward percolation of the ore-forming solutions. The absenceof mineralisation in the cover rocks suggests that the main min-eralisation event took place during peneplanation of the EarlyProterozoic rocks. Evidence in support of a downward percolat-ion of the mineralising solutions includes ponding effects withuranium enrichment on the upper side of steeply dipping,impermeable barriers in the ore zones, bottoming out of the orezones against impermeable mafic intrusives as at Jabiluka One,and Nabarlek and limited vertical extent of all the ore bodies.In that photosynthesis probably took place from at least 2500 Ma(Cloud, 1976) it is likely that younger meteoric waters wereoxidising to varying degree as suggested by the red bed character-istics of the Kombolgie sandstones. Under these situations itis likely that U was transported as the U complex. As Th is

4+diadochic with U (Naumov, 1959), the U/Th ratio is a criticalparameter in deciding the valence state in which U is beingtransported. That the uranium was probably transported in thehexavalent state is supported by the marked increase in U/Thratios within the ore bodies (>500) as compared to the granitoids(<1.1) granitic pegmatites (-12.5) and black shales (-0.3)(Ferguson & Winer, 1980; Ewers, 1982). The presence of

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carbonates and micas in the metasediments would buffer groundwaterpH to neutral or slightly alkaline. Under these conditionsphosphate, carbonate and hydroxy uranyl complexes would be themost important (Langmuir, 1978). With increasing temperature,they are minor species at all pH's at temperatures of 100 C(Langmuir, 1978). Since P is not an important element in anyof the rocks associated with the ore zones, whereas the REE'sshow enrichment in the ore zones and are known to be transportedby carbonate complexes (Kosterin, 1959) uranyl carbonatecomplexes are preferred. That C0„ was present in the ore zonesis supported by the presence of chert after carbonate - acondition resulting from increased P„„ (von Engelhandt, 1977).ou Q

Experimental studies indicate that organic material and claysreadily adsorb large quantities of uranium in the pH range 5 to8.5 (Rozhkova et al., 1959; Muto et al., 1965; Doi et al.,1975). Muto et al., (1965) found that uranium, presumed to be ina metastable state when adsorbed on clays, was released andformed in time a stable uranium mineral and that, in the process,the clays recovered their sorptive capacity. Muto et al., (1965)quote an enrichment factor of 10,000. The experimental work ofGiblin (1980) shows that the adsorption capacity of kaolin, knownto have the least effective sorption properties of all clayminerals, reached a maximum Kd of 35,000 at 25°C and pH 6.5. Inorder for desorption to take place the surface charge of theadsorbed uranium ion must be made incompatible with the negativesorbent surface of the clay or clay-sized chlorite. Thisincompatibility can be brought about by increasing the C0„ contentat.constant pH, raising the pH (Langmuir, 1978), or varying thetemperature (Giblin, 1980). The accumulation of organic materialwithin the breccia zones would create an anaerobic environment inwhich CH. and CO- might be present at a pH between 5 and 7. Itis postulated, that while the clays and organic material wereaccumulating within the breccia zones, the plumbing system ofthese structures was such that low-temperature CO^-rich ground-water, carrying uranium as uranyl carbonate complexes, passedthrough them. It is likely that these meteoric solutions werecapable of transporting the other elements that are character-istically enriched in the ore deposits, namely, W, As, Nb, Mo, Li,Sc, Co, Cu, Au, Mg, B and REE's ; although the ore zones couldhave partly inherited this geochemistry from the carbonaceous-rich

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residue produced during development of the solution breccias.The main source of U was probably derived from dissolution ofuraninite within the Archaean and Lower Proterozoic granitoidsand their extrusives with a further contribution from the meta-sedimentary rock pile. The initial Eh and pH of the solutionscould have evolved significantly before they reached the sites ofuranium deposition in breccia zones. On entering the brecciazones reduction in pH would enhance the adsorption of U ontoparticulate matter. The reduction of uranyl complexes may have

2 +been facilitated by the oxidation of Fe in clay, chlorite orother Fe-bearing minerals (Ewers & Ferguson, 1980).

The breccia zones developed during peneplanation, wouldcontinue to act as depositional sites for uranium after the onsetof Middle Proterozoic sedimentation, provided the followingconditions were met:

uranium continued to be leached from provenance rocksby groundwater;

access of these uranium-bearing waters into thestructures remained possible.

Presumably the structures would initially remain open to theintroduction of surficial waters and the oxidation-reauctionboundary would remain substantially in the same position withinthe structure. uranium deposition would only stop once suffic-ient sedimentation had taken place to result in the followingchanges :

Compaction of the regolith, particularly over the topo-graphic lows which must have overlain the breccias,thereby sealing the structure and preventing further accessof uranium-bearing waters.

Loading and dewatering of clays within the trap whichwould reduce permeability and develop chlorites fromclays (Rowsell and De Swart, 1976) .

Uranium could have been added to that already present in theopen structures within the Early Proterozoic rocks during theearlier part of the Middle Proterozoic sedimentation. Otherauthors have suggested that mineralisation could be formed during

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the early deposition of the cover rocks (Kalliokoski et al., 1978)and it would seem to be a logical extension of the overall modelpresented here.

It is suggested that once the deposits were effectivelysealed, all subsequent events recorded within the deposits,including the range of isotopic dates and high temperaturefeatures, represent reworking in situ. In the ARUF high heat-flow conditions were probably present in the area due to igneousactivity associated with the pre-Kombolgie Oenpelli Doleriteat 1,688 Ma and intercalated lava flows in the KombolgieFormation at 1,650 Ma (Page et al., 1980). It is probable thatthe heat associated with the latter igneous event resulted in the-270 C temperature recorded in the ore-zone from stable isotopedata; additionally, amorphous uranium oxides could havecrystallized to a disseminated euhedral form about 1,600 Ma.This euhedral uraninite is particularly well developed at Rangerand Ranger also records the -1,600 Ma event very clearly.

Russian studies of hydrothermal uranium deposits (Tugarinovand Naumov, 1969) indicate that the formation of uraninite occursabove 250 C whereas pitchblende, the massive microcrystallineand commonly colloform variety of uraninite, forms at temperaturesnot exceeding 220°C and probably less than 200°C. The -1,600 Madates recorded within the ore-zones would belong to this period.It is suggested that the Type II fluid inclusions found atNabarlek and Jabiluka (Ypma and Fuzikawa, 1980) may represent theoriginal mineralising groundwater.

Pétrographie work shows that solution of the disseminateduraninite and its redeposition as stringers and colloform uranin-ite has occurred in the ARUF deposits (Ewers and Ferguson, 1980;Binns et al., 1980) and it would seem that the conditions requiredfor this to take place may be very mild. The -900 Ma event canbe correlated with epeirogenesis and development of Adelaidean(Upper Proterozoic) sedimentation to the south of the Pine CreekGeosyncline. Similarly the -500 Ma dates associated with theSAVUF deposits could be attributed to epeirogenesis associatedwith the development of the early Palaeozoic Daly River Basin tothe south of the PCG. Hydrostatic imbalance at high levels inthe crust would be well-developed during periods of thermal activ-

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ity associated with epeirogenesis which could subsequentlyremobilise the uranium.

To preserve these ore-zones it was necessary that impermeablecover rocks develop rapidly. The dominantly coarse-elasticswith lesser intercalated volcanic units of the overlying MiddleProterozoic Formations appear to be excellent cover-rock protect-ion once the depth of sedimentation was sufficient to seal theuranium deposits.

DISCUSSIONThe events described for the period 2,200-1,700 Ma in the PCG

do not appear to be unique to that period and could equally applyto low-temperature vein-type deposits in younger rocks. Duringthe post 2,200 Ma period, the essential requirements of uraniumsource, transport, deposition and preservation that account forvein-type uranium deposits in Early Proterozoic rocks wouldappear to apply up to the present. The only apparent post-1,700Ma variable compared to the 2,200-1,700 Ma period is the hydro-sphere, which was progressively evolving an oxygenated atmosphere(Boichenko et al., 1975). The transitional oxidising atmospherein the period 2,200-1,700 Ma whilst sufficiently oxidising totransport the uranyl ion in the hydrosphere, met rapidly reducingconditions short distances into the lithosphère. This allowedprecipitation of UO2 and, at the same time reducing conditionsencountered in the lithosphère stabilised the uraninite. Withrapid development of cover rocks the uranium ore-bodies werepreserved.

It is suggested that in the post 1,700 Ma period the atmosph-ere was sufficiently oxidising so that groundwater conditionswere generally inadequate for the reduction and precipitation ofuranium on any large scale. The ultimate sink for uranium underthese circumstances would have been the oceans, where uranium isadded to sea-water, altered floor basalts, black shales and otherpelagic sediments (Fyfe, 1979). This broad dissemination ofuranium in the marine environment did not favour the formation ofextensive vein-type uranium deposits in the post 1,700 Ma period.

No uranium deposits have been discovered to date in theMiddle Proterozoic cover rocks in the PCG and workers in

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Australia have not had the opportunity to study this type ofmineralisation. The reader is referred to the companion papersin this volume concerning uranium mineralisation in the coverrocks in some uranium deposits in northern Canada.

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NEEDHAM, R.S., CRICK, I.H., and STUART-SMITH, P.G., 1980:Regional Geology of the Pine Creek Geosyncline. In:Ferguson, John, & Goleby, A.B. (Eds) Uranium in the PineCreek Geosyncline. IAEA, Vienna, Proc. Ser., 1-22.

NEEDHAM, R.S., and STUART-SMITH, P.G., this volume: Geology ofthe Alligator Rivers Uranium Field. In: Ferguson, John,& Goleby, A.B. (Eds) Uranium in the Pine Creek Geosyncline.IAEA, Vienna, Proc. Ser., 233-258.

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201

NEEDHAM, R.S., 1982: Nabarlek Region, Northern Territory.1:100 000 geological map commentary, 5573, 5572. Bur. Min.Res. Aust.

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PAGE, R.W., 1976: Geochronology of Archaean and Lower Proteroz-oic rocks in the Rum Jungle-Alligator Rivers area, NorthernTerritory, Australia. 25th Int. geol. Congr., Section1A,Abstracts.

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PLUMB, K.A., DERRICK, G.M., NEEDHAM, R.S., and SHAW, R.D., 1981:The Proterozoic of Northern Australia. In: Hunter, D.R.(Ed.) Precambrian of the Southern Hemisphere. ElsevierScientific Publ. Co., Amsterdam. 205-307.

PRETORIUS, D.A., 1975: The depositional environment of theWitwatersrand goldfields: a chronological review ofspeculations and observations. Minerals, Science andEngineer ing , _7, 18-47.

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RICHARDS, J.R., BERRY, H., and RHODES, J.M., 1966: Isotopic andlead-alpha ages of some Australian zircons. J. geol. Soc.Aust., 13, 69-96.

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ROBERTS, W. M. B., 1960: Mineralogy and genesis of White's Orebody,Rum Jungle uranium field, Australia. Ne we s J b . M i n e r .Geol. Palaont. , 94, 868-889.

ROBERTSON, D.S., 1974: Uranium in quartz-pebble conglomerates:Report of working groups. IAEA, Vienna, Proc. Ser.No. 571/PUB/374, 707-711.

ROWSELL, D.M., and DE SWART, A.M.J., 1976: Diagenesis in Capeand Karroo sediments, South Africa, and its bearing ontheir hydrocarbon potential. Trans, geol. Soc. S. Af r . ,?£, (1) , 81-145.

ROZHKOVA, E.V., RASUMNAYA, E.G., SEREBRYAKOVA, M.B., andSHCHERBAK, 1958: The role of sorption in the process ofuranium concentration in sedimentary rocks. Proc . of theInternational Conference on Peaceful Uses of Atomic Energy,U.N., Geneva, 1959, 420-431

RYALL, W.R., 1981: Mercury in some Australian uranium deposits.Econ. Geol. , 76 , No. 1, 157-160.

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SANGSTER, D.F., 1976: Sulfur and lead isotopes in stratabounddeposits. In: Wolf, K.E. (Ed.) Handbook of Strataboundand Stratiform Ore Deposits. Elsevier , Amsterdam, 219-266.

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? 14SHIRVINGTON, P.J., 1980: U/ ÖU activity ratios in clay as afunction of distance from primary ore. In: Ferguson,

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SNELLING, A.A., 1980: Uraninite and its alteration products,Koongarra Uranium deposit. In: Ferguson, John, & Goleby,A.B. (Eds) Uranium in the Pine Creek Geosyncline. IAEA,Vienna, Proc. Ser., 487-498.

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URANIUM MINERALIZATIONAT TUREE CREEK,WESTERN AUSTRALIA

G.N. LUSTIG*, G.R. EWERS**,C.R. WILLIAMS*1", J. FERGUSON**

* Pancontinental Mining Ltd,Sydney, NSW

** Bureau of Mineral Resources,Canberra City, ACT

Australia

Abstract

Uranium mineralisation which appears to be unconformity-related has been discovered at a contact between Middle Proter-ozoic sandstone and Early Proterozoic shale, greywacke, anddolomite in the Turee Creek area, W.A. The nature of this con-tact is controversial; it may represent a fault or anunconformity along which there has been minor movement. Althoughthe mineralisation is still being evaluated and the ore controlshave yet to be defined, a small uranium deposit consisting ofapproximately 643 000 tonnes grading 0.124% U.,ug has already beendelineated. The mineralisation consists of uraninite, carnotite,phosphuranylite, and metatorbernite, and is hosted by clay zones,hematitic and/or carbonaceous shale and.their brecciatedequivalents, and chert breccias which form a sequence of uncertainage in the contact zone. Relationships are complex due to rapidfaciès changes within and between drill sections, and a tendencyfor units to pinch out along strike and down dip.

INTRODUCTIONThe Turee Creek area is in the Pilbara region of Western

Australia, approximately 1200 km north-northeast of Perth (Fig.1). Exploration for uranium in this area began in approximately1973, with the initial target being possible "roll-front" depositswithin the Middle Proterozoic sandstones. Although sub-economicoccurrences of sandstone-hosted mineralisation were located, the

Present address: Hunter Resources Ltd, 10 Spring Street, Sydney NSW 2000, Australia.

207

Middl.

E«rlyProtarozoic

LEGEND

I | Bangemall Group

P~"| Bresnahan Group

frV| Manganesp Group

p| Wyloo Group

| j Hamvrsley ft Tür« Ck Grpa

jv;j Fort«8cu« Group

Archaan 1+ •! Granlta, matasadimants,b_d rnalavotcanic«

Pig. 1 Geological map of the Pilbara Region (modified after Goode, 1981)

emphasis switched to the Early Proterozoic sediments because ofsome similarities with the Alligator River region of the Northern

Territory.Initial prospecting involved ground follow-up of airborne

radiometric anomalies, with sub-economic uranium mineralisationbeing located within shales and greywackes of the Early ProterozoicMount HcGrath Formation in the Angelo River area (Fig. 2) westof the Turee Creek homestead markeü in Figure 1. The mostsignificant mineralisation was discovered in 1980-81 arid, althoughcurrently uneconomic, investigations are continuing. Theseuranium deposits are associated with the contact between theMiddle Proterozoic sandstones and the Early Proterozoic shales,greywackes, and dolomites. The BMR's involvement in a petrolog-ical, geochemical, and stable isotope study of the Angelo Rivermineralisation began in late 1982, and the results obtained thusfar are of a preliminary nature.

The uranium deposits at Angelo River represent a type ofunconformity-related mineralisation, distinctly different fromboth the Alligator River and Athabasca types of mineralisation.The mineralisation is not clearly located within the overlyingsandstone nor the underlying flysche faciès sediments, but withina contact unit of disputed sedimentary or tectonic origin. Therelationship between this unit and either sequence is uncertain.

208

\

LEGEND

MIDDLE PROTEROZOIC

EARLY PROTEROZOIC

Ed Dolarite

| Ebi I Kundarong Sandstona

Pbc | Cherrybooka Congtomarata

Ewo I Aghburlon Formation

E«d I Duck Creak Dolomit«

e.m| Mount McGrath Formation

SYMBOLS

Brasnahan Group

Wyloo Group

—————— Gaological contact

—•*••!?••• Fault with ralatlva movemant

—— Fault/unconformity!?]

——|—— Anticline

———|—— Syncllna

-J- Strika t dip of badding

^—————• Badding trand Unas

Pig. 2 Regional Geology, Angelo River Area

REGIONAL GEOLOGICAL SETTINGThe Turee Creek area is located at the southern margin of the

Hamersley Basin (Fig. 1). The area is underlain by EarlyProterozoic sediments and minor volcanics of the Wyloo Group,which are unconformably overlain by arenites, felclspathicarenites, and conglomerates of the Middle Proterozoic BresnahanGroup (Daniels, 1968) .

The Wyloo Group was deposited in an elongated trough alongthe south-western margin of the Hamersley Basin (Fig. 1) and isseparated from the rest of the basin by an active zone of basementhighs referred to as the Paraburdoo Hinge Zone (Goode, 1981).Folding is thought to have controlled the configuration of thebasin, and was active both prior to and during deposition.

Wyloo Group sedimentation commenced after a brief period ofsubaerial exposure, following the dominantly chemical sediment-ation of the Hamersley Group. The lowermost clastic units arethought to be terrestrial in origin, with later transgressive

209

marine deposition. Shallow marine conditions existed duringcarbonate deposition. The Early Proterozoic sedimentary sequencewas subsequently folded and metamorphosed to greenschist faciesduring the Ophthalmian Orogeny (1700-1800 Ma).

The Middle Proterozoic Bresnahan Group, possibly represents amolasse sequence, deposited in an adjoining basin developed as aresult of the Ophthalmian Orogeny.

GEOLOGICAL SETTING OF THE DEPOSITSStratigraphy and lithologies

The oldest recognised unit of the Wyloo Group in the AngeloRiver area has been correlated with the Mt McGrath Formation(Table 1). It consists of a succession of greywacke and shalewith minor dolomite, which represents a turbidite sequence, over-lain by a sequence of sandstone, dolomite and dolomitic shale,which passes into a thicker sequence of shales with minor dolomite.Near the top of the sequence is a volcanic sedimentary breccia.The overlying Duck Creek Dolomite occurs at several locationswithin the area, and consists of dolomite, chert breccia and minorcarbonaceous shale. The dolomites are frequently silicified atsurface, and ironstones have been developed on many outcrops.The Ashburton Formation forms the uppermost unit of the Wyloo Groupand consists of interbedded shale, mudstone, siltstone and grey-wacke. The greywackes are micaceous, a feature which appears tobe particularly diagnostic of this formation.

The clastic sediments in the Wyloo Group are characterised byfine grained assemblages of quartz, chlorite, K-mica, possiblyclay minerals, and minor opaques. Variations in the relativeabundance of any of these minerals can give rise to laminations.The opaques may contain carbonaceous material but usually consistof hematite with some limonite staining along fractures. Veryfine grained pyrite and chalcopyrite are rare, as are accessoryminerals such as tourmaline and zircon. Brecciation of WylooGroup lithologies in the Angelo River area is common.

The Bresnahan Group sediments, which unconformably overliethe Wyloo Group, consist of a basal conglomerate belonging tothe Cherrybooka Conglomerate passing upwards into a unit known asthe Kunderong Sandstone. The Cherrybooka Conglomerate occurs atthe western end of the Angelo River area "and contains cobbles

210

Table 1 - Stratigraphie units of the Turee Creak Area.

AGE

CAINOZOIC

MIDDLEPROTERO -

ZOICCarpentarian]

EARLYPROTERO-

ZOIC

QUATERNARY

TERTIARY

STRATIGRAPHICUNIT

BR

ES

NA

HA

NG

RO

UP

WY

LOO

G

RO

UP

KUNDERONGSANDSTONE

CHERRY -BOOKA

CONGLOM.ASHBURTONFORMATION

DUCK CRK.DOLOMITE

MT.McGRATHFORMATION

LITHOLOGY

Colluvium, alluvium—————————— -UNCONFORMITY ——————— —Colluvium, calcrete, ironstone, silcrete, sandstone.~ ——————— -~ UNCONFORMITY ———————— "

Quartz arenite, sub-arkose, arkose

Conglomerate——————————— UNCONFORMITY —————————

Greywacke, siltstone, shale

Dolomite, dolomitic shale, chert

Greywacke, siltstone, shale, sandstone,carbonaceous shale, dolomite.

and coarse pebbles of quartz, quartzite, hematitic chert,jaspilite and banded iron formation occurring within a coarsesand matrix. The Kunderong Sandstone is the dominant MiddleProterozoic unit within the Turee Creek area. It consists ofa sequence of medium to coarse grained, and sometimes pebbly,arenite, feldspathic arenite, and arkose. Coarse cross-beddingis common. The feldspar is mainly microcline and is either palepink, due to fine grained hematite, or buff white where sericit-ised. The matrix usually contains fine K-mica, chlorite andminor opaques, and detrital tourmaline is common. Hematitic,chloritic, and kaolinitic alteration are widespread suggestinga complex redox history and resulting in some rocks developing amottled appearance.

At the Angelo B-zone (Figs 2, 4), the contact between theKunderong Sandstone and Mt McGrath Formation is marked by a thicksilicified breccia which has been alternately described as asilicified fault breccia, a Middle Proterozoic silcrete, orsilicified Kunderong Sandstone. The interpretation of this unitis controversial and is largely dependent on whether theBresnahan-Wyloo boundary is viewed as a faulted contact or anunconformity. The silicified zone is underlain by a clay zone.

At the base of the Kunderong Sandstone in the Angelo A-zone(Figs 2, 3) a sequence of shale/breccia units consisting ofhematitic and/or carbonaceous shale, their brecciated equivalents,and mixed breccias containing chert, shale, silicified dolomite(?) and sandstone fragments in a silty matrix are developed.

211

NWLEGEND!•:'.'•] Kunderong Sandsion«

BH Clly

|J2| Carbonacaous shala braccla

l~3 Hamatltlc chart braccla &carbonacaous shala braccia

Mt. F H Dolomit«IcGrath __

Fm. |_Ü3 Graywacka

Uranium minaralisation

Fig. 3 Generalised cross-section of Angelo River A-Zone

NW SE

KUNDERONGSANDSTONE

L E G E N D|:'.-!!l Sandstona

P'tj Bracclatad Bandston*

£[j Slllclflad braccla

fcj::] Clay Icarbonacaous)

EB1 Clay

Mt. McGrathFormation

p^j Dolomltic shala

p=rj Dolomlta

^ Graywacke/slltstonerrnYM Uranium mlnarallzatlon

Pig. 4 Generalised cross-section of Angelo River B-Zone

Relationships are complex with rapid faciès changes within andbetween drill sections, and a tendency for units to pinch outalong strike and down dip. These units have been considered tobe part of the basal Bresnahan Group, or alternatively as WylooGroup sediments within a fault zone. Some of the brecciationappears to be tectonic, but other breccia units are clearly ofsedimentary origin.

212

Structural GeologyThe dominant structure within the Angelo River area is the

contact between the Middle Proterozoic Bresnahan Group, and theEarly Proterozoic Wyloo Group (Fig. 2). Initial mapping in theTuree Creek area (Daniels, 1968} has shown this contact as a fault.Subsequent work by Pancontinental geologists has led to variedopinions. Some have considered the contact to be an unconformity,with minor movement along the unconformity surface. Curtis(1982) concurred with this interpretation, and felt that thecarbonaceous unit associated with mineralisation at the contactrepresents restricted marine embaywents existing at the time ofinitial alluvial sedimentation in the Bresnahan Basin. Hironoand Suzuki (1983) have interpreted silicifiea sandstone brecciasthat occur at several locations along the contact as evidence ofpost-Kunderong faulting.

The contact is a focus of intense shearing and brecciationwithin the Wyloo Group sediments, and angular relationshipsexist between the unconformity/fault surface at several locations.The regional strike of the Kunderong Sandstone is approximately atright angles to the Angelo River contact, but the strike commonlyswings to a sub-parallel orientation within 100 to 300 m of thecontact (Fig. 2).

A prominent east-west oriented fault, locally known as"Frank's Fault" intersects the main Early Proterozoic-MiddleProterozoic contact west of the mineralised "A-Zone". The faultdips at approximately 70 degrees to the south ana has downthrownthe southern block relative to the northern block.

The Early Proterozoic sequence has been folded into a broad,northwesterly plunging anticline, that is cored by the Mt McGrathFormation. Folding in the Bresnahan Group occurs near thecontact which has resulted in synclinal anticlinal structuresparallel to the contact. A large northeasterly plunging synclinewithin the Kunderong Sandstone is present at the eastern end of .the Angelo River area.

Uranium MineralisationAlthough there is mineralisation within the Wyloo Group

sedimentary rocks away from the contact, most of the known uraniummineralisation is within the contact zone. As stated previouslythe nature of this contact is somewhat controversial, and there-

213

fore some uncertainties about the stratigraphie position of themineralisation exist. Along the contact there are two mainmineralised zones, the Angelo A-Zone ana the Angela B-Zone(Fig. 2).

The Angelo A-Zone (Fig. 3) is a small uranium deposit con-sisting of approximately 643 000 tonnes grading 0.124% U,0,,.The deposit is approximately 400 m long, with a maximum thicknessof approximately 30 m. Uranium mineralisation is hosted byhematitic and/or carbonaceous shale, their brecciated equivalents,and chert breccias that form a sequence of uncertain age withinthe contact zone. Uranium mineralisation is irregular indistribution and commonly neither the mineralisation nor the hostrocks can be correlated from hole to hole.

The Angelo B-zone was discovered in 1980, and due to severedrilling problems has been incompletely delineated. Mineralis-ation has a maximum width of 8.5 m with a grade of 0.047% U,00J O

and is associated with clay (which is in part carbonaceous) andbrecciated Kunderong Sandstone. The lower horizon has a maximumintersection of 10.5 m averaging 0.438% U^Oo and mineralisation isassociated with cross-cutting iron oxide veins within a clay zonewhich underlies a silicified breccia of disputed origin. U-Pbisotope data indicate that the age of U mineralisation fromthis lower zone is approximately 1015 +_ 30 Ma (Pigeon, 1981).

Uraninite, carnotite, phosphuranylite and metatorbernitehave been identified from the Angelo A-zone and B-zone. Auto-radiographs of thin sections prepared from core containing>100 ppm U indicate the following features. In brecciated WylooGroup sediments, U occurs finely disseminated throughout shalefragments and matrix material. The breccia fragments are notalways mineralised, but the fact that some are enriched is clearevidence to suggest a syngenetic concentration in the EarlyProterozoic sequence. In mineralised zones, U is usually dist-ributed as secondary minerals either in late fractures or asisolated grains. Uranium also occurs in fractures anddisseminated form associated with opaques. In many instances,these opaques are secondary iron oxides which have eitherscavenged U or have associated secondary U minerals, though somemay represent as yet unconfirmed uraninite concentrations.Although sulphides are not generally related to the U distri-bution, the concentration of U around the margins of some

214

Table 2 - Correlations on trace element data at99 percent confidence limits for samplescontaining >50 ppm U (*denotes samplepopulation too small, <10 samples).The most significant correlations (i.e.where I r I >0.70) are underlined.

ELEMENT CORRELATES WITH

Ba SrLi F Zn Rb Zr U GaRb B Zr_ Li Ga F U Se Zn Nb ThSr BaPb As U Ce Nd LaTh Nb Zr Ga B RbU As Rb Co Pb LiZr Mb B F Th Rb Ga LiNb Zr Th F Ga B Rb VYLa Ce Nd F PbCe La Nd F Pb BNd Ce La F PbSc B Ga Rb Cu NiV Ga B NbCo Ni As UNi Co SeCu SeZn Li RbSnMo *Ga B _Z_r Nb Se Th Rb F V LiAs U Co PbSF :Z_r LJ. Nb Ce Nd La Rb GaB Se Ga Zr Rb Nb Th V Ce

sulphide grains indicates that localised reducing conditionsmay have exerted some influence.

At the time of writing, only trace element data are avail-able and the interpretation of this data is at an early stage.The results of a correlation matrix calculated for those sampleswhere U concentrations are in excess of 50 ppm are summarised inTable 2. Correlations at the 99 percent confidence limit (for

215

48 samples, r -0.37) have been ranked in descending order accord-ing to the absolute value of the correlation coefficient. Themost significant correlations (i.e. where I r I >0.70) are under-lined. Uranium correlates, though not strongly, with elementshaving different geochemical affinities (i.e. some arechalcophile, others are lithophile). The absence of strongcorrelations involving U, particularly with respect to Pb,probably reflect the mobility of U in a deeply weathered envir-onment as indicated by the widespread development of secondaryU minerals and both a primary and radiogenic source for the Pb.Surprisingly, no element correlates with S. Of these 48samples containing >50 ppm U, 39 samples are either brecciasor have been brecciated, and they encompass the full range oflithologies represented at Turee Creek, though shale brecciaspredominate. Twenty-five samples are either known orsuspected to be carbonaceous and 24 are hematitic.

CONTROLS ON ORE-FORMATIONFor a variety of reasons, an ore genesis model for the

mineralisation at Turee Creek cannot be advanced at this stage.The true nature of the Bresnahan-Wyloo contact (i.e. whetherit is a fault or unconformity) has not been resolved, and themineralisation is associated with lithologies which are complexand are of uncertain age. Petrological, geochemical, andstable isotope studies are also at an early stage.

REFERENCESCURTIS, L.W., 1982: Initial findings of a study of drill core

samples from the Angelo Creek uranium prospect, Turee CreekArea, Western Australia, unpublished consulting report forPancontinental Mining Ltd.

DANIELS, J.L., 1968: Explanatory notes on the Turee Creek1:250 000 Geological Sheet, West. Aust. Geol. Surv.

__________ 1975: Bresnahan and Mount Minnie Basins, West.Aust. Geol. Surv. Mem, 2, p!43-147.

GOODE, A.D.T., 1981: Proterozoic geology of Western Australia inD.R. Hunter (Ed.) Precambrian of the southern hemisphere:Ams ter d am, Els eyi er, pl05-203.

216

HIRONO, S. and SUZUKI, S., 1983: The 1982 report of mineralogicalstudies on rock samples from Turee Creek Western Australia,unpublished PNC Exploration (Australia) Pty Ltd report.

PIGEON, R.T., 1981: Report on lead isotopic composition of rockpowders submitted by Pancontinental Mining Ltd, unpublishedconsulting report for Pancontinental Mining Ltd.

217

UNCONFORMITY-RELATED URANIUM DEPOSITSIN THE ATHABASCA BASIN REGION,SASKATCHEWAN

V. RUZICKAGeological Survey of Canada,Ottawa, Ontario,Canada

Abstract

Uranium deposits related to the sub-Athabasca unconformitycontain large tonnages of high grade uranium resources. Theirdistribution is structurally and lithologically controlled by theunconformity, steep faults, pelitic rocks in the crystalline basementand clastic sedimentary rocks in the cover. The deposits containmonominerallic or polymetallic mineralizations, which are surroundedby alteration haloes. The deposits formed 1050 to 1300 Ma ago underhydrothermal conditions.

INTRODUCTION

Discoveries of uranium deposits related to the sub-Athabascaunconformity in the late 1960s triggered intense exploration activityand extensive geological studies not only in Saskatchewan and Canadabut also around the world.

Economic importance of this deposit type attracted in the late1970s more than 60 companies which operated more than 150 explorationprojects with a total annual budget exceeding 60 million Canadiandollars.

As of the end of 1982 uranium resources of the Athabasca regionconstituted almost a half of Canada's uranium endowment in theReasonably Assured and Estimated Additional low price resourcecategories.

GEOLOGY OF THE ATHABASCA REGION

The Athabasca region is a part of the Churchill GeologicalProvince of the Canadian Shield. It is built up of Archean andAphebian crystalline basement and younger, Helikian, supracrustalrocks.

219

The crystalline basementLewry and Sibbald (1979) subdivided the crystalline basement

complex into lithostructural domains and these domains were groupedin major crustal units designated from northwest to southeast asWestern Craton, Crée Lake Mobile Zone and Rottenstone Complex(Fig. 1).

The Western Craton was interpreted as a relatively stableforeland bordering in the northwest the Hudsonian erogenic belt.This craton is built up of mainly Archean katazonally metamorphosedrocks with some remnants of Aphebian supracrustal rocks. It isbounded north and northwest of the Athabasca Basin region by theMacdonald Fault and the Thelon Front and in the southeast by theVirgin River Zone. During the Hudsonian Orogeny the craton wasaffected by intense brittle to ductile deformation and by retrogrademetamorphism.

The Crée Lake Mobile Zone, which is part of the Hudsonianorogenic belt, comprises a core of Archean granitic gneisses andAphebian shelf to miogeosynclinal metasedimentary rocks includinggraphitic and pyritic metapelites. It consists of Wollaston,Mudjatic and Virgin River domains.

The Wollaston domain, which is the southeastern marginal part ofthe Crée Lake Mobile Zone, contains elongated Archean granitic domesflanked by Aphebian metasedimentary rocks including quartzconglomerates, quartzites, aluminous, graphitic, pyritic and non-graphitic pelites and semipelites, calc-silicate rocks, banded ironformation, volcanic arkoses and greywackes. The lowermost horizon inthe metasedimentary sequence, the Wollaston Group, are graphiticmetapelites. They are fine- to medium-grained rocks consistingchiefly of quartz, feldspars, biotite and graphite (Sibbald, 1979).Garnet, tourmaline and pyrite are present in lesser amounts. Thishorizon underlies for example Collins Bay 'A1 and 'B1 uraniumdeposits. The Wollaston Group contains several such pelitichorizons, which usually contain graphite. These horizons are inter-calated in the vicinity of several uranium deposits commonlywith calc-silicate rocks containing locally scapolite and albite.These scapolite/albite rocks in the Wollaston domain are interpretedby some geologists (e.g. Ray, 1978; Chandler, 1978) as having beenformed by evaporitic processes.

The most common rocks of the Mudjatic domain are Archeangranitoid gneisses locally containing pegmatitic intercalations.

220

10460

U DEPOSITS

1 Eagle Point

2 Collms Bay A

3 Collms Bay B

4 Rabbit Lake

5 Midwest Lake

6 Cigar Lake

7 Key Lake

8 "D"

9 Peter

10 Ace-Fay-Verna11 Maurice Bay12 Fond-du-Lac

HELIKIAN

E ^ Athabasca Group

APHEBIAN-HELIKIAN

I____I Martin FormationAPHËBIAN

|p r :| Wollaston Lake-Virgin River Belt

ARCHEANI; .'"T;!) LATE Tazin Belt, LaRonge Bait, Granite

| I MIDDLE Fontaine Belt, Rottenstone Domain

f "] EARLY Black Lake Wedge , Clearwater Block

Major Shear Zonekm

Figure 1. Major crustal units and lithostructural domains of the Athabascaregion (modified from Lewry and Sibbald, 1979), Circled numbers:

(1) Rabbit Lake ~ Collins Bay area(2) McClean Lake - Midwest Lake -

Waterbury Lake area(3) Key Lake area

(4) Car swell Structure(5) Maurice Bay area(6) Fond-du-Lac area

221

The Virgin River domain is a mobile belt of Archean granitoidcores and Aphebian supracrustal rocks; it is bounded in the northwestby the Virgin River — Black Lake Shear Zone. It represents a majorHudsonian mobile front, analogous to the Grenville Front at thesoutheast margin of the Superior Structural Province (Lewry andSibbald, 1979) .

The Rottenstone Complex, which borders the Crée Lake Mobile Zoneto the southeast, comprises a broad belt of plutonic rocks of mainlyHudsonian age (Lewry and Sibbald, 1979) and older intrusive rocks.The Wathaman Batholith, which is a major intrusive body in thiscomplex, originated during a tectonic event, which took place about1880 Ma (Bell and Macdonald, 1982). The Peter Lake domain, separatedfrom the Wollaston domain by the Needle Falls Shear Zone, comprises avariety of granitic to basic plutonic rocks; the basic rocksrepresent the oldest intrusive suite in the complex (2177 ± 110 Ma;Ray and Wanless, 1980).

Geochronology of the crystalline basementBell and Macdonald (1982) summarized and interpreted results of

geochronological studies on the Precambrian in Saskatchewan andconcluded, that: (a) the major plutonic events took place in fourperiods: at about 2510 Ma (e.g. intrusion of 'Wollaston Inliers1);2,180 Ma (e.g. formation of Donaldson Lake gneiss); 1,880 Ma(e.g. intrusion of Wathaman batholith); and 1740 Ma (the "main"Hudsonian event); (b) the felsic hypabyssal rocks were emplaced atabout 1580 Ma; (c) the three main metamorphic episodes culminated at1875 ± 20 Ma; 1762 Ma; and especially 1680 - 1655 Ma; the latterepisode represents an important and distinct anatectic event; (d) thesupracrustal rocks of the Wollaston Group were deposited1875 - 2500 Ma ago; and (e) the intrusion of lamprophyre dykes tookplace at about 1740 Ma ago.

The Helikian supracrustal rocksPrior to deposition of the Helikian sedimentary rocks in the

Athabasca Basin the crystalline basement complex underwent chemicalweathering in a warm, moist climate (Macdonald, 1980) . Thisweathering produced a regolith similar to modern lateritic soils upto several tens of metres thick (Ruzicka, 1975) . The Helikiansupracrustal sequence (Athabasca Group) was deposited on thisregolith or locally, where the regolith was eroded, on the unalteredbasement. The sedimentation took place in three structural sub-

222

Table 1: Lithostratigraphic units of the Athabasca Group(After Ramaekers, 1979 and 1980) .

MaximumThickness

§< Formation metres Lithologyo£, Carswell 500 Dolomite, some chert•§ Douglas 2000 (?) Psammites, pelitesenu Tuma Lake 80 Pebbly sandstone£ Otherside 350 Sandstone, siltstone•£ Locker Lake 120 Pebbly sandstone,

conglomerate{g Wolverine Point 700 Psammites, pelites,••H phosphoritesIH Lazenby Lake 118 Pebbly sandstone,"rg sandstone

Manitou Falls 1400 Sandstone, basalconglomerate

Fair Point 300 Conglomeratic quartz-rich sandstone

with clay matrix

basins (Ramaekers, 1980) and reflected a general uplift in theWollaston Fold Belt and perhaps along some of the faults in theeastern portion of the Athabasca Basin (Ramaekers, 1979) . Thereforethe eastern part of the basin, i.e. that in the vicinity of theuplifted area, contains mainly fluviatile deposits of detritus,whereas the distal (i.e. western) part contains lacustrine or shallowmarine sediments. The Athabasca sedimentary sequence resembles atypical molasse assemblage which, after deposition underwent, severediagenetic changes (ibidem).

The Athabasca Group has been subdivided (Ramaekers, 1979and 1980) into nine formations (Table 1) . In the eastern part thebasal unit, the Manitou Falls Formation, is made up of quartz sand-stones with variable amounts (up to 20 percent) of interstitial clay.The source of the detritus was apparently to the northeast or toeast. The Manitou Falls Formation has a maximum thickness of about1400 m.

The basal rocks of the Athabasca Group in the western part ofthe Athabasca Basin were designated as the Fair Point Formation.This formation, about 300 m thick, is built up of conglomeratic sand-stone with clay-rich matrix. The detritus was derived from a nearbysource and deposited in, apparently, lacustrine or shallow marineenvironment.

The Manitou Falls Formation is overlain by the Lazenby LakeFormation and the Wolverine Point Formation, which consist of pebblyquartz sandstone and of pelitic sedimentary rocks respectively with amaximum thickness about 700 metres.

223

The Locker Lake Formation which overlies the Wolverine PointFormation in the western portion of the Athabasca Basin consists ofpebbly poorly sorted sandstone, which is about 120 m thick.

The uppermost members of the Athabasca Group, namely theOtherside, the Tuma Lake, the Douglas and the Carswell Formationswere deposited apparently in a paralic environment. Most of themconsist of sandstone with abundant interstitial clay; in the CarswellFormation carbonate prevails. The combined maximum thickness ofthese members exceeds 600 metres.

The sedimentation within the Athabasca Basin was intermittentlyaccompanied by explosive volcanism; ash beds occur, for instance, inthe Wolverine Point Formation (Ramaekers, 1980)!

Locally the clastic sedimentary rocks contain substantialamounts of phosphates (e.g. in the upper part of the Wolverine PointFormation; Ramaekers, 1980), organic substance, such as kerogen(e.g. in the basal part of the succession; Ruzicka, 1975; Ruzicka andLeCheminant, 1984) or layers of sulphides (e.g. in the BeartoothIsland area).

The deposition of the Athabasca Group rocks took place probablyduring the 1340 to 1450 Ma period (Bell and Macdonald, 1982) asindicated by the Rb-Sr isochron on tuffaceous shales from theWolverine Point Formation and diabase dykes cutting the AthabascaGroup sedimentary rocks.

Structural features of the Athabasca regionThe structural pattern of the crystalline basement is dominated

by northeasterly trending geological units, most of which are boundedby major faults or shear zones. The Virgin River Shear and Black LakeFault Zones separate the Western Craton from the Crée Lake MobileZone. The Needle Falls Shear Zone separates the Crée Lake MobileZone from the Rottenstone Complex. The northeasterly striking faultsystem is complemented by northwesterly and northerly striking faultsets.

Basic intrusive rocks, which were emplaced after the depositionof the Athabasca Group sedimentary rocks, heal some of the north-westerly and northerly trending faults.

A multi-ring structure, the Carswell Structure (Currie, 1969),occurs in the western part of the Athabasce Basin. The origin of thisstructure is not unanimously explained: it is interpreted either ashaving been formed by a meteoritic impact (Innés, 1964) or as a

224

cryptovolcanic feature (Currie, 1969); the latter interpretationappears to be more reasonable.

URANIUM METALLOGENY OF THE ATHABASCA REGION

Two major important episodes dominate the uranium metallogeny ofthe Athabasca region: (1) Mineralization associated with theHudsonian orogeny about 1740 Ma; and (2) mineralization associatedwith the tectonic events about 1240 Ma.

Processes associated with the first episode were active mainlywithin the crystalline basement rocks whereas the mineralization ofthe second episode was specially related to the sub-Athabascaunconformity. Uranium deposits of the first episode occur in theBeaverlodge area, north of the Athabasca Basin (Tremblay, 1972) .Mineralization in deposits of the second period occurs in the alteredcrystalline basement rocks below, within the altered sedimentaryrocks above the unconformity or, most commonly, within the alteredinterface between the crystalline basement rocks and overlyingclastic sediments, i.e. along the sub-Athabasca unconformity.

The first episode in uranium metallogenyTremblay (1972) recognized four stages in formation of the

Beaverlodge uranium deposits, which are hosted by crystallinebasement rocks of the Western Craton: (i) Deposition of uranium-bearing sediments; (ii) mobilization of uranium and its concentrationduring granitization; (iii) remobilization of uranium and itsconcentration during mylonitization (about 1750 Ma ago) and(iv) remobilization and concentration during late fracturing (about1240 Ma and later). He regarded the granitic and argillaceous rocksof the Archean Tazin Group as the ultimate source of uranium in theBeaverlodge deposits.

Bell and Macdonald (1982) recognized two periods of prominentmineralization in the crystalline basement rocks in Saskatchewan:(1) Syngenetic mineralization in pegmatites with absolute age ofuraninite about 1860 Ma (which would correspond with Tremblay'ssecond stage) and (2) epigenetic mineralization in pitchblende veinswith absolute age about 1740 Ma, (which is indentical with Tremblay'sthird stage of mineralization in the Beaverlodge area) . These twoperiods coincide with igneous and metamorphic events in the regionnamely: the syngenetic mineralization with the "Wathaman Plutonism"(c. 1880 Ma) and "Wathaman Peak" of metamorphism in metasediments(1875 ± 20 Ma); the epigenetic mineralization with the emplacement of

225

"Younger Granites" along with lamprophyre dykes (1740 Ma) and withthe second ("Main") peak of me t amorph ism (c. 1762 Ma). The secondperiod, i.e. the epigenetic mineralization, represents a majormetallogenic event associated with magmatic and post-magmaticprocesses of the Hudsonian orogeny. These processes includedextensive mylonitization (Tremblay, 1972), feldspathic (commonlysodic) metasomatism (Robinson, 1955), carbonatization andhematitization of the rocks hosting uranium and/or polymetallicdeposits (Ruzicka, 1971) . Similar processes as those associated withthe "Younger Granites" or the "Main" peak of (Hudsonian) metamorphismare recognizable in other regions of the Canadian Shield.Langford (1977), however, speculated that the Beaverlodge pitch-blende deposits originated from uranium-bearing meteoric and near-surface ground water.

Uranium mineralization of the first period of Bell and Macdonaldis widespread in Churchill Structural Province, but the mineralizedpegmatites and mineralization in them are very irregularlydistributed and, as a rule, of low grade.

Uranium mineralization of the second period of Bell andMacdonald occurs in fractures in certain lithostratigraphic units(e.g. Fay Complex of Tremblay, 1972) which were affected bymetasomatic and hydrothermal processes. The main mineral assemblagesare either pitchblende and U-Ti phases ('brannerite1) or (rarely)uranium-polymetallic associations. The ultimate source of uranium inthese deposits is unknown: it might be deep-seated fluids,granitized and mylonitized uranium-bearing sediments(Tremblay, 1972), residual evaporitic brines, connate fluids orcrystalline basement rocks.

Unaltered crystalline basement rocks underlying the AthabascaGroup locally contain elevated amounts of uranium and associatedelements, such as nickel, cobalt and molybdenum (Table 2). Theserocks might be sources not only of the Hudsonian epigeneticmineralization, but also of the mineralization associated with thesub-Athabasca unconformity.

The second episode of uranium metallogenyThe main ore-forming processes of the second episode of uranium

mineralization took place after deposition of the Athabasca Grouprocks, i.e. during their diagnesis, under elevated thermalconditions. They were accompanied by retrograde metamorphism of thehost rocks (e.g. argillization, depletion of graphite), magnesianmetasomatism and redistribution of iron, silica and aluminum oxides.

226

Table 2:

ElementÇaNaMgTiMnAgAsBBaBeCeCoCrCuLaMoNiPbSbSnSrUVYYbZnZr

Some elemental constituents from crystallinebasement rocks, Athabasca Basin.

Sample analyses in ppm1 2 3730329

>200009000110<5

<2000981306

5002017014240<50190<700<500<20030048330417

<200380

17000>10000>20000100002600

<5<2000

53140

5<200

5214018

<100<50110<700<500<2001501421050<4

<200200

340720

>200006900510<5

<200062680

320057120180

<100<50220<700<500<2008917320<40<4

<200210

Sample 1: Graphitic pelite from the footwall of theDeilmann ore body.

Sample 2: Unaltered garnetiferous gneiss from thefootwall of the Midwest Lake deposit.

Sample 3: unaltered graphitic pelite from the footwallof the Midwest Lake deposit.

Analytical method: Optical emission spectroscopy using12B-DR spectroscope (samples 1-3)

As a rule illite and/or chlorite form haloes around themineralization, whereas kaolinite occurs in a greater distance fromthe mineralization. The host rocks in the vicinity of themineralization are hematitized and higher up in the sandstonelimonitized. Graphite in the pelite immediately below themineralization is usually depleted. The sandstone above the zone oflimonitization is often silicified (Fig. 2).

Distribution of the ore bodies is, as a rule, structurallycontrolled by intersections of strike faults with the unconformity

227

KAOLINITE :••I

IN THE MATRIX

^^i^é'Ê^^^H^MA'\tZED SEDIMENTARY 'aOCK^^^^^(^^fp-&^• ? '-'] <>£$^:i^?-b^^^Äl ftiÄMÄiÄEAÄfmmwmwmmmwmïfffm i L L i T E •f^mm-^mmmmm^m-:ï^miïm^mmï^ E N V E L O P E mmïïï^mMMm^Mï

muèiwïïmmïmm^wï$fmmmmmmm

Q U A T E R N A R Y A P P R O X I M A T E S C A L E I ? 1.° 2,° 3.° M E T R E S

HELIKIANp;-:;v-.;-:.-;j SED. R O C K S OF THEfob&a A T H A B A S C A GROUP

APHEBIAN (?)^ 1 REQOLITH

K^JyJ METASEDIMENTARYf^^Sj ROCKS

ARCHEAN (?)1+ + + +1 GRANITOIDS

M I N E R A L I Z A T I O N

FAULT

A P P R O X I M A T E LIMITS OFALTERATION HALOES

Figure 2. A generalized cross section through a mineralized body associatedwith the sub-Athabasca unconformity.

228

and lithologically by graphitic pelite or semipelite horizons in thecrystalline basement.

Age of mineralization, as determined on seven pitchblendesamples (Tremblay, 1982) , spans an interval from 1075 to 1320 Ma.The mineralization is either monomineralic (pitchblende) orpolymetallic (U, Ni, Co, Bi, Cu, Pb, As, Y, Zn, Mo, Ag, Au in variousproportions).

Mineralogical geothermometric analyses of samples from the KeyLake deposit indicate (Dahlkamp and Tan, 1977; Pechmann andVoultsidis, 1981; Ruzicka and Littlejohn, 1982; Ruzicka, 1982) thatthe uranium minerals crystallized at temperatures from 135° to 150°C,Pagel (1975) and Pagel et al. (1980) on the other hand determined, onthe basis of fluid inclusion studies on samples from the Cluff Lakearea, that temperature of mineralization was probably in the rangefrom 60° to 260°C.

URANIUM DEPOSITS

The uranium deposits related to the Aphebian-Helikianunconformity at the base of the Athabasca Basin occur beneath a coverof rocks of the Helikian Athabasca Group or, where these rocks wereeroded, under unconsolidated Quaternary deposits. The cover rocksabove the known deposits vary in thickness from nil at Rabbit Lake to10 metres at Collins Bay and Key Lake, 165 metres at McClean Lake,210 metres at Midwest Lake and 450 metres at Cigar Lake.

Individual deposits contain resources comprising tonnages ofuranium from a few hundred to several ten thousands tonnes in oresgrading from 0.2% to more than 10% U (Table 3).

Areal distribution of the deposits is controlled by structuraland lithological features of the crystalline basement and thesedimentary cover.

The Athabasca region has been subdivided in this paper intoareas as follows (Fig. 1) : (1) *Rabbit Lake — Collins Bay;(2) McClean Lake — Midwest Lake — Waterbury Lake; (3) Key Lake;(4) Carswell Structure; (5) Maurice Bay; and (6) Fond du Lac.

RABBIT LAKE - COLLINS BAY AREA

The most important deposits in this area are: Rabbit Lake,Collins Bay "A 1, Collins Bay 'B' and Eagle Point. In addition tothese deposits the Raven, Horseshoe and West Bear deposits containidentified uranium resources.

*Numbers in brackets refer to circled numbers in Figure 1.

229

K)U>OTable 3: Published uranium resources in

associated with the sub-Athabascaselected uranium deposits

unconformity.

Area/DepositRabbit Lake -Collins Bay area

Rabbit LakeCollins Bay BWest BearMcClean Lake —

Midwest Lake —Waterbury Lake area

McClean LakeMidwest LakeKey Lake areaKey Lake

Carswell StructureDMaurice Bay areaMaurice Bay

Fond-du-Lac areaFond-du-Lac

Ore U UTonnes % Tonnes Reference

5 655 000 0.34 19 230 1) Clark et al., 1982N.A. N.A. 11 293 Denner an<3 Reeves, 1981N.A. N.A. 425 Tremblay, 1982

353 800 1.531 200 000 1.60

3 277 800 2.12

123 000 4.25

N.A. <0.42

5 384 Saracoglu et al., 198319 250 Canada Wide Mines (1980)

69 228 Saskatchewan MiningDevelopment Corporation (1983)

5 211 Bayda (1978)

577 Lehnert-Thiel andKretchmar, 1979

N.A. <0.21 385 Tremblay, 1982

N.A. Information not available1) Production has not been excluded.

Lampin Lak«x^>*~i

SOUTH LAMPIN '

c*=S%CO

FIRST GRAYLING

OLE PQ9NTIKE ISLAND

COLLINS BAY A. D ZONE —

/ACOLLINS BAY B ZONEa - 58 15

_ «HOOK/LAKE-rfABEMT LAKE

JOHN)'BEAVER LAKE *•

AHORS

WOLLASTON LAKE

•STAND UP TO REASON

— 58L.'."3 ATHABASCA GROUP (HELIKIAN)

A U DEPOSIT

• U OCCURRENCE

Milchlell L a k e

ELDORADO* W EST BEAJ<CONWEST)

0MILES i_

KM 0

10

57 45

104 45' 30 103 15

Figure 3. Uranium occurrences in Rabbit Lake ~ Collins Bay area.

231

NW

Q U A T E R N A R Y

F > l OVERBURDEN

HELIKIANl l A T H A B A S C A GROUP

APHEBIANL'\»j UPPER GNEISSES

HHIJ MASSIVE META-ARKOSES

| | P L A G I O C L A S I T E

M A S S I V E DOLOMITE

mH REGOLITH

INTRUSIVEl.'tyy BIOTITE MICROORANITE

NW

/ FAULT

REGOLITH

L O C A L C H L O R I T I Z A T I O N

I I P E R V A S I V E C H L O R I T I Z A T I O N

| I G R A P H I T I C CHLORIT IC A L T E R A T I O N

DOLOMITIZATION AND CHLORITIZATION

PERVASIVE DOLOMITIZATION

/ FAULT

met res

Figure 4. Geological section through the Rabbit Lake deposit; (a) lithology;(b) alteration. After Sibbald m Heine (1981).

The Rabbit Lake depositThis deposit is located about 5 km west of the west shore of

Wollaston Lake and 354 km north-northeast of La Ronge (Fig. 3).Uranium mineralization in the Rabbit Lake deposit is hosted by a

suite of Aphebian rocks of the Wollaston Group (Hoeve andSibbald, 1978). Sibbald (1978) recognized in this suite three rocks

232

Figure 5. Radiating crystals of sklodowskite in quartz;Rabbit Lake deposit, Saskatchewan. Transmitted light.

units: (1) soda-rich feldspathic quartzite at the base; (2) anassemblage of calc-silicate rocks, feldspathic quartzite, marble andgraphite-bearing rocks; (3) layered gneiss made up of quartzo-feldspathic rocks intercalated with amphibolite. This suite isintruded by fine-grained granites (microgranite), (Fig. 4).

In the area of the deposit the Aphebian rocks trend north-easterly (065°) and dip southeasterly (65°); the host rocks representa northwestern limb of a major syncline. They are dislocated by the065°-trending and 30° SE-dipping Rabbit Lake Fault.

The host rocks are strongly altered. Chloritic alteration tookplace in the pre-mineralization period during the late stages ofHudsonian Orogeny, kaolinization and illitization during themineralization period and argillization mainly due to weatheringduring the post-mineralization period until present time(Tremblay, 1982).

The ore body is cigar-shaped and in its longest dimension550 metres long. In the horizontal plane projection the ore zone is215 to 365 metres long and 90 to 125 metres wide; it extends to adepth of 150 metres.

233

The mineralization consists mainly of pitchblende and coffinite,but sklodowskite (Mg (U02)2(Si03)2(OH)2*5H2O) is a common secondarymineral (Fig. 5) along with minor amounts of kasolite,woelsendorfite, uranophane and boltwoodite.

Minor amounts of other metallic minerals are associated withuranium mineralization: galena, pyrite, rutile, nickeline andmarcasite. Calcite and phyllosilicates are most common gangueminerals.

Elemental assemblages associated with uranium in the Rabbit Lakedeposit include vanadium, molybdenum, nickel, copper, lead, barium,boron, magnesium and phosphorus in amounts exceeding the clarkes ofthese elements.

The minimum absolute age for the pitchblende mineralization hasbeen determined by U-Pb method as 1281 ± 11 Ma (Gumming andRimsaite, 1979); however uranium has been later rejuvenated severaltimes.

Decrepitation tests on dolomite, which accompanied uraniummineralization as a gangue in the Rabbit Lake deposit, indicatedtemperature of crystallization at 245°C and on fluid inclusions ineuhedral quartz at 135° to 160°C and allowing for effects of pressureat 180° to 225°C (Little, 1974).

A similar type of mineralization was noticed about 21 kmnortheast of Rabbit Lake on Spurjack Island (Chandler, 1978).

The Collins Bay 'A1 ZoneThe Collins Bay 'A' Zone is located about 10.5 km north of the

Rabbit Lake deposit near the eastern edge of the Athabasca Basin(Jones, 1980; Tremblay, 1982). Its uranium-polymetallic mineraliza-tion occurs in the hanging wall of the Collins Bay Fault under overburden and a small remnant of a sandstone layer of the AthabascaGroup rocks. The ore body is enclosed in clay, which graduallychanges with depth to clay-altered Aphebian rocks (regolith) freshgraphitic and/or biotitic paragneiss and Archean (?) granitoid gneiss(Fig. 6). The Aphebian rocks are folded into a northeasterlytrending anticline. Distribution of the mineralization isstructurally controlled by intersections of the northeasterlytrending and southeasterly dipping Collins Bay Fault, the alteredzone along the sub-Athabasca unconformity, and by northwesterlytrending cross faults. The high grade core of the Collins Bay'A' Zone has a lensoid shape; it is about 50 metres long, up to30 metres wide and up to 22 metres thick. Some drillhole inter-

234

W

W a t e r

Q U A T E R N A R Y

[~ '• <] O V E R B U R D E N

H E L I K I A N[ ) A T H A B A S C A GROUP

APHEBIANjj| j P A R A G N E I S S (GRAPHITIC)

L-ÏH3 P A R A G N E I S S

HIGH GRADE CORE

C L A Y A L T E R A T I O N

COLLINS BAY FAULT

ARCHEAN(?)

G R A N I T O I D G N E I S S

Figure 6. Generalized cross section through the Collms Bay 'A' Zone.After Jones (1980).

Table 4: Analyses of selected drill core samples fromthe Collins Bay 'A' Zone!

Constituents

Sample No1234561

U Ni Pbppm

Ag Au2.33

59.7864.8761.2213.4053.1735.19

0.561.070.961.205.091.693.66

0.209.189.118.891.186.167.08

3.7729.1430.5125.3723.6643.54

1,349.8

0.0310.6324.3133.46

1.9215.9410.35

Samples 1 to 5 were:- randomly collected from the central portion of the

'A1 zone;Sample 6 and 7 were:

randomly collected from the northeastern portion ofthe 'A1 zone;

235

sections contained uranium in excess of 50 percent U over more than10 metres.

The ore consists of pitchblende associated with rammelsbergite,pararammelsbergite, some galena, sphalerite and traces of silver andgold. (Table 4).

Collins Bay 'B1 ZoneThe Collins Bay 'B1 Zone is located about 8 km north of Rabbit

Lake deposit (Jones, 1980; Tremblay, 1982).Uranium mineralization occurs mainly in clay altered sandstone

of the Athabasca Group, which unconformably overlies highly alteredArchean granitic gneiss. The Aphebian paragneiss, which is locallygraphitic, occurs mainly to the east of the mineralization and onlyat the northern end of the deposit is with it in contact. The orebodies of the 'B1 Zone follow the trend of the Collins Bay Fault.

The ore body is about 1200 metres long, up to 150 metres wideand in average 17 metres thick. It contains high grade pods in thenorthern, central and southern part of the deposit with ore gradesexceeding 20% U and 20% Ni over 2 metres.

In addition to pitchblende and coffinite the mineralizationincludes gersdorffite, skutterudite, nickeline, millerite, galena,chalcopyrite and pyrite (Fig. 7). Elemental assemblages in theCollins Bay 'B1 zone include Ag, As, Co, Cr, Cu, Ni, Pb, Ti, V and Zr(Table 5).

Several small ore bodies occur between the Collins Bay'A1 and 'B1 Zones. Their distribution is controlled by intersectionsof the Collins Bay Fault with the sub-Athabasca unconformity. Theseore bodies are called Collins Bay 'D1, 'E1 and 'F1 Zones. Theirmineralization is similar to the 'A' and 'B1 Zones (Fig. 8).

Eagle Point depositThe Eagle Point deposit occurs about 2 km northeast of the

Collis Bay 'A' Zone (Fig. 3). Its host rocks are Aphebianmetasediments such as feldspar-cordierite-biotite gneiss, pyriticgarnetiferous gneiss, feldspathic quartzite and graphiticparagneiss, which are intercalated with granitic pegmatoids. Thehost rocks exhibit monoclinal layering and faulting and strongargillization and hematitization or limonitization which followfaults, bedding-planes or foliation.

236

G a l e n am.» —?-

P i t c h b l e n d e „

Figure 7. Photomicrograph of pitchblende associatedwith gersdorffite and nickeline; Collins Bay 'B1 Zone.Reflected light.

Figure 8. Photomicrograph of pitchblende, partly coated bygalena, associated with gersdorffite and nickeline. Collins Bay'D1 Zone. Reflected light.

to

0)x:4J

§

vu

0)<UrHn.g^.

<"oo<U rHi-l0 -U Cx:

Hj "i — *m

i— 1»ri -P

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QJil __,

u^<D ro' — 1W ^

U

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3

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238

. 83

Table 6: Elemental assemblage in a weakly mineralizedpart of the Eagle Point deposit.

Sample1

Si02A1203Fe203CaOK2ONa2O %MgOTi02P20SMnO

AsBaCoCrCuNbNiPb ppmSrUYZnZrRbFCl

N.B.: N.F. — not foundSamples 1 and 2 collected from drill core andanalyzed using 12B-DR

Uranium mineralization occurs in lenses arranged in narrow zonesand consists mainly of pitchblende, which occurs in small cubes,veins, botryoids or associated with carbon, coffinite, uranophane andboltwoodite (Figs. 9 & 10). Its distribution is controlled by thealteration zones. Gersdorffite, millerite, chalcocite and galenaoccur locally in small amounts. Elemental assemblage in a weaklymineralized part of the deposit includes Cr, Pb, Ni, Zr, Rb, F and Cl(Table 6) .

240

174.012.00.40.253.40.22.50.380.050.06

NF340109010102020NF

102080180200900200

284.08.50.40.151.70.21.90.160.04NF

NF100

NF50401020410

NF12101021080500400

Unlike the Collins Bay zones the Eagle Point deposit occursentirely in the altered Aphebian basement rocks below the sub-Athabasca unconformity, is stratabound and its mineralization isconfined to at least fifteen sheet-like zones. The general strike ofthe rocks and mineralized zones is east-northeast and dips are40° to 70° SE. The mineralization has been so far identified along adistance of about 2000 m, up to 200 m width and to 450 m depth downthe dip.

Other deposits of the Rabbit Lake — Collins Bay areaIn addition to the Collins Bay zones and Rabbit Lake and Eagle

Point deposits, the Rabbit Lake — Collins Bay area includes the WestBear, Raven and Horseshoe deposits and several small uraniumoccurrences (Fig. 3).

The West Bear deposit occurs about 43 km southwest of the RabbitLake mine. Distribution of the mineralization in the deposit iscontrolled by the sub-Athabasca unconformity, which is beneath over-burden and 15 to 20 metre thick sandstone of the Athabasca Group.Most of the mineralization is in the altered pyritic and graphiticsemipelites of the Wollaston domain just below the unconformity.Some mineralization occurs in the sandstone immediately above theunconformity and within the clay alteration zone at the unconformity.In addition to uranium the mineralization contains higher amounts ofarsenic, lead, nickel, vanadium, zirconium and silver (Table 7).

The Raven and Horseshoe deposits occur about 5.5 km southeast ofthe Rabbit Lake mine (Fig. 3). The host rocks of both deposits aremainly altered graphitic guartzites and biotite gneisses with minoramphibolites and calc-silicate rocks of the Wollaston Group. Thepitchblende and uranophane mineralization is irregularly distributedin fractures and disseminated in the altered host rocks.

MCCLEAN LAKE - MIDWEST LAKE - WATERBURY LAKE AREA

Deposits with identified uranium resources in this area are:McClean Lake, Dawn Lake, Midwest Lake and Cigar Lake. In additionseveral occurrences, such as JEB and Mallen Lake near the McCleanLake deposit and BJ near McArthur River, occur in this area(Fig. 11).

The McClean Lake deposit occurs about 11 km northwest of theRabbit Lake mine (Brummer et al., 1981; Saracoglu et al., 1983;Tremblay, 1982; and Fig. 11). The deposits contains three east-

241

(O-ʻto Table 7: Spectroscopic analyses of selected samples from the West Bear deposit,Sample

ConstituentSiAlFeCa %NaMgTiPMnAgAsBiCeCoCrCuLaNbNd PpmNiPbSbScSuSrUVYYbZnZr

N.A. - not availableN.F. - Not found

>2.01.5>2.00.0330.00540.670.11N.A.0.031<5

19 000N.A.<20036091570<100<50N.A.1 700

>100 000<500N.A.<2002470<2067<4

15 000100

>20.02.0>3.00.3N.F.

0.1 - 0.51.0N.F.0.2100

10 0002 000N.F.1 0002001 500N.F.N.F.N.F.

2000 - 500020 000N.A.100N.A.1 00085 7002 000100N.A.N.F.3 000

15.02.0>3.00.5N.F.

0.2 - 0.51.0N.F.0.170

15 000700N.F.700150500N.F.N.F.N.F.

1000 - 300015 000N.A.100N.A.1 000125 0005 000200N.A.N.F.1 500

Sample 1:Sample 2 & 3:

analyzed using 12B-DRanalyzed using SQ23

5830 •

30I

IS1 04

10335

-58°30

Q5

MALLE NLAKE

58 .

JEB

tf«

TOR WA L T iMcCLEAN .NORTH

McCLEAN SOUTH

à. U DEPOSIT

• U OCCURRENCE

KM 0 5 10

OJFigure 11. Uranium occurrences in McClean Lake - Midwest LakeWaterbury Lake area.

58

north-east trending mineralized zones, each of which consists ofseveral ore lenses. These zones occur at the sub-Athabascaunconformity which is about 165 metres below the surface. Thefootwall of the zones consists of Aphebian rocks including graphiticand pyritic metapelites, feldspathic quartzite and amphibolite.The hanging wall consists of coarse or pebbly sandstone of theManitou Falls Formation. Uranium mineralization, pitchblende andcoffinite, is commonly associated with illite, chlorite and metallicminerals such as pararammelsbergite, nickeline, gersdorffite,safflorite, chalcopyrite, siderite, hematite, goethite, bravoite andpyrite (Brummer et al., 1981).

The Dawn Lake deposit is located about 20 km west of the RabbitLake mine (Clarke and Fogwill, 1981; Tremblay, 1982; and Fig. 11) .Its four mineralized zones are designated as 14, 11, 11A and 11B(Fig. 12). The mineralization, consisting mainly of pitchblende(massive, botryoidal and sooty), some uranoan carbon and secondaryuranium minerals, is accompanied in one of these zones with cobaltand nickel arsenides and copper sulphide, whereas in the other zonesthe uranium mineralization is monomineralic. The mineralizationoccurs in the altered basement rocks (Zone 11B), within theargillized interface along the unconformity (Zones 11A and 14) andwithin the sedimentary rocks of the Manitou Falls Formation(Zone 11). The basement host rocks are chloritized pelites withvariable amounts of disseminated graphite, biotite, pyrite andlocally cordierite; calc-silicate rocks and granitic pegmatiteslocally contain tourmaline. The argillized interface along the sub-Athabasca unconformity includes weathered zone of the basement rocks(saprolite or regolith) and products of diagenetic alteration. Therocks of the Manitou Falls Formation include basal andintraformational conglomerate, sandstone and clay intercalations.Distribution of the mineralization is structurally controlled byintersections of the sub-Athabasca unconformity with strike- andcross-faults (Fig. 12).

The Midwest Lake deposit is situated 25 km northwest of theRabbit Lake mine (Fig. 11; Scott, 1981; Tremblay, 1982) .

The deposit occurs along the sub-Athabasca unconformity, whichis about 195 metres below the surface at the centre of the deposit,and extends along an intersection of this unconformity with anortheasterly trending fault zone (Figs. 13 and 14). It is about900 metres long, up to about 110 metres wide and 36 metres thick.

244

LONGITUDINAL SECTION

11AZONE SCALE i———£——Too

QUATERNARY[' ' | OVERBURDEN

HELIKIAN| ] A T H A B A S C A GROUP

ARCHEAN AND APHEBIANMETAMORPHIC BASEMENT (UNDIVIDED)

METAPELITE

$ CALC-SILICATE ROCKS

+\ GRANITE PEGMATITE

U MINERALIZATION

// FAULT

DAWN LAKE

FOLD A X I S

Figure 12. Dawn Lake deposit. Modified after Clarke and Fogwill (1981).

245

S E C T I O NA

MINK ARM OFMIDWEST L A K E

[ j S A N D S T O N E ORE

M A S S I V E O R E

F A U L T

S C A L E

0 20 40 60 80 100

M E T R E S

PLAN VIEW OF O R E B O D Y

Figure 13. Plan view of Midwest Lake deposit.(After 'Canada Wide Mines - Midwest Lake Uranium Project';Canada Wide Mines, 1980).

The uranium mineralization occurs (i) in a massive or spheruliticform (Fig. 15); (ii) in a mixture of pitchblende with coffinite; and(iii) in a sooty form (Ruzicka and Littlejohn, 1981). Associatednonradioactive minerals are nickeline, millerite, maucherite,rammelsbergite, gersdorffite, nickelhexahydrite, retgersite,calcite, chlorite, halloysite, hematite and goethite. Mineralizationin the sandstone consists mainly of massive and sooty pitchblendeoccurring interstitially or as fracture filling in quartz grains(Fig. 16).

A generalized conceptual mineral succession postulates fivestages of mineralization (ibidem; Fig. 17): (1) massive orspherulitic pitchblende; (2) first generation of nickel arsenides andsulphides; (3) second generation of nickel arsenides and sulphidesand pitchblende-coffinite assemblage; (4) sulpharsenides and sootypitchblende; (5) gangue and post-ore minerals. The main elementalconstituents in the Midwest Lake deposit are, in addition to uranium,nickel, cobalt, arsenic, molybdenum, titanium, vanadium and zirconium(Table 8).

In addition to concentrations in the main ore bodies at theunconformity some mineralization was deposited in fracturesconnecting the ore bodies with the surface and later incorporated in

246

•• rV? =fff.P-e\jyytfYfiD'O- -TW

S C A L E0 5 10 iO 30 40 50

SANDSTONE ORE

M A S S I V E ORE

// FAULT

QUATERNARYPPUl O V E R B U R D E N

HELIKIANpill ATHABASCA GROUP-SANDSTONEf-v-r-r'v-r-r-l

[5g|g| A T H A B A S C A GROUP-CONGLOMERATE

ARCHEANfS^fjj GNEISSr '-^ (BIOTITE.PELDSPAR.CHLORITE.GRAPHITE»

Figure 14. Cross section through Midwest Lake deposit. (After 'Canada WideMines - Midwest Lake Uranium Project', Canada Wide Mines, 1980).

247

248

MINERAL ASSEMBLAGE

Rammelsbergite

Pitchblende andCoffinlte

Pitchblende (sooty)

Calclte.Hematite.Chlorlte, Goethlte,Halloyslte .Retgersl te

Nfckel-hexahydrlte

Figure 17. Generalized mineral succession in the Midwest Lake deposit.Dashed line (----) indicates replacement process; dotted line (.....) indicates that theyounger mineral has been derived from another mineral(s).

glacial drift. In this way the surficial dispersion haloes indicatedpresence of concealed ore bodies and thus exploration targets.

K-Ar analyses of sericitic and chloritic clay collected from themineralized zone indicated absolute age of 1212 ± 33 Ma(Stevens et al., 1982), which can be considered as the main age ofthe Midwest Lake deposit.

The Cigar Lake deposit occurs near the southwestern shore ofWaterbury Lake (Fig. 11). Its mineralization is structurallycontrolled by a mylonite zone in the basement rocks, by an alteredzone at the sub-Athabasca unconformity and by graphitic metapelitesof the Wollaston Group (Ruzicka and LeCheminant, 1984). The mylonitezone has been subjected to faulting and cross-faulting. Retrogrademetamorphism resulted in extensive clay (kaolinite, illite andchlorite) alteration of the host rocks. Two phases of uraniummineralization occur in the deposit: (i) a polymetallic, which

249

to

Table 8: Selected elemental constituents in selected drill core samples from the Midwest Lake deposit.(From: Ruzicka and Littlejohn, 1981 and Canada Wide Mines Limited, 1980).

Constituents in PercentSample

ElementAgAsBBaBeBiCoCrCuMoNiPbSrTiU3)VZnZr

1)10.00687.20.0068<0.0005<0.0003

n.a.0.22<0.00050.0330.49

>2.00.0820.00280.240.020.014<0.020.025

1)2

<0.0005<0.200.0240.00820.0004n.a.

0.0130.0190.0024<0.0050.02<0.070.00520.930.0070.062<0.020.032

2)30.015

>10.0n.a.n.a.n.a.n.a.1.00.0150.15n.a.

> 0.73.0n.d.0.2

27.20.3n.d.

< 0.005

2)4n.d.n.d.n.a.n.a.n.a.n.d.0.30.0030.007n.a.0.2n.d.n.d.0.054.80.015n.d.

0.03

2)50.0033.0n.a.n.a.n.a.0.10.50.0150.007n.a.

>1.5n.d.0.0072.02.20.3n.d.0.3

1)6

<0.00050.230.0680.00780.0016n.a.0.0030.0330.0014<0.0050.35<0.070.00290.660.140.49

<0.020.02

2)70.00155.0n.a.n.a.n.a.

0.150.20.010.15n.a.

> 1.5n.d.0.0031.014.40.3n.d.0.5

2)8n.d.n.d.n.a.n.a.n.a.n.d.

< 0.050.020.02n.a.

< 0.005n.d.n.d.0.7

17.30.1n.d.0.15

1)9<0.00050.350.00850.0033<0.0003

n.a.0.29<0.00050.00073<0.0050.12<0.07<0.0010.0660.0090.0055<0.020.014

4)10tr.1.68n.a.n.a.n.a.0.0170.190.0190.080.0230.940.04n.a.

0.790.250.110.11n.a.

4)110.00689.62n.a.n.a.n.a.0.0570.110.0230.420.1364.801.19n.a.

0.7311.790.340.52n.a.

tr. - tracen.a. - analysis not availablen.d. - not detected (i.e. below the detection limit)1. Analyzed using 12B-DR2. Analyzed using 5Q-233. Analyzed by Neutron Activation4. Analyzed by a commercial laboratory

Location of Samples:DDH 78-40: sandstone58 18'N, 104 06'W: chloritizedDDH 78-42: regolithDDH 78-40: sandstoneDDH 78-18: regolithDDH 78-18: regolithDDH 78-18: sandstoneDDH 78-70: sandstoneDDH 78-15: sandstone

1234567891011

Typical sandstone oreTypical massive ore

includes, in addition to uranium, Ni, Co, Pb, C, Mo, As, Zr and Zn;this phase prevails at the base of the ore bodies; (ii) amonomineralic, which consists of pitchblende in botryoidal, massive,granular, brecciated or finely disseminated forms; this phase usuallyoccurs in the upper portions of the mineralized bodies, i.e. in thesandstone.

The main mineralized zone occurs about 450 metres below thesurface, but lesser amounts of mineralization were encountered about100 metres below the present surface.

KEY LAKE AREA

The Key Lake area is located about 240 km north of La Ronge andabout 180 km south-southwest of the Rabbit Lake mine (Fig. 1). Theuranium mineralization occurs in two ore bodies: Gärtner andDeilmann (Gatzweiler et al., 1981; Tremblay, 1982; and Fig. 18).The ore bodies are closely associated with the sub-Athabascaunconformity, basement tectonic features, such as several layers ofgraphitic metasedimentary pelitic rocks and a major reverse faultzone. The fault zone is parallel with the foliation of the basementrocks, i.e. 060° to 070°, dips 40 to 60 degrees to the north-west anddislocates both the basement and cover rocks vertically up to 30 m.The graphitic pelites are part of the Aphebian Wollaston Group,comprising biotite-plagioclase-quartz-cordierite gneiss, garnet-quartz-feldspar-cordierite gneiss, amphibolite, calc-silicate rocks,migmatite and granite pegmatite. The Wollaston Group rocksunconformably overlie Archean granitic rocks, which occur as north-easterly elongated domal structures. The Aphebian sediments are atthe sub-Athabasca unconformity, are strongly altered and containchlorite and illite. Some of the alteration is, apparently,regolithic, but some is diagenetic hydrothermal or related totectonic movements along the fault zone. The sedimentary rocks ofthe Athabasca Group rest unconformably on the basement rocks and areup to 60 metres thick in the area of the deposits. They areconglomeratic sandstone at the base and quartz sandstone in the restof the sequence. The sedimentary rocks are altered with hematite,kaolinite and chlorite.

The uranium mineralization occurs in at least four main genera-tions (Ruzicka and Littlejohn, 1981): (i) as thorium-free euhedralcrystals of a alpha U30? compound; (ii) as massive, commonlybotryoidal, pitchblende; (iii) as pitchblende-coffinite aggregates;and (iv) as sooty pitchblende. The most common mineral assemblagesof the deposits consist of pitchblende-coffinite aggregates with

251

105 45

1OOO 2OOOMetres

AthabascaGroupAphebian — — — Geological Contact

Approximate Location ofAthabasca Unconformity

Archean L a k e

Fault

Ore Zone

Camp

Figure 18. Geological setting of the Key Lake deposit. (After Tremblay, 1982).

Figure 19. Snowflakes of gersdorffite with dark cores of bravoitefrom Deilmann ore body, Key Lake deposit. Reflected light;cp = chaZkopyrtte.

252

gersdorffite and sooty pitchblende. In addition to clay, calciteand/or siderite are the main gangue minerals. Associated with theuranium mineralization are also millerite, nickeline, bravoite(Fig. 19), rammelsbergite, chalcopyrite and galena. According toanalyses of selected samples the elemental assemblages of themineralization consist of arsenic, cobalt, copper, nickel, lead,titanium and vanadium (Table 9).

U-Pb isotope analyses on 34 samples of pitchblende from the KeyLake deposit indicated three main ages of mineralization: ~1,228 Ma;~960 Ma; and ~89 Ma (Gatzweiler et al., 1979); Wendt et al. (1978)reported an age of 1270 Ma also on pitchblende. The ~1,228 Ma agefits well with the absolute ages of the mineralizations obtained onother sub-Athabasca unconformity related deposits.

The associations of pitchblende/bravoite and pitchblende/alpha-triuranium heptaoxide, found in the Key Lake deposits, indicate atemperature of crystallization between 135° and 150°C(Dahlkamp et al., 1977; Pechmann and Voultsidis, 1981; Ruzicka andLittlejohn, 1982).

The Gärtner and Deilmann deposits are 1450 and 1200 metres longrespectively, their horizontal width is up to 100 metres and theirvertical dimension is up to 150 metres.

CARSWELL STRUCTURE

The Carswell Structure is located in the western part of theAthabasca Basin, about 140 km southwest of Uranium City (Fig. 1).The central core of the structure comprises Aphebian gneisses andgranitoids, classified by Alonso et al. (in press) into two groups:an older Earl River Complex (2005 to 1960 Ma) and a younger PeterRiver Gneiss. This core is surrounded by Helikian sedimentary rocksof the Athabasca Group: William River Subgroup, consisting ofconglomerate, sandstone and peli-te; Douglas Formation with sandstone,siltstone and mudstone; and Carswell Formation consisting mainly ofdolomite (ibidem). These rocks are intruded by diabase dykes (datedby Wanless et al., 1979, as 949 ± 33 Ma). The youngest, volcanic-like rocks, are Cluff Breccia, dated on two samples by a K-Ar methodas 486 ± 55 Ma and 469 ± 28 Ma (Currie, 1969).

The known deposits occur in the south-central part of theCarswell Structure, in the vicinity of Cluff Lake (Fig. 20). Todate, uranium resources have been identified in the followingdeposits: D, N, R, OP, Claude and Peter River (Fig. 21).

253

to

Table 9: Selected elemental constituents in selected drill core samples from the Key Lake deposit.(From: Ruzicka and Littlejohn, 1981) .

Constituents in PercentSample 1)Element 1

AgAsCaCoCrCuMnMoNiPbSrTiu3)VZnZr

n.an.dTh1.2.3.

< 0.0005< 0.200.0730.0020.0170.00140.011

< 0.0050.019

< 0.070.030.900.00480.033

< 0.020.058

<<<<<<<

<<

1)20.00050.20.0170.0010.00050.00070.0020.0050.00220.070.0050.0290.01550.0020.020.0055

2)3n2.10.0.

< 0.0.0.n

> 0.2.0.0.

23.0.

< 0.< 0.

.d. <001001 <055. a.7 >012011 <005 <

2)400300000100

20000

. - analysis not available

. - not detected (i.e.was not detected in any

below the detection limit)sample

Analyzed using 12B-DRAnalyzed using 5Q-23Analyzed by Atomic EnergyNeutron Activation

Canada Ltd. by

2)

.001

.7

.0

.2

.001

.02

.3n.a..7.0.03.15.4.07.1.005

503

100

< 000

> 0300

310

< 0< 0

Location1.2.3.4.5.6.7.8.

DDHDDHDDHDDHDDHDDHDDHDDH

.03

.0

.0

.07

.001

.07

.3n.a..7.0.07.2.6 '.1.1.005

2)60.03

> 10.01.50.3

< 0.0010.20.05n.a.

> 0.73.00.020.2

30.80.07

< 0.1< 0.005

2)70.03

> 10.01.50.30.030.070.02n.a.> 0.71.5

< 0.0010.15

32.10.15

< 0.1< 0.005

of Samples:19621967505250525057506250505057

(Deilman)(Deilman)(Gärtner)(Gärtner)(Gärtner)(Gärtner)(Gärtner)(Gärtner)

graphiticsandstoneregolithregolithregolithregolithregolithregolith

2)80.02

10.010.00.10.0010.030.2n.a.0.75.00.150.17

26.90.070.10.005

schistwith pitchblendewith pitchblendewith pitchblendewith pitchblendewith pitchblendewith pitchblende

5830-

o i5815

-5830

-5815

HELIKIANCARSWELL FORMATION

DOUGLAS FORMATION

I I WILLIAM RIVER SUBGROUPAPHEBIAN

F7%] METAMORPHIC BASEMENT

A U DEPOSIT

MAIN FAULT

Figure 20. Geology of Cars-well Structure. (Simplified after Alonso et aL, in press).

The D deposit occurs along the sub-Athabasca unconformity(Ruzicka, 1975). The whole sequence is overturned so that the rocksof the Peter River Gneiss Formation rest upon the sedimentary rocksof the William River Subgroup. The paleosurface of the basementrocks is deeply weathered and has a thick regolith. The highesturanium concentration occurs in the basal sequence of the AthabascaGroup. It is confined almost entirely to bituminous pelite, which

255

10930

5 8 2 5 — i

HELIKIAN

[ | WILLIAM RIVER SUBGROUPAPHEBIAN

tr^a PETER RIVER GNEISS

Ë----3 EARL RIVER COMPLEX

A U DEPOSIT

• U OCCURENCE

Figure 21. Geology and distribution of uranium occurrences in a part of Cars-wellStructure m the vicinity of Cluff Lake. (After Alonso et al., in press).

grades transitionally to sandstone. The main uranium ore mineral ispitchblende; coffinite and uranoan carbon occur in lesser amounts.The associated polymetallic mineralization includes pararam-melsbergite, native selenium and, selenides of lead, bismuth, nickeland cobalt, native gold, altaite, galena, chalcopyrite, pyrrhotiteand pyrite.

U-Pb isotope analyses indicate absolute age of the 'D1 depositas 1100 Ma (Tapaninen, 1975) , but Gancarz (1979) using U-Pb andPb-Pb methods suggested two main phases of mineralization (1050 Maand 800 Ma) and a remobilization phase (234 Ma); the remobilizationphase coincides with the range of the Hercynian uranium metallogenicepoch in Europe (Ruzicka, 1971; Ball et al., 1982).

The D deposit has been depleted. It was a high grade ore body(Table 3), 140 metres long, 12 metres wide and 12 metres thick.

256

MAIN ZONESW

B ZONE

MAURICE CREEK ZONE

QUATERNARYI" ,~^'j Overburden

HELIKIAN

[""""I ATHABASCA GROUP ,Sandstone

APHEBIAN (?)

Gneiss.locally graphitic

ARCHEAN (?)

F^I Qranltoids

Faults

U Mineralisation

Figure 22. A sketch (not to scale) showing distribution of mineralized zones in MauriceBay area. Modified from Lehnert-ThieZ et al., 1981 and Uranerz Exploration and MiningLimited, personal communication.

The remaining deposits in the Carswell Structure aremonometallic and consist mainly of pitchblende and coffinite. The N,R, OP, Claude and Peter River deposits occur in quartzo-feldspathicgneiss, metapelite and quartzite of the Peter River Gneiss Formation(Fig. 21). Their mineralization is structurally controlled bysteeply dipping faults and subhorizontal mylonite zones.

MAURICE BAY AREA

The Maurice Bay area is located about 80 km west-southwest ofUranium City on the north shore of Lake Athabasca and northwest rimof the Athabasca Basin (Fig. 1).

Uranium mineralization occurs in the Main Zone, the A Zone, theB Zone, and in a few additional zones. All the mineralized zones arespatially related to the sub-Athabasca unconformity (Lehnert-Thieland Kretschmar, 1979; Lehnert-Thiel et al., 1981; Tremblay, 1982).

The basement rocks in the Maurice Bay area are granitizedmetasediments, apparently a part of the Tazin Belt (Tremblay, 1982) ,which are the main host rocks of the Beaverlodge uranium deposits.The basement rocks were affected by Hudsonian Orogeny and are locallyalbitic and some are graphitic. The tops of the basement rocks at theunconformity (regolith) are strongly hematitized and chloritized

257

oo

Figure 23. Photomicrograph of pitchblende (P) associatedwith chlorite (Chl) and pitchblende-chlorite mixture(P ± Chl) in sandstone, Maurice Bay deposit (Main Zone)(Q) = quartz grains. Reflected light.

Figure 24. Photomicrograph of mineralizationFond-du-Lac deposit. Q = quartz, Fe = h'monite,P = pitchblende. Reflected light.

from

(Fig. 22). The sedimentary rocks of the Athabasca Group whichoverlie unconformably the basement rocks, are at the unconformitychloritized and limonitized, brecciated and, further upward,silicified.

The mineralization in the Main Zone is controlled by a steeplydipping fault and occurs in both the altered basement rocks and inthe altered sandstone. In the A Zone the mineralization occurs alongfractures and disseminated in the basement rocks and the sandstone.In the B Zone the mineralization occurs entirely in the alteredsandstone. In one of the additional small zone mineralization occursas subhorizontal lens in the sandstone near the surface. The largestresources (Table 3) are in the Main Zone, which is 1500 metres long,up to 200 metres wide and up to 10 metres thick. Pitchblende andcoffinite are the main ore minerals (Fig. 23); they are commonlyassociated with chlorite and/or limonite (Table 10).

Uranerz Exploration and Mining Limited reported (verbalcommunication) absolute age of most of the uranium mineralization(determinted by U-Pb method in W. Germany) as 1,200 Ma, but some ofthe pitchblende from the Main Zone reportedly yielded 2500 Ma.Absolute age of mineralization in one of the small zones was reportedby Uranerz as 250 Ma.

Table 10: Selected elemental constituents in selected drill core samplesfrom the Main Zone, Maurice Bay deposit. (From: Ruzicka andLittlejohn, 1981).

ElementAgAsCoCrCuMoNiPbScTi

Sample

VZnZr

Constituents in Percent1)1n.d.n.d.n.d.

0.010.001n.a.

0.020.2n.d.

0.35.840.01n.d.

0.015

1)2

n.d.n.d.n.d.

0.030.05n.a.

< 0.00151.5

< 0.00070.3

32.50.07n.d.

< 0.005

2)3

< 0.005< 0.2< 0.001< 0.0005< 0.007< 0 .005< 0.001< 0.07

n.a.0.480.14

< 0.0020.110.0096

1)4

n.d.n.d.n.d.n.d.

0.005n.a.

0.02n.d.n.d.

0.17.700.005n.d.n.d.

1)5

n.d.n.d.n.d.n.d.

0.007n.a.

< 0.0015n.d.n.d.

0.111.1

n.d.n.d.n.d.

n.a. - not availablen.d. - not detected

(i.e. below the detection limit)Th was not detected in any sample1. Analyzed using 5Q-232. Analyzed using 12B-DR3. Analyzed by Neutron Activation

Location of Samples:1. DDK MB-21: sandstone2. DDK MB-87: regolith3. DDK MB-24: altered sandstone4. DDK MB-46: regolith5. DDH MB-98: regolith

259

FOND-DU-LAC AREA

The Fond-du-Lac area is located at the south shore of theAthabasca and at the northern rim of the Athabasca Basin (Fig. 1) .The Fond-du-Lac deposit, the only known deposit in this area, occursabout 90 km east-southeast of Uranium City (Homeniuk et al., 1980;Tremblay, 1982) .

Most of the uranium mineralization of the Fond du Lac depositoccurs in the sandstone of the Athabasca Group within 35 metres abovethe sub-Athabasca unconformity (Homeniuk et al., 1980). The hostrocks, which contain carbonate cement, are hematitized, limonitizedand chloritized. The main ore mineral, pitchblende, occurs ascoating on quartz grains or in the sandstone matrix as a mixture withlimonite (Fig. 24). Coffinite is less abundant. According toHomeniuk et al., (1980) the main mineralization took place1100-1200 Ma ago and its rejuvenation at 215 Ma and 80 Ma.

A CONCEPTUAL GENETIC MODEL FOR THE SUB-ATHABASCAUNCONFORMITY-RELATED DEPOSITS

Conceptual genetic models for the sub-Athabasca unconformity-related deposits have been proposed by several authors(e.g. Knipping, 1974; Little, 1974; Morton, 1977; Langford, 1977;Hoeve and Sibbald, 1976, 1978; McMillan, 1978; Dahlkamp, 1978;Munday, 1979; Ruzicka, 1979, 1982; Kirchner et al., 1979;Pagel et al., 1980; Ramaekers, 1980; de Carle, 1981; Clark et al.,1982; Tremblay, 1982) . Some of the authors consider that theconcentration of uranium took place in several stages and thatseveral processes were involved in formation of the deposits.

Taking into account similarities and dissimilarities among thesedeposits, field and laboratory observations, and studies on similardeposits outside Canada (e.g. Ferguson, et al., 1980; Crick andMuir, 1980; Binns et al., 1980; Brookins, 1980) the followingfactors for establishing a conceptual model are recognized:

(1) The main episode of uranium mineralization in all the sub-Athabasca unconformity-related deposits took place from1050 to about 1300 Ma ago;

(2) most of the pitchblende, the main ore-forming mineral, wasdeposited under temperatures between 130° and 260°C;

(3) the mineralization was associated with retrogrademetamorphism, which commonly resulted in clay alteration;tourmalinization of the host rock and magnesian metasomatismwere common companions of these processes;

260

(4) deposition of the mineralization was structurallycontrolled by the sub-Athabasca unconformity and steepfaults intersecting this unconformity; the host rocks were,as a rule, dislocated along these faults;

(5) at least one additional period of uranium mineralization(remobilization ?) took place about 200 to 300 Ma ago,i.e. during the time of Hercynian Orogeny;

(6) uranium might have been derived from several sources. Someunaltered basement rocks, such as granitoids andmetapelites, contain above normal amounts of uranium andassociated elements which constitute the ore bodies; theseelements could have been incorporated into mineralizingsolutions by leaching. Altered sedimentary rocks of theAthabasca Group are depleted of some elements, such as U,Ni, As and Co; these elements might have been incorporatedin connate waters or residual brines and thus might becomeanother possible source for the mineralization;

(7) hydrothermal solutions associated with diagenetic, tectonicor deep seated processes acted as an important factor in theformation of the ore;

(8) remobilization and redistribution of the mineralization dueto hydrodynamic conditions are continuing processes, whichhave caused modification of the deposits and development ofdispersion haloes in their vicinity.

Therefore formation of the uranium/polymetallic depositsassociated with the sub-Athabasca unconformity should be classifiedas polygenetic.

ACKNOWLEDGEMENT

The author acknowledges co-operation of the following companiesin supplying information on their properties: Amok Ltée,Asamera Inc., Canadian Occidental Petroleum Ltd., EldorResources Ltd., former Gulf Minerals Canada Ltd., Key Lake MiningCorporation, S.E.R.U. Nucléaire (Canada) Ltée, Saskatchewan MiningDevelopment Corporation and Uranerz Exploration and Mining Ltd.Minerals shown on photomicrographs were identified in laboratories ofthe Geological Survey of Canada by A.L. Littlejohn andG.M. LeCheminant.

261

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Brookins, D.G., 1980: Syngenetic model for some early Proterozoicuranium deposits; evidence from Oklo; in 'Uranium in the PineCreek Geosyncline1 edited by J. Ferguson and A.B. Goleby;International Atomic Energy Agency, Vienna; p. 709-720.

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Canada Wide Mines Ltd., 1980: Preliminary Environmental ImpactStatement, 1980; A report t •> Atomic Energy Control Board,Canada. Unpublished.

Chandler, F.W., 1978: Geology of part of the Wollaston Lake FoldBelt, North Wollaston Lake, Saskatchewan; Geological Survey ofCanada, Bulletin 277, 25 p.

Clark, R.J. McH., Homeniuk, L.A. and Bonnar, R. , 1982: Uraniumgeology in the Athabasca and a comparison with other CanadianProterozoic Basins; in Canadian Mining and MetallurgicalBulletin, Volume 75, Number 840, April, 1982, p. 91-98.

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Clarke, P.J. and Fogwill, W.D., 1981: Geology of the Asamera DawnLake Uranium Deposits Athabasca Basin, Northern Saskatchewan;in Canadian Institute of Mining and Metallurgy Geology DivisionUranium Field Excursion Guidebook, Sept. 8-13.

Crick, I.H. and Muir, M.D., 1980: Evaporites and uraniummineralization in the Pine Creek Geosyncline; in "Uranium inthe Pine Creek Geosyncline1 edited by J. Ferguson andA.B. Goleby; International Atomic Energy Agency, Vienna;p. 531-542.

Gumming, G.L. and Rimsaite, J., 1979: Isotopic studies of lead-depleted pitchblende, secondary radioactive minerals andsulphides from the Rabbit Lake uranium deposit, Saskatchewan;Canadian Journal of Earth Sciences, v. 16, p. 1702-1715.

Currie, K.L., 1969: Geological Notes on the Carswell CircularStructure, Saskatchewan (74K); Geological Survey of Canada,Paper 67-32, 60 p.

Dahlkamp, F.J., 1978: Geological Appraisal of the Key LakeU-Ni Deposits, Northern Saskatchewan; Economic Geology, v. 73,p. 1430-1449.

Dahlkamp, F.J. and Tan, B., 1977: Geology and mineralogy of the KeyLake U-Ni deposits, northern Saskatchewan, Canada; in Geology,Mining, and Extraction Processing of Uranium, ed. M.J. Jones;London, Institute of Mining and Metallurgy, p. 145-158.

de Carle, A.L., 1981: Geology of the Key Lake Deposits - anoverview; in Canadian Institute of Mining and MetallurgyGeology Division Uranium Field Excursion Guidebook,Sept. 8-13, p. 109-130.

Denner, M.J. and Reeves, P.L., 1981: Uranium Reserves and Potentialof Northern Saskatchewan; Saskatchewan Mineral Resources,Miscellaneous Report 81-5, 22 p.

Ferguson, J., Ewers, G.R. and Donnelly, T.H., 1980: Model for thedevelopment of economic uranium mineralization in the AlligatorRivers Uranium Field; rn 'Uranium in the Pine CreekGeosyncline' edited by J. Ferguson and A.B. Goleby;International Atomic Energy Agency, Vienna; p. 563-574.

Gancarz, A.J., 1979: Chronology of the Cluff Lake Uranium Deposit,Saskatchewan, Canada; International Uranium Symposium in thePine Creek Geosyncline, N.W. Australia, June; ExtendedAbstracts.

263

Gatzweiler, R., Lehnert-Thiel, K., Glasen, D., Tan, B.,Voultsidis, V., Strnad, J.G., and Rieh, J. , 1979: The Key LakeUranium-nickel deposits; Canadian Mining and MetallurgicalBulletin, v. 72, p. 73-79.

Gatzweiler, R. , Schmeling, B. and Tan, B., 1981: Exploration of theKey Lake uranium deposits, Saskatchewan, Canada; in Uraniumexploration case histories; International Atomic EnergyAgency, Vienna, 1981, AG-250/5, p. 195-220.

Heine, T., 1981: The Rabbit Lake deposit and the Collins Baydeposits; ijn CIM Geology Division Uranium Field ExcursionGuidebook, September 8-13, 1981, Saskatchewan, p. 87-102.

Hoeve, J. and Sibbald, T.I.I., 1976: The Rabbit Lake Uranium Mine;in Proceedings of a symposium on Uranium in Saskatchewan,ed. C.E. Dünn, Saskatchewan Geological Society, SpecialPublication no. 3, p. 331-354.

____________ 1978: On the genesis of Rabbit Lake and otherunconformity type uranium deposits in northern Saskatchewan,Canada; Economic Geology, v. 73, no. 8, p. 1450-1473.

Homeniuk, L.A., Bonnar, R. and Clark, R.J. McH., 1980: The Fond-du-Lac Uranium Deposit, Saskatchewan; Canadian Mining andMetallurgy, Toronto Meeting, Ontario, April, v. 73, Abstract.

Innés, M.J.S., 1964: Recent advances in meteorite crater research atthe Dominion Observatory, Ottawa, Canada; Meteorics, v. 2,p. 219-249.

Jones, B., 1980: The Geology of the Collins Bay Uranium Deposit,Saskatchewan; Canadian Mining and Metallurgical Bulletin,v. 73, no. 818, p. 84-90.

Kirchner, G., Lehnert-Thiel, K., Rich, J. and Strnad, J.G., 1979:Key Lake and Jabiluka: Models for Lower Proterozoic UraniumDeposition; International Uranium Symposium of the Pine Creekgeosyncline, N.W. Australia, June; Extended Abstracts.

Knipping, H.D., 1974: The concepts of supergene versus hypogeneemplacement of uranium at Rabbit Lake, Saskatchewan, Canada; iriFormation of Uranium Deposits, International Atomic EnergyAgency, Vienna, p. 531-548.

Langford, F.F., 1977: Surficial origin of North American pitchblendeand related uranium deposits; American Association of PetroleumGeologists Bulletin, v. 61, no. 1, p. 28-42.

264

Lehnert-Thiel, K. and Kretschmar, W., 1979: The Geology of theMaurice Bay Deposit; Canadian Mining and Metallurgy, WinnipegMeeting, Manitoba, September, Abstract.

Lehnert-Thiel, K., Kretschmar, W. and Petura, J., 1981: The Geologyof the Maurice Bay Uranium Deposit; iji Canadian Institute ofMining and Metallurgy Geology Division Uranium Field ExcursionGuidebook, Sept. 8-13.

Lewry, J.F. and Sibbald, T.I.I., 1979: A review of pre-Athabascabasement geology in Northern Saskatchewan; rn UraniumExploration Techniques, Proceedings of a Symposium held inRegina, 1978, November 16-17, edited by G.R. Parslow,Saskatchewan Geological Society, Special Publication Number 4,p. 19-58.

Little, H.W., 1974: Uranium in Canada; in Report of Activities,Part A, Geological Survey of Canada, Paper 74-1A, p. 137-139.

Macdonald, C.C., 1980: Mineralogy and geochemistry of a Precambrianregolith in the Athabasca Basin; Unpublished M.Sc. thesis,University of Saskatchewan.

McMillan, R.H., 1978: Genetic Aspect & Classification of ImportantCanadian Uranium Deposits; Canadian Mining and MetallurgicalBulletin, v. 71, p. 61-67.

Morton, R.D., 1977: The western and northern Australian uraniumdeposits — exploration guides or exploration deterrents forSaskatchewan?; in Proceedings of a Symposium on Uranium inSaskatchewan, ed. C.E. Dünn, Saskatchewan Geological Society,Special Publication no. 3, p. 211-255.

Munday, R.J., 1979: Uranium mineralization in northern Saskatchewan;Canadian Mining and Metallurgical Bulletin, v. 72, April,p. 102-111.

Pagel, M., 1975: Cadre géologique de gisements d'uranium dans lastructure Carswell (Saskatchewan-Canada): Etude des phasesfluides; Thèse de Docteur de Spécialité (3e cycle), Universitéde Na,ncy.

Pagel. M., Poty, B. and Sheppard, S.M.F., 1980: Contribution to someSaskatchewan uranium deposits mainly from fluid inclusion andisotopic data; in Uranium in the Pine Creek Geosyncline,Proceedings of the International Uranium Symposium on the PineCreek Geosyncline, Sydney, 4-8 June 1979, jointly sponsoredby BMR and CSIRO in co-operation with IAEA; InternationalAtomic Energy Agency, Vienna, p. 639-654.

265

Pechmann, E. von and Voultsidis, V., 1981: Further results on thepetrography and mineralogy of the Key Lake uranium-nickelorebodies, Saskatchewan, Canada; in Abstracts CIM GeologyDivision Uranium Symposium and Field Tours, Saskatchewan,September 8-13, 1981, p. 14-15.

Ramaekers, P., 1979: Stratigraphy of the Athabasca Basin; in. Summaryof Investigations 1979, Saskatchewan Geological Survey,Miscellaneous Report 79-10, p. 154-160.

______________ 1980: Stratigraphy and Tectonic History of theAthabasca Group (Helikian) of Northern Saskatchewan; rn Summaryof Investigations, 1980; Saskatchewan Geological Survey,Miscellaneous Report 80-4, p. 99-106.

Ray, G.E., 1978: Reconnaissance Geology: Wollaston Lake (West) Area(Part of NTS Area 64L) ; in Summary of Investigations 1978,Saskatchewan Geological Survey, Miscellaneous Report 78-10,p. 25-34.

Ray, G.E. and Wanless, R.K., 1980: The age and geological history ofthe Wollaston, Peter Lake, and Rottenstone domains in northernSaskatchewan; Canadian Journal of Earth Sciences, Volume 17Number 3, p. 333-347.

Robinson, S.C. , 1955: Mineralogy of Uranium Deposits, Goldfields,Saskatchewan; Geological Survey of Canada, Bulletin, 31,218 p.

Ruzicka, V., 1971: Geological comparison between East European andCanadian uranium deposits; Geological Survey of Canada,Paper 70-48, 195 pp.

____________ 1975: Some metallogenic features of the "D" uraniumdeposit at Cluff Lake, Saskatchewan; in Report of Activities,Geological Survey of Canada, Paper 75-1C, p. 279-283.

____________ 1979: Uranium and thorium in Canada, 1978; inCurrent Research, Part A, Geological Survey of Canada,Paper 79-1A, p. 139-155.

______________ 1982: Studies on uranium metallogenic provinces inCanada; in Proceedings of the Symposium on Uranium ExplorationMethods Paris lst-4th June 1982; Nuclear Energy Agency,Organization for Economic Co-operation and Development;p. 143-155.

Ruzicka, V. and LeCheminant, G.M., 1984: Uranium deposit research,1983; rn Current Research, Part A, Geological Survey of Canada,Paper 84-1A, p. 39-51.

266

Ruzicka, V. and Littlejohn, A.L., 1981: Studies on uranium inCanada, 1980; in Current Research, Part A, Geological Survey ofCanada, Paper 81-1A, p. 133-144.

____________ 1982: Notes on mineralogy of various types ofuranium deposits and genetic implications; in Current Research,Part A, Geological Survey of Canada, Paper 82-1A, p. 341-349.

Saracoglu, N., Wallis, R.H., Brummer, J.J. and Golightly, J.P.,1983: The McClean uranium deposits, northern Saskatchewan —discovery; The Canadian Mining and Metallurgical Bulletin,April, 1983, p. 1-17.

Saskatchewan Mining Development Corporation, 1983: Annual Report,1982; Regina, 1983

Scott, F., 1981: Midwest Lake uranium discovery; in UraniumExploration Case Histories; International Atomic EnergyAgency, Vienna, 1981, AG-250/1, p. 221-242.

Sibbald, T.I.I., 1978: Uranium Metallogenetic Studies, Rabbit Lake:Geology; in Summary of Investigations 1978, SaskatchewanGeological Survey, Miscellaneous Report 78-10, p. 56-60.

________________ 1979: NEA/IAEA Test-Area: Basement Geology; irtSummary of Investigations 1979; Saskatchewan GeologicalSurvey, Miscellaneous Report 79-10, P. 77-85.

Stevens, R.D., Delabio, R.N., and Lachance, G.N., 1982: Agedeterminations and geological studies, K-Ar isotopic ages,Report 15; Geological Survey of Canada, Paper 81-2, p. 34-35.

Tapaninen, K., 1975: Geology and metallogenesis of the Carswell areauranium deposits; Canadian Institute of Mining and Metallurgy,Edmonton Meeting, Alberta.

Tremblay, L.P., 1972: Geology of the Beaverlodge mining area,Saskatchewan, Canada Geological Survey of Canada, Memoir 367,265 p.

____________ 1982: Geology of the uranium deposits related to thesub-Athabasca unconformity, Saskatchewan; Geological Survey ofCanada, Paper 81-20, 56 p.

Wanless, R.D., Stevens, R.D., Lachance, G.R., and Delabio, R.N. ,1979: Age determinations and geological studies, K-Ar isotopicages, Report 14; Geological Survey of Canada, Paper 79-2.

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267

URANIUM GEOLOGYBEAVERLODGE AREA

D.M. WARDEldor Resources Ltd,Saskatoon, Saskatchewan,Canada

Abstract

A brief summary is presented of the geology of the Beaver-lodge Area with particular emphasis placed on the uranium miningproperties of Eldorado Resources Limited. The deformed meta-sedimentary and volcanic rocks within the area are of Aphebianand Helikian age. These rocks have undergone at least threeperiods of orogeny, namely, the Kenorian (-2560 m.y.),Hudsonian (-1750 m.y.) and Grenvillian (-1000 m.y.). The finalphase of the Hudsonian Orogeny caused the activation of majorfaults with initial emplacement of epigenetic pitchblendedeposits taking place and with probable widespread remobilisationduring the Grenville Orogeny. Dates as recent as 200 m.y. havealso been recorded for the uranium mineralisation.

Genesis of the numerous uranium deposits in the BeaverlodgeArea remains uncertain. It is generally considered that thedeposits are related to metamorphic events and are the result ofremobilisation within and from donor metasediments.

INTRODUCTIONThe purpose of this paper is to provide a brief summary of

the geology of the Beaverlodge area of northern Saskatchewanwith particular emphasis on the uranium mining properties ofEldorado Resources Limited (formerly Eldorado Nuclear Limited).An attempt has been made to accentuate some of the morecontroversial aspects of the geological record and to summarizeas briefly as possible the concepts of various proposed geneticmodels.

Only the most recent and major publications on the Beaver-lodge area have been included in the selected references and thereader is referred to those publications for a more completerecord. All geological maps and records of the Eldorado mining

269

operations are maintained in storage in the National Archives, inOttawa.

Total production of uranium from the Beaverlodge districtover a lifetime of 32 years, commencing in 1950 and terminatingin June 1982, has exceeded 30,500 tonnes of U000, °f which in

J O

excess of 21,000 tonnes were produced from the mines of EldoradoResources. Although this volume of production is comparable tothe reserves of many of the more recent discoveries associatedwith the Athabasca Basin, the long production life coupled withthe large number of geologists who have been associated with theBeaverlodge area has resulted in the accumulation of a consider-ably greater technical data base. As a consequence there maybe a tendency to assume that the geology of uranium deposits ofthe Beaverlodge area is more complex than that associated withthe Athabasca Basin.

GENERAL GEOLOGY OF THE ARLAThe Beaverlodge area is located within the Canadian Shield,

north of the northwest edge of the Athabasca Basin and north ofLake Athabasca in the province of Saskatchewan. The area issituated in the western Craton subdivision of the ChurchillStructural Province. The rocks within the area have beensubdivided into three groups (Tremblay 1972) which include thefolded and metamorphosed Aphebian meta-sedimentary rocks of theTazin Group, Aphebian meta-sedimentary rocks of the Murmac BayFormation and Helikian sedimentary and volcanic rocks of theMartin Formation (Fig. 1).

Rocks of the area have undergone at least three periods oforogeny. The earliest was the Kenoran (about 2560 m.y. ago),which was followed by the most prominent the Hudsonian (about1750 m.y. ago) and finally by the Grenville (about 1000 m.y. ago).The Kenoran orogeny is considered to have resulted in meta-morphism to produce Archean gneiss domes and granulites plussyngenetic uranium in pegmatites. The Hudsonian orogeny resultedin widespread metamorphism and mylonitization of the Tazin Groupand Murmac Bay Formation with the later stages having marked aperiod of active mountain building, the deposition of the MartinFormation sediments, and eruption of the Martin volcanic rocks.The final phase of the Hudsonian also caused the activation ofthe ma3or faults including the St Louis, Black Bay and ABC faults.

270

HP—T~l mtttieniitie granittI •*• *l fildipir roch

Pig* 1 Generalized geology of the Beaverlodge Area

271

Initial emplacement of epigenetic pitchblende deposits occurredat this time while, the Grenville orogeny is credited forfurther widespread remobilisation.

HISTORICAL REVIEWDuring the gold rush in the 1930's, pitchblende was

discovered at the property of Nicholson Mines Ltd, on the northshore of Lake Athabasca. The occurrence remained a mineralog-ical curiosity until Eldorado Mining and Refining (now EldoradoResources) staked mineral claims in the Fish Hook Bay regionimmediately east and north of the Nicholson property. Eldoradothen commenced prospecting for uranium throughout the area andsucceeded in identifying hundreds of uranium showings, which ledto the driving of an exploration adit near Martin Lake in 1948and the sinking of two prospect shafts in 1949, the Eagle and Aceshafts (Fig. 1).

The greatest success from underground exploration and devel-opment was achieved from the inclined Ace shaft situated parallelto, and in the footwall of the St Louis Fault. After a decisionwas made to go into production the Ace shaft was deepened to avertical depth of approximately 240 m and a vertical productionshaft (Fay) was collared in 1951, approximately 1200 m west of theAce shaft. Further underground shaft development included theVerna shaft approximately 900 m east of the Ace shaft in thehanging wall of the St Louis fault and two Winze shafts todevelop the Fay and Verna orebodies at depth. The Fay Winzeshaft achieved a vertical depth of approximately 1.8 km belowsurface. Lateral underground development from these shaftstotalled in excess of 136 km by the time the mine closed.

Several satellite mines were developed on the Eldoradoholdings and provided supplementary ore at various times through-out the life of the property. These included the undergroundHab, Dubyna and Martin Lake mines and small open pits in theVerna, Eagle, West Fay, and Dubyna areas, the largest pit beingthe Böiger pit located east of the Verna shaft.Increasing production costs coupled with declining market pricesand ore reserve grade necessitated the termination of productionfrom the area in June 1982.

Production for the Eldorado properties is summarised intable 1. If production is included from other operations the

272

TABLE 1ALL TIME MINE PRODUCTIONELDORADO RESOURCES LIMITEDBEAVERLODGE OPERATION

Mine Average grade Tonnes

Fay 0.28 12,305Verna 0.19 6,934Hab 0.43 934Open Pits 0.20 657Dubyna 0.22 195Martin Lake 0.30 7Eagle 0.10 1Total 0.25 . 21,033

total for the district becomes 33,713 tonnes U000 with most of3 Othe additional production coming from the Gunnar Mine at thesouth end of the Crackingstone peninsula and from small under-ground orebodies located west of the Black Bay Fault.

REGIONAL GEOLOGY AND STRATIGRAPHYFigure 2 is a stratigraphie column of geology and stratigr-

aphy of the area containing uranium deposits of Eldorado's miningoperations. Most of the rock nomenclatures used throughout thehistory of the operations have been included for cross referencepurposes.

The Tazin Group comprises a large variety of metamorphicgranulite, sedimentary and volcanic rocks. The major uraniumhost rocks are contained within this group and in a broad zoneof mylonitization and faulting. Although significant in surfaceexposure, the Murmac Bay Formation did not contribute in asignificant way to the uranium production for the area. TheMartin Formation is made up of the remnants of a Helikian supra-crustal basin lying unconformably on the Tazin Group. The rocksconsist of a succession commencing with a basal angular conglom-erate through a rounded upper conglomerate grading into anarkosic sandstone which in turn grades into a sequence of red-bedsiltstones. The Formation also contains andésites and basalticflows. A small amount of uranium production was achieved from

273

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Z2S_lUJX

z

CDUlXa<

ELDORADO RESOURCES LIMITEDBEAVERLODGE OPERATION

X

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1490 •

1799 •1 630 •«

1975 •

2100 •

23003200 ••

GEOLOGY LEGEND

MARTIN FORMATION-1400

Diabase

Stltstone and Shale

>Andésite, Porphyry, Basalt**

Arkose and Sandstone

Conglomerate

i8oo TAZIN GROUPMETASEDIMENTS

%Pegmatite, Pegmatitic Alaskite

Orange Mylonite, Feldspar rock.Mylomtic Alaskite

Mylomtic Mica Schist/Para Gneiss,Argillite, Paraschist* Meta Argillite. Migmatite

Epidotic Argillite, Greenschist, „. . ,. ... ., ,_ , , Siliceous Argillite, Ultra MylonitePhyllomtic Amphibolite

Quartzite Silica, Siliceous Mylonite

Diopside, Cale-Silicate rocks Dolomite

-2100 n/MUAi rtcnu i kvc r'ucicc

Alaskite,* Leucocratic Gneiss ... . fn „Mylonite «4O% Silica)

(Mafics <I5%)

Amphibolite

22?9 FOOT BAY GNEISS

Granitic crush rock Ultra Mylonite

ÄCataclasite, Augen Gneiss •• Amphibolite(Mafics >I5%) Kakinte

Pig. 2 Stratigraphie column

the basaltic-flows and from within the basal conglomerates, attopographic lows in the unconformity.

Structural features of the area include the St Louis Fault(Fig. 1) which strikes northeast and dips 50° to the southeast andalong which the Tazin host rocks for the ma3or uranium producingdeposits are distributed. North of the St Louis fault is theDonaldson Lake anticline, a broad fold structure which plungesgently to the southwest with its southeast limb lying parallel tothe St Louis fault and its northwest limb being offset by theBlack Bay fault (Section A-A1, Fig. 1). The Black Bay fault alsostrikes northeast, is situated 6 km west of the St Louis fault,and is slightly offset by the St Louis fault. These two fault

274

structures are connected by the ABC fault which dips approximately45 to the southwest, abuts the Black Bay Fault at the southwestend of Fredette Lake and splays off the St Louis fault in thevicinity of the old Eldorado townsite. The other majorstructural feature of the area is the Martin Lake syncline whichis a synclinal trough with maximum dips up to 75° and whichplunges to the north where it is offset by the ABC fault.

Although uranium mineralisation occurs in nearly all Tazinand Martin rocks, the major concentrations lie within meta-sediments of the Tazin Group distributed along the St Louis fault.Lesser amounts of uranium ore production has been achieved frombasal Martin rocks west of the Fay Mine shaft and very minoramounts from volcanic rocks of the Martin Group.

ST LOUIS FAULT AREAThe majority of rocks seen underground and on surface in the

immediate vicinity of the St Louis Fault are part of the TazinGroup. Figure 3 shows generalised sections of geology anduranium mineralisation on both sides of the fault. In generalterms the section commences with the Foot Bay Gneiss. This unit,which forms the core of the Donaldson Lake Anticline, isrepresented in the lower levels of the Fay Mine in the extremefootwall of the fault, as an augen gneiss, whose texture isprimarily a result of cataclastic deformation. It varies ingrain size with plagioclase augen ranging up to 3-4 cm indiameter set in a fine quartzo-feldspathic matrix. The unitdoes not host any significant uranium mineralisation and due toits distance from the fault is not often encountered in the mineworkings.

The second member of the Tazin Group is known as theDonaldson Lake Gneiss. This unit has also undergone severecataclastic deformation and within the mine is represented by afine grained highly laminated mylonite of granite composition.The laminations tend to be relatively flat lying where they areencountered in the extreme footwall of the fault but tend tosteepen as they approach the fault. Only occasionally do thesemylonites host uranium mineralisation and then usually at thecontact with overlying units. Near surface and away from theSt Louis fault however, this unit is not so highly mylonitised and

275

HANGING WALL DEPOSITS VERNA MINE

*c——~S ^-^

MW

HANGING WALL AND FOOTWALL DEPOSITSACE-FAY MINE

M a r t i n F o r m a t i o n

m e t a s o m a t i c g r a n i t ef e l d s p a r rockorange my lon i te

I I mica seh i s t ,a rg i l l i te

P-I Ï-IJ ep ido t i c a m p h i b o l i t e

s 11 ica ,si l i ceous my lon i t e

IDL I O o n a l d s o n L a k e g n e i s s

_X p i tchb lende orebody

Pig. 3 Sections showing generalized geology in hanging wall andfootwall of St. Louis Fault

hosts the Dubyna deposit, the 38 zone of the Hab Mine, and thedeposits of National Exploration.

The next series of rocks is a complex sequence which hoststhe majority of the uranium deposits. It occurs for a consider-able distance (>5 km) along the footwall of the St Louis faultwhere it hosts the deposits of the Fay Mine ana in a wedge locatedin the hangwall of the fault, which is formed by the St Louis, andLarum faults, where it hosts the Verna mine and related deposits.The sequence consists mainly of what is generally referred to asa mylonitic mica schist. It is composed primarily of albitefeldspar with accessory chlorite and muscovite and has beenintensely mylonitized. By far the greatest quantities of uraniumoccur in this rock as branching networks of veins usually display-ing cymoidal structural patterns. Near the base of the sequencethere usually occurs an amphibolitic unit which is thought to be

276

metavolcanic rock. It is represented by dark green to blackphyllonitic and epidotic amphibolite or chlorite-epidote rock.The unit is characteristically laced with a network of fineepidote veinlets, is associated with relatively large bands ofsilica or siliceous mylonite, and is the second most productiverock for uranium mineralisation. Although it provides anexcellent marker horizon for the base of the uranium producingsequence it does occasionally occur well up within the mylonitic

imica schist especially in the hanging wall of the fault.The uppermost unit of the Tazin Group in the vicinity of the

St Louis fault is a controversial rock indeed. It occurs as amylonite which is granitic in composition and frequently difficultto distinguish from the mylonites of the Donaldson Lake Gneiss.It contains megascopic porphyroclasts of plagioclase lying in afine grained quartzo-feldspathic matrix, in which the feldsparcontains fine inclusions of hematite giving the rock a character-istic orange colour. Due to this characteristic colour andtexture, the rock has been referred to as feldspar rock or orangeporphyroclastic mylonite by mine geologists and as a metasomaticquartz feldspar granite by others. The unit is much morebrittle than the underlying mylonitic mica schist and thereforetends to host uranium deposits in lens-shaped stockworksconsisting of tiny cross cutting fractures or breccia zones.Controversy exists as to whether the unit is an alteration ofunderlying units resulting from metasomatism or albitization ofthe underlying units adjacent to the fault, or is perhaps anancient paleoweathering of the underlying units. Uraniumdeposits occurring in this unit tend to be low grade, irregularin shape and extremely erratic in mineral content. Most of theproduction from the very deepest levels of the mine, along thefootwall of the fault was derived from this unit.

Martin Formation rocks also occur on surface and in theunderground workings in the St Louis Fault area. The mostcommon occurrence are the coarse angular basal conglomerateswhich occur in the hanging wall of the St Louis Fault, west ofthe Larum Fault. Uranium was mined from the conglomerate intopographic lows generally formed by down faulted blocks, which,within the limits of the underground workings, extended to adepth of 150 m. At one location uranium was mined adjacent to aMartin amygdaloidal basalt dyke in the footwall of the fault at a

277

depth of approximately 240 m. In access crosscuts driven southfrom the Fay Shaft toward the main footwall ore zone, porphyriticandésite dykes of the Martin Formation are exposed. Thisintrusive unit appears to occur as a single narrow dyke extendingup to within 840 m of surface and branches at depths into severaldykes at approximately 1060 m from surface, which is the limit ofthe exposure. In the immediate footwall and adjacent to thefault plane, Martin rocks have been seen to occur to depth inexcess of 1.8 km. Considerable controversy exists as to whetherthese rocks actually are Martin or a tectonic breccia. Howeverlarge blocks of polymict conglomerate and bedded and cross beddedarkosic sandstone containing lenses of siltstones have been mappedfrom surface to the deepest levels of the mine. Uranium hasbeen known to occur in close proximity to the Martin rocks in thefootwall of the fault but rarely within the rocks themselves.

Uraninite is the most abundant and widespread uranium bearingmineral in most of the host rocks. In the mylonitic mica schistand associated amphibolites, and especially in the lower levelsof the Fay Mine minor uranium depleted brannerite occurs as theextremities of the ore bearing structures. In the uppermostunit of the Tazin Group (orange mylonite-feldspar rock) bran-nerite is the most abundant ore bearing mineral where it occursin networks of Mg-chlorite veins at the core of the ore bearingstructures. At the ends of the structures the brannerite givesway to a uranium depleted variety of brannerite and uraniumbecomes enriched in a siliceous pitchblende rock, while theextremities of the structure are relatively brannerite free.

ORE GEOLOGYWith the exception of surface production from the Eagle

area, from Hab Mine and from Dubyna Mine all uranium productionfor the Eldorado Beaverlodge operation has been achieved frommylonitised metasediments and granitic rocks located within 400 mof the St Louis Fault. Figure 4 is a simplified stratigraphiesection indicating the proportion of total production plusremaining drill indicated resources for major rock units.

Upon termination of production from the operation an estimatewas made of remaining reserves and resources. Reserves areclassified as all uranium bearing rock from which production was

278

MARTIN FORMATIONz

uX

RELATIVE U CONTENT

CONGLOMERATEARKOSEANDESITE / BASALT

TAZIN GROUP (MYLONITIZED)

mtuxo.

METASOMATIC GRANITEFELDSPAR ROCKORANGE MYLONITE

MICA SCHISTARGILLITE

EPIDOTIC AMPHIBOLITESILICA , SILICEOUS MYLONITE

2O%

48«

26%

DONALDSON LAKE GNEISS 5%

UlOc

FOOT BAY GNEISS

Pig. 4 Proportion of totalUranium content in stratigraphie column

being achieved and for which mining development had been completedat the time of closure. Resources, on the other hand includesreserves plus all drill indicated uranium, whether under develop-ment or having been mothballed for possible future development,should economics warrant it. Therefore, in order to provide anestimate of the quantities of uranium within each unit of thestratigraphie section, production plus resources has been used.

By far the greatest amount of uranium was contained withinthe top 3 units of the Tazin Group. The largest producers werethe vein type deposits occurring in the mica schist and at thebase of the mica schist within the metavolcanics (epidotes argill-ites) and associated silica units. The veins generally displayedbranching cymoidal structural patterns both longitudinally andvertically, with those located in the footwall of the St LouisFault being the most elongated, continuous and of highest grade.Individual ore veins ranged from less than 0.1 m to more than1.5 m in width.

279

A considerable amount of uranium was produced from the upperunit of the Tazin Group, the mylonitized metasomatic granite ororange mylonite. Due to its more brittle nature this unit tendedto provide a stockwork of tiny cross cuttings fractures hostinguranium mineralisation rather than the more continuous vein of themica schist units. This unit generally lies stratigraphicallybelow the Martin-Tazin unconformity in the footwall of the StLouis Fault both on surface and at depth. Production from theupper unit of the Tazin therefore also includes some of thematerial from the base of the Martin rocks west of the Fay Shaft,in the hanging wall of the fault, where the stockworks orbreccias occurred in downfaulted topographic lows in the Martin-Tazin unconformity.

Most orebodies distributed along the St Louis fault plungedto the west and displayed a remarkable continuity with depth. Atthe time of closure the deepest production was being achieved froma stockwork breccia deposit in the footwall of the St Louis Faultat a vertical depth in excess of 1800 m.

FACTORS CONTROLLING MINERALISATIONMany factors have been reported to have a major controlling

influence on uranium mineralisation in the Beaverlodge area.The most dominant of those factors are listed as follows:

A. All Tazin Group host rocks are highly mylonitized butretain a recognisable stratigraphie order.

B. All uranium deposits are spacially related to majorfaults and occur in secondary structures.

C. Uranium bearing structures are confined to elongatedzones that are both stratiform and crosscutting andform cymoidal fracture patterns.

D. Wall rocks of uranium bearing structures are hematitised.E. All uranium deposits are spacially related to the

Martin-Tazin contact.F. Rocks in the vicinity of uranium deposits have been

subjected to some late stage soda-metasomatism in theform of albitization.

Although graphite is an important association with uraniumdeposits in the Athabasca basin, its presence in the Beaverlodgedeposits does not appear to be as significant. First it is

280

characteristically associated with ore bearing structures at thebase of the mylonitic mica schsits, within and adjacent to theepidotic amphibolite. Secondly, at one known location, it occurswithin an ore-bearing shear zone immediately below the MartinFormation west of the Fay Shaft. The third notable occurrenceis in ore bearing fractures in the uppermost unit (mylonite) ofthe Tazin Group in the Eagle Mine area, situated on the crest ofthe Donaldson Lake anticline. In all cases the graphite occurson slickenside faces but is not considered to be sufficientlyubiquitous to be a major factor in the control of uranium miner-alisation in the Beaverlodge area.

In addition to the above, geochronology of uranium depositsin the Beaverlodge area has been the subject of considerablework by a number of geologists. To address this subject indetail is well beyond the scope of this paper. In summary there-fore, age dates of epigenetic pitchblende mineralisation arehighly discordant. The earliest dates that have been recordedare in the order of 1700 to 1800 m.y. and coincidental with theHudsonian orogeny. Age determinations have also suggestedclusterings in the 900 to 1100 m.y. area and as recent as 200 m.y.

Statistics, group summaries with corresponding histograms,for a total of approximately 120 age determinations were made byRobinson (1955), Koeppel 1968 and Slater 1982 using Pb206/U238,Pb207/U235 and Pb207/Pb206 isotopic compositions.Histograms illustrate the suggestion of the above-mentionedclusterings, while the statistical summaries and plotssuggest that the two methods using the uranium-lead isotopes arecomparable but the lead-lead isotopes do not correlate as wellwith the other two and on the average indicate considerably olderages. The effects of episodic lead loss, and consequentlythe discordancy of the uranium ages, are therefore illustrated.

Fluid inclusion studies in an attempt to determine temperat-ure of formation have also been quite extensive and wide rangingin reported results. Temperatures have ranged from greaterthan 400°C to as low as 60-70°C. Sassano (1972) suggested thatdeposition may have occurred over a long period of time andperhaps that temperatures of formation have gradually decreasedover that time. It was not uncommon in the deeper levels of theFay Mine in particular to encounter pockets of highly saline

281

(CaCl) water under extremely high pressure. It was generallyfelt that these waters represented remnants of originalformational waters or hydrothermal fluids.

CONCEPTS OF GENETIC MODELSGenetic Models proposed for uranium deposits in the

Beaverlodge area have ranged from exclusively hypogene to exclus-ively supergene, depending largely what factors controllingmineralisation are considered to be significant. Neithergeochronological nor fluid inclusion studies have been able toprovide a definitive guide to a specific genetic model. Agedates of uranium mineralisation have, however, defined possibleperiods of remobilisation of uranium, the oldest coincidingroughly with the late stages of the Hudsonian orogeny, and themost recent being within the last 200 m.y. Fluid inclusionstudies also have provided a wide range of results, which for anysingle period could be interpreted to be a result of either deep-seated hydrothermal fluids or of relatively cooler surface waterheated purely by diagenesis.

What is most common to nearly all models is that uraniumprobably originated from weathering of the pre-existing Archeanterrane which contained uranium in granitic and pegmatitic rocks.The uranium was concentrated syngenetically in Apinebian pelitesand was then mobilised during subsequent metamorphic events.Genetic models vary at this point with respect to how and duringwhat metamorphic event the ma3or period of mobilisation occurredto bring the uranium to its present site of deposition. Proposalsrange from uranium being moved during and immediately followingmetasomatism via deep thrust faults produced by wide and deepzones of mylonitisation, to during the mylonitisation.

If the uranium was deposited syngenetically in Tazin sedi-ments, the present vein systems might reflect old stream channelsor shore lines in the Tazin sedimentary sequence. This wouldhelp to explain the remarkable continuity down plunge of thestratiform vein systems along the St Louis Fault as shown infigure 3.

If the uranium was moved up via deep thrusts associated withmylonitisation, the fault association and specific alterationhalos would be explained.

282

A supergene model would have uranium released through weather-ing of the Aphebian metasediments and older Archean rocks probablyduring Hudsonian times. Uranium solutions would migrate down-wards via open fractures in Tazin meta-sediments immediately belowelongated fracture controlled basins or valleys on the Martin-Tazinunconformity. Late tectonic activity during the Hudsonianorogeny and related to faulting, folding and subduction of Martinrocks along the northeast trending St Louis and related faults,resulted in the remobilisation of uranium downwards in tensionalfracture zones to its present depositional site.

The greatest amount of remobilisation would occur in theimmediate vicinity of the fault resulting in the greatest concen-tration and greatest preservation of the uranium mineralisation atthis location.

Areas away from the fault such as across the west of theDonaldson Lake anticline would contain erratic mineralisation infractures which tend to accumulate in elongated zones parallelwith the axis of the pre-Martin valleys. Most mineralisationseen at the present surface in these areas would thereforerepresent roots of eroded deposits, explaining the lack of struct-ural continuity.

In summary the complete genesis of the numerous uraniumdeposits in the Beaverlodge area remains uncertain. Mostworkers agree however, that the deposits are related to metamor-phic events and are the result of remobilisation within and fromdonor metasediments.

CONCLUSIONSThe Beaverlodge mining district has played a major role in

the history of Canadian uranium mining. Had the richer depositsassociated with the Athabasca Basin been discovered first, theBeaverlodge story might never have occurred. However, greaterbedrock exposures of mineralisation has naturally attractedattention to the Beaverlodge area and it was this attention thatlead to discoveries in the Athabasca Basin. It appears thatthrough mining and continuing discovery of new deposits in theAthabasca Basin that the interpretation of the geology of theassociated deposits is becoming increasingly more complex. There-fore, what might have initially been interpreted as substantial

283

geological differences between the Beaverlodge deposits and theAthabasca deposits will very likely be interpreted as beingsimilar and indeed merely as variations within a common geologicalenvironment.

ACKNOWLEDGEMENTSThe author wishes to thank the management of Eldorado

Resources Limited for granting permission to publish this paper.The contribution of our 85 geologists who were involved in theexploration, development and production of the Eldoraao Beaver-lodge operation are gratefully acknowledged.

SELECTED REFERENCESKOEPPEL, V., 1968: Age and History of the Uranium Mineralization

of the Beaverlodge Area, Saskatchewan. Geol. Surv. Canada,Paper 67-31.

SASSANO, G., 1972: The Nature and Origin of the UraniumMineralization at the Fay Mine, Eldorado, Saskatchewan.Ph.D. Thesis, University of Alberta.

SLATER, J.R., 1982: Some Relationships Between Deformation andOre Genesis at the Fay Mine, Eldorado, Saskatchewan.Unpublished M.Sc. Thesis, University of Alberta.

SMITH, E.E.N. (in press): Geology of the Beaverlodge OperationsEldorado Nuclear Limited. C.I.M. Spec. Publ., Geologyof Canadian Uranium Deposits.

TREMBLAY, L.P., 1972: Geology of the Beaverlodge Mining Area,Saskatchewan. Geol. Surv. Canada, Memoir 367.

284

GEOLOGY AND DISCOVERYOF PROTEROZOIC URANIUM DEPOSITS,CENTRAL DISTRICT OF KEEWATIN,NORTHWEST TERRITORIES, CANADA

A.R. MILLERGeological Survey of Canada,Ottawa, Ontario

J.D. BLACKWELLCominco Ltd,Vancouver, British Columbia

L. CURTISL. Curtis and Associates Inc.,Toronto, Ontario

W. HILGERUrangesellschaft Canada Ltd,Toronto, Ontario

R.H. McMILLAN, E. NUTTERWestmin Resources Ltd,Toronto, Ontario

Canada

Abstract

Proterozoic supracrustal successions, Central District ofKeewatin, are similar to the Proterozoic basins of northernSaskatchewan which contain economic uranium deposits. Becauseof the geological similarities, this district experienced adramatic increase in uranium exploration during the 1970's.Deposits of varying type were discovered in this new uraniumprovince indicating a potential comparable to the NorthernSaskatchewan uranium province. In this paper, examples of threeof the more -important uranium deposit types are described.

Early Aphebian shallow marine pelitic to psammitic meta-sediments of the upper Amer group host subeconomic uranium conc-entrations. Mineralisation interpreted as syngenetic to earlydiagenetic is restricted to drab to red fine clastic sedimentsthat display a moderate magnetic susceptibility.

Late Aphebian structurally controlled depressions, BakerLake Basin, Yathkyed Lake and Angikuni Lake subbasins, are

285

filled with continental sediments and volcanics of the DubawntGroup. Structurally controlled uranium mineralisation is mostcommon adjacent to the margins of these basins. The LacCinquante deposit located in the northeastern Angikuni Lakesubbasin is hosted in Archean metasedimentary and metavolcanicrocks immediately adjacent to the Dubawnt unconformity. Mineral-isation is localised in or in close proximity to a highly fract-ured and brecciated unit consisting of interbedded chloritictuffaceous and sulphide + graphite metasediments. In theeastern Baker Lake Basin uranium mineralisation is localised inthermal altered Kazan Formation arkose near alkaline dykecomplexes.

Conglomerate and sandstone of the Helikian Thelon Basin lieunconformably on a highly weathered basement complex. The LoneGull deposit is hosted in retrograded and altered psammitic topelitic metasediments and undeformed granite near the erosionaledge of the basin. Mineralisation is structurally controlledand associated with zones of intense K- and Mg-metasomatism.These features parallel those of the unconformity-relateddeposits of N. Saskatchewan,

INTRODUCTIONThe Baker-Thelon uranium district lies within one of the

most remote areas of Canada, in the Districts of Keewatin andMackenzie, Northwest Territories. This is a subpolar region,beyond the northern limit of forest cover, marked by subdued top-ography and extensive lake and river systems which can be icecovered for nine months of the year. Existing transport-ation infrastructure is minimal, comprising only float aeroplaneaccess, a single land air strip at the village of Baker Lake,and barge access 280 km inland from Hudson Bay through Chester-field Inlet and Baker Lake. The village of Baker Lake is situated640 kilometres north of the port and railway terminal Churchill,Manitoba.

In spite of the remote location and high costs of explorat-ion, the region has been subjected to several periods of intenseuranium exploration by both private and government-owned miningcompanies. The most recent exploration period dating from 1974to the time of writing, is largely the result of the recognition

286

of marked regional geological similarities to the productiveAthabasca Basin (700 kilometres to the southwest) and the PineCreek Geosyncline region in the Northern Territory of Australia(Curtis and Miller, 1980), and the fierce competition for landin the Athabasca Basin, which forced companies with a commitmentto uranium exploration but a restricted land position northward.Earlier discoveries in the Baker-Thelon area, made during theperiod 1968 to 1970, had demonstrated that uranium mineralisationwas present, though not as economically viable deposits, andtogether with the rapidly accumulating data from discoveries inSaskatchewan and Australia, set the stage for a major effortin the region and the establishment of a new camp (Bundrock,1981) .

This paper outlines the igneous and sedimentary record ofProterozoic successions in this district and uses this record asthe framework to outline the exploration history and geology ofthe most important uranium prospects and deposits. The geologyand uranium metallogeny are presented in three sections:A) Early Aphebian metasedimentary belts: i) Amer group consist-ing of shallow marine to continental deposits and ii) undiffer-entiated metasedimentary belts; B) Late Aphebian Baker LakeBasin and subbasins: Dubawnt Group continental sedimentaryand volcanic rocks: C) Helikian Thelon Basin: Dubawnt Groupcontinental and shallow marine sedimentary rocks.

A) Early Aphebian Metasedimentary beltsAmer groupEarly Aphebian metasedimentary belts are principally found

in the region bounded by the Baker, Aberdeen and Amer lakes(Figure 1). These belts are largely confined to the Armit LakeBlock (Heywood and Schau, 1978) which is bounded to the north bythe Amer-Meadowbank Fault and to the south by the ChesterfieldFault Zone (Schau et al., 1982).

The Amer group is preserved in a west-southwest trendingsouthwest plunging synclinorium that is covered in the southwestby the Helikian Thelon Formation. The belt is exposed for morethan 120 km with an average width of about 50 km. The Amer groupis interpreted as early Aphebian based on its correlation withthe Hurwitz Group of southern Keewatin (Bell, 1969, 1971).However the age of the Amer group is poorly bracketed between

287

(O<x>oo 66

106

66°

cSg-rff'ï*-V? J. t V*"gV'^f^' *

104102° 100

ORDOVICIAN

Fossiliferous limestone

HELIKIAN

v l Basalt

Dolostone, siliceous dolostone

T" I Thelon Formation: sandstone, pebbly sandstone, conglomerate

APHEBIAN

Porphyritic diorite-gabbro (includes McRae Lake dyke)

Granite: In part fluorite bearing and rapakivl textured

Syenite: quartz syenite; includes Martell intrusive suite

Amer group and equivalents

APHEBIAN and/or ARCHEAK

Pitz Formation

Christopher Island and KunwaX formations

South Channel and Kazan formations

DubawntGroup

Orthoquartzite, quartz pebble conglomerate,muscovite schist and phyllite,

Basic metavolcanics, metapelites, calc-silicate rocks,graphitic schists, metagreywacke, iron formation

Migmatite, irregularly layered granitoid gneiss, paragneiss;amphibolite, cataclastlc augen gneiss

ARCHEAN

Ultramafics: komatiites, serpentenites

Basic metavolcanics, massive and pillowed flows, pyroclastics,tuff, and agglomerate; metagreywacke, mixed basic and intermediatemetavolcanics, amphibolite. carbonate iron formation, calc-sllicate rocks;quartzite, quartz pebble conglomerate, black slate, includes some gabbro

s^ Fault

(B) Baker Lake Basin

(Y) Yathkyed Lake sub basin

(A) Angikuni Lake sub basin

TF Tulennalu Fault

AM F Amer-Meadowbank Fault

C F Chesterfield Fault Zone

if Uranium deposit, prospect

Location of the Thelon and Baker Lake Basins,Districts of Keewatm and Mackenzie,

Northwest Territories, Canada

**drpi ~-%CV

toooFig 1. Geology of Central District of Keewatin, N.W.T. (from Miller ana

LeCheminant, in press)

K)VOO

Table 1. Stratigraphie column of the Amer group, Central District of Keewatin

Lithologyquartz arenite, arkose, maroonsandstone, shale and siltstonerare stromatolitic dolostone.

with

upper clasticsequence

fine grained pink feldspathicarenite, interlaminated arenite andsiltstone, calcareous trough-crossbedded arenite, local conglomeraticbreccia; capped by massive tolaminated siltstonegrey black, green and maroon shalesand siltstone, interbedded finegrained feldspathic and quartzarenite, thin stromatolitic dolostonelaminated and locally stromatoliticdolostone, with minor intercalatedshale, fine-grained quartz arenite,chert and conglomerate; very localvesicular mafic flows and sills.pyritic grey to black carbonaceoussiltstone and shale, interbeddedchert and fine grained quartzarenite.basal thicK quartz arenite and quartz-

lower clastic pebble orthoconglomerate with minorsequence shale and iron formation.

Remarkpreserved only at the western end of thebelt

cyclic with overall upward fining trend;main uraniferous unit at interfacebetween arenite and siltstone

upward coarsening trend

transitionalsequence

fluvial to near shore marine

2801+24/-21 Ma (U/Pb, zircon) from the basement complex adjacentto the Amer group (Miller and LeCheminent, in press) and1849+18 Ma (U/Pb zircon) from an undeformed quartz syenite thatintrudes Amer metasediments (Telia and Heywood, 1978).

Stratigraphy and sedimentology of the Amer group have beendescribed by Heywood (1977) , Tippett and Heywood (1978) , Knox(1980), Patterson (1981) and Telia et al. (1983); see Table.1.The group consists predominantly of two clastic sequences, alower white orthoquartzite and an upper interbedded sequence offeldspathic sandstone, siltstone, mudstone and arkose. Thesetwo sequences are separated by a transitional sequence thatincludes thin discontinuous dolomitic limestone at the base andintercalated mudstone-siltstone at the top. Minor mafic igneousunits occur along the fold belt at approximately the samestratigraphie level as thin locally conformable flows (Patterson,1981) and as gabbroic sills (Telia et al., 1983).

Metamorphic assemblages in the northeast segment of the foldbelt indicate upper greenschist-lower amphibolite faciès regionalmetamorphism (Patterson, 1981) with the grade decreasing to sub-greenschist in the south-western end of the belt (Knox, 1980,Telia et al., 1983). Deformational events affecting the meta-sedimentary sequence are: i) west trending folds and northerlyverging thrust faults, ii) southwest trending folds, andiii) northwest trending normal faults (Tippett and Heywood, 1978;Knox, 1980; Patterson, 1981; Telia et al., 1983).

Exploration for and geology of uranium mineralisation inthe Amer groupThe discovery of economic uranium deposits in Wollaston Group

metasedimentary rocks adjacent to the Athabasca Basin,N. Saskatchewan in 1968, prompted Aquitaine Company of Canada toconduct reconnaissance airborne surveys in other geologicallysimilar regions. Reconnaissance airborne radiometric and magneticsurveys were conducted over the northeastern portion of the Amerbelt because of the abundant outcrop compared to the southwesternportion. Radiometric anomalies were identified which initiateda program of ground radiometric surveys, prospecting, soil geo-chemistry and diamond drilling (Aquitaine Company of Canada,1970; Laporte, 1974). Re-evaluation of the northeast Amerbelt by Cominco in 1977 involving detailed mapping, radiometric

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S(r«tibouA4 urinium m Ann groupclMlIC W

Eirly Aphchiu

Bisemenl complex

Rtgolith

O C_^ Uranium occurrenceAmgr group

tndmelasedirnonttry bt/ti

Fig 2. Schematic cross section, Schultz andAmer Lakes, Northeast Thelon Basin

surveys and diamond drilling indicated subeconomic uraniumconcentrations. The discovery of uranium mineralisation byWestmin Resources Ltd in the southwestern portion of the Amergroup has influenced the exploration for unconformity-relatedmineralisation in that region.

Stratiform and stratabound uranium with minor base metals ishosted by several lithologies within drab and red fine clasticunits situated near the boundary of the transitional and upperunits of the Amer group (Figure 2). Much of the uranium miner-alisation is restricted to this stratigraphie position along thelength of the synclinorium (Miller and LeCheminant, in press).Mineralised zones, lenticular in plan and section, are comprisedof stacked discontinuous concordant psammitic and pelitic beds.These zones may have a composite strike length to 1000 m andthickness of 3-5 m. Mineralised zones show variable grades of0.05 to 0.18% LUOo over 1-2 m (Laporte, 1974; Lord et al., 1981),J o

Mineralisation is contained in a wide variety of lithologiesof the upper Amer group: siltstone, sandy siltstone, feldspathicsandstone and calcareous equivalents (Knox, 1980; Taylor, 1978;Curtis and Miller, 1980). Rapid lateral and vertical litholog-ical variations of the interstratifled mudstone-siltstone-feld-spathic sandstone are interpreted to have been deposited in atidal flat-restricted bay environment. Mineralisation localisedwithin siltstone, along silt-sand interfaces or within sandy bedsexhibits a consistency of both macro- and microscopic features,

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alteration assemblages and elemental associations along theentire length of the Amer group. The mineralised lenses have anelevated magnetic susceptibility and a red coloration whichcontrasts with drab hues common in adjacent strata. Uraninite,coffinite and subordinate U-Ti-Si compounds occur as ultra fine(1-10 microns) disseminated grains replacing framework feldsparand interstitial to quartz-feldspar framework grains in psammiticbeds and as disseminated and spheroidal aggregates in peliticbeds (Curtis & Miller, 1980). Disseminated pyrite, galena,chalcopyrite, bornite, molybdenite,trace arsenopyrite andcobaltite are associated with uranium mineralisation. Radio-active strata are characterised by goethite and celadoniteinterstitial to and replacing feldspar framework grains, hematite,disseminated authigenic subhedral to euhedral magnetite andinterstitial iron-chlorite and quartz. Complex metal assembla-ges and early diagenetic alteration support the interpretationthat the stratiform mineralisation is comparable to sandstone-type mineralisation which is controlled by initial porosity,permeability and redox barriers.

Mineralised zones explored to date do not constituteeconomic ore bodies because of i) remote location, ii) size,form and grade of the lenses, and iii) geometry of the strati-form lenses due to multiple deformation. However potentiallyeconomic zones of uranium mineralisation may occur in foldclosures where mining thickness could be created due torecumbent folding.

Exploration conceptsThe following exploration concepts have been useful in

prospecting and delineating the stratiform mineralisation in theAmer group: a) recognition of the mineralisation to be restric-ted to a specific lithostratigraphic position and environment,b) recognition of a primary sedimentologic control, andc) coincidence of mineralised strata with moderate magneticsusceptibility.Undifferentiated metasedimentary belts

Several discrete sedimentary and volcanic-bearing sequencescontaining orthoquartzite strata are recognised south and south-east of the Amer group (Ashton 1981, 1982; Schau et al., 1982)and are termed Archean and/or Aphebian (Figure 1). Polyphase

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toVD-P- Table 2. Stratigraphy of the Dubawnt Group, Baker Lake Basin and subbasins

Unit

Mackenzie Diabase

Thelon Formation

Basic Intrusions(McRae Lake dyke)

Granite

Basalt

Pitz Formation

Kunwak Formation

Christopher Island Formationand related alkaline intrusivecomplexes (includes KartellSyenites)

Basal clastic successions(Kazan Formation, SouthChannel Formation, AngikuniFormation)

Lithologies

medium to coarse grained gabbro and quartzgabbro

conglomerate, sandstone, pebbly sandstone

porphyritic leucogabbro; medium grainedgabbro and quartz monzonite

alkali-feldspar granites; porphyriticbiotite-hornblende, biotite and biotite-muscovite granites

black amygdaloid basalt

rhyolite, minor rhyodacite and dacite;rhyolite breccia and conglomerate, redlithic sandstone and siltstone

conglomerate, lithic sandstone, siltstone,and mudstone; minor amygdaloidal dacite

trachybasalt, trachyandesite and trachyte;volcaniclastic conglomerates, wacke,sandstone and siltstone

medium to coarse grained syenite-quartzsyenite; minor biotite pyroxenite;biotite lamprophyre

polymictic conglomerate, red to greyarkose and arkosic wacke, siltstoneand mudstone

Remarks

NW trending: 1204 Ma (Rb/Sr)

NE trending dyke-like intrusions:1735 Ma (K/Ar)

circular to multilobate; fluorite-bearing; granophyric, miarolitic torapakivi textures

intercalated with laminated green siltstone

fluorite and topaz-bearing; interflowquartz-rich sediments represent debrisflow and stream channel deposits

alluvial fan, braided river and lacustrinedeposits

subaerial potassic alkaline lavas andpyroclastics with interflow fluvialdeposits: 1786 Ma (Rb/Sr) to 1830 (K/Ar)

small stocks, sills and dykes: 1832 Ma(K/Ar)

alluvial fan and braided fluvial deposits,minor eolian deposits; clastic wedgesderived from east and southeast.

folding, northerly directed thrusting and lithological differenceshave made stratigraphie correlations with the Amer grouptentative. Some of these sequences may be equivalent to theArchean Prince Albert Group, Melville Peninsula (Schau et al.,1982). Scattered uranium occurrences are known throughout thesesequences however their position in the stratigraphy, alterationand associated minerals is poorly known (Curtis & Miller, 1980).

B) Late Aphebian Baker Lake Basin and subbasinsSedimentary and volcanic units of the Dubawnt Group were

divided into six formations by Donaldson (1965) and two format-ions of more local extent were established (Blake, 1980;LeCheminant et al., 1979a; see Table 2). The most completerecord of the Dubawnt Group is preserved in the Baker Lake Basin.This complex graben, 50 km in average width, can be traced morethan 300 km from eastern end of Baker Lake to Tulemalu Lake(Figure 1). Isolated Dubawnt Group successions west ofTulemalu Lake and southeast of Dubawnt Lake may be faultedremnants of a southwestern extension of the basin. West ofForde Lake, the Baker Lake Basin is bounded by Tulemalu Fault.To the northeast the basin extends across the projection of thisfault. East of the Tulemalu fault the boundaries of the BakerLake Basin are generally parallel to earlier structural trends inthe basement and are defined by unconformities or steep normalfaults. Here the basin has a generally asymmetric cross-sectionwith northwest dipping basal red beds sequences overlain by athin veneer of alkaline volcanic rocks (Donaldson, 1965, 1967;Blake, 1980).

Two smaller intracratonic basins, the Yathkyed Lake subbasin(Curtis and Miller, 1980) and the Angikuni Lake subbasin (Millerand LeCheminant, in press) are southeast of the Tulemalu fault.Structurally and stratigraphically these subbasins are similar toeastern parts of the Baker Lake Basin (Bade and Blake, 1977;Blake, 1980).

Initial development of the Baker Lake Basin and subbasinswas strongly influenced by the pre-existing tectonic frameworkparticularly major faults. Basal redbeds of the South Channeland Kazan formations were deposited in response to faulting andshedding of debris from adjacent highland areas. These format-ions are interpreted as alluvial fan-braided river deposits

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having a source area to the south and southeast (Donaldson, 1965,1967) .

Redbed sedimentation was succeeded by widespread voluminoussubaerial volcanic eruptions that produced potassic alkalinelavas, pyroclastics and comagmatic stocks, sills and plutons ofthe Christopher Island Formation. Alkaline flows are inter-bedded with debris flows and fluvial deposits which display thecomplex interaction between volcanism, faulting and fluvialdeposition. The Christopher Island Formation is not diachronous(Curtis and Miller, 1980) as it is probably the only valid time-stratigraphic event in the Baker Lake Basin, instead its relation-ship to a wide variety of underlying units reflects therelatively independent histories of deposition in the varioussubbasins of the Baker Lake Basin (Blackwell, in prep.).Volcanic rocks of the Christopher Island Formation are alkaline,with shoshonitic affinities (Blake, 1980) and are characterisedby high K20 + Na20,K 0:Na20 >1, low Fe, Ti contents, high Ba andP contents and erratic but high U and Sr contents. Coveringan area of 5 x 10 km , the Christopher Island constitutes thelargest alkaline province in the world. Whole rock Rb/Sr andmineral K/Ar radiometric dating indicate a minimum age foralkaline volcanism of about 1830 Ma (LeCheminant et al., 1979b).

Continued block faulting following the waning of alkalinevolcanism created local subbasins that were infilled with KunwakFormation red clastic sediments. Depositional environmentsrange from alluvial fan to braidplain to lacustrine (LeCheminantet al., 1981).

Large scale magmatism resumed with rhyolitic eruptions andemplacement of epizonal granite plutons. Pitz Formation iscomprised of high-K, silica-rich, topaz-bearing rhyolite lavas,lithic-crystal tuffs with intercalated quartz-rich volcaniclasticrocks. This sequence records a history of contemporaneoussilicic volcanism, mass wasting and stream-channel deposition.Pitz volcanic rocks are comparable to topaz rhyolites as definedby Burt and Sheridan (1980) and Burt et al., (1982). Elongatemultilobate plutons of rapakivi granite intrude Dubawnt Grouprock west of Tebes3uak Lake. Northeasterly trending bodiesof porphyritic leucogabbro-quartz monzonite, including the McRaeLake dyke, intrude the latter rocks and have yielded a whole

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rock K/Ar date of 1735 ± 52 Ma. The youngest intrusions in thearea are regionally extensive MacKenzie diabase dykes(whole rock Rb/Sr 1204 + 39 Ma, Patchett et al., 1978) whichintrude isolated cappings of Thelon Formation west of TulemaluFault (Figure 1). In summary the Baker Lake Basin and subbasinsdeveloped in a continental extensional regime at about1.9-1.8 Ga.

Uranium mineralisation has been recognised in each formationof the Dubawnt Group and basement complex to the Baker Lake Basinand subbasins (Miller and LeCheminant, in press). Zones ofbrittle failure are potentially the best exploration targets andhave yielded the largest number of occurrences (Miller, 1980)with the highest uranium concentrations. Structurally controlledmineralisation is spatially related to: i) faulted marginsof the basins, ii) fault and cataclastic zones cutting oradjacent to the basal unconformities, iii) specific lithologiesin the Archean-Aphebian basement and Dubawnt Group, andiv) structures that have suffered repeated movement. The mostsignificant fracture controlled uranium deposit discovered todate, Lac Cinquante deposit, is situated near the northeasternrim of the Angikuni Lake subbasin.

Exploration and Geology of the Lac Cinquante deposit,Angikuni Lake subbasinThe Lac Cinquante deposit contains drill indicated reserves

of 5.3 million Kg tUOg (Northern Miner, 1982). The initialdiscovery of mineralisation on the property was made by Pan OceanOil Ltd. in 1974 during a reconnaissance airborne radiometricprospecting program. At the time of discovery, only 1:1,000 000scale maps were available in the region of Yathkyed-Angikunilakes; however the rims of these subbasins were targeted basedupon the success of Pan Ocean's exploration along the easternmargin of Baker Lake Basin in the late 1960's. Uranophane-bearing conglomerate was detected during the airborne survey andprovided the impetus for ground acquisition. Follow-up workincluded regional airborne magnetic and electromagnetic surveysand detailed ground scintillometer, magnetic and VLF surveys. Onthe Lac Cinquante property, detailed ground prospecting revealednumerous fracture controlled uraninite-hematite-carbonate veinswithin Archean metavolcanic rocks which are basement to theDubawnt Group on the property. The key to ongoing exploration

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was the recognition by Pan Ocean personnel of the associationbetween VLF conductors and uraninite-bearing vein systems.

The northeast terminus of the Angikuni Lake subbasin isunderlain by an Archean greenstone belt which on a regional scaleis subordinate in volume to granite gneiss and granodiorite.This greenstone belt is correlated with the Henik Group ofsouthern Keewatin (Bade, 1980). Fault and mylonite zonestrending E-W, NE and NW transect this basement complex andcontrol the distribution of Dubawnt Group rocks (Bade, 1980).

The deposit is hosted in a sequence predominantly ofpropylitized pillowed and massive basic metavolcanics, meta-gabbros and subordinate massive and fragmental felsic meta-volcanics with related metasediments. Few lithological markersare apparent in this sequence, the one exception to this general-isation is a thin, 1-2 metre, tuffaceous chert and chert-graphite-sulphide unit which occurs discontinuously through theprincipal zone of mineralisation. This zone is termed the MainZone. Similar cherty carbonate and graphitic metasediments ofpossible exhalative affinity have been noted in outcrop and indrill core elsewhere in the district.

Greenschist grade basaltic to andesitic volcanics consistof chlorite, epidote, relict plagioclase (An3 5) with minorquartz, clino-amphibole, albite, carbonate, pyrite, sphene,anatase and sericite. Interbedded tuffaceous metasedimentscontain quartz, chlorite, sericite and carbonate with minorpyrite and Fe-Ti oxides. The chert-sulphide marker unit, some-times associated with U-Mo mineralisation, consists of chlorite,quartz, a carbonaceous chlorite mix, sericite, pyrite and rarechalcopyrite and sphalerite.

The contact between the greenstone and overlying DubawntGroup is marked by an increase in hematite and to a lesserextent carbonate. This hematite-carbonate paleo-weatheringprofile is up to 5 metres thick with pronounced variations thatreflect pre-Dubawnt paleo-topography.

Coarse grained conglomerate of the late Aphebian DubawntGroup overlies the basement complex. Boulder to pebble sizedgranitic to granodioritic gneiss framework clasts characterisethe conglomerate although lithologies of the greenstone terrain

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are present. Basal conglomerate is overlain conformablyand gradationally by pink to red arkosic grits, siltstone, mud-stone and conglomerate.

Minor clasts of Dubawnt mafic volcanic rocks in the basalred bed succession indicate alkaline volcanism accompaniedalluvial fan-braided river sedimentation. Because of this minorvolcanic component, the basal redbeds have been mapped asChristopher Island Formation (Bade, 1980) but they occupy thesame stratigraphie position and display essentially the samelithological relationships as South Channel and Kazan formationsin the Baker Lake Basin (LeCheminant et al., 1976, 1977, 1979a,b;Donaldson, 1965). Christopher Island Formation phlogopite-phyric lavas, lapilli tuff and intercalated volcanic-richsediments overlie the redbeds (Blake, 1980). The alkalinevolcanics display elevated U, Th, Ni, Cr, Ba, and Sr contents.Lamprophyres and sill-like intrusions comagmatic with volcanicrocks occur throughout the stratigraphy.

In the vicinity of the deposit, major penetrative faultsand shears are subparallel to WNW foliation. The principalconductive zones, one of which coincides with the ore deposit,also strike in this direction. Extensive shearing sub-parallelto foliation is indicated by transposition of pillows andlocally extreme stretching of pillows. Although the Main Zonedirection is indicated by VLF conductor axis at 290°-300°,most of the surface mineralisation spatially associated with theMain Zone is controlled by a set of cross fractures, termed gashveins, trending 040 -060 . These vein systems composed ofcarbonate + hematite + uraninite with subordinate chalcopyrite,pyrite and rare arsenopyrite are discontinuous and of variablewidth, 1-15 cm.

The Main Zone structure, approximately 1100 m long,coincides with a band of volcanogenic and volcaniclastic meta-sediments ranging from 1 to 8 metres and averaging 2.5 metres inthickness. Mineralisation occurs within this stratigraphieunit or in very close proximity to it and has the form of plungingpods of uraninite veins (Miller et al, in press). The ore zoneis 300-400 m long with an average width of 1 metre and has beendrill tested to 270 m depth. Extensive fracturing and localisedcataclastic textures characterise the Main Zone and brecciationis typical of the cross-cutting 'gash veins'.

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A complex metal assemblage, U + Mo + Ag + Pb + Cu + (Zn)characterises the Lac Cinquante Main Zone ore body. This typeof mineralisation is similar to structurally controlled occurr-ences in the Baker Lake Basin (Miller, 1980}. U, Mo and Agare potentially economically recoverable in the deposit.Hematite, chlorite, Fe-carbonate and albite are the commonestgangue minerals forming diffuse envelopes around veins andfilling veins. Uraninite occurs as vein and fracture fillingsin brecciated metavolcanics and metasediments, as selvages tohematitised volcanic fragments in breccias and disseminated inpervasively altered wall rock. Very fine grained felty molyb-denite accompanies uraninite and occurs with minor quantities ofchalcopyrite. The mineral containing silver has not beenidentified.

Summary and Genesis of the Lac Cinquante DepositThe deposit is a structurally controlled vein system cont-

aining complex U + Mo + Ag + Pb + Cu + (Zn) mineralisation.The cherty, sulphidic and graphitic interflow sediments may wellhave influenced the localisation of the host structures andcreated a local reducing environment for the precipitation ofmineralisation. These earlier sediments may have containedtraces of primary pyrite + sphalerite + chalcopyrite mineral-isation .

The Main Zone mineralisation has been emplaced in a WNWstriking structure which intersected the late Aphebian unconform-ity. Cross-cutting NE and ENE veins were also mineralisedduring the main event. The mineralising fluids were oxidisingand characteristically CO„-rich and were enriched in U, Mo, Ag,Cu, Pb and to a lesser extent As, and Na.

Explorationists in this region have observed the preferent-ial association of uraninite veins in greenstones near the basalDubawnt Group unconformity as compared to granite and granitegneiss and this has been used as an empirical exploration guidein the region. One of the genetic implications of this obser-vation could be that the ultimate source of the uranium is theoverlying Dubawnt Group volcanics and clastic sediments, whichas previously noted are high in uranium and also coincidentallycontain U-Mo mineralisation. Basic metavolcanics and interflowsediments appear to be more favourable hosts than granites or

300

granite gneisses and this may signify a redox effect which is notyet understood. The mineralisation and its structural settinghas invited comparisons between the Lac Cinquante deposit andthe Beaverlodge vein-type deposits in Northern Saskatchewan(Tremblay 1972, 1978; Miller, 1980; Miller and LeCheminant, inpress).

Exploration ConceptsThe following exploration concepts have proved useful in

prospecting and outlining the deposit: (a) recognition that alate Aphebian unconformity truncates a sequence of Archean meta-volcanic and metasedimentary rocks; (b) evidence of enrichmentin uranium in the overlying late Aphebian sequence; (c) recog-nition of dominant structural control; (d) coincidence of VLFconductors with mineralised structures; and (e) some litholog-ical (redox?) control of mineralisation by cherty-sulphide andgraphitic interflow sediments. These same units may also haveexerted an influence on the localisation of structures.

Exploration and geology of epigenetic strataboundmineralisation, Baker Lake BasinIn 1974-75 Cominco Ltd entered a joint venture with Pan

Ocean Oil Ltd to explore the eastern Baker Lake Basin (Laporteet al., 1978). Radiometrie anomalies detected by airbornesurveys were followed up with regional and detailed geologicalmapping, prospecting, geochemical surveys, a variety ofgeophysical surveys and diamond drilling.

Uranium mineralisation in Kazan Formation arkose and SouthChannel conglomerate has been described in detail by Stanton(1979) and Miller (1980). Mineralisation comprising dissemin-ated uraninite, coffinite, U-Ti-Si compounds, digenite,covellite, chalcopyrite, bornite, native copper and native silveroccurs within laminar and cross-bedded arkose, arkosic siltstoneand conglomerate peripheral to crosscutting xenolithiclamprophyre dykes. The outer portion of the thermally alteredsediments commonly grey, mottled pink to cherry red contain thehighest grades of uranium-copper-silver mineralisation: uranium,to 0.31%, copper to 3.5%, silver to 15 ppm and anomalous concen-trations of ,V, Mo, and Ba (Miller, 1980; Stanton, 1979).However the epigenetic cross-cutting mineralised zones are small,irregular, discontinuous along strike and confined to arkose and

301

conglomerate beds having higher porosity and permeability.Alteration assemblages include albite, hematite, anatase,carbonate minerals and quartz. Miller (1980) considered themineralisation process to be a two-stage event. Metallic,sulphide influx and precipitation occurred due to convective flowof groundwater accompanying dyke emplacement and thermal and Ehgradients in the outer portion of the thermal aureole. Later,uranium was precipitated from oxygenated groundwater in zones ofpre-existing sulphides. Stanton (1979) considered mineralisationto occur entirely during dyke emplacement and resulted from redoxreactions between the lamprophyre dyke and heated metal-bearinggroundwater.

Exploration conceptsThe following concepts proved useful in prospecting for

epigenetic mineralisation within redbeds: a) distinctive colourchanges; b) localisation of mineralisation to specific poroushorizons in the arkosic sequence; and c) proximity of alkalineintrusive bodies.

C) Helikian Thelon BasinThe Thelon Formation as outlined by Wright (1967) and

Donaldson (1969), is largely preserved in an arcuate northeasttrending basin which lies principally to the west and northwestof the early Aphebian belts and late Aphebian basins (Figure 1).

Thelon sediments lie unconformably upon a diverse basementcomplex of Archean tonalitic to granitic gneisses, Aphebianmetasediments of the Amer group and interpreted equivalents andlate Aphebian continental sediments and volcanics of the DubawntGroup. Basement lithologies at and adjacent to the perimeter ofthe Thelon Basin display intense hematization and argillizationand record a period of lateritic weathering prior to and duringinitial Thelon sedimentation (LeCheminant et al., 1983). Depthof weathering is variable based on lithology but feldspathicmetasediments and granitoid gneiss can be weathered to depthsgreater than 50 m.

Sedimentology of the Thelon Basin has been studied byDonaldson (1969) and Cécile (1973) . Cécile (1973) subdividedthe basin on the basis of various physical and sedimentologicparameters into four 'faciès' that broadly mirror the preserved

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form of the basin. The basal 'faciès1 of the Thelon Formationis commonly conglomeratic and interstratified with pebbly arkoseand siltstone. Basal coarse to medium grained siliciclasticscan be cemented by authigenic clay + quartz or syngenetic to earlydiagenetic uraniferous fluorapatite, goyazite and illite.Uraniferous phosphates have been found throughout an extensivestratigraphie interval in the Thelon Basin, from the sub-Thelonsaprolite to more than 300 metres above the basal unconformity(Westmin Resources private reports). A minimum age for theThelon Formation of 1660 Ma (Pb/Pb, fluorapatite) was determinedfrom a phosphate-cemented pebbly arkose overlying the weatheredbasement complex (Miller, 1983). Ninety percent of the basin ischaracterised by mature sandstone, quartz arenite and lithicsubarenite which are interpreted as continental to nearshoremarine in origin (Donaldson, 1969). This thick sandstonesequence corresponds to the upper three "faciès1 of Cécile (1973).Multicoloured siltstone-sandstone intercalate with and increasein abundance near the top of the sandstone sequence. These fineelastics are overlain by shallow marine stromatolitic andoolitic dolostone-siliceous dolostone. Basalt is spatiallyassociated with the upper most lithologies, however, itsstratigraphie relationship is uncertain.

Exploration and geology of the Lone Gull depositIn 1973, Urangesellschaft Canada Ltd began a grassroots

uranium exploration program in the Central District of Keewatinfor unconformity-related mineralisation based on the available1:1,000,000 map of Wright (1967) and on then current conceptspertaining to this deposit type. Geochemical lake water andlake sediment surveys and a helicopter-mounted airborne systemwere undertaken on permits obtained in 1974 (Bundrock, 1981). Asignificant airborne uranium anomaly was located in highlyglaciated and drift covered terrain near the erosional edge ofthe southeastern Thelon Basin, south of Schultz Lake. Groundfollowup detected a uraniferous mudboil. In addition thedrainage geochemical survey disclosed anomalous radon in a lake,3 km south of the deposit. This led to an intensified explor-ation program in 1975 and 1976 that incorporated prospecting,detailed mapping, geomorphology surveys, soil geochemistry,scintillometer, magnetometer, VLF and IP surveys. Explorationdiamond drilling, initiated in 1977, intersected ore grade

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mineralisation of 1.0% U000 over 33 m (Northern Miner, 1978).3 oSince then the deposit, called Lone Gull, has undergone extensiveexploration drilling to 1983. Exploration has been focused ontargets that display multiple coincidences of VLF, gravity,resistivity, radiometric and EM-16 anomalies in altered meta-sediments. The Lone Gull deposits contain 17 million kg U^Ogat the present stage of exploration (Hilger et al., 1982).

The Lone Gull deposit is hosted by clastic metasedimentaryrocks, of the informally called Judge Sissions Lake belt, andunmetamorphosed fluorite-bearing granite which is intrusive intothis metasedimentary sequence (Miller and LeCheminant, in press).In the immediate area, shallowly north dipping metasedimentaryrocks can be subdivided into a lower group characterised byimmature clastic sediments and an overlying group comprisingsupermature orthoquartzite that resembles orthoquartzite of theAmer group. The lower immature metasedimentary sequence isdominated by drab green to green-black sulphide-bearing biotite +muscovite feldspathic wacke, lithic feldspathic wacke andepidote-bearing equivalents. Pelite and lean oxide faciès ironformation intercalate with the psammitic units and increaseproportionally eastward through the deposit area. Metamorphicassemblages indicate upper greenschist-lower amphibolite regionalmetamorphism followed by retrogression to lower greenschistfaciès. Phlogopite-phyric lamprophyres, syenitic dykes, andnon-foliated, medium grained, sulphide- and fluorite-bearing sub-porphyritic to porphyritic granites intrude this metasedimentarysequence. Differences in the structural style and in thelithological assemblage of the immature metasedimentary sequencecompared to the Amer group to the north suggest that these meta-sediments are older than the Amer group, and could be Archean.

Basal silicified conglomerate and sandstone of the HelikianThelon Formation rest unconformably on weathered orthoquartzite,approximately 2 km north of the mineralised zone. In this areahematized orthoquartzite reflects pre-Thelon paleo-weathering.Northwest trending Mackenzie diabase dykes intrude all of theabove lithologies.

Potential uranium ore, occurring as a series of three podshaving a combined strike length of 1900 m (Fuchs et al., inpress), is controlled in part by a well defined east-northeast

304

j* , "l Fluorite granite

p^<:l Aphebian metasedimentsmuscovite-biotite teldspathic wackechlontoid-garnet metapehtelean oxide iron formation

| | Uranium ore zone

[fftj3y| Overburden

..••'* Interpretative alteration boundary

Fault

100

250m100

Fig 3. Interpretative drill section through theLone Gull uranium deposit (from Millerand LeCheminant, in press)

trending steeply north-dipping normal fault and psammiticlithologies that are highly susceptible to alteration.Alteration, present as an envelope about the major structure, ischaracterised by a pervasive illite, Mg- and Fe-chlorite, minorsmectite group clays, hematite and limonite (Miller andLeCheminant, in press; Figure 3). Mineralisation is mostcommonly present as fractures cross-cutting altered metasedimentarylayering and granite and disseminations through intensely clayaltered rocks. Multi-generation uraninite and coffinite areassociated with minor and variable galena, molybdenite, pyrite,chalcopyrite, clausthalite and nickel, bismuth and bismuth-silvertellurides. Anomalous contents of boron and vanadium areassociated with mineralisation.

Summary of Exploration conceptsThe Lone Gull deposit exhibits several features in common

with unconformity-related deposits in Northern Saskatchewan(Tremblay, 1978; Hoeve, 1978) except for the absence ofgraphitic metasedimentary host rocks (Miller and LeCheminant, inpress). Thus the following concepts have proved useful indelineating and exploring the Lone Gull deposit: i) positionnear the erosional edge of a Helikian sandstone basin; ii)spatial association of mineralisation with reactivated majorstructures; iii) a comprehensive geophysical program to

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delineate the response of each system and coincidence of variousmethods ;

Southwestern Amer groupThe southwestern portion of the Amer group and overlying

Thelon Formation sediments have been the focus of continueduranium exploration utilizing unconformity-related models becausethis region possesses many features similar to the southeasternAthabasca Basin and East Alligator River uranium field, Australia,This region of the Thelon Basin has been extensively faulted byNE trending structures as the Amer mylonite zone and parallelfaults (Geol. Surv. Can. Maps 1566A, 1567A) and NW trendingfaults. Kirchner et al. (1980) suggested that lower Proterozoicpelites syngenetically enriched in uranium are an importantcomponent in the generation of unconformity-related deposits inthe Athabasca region. Stratiform mineralisation in upperAmer group pelites-psammites may represent a similar favourablesituation. Carbonaceous/graphitic mudstone and siltstone inthis same sequence may represent reduced strata that could reactwith oxidised uraniferous solutions. Numerous fracture-controlled occurrences with a uraninite-illite signature havebeen found in this region and these suggest similarities to theunconformity-related environment present in Northern Saskatchewan.

CONCLUSIONNumerous factors interact to determine area selection by

the exploration geologist, however the Baker-Thelon regionrepresents a target selection based almost exclusively on geo-logical criteria. The region is remote, did not have thebenefit of early prospecting discoveries, but did have a reliablegeological base of maps and reports by officers of theGeological Survey of Canada (Wright, 1967; Donaldson, 1965,1966). The importance of having an established geologicalframework cannot be overstated, and coupled with interpretativepapers such as those by Donaldson (1967) and Fraser et al (1970),provided the analogical link to the Athabasca Basin inSaskatchewan and the East Alligator River area in northwesternAustralia. The discovery of the Rabbit Lake, Cluff Lake andKey Lake deposits in Saskatchewan, and the Jabiluka, Koongarraand Ranger deposits in Australia allowed geologists to focus inon areas where higher metamorphic rank Lower Proterozoic supra-

306

crustal rocks are overlain with profound unconformity by undist-urbed Middle Proterozoic fluvial quartz arenite units, withthat unconformity surface marked by intense regolithic alteration.That relationship exists in the Baker-Thelon region, witnessedby the relative distributions of the Thelon Formation overlyingthe Amer Group and the informally named Judge Sissons Lake beltsupracrustal rocks in the Shultz Lake area.

The discovery of the unconformity-related Lone Gull depos-its confirm the validity of exploration by geological analogywithin this geological setting. Notwithstanding the fact thatan unconformity-type deposit occurs where it was predicted to be(and deposits of this type, with their higher grades and largetonnages represent t.he prime exploration target) , other aspects ofthe Baker-Thelon region add strength to this area being selectedas a prime exploration target.

1. The presence of moderate to high metamorphic rank "basementrocks" of Archean and Lower Proterozoic ages, with preservedbelts of Lower Proterozoic supracrustal sequences containingstratabound uraniferous horizons provide a "protore" situationfor subsequent upgrading at the Lower-Middle Proterozoic uncon-formity.

2. The presence of numerous vein-type occurrences in adjacentbasement terranes and regional relationships to those inSaskatchewan with the uranium veins in the Beaverlodge area immed-iately adjacent to the lucrative Athabasca Basin camp (Miller,1980) suggest widespread uranium mobility.

3. The presence of uranium-enriched subaerial volcanic rockssuch as the Christopher Island and Pitz formations, or theuraniferous fluorite-bearing granites of the Nueltin-type, whichduring lithification and subsequent weathering provided a sourceof labile uranium for subsequent remobilization and re-deposition.

4. The multiple episodes of weathering and saprolite devel-opment recorded in the Dubawnt Group stratigraphie record, andthe flat-lying nature of this rock package, provides larger,areally extensive tracts of prospective ground.

307

REFERENCES

AQUITAINE COMPANY OF CANADA LTD, 1970: Assessment report on PROand BRO claim groups: ground radiometric surveys and dia-mond drill report by G. Chambrias available from IndianAffairs and Northern Development Mining Division files# N019953.

ASHTON, K.E., 1981: Preliminary report on geological studies ofthe "Woodburn Lake Group" northwest of Tehek Lake, Districtof Keewatin; in Current Research, Part A, GeologicalSurvey of Canada, Paper 81-1A, 261-274.

__________ 1982: Further geological studies of the 'WoodburnLake Group' northwest of Tehek Lake, District of Keewatin;in Current Research, Part A, Geological Survey of Canada,Paper 82-1A, 151-157.

BELL, R.T., 1969: Study of the Hurwitz Group in the eastern partof the Rankin-Ennadai belt, District of Keewatin (65H easthalf, 651 east half); Geological Survey of Canada, Paper69-1A, 147-148.

________ 1971: Geology of Henik Lakes (east half) andFerguson Lake (east half) map-areas, District of Keewatin(with a contribution by J.L. Talbot); Geological Survey ofCanada, Paper 70-61, 31.

BLACKWELL, J.D., in prep.: The Geology of the Middle ProterozoicChristopher Island Formation and Related Uranium Occurren-ces; Ph.D. Thesis, University of Western Ontario.

BLAKE, D.H., 1980: Volcanic Rocks of the Paleohelikian DubawntGroup in the Baker Lake-Angikuni Lake Area, District ofKeewatin, N.W.T.: Geological Survey of Canada, Bulletin309, 39 pages.

BUNDROCK, G., 1981: From armchair geology to a deposit in a newuranium province; in Uranium Exploration Case Histories;International Atomic Energy Agency, Vienna, 1981, 243-277.

BURT, D.M., SHERIDAN, M.F., BIKUN, J.V. and CHRISTIANSEN, E.H.,1982: Topaz Rhyolites-Distribution, Origin, and Signif-icance for Exploration; Econ. Geol., 77, 1818-1836.

BURT, D.M. and SHERIDAN, M.F., 1980: Uranium Mineralisation inFluorine-enriched Volcanic Rocks; U.S. Dept Energy OpenFile Report GJBX-225(80), 494.

308

CECILE, M.P., 1973: Lithofacies analysis of the ProterozoicThelon Formation, Northwest Territories (including computeranalysis of field data); unpub. M.Sc. Thesis, CarletonUniversity, 119 pages.

CURTIS, L. and MILLER, A.R., 1980: Uranium geology in the Amer-Dubawnt-Yathkyed-Baker Lakes Region, Keewatin District,N.W.T. Canada. Uranium in the Pine Creek Geosyncline,Proceedings of the International Uranium Symposium on thePine Creek Geosyncline; International Atomic Energy AgencyVienna, 1980, 595-616.

DONALDSON, J.A., 1965: The Dubawnt Group, Districts of Keewatinand Mackenzie. Geological Survey of Canada, Paper 64-20.

__________________ 1966: Geology, Schultz Lake, District ofKeewatin. Geological Survey of Canada, Map 7-1966.________ 1967: Two Proterozoic clastic sequences:sedimentological comparison. Proceedings GeologicalAssociation Canada, Vol. 18, 33-54.

____________ 1969: Descriptive notes (with particular ref-erence to the late Proterozoic Dubawnt Group) to accompanya geological map of central Thelon Plain, Districts ofKeewatin and Mackenzie (65M, N W%, 66B,C,D,75P E^,76A Eh)•Geological Survey of Canada, Paper 68-49.

BADE, K.E., 1980: Tulemalu Lake, District of Keewatin, N.W.T.1:125,000 Geological Survey of Canada Open File 553.

BADE, K.E. and BLAKE, D.H., 1977: Geology of the Tulemalu Lakemap-area, District of Keewatin; Report of Activities,Part A, Geological Survey of Canada, Paper 77-1A, 409-410.

FUCHS, H., HILGER, W., PROSSER, E. and STUART, M., in press:Exploration of the Lone Gull property, Baker Lake areaDistrict of Keewatin, Northwest Territories, CIM Bull.

HEYWOOD, W.W., 1977: Geology of the Amer Lake map-area, Districtof Keewatin; Report of Activities, Part A; GeologicalSurvey of Canada, Paper 77-1A, 409-410.

HEYWOOD, W.W. and SCHAU, MIKKEL, 1978: A subdivision of thenorthern Churchill Structural Province. Current Research,Part A, Geological Survey of Canada, Paper 78-1A, 139-143.

309

HILGER, W., STUART, M., PROSSER, E. and FUCHS, H., 1982:Exploration of the Lone Gull property, N.W.T.; CIM Bull.,7J3, #839, p!30.

HOEVE, J., 1978: Classification of uranium deposits in northernSaskatchewan; in MAC Short Course in Uranium Deposits:Their Mineralogy and Origin, edited by M.M. Kimberley,Min. Assoc. Can. _3' 397-402.

KIRCHNER, G., LEHNART-THIEL, K., RICK, J. and STRAND, J.G., 1980:The Key Lake Ni-U deposits: A model for Lower Proterozoicuranium deposition. Uranium in the Pine Creek Geosyncline,Proceedings of the International Uranium Symposium on thePine Creek Geosyncline; International Atomic EnergyAgency, Vienna, 1980, 617-630.

KNOX, A.W., 1980: The uranium mineralisation of the Amer Group(Aphebian); Regional Geology, Geochemistry and Genesis,66G/1, 66G/2, GGH/5, 66H/6. District of Keewatin, N.W.T.;unpub. M.Sc. Thesis, University of Calgary, 159 pages.

LAPORTE, P.J., GIBBINS, W.A., HURDLE, E.J., LORD, C., PADGHAM,W.A., and SEATOÏM, J.B., 1978: Northwest Territories,Mineral Industry Report, 1975 Canada Indian Affairs andNorthern Development, EGS 1978-5.

LAPORTE, P.J., 1974: Mineral Industry Report 1969 and 1970vol. 2 of 3, Northwest Territories, east of 104°; Indianand Northern Affairs EGS 1974-1.

LeCHEMINANT, A.N., ASHTON, K.E., CHIARENZELLI, J., DONALDSON,J.A., BEST, M.A., TELLA, S. and THOMPSON, D.L., 1983:Geology of Aberdeen Lake map area, District of Keewatin:Preliminary report; i_n Current Research, Part A, GeologicalSurvey of Canada, Paper 83-1A, 437-448.

LeCHEMINANT, A.N., IANNELLI, T.R., ZAITLIN, B., and MILLER, A.R.,1981: Geology of the Tebesjuak Lake map area, District ofKeewatin: A progress report: in Current Research, Part B,Geological Survey of Canada, Paper 81-1B, 113-128.

LeCHEMINANT, A.N., LAMBERT, M.B., MILLER, A.R., and BOOTH, G.W.,1979a: Geological studies: Tebesjuak Lake map area,District of Keewatin; jin Current Research, Part A, Geolog-ical Survey of Canada, Paper 79-1A, 179-186.

310

LeCHEMINANT, A.N., LEATHERBARROW, R.W., and MILLER, A.R., 19795:Thirty Mile Lake (65P, W^) map-area, District of Keewatin,in Current Research, Part B, Geological Survey of Canada,Paper 79-1B, 319-327.

LeCHEMINANT, A.N., BLAKE, D.H., LEATHERBARROW, R.W., anddeBIE, L., 1977: Geological studies: Thirty Mile Lake andMacQuoid Lake map-areas, District of Keewatin; GeologicalSurvey of Canada, Report of Activities, Part A, Paper 77-1A,205-208.

LeCHEMINANT, A.N., HEWS, P.C., LANE, L.S. and WOLFF, J.M., 1976:Macquoid Lake (55M West Half) and Thirty Mile Lake (65PEast Half) map-areas, District of Keewatin; GeologicalSurvey of Canada, Report of Activities, Part A, Paper 76-1A,383-386.

MILLER, A.R., 1980: Uranium Geology of the Eastern Baker LakeBasin, District of Keewatin, Northwest Territories;Geological Survey of Canada, Bulletin 330, 63 pages.

MILLER, A.R., and LeCHEMINANT, A.N., in press: Geology anduranium metallogeny of Proterozoic supercrustal successions,Central District of Keewatin, N.W.T. with comparisons toNorthern Saskatchewan in CIMM Special Volume on Uraniumgeology of Northern Saskatchewan.

MILLER, A.R., STANTON, R.A., CLUFF, G.R. and MALE, M.J., in press:Uranium deposits and prospects of the Baker Lake Basin andsubbasins, Central District of Keewatin, NorthwestTerritories, in CIMM Special Volume on Uranium Deposits ofCanada.

NORTHERN MINER, 1978: Urangesellschaft "U" find in Baker Lakearea, N.W.T. Northern Miner, Sept 21, 1978, 1.

NORTHERN MINER, 1982: Aber ford in rapid expansion with acquisit-ion of Marathon, Northern Miner, Vol. 68, 32, 1.

PATCHETT, P.J., BYLUND, G., and UPTON, B.G.J., 1978: Paleomag-netism and the Grenville Orogeny: New Rb-Sr ages fromdolerites in Canada and Greenland. Earth and PlanetaryScience Letters, Vol. 40, 349-364.

PATTERSON, J.G., 1981: Amer Lake: An Aphebian fold and thrustcomplex; unpub. M.Sc. Thesis, University of Calgary,106 pages.

311

SCHAU, MIKKEL, TREMBLAY, F., and CHRISTOPHER, A., 1982: Geology'of Baker Lake map area, District of Keewatin: a progressreport: in Current Research, Part A, Geological Survey ofCanada, Paper 82-1A, 143-150.

STANTON, R.A., 1979: Genesis of Uranium-Copper Occurrence 74-1E,Baker Lake, Northwest Territories; unpublished M.Sc. Thesis,University of Western Ontario, 167 pages.

TAYLOR, M.A., 1978: An investigation of the uranium mineralis-ation at the Sandhills Showing, District of Keewatin, North-west Territories; unpubl. B.Sc. Thesis, Queen's University,51 pages.

TELLA, S., ASHTON, K.E., THOMPSON, O.K., and MILLER, A.R., 1983:Geology of the Deep Rose map-area, District of Keewatin,N.W.T. in Current Research, Part A, Geological Survey ofCanada, Paper 83-1A, 403-409.

TELLA, S., and HEYWOOD, W.W., 1978: The structural history ofthe Amer Mylonite Zone, Churchill Structural Province,District of Keewatin; Current Research, Part C, GeologicalSurvey of Canada, Paper 78-1C, 79-88.

TIPPETT, C.R ., and HEYWOOD, W.W., 1978: Stratigraphy andstructure of the northern Amer group (Aphebian), ChurchillStructural Province, District of Keewatin; GeologicalSurvey of Canada, Current Research, Part B, Paper 78-1B,7-11.

TREMBLAY, L.P., 1972: Geology of the Beaverlodge Mining area,Saskatchewan; Geological Survey of Canada, Memoir 367,265 pages.

TREMBLAY, L.P., 1978: Geologic setting of the Beaverlodge-typeof vein-uranium deposit aad its comparison to that of theunconformity-type; in MAC Short Course in uraniumDeposits; Their Mineralogy and Origin edited byM.M. Kimberley, Mm. Assoc. Can. 3, 431-456.

WRIGHT, G.M., 1967: Geology of the southeastern barren grounds,parts of the Districts of Mackenzie and Keewatin (Operat-ions Keewatin, Baker, Thelon); Geological Survey ofCanada, Memoir 350, 91 pages.

312

THE RIO PRETO URANIUM OCCURRENCES,GOIAS, BRAZIL

P.M. DE FIGUEIREDO FILHONUCLEBRÄS - SUPPM,Rio de Janeiro,Brazil

Abstract

The pre-Arai ( 1 1,200 Ma ) discordance is interpreted as asurface from which uranium was leached from older (3,200 -2,500 Ma)granites and gneisses, and transported by surface and ground waters.Prior to their metamorphism, the graphitic schists of the TicunzalFormation possessed the oxi-reduction conditions necessary to fixthe uranium available at that time.

The Basal Complex consists of Archaean and lower Proterozoic rp_cks ( 3,200 - 2,500 Ma ) including gneissic granites and orth£gneisses. The Ticunzal Formation ( 2,000 Ma ) which overlies theBasal Complex consists of paragneisses and biotite-muscovite graph_ite schists. The Basal complex and the Ticunzal Formation are incontact with granitic bodies, and together these underlie discordaiitly the metasediments of the Arai and Bambui groups which are like_wise separated by a discordance. The Arai and Bambui groups are ofUpper Proterozoic age and are sterile with respect to uranium.

INTRODUCTIONAerogammaspectrometric surveys carried out in 1983 over appr£

ximately 46,000 km2 of the northeastern part of the State of Goiasrevealed hundreds of radiometric anomalies in basement rocks. Themorphology of the region is characterized by a dissected plateauand ranges of Proterozoic quartzites and metaconglomerates attribu^ted to the Arai Group. These anomalies were evaluated during geolo^gical reconnaissance, regional and local geological mapping, geophy_sical surveys and geological drilling. A number of uranium occur_rences in Proterozoic metamorphic and metasomatic rocks were found,the ages of which vary from 1,700 to 2,200 Ma. During the Uruassuan(1,400 - 1,000 Ma) and Brazilian (1,000 - 500 Ma ) cycles the rockswere rejuvenated isotopically.

Various types of uranium mineralization in veins have been ideiitified. These include the classical types of occurrence such as th£se associated with pegmatites, with rocks attributed to hydrothermal

313

origin and with tectonic structures, along with mineralization typesassociated with unconformities such as those defined in the class^fication of Me Millan (1978) for Canadian deposits and by Fergusonand Rowntree (1980) for the Alligator River uranium occurrences.

This paper summarizes the regional geological panorama of theCampos Belos-Cavalcante-Rio Preto areas of the State of Goias covering some 10,000 km2 of the state, and comments on the smalluranium deposits around Rio Preto which are interpreted as being ofthe "unconformity" type.

GEOLOGICAL SETTINGGeomorphology

The regional uplift which initiated thepresent erosional cycleof the basins of the rivers Parana and Preto in the northeasternpart of the State of Goiâs began after the Lower Cretaceous. Accor_ding to Bruni and Schobbenhaus Filho (1976), this surface can beequated to the Gondwana Surface of King (1956), the remnants ofwhich include the Chapada dos Veadeiros and the Serra Gérai do Para^na. This regional uplift has a NE-SW trending axis with outcrops ofthe Archaean and Lower Proterozoic basement preserved in a numberof inliers at elevations of between 400 and 500 m, surrounded by theUpper Proterozoic quartzite uplands of the Serra do Santana whichattain altitudes of 1,000 m to the north and 1,700 m to the south.

The most significant of these inliers are those of Campos Belo_s_Cavalcante and Rio Preto (Figure 1). The former is drained by theRio Parana while the latter through the Rio Preto system drains itito the Rio Maranhäo to the northwest of the area before it receivesthe waters of the Rio Parana.

The orientation of the inliers shows a distinct northeasterlytrend, the eastern border being controlled by large scale thrustfaults and the northwestern one by broad synclines and anticlineswith axes trending N-S. In the more eroded westerly areas of Campos Belos, the Parana River flows over older rocks, leaving isola_ted relicts of quartzitic rocks.

Within the eroded parts of both the Campos Belos-Cavalcante in_lier as well as that of Rio Preto, there occur four types of geomor_phological structures: (1) alignments of crests of the older faults,whether fault scarps or fault-line scarps with elevations between500 and 600 m; (2) elevated and rounded forms composed of granjL^toids attaining heights of 800 m; (3) hogbacks of quartzites whichprotect the relict hills attaining altitudes of about 700m; (4) the

314

LOCATION INSOUTH AMERICA

I3°20'

HW47«00'

Fig. 1 - REGIONAL GEOLOGY OF THERIO PRETO-CAVALCANTE-CAMPOS BELOS AREAS

STATE OF GOIAS, BRAZIL

315

Tertiary surface composed of detritus and latérite mapped by Sousaet al. (1980).

Regional GeologyThe principal elements of the Precambrian geology and strat_i_

graphy of Goiâs were defined by Barbosa et al. (1969). The regionalmapping of the Cavalcante-Rio Preto area of Goias by Sousa et al.(1980) was based on this work and the stratigraphy was accordinglyadapted. The mapping of Sousa et al. (1980) has served as a basefor more recent studies and specifically those related to uraniumprospection and exploration.

The oldest rock unit in the area is the Basal Complex whichcontains rocks of Archaean and Lower Proterozoic age. Sousa et al.(1980) included within this the coarse grained granite gneisses whichare grey in colour, with K-feldspar porphyroblasts as well as ortho_gneiss of granodiorite-tonalite composition, (typically coarse gra^ned and leucocractic with a slightly conspicuous foliation). Agesof this complex very from 3,200 to 2,500 Ma (Reis Neto, In: Andradeet al. 1981). Subordinately, there occur nuclei of migmatites andveins of pegmatite. According to Figueiredo and Oesterlen (1981) ,the uranium mineralization which occurs to the west of Campos Belosis associated with rocks of the Basal Complex.

Of the principal groups of rocks comprising the Basal ComplexMarini et al. (1978) and Sousa et al. (1980) drew attention to anewunit of metasedimentary rocks which was named the Ticunzal Format^on, divisable into two members. The lower member is composed of finegrained biotite paragneiss which is light grey in colour, garnetif£rous, finely foliated, generally cataclastic and intercalated withbiotite-tnuscovite schist ( which can be graphitic ). Amphibolitesalso occur as well as pegmatite veins and basic dykes. Mineralssuch as chlorite, garnet and tourmaline usually occur. The mostcommon assessory minerals are zircon, rutile, apatite, goethite andopaques. The upper member, separated from the underlying unit by agradational contact, is composed of muscovite-biotite schist, gen_erally graphitic; schist containing garnet and tourmaline and usuallyquartzose, passing to quartz-schist vertically. Secondary mineralsinclude epidote, chlorite and sericite while the more common asses^series include zircon, apatite and opaques. Also present in thismember are veins of pegmatite and quartz. This Ticunzal Formationis the host of the uranium mineralization in the Rio Preto region.This formation is dated at about 2,000 Ma (Reis Neto, in Andrade etal., 1981).

316

Normally, two schistositles are clearly visible. S, is markedmainly by lenses of quartz, sericite/muscovite and biotite derived,probably, by metamorphic segregation. This surface may coincidewith the stratification plane of the original rock (So), an hypothesis which is reinforced by the tectonics of isoclinal folds whichgave rise to this S,.

The S~ surface indicates the most recent foliation plane. Onmany occasions, various stages of crenulation development can beobserved with reorientation of the micaceous minerals and graphiteto the extent that, in some places, there has occurred a total tran_sposition of the older shistosity (S,). In many cases S~ is defi^ned in thin section by narrow zones of opaque minerals.

The rocks of the Ticunzal Formation have been metamorphosed tobiotite faciès. Indicators of partial retrograde metamorphism arecommon. These include: chloritization of the biotite, alteration ofthe garnet to biotite and chlorite and sericitization of the plagioclase.This retrograde metamorphism is attributed to generalized catacljisis which affected the whole area.

The rocks of the Basal Complex and the TicunzalFormation are cutby bodies of granitic composition which Sousa etal. (1980) referredto as "acid intrusives". These include bodies which have been intru_ded only into the Basal Complex as well as younger bodies which cutthe Ticunzal Formation. The older intrusives can be separated intothose which contain tin mineralization and those which do not.

The rocks associated with the tin mineralization are restri£ted to certain bodies such as those of Sucuri, Mangabeira, FedraBranca and Riacho dos Cavalos. These are light grey in colour, fi^ne to medium grained and composed of quartz, feldspar and biotite.The rocks are of porphyroblastic texture and contain porphyroblast sof blue quartz and euhedral as well as subhedral crystals offeldspar. Hydrothermal pneumatolytic alteration phenomena areobserved. The rocks occur in the form of greisens mineralized withcassiterite and to a lesser extent with tantalite. According toUrdininea (1977), there is likely to exist a relationship betweenthe tin and tantalite mineralization and the gold which occursthroughout the area. Attention was drawn to geochemical sampleswith anomalous uranium values suggesting a probable association ofthis element and its remobilization with the same mineralizingphenomenon.

The granitic bodies which do not contain tin mineralizationinclude the Teresina Granophyre, the Caldas Granite, the Morro daMangabeira, Serra dos Mendes and the Serra do Mocambo. According to

317

Reis Neto ( In: Andrade et al., 1981 ), some radiometric datingssuggest an age of 2,500 Ma for the Teresina Granophyre. Urdininea(1977), found anomalous uranium values in stream sediments derivedfrom these areas.

The younger "acid intrusives" are those which cut the TicunzalFormation and have a granitic-granodioritic composition. Accordingto Andrade et al. (1981), the emplacement of these bodies was rel£ted to diapiric processes. They have been attributed an age of1,600 Ma by Reis Neto ( In: Andrade et al., 1981 ).

The rocks of the Basal Complex, the Ticunzal Formation, andthe acid intrusives are separated from the metasediments of theArai Group by an unconformity representing an erosional phase. TheArai Group has been divided into two formations: The Arraias Formjition and the Trairas Formation. The Arraias Formation consists offine-grained quartzites, well stratified and laminated. Fine tomedium-grained quartzites and medium to coarse-grained quartziteswith cross bedding and ripple marks and a level of calcareous quartzschist also occur. In several places a basal conglomerate having aschistose quartz matrix with pebbles of quartz and quartzite hasalso been observed. Volcanic rocks of acid to intermediate compos^tion and represented by rhyolites, dacites and andésites are intercalated in the sequence, especially in the region of Cavalcante.

The Trairas Formation consists of quartz-schists with lensesof friable quartzite and beds of metamorphosed siltstone. The AraiGroup is about 2,240 m thick and the youngest date determined fromthe volcanic rocks is 1,200 Ma ( Sousa et al., 1980 ).

The Arai Group constitutes a sedimentary cover overlying theunconformity under which the uranium occurrences are found in theareas of Rio Preto as well as Campos Belos. These sediments arereponsible for the topography of the Chapada dos Veadeiros, Chapadade Ägua Doce, the serras of Caldas, Forquilha, Ticunzal and others,as well as for the spurs which occur at the margins of the inliersunder consideration.

Metasediments of the basal formation of the Bambui Group over_lie unconformably the rocks of the Arai Group. However, the occur_rence of the former is restricted throughout the region under study.

Deposits of detritus and latérite of probable Tertiary ageoccur on the eroded parts of the Basal Complex forming terraces andplateaus of small extension as well as on the tablelands composedof quartzite of the Arraias Formation.

The recent sediments are composed of bedded fine - to medium-grained loose sands which are white in colour. These form deposits

318

SSE

350 J

L E G E N D

t A OUARTZ - 8IOTITE SCHIST(TICUNZAL FORMATION)

GNEISS

LOG

SO/ SI - N 7 0 E / 8 0 N W

URANIUM MINERALIZATION

Fig. 2-SCHEMATIC SECTION OF THE MODE OF OCCURRENCEOF URANIUM ORE IN INOO2, RIO PRETO PROJECT,

GOIAS, NUCLEBRAS-SUPPM

on the bed and margins of some stretches of the Preto, Claro,Pedras, das Almas and Parana rivers ( Sousa et al., 1980 ).

das

GENERAL CHARACTERISTICS OF THE URANIUM MINERALIZATIONFeatures of the uranium mineralization in the Rio Preto area

have been summarized by Figueiredo Filho et al. (1982) as follows :

Lithological ControlsThe uranium mineralization occuring in the Rio Preto area is

restricted to the Ticunzal Formation.This mineralization is found most commonly in quartz-muscovit^

biotite schist which may be graphitic. The levels of schists coiltain little biotite are sterile with respect to uranium. However ,this does not mean that the biotite-rich schists necessarily containuranium. Quartz veins, composed generally of smoky quartz, arefound frequently adjacent to the mineralization.

Another type of uranium mineralization is that which occurs inthe contact between a biotite schist of metasomatic origin and agneiss belonging to the lower member of the Ticunzal Formation.

319

Structural ControlsThe principal control over the mineralization verified in sur_

face studies is the schistosity plane So/S, (Figure 2) since nearlyall the radioactive bands follow this trend. Besides this form ofoccurrence, subsurface studies have revealed that mineralization ispresent frequently in fractures.

Another important structural control is the contact zonebetween the metasomatic rocks and the gneisses of the Ticunzal For_mat ion.

Mineralogical AssociationBiotite, pyrite, chalcopyrite, chlorite and hematite nearly

always accompany the uranium mineralization. However, it can notbe stated that these minerals are always present along with signif^cant uranium grades.

Grades, Thickness and RadioactivityThe best occurrences in the area of the Rio Preto Project are

occurrences 001 and 002. These are small deposits less than 1,000tonnes of U«0„ each. On the basis of drilling results, the gradesvary from 600 to 2,000 ppm U_0g and less than 100 ppm Th02. Themineralized thicknesses are about 0,85 m. The radiometric values ofgamma logs of these two occurrences are generally high with maximaof 20,000 cps at occurrence 001 and 40,000 cps at occurrence 002.

MINERALIZATION MODELThe eleven most promising uranium occurrences in the Rio Preto

area are associated with two distinct lithological contexts whichfall into the same metamorphic rock domain. These consist of gra^phitic schists and paragneisses of the Ticunzal Format ion»describedby Sousa et al. (1980).

In the first case the uranium concentrations are conditionedby the presence of schistose rocks, grey to dark grey in colour inthe form of a dyke. These rocks possess sharp contacts and areenclosed in the paragneisses referred to above. In many cases, bothrock-types have been affected by cataclasis. At depth, the uraniummineralization in this lithological context is very irregular anddiscontinous. It occurs exclusively in the form of uraninite assmall fracture fillings in the contact zone with the gneiss countryrock. This rock is the product of metasomatic alteration of thecountry rocks. Recent pétrographie and chemical studies carried outon surface samples and drill cores have shown this to be a biotite

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schist, sometimes containing abundant hornblende and apatite. Biotj^te, present as a major constituent is neoformed.

The second and more usual lithological context is invariably abiotite-muscovite schist, generally graphitic and cataclastic. Thisschist encases pegmatite injections and veins of quartz which isnearly always of the smoky variety. In this rock-type, the uraniummineralization occurs as uraninite, sometimes in fractures and atother times, distributed along the schistosity planes. In both coi\texts (metasomatic rocks and graphitic schists), autunite and morerarely renardite, torbernite, rutherfordine and sabugalite are thesecondary uranium minerals which occur at the surface.

According to Marini (1978), the controls over the uraniummineralization are essentially stratigraphie in the graphiticschists. This contrast with earlier views developed during theinitial stages of evaluation which considered that the uranium minjsralization was controled structurally by faults. However, thisauthor did not comment on the origin of the uranium although itsconcentration was attributed to tectonic mechanisms and folding inwhich graphite played the role of a chemical barrier for mineralizedsolutions. Rodrigues (1978) proposed an epigenetic origin for theuranium which may have migrated through intensely cataclased zonesin the graphitic schists.

As a result of the integration of the various phases of theRio Preto Project, it is reasonable to consider the hypothesis thatthe pre-Arai unconformity (1,200 Ma) was a surface over which ura_niurn leached from the older gneisses and granites (2,500 Ma) wastransported by surface and ground waters. The anomalous geochem^cal uranium values adjacent to the older granites and gneisses des_cribed by Urdininea (1977) support the hypothesis that these rockswere the source rocks of the uranium.

The graphitic schists possessed the physical and chemical cha^racteristics which permited redox reaction and the fixation of anyfree uranium available at that time. The Ticunzal Formation is thehost rock of the uranium mineralization while the paragneisses andquartzose schists are mineralized only slightly.

Another possibility worth considering involves the contribuit^on of fluids having hydrothermal characteristics in the phase ofuranium concentration. This view takes into account the frequentoccurrence of hematite and the argilization of the mineralized sec_tions having significant uranium grades along with sulphides (te ) associated with uraninite.

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REFERENCESANDRADE, S.M., SOUSA, E.L., OLIVEIRA, A.G. and SANTOS, A.R., 1981 :

Projeto Rio Preto - Mapeamento Geologico 1:25.000Relatorio Final de Fase. NUCLEBRÂS-EGOI.PM - CompanyReport - Restricted.

BARBOSA, 0., BAPTISTA, M.B., BRAUN, O.P.G., DYER, R.C. andCOTTA.J.C., 1969 : Projeto BrasiliaGoias, Restricted Report.PROSPEC to Departamento Nacional da Produçao MinéralRio de Janeiro, 225 pp.

BRUNI, M.A.L. and SCHOBBENHAUS FILHO, C., 1976 : Carta Geologica doBrasil ao Milionésimo-Folha Brasilia - SD. 23, explicative text and map - 163 pp. MME/DNPM.

FERGUSON, J. and ROWNTREE, J.C., 1980 : Vein-type Uranium Depositsin Proterozoic Rocks - Renue de L'Institut Françaisda Pétrole. XXXV, (3), 485-496.

FIGUEIREDO, A.M. and OESTERLEN, P.M., 1981 : Prospecçâo de urâniono Estado de Goias. Rev. Bras. Geociências. 11 ,(3), 147-152.

FIGUEIREDO FILHO, P.M. de, OLIVEIRA, A.G. LIBERAL, G.S. andABRAHAO,J.R.S., 1982 : Projeto Rio Preto - Relatorio Final,NUCLEBRÂS-EGOI.PM. Company Report, Restricted.

KING, L.G., 1956 : A Geomorfologia do Brasil Oriental - Revis taBras. Geogr., XVIII, (2) - Rio de Janeiro.

MARINI, O.J., 1978 : Projeto Rio Preto - Relatorio Mensal - Novembro 1978. NUCLEBRÂS-DRM-SPPM-ERGOI -Company Report,Res trie t ed .

MARINI, O.J., LIBERAL, G. da S., REIS, L.T. dos, TRINDADE, C.A.H. ,and SOUZA, S.L. de, 1978 : Nova unidade litoes trat_i_grafica do Pré-Cambriano do Estado de Goias - In :"XXX Congresso Brasileiro de Geologia", Recife, Resumo das Comunicaçoes, p. 126.

Me MILLAN, R.H., 1978 : Genetic Aspects & Classification of Impor_tant Canadian Uranium Deposits. Mineral. Assoc. Çanada. Shorth Course Handbook, _3_: 187-204.

RODRIGUES, C.S., 1978 : Projeto Rio Preto - Relatorio Final de Etapa I - Poços e Trincheiras - NUCLEBRÂ'S - DRM - SPPM-ERGOI. Company Report, Restricted.

SANTOS, A.R. and VASQUES, R.C., 1980 : Projeto Cavalcante, SÏnteseGeologica -Relatorio Final de Fase - NUCLEBRÂS-EGOI .PM.Company Report, Restricted.

SOUSA, E.L., ANDRADE, S.M. and OLIVEIRA, A.G., 1980 : Projeto Campos Belos/Cavalcante - Mapeamento Geologico 1:100.000

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Relatorio Final de Fase - NUCLEBRÂS-EGOI.PM - CompanyReport, Restricted.

URDININEA, J.À., 1977 : Projeté de Geoquïmica Regional Campos Be_los Cavalcante - Goiäs. Relatorio Final da GEOMITECGeologia e Mineraçâo Trabalhos Tecnicos Ltda. pajraEMPRESAS NUCLEARES BRASILEIRAS S/A - NUCLEBR&S. 170p,maps and anexes.

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PROTEROZOIC UNCONFORMITYAND STRATA-BOUND URANIUM DEPOSITSEpilogue

J. FERGUSONBureau of Mineral Resources,Canberra City, ACT,Australia

No detailed attempt is made to synthesise the diverse stylesof mineralisation found in the Proterozoic rocks which are pres-ented in this collection of papers. It appears to me unlikelythat a common thread would link all these deposits into a singlegenetic framework. Although a number of these deposits appearto have a similar history of development when viewed in the grossaspect.

The question could be asked as to why the Proterozoic wassuch a favourable time period for the development of uranium orebodies, in particular the high-grade ones, as well as a number ofother mineral commodities. The sceptic may reply that theProterozoic covers a large time span amounting to a third ofearth's history and nearly half of the geological record. TheEarly to Middle Proterozoic period appears to have been essentialto the development of the high-grade unconformity related uraniumdeposits. Further exploration is needed before it can beestablished whether the Zambia-Zaire, Labrador and Olympic Damstratiform types of uranium deposits should be restricted to aparticular era of the earth's history. The uranium depositscovered by this collection of papers group themselves into twomain types, being:

a) dominantly stratiform typesb) stratabound and unconformity-related types

I have chosen to look at these two groupings separately. Thenotable absence of a paper on the Franceville Region, Gabon, inthis treatise is unfortunate and stems from the difficulty incommunicating with authors in remote parts of Africa.Fortunately reference can be made to a range of publishedmaterial on these deposits and this is included in the discussionpresented here.

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STRATIFORM TYPES

Regional SettingA number of regional contrasts are offered by the three

stratiform provinces, namely, Zambia-Zaire, Aillik Group-Labrador, and Olympic Dam Deposit-South Australia. The Labradordeposits are hosted in Lower Proterozoic rocks whereas the othertwo occurrences are hosted in Middle Proterozoic rocks. Therocks range from undisturbed and unmetamorphosed as in theOlympic Dam Deposit to highly deformed and metamorphosed as isthe case for the Labrador Province. The Zambia-Zaire Provinceoffers an interesting contrast in that the individual depositsrange from an unmetamorphosed condition grading through todeposits occurring in lower amphibolite grade.

Contrasting rock types host the uranium mineralisation inthe three provinces,they span the compositions from felsicvolcaniclastics, argillites, arenites, rudites and calcareousrocks. In all three provinces the uranium mineralisation occursin the basal sections which unconformably overlie high uraniumgranitoids of the basement complexes. In these provinces thereis no relationship to a major unconformity with the overlyingrocks. Dolerite dykes cut the mineralised zones in two of thedeposits and additionally lamprophyres are found in the alkalineigneous province of the Aillik Group, Labrador. Pre-Adelaideanlamprophyric rocks are found at the Cattle Grid, Cu-deposit,Mt Gunson area some 85 km from the Olympic Dam deposit(J. Knutson pers. comm.).

The rocks hosting the uranium mineralisation in the threeprovinces occur in anorogenic environments related to extension-al tectonics with rifting having been identified in two cases.

Geological setting of depositsIn all three situations the initial uranium concentration,

that in some places produced protore and elsewhere ore-grade,appears to have been stratiform and syngenetic with some devel-opment of epigenetic mineralisation. The epigenetic mineral-isation was usually accompanied by upgrading of uranium values.This transgressive epigenetic uranium appears in the post-lithification situation in the Labrador and Zambia-ZaireProvinces and appears to have been developed shortly after thesyngenetic mineralisation. The transgressive mineralisation is

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related to fractures, faults, breccias, joints, shears andmylonites. In all situations the main primary mineralisationis low thorium uraninite; minor coffinite, brannerite andsoddyite are present in some situations. In the Zambia-Zaireand Olympic Dam Deposit multi-element mineralisation is present,in particular Cu, rare earth elements, gold + silver - thesedeposits are all characterised by high U/Th ratios.

The role of metamorphism in concentrating uranium appearsto have been insignificant as evidenced by the Zambia-ZaireProvince where no marked change is discernable within the differ-ent metamorphic grades encountered. It is suggested that ifuranium mobilisation occurred it could only be over shortdistances and may account for the resetting of uraninite ages.

Source, Transport and DepositionIn all three situations the deposits have a close spatial

relationship with the basement and initial mineralisation isthought to have been syngenetic. It would appear likely thatthe syngenetic mineralisation, developed in the Lower KatangaSystem of the Zambia-Zaire Province, was derived from uraniumenriched basement. A likely source for the uranium is thegranitoids whereas the copper probably owes its origin to maficrocks within the basement complex. The mineralisation assoc-iated with both the Labrador and Olympic Dam Deposit appear tobe related to alkaline igneous activity which is identified inthe Aillik Group by the mineralisation being contained in oradjacent to alkaline felsic volcanogenic rocks. In the case ofthe Olympic Dam Deposit, these alkaline igneous rocks have notbeen identified but the high uranium granitoid basement rocksat the nearby Cattle Grid copper deposit, Mt Gunson area, containlamproitic rocks (pers. comm. J. Knutson). Alteration hasplayed a significant role in the mobilisation of uranium at theOlympic Dam and Aillik Group Deposits being characterised byalkaline metasomatism; K being dominant in the former and Nain the Aillik Group Deposits. In both situations this alkalinemetasomatism is probably related to the alkaline igneousactivity with the possibility of a meteoritic interaction.The resetting of uranium isotopic values is a feature of mosturanium deposits, particularly those that have undergone meta-morphism, orogenesis, epeirogenesis and alteration produced by

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groundwaters. This holds for Labrador and, in the case of theZambia-Zaire Deposits is pertinent to those that have undergonemetamorphism.

It is suggested that the copper, iron and gold at theOlympic Dam Deposit probably originated from a mafic source.The uranium was probably transported in the oxidised state asuranyl complexes as the ore deposits are characterised by lowthorium contents.

In the Labrador and Zambia-Zaire situation carbonaceousmatter and sulphides, which may have acted as reductants, arepresent in the host rocks. Evans (1980) also suggests absorpt-ion of uranyl complexes onto Fe , Mn and Ti hydroxides withredox reactions causing precipitation of uraninite. The absenceof reductants at the Olympic Dam Deposit suggests that themechanism of precipitation of the oxidised uranium may have beenbrought about by oxidation of ferrous to ferric iron. Latercirculation by mineralising fluids in these deposits alsoresulted in transgressive uranium being deposited.Discussion

The role of alkaline igneous activity seems a likelymechanism for the development of uranium mineralisation in theOlympic Dam and Labrador Deposits, but is not obvious in theZambia-Zaire Province. Although the structural setting of thesedeposits is very different, they share the aspect of having beendeveloped in a continental regime during anorogenic periods.For the development of uranium deposits of this type the follow-ing appears to be essential:1) Establishment of a uranium province either by an enriched

basement complex or association with U-enriched alkalineigneous activity.

2) Transport as uranyl complexes.3) A suitable environment for syngenetic concentration.4) Mobilisation of syngenetic uranium mineralisation producing

some transgressive zones of epigenetic uranium enrichment.5) Sealing of deposit so that present-day oxidising ground

and surface waters do not disperse the uranium bysolution and transport.

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STRATABOUND AND UNCONFORMITY-RELATED TYPES

Regional SettingIn most situations the unconformity, when present, separates

the Lower Proterozoic from the Middle Proterozoic as is the casefor the extensive Alligator Rivers Uranium Field and those ofthe Athabasca Basin Region. Others that share this time breakare the Beaverlodge Deposits, Brazilian Deposits, Turee Creek andThelon Basin Deposits. Elsewhere the unconformity separatesthe Archaean and Lower Proterozoic; for example, the LateAphebian Basin mineralised zones and the Lianshanguan Deposit.Stratabound deposits that appear to have no spatial relationshipto unconformities include those of the Dripping Spring Quartzite,Amer Group, and the Franceville Deposits (Diouly-Osso & Chauvet,1979). In fact the Amer Group Deposits more correctly belongto the stratiform type but because the other two types ofdeposits in the Central District of Keewatin fall within thiscategory they have been included here.Mineralisation occurs in diverse lithologies which display vary-ing degrees of metamorphism and alteration. In all situationsthe host rocks are either Lower or Middle Proterozoic in age,except for the Late Aphebian Basins, here the mineralisation ispresent not only in the Lower Proterozoic rocks but also inmetastrata of the Archaean basement.

In the unconformity-related situations, the mineralisationis either restricted to the lower sequences of rocks or, alter-natively, present in rocks both above and below the unconform-ity. Examples of the first type include the deposits of thePine Creek Geosyncline, the Brazilian Deposits and the ThelonBasin. Examples of the cover rocks containing mineralisationinclude Late Aphebian Basins, Beaverlodge area, Athabasca BasinRegion and Turee Creek.

With the exception of the stratiform and stratabound AmerGroup mineralisation, the economic grades of uranium occupyprominent dislocation features, for example, fractures, faults,schistosity planes, breccias, shears and mylonitised zones.Metamorphic grade in the hosting rocks is generally greenschistto amphibolite faciès, although notable exceptions includezeolite faciès in the Franceville and Dripping Spring QuartziteDeposits. Additionally, the Amer Group uranium mineralisation

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is contained in rocks ranging from zeolite through greenschistto lower amphibolite grade. The uranium is usually strataboundand in some local situations also has a stratiform disposition.In most of these deposits the mineralisation post-dates themetamorphism and deformation. However, a notable exception isthe Lianshanguan Deposit where the early syngenetic uraniummineralisation has been migmatised.

The host rocks to these uranium deposits are dominantlymeta-terrigenous rocks that are usually carbonaceous. Otherrock types include metavolcanics, granitoids, volcaniclasticrocks, metaregolith and carbonates.

Basement granitoids have a close spatial relationship withmost of these uranium deposits and furthermore, where present,display anomalously high uranium content. In a number ofdeposits there is a spatial association of uranium mineralisationwith dolerite intrusives and alkaline igneous rocks. In allsituations the uranium deposits occur in tectonic regimesdeveloped during anorogenic periods produced by extensionaltectonics where continental and/or marine sequences dominate.Geological Setting of Deposits

In some of these deposits there was initial syngeneticconcentration of uranium in terrigenous sediments (Franceville,Amer Group, Beaverlodge, Lianshanguan) or volcaniclastic rocksas in the Dripping Spring Quartzite Deposits. This syngeneticuranium does not produce economic grades. However, diageneticprocesses have upgraded some of the syngenetic concentrations.Epigenetic processes superposed on post-lithificationsyngenetic mineralisation is the main factor responsible forupgrading these uranium deposits to economically viable grades.Most of the deposits are characterised by multiple resetting ofthe uranium mineralisation ages. Generally these mineralresetting ages correspond to periods of metamorphism, igneousevents and epeirogenesis. Uraninite is the main primaryuranium mineral in all situations, with some deposits containingminor coffinite and brannerite.

A feature of these uranium deposits is that uranium caneither occur with other economically viable elements or in themono-element situation. Even within a single province it ispossible to find both situations. Common associated economic

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elements can be any of a combination of the following: Cu, Ni,Pb, Mo, As, Co, Ag, Au, Nb, Zn, U, Sn, Bi and Li.

Within the ore zones metasomatism is usually present andis usually accompanied by the introduction of one or more of thefollowing major elements: Na, K, Mg and Fe.

The gangue mineralogy in these uranium deposits is widelydiverse,chlorite and iron oxides and hydroxides (mainly hematiteand limonite) are common to most of these deposits but can alsobe mutually exclusive within individual deposits. Otherrelatively common minerals include clays, graphite (and carbon-aceous matter), alkali feldspars, chert, quartz, carbonates andtourmaline. Common opaques include galena, pyrite, chalcopyrite,covellite, and sundry nickel sulphides.

In the unconformity-related uranium deposits the depths towhich mineralisation is found below the unconformity is usuallyshallow, being less than 300 m. The Beaverlodge Deposits arean exception as here mining has taken place to depths of 1800 m.Source, Transport and Deposition

Most of these deposits are juxtaposed with crystallinebasement rocks and where information is available, these rocksare shown to be enriched in uranium and appear in most circum-stances to be the primary source of uranium found in the deposits.In all occurrences the high U/Th ratios present within themineralised zones suggest that uranium was transported in theoxidised state. The labile uranium extracted from the granit-oid basement rocks usually results in mild syngenetic enrichmentof the sediments and regoliths derived from these sources.In the case of the Amer Group deposits, the syngenetic enrich-ment approaches economic grades, but this is the exception ratherthan the rule. Elsewhere it has been necessary to upgrade theinitial syngenetic mineralisation in order to achieve economicgrades. In most syngenetic situations uranium appears to havebeen precipitated by reduction in the presence of carbonaceousmatter. Diagenetic upgrading of some deposits has also takenplace. Ore grade concentration of uranium has in general beenbrought about by epigenetic remobilisation with depositionhaving taken place in zones of disruption. The low temperatureepigenetic movement of uranium generally appears to have beengenerated by surface and groundwater flow initiated by epeiro-

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genesis. The higher temperature epigenetic solutions appearto have been mobilised by metamorphism, granite intrusion, mig-matisation and other igneous events. In some situations stableisotope data suggest the reductant appears to have been biogeniccarbonaceous matter and/or bacterial sulphate. In a few occurr-ences it has been suggested that the absorption of uranium onto

2+clays or Fe -Mn-Ti hydroxides was coupled with later redoxreactions involving these minerals resulting in the precipitationof uranium.

Fluid inclusion and stable isotope data suggest a widevariation of temperatures recorded by the mineralisingsolutions in the general range 60-300 C. Temperatures at boththese extremes have been recorded within individual deposits.There is a wide variation in uraninite ages within individualdeposits, within a province ages can range from Middle Proteroz-oic to Mesozoic.

DiscussionIt would appear that once a province has been enriched in

uranium that all the succeeding sedimentary rocks derived fromthe basement rocks are also uranium enriched, although sometimesonly mildly so. This feature also generally applies to thelater development of igneous rocks within the province. Thelabile nature of uranium in the post-2200 Ma period is veryevident judging from syngenetic and epigenetic upgrading foundin most of these Proterozoic uranium deposits. In the syn-genetic situation psammitic, pelitic and volcanogenic rocks hostthe mineralisation. In these situations there is a closespatial relationship with carbonaceous matter. The largecompositional variety of rock types that host the epigeneticuranium deposits at first suggest a wide spectrum of depositionalcontrols. In these situations structure appears to be the maincontrol of uranium mineralisation rather than the primary hostrock lithology. All the epigenetic mineralisation zones sharethe feature of being associated with low temperature paragenesesapparently brought about by wall rock-solution reaction. Orezones are usually characterised by the presence of chlorites,clays, iron oxides, graphite and carbonaceous matter. It couldbe argued that in situations where graphite/carbonate matter isabsent that this material was initially present and has sub-

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sequently been consumed. The alternative suggestion to uraniumbeing precipitated by organic reduction is one of absorption ofuranium from solution onto particulate matter with redox reactioninvolving Fe causing deposition of uraninite.

The generally shallow depths to which mineralisation isencountered in the unconformity-related deposits has led to thesuggestion that these deposits were produced by surface-relatedsupergene processes. The Beaverlodge Deposits are the except-ion as mining has proven ore to depths of 1800 m. The fact thatthe Beaverlodge Deposits are spatially related to major faultsmay account for the great depths of mineralisation encountered.If one is to adopt a supergene model it could be argued that thismineralisation could be brought about by downward percolation ofuranium solutions via deep tensional fractures related to majorfaults. An alternative interpretation could account for thismineralisation to be produced as a result of remobilisation ofsyngenetic uranium during metamorphic events.

The grade of metamorphism does not always play a significantrole in the location of these deposits as hosting terranesrange from zeolite to amphibolite faciès. Within single geo-logical provinces uranium deposits occur in rocks that haveundergone metamorphic grades ranging from greenschist to upperamphibolite. In the case of the Pine Creek Geosyncline uraniummovement does not appear to be related to this metamorphicgradient. It is to be remembered that in a number of depositsthe uranium mineralisation post-dates the regional metamorphismbeing contained in zones of dislocation. In some terranesmetamorphism does appear to have played a role in the generationand activation of solutions that may have been responsible forthe scavenging of uranium with precipitation taking place intraps under epigenetic conditions. Metasomatism can also resultfrom the solutions associated with those metamorphic processes.

In all situations the high U/Th ratios recorded in the orezones strongly suggest that the uranium was transported in theoxidised state.

The multiple ages recorded in the ore mineralisation ofthese deposits indicates high mobility of uranium under a numberof geological conditions. This feature is the norm rather thanthe exception and evaluation of the timing of ore-forming events

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should be treated with great caution. Uraninite dissolutionand re-precipitation under low temperature groundwater situationspossibly related to epeirogenesis is a common feature. Thelarge range of temperatures deduced from fluid inclusion andstable isotope data equally has to be treated with great cautionas most of these deposits occupy open systems. It is imperativeto identify which gangue minerals are related to uranium miner-alisation and to identify the geological events. The range oftemperatures deduced from fluid inclusion and stable isotopedata is simply reflecting different periods of activity only someof which might be related to uranium mineralisation. Unfortun-ately stable isotope data is presently limited; where sulphurisotopes are available there is the suggestion of bacterialsulphate reduction as well as sedimentary source input. In theAlligator Rivers Uranium Field biogenic carbonaceous matter hasalso been identified.

Any account of the behaviour of uranium transportingsolutions would also have to take account of the frequentassociation of other economic elements found in zones of uraniummineralisation. Where experimental work is available it wouldappear that most, if not all, of these elements can be transport-ed in oxidising solutions. Where determined, the lead isradiogenic which explains the correlation of this element withuranium.

The role of the unconformity in the development of thesedeposits is a tantalising one. This enigma is caused mainly bythe fact that uranium mineralisation is either confined torocks below the unconformity or is found both above and belowthe unconformity. A few workers have suggested that the coverrocks are the main source of the uranium mineralisation. Otherinterpretations see the unconformity surface serving as anaquifer along which uranium-rich solutions were transported toproduce the ore grades encountered. Yet other workers regardthe rocks above the unconformity as having acted as a passiveimpervious cap shielding uranium deposits from subsequentweathering and transport. Those who argue for the protectiverole of the cover rocks explain the situation where there ismineralisation within these rocks as having resulted fromremobilisation from below the unconformity. In this model it

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is necessary for faulting and brecciation to have taken place inthe cover rocks, these zones of dislocation then acted asavenues for solutions which derived their uranium contentfrom the ore zones below the unconformity. It is also necessaryto fix the uranium in the cover rocks, the absence of a suitablereductant in the Kombolgie cover rocks of the Alligator RiversUranium Field may explain why there is chloritisation of theKombolgie but no associated uranium.

CONCLUDING REMARKSThe high solubility of uranium in oxidising environments

appears to be responsible for the diverse styles of mineralis-ation outlined in this collection of papers. What has to beanswered is whether there is a real time dependency in thedistribution of these Proterozoic uranium deposits. In orderto make this assessment it is as well to look at the uraniumcycle.

During the 3800-3000 Ma period, the continental crust dev-eloped by generation of sodic plutonism characterised bytonalitic and frondhjemitic magmas derived from a peridotiticupper mantle source (Glikson, 1979; Button, 1979). These Na-rich and uranium-depleted sialic rocks occur in close associat-ion with the early greenstone belts. In southern Africa, thepotassic-rich volcanic and plutonic rocks developed at 3000 Ma,whereas in most other areas this development took place at 2700Ma, corresponding to a major global thermal peak. With theadvent of the development of K-rich granitoids in the period3000-2700 Ma, uranium, owing to its incompatible behaviour, waspartitioned into these magmas by partial melting and fraction-ation of an undepleted upper mantle source. Some K-richgranitoids are richer in uranium than others, so it wouldappear that the upper mantle source peridotites were probablyinhomogeneous, at least in their uranium content. Where theuranium content of these granitoids exceeded 6 ppm, theyrepresent potential provenance areas for the quartz-pebbleconglomerate deposits. In that K-rich granitoids were develop-ed at an earlier stage in southern Africa (i.e. at 3000 Ma), itis not surprising that the uraniferous quartz-pebble conglomer-ates of the Dominion Reef and Witwatersrand Systems fall intothe older time slot of about 2800-2600 Ma (Pretorius, 1975)

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whereas the Blind River deposits developed between 2500-2200 Ma(Robertson, 1974). The low oxidation state of the atmosphereat this time would have allowed detrital transport of uraninitein the reducing environs of the bedload of streams to producethe quartz-pebble conglomerate uranium deposits.

The development of red beds in the post-2200 Ma era (Cloud,1976) indicates uranium, readily leached from the enrichedprovenance rocks, could be transported as the uranyl ion in thenear-surface oxidizing atmosphere. A feature of the Proterozoicunconformity and stratabound uranium deposits are the large U/Thratios, which are generally greater than 100 and in the high-grade ore zones can exceed 1000. These high ratios supportthe uranyl transport concept as Th is diadochic with U

The high-grade unconformity-related uranium deposits weredeveloped in the post-2200 Ma period being either hosted inLower or Middle Proterozoic rocks. It would appear from knowndiscoveries that this time constraint is valid in the productionof these deposits.

In order to account for this time constraint it could beargued that earlier detrital uraninite was concentrated in pre-2200 Ma sediments, as is the case for the quartz-pebbleconglomerates, and that this syngenetic mineralisation waslater mobilised and concentrated by oxidising solutions relatedto tectonic processes. An alternative hypogene argumentwould be to suggest that igneous activity in the Lower andMiddle Proterozoic was U-enriched. However, some of theseU-deposits lack a time and spatial association with igneousevents. If a supergene model is adopted the following argumentcould be advanced to account for the skewed time distribution ofunconformity-related deposits (Ferguson & Rowntree, 1980;Robertson et al, 1978) .

As continental crustal development in post-2200 Ma timesappears mainly to follow uniformitarian lines, the only variablewhich could explain the concentration of vein-type uranium bysupergene processes in the 2200-1400 Ma period appears to be asteadily evolving atmosphere. It is suggested that during thesetimes the hydrosphere was sufficiently oxidising for uranyltransport, but that rapidly reducing conditions were met shortdistances into the lithosphère. Reduction resulted in precip-

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itation of uranium as U02 from meteoric water into suitablestructural traps, which were largely developed during periods ofprolonged erosion. The structural traps may also have beenactive during the early sedimentation of the Middle Proterozoiccover rocks. Rapid development of impermeable cover rockspreserved the uranium deposits from subsequent weathering andtransport. In the post-1400 Ma period the atmosphere wassufficiently oxidizing so that reduction in groundwater situat-ions was generally inadequate for the precipitation of uraniumon any large scale other than in restricted continental sand-stone situations. The ultimate "sink" for uranium under thesecircumstances would have been the oceans where it is found toconcentrate in sea-water, altered floor basalts, black shalesand other pelagic sediments. This broad dissemination ofuranium within the marine environment did not allow any extens-ive uranium deposits to form in the post-1400 Ma period.

A similar supergene genetic argument could be advanced forthe dominantly stratiform U-deposits to explain their distribut-ion in Lower and Middle Proterozoic rocks. A hypogene processcan also be advanced for the Aillik and Olympic Dam Deposits;if this genetic model is correct then it is difficult to under-stand why uranium associated with alkaline igneous rocks shouldnot in fact occur throughout the geological time scale. Theapparent time skewness shown for these deposits may only be areflection of yet undiscovered younger occurrences.

It is clear that once a province has been enriched inuranium - whether these are basement rocks or alkaline igneousrocks that all rocks post-dating this event in the province areusually anomalously enriched in uranium. This later enrichmentresults from the highly labile behaviour of U under oxidisingconditions whether they be hypogene or supergene. What isrequired in exploration to locate ore bodies is identifying thestructural traps which can be either syngenetic or epigenetic.

ACKNOWLEDGEMENTG.R. Ewers kindly reviewed this Chapter.

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REFERENCESCLOUD, P., 1976: Major features of crustal evolution.

Du Toit Memorial Lecture 14, Geol. Soc. Sth Africa,Annexure to Volume 79.

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ROBERTSON, D.S., TILSLEY, J.E. & HOGG, G.M., 1978: The time-bound character of uranium deposits. Econ. Geol., 73,1409-1419.

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