magmatic ni-cu-platinum-group element deposits of the thompson nickel belt

7/27/2019 Magmatic Ni-cu-platinum-group Element Deposits of the Thompson Nickel Belt

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Layton-Matthews, D., Lesher, C.M., Burnham, O.M., Liwanag, J., Halden, N.M., Hulbert, L., Peck, D.C., 2007, Magmatic Ni-Cu-platinum-group elementdeposits of the Thompson Nickel Belt, in Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, theEvolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, p.409-432.

MAGMATIC NI-CU-PLATINUM-GROUP ELEMENT DEPOSITS OF THE

THOMPSON NICKEL BELT

DANIEL LAYTON-MATTHEWS1,2 , C. MICHAEL LESHER 1, O. MARCUS BURNHAM1,3,JANICE LIWANAG 4, NORMAN M. HALDEN4, LARRY HULBERT5, AND DAVID C. PECK 6

1. Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 6B52. Current Address: Department of Geological Sciences and Geological Engineering, Queen's University,

Kingston, Ontario K7L 3N6

3. Current Address: Ontario Geoscience Laboratories, Willet Green Miller Centre, 933 Ramsey Lake Road,Sudbury, Ontario P3E 6B5

4. Department of Geological Sciences, Wallace Building, University of Manitoba, Winnipeg, Manitoba R3T 2N25. Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8

6. Anglo American Exploration (Canada) Ltd., Suite 800 - 700 West Pender Street, Vancouver, British Columbia V6C 1G8Corresponding author’s email: [email protected]

Abstract

The Circum-Superior Boundary Zone in Manitoba hosts several world-class magmatic Ni-Cu-platinum-group ele-ment (PGE) deposits. Operating mines in the Thompson Nickel Belt (TNB) include the Thompson T1, T3, andBirchtree mines. In 2004, the International Nickel Company (Inco) reported proven and probable reserves of 27 Mtgrading 2.10% Ni and 0.14% Cu. These deposits are locally concentrated in a narrow, northeast-trending tectonic beltthat represents collisional remnants of Archean Superior Province rocks, autochthonous Proterozoic supracrustalsequences (Ospwagan Group, TNB), and allochthonous Proterozoic rocks related to the Trans-Hudson Orogen. Since

the discovery of Ni-Cu-(PGE) mineralization by Inco in 1955, it has been recognized that the TNB deposits exhibitmany fundamental characteristics of other major magmatic Ni-Cu-(PGE) districts.Mineralization within the TNB is associated with variably serpentinized ultramafic intrusions that range in compo-

sition from dunite to pyroxenite and are hosted by clastic and chemical sediments of the Ospwagan Group. The serpentinized intrusions are generally lensoid to tabular in shape, which reflects the multiple phases of deformation expe-rienced by the host Ospwagan Group during orogenesis. Ore within the TNB is found as ‘primary’ massive sulphidemineralization within the ultramafic bodies (i.e. Pipe and Birchtree deposits) or, more commonly, as ‘secondary’ mas-sive sulphide mineralization within metasedimentary rocks of the Ospwagan Formation. Serpentinized intrusions occur at specific stratigraphic levels within the Ospwagan Group. The consistent stratigraphic correlations between ultramaficintrusions, S-rich chemical sedimentary rocks, S isotopic data, and Ni-Cu-(PGE) sulphide mineralization collectivelysuggest that the mineralization formed by assimilation of S-rich sedimentary rocks by high-temperature ultramafic mag-mas. Post-ore deformation and metamorphism have significantly modified the primary characteristics of many of theTNB ore deposits.

Résumé

La zone périphérique de transition de la Province du lac Supérieur au Manitoba renferme plusieurs gisements mag-matiques de Ni-Cu-éléments du groupe du platine (ÉGP) de classe mondiale. Les mines en exploitation dans la cein-ture nickélifère de Thompson (CNT) sont les mines Thompson T1 et T3 et la mine Birchtree. En 2004, l’International

Nickel Company (Inco) a signalé des réserves prouvées et probables de 27 millions de tonnes de minerai renfermant2,10 % de Ni et 0,14 % de Cu. Ces gisements sont concentrés localement dans une étroite zone tectonique d’orienta-tion nord-est (correspondant à la CNT), qui constitue un assemblage vestigial de collision formé de roches archéennesde la Province du lac Supérieur, de séquences de roches supracrustales autochtones du Protérozoïque (Grouped’Ospwagan, CNT) et de roches allochtones du Protérozoïque associées à l’orogène trans-hudsonien. Depuis la décou-verte des minéralisations de Ni-Cu-ÉGP par l’Inco en 1955, on a reconnu que les gîtes de la CNT présentent un grandnombre des caractéristiques fondamentales qui définissent les autres principaux districts de minéralisations magma-tiques de Ni-Cu-(ÉGP).

Dans la CNT, les minéralisations sont associées à des intrusions ultramafiques serpentinisées à des degrés diversdont la composition varie de la dunite à la pyroxénite et qui sont encaissées dans les roches sédimentaires clastiques etchimiques du Groupe d’Ospwagan. Les intrusions serpentinisées sont généralement de forme lenticulaire à tabulaire,ce qui est un reflet des multiples phases de déformation qu’a subies le Groupe d’Ospwagan pendant l’orogenèse. Dansla CNT, le minerai se présente sous forme de minéralisations « primaires » de sulfures massifs à l’intérieur de corps

ultramafiques (p. ex. les gisements de Pipe et de Birchtree) ou, plus couramment, sous forme de minéralisations « sec-ondaires » de sulfures massifs dans les roches métasédimentaires du Groupe d’Ospwagan. Les intrusions serpentiniséesse situent à des niveaux stratigraphiques spécifiques dans le Groupe d’Ospwagan. Les corrélations stratigraphiquescohérentes entre les intrusions ultramafiques, les roches sédimentaires chimiques riches en S, les données sur les iso-topes du S et les minéralisations sulfurées de Ni-Cu-ÉGP suggèrent collectivement que la minéralisation s’est formée

par assimilation de roches sédimentaires riches en S par des magmas ultramafiques de haute température. La déforma-tion et le métamorphisme postérieurs à la formation des minerais ont considérablement modifié les caractéristiques pri-maires d’un grand nombre des gîtes de la CNT.



Introduction

The Thompson Nickel Belt(TNB) represents the remnants of a

Neo-Archean - Paleoproterozoiccontinental margin and lies withinthe Circum-Superior BoundaryZone (CSBZ) that separates 3.1 to2.6 Ga autochthonous ArcheanSuperior Province rocks in the eastfrom 1.92 to 1.83 Ga allochthonousProterozoic domains of the Trans-Hudson Orogen (THO) to the west(Fig. 1 inset). The TNB has long

been defined as a narrow, north-east-trending (030°) geologicaldomain that locally hosts mag-matic Ni-Cu-(PGE) mineralization(McRitchie, 1995). The TNB con-stitutes part of the larger Circum-Superior Boundary Zone (CSBZ),where the TNB component of the

CSBZ extends from approximately50 km northeast of Thompson toapproximately 125 km southwestof Thompson, from where it is cov-ered by Paleozoic carbonate plat-form rocks (Fig. 1). Beneath thePaleozoic platform, the geophysi-cal expression of the TNB contin-ues approximately 275 km south-ward towards the Saskatchewan

border, where the Circum-Superior Boundary Zone is interpreted tocontinue into North Dakota to adepth of 500 to 2000 m beneath

Paleozoic and Mesozoic rocks. Tothe northeast of the exposed TNB,the CSBZ trends east through the

Ni-Cu-(PGE)-bearing Fox River Belt and Sutton Inlier, and contin-ues north through the Belcher Islands, the Ottawa Islands, andeast through the Ni-Cu-(PGE)-

bearing Cape Smith Belt (Lucas et al., 1990).The TNB hosts several past and current producing world-

class magmatic Ni-Cu-(PGE) deposits (Table 1; Fig. 1).Although the deposits of the TNB have been structurally andmetamorphically modified, they exhibit many fundamentalcharacteristics of other major magmatic Ni-Cu-(PGE) dis-

tricts. Over the past fifty years, exploration has concentratedon the exposed section of the northern Thompson NickelBelt (NTNB) and central Thompson Nickel Belt (CTNB),

between Moak and Muhigan Lakes. More recently, explo-ration has been extended to include the Paleozoic-coveredsouthern Thompson Nickel Belt (STNB).

Inco reports provide some of the earliest descriptions of the Ni-Cu-(PGE) deposits in the Thompson area, but most of these reports remained confidential until the early 1980s(Peredery, 1979, 1982). Inco geologists identified two typesof ultramafic bodies: 1) “ultramafites”, the ore-hosting

metadunitic to metaperidotitic rocks in the TNB, and2) “ultramafic amphibolites”, metagabbroic rocks that occur in close proximity to the ultramafic bodies (terminologyafter Peredery et al., 1982). Several of the TNB ultramafic

bodies have been studied as part of masters and doctoral the-ses or government directed studies, including the

Manibridge ultramafic body (Coats and Brummer, 1971;Coats et al., 1976), Reservation 34 ultramafic bodies (Bliss,1973), West Manasan and Pipe II ultramafic bodies (DeSaboia, 1978), and the Bucko ultramafic body (Good and

Naldrett, 1993). Several workers have summarized and clas-sified the TNB ultramafic rocks as a group (Coats, 1966;Scoates, 1971), but it was not until the work of Macek andBleeker (Macek, 1986; Bleeker, 1990) and the TNB workinggroup (Macek et al., 2002), which expanded on Inco’s strati-graphic correlations, that a stratigraphy and tectonostrati-graphic model for the genesis of the TNB ultramafic bodiesand the Ospwagan Group were developed.

D. Layton-Matthews, C.M. Lesher, O.M. Burnham, J. Liwanag, N.M. Halden, L. Hulbert, and D.C. Peck

410

S M B 9 2

Grand Rapidsrand Rapids

Snow Lakenow Lake

Pontononton

Wabowdenabowden

THOMPSONHOMPSON 55.85.8oN

97.97.9oW

Eastervilleaste rville

Kisseynew

Flin Flon

Pikwitonei

C h

u r c h i l l

- S u p e r i o

r

B o u n d a

r y Z o n e

Gods Lake

Molson Lake

kilometres

010 5020304050

PaleozoicCover

Nickel Belt

SOUTHER NOUTHERN

CENTRALENTRAL

NORTHERNORTHERN

Figure 2

Figure 3

Mines and Deposits

1

23

4

56

7

1. Moak2. Thompson3. Birchtree

4. Pipe

5. Soab North6. Soab South7. Manibridge

391

280

6

60

6

39

373

Figure locations

Map Area

FIGURE 1. General geological map of the Thompson Nickel Belt. Areas of Figures 2 and 3 are outlines indashed lines.



Magmatic Ni-Cu-Platinum-Group Element Deposits of the Thompson Nickel Belt

4

The regional geologic setting and tectonic evolution of the

TNB have been summarized by Kornik and MacLaren(1966), Gibb (1968; 1982), Weber and Scoates (1978), Green(1981), Bleeker (1990), Machado et al., (1990), Weber (1990), Lewry et al., (1994), and White et al., (1999; 2002).Additional geological information are available from theManitoba Geological Survey bedrock maps and the recentlycomplied series of fifteen maps of the Thompson Nickel Belt(Macek et al., 2001, and references therein). This map-serieswas a collaborative project involving the ManitobaGeological Survey, Inco Limited, Falconbridge Limited, andHudson Bay Exploration and Development Company

Limited that covered an area from Moak Lake (northeast of

Thompson) to Bracken Lake (north of Grand Rapids).The aim of this manuscript is to provide a current synopsisof the geology and exploration models for the Ni-Cu-(PGE)deposits in the TNB. It is derived largely from the work of theforegoing authors, but includes some of the data and inter-

pretations generated in the recently completed CanadianMining Industry Research Organization (CAMIRO)Thompson Nickel Belt project (Burnham et al., 2003).

Exploration History

Fraser (1985) provides a complete account of theInternational Nickel Corporation (Inco) Ltd.’s early explo-

NameSize of UM body

(DDH intersectionwidth, m)

Deposit StatusLocation

UTM(NAD83)

Description

Mel Zone n/a Explored Lease 576762.69 E6203813.21 N

Moak 250-1000 Explored Lease 588635.39 E6199717.48 N

Mystery Lake 250-1000 Explored Lease 577874.66 E6187672.92 N

Thompson Area

Thompson 1C 10-250 Present Producer 571987.65 E6175512.04 N

Thompson 1D n/a Present Producer n/a

Thompson South Pit n/a Past Producer n/a

Birchtree 10-250 Present Producer 567401.25 E6173472.14 N

Pipe I 10-250 Explored Lease 553193.46 E6150384.44 N

Pipe II 250-1000 Past Producer n/a

Pipe Deep n/a Explored Lease n/a

Hambone 10-250 Explored Lease 573033.62 E6127480.23 N

Grass 10-250 Explored Lease 541194.64 E6121504.84 N

Soab (North & South) 1-10 Past Producer 538042.03 E6120827.49 N

Bowden n/a Explored Lease 522859.16 E6086262.75 N

Discovery n/a Explored Lease 524292.08 E6084007.59 N

Bucko n/a Past Producer 522060.43 E

6081418.11 N

Resting Lake n/a Explored Lease 518593.22 E6079436.51 N

Manibridge n/a Past Producer 510493.18 E6061680.43 N

Minago n/a Explored Lease 487790.01 E5993365.93 N

William Lake n/a Explored Lease 474922.46 E6075387.78 N

1. Naldrett, 20042. Bleeker, 1990

3. Crowflight Minerals Ltd., press release, Sept. 20, 2005n/a = not available

Mineral Resource - 0.74% Ni over 32 m

Mineral Resource - 18.8 Mt at 1% Ni or 2.5 Mt @ 2.23%

Ni and 0.17% Cu. Includes 11.7 m @ 5.1% Ni, 0.40%Cu, and 1.62 g/t PGE3

Mineral Resource - 90 Mt @ 0.30% Ni-Cu

Mineral Deposit - 1.27 Mt @ 2.55% Ni and 0.27% Cu

Mineral Resource - 20.5 Mt @ 1.02% Ni

Mineral Resource - low grade

Mineral Resource - 0.9 Mt grading up to 1.5% Ni

Mineral Resource - 87.9 Mt @0.627% Ni

Mineral Resource - 4.5 Mt @ ~1% Ni

Mineral Deposit - n/a

Mineral Deposit - ~18 Mt2

Mineral Resource - 3.27 Mt @ 0.81% Ni and 1.09 [email protected]% Ni in north Creek zone

Mineral Resource - 3.6 Mt @ 2.32 Ni, 0.1% Cu

Mineral Deposit - 4.5 Mt

Mineral Deposit - 19 Mt @ 2.5% Ni



Mineral Resource - n/a

Mineral Resource - 45 Mt @ 0.7% Ni, widths up to 90 m

Mineral Resource - 227 Mt @ 0.6% Ni

Combined Resource - 150 Mt @ 2.32% Ni, 0.16% Cu,0.046% Co, 0.83 g/t PGE1

TABLE 1. Mineral resources and deposits of the Thompson Nickel Belt (after Bamburak, 1980; Peredery, 1982; McRitchie, 1995).



ration efforts and their discovery of Thompson Ni deposits inthe immediate vicinity of the City of Thompson, Manitoba.Exploration in the TNB area started in 1946, when Inco

began a 10 year geophysical and geological exploration pro-gram, leading to the discovery of the Moak property in 1955.By the beginning of 1956, over 72 km of exploration dia-mond drilling had been completed and in February 1956 the

main Thompson orebody was discovered. This discovery ledto the construction of the current mining complex and theCity of Thompson (Fraser, 1985). The Thompson mine offi-cially opened on March 25, 1961, becoming the Thompson

Nickel Belt’s first active mine. Additional resources werediscovered on the Pipe I property south of Upper OspwaganLake. By 1961, Inco had 8000 active claims in the TNB, andhad conducted an extensive geophysical program thatincluded 128,000 km of aeromagnetic surveys, 112,000 kmof airborne electromagnetic surveys, and 17,600 km of ground geophysical surveys, by which time the Pipe II

deposit had been discovered. Production from an open pit atPipe II began in 1969, with production continuing until itsclosure in 1984. From 1961 to 1971, Inco brought theThompson, Birchtree, Pipe I, Pipe II, Soab North, and SoabSouth mines into production (Table 1). In response to falling

Ni prices, Inco closed all but the Thompson and Pipe open pit mines in the TNB by 1977. In 1986, the Thompson Open

Pit North was brought into production, and mining of theThompson 1C orebody between the 732 and 975 m levels,reactivation of the Birchtree mine, and construction of theThompson Open Pit South soon followed. In 1990, Incoannounced plans to mine the Thompson 1D orebody to the1100 m level and to deepen the Birchtree shaft from the 1045m level to the 1295 m level (Bamburak, 1980; Hulbert et al.,2005). The deepening of the Birchtree mine was completedin 2002, nearly doubling daily production, increasingreserves to 13.6 Mt of 1.79% Ni, and extending Birchtree’smine production life to 2016. Recently, mineralization and


412

LEGEND

INTRUSIVE ROCKS

Gabbro of MacKenzie dyke swarm

Pegmatite

Metadiabase dykes of Molson Dyke swarm

BURNTWOOD GROUP

Ultramafic rocks - sills in Ospwagan Group

undivided

undivided - paragneiss

GRASS RIVER GROUP

OSPWAGAN GROUP SUPRACRUSTAL ROCKS

Bah Lake Assemblage- metavolcanic rocks

Setting Formation- metaquartziteand metapelite

Pipe iron formation- sulphide facies

Pipe Formation- undivided iron formation,chert, metapelite, semipelite,

dolomite marble, calc-silicate

Thompson Formation- marlstone or marble

Manasan Formation- clastic rocks

ARCHEAN BASEMENT

Enderbitite gneiss

Migmatite - Gneiss

Metagabbro

5 6 5 0 0 0

5 5 5 0 0 0

6 155 000

6 165 000

6 175 000

Thompson

ThompsonMine complex

T1 & T2

Headframes

BirchtreeMine &

Headframe

Pipe MineHeadframe& open pit 0 2.5

km

5

FIGURE 2. Interpreted bedrock geological map from the Thompson to the Pipe deposits in the Northern Thompson Nickel Belt (modified after Macek et al.,2001). Mine locations indicated by black boxes.




4

host rocks similar to those of TNB(i.e. Ospwagan Group; see below)were discovered north and west of Mystery Lake. The current operat-ing mines in the TNB include theThompson T1, T3, and Birchtreemines (Figs. 1, 2).In 2004, proven and probable

reserves of 27 Mt grading 2.10% Niand 0.14% Cu were reported(International Nickel CompanyLimited, Annual Report, 2004) for the active mines in the TNB.

In the Paleozoic-covered south-ern Thompson Nickel Belt (STNB),Falconbridge conducted a multi-year (1991-2002) (FalconbridgeLimited, Annual Report, 1996,2004) exploration program thatidentified significant Ni mineral-ization in the William Lake area(Figs. 1, 3). The principal resourcewas one of several Ni occur-rences that are spread over a dis-tance of approximately 20 km. In1994, Falconbridge completedextensive airborne electromagnetic(GEOTEM) and magnetic surveysto improve the regional delineationof the more prospective zones onits properties. Several other compa-nies have recently operated, or areactively operating, exploration programs in the southwestextension of the TNB, most notably Hudson BayExploration and Development Company Ltd. (Minago River area), Cominco Ltd. (mid-1990s; Winnipegosis Belt), andCrowFlight Minerals Inc. (Wabowden area).

Stratigraphy

The Circum-Superior Boundary Zone in northernManitoba has been interpreted to represent part of a forelandmargin of the Superior Craton that is marked by 100 kmscale re-entrants, such as the Winnipegosis Komatiite Belt,the Fox River Belt (Lucas et al., 1990), and the TNB (Whiteet al., 2002). The recognition that the Ni-Cu-(PGE)-bearingultramafic bodies occur at specific stratigraphic levels withinthe foreland margin (Pipe Formation of the OspwaganGroup) has been the historical focus of exploration of mag-matic ore deposits in the TNB. The stratigraphic framework,

formational thickness, and sedimentary characteristics of theOspwagan Group are presented in Figure 4.The Ospwagan Group (Scoates et al., 1977) is interpreted

to have been deposited near a passively rifted margin on acontinental platform that experienced a subsequent period of active rifting and ultramafic to mafic magmatism, as repre-sented by the boudinaged, mineralized, and nonmineralizedultramafic sills. The following sections review the stratigra-

phy of the Ospwagan Group, as understanding the level of emplacement of the ultramafic intrusions has been criticallyimportant in mineral exploration in the TNB (Bleeker, 1989).

Basement Gneisses

The Ospwagan Group unconformably overlies felsic tointermediate migmatitic gneisses that record at least one gen-

eration of deformation prior to the deposition of theOspwagan Group (Figs. 5A,B). The basement gneisses con-tain relics of granulite facies metamorphism or retrogressed

pseudomorphs after ortho- and clinopyroxene (Bleeker,1990). Gneiss compositions vary in the TNB and likelyreflect protolith inhomogeneity. Generally, the gneisses aretonalitic in composition, and exhibit varying degrees of mylonitization and anatexis. Biotite-rich varieties containgarnet and sillimanite porphyroblasts.

Ospwagan Group

The Ospwagan Group consists of a sequence of clasticand chemical sediments separated from underlyingmigmatitic Archean basement by an angular unconformity. It

has been divided into four formations, from oldest toyoungest (Bleeker, 1990): basal conglomerates andquartzites of the Manasan Formation; impure calc-silicatesof the Thompson Formation; semipelites, sulphidic sedi-ments, and iron formations of the Pipe Formation; and inter-layered quartzite and semipelites of the Setting Formation.The Setting Formation is overlain by mafic-ultramaficmetavolcanic rocks (Bah Lake Assemblage: Zwanzig, 2004)and/or a younger suite of clastic metasedimentary rocks(Grass River Group: Zwanzig and McRitchie, 1997). Nickel-Cu-(PGE) mineralization in the TNB is associated almost

4 7 0 0 0 0

4 7 5 0 0 0

5 970 000

5 965 000

LEGEND

INTRUSIVE ROCKS

Pegmatite

Metadiabase dykes- Molson Dyke swarm

Ultramafic rocks- sills in Ospwagan Group

OSPWAGAN GROUPSUPRACRUSTAL ROCKS

Setting Formation- metaquartzite/pelite

- sulphide-faciesiron formation

Pipe Formation- undivided iron formation,metapelite, calc-silicate

Thompson Formation- marlstone or marble

Manasan Formation- clastic rocks

ARCHEAN BASEMENT

Migmatite - Gneiss

km

0 2.5

William Lake Dome

FIGURE 3. Interpreted bedrock geological map of the William Lake area in the southern Thompson Nickel Belt(modified after Macek et al., 2001).



exclusively with, or localizedwithin, ultramafic bodies within thelower part of the Ospwagan Group,in particular the Pipe Formation(Bleeker, 1990).

Manasan Formation

The contact of the Ospwagan

Group with the underlying base-ment is angular to disconformable,and in some locations is marked bya 5 to 10 cm thick sillimanite-richlayer, interpreted by Bleeker (1990)to represent a metaregolith. Twomembers have been identified inthe Manasan Formation: M1 andM2. The M1 basal quartzite and

basal conglomerate fine upwardsand gradationally increase in phyl-losilicate content toward the M1-M2 transition (Fig 5C,D). It is light

beige to grey, and subarkosic toarkosic in composition with 85 to90 wt.% SiO2. The overlying M2unit, known as the Manasan semi-

pelite, contains silt-sized micro-cline and quartz grains in an argilla-ceous matrix, is light to dark greyin colour, and is interpreted as anargillaceous wacke.

The angular unconformity is evi-dence that the Archean basementwas exposed during the LateArchean to Early Proterozoic. The

progression from clastic quartzite

(M1) to argillaceous wacke (M2)has been interpreted as a transgres-sive event in response to a passivemargin subsidence (Bleeker, 1990).

Thompson Formation

The Thompson Formation marksthe onset of chemical sedimenta-tion in the Ospwagan Group, andhas been subdivided into three rec-ognizable members: T1 semipelitewith sodic plagioclase, T2 semi-

pelite with calcic plagioclase, andT3 dolomitic marble.

The T1 unit comprises micro-cline, diopside, hornblende, and biotite that form alternating lami-nae of microcline-rich and diop-side+carbonate+biotite+quartz lay-ers. Leucosome produced during

peak metamorphism is pink andcomprises K-feldspar with minor quartz (Fig 6A,B). The T2 unit wasfirst defined at the Thompson Open Pit (1C), where it super-ficially resembles the M2 semipelite; however, diopside andcalcic plagioclase are more abundant in T2. Leucosome

veins within the T2 member are white and contain abundant plagioclase (Bleeker, 1990). The T3 member consists of dolomitic marble with interlayered ‘chert’ bands. The T3


414

1,2,3

S2

S1

P3

P2Thompson,

Willam Lake

Pipe, BirchtreeP1

T3

T2

T1

M2

M1

A

O S P W A G A

N G R O U P

Member

BAH LAKE

ASSEMBLAGE

Mafic to

Ultramafic

Volcanics

SETTING

FORMATION

Upper

Clastic

Sediments

PIPE

FORMATION

Pelitic Sediments

and

Iron Formations

THOMPSON

FORMATION

Calcareous

Sediments

MANASAN

FORMATION

ARCHEAN

BASEMENT

Lower ClasticSediments

Formation

SedimentarySulphide

(modal increase)

Mafic and Ultramafic flows -

massive and pillowed

Pebble conglomerates

Quartzite

Pelitic schist

Nickel sulphide deposit

Silicate and sulphide facies iron-formation, sedimentary sulphides,and red chert

Dolomictic marble

Semipelite with calcic plagioclase

Semipelite with sodic plagioclase

Semipelite

Subarkosic to arkosic

Retrogressed gniesses

LEGEND

FIGURE 4. Reconstructed Ospwagan Group lithostratigraphy for the Thompson Nickel Belt (modified after Bleeker, 1990). Stratigraphic locations of ore deposits are indicated by pink arrows.




4

FIGURE 5. Photographs of the Ospwagan Group. (A) Angular relationship between basement gneisses (right) and overlying Manasan Formation (left) at thesouthern margin of the Thompson South Pit. (B) Same contact as is shown in photograph A but at Pipe Pit; the contact (hammer parallel to contact, Manasanon top, basement on bottom) has been obscured by deformation. (C) The Manasan Formation at Pipe Pit M1/M2 contact. Contact is located at the lens cap,with the M1 unit to the right and the M2 to the left. (D) M2 unit at Pipe Pit with pegmatitic leucosome.

A B

C D



unit usually has a pale green colour, with varying propor-tions of fosteritic olivine, clinopyroxene, and calcic amphi-

bole.The Thompson Formation is interpreted to represent wan-

ing clastic sedimentation and waxing chemical sedimenta-tion. This is consistent with continued transgression and thegeneration of a starved basin (Bleeker, 1990).

Pipe Formation

The Pipe Formation consists of a sequence of chemicaland chemical-clastic sedimentary rocks that can be subdi-vided into three members: P1 silicate- and sulphide-faciesiron formation and red chert, P2 pelitic schist, and the P3

pelitic schist with calc-silicate intercalations, silicate-faciesiron formation, dolomitic marble, and chert with calc-silicateintercalations.

The Pipe Formation is named after the type locality at

Pipe Lake, which has since been drained and mined by open pit (Pipe II Pit). The glacially polished surface exposures inthis area are exceptional and the mapping by Bleeker (1990)at Pipe Pit has formed the basis for defining most of the PipeFormation stratigraphy.

The P1 member comprises pelitic schists that have a gra-dational lower contact with T3 calcareous sediments. The P1member contains abundant graphite and moderate amountsof sedimentary sulphides. Sulphide-facies iron formation(known as the lower sulphide iron formation) is character-ized by thin pyrrhotite layers intercalated with graphiticchert or schist (Fig 6C). Sulphide-facies iron formations are

overlain by hornblende+biotite±grunerite±garnet silicate-facies iron formation (lower silicate-facies iron formation)with minor disseminated sulphides. Some layers within thesilicate-facies iron formation contain up to 20% (modal)

magnetite (Fig. 6D).The P2 member is characterized by muscovite-biotite or

biotite schists with abundant aluminous porphyroblasts (gar-net, staurolite, or sillimanite, depending on metamorphicgrade), and represents the main parcel of pelitic schists in theTNB. P2 schists are intercalated with thin layers of chert,calc-silicate, and silicate-facies iron formation (Fig. 7A).The uppermost part of the P2 member is characterized by anincreasing abundance of sulphide-rich lamellae and semi-massive to massive pyrrhotite-pyrite layers (known as theupper sulphide-facies iron formation) (Fig. 7B).

The thick succession of pelitic schists of P2 grade intocyclic units of silicate-facies iron formation and layeredquartzites of the P3 member. The lower section of the P3member is garnet- and biotite-rich near the base and theseminerals decrease in abundance up section, likely reflectinga reduction in the pelitic component (Fig. 7C). The upper-most part of the Pipe Formation is marked by P3 upper sili-cate-facies iron formation that comprises quartz, biotite, gar-net, calcic amphibole, ortho- and/or clinopyroxene, fayalite,carbonate, and magnetite with minor graphite and sulphide(Fig. 7D).

The Pipe Formation is interpreted to represent periods of prolonged chemical sedimentation and nondeposition of clastic material, alternating with periods of turbiditic sedi-


416

A B

FIGURE 6. Photographs of the Ospwagan Group. (A) ‘Skarns’ of the Thompson Formation (T1) from Manasan quarry. (B) Thick sequence of ThompsonFormation from the southern margin of Thompson south pit.




4

FIGURE 6 CONTINUED. (C) Sulphide-facies iron formation (known as the lower sulphide iron formation) with thin pyrrhotite layers intercalated with graphiticchert or schist from Pipe Pit (D) Isoclinally folded lower silicate-facies iron formation from Pipe Pit.

C D

FIGURE 7. Photographs of the Ospwagan Group from Pipe Pit. (A) P2 member of pelitic schists with sulphide-rich lamellae and semimassive to massive pyrrhotite-pyrite layers. (B) Muscovite-biotite or biotite schists of the P2 member with abundant aluminous porphyroblasts (garnet, staurolite, or sillimanite,depending on metamorphic grade). Commonly intercalated with thin layers of chert, calc-silicate or silicate-facies iron formation.

A B



mentation (Bleeker, 1990).

Setting Formation

The Setting Formation is characterized by the absence of

iron formations and is composed of interlayered quartzitesand pelitic schists that coarsen upward into chemicallyimmature wackes and, locally, pebbly conglomerates withoccasional graded beds. The Setting Formation has beenreferred to as the “core quartzites” (Fig. 8A), as they repre-sent the core of the downward-facing sequence of theThompson mine structure (Bleeker, 1990). Minor mafic vol-canic rocks (flows and tuff) are intercalated with the clasticrocks near the top of the Setting Formation. The SettingFormation is interpreted to represent a magmatically activeforedeep, with clastic material derived from a tectonicallyactive hinterland (Bleeker, 1990).

Bah Lake Assemblage

The mafic metavolcanic rocks overlying the SettingFormation have been referred to as the Ospwagan Formation(Bleeker, 1990), but have recently been renamed the BahLake Assemblage (Zwanzig, 2004). Although these rockswere interpreted by Bleeker (1990) to stratigraphically over-lie the Setting Formation, many of the diamond-drill coreintersections of mafic rocks cannot be stratigraphically cor-related with any confidence. Nevertheless, these rocks have

been separated into three broad members based on petro-graphic textures and chemical composition: BL1 metabasalt,BL2 magnesian metabasalt, and BL3 metapicrite (metamor-

phic olivine and 12-18 wt.% MgO; see discussion by Kerr and Arndt, 2001).

The paucity of continuous exposures of the Bah LakeAssemblage makes inferences about stratigraphic correlation

between the members (BL1, BL2, and BL3) difficult. The best exposures of the Bah Lake Assemblage are sporadicallylocated between Setting and Mystery lakes, and consist pre-dominantly of pillowed and massive flows (Fig. 8B,C), but

basal units commonly contain heterolithic mafic breccias.Metabasalts of the BL1 member are pillowed and massive

flows with minor fragmental rocks. In outcrop, they are finegrained, grey to black in colour, and are commonly foliated.The BL2 member is often spatially associated with BL1, butis more magnesian in composition. BL2 magnesianmetabasalts are commonly pale grey, massive with pillow-like structures, and are generally characterized by abundant

porphyroblastic olivine. Metapicrites of the BL3 member areclosely associated with the BL1 and BL2 members at Upper

and Lower Ospwagan Lakes, forming pillowed to massiveflows with a characteristic pale green colour. Pillowedsequences have distinct pale cores with dark green selvages(Fig. 8D). In almost all instances, volcanic rocks of the BahLake Assemblage have been metamorphosed to amphibolitefacies (Peredery, 1979).

Ultramafic Units

The stratigraphic relationships between the ultramafic bodies of the TNB and the host Ospwagan Group metasedi-mentary and metavolcanic rocks have been difficult to estab-


418

FIGURE 7 CONTINUED. (C) Layered quartzites of the P3 member. (D) Upper silicate-facies iron formation of the P3 member.

C D




4

A B

FIGURE 8. Photographs of the Ospwagan Group and Thompson Nickel Belt volcanic lithologies. (A) ‘Core quartzite’ of the Setting Formation from SettingLake. (B) Pillowed metabasalts from Ospwagan Lake. (C) Pillowed metabasalts from the Mid Lake area. (D) Pillowed ultramafic volcanic rocks in theMystery Lake area.

C D




420

P i p e ( 8 6 2 3 2 )

3 7 0 0

3 9 0 0

3 8 8 0

P y r o x e n i t e

3 8 0 0

D E P T H ( m )

D E P T H ( m )

H a r z b u r g i t e

H a r z b u r g i t e

D E P T H ( m )

D E P T H ( m )

M g O

( w t . % )

1 2 0 0

1 3 0 0

1 4 0 0

1 5 0 0

1 6 0 0

1 7 0 0

1 8 0 0

1 9 0 0

2 0 0 0

2 1 0 0

2 2 0 0

2 3 0 0

2 4 0 0

2 5 0 0

2 6 0 0

M g O

( w t . % )

M i d L a k e ( 8 6 2 2 0 )

0

5 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

W i l l i a m L a k e ( W L 9 6 - 1 6 8 )

1 5 0

6 5 0

0

5 0

M g O

( w t . % )

0

5 0

0

5 0

M g O

( w t . % )

S o u t h S p u r ( 8 6 2 2 7 )

7 0 0

8 0 0

9 0 0

1 0 0 0

1 1 0 0

O l i v i n e p y r o x e n i t e

D u n i t e

P a l e o z o i c l i m e s t o n e

P a l e o z o i c s a n d s t o n e

W e

a t h e r e d a l t e r e d u l t r a m a f i c

O r a

n g e p e g m a t i t e

A l t e r e d u l t r a m a f i c

O r a


H e m - A l t e r e d u l t r a m a f i c

H e m - A l t e r e d p e g m a t i t e

H e m - A l t e r e d u l t r a m a f i c

O r a



O r a



W h

i t e p e g m a t i t e


W h

i t e p e g m a t i t e


D u n i t e

O r a


D u n i t e

O r a


D u n i t e

D u n i t e / T r e m o l i t i c

a l t e

r e d u l t r a m a f i c

O r a



B i o

t i t e g n e i s s / s c h i s t

G a r n e t a m p h i b o l i t e

E o H

B i o t i t e S c h i s t

O l i v i n e p y

r o x e n i t e

M a s s i v e s

u l p h i d e v e i n

A m p h i b o l i t e

D u n i t e

D u n i t e

S u l p h i d i c

m e t a s e d i m e n t &

D u n i t e

M a s s i v e s

u l p h i d e z o n e

w i t h i n d u n

i t e

P e r i d o t i t e

S u l p h i d i c

g a r n e t - b i o t i t e s c h i s t

B i o t i t e - g a r n e t s c h i s t

S u l p h i d i c

b i o t i t e s c h i s t


P e r i d o t i t e

D u n i t e

P e g m a t i t e

u l t r a m a f i c s c h i s t






P e r i d o t i t e

D u n i t e

B i o t i t e s c h i s t

F I G U R E

9 .

G r a p h i c a l l o g a n d g e o c h e m i c a l p r o f i l e t h r o u g h t h e W i l l i a m L a k e ( D D H W l 9 6 - 1 6 8 ) , M i d L a k e ( D D H 8 6 2 2 0 ) , S o u t h S p u r ( 8 6 2 2 7 ) , a n d P i p e ( 8 6 2 3 2 ) u l t r a m a f i c b o d i e s f r o m t h e n o r t h e r n , c e n t r a l , a n d

s o u t h e r n T h o m p s o n N i c k e l B e l t . M g O d a t a r e f l e c t p r i m a r y c o m p o s i t i o n a l v a r i a t i o n w i t h i n t h e u l t r a m a f i c b o d y ( m o d i f i e d a f t e r B u r n h a m e t a l . , u n p u b .

C A M I R O r e p o r t , 1 9 9 9 ) .




42

lish because of their limited exposure, variable host-rock metamorphism, and discontinuous nature. Owing to their high degree of serpentinization, the ultramafic bodies in theexposed northern and central TNB have experienced intenseglacial erosion and form recessive topographic features thatare now partially to completely filled by lakes. Extensivedrilling based on geophysical surveys by Inco, Falconbridge,and Hudson Bay Mining and Smelting have shown that most

ultramafic bodies occur as discrete and disjointed boudin(Figs. 2, 3). The geometry and internal structure of many of the ultramafic bodies is still poorly constrained and, in manycases, only one-dimensional information is available.

Ultramafic bodies are generally lensoid to tabular in shapeand have tectonized contacts with the country rocks.Although the morphology of the ultramafic bodies is a prod-uct of tectonism, deformation is largely restricted to the mar-gins, with the degree of primary textural preservationincreasing away from the margins toward the cores of theultramafic bodies.

The ultramafic bodies appear to have experienced multiple phases of deformation, similar to the surrounding coun-

try rocks, indicating that the ultramafic bodies wereemplaced after, or perhaps during, sedimentation but prior toinitial phases of deformation (Cumming et al., 1982;Peredery, 1982; Bleeker, 1989, 1990). In most cases, tecton-ism and pegmatite intrusions along the margins of the ultra-mafic bodies have obscured original contacts. However, inrare examples, primary intrusive contacts between the ultra-mafic rocks and the surrounding sediments are preserved.

Larger, less tectonized ultramafic bodies commonlyexhibit an asymmetric variation in cumulus olivine contentand whole-rock MgO content (Fig. 9) characterized by a thin

pyroxenitic basal zone that grades into a thicker lower zoneof chromite-bearing olivine peridotites and dunites that

become progressively less olivine-rich and grade upward

into a thinner pyroxenitic upper zone. Similar asymetrictrends have been observed in many intrusive and extrusiveultramafic and mafic bodies in less deformed areas such asKambalda, Western Australia (Lesher et al., 1981;Donaldson, 1983; Lesher, 1989); Dumont, Quebec (Duke,1986); Pechenga, Russia (Hanski and Smolkin, 1989);Mouth Keith, Australia (Hopf and Head, 1998); andForrestania, Western Australia (Porter and Mckay, 1981),where the upward decrease in olivine content correlates withstratigraphic younging direction. In areas of the TNB wherethe facing direction of the host sedimentary rocks are wellestablished, the petrographic and geochemical variationswithin the ultramafic bodies are consistent, indicating con-cordant to semiconcordant emplacement into the sedimen-

tary rocks. Unfortunately, insufficient mineral compositionaldata are available to establish whether the olivine-rich lower portions reflect flow differentiation of intratelluric olivine or preferential accumulation of in situ crystallized olivine.

Some ultramafic bodies are more strongly differentiated.Within many intrusions from the central TNB, the upper stratigraphic portions grade from chromite-undersaturatedolivine cumulate rocks into plagioclase- and garnet-bearingamphibolite horizons that can be traced over several 100s of metres. The plagioclase- and garnet-bearing amphibolitehorizons most likely represent more evolved liquids

(basaltic/gabbroic) that solidified in the upper parts of the silland were subsequently metamorphosed to amphibolite.

Mineralogy and Petrology

The ultramafic bodies in the TNB have experienced defor-mation and metamorphism under upper amphibolite faciesand serpentine±carbonate alteration that resulted from theinfiltration of post-peak metamorphism volatile-rich fluids

from metasedimentary country rocks. Although almost alligneous textures and minerals have been destroyed along thedeformed margins of the bodies, primary igneous texturesand relict magmatic minerals are commonly preserved in thecores of the ultramafic bodies. However, as even the mostdeformed rocks in the TNB can be identified on the basis of metamorphic mineralogy and whole-rock geochemical com-

position, all of the rocks have been classified in terms of thetextures and mineralogy of the igneous protoliths.

On the basis of relict igneous textures and/or whole-rock geochemistry, the ultramafic rocks in the TNB can been sub-divided into three main lithologies: metadunites, metaperi-dotites (including harzburgites and rare wehrlites), andmetapyroxenites (both orthopyroxenites and clinopyroxen-ites). Many of the ultramafic bodies appear to have pre-served a primary igneous zoning and layering, where olivinemodal abundances vary systematically through individualdrill-core intersections.

Metadunites

Metadunites in the TNB are generally serpentinizedolivine accumulate rocks with >90% fine- to medium-grained serpentine pseudomorphs after olivine in a fine-grained matrix of amphibole±chlorite±serpentine±chromite.Despite pervasive alteration to mesh-textured serpentine,altered olivine grains commonly preserve part of their origi-nal morphology, including grain outlines and irregular frac-

ture patterns. Olivine grains are generally equant to elongate,with either curvilinear grain boundaries that meet in triple- point junctions and define a close-packed mosaic texture(Fig. 10A) or with ovoid, occasionally idiomorphic shapes(Fig. 10B). Olivine grain sizes vary between 0.2 and 10 mm,

but are generally ~1 mm. Some samples exhibit a weak ori-entation or flattening of olivine grains. Magnetite definingthe rims of altered olivine grains indicates that the dunitesinitially contained <10% intergranular material. In additionto the fine-grained magnetite generated during serpentiniza-tion of olivine, the most altered dunites commonly containstockworks of magnetite and/or carbonate veins. Most of themetadunites appear to have originally contained minor amounts of fine-grained euhedral intercumulus chromite

(Fig. 10C,D). It is unclear if these primary relict chromitesexperienced Fe-Mg interchange with olivine during cooling.However, due to the small proportion of chromite, it seemsunlikely that olivine chemistry would have been substan-tially altered by Fe-Mg interchange.

Unserpentinized dunite cores occur in the central parts of many of the TNB ultramafic bodies. Such dunites are com-

posed of more than 95% olivine with almost complete boundary contact between the grains and relatively littleintergranular material (Fig. 10A,B), suggesting that theymight be less altered not only because they are in the coresof the bodies and therefore further from the infiltrating-react-



ing fluids, but also because they were less permeable thanthe other rocks. Unaltered dunites commonly contain fine-grained disseminated chromite (1-2%, 60-100 µm), whichmay occur both as subhedral to anhedral inclusions withinthe olivine grains and as euhedral intercumulus grains. Inaddition to coarse chromite inclusions, many of the relictolivines within the dunites contain finely disseminatedchromite micro-inclusions or inclusion trails, from which theorigin of the relict olivine may be discerned. In many cases,the compositions of these micro-inclusions are similar to

those of the cores of the coarse chromite inclusions andintercumulus grains, and coarse-grained intercumuluschromites inclusions from relatively unmetamorphosedkomatiite flows (Barnes and Brand, 1999), suggesting thatthey have not interacted with either serpentinizing fluids or any trapped melt phases. On this basis, the chromite micro-inclusions are interpreted to represent primary igneous inclu-sions, the unaltered olivines in these samples are inferred to

be of magmatic origin, and their compositions are interpreted to reflect the compositions of the parental magmas. In


422

FIGURE 10. Photomicrographs of textures and mineralogy of olivine cumulate rocks from the Thompson Nickel Belt. (A) Olivine cumulate, Mid Lake (DDH86291) PPL. (B) Partially serpentinized ovoid to idiomorphic, cumulate olivines PPL. (C) Subhedral to anhedral chromite inclusions within olivine grains,Hambone East (DDH 74289) RL. (D) Euhedral intercumulus chromite grains and sulphide bleb, Mid Lake (DDH 86291) RL. (E) Serpentinized metaperi-dotite, South Spur (DDH 86227) PPL. F) Partially serpentinized peridotite, North Manasan (DDH 89299) PPL = plane polarized light, TL = reflected light.

Ccp = chalcopyrite, Chr = chromite, Pn = pentlandite, Po = pyrrhotite.

A B

C D

E F




42

a number of cases, the micro-inclusions are either signifi-cantly more aluminous or ferric iron-rich than would beexpected for magmatic chromite grains (Fig. 10C). In suchcases, the inclusions (and by association the host olivines)are inferred to have formed through dehydration reactionsinvolving alteration minerals formed at lower metamorphicgrades, the relict olivines are inferred to be of metamorphicorigin, and their compositions can not be used to estimate the

compositions of the parental magmas.

Metaperidotites

Metaperidotites in the TNB are composed of 40 to 90%fine- to medium-grained, variably serpentinized subhedralolivine grains in a matrix of fine-grained chlorite±calcicamphibole±serpentine with trace amounts of fine-grainedchromite (Fig. 10E). The replacement of the interstitial mate-rial by chlorite±amphibole±serpentine suggests that many of the peridotites were originally composed of olivine and

pyroxene (O’Hanley, 1996) and the presence of calcicamphibole in these intergrowths indicates that the pyroxenewas originally Ca-rich clinopyroxene (i.e. that the rockswere originally wehrlites) rather than Ca-poor orthopyrox-ene (i.e. the rocks were originally harzburgites). Althoughkomatiitic olivines crystallized in rapidly cooled extrusiveenvironments may contain significant amounts of Ca (seereview by Lesher, 1989), the olivines in the ultramafic intru-sions in the TNB appear to have crystallized in slowlycooled intrusive environments and have low Ca contents.The high Fo contents of the relict igneous olivines (up toFo94) suggest that they crystallized from magmas contain-ing up to 22% MgO, indicating that the magmas were trulykomatiitic.

Although orthopyroxene-bearing olivine cumulate rockshave been reported in the ultramafic bodies of the TNB by

previous workers, predominantly at the Thompson mine (De

Saboia, 1978; Peredery, 1979, 1982; Paktunc, 1984; Paktuncet al., 1984), very few harzburgites were identified either petrographically or geochemically in the 2093 samples of ultramafic rocks that were studied in CAMIRO project 97E-02 (Burnham et al., 2003). Those identified were generally

poikilitic olivine mesocumulates, in which the orthopyrox-ene forms oikocrystic grains enclosing cumulus olivine. The

paucity of orthopyroxene outside the Thompson mine areamay reflect either the higher metamorphic grade in theThompson area or a difference between the amount of wall-rock assimilation during the formation of the Thompson andother ultramafic bodies. Whereas the ultramafic bodieswithin the Thompson mine experienced upper amphibolite-to granulite-facies metamorphism, during which the assem-

blage olivine±talc±anthophyllite or chlorite would have beenunstable and reacted to form either orthopyroxene or orthopyroxene±olivine±spinel (Paktunc, 1984), the majorityof the ultramafic bodies in other parts of the TNB experi-enced only middle to upper amphibolite-facies metamor-

phism, during which olivine±talc, olivine±anthophyllite, or chlorite are stable phases. Alternatively, because most of theTNB ultramafic bodies outside the Thompson mine area (i.e.northern TNB) are either nonmineralized or subeconomi-cally mineralized intrusions, the presence of orthopyroxenein the Thompson ultramafic bodies may reflect greater felsi-fication of the magma via assimilation of wall rocks, poten-

tially inducing sulphide saturation and orthopyroxene crys-tallization (e.g. Irvine, 1975).

Partially serpentinized peridotites, in which the cores of olivine grains are partially preserved, have been documentedin only a few ultramafic bodies (e.g. North Manasan andSpur South: Fig. 10F). These rocks are characterized by<90% fine- to medium-grained, partially serpentinizedolivines within a matrix of >10% fine-grainedchlorite±amphibole±serpentine. The relict olivines areovoid, between 0.1 and 3 mm in diameter, and containopaque micro-inclusions similar to those present in thedunites.

Metapyroxenites

Olivine metapyroxenites occur along the margins of theultramafic bodies and as layers within the bodies. They arecomposed primarily of fine- to medium-grained olivineand/or pyroxene altered to either mesh-textured serpentine or

porphyroblastic calcic amphibole in a matrix of acicular chlorite±amphibole±serpentine. In most cases, olivine iscompletely serpentinized. However, relict olivine fragmentsare preserved in olivine pyroxenites along the margins of some ultramafic bodies.

Although pure orthopyroxenite and clinopyroxenite layersare preserved in a few of the ultramafic bodies, such layersare relatively rare in the TNB. In most cases, the pyroxeniteshave been altered to an assemblage of either tremolite or hornblende±plagioclase (clinopyroxenites) or tremolite±anthophyllite±orthopyroxene (orthopyroxenites) and canonly be easily recognized from their whole-rock chemicalcompositions.

Structure

Several models have been proposed to explain the struc-ture of the TNB (e.g. Fueten and Robin, 1989; Bleeker,

1990; Fueten, 1992; White et al., 1999; Gapais et al., 2005).The main differences between the models are the relativeweights given to the fold geometry of stratigraphic units ver-sus finite strain and kinematic analysis. Interpretations of superimposed fold structures in terms of successive defor-mation phases has led to the development of a model involv-ing early nappe tectonics followed by northeast-directedtranspression (Bleeker, 1990). However, strain and kine-matic analysis has led to the development of a southeast-side-up kinematic model (Fueten and Robin, 1989). Bothapproaches are limited by the paucity of outcrop in the TNBand given that the different models arise from the applicationof a different range of structural techniques, a consensus onthe evolution of the TNB has not been reached to date.

Geochronological data (Machado et al., 1990) are notcompatible with a simple nappe model, because the disper-sion of ages (1836-1720 Ma) militated against a single major metamorphic melting event of regional extent, as may beexpected and has been observed in areas of known thrust tec-tonics. Nevertheless, a protracted history of polyphase defor-mation similar to that observed in the adjacent part of theTrans-Hudson Orogen, and characterized by more than one

period of metamorphism related to multiple continental col-lisions, remains a viable tectonic model (Zwanzig, 1999, andreferences therein). Although an alternative, transpressional




42

tectonic model is easier to reconcile with the geochronolog-ical data (Gapais et al., 2005), a complete transpressionalmodel for the TNB has not been presented using high-reso-lution geochronological methods. A comparison of differingtectonic models for the TNB is presented in Table 2.

Mineral Deposits and Occurrences

Sulphides occur in many of the rocks in the Thompson

Nickel Belt, but are most abundant in sulphide-facies ironformations and associated metasedimentary rocks of thePipe Formation and in ultramafic bodies intruded into thePipe Formation. Because many sulphide phases, particularlychalcopyrite and pyrrhotite, are more ductile at high temper-atures compared to many silicate phases, sulphide-rich rocksare more easily deformed than associated silicate-rich rocks(see reviews by Marshall et al., 2000). Although the Ni-Cu-(PGE) sulphides in the TNB should have homogenized tomonosulphide solid-solution (MSS) at such high metamor-

phic grades (upper amphibolite to granulite facies) andrecrystallized to low-temperature mineral assemblages dur-ing retrograde metamorphism, macroscopic and microscopicsulphide deformation structures, fabrics, and textures arewell preserved in the TNB, recording a complex polyphasedeformational history (see Bleeker, 1990). Although miner-alization in the TNB is more deformed than those in mostother magmatic Ni-Cu-(PGE) deposits, and in many casesthe mineralization is now hosted within metasedimentaryrocks, the mineralogy and geochemistry are similar to other komatiite-associated magmatic Ni-Cu-(PGE) deposits(Lesher, 1989; Lesher and Keays, 2002). Primary (i.e. non-supergene) ores are dominated by pyrrhotite-pentlandite,

pyrrhotite-pentlandite-pyrite, and less common pentlandite- pyrite and pentlandite-pyrite-millerite assemblages (Bleeker,1990; Liwanag, 2000). Chalcopyrite, magnetite, andchromite are ubiquitous minor phases.

Thompson Deposit

The Thompson ore is stratabound and occurs within the pelitic schist unit of the Pipe Formation (Fig. 2). TheThompson structure consists of nearly upright, variably

plunging, high-amplitude folds interpreted to have formedduring an F3 folding event in the Thompson Nickel Belt(Bleeker, 1990). Reworked basement gneisses occur on theouter limbs of the structure, while the Ospwagan Groupsupracrustal rocks occur in the core.

Birchtree and Pipe Deposits

The Birchtree and Pipe deposits share similar lithologicaland structural settings. The ultramafic rocks of the Birchtree

deposit are interpreted to be part of a discontinuous array of ultramafic bodies that stretch from the Pipe II open pit to theBirchtree mine (Bleeker, 1990) (Figs. 1, 2). The discontinu-ous ultramafic bodies from the Birchtree deposit and thePipe deposits occur in similar stratigraphic positions withinthe Ospwagan Group supracrustal rocks, i.e., within thegraphitic sulphide-facies iron formation of the P1 member of the Pipe Formation. The massive and semimassive sulphidemineralization in the Birchtree and Pipe deposits has under-gone a lesser degree of deformation relative to that in theThompson deposits. Massive sulphide accumulations withinthe Birchtree and Pipe deposits tend to occur as ultramafic

breccias consisting of peridotitic to dunitic clasts in a sulphide matrix. These breccias are typically proximal to ultra-mafic boudins and are not stretched within the metasedi-mentary horizon to the same extent as in the Thompsondeposits.

William Lake Deposit

The William Lake deposit lies beneath Paleozoic carbon-

ate rocks in the southern portion of the Thompson NickelBelt (Fig. 1, 3). Intrusion-hosting metasediments includequartzites, pelites, calcareous metasediments, iron forma-tions, and graphitic schists that are similar to the Thompsonand Birchtree deposits. As such, the ultramafic bodies areinterpreted as being hosted by the Ospwagan Groupmetasediments (Macek et al., 2002). The ultramafic bodiesconsist of pyroxenite, peridotite, and dunite. No work has

been published to place the exact stratigraphic position of theses ultramafic bodies within the Ospwagan Group lithos-tratigraphic section of Bleeker (1990), however, maps pub-lished by the TNB working group (Macek et al., 2002), indi-cate that they reside within the Pipe Formation. The ultra-mafic bodies and the hosting metasediments are concen-trated on the southeastern margin of a granitic intrusion, theWilliam Lake Dome (Fig. 3). Numerous centimetre- tometre-scale granitic pegmatite veins, interpreted to berelated to the intrusion of the William Lake Dome, cross-cutthe older Ospwagan Group rocks and the ultramafic bodiesin many drill-core samples (Fig. 9).

The mineralization in the William Lake area typicallyoccurs as disseminated sulphides within the ultramafic bod-ies, and locally grades to centimetre- to decimetre-scale mas-sive and semimassive sulphide accumulations toward the

base of bodies. Banded sulphides, sulphide stringers, anddisseminated sulphides within metapelites and iron forma-tions are commonly encountered in diamond-drill cores.

Mineralization Types

Komatiite-associated Ni sulphide deposits, such as thosein the TNB, are part of a continuum of lithotectonic associa-tions in the family of magmatic Ni-Cu-(PGE) deposits,which contains a variety of mineralization types traditionallyclassified by ore morphology and ore host rocks (Lesher andKeays, 2002): Type I stratiform basal massive/net-tex-tured/disseminated mineralization, Type II stratabound inter-nal disseminated mineralization, Type III stratiform ‘reef-style’ mineralization, Type IVa Ni-enriched metasedimentmineralization, Type IVb hydrothermal mineralization, andType V tectonic ‘offset’ mineralization.

Deformation within the Thompson Nickel Belt mineral-

ization has led to more detailed classifications based on oretextures (disseminated versus inclusion-bearing versus mas-sive) and host rocks (i.e. ultramafic versus metasediment)(Zurbrigg and Patterson, 1963; Peredery, 1982; Bleeker,1990; Larocque et al., 1998; Liwanag, 2000). The classifica-tion scheme presented in this review reflects the most recentdescriptions of macroscopic, mesoscopic, and microscopictextures in sulphide mineralization, and the character of thehost rock in which the sulphide mineralization resides(Bleeker, 1990; Liwanag, 2000). The abbreviations for thenomenclature in this classification scheme include a prefixindicating the texture of the sulphides (D = disseminated,



HD = heavily disseminated, N = net textured, SM = semi-massive, M = massive, L = laminated), the uneconomic or economic mineralization (S and $, respectively), and a suffixindicating the nature of the host rock (U = ultramafic, UB =ultramafic breccia, SED = metasediments) (Table 3). Figure11 shows the general location of the sulphide types in rela-tion to the TNB ore deposit model of Bleeker (1990). In gen-

eral, the “intraparental” ores of Bleeker (1990) and D$-U,L$-U, and M$-UB ores of Liwanag (2000) correspond toTypes I-II ores of Lesher and Keays (2002), whereas the“extraparental” ores of Bleeker (1990) and D$-SED, SM$-SED, and M$-SED of Liwanag (2000) correspond to Type Vores of Lesher and Keays (2002).

Barren disseminated sulphides in metasediments (DS-SED) are composed of primarily fine- to medium-grained,foliated pyrrhotite patches or euhedral grains of pyrite, or

both (Fig. 12A). Sulphides adjacent to or near ultramaficrocks may also contain very fine-grained, angular to rounded

patches of pentlandite and/or chalcopyrite.Barren semimassive and massive sulphides in metasedi-

ments (SMS-SED and MS-SED) occur as millimetre- to

decimetre-scale layers of pyrrhotite (Fig. 12B). Pyrrhotite istypically fine-grained and annealed, and comprises bothhexagonal and monoclinic polymorphs. The sulphides do notcontain macroscopic pentlandite and are normally graphitic,commonly containing up to 25% graphite. Graphite occursas foliated single inclusions in sulphides or as aggregatesassociated with foliated biotite flakes and rounded to sub-rounded quartz and plagioclase. Chalcopyrite, pyrite, andmagnetite are present in minor to trace amounts.Chalcopyrite typically occurs as very fine-grained blebswithin pyrrhotite and as ‘porous’ masses adjacent to silicate

inclusions. Pyrite occurs as subhedral aggregates replacing pyrrhotite. Magnetite occurs as randomly distributed, fine-grained, anhedral grains.

Heavily disseminated or mineralized semimassive to mas-sive sulphides in metasediments (HD$-SED, SM$-SED, andM$-SED) occur at the contacts between ultramafic bodiesand barren metasediments (interpreted to be largely in place)


426

Sulphide Sulphide Rock Sulphide

Abundance Texture Texture Distribution Meta-

Sediment

Ultramafic

Breccia

Ultramafic

Rock

Intraparental*

DS-SED D$-UB

D$-SEDHeavy Disseminated HD$-UB HD$-U

Patchy Disseminated PD$-UB PD$-U

Blebby Disseminated B$-U

LS-SED ––

L$-SED

Net-Textured meso- to adcumulate intercumulus –– –– N$-U

Semi-Massiveschistose, gneissic,

myloniticintergranular

SMS-SED

SM$-SEDSM$-UB ––

MS-SED M$-UB

M$-SED

diverseinterstitial to

intercumulus

HDS-SED

HD$-SED

––

40-70%

>70% Massivecataclastic,

blastomylonitic –– ––

Layered laminated to bandedinterstitial to

intergranular

Notes: All textures and abundances are gradational. Abbreviations based on textural type: B = brecciated,D = disseminated, HD = heavily disseminated, L = laminated, M = massive, N = net textured, PD = patchy

disseminated, SM = semimassive; mineralization status: $ = mineralized and S = unmineralized; and a suffix

indicating the nature of the host rock: U = ultramafic, UB = ultramafic breccia, SED = metasediments.

* nomenclature after Bleeker, 1990.

Host Rock

Extraparental*

≤10% Disseminated diverse interstitial D$-U

10-40%

TABLE 3. Sulphide ore types, textures, distributions, and host rocks in the Thompson Nickel Belt (adapted and expanded fromBleeker (1990) and Liwanag (2000).

DS-SED

D$-SED

M$-SED

MS-SED

HD$-SED

D$-SED

M$-SEDHD$-SED

D$-SEDHD$-SED

HD$-UB

B$-U

SM$-UBM$-UB

PD$-UB

Ultramafic bodies

Sulphide mineralization

Sedimentary sulphidesHDS-SED D$-U

D$-UB

not to scale

FIGURE 11. Schematic geometry of ultramafic bodies and mineralization inthe Thompson Nickel Belt (adapted from Bleeker, 1990). Arrows indicategeneralized locations of mineralization types presented in Table 3.Abbreviations based on textural type, host rock, and mineralization status:D = disseminated, HD = heavily disseminated, N = net textured, SM =semimassive, M = massive, L = laminated, PD = patchy disseminated, S =mineralized and $ = nonmineralized, and a suffix indicating the nature of the host rock: U = ultramafic, UB = ultramafic breccia, SED = metasedi-ments.




42

FIGURE 12. Textures repre-senting sulphides in metasedi-mentary and ultramafic rocks(Photographs taken fromLiwanag 2000). (A) D$-SED:mineralized biotite schist fromthe Thompson 1D orebody.(B) MS-SED: Thompson T1mine, showing massive

pyrrhotite with foliated single

crystal inclusions of graphite and biotite, and ovoidaggregates of plagioclase,quartz, graphite, and biotite.(C) M$-SED: Thompson T1mine, showing coarse-grainedannealed pyrrhotite and lighter coloured pentlandite eyes.Inclusions are foliated maficmica and aggregates of bio-tite, quartz, and plagioclase.(D) M$-SED: Thompson 1Corebody showing coarse

pyrrhotite and pentlandite eyesoriented parallel to foliatedmica inclusions and schistoseaggregates (highlighted by

dashed lines). (E) D$-U:Birchtree mine showing serpentinized peridotite contain-ing interstitial sulphides.(F) M$-UB: Birchtree mineshowing centimetre-scaleinclusions of serpentinite (um)and single crystal inclusions of mafic mica grains, quartz, andfeldspar in massive pyrrhotite(po). Pentlandite (pn) occursas medium-grained lightcoloured ‘eyes’. (G) M$-UB:Thompson T1 mine, sampleconsisting of medium-grainedannealed pyrrhotite and pent-landite with inclusions of

pelitic schist fragments and

serpentinized peridotite.

A

B

C

D

E

GF



and isolated within barren metasediments (interpreted tohave been mobilized from ultramafic contacts). They arecomposed of mainly pyrrhotite and pentlandite (Fig. 12C)with minor amounts of chalcopyrite, pyrite, magnetite,chromite, and gersdorffite [NiAsS]. Pyrrhotite occurs asannealed polygonal grains. Pentlandite occurs as i) very finelamellae; ii) elongated, wispy ‘chains’; and iii) coarse-grained, subangular to subrounded ‘eyes’ that are typically

<1 to 5 mm in size, but range up to 2 cm in some horizons.In many massive sulphide horizons in the Thompson mine,

pentlandite eyes are often segregated into centimetre-scale bands within a coarse-grained, annealed pyrrhotite matrix(Fig. 12D). Pyrite typically occurs as aggregates of subhe-dral grains that replace both pentlandite and pyrrhotite.Gersdorffite occurs as very fine, anhedral to subhedralgrains. Magnetite and chromite occur as very fine-grainedeuhedral to subhedral grains randomly oriented in a sulphidematrix. Fine- to coarse-grained chalcopyrite typically occursadjacent to pentlandite in stringers that project into themetasedimentary rocks along the contact. Chalcopyrite isalso concentrated adjacent to silicate inclusions.

Mineralized disseminated and net-textured sulphides inultramafic rocks (D$-U and N$-U) range from fine-grainedinterstitial disseminations through lobate intercumulusgrains to coarse-grained interstitial networks (Fig. 12E).Disseminated sulphides typically occur in the interior partsof ultramafic bodies, whereas net-textured sulphides typi-cally occur in the exterior parts of the bodies. The sulphidescomprise mainly pyrrhotite and pentlandite with minor chal-copyrite and magnetite. Pentlandite and chalcopyrite typi-cally form subangular to subrounded patches along the edgesof pyrrhotite grains. Magnetite typically occurs as fineanhedral grains adjacent to sulphides and as lamellae in sili-cates.

Mineralized semimassive and massive sulphides in ultra-

mafic breccias (SM$-UB and M$-UB) form the matrix component of the breccias (Fig. 12F). Breccia fragments are fine-to very coarse-grained, tectonized inclusions of the hostultramafic rocks. The textures of the sulphides are similar ona microscopic scale to those in metasedimentary rocks (seeabove). The sulphide mineralogy is dominated by pyrrhotiteand pentlandite, with minor amounts of chalcopyrite, pyrite,chromite, and gersdorffite. Pyrrhotite occurs as annealedmedium- to coarse-grained polygonal grains. Pentlanditeoccurs as i) very fine lamellae; ii) elongated, wispy ‘chains’;and iii) coarse-grained, subangular to subrounded ‘eyes’.Pentlandite eyes are typically <1 to 5 mm in size.Gersdorffite occurs as very fine, anhedral to subhedralgrains.

Textural Variations between Ore Deposits

Mineralized semimassive to massive sulphide samplesfrom the Birchtree, T1, 1C, 1D, and William Lake depositswere characterized in terms of host rocks, proximity to ultra-mafic boudins, inclusion populations, and macroscopic tex-ture of pentlandite (Liwanag, 2000). Ores from the Birchtreemine are semimassive and massive sulphides in ultramafic

breccias that occur in ultramafic boudin necks and at the bases of ultramafic bodies. They contain randomly orientedto oriented, medium- to coarse-grained, brecciated ultramaficinclusions and minor pelitic inclusions. Pyrrhotite, the prin-

cipal sulphide phase, is associated with up to 10% fine- tomedium-grained pentlandite (Fig. 12F). Owing to their closer

proximity to ultramafic boudins, the Birchtree samples areinterpreted to be closer in textural character to ‘primary’magmatic massive sulphides compared to the samples fromThompson (Peredery, 1982; Bleeker, 1990; Liwanag, 2000).

Ores from the Thompson mine exhibit a spectrum of tex-tures that vary between the T1 fold nose, the 1C orebody, andthe 1D orebody, which reflect the highly tectonized nature of the mineralization. T1 ores are semimassive and massivesulphides in metasediments (SM$-SED and M$-SED), butare texturally similar to the Birchtree mineralization(Liwanag, 2000). Most mineralization is near ultramafic

boudins, but some occur as massive sulphide lenses hosted by pelitic schists distant from ultramafic boudins. T1 orestypically contain randomly oriented to oriented, medium- tocoarse-grained ultramafic and pelitic inclusions, and containup to 25% medium- to coarse-grained pentlandite grains(Fig. 12G). Single crystal inclusions also occur in the T1massive sulphides; the most abundant crystal inclusions arefine- to medium-grained foliated biotite flakes, and roundedquartz and plagioclase grains.

Ores from 1C are dominantly massive sulphides inmetasediments (M$-SED), but also include massive sul-

phides in ultramafic breccias, and net-textured and dissemi-nated sulphides in ultramafic boudins (M$-UB, N$-U, andD$-U). Thompson 1C massive sulphides in metasedimentsdiffer from T1 ores in that the pentlandite grains tend to becoarser and distributed into parallel, centimetre-scale bandswithin the ore horizons (Fig. 12D). Single crystals of biotite,quartz, and plagioclase dominate the inclusion population.

Ores from the 1D orebody are primarily massive sulphides in metasediments (M$-SED), which are hosted by pelitic and quartz-rich metasediments that have beenintruded by late pegmatitic granite (Fig. 12G). The 1D ore-

body has undergone extreme and complex deformation(Bleeker, 1990). The 1D ores are unique compared to mostother ores in the TNB; undulating contacts between rock units, numerous metre-scale shear zones, and abundant peg-matitic/tonalitic inclusions indicate more complex deforma-tion. These ores are interpreted to be relatively more distal toultramafic boudins than T1 and 1C ores; no interpreted ultra-mafic parent rocks have been observed within 250 m alongstrike of the 1D orebody (Liwanag, 2000). These ores typi-cally contain up to 40%, well rounded, medium- to coarse-grained pegmatitic/tonalitic inclusions, and up to 10%, fine-to medium-grained pentlandite grains (Fig. 12G). Locally,the pentlandite grains are as coarse grained as in 1C ores,however, they are interpreted to occur in less deformed por-

tions of the orebody.Mineralization in the William Lake area occurs primarily

as disseminated sulphide within ultramafic boudins (D$-U)and has textures similar to those in other stratabound dis-seminated Ni-Cu-(PGE) deposits, such as Mt. Keith (Grovesand Keays, 1979) and Dumont (Eckstrand, 1975).Mineralization occupies interstitial spaces between olivineand occurs as very fine (μm-sized) inclusions in olivine. Thissuggests that these rocks were saturated in sulphide at anearly to intermediate stage of their crystallization history.Minor massive sulphide hosted in ultramafic breccias andmetasedimentary rocks (M$-UB, M$-SED) in the William


428




42

Lake area have macroscopic textures similar to those atBirchtree, T1, and 1C, but are much less deformed than thoseat 1D.

Exploration Methods

Historically, Inco Ltd. has been the largest land claimholder within the northern and central TNB. As such, Incohas been continually exploring the Thompson Nickel Belt

over the past 50 years in a search for new sources of Ni feed.Early work by Inco geologists indicated that the ultramaficintrusion-hosting Ospwagan Group rocks have been meta-morphosed to schists and gneisses and that the complexdeformation history has produced complex folding patterns(summarized by Peredery et al., 1982). The realization thatthe Ni-Cu-(PGE) deposits in the TNB are associated withultramafic intrusions that occur within a specific horizons inthe Ospwagan Group rocks has provided a working explo-ration model that uses an integrated approach combininggeology and geophysics. Preliminary maps using this inte-grated approach have been recently published for the TNBfrom Moak Lake in the north to Lamb Lake in the southernThompson Nickel Belt (Macek et al., 2001).

Using this exploration approach, audio magnetotelluric(AMT) surveys are used to interpret the geology to depths of two kilometres. Promising stratigraphic intervals selectedfrom AMT surveys are then investigated by surface UTEMsurveys to a depth of approximately 600 m. Anomalous con-ductivity occurring in the favourable stratigraphic horizon(i.e. Pipe Formation sulphide-facies iron formation) is tested

by diamond-drill holes, followed by borehole electromag-netic surveys (Inco Ltd., Annual report 2004).

Underground exploration within the operating mines inthe TNB is influenced by the complex deformation history of the northern TNB. At this scale, exploration includesdetailed structural analysis of the host rock and sulphide

mineralization. Geological interpretations are tested byexploration diamond drilling supported by in-hole electro-magnetic surveys (Inco Ltd., Annual Report, 2004).

Exploration in the southern TNB, using traditional geophysical techniques, is hampered by the southward-thicken-ing Paleozoic cover rocks and the southwestern (south of Grand Rapids, Manitoba) formational brines (Gascoyne etal., 1987; McRitchie, 1995).

Knowledge Gaps

Timing and Level of Emplacement of Ultramafic Sills intothe Ospwagan Group

The ultramafic compositions of the intrusions make them

difficult to date absolutely, as they do not contain primaryminerals suitable for radiometric dating. The only intrusionthat has been dated is a differentiated ultramafic-mafic bodyat Setting Lake, which yielded a U-Pb zircon age of 1880 ±2 Ma (Hulbert et al., 2005).

The ultramafic bodies occur throughout the lower andcentral parts of the Ospwagan Group stratigraphy up to thelower member of the Setting Formation. This may indicatethat they were emplaced during the deposition of the SettingFormation or that their emplacement was limited to lower levels for physical reasons (e.g. buoyancy, rheology, and/or tectonics). This suggests that they must be younger than the

2.6 Ga age of the youngest detrital zircon in the SettingFormation (Burnham et al., 2003).

Thus, the only constraints on the absolute age for theultramafic bodies as a group are that they may have beenemplaced between 2.6 and 1.80 Ga. Given the magnitude of magmatism indicated by the prevalent 1880 to 1890 Mamafic-ultramafic ‘Molson’ dykes in the adjacent Superior Province, it is most likely that the ultramafic sills wereemplaced during this event. However, the absence of absolute age dates for the ultramafic intrusive event(s) and afirm understanding of the nature of the intrusive event(s) (i.e.multiple emplacement events and/or multiple levels in theOspwagan Group), combined with the complex deforma-tional history, hampers the development of more detailedstratigraphic interpretations.

Tectonic Structure and History

Contrasting interpretations of the TNB structure and tec-tonic history have recently been presented (Bleeker, 1990;Zwanzig and McRitchie, 1997; Zwanzig, 1999; Gapais et al.,2005). Structural mapping using stratigraphic markers (e.g.Bleeker, 1990; Zwanzig and McRitchie, 1997) suggest thatthe TNB structure is dominated by upright folds superposedon westward-verging isoclinal recumbent folds. However,strain and kinematic analysis of shear zones supported by U-Pb ages of syn-kinematic pegmatite intrusions (e.g. Fueten,1992; Potrel et al., 2000; Gapais et al., 2005) suggests thatthe structure is dominated by north-northeast-trendingreverse shear zones (southeast-side up) in a transpressionaltectonic zone. The progressive, polyphase nature of thedeformation in the TNB is the likely cause of inconsistencies

between the two models; however, more detailed work isstill required before a coherent model will emerge for thetectonic evolution of the TNB.

Host Units

The high MgO and cumulate olivine contents of the ultra-mafic intrusions could be attributed to emplacement asolivine-magma mushes, accumulation of intratelluric olivine

phenocrysts, or accumulation of olivine in situ. Althoughinsufficient mineral chemical data are available to distin-guish between the latter two possibilities, the density of akomatiitic magma containing abundant olivine would bemuch greater than the density of the sedimentary section,even if consolidated, so the first possibility seems unlikely.If so, then the ultramafic intrusions represent dynamic flow-through feeder sills, similar to the lava channels in extrusivekomatiitic systems (see discussion by Lesher, 1989).

Ore Genesis and Crustal Contamination Signatures

The consistent stratigraphic correlations between ultra-mafic bodies, S-rich sedimentary rocks, and Ni-Cu-(PGE)sulphide mineralization (Peredery, 1982), and the large com-

ponent of crustal sulphur in the ores (Eckstrand et al., 1989)suggest that the majority of the sulphides formed via meltingof the S-rich sedimentary rocks. The large degree of crustalcontamination (up to 30%: Lesher and Burnham, 2001;Lesher et al., 2001; Burnham et al., 2003) in the host unitsindicates that large amounts of silicates were assimilated, butthe solubility of S in even komatiitic magmas is quite low(Shima and Naldrett, 1975), so it is most likely that the sul-



phide simply melted, remaining as a ‘xenomelt’ (see discus-sion by Lesher and Burnham , 2001) at or near the bases of the sills. This interpretation is supported by the high (nor-mal) PGE contents of some of host rocks (Lesher et al.,2001), which requires that the magmas did not equilibratewith sulphides prior to emplacement. Ironically, geochemi-cal models of the ores suggest that there was too much S inthe system and not enough magma, and that the deposits

would have had higher metal contents (especially PGE andCu) if less S had been added or if more magma had inter-acted with the sulphides (see discussion by Lesher et al.,2001). Nevertheless, much more work is required to estab-lish the nature of contamination (wholesale versus incongru-ent versus volatilization), the degree of contamination, andthe amount of S required to generate an economic deposit.

Deformation during High-Grade Metamorphism

The Ni-Cu-(PGE) mineralization in the TNB is oftendetached from what is inferred to be the primary ore host(i.e. ultramafic rocks). This tectonic detachment has beendescribed as simple boudinage (e.g. Bleeker, 1990).However, during this mobilization of the sulphide mineralsfrom their ultramafic hosts, fluids generated from the meta-morphism of the ore zones and their host rocks, appear tohave mobilized many elements away from the primary oreenvironment, not only Ni (Bleeker, 1990) but particularlyAu, Pd, and Cu (Burnham et al., 2003). On the basis of experimental studies, it has been suggested that many ele-ments thought to be chloride-immobile (i.e. PGE) at lowtemperature (i.e. ~300ºC; Wood, 1987, 1996; Pickrell et al.,1996) may be very mobile in chloride solutions at near-mag-matic conditions (Hanley et al., 2005). Additionally, at lower temperatures, the PGE may be transported in significant con-centrations via bisulphide complexation. Although mostother analyzed elements are present in the TNB ores are at

predicted abundances (Burnham et al., 2003), it is possiblethat other, more volatile elements were added to the miner-alization during high-grade metamorphism. Future work onthe TNB must establish the extent of element mobility, theextent of structural and metamorphic/metasomatic mobiliza-tion within regional and mine scales, and the volume of flu-ids that may have been generated during metamorphic dehy-dration of the host rocks. Such information is critical inunderstanding genetic links between the different deposits inthe TNB (e.g. Thompson, Pipe, and Birchtree mines).

Acknowledgements

This synopsis reflects our participation CAMIRO Project97E-02 and includes information summarized by the

authors, but also includes insight gained from other projectcollaborators, including Nuno Machado, Dianne Michalak,Alain Potrel, Kim Toope, Herman Zwanzig, Kevin Ansdell,Chris Böhm, Carl Chandler, Eric Ducharme, Denis Gapais,Reid Keays, Jürgen Kraus, Josef Macek, Peter Theyer, Mark Pacey, Paul Lenton, and John Broome. We have also bene-fited from discussions with and assistance from PaulGolightly, Patti Tirshman, Larry Larson, Scott Mooney,Dave Owens, John Pearson, Jamie Robertson, Rick Somerville, Gary Sorensen, Rob Stewart, and Kevin Wells.

We are very grateful to the sponsors of the CAMIROProject 97E-02, including Billiton Ltd., Cominco Ltd.,

Falconbridge Ltd., Hudson Bay Exploration andDevelopment Ltd., Inco Ltd., and WMC Ltd., for their strongand enthusiastic scientific, logistical, and financial support.

We would also like to thank A. Naldrett and J. Hanley for their constructive reviews of the manuscript.

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magmatic ni-cu-platinum-group element deposits of the thompson nickel belt

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