Precambrian Research, 25 (1984) 119--133 119 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

DISCOVERY, GEOLOGY AND GENESIS OF COPPER---COBALT MINERALISATION AT CHAMBISHI SOUTHEAST PROSPECT, ZAMBIA

V.D. FLEISCHER*

Geological Services, Z.C.C.M. Ltd., P.O. Box 1, Kalulushi (Zambia)

ABSTRACT

Fieischer, V.D., 1984. Discovery, geology and genesis of copper--cobalt mineralisation at Chambishi Southeast prospect. Zambia. Precambrian Res., 25: 119--133.

Grid drilling on a broad grid, stepping out down-dip from subcropping Lower Roan has made an important intersection of 10.7 m of copper--cobalt mineralisation. This adds significant tonnage to the 55 Mt grading 2.4% Cu already known at Chambishi Southeast. The host rock is the Lower Roan ore shale surrounding a dolomite biotherm which caps a basement palaeohigh. Mineralisation consists of chalcopyrite with minor bornite, carrollite, cobaltiferous pentlandite and linnaeite, disseminated along bedding planes in the shale as well as in an underlying channel sandstone. Moving away from the palaeohigh, pyrrhotite and pyrite surround the ore, giving a distinct mineral zoning with carrollite hugging the palaeohigh. Gangue minerals are quartz, feldspar, mica, car- bonate and tourmaline. The palaeogeographic setting of the carbonaceous shale com- bined with high carbonate and tourmaline contents points to a supersaline sabkha-type environment along the edge of a palaeohigh on which algal reefs formed. The sulphides had a syn-sedimentary origin with the metals coming from erosion of the hinterland and out of the sea water.

INTRODUCTION

In a relatively small area within the Chambishi basin completely blind ore shale and Chibuluma-type copper--cobal t sulphide mineralisation has recent- ly been discovered at the Chambishi Southeast prospect (Figs. 1 and 2).

Surface geological mapping commenced here in the late 1920's and was followed by diamond drilling which probed down-dip from the shallow sub-outcrop of the prospective Lower Roan beds into the deeper parts of the basin. The drillhole spacing was on 400 m centres; this is regarded on the Copperbel t as a tight offset grid. This drilling outlined the Southern Area strip of copper--cobal t mineralisation, but thicknesses and grades were unattractive.

The exploration strategy was then revised to a broader grid in order to

*Present address: 14 Camelot, 37 Helen Road, Strathaven Ext. 22, Johannesburg, South Africa.

0301-9268/84/$03.00 © 1984 Elsevier Science Publishers B.V.

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Fig. 1. Copperbelt index plan showing shale "belt" and location of Chambishi Southeast

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CHAMBISHI BASIN SHOWING SURFACE GEOLOGY.

Fig. 2. Chambishi basin showing surface geo logy .

search for sizeable ore occurrences over the remaining areas of the ore shale. In 1975, drillhole NN42 intersected more promising copper-cobal t sulphides north of an extensive bioherm which lay directly over a pronounced base- ment high. This is termed the Northern Area (Fig. 2) and is the subject of this paper. From the present drillhole data, resource calculations indicate the presence of at least 55 Mt of mineralised ore shale in the Northern Area that averages 2.4% total copper, with significant cobalt, all as sulphide.

In 1982, continuing the strategy of stepping out by grid drilling, drillhole NN75 (Fig. 2) intersected 10.7 m true thickness of sulphide ore shale with 2.11% total copper and 0.09% total cobalt, indicating the likely presence of

122

another significant oreshoot. RCB2 was a test hole sited to explore both structure and mineralisation in the deeper parts of the Chambishi basin and it also gave an encouraging intersection, albeit at a depth of nearly 1300 m (Fig. 3). To the east in hole NN66, only barren pyrit ic/pyrrhoti t ic miner- alisation was intersected suggesting that no base metal continui ty exists between the RCB2 and Northern Area deposits.

REGIONAL GEOLOGY

There have been many descriptions of the general geological setting of the Zambian Copperbelt (Mendelsohn, 1961; Garlick and Fleischer, 1972; Fleischer et al., 1976; Garlick, 1981).

Copper and cobalt orebodies are restricted to certain shales and arenites of the Lower Roan Formation which lies at the base of the Katanga Super- group. Deposit ion of the Katanga Supergroup had started by 1000 Ma. The Lower Roan Format ion outcrops along the margins of structural depressions on either side of the Kafue anticline (Fig. 1). It consists of arenites and argil- lites, both containing carbonate and sulphate, deposi ted in a marginal marine environment. Algal bioherms are also present. Most of the exploited ore- bodies lie in the shale "be l t " (Fig. 1), which runs northwestwards into Zaire along the southwestern margin of the Kafue anticline. The shale "be l t " is characterised by the ore shale facies in the Lower Roan Formation.

No tewor thy are the biothermal dolomites which always occur over base- ment palaeohighs and interrupt the cont inui ty of ore shale mineralisation. It was only after tromatoli tes were discovered at Mufulira (Malan, 1964) that closer at tent ion was paid to a large bioherm found north of the thin strip of mineralisation named the Southern Area. Parts of this bioherm, especially in the southern section, display stromatolitic structures. However, it is not certain whether the rest o f the dolomite is truly biohermal, as it is fairly massive and mainly consists of dolomite with anhydrite, suggesting an evaporitic environment. The overlying Upper Roan Formation consists of dolomites, dolomitic argillites and some anhydrite.

The Katanga Supergroup unconformably overlies the Muva Supergroup, the Lufubu schists and gneisses, and various granites. The quartzites and schists of the Muva Supergroup were deposited after the Eburnian orogeny and were involved in the Kibaran orogeny. The Lufubu schists and gneisses were basement to the Muva sediments and were involved in the Eburnian orogeny. The granites are of Eburnian and Kibaran age. All Katangan and pre-Katangan rocks were deformed b y Pan-Afxican events which formed the Lufilian Arc. The Kafue anticline lies along the arc.

GEOLOGY OF THE PROSPECT AREA

The generalised stratigraphic column for Chambishi Southeast (Fig. 4) shows the similarity between this prospect and other Copperbel t occurrences

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The footwall quartzites and grits and the basal conglomerate of the Lower Roan Formation are barren of mineralisation. The critical mineralised sequence (CMS) embraces two lithostratigraphic units. The upper unit is the main ore shale, and consists of carbonaceous shale, sittstone and argil-

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lite, with interbedded dolomite. There is a conglomerate at the base. Below this are locally mineralised quartzites like those at Chibuluma. This Chibul- uma-type mineralisation consists of cyclical sequences of cobalt-rich sulphides, sericitic arenites and feldspathic arenites (Fleischer et al., 1976, p. 299). Above the CMS lie argillites and quartzites, followed by the chemical sed- iments of the Upper Roan Formation.

The lateral facies changes associated with the ore shale are shown in Fig. 5. A palaeohigh capped by biohermal dolomite is surrounded by the ore shale. Isopachytes of the cupriferous part of the ore shale (Fig. 6) show that the cupriferous zone which carries cobalt lies within an embayment against a palaeohigh and peters out to the north, east and south.

The Lower Roan sediments are relatively flat-lying and have dips of generally <15 ° (Fig. 3). The dip is generally to the west (Fig. 2} and to the west of the Northern Area is a deep structural trough (Fig. 7).

MINERALISATION

The mineralisation occurs in sulphide form and the multi-metal Fe--Cu-- Co ore minerals are disseminated, with distinct concentrations along the bed- ding planes. The bulk of the mineralisation is in the ore shale although copper sulphide is present in the underlying grits and quartzites, particularly in palaeo-channels. These channels are normally restricted in strike length and the copper grades are commonly lower than in the ore shale where tested.

Ore mineralogy

Chalcopyrite is the major sulphide, with minor but significant carrollite, cobaltiferous pentlandite and minor bornite/linnaeite in certain areas. Beyond the copper prospects, copper--cobalt su!phides give way to pyr- rhotite and pyrite (Fig. 8). Average copper sulphide grain size ranges from 5 to 1500 /~m, with a concentration between 25 and 400/~m. Average trace element analyses on ore shale samples are given in Table I. Initial metallurgi-

TABLEI

Average trace element analysis at the Northern Area prospect a

Element Range (ppm)

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aData obtained by chemical analyses on 500 composited drillcore samples from 16 holes in the mineralised ore shale formation. Analysed by Mining Industry Technical Services, 1982.

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cal work shows that the copper sulphides respond well to flotation, giving recoveries similar to those presently obtained from Nkana/Chambishi.

Gangue mineralogy

X-ray diffractometry and transmitted light microscopy were used to provide data on gangue minerals in the ore shale at the Northern Area (Table II).

TABLE II

Mineral Range of weight %

Quartz 2 0 - 3 3 Feldspar 2--5 Micas 24--34 Carbonates 28--32 Tourmaline 1--4 Accessories 1--2

Data from Kostik and Webber (unpublished ZCCM report, 1982).

PALAEOGEOGRAPHY AND ORE GENESIS

The barren footwall quartzites, grits and conglomerates have been inter- preted as terrestrial talus screes, valley boulder conglomerates and aeolian sands deposited in an arid environment (Fleischer et al., 1976, p. 227). However, Van Eden (1974) argued that they are of marine origin. The over- lying channelised sandstones which host the Chibuluma-type mineralisation were deposited during a major marine transgression, possibly on deltas. The presence of organic carbon, sulphate and carbonate in the ore shales and their stratigraphic position suggest that they were deposited in hypersaline conditions in a marginal marine, possibly sabkha setting. The high content of boron, contained in tourmaline, may also reflect the supersaline condi- tions, but could also be the result of hydrothermal activity. The ore shales were deposited in restricted anoxic basins adjacent to algal bioherms. The overlying sediments, mainly chemical dolomites, reflect continued marine transgression over the shore-line deposits below.

Any genetic model for the mineralisation must take into account the following factors:

(1) The footwall clastic quartzites, grits and conglomerates are totally barren. There is no geochemical enrichment of copper or cobalt in these sediments, although several zones are highly porous and permeable.

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(3) Copper--cobal t mineralisation occurs as fine disseminations and con- centrations along the bedding planes largely confined to the ore shale with lesser amounts in the underlying deltaic sediments.

(4) Copper--cobal t mineralisation always occurs adjacent to basement palaeohighs and algal reefs.

(5) The algal reefs (and associated dolomite) are proximal to the cupri- ferous/cobalt iferous ore shale, yet are essentially barren although minor amounts of bornite have been recorded over narrow widths. The reefs are partly porous and are the ideal site for epigenetic mineralisation.

The evidence favours a syngenetic or syn-sedimentary origin for the sulphides (Fig. 9). In this model the Fe--Cu--Co metals were derived from the provenance area and the supersaline sea-water. Anomalous concentrations of metals in the source area could have been related to rifting or subduction. The conversion of the metals to sulphides took place after bacterial reduc- tion of sulphides was induced by high pH and low Eh conditions in the shallow water or shelf zone. Textural transformation and replacement of some sulphides by others was caused by post-mineralisation events including the Pan-African orogeny.

ACKNOWLEDGEMENTS

I am indebted to the Zambia Consolidated Copper Mines Limited for permission to publish this paper.

Chad Kaunda assisted in some compilations and the figures were prepared by Phineas Hamabele, Morris Mwangha and Enos Chola. I thank Regina Champo for typing the manuscript several times wi thout complaint and Joseph Kalumbi in the printing and dark rooms. The paper benefited from reviews by C.J.V. Wheatley and K.J. Maiden.

This paper is dedicated to the Father of Zambian Geology, Mr. W.G. Garlick, former Consulting Geologist of Roan Consolidated Mines Limited, now merged with Nchanga Consolidated Copper Mines Limited to form Zambia Consolidated Copper Mines Limited.

REFERENCES

Fleischer, V.D., Garlick, W.G. and Haldane, R., 1976. Geology of the Zambian Copper- belt. In: K.H. Wolf (Editor), Handbook of Strata-bound and Stratiform Ore Deposits, Vol. 6. Elsevier, New York, pp. 304--323.

Garlick, W.G., 1981. Sabkhas, slumping and compaction at Mufulira, Zambia. Econ. Geol., 76: 1817--1847.

Garlick, W.G. and Fleischer, V.D., 1972. Sedimentary environment of Zambian copper deposition. Geol. Mijnbouw, 51 : 277--298.

Malan, S.P., 1964. Stromatolites and other algal structures at Mufulira Northern Rhodesia. Econ. Geol., 50: 397--415.

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Mendelsohn, F. (Editor), 1961. The Geology of the Northern Rhodesian Copperbelt . Macdonald, London, 523 pp.

Van Eden, J.G., 1974. Depositional and diagenetic environment related to sulfide miner- al izat ion,Mufulira, Zambia. Econ. Geol., 69: 59--79.