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Chibuluma |
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Zambia |
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Cu Co
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Super Porphyry Cu and Au
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The Chibuluma East, West and South copper-cobalt deposits are located some 10 km to the SW, 12 km WSW and 18 km SW of Nkana respectively, in the Zambian Copper Belt of northern Zambia (#Location: Chibuluma South 12° 54' 49"S, 28° 4' 49"E; Chibuluma West 12° 49' 52"S, 28° 6' 20"E; Chibuluma East 12° 50' 29"S, 28° 8' 27"E).
The Chibuluma East and West deposits were discovered in 1939 and 1941 respectively. Chibuluma East commenced production in 1956. The Chibuluma West mine, 2 km to the west, began operations in 1963. Discovery and production was by Roan Selection Trust (RST). The Chibuluma South orebody was discovered by RST in 1969, as a strike extension to the small Chifupu deposit (discovered in 1967), which was ~1.7 km to the SW. The Chibuluma mines were nationalised in 1970 and became part of the state owned Zambian Consolidated Copper Mines (ZCCM), before being privatised and sold to Metorex Limited in 1997. The Chibuluma East mine was closed prior to 1997, while the Chibuluma West mine ceased operation in 2005. The Chibuluma South mine achieved steady state production as an underground mine by mid 2007.
For details of the regional setting of Chibuluma, the Chambishi-Nkana basin, the Central African/Zambian Copper Belt and the Lufilian Arc, see the separate Zambian Copperbelt record.
The Chibuluma deposits are hosted by the Neoproterozoic Lower Roan Subgroup of the Katanga Supergroup, within the Chambishi-Nkana basin, located on the mid-southwestern flank of the 'Kafue Anticline'. The 'Kafue Anticline' is a late-tectonic structural feature within the Domes Region of the Lufilian Arc, centred on a basement high of Palaeo- and Mesoproterozoic gneisses and schists, over which the Katangan sedimentary rocks were draped. The Chambishi-Nkana basin is essentially a NW-SE elongated, doubly plunging, structural basin, predominantly surrounded by pre-Katangan basement. The basinal structure cuts across the generally NW-SE trending facies boundaries within the Lower Roan Subgroup. The Chibuluma East and Chibuluma West deposits lie on the southwestern margin of the Chambishi-Nkana structural basin, ~10 and ~14 km to the NW of the main Nkana Central headframe. Chibuluma South is 10 km south of Chibuluma West, on the opposite margin of the NW-plunging anticlinal basement nose that forms the southwestern margin of the Chambishi-Nkana structural basin.
These deposits are hosted by Lower Roan Subgroup sedimentary rocks that are further removed from the shoreline of the Roan sedimentary basin than Nkana-Mindola and other major deposits of the Zambian Copper Belt. The basement to the Lower Roan Group is composed of Palaeoproterozoic Lufubu System quartz-biotite schists, quartz-microcline-biotite granite gneisses and related granites, and Mesoproterozoic Muva conglomerate, quartzite and quartz-schist. At Chibuluma South, the immediate basement is described as a variably foliated, coarse-grained, grey quartz-biotite-microcline-porphyritic granite, with pink phenocrysts and aligned biotite clots, and local leuco-granite dykes.
The unconformity surface below all three of these deposits includes hills and ridges of granite gneiss, that have influenced the shape and distribution of the host sequence and mineralisation, and subsequent deformation.
The stratigraphy of the Lower Roan Subgroup within the Nkana-Chambishi Basin can be summarised as follows (after Garlick, in Fleischer, Garlick and Haldane, 1976; Metorex, 2010), from the base:
Mindola Clastics (Footwall) Formation at Chibuluma East and West is subdivided into,
Basal Conglomerate, 0 to 1.5 m thick - angular boulders of granite and quartz-biotite gneiss in a sandy matrix, flanking basement hills, and at Chibuluma West, filling gullies between hills.
Aeolian Quartzite, 0 to 130 m thick - well banded feldspathic arenite with large scale cross bedded cossets up to 5 m thick and at angles of over 30°. Quartz, albite, minor biotite, iron oxide and rutile with variable carbonate and anhydrite cement, are the usual constituents. At Chibuluma West, the basement projects above the top of this unit.
Aqueous Arkose, 1 to 5 m thick - overlying an undulating, and in places, deeply pot-holed surface in the aeolianites, are feldspathic grits with pebbles up to 7 cm in diameter, crossbedded arkoses and minor shale beds. Small scale cross bedding, ripple marks, pebbles, gritty and muddy layers are common.
At Chibuluma South, the Footwall Formation, below the Ore Member, comprises,
Basal Conglomerate, 0 - 1.5 m thick - biotite-quartz schists and biotite quartzites, usually containing unsorted boulders and pebbles of basement rocks.
Footwall Sandstone, 10 to 100 m thick - dark pink and grey, mottled and laminated, feldspathic, argillaceous sandstones which are interbedded with biotite quartzites and biotite-quartz schists, and occasional 2 m thick beds of conglomerate with well-rounded cobbles of granite, which are matrix-supported in feldspathic argillaceous sandstone. There are occasional minor interbeds of dark and pale, hard, feldspathic quartzite.
Chibuluma Ore Member, generally 0 to 7 m, but locally up to 30 m thick - a layer of arkose with scattered quartz and quartzite pebbles is discontinuous at the base and generally carries disseminated Cu and Fe sulphides. The overlying sedimentary rocks are feldspathic arenites with sulphides, and argillaceous material now represented by sericite and biotite. Crossbedding is common in places. The host rock to the orebody is a pebbly sericitic feldspathic sandstone or grit with schistose shales. Lithologically it is very similar to the underlying aqueous arenites, except that where mineralised it is conspicuously of a more schistose nature with a higher mica content (Garlick 1962). Where not mineralised it is a pebbly arkose.
The Chibuluma East orebody has a strike length of 330 m and persists down dip for ~1500 m to a depth of 900 m, with a maximum thickness of 23 m, averaging 7.5 m. At surface the deposit dips at ~10°N. It was deposited in a channel cutting into the underlying Aqueous Arkose. At the base of the Chibuluma East orebody, there are marker beds of sulphidite (rock with >33% sulphide), each 7 to 120 cm thick, consisting of cobaltiferous pyrite, carrollite and minor chalcopyrite in a matrix of detrital quartz and feldspar. These sulphidite bands extend over the whole length of the orebody and grade upwards into normal disseminated chalcopyrite. Conglomeratic pebbles within the sulphidite are matrix supported. Dense intergranular and authigenic tourmaline is common within sections of the sulphidite. The overlying disseminated sulphides are evenly distributed along any one bed, but vary considerably across the sequence. In some sections, the chalcopyrite zone is separated from barren intervals by a bornite-chalcopyrite phase. There is a general zoning across the orebody, parallel to strike, of barren to chalcopyrite to pyrite to chalcopyrite to barren.
At Chibuluma West, the orebody is similar, except for the absence of sulphidite layers, and that the ore is bounded by basement hills which protrude through the Chibuluma Ore Formation. The ore bearing rocks lap onto basement, occurring in a 'moat' on the north, east and south sides of a basement hill. To the north and east, the ore rests on barren aqueous arenites, but to the south, the 'moat' was deep, and the ore rests on the aeolian arenites and to the north on the scree slopes of the basement hill. Mineralisation can be very rich with intersections of up to 25 m @ 15% Cu. The underlying arenite may also locally be mineralised. At the orebody fringes, distant from basement ridges the ore grades abruptly into disseminated pyrite with lenses of chalcopyrite, which within 30 m fades into barren arkose, that on the southern fringe is a dark flinty quartzite. Where resting on basement the immediately underlying scree and basement schist may be mineralised for up to 7 m below the unconformity.
At Chibuluma South, the mineralisation is predominantly copper with only very minor cobalt. The orebody is hosted by the competent Orebody Quartzite unit, directly overlain by argillites and dolomites of the Upper Roan Subgroup. The Orebody Quartzite is described as a 0 to 30 m thick unit composed of hard, creamy and pale grey, coarse- to medium-grained, well sorted, feldspathic quartzite, which may be argillaceous and pebbly, with muscovite clots, and sometimes small vugs after carbonate. The orebody occurs over a strike length of 300 m, dipping at ~38°NW, and varies in thickness from a few metres to locally >30m. It persists to a maximum depth of 600 m where it pinches out against a basement high. Mineralisation occurs as oxide (dominantly malachite) to a depth of 60 m and as sulphides (bornite, chalcopyrite and chalcocite) below that level. Supergene chalcocite occurs immediately below the oxide cap in the sulphide zone. Bornite dominates in the thickest and richest central portion of the orebody and is the predominant sulphide mineral with primary chalcocite below the 400 m level. Chalcopyrite accounts for ~20% of the sulphides and becomes more evident towards the fringes. Pyrite dominates in the barren margins. Cobalt mineralisation is patchy.
Hangingwall Quartzite, 0 to 9 m thick - a hard pyritic feldspathic arenite overlies the orebodies, followed by pyritic gritty arenites and argillaceous interbeds. In places this unit has disseminated chalcocite or chalcopyrite, locally forming a second patchy ore horizon. At Chibuluma South the hanging wall sequence is described as a 0 to 30 m thick suite of pink and green banded chlorite-dolomite and quartz schists, interbedded with dark pink feldspathic argillaceous quartzites, overlain directly by the Upper Roan Subgroup to the west.
Hangingwall Conglomerate, up to 9 m thick - this unit has been correlated with the Lower Conglomerate (of the Mindola Clastics Formation) at Nkana to the east. At Chibuluma East it is a compact porous conglomerate, in places directly overlying the orebody where the intervening quartzite was eroded. It contains considerable disseminated pyrite. It is absent at Chibuluma West, where a possible tectonic breccia forms the contact with the overlying Upper Roan Subgroup.
The Hangingwall Conglomerate is overlain directly by the Upper Roan Subgroup, with a chlorite and talc schist at the base, probably a tectonic feature.
At Chibuluma West the Lower Roan-Upper Roan contact is marked by a dolomitic breccia composed of angular to rounded nodules and pebbles of dolomite
and talc-carbonate rock set in a matrix of talc, carbonate and albite, known as the talc-albite-carbonate breccia (Whyte and Green, 1971).
In the Chambishi-Nkana basin, the Upper Roan is made up of a sequence, from the base, of interbedded schist and quartzite; cherty dolomite; sandy talc-schist; and up to 400 m of white to pink dolostone, that includes some cherty layers with talc, and shale bands with disseminated pyrite. Around 100 m above the base of the latter dolostone unit, in the northern and western parts of the structural basin, an ~170 m thick metagabbro sill occurs, plus ~52 m of granophyre and ~8 m of magnetite. At Chibuluma West, this intrusion has been extensively altered to amphibolitic scapolitised rocks and in places has also been albitised, with a common assemblage of albite, scapolite, hornblende, chlorite, carbonate and accessory opaque minerals with minor epidote. At Chibuluma South, the Upper Roan Subgroup is described as >250 m of dolostones, dolostone breccias and chlorite-dolomite schists intruded by massive metagabbro and gabbro-dolomite breccias.
Mineralisation at Chibuluma East and West is entirely within the Footwall Formation, which is overlain directly by the Upper Roan Subgroup. The 'Ore Shale' that is characteristic of the main Copper Belt deposits was not developed at Chibuluma and the Hangingwall Formation of the Lower Roan Subgroup pinches out ~2 km to the east of Chibuluma East. The mineralisation at Chibuluma East and West is capped by up to 15 m of quartzites and conglomerates, followed by the Upper Roan Subgroup carbonates.
Chibuluma is an example of a Footwall Orebody (e.g., below the Nkana Southern Orebody - see the separate Nkana-Mindola record), which are usually characterised by lower tonnages, but higher grades. Footwall orebodies on the Copper Belt are generally characterised by being markedly more transgressive, not confined to any particular stratigraphy and may even extend into the basement.
Published production, reserve and resource figures include:
Chibuluma East & West
total production + resource as at 1988 - 28.4 Mt @ 4.12% Cu, 0.19% Co (Freeman, 1988);
total production to 1988 - 20 Mt @ 4.53% Cu, 0.19% Co (Freeman, 1988);
Chibuluma West - remaining down dip resource 1998 - 0.92 Mt @ 5.33% Cu, 0.11% Co (Crew Development Corp., 1998);
Chibuluma East - original pre-mining resource - 28 Mt @ 4.1% Cu (Arthurs, 2009);
Chibuluma West - original pre-mining resource - 8.4 Mt @ 3.2% Cu, 0.19% Co (Arthurs, 2009);
Chibuluma South (after Metorex, 2010) in 2000
original pre-mining ore reserve - 9.3 Mt at 3.6% Cu;
Chibuluma South (after Metorex, 2010) at the end of 2009
measured + indicated resource - 4.9 Mt @ 4% Cu;
inferred resource - 6.9 Mt @ 3.8% Cu;
proved + probable reserve - 3.9 Mt @ 3.6% Cu;
Chibuluma is currently held by Metorex (85%) and Zambian Consolidated Copper Mines Ltd, 15%.
The most recent source geological information used to prepare this decription was dated: 2010.
Record last updated: 13/8/2013
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Chibuluma South Chibuluma West Chibuluma East
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Selected References:
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Fleischer V D, Garlick W G, Haldane R 1976 - Geology of the Zambian Copperbelt (Exerpt covering Chibuluma and Chibuluma West): in Wolf K H, (ed), Handbook of Strata-bound and Stratiform Deposits; II. Regional Studies and Specific Deposits Elsevier, Amsterdam v6, Cu, Zn, Pb and Ag Deposits pp 298-304
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Selley D, Broughton D, Scott R, Hitzman M, Bull S, Large R, McGoldrick P, Croaker M and Pollington N, 2005 - A new look at the geology of the Zambian Copperbelt: in Economic Geology, 100 Anniversary Volume, Society of Economic Geologists, pp. 965-1000
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Whyte R J and Green M E, 1971 - Geology and Palaeogeograhy of Chibuluma West Orebody, Zambian Copperbelt: in Econ. Geol. v.66 pp. 400-424
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Winfield O 1961 - Chibuluma: in Mendelsohn F (ed), Geology of the Northern Rhodesian Copperbelt McDonald, London pp 328-342
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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