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Kabwe, Broken Hill |
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Zambia |
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Zn Pb
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Kabwe zinc-lead deposit (also been known as Broken Hill) is located ~110 km NNE of Lusaka in Zambia, some 200 km to the south of the Zambian Copper Belt (#Location: 14° 27' 20"S, 28° 25' 59"E).
The deposit is hosted with rocks of the Neoproterozoic Katangan Supergroup, overlying quartzite, schist, ironstone and banded iron formation basement of Mesoproterozoic Muva Group (1.941 to 1.355 Ma; Cahen et al., 1984, Rainaud et al., 2002) and Palaeoproterozoic granite gneiss with minor amphibolite, schist, quartzite, pegmatite and xenolithic metasediments commonly referred as the "Granite-Gneiss Complex" (2050 to 1865 Ma; Arthurs et al., 1995, Cairney and Kerr, 1998).
The Katangan Supergroup metasedimentary cover rocks in the Kabwe area are considered to have been deposited between 880 to 600 Ma, and comprise meta-arkose, quartzite and conglomerate at the base, followed by phyllite, meta-argillite, meta-siltstone, quartzite and dolomite-marble with associated tremolite schist, microdiorite, gabbro, amphibolite and epidosite (Moore, 1964, Cairney and Kerr, 1998). In the Kabwe area has been subdivided into the Kangomba and Nyama Formations, which are considered to be equivalents of the Roan and Nguba Groups of the Zambian Copperbelt (see the separated Zambian Copperbelt record).
The Kangomba Formation commences with a quartzitic-micaceous member and an overlying, mainly dolomitic member, which are considered to correspond to the Lower and Upper Roan Subgroups respectively (Moore, 1964, Cairney and Kerr, 1998), suggesting deposition between 880 and 765 Ma (Armstrong et al., 1999, Key et al., 2001).
The lower siliciclastic Kangomba Formation is, in turn, divided into two lithological units, a basal arkosic sandstones and feldspathic quartzite with minor conglomerates and pelites, overlain by pelites which are indistinguishable from those above overlying host Kabwe Dolomite. The upper Kangomba Formation commences with the ~1150 m Kabwe Dolomite, which is composed of massive dolomite with dolomitic limestone lenses. These are overlain by a suite of pelites, mainly phyllites and meta-siltstones, with interbedded dolostones, locally grading into interbedded pelites and quartzites.
The overlying Nyama Formation is predominantly composed of dolostone and calcite marble and phyllite (Kamona and Friedrich, 2007). It has a lower unit of pelites with thin limestones with local pelitic partings, overlain by pelites with minor quartzite and dolostones It is also possible that the Nyama Fm is partly equivalent to the Upper Roan Subgroup (Barr et al., 1978).
The post-Katangan Karoo Supergroup comprises a series of Carboniferous to Jurassic conglomerate, sandstone and mudstone (Mulcahy, 1991).
The orebodies occur as six main pipe like massive sulphide bodies with oxide/carbonate rims. They are hosted within light grey massive dolomite of the Upper Proterozoic Damaran Supergroup. The pipes strike approximately normal to the host dolomites, and plunge at from 30û to almost vertical. The three largest pipes have total down plunge dimensions of 800, 500 and 400 m. Widths range from 100 to 200 m in the larger examples and thicknesses from 20 to 40 m.
The main pipe like massive orebodies occur within the light grey massive dolomite of the 'Dolomite Formation'. They strike WSW, which is approximately normal to the strike of the Dolomite Formation, and plunge to the ENE at angles ranging from 30 degrees to almost vertical. There are six main ore pipes, the three largest of which have total down plunge dimensions of 800, 500 and 400 m. Widths range from 100 to 200 m in the larger examples and thicknesses from 20 to 40 m. It is suggested in Pelletier (1964) that the pipes follow the dominant fracture and cross fold directions.
In places the boundary between the ore and the dolomite wallrock may be sharp, although the dolomite is often brecciated adjacent to the orebodies, with the zone of brecciation inter-connecting most of the main ore pipes. This breccia comprises angular dolomite fragments from 1 to 50 cm across in a matrix of dark brown, fine grained, quartz, limonite and ankerite. The matrix of the breccia is mineralised with up to 11% Zn. This mineralised breccia grades progressively outwards into a crackle breccia before passing into massive host dolomite, with the matrix becoming light brown and limonitic towards the contact.
The orebodies comprise a core of massive very high grade, coarse sulphide, generally 33 to 34% Zn (although the almost exclusively sphalerite No. 8 has 63% Zn) and 14 to 28% Pb. This sulphide core is usually almost exclusively sphalerite, galena and pyrite. There is an antipathetic relationship between galena and pyrite. Ore may be sphalerite and galena or sphalerite and pyrite, but not galena and pyrite. Other sulphides may be chalcopyrite, bornite, tetrahedrite and chalcocite, but are usually < 0.1%.
Within the sulphide ore, sphalerite has an average grain size of 1 mm (although in the No. 2 orebody it is 2 to 10 cm), while galena averages 1 to 2 mm. Pyrite is usually around 1 mm. The sphalerite has 0.8 to 1.1% Fe, 0.02 to 0.05% Mn and 0.07 to.0.11% Cd. Galena has 0.05 to 0.07% Ag and 0.02% Se.
The margins of the ore pipes are invariably 'oxidised' over a width of 10 m. If the pipe is <20 m thick, it has no sulphides. These 'oxide' shells comprises willemite (Zn2SiO4), smithsonite (ZnCO3), and cerussite (PbCO3), accompanied by lesser anglesite, pyromorphite, vanadinite, descloizite, mottramite, quartz, hematite, goethite, limonite and sporadic Cu minerals. The grade of this zone varies from 17 to 20% Zn and up to 15% Pb. The outer shell of this zone is commonly a very hard brown jasper like mixture of quartz and limonite a few metres wide. The 'oxide' (or 'silicate') zone typically contains many large, open vughs and fissures which may be partially filled with an unconsolidated mud comprising angular dolomite fragments (5 to 10 cm across) in a slurry of very fine grained iron oxides, silicates and dolomite. This mud commonly has very fine 'varve like' bands. It is strongly mineralised and classed as high grade ore.
Large open fissures are also found in the massive dolomite, lined with euhedral dolomite crystals, each averaging 1 m in diameter and locally coarse grained sphalerite. Dolomite crystals of these dimensions are not uncommon and are referred to as 'giant grained dolomite'. One orebody in a zone of 'giant grained dolomite' was 0.5 Mt @ 63% Zn and 0.9% Pb.
The upper sections of the ore shoots have been completely altered to the silicate and carbonate ore, similar to that flanking the sulphides at depth. In deep caves and crevasses, numerous bones of animals have been found, making up a 'bone breccia' within this ore type.
Age dating of the sulphides suggests formation at 712 Ma, younger than the hosts but older than the main Pan African Lufilian Orogeny.
The total deposit is estimated to have contained (USGS Mineral Resources On-Line Spatial Database):
15.3 Mt @ 13% Zn, 5.4% Pb, 6 g/t Ag.
The most recent source geological information used to prepare this decription was dated: 2007.
Record last updated: 31/1/2015
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.
Kabwe
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Haest, M. and Muchez, P., 2011 - Stratiform and vein-type deposits in the Pan-African orogen in central and southern Africa: evidence for multiphase mineralisation: in Geologica Belgica v.14, pp. 23-44,
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Hitzman M W, Reynolds N A, Sangster D F, Allen C R, Carman C E 2003 - Classification, genesis, and exploration guides for nonsulfide zinc deposits: in Econ. Geol. v98 pp 685-714
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Kamona A F, Leveque J, Friedrich G, Haack U 1999 - Lead isotopes of the carbonate-hosted Kabwe, Tsumeb, and Kipushi Pb-Zn-Cu sulphide deposits in relation to Pan African orogenesis in the Damaran-Lufilian Fold Belt of Central Africa: in Mineralium Deposita v34 pp 273-283
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Kamonaa A F and Friedrich G H, 2007 - Geology, mineralogy and stable isotope geochemistry of the Kabwe carbonate-hosted Pb-Zn deposit, Central Zambia : in Ore Geology Reviews v30 pp 217-243
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Kampunzu A B, Cailteux J L H, Kamona A F, Intiomale M M and Melcher F, 2009 - Sediment-hosted Zn–Pb–Cu deposits in the Central African Copperbelt: in Ore Geology Reviews v.35 pp. 263-297
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Schneider J, Melcher F and Brauns M, 2007 - Concordant ages for the giant Kipushi base metal deposit (DR Congo) from direct Rb-Sr and Re-Os dating of sulfides : in Mineralium Deposita v42 pp 791-797
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