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Kalahari Copper Belt - Klein Aub, Witvlei, Dordabis, Oamites |
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Namibia |
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Cu Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Klein Aub, Witvlei, Dordabis and Oamites stratabound, sediment hosted copper-silver deposits define the Kalahari Copper Belt, which stretches discontinuously for ~800 km along the south-eastern margins of the Damaran/Katangan rift basin from central Namibia, extending into northern Botswana.
See the separate record for the continuation of the Kalahari Copper Belt in Botswana including the Lake N'Gami, Ghanzi and Boseto deposits.
Tectonic and Geological Setting
These deposits are hosted within the Neoproterozoic Damara Supergroup which occupies the western section of the Damaran/Katangan rift basin, bounded by the Archaean to early Palaeoproterozoic basement rocks of the Kalahari and Congo cratons to the SE and NW respectively. The Damaran/Katangan sequences are fringed by, and separated from, the two older blocks of cratonic basement by the Mesoproterozoic Kibaran and Irumide successions, comprising thick (>10 km) sequences of variously metamorphosed conglomerates, sandstones, shales with lesser dolerite flows, and overlying graphitic shales, stromatolitic limestones and dolostones. These Mesoproterozoic rocks, deposited from ~1.4 to 1.0 Ga, represent an earlier Kibaran rift basin/orogen, underlying and broadly co-extensive with the Damaran/Katangan rift (Petters, 1986 and sources quoted therein).
Exposure of the Damara Supergroup is bounded to the east by the Cenozoic cover of Kalahari Sands which separates it completely from the Katanga Supergroup. To the west, exposure is terminated by the Atlantic Ocean, although the sequence may be followed for some way to the north and south along the coast in the Atlantic Arm of the rift zone. The other half of the Atlantic Arm is represented by the Araçuaí Basin in Brazil (see the Rio Pardo - Rio do Peixe Bravo District record)
The western Damaran/Katangan Rift Zone commenced with a series of sub parallel fault bounded troughs separated by basement ridges and filled with coarse clastics and variable amounts of volcanic rocks during the extensional tectonic 'rift phase'. This was followed by the onset of the 'sag phase' (or thermal subsidence) which resulted in an overlying carbonate dominated succession developing across the whole zone and lapping onto the adjacent cratons. A renewed phase of extension is evident late in the sequence resulting in a thick clastic sequence and associated mafic volcanics in the southern half of the rift zone, corresponding to a previous broad basement ridge where deposition had until that stage been minimal. This was followed by further 'sag phase' deposition into the lower Phanerozoic which was interrupted in the main rift zone by basin inversion resulting in intense thrust and nappe development in several sections of the Damaran/Katangan Rift, particularly the southern margin. This compressional phase was both normal to the rift zone (NW-SE), and lateral (NE-SW) with a dextral sense of movement, and is also referred to as the Pan African Damaran-Lufilian orogeny.
Each of the tectono-stratigraphic elements of the Damaran section of the Damaran/Katangan Rift Zone is separated by a major linear zone of faulting/shearing/mylonitisation, across which many fundamental changes occur, such as deformational, geophysical and lithological character. Each of these elements also has a particular structural and geophysical characteristic. The Central Zone for instance has been influenced by low pressure metamorphism, and with the Okahandja Lineament Zone is the main focus of granitic development. In contrast, the Southern has been affected by low temperature and high pressure metamorphism. The Southern Marginal Zone is the focus of intense thrusting involving all units from the Mesoproterozoic basement to the Neoproterozoic to Cambrian Nama Group. For more detail see Miller (1983). Each of these tectono-stratigraphic elements represents a different depth and/or lateral slice of the crust brought to the surface and juxtaposed by thrusting during the compressional phase of the Pan African Damaran-Lufilian orogeny.
The Kalahari Copperbelt mineralisation is hosted by a string of basins within the Southern Marginal Zone, bounded by Archaean to Mesoproterozoic basement, both to the NW and SE.
The sequences within and adjacent to the Damaran section of the Damaran/Katangan Rift Zone may be summarised as follows:
ARCHAEAN TO MESOPROTEROZOIC
• Basement is composed of Palaeoproterozoic reworked Archaean gneisses of the Angola Shield, to the north, which includes the 18 000 km2 Kunene Anorthosite Complex, consisting of several distinct anorthosite and leucotroctolite intrusions, the earliest of which is dated at ~1363 Ma. The complex intrudes a sequence of Neoarchean to Palaeoproterozoic granite-gneiss basement rocks and Palaeo- to Mesoproterozoic supracrustal rocks which include 1810 Ma mafic granulites with metamorphic ages of 1520 to 1450 Ma, para- and orthogneiss as well as upper amphibolite facies 1334±21 Ma metasedimentary and volcanic rocks.
• Palaeoproterozoic to Mesoproterozoic rocks of the Rehoboth Terrane underlie the Damaran sequence to the south. The Rehoboth sequence has a possible Archaean foundation, and is generally poorly deformed. It commences with the Rehoboth Magmatic Arc, composed of as much as 5000 to 15 000 m of felsic and andesitic volcanics with intercalated quartzite, phyllites and conglomerates. The basal Elim Formation is largely composed of metabasaltic rocks, with an age of 1870±6 Ma. Overlying volcanic rocks include 1769±6 Ma andesites and 1753±6 Ma meta-rhyolites (Van Schijndel et al., 2014). Note however that Van Niekerk (2009) regard the Elim Formation as part of the uppermost Transvaal Supergroup, which also underlies the Kheis Supergroup to the SE.
The overlying Mesoproterozoic Sinclair Sequence is composed of four cycles of rift facies clastic sedimentation (sandstones and shales) and bimodal volcanism, and forms an arcuate chain rimming the northwestern margin of the Kalahari Craton, and contains numerous vein type copper-lead-zinc and gold occurrences. The youngest unit comprises 750 m of younger quartzites and slates overlying acid lavas and pyroclastics dated at 1050 to 1100 Ma, which unconformably overlie 1250 to 1050 Ma aged granitic rocks. The upper Sinclair underlies or may be equivalent to the Dordabis (or Tsumis) Group.
Unconformity
MESO- to NEOPROTEROZOIC
• Dordabis (or Tsumis) Group - The earliest stages of the Damaran appears to have been triggered by uplift along the Rehoboth and Irumide zones associated with the latter stages of the Kibaran event. This resulted in mafic volcanism and coarse cratonic sedimentation along the major structural feature defining the southeastern margin of the rift zone. Sedimentation and volcanism of the Doornpoort and Klein Aub Formations in Namibia (and the Ghanzi Group in Botswana) represented a transition from the Sinclair of the Rehoboth Province to the Damaran, mainly on the Southern Foreland and Platform. The Grauwater and Nukopf Formations in Namibia (Kgwebe Group in Botswana), which are found unconformably below the Doornpoort Formation contain abundant basic and acid volcanics, quartzites and conglomerates. The Doornpoort Formation overlies a 1090 Ma granitic intrusive which cuts the Grauwater and Nuckopf Formations (Maiden, et al., 1984).
Doornpoort Formation, around 1500 m thick at Klein Aub and in Botswana, but much thicker at Witvlei - red poorly sorted pebble to boulder conglomerates predominating at the base, with intercalated dirty quartzite, fining upwards to include minor argillites. This is interpreted as a piedmont to alluvial fan environment. Casts and pseudomorphs after gypsum and anhydrite occur locally within these red beds. Basic lavas and lesser acid pyroclastics are found in the lower parts of the unit, which has a gradational contact with the overlying Klein Aub Formation which represents a major environmental change.
Klein Aub Formation, around 2000 m thick - dark coloured laminated mudstone and siltstone, green to grey crossbedded fine sandstone, minor conglomerates and siltstone, and minor limestone. These apparently represent lacustrine deposition.
The Doornpoort and Klein Aub Formations are restricted to the Southern Foreland and Platform and are estimated as having been deposited between 1100 and 950 Ma (Maiden, et al., 1984 and Borg and Maiden 1989).
Unconformity
NEOPROTEROZOIC
Damara Supergroup
• Nosib Group, up to 7000 m thick - typically a pinkish, coarse to medium grained, massively to thinly bedded, poorly to moderately sorted feldspathic quartzite, locally also having basal conglomerates and grading laterally into grits or feldspathic or lithic greywacke. In many places it has scattered pebbles, and heavy mineral laminae are common, while crossbedding is relatively widespread.
The distribution of the Nosib Group is variable, reflecting the distribution of semi-parallel and semi-isolated fault bounded troughs and ridges within the rift zone. Each of the main tectonic subdivisions of the Damaran section of the rift zone elements is separated by a major linear zone of faulting/shearing/mylonitisation, across which many fundamental changes occur, such as deformational, geophysical and lithological character.
The Nosib Group is present in the Southern Foreland and Platform, (around 5000 m thick), the Southern Margin Zone, (3000 to 6000 m thick), possibly continuous with the SF; the Northern Platform, (around 1200 m thick), the Eastern Kaoko Zone, (around 5000 m thick), the Northern Zone, (more than 6500 m thick), and only over the SW section of the northern Central Zone where it was thin. A SW pointing wedge of the Central Zone was basement high as far west as Usakos separating deposition on the Northern Zone from the Southern Central Zone, where up to 3000 m of the Nosib Group is mapped. The lower Nosib Etusis Formation is not present within the northern Central Zone between the Omaruru Lineament and the Otjihorongo Thrust where the total Nosib sequence thins. There was no Nosib Group in the Southern Zone, the Okahandja Lineament Zone, or the Central Kaoko Zone. A thick 'turbidite unit' occupies the Southern Kaoko Zone, and Western Kaoko Zone.
While the shallow water clastic sequence described above is characteristic of the Nosib Group, other lithologies are present. Within the Northern Zone, only, there are three upper Nosib to lower Swakop Group volcanic units, bimodal in the west. These are the Austerlitz and Summas rhyolitic ignimbrites and thinner upper vesicular basalts of the Naauwpoort Formation in the SW, and the Askevold Formation in the Tsumeb area to the NE. These volcanics are up to 6600 m thick and in places interfinger with the lower Swakop Group sediments, as high as the base of the Chuos Formation. They are concentrated along the faulted margins of the grabens in which the Nosib Group was deposited and are dated between 840 and Ma 730.
In the Northern Zone, the upper Nosib Group contains a 'glaciogene' mixtite which is a poorly sorted matrix supported conglomerate much of which is a pebbly lithic greywacke to pebbly arkose with layers of disseminated hematite and magnetite in its upper sections.
In the Central Zone, and the Southern Margin Zone, the upper Nosib Group sediments of the Khan and Duruchaus Formations respectively, contain carbonatic to marly facies. The Khan Formation comprises feldspathic quartzites, pyroxene gneisses and amphibolites, with rare thin conglomeratic layers and calc-silicates towards the top. The Duruchaus Formation has been subjected to less intense thermal metamorphism and comprises phyllite, quartzite, conglomerate and dolomite. Both units provide varying evidence of evaporites.
The Nosib Formation was deposited in three grabens separated by basement ridges within the main Damaran/Katangan Rift Zone and on three platform areas, one on a basement ridge between two grabens, the other two on the adjacent cratonic margins of the rift zone. Deposition of the Nosib Group is estimated as being between 1000 and 730 Ma.
• Otavi and Swakop Groups, have thicknesses of up to 5000 m on the Northern Platform, 2000 m in the Northern Zone, and Central Zone; and 7500 m in the Southern Margin Zone - These two groups are generally correlated in the three regions. They both have a high content of carbonates, overlie the Nosib Group and contain some common correlatable units. The thick sequence of schists, quartzites and lesser calc-silicates and amphibolites, that constitute the Kuiseb Formation has equated with the upper most Otavi Group carbonates, although it is possible that these are in fact post Swakop/Otavi Group rocks associated with a fresh burst of rifting, probably equivalent to the lower Mulden and lowermost Nama Groups.
The Otavi Group is very widespread but restricted to the Northern Platform to the east of the East Kaoko Zone, to the north of the Northern Zone and as a rim on the southern margin of the Kamanjab Inlier. It is areally more extensive than the underlying Nosib Group and transgresses well onto the Angola-Kasai Craton. As such it is largely the 'steers horn' section of the Damaran 'sag phase' sequence, while the Swakop Group is the main basinal 'sag phase' succession.
The Swakop Group in the Southern Margin Zone, commences with the:
Kudis Subgroup, 3000 m thick, comprising a basal thick bedded marble which rests unconformably on both Nosib Group and pre Damaran basement and is tectonically intercalated with quartz-mica schist, partly due to tectonic imbrication. These are followed by a thick quartzite and a turbiditic quartzite-graphite schist facies, graphitic schists with interbedded quartz-mica schist, marble, quartzite, conglomerate and local pyritic cherts. In places the Corona Formation interfingers with the lower sections of this unit.
Vaalgras Subgroup, (after Hoffmann 1983 and Miller 1983), composed of a lower, thick, extensive, (glaciogene?) mixtite, with numerous interbedded amphibolites and mafic schists and thin horizons of ferruginous schist grading to banded iron formations. These are overlain by monotonous, coarsely garnetiferous, massive, green chloritic schists which grade upwards into grey mica schists, quartzites, grey micaceous dolomite, local pebbly facies and intercalated amphibolites, sometimes with associated magnetic quartzites, followed in turn by dark amphibole schist with minor mica schist, local lenticular marble units and one or more thin rhyolitic bands. The sequence is capped by a laterally discontinuous ubit of well layered, generally very micaceous quartzite, quartz schists and thinly interlayered mica schists.
LATE NEOPROTEROZOIC to CAMBRIAN
• Kuiseb Formation, is mapped in the Southern Zone, where it averages ~3000 m in thickness, continuing into the Okahandja Lineament Zone where it reaches 10 000 m thick; the Central Zone, where it is thinly (generally <3000 m) and sporadically developed, and rests directly on Pre Damaran Basement; and the Northern Zone, where it may again be up to 10 km thick. The Kuiseb Formation is again evident in the Western Kaoko Zone. It comprises a monotonous sequence of quartz-plagioclase-mica schist, made up of interbedded quartz rich and mica rich layers of variable thickness, and lesser quartzites. Thick portions of the sequence are either predominantly mica or quartz rich. Minor components are garnet-cordierite gneiss, the amphibolites of the Matchless Member (restricted to the Southern Zone, where it occurs as a linear zone from 2 to 10 km wide and 350 km long), thin graphitic schists at various levels, minor marbles and calc-silicate lenses. The degree of deformation and schistosity increases from north to south, although the grade of thermal metamorphism increases from south to north. The Kuiseb Formation is interpreted as a greywacke shale sequence, metamorphosed during basin inversion which resulted in intense thrusting and shearing on the southern margin of the Southern Zone, folding throughout the Southern and Okahandja Lineament Zone, and thermal activity in the Central Zone, forming extensive s-type granites.
• Mulden Group, up to 5500 m thick, found on the Northern Platform, where the Otavi Group is unconformably overlain by the Mulden Group, which is also evident on the margins of the Northern Zone, and the Eastern Kaoko Zone. This group appears to represent an initial thicker coarse clastic phase, followed by finer grained clastics and carbonates. Age dating suggests that the Mulden Group was deposited from 580 to 530 Ma (Cahen and Snelling 1984).
• Nama Group, which is very extensively developed on the Southern Platform, and over a wide area of the adjacent Kaapvaal/Kalahari Craton to the south. It was deposited between 650 and 520 Ma (Cahen and Snelling 1984, Miller 1983), and comprises the:
Kuibis Subgroup - basal tightly folded dolomite, limestone and shale, followed paraconformably by 2 cycles of basal conglomerate followed by feldspathic sandstone, orthoquartzite, shale and limestone. A green shale facies is found in an equivalent position basinwards, to the west. Ediacaran faunas indicate an age of 650 to 570 Ma.
Schwarzrand Subgroup - thick limestones in the south and alternating shallow water sandstone and green shale that becomes increasingly mature upwards. Red beds occur above an extensive unconformity near the top of the unit. Fossils indicate a Cambrian age near the top.
Fish River Subgroup - unconformably above the Schwarzrand Subgroup, this unit comprises mainly red, medium to coarse grained feldspathic sandstone with minor shale.
It is possible that the Nama Group, which is restricted to the Southern Platform and adjacent Kalahari Craton is equivalent to sections of the Kuiseb Formation in the main Damaran Rift Zone and the Mulden Group, which is only found on the Northern Platform and Angola-Kasai Craton.
Klein Aub
The Klein Aub deposit is located within the Southern Foreland and Platform section of the Damaran/Katangan Rift Zone, some 150 km to the SW of Windhoek in Namibia. It was discovered during the 1960s by General Mining Corporation of South Africa following a diamond drilling program in the vicinity of outcropping copper mineralisation. Mining commenced in 1966 and continued until 1987 when the mine closed. According to Borg and Maiden (1986), production from 1967 had been 7.5 Mt @ 2% Cu and 45 g/t Ag, while Ruxton (1986) states that in 1980 the reserves were 6 Mt @ 1 to 2% Cu and 50 to 100 g/t Ag. The mine has been closed, the shafts sealed and the head frames and mill removed.
Geology - The Klein Aub deposit is hosted by a suite of alluvial fan conglomerates, sandstones and carbonatic argillites, fining upwards, with the lower sections including intercalated basic volcanics. Mineralisation is found at the transition from the predominant red beds of the lower part of the Klein Aub Formation sequence to the overlying, finer reduced sediments.
The northern part of the Klein Aub Formation exposure is unconformably underlain by the Rehoboth basement inlier, which consist of rocks of the Elim Formation (Rehoboth Sequence), the Piksteel Granodiorite and the Kobos Granite. The basement is overlain by Mesoproterozoic rocks of the Sinclair Sequence, which include the lower Nückopf and Grauwater Formations, comprising mainly rhyolithic volcanic rocks with subordinate conglomerate, quartzite and minor basalt (Borg, 1995; Borg and Stanistreet, 1996).
To the south, the Klein Aub Group again unconformably overlies metasedimentary and meta-volcanic, gneissic and granitic basement rocks of the Palaeo- to Mesoproterozoic 1800 and 1000 Ma Rehoboth and Sinclair Sequences, and is overlain by sedimentary rocks of the Neoproterozoic to Cambrian Nama Group, and by nappes of the Naükluft Nappe Complex.
The host rocks belong to the Meso- to Neoproterozoic Dordabis Group which marks the transition from the Mesoproterozoic largely igneous Sinclair Supergroup to the Neoproterozoic Damara Supergroup. The mineralised section is located on the Southern Foreland and Platform of the Damaran Rift Zone where these sediments lap over onto the Rehoboth Province of the Kaapvaal/Kalahari Craton to the south. The Klein Aub deposit lies on the southern margin of the Rehoboth Inlier of Sinclair Supergroup and Rehoboth Magmatic Arc rocks which separates it from the main Damaran/Katangan Rift.
The earliest stages of the Damaran/Katangan Rift Zone appears to have been triggered by uplift along the Rehoboth and Irumide zones associated with the latter stages of the Kibaran event. This resulted in mafic volcanism and coarse cratonic sedimentation along the major structural feature defining the southeastern margin of the rift zone. Sediments and volcanics of the Dordabis/Tsumis Group Doornpoort and Klein Aub Formations were deposited on the Southern Foreland and Platform of the Damaran/Katangan Rift Zone in Namibia. These represent a transition from the Grauwater and Nükopf Formations of the Sinclair Supergroup which was laid down within the pre Damaran Rehoboth Province.
The Grauwater and Nükopf Formations, which are found unconformably below the Doornpoort Formation, contain abundant basic and acid volcanics, quartzites and conglomerates. The acid porphyry, rhyolitic, ignimbritic and pyroclastic volcanics of the Grauwater and Nükopf Formations, according to Borg and Maiden (1987), are characteristic of caldera type volcanism. Radiometric dating of zircons from these volcanics yield ages of 1230 to 1080 Ma. The Doornpoort Formation overlies a 1090 Ma granitic intrusive which cuts the Grauwater and Nückopf Formations (Maiden, et al., 1984). In the immediate Klein Aub area, the Doornpoort Formation oversteps the Grauwater and Nükopf Formations and lies directly on the older basement.
The Klein Aub Formation is followed disconformably by the Nosib Group of the Damara Supergroup, which is in turn unconformably overlain by the late Neoproterozoic to lower Cambrian Nama Group.
The Doornpoort and Klein Aub Formations represent an upward change from eluvial piedmont sedimentation immediately above the Rehoboth Inlier basement, to sheet flood and braided stream sediments of alluvial fan deposition, followed by shallow subaqueous lacustrine pyritic meta-sandstone, dark grey and black pyritic slate, detrital meta-carbonate and laminated slate of the Klein Aub Formation (Borg and Maiden 1987).
The Doornpoort and Klein Aub Formations are restricted to the Southern Foreland and Platform and are estimated as having been deposited between 1100 and 950 Ma (Maiden, et al., 1984 and Borg and Maiden 1989).
The stratigraphy of the Dordabis/Tsumis Group in the Klein Aub area is as follows:
• Doornpoort Formation, around 1500 m thick at Klein Aub and in Botswana, but much thicker at Witvlei to the NE where it may be up to 3500 m or more thick - commences with red poorly sorted (sand size to 0.5 m) angular basement derived clasts forming a pebble to boulder conglomerates predominating at the base, with intercalated dirty quartzite, fining upwards to include minor argillites. This unit which is characterised by large scale trough cross bedding, poor sorting, rapid grain size variations and red colouration is interpreted as a transition from a piedmont to an alluvial fan environment. Casts and pseudomorphs after gypsum and anhydrite occur locally within these red beds. There is a gradational contact with the overlying Klein Aub Formation marking a major environmental change.
Locally basic lavas and lesser acid pyroclastics are found in the lower section of this formation. These volcanics are interpreted as extrusive which have been introduced along the deep tapping faults that controlled the block faulting and periodic uplift which resulted in the coarse clastics of the Doornpoort Formation with which they are interbedded.
The basic lavas have a tholeiitic composition and comprise both amygdaloidal and porous varieties, the latter including flow top breccias, tuffs, sediment filled contraction fractures, tectonically brecciated varieties and strongly amygdaloidal zones. In the brecciated domains feldspar has been replaced by epidote, quartz and subordinate chlorite, muscovite, carbonate and hematite. This alteration accounts for approximately 20% of the of the outcropping basalt. Relatively little basalt outcrops in an anticlinal core to the east of Klein Aub, although geophysical data suggests a larger volume of basic rock down dip to the south.
• Klein Aub Formation, around 2000 m thick - commences with a thick (from 400 m to 1000 m) coarse conglomeratic unit with subrounded to rounded clasts of both basement and sediments in a yellow to red arkosic matrix. This basal conglomerate is intercalated with medium to coarse grained yellow to brown to red poorly sorted arkose. The conglomerates and sandstones have coarse trough cross bedding and planar beds with trough cross stratification. The conglomerates and sandstones are followed by a succession of intercalated dark coloured laminated mudstone and siltstone, green to grey crossbedded fine sandstone, minor conglomerates and siltstone, and minor limestone. These apparently represent lacustrine deposition. Three lacustrine successions were recognised by Ruxton (1986), two transgressive and one regressive. The initial transgressive event is confined to the Klein Aub mine area and consists of fluvial sandstones with several thin intercalated lacustrine bands. Towards the base, argillites are widely spaced (20 to 30 m apart) containing fine grained red sandstones and siltstones. At the top of the Klein Aub Formation, the Kagas Member consists of quartzite and green slate, overlain by argillite, sandstone, marl and limestone, with beds of green to black, organic-rich dolomitic argillite (siltstones and mudstones) which host the copper-silver deposits at the Klein Aub Mine. These organic argillite bands, which can be traced laterally into grey siltstones and fine grained sandstones, include finely laminated sections containing Cu and Fe sulphides, replacive and displacive carbonate cements and algal mat laminations. The bands are from 1 to 3 m thick separated by 10 to 15 m of red or green fluvial sandstone, and contain small scale ripple cross stratification and small flute casts (Borg and Maiden, 1986).
In the transition zone from the red continental to green lacustrine argillites there is a variegated red to green siltstone to mudstone band containing laminar and nodular limestone which forms a persistent marker bed within the mine.
The first and second transgressive phases are separated by a break which may be either faulted or a low angle local unconformity. This second phase and the following regressive phase contain sediments similar to those described above.
In the mine area the host sequence dips at 50° to the SSE, away from the basement rocks of the Rehoboth Inlier. Folding is very minor, with faulting being dominant.
Dextral wrench-thrust faulting is represented in the district, particularly near the base of the Kagas Member within the mine area, where a thrust juxtaposes red coarse footwall clastics over the reduced host sequence. This faulting is believed to parallel the major NE-SW fault direction, which follows the main Damaran/Katangan Rift margin.
In the mine area the thrust fault is almost bedding parallel, cutting the ore bearing sequence at about 15° to bedding, transgressing down sequence with depth. It is marked by a zone of brecciation containing tectonic clasts of the adjacent lithologies which are up to 10 cm in diameter (Holcombe 1977). A slight flexure in the host argillites within 10m of the fault breccia marks an incipient drag fold related to the thrust.
Mineralisation - The following is largely taken from Ruxton (1986), Borg and Maiden (1987) and Borg and Maiden (1989), with modifications from Maiden (2011). Mineralisation within the Klein Aub area is present at a number of stratigraphic and structural positions. These include:
i). Fractured, brecciated and altered meta-basalts in the lower Doornpoort Formation. Mineralisation occurs as native copper fillings of amygdales, in flow top breccias and in shear zones in altered domains of basaltic lava. Single lumps of native Cu up to 1.5 tonnes have been found;
ii). Zones developed along some faults; and
iii). Generally strata-bound developments in up to seven beds of green to grey to black argillite, separated by beds of pyritic quartzite in the Kagas Member of the Klein Aub Formation. These green and dark grey reduced argillite bands belong to the first lacustrine transgressive sequence, usually containing some graphite, which immediately overlie coarse red clastics of the Doornpoort and basal Klein Aub Formations, and form the dominant host rock, separated by low grade pyritic quartzite and lesser marble. The red sediments are unmineralised. Not all reduced laminites are mineralised and, in places, close to highly mineralised zones, adjacent reduced alluvial sandstones may contain Cu sulphides as nodular and lenticular aggregates and as cement to the detrital grains.
Macroscopically, copper mineralisation at Klein Aub, and in most other parts of the Namibian segment of the Kalahari Copperbelt, is strata-bound. On a regional scale, a broad zone of sub-economic mineralisation is developed within the lower dark slates and interbedded quartzites of the Klein Aub Formation. This zone, which embraces the Klein Aub deposit, is intermittently developed over a strike length of 80 km.
Geologic mapping (e.g., Handley, 1965) has also indicated that the distribution of copper mineralisation is related to the Klein Aub fault, a strike-parallel reverse structure that post-dates cleavage. The nature of the fault structure and the ore petrology of the mineralogically zoned orebodies indicates reverse dextral oblique slip movement on the fault. This fault has dragged and cut off the mineralised bands, abruptly terminating the orebodies down dip. The geometry of the orebodies is also controlled by subordinate faults and shears of the same system (Borg et al., 1987).
In detail two styles of mineralisation are present:
i). The first is stratabound, hiosted by argillites that in detail comprise interbedded calcite-quartzite, interlaminated dolomitic siltstone/mudstone, silicified chloritic mudstone, thin chloritic layers and rare bands of calcite nodules. Chalcocite occurs as disseminated grains in coarser laminae of reduced dolomitic siltstone and quartzite. The sulphides are essentially interstitial, apparently cementing detrital grains. Both clastic and calcareous laminates host Cu sulphides, although the latter are usually of lower grade. In addition Cu minerals are present as pseudomorphed replacements of early coarser pyrite.
ii). The second is transgressive, apparently emplaced during the Pan African event. Much of the sulphide mineralisation of the deposit (~60%) occurs in small 2 to 3 mm joints and fractures as cleavage parallel lenticles, and in large scale crosscutting quartz-carbonate veins and blows up to 0.5 m across (both parallel and transgressive to bedding), controlled by brittle fractures, and tectonic breccia zones, with associated hematite, calcite, quartz and chlorite. The best Cu grades are apparently in areas of more intense deformation with tectonically induced permeability, principally adjacent to the main fault. Joints, fractures and veins only contain copper in the mineralised parts of the stratabound argillite bands. Hematite and Cu sulphides also concentrate in the axes of small scale pinch and swell folds and occur intergrown with metamorphic chlorite which developed during cleavage formation. The association of hematite and Cu sulphides is common, generally associated with structural and metamorphic textures, but also forming random displacive/replacive aggregates.
At Klein Aub, there are 6 to 10 mineralised stratabound and structural bands, each 1 to 3 m thick with a maximum strike length of 1.5 km, which make up the ore deposit. These bands are each generally continuous over a strike length of 1 km and up dip from the thrust fault for 500 m.
Chalcocite is the most abundant Cu mineral at Klein Aub, accounting for more than 85% of the total Cu sulphides. Minor copper minerals include bornite and chalcopyrite with lesser chalcocite, djurleite, digenite, hematite, bornite, chalcopyrite, pyrite, covellite, native copper, cuprite, magnetite, native silver, malachite, galena and wittchenite in decreasing order of abundance.
Mineralisation is zoned regionally, passing upward from the root zone immediately above the fault, via a cuprite zone (where grades of 6 to 8% Cu are found), with rare native Cu close to the thrust fault, through the main chalcocite orebodies toward narrow chalcopyrite-bornite fringes. Within the ore bands, grades decrease up dip away from the thrust. Chalcocite is the main sulphide mineral at Klein Aub, and is normally associated with hematite and minor pyrite (Borg and Maiden, 1989), with chalcocite nodules sometimes rimmed by hematite (Holcombe 1977). The updip fringes, with their rapid gradation from the main chalcocite zone into bornite, then chalcopyrite, constitutes only a minor portion of the total ore. Even farther updip, the copper mineralisation grades into diagenetic pyrite. Pyrite also occurs as cubes, up to 10 mm across, in the arenites that are intercalated with the argillite bands. Here the (diagenetic) pyrite cubes have quartz-calcite-filled pressure shadows that are the result of regional tectonometamorphism. The "hard" pyrite is locally replaced by "soft" chalcocite, pseudomorphing the original pyrite cubes with blurred hematite halos from iron liberated from the replaced pyrite lattice. Borg and Maiden (1989) argue that "soft" chalcocite would have been deformed or smeared out during ductile deformation under greenschist facies conditions and hence this ore texture documents the post- (ductile-)deformational origin of the copper mineralisation at Klein Aub.
There is a close correspondence between Cu and Ag grades, suggesting that the Ag is held within the chalcocite. Pb, Zn and Co are very low throughout the orebody (Borg and Maiden 1989).
According to Ruxton (1986) the most striking control on ore is the association of mineralisation and coarse alluvial fan conglomerate facies. At Klein Aub, the Cu ores lie adjacent to an underlying thick conglomeratic facies development, which represents the zone of maximum sedimentation during the alluvial fan formation. This association is also observed at Witvlei. A contour map of Cu grade related to thickness of Band 3 in the Klein Aub mine defines ore shoots and low grade (1% Cu over 1 m thickness cutoff) areas within the broad mineralised blanket. Correlating this data with palaeocurrent information shows a close match between palaeocurrent/facies development direction and ore shoot elongation (Ruxton 1986).
The permeable domains of the altered meta-basalts, which have a tholeiitic composition, account for approximately 20% of the outcrop and have been altered such that the plagioclase has been replaced by epidote and quartz with subordinate chlorite, muscovite, carbonate and hematite. Fractures and amygdales are generally filled with quartz, carbonate and epidote. The unaltered basalts have a normal content of around 70 ppm Pb, 120 ppm Zn and 70 ppm Co, while the Cu value is around 80 ppm Cu. In the altered basalts however the Pb is higher at 106 ppm, Zn is markedly lower at 10 ppm, Co is also reduced at 24 ppm and Cu is depleted being typically 15 ppm. The altered basalts also have a higher Ca, but lower Mg and Na content in comparison to the unaltered domains.
It has been suggested by Borg and Maiden (1987) that the depleted Cu from the altered basalts is a potential source of the transgressive mineralisation within the Klein Aub orebody. This is postulated as having occurred during the metamorphism of the host sequence and basalts accompanying the Damaran Orogeny, or Pan African event. Mineralisation was deposited in brittle fracture tectonically imposed permeability. The stratabound mineralisation, originally considered to be late diagenetic by Borg and Maiden (1987), is generally subeconomic, and was interpreted to have occurred within the host Klein Aub sediments prior to the loss of permeability and subsequent compaction. The combination of the low grade stratabound and the overprinted, higher grade, tectonically controlled, transgressive mineralisation (not remobilised, but brought in from the underlying basalts via the fault zone), they claim, resulted in the ore grade.
However, Maiden (2010) and Sillitoe et al. (2010) observe that there is no mineralogical or geochemical difference in composition between the stratabound and transgressive mineralisation in the deposits of the Kalahari Copperbelt of Botswana and Namibia. These authors also report that identical sulphide assemblages and textures characterise both the disseminated and accompanying veinlet mineralisation throughout the zoned sulphide assemblages, taken to imply contemporaneity of the two styles. Maiden (2010) suggests the stratabound mineralisation could represent copper in reactive laminae, replacing precursor minerals such as diagenetic sulphides, carbonate and/or feldspar. He also suggests here is no evidence that the cleavage-parallel lenticles and the vein and breccia-hosted copper have been remobilised from the stratabound laminae (as earlier postulated), and that there is overwhelming evidence that all the copper was emplaced late in the geologic history, most likely during basin inversion, metamorphism, and even during the terminal stages of orogeny (Sillitoe et al., 2010).
Oamites
The Oamites deposit is hosted by sediments of the Neo- to possibly late Mesoproterozoic, Damaran, Nosib Group, on the southern margin of the main Damaran/Katangan Rift, just to the SW of Windhoek. It lies within a zone of major thrusting, and may alternatively be within basement sediments of the Sinclair succession. Dips are near vertical in the mine area but shallow to the north.
Mineralisation is hosted by a 20 to 50 m thick unit of sandstone, conglomerate and thin dolomitic marble, sandwiched between the underlying 400 m thick amphibole-biotite schist (suggested metamorphosed andesitic volcanics) and overlying intercalated conglomerates and amphibole-biotite schists (interpreted as alternating sandstones and shales). The footwall sediments are pink, while the ore bed is reduced. The orebed itself thins to the east, an interpreted original sedimentary feature.
Mineralisation is best developed in the thicker sections of the ore bed, with the 1% Cu contour corresponding to the 20 m isopach. The orebody is of the order of 600 x 400 x 15 m, to a maximum of 55 m thick. Some copper mineralisation occurs within the schists below the ore bearing unit, while non-cupriferous pyrite is found in the rocks above. Sulphides constitute about 5% of the orebody, comprising 40% chalcopyrite, 30% bornite, 20% pyrite and 10% chalcocite. Trace amounts of Pb, Zn, Ni and Co are recorded, but seldom exceed 0.2%. The mineralogical zoning upwards and towards the 'shore' (ie. eastward) is pyrite to chalcopyrite to bornite to bornite-chalcocite. Pb is antipathetic to Cu and increases towards the west. Native Ag accompanies the Cu and occurs in argentiferous galena on the western fringes. Sulphides are present as 10 to 30 µm grains, tiny shreds and as stringers. Replacement and pseudomorphism indicate that pyrite was replaced by chalcopyrite, this by bornite and then chalcocite. Ag occurs as distinct grains up to 35 µm across and as rim replacements of chalcopyrite and pyrrhotite. The main source of information has been Holcombe (1977).
Witvlei
The Witvlei deposits are located ~150 km east of Windhoek, within a northeasterly trending belt of Meso- to Neoproterozoic sediments of the Sinclair Sequence Doornpoort/Eskadron Formations. This succession is made up of altered andesitic volcanic and volcaniclastics rocks, a series of red to grey alluvial fan breccias, sandstones and siltstones intercalated with aeolian, playa, lacustrine as well as shallow water carbonate sediments.
This succession has been extensively folded around NE-SW trending axes, with antiformal cores of unconformably underlying Palaeoproterozoic Rehoboth Sequence basement rocks composed of dioritic intrusive, mafic to intermediate volcanic and volcaniclastic rocks.
The Eskadron Formation sedimentary package is bounded to the north and south by NE-trending thrust faults that separate the sequence from younger Damaran-aged Neoproterozoic Nosib Group sediments.
Copper mineralisation is typically hosted within argillites and local marls of the Eskadron Formation. The area is masked to a large extent by a veneer of Tertiary to Quaternary age Kalahari sediments.
Stratabound copper mineralisation found within the belt is generally associated with Sinclair Sequence volcanic rocks and Eskadron Formation clastic red bed sequences. Two main prospects are known:
Malachite Pan, where 25 mineralised bands have been identified, 15 of which were delineated as having sufficient grade and lateral continuity to be extracted using underground mining methods. A non-Jorc compliant resource of 2.98 Mt @ 2.1% Cu was delineated over an average width of 2.36 m.
Witvlei Pos, which is located some 9 km south of Malachite Pan. A non-Jorc compliant resource of 2.85 Mt @ 1.52% Cu was delineated over
an average width of 2.12 m.
The deposits of the Witvlei area, comprise multiple bands of copper-bearing slate and fine quartzite, which in detail, exhibit textures similar to those of the Klein Aub deposit.
Dordabis
The Dordabis deposits are located approximately 90 km southeast of Windhoek and 125 km SW of Witvlei. The deposit area is characterised by a series of NE-trending belts of Meso- to Neoproterozoic Sinclair-age volcaniclastic sediments of the Marienhof and Eskadron Groups and the Neoproterozoic Damaran age metasediments of the Nosib and Nama
Groups that are separated by low angle NE-trending thrust faults formed during the Damaran Orogeny. A number of phases of folding with varying intensities are known, dominated by NE-trending fold axes within the Sinclair Sequence equivalents (e.g., the Marienhof Group) which displays the most intense deformation. Much of the area is masked by Tertiary to Quaternary Kalahari Sequence cover.
Two styles of copper mineralisation are recognised in the district:
i). Native copper within basalt, occurring in vesicles, along fractures and shears and in flow-top and tectonic breccias. The meta-basalts have been strongly altered to epidote-chlorite-quartz-muscovite-hematite-carbonate assemblages, with Cu associated with high Ag and Ba contents. Single sheets of up to 1.5 t of native copper have been extracted from the basalt.
ii). Sediment hosted copper mineralisation such as the historical Koperberg Cu occurrence where mineralisation is hosted within a series of stacked mineralised argillite horizons that are tightly infolded into a synform that plunges steeply to the north at 70°. Mineralisation occurs as disseminations along sand and silt laminae in slate, as cleavage parallel lenticle, as nodular aggregates in quartzite and as brittle fracture fillings. It is dominated by chalcocite with lesser chalcopyrite and bornite in sulphide zones below a depth of 25 to 30 m. Malachite dominates secondary oxide copper mineralisation at Koperberg that can be clearly seen coating well defined axial planar cleavage surfaces. Mineralisation covers a strike length of 1.5 km, containing a resource of 0.29 Mt @ 1.7% Cu to a depth of 50 m. Mineralisation was confined to six slate bands that average 2 m in thickness, with localised thickening up to 9 m.
Deposits sizes are as follows:
Klein Aub - total production between 1966 and 1987 of 7.5 Mt @ 2% Cu, 45 g/t Ag,
Oamites - total production + resources (in 1992) of 8 Mt @ 1.3% Cu, 12 g/t Ag,
Witvlei possibly reources in 1990 of 5 Mt @ 1.5 to 2% Cu, up to 30 g/t Ag.
van Schijndel, V, Cornell, D.H., Freib, D., Simonsen, S.L. and Whitehouse, M.J., 2014 - Crustal evolution of the Rehoboth Province from Archaean to Mesoproterozoic times: Insights from the Rehoboth Basement Inlier; Precambrian Research, v. 240, pp. 22–36.
The most recent source geological information used to prepare this decription was dated: 2010.
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.
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Maiden K J and Borg G, 2011 - The Kalahari Copperbelt in Central Namibia: Controls on Copper Mineralization: in SEG Newsletter, No.87 pp. 1, 14-19
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Sillitoe R H, Perello J and Garcia A, 2010 - Sulfide-Bearing Veinlets Throughout the Stratiform Mineralization of the Central African Copperbelt: Temporal and Genetic Implications : in Econ. Geol. v105 pp 1361-1368
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