DEPOSIT DESCRIPTIONS - MODULE 1
This tour, which was developed, organised, managed and led by TM (Mike) Porter of Porter GeoConsultancy Pty Ltd (PGC), as a joint venture with the Australian Mineral Foundation Inc. (AMF), included:
MODULE 1 - PROTEROZOIC COPPER, ZINC & LEAD
PART A - Zambian Copper Belt
Sunday 10 to Saturday 16 June 2001
PART B - Zinc, Lead & Copper in South Africa & Namibia,
Sunday 17 to Sunday 24 June 2001
|For information on the remainder of the tour,
see the Deposit Descriptions for Module
Module 1, Part A, was managed by PGC (in joint venture with the AMF), in cooperation with AMIRA International to double as the non-confidential segment of its Sponsors Field Meeting for research project P544, "Proterozoic Sediment Hosted Copper Deposits".
MODULE 1 - PART A - Zambian Copper Belt
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The Zambian Copperbelt is part of the larger Central African Copperbelt, and extends north into the neighbouring Democratic Republic of Congo (DRC), to continue as the Congolese Copperbelt. It closely coincides with the southern half of a complex, arcuate structural zone, the late Neoproterozoic to early Cambrian Lufilian Arc. Although part of the same curvilinear trend, the Zambian Copperbelt deposits, are hosted by non-evaporitic, rift facies siliciclastic rocks that are not temporally or lithologically equivalent to the mainly carbonate hosts of the Congolese Copperbelt. In addition, the structural architecture in Zambia is characterised by thrusted Palaeo- to Mesoproterozoic metamorphic basement inliers and folded Neoproterozoic sedimentary host rocks, in contrast to the thin skinned deformation of the hosts in the DRC, without involvement of basement.
Mineralisation within the Central African Copperbelt is hosted within the Neoproterozoic Katanga Supergroup, an intracontinental rift basin sequence, comprising the Roan, Nguba and Kundelungu groups. A period of extension commenced after ~900 Ma, and in Zambia, the sedimentary sequence began after 880 Ma, with the Lower Roan Subgroup oxidised rift facies clastic rocks, deposited in a series of restricted sub-basins controlled by extensional normal faults, including the thin reduced argillites of the Copperbelt Orebody Member near the middle of the unit. The succeeding sag phase Upper Roan Subgroup, is dominantly made up platformal mixed carbonate and clastic rocks, but also included a <500 m thick evaporite/salt bed. To the north, in the DRC, the sag phase sequence transgresses beyond the main rift margin, to become the basal unit of the Roan Group, where the Upper Roan Subgroup equivalents are divided into three. The lowest of these, the R.A.T. Subgroup, comprises the red, silt- and sand-sized residue after dissolution of a thick (>500 m) evaporite/salt bed deposited in a restricted sub-basin, separated from the open sea by a stromatolitic reef. This is overlain by the Mines Subgroup dolostones and shales, including a number of reduced units near the base. The uppermost of the three Roan Group units in the DRC is the Dipeta Subgroup, comprising a lower regressive suite, including oxidised, evaporitic rocks, followed by transgressive carbonate rocks with some reduced intervals.
During deposition of the late Upper Roan and Dipeta subgroups, a period of 'renewed extension' commenced, reflected by a ~30 to 100 km wide by ~500 km long corridor of gabbroic and lesser felsic intrusions, and an associated volcanic belt of thin mafic lavas and tuffs emplaced from ~765 to 735 Ma. During the same event, an arcuate metamorphic core complex formed, embracing the gabbroic intrusive corridor, marked by a series of basement domes capped by mylonitic detachments. During this period, deposition transgressed further onto basement, and the Dipeta Group passed upwards into a sequence of carbonaceous argillites of the Mwashya Subgroup, the uppermost unit of the Roan Group in both Zambia and DRC. This was overlain by glacial diamictites and interbedded pyritic black argillites of the basal Nguba Group Grand Conglomérat. The remainder of the Nguba Group carbonate and siliciclastic rocks were overlain in turn by the Kundelungu Group basal diamictites and then carbonate rocks, passing up into siliciclastic rocks.
The main Katanga Supergroup deposition was terminated by the Lufilian orogenic event after ~600 Ma, with NE vergent D1 folding and thrusting and D2 strike-slip folding to produce the Lufilian Arc. Prior to this event, possibly during late 'renewed extension', uplift of the Domes core complex to the SW, coupled with deepening of the basin to the NE (in response to extension), and dissolution of the R.A.T. Subgroup salt beds (as the increasing temperature and pressure allowed fluids to penetrate the salt beds), led to northward mass gravity gliding of the Upper Roan/R.A.T., Mines and Dipeta subgroup successions towards the main basin centre. This gravity gliding, slumping, salt dissolution and diapirism, produced a regional scale megabreccia of large clasts up to several km in length of Mines Subgroup rock in a matrix of R.A.T. Subgroup and comminuted Roan Group rock fragments to rock flour.
During the late Lufilian event, a complex of anorogenic granites was emplaced in the inner Lufilian Arc to the SW from ~560 to 530 Ma, with minor coeval mafic to ultramafic intrusions. This was followed after ~530 Ma by orogenic relaxation that lasted until after 500 Ma.
Stratabound and vein ore emplaced after 880 Ma is found in rocks from the Palaeoproterozoic Lufubu Metamorphic Complex to the Kundelungu Group. However, large, stratabound sediment hosted Cu-Cu±U deposits are restricted to the stratigraphic interval from the Lower Roan Subgroup to the basal Nguba Group, specifically where it occurs in association with rocks that contain (or contained) in situ or mobile hydrocarbons at the following stratigraphic levels: i). the Copperbelt Orebody Member (including secondary positions within 100 m above and below this member), where the great bulk of the Zambian Copperbelt deposits are hosted; ii). the base of the Mines Subgroup (including a subsidiary position within ~100 m higher in the sequence) in the Congolese Copperbelt, accounting for the bulk of ore in the DRC; iii). the upper Dipeta Subgroup (e.g., Mutanda, Deziwa; iv). Mwashya Subgroup (e,g, Frontier, Kansanshi); and v). Grand Conglomérat of the Nguba Group (e.g., Kamoa, Fishtie).
However, almost invariably, ore is only found at the first reduced unit overlying an oxidised arenaceous (or otherwise permeable) sequence, separating it from basement, e.g., ore is only found within the Mwashia Subgroup or Grand Conglomérat where no reduced Roan Group unit exists in the underlying sequence (e.g., Frontier, Kamoa).
Regional scale alteration includes an early i). calcic-magnesian phase that redistributed carbonate and anhydrite from the evaporite beds to porous rocks throughout the basin; ii). intense potassic alteration that overlapped the Ca-Mg stage, and preceded and overlapped ore; and iii). a later, but multipulse sodic-calcic phase that pervasively overprinted early ore and predominantly affected Nguba and Kundelungu group rocks, but also locally Roan Group lithologies.
Sulphide mineralisation was emplaced in a number of generations, including: i). early pyrite, developed either during early diagenesis in association with bacterial sulphate reduction (BSR) in carbonaceous argillites, or late diagenesis after emplacement of oil or gas reservoirs. ii). early diagenetic pyrite was overprinted and partially replaced by weak regional copper mineralisation derived from diagenetic, saline K-Mg brines circulating and scavenging metal from the basin; iii). significant, late diagenetic and pre-folding, generally fine-grained, disseminations (mostly bedding controlled) and veins containing Cu-Co sulphides and pyrite, concentrated in fluid traps along basement growth faults. These sulphides usually have isotopic signatures suggesting low temperature BSR associated with brines at temperatures of from 80 to 150°C; iv). early tectonic syn-folding sulphides, occurring as coarser veins and lesser disseminations of Cu-Co sulphides associated with brines at temperatures of 150 to 250°C that are replacing earlier, and generating new, sulphides by thermochemical sulphate reduction (TSR); v). post-folding, coarser veins and lesser disseminations of copper sulphides, that formed at the end the main Lufilian Orogeny during orogenic extensional relaxation. This generation was associated with calcic-sodic brines at temperatures of 250 to 400°C, replacing existing sulphide and producing ones by TSR, concentrated in reduced rocks, including those with pre-existing mineralisation. These brines were also responsible for IOCG and hybrid IOCG-sediment hosted copper mineralisation.
Most significant sediment hosted Cu-Co deposits in the Central African Copperbelt can be shown to contain early pyrite, overprinting pre-folding, early to late diagenetic Cu-Co sulphides in disseminations and veins, and syn- to post-folding coarser veins and lesser disseminations. From the evidence available it can be inferred that significant Cu-Co mineralisation was emplaced by a major influx of metalliferous brines scavenged from both the basement and Katanga basin during: i). the renewed extension coeval with or soon after deposition of the Grand Conglomérat of the Nguba Group, at ~765 Ma; ii). during the early stages of the Lufilian compression from ~585 to 575 Ma; and iii). during orogenic relaxation from ~530 to ~500 Ma. Not all deposits have experience all three generations (e.g., Kansanshi), although most of the large high grade deposits are the product of at least two.
The Central African Copperbelt is the largest and highest grade sediment-hosted stratabound copper province known in the world. Mineralisation is hosted in a variety of rocks and fluid traps, and emplaced by a number of events spread over a period of ~200 Ma. The key however, is the large amount of Cu-Co, necessarily scavenged from a very large volume of rock with lithologic and structural permeability. The structural preparation of basement permeability, and the generation and circulation of brines required both a large scale source of salts/evaporites, and significant pulses of energy to promote basement faulting (both listric and compressional detachments) and to elevate thermal gradients. These energy pulses were provided by the 'renewed extensional' event, particularly the metamorphic core complex and gabbroic magmatism, and the Lufilian compression and related anatectic magmatism. Without one or both of these two events, the result may have been a regionally distributed low grade to anomalous copper shale.
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Kitwe Seminar - The Setting, Geology & Metallogeny of the Zambian Copper Belt
The seminar included presentations covering the following topics:
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Mufulira - Cu-Co
The Mufulira stratabound, sediment-hosted, copper-cobalt deposit is located 30 km north of the town of Kitwe, in the Zambian Copper Belt of central Zambia. It is also one of the 'classic' Zambian Copperbelt deposits, located on the eastern margin of the Kafue Anticline, hosted by the Lower Roan Subgroup of the Neoproterozoic Katangan Supergroup. Copper sulphides occur at 21 distinct stratigraphic positions within the Mufulira mine, of which only three are economically exploited, termed the 'A', 'B' and 'C', within the 30 to 80 m thick "Ore Formation". The Ore Formation overlies the westward thinning "Footwall Formation", which is equivalent to the Mindola Clastics Formation on the western flank of the Kafue Anticline. The 'C' Orebody is the lowest and most extensive, having lateral dimensions of 5800 m, continuous down dip for >1300 m, to at least the 1500 m level, and is up 23 m thick, averaging ~14 m.
- The setting & geology of the Damaran-Katangan System in Africa and the place of the Lufilian Arc & Copper-Belt within it.
- The tectonics, geology, structure, metallogeny and mineralisation of the Copper Belt in Zambia and the Cupriferous Arc in the neighbouring Democratic Republic of the Congo (DRC) and how they are related.
- Descriptions of the important DRC deposits and their characteristics.
- The stratigraphy, structure, occurrence and controls of copper mineralisation within the Copper Belt in Zambia.
- Details of the main deposits that are not to be visited and how they compare to those orebodies on the itinerary.
- Aspects of exploration on the Copper Belt - the expression of mineralisation in outcrop, geochemistry, geophysics, etc.
The ore at Mufulira is almost exclusively within 'arenites' and not in the intervening and overlying finer sedimentary rocks. Copper sulphides occur as discontinuous bands of 'fly speck' disseminations following bedding planes and as irregular angular clots of weak to dense interstitial disseminations of sulphides whose elongation is influenced by bedding, but may also be markedly transgressive. Chalcopyrite dominates in the 'C' Orebody (although in sections this grades to bornite dominant), whilst in the 'B' Orebody, the predominant ore mineral is bornite. The uppermost 'A' Orebody is also the highest grade, with bornite and chalcocite. Total production + resources were estimated in 1988 as 335 Mt @ 3.3% Cu. ..... MORE
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Nkana - Cu-Co
The Nkana-Mindola stratabound, sediment-hosted, copper-cobalt deposits are exploited from four open pit and four underground mines, from north to south, the North Mindola, Mindola, Nkana Central and Nkana SOB (Southern Ore Body). Mineralisation, is more or less continuous over a strike length of 14 km, with narrow 'barren gaps', on the SW margin of the Kafue Anticline, immediately to the east of the town of Kitwe and is hosted within the 21 to 23 m thick Copperbelt Orebody Member, or 'Ore Shale'. The Copperbelt Orebody Member is the basal unit of the Kitwe Formation, predominantly composed of dark grey dolomitic argillite, overlain by quartzites and dolomitic argillites of the main bulk of the Kitwe Formation. It overlies an eastward thinning sequence of oxidised sandstones and conglomerates that comprise the Mindola Clastics Formation, which rest on basement gneisses.
Orebodies are distributed throughout the Copperbelt Orebody Member. Overall, the best grades of primary mineralisation (~4% Cu) are found to the NE in the Mindola orebody, dropping off to nearer 2.2% Cu at depth to the south in the SOB (South Orebody), where the host then passes into black carbonaceous shale, with only low grade chalcopyrite, carrollite and pyrite, then to pyrite with scattered chalcopyrite. To the north and up-dip of this facies gradation, the primary ore is chalcopyrite dominant, passing into a bornite-chalcopyrite in the Mindola deposit. At Mindola, bornite accompanies chalcopyrite at the base of the ore zone, decreasing upwards. Total production + resource were estimated in 1988 as 690 Mt @ 2.5% Cu. ..... MORE
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Nchanga - Cu-Co
The Nchanga-Chingola stratabound, sediment-hosted copper-cobalt deposits are located on the northern section of the western margin of the Kafue Anticline in Zambia. Mineralisation is hosted by the Neoproterozoic Lower Roan Subgroup, where its lower members lap onto a palaeo-basement high that corresponds to the core of the current Kafue Anticline. Primary copper mineralisation is found at five overlapping levels over a total stratigraphic interval of 150 m, occurring as 'Footwall ore' in the Mindola Clastics Formation, the lowermost unit of the Lower Roan Subgroup, in the Lower Banded Shale (equivalent to the regional Copperbelt Orebody Member) at the base of the overlying Kitwe Formation, and in succeeding members of the latter formation. Mineralisation, which is generally found at progressively higher stratigraphic positions from south to north, is hosted by a variety of lithologies, including micaceous and dolomitic feldspathic quartzite, coarse grained arkose and greywacke, and, to a lesser extent, by siltstones and shales which cap these arenite units. Primary sulphides, which include chalcopyrite, bornite, chalcocite and pyrite, occur mainly as disseminations within the arenitic units and as disseminations orientated along bedding or cleavage planes, and in quartz veins, within the finer facies. Much of the sulphides have been oxidised to depths of over 300 m, with malachite and secondary chalcocite being ubiquitous. Total production + resources at the Nchanga-Chingola cluster is 1080 Mt @ 2.16% Cu. .... MORE
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Konkola - Cu-Co
The Konkola-Kirila Bombwe and Konkola Deep stratabound, sediment-hosted, copper-cobalt deposits, which are ~15 km to the north of Nchanga on the northwestern extremity of the Kafue Anticline in Zambia, are part of the Konkola Copper Mines operation which also includes Nchanga-Chingola. The Konkola mineralisation continues north at depth, becoming the neighbouring Lubambe (previously Konkola North) mine that emerges on the eastern margin of the Konkola Dome to the NW, following the rim of dome northwestward into the DRC, as the Musoshi deposit. The ore is predominantly hosted by banded siltstones and sandy shales of the up to 60 m thick Lower Roan Subgroup Copperbelt Orebody Member, equivalent to the Lower Banded Shale facies that hosts some of the ore at Nchanga. Total production + resources from Kirila Bombwe to Musoshi is estimated to total >1000 Mt @ ~3% Cu. ..... MORE
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Chambishi - Cu-Co
The Chambishi Main, West and Southeast sediment hosted copper-cobalt deposits are developed within the Lower Roan Group in the Zambian Copper Belt. They are located on the southwestern margin of the 'Kafue Anticline', approximately half way between the Nkana-Mindola and Nchanga mining complexes in northern Zambia. They are ~360 km north of Lusaka and 28 km NW of Kitwe. Chambishi Southeast is centred on a point ~10 km NW from the Mindola mine, while the Main and West deposits, which are separated by a narrow barren gap, are a further ~7 km to the NW (#Location: 12° 39' 36"S, 28° 3' 21"E).
The Chambishi Main deposit was discovered in 1902, with underground access being developed in 1927, although operations were suspended in 1931 due to depressed copper prices. Operations were not restarted until an open pit was initiated in 1963, with the first ore treated in 1965, to be supplemented by underground production from 1972. The open pit was closed in 1978, and the underground mining were suspended in 1987. Non-Ferrous Corporation Africa (NFCA) aquired the operation in 1998, and commissioned a new undeground mine in 2003. Construction commenced on the Chambishi West underground mine in 2007, with first production in 2010. Exploration at Chambishi Southeast has sporadically taken place since 1903, with the main discovery drilling from 1975 to 1982. Production was scheduled to begin in 2016.
For details of the regional setting of Chambishi, the Chambishi-Nkana basin, the Central African/Zambian Copper Belt and the Lufilian Arc, see the separate Zambian Copperbelt record.
The Chambishi deposits are hosted by the Neoproterozoic Lower Roan Group Ore Formation 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 Group. The Chambishi deposits lie on the northeastern and northern margins of the Chambishi-Nkana structural basin.
Geology & Mineralisation
The stratigraphy of the Lower Roan Group at Chambishi can be summarised as follows, from the base, where it unconformably overlies the Palaeo- to Mesoproterozoic basement complex, which is predominantly a grey, microcline-biotite-granite and numerous aplite dykes, with a few Lufubu schist xenoliths, overlain by Mesoproterozoic Muva conglomerate, quartzite and quartz-schist (after Garlick in Mendelsohn, 1961):
Mindola Clastics Formation or Footwall Formation, 0 to 150 m thick - at surface the Mindola Clastics Formation is thin, but thickens down-dip, and comprises:
Basal Conglomerate, 0 to 65 m thick - a sporadically developed boulder bed near the surface, where it is a metre or so thick, overlying Muva schist east of the orebody, increasing to 65 m in thickness above granite basement deeper in the basin, under the West Orebody.
Footwall Aeolian Quartzite, up to 33 m thick - quartzites with large scale cross bedding (most likely aeolian), with upper beds which are coarser grained, feldspathic to arkosic and with considerable small scale crossbedding suggesting a provenance from the SW. Includes a layer of anhydrite lenses. Laps against granitic basement highs in many places where the Basal Conglomerate is absent.
Footwall Transition Arenites, 0 to 18 m thick - arkose, grading upwards into argillaceous arenites and thin argillites. The unit contains local, thin zones of disseminated chalcopyrite and pyrite, and a few fekdspar, quartz and sulphide veins.
Cobble Conglomerate, 0 to 20 m thick - a conglomerate with granite and quartzite cobbles up to 8 and rarely 25 cm in diameter in a carbonate and anhydrite-rich sandy matrix which leaches to a porous manganiferous material near surface. A few drill holes intersected traces of disseminated bornite and chalcopyrite in parts of this member.
Arkose and Argillite, 12 to 25 m thick - poorly bedded, white to pink arkose and layers of grey, generally schistose sandy argillite. Under the shallower parts of the Main orebody and farther east the argillite beds are absent.
Footwall Conglomerate, 0.5 to 10 m thick - this member has a variable composition and thickness. Where thickest it is a coarse arkose with pebbles and rare cobbles, and near the surface large fragments of granite and schist. In places there are scree like granitic deposits flanking granitic palaeo-hills. Elsewhere it is 1 to 3m thick and comprises light grey and rarely pink, poorly sorted, porous, pebbly arkose. In many drill holes, down dip this member is a thin gritty arkose or feldspathic quartzite, difficult to differentiate from the underlying quartzites of the 'Arkose and Argillite' and 'Footwall Quartzite' members.
Kitwe Formation, which commences with the:
Copperbelt Orebody Member, previously known as the Ore Formation or Ore Shale, 0 to 30 m thick - where fully developed, the basal 1 to 3 m of the Copperbelt Orebody Member is a dolomitic-schist, a rock rich in carbonate which is intensely contorted and schistose and cut by quartz-dolomite-anhydrite veins. The major part of the Copperbelt Orebody Member is a fine grained, biotite-quartz argillite, with well developed banding due to differences in grain size and the varying minor dolomite content. A pronounced flow-cleavage is parallel to the axial planes of small and large drag folds. In the west, the cleavage is less prominent. Over considerable areas small lenticules of dolomite, anhydrite and minor quartz, commonly with sulphides, are prominent in the upper parts of the Copperbelt Orebody Member. The upper 3 to 6 m is commonly more sandy, and may show graded bedding. Near the surface and near granitic promontories, cross bedded arkosic lenses in scours herald the approach of the hangingwall arenites. Down dip, a brownish vuggy dolomite bed up to 1 m thick occurs 2.5 m from the top. West of the Chambishi West Orebody, the Copperbelt Orebody Member becomes black, carbonaceous and pyritic.
In the folded section of the Main Orebody, and in the West Orebody, the Copperbelt Orebody Member averages close to 30 m in thickness, but down dip, thins to 20 m. Against resistant granite ridges, both east and west of the Main Orebody, the shale thins to 6 m, as the lower beds become sandy or arkosic, which in the absence of 'Footwall Conglomerate' may not be distinguished from footwall arenites. Rarely is argillite found in contact with granitic basement. East of the easternmost granitic promontory, the Copperbelt Orebody Member is 6 to 9 m thick for 1.5 km, beyond which it becomes sandy or quartzitic and may not be differentiated from the overlying or underlying arenites.
In the Main Deposit, where the Copperbelt Orebody Member is above the eastern granite ridge, it is barren and contains numerous arkosic layers, which thin down-dip and to the west, accompanied by the appearance of disseminated chalcocite in the lower third of the Copperbelt Orebody Member. Chalcocite gives way to bornite down-dip, while in the basal dolomite-schist, chalcopyrite appears, accompanied by abundant bornite. Coarse aggregates of both bornite and chalcopyrite are distributed along bedding planes, with associated cross-cutting quartz-dolomite veins carrying the same sulphides. The basal dolomitic-schist thickens down-dip and to the west, to become a 3 m thick layer with grades of up to 10% Cu. This high grade compensates for the progressive down-dip decrease in copper grade in the overlying argillite, as bornite passes into chalcopyrite. Further down-dip and to the west, chalcopyrite in the upper Copperbelt Orebody Member gives way pyrite. Bornite again appears as the western granite ridge is approached, and over which the Copperbelt Orebody Member thins to ~6 m, and is barren for a strike interval of ~200 to 300 m. Grades in the chalcopyrite, chalcopyrite-bornite and bornite zones generally average ~2%, 2 to 4% and >4% Cu respectively.
The Main orebody is laterlly limited by the two basement granite ridge. It has a strike length of ~800 m, expanding to 1500 m at a depth of ~300 m, with a thickness of up to 30 m, averaging 8 m, and an overall dip of 15 to 75°W with a series of drag fold-related reversals (Fleischer et al., 1976; SRK 2012).
The West Deposit, occurs to the west of the 200 to 300 m wide barren gap over the western granite ridge. As the Copperbelt Orebody Member thickens again, the sulphide mineralogy is dominated by chalcopyrite disseminations, with only minor bornite appearing again. The deposit has a strike length of ~1800 m, persists down dip for ~600 m, varies from 2 to 17 m in thickness, averaging 8 m, and is lower grade, containing ~2% Cu. A small ore lense is also found in arkose below the Cobble Conglomerate where these beds abut the western granite ridge. While the eastern margin of the West Deposit is influenced by the barren gap over the western granite ridge, its western margin corresponds to a facies change within the Copperbelt Orebody Member to a thick pyritic and carbonaceous shale, that persists to the western margin of the Nkana-Chambishi structural basin (Fleischer et al., 1976; SRK 2012).
There is an ~20 km long, largely barren gap to the south, separating the Chambishi Main deposit and Mindola. Within the middle of this interval, the Chambishi Southeast resource has been outlined, comprising two orebodies distributed over an 8 km by 1 to 2 km NW-SE trending zone. The North body is 4500 m long, 570 to 1240 m wide, dips at 5 to 15°NW and is 1.4 to 23 m thick, averaging 10 m, with ~55 Mt @ 2.3 to 2.4% Cu, 0.074% Co. The south body is 3540 m long, 800 to 1600 m wide and lower grade, with ~50 Mt @ 1.6% Cu, 0.0125% Co. The deposits are flat lying to shallowly dipping, and completely blind. They are located on the flanks of a palaeo-hill over which the Ore Formation becomes dolomitic. Cobalt mineralisation appears within the Copperbelt Orebody Member on the flanks of the basement palaeo-hill. Away from the palaeo-hill, the Mindola Clastics Formation again comprises quartzites, grits and conglomerates, and the overlying Ore Formation consists of interbedded dolomite shale/siltstones and argillites with organic carbon. The mineralisation occurs as fine disseminations (5 to 1500µm, with most between 25 and 400µm) or concentrations of chalcopyrite, distributed along bedding planes with minor amounts of carrollite, cobaltiferous pentlandite and skutterudite, and bornite/linnaeite in certain areas. Beyond the copper mineralisation, sulphides are principally pyrrhotite and pyrite (Garlick, 1961; Fleischer et al., 1976; Fleischer, 1984; SRK 2012).
The remainder of the Kitwe Formation, above the Copperbelt Orebody Member, was previously known as the "Hanging wall Formation", which is 40 to 80 m thick, and is subdivided into,
Hangingwall Quartzite, 3 to 12 m thick - an eastward thickening unit of white feldspathic quartzite and arkose beds with streaks of detrital iron oxides and silty and argillaceous layers.
Interbedded Quartzite and Argillite, 25 to 35 m thick - over the Main and West Orebodies this comprises a 25 m thick argillite unit with two quartzite members near the middle, each of around 3 m thickness. Above these middle quartzites, there are 12 to 15 m of schistose shale and dolomite. To the east, the quartzites thicken at the expense of the intervening argillites.
Upper Quartzite, 12 to 25 m thick - a coarse white to pink feldspathic quartzite, with in places a few pebbles. Near the base and top there are some thin silty and argillaceous beds. Magnetite and other detrital minerals mark the cross bedding, with intense concentrations on lower foresets and bottomsets. Generally it is 14 to 15 m thick, but thickens to the east due to an increase in the arenaceous content of the underlying and overlying units. It is also characterised by a chalky white appearance from the weathering of its high feldspars content.
Upper Roan Group, averaging 350 m thick - comprising, ~25 m of interbedded schist and quartzite; 12 to 24 m of cherty dolomite, 75 to 90 m of 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 Nkana-Chambishi Basin, a ~170 m thick metagabbro sill is underlain by ~52 m of granophyre, ~8 m of magnetite-rich rock and a further 35 m of gabbro, enclosed within Upper Roan Group carbonates. Chlorite-amphibole shear zones within the gabbro, contain dolomite, sparse pyrite, pyrrhotite and minor chalcopyrite.
Mwashia Group, 550 m thick - comprising ~300 m of grey argillite, overlain by ~240 m of black carbonaceous argillites, with substantial disseminated pyrite and minor chalcopyrite.
Nguba Group, ~1000 m thick - diamictite, dolomite and shale.
The Lower Roan is strongly deformed in the vicinity of Chambishi Main and West, the main structure being the Chambishi Monocline, which parallels the Kafue Anticline margin in the vicinity of the western granite ridge. Conspicuous east-west drag folds associated with this structure are seen to the east of the ridge. The most intense folding is in the Ore and Hangingwall Formations, with the Upper Quartzite in the upper limb commonly thrust over the adjacent syncline. Fold geometries and plunges are influenced by the granite ridges also.
Published production, reserve and resource figures include:
Total production + resource as at 1988 - 240 Mt @ 2.3% Cu (Freeman, 1988);
Total production to 1988 - 33 Mt @ 2.88% Cu (Freeman, 1988);
Mineral resources at the end of 1997 (Northern Miner, Jan 26, 1998) were:
Chambishi Main Mine, proved reserves - 33.5 Mt @ 2.55% Cu;
Chambishi West Mine, resources - 47 Mt @ 2.27% Cu;
Chambishi Southeast Mine, resources - 69.7 Mt @ 2.59% Cu, 0.13% Co;
Total remaining ore reserves and mineral resources at 31 December 2011 (China Non Ferrous Metal Industries, 2012) were:
Chambishi Main Mine
sulphide, proved + probable reserves - 8.7 Mt @ 1.92% Cu;
sulphide, measured + indicated resources - 10.7 Mt @ 2.5% Cu;
sulphide, inferred resources - 8.1 Mt @ 2.42% Cu;
Chambishi West Mine
proved + probable reserves - 25.3 Mt @ 1.44% Cu;
oxide, indicated resources - 6.2 Mt @ 1.11% Cu;
sulphide, measured + indicated resources - 25.3 Mt @ 2.06% Cu;
inferred resources - 17.3 Mt @ 2.09% Cu;
Chambishi Southeast Mine
probable reserves - 29.7 Mt @ 1.44% Cu, 0.10% Co;
sulphide, indicated resources - 35.4 Mt @ 2.30% Cu, 0.12% Co;
sulphide, inferred resources - 125.6 Mt @ 1.82% Cu, 0.10% Co.
TOTAL measured + indicated resources - 77.6 Mt @ 2.16% Cu;
TOTAL inferred resources - 151.0 Mt @ 1.88% Cu.
For detail consult the reference(s) listed below.
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Chibuluma - Cu-Co
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%.
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An extensive core storage facility at Kalulushi houses a collection of drill core that may be used to illustrate the stratigraphy of the Copper Belt. Although surface exposures will be inspected during the tour, outcrop in general is poor and the core provides a convenient substitute for surface traverses.
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MODULE 1 - PART B - Zinc, Lead & Copper in South Africa & Namibia
of large zinc, lead and copper deposits are known over an interval
of 500 km on the south-western and western margins of the Kaapvaal/Kalahari
Craton in the Northern Cape Province of South Africa and in southern
Namibia. These are found in both the late Palaeo- and Meso-Proterozoic
metamorphics of the Namaqua Mobile Belt and in the late Neo-Proterozoic
sediments and volcanics of the Gariep Province in Namibia.
Mobile Belt is part of an extensive U-shaped fringe that extends
around the southern, western and northern margins of the Kaapvaal/Kalahari
Province, from the Indian Ocean, to the Atlantic and back into
central Africa and is composed of late Palaeo-Proterozoic and
Meso-Proterozoic gneisses, schists and granitoids after volcanics,
sediments and intrusives that were metamorphosed in the late
Meso-Proterozoic from 1200-1000 Ma. These rocks in part form
the basement to the Damara-Katangan that hosts the deposits of
Module 1 Part A.
an inlier on the western margin of the Kalahari Craton along
the Atlantic coast, is part of the Damaran-Katangan System, resting
unconformably on Namaqualand metamorphics and intrusives. These
comprise a variety of sediments from mixtites through arenites
and argillites to carbonates, with variable but generally lesser
mafic rocks, and felsite-rhyolite volcanics.
segment of the Kalahari Craton to the north and east of the Namaqua
Mobile Belt and Gariep Complex is occupied by a thick sequence
of poorly deformed volcanics and intrusives, the Rehoboth Complex
of granitic, intrusives cutting volcanics of from andesitic to
acid composition with intercalated sediments ranging from 1700
to possibly 1050 Ma in age. These are in part equivalent to the
metamorphosed volcanics and sediments of parts of the Namaqua
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Black Mountain/Swartberg - Cu-Pb-Zn
The Black Mountain or Swartberg deposit is some 6 km west of the Broken Hill deposit within the Aggeneys District of the Northern Cape Province of the Republic of South Africa.
The deposit occurs on Swartberg (or Black Mountain), a hill characterised by a black colouration due to the magnetite (and to a lesser extent the manganese) content of the exposed mineralisation.
Black Mountain comprises two conformable 'orebodies', separated by 0 to >30 m of waste. Both 'orebodies' comprise large high-grade sulphide lenses, enveloped by lower-grade magnetite-rich rocks, which, in turn, are over- and underlain by weak schist and quartzite. The main sulphide minerals are galena, sphalerite, chalcopyrite and pyrrhotite.
The host sequence has been subjected to four phases of deformation, resulting in variable dips and thicknesses. A tight, isoclinal F2 fold nose has been developed in the vicinity of the orebody. Higher grade, coarser, mineralisation is present as an elongate body concentrated down the plunge of the F2 fold axis and occurs in garnet-quartzite, magnetite-quartzite and amphibole-magnetite rocks. In this zone ore occurs as heavily disseminated pyrite and ore sulphides (galena, sphalerite and chalcopyrite) in magnetite and amphibole rich quartzose metamorphics, with finer grained, banded sulphides on the limbs aligned with the foliation of the magnetite-quartzite and amphibole-magnetite hosts.
There is a zoning from a Cu bearing garnet-quartzite, to chalcopyrite-galena rich magnetite-quartzite to an upper galena-sphalerite dominant amphibole-magnetite in the core of the fold hinge,
The regional geology is as described in the Aggeneys (Broken Hill) record.
At Black Mountain, the host Aggeneys Ore Formation overlies a sequence of variable interbanded schists, quartz schists and quartzites. The schists are predominantly muscovite, biotite and sillimanite bearing, with quartz in varying amounts, becoming dominant in the quartz schist layers. Garnet is present in some of the quartzites. It is possible that, as at Broken Hill the underlying schists and quartzites are separated from the Aggeneys Ore Formation by a folded thrust/shear which would also account for the marked transgressive nature of the contact.
These underlying schists and quartzites in turn overlie coarse pink leucocratic gneisses with a granitic texture, and amphibolites.
Within the Black Mountain deposit the Aggeneys Ore Formation comprises the following generalised sequence, from the interpreted stratigraphic base (after Black Mountain Mineral Development Co., 1988, and Ryan, et al., 1986):
► Lower Orebody, approximately 5m thick - this zone of mineralisation is only present on the lower limb of the main F2 fold which hosts the main zone of mineralisation at Black Mountain, occurring immediately above the postulated thrust/shear separating it from the underlying barren schists and quartzites.
This unit is predominantly a baritic schist with disseminated magnetite and sulphide minerals, of which pyrite is dominant with subordinate galena and sphalerite, and traces of chalcopyrite. In the vicinity of the F2 fold closure the Lower Orebody grades laterally into an amphibole-magnetite quartzite and garnet quartzite also. The garnet quartzite is identical to that described below, although the amphibole magnetite rock differs from that in the Upper Orebody in that it consists of dominant grunerite and subordinate spessartine, with magnetite being rarer.
The Lower Orebody may either be a separate unit, or part of the Upper Orebody which has been thrust into its present position. It is similar in composition to the barite-magnetite schist which is found near the Upper Orebody position (see below).
► Mixed zone of Schists and Garnet Quartzite, 30 to 65m thick - this interval includes a range of lithologies forming a progression from garnet-quartzite to garnet-quartz schist to banded schist with increasing distance from the nose of the F2 fold where the main high grade mineralisation is concentrated. The main rock types are as follows, not necessarily described in the order in which they appear.
• Garnet-Quartzite - composed predominantly of fine to medium grained quartz, generally around 80%, with subordinate almandine garnet (Fe rich) and biotite, and accessory cordierite (Mg, Al) and sillimanite. Magnetite increases towards the margins with the magnetite quartzite. The rock is generally massive with indistinct banding in the core of the unit away from the gradational contacts with the schists. The garnet quartzite is thickest, and is concentrated in the vicinity of the high grade mineralisation in the nose zone of the F2 fold. Towards the NW, up dip along the axial plane of the F2 fold, there is a gradual decrease in the ferro-magnesian minerals and it grades into a glassy quartzite (see plan no. AFRa azk). The adjacent magnetite quartzite, a lithotype of comparable competence, displays far less thickening in the same structural position.
In outcrop the garnet quartzite appears as a foliated pink and black medium grained quartzite with a black surface staining/varnish.
• Garnet-Quartz Schist - which has a composition intermediate between the garnet quartzite and the banded quartz schist. The K-feldspar, sillimanite and muscovite contents are higher than in the garnet quartzite, although it contains 40 to 70% quartz and garnet. Traces of pyrite are disseminated throughout the rock, particularly near the gradation to garnet quartzite, where sporadic minor chalcopyrite concentrations are also present.
• Banded Quartz Schist - is characterised by the alternation of quartzose and schistose material, the former being similar to the white quartzite, while the latter is an aluminous schist.
► Barite-Magnetite Quartzite and Barite-Quartz Schist, 0 to 25m thick - these two lithologies are separate and grade laterally into one another.
• Barite-Magnetite Quartzite - has from 25 to 70% barite accompanied by quartz and around 10% magnetite with accessory fine grained orange garnet and micas. Where it approaches the lateral extremities of the ore zone it is coarse grained and exhibits a moderate compositional banding. The sulphide content is negligible.
• Barite-Quartz Schist - this lithotype is similar to the Lower Orebody host. It appears to represent a gradational change from the barite-magnetite rock, with a decrease in barite and magnetite relative to quartz, while muscovite, biotite and chlorite increase until the rock has a schistose texture.
► Magnetite-Quartzite, 0 to 20m thick - composed dominantly of quartz and magnetite which together make up 80 to 100% of the rock, with accessory garnet (almandine-spessartine), biotite, chlorite and rare apatite. Towards the extremities of the magnetite-quartzite on the limbs of the F2 fold magnetite decreases relative to quartz. On the main sections of the two limbs it displays a millimetric compositional banding, which is obliterated in the nose zone by rotation.
► Amphibole-Magnetite Rock, 0 to 25m thick - this unit, which is also concentrated in the nose of the F2 fold, tapering and lensing out on the limbs, comprises magnetite with cummingtonite/grunerite, accompanied by, in decreasing order of abundance, pyroxmangite, quartz, hedenbergite, garnet (spessartine rich), fayalite and apatite.
Laterally in the lower fold limb this has been interpreted as grading into the barite-magnetite quartzite described above.
► Leptite and Amphibolites, 1 to 30m thick - are found discordantly cutting the Upper Orebody zone. The leptite is a medium grained quartzo-feldspathic rock composed predominantly of quartz and microcline with subordinate plagioclase and muscovite. The amphibolite, which almost invariably accompanies or borders the leptite, is predominantly hornblende.
The regional structural features and deformation stages are as described at Aggeneys/Broken Hill deposit.
Within the mineralised zone there is a strong foliation that has been folded by both F2 and F3 structures. This foliation is represented by both the schistosity and by compositional banding within the garnet quartzites, magnetite quartzites and magnetite amphibole rocks. It is uncertain whether this foliation represents the original bedding or is a regional F1 metamorphic/structural feature.
The isoclinal F2 folding is best displayed by the main tight fold whose nose is occupied and intimately followed by the mineralisation. This fold is mappable at the surface, with abundant clear parasitic folds in the folded magnetite-quartzite whose vergence supports the interpreted fold closure. Strong L2 mullions are also present within the amphibole-magnetite in the nose of the fold. This fold is corroborated by the drill intersections.
Mineralisation ocurs in two positions, as preserved at present. These are the,
• Lower Orebody - which is found at the interpreted base of the Aggeneys Ore Formation, possibly following a thrust contact with the underlying schists and quartzites. It is predominantly a baritic schist with disseminated magnetite and sulphide minerals, of which pyrite is dominant, with lesser galena and sphalerite, and traces of chalcopyrite. Towards the nose of the F2 fold it grades into amphibole-magnetite rock and garnet quartzite. All three lithologies are mineralised.
The garnet quartzite envelopes the magnetite rich rocks in the hinge zone. The dominant sulphide is pyrite, followed by chalcopyrite with accessory galena, pyrrhotite and sphalerite.
The amphibole magnetite rock of the Lower Orebody differs from that of the Upper Orebody in that it has dominant grunerite, subordinate spessartine and rare magnetite. The main sulphide minerals are sphalerite and chalcopyrite, in contrast to the dominant galena in the Upper Orebody.
Down dip to the southeast the amphibole magnetite rock grades rapidly into the baritic, sulphidic schist over a lateral distance of a metre or two. This latter rock type often becomes a massive sulphide. Pyrite is dominant, with subordinate galena and sphalerite, and traces of chalcopyrite. Barite is irregularly distributed, reaching 40% in places, while gahnite is a common gangue silicate, together with quartz, white mica and hematite.
• Upper Orebody - Mineralisation is hosted by the garnet quartzite, magnetite quartzite and amphibole magnetite rock. Galena and sphalerite are contained throughout the amphibole magnetite rock and the magnetite quartzite, mainly as low grade disseminations, but also within the high grade core.
Within the amphibole magnetite rock the principal sulphide is pyrrhotite, followed by pyrite, while galena and sphalerite are the most abundant base metal sulphides. Chalcopyrite is an accessory, with minor amounts of arsenopyrite.
Within the magnetite quartzite the dominant sulphide present is pyrite, accompanied by subordinate pyrrhotite. Galena is the main base metal sulphide, followed by chalcopyrite and accessory sphalerite. The greatest concentrations lie adjacent to the high grade mineralisation within the garnet quartzite.
The dominant sulphide within the garnet quartzite is generally chalcopyrite which is disseminated, and not ubiquitously or regularly distributed through the host rock. Although the sulphide content is variable, there is a general increase in sulphide towards the core of the F2 fold closure where concentrations approaching massive sulphides are found locally, with pyrite being the dominant sulphide and subordinate chalcopyrite. Away from the high grade core the sulphide minerals decrease in all directions, with pyrite persisting beyond chalcopyrite. Minor sphalerite, galena and pyrrhotite are encountered near the contact with the magnetite quartzite.
Massive sulphides are virtually unknown at Black Mountain, with predominantly disseminated pyrite and ore sulphides, within magnetite and amphibole rich quartzose metamorphics. Much of the ore is finely banded on the limbs of the F2 ore fold, with sulphides often present as small grains aligned along the micro banding of the magnetite quartzite and amphibole magnetite rock. In the vicinity of the hinge zone however the ore is recrystallised to form coarse aggregates.
The high grade rod like core of mineralisation follows the plunge of the F2 antiform in the fold hinge. This core has been traced for some 1300 m down plunge, within all three lithologies. In detail this mineralisation displays asymmetry with respect to the F2 hinge trace, corresponding to the maximum development of the garnet quartzite.
There is a vertical 'stratigraphic' zonation of metals, or alternatively a zonation from the outer to the core sections of the fold hinge. The stratigraphically lower garnet quartzite on the outer sections of the hinge is Cu rich, followed by the galena-chalcopyrite in the magnetite quartzite and galena-sphalerite in the upper amphibole magnetite rock in the core of the fold.
There appears to be a strong relationship between the distribution of sulphides, ore, magnetite and garnet and the presence of a tight isoclinal F2 fold within a major shear/thrust zone. In detail there is a high grade core of mineralisation forming a rod like body that is 100 to 200 m long, closely following the F2 fold hinge down plunge for more than 1300 m. Within this core there appears to be a lithological control on the sulphide zonation. Away from the high grade core, lower grade finely banded to disseminated sulphides follow the magnetite and amphibole bearing lithotypes, but like those rock types, pinch out on the limbs. On a broader scale, the magnetite rich rocks also lens out on the limbs of the F2 structure over a distance of a km or so. The stratigraphic equivalents of the ore bearing lithotypes do contain weak mineralisation and occasional more intense developments sporadically between Black Mountain and Broken Hill.
Reserves, Resources and Production
The deposit has been developed and ore is extracted via a shaft to truck to the Black Mountain Mining treatment facility at Broken Hill, 6 km to the east. Mining capacity has been ~350 000 tpa of ore to produce 13500 tpa of metal in concentrate. Plans are underway (2018) to deepen the shaft and increase capacity to 1.6 mtpa of copper and lead ore, and 60 ktpa - 70 ktpa of metal in concentrate (Vedanta Resources website viewed Jan, 2019).
The deposit had an original geological resource of 82 Mt @ 0.75% Cu, 2.7% Pb, 0.6% Zn, 30 g/t Ag (a cut-off of 0.5% Cu equiv.; Ryan, et al., 1986).
Remaining Ore Reserve and Mineral Resources at the end of 2018 (Vedanta Resources Annual Report, 2018) were:
Proved + Probable Reserve - 2.33 Mt @ 0.62% Zn, 3.26% Pb;
Measured + Indicated Resource - 35.68 Mt @ 0.84% Zn, 3.70% Pb;
Inferred Resource - 26.49 Mt @ 2.19% Zn, 3.04% Pb.
NOTE: Reserves are additional to resources.
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Broken Hill - Pb-Zn-Cu
The Broken Hill mine at Aggeneys has exploited one of the three main ore deposits (Black Mountain, Broken Hill and Gamsberg) within the Aggeneys District of the Northern Cape Province of the Republic of South Africa. It lies some 16 km to the west of Gamsberg and 6 km east of Black Mountain.
All three are hosted by the Bushmanland Group of the Namaqualand Metamorphic Complex, which in turn lies within the broader Namaqua Mobile Belt on the south-western and southern margin of the Kaapvaal/Kalahari Craton. This complex comprises an older metamorphic suite (locally a quartzo-feldspathic augen gneiss) dated at around 2000 to 1900 Ma, overlain by a 1700-1600 Ma (?) supra-crustal succession. This latter succession comprises a lower intrusive to extrusive l euco-gneiss, the Hoogoor Suite, and an upper schist/quartzite succession, the Bushmanland Group. The latter is extensive, and is largely preserved as infolded enclaves or thrust slivers, and is relatively thin, generally less than 1000m thick. At Aggeneys the Bushmanland Group comprises a lower 80 m thick aluminous schist, the Namies Schist composed of quartz-muscovite-k feldspar, locally with up to 25% sillimanite and biotite. This is overlain by a 5 to 900 m thick white crystalline white to grey quartzite, followed by the up to 200 m thick Aggeneys Ore Formation and amphibolite, leucocratic grey gneiss and conglomerates.
The Aggeneys Ore Formation is strongly deformed and in general comprises a footwall schist of sillimanite-quartz-biotite with minor garnet. This is followed by a ferruginous garnet-quartzite composed of bands of garnet and magnetite in a matrix of quartz. This zone is in turn followed by an interval containing magnetite-quartzite (medium grained magnetite & quartz) and amphibole-magnetite (quartz, spessartine, magnetite, ortho-pyroxene, grunerite, cummingtonite & fayalite) with some bands of ferruginous garnet-quartzite. The lower massive sulphide falls within this interval. Above this amphibole-magnetite is the main massive sulphide body and sulphide-quartzite. This is in turn overlain by magnetite-quartzite and amphibole-quartzite, another zone of ferruginous garnet-quartzite and the hangingwall sillimanite-quartz schist.
The massive sulphides (>25% sulphide) generally contain <10% magnetite and range from banded to brecciated textures to heavy disseminations. Pyrrhotite and galena dominate, followed by sphalerite, chalcopyrite and pyrite. The gangue is predominantly quartz, with variable garnet and some barite.
Reserve and reource figures include:
Initial pre-mining resource - 85 Mt @ 0.34% Cu, 3.6% Pb, 1.8% Zn, 48 g/t Ag,
including the reserve of 38 Mt @ 0.45% Cu, 6.4% Pb, 2.9% Zn, 82 g/t Ag.
Reserves in 2000 were ~8.7 Mt @ 0.5% Cu, 5.5% Pb, 2.9% Zn, 78 g/t Ag.
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Gamsberg - Zn
The Gamsberg deposit is located some 16 km to the east of Aggeneys/Broken Hill in the Northern Cape Province of South Africa.
The project is owned and managed by Vedanta Resources - Black Mountain Division.
Mineralisation occurs within the Gams Ore Formation, which is a direct correlative of the Aggeneys Ore Formation that hosts the ore at Broken Hill, and is a member of the Bushmanland Group. The Gams Ore/Iron Formation comprises three units, namely the:
i). A Member a lower member composed of a diverse suite of quartz-felspar-garnet-clinopyroxene rock, garnet-clinopyroxene-feldspar marble and garnet-clinopyroxene-quartz-magnetite rocks;
ii). B Member a middle sulphide zone with quartz-garnet-amphibole rocks and graphitic quartz-sillimanite-muscovite-feldspar containing major amounts of pyrite, pyrrhotite, sphalerite and galena; and
iii). C Member an upper unit of diverse garnet, pyroxenoid, clinopyroxene, orthopyroxene, amphibole, quartz, apatite, carbonate, magnetite, hematite and barite bearing rocks.
It is underlain by a thick, massive recrystallised white quartzite with minor schists and conglomerate.
Deformation is complex, with the Gamsberg mineralisation falling within a major 'sheath fold' several kilometres across, on whose margin the deposit is located at the contact between the underlying quartzite and overlying mafic gneisses of the Nousees/Koeris Formation. This structure is on the 'underlimb' of a large nappe structure to the north of the thrust/shear that encloses the Broken Hill and Black Mountain deposits. The main 'ore' shoot at Gamsberg is elongated parallel to the isoclinal sheared nose of one margin of the sheath fold, whose axis it follows down dip possibly for >1 km. In the sheared fold nose the ore terminates against a coarse mylonite, while laterally ore persists for several hundred metres before reaching a grade boundary on its opposite margin.
The mineralised sulphide zone of the Gamsberg Iron Formation is intermittently present within the sheath fold structure and is generally weakly mineralised containing 1 to 4% Zn. The intervals of weaker mineralisation include a number of higher grade 'ore shoots' with >7% Zn embracing smaller cores of >10% Zn.
The sphalerite rich pyrrhotitic-pyritic ore is found towards the centre of the Gams Iron/Ore Formation, flanked by iron sulphides, predominantly pyrite below and pyrrhotite above. These sulphides are in turn sandwiched by two magnetite to hematite rich zones towards the outer margins of the Gams Iron Formation.
The ore zone sphalerite occurs as intergranular disseminations within a quartz-sericite-sillimanite rock grading up with increasing zinc grade to a quartz-garnet-grunerite host. An impediment to the development of Gamsberg has been the high Mn content of the sphalerite, which contains 2 to 3% Mn within its lattice.
The regional geology is as described in the Aggeneys (Broken Hill) record.
The geological succession in the Gamsberg area is as follows, from the base:
► Haramoep Gneiss, unknown thickness - predominantly a pink medium to fine grained quartz-feldspar gneiss with an aplitic appearance, although it often grades into types with poorly developed augen or greyish varieties with feldspar porphyroblasts. Foliation is generally only poorly developed except in more biotitic varieties. In general it is composed of quartz with major to subordinate feldspar which include microcline, microcline-perthite and plagioclase. The characteristic pink colour is due to the K-feldspar. Micas are generally present in varying, though minor amounts, and include chloritised biotite, muscovite and sericite. The upper contact with the Namies Schist is generally sharp and conformable, although in certain restricted areas it has the appearance of being intrusive. The Haramoep Gneiss represents the Hoogoor Suite at the base of the Bushmanland Group in this area.
► Wortel Formation - subdivided into,
• Namies Schist, 70 m thick - a well foliated, quartz-biotite-sillimanite schist. The schistosity is imparted by biotite and lesser parallel muscovite, with flattened nodular clusters of sillimanite. The quartz content of the schist gradually increases upwards in the stratigraphy until dark prominent quartzite bands alternate with thin layers of quartz-biotite-muscovite-sillimanite schist. This then passes progressively into the white, pure ortho-quartzite of the Zuurwater Quartzite.
• Zuurwater (or Pella) Quartzite, 250 to 375 m thick - which is in turn subdivided into,
- White Quartzite Member, 200 to 250 m thick - a milky white to light grey, macroscopically recrystallised quartzite. Microscopically it is coarse grained and saccharoidal with all primary features, structures and secondary cement having been obliterated by recrystallisation. Accessory minerals include muscovite, apatite, zircon, rutile and occasional pyrite. It has a conformable gradational contact with the Pelitic Schist Member.
- Pelitic Schist Member, 10 to 25 m thick - a thin irregular unit with common pinch and swell structures and intercalated micaceous dark quartzite layers. The mineralogical makeup of the rock varies considerably along strike from quartz-biotite-muscovite-sillimanite-garnet (almandine) schist in the west, to a quartz-muscovite schist in the east. It passes conformably and gradationally upwards into the Dark Quartzite Member.
- Dark Quartzite Member, 40 to 100 m thick - a coarse grained recrystallised quartzite with no remaining primary texture. It comprises glassy quartz with accessory zircon, biotite, apatite, muscovite, sericite, sillimanite, hematite and magnetite. Pyrite is rare, occurring locally near the stratigraphic top of the unit. The dark colouration is due to disseminated magnetite and earthy hematite concentrated along the quartz grain boundaries. In the area of its maximum development, prominent bands of attenuated white and dark quartzite are found in a light grey recrystallised quartz matrix. Intercalated 1 to 2 m thick bands of biotite-muscovite-sillimanite schist are observed locally.
To the north of the Aggeneys-Gamsberg area there are a number of additional formations between the Zuurwater (Pella) Quartzite and the Gams Iron Formation. While this contact is commonly a thrust plane in the Aggeneys-Gamsberg area, this is not always the case. Therefore if the mapping of Colliston, et al., (1986) and Strydom, et al., (1987) is correct, there was a hiatus or a thinning of the sequence southwards towards the Aggeneys-Gamsberg area and the position of the subsequent Swartberg-Zuurwater Thrust/shear Zone during the deposition of the Bushmanland Group.
► Gams Iron Formation (equivalent to the Aggeneys Ore Formation), 0 to 80 m thick - comprising,
• A Member, 0 to 10 m thick - which has four beds, as follows,
- A1 Bed, averaging 2 m thick - quartz-biotite-muscovite-sillimanite schist, well foliated, with accessory garnet (almandine), tourmaline, apatite, zircon and hematite.
- A2 Bed, 0 to 1 m thick - an impersistently developed zone of clinopyroxene-grunerite-garnet-magnetite rock, which follows the A1 Bed with a sharp contact. Generally at surface it has a black coating of manganese and iron oxides. In the southern and north eastern parts of the Gamsberg area this unit thickens into an up to 15m, more persistent garnet rich rock. In general it is massive, medium grained and poorly banded to the west, while in the east it is fine grained and moderately banded. The bands are 1 to 4 mm thick comprising alternating magnetite poor and rich layers. The clinopyroxene is ferro-augitic, while the garnet is primarily andradite.
- A3 Bed, unstated thickness - overlies the A2 Bed with a gradational contact, and comprises a leucocratic, banded, medium grained, crystalline, calcite-diopside-garnet marble, with the carbonate content increasing upwards, with mafic minerals correspondingly decreasing. The banding is due to alternating carbonate rich and poor (mainly K-feldspar) layers. It does not outcrop and is only impersistently developed, passing laterally into dolomite rich strata. Ferroan and manganoan calcite is the dominant carbonate, and grossular the principal garnet. While the A2 Bed is rich in Fe oxides, accessory sulphides are found in A3.
- A4 Bed, 1 to 5 m thick - generally a garnet-muscovite-sillimanite quartzite, forming a transition from the underlying calcareous unit and more siliceous lithologies. This unit exhibits rapid mineralogical changes both vertically and laterally. Typical assemblages include quartz-biotite-muscovite-sillimanite schist; garnet-sillimanite quartzite, muscovite-microcline quartzite and muscovite quartzite. At the surface the resistant quartzitic units are covered by a dark brown to black coating of mixed manganese and iron oxides, with a cellular texture where they contain original Fe sulphides. Texturally these rocks are fine grained to hornfelsic in places, with either moderately to poorly developed or prominent banding. The latter is found in the garnetiferous quartzite which has alternating garnet rich and poor bands. The garnet is generally spessartine.
• B Member, 0 to 50 m, averaging 25 m thick, - this contains the main ore zone. In places this unit grades laterally into a hematite bearing, medium grained sillimanite-quartzite with no sulphides. The B Member has been divided into two beds, as follows,
- B1 Bed, 0 to 30 m thick - fine grained, poorly to moderately banded, dark, dense quartz-sericite-sillimanite schist. The banding is generally contorted, consisting of slight variations in silicate mineralogy and bands of sulphide minerals. It is accentuated by parallel streaks of sericite and sillimanite, forming a poorly developed schistosity. Towards the top poorly mineralised leucocratic, fine grained sericitic quartzite, occasionally holding potash feldspar, is interbedded with the schist. In some drill intersections a cordierite rich rock (2 to 4 m thick) composed of flattened spheroidal aggregates 10 to 15cm in diameter, embedded in a quartz-sillimanite-sericite matrix, occurs above the schist. The average opaque content of this unit is 10 to 40%, comprising in decreasing order of abundance, pyrite, pyrrhotite, marcasite, sphalerite, galena, and accessory chalcopyrite, alabandite, magnetite, ilmenite and graphite.
- B2 Bed, 8 to 16 m thick - conformably overlies the B1 Bed, with the contact taken as the first appearance of major amounts of garnet and grunerite. It is generally a medium grained quartz-garnet-grunerite rock, with well developed banding of alternating garnetiferous, apatite and grunerite rich layers. Poorly and well mineralised streaks generally parallel the silicate mineral banding. Schistosity is feebly developed to non-existent. Lithological changes are marked by the appearance of orthopyroxene and clinopyroxene towards the top of the unit. Together with the C Member, this bed is absent in numerous localities. It is generally mineralised, with pyrrhotite and sphalerite being present in major amounts, marcasite and pyrite in subordinate quantities and magnetite, ilmenite and chalcopyrite as accessories.
• C Member, 0 to 9 m thick - the contact between the B and C Members is gradational on a decimetric scale, being marked by the change from predominantly contained sulphide to iron oxide (magnetite and hematite) mineralisation. In the western section of the prospect it has been divided into two beds, although lateral changes in the mineralogy are common. The two beds are as follows,
- C1 Bed, thickness not stated - a magnetite-clinopyroxene-grunerite-garnet rock, which is very similar to the B1 Bed, except for the absence of orthopyroxene and the presence of Fe oxides rather than sulphides. It is medium to fine grained with a well developed mineralogical banding of 0.8 to 3cm in thickness. It persists discontinuously along strike, with this mineralogy being best developed where in contact with mineralised B2 Bed. Laterally it changes to a banded magnetite-hematite quartzite, with or without interbedded barite, a quartz-grunerite-garnet-magnetite rock, a magnetite-hercynite-garnet-sillimanite-quartz rock, or to a garnet quartzite with no iron oxides.
In the south eastern exposures of the C Member a barite horizon is developed at the stratigraphic level of the banded magnetite-hematite quartzite of the C1 Bed. Barite occurs as intercalated lenticular bodies in some places, and in others as persistent massive layers of 1 to 2.5 m in thickness, oriented parallel to the regional banding. In the exposures of massive barite little or no banding is seen, apart from thin interbedded quartzitic seams.
- C2 Bed, thickness not stated - a garnet-magnetite-grunerite rock with a gradational contact with the underlying C1 Bed, marked by an increase in garnet and decrease in mafics, together with a considerable increase in Ca. The latter change is reflected in the presence of andradite garnet. It is well banded, with layers of massive garnet alternating with magnetite and quartz rich seams. Towards the top there is a considerable increase in calcareous minerals, until a banded (5 to 10 mm) medium grained calcite-fayalite-hedenbergite-garnet (andradite) rock is locally developed. This comprises layers of massive yellow manganiferous andradite alternating with seams of quartz, magnetite, pyroxene and possibly manganiferous wollastonite producing a prominent banding. Ferroan calcite, fayalite, hedenbergite, minor johannsenite, magnetite and accessory sulphides, together with much yellow garnet constitute the lithotypes of the top of the C2 Bed.
► Koeris Formation, or Nousees Mafic Gneiss, 400 to 500 m thick - the Gams Iron Formation is overlain by a succession of quartz-muscovite schist, lenses of conglomerate and bands of micaceous quartzite. Amphibolite is interlayered with and stratigraphically overlies the quartz-muscovite schist. In the Gamsberg area this has been divided into two separate units as follows,
• Psammitic Schist Member, which has limited distribution within Bushmanland. It is found at the base of the Koeris Formation and comprises leucocratic rock types, mainly well foliated quartz-muscovite schist and quartz-biotite-muscovite-sillimanite schist holding interbedded bands (1 to 10 m thick) of greyish medium grained quartzite. Quartz-muscovite schists predominate in the western section of the prospect, locally with flattened oriented quartz-sillimanite nodules (0.5 to 3 cm long), while in the central and eastern sections, equigranular, medium grained quartz-muscovite-feldspar schist and feldspathic quartzite, both with varying biotite, predominate. The feldspars in the latter rock types are mainly potash, with lesser plagioclase.
Locally the schists and quartzites exhibit faint relics of original bedding and cross bedding. Conglomerate bands of varying thickness and strike continuity consist mainly of white and dark quartzite pebbles (occasionally holding hematite-quartzite fragments) in a quartz-muscovite matrix. The hematite-quartzite pebbles are taken to be from the Gams Iron Formation, although abundant older iron formations are widespread in the older basement. We did not see any of these iron formation pebbles during the visit.
The pebbles and cobbles of these conglomerates have been structurally attenuated, with long dimensions of from 1 to 25 cm. In the western section of the prospect area 0.5 to 2 m thick lens of sheared, banded hematite-quartzite are interbedded with the quartz-muscovite schist. The middle and top part of the psammitic schists contain intercalated bands of the Basic Gneiss Member.
• Basic Gneiss Member, which has been locally subdivided into two units, both characterised by hornblende and/or clinopyroxene, as follows,
- Quartz-Feldspar-Amphibole Gneiss - occurring as discontinuous bands (5 to 20 m long) of variable thickness (1 to 2 m thick) and texture, intercalated with the psammitic schist. In places they are medium grained and gneissic with moderate to poorly developed foliation, imparted by platy hornblende crystals. Elsewhere a tough, massive, medium to fine grained equigranular variety of similar composition displays felsitic textures. Some are epidosites, holding abundant epidote instead of hornblende. This mafic rock in places is interstitial to the pebbles and cobbles of the conglomerate, but elsewhere appears to be intrusive into the amphibolite.
- Amphibolite - the major development of this lithotype is in the upper part of the succession, where it is irregularly distributed within the Psammitic Schist Member. Minor bands and lenses of similar composition are found at all levels from as low as the Namies Schist and Pella Quartzite. It occurs as mainly conformable bands and 'sills' from 5 to 200 m thick, with strike lengths of from 10 to 5000 m. Local crosscutting relationships are common. The contacts are sharp, with an increased biotite contact towards the margins. The rocks are generally equigranular and fine to medium grained, although a distinct banding is present in some outcrops involving alternating plagioclase and hornblende rich layers. Hornblende alignment defines the foliation. It comprises mainly common hornblende (± cummingtonite) and plagioclase, with accessory sphene, almandine, magnetite and pyrite. Amphibolites within the Haramoep Gneiss contain abundant microcline as well and as such may be differentiated.
Some of the conformable amphibolite display abundant amygdale like structure, and are assumed to be metamorphosed mafic volcanics. Substantial areas of outcrop of this variety are present in the core of the Gamsberg sheath fold. Dating from this locality has yielded ages of 1600 Ma.
Major Element and Mineral Zonation
The zonation across the Gams Iron Formation within the Gamsberg deposit is as follows, from the base,
► A Member,
• A2 Bed - characterised by almandine-spessartine garnets (Fe, Mn), ferro-augitic (Fe, Mg, Ca) to ferro-silitic clinopyroxene and manganoan calcite (Ca, Mn), with the main opaque being magnetite. This unit has high Si, Mn, Fe and Ca with lesser Mg.
• A3 Bed - is more calcareous with lesser silica and little Fe. The silica gangue is replaced by pyrite, minor pyrrhotite and no magnetite. It carries grossular garnet (Ca, Al) and diopsidic pyroxene (Ca, Mg).
• A4 Bed - is aluminous, siliceous, Mn and Fe rich and in part graphitic. It carries spessartine garnet (Mn) and the pyroxenes are ferro-augitic (Fe, Mg, Ca) to ferro-silitic. As with A3 there is no magnetite, but increased disseminated pyrite and pyrrhotite.
► B Member,
• B1 Bed - fine grained quartz muscovite (K, Al), sillimanite (Al), schist overlain by cordierite (Mg, Al) hornfels in a groundmass of quartz, muscovite (K, Al) and sillimanite (Al). The cordierite 'hornfels' at the top represents an Mg rich zone in the overall potassium bearing, aluminous and siliceous unit. Pyrite comprises 35% of the opaque minerals at the base of the unit. Zn increases towards the top of the unit, while the sulphur content reaches its maximum in the upper half. The unit is therefore Fe and Zn rich, but these are present as sulphides rather than silicates.
• B2 Bed - banded quartz garnet grunerite (Fe) rock and quartz orthopyroxene garnet olivine amphibole rock with alternating garnetiferous, apatite and grunerite rich layers and contrasting sulphides. Pyroxenes consist of manganoan ortho-ferro-silite (Mn, Fe) and eulite while garnets are mainly almandine-spessartine (Fe, Mn, Al) and the olivine is Fe rich. This unit is therefore P, Fe and Mn rich and siliceous. Pyrite decreases to 5% of the opaque minerals at the top, with a corresponding increase in pyrrhotite. Zn (up to 10%) and Cu (up to 200 ppm) are highest in this unit, while sulphur progressively decreases towards the top. Minor magnetite appears in the upper sections where it occurs in part within sphalerite aggregates.
► C Member,
• C1 Bed - which is predominantly pyroxenoid, primarily pyrox-ferroite and pyroxmangite (Fe, Mn); amphibole; garnet, mainly spessartine-almandine (Fe, Mn, Al); and clinopyroxene rocks where in contact with ore, grading laterally into banded magnetite hematite quartzite. It is siliceous with Fe and Mn silicates, magnetite, minor hematite and few sulphides.
• C2 Bed - which is Mn rich, comprises a garnet pyroxene rhythmite of andraditic garnet (Ca, Fe) with quartz, magnetite, hematite and rhodonite (Mn).
Rozendaal (1977) undertook a structural analysis of the immediate Gamsberg area in which he recognised five periods of deformation. In this he disagreed with Joubert who had earlier studied the regional structure as repeated in Joubert (1986). Joubert interpreted four stages of deformation. He had ascribed the F1 phase to an episode of isoclinal and translational folding responsible for the main mineralogical banding which parallels the gross lithological banding (bedding) and is seen to be folded by the subsequent deformations. Rozendaal interprets this mineralogical banding to be primary bedding.
Rozendaal (1977) has reasoned that the main structure at Gamsberg, is due to interference produced by the interaction of his F1 (flat lying WNW-ESE trending axial plane) and F2 (vertical NE-SW trending axial plane) folding (Jouberts F2 and F3) and subsequently modified by F5 open folding. This interpretation of the main Gamsberg structure is contradicted by Colliston, et al., (1991) as described below.
Rozendaal (1977) noted that the earliest schistosity S1 caused disruption of the main compositional banding within the schists, and caused marked thinning and boudinage of the more competent layers. He also noted that this schistosity, where developed, is generally parallel to sub-parallel to the gross bedding (as defined by the main unit boundaries) and compositional banding on the fold limbs, but cut the gross bedding in the north western fold closure (his F1 nose). He recorded conspicuous lineations, generally parallel to the intersection of the main compositional banding and the principal cleavage. These included foliation intersections, mineral lineations, attenuated pebbles, parasitic fold axes and crenulations. He also noted that a prominent feature of the major (his F1) fold is the evidence of thrusting in several localities at the top of the overturned limb. This was also obvious during the visit, where a pronounced coarse mylonite zone was seen to accompany the attenuation and 'pinching out' of the Gams Iron Formation at the north western fold closure adjacent to the main high grade mineralisation. This mylonite zone is composed of coarse crystalline quartzite boudins set in a darker magnetite-hematite quartzite matrix. The boudins are rod shaped, with 'S' shaped cross sections. This zone may be traced for 150m in outcrop and has laterally gradational boundaries with the enclosing lithologies.
Rozendaal (1977) reported that the upper overturned limb of the major fold is strongly attenuated, as implied by the frequent 'lensing out' of the Gams Iron Formation on the northern contact between the Pella (Zuurwater) Quartzite and the Koeris Formation (Nousees Mafic Gneiss).
Rozendaal (1977) noted a tightly spaced steep NE-SW trending penetrative S2 cleavage in the amphibolite and psammitic schists of the Koeris Formation which he believed were reflecting his F2 folding. He also differentiated a few examples of an L2, which was generally parallel to his L1. He defined his NE-SW trending F3 as being responsible for the open folding apparently overprinting the main upper and lower limbs of the main sheath fold, although no cleavage was recorded for this phase. The plunge of these folds is variable and they die out with depth. His F4 was responsible for local small scale folds in the quartzite bands and for kink folds with associated fracture cleavage in schists, while F5 was responsible for subsequent mild buckling.
On the basis of regional mapping Colliston, et al., (1991), attribute the majority of the deformation in their Aggeneys Terrane to progressive ductile shearing, resulting in a series of shear/thrusts, nappes and associated sheath folds. They have interpreted the main structure at Gamsberg as being a major sheath fold, which is more believable than the interference of two fold phases as postulated by Rozendaal. In general it would appear that most of the observations of Joubert (1986) and are not inconsistent with this conclusion. Furthermore, in the mapping of Colliston, et al., (1986), they place a thrust fault at the boundary between the Zuurwater Quartzite (of their Wortel Formation) and the Gams Iron Formation (or their Gams Formation). This structure would appear to be a decollement shear developed within the sheath fold at the contact of the two units of different competence. The schists of the A1 Bed in the Gams Iron Formation may represent this decollement surface. Colliston, et al., (1991) interpret the inlier to the west in the vicinity of the Big Syncline prospect as being two parallel telescoped nappes separated by thrusts to the north of the major Swartberg-Zuurwater Thrust/shear Zone. It would then appear, to be consistent, that the Gamsberg sheath fold is part of the lower sheared synform of a similar south-westward transported nappe structure which has subsequently been gently folded. The upper limb of the sheath fold as observed by Rozendaal (1977) was in the process of being more extensively sheared and attenuated.
The higher grade mineralisation at Gamsberg is developed as an elongate shoot within the B Member of the Gams Iron Formation in the western hinge zone of the major sheath fold. It occurs on the lower limb of the fold in which the immediately adjacent upper limb has been attenuated and sheared out along a mylonite zone. The orebody appears to be best developed adjacent to this sheared out closure, and absent on the adjacent upper limb. In the upper limb, the Gams Iron Formation is only sporadically preserved, having been either sheared out due to attenuation (or eroded prior to the deposition of the Koeris Formation). On the lower limb away from the fold axis the B Member of the Gams Iron Formation contains values of the order of 1% Zn (this estimate is based on very sparse information), with sporadic higher grade patches of up to 4% Zn.
In the nose zone of the fold with which the main higher grade mineralisation is associated, abundant parasitic folds are developed on a variety of scales, with axes plunging at around 30°to the ENE. These axes are parallel to the main fold closure and the mineralised shoot, and having the appropriate vergence to support the interpreted structure.
Studies by Rozendaal suggest the metamorphism within the Gamsberg deposit took place at temperatures of from 630 to 670°C and pressures of 2.8 to 4.5 Kb. Pb-Pb isotope dating indicates an age of 1250 to 1300 Ma in contrast to an age of 1600 to 1650 Ma for the host rocks.
The detailed structural setting at Gamsberg therefore differs from that at Broken Hill and Black Mountain, which occur within the sheared isolated limb and a fold nose respectively within a major regional thrust shear zone. However it may be similar to that at Big Syncline area where further sheath folds are reported. Never the less all three are found within zones of major dilation in some of the most intensely deformed positions within the Bushmanland/Aggeneys Terrane closely associated with sheared fold hinges.
The major mineralisation at Gamsberg is all hosted by the B Member of the Gams Iron Formation at Gamsberg. This comprises the lower 0 to 30 m of fine grained dark quartz-muscovite-sillimanite schist of the B1 Bed, the overlying 2 to 4 m thick cordierite 'hornfels' and the 8 to 16 m thick B2 Bed which comprises fine to medium grained quartz-garnet-grunerite rock and grunerite-orthopyroxene-clinopyroxene rock. Most rocks within the ore zone now have sand sized grains.
The mineralisation is present as a main high grade core of 8 to 10 Mt @ >10% Zn, surrounded by a lower grade zone, which with the core amounts for 30 Mt @ around 8% Zn. This high grade core is apparently restricted in strike length, but is elongated closely following the sheared off northwestern hinge zone of the sheath fold. Away from this structure the mineralisation drops of to levels of 1 to 4% Zn, with the western ore zone which is 300 to 400 m from the fold axis and the high grade shoot averaging 4% Zn.
Within the high grade ore shoot there is a higher grade hangingwall zone of limited tonnage which has 15 to 25% Zn, and a lower grade footwall zone with 4 to 7% Zn. The hangingwall zone is too small and irregular to be susceptible to selective mining. These observations are not recorded in the literature, but were ascertained during the visit.
Within the Gams Iron Formation there is a well established vertical mineral zonation. As at Broken Hill/Aggeneys the main base metal sulphide zone is bracketed by magnetite and/or hematite rich rocks above and below, namely within the underlying A2 Bed and the overlying C Member beds. Pyrite is found in the A3 and A4 Beds below the main base metal sulphides, and in the lower mineralised zone in the B1 Bed. The sulphur content decreases upwards progressively from the middle of the B1 bed as pyrite is replaced by pyrrhotite as the dominant Fe sulphide in the B2 Bed, before minor magnetite appears in the upper B2 Bed, and dominates with hematite in the C1 Bed.
Zinc gradually increases upwards from the base of the B1 Bed to the top of the B2 Bed.
The opaque minerals within the base metal mineralised interval comprise, in decreasing order of abundance, pyrite, pyrrhotite, marcasite, sphalerite, galena and accessory chalcopyrite, alabandite, magnetite, ilmenite and graphite.
The distribution and form of the various sulphides is as follows, from Rozendaal (1975, 1980 and 1986):
• Pyrite - is present as discrete disseminated grains (0.01 to 0.5 mm), networks of grains and massive clusters (>10 mm) as a replacement of gangue. It comprises the bulk of the sulphides (more than 35%) within the B1 Bed, dropping off to about 5% at the top of the B2 Bed. Galena, sphalerite and pyrrhotite partially envelop and sometimes appear to replace pyrite. The pyrite idioblasts are of varied size, related directly to the intensity of folding. Pyrite and the other sulphides are concentrated in hinge zones of minor folds, in pressure shadows and fractures; in these locations the pyrite is coarser than in the normal banded ore matrix.
• Pyrrhotite - occurs as massive aggregates (>10 mm) as well as disseminated xenoblastic grains (0.01 to 2 mm) dominantly associated with sphalerite. These two sulphides jointly occupy interstices between, and also partially or completely envelop, pyrite. Pyrrhotite selectively replaces muscovite and chlorite along cleavage planes and partially includes garnet and ilmenite grains. The distribution of pyrrhotite is the inverse of pyrite.
• Marcasite - replaces pyrrhotite as blades along crystallographic planes and also occurs as veinlets (0.1 to 0.5 mm) intergrown with pyrite. Pyrrhotite appears to alter to marcasite and marcasite to pyrite from the top to the base of the mineralised unit.
• Sphalerite - is the most significant economic base metal. It is present as small (0.001 to 0.1 mm) xenoblastic grains dispersed intergranularly in gangue and as massive intergranular aggregates (0.5 to 5 mm) associated with pyrrhotite and marcasite, occupying interstices between or partly enclosing pyrite grains. The sphalerite is generally coarse grained and matured due to annealing (Rozendaal 1975). Sphalerite grains may contain wispy concave galena particles, pyrrhotite blebs and chalcopyrite as fine ex-solution blebs and chains of blebs (0.001 to 0.1 mm) aligned parallel to the crystallographic direction of the host.
Most of the sphalerite is manganiferous and marmatitic, but honey coloured varieties with negligible Fe and Mn are present in places. Sphalerite increases from minor amounts at the base of the B1 Bed to peak concentrations at the top of the B2 Bed. Cd is a substitution element in sphalerite, with 100 to 130 ppm generally being present within the mineralised layer.
• Galena - is present as disseminated sub-idioblastic to xenoblastic grains (0.001 to 0.1 mm), dispersed intergranularly with gangue and sulphides, as well as massive vein like aggregates (>2 mm) filling fractures and mineral interstices. Silver occurring as a substitute element in galena, has an average concentration in the mineralised zone of 5 to 7 ppm.
Galena displays textures indicative of ductile behaviour and annealing. Small wispy concave particles and cusps of galena found in sphalerite do not (according to Rozendaal 1986) represent original inclusions in sphalerite, but are concentrations of the mineral forming phase boundary triple junctions with sphalerite, which is indicative of annealing, not replacement.
Galena is particularly well concentrated in hinge zones of minor folds, in pressure shadows and in fractures. Rozendaal (1975) regards the coarse grained galena as representing original fine grained material being squeezed out of its matrix and mobilised into cracks and cavities during metamorphism.
• Chalcopyrite - is present as fine ex-solution blebs (0.001 to 0.01 mm) within massive sphalerite, the distribution trend of these two minerals being sympathetic. The general concentration of Cu within the mineralised zone ranges from 50 to 300 ppm.
• Magnetite - while being found in the footwall in the A2 Bed, is absent from the lower part of the mineralised zone, but is a minor constituent in the upper zinc rich portion where it occurs as sub-rounded grains embedded in massive aggregates of sphalerite. In the C1 and C2 Beds magnetite becomes the dominant opaque mineral, which occurs throughout the C Member as individual grains and massive aggregates, ranging in size from 0.05 to 5 mm, displaying varying degrees of martitisation along crystallographic directions, intergrown hercynite grains and sporadic ilmenite lamellae. Hematite is present in this member as oriented idioblastic specular grains, 0.5 to 1.5 mm in size.
• Graphite - is found predominantly in the basal quartz-muscovite-sillimanite schist of the B1 Bed in which it forms xenoblastic flakes (0.002 to 0.2 mm), aligned parallel to the foliation planes or interlayered with slightly elongated pyrite and sphalerite grains.
• Barite - is present mainly as recrystallised white to greyish pink, coarse grained massive bands, interbedded with, above and below the magnetite-hematite quartzite. The barite bands increase in thickness from south to east as the underlying mineralised B member thins and eventually peters out in the southeastern corner of the Gamsberg Iron Formation outcrop. This inverse relationship of sulphide and sulphate is manifested along strike and down dip. Ba levels are very low (250 to 1000 ppm) in both the B and C Members away from the major barite developments.
The main barite outcrop is either massive white or massive black crystalline barite or as a banded black and white variety. Drill intersections of the barite band assayed around 0.8% Pb, 0.4% Zn over a 7 m interval. In outcrop a thin 1.5 m thick gossan is found in the immediate footwall of the barite unit. The barite zone has a strike length of 1 km.
The main mineralised zone ranges from fine disseminated sulphide grains (generally <2 mm) and larger aggregates (up to 1 cm), aligned parallel to the main foliation, to irregularly contorted bands and lenses (1 mm to 1 cm thick) of massive sulphide and gangue. These are cut by massive crosscutting veins (mm's to 30 cm) of fine to coarse sulphides and aggregates. The sulphide content ranges from <10% up to around 80% within the B Member.
The irregular contorted texture of the ore is not a consistent fold pattern, but is very irregular, with a knotted appearance in paces, as well as producing concentric circular structures on the core face.
The well developed banding at Gamsberg contrasts to the virtual absence of banding of the sulphides at Broken Hill/Aggeneys.
In some intervals there are round nodules (around 1 cm) of galena and apatite surrounded by sphalerite bearing meta-pelite.
Rozendaal and subsequent workers have interpreted the main foliation within the ore to be bedding. They claim that 90% of the sulphides within the mineralised zone are recrystallised original syngenetic minerals, overprinted by <10% remobilised sulphides and minor very late poikiloblastic coarse sulphides. They maintain that the sulphides have been recrystallised with the host lithologies without substantial transport. However as with other examples this does not address the concentration of mineralisation within the high grade fold axis shoot, the microscopic relationships between the various sulphides and the constituents of the mineralised interval and the probability that different minerals (sulphides and silicates) display different behaviours and properties during metamorphism (eg, in situ recrystallisation, versus pressure solution and transport, etc.).
Surface Expression and Geochemistry
A diagnostic gossan with a strike length of 5.3 km marks the surface exposure of the main mineralised band within the Gams Iron Formation. The strike length of the high grade shoot of mineralisation on which the reserves are based however is of the order of several hundred metres.
In outcrop the gossan has a sharp contact with the underlying quartzite although faint boxworks persist into that unit below the Gams Iron Formation. The gossan passes upwards into a black banded magnetite quartzite of the C1 Bed.
The gossan directly overlies the mineralised band and where surface features permit, simulates the shape and structure of the latter. According to Rozendaal (1975), macroscopically there are two major types or 'end members' of gossan at Gamsberg, with numerous intermediate variations, namely:
• A mainly limonitic cellular type with sponge and boxwork structures.
• A fine grained massive limonitic jasper occurring as sprawling masses and ragged edged seams and patches which develop from the cellular or intermediate varieties and merge into the massive jasper types again.
The original textural features of the un-oxidised mineralisation, ie. banding and schistosity are generally obscured or poorly preserved in the massive jaspilitic gossan, although these features are generally observable within the cellular gossan. Macroscopically this layering is visible as coarse and fine grained seams of quartz.
On the unbroken surface, the gossan is covered by a patina of iridescent smeary crusts of both ferruginous and manganiferous oxidation products, usually darker than the underlying rock. The latter presents a range of colours from yellow to dark brown to maroon to matte black. On the Cretaceous peneplain, the gossan is coated with exotic yellow to ochreous powdery gypsum and calcite (Rozendaal 1975).
Mineralogically the gossan consists of secondary limonitic, goethitic and hematitic jasper as well as kaolinised feldspar, garnet as pseudomorphs, silicified amphibole, ferruginised micas and major quartz, with accessory rutile, tourmaline and sillimanite.
The gossan capping persists for a depth of around 110 m below the surface, with the gossan-sulphide interface at a constant elevation above sea level. A relatively sharp 3 to 4 m vertically thick transition zone separates the oxidised and fresh sulphides.
Geochemical soil sampling values vary with the topographic elevation. In general at higher elevations assays averaged 700 to 900 ppm Pb, 150 to 200 ppm Zn. In lower areas these values become 100 to 400 ppm Pb, 1000 to 2400 ppm Zn, showing an inversion of the relative abundance of the two metals. This reflects the concentration of Pb as anglesite and plumbojarosite at the Cretaceous surface in contrast to the dispersion of the more mobile zinc sulphates and carbonates. Cu varies from 30 to 350 ppm. Chip samples of the gossan range up to 7.75% Pb and 0.3% Zn.
Sampling of the Gams Iron Formation within the 5.3 km outcrop of the mineralised unit return values seldom less than 1000 ppm Zn (except in the areas of greater elevation).
Reserves and Resources
The measured and indicated resource was 140 Mt @ 5.8% Zn, 0.5% Pb, within a geological resource of 170 Mt (when visited in 2001).
Remaining Ore Reserve and Mineral Resources at the end of 2018 (Vedanta Resources Annual Report, 2018) were:
Proved + Probable Reserve - 53.81 Mt @ 6.63% Zn, 0.51% Pb;
Measured + Indicated Resource - 97.91 Mt @ 6.20% Zn, 0.54% Pb;
Inferred Resource - 64.36 Mt @ 7.81% Zn, 0.52% Pb.
NOTE: Reserves are additional to resources.
This description is based on a visits in 1992 and 2001 and information from Rozendaal (1975, 1977, 1980 and 1986). As such it may be dated. For more up to date detail and alternate interpretations, see the references listed below.
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Rosh Pinah - Zn-Pb
The Rosh Pinah Zn, Pb, Ag deposit is located in the Namib Desert of south western Namibia, in southern Africa.
It is basically a stratabound deposit, hosted by the Rosh Pinah Formation arkoses and quartzites, in the lower sections of the late Proterozoic to early Palaeozoic Gariep Complex, an equivalent of the Damaran Supergroup. In this area the basement to the Gariep Complex is made up of Richtersveld Sub-province (Vioolsdrif Terrane) igneous rocks of the Namaqualand Metamorphic Complex.
The type section of the Rosh Pinah Formation, which forms the base of the Gariep Complex, comprises a thin basal mixtite/conglomerate overlain by a thin unit of mafic volcanics. These are succeeded by thicker felsite and rhyolite lavas, pyroclastics and epiclastics, followed by a thin dark argillitic arkose, more felsic volcanics and then the main thick sequence of arkoses, comprising arkose, graded quartzite, argillite and dolomite. A further felsic volcanic unit is found near the top of these arkoses. These rocks of the Rosh Pinah Formation are overlain by further formations composed of limestones, conglomerates, grit, sandstone and argillite.
In the mine area the Rosh Pinah Formation is 1220m thick. The ore zone is in the lower sections of the Formation, underlain by at least 200m of quartzites. The ore occurs just below the dark argillitic arkose and felsites which occupy the middle and upper section of the formation.
The ore bed consists of a well banded to massive carbonaceous cherty zone or micro-quartzite, in places grading into an argillite; various carbonate bearing rocks; sugary quartzite; lenses and bands of massive mixed pyrite, sphalerite and galena; argillite and intercalations of generally poorly mineralised quartzite. The microquartzites are fine grained and dark due to their carbonaceous content. Barium rich carbonate is an important constituent in places. The lower sections of the ore bed are generally Zn rich micro-quartzite, overlain by further micro-quartzites or carbonates with a higher Pb:Zn ratio, while the hangingwall is another micro-quartzite grading to argillite. Most ore is within the micro-quartzites and seldom in the argillites.
The ore minerals are generally present as intergranular disseminations and discrete blebs associated with a fine grained sugary quartz-carbonate matrix, or as thin bands from 1 mm to a few cm's thick of massive sulphide. Irregular barite-carbonate or dolomite lenses are present in the central or lower part of the ore bed.
Massive sulphide bands may be up to a few metres thick in sections of the mine within micro-quartzites and occasionally argillites, and may grade laterally into disseminated ore within the micro-quartzites or carbonates.
In contrast to the hangingwall quartzite which is generally little fractured, the footwall quartzite is intensely fractured forming a breccia, which is silicified and carries sulphide and carbonate veining.
The ore deposit comprises a number of lenses distributed over an area of some 2 x 3 km, each of 150 000 t to 2 to 3 mt. The structure is variable with orebodies ranging from steeply dipping to flat to folded.
The operation is owned (in 2001) and managed by Iscor Limited Base Metals strategic business unit and lies near the southern border of Namibia with South Africa.
Production since 1969 has been 14.5 Mt @ 7% Zn, 2% Pb, 0.1% Cu, 11 g/t Ag. In 1999 proven and probable reserves were quoted at 6.59 Mt @ 8.7% Zn, 2.5% Pb, within a resource of 15 Mt @ 7.5% Zn, 2.2% Pb.
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A half field traverse of the host lithologies/sequence of the Rosh Pinah District is undertaken.
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For more information contact: T M (Mike) Porter, of Porter GeoConsultancy (email@example.com)
This was another of the International Study Tours designed, developed, organised and escorted by T M (Mike) Porter of Porter GeoConsultancy Pty Ltd (PGC) in joint venture with the Australian Mineral Foundation (AMF). While the reputation and support of the AMF contributed to the establishment of the tours, after it ceased trading at the end of 2001, PGC has continued to develop, organise and manage the tour series.
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