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Witwatersrand, Evander, East Rand, Central Rand, West Rand, Carltonville, Klerksdorp, Welkom |
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Super Porphyry Cu and Au
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The Witwatersrand gold - uranium mines in north-central South Africa, between Johannesburg in Gauteng and Welkom in the Free State, have contributed over 50% of all of the gold produced in the world to date. These mines are distributed around the arcuate, active, fault bounded, north to north-western to western margins of the northeast-southwest elongated, 390 x 250 m Witwatersrand Basin which has a gently downwarping, more passive SE margin.
Production from the basin has come from more than 150 mines which have extracted over 4.5 Gt of ore to yield more than 42 000 t of Au at an average grade of 9.3 g/t Au. It has also produced nearly 145 000 t of U3O8 from 675 Mt of ore in 31 mines at an average grade of 0.215 kg/t U3O8. These figures relate to the period from 1887 to 1987 for Au and 1952 to 1987 for U (Pretorius 1991).
The mines are distributed within a series of gold fields that are, in anti-clockwise order from the north-east: the Evander, the over a gap of approximately 60 km across a basement granite to the contiguous East Rand, Central Rand (in the Johannesburg area) West Rand, South Deeps, Western Areas and Carltonville gold fields which represent around 150 km. After a gap of 80 km, the Klerksdorp gold field is located to the south-east of Klerksdorp. Approximately 110 km to the south the Welkom gold field covers and area of around 70 x 20 km centred on the city of Welkom on the southern tip of the basin.
The Witwatersrand Basin is one of five basins that occupy the central portion of the Archaean Kaapvaal craton. The axes of these generally northeast-southwest elongated basins progressively migrated from south-east to the north-west. They comprise the i). Pongola, represented by limited remnant sediments which are broadly contemporaneous with the lower parts of the Witwatersrand Basin to the north-west; ii). Witwatersrand (approximately 2985 to 2715 Ma); iii). Ventersdorp (approximately 2.7 to 2.65 Ma); iv). Transvaal (2.25 to 2.0 Ga); and v). Waterberg Basins (1.9 to 1.75 Ga).
The oldest basement rocks beneath these basins includes the ~3.64 Ga Ancient Gneiss Complex in Swaziland to the east, on the southern margin of the Barberton greenstone belt which represents 3.49 to 3.42 Ga basic to ultra-basic magmatism following 3.55 to 3.52 Ga tonalitic intrusion. Granite-greenstone and granitoid terranes, including the Murchison, Pietersberg and Giyani belts are extensively exposed or sub-cropping to the east and north of the Witwatersrand Basin as well as those immediately underlying the basin. The Murchison greenstone terrane to the north includes granitoids dated at 2901±12 Ma with some dates at 2820 to 2811 Ma. The Amalia-Kraaipan granitoid-greenstone to the west of the Witwatersrand Basin includes 3033±1 Ma mafic intrusives, a 2930 Ma volcanic arc and high level granitoids as young as 2767±6 Ma. The Kaapvaal greenstone belts include banded iron formations and pass upwards into a clastic sequence. A change from compressional to transtensional tectonics at around 3.1 Ga corresponded to the development of orogenic greenstone belt gold deposits of the Barberton gold field.
This change of tectonic regime marked the commencement of intracratonic basin formation. The lowest unit of this domain commences with a thin basal siliciclastic unit, overlain by a 13 000 sq. km, 2250 m thick, 3086±3 Ma to 3074±6 Ma bimodal volcanic sequence which comprises the Dominion Group. This sequence comprises 45% andesitic to rhyolitic rocks; 35% conglomerate, feldspathic quartzites and shaly quartzites; and 20% shales. The basal siliciclastics include a mineralised conglomerate with abundant uraninite and pyrite with relatively low gold. The Dominion Group was followed after a break of 100 m.y. by the lower unit of the Witwatersrand Supergroup.
The Witwatersrand Supergroup is divided into two groups:
West Rand Group - which extends over an area of 43 000 sq. km, is up to 5150 m thick and rests on the Dominion Group above an angular unconformity. It is thickest in the Klerksdorp area in the west, and thins to the north-east. Age dating restricts the deposition to between 2985±14 Ma and 2914±8 Ma, the age of volcanics in the upper part of the group. It has been sub-divided into three sub-group, based on varying sandstone/shale ratios and basinwide disconformities, namely the i). Hospital Hill Sub-group, is primarily composed of shallow marine deposits, mainly shales and orthoquartzites, comprising up to 2500 m of conglomerates, sub-greywacke, sub-arkose, ortho-quartzites, siltstones, shales, ferruginous shale and banded ironstone; with no volcanics, approximately 50% quartzites/conglomerates and 50% shales; the ratio of sandstone/shale increases upwards, with the sandstone being predominantly quartz-arenite; the unit contains no economic reefs; ii). Government Sub-group, characterized by more diverse lithologies which formed in a far greater variety of depositional settings, and include diamictites (possibly of glacial origin), interbedded beds of fluvial conglomerates and quartzite, shelf shales and quartzite, and iron formations. The base of the Subgroup is represented by a widely correlated diamictite which lies on a major, regional unconformity; the unit contains three subeconomic auriferous reefs; iii). Jeppestown Sub-group, which commences with a quartzite overlying a major regional unconformity and represents a return to a predominance of shelf deposits. It is up to 1200 m in thickness and comprises interbedded conglomerates, feldspathic quartzites, siltstones, shales, ferruginous shale, calcareous shale and andesitic volcanics; comprising approximately 30% volcanics, 30% quartzites/conglomerates and 40% shales. Two sub-economic reefs have been recognised within the unit.
Central Rand Group - which is up to 2880 m thick in the central part of the basin around the Vredofort structure and unconformably overlies the West Rand Group, covering an area of 9750 sq. km with a basinal extent of around 290 x 150 km. Similar to the West Rand Group, this group is composed of a series of cycles, each comprising fluvially dominated coarse grained siliciclastics overlying an erosional surface, although in contrast to the West Rand Group, the coarser lithologies predominate with subordinate shale or mudstone. Age constraints on the group places deposition between a lower limit of 2902±13 Ma and minimum age of 2840±3 to 2780±3 Ma. Two sub-groups have been defined, namely the:
Johannesburg Sub-group - The base of this unit is the first prominent conglomerate overlying quartzite of the Maraisburg Formation at the top of the Jeppestown Sub-group. This conglomerate is widely recognised and occurs as the North Reef in the Central Rand Gold Field (GF); Rock Tunnel Reef in the West Rand GF; North leader in the Carletonville GF; and the Ada May Reef in the Klerksdorp and Welkom GFs. This conglomerate and the overlying quartzite, which together comprise the Blyvooruitzicht Formation, are both locally truncated by an erosion surface upon which another widespread conglomerate was deposited.
The overlying Main Formation contains the economically important Main Reef, Carbon Leader and Main Reef Leader in the Central and West Rand and at Carletonville, and has been correlated by the prominent, shale-filled erosion channels (Green Bar of Carletonville, Black Bar of the Central Rand), which have been recognized in almost all of the goldfields. Although widespread, this unit was truncated by the overlying sequence in the eastern portions of the Witwatersrand basin.
The overlying Randfontein Formation consists of quartzite and conglomerates, which along the northern margin of the basin contains, at its base, the Nigel Reef of the East Rand, the South Reef of the Central Rand and the Middelvlei Reef in the Carletonville area. Several conglomerates are developed within the succeeding quartzite, which degenerated to pebbly quartzites regionally. This sequence of conglomerates and quartzite was accordingly grouped together in a single formation, termed the Randfontein Formation.
The Randfontein Formation is followed by the sequence of pebbly quartzite and locally developed conglomerates of the Luipaardsvlei Formation, which can be correlated with most of the Krugersdorp Formation of the Carletonville GF, the Stilfontein Formation of the Klerksdorp GF and the Welkom Formation of the Welkom GF, with some small changes in boundaries. This unit is absent on the East Rand, as it has been erosionally truncated prior to deposition of the overlying formation.
An extensive unconformity marks the top of the Luipaardsvlei Formation and the base of the Krugersdorp Formation which contains the Bird Reef and its correlatives Vaal and Basal/Steyn Reefs. This basal conglomerate is overlain by quartzite with locally developed pebbly quartzite or conglomerates, although locally, it fines upwards to siltstone or shale (e.g. Khaki Shale of Welkom). In the eastern sections of the basin, the formation contains the mafic Bird volcanics.
The succeeding Booysens Formation contains the only widespread shale sequence in the Central Rand Group and marks the top of the Johannesburg Subgroup. Both the upper and lower boundaries are generally transitional. The shale coarsens up into the locally prominent orthoquartzite of the Doornkop Member. The marks the top of the Johannesburg Subgroup.
The Turffontein Sub-group - The base of this unit is defined by the widespread unconformity-based conglomerate of the Kimberley Formation which comprises the 'B', Kleinfontein, 'C' and 'LK1' Reefs, and consist primarily of quartzites and conglomerates, deposited in a number of cycles of deposition and erosion. The the Beatrix Reef occurs in conglomerates of one these cycles. Shale-filled erosion channels occur at the base of several of these cycles. The upper boundary of the formation is defined by another widespread unconformity.
The basal conglomerate of the overlying Elsburg Formation contains the Intermediate Reef of the Evander GF, the UE1A of the West Rand GF, the Denny's Reef of the Klerksdorp GF and the VS5 of the Welkom GF. This basal conglomerate is overlain by upward coarsening quartzite throughout the basin. The lower, predominantly quartzitic portion, locally contains economic conglomerates which become increasingly more abundant upwards in the succession. Conglomeratic rocks dominate the upper portion of the sequence, which has been termed the partially equivalent Mondeor Formation. In the Welkom GF, the transition from the Elsburg to the Mondor Formation is characterised by a transition from predominantly oligomictic to polymictic conglomerates.
The top of the Witwatersrand Supergroup is defined by the unconformity which marks the base of the Ventersdorp Contact Reef (Venterspost Formation), where developed, or the base of the volcanic rocks of the Ventersdorp Supergroup.
Ventersdorp Supergroup, which is 5000 to 7800 m thick and forms an elliptical, north-east elongated basin covering an area of approximately 300 000 sq. km. It conformably overlies the clastics of the Witwatersrand Supergroup, is predominantly composed of andesitic composition rocks dated at 2720 to 2650 Ma and is divided into three groups, namely the: i). Klipriviers Group - around 1830 m of tholeiitic lavas which range from basaltic komatiite at the base, overlain by a succession of amygdaloidal basaltic lavas with an increasing Ni, Cr and Mg content with height within the sequence. It covers a very extensive area from south-west Botswana to the northern Cape Province. ii). Platberg Group - which is largely confined to narrow graben structures and is dominated by the 2.709 Ga quartz-porphyry rhyolites of the Makwassie Formation iii). Pniel Group - which includes arenaceous sedimentary rocks overlain by a further suite of mafic volcanics of the Allanridge Formation, which is predominantly fine grained amygdaloidal porphyritic tholeiites which filled small basins in the underlying sedimentary units.
Transvaal Supergroup, which is up to 15 000m thick and is composed of two main units, the lower 2642±2 to 2432±31 Ma Chuniespoort Group comprising predominantly carbonatic sediments, including dolomitic limestone, quartzite and shale, with lesser basaltic and rhyolitic volcanics, and tilloids, separated by a ~80 m.y. hiatus from the overlying Pretoria Group, a volcano-sedimentary succession with alluvial fan and fan delta deposits with minor marine components comprising up to 7000 m of andesitic lavas, banded iron formation and carbonates. The base of the Chuniespoort Group is marked by the Black Reef Quartzite Formation, a laterally persistent basal conglomerate and sandstone unit that rests on the post-Ventersdorp erosion surface and contains the Black Reef conglomerates which are indistinguishable from the auriferous, uraniferous and pyrite rich Witwatersrand reefs.
Deposition of the Pretoria Group was followed after a hiatus by the generally contemporaneous extrusion of the Rooiberg Group felsic volcanics pile and the intrusion of the 2059±1 Ma Rustenberg Suite of up to 9000 m thick gabbro, norite, pyroxenite, anorthosite, harzburgite and diorite mafic to ultramafic layered complex and the 2054±2 Ma Lebowa Suite granitic to granophyre phase. Both intrusive phases form the Bushveld Igneous Complex which covers an area of some 70 000 sq. km in four generally circular overlapping lobes to the north of the Witwatersrand Basin.
The structural pattern of the central eastern section of the Witwatersrand Basin has been modified by the ~75 km diameter Vredefort Dome that has a core of older Archaean basement and is interpreted to represent a 2023±2 Ma meteorite impact.
Waterberg Supergroup, up to 8 000m thick, distributed over an area of around 40 000 sq. km - was deposited along the northern margin of the Kaapvaal Craton between 1.9 and 1.75 Ga and comprises marine and continental volcano-sedimentary successions, including conglomerate-quartzite units, red-beds, shales and thin units of trachyandesitic and quartz-porphyritic lava.
Seismic profiling shows that the northern and western margins of the Witwatersrand Basin are characterised by thrusting and faulting, and block faulting throughout the basin. There are several stages evident in the evolution of the Witwatersrand Basin, which was initiated as a rift at ~3.07 Ga, during Dominion Group times, followed by post-rift thermal subsidence during deposition of the early parts of the West Rand Group. Thermal subsidence is interpreted to have been completed by late West Rand Group times (~2.9 Ga), although minor volcanic interludes within the West Rand Group sequence may indicate phases of extension. The onset of compression and thrusting outside of the thermal basin generated a flexural load and clastic input into the basin as it evolved from thermal sag to foreland basin during the late West Rand Group (~2.95 Ga). Foreland basin development culminated during deposition of the Central Rand Group, with an increasingly coarse-grained clastic input, and thrust systems that progressively encroached on the basin margins, profoundly influencing structural styles. Thrusting was interrupted during the early Ventersdorp Supergroup by the accumulation of Klipriviersberg Group basic volcanic rocks (2.71 Ga). Thrusting recommenced at the end of Klipriviersberg Group. The basin then returned to an extensional tectonic setting during the Platberg Group rifting (2.709 Ma), with major preserved grabens filled with immature Platberg Group diamictites and other mass flow sediments. This extension represents a major rift event across much of the region, with post-rift thermal subsidence during the late Platberg Group. The overprint of Platberg Group extensional faults breaks up the structural continuity of the Central and West Rand Group sediments within, and adjacent to, the basin, making it is difficult to differentiate the earlier compressional thrust-fold structures.
The overlying Transvaal Supergroup is relatively un-deformed, with structural deformation characterised by broad, open to overturned folds, low angle southerly dipping faults and small periclinal folds. Faulting of the underlying Ventersdorp and Witwatersrand Supergroups is developed both parallel and normal to the basin margins. These faults have been the locus of normal displacement of the Klipriviersberg Group volcanics by as much as nbsp;m, as well as forming the margins of the Platberg Group grabens. Many of these same faults were also active during the deposition of the Witwatersrand Supergroup and controlled the thickness and distribution of the units of the sequence. Monoclines are associated with some of these faults, with drastic thickness changes evident across the faults perpendicular to strike, but little to no change parallel to strike. In addition, significant erosion is indicated with the Klipriviersberg Group volcanics overstepping the monoclines and associated faults to lie directly on West Rand and Basement rocks within 3 to 4 km outboard of the monoclines. The resultant apparent vertical displacement across some of these faults in the Johannesburg area during, and just prior to deposition of the Central Rand Group, was as much as 7000 m, with an implied reverse component of movement, downside into the basin. Some of these same faults had a moevement in the opposite sense, ie. downthrow outboard of the basin, by as as much as 2.5 km during deposition of the Platberg Group. The development of these monoclinal structures resulted in steep to overturned dips and local thrust displacement of the reefs. In general, along these structures, dips decrease stratigraphically upwards. In addition, the faults normal to the basin margins have displaced the margins of the basin and form many of the boundaries of the individual gold fields.
Gold has been mined from at least 30 reefs within the Witwatersrand Basin sequence, 95% of which are within the Central Rand Group, with a general upward decrease in both total Au yield and average grade (eg. the Carbon Leader Reef at the base of the Central Rand Group averages 25 g/t Au, while the Venterdorp Reef at the top of the same group carries 5 to 12 g/t Au). The most important reefs within the Central Rand Group are at 6 stratigraphic positions, 3 within the Johannesburg and 3 within the Turffontein Sub-group. In additon there are three sub-economic reefs within the Government Sub-group and a further two in the Jeppestown Sub-group, both within the West Rand Group. The reefs are typically composed of a variety of fluvial lithofacies ranging from clast-supported oligomictic conglomerate to loosely packed conglomerate, pebbly arenite or pebble-lag surface associated with trough cross-bedded quartz-arenite. Rarely, ore occurs within debris-flow lithofacies.
The individual orebodies range in thickness from a few tens of centimetres to several metres and are confined to the interval between a basal degradation surface, which is normally an angular unconformity, and an upper planar bedding surface marking the contact with overlying quartzwackes or siltstones. Both the upper and lower margins of the orebodies are marked by a sharp change in gold content from several grams per tonne to <20 ppb. The orebodies occurs as lens-like fluvial fluvial bar and channel beds with unimodal palaeo-current directions. Thicker bodies occur as multi-channel sequences of conglomerate and quartz-arenite representing flood and waning stage flows. The depositional environment of the orebody hosts range from proximal alluvial fans, to terraced fluvial, braid plain and braid delta, merging into shoreline environments. On a regional scale the orebodies are distributed along the margin of the Central Rand Group basin, at the main entry points of complex river systems into the original basin. On a mine scale, the highest grades are usually, but not exclusively in the channel facies, with higher grades where the channels are thickest. Individual orebodies may be of the order of 3 km or more wide and 5 to 10 km long.
Gold, together with uraninite and pyrite are spatially associated with detrital minerals such as zircon and chromite which are concentrated on degradation surfaces marked by pebble lags at the base of clast supported conglomerates, but also on crossbedded foresets, bottomsets and coset boundaries. Uranium occurs as uraninite, brannerite and leucoxene, with a distinctive decrease in the uraninite:brannerite ratio up-section, from 8.7 in the Steyn Reef towards the base of the section, to only brannerite in the younger Beatrix Reef, both examples from the Welkom District. There is also a systematic trend in U:Au ratio up-section, from the gold-rich Central Rand Group reefs to theDominion Reef which does not contain significant gold. In addition, there is a systematic trend in the U:Au ratio doen the palaeoslope in the Welkom Goldfield, with uraninite being enriched in the more distal facies and the ratio ranging from 0.001 on the basin margin to 10 in the distal facies. Pyrite is the most common iron minerals, with iron oxides (mainly magnetite) only being found in the shale facies.
There is an intimate association between gold and carbonaceous matter, which occurs as stratabound seams and a spherical glassy globules. The carbon seams are absent from proximal high energy deposits, and preferentially in more distal environments, and occur in conglomerates and sandstones that contain rounded pyrite. These seams are found along micro-fractures and appear to represent hydrothermal pyrobitumen.
Gold is found in association with carbon and/or pyrite within siliceous quartz pebble and cobble (rounded white and smoky quartz) conglomeratic rocks, banded pyritic quartzites and orthoquartzites, as well as in low energy environments of distal fans where thin bituminous bands carry fine gold and uranium. Within these rocks gold is present in three separate forms; as fine free gold in the clastic matrix with quartz, chlorite, pyrophyllite, fuchsite or sericite; with thucholite (bituminous material); or as a coating on pyrite.
Major mines operating in 2007 included:
Driefontein - in the Carltonville District, Jauteng Province.
Measured + indicated resource figures at 31 December 2006 were (Gold Fields, 2007): 111.6 Mt @ 12.5 g/t Au for 1390 t Au.
Kloof - in the Carltonville District, Jauteng Province.
Measured + indicated resource figures at 31 December 2006 were (Gold Fields, 2007): 177.6 Mt @ 12.0 g/t Au for 2135 t Au.
South Deep - in the Carltonville District, Jauteng Province.
Measured + indicated resource figures at 31 December 2006 were (Gold Fields, 2007): 288.7 Mt @ 7.2 g/t Au for 2077 t Au.
Beatrix - in the Welkom District, Free State Province.
Measured + indicated resource figures at 31 December 2006 were (Gold Fields, 2007): 84.2 Mt @ 6.9 g/t Au for 580 t Au.
Vaal Rivers Operation, in the Klerksdorp District, is located near the towns of Klerksdorp and Orkney in the North West and Free State provinces of South Africa, which includes the Great Noligwa, Kopanang, Tau Lekoa and Moab Khotsong mines.
In order of importance, the Witwatersrand reefs mined at the Vaal River operations are the Vaal Reef, the Ventersdorp Contact Reef (VCR) and the C Reef:
• the Vaal Reef contains approximately 85% of the reserve tonnage with mining grades between 10 and 20 g/t and comprises a series of oligomictic conglomerates and quartzite packages developed on successive non-conformities. Several distinct facies have been identified, each with its own unique gold distribution and grade characteristic;
• the VCR has a lower grade than the Vaal Reef, and contains approximately 15% of the estimated reserves. The economic portion is concentrated in the western part of the lease area and can take the form of a massive conglomerate, a pyretic sand unit with intermittent pebble layers, or a thin conglomerate horizon. The reef is situated at the contact between the overlying Kliprivierberg Lavas of the Ventersdorp Super Group and the underlying sediments of the Witwatersrand Super Group, which creates a distinctive seismic reflector. The VCR islocated up to 1km above the Vaal Reef; and
• the C Reef is a thin, small-pebble conglomerate with a carbon-rich basal contact, situated approximately 270m above the Vaal Reef. It has less than 1% of the estimated reserves with grades similar to those of the Vaal Reef, but more erratic. The most significant structural features are the north-east striking normal faults which dip to the north-west and south-east, resulting in zones of fault loss (AngloGold Ashanti, 2007).
Measured + indicated + inferred resources at 31 December, 2006 were (AngloGold Ashanti, 2007):
Great Noligwa - 23.72 Mt @ 13.94 g/t Au for 330.6 t Au,
Kopanang - 20.60 Mt @ 16.57 g/t Au for 341.4 t Au,
Moab Khotsong - 19.66 Mt @ 18.24 g/t Au for 358.6 t Au,
Tau Lekoa - 41.84 Mt @ 5.31 g/t Au for 222.4 t Au.
West Wits Operation, in the Carltonville District, is situated near the town of Carletonville in North West Province, south-west of Johannesburg, straddling the boundary with the province of Gauteng of South Africa, which includes the Mponeng, Savuka and TauTona mines.
Two reef horizons are exploited at the West Wits operations: namely the Ventersdorp Contact Reef (VCR), which is located at the top of the Central Rand Group, and the Carbon Leader Reef (CLR) near the base. Due to non-conformity in the VCR, the separation between the two reefs dencreases from west to east, from 900 to 400 m. TauTona and Savuka exploit both reefs while Mponeng only mines the VCR. The structure is relatively simple with fault displacements of greater than 70 m being rare. The CLR consists of one or more conglomerate units and varies from several cm to more than 3 m in thickness. Regionally, the VCR dips at approximately 21°, but may vary between 5 and 50°, accompanied by changes in thickness of the conglomerate units. Where the conglomerate has the attitude of the regional dip, it tends to be thick, well-developed and accompanied by higher gold accumulations. Where the attitude departs significantly from the regional dip, the reef is thin, varying from several centimetres to more than 3 m in thickness (AngloGold Ashanti, 2007).
Measured + indicated + inferred resources at 31 December, 2006 were (AngloGold Ashanti, 2007):
Mponeng - 24.08 Mt @ 8.75 g/t Au for 210.8 t Au,
Savuka - 0.95 Mt @ 5.69 g/t Au for 5.4 t Au,
TauTona - 13.76 Mt @ 11.27 g/t Au for 155.1 t Au.
Harmony Freegold Operations - comprises a series of mines located in the Free State Province, on the south-western edge of the Witwatersrand basin. These include the Tshepong, Bambanani, Eland and St. Helena Mines which are located in and around Welkom, while the Joel Mine is approximately 50km south of Welkom. Mining at Tshepong Mine and Bambanani Mine, is primarily conducted on the Basal Reef, with limited exploitation of secondary reefs. Mining at Joel Mine is primarily conducted on the Beatrix-VS5 Composite Reef. The reefs generally dip towards the east or northeast while most of the major faults strike north-south (SRK, 2007).
In 2006 reserve and resource figures (SRK, 2007) totalled:
Proved + probable reserves - 56.710 Mt @ 7.5 g/t Au for 423 t Au,
Measured + indicated resources - 71.903 Mt @ 11.8 g/t Au for 633 t Au (?),
Inferred resources - 72.42 Mt @ 9.1 g/t Au for 657 t Au.
Harmony West Wits Operations are some 85 km SW of Johannesburg Gauteng and North West Provinces and comprises the Elandsrand, Cooke 1, Cooke 2, Cooke 3 and Doornkop Mines. Three main reefs have been identified in the district, the Ventersdorp Contact Reef (VCR), the Carbon Leader Reef (CLR) and the Mondeor Reef. Only the VCR is economic to mine and has been exploited at depths of from 1600 to 2800 m below the surface, extending to 3300 m at Elandsrand Mine. The VCR and CLR consist of
narrow (20 to 200 cm) tabular orebodies of quartz pebble conglomerates hosting gold, with extremelateral continuity.
At the Elandsrand Mine, the vertical separation between the VCR and CLR increases east to west from 900 to 1300 m due to the angle between the VCR unconformity surface and the regional strike and dip. The CLR strikes west-southwest and dips to the south at 25°. The VCR strikes ENE, with a regional dip of 21° SSE. Local variations in dip are largely due to the terrace-and-slope paleotopography surface developed during VCR deposition. The dip of the VCR at Deelkraal Mine is relatively consistent at 24°
The Randfontein Section (collectively the Cooke 1, 2, 3 and 4 Mines and Doornkop Mines) is situated in the West Rand Goldfield of the Witwatersrand Basin, where the Witpoortjie and Panvlakte Horst blocks are superimposed over
broad folding associated with the southeast plunging West Rand Syncline. The structure in the north part of the Randfontein Section is dominated by a series of NE trending dextral wrench faults. The Randfontein Section contains six identified main reef groupings: the Black Reef; the VCR; the Elsburg Formations; the Kimberleys; the Livingstone Reefs; and the South Reef. Within these, several economic reef horizons have been mined at depths of between 600 and 1260 m below the surface. The reefs comprise fine to coarse grained pyritic mineralisation within well developed thick quartz pebble conglomerates or narrow single pebble lags, which in certain instances are replaced by narrow carbon seams (SRK, 2007).
In 2006 reserve and resource figures (SRK, 2007) totalled:
Proved + probable reserves - 40.252 Mt @ 7.3 g/t Au for 293 t Au,
Measured + indicated resources - 63.289 Mt @ 10.3 g/t Au for 652 t Au,
Inferred resources - 50.74 Mt @ 8.8 g/t Au for 446 t Au.
Harmony Target Operations, are located in the Free State Province, some 270 km SW of Johannesburg. The gold mineralisation exploited by the Target Mine is hosted within a succession of Elsburg and Dreyerskuil quartz pebble conglomerate reefs hosted by the Van Heeversrust and Dreyerskuil Members of the Eldorado Formation, respectively, while additional resources occur in the Big Pebble Reefs (BPR) of the underlying Kimberley Formation. All of these units are within the Turffontein Sub-group of the Central Rand Group. The bulk of the resources in the Target Mine are within the Eldorado fan, a structure with dimensions of some 135 m vertically, 450 m down-dip and 500 m along strike. This fan is connected to the subsidiary Zuurbron fan, located between the Target Mine and Loraine, by a thinner and lower grade sequence of Elsburg reefs termed the Interfan area. To the north of the Eldorado fan, a number of fans are known, including the Siberia and Mariasdal fans. A number of faults displace the reefs of the Target Mine, the most prominent of which are the north-south trending Eldorado fault and the east-west trending Dam and Blast faults which offset and divide the mine into a number of mining areas, the Paradise, Siberia and Mariasdal areas. An additional north-south trending fault, the Twin fault has uplifted the distal portions of the reefs. North of the Mariasdal fault, the reef horizons are at depths of >2500 m below surface. Mineral Resources have been delineated over a strike lenght of up to 15 km north of the Target Mine. Approximately 40 km north of Target Mine, drilling has intersected gold bearing reefs in the Oribi area close to the town of Bothaville, where mineral resources have been delineated on the VCR and Elsburg Reefs at depths of ~2750m below surface (SRK, 2007).
In 2006 reserve and resource figures (SRK, 2007) totalled:
Proved + probable reserves - 19.281 Mt @ 7.0 g/t Au for 135 t Au,
Measured + indicated resources - 133.624 Mt @ 7.4 g/t Au for 985 t Au,
Inferred resources - 127.203 Mt @ 7.0 g/t Au for 895 t Au.
Harmony Free State Operations, are located in the Free State province, some 270 km SW of Johannesburg, within the Free State Goldfield on the SW edge of the Witwatersrand Basin. They are located on the SW and SE limb of a synclinal closure, with the Brand, Saaiplaas and Masimong shafts occupying northerly extensions of the same structure. The reefs dip inwardly from their sub-outcrop positions in the east and south of the mining area to a position close to the western boundary of the original Harmony mine, where the reefs abut against the De Bron fault. To the west of the De Bron faulted zone, faulting is generally more intense (SRK, 2007).
In 2006 reserve and resource figures (SRK, 2007) totalled:
Proved + probable reserves - 25.548 Mt @ 5.0 g/t Au for 125 t Au,
Measured + indicated resources - 37.835 Mt @ 7.2 g/t Au for 272 t Au,
Inferred resources - 132.2 Mt @ 6.0 g/t Au for 795 t Au.
Harmony Evander Operations, are located in Mpumalanga Province, some 120 km ESE of Johannesburg and include the Evander 2, 5, 7 and 8 Mines, and the Rolspruit and Poplar Projects. These operations are situated within the Evander basin, a discrete easterly extension of the main Witwatersrand Basin, where the only economic reef mined is the Kimberley Reef. In addition to the faulting of the reef horizon, there are numerous dykes and sills that complicate continuity, the most significant of which is an extensively developed dolerite footwall sill that occasionally intersects the Kimberley Reef, causing displacements (SRK, 2007).
In 2006 reserve and resource figures (SRK, 2007) totalled:
Proved + probable reserves - 56.169 Mt @ 7.6 g/t Au for 450 t Au,
Measured + indicated resources - 55.455 Mt @ 12.4 g/t Au for 685 t Au,
Inferred resources - 29.32 Mt @ 10.9 g/t Au for 335 t Au.
Harmony Orkney Operations, are located in the North West Province, near Klerksdorp, some 175 km SW of Johannesburg and comprise the Orkney 2, 4, 6 and 7 Mines, where the Vaal Reef is the most significant reef mined. This reef strikes NE, dips SE and is heavily faulted to form aseries of graben structures. Dips are generally <30° but vary locally in direction and magnitude to >45°. The VCR Elsburg Reefs are also exploited. There are several major faults in the lease area, typically with throws of tens of metres which further divide the reef into blocks of up to 100 m in width (SRK, 2007).
In 2006 reserve and resource figures (SRK, 2007) totalled:
Proved + probable reserves - 6.751 Mt @ 6.0 g/t Au for 40 t Au,
Measured + indicated resources - 27.271 Mt @ 9.6 g/t Au for 262 t Au,
Inferred resources - 15.93 Mt @ 6.1 g/t Au for 96 t Au.
The most recent source geological information used to prepare this decription was dated: 2007.
This description is a summary from published sources, the chief of which are listed below.
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Selected References:
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Agangi A, Hofmann A and Wohlgemuth-Ueberwasser C C, 2013 - Pyrite Zoning as a Record of Mineralization in the Ventersdorp Contact Reef, Witwatersrand Basin, South Africa: in Econ. Geol. v.108 pp. 1243-1272
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Antrobus E S A 1986 - The South Rand Goldfield: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v1 pp 689-703
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Antrobus E S A, Brink W C J, Brink M C, Caulkin J, Hutchinson R I, Thomas D E, van Graan J A, Viljoen J J 1986 - The Klerksdorp Goldfield: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v1 pp 549-598
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Berlenbach J W, 1995 - Aspects of bedding-parallel faulting associated with the Ventersdorp contact reef on the Kloof gold mine : in S. Afr. J. Geol. v98 pp 335-348
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Berlenbach J W, 1995 - Underthrusting in the Kloof gold mine : in S. Afr. J. Geol. v98 pp 35-42
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Drennan G R, Robb L J, Meyer F M, Armstrong R A and de Bruiyn H, 1990 - The nature of the Archaean basement in the hinterland of the Witwatersrand Basin; II, A crustal profile west of the Welkom Goldfield and comparisons with the Vredefort crustal profile: in S. Afr. J. Geol. v93 pp 41-53
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Ebert L B, Robbins E I, Rose K D, Kastrup R V, Scanlon J C, Gebhard L A and Garcia A R 1990 - Chemistry and palynology of carbon seams and associated rocks from the Witwatersrand goldfields, South Africa : in Ore Geology Reviews v5 pp 423-444
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Els B G 1991 - Placer formation during progradational fluvial degradation: the Late Archean Middelvlei Gold Placer, Witwatersrand, South Africa: in Econ. Geol. v86 pp 261-277
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Els B G and Beukes N J, 1987 - Sedimentology of the Middelvlei Placer, Doornfontein Gold Mine, Witwatersrand Supergroup: in S. Afr. J. Geol. v90 pp 72-85
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Els B G, 1988 - Textural and mineralogical composition of some fluvial Witwatersrand rocks of the West Wits Line Goldfield: in S. Afr. J. Geol. v91 pp 543-549
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Engelbrecht C J, Baumbach G W S, Matthysen J L, Fletcher P 1986 - The West Wits Line: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v1 pp 599-648
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Frimmel H E 2005 - Archaean atmospheric evolution: evidence from the Witwatersrand gold fields, South Africa: in Earth Science Reviews v70 pp 1-46
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Frimmel H E, le Roex A P, Knight J, Minter W E L, 1993 - A case study of the postdepositional alteration of the Witwatersrand Basal Reef gold placer: in Econ. Geol. v88 pp 249-265
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Gartz V H, Frimmel H E 1999 - Complex metasomatism of an Archean placer in the Witswatersrand Basin, South Africa: the Ventersdorp contact reef - a hydrothermal aquifer?: in Econ. Geol. v94 pp 689-706
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Gray N K, Tucker R F, Kershaw D J 1994 - The Sun Project - I. Discovery of a major new Witwatersrand goldfield: in XVth CMMI Congress, Johannesburg, SAIMM, 1994 v3 pp 95-102
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Hallbauer D K 1986 - The mineralogy and geochemistry of Witwatersrand pyrite, gold, uranium, and carbonaceous matter: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v1 pp 731-752
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Henckel J and Schweitzer J K, 1990 - The identification of provenance-controlled facies by geochemical methods on a portion of the Vaal Reef, Klerksdorp Goldfield : in S. Afr. J. Geol. v93 pp 602-610
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Hutchinson R W and Viljoen R P, 1988 - Re-evaluation of gold source in Witwatersrand ores : in S. Afr. J. Geol. v91 pp 157-173
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Jolley S J, Freeman S R, Barnicoat A C, Phillips G M, Knipe R J, Pather A, Fox N P C, Strydom D, Birch M T G, Henderson I H C, Rowland T W, 2004 - Structural controls on Witwatersrand gold mineralisation: in J. of Structural Geology v26 pp 1067-1086
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Killick A M and Roering C, 1994 - Bedding-parallel simple shear deformation at Randfontein Estates gold mine, Cooke section, Witwatersrand Basin, South Africa : in S. Afr. J. Geol. v97 pp 228-231
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Large R R, Meffre S, Burnett R, Guy B, Bull S, Gilbert S, Goemann K and Danyushevsky L, 2013 - Evidence for an Intrabasinal Source and Multiple Concentration Processes in the Formation of the Carbon Leader Reef, Witwatersrand Supergroup, South Africa: in Econ. Geol. v.108 pp. 1215-1241
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Malitc K N, Merkle R K W, 2004 - Ru-Os-Ir-Pt and Pt-Fe alloys from the Evander Goldfield, Witwatersrand Basin, South Africa: detrital origin inferred from compositional and osmium-isotope data - **CURRENTLY UNAVAILABLE**: in The Canadian Mineralogist v42 pp 631-650
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Meyer F M, Robb L J, Oberthur T, Saager R and Stupp H D, 1990 - Cobalt, nickel, and gold in pyrite from primary gold deposits and Witwatersrand reefs: in S. Afr. J. Geol. v93 pp 70-82
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Minter W E L and Loen J S, 1991 - Palaeocurrent dispersal patterns of Witwatersrand gold placers : in S. Afr. J. Geol. v94 pp 70-85
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Minter W E L, Goedhart M, Knight J, Frimmel H E 1993 - Morphology of Witwatersrand gold grains from the Basal Reef: Evidence for their detrital origin: in Econ. Geol. v88 pp 237-265
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Minter W E L, Hill W C N, Kidger R J, Kingsley C S, Snowden P A 1986 - The Welkom Goldfield: in Anhaeusser C R, Maske S, (eds), Mineral Deposits of Southern Africa Geol. Soc. of South Africa, Johannesburg v1 pp 497-539
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Myers R E, McCarthy T S, Stanistreet I G 1990 - A tectono-sedimentary reconstruction of the development and evolution of the Witwatersrand Basin, with particular emphasis on the Central Rand Group: in S. Afr. J. Geol. v93, no. 1 pp 180-201
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Nwaila, G.T., Bourdeau, J.E., Jinnah, Z., Frimmel, H.E., Bybee, G.M., Zhang, S.E., Manzi, M.S.D., Minter, W.E.L. and Mashaba, D., 2021 - The Significance of Erosion Channels on Gold Metallogeny in the Witwatersrand Basin (South Africa): Evidence from the Carbon Leader Reef in the Carletonville Gold Field: in Econ. Geol. v.116, pp. 265-284.
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Oberthur T, Saager R 1986 - Silver and Mercury in Gold particles from the Proterozoic Witwatersrand placer deposits of South Africa: metallogenic and geochemical implications: in Econ. Geol. v81 pp 20-31
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Parnell J 2001 - Paragenesis of mineralization within fractured pebbles in Witwatersrand conglomerates: in Mineralium Deposita v36 pp 689-699
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Phillips G N and Law J D M 1994 - Metamorphism of the Witwatersrand gold fields: A review: in Ore Geology Reviews v9 pp 1-31
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Phillips G N, Law J D M, Myers R E 1990 - The role of fluids in the evolution of the Witwatersrand Basin: in S. Afr. J. Geol. v93, no. 1 pp 54-69
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Phillips G N, Myers R E 1989 - The Witwatersrand Gold Fields: Part II. An origin for Witwatersrand gold during metamorphism and associated alteration: in Econ. Geol. Mono. 6 pp 598-608
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Phillips G N, Myers R E, Law J D M, Bailey A C, Cadle A B, Beneke S D, Guisti L 1989 - The Witwatersrand Gold Fields: Part I. Post depositional history, synsedimentary processes and gold distribution: in Econ. Geol. Mono. 6 pp 585-597
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Phillips G N, Zhou T and Powell R, 1997 - Metamorphic temperature variations among Witwatersrand goldfields; evidence from the pyrophyllite-chloritoid-chlorite mineral assemblage : in S. Afr. J. Geol. v100 pp 393-404
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Phillips GN, Law J D M, and Stevens G, 1997 - Alteration, heat, and Witwatersrand gold; 111 years after Harrison and Langlaagte: in S. Afr. J. Geol. v100 pp 377-392
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Pretorius D A 1991 - The sources of Witwatersrand gold and uranium: A continued difference of opinion (extracts): in Econ. Geol. Monograph 8 pp 139-145, 160-163
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Pretorius D A 1986 - The goldfields of the Witwatersrand Basin: in Anhaeusser C R, Maske S, (eds), Mineral Deposits of Southern Africa Geol. Soc. of South Africa, Johannesburg v1 pp 489-493
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Pretorius D A 1974 - The nature of the Witwatersrand gold-uranium deposits: in Wolf K H, (ed), Handbook of Stratabound and Stratiform Ore Deposits, Elsevier v7 pp 29-88
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Robb L J and Meyer F M, 1996 - The Witwatersrand Basin, South Africa: Geological framework and mineralization processes : in Ore Geology Reviews v10 pp 67-94
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Robb L J and Meyer M, 1987 - The nature of the Archaean basement in the hinterland of the Witwatersrand Basin; I, The Rand Anticline between Randfontein and Rysmierbult : in S. Afr. J. Geol. v90 pp 44-63
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Robb L J, Meyer F M 1994 - Geological environment and mineralisation processes during the formation of the Witwatersrand Au-U deposits: in XVth CMMI Congress, Johannesburg, SAIMM, 1994 v3 pp 3-18
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Robb L J, Meyer F M, Ferraz M F and Drennan G K, 1990 - The distribution of radioelements in Archaean granites of the Kaapvaal Craton, with implications for the source of uranium in the Witwatersrand Basin : in S. Afr. J. Geol. v93 pp 5-40
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Stevens G, Boer R and Gibson R, 1997 - Metamorphism, fluid-flow, and gold mobilization in the Witwatersrand Basin; towards a unifying model: in S. Afr. J. Geol. v100 pp 363-375
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Stewart R A, Reimold W U and Charlesworth E G, 2004 - Tectonosedimentary model for the Central Rand Goldfield, Witwatersrand Basin, South Africa: in S. Afr. J. Geol. v107 pp 603-618
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Tainton S 1994 - A review of the Witwatersrand Basin and trends in exploration: in XVth CMMI Congress, Johannesburg, SAIMM, 1994 v3 pp 19-45
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Tucker R F, Schlegel G C-J, Wagener J H F, Gray N K 1994 - The Sun Project - II. Scientific and technological innovations in a Witwatersrand exploration venture: in XVth CMMI Congress, Johannesburg, SAIMM, 1994 v3 pp 103-116
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Tweedie E B 1986 - The Evander Goldfield: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v1 pp 705-730
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Tweedie K A M 1986 - The discovery and exploration of Beisa and Beatrix gold and uranium mines in the southern extension of the Welkom goldfield: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v1 pp 541-547
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