Summary:
South Africa’s gold production is centred overwhelmingly on the Witwatersrand Basin, a 350km arcuate basin that stretches to the east and west of Johannesburg and southwards into the Free State. This basin comprises seven major discrete gold fields and has historically produced over 247 million of ounces of Gold.
The Witwatersrand Basin Project (WBP) comprises five distinct project areas and overlays different reef horizons of the Central Rand Group. The five principal auriferous conglomerate units in the Central Rand Gold Field are the Main Reef, Main Reef Leader, South Reef and Bird Reefs, which occur as part of the Johannesburg subgroup. The fifth conglomerate unit, Kimberley Reefs, are locally important and forms part of the Turffontein subgroup, which overlies the Johannesburg subgroup.
The WBP has been split in two, i.e. the east and west sides. The re-evaluation focussed on the eastern portion of the project, namely the Bird Reef (BR), Main Reef Leader (MRL) and Main Reef (MR) mineral resources. This dividing line between the East and West is defined by a large fault that displaces the reef horizons with approximately 350m on surface. The area now outlined as WBP East was historically mined by Rand Leases.
All three reefs (BR, MRL and MR) have extensive lengths of outcrop, with an approximate length of 2.2km outcrop inside West Wits’ Mining Right boundary. Historically, opencast mining already exploited the reefs on outcrop position. From the outcrop, the reefs dip at 35° south-southwest where they have been extensively mined since the early history of mining on the Witwatersrand.
The Witwatersrand Supergroup consists of the lower West Rand Group, comprising mainly shale with subordinate quartzite, and the upper, predominantly arenaceous Central Rand Group, in which the majority of the gold bearing conglomerates is located. Conglomerates comprise approximately 600 m or 8% of the total thickness of the Witwatersrand Supergroup (Pretorius, 1964). The majority of the Witwatersrand conglomerates occur in the Central Rand Group. The Central Rand Group contains the Kimberley Reefs which are the target reefs for mining at Qala Shallows.
The Kimberley Reef package consists of up to 17 individual conglomerate bands, separated by quartzites and grits, which vary in thickness from 0.45 m to 36 m (Pretorius, 1964). Individual conglomerate bands are lenticular and do not persist for great lengths along strike (Clay, 1988a). The average pebble size is considerably larger than in any of the underlying reefs (Clay, 1988a). The Kimberley conglomerates have only been mined economically in the western section of the Central Rand (Pretorius, 1964), but have recently been under investigation in the 3Cs (Crown Mines, Consolidated Main Reef and City Deep) area for possible open-cast mining operations. The grades associated with the Kimberley Reef packages are distributed erratically, both vertically and laterally. Gold accumulation increases proportionally with channel width (Clay, 1988b).
The Kimberley Reefs were mined extensively in the old DRD and RL gold mines. These reefs are found near the base of the Turffontein Subgroup and outcrop at surface through the centre of DRD and near the southern boundary of Rand Leases. Although numerous reefs contain gold values, mainly the K9B (Bottom) and K9A (Top) were exploited on both properties and the K8 towards the west of DRD. The K8 is commonly scoured by the K9B Reef but where the former is present a high gold tenor is associated with a small pebble conglomerate. The K9A Reef is a thin more erratically mineralised, medium pebble conglomerate some 10 m above the consistently mineralised, robust, large pebble K9B conglomerate. The K9B reef occurs stratigraphically below the K9A Reef.