Summary:
Tshepong South is situated in the Free State Goldfield, on the southwestern margin of the Witwatersrand Basin of South Africa, one of the most prominent gold provinces in the world. The major gold bearing conglomerate reefs are mostly confined to the CRG of the Witwatersrand Supergroup.
The general orientation of the Witwatersrand Supergroup succession in this goldfield is interpreted as north-trending, within a syncline that is plunging to the north. The syncline has been divided by faults into the Odendaalsrus, Central Horst and Virginia sections. The Tshepong South mining right area is also affected by the Tribute fault (~270m) and Arrarat Fault (~180m) to the far south-east of the shaft.
Tshepong South exploited primarily the Basal Reef, which occurs within the Harmony Formation of the Johannesburg Subgroup of the CRG.
Mineralization also occurs within the stratigraphically higher A and B reefs of the Kimberley (formerly Aandenk) Formation, within the Turffontein subgroup of the CRG. However, only the B Reef can be economically extracted.
Mineralization is associated with the presence of medium to coarse, clast-supported oligomictic pebble horizons. The presence of allogenic pyrite and detrital carbon is also common.
The principal gold-bearing orebody is the stratiform and strata-bound Basal Reef (known as the Basal Reef Zone or BRZ). This unit comprises a thin conglomerate at the base of the BRZ, overlain by clean “placer” quartzites. The Basal Reef is underlain by a thick series of siliceous and argillaceous quartzites comprising the Welkom formation and overlain by shales and quartzites of the Harmony formation, both of the Johannesburg sub-group of the Central Rand Group. Although not apparent within the mine lease area, the Basal Reef sits unconformably on the Welkom formation.
The Basal Reef dips towards the east at 25° in the north and up to 45° in the south. The Lower Cycle Black Chert facies predominates in the north with a north-west south-east value trend. The reef consists of an oligomictic small pebble matrixsupported conglomerate lag with fly-speck carbon contact. The rest of the reef package constitutes barren siliceous fine-grained reef quartzite. The entire reef package reaches up to 160cm thick and is overlain by 1cm to 30cm of lower khaki shale. This in turn is overlain by the approximately 3-4m thick waxy brown leader quartzite, above which lies the 3-4m thick upper khaki shale.
The Upper Cycle Black Chert facies Basal Reef prevails in the south of the lease area, and consists of a slightly polymictic (yellow shale specks present), matrix-supported medium-pebble conglomerate with a more gradational contact absent of carbon where mineralisation is associated with fine disseminated and buckshot pyrite. The conglomerate is slightly thicker compared to the Lower Cycle, but is also overlain by barren reef quartzite, the entire package being characteristically up to only 40cm thick. The lower khaki shale is up to 1m thicker.
The Central Rand Group itself is overlain in turn by lavas and sediments of the Ventersdorp System and the more recent sediments of the Karoo Group.
Mineralization also occurs within the stratigraphically higher A and B reefs of the Kimberley (formerly Aandenk) Formation, within the Turffontein subgroup of the CRG. However, only the B Reef can be economically extracted.
Mineralization is associated with the presence of medium to coarse, clast-supported oligomictic pebble horizons. The presence of allogenic pyrite and detrital carbon is also common.
The B Reef occurs approximately 145m stratigraphically above the Basal Reef and varies in thickness from 30cm to 170cm. The conglomerate varies in character depending on the facies, with B1 being a small to medium pebble conglomerate and usually no more than 30cm thick with abundant carbon. The B2 facies is a small pebble lag in an argillaceous quartzite, with little to no mineralisation. B3 facies is a 20 to 150cm thick conglomerate, mature, well packed, with pebble sizes varying from small to cobble size, very polymictic, normally with abundant pyrite and some carbon. This is the most common facie.