Driefontein is a deep level underground gold mine located along the northern and southwestern margins of the Witwatersrand Basin in South Africa. This mine is typical of the many Witwatersrand Basin operations, which have been the primary contributors to South Africa’s production of a significant portion of the world’s recorded gold output since 1886.

The orebodies at Driefontein are laterally continuous with relatively long-range predictability. This lends to clear patterns of mineralisation governed by sedimentary characteristics.

The principle mining at Driefontein takes place on the Multiband Carbon Leader Reef. In most cases, the reef was deposited along a structurally controlled basin edge, which created discrete unconformable surfaces of deposition. This led to typical main channel, wide reef packages, which are generally >2m thick and bottom loaded.

The Witwatersrand Basin comprises a 6,000-meter vertical thickness of sedimentary rocks, extending laterally for some 350 kilometres northeast to southwest by some 120 kilometres northwest to southeast, generally dipping at shallow angles toward the centre of the Witwatersrand Basin. The Witwatersrand Basin outcrops at its northern extent near Johannesburg, but to the west, south and east it is overlaid by up to 4,000 meters of volcanic and sedimentary rocks. The Witwatersrand Basin is Archaean in age, meaning the sedimentary rocks are of the order of 2.8 billion years old.

Gold mineralisation occurs within laterally extensive quartz pebble conglomerate beds called reefs, which are developed above unconformable surfaces near the basin margin. As a result of faulting and primary controls on mineralisation processes, the goldfields are not continuous and are characterised by the presence or dominance of different reef units. The reefs are generally less than two meters in thickness and are widely considered to represent laterally extensive braided fluvial deposits or unconfined flow deposits, which formed along the flanks of alluvial fan systems around the edge of an inland sea. Dykes and sills of diabase or dolerite composition are developed within the Witwatersrand Basin and are associated with several intrusive and extrusive events.

Gold generally occurs in native form, often associated with pyrite, carbon and uranium. Pyrite and gold within the reefs display a variety of forms, some obviously indicative of detrital transport within the depositional system and others suggesting crystallisation within the reef itself.

As early as 1923, the presence of uranium was noted in the Witwatersrand reefs. It was found that on average the reefs contain about 0.03% uranium and as a by-product of gold relatively low uranium grades can be recovered. Notwithstanding different opinions as to the origin of the uranium in the reefs, most theories accept localisation of both gold and uranium a function of sedimentary textures. Metal concentrations are directly related to the reefs. Exploration programmes and eventual evaluation of gold and uranium according to a placer philosophy, prove to be highly successful.

The most fundamental controls of gold and uranium distribution are the primary sedimentary features such as facies variation and channel directions. Consequently, the modeling of sedimentary features within the reefs and the correlation of payable grades within certain facies is key to in situ reserve estimation, as well as effective operational mine planning and grade control.

Scattered stoping, mini-longwall stoping with closely spaced dip pillars (140m x 40m and 130m x 30m regional pillars) and surface rock-dump mining.

Driefontein has three metallurgical plants exploiting the Carbon Leader Reef, the Ventersdorp Contact Reef and the Middelvlei Reef.

The Driefontein 1 Plant treats underground ore and has a processing capacity of 240,000tpm. The upgraded CIP circuit at the No 1 Plant consists of a semi-autogenous grinding (SAG) mill circuit followed by cyanide leaching, CIP and a central elution facility.

The Driefontein 2 Plant processes SRD material, which is delivered by rail and truck. Plant flow incorporates two SAG mills and a ball milling circuit, cyanide leaching and a CIP plant. A CIL circuit was commissioned in 2014 at the No 2 Plant to improve recoveries by replacing the aging CIP circuit.

The Driefontein 3 Plant was originally designed as a uranium plant but was converted to process low-grade surface rock in 1998. Similar to the No 2 Plant, SRD ore is delivered by rail and truck. The plant has four SAG mills followed by cyanide leaching and a CIP circuit.