The Witwatersrand Basin comprises a 6,000 metre vertical thickness of sedimentary rocks, extending laterally for some 350 kilometres northeast to southwest by some 1,200 kilometres northwest to southeast, generally dipping at shallow angles toward the centre of the basin. The basin outcrops at its northern extent near Johannesburg, but to the west, south and east it is overlaid by up to 4,000 metres 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.

Regionally, gold mineralisation occurs within laterally extensive quartz pebble conglomerate horizons 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 individual reefs that make up the stacked Upper Elsburg reef package that defines the South Deep ore body are generally less than two metres 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.

The most fundamental controls of gold distribution are the primary sedimentary features such as facies variation and channel directions. Consequently, the modelling 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 reef definition drilling programmes, operational mine planning and grade control.

Gold mineralisation at South Deep is hosted by conglomerates of the Upper Elsburg reefs and the Ventersdorp Contact Reef (VCR). The Upper Elsburg reefs sub-crop against the VCR in a north-easterly trend, which defines their western limits. To the east of the sub-crop, the Upper Elsburg reefs are preserved in an easterly diverging sedimentary wedge attaining a total thickness of approximately 120 metres, which is subdivided into the lower “Individuals” and the overlying “Massives”. To the west of the sub-crop, only the VCR is preserved.

The stratigraphic units at South Deep generally dip southward at approximately 12 to 15 degrees and the gold-bearing reefs occur at depths of 1,500 metres to 3,500 metres below surface. In general, the gold mineralisation hosted by the conglomerates is laterally continuous with long range predictability and clear patterns of predictable mineralisation governed by sedimentary characteristics.

Production at South Deep is currently derived from the Upper Elsburg Reefs. In general terms, the Upper Elsburg succession represents an easterly prograding sedimentary sequence, with the Massives containing higher gold grades and showing more proximal sedimentological attributes in the eastern sector of the mining authorisation than the underlying Individuals. The sedimentary parameters of the Upper Elsburg reef units influence the overall tenor of the reefs with gold grade displaying a gradual, general decrease toward the east, away from the sub crop.

The North-South trending “normal” West Rand and Panvlakte faults, which converge on the Western side of the lease area, are the most significant large-scale faults in the area and form the western limit to gold mineralisation for the mine.

South Deep is an underground mechanised mine, using an owner mining workforce. Reserves are accessed through de-stress and shadow development cuts to manage rock stress and seismic activity. A number of selective mining methods, including drifts and benches, are employed but long-hole stoping with paste fill is the primary bulk mining method. Significant focus is on frontline coaching to improve compliance to plan and productivity. Mining method and extraction sequence optimisation is ongoing.

The workings are accessed from the surface through two shaft systems, the Twin Shaft complex (main and ventilation shafts), of which the main shaft comprises a single-drop to 110A level, a depth of 2,998m, the vent shaft to 110 level at a depth of 2,947m and the South Shaft complex, which is a subvertical system (three operating shafts) to 95 level at a depth of 2,786m.

The mine is divided into three main areas:

1. Current Mine (CM), characterised by selective mining methods scattered over a large area originally exploited by means of conventional tabular mining. CM is accessed from four active levels (90, 93 and 95) from both the South Shaft and Twin Shaft complexes.

2. The North of Wrench (NoW), directly south and down dip of CM, comprises six mining corridors separated by regional pillars that extend southwards to the Wrench fault. A bulk non-selective mining method is applied, resulting in a higher Resources to Reserves conversion ratio.

3. The South of Wrench (SoW) east and west areas, situated south and down dip of NoW, will be mined in the same manner as NoW.

South Deep has one active TSF, the Doornpoort TSF, and four dormant TSFs, known as TSF 1, 2, 3 and 4. TSF 2 is undergoing remining using hydraulic mining methods (ie using a water cannon). The top portion of TSF 1 has also been remined to date. TSFs 1 and 2 were commissioned in 1968 and operated as upstream raised paddock dams as is typical in South African gold mines. These TSFs cover a combined footprint of 69ha and have a maximum height of 47m. TSFs 3 and 4 were commissioned in 1982 and are also upstream raised paddock dams. These TSFs cover a combined footprint of 100ha and have a maximum height of 41m. Deposition on these TSFs ceased in 2011.

The Doornpoort TSF was commissioned in April 2011 and three upstream wall raises have been constructed. The Doornpoort TSF incorporates a gravity decant system with drainage structures placed beneath the tailings itself. This facility has a remaining LoM storage capacity of ~149Mt (Phase 2).

Milling
When ore enters the plant from underground it first needs to be milled to reduce its size so that the gold is separated from the host rock. South Deep’s milling circuit consists of a single-stage semi-autogenous grinding mill (SAG) and a pebble crusher for primary milling, followed by secondary milling using two overflow ball mills. These mills are essentially rotating drums that have steel balls inside which grind the rock into particles that are finer than 0.075mm in size.

The South Deep processing plant consists of a conventional semi-autogenous grinding (SAG)/ball milling circuit, a gravity gold recovery circuit and a conventional leach/Carbon in Pulp (CIP) circuit. Final product from both the gravity and CIP circuits is smelted into bullion.

For pre-processing tailings, the South Deep plant also includes a tailings retreatment section that consists of a thickener followed by a dedicated CIL circuit.

Cyanide is used for gold dissolution and lime is added to ensure protective alkalinity. An eight-stage, carousel-type carbon-in-pulp (CIP) circuit is used for gold adsorption. Each tank in the train is 200 cubic metres. Carbon stripping is achieved using the AARL elution system, while carbon regeneration is carried out in a rotary kiln at a rate of 500 kg/hr.

Gold is recovered from the solution by using electro-winning Sludge Reactors, and then dried. This is followed by smelting in an induction furnace to produce gold bul ........