South Deep is located in the Far West Rand Goldfield on the north-western rim of the Witwatersrand Basin. This basin comprises a 6,000 metre-thick sequence of predominantly clastic sedimentary rocks, the upper part of which, the Central Rand Group, is characterised by the occurrence of auriferous and uraniferous quartzpebble conglomerates (reefs) that are sporadically interspersed between finer grained quartzitic units. All major reef units are developed above unconformity surfaces. The angle of unconformity is typically greatest near the basin margin and decreases toward more distal areas.

The reefs are considered to represent extensive fluvial deposits into a yoked basin, some 350 kilometres long in an east-north-easterly direction, and 200 kilometres wide in a northnorth-westerly direction. The reefs are continuous as a consequence of the regional nature of the erosional surfaces. Preferential reef development within channel systems and sedimentary features such as facies variations and channel frequency assist in mapping out local gold distributions.

The gold is deemed to be primarily of detrital origin, deposited syngenetically with the conglomerates. Although the gold generally occurs in native form and is usually associated with pyrite and carbon, most of it has been subsequently modified and locally remobilised during secondary hydrothermalism. The most fundamental control to the gold distribution remains the association with quartz-pebble conglomerates on intrabasinal unconformities. The Modified Palaeoplacer Model is the favoured mineralisation model that is currently in use.

South Deep’s mining methods are unique and require ongoing optimisation and refinement. The mining value chain has significant potential for productivity and efficiency improvements, as the mining methods are bedded down. The high profile destress method was successfully implemented during 2017, and the embedding of this method continues.

Current production is sourced from two primary mining areas, Current Mine (CM) and North of Wrench (NoW). CM is characterised by selective mining methods scattered over a large area whereas the NoW area focuses on six corridors migrating southwards with bulk non-selective mining methods.

The CM area is defined by the previously conventionally mined area that serves as destress for mechanised massive mining above and below these horizons in the remaining Elsburg reefs. Mining activities are largely scattered over the whole of the CM area. These areas are accessed with normal development from existing ramps and extracted with drifts, benches and long-hole stopes. Due to the nature of mining here and risks associated with mining between other old excavations, the resource to reserve conversion is generally low, however, feasible mining and extraction methods have the potential to mine these areas, and will be further assessed and developed where appropriate.

The NoW area is largely unmined, and an improved mining method is applied here designed for the efficient extraction of the thick ore body without being impacted by historical excavations, backfilled areas or old pillars. Applying an optimal bulk extraction method here maximises the gold ounce extraction and reduces mining complexity. Owing to South Deep’s depth, a destress cut is required to lower the in situ stress, such that mechanised bulk mining can be conducted. This cut also serves as access for bulk long-hole stoping. The 5.5 metre high destress cut is mined horizontally into the ore body with mechanised equipment similar to a room and pillar layout. From the destress cut, a long-hole stoping method is applied to extract the remainder of the Elsburg reefs to the cut above or the limit of the target zone. Thinner target areas for stoping above and below the destress cuts are mined with drifts and benches similar to CM. The combination of these methods, applied in NoW, significantly improves the resource to reserve conversion rate and forms the basis of South Deep’s future.

All stoping areas are backfilled after completion to increase geotechnical stability of surrounding rock mass.

The milling circuit includes a semi-autogenous grinding (SAG) mill for primary and secondary milling, which is conducted in two overflow ball mills. Classification is done using a cluster of cyclones. Four Knelson concentrators facilitate the recovery of free gold and the concentrate is tabled in the smelthouse using a Gemini Table. The cyclone overflow is thickened before the slurry reports to the leach circuit, which has 10 mechanically agitated tanks, each with a 3,000 cubic metres volume.

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 bullion. Concentrates from the Gemini Table are calcined and smelted in the induction furnace. The gold bars are marked and dispatched to the Rand Refinery.