South Africa region - Regional geology
South Deep is located in the 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 quartz-pebble 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 most fundamental control to the gold distribution remains the association with quartz-pebble conglomerates on intra-basinal unconformities. The modified palaeoplacer model is the favoured mineralisation model that is currently in use.
The Witwatersrand Basin 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 north-north-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. Refer to the asset fundamentals table above for information on local geology and mineralisation style.