Summary:
South Deep is hosted in the Central Rand Group of the Witwatersrand Basin, which comprises gold-bearing quartz-pebble conglomerates deposited along unconformities. These conglomerates are interpreted as fluvial in origin and are laterally continuous, with gold localised in high-energy channel systems. The basin is a synclinal structure with a broad east-west orientation, and mineralisation at South Deep is typically associated with intraformational erosional surfaces within the conglomerate reefs.
Gold mineralisation in the Witwatersrand Basin occurs within uraniferous quartz pebble conglomerate horizons, termed reefs. These reefs occur within seven separate goldfields located along the eastern, northern, and western margins of the basin known as the Evander, East Rand, Central Rand, West Rand, Far West Rand, Klerksdorp and Free State Goldfields respectively. Typically, within each goldfield there are one or two major reef units present, which may be overlain by one or more secondary reef units. As a result of geological faulting and other primary controls on mineralisation, the goldfields are not continuous and are characterized by the presence or dominance of different reef units. The reefs are generally less than 2 m 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 what was effectively an ancient inland sea.
The gold is deemed 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 remobilized on a very local scale, typically less than a metre, by secondary hydrothermal processes.
Most early theories considered that the gold was deposited syngenetically with the conglomerates, but subsequent research has confirmed that metamorphism within the Witwatersrand Basin caused some limited post depositional redistribution of gold. Other experts regard the gold to be totally epigenetic and deposited solely by hydrothermal fluids, sometime after deposition of the reef sediments.
Despite these varied viewpoints, the most fundamental control to the gold distribution remains the association with quartz pebble conglomerates on intra-basinal unconformities. The reefs are extremely continuous due to the regional nature of the erosional surfaces. Bedrock (footwall) controls govern the distribution of many of the reefs. Preferential reef development within channel systems and sedimentary features such as facies variations and channel frequency assist in mapping out local gold distributions. The drilling, mapping, identification and modelling of erosional and sedimentary features is the key to developing meaningful geological and geozone models required to underpin robust Mineral resource estimation.
There are five primary reefs developed within the Central Rand Group. In chronological order, from older to younger, they are: South Reef, Kimberly’s, Lower Elsburg, Middle Elsburg and Upper Elsburg. Deposition of these reefs commenced at the north with the alluvial fans of the various reefs progressing southwards with time.
The Upper Elsburg reef at South Deep is anomalous with respect to the other Witwatersrand reefs developed within the basin due to its massive, divergent nature at depth below the VCR. The other reefs are typical Witwatersrand deposits representing narrow, tabular orebodies with distinct channelisation consistent with fluvial dynamics of the Witwatersrand.
The reefs at South Deep are typically massive in nature with little or no evidence of channelisation and are attributed to a sheet flood type deposit in a very high energy environment. This is believed to be a result of rapid and frequent movement of the syn-sedimentary, east-west trending faults in the vicinity giving rise to a high energy fluvial environment.
In the western half of the South Deep mining right, the VCR reef occurs as a single reef horizon at the base of Ventersdorp lava that overlies footwall lithologies of the Central Rand Group. The Upper Elsburg reef, which subcrops with the VCR in a north-northeast trend, comprises multiple stacked reef horizons that form part of an easterly divergent clastic wedge. This wedge attains a maximum thickness of approximately 120 m to the east and truncates against the VCR to the west.
The Upper Elsburg reef comprises up to 16 stratigraphic horizons with alternating quartzite and conglomeratic units. This divergent wedge is believed to be the resultant of rapid movement on syn-sedimentary, east-west trending faults resulting in a series of uplifts with associated transgressions and regressions giving rise to the alternating conglomerate and quartzite units and the staggered nature of the sub-cropping of the various units against overlying units. Due to the high energy environment, a sheet flood type deposit has developed resulting in areas of elevated grades, termed pay trends, being orientated in a north-south direction which is in contrast with a typical Witwatersrand deposit with pay trends aligned to the palaeocurrent direction where distinct channels are present.
The mine is also traversed by a series of intrusive dykes and geological faults which are of differing ages. The dykes are generally north-south and east-west trending. The younger north-south dykes are approximately 30 m thick and are not associated with any notable displacement. The east-west dykes are commonly associated with displacement in the magnitude of a few metres and range in thickness from 0.5 m to 3.0 m.
Faulting also has two prominent trends, namely east-west and north-south, with the latter being older and displaced by the east-west faults. The faults are steeply dipping with a normal throw and an average magnitude of vertical displacement that rarely exceeds 5 m. However, the most significant displacement is in the lateral sense which is clearly evidenced by displacement of the various dykes and to a lesser extent variance in channel width across faults. The minor faults are generally confined to the reef horizon and do not penetrate significantly into the footwall. The Wrench Fault has an approximate 180 m right lateral movement and between 110 m and 160 m upthrow to the south. The upthrow to the south benefits the mine by enabling a significant portion of the orebody to be accessed by current infrastructure.
Deposit types
Gold mineralisation occurs within uraniferous quartz pebble conglomerate horizons, termed reefs. The reefs are widely considered to represent laterally extensive braided fluvial deposits or unconfined flow deposits which formed along the flanks of alluvial fan systems or deltas. All major reef units are developed above stratigraphic unconformity surfaces.
The Upper Elsburg formation conglomerates constitute the target economic horizon at South Deep and constitute 100 per cent of the South Deep Mineral reserve ounces. The Upper Elsburg conglomerates sub crop or truncate below the Ventersdorp Contact Reef (VCR) in a north-north-west trend and are anomalous with respect to the other Witwatersrand reefs as they comprise multiple stacked reef horizons forming an easterly-divergent massive clastic wedge which attains a maximum thickness of approximately 120 m to 130 m in the vicinity of the eastern boundary of the mining right area. In the western half of the South Deep mining right, the Ventersdorp Contact Reef occurs as a single reef horizon that overlies the Turffontein Subgroup and is covered by the Ventersdorp lavas and the VCR contributes to the Mineral resource only at this juncture.
Commodity Production
In 2024, in addition to ore treated from underground - 1,618 kt, South Deep processed 1,173 kt from hydraulic mining tailings re-processing, and 211 kt of Waste was treated.
| Commodity | Units | 2025 | 2024 | 2023 | 2022 | 2021 | 2020 | 2019 | 2018 | 2017 | 2016 | 2015 |
|
Gold
|
kg
| ....  | .... | .... | .... | .... | .... | 6,907 | 5,220 | 8,748 | 9,032 | 6,160 |
All production numbers are expressed as metal in doré.
^ Guidance / Forecast.