Summary:
Moab Khotsong is situated within the Klerksdorp Goldfield on the western margin of the Witwatersrand Basin of South Africa, one of the most prominent gold provinces in the world. The major gold bearing conglomerate reefs are mostly confined to the Central Rand Group (CRG) of the Witwatersrand Supergroup.
The geology at Moab Khotsong is structurally complex, with large fault-loss areas between the three mining areas (top mine (Great Noligwa), middle mine and lower mine (growth project and Zaaiplaats project in execution phase).
The Vaal Reef is the primary economic horizon at Moab Khotsong. A secondary economic horizon, the C-Reef, contributes less than 5% of total mining volumes. Both reefs are narrow tabular deposits forming part of the Witwatersrand Supergroup and are stratigraphically located near the middle of the Central Rand Group. The Vaal Reef lies approximately 255m below the C-Reef.
The CRG is up to 2,100m thick in the Vaal River area and the general orientation of the Witwatersrand Supergroup succession in this goldfield is interpreted as southwest-trending and southeast dipping. A series of northeast-trending faults including the Buffelsdoorn, the Kromdraai, the Buffels East and the Jersey Faults, is a key feature of the Klerksdorp Goldfield and the key structural features at Moab Khotsong are related to this series of faults.
Moab Khotsong exploits gold mineralisation occurring in the VR. This reef is stratigraphically located near the top of the Johannesburg Sub-group, within the CRG. The VR ranges in depth at Moab Khotsong from 1,500m BMD to 3,400m BMD. Gold mineralisation also occurs in the stratigraphically higher C-Reef, which lies approximately 225m above the VR. However, the CReef typically contributes less than 5% to the mining production.
The geology at Moab Khotsong is structurally complex with large fault-loss areas between the three mining areas (top mine, middle mine and Zaaiplaats). The geological setting is one of crustal extension, dominated by major south-dipping fault systems with north-dipping Zuiping faults wedged between the south-dipping faults. The De Hoek and Buffels East faults are structural bounds for the reef blocks of the middle mine to the north-west and south-east respectively. The northern boundary of Moab Khotsong’s middle mine is the north-dipping Zuiping fault. Moab Khotsong requires a reduced drill spacing pattern of the order of 50m x 50m, which allows for accurate delineation of the structurally bound mineable blocks so that accurate and efficient mine designs can be implemented to ensure optimal extraction and maximum orebody use.
The mineralisation model adopted for the deposit is that of gold precipitation in the conglomerates through the actions of hydrothermal fluids. The fluids precipitated gold and other elements through reactions that took place at elevated temperatures (300-350oC). Migrating liquid and gaseous hydrocarbons precipitated as solid hydrocarbon (carbon), which was then mesophased through metamorphism and structural deformation. Carbon was preferentially precipitated in beddingparallel fractures that most commonly followed the base of the Vaal Reef package (A-bottom sub-facies); however, gold and uranium mineralisation is also commonly observed within the A-middle and A-top sub-facies of the Vaal Reef. Gold was precipitated very soon after the carbon, giving the critical gold-carbon association that characterises many of the highgrade Vaal Reef localities.
Carbon was preferentially precipitated in bedding-parallel fractures that most commonly followed the base of the Vaal Reef package (A-bottom sub-facies), however, gold and uranium mineralisation is also commonly observed within the A-middle and A-top sub-facies of the Vaal Reef. Gold was precipitated very soon after the carbon, giving the critical gold-carbon association that characterises many of the high-grade Vaal Reef localities.
A geological model is employed to delineate variations (either lateral or vertical) in characteristics of the Vaal Reef and C Reef. The current geological model thus sub-divides these two reefs into homogeneous zones based on geological and grade characteristics.
The Vaal Reef consists of a thin basal conglomerate (the C-facies) and a thicker sequence of upper conglomerates (A-facies). These two sedimentary facies are separated by the B-facies, which is a layer of barren orthoquartzite. The A-facies is the primary economic horizon at Moab Khotsong; however, remnants of the C-facies are sporadically preserved below the A-facies. High gold values in the Vaal Reef are often located at the base of this unit and are associated with high uranium values and the presence of carbon. Uranium is an important by-product recovered from the Vaal Reef.
The C Reef is mined on a limited scale in the central part of top mine where a high-grade, north-south trending sedimentary channel, containing two economic horizons, has been exposed. To the east and the west of this channel, the C Reef is poorly developed with limited areas containing economic concentrations of gold and uranium. As with the Vaal Reef, high uranium values are also often associated with high gold values. A carbon seam, with a thickness of 5mm to 20mm, commonly
occurs at the base of the conglomerate.
To the north of the mine, the C Reef sub-crops against the Gold Estates Conglomerate Formation and, in the extreme south of the mine, the C Reef has been eliminated by a deep Kimberley erosion channel and the Jersey fault.