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 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.
The Moab Khotsong deposit is classed a meta-sedimentary gold deposit. Folding and basin edge faulting have been important controls for sediment deposition and gold distribution patterns within the Witwatersrand Basin and fold trends have been employed in the economic evaluation of various reef horizons.
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.
Vaal Reef Lithology
The Vaal Reef consists of a thin basal conglomerate, termed the C-Facies, and a thicker upper sequence of conglomerates, termed the A-Facies. The A-Facies and the C-Facies are separated by the B-Facies, which is a barren orthoquartzite. The A-Facies is the primary economic horizon at Moab Khotsong; however, remnants of the C-Facies are sporadically mined where it is preserved below the A-Facies.
The Vaal Reef mining unit can reach thicknesses of up to 280cm, while the average channel width is approximately 90cm.
C Reef Lithology
The C Reef is an approximately 10cm thick conglomerate, which has a 5mm - 20mm thick carbon seam commonly occurring at the base. 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.
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.
Mineralization
The gold mineralization in the Witwatersrand deposits is believed to have followed an episode of deep burial, fracturing and alteration. The mineralization model is that Archean gold bearing hydrothermal fluid was introduced into the conglomerates and circulated throughout in hydrothermal cells. The fluids precipitated gold and other elements through reactions that took place at elevated temperatures along the reef horizons, which was the more favorable fluid conduit.
Vaal Reef Mineralization
The Vaal Reef A-facies is dominated by silicate phases including quartz (~84 %), chlorite (~5 %), muscovite (~4 %) and zircon (~0.22 %), as well as sulphide phases of pyrite (~5 %). The remainder of the conglomerate consists of an array of accessory mineral phases which include uraninite, coffinite, brannerite, as well as various minor sulphide and oxide phases. The matrix of the conglomerate is made up of muscovite, chlorite, re-crystalized quartz and pyrite.
High gold values in the Vaal Reef are often located at the base of this unit and are associated with high uranium values as well as with the presence of carbon. It is interpreted that carbon was preferentially precipitated in bedding–parallel fractures that most commonly followed the base of the Vaal Reef package and that gold was precipitated very soon after the carbon, giving the critical gold-carbon association that characterizes many of the high-grade Vaal Reef localities. Uranium is an important by-product which is also recovered from the Vaal Reef.
C Reef Mineralization
The Crystalkop formation attains a maximum thickness of about 9m, and contains a lower greenish, fuchsite-rich, tinged orthoquartzite overlain by a coarser grained protoquartzite that can be confused with Kimberley Channel deposits. The C Reef is present at the base of the Crystalkop formation and is typically a thin (<5cm ) oligomictic conglomerate with a well-developed carbon seam at the base. Although grades can be similar to that of the Vaal Reef the C Reef is only preserved in a small portion of the mine as it is truncated in the north by the Gold Estates Conglomerate and in the south by a large Kimberley channel (Watts, M, 2010).
The C Reef comprises of the following minerals: quartz (74.34 to 80.32 weight %), muscovite (7.69 to 10.54 weight %), pyrophyllite (2.89 to 8.35 weight %) and pyrite (5.21 to 7.73 weight %). A wide variety of accessory minerals includes zircon, chlorite, gersdorffite, sphalerite, cobaltite, galena, rutile, chromite, magnetite, ilmenite, calcite, siderite and florencite. Uranium bearing minerals such as uraninite, coffinite and brannerite occur in relevant amounts (Pienaar, D, 2016).
Gold is present predominantly in the form of pure gold, containing ~ 11 to ~ 18 weight % silver (Harris, 1990; Petruk, 2000; Zhou and Cabri, 2004; Zhou et al., 2004; Zhou and Gu, 2008).