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 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 (particularly middle mine) requires a reduced drill spacing pattern of the order of 50mx50m, 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 (300-350°C). Migrating liquid and gaseous hydrocarbons precipitated as solid hydrocarbon (carbon), which was then mesophased through metamorphism and structural deformation.

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 subdivides 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.

The tabular nature, along with the depth and structural complexity of the orebody dictates the mining method utilised at Moab Khotsong mine. Mining at Moab Khotsong is based on a scattered mining method together with an integrated backfill support system that incorporates bracket pillars. The economic horizons are exploited between 1,791m and 3,052m below surface.

Moab Khotsong’s mineral processing is done through the Great Noligwa gold plant with design capacity exceeding the maximum planned production volume from the operation. The plant uses the reverse gold leach method which recovers gold and uranium through gold cyanide and acid uranium leaching.

The operation has a dedicated ore processing plant within close proximity to Moab Khotsong and tailings are pumped to existing tailings storage facilities. Most of the waste rock is separated from reef ore underground and accounted for separately. All waste and reef is delivered to the metallurgical plant.

Noligwa gold plant was commissioned in 1971 and treats approximately 200 000 tons per month of reef material from the Great Noligwa, Kopanang and Moab Khotsong mines. Following milling in two run-of-mine mills, the slurry is thickened before being pumped to the uranium section for uranium extraction.

The uranium plant residue at pH 1.5 -2.5 is returned to the go ........