Mponeng Mine is located on the northwestern rim of the Witwatersrand Basin. The VCR is the main reef horizon mined at Mponeng Mine. The VCR forms the base of the Ventersdorp Supergroup, which caps the Witwatersrand Supergroup through an angular unconformity. The overlying Ventersdorp Lavas halted the deposition of the VCR, preserving it in its current state. The CLR, previously mined at TauTona and Savuka Mines, is found within the Witwatersrand Supergroup and lies 800m beneath the VCR at Mponeng. The VCR is preserved across the Mponeng lease area and dips approximately 22° in a south-southeast direction. The VCR was deposited on the uneven footwall strata due to uplift and is now represented by a shallow angular unconformity. The footwall lithologies to the VCR therefore vary across Mponeng Mine as the unconformity cuts deeper in an easterly direction into older strata of the Witwatersrand Supergroup. Fluvial action during deposition of the VCR continually eroded and reworked the conglomerate, creating steep slopes and embayments between relatively undisturbed terraces.

The CLR conglomerate was deposited by a number of sedimentary cycles. Erosion and reworking of the conglomerate and quartzite sediments has resulted in the preservation of the CLR within the Central Rand Group of the Witwatersrand Supergroup.

The VCR consists of a quartz pebble conglomerate, which can be up to 3m thick in places. The footwall stratigraphy, following periods of uplift and erosion, controlled the development and preservation of the VCR. This is characterised by a series of channel terraces preserved at different relative elevations, and the highest gold values are preserved in these channel deposits. The different channel terraces are divided by zones of thinner slope reef, which are of lower value and become more prevalent on the higher terraces and on the harder footwall units. The relatively argillaceous protoquartzites of the Kimberley Formation in the central portion of Mponeng are covered by the best preserved VCR conglomerates. The Elsburg Formation in the west is relatively more durable while the eastern side of the mine is dominated by shales and siltstones of the Booysens Formation. No VCR is preserved on the Krugersdorp Formation on the far eastern side of Mponeng.

The CLR is the other gold bearing reef reported as part of the Mineral Resource for Mponeng. The CLR is located near the base of the Johannesburg Subgroup, which forms part of the Central Rand Group of the Witwatersrand Supergroup. The CLR was historically mined extensively at Savuka and TauTona Mines and the remaining portions thereof have now been transferred to Mponeng Mine. The CLR consists of, on average, a 20cm thick, tabular, auriferous quartz pebble conglomerate and three sedimentary facies. Economically, the most important facies is Unit 1, which overlies Unit 2. Unit 1 is a complex channel deposit that is only present along the eastern side of the lease area. Unit 2 can be up to 2m thick. Unit 3 is exposed in the southern edges of the lease area and is the oldest of the conglomerates.

Gold mineralisation followed an episode of deep burial, fracturing and alteration. A variant of Archaean 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 horizon, which was the more favourable fluid conduit. In the case of the VCR, the resulting gold grades are mostly uniformly distributed throughout the reef package.

CLR mineralisation associated with the conglomerate occurs in the form of fine layers and stringers of pyrite rather than finely disseminated pyrite around the pebbles. Flyspeck carbon can be frequently found at the base of the conglomerate. The hydrocarbon also precipitated in thin, flat veins, usually at the base of the Carbon Leader conglomerate where the majority of the gold is concentrated.

The VCR displays strong alteration features, which can be explained by the hydrothermal fluids that infiltrated the reef and have overprinted the original mineral assemblage. Portions of the reef contain authigenic sulphides such as pyrite, pyrrhotite, chalcopyrite, sphalerite and galena, incorporated in the conglomerate matrix. Gold associations with these mineral assemblages indicate a strong correlation of gold mobilisation and redistribution at the time of the hydrothermal fluid influx. There is also a strong association of gold with a chloritisation event focused along the reef horizon. The chlorite alteration gives a dark colouration to the reef. Gold was precipitated by cooling and reactions between the fluids and wallrock, in this case pyritic conglomerates. Gold mineralisation was enhanced in certain areas of high fluid throughput, which were often the sites of high carbon precipitation and early alteration in the case of the CLR.

Both the VCR and the CLR have been subjected to faulting and are intruded by a series of igneous dykes and sills of various ages that cross-cut the reefs. There is an inherent risk in mining through these faults and intrusives and a key objective of Mponeng Mine geologists is to identify these geological features ahead of the working face to assist with deciding on the best way to approach and mine through these structures.

Mponeng Mine is a deep level underground gold mine operating between 3,160m and 3,740m below mine datum (BMD*) and is currently the deepest mine in the world with development at 3,841m BMD. Future mining is planned to deepen the shaft bottom to 4,227m BMD. All production is currently from the VCR with future expansion planned on both the VCR and the CLR horizons.

For the exploitation of the ever deepening Mineral Resource, and the need for flexibility in a mine of this nature, the sequential grid mining method was adopted. This has been proven as the best method suited to safe, deep level gold mining associated with seismicity

The primary reef mined is the Ventersdorp Contact Reef (VCR). The CLR, that was historically mined at the now closed TauTona Mine, is planned to be mined in the Mponeng LOM extension project. A sequential grid mining method is employed to extract the gold from the deep, narrow, tabular orebody. The grid is predeveloped through a series of haulages and crosscuts. Stoping takes place by means of breast mining using conventional hand held drill and blast techniques. The SMU is 100 x 100m.

Mponeng has its own processing plant situated adjacent to the mine. Ore and waste material is hoisted separately with ore being delivered to the plant by means of a conveyor belt and the waste rock going to the low grade stockpile.

Ore is treated and smelted at the Mponeng Gold Plant, which also processes low grade ore from stockpiles adjacent to the shaft. Ore is initially ground down by means of semi-autogenous milling, after which a conventional gold leach process incorporating liquid oxygen injection is applied. The gold is then extracted by means of CIP technology. The plant conducts electro-winning and smelting using induction furnaces.