Summary:
Mponeng is situated on the northwestern margin of the Witwatersrand Basin of South Africa, one of the prominent gold provinces in the world. There are seven gold-bearing conglomerates within the lease area, of which only the Ventersdorp Contact Reef (VCR) and Carbon Leader Reef (CLR) are economically viable.
The Ventersdorp Contact Reef (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 Carbon Leader Reef (CLR), previously mined at Tau Tona and Savuka mines, is found within the Witwatersrand Supergroup. The CLR lies 900m beneath the VCR on Mponeng. The VCR is preserved across the Mponeng lease area and dips at approximately 22 degrees in a SSE direction.
The VCR was deposited on 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 embayment’s between relatively undisturbed terraces.
The CLR conglomerate was deposited by several 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.
Deposit type
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, which 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.
VCR is poorly 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 total 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 of rocks.
The CLR has historically been mined extensively at Savuka and Tau Tona mines and the remaining portions thereof have now been transferred to Mponeng Mine. The CLR in the West Wits 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 West Wits 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.
Mineralisation style
Gold mineralisation followed an episode of deep burial, fracturing and alteration. A variant of 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 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 precipitated also in thin, flat veins, usually at the base of the Carbon Leader conglomerate, and this is 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 on the original mineral assemblage. Portions of the reef contain authigenic sulphides such as pyrite, pyrrhotite, chalcopyrite, spahelerite 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 coloration 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.