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. 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 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.

Mponeng is a deep-level mine.

The orebody is extracted mostly by breast-mining methods with associated waste mining in addition to the reef being extracted. The dilution from these waste sources is captured and incorporated in the tonnage calculation, with historical performance being the benchmark. The mine exploits the Ventersdorp Contact Reef as its primary orebody. The economic reef horizons are mined between 3 160m and 3 740m below surface.

In addition to the in-stope dilution sources being accounted for allowance is also made for dilution from development waste sources to mill by both schedule results and factors based on history. Widths used are based on the channel width of the orebody being mined and are aligned with the mining method (stoping and ledging) and historical achievements.

Due to the variability of the VCR with respect to value and the seismic risk associated with deep level mining the sequential grid mining method is used at Mponeng.

(a) Some design criteria include the following for the VCR ore body:
• Breast mining to strike spans of 180m with 30m wide dip stabilising pillars orientated on true dip;
• Major geological features are bracketed;
• Incorporation of 30m strike stability pillars at a maximum dip spacing of 100m (skin to skin) on the side of the raise where the mining is conducted last in order to minimise closure in the stoping areas below 109 level;
• Rock engineering requirements are adhered to.

(b) Some design criteria include the following for the CLR ore body:
• Breast mining to strike spans of 180m with 40m wide dip stabilising pillars orientated on true dip;
• Major geological features are bracketed;
• Incorporation of 30m strike stability pillars at a maximum dip spacing of 100m (skin to skin) on the side of the raise where the mining is conducted last in order to minimise closure;
• Rock engineering requirements are adhered to.

(c) On the VCR horizon above 109 level the access haulages have all been developed in the hanging wall with the exception of 99 level. For the areas below 109 level all the haulages are being developed in the footwall. In the case of the CLR all haulages will be developed in the footwall.
• Middling to reef 85-150m for the VCR and 70m for footwall placement in the CLR orebody;
• Where possible the VCR haulages are placed out of the Booysens shale, however, where these shales are traversed allowance is made for reduced rate of advance to account for delays due to additional support requirements.

(d) The overall mining sequence is an inverted Christmas tree, however, within each raise the face configuration is underhand when mining towards the west and overhand (top panels leading) when mining towards the east (the bottom panels leading). This is, however, governed by the presence of large geological structures within the raise line.

This design and schedule are the basis of the mine plan and the Ore Reserves declared. With the exploitation of ever deepening resources and the need for flexibility on a mine of this nature the sequential grid mining method was adopted. This has been proven as the method best suited to the deep-level gold mining with its associated seismicity and therefore flexibility requirements.

Mponeng has its own processing plant situated adjacent to the mine. Ore and waste material are 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 mined is treated and smelted at the Mponeng gold plant, which also processes low grade ore from the stockpile adjacent to the shaft.

The 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 recovered by means of carbon in pulp (CIP) technology together with electrowinning and smelting processes.

PLANT
Lime is added to the thickener feed to achieve a pH of 10.2 reporting to the pre-leach tank. The leach section consists of 10 leach tanks in series. The first tank is used for pre-oxidation while cyanide is added to the second leach tank. Cyanide dosing control is affected by means of an on-line ........