Tshepong North is situated in the Free State Goldfield, on the southwestern 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 CRG of the Witwatersrand Supergroup.

The general orientation of the Witwatersrand Supergroup succession in this goldfield is interpreted as north-trending, within a syncline that is plunging to the north. The syncline has been divided by faults into the Odendaalsrus, Central Horst and Virginia sections. The Tshepong North mining right area is also affected by the Ophir and Dagbreek faults.

Tshepong North exploited primarily the Basal Reef, which occurs within the Harmony Formation of the Johannesburg Subgroup of the CRG.

Mineralization also occurs within the stratigraphically higher A and B reefs of the Kimberley (formerly Aandenk) Formation, within the Turffontein subgroup of the CRG. However, only the B Reef can be economically extracted.

Mineralization is associated with the presence of medium to coarse, clast-supported oligomictic pebble horizons. The presence of allogenic pyrite and detrital carbon is also common.

The principal gold-bearing orebody is the stratiform and strata-bound Basal Reef (known as the Basal Reef Zone or BRZ). This unit comprises a thin conglomerate at the base of the BRZ, overlain by clean “placer” quartzites. The Basal Reef is underlain by a thi ........

The Tshepong North is a mature underground operation miningat moderate depths of between 1 600m and 2 400m below surface. The mine is spilt in upper mine and lower mine sections (decline system) of which the production is a 50/50 ratio from both sections.

The SGM mining methods are suitable for underground, narrow reef mining. A common feature of the SGM method is the layout of the primary and secondary development. Primary development is done off-reef (in waste rock), while secondary development is done on-reef (in the mineralized zone).

SGM is the preferred mining method used at the Tshepong North. This method makes use of dip pillars and reduced mining spans with pre-developed tunnels, aimed at further control of stresses experienced in rock movement. The SGM sequence is a V-shaped configuration, colloquially referred to as the “inverted Christmas tree”.

The SGM is employed for a deeper mining approach and offers various advantages, the critical one being increased safety. A noticeable characteristic of the SGM method is that mining from the raises is advanced in only one direction at a time, which is directed towards the stabilizing or regional pillars. This SGM mining sequence eliminates the creation of remnant pillars reducing the risk of seismicity.

While Tshepong North business plan is based primarily on the SGM methodology and sequencing, there are sections of the mine that are operating using the breast, undercut and open ........

Ore Reception:
The use of parallel processing starts at the ore receive bin, where there are two unloading stations for the railcars that bring the ore in from the various shafts and rock dumps. A unique feature of the Plant is the Ore Reception facility, which has been designed to eliminate dead storage space, a serious constraint in railway ore storage bins. Ore is transported by rail to the Plant. The railway hoppers discharge individually into one of the two concrete, rail-lined inverted cones, 12m in diameter and 8m deep. The apex of the cone is 57o. The ore is rapidly withdrawn from the apex of the cone via a shuttle belt feed conveyor feeding onto a main silo feed conveyor. The twelve ore storage silos are constructed from concrete and are also rail-lined. Each silo has a live storage of approximately 3 000 tons. Ore is discharged onto the mill feed conveyor via a pneumatically-operated Langlaagte chute. When filled to full capacity, the twelve ore silos provide approximately 60 hours storage for the six Run-of-Mine (ROM) mills.

There are six parallel and independent milling lines with each one having a conveyor running underneath 2 silos (A and B) and taking the ore up into one of the six run-ofmine (ROM) mills. Generally, ore is drawn only from one of the two silos for each mill whilst the other is being filled with ore. Hence, one silo is discharging onto the mill feed belt, and the other refilled.

Milling:
The ore is taken up the slow-moving conveyers from underneath the silos and discharged directly into the feed hopper for the ROM (Run Of Mine) mills. Fully autogenous (FAG) milling is a milling process in which the entire ROM ore stream is fed directly into the mills. The grinding media is generated within the mill from suitably sized pieces of ROM ore itself, supplemented by waste rock dump material. The feed rate to the mills is between 90 and a 100t/h.

The milling circuit consists of six single stage ROM mills that are controlled on maximum power, utilising programmable logic controllers (PLCs). Variations in mill load are measured by load cells situated under the outlet trunnion bearings. Each ROM mill is 4.9m diameter by 10m long and powered by 3.3MW motors and grinds the ore to between 68 and 73% minus 75µm.

For control purposes, the mill feed belts and the mill discharge pumps both have variable speed drives. Each mill is in closed circuit with a 1200mm primary cyclone with mass flow measurement on the feed. The primary cyclone overflow is screened on a 600µm linear screen for the removal of coarse woodchips and tramp steel. This has the purpose of preventing gold losses and carbon contamination in the downstream CIP circuit. Cyclone overflow, which has a low density, is pumped out to the thickeners. The current cyclone overflow size is 68% at -75µm.

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