Both the Merensky and UG2 Reefs are exploited. The Merensky Reef is generally composed of an upper feldspathic pyroxenite, overlying a thin basal chromitite stringer, followed by an anorthosite to norite footwall. Locally this is termed a “pyroxenite reef”. Occasionally a pegmatoidal pyroxenite and a second chromitite stringer may be developed between the feldspathic pyroxenite and the footwall units. This is termed a “pegmatoid reef”. As an aid to mining operations the Merensky Reef is further defined as being “A”, “B” or “C” Reef where it rests on specific footwall units – locally called Footwall 1, 2 and 3 respectively.

The UG2 Reef is defined as a main chromitite layer, with most of the PGM and base metal mineralisation confined to this unit, followed by a poorly mineralised pegmatoidal pyroxenite footwall. The hangingwall to the main chromitite layer is a feldspathic pyroxenite containing up to four thin, poorlymineralised chromitite layers.

Both mineralised horizons dip gently away from the sub-outcrop in a north-easterly direction at 10° to 12°. The vertical separation between the Merensky and UG2 Reefs varies from about 125m in the south to 45m in the north of the mining area. The reefs may be disrupted by minor and major faults, lamprophyre, syenite and dolerite dykes, late stage ultramafic replacement pegmatoid bodies and potholes. The latter features are generally circular in shape and represent “erosion” or “slumping” into the footwall units. They vary in size from a few metres to tens of metres across and up to tens of metres in depth.

The Merensky and UG2 Reefs are mined concurrently at Impala. The mining method is predominantly conventional breast mining. Stoping at the operations is carried out through conventional double-sided breast mining in accordance with Impala’s best practice principles. The access haulages are developed in opposite directions from cross-cuts connected to a central shaft position, following the two reef horizons on strike in the footwall of the reefs and are defined as half levels. Footwall drives are developed at approximately 18m to 30m below the reef horizon with on-reef raise/winze connections being between 180m and 250m apart. Panel face lengths vary from 15m to 28m for both Merensky and UG2 Reefs, with panels being typically separated by 6m x 3m grid pillars with 2m ventilation holes. Stoping widths are approximately 1.3m and 1.1m for conventional Merensky and UG2 Reefs, respectively, depending on the width of the economical reef horizon. Mechanised (trackless) bord and pillar mining occurs in selected Merensky Reef areas at 14 Shaft. The average stoping width of the mechanised panels is about 1.9m.

Processing receives ore from the shafts which is allocated to either the UG2 Plant, for the higher chromium grade material, or the Central Concentrator for Merensky ore. Between 89% and 91% of the PGMs from the Merensky ore are recovered at mass pulls ranging from 5% to 7% utilising 10 primary mills, feeding two, nine stage, tank cell flotation banks. Tailings from this section are milled for further liberation and floated in conventional cells to achieve the aforementioned recovery

Approximately 79% to 81% of the PGMs are recovered from the UG2 ore at a mass pull of 2% to 3%. The PGM recovery from UG2 ore is performed utilising a more complex circuit configuration in order to reduce the amount of chromium reporting to the concentrate stream. The MF2 plant, also situated at the Central Concentrator, utilises three primary mills that can accommodate any Merensky ore spill over, as well as UG2 ore. This allows for flexibility in the ore split received from the mining operation ........