Nickel, copper, cobalt, PGM and chromite mineralisation is hosted by the Uitkomst Complex, a layered mafic-ultramafic, Bushveld satellite intrusion.

The Uitkomst Complex is a Bushveld-age layered, mafic-ultramafic body intruded into the basal sediments of the Transvaal Supergroup, which lies unconformably on an Archean granitic basement. The complex is a long linear body, which outcrops in the Slaaihoek valley for approximately 8 kilometres and dips below an escarpment where it has been drilled at depth for an additional 4 kilometres. The complex, which dips at approximately 4 degrees to the north-west, is still open-ended.

From the base to top, the stratigraphy of the Uitkomst Complex comprises the Basal Gabbro Unit (up to 15 metres thick), the Lower Pyroxenite Unit (average 35 metres), the Chromititic Peridotite Unit (30 to 60 metres), the Massive Chromitite Unit (up to 10 metres), the Peridotite Unit (330 metres), the Upper Pyroxenite Unit (65 metres), the Gabbronorite Unit (250 metres), and the Upper Gabbro Unit (50 metres). The complex and surrounding sediments are intruded by numerous diabase sills up to 30 metres in thickness.

There are five sulphide zones in the Uitkomst Complex: the MSB, situated at and below the base of the complex, but which has now been mined out; the Basal Mineralised Zone (BMZ) within the Basal Gabbro; the MMZ, occurring within the Lower Pyroxenite, the PCMZ, which occurs with the Chromititic Peridotite (PCR), and the Peridotite Mineralised Zone (PRDMZ), which occurs in the Peridotite Unit, but which is not yet included in the mine’s resource base.

In addition, the Massive Chromitite Unit (MCHR), situated at the top of the PCR Unit, is mined in the open-pit area. The dominant sulphide minerals are pyrrhotite, pentlandite and chalcopyrite; cobalt is mostly in solid solution in the pentlandite, and the PGMs occur as separate minerals, merenskyite being dominant.

• MMZ: Main Mineralized Zone. Head grades 0.3-0.7% Ni, averaging approximately 0.37% Ni;
• PCMZ: Peridotitic Chromititic Mineralized zone. Chrome-rich ore with grades of 0.2 1% Ni, 0.23 % Ni average, chrome grades of 10-15% Cr2O3;
• Massive chromitite (often called PCR): stockpiled for a separate chrome washing plant, currently mothballed;
• Basal Mineralized Zone: unexploited at present.

The primary minerals originally constituting the pyroxenites and peridotites of the Lower Pyroxenite that hosts the MMZ are mainly clinopyroxene, olivine, and plagioclase. Hydrothermal alteration has resulted in extensive alteration of these minerals to amphibole, serpentine, biotite, and talc. Contamination of the ultramafic suite by country rocks accounts for the presence of most of the calcite, dolomite quartz, and plagioclase. Talc content is highly variable within the deposit and irregular (Brits, 2008).

Severely altered talc-rich zones are often associated with pyrite-rich zones, and due to their fast flotation kinetics, flotation of either mineral complicates the flotation process, resulting in lower pentlandite recoveries, dilution of the concentrate with pyrite, and reduced concentrate quality due to higher MgO levels.

Nickel is mainly contained within pentlandite, although a significant proportion (as much as 15%) occurs in solid solution within pyrrhotite, and 1-2% within chlorite. Copper occurs almost exclusively within chalcopyrite, with some occurring as bornite (1-2%).

The MMZ in many ways resembles the Merensky Reef, though having substantially lower PGMs but higher base metal sulphides (typically 5-8 %) with traces of PGMs (predominantly merenskyite). The PCMZ resembles the UG2 Reef, with chrome grades of 7-15% Cr2O3. From the geologist’s perspective, the ores are effectively the same with the exception of the chrome grades. The boundaries of the two ore types are not clearly delineated, making segregation of ore and prevention of cross-contamination challenging.

From a processing perspective, however, the ores are significantly different. Target liberation grind for MMZ ore is 67% -75µm, although recoveries are relatively insensitive to grind. PCMZ ore is extremely sensitive to grind, with target grind of 80% -75 µm, and drastic losses in recovery at lower grind values. Misplaced ore thus directly affects plant performance.

Mining operations to date comprise a mechanised underground (Bord and Pillar and Long Hole Open Stoping) and open-pit mining operation which feeds two concentrators.

Concentrator process is crush, mill and float.