Summary:
The Bushveld Complex is approximately 2.060 million years old and is a mafic to ultramafic rock sequence. The Rustenburg Layered Suite (RLS) is the world's largest known mafic igneous layered intrusion containing about 80% of the world's known reserves of PGM's (Crowson, 2001 quoted in Cawthorn, 2010). In addition to PGM's, extensive deposits of iron, tin, chromium, titanium, vanadium, copper, nickel, and cobalt also occur. The Bushveld Complex extends approximately 450km east to west and approximately 250km north to south. It underlies an area of some 67,000km2, spanning parts of Limpopo, North West, Gauteng, and Mpumalanga Provinces.
Interlayered in the Upper Critical Zone of the Bushveld Complex's RLS, the Merensky and Upper Group No. 2 (UG2) Reefs are preserved as narrow tabular structures. The Rustenburg Operations are situated on the western limb of the Bushveld Complex and produce the PGMs and associated Base Metals from the mining and processing of the Merensky and UG2 Reefs.
The Main Zone predominantly comprises gabbro-norite and norite rock types, whereas, in the Upper Critical Zone, pyroxenite, norite, anorthosite, and chromitite lithologies are found.
The Upper Critical Zone stratigraphy of the RLS, which contains the units of economic interest, the Merensky and UG2 Reefs, comprises well-developed cyclic units divided into six sub-units as follows:
- Bastard Pyroxenite;
- Merensky Reef;
- Merensky Footwall;
- UG2 Hangingwall;
- UG2 Chromitite Layer/Reef;
- UGI Chromifite Layer.
In Rustenburg Operations, there are local variations in thicknesses of individual stratigraphic units within the Boschfontein farm in the far west and the Hoedspruit farm in the east.
The Giant Poikilitic Anorthosite (GPA) generally defines the start of the Critical Zone, which normally occurs 5m to 10m above Bastard Pyroxenite and approximately 20m to 25m above Merensky Reef. The GPA is normally about 7m to 10m in thickness.
Localised geological discontinuities associated with the Merensky and UG2 Reefs include potholes, faults, joints, shears zones, dykes and IRUP bodies. These are the main structures that impact to the Material asset.
Across the PGM operations, the UG2 Reef occurs vertically between 90m and 180m below the Merensky Reef. The Merensky and the UG2 Reefs are affected by structural and other geological features, including potholes and iron-rich ultramafic pegmatoids (IRUPs), which result in geological losses and have an impact on mining.
Merensky Reef
The Merensky Reef mineralogy comprises major silicate minerals: pyroxene, plagioclase, and biotite. These minerals form secondary minerals such as talc and chlorite in structurally disturbed and weathered areas. PGM mineralization is closely related to thin chromite layers (1mm to 5cm thick). PGM and sulphide mineralization can also occur in the immediate footwall rocks.
The dominant platinum group minerals are -30% Pt-Pd sulphides (braggite-cooperite), -11% PGM tellurides and arsenides, -6% sperrylite and minor PGM alloys. Platinum group mineral grain sizes have two size ranges in the Merensky Reef: 10 to 30µm and 50 to 350µm.
The platinum-group minerals of the Merensky Reef occur in three textural associations:
• Enclosed in or attached to base metal sulphides (38-97 %). This is a common occurrence on the western limb.
• Enclosed in silicate (3 62%) and further north along the western limb past the regional Swartklip facies (62%).
Enclosed in/or attached to chromite or Fe-oxide.
UG2 Reef
The UG2 Reef consists predominantly of chromite (60 to 90% by volume) with lesser silicate minerals 5 to 30% pyroxene and 1 to 10% plagioclase. Other minerals, present in minor concentrations, can include the silicates: phlogopite and biotite, the oxides: ilmenite, rutile and magnetite, and base metal sulphides. Secondary minerals include quartz, serpentine and talc. The Cr2O3 content of the UG2 Reef varies from 30 to 35%. The PGMs present in the UG2 Reef is highly variable, but generally, the UG2 Reef is characterised by the presence of various PGM sulphides, comprising predominantly laurite (RuOsir sulfide), cooperite (PtS), braggite (Pt, Pd. NIS), and an unnamed PtRhCUS. The PGMs only reach an average size of approximately 12 µm, with particles larger than 30 um being extremely rare.
Most of the PGMs occur in association with the base metal sulphides and silicates. It is only the mineral laurite which exhibits a preferred association with the chromite grains. Both the grain size and associations are extremely important as these affect the metallurgical behaviour during subsequent processing. The major base metal sulphides constitute chalcopyrite, pentlandite and pyrrhotite. The base metal sulphides occur almost entirely within the interstitial silicate and are only very rarely enclosed within the chromite particles. The grain size of the base metal sulphides rarely exceeds 30 µm.
The distribution of grade within the layer is not uniform however PGMs are generally concentrated at the upper and lower contacts of the main chromitite, with lesser concentrations in the Leader layers. The highest PGM concentration is generally recorded at the base of the UG2 Reef chromitite.
In the UG2 Reef, Cu, Ni and Sulphur values are extremely low.