The layered sequence, the Rustenburg Layered Suite, comprises five major sub divisions, ie the Marginal, Lower, Critical, Main and Upper zones. Two horizons within the Critical Zone, namely the Merensky Reef and the Upper Group 2 (UG2) Reef, host economically exploitable quantities of PGMs. These two horizons, along with other layers which can be traced for hundreds of kilometres around the complex, are the focus of Implats’ operations. The PGMs – platinum, palladium, rhodium, ruthenium and iridium – as well as the associated gold, copper, nickel, cobalt, chromite and other minor metals and compounds, are mined and recovered concurrently.
The Great Dyke
The Great Dyke is a 2.5 billion year-old layered mafic-ultramafic body intruded into Archaean granites and greenstone belts. It is highly elongated, slightly sinuous, 550km long, north-northeast trending with a maximum width of 12km and bisects Zimbabwe in a north-northeasterly trend and is divided vertically into a lower ultramafic sequence, comprising cyclic repetitions of dunite, harzburgite, pyroxenite and chromitite, and an uppermafic sequence consisting mainly of olivine gabbro, gabbronorite and norite. It is U-shaped in section with layers dipping and flattening towards the axis of the intrusion. Much of the mafic sequence has been removed by erosion and at the present plane of erosion the Dyke is exposed as a series of narrow,contiguous layered complexes or chambers. These are, from north to south, Musengezi, Hartley (comprising the Darwendale and Sebakwe sub-chambers) and a southern chamber comprising the Selukwe and Wedza sub-chambers.
The Main Sulphide Zone (MSZ), host to economically exploitable PGMs and associated base metal mineralisation, is located 10m to 50m below the ultramafic/mafic contact in the P1 pyroxenite. The PGMs, along with gold, copper and nickel, occur in the MSZ. The grade profiles vary between areas and the platinum and palladium peaks are some what offset. Typically, the MSZ consists of a 2m to 10m-thick zone containing 2% to 8% of iron-nickel-copper sulphides disseminated in pyroxenite. The base of this nickel copper-rich layer is straddled by a 1 to 5m-thick zone of elevated precious metals (Pt, Pd, Rh and Au). The base metal zone contains up to 5% sulphides, while the sulphide content of the PGM zone is less than 0.5%.