The magmatic controls on Ni-Cu-PGE mineralisation in the Platreef, northern limb of the Bushveld Complex, South Africa

The Platreef, northern limb of the Bushveld Complex, South Africa, is widely regarded as one of the world’s largest resources of platinum-group elements (PGE), and also contains significant amounts of base metal (Ni, Cu, Co) mineralisation. Although it is widely accepted that the Platreef formed via the emplacement of a series of discrete magmatic injections, there remains little understanding of the role that this primary magmatic stratigraphy plays in controlling the distribution of PGE-Ni-Cu-Co mineralisation.

This study uses petrology, bulk geochemistry (including the long-neglected method of Niggli Numbers), and mineral chemistry to examine the magmatic stratigraphy of the Platreef, and potential controls on Ni-Cu-PGE mineralisation along the northern limb. Niggli Numbers are shown to be an effective tool for identifying local contamination, with Niggli c > 20, and mg values > 0.8 strongly indicating dolomitic contamination at Sandsloot, where grades are also significantly elevated compared to at Tweefontein and Overysel.

When examining the Platreef in-depth on the farms Tweefontein and Sandsloot, strikingly different stratigraphies, Ni-Cu-PGE distributions and associated host lithologies are evident. At Tweefontein, differences in pyroxene geochemistry, petrology and bulk geochemistry (e.g., REE λ shape coefficients, Cr/MgO, Eu/Eu* and Ca/Al2O3 ratios) define three magmatic packages of Platreef, Main Zone and Lower Zone affinity, with the uncontaminated Upper Platreef unit hosting PGE mineralisation. At Sandsloot, the same methods are employed to identify six discrete units, with significant PGE and Ni-Cu mineralisation observed in the strongly contaminated PGE-Reef, and Base Metal Zone respectively. Multi-stage emplacement models are proposed at both locations, with the nature of the country rocks, and order of emplacement of the PGE-fertile and/or barren units facilitating increased complexity in the Platreef. Radiogenic isotope results (Sr-Nd-Hf-Pb) cannot constrain a magma source and/or crustal contaminant unique to the PGE-rich horizons alone at this time.

An exploration framework for the Platreef is provided, which outlines the key processes to be taken in consideration during exploration for further Ni-Cu-PGE prospects along the northern limb.