Magmatic Stratigraphy of the Deep Platreef at Sandsloot, Northern Bushveld Complex: Carbonate Contamination and Controls on Ni-Cu-Platinum Group Element Mineralization

Abstract

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 (PGEs), critical metals that are essential for the growth of many sustainable technologies. In this study, the petrology, bulk geochemistry, and mineral chemistry of drill holes from the deep Platreef at Sandsloot, downdip from the established Sandsloot open pit, are examined, with the aim of establishing the magmatic stratigraphy. This newly described sequence is composed of up to six discrete pyroxenitic packages, with significant PGE and base metal mineralization observed in two consistent units: the PGE reef and the Base Metal zone. Varying CaO/Al2O3 ratios and mineral and alteration assemblages are associated with different units, alluding to varying degrees of carbonate contamination and hydrothermal alteration. Variations in parental magma compositions for the identified units are equivocal. The PGE reef and Ni-rich Base Metal zone are strongly associated with contamination from the underlying Malmani subgroup dolomites (indicated by CaO/Al2O3 ratios ≥ 2), and although this is not exclusive to the mineralized horizons, the highest PGE grades all show this signature. The stratigraphy of the deep Platreef at Sandsloot is difficult to correlate with other sections of deep Platreef/Critical zone in the northern Bushveld, which is likely a function of complex and localized contamination, and an emplacement history of discrete sills/fingers of barren and PGE-rich magmas. Notwithstanding the localized contamination, it is apparent that only some of the magmatic pulses that formed the Critical zone succession in the northern limb were PGE rich.