The University of the Witwatersrand (Wits) has published research on platinum deposits in the Bushveld Complex, which stretches across South Africa’s Limpopo and North West provinces.
The research, published in the Scientific Reports journal, states that most of the world’s economically viable platinum deposits occur as ‘reefs’ in layered intrusions – thin layers of silicate rock that contain sulphides enriched in platinum group elements, which are vital for the sustainable development of modern human society.
There are two competing ideas of how platinum deposits formed. The first involves gravity-induced settling of crystals on the chamber floor, while the second implies that the crystals grow in situ, directly on the floor of the magmatic chamber.
Through examining the Merensky reef of the Bushveld Complex, which hosts the lion’s share of the world’s platinum and other noble metals, Wits’ School of Geosciences academic Dr Sofya Chistyakova and her collaborators have established that the crystals grow in situ, with its high platinum status being attained while all its minerals were crystallising along the cooling margins of the magma chamber.
One of the remarkable features of the Bushveld Complex is that, at the time when the Merensky reef was forming, some portions of its chamber floor were highly irregular owing to circular depressions or potholes.
These potholes are best exposed in underground exposures of platinum mines.
“Our key discovery is that the entire Merensky reef package in these potholes may develop as a ‘rind’ covering all the chamber floor depressions and culminations, even where these are vertical or overhanging,” says Chistyakova.
This finding points to no possibility that the Merensky reef was formed by crystal settling on the chamber floor because sinking crystals cannot penetrate the solid rocks that form a pothole’s overhangs.
This strongly supports the concept that the silicate minerals and sulphides of platinum deposits grow directly at the floor of magmatic chambers.
“This is the most fundamental conclusion of our work and it can probably be applied to platinum deposits in other layered intrusions, as well as potentially extended to other types of magmatic deposits, for example, chromite and iron/titanium/vanadium magnetite ores in mafic-ultramafic complexes,” says Chistyakova.