Supergene leaching and formation of platinum in alluvium: evidence from Serro, Minas Gerais, Brazil

TLDR: In this article, the Corrego Bom Sucesso alluvial deposit near Serro, Minas Gerais, Brazil, was used for the first discovery of the element palladium, and an X-ray mapping of an arborescent Pt-Pd nugget from the historical occurrence documents selective palladium depletion.

Abstract: The Corrego Bom Sucesso alluvial deposit near Serro, Minas Gerais, probably provided the specimens from which the element palladium was first discovered. Its Pt–Pd nuggets are characteristically botryoidal, arborescent and coralloidal, and exhibit an external halo with the composition of palladiferous platinum to virtually pure platinum. X-ray mapping of an arborescent Pt–Pd nugget from the historical occurrence documents selective palladium depletion, similar to the high-fineness gold haloes developed on detrital Au–Ag grains under supergene conditions. The Pd-depleted alteration zone truncates inclusions of crystals stoichiometrically close to PdPt within the Pt–Pd aggregate. Selective metal leaching in the weathering environment can account for the frequently observed Pt-enriched rims on alluvial platiniferous alloys which may lead to the formation of platinum nuggets at an advanced degree of weathering.