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

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.