On the association of palladium-bearing gold, hematite and gypsum in an ouro preto nugget

An ouro preto (black gold) nugget from Gongo Soco, Minas Gerais, Brazil, has a mineral assemblage of hematite and gypsum hosted by Pd-bearing gold. The hematite inclusion is microfractured and stretched. Scattered on the surface of the gold is a dark-colored material that consists partially of Pd-O with relics of palladium arsenide-antimonides, compositionally close to isomertieite and mertieite-II. The Pd-O coating has considerable amounts of Cu, Fe and Hg, and a variable metal:oxygen ratio, from O-deficient to oxide-like compounds. The existence of a hydrated Pd-O compound is suggested, and its dehydration or deoxygenation at low temperatures may account for the O-deficient Pd-rich species, interpreted as a transient phase toward native palladium. Although gypsum is a common mineral in the oxidized (supergene) zones of gold deposits, the hematite-gypsum-bearing palladian gold nugget was tectonically deformed under brittle conditions and appears to be of low-temperature hydrothermal origin.