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ON THE ASSOCIATION OF PALLADIUM-BEARING GOLD, HEMATITE AND GYPSUM IN AN OURO PRETO NUGGET

Alexandre Raphael Cabral^1,, Bernd Lehmann¹, Rogerio Kwitko-Ribeiro^2,, Richard D. Jones³ and Orlando G. Rocha Filho⁴

¹ Institut für Mineralogie und Mineralische Rohstoffe, Technische Universität Clausthal, Adolph-Roemer-Str. 2A, D-38678 Clausthal-Zellerfeld, Germany
² Centro de Desenvolvimento Mineral, Companhia Vale do Rio Doce, Rodovia BR 262/km 296, Caixa Postal 09, 33030-970 Santa Luzia – MG, Brazil
³ 1636 East Skyline Drive, Tucson, Arizona 85178, U.S.A.
⁴ Mina do Gongo Soco, Companhia Vale do Rio Doce, Fazenda Gongo Soco, Caixa Postal 22, 35970-000 Barão de Cocais – MG, Brazil

E-mail addresses: cabral{at}min.tu-clausthal.de, rogerio.kwitko{at}cvrd.com.br

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.

Keywords: ouro preto, hematite-gypsum mineral assemblage, Pd-bearing gold, Pd-O coating, isomertieite, mertieite-II, Gongo Soco, Minas Gerais, Brazil.

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