Arborescent palladiferous gold and empirical Au_2Pd and Au_3Pd in alluvium from southern Serra do Espinhaco, Brazil

摘要

Arborescent crystals of palladiferous gold are identified for the first time in platiniferous alluvium from Corrego Bom Sucesso, Serro region, in southern Serra do Espinhaco, Minas Gerais, Brazil. The Corrego Bom Sucesso alluvium is regarded as the type locality of Pd, being also famous for dendritic Pt-Pd-Hg nuggets. Gold composition ranges from 66.5 to 95.4 at percent Au (77.6-97.9 wt percent Au) and from 4.4 to 29.8 at percent Pd (2.4-18.8 wt percent Pd), with trace amounts of Ag. Two near-stoichiometric alloys, Au_2Pd and Au_3Pd, determined by electron-microprobe analysis, may correspond to unnamed mineral phases. The arborescent crystals of palladiferous gold are well preserved and as delicate as the arborescent Pt-Pd-Hg alloys in the alluvium, thereby suggesting that they all formed under the same oxidising low-temperature conditions. The recognition of arborescent palladiferous gold represents a critical mineralogical link with the Serra Pelada Au-Pd-Pt deposit in northern Brazil and the unconformity-related Au-Pd-Pt occurrences in southern Devon, England.