Platinum-palladium nuggets and mercury-rich palladiferous platinum from Serro, Minas Gerais, Brazil

Arborescent, botryoidal and reniform grains of platinum and palladium from Corrego Bom Sucesso, Serro, Minas Gerais, Brazil, were investigated by scanning electron microscopy and electron-microprobe analysis. The nuggets reach a maximum length of 2.5 mm and exhibit core-to-rim compositional zoning in platinum, palladium and mercury contents. One grain is noteworthy for a marginal halo of mercury-rich palladiferous platinum and internal vermiform lamellae enriched in mercury and gold. The mercury-rich palladiferous platinum has 10-20 wt% Hg and an empirical stoichiometry close to (Pt,Hg)(2)Pd. Hexagonal crystals occur in palladiferous platinum forming a peripheral zone of an arborescent grain. These crystals are compositionally analogous to stoichiometric PdPt and seem to represent an ordered phase distinct from either platinum or palladium. Most Pt-Pd grains have dissolution pits, suggesting that they are primary nuggets that have been weathered. Anatase, with or without intergrowth of gold, and Na-K-Cl-bearing aggregates postdate the Pt-Pd nuggets. Selenium is a minor alloying constituent (up to 0.3 wt%) in the Pt-Pd nuggets. The ratio S:Se is <1 and suggests fractionation of selenium from sulfur under low-temperature oxidizing conditions. Such oxidizing conditions are reflected in the Fe-poor bulk composition of the alloys. The Pt-Pd nuggets from Corrego Bom Sucesso are consistent with the observation that almost all economically important Brazilian Pd-Pt-bearing deposits associated with gold or hematite (or both) have a distinctive seleniferous signature.