期刊
CANADIAN MINERALOGIST
卷 44, 期 -, 页码 385-397出版社
MINERALOGICAL ASSOC CANADA
DOI: 10.2113/gscanmin.44.2.385
关键词
platinum; palladium; mercury-rich palladiferous platinum; PdPt; alluvial nuggets; Corrego Bom Sucesso; Serro; Minas Gerais; Brazil
类别
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.
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