385 § E-mail address: alexandre-raphael.cabral.1@ulaval.ca or cabral70tricolor@yahoo.com.br The Canadian Mineralogist Vol. 44, pp. 385-397 (2006) PLATINUM–PALLADIUM NUGGETS AND MERCURY-RICH PALLADIFEROUS PLATINUM FROM SERRO, MINAS GERAIS, BRAZIL AlexAndre rAphAel CABrAl § And GeorGes BeAUdoIn Département de géologie et de génie géologique, Université Laval, Pavillon Adrien-Pouliot, Québec, QC G1K 7P4, Canada roGerIo KWITKo-rIBeIro Centro de Desenvolvimento Mineral, Companhia Vale do Rio Doce, Rodovia BR 262/km 296, Caixa Postal 09, 33030-970 Santa Luzia – MG, Brazil Bernd lehMAnn Institut für Mineralogie und Mineralische Rohstoffe, Technische Universität Clausthal, Adolph-Roemer-Str. 2A, D–38678 Clausthal-Zellerfeld, Germany José CArlos polÔnIA Rua Paris 105, 34800-000, Caeté – MG, Brazil MArC ChoQUeTTe Département de géologie et de génie géologique, Université Laval, Pavillon Adrien-Pouliot, Québec, QC G1K 7P4, Canada ABsTrACT Arborescent, botryoidal and reniform grains of platinum and palladium from Córrego 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 note- worthy 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 inter- growth 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 relected in the Fe-poor bulk composition of the alloys. The Pt–Pd nuggets from Córrego 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. Keywords: platinum, palladium, mercury-rich palladiferous platinum, pdpt, alluvial nuggets, Córrego Bom sucesso, serro, Minas Gerais, Brazil. soMMAIre des grains arborescents, botryoïdaux et réniformes de platine et de palladium provenant de Córrego Bom sucesso, serro, Minas Gerais, Brésil, ont fait l’objet d’analyses avec un microscope électronique à balayage et une microsonde électronique. les pépites atteignent une longueur maximale de 2.5 mm et montrent une zonation du noyau à la bordure en pt, pd et hg. Un grain est particulier pour son liseré de platine palladifère riche en mercure et des lamelles internes vermiformes enrichies en hg et Au. le liseré contient 10–20% de hg par poids, et possède une stoechiométrie empirique proche de (pt,hg) 2 pd. des cristaux hexa- gonaux de platine palladifère forment une zone périphérique d’un grain arborescent. Ces cristaux ont une composition analogue