Alluvial platinum‐group minerals from the Manampotsy area, East Madagascar

Abstract

Two platinum‐group mineral (PGM) concentrates collected from the same placer in the Manampotsy area located 120 km southeast of Antananarivo, Madagascar, were studied. Nuggets, which ranged in size from 0.1 to 1 mm, were characteristically rounded but a complex aggregate was also observed. Except for the latter which was multiphase (composed of laurite, Ir‐Os and Pt‐Fe alloys), they consisted of isoferroplatinum (Pt3Fe) with variable contents of Cu, Rh, Pd, Ir and Os in solid solution. Two varieties of inclusions were encountered within the nuggets, PGM and silicates, but no base‐metal sulphide was found. The most common PGM inclusions were Os‐Ir‐(Ru) alloys occurring as laths corresponding either to trapped minerals or to exsolution. Other minerals, occurring either isolated or as complex assemblages, were: (1) alloys — unnamed Pt3Cu and (Pd,Pt,Au)2Cu; (2) sulphides — laurite, erlichmanite, cooperite, braggite, kashinite, cuprorhodsite and several unnamed species; (3) sulfarsenides — hollingworthite; (4) tellurides — keithconnite and unnamed (Rh,Pd)3Te, and telluro‐arsenide — vincentite? Complex PGM assemblages, forming rounded multiphase aggregates, resulted from fractional crystallization in a closed system of PGE‐rich liquid droplets, mainly composed of Pd, Pt, Ir, Rh, Au, Cu, Ni, Te, S, trapped in isoferroplatinum grains at high temperature. Two types of silicate inclusion were found: (1) inclusions consisting of one or two minerals (amphibole, albite or two‐phase amphibole‐clinopyroxene and amphibole‐K‐feldspar inclusions; and (2) mixed inclusions of glass (Si, Al, and alkali‐rich) and amphiboles. No inclusion exceeded 50 μm in size. The nature and characteristics of the inclusions indicate that the nuggets formed in a magmatic environment. The presence of a large variety of PGM included in the Pt‐Fe nuggets suggests that the original mineralization consisted principally of Pt (as Pt3Fe, the only mineral found in the concentrates) with lesser amounts of other PGE (dominantly Pd, with Os, Ru, Rh). Mineralogical data indicate that this mineralization could have been formed in an ‘Alaskan‐type’ complex.