Platinum nuggets from the Kompiam area, Enga Province, Papua New Guinea: evidence for an Alaskan-type complex

Abstract

¶Platinum nuggets recovered from a placer located about 6.5 km NNW of Kompiam (Enga Province, New Guinea Highlands) show characteristics typical of PGE mineralization from Alaskan-type intrusions. They are composed of (Pt, Fe) alloys with average (Fe+Cu) concentrations varying from 14 to 17 at.%, that are significantly lower than required for isoferroplatinum indicating a disordered structure. Rare nuggets contain exsolutions of native osmium. Platinum crystallized after chromite and some silicates (diopsidic clinopyroxene and amphibole). Chromite included in nuggets shows a high Fe3+/M3+ ratio and high TiO2 content. The TiO2 concentration in chromite can be used as a discriminating factor to distinguish between PGM originating from ophiolitic complexes and those related to Alaskan-type complexes. The chemical composition of the alloys indicates a source strongly depleted in Ru and Pd while enriched in Pt with respect to chondrites. However, this PGE fractionation cannot be attributed to immiscibility or sulphuration processes. Two types of silicate inclusions were observed in the Pt nuggets: (1) monomineral inclusions of a Na-, Ca-, Mg-, Al-silicate (amphibole?), plagioclase of the composition An20–50, and rare diopside representing cumulate mineral phases caught by the ore-forming liquid; (2) hydromagmatic inclusions of ore-forming silicate liquid manifested by skeletal clinopyroxene crystals, pargasitic amphibole and albite. The chemical composition of clinopyroxene crystallized from the hydromagmatic liquid indicates that the PGE transporting system had an alkaline nature. It is therefore affirmed that the platinum nuggets from the Kompiam area originated from an Alaskan-type complex, probably represented by the Maramuni diorite associated with andesitic lavas and pyroclastics. Platinum-bearing placers in Papua New Guinea are thus derived from two magmatic sources: ophiolitic complexes and Alaskan-type intrusions.