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

摘要

Platinum nuggets recovered from a placer locatedabout 6.5km NNW of Kompiam (Enga Province, New GuineaHighlands) show characteristics typical of PGE mineralizationfrom Alaskan-type intrusions. They are composed of (Pt, Fe)alloys with average (Fe + Cu) concentrations varying from 14 to17 at.%, that are significantly lower than required forisoferroplatinum indicating a disordered structure. Rare nuggetscontain exsolutions of native osmium. Platinum crystallized afterchromite and some silicates (diopsidic clinopyroxene andamphibole). Chromite included in nuggets shows a highFe~(3+)/M~(3+) ratio and high TiO_2 content. The TiO_2concentration in chromite can be used as a discriminating factorto distinguish between PGM originating from ophioliticcomplexes and those related to Alaskan-type complexes. Thechemical composition of the alloys indicates a source stronglydepleted in Ru and Pd while enriched in Pt with respect tochondrites. However, this PGE fractionation cannot be attributedto immiscibility or sulphuration processes. Two types of silicateinclusions were observed in the Pt nuggets: (1) monomineralinclusions of a Na-, Ca-, Mg-, Al-silicate (amphibole?),plagioclase of the composition An_(20-50), and rare diopsiderepresenting cumulate mineral phases caught by the ore-formingliquid; (2) hydromagmatic inclusions of ore-forming silicateliquid manifested by skeletal clinopyroxene crystals, pargasiticamphibole and albite. The chemical composition ofclinopyroxene crystallized from the hydromagmatic liquidindicates that the PGE transporting system had an alkaline nature.It is therefore affirmed that the platinum nuggets from theKompiam area originated from an Alaskan-type complex,probably represented by the Maramuni diorite associated withandesitic lavas and pyroclastics.Platinum-bearing placers inPapua New Guinea are thus derived from two magmatic sources:ophiolitic complexes and Alaskan-type intrusions.