PLATINUM-GROUP MINERALS IN GOLD-BEARING PLACERS ASSOCIATED WITH THE VELUCE OPHIOLITE COMPLEX, YUGOSLAVIA

The first comprehensive investigation of the platinum-group minerals ( PGM) in the gold-bearing placers associated with the Veluce ophiolite complex, Yugoslavia, has shown Ru-Os-Ir and Pt-Fe alloys as the main m ineral phases, in the approximate ratio 2:1. A variety of PGM included in individual grains of Ru-Ir-Rh and Pt-Fe alloys are represented by alloys in the systems Ru-Os-Ir, Ru-Ir-Pt, Ir-Os-Rh and Pt-Cu +/- (Sb) as well as Rh-Pd and Rh-Ru arsenides, laurite, hollingworthite, irarsi te, sperrylite, keithconnite, Pd antimonides, iridarsenite(?) (Ir,Rh,P t)(As,Te)(2) unnamed (Ru,Rh,Ir)(Te,Sb,As)(2) and ruthenium oxide(?). O nly platarsite is found as an inclusion in a discrete irarsite aggrega te. Inclusions of PGM within Ru-Os-Ir and Pt-Fe alloys were trapped ei ther as already solidified mineral grains or they crystallized from tr apped, PGE-rich liquid droplets. Likewise, crystallographically orient ed intergrowths among PGM also indicate their origin by exsolution fro m solid solutions. Textural and compositional evidence suggests crysta llization of the PGM at magmatic conditions with limited replacement o f the primary Ru-Os-Ir alloys by laurite, hollingworthite, irarsite an d ruthenium oxide(?). The resemblance of compositions of the PGM in ch romitites with those in the placers indicates the ultramafic suite of the ophiolite complex as the likely source for the alluvial PGM minera lization. It appears that most of the PGM precipitated directly from t he silicate melt under relatively low f(S-2) but notable contributions of As, Sb and Te in deposition of the platinum-group elements (PGE).