Platinum-group minerals in gold-bearing placers asoociated with the veluće ophiolite complex, Yugoslavia

The first comprehensive investigation of the platinum-group minerals (PGM) in the gold-bearing placers associated with the Veluće ophiolite complex, Yugoslavia, has shown Ru-Os-Ir and Pt-Fe alloys as the main mineral 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, irarsite, sperrylite, keithconnite, Pd antimonides, iridarsenite(?) (Ir,Rh,Pt)(As,Te)2, unnamed (Ru,Rh,Ir)(Te,Sb,As)2 and ruthenium oxide(?). Only platarsite is found as an inclusion in a discrete irarsite aggregate. Inclusions of PGM within Ru-Os-Ir and Pt-Fe alloys were trapped either as already solidified mineral grains or they crystallized from trapped, PGE-rich liquid droplets. Likewise, crystallographically oriented intergrowths among PGM also indicate their origin by exsolution from solid solutions. Textural and compositional evidence suggests crystallization of the PGM at magmatic conditions with limited replacement of the primary Ru-Os-Ir alloys by laurite, hollingworthite, irarsite and ruthenium oxide(?). The resemblance of compositions of the PGM in chromitites with those in the placers indicates the ultramafic suite of the ophiolite complex as the likely source for the alluvial PGM mineralization. It appears that most of the PGM precipitated directly from the silicate melt under relatively low f(S2) but notable contributions of As, Sb and Te in deposition of the platinum-group elements (PGE).