In situ alteration of platinum-group minerals at low temperature; evidence from serpentinized and weathered chromitite of the Vourinos Complex, Greece

Abstract

Chromitite samples from the Vourinos ophiolite complex, in northwestern Greece, are found to contain platinum-group minerals (PGM) derived from the in situ alteration of primary (magmatic) PGM at low temperature. Primary laurite and ruthenian pentlandite located adjacent to cracks in partially serpentinized chromitite are replaced along their rim by metallic Ru and awaruite. Laurite in serpentine displays a rim depleted in S, Ir, and Os, and enriched in Fe and Ni. Several PGE alloys enriched in Ir, Pt, and Pd (Ir - Ph - Fe - Ni - Cu, Ni - Fe - Pt, Cu - Pt - Pd), and an unknown Ru - Os - Ir - Fe compound, were discovered in strongly weathered chromitite from Agriatses, usually associated with chlorite, ferrian chromite, and abundant limonite. Experimental monitoring of the fluorescence effect due to chromite in the analysis of various PGM leads to the inference that the Ru - Os - Ir - Fe compound contains oxygen. It was not possible to establish if the mineral is an intergrowth of Ru-rich alloy with Fe-hydroxide (goethite or limonite) or a true PGE oxide. On the basis of the restricted occurrence of these PGM to weathered chromitite, as well as paragenetic and compositional considerations, the secondary assemblage seems to have formed at low temperatures by in situ desulfurization of a PGM sulfide precursor, during serpentinization, followed by oxidation of the desulfurized PGM, under conditions of weathering.