IN-SITU ALTERATION OF PLATINUM-GROUP MINERALS AT LOW-TEMPERATURE - EVIDENCE FROM SERPENTINIZED AND WEATHERED CHROMITITE OF THE VOURINOS COMPLEX, GREECE

Chromitite samples from the Vourinos ophiolite complex, in northwester n Greece, are found to contain platinum-group minerals (PGM) derived f rom the in situ alteration of primary (magmatic) PGM at low temperatur e. Primary laurite and ruthenian pentlandite located adjacent to crack s in partially serpentinized chromitite are replaced along their rim b y metallic Ru and awaruite. Laurite in serpentine displays a rim deple ted in S, Ir, and Os, and enriched in Fe and Ni. Several PGE alloys en riched in Ir, Pt, and Pd (Ir - Ph - Fe - Ni - Cu, Ni - Fe - Pt, Cu - P t - 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 monit oring of the fluorescence effect due to chromite in the analysis of va rious PGM leads to the inference that the Ru - Os - Ir - Fe compound c ontains 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 thes e PGM to weathered chromitite, as well as paragenetic and compositiona l considerations, the secondary assemblage seems to have formed at low temperatures by in situ desulfurization of a PGM sulfide precursor, d uring serpentinization, followed by oxidation of the desulfurized PGM, under conditions of weathering.