Contrasting platinum-group mineral assemblages from two different podiform chromitite localities in the Pindos ophiolite complex, Greece

Platinum-group minerals (PGM) have been identified in chromitite from two localities in the Pindos ophiolite complex in Greece, Milia and Korydallos. At Milia, the PGM are predominantly Os, Ir, and Ru, i.e., IPGE-bearing, as is typical of many chromitites in ophiolitic complexes; they consist mainly of IPGE alloys and sulfarsenides rather than laurite. At Korydallos, the assemblage is dominated by Pt- and Pd-bearing PGM characteristic of rarer Pt- and Pd-enriched ophiolites. These PGM include Pt–Pd alloys with the base metals Ni, Cu, and Fe, and traces of Au, PGM arsenides and sulfarsenides, sperrylite and members of hollingworthite – irarsite – platarsite solid-solution series. The base-metal-enriched minerals at Korydallos include a Cu-rich Pd–Pt alloy and a Ni–Fe alloy with 1–2% Pd. The association of Pt- and Pd-bearing PGM with base metals suggests that these PGE were collected by small quantities of immiscible sulfide liquid. The PGM are irregular in shape and are located almost exclusively in the altered silicate matrix interstitial to the chromite grains, on the edges of chromite grains, and in veins of silicate cross-cutting chromite. We suggest that the IPGE-bearing and Pt- and Pd-bearing PGM alloys and sulfarsenides are secondary and were formed by loss of sulfur and gain of arsenic during serpentinization. A further stage of alteration produced oxides and hydroxides of Pd around the PGE-bearing alloys and arsenides