Formation of a secondary platinum-group mineral assemblage in chromitites from the Herbeira ultramafic massif in Cabo Ortegal, NW Spain

Many different PGM showing various stages of alteration from primary magmatic PGE concentrations have been found in chromitites from Cabo Ortegal with anomalous values of < or = 13 ppm Sigma PGE. Laurite, common in chromitites from elsewhere, is rare in these samples but is always enclosed within chromite grains where it is protected from alteration. Most of the PGM occur either at the edge of chromite grains or within altered silicate interstitial to the chromite grains. Most of the PGM are associated with sulphides (pentlandite, chalcopyrite and millerite) and these are interpreted as having exsolved from primary magmatic sulphides; they include Pt- and Pd-sulphides of the braggite solid solution series. PGE-rich alloys, amalgams, arsenides, bismuthides, tellurides and antimonides, including tetraferroplatinum, potarite, zvyagintsevite, sperrylite, irarsite, hollingworthite and Ru-Os-bearing Fe-Ni-sulphides are often associated with the Pt- and Pd-sulphides and may also be alteration products. On the edge of chromite grains, where alteration has been most intense, the sulphides are sometimes surrounded by replacement haloes of Pt- and Pd-bearing oxides forming a further alteration stage in this sequence of PGM genesis.