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

Abstract

Many different platinum-group minerals (PGM) displaying stages of alteration from primary magmatic platinum-group element (PGE) concentrations have been located in chromitites from Cabo Ortegal, NW Spain containing anomalous values of up to 13 ppm total 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. However the majority of the PGM are located either at the edge of chromite grains or within altered silicate interstitial to the chromite grains and show complex and irregular secondary alteration textures. Most of the PGM are associated with sulphides, namely pentlandite, chalcopyrite and millerite, and these PGM are interpreted as having formed from PGE exsolved from primary magmatic sulphide and include Pt- and Pd-sulphides belonging to the solid solution braggite 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 it is suggested that they are also alteration products. On the edge of chromite grains, where alteration has been most intense, Pt- and Pd-sulphides are sometimes surrounded by replacement haloes of Pt- and Pd-bearing oxides which form a further alteration stage in this sequence of PGM genesis.