Platinum-group minerals as indicators of sulfur fugacity in ophiolitic upper mantle: An example from chromitites of the Ray-Iz ultramafic complex, Polar Urals, Russia

The Ray-Iz ophiolite complex (Polar Urals, Russia) contains large chromite deposits associated with concordant to discordant bodies of dunite emplaced within harzburgitic mantle tectonite. Primary inclusions (1-25 mu m) of pl atinum-group-minerals (PGM) occur in the chromite, and consist of laurite, erlichmanite, and Os-Ir alloys, accompanied by cuproiridsite (Ir2CuS4), kas hinite (Ir2S3), rhodian pentlandite, unknown sulfides with stoichiometries varying from (Ni>Fe greater than or equal to Cu)(2)(Ir>Rh)S-3 to (Ni>Fe gre ater than or equal to Cu)(2)(Ir>Rh)S-4, irarsite, cherepanovite (RhAs), and unknown (Rh,Ni)(2)As. The PGM paragenesis indicates deposition through an unusually wide range of f(S-2) and T compared with mantle-hosted chromitite s from other ophiolite complexes. This wide range is ascribed to the crysta llization of PGM and chromite down to a relatively low temperature (T), ena bling the relative increase of f(S-2). Such f(S-2)-T conditions, previously observed in chromitites of Tiebaghi (New Caledonia) and Kempirsai (Souther n Urals, Kazakhstan), seem to be peculiar to a chromite-forming system in f luid-metasomatized upper mantle of ophiolite complexes.