Zoning of laurite (RuS2)-erlichmanite (OsS2): implications for the origin of PGM in ophiolite chromitites

We have investigated several chromite deposits in the Mayari-Baracoa Ophiolite Belt (eastern Cuba) and in the Dobromirtsi metamorphosed ultramafic (ophiolitic) massif (SE Bulgaria) with regard to zoning in platinum-group minerals (PGM) of the laurite (RuS2)-erlichmanite (OsS2) solid solution series. We found several zoned laurite-erlichmanite grains all included in unaltered chromite crystals. On the basis of internal ordering and compositional variations, three different patterns of zoning have been distinguished: (i) grains with Os-poor (laurite) core and Os-rich rim (normal zoning), (ii) grains with Os-rich core and Os-poor rim (reverse zoning) and (iii) grains made up of a complex intergrowth of Os-rich, Os-poor laurite and erlichmanite (oscillatory zoning). The origin of zoning is interpreted mainly as a result of changes in f(S-2),f(O-2) and to a lesser extent in melt temperature, before PGM trapping in chromite. A possible case of heterogeneous physicochemical environment in which such changes can take place is when chromite forms during magma mingling of silicate melts in the upper mantle. The preservation of laurite-erlichmanite zoning is attributed to the low diffusion coefficient of Ru and Os in pyrite-type structures.