Abstract
A number of platinum-group mineral (PGM) grains with characteristic optical and geochemical properties are found in the recently discovered PGE-mineralized chromitite at Wadi Hilti in the deeper mantle section of the northern Oman ophiolite. They usually occur as polygonal inclusions within fresh chromian spinel. They have exclusively Ru-Os-Ir compositions with low totals on conventional microprobe analysis and show the same Ru-Os-Ir ratios as Os-rich laurite, (Ru, Os, Ir)S2, commonly found in the chromitite. Qualitative and quantitative analyses show that oxygen is one of major constituents of these PGM grains, and they are Ru-Os-Ir oxides. They are characteristically lower in Fe-Ni contents than Ru-Os-Ir oxides ever reported. The characteristic radial-fibrous textures as well as a hollow space around the polygons possibly mean their secondary origin with significant volume reduction. The similarity of element ratios and mode of occurrence between the PGE oxides and laurite indicates that the former have been produced by desulfurization/oxidation of the latter. Formation of the PGE oxides from Oman has been performed with an increase of Eh at a very early stage of serpentinization.
