Occurrence of rare Ru-Fe-Os-lr-oxide and associated Platinum-group minerals (PGM) in the chromitite of Mugla ophiolite, SW-Turkey

摘要

A number of podiform chromitites, associated with different ophiolites, proved to contain minute grains, generally less than 15 microns in size, of unusual Platinum-group mineral (PGM) composed of Ru-Fe-Os-Ir and O (GARUTI & ZACCARINI 1997, GARUTI et al. 1999a, b, ZACCARINI et al. 2005, PROENZA et al. 2007, 2008, TSOUPAS & ECONOMOU-ELIOPOULOS 2008). In this contribution, we describe a further occurrence of oxidized PGE compounds from chromitites of the Mugla ophiolite complex, located in SW Turkey. A number of grains, with a size comprised between 20-60 jam, were found in heavy concentrates obtained using the hydroseparation technique. The electron microprobe analysis of these grains, liberated from their including matrix, are not affected by spurious fluorescence caused by the direct or secondary excitation of the adjacent phases and confirms the presence of oxygen as major constituent of the PGM. Some grains contain Si and Mg up to 9.1 and 10.4 at percent, respectively. The Si and Mg are positively correlated (r = +0.92) but do not correlate with any other elements including oxygen. This evidence strongly supports the observation that the oxidized PGM contain sub-microscopic particles of chlorite or serpentine. In all the analyzed grains the content of PGE is higher than that of Fe, and their compositions in terms of Ru-Os-Ir overlap the field of coexisting laurite. This observation confirms the conclusion that the oxidized PGE compounds derived from desulfurization of laurite at low temperature, with substitution of the removed S by Fe and O. In contrast with the chromitites from Vourinos where these oxides formed under weathering conditions (GARUTI & ZACCARINI 1997), the paragenetic assemblage (ferrian chromite, serpentine, chlorite) of the Ru-Fe-Os-Ir oxides of Mugla indicates that the origin of these PGM is compatible with rodingitization and serpentinization under hydrothermal conditions as it was proposed for the chromitites of the Urals (GARUTI et al. 1997, ZACCARINI et al. 2004) or under metasomatic fluids released along shear zone, as suggested for the Veria chromitites (TSOUPAS & ECONOMOU-ELIOPOULOS 2008).