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Ibrahim Uysal, Karadeniz Technical University, Department of Geology (Turkey)
Federica Zaccarini, Department of Applied Geological Sciences and Geophysics, University of Leoben (Austria)
M. Burhan Sadiklar, Karadeniz Technical University, Department of Geology (Turkey)
Mahmud Tarkian, Institute of Mineralogy and Petrology, University of Hamburg (Germany)
H. Juergen Bernhardt, ZEM, Institute of Geology, Mineralogy and Geophysic, University of Ruhr (Germany)
Giorgio Garuti, Department of Earth Sciences, University of Modena and Reggio Emilia (Italy)
Simona Bigi, Department of Earth Sciences, University of Modena and Reggio Emilia (Italy)
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A number of podiform chromitites, associated with different ophiolites, such as Vourionos and Veria (Greece), Loma Peguera (Dominican Republic), Ray-Iz (Polar Urals) and with the Nurali lherzolite-gabbro complex (Southern Urals), 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. These PGM occur along cracks and fissures of the host chromite and always associated with altered minerals, mostly chlorite, serpentine and ferrian chromite. These PGM are characterized by peculiar optical properties and micro-hardness suggesting that they might represent a new mineral species of Platinum group elments (PGE)-oxides. However, it was not possible to support this conclusion with structural data, owing to the small size of the grains. The presence of oxygen in these PGM was determined by in-situ electron microprobe analysis, although results were affected by spurious fluorescence from direct or secondary excitation of the adjacent minerals, because of the small size of the analyzed grains.
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 m, were found in heavy concentrates obtained using the hydro-separation technique. The electron microprobe analysis of these grains, liberated from their including matrix, are not affected by spurious fluorescence 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%, respectively. The Si and Mg are positively correlated (r = +0.96) 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. As a consequence, part of the analyzed oxygen may derive from excitation of these silicate inclusions. If the weight percent Ru, Fe, Os, Ir are recalculated as ideal oxides with a constant valence of +4 for the PGE and +3 for Fe, the analyses totals are close to 100 wt%. In all the analyzed grains the content of PGE is higher than those 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 desulfuration 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, 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 or under metasomatic fluids released along shear zone, as suggested for the Veria chromitites.
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