Os-Ir-Ru alloys and Ru-Os sulfides from clinopyroxenite-dunite complexes: a combined EMPA, LA MC-ICP-MS and N-TIMS study

The Kondyor, Inagli, Bor-Uryah and Guli clinopyroxenitedunite complexes, located in the limits of the Siberian Craton, are associated with platinum-group elements (PGE) placer deposits. The compositionally diverse platinum-group minerals (PGM) from these placer deposits have been studied by a number of modern techniques including SEM, EMPA, LA MC-ICP-MS and N-TIMS. Detrital Pt-Fe alloy grains constitute the majority of PGM at Kondyor and Inagli, whereas Os-rich alloys [i.e., osmium Os, iridian osmium (Os,Ir)], osmian iridium (Ir,Os) and Ru-Os sulfides are subordinate species. In contrast, Os-rich alloy grains predominate at Guli, whereas the detrital PGM assemblage at Bor-Uryah contains equal amounts of Pt-Fe and Os-Ir alloy grains. Since the concentration of Re in all PGM grains is less than 0.05 wt.%, the isotopic effect resulting from the radioactive decay of 187Re can be considered negligible. Consequently, the value of 187Os/188Os in the detrital PGM corresponds to that in the source area at the time of PGM formation. The ¹⁸⁷Os/¹⁸⁸Os value measured by LA MC-ICPMS and N-TIMS in osmium (80-100 wt. % Os) varies from 0.12326 ± 0.00001 to 0.12529 ± 0.00001. The 187Os/188Os value in PGMs with Os contents between 50 and 79 wt.% (i.e., PGE alloys of the system Os-Ir-Ru and Os-Ru-Ir-Pt) ranges from 0.12309 ± 0.00002 to 0.12775 ± 0.00001. The ¹⁸⁷Os/¹⁸⁸Os values in PGM, which contain Os in the range 1- 31 wt. % (i.e., laurite, rutheniridosmine and erlichmanite) show a less pronounced variation, from 0.12333 ± 0.00015 to 0.12462 ± 0.00009. In contrast to PGM from ophiolites, the main set of PGM in this study (n=96) closely matches the present-day ¹⁸⁷Os/¹⁸⁸Os values for depleted mid-ocean ridge mantle (DMM; 0.1246 ± 0.0014, [1]).