PGE mineralization and melt composition of chromitites in Proterozoic ophiolite complexes of Eastern Sayan, Southern Siberia

The Ospino-Kitoi and Kharanur ultrabasic massifs represent the northern and southern ophiolite branches respectively of the Upper Onot ophiolitic nappe and they are located in the southeastern part of the Eastern Sayan (SEPES ophiolites). Podiform chromitites with PGE mineralization occur as lensoid pods within dunites and rarely in harzburgites or serpentinized peridotites. The chromitites are classified into type I and type II based on their Cr#. Type I (Cr# = 59–85) occurs in both northern and southern branches, whereas type II (Cr# = 76–90) occurs only in the northern branch. PGE contents range from ∑PGE 88–1189 ppb, Pt/Ir 0.04–0.42 to ∑PGE 250–1700 ppb, Pt/Ir 0.03–0.25 for type I chromitites of the northern and southern branches respectively. The type II chromitites of the northern branch have ∑PGE contents higher than that of type I (468–8617 ppb, Pt/Ir 0.1–0.33). Parental melt compositions, in equilibrium with podiform chromitites, are in the range of boninitic melts and vary in Al2O3, TiO2 and FeO/MgO contents from those of type I and type II chromitites. Calculated melt compositions for type I chromitites are (Al2O3)melt = 10.6–13.5 wt.%, (TiO2)melt = 0.01–0.44 wt.%, (Fe/Mg)melt = 0.42–1.81; those for type II chromitites are: (Al2O3)melt = 7.8–10.5 wt.%, (TiO2)melt = 0.01–0.25 wt.%, (Fe/Mg)melt = 0.5–2.4. Chromitites are further divided into Os-Ir-Ru (I) and Pt-Pd (II) based on their PGE patterns. The type I chromitites show only the Os-Ir-Ru pattern whereas type II shows both Os-Ir-Ru and Pt-Pd patterns. PGE mineralization in type I chromitites is represented by the Os-Ir-Ru system, whereas in type II it is represented by the Os-Ir-Ru-Rh-Pt system. These results indicate that chromitites and PGE mineralization in the northern branch formed in a suprasubduction setting from a fluid-rich boninitic melt during active subduction. However, the chromitites and PGE mineralization of the southern branch could have formed in a spreading zone environment. Mantle peridotites have been exposed in the area with remnants of mantle-derived reduced fluids, as indicated by the occurrence of widespread highly carbonaceous graphitized ultrabasic rocks and serpentinites with up to 9.75 wt.%. Fluid inclusions in highly carbonaceous graphitized ultrabasic rocks contain CO, CO2, CH4, N2 and the δ13C isotopic composition (−7.4 to −14.5‰) broadly corresponds to mantle carbon.