Manifestation of various stages PGE mineralization in the different locations Ospa-Kitoy ophiolite massif (East Sayan, Russia).

A study of chrome-spinels and PGE mineralization (PGM) from the podiform chromitites has been carried out on the area of four locations of the Ospa-Kitoy ophiolite massif (northern and southern branches East Sayan ophiolite). It has been established that different PGM assemblages formed at different stages of formation of the Ospa-Kitoy ophiolite massif, at various temperature and fluid regimes, are present at four sites. The chromite pods show both disseminated and massive structures. There are veins of massive chromitites, 0.01-0.5 m thick and 1-10 m long, rarely disseminated, schlieren, and rhythmically banded ores, which are discordant to the host ultramafic rocks. (Os-Ir-Ru) alloys occur as inclusions in the Cr-spinel or intergrowth with them (fig 3a). In addition, FePt3 alloys are found in the PGM assemblage. In such grains, decomposition structures of solid solutions represented by osmium lamellas can be observed. Polyphase PGM assemblage: (Os, Ir, Ru), (Ni, Fe, Ir),  (Ir, Ru, Pt)AsS, CuIr2S4, (Os, Ru)As2, Rh-Sb,  PtCu, and Pd5Sb2 are localized in serpentine, in close association with sulfides, sulfoarsenides, arsenides of nickel.

Figure 1. Chromitite bodies and PGE mineralization in Ospa-Kitoy ophiolite massif: 1 – Harh mountain (north branch of the ophiolites); 2 –  lake Sekretnoye (apically Zun-Ospa river); 3 – stream Zmeevikovyi (south branch of the ophiolites); 4 – Harh-Ilchir site (south flank Harh mountain).

Figure 2. Composition of  Os-Ir-Ru alloys: 1 – Harh mountain, 2 – lake Sekretnoye site, 3 – stream Zmeevikovyi.

Based on chemical and microtextural features of the PGM´s and assemblage with magmatic and hydrothermal minerals in the chromitites, it is established that each studied location of chromitites at different stages of PGM formation are exhibited. High-temperature magmatic Os-Ir-Ru alloys are widely exhibited in the Harh and Zmeevikovyichromitites. In the Harh-Ilchir site, there is no magmatic PGM and are established sulfoarsenides and arsenides Ru, Ir, which are formed from the residual fluid phase in the late magmatic stage. Chromitites in the lake Sekretnoye MPG are contained high-temperature magmatic (Os-Ir-Ru) alloys, and there are signs of PGE remobilization with Os⁰ , Ru⁰ , (Ir-Ru) alloys. Remobilization processes during serpentinization and fluid interaction of peridotites and chromitites.

In addition, it should note that the PGM assemblage of the Zmeevikovyi and Harh-Ilchir locations has been undergone by influence metamorphogenic fluids with increased activity of O₂, As, Sb. and these minerals can be formed directly in hypergenic environments. PGM҆'s such as (Ru, Rh, Pt)Sb, Rh-Sb were created at this stage.

Figure 3. BSE images of primary and secondary PGM: Harh location: а) individual grain of magmatic (Os-Ir-Ru) with microinclusion native Os; b) remobilized polyphase aggregate native Os, (Ir-Ru) (CuIr₂S₄); location Sekretnoye lake: с) inclusion magmatic (Os-Ir-Ru) in the chromite grain; d) remobilized polyphased aggregate (Ir-Ru), (Rh-Sb); location stream Zmeevikovyi: e) idiomorphic magmatic grain (Os-Ir) replaced by (Ir,Ru)AsS, with separation remobilized (Os,Ir);  Harh-Ilchir site: f) inclusion of Pd₅Sb₂ in the heazlewoodite (Hzl).

Analytics  made in Analytical Centre SB RAS. Supported by RFBR  19-05-00764а and  Russian Ministry of Education and Science.