Abstract:We describe assemblages of platinum-group minerals (PGM) and associated PGE-Au phases found in alluvium along the River Bolshoy Khailyk, in the western Sayans, Russia. The river drains the Aktovrakskiy ophiolitic complex, part of the Kurtushibinskiy belt, as does the Zolotaya River~15 km away, the site of other placer deposits. Three groups of alloy minerals are described: (1) Os-Ir-Ru compositions, which predominate, (2) Pt-Fe compositions of a Pt 3 Fe stoichiometry, and (3) Pt-Au-Cu alloys, which likely crystallized in the sequence from Au-(Cu)-bearing platinum, Pt(Au,Cu), Pt(Cu,Au), and PtAuCu 2 , to PtAu 4 Cu 5 . The general trends of crystallization of PGM appear to be: [Os-Ir-Ru alloys] → Pt 3 Fe-type alloy (with inclusions of Ru-dominant alloy formed by exsolution or via replacement of the host Pt-Fe phase) → Pt-Au-Cu alloys. We infer that Rh and Co mutually substitute for Fe, not Ni, and are incorporated into the pentlandite structure via a coupled mechanism of substitution: [Rh 3+ + Co 3+ + → 3Fe 2+ ]. Many of the Os-Ir-Ru and Pt-Fe grains have porous, fractured or altered rims that contain secondary PGE sulfide, arsenide, sulfarsenide, sulfoantimonide, gold, Pt-Ir-Ni-rich alloys, and rarer phases like Cu-rich bowieite and a Se-rich sulfarsenide of Pt. The accompanying pyroxene, chromian spinel and serpentine are highly magnesian, consistent with a primitive ultramafic source-rock. Whereas the alloy phases indicate a highly reducing environment, late assemblages indicate an oxygenated local environment leading to Fe-bearing Ru-Os oxide (zoned) and seleniferous accessory phases.

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Abstract:We report the results of a mineralogical investigation of placer samples from the upper reaches of the Sisim watershed, near Krasnoyarsk, in Eastern Sayans, Russia. The placer grains are predominantly Os-Ir-(Ru) alloys (80%) that host various inclusions (i.e., platinum-group elements (PGE)-rich monosulfide, PGE-rich pentlandite, Ni-Fe-(As)-rich laurite, etc.) and subordinate amounts of Pt-Fe alloys. Analytical data (wavelength-and energy-dispersive X-ray spectroscopy) are presented for all the alloy minerals and the suite of micrometer-sized inclusions that they contain, as well as associated grains of chromian spinel. The assemblage was likely derived from chromitite units of the Lysanskiy mafic-ultramafic complex, noted for its Ti-(V) mineralization. In the Os-Ir-(Ru) alloys, the ratio Ru/Ir is ≤1, Ir largely substitutes for Os, and compositional variations indicate the scheme [Ir + Ru] → 2Os. In contrast, in the laurite-erlichmanite series, Ir and Os are strongly and positively correlated, whereas Ir and Ru are negatively correlated; Ru and Os are inversely correlated. These compositions point to the scheme [Os 2+ + 2Ir 3+ + ] → 4Ru 2+ or alternatively, to Os 2+ + Ir 2+ → 2Ru 2+ . We deduce a potential sequence of crystallization in the parental rock and address the effects of decreasing temperature and increasing fugacity of sulfur and arsenic on the assemblage. Inclusions of Ti-rich minerals in the alloy grains are consistent with the Lysanskiy setting; the complete spectrum of chromite-magnesiochromite compositions indicates that an important part of that complex was eroded. A localized fluid-dominated micro-environment produced the unique association of laurite with monazite-(Ce), again considered a reflection of the special attributes of the Lysanskiy complex.