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 [Os2+ + 2Ir3+ + □] → 4Ru2+ or alternatively, to Os2+ + Ir2+ → 2Ru2+. 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.
Highlights
The Sisim placer zone contains various deposits of minerals and unusual phases rich in the platinum-group elements (PGE)
In placer samples of chromian spinel from the Sisim zone, we find a broad range of compositions along the magnesiochromite–chromite solid solution, with Mg# [100 Mg/(Mg + Fe2+ + Mn)] in the range 76–3 and a MgO content up to 16.6 wt % (Table 1)
The chromite grains analyzed in the lode serpentinite are fairly similar in composition the detrital grains recovered from the Sisim placer zone (Figure 4)
Summary
The Sisim placer zone contains various deposits of minerals and unusual phases rich in the platinum-group elements (PGE). The deposits are found in an 80 × 30 km area in the southern portion of the huge Krasnoyarskiy kray territory in central Siberia, not far from Krasnoyarsk in the southwestern part of the Eastern Sayans, Russia (Figure 1). Minerals 2018, 8, x FOR PEER REVIEW Federation. The Au–PGE-bearing placers are developed alluvially along the River Sisim and its tributaries, Thewhich. Au–PGE-bearing are developed alongKotel’, the River tributaries, among are the riversplacers Ko, Levyi.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
