First findings of platinum group minerals from ultramafites of the Idzhim mafic-ultramafic massif (Western Sayan)

In plastically deformed harzburgite and dunite of the Idzhim mafic-ultramafic massif, which is part of the Kurtushiba ophiolite belt of the Western Sayan and is one of largest massifs of this belt, platinum group minerals (PGM) were identified for the first time. They were found in pentlandite, awaruite and nickel arsenides (NiS, Ni2S) in form of finely dispersed inclusions, the diagnosis of which, due to their small size, was carried out only qualitatively. Native osmium, Ir-bearing osmium, native ruthenium, garutiite, tetraferroplatinum, unnamed (Pt,Ir)Fe and (Ni,Cu,Pd,Pt)2-3Fe phases, zaccarinite, Ir-bearing erlikmanite and unnamed sulfides with crystal chemical formula Me2S were quantitatively identified and characterized. All PGM grains found are predominantly localized either in peripheral parts of grains of sulfides, awaruite and wairauite, or in silicate matrix in immediate vicinity of these minerals. The platinum group elements (PGE) content and their distribution in restite ultramafic rocks was apparently controlled by partial melting of primary peridotite substrate. During partial melting, the extraction of sulfur and Pt-group platinoides (Pt, Pd, Rh) into the silicate melt led to a decrease in S2 fugacity and the accumulation of Ir-group platinoides (Os, Ir, Ru) in monosulfide solid solution (mss), from which subsequently primary Os-Ir-bearing pentlandite crystallized. The subsequent transformation of this sulfide led to appearance of PGE-containing awaruite and nickel arsenides, as well as to everything discovered diversity of identified PGM.