First Findings of the Platinum-Group Minerals from Ultramafites of the Idzhim Mafic-Ultramafic Massif (Western Sayan)

Platinum-group minerals have been first identified 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 one of largest massifs of this belt. The minerals have been found in pentlandite, awaruite, and nickel arsenides (NiS, Ni2S) in the form of finely dispersed inclusions, the diagnosis of which, due to their small size, has been 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, zaccariniite, Ir-bearing erlichmanite, and unnamed sulfides with the crystal chemical formula Me2S have been identified. All the platinum-group mineral grains found are mainly localized either in peripheral parts of sulfides grains, awaruite, and wairauite, or in a silicate matrix in the immediate vicinity of these minerals. The content of platinum-group elements and their distribution in restite ultramafic rocks have been apparently controlled by partial melting of the primary peridotite substrate. Upon partial melting, the extraction of sulfur and Pt-group platinoides (Pt, Pd, and Rh) into the silicate melt has led to a decrease in the S2 fugacity and the accumulation of Ir-group platinoides (Os, Ir, and Ru) in the monosulfide solid solution (mss), from which primary Os‒Ir-bearing pentlandite has then crystallized. The subsequent transformation of this sulfide has led to the appearance of platimun-group-containing awaruite and nickel arsenides.