Vol 8, No 2 (2022)

The article is devoted to tomamaeite Cu₃Pt from inclusions in a native osmium grain from alluvial river sediments in the Nizhnie Sergi region, Sverdlovsk oblast. It is the fourth finding of this mineral in the Urals. In addition to tomamaeite, native osmium Os_0.36Ir_0.33Ru_0.31 contains the inclusions of digenite (or, possibly, roxbyite), millerite, Fe-Ni-Pt alloy, an unidentified Pd mineral, magnesiohornblendite, and lowCa pyroxene. In composition, tomamaeite is close to the ideal formula Cu₃Pt and contains minor amounts of (wt. %) Pd (up to 2.0), Ir (up to 1.7), Ru and Ni (up to 0.6), and Fe (up to 0.2). In addition to native osmium, two grains of native ruthenium Ru_0.54Os_0.23Ir_0.23 and Ru_0.40Os_0.31Ir_0.29 are found in sediments. Our data suggest that the PGE minerals in river sediments could be sourced from mafic-ultramafic massifs located in the Nizhnie Sergi area. The formation of tomamaeite and coexisted minerals is related to the process of postmagmatic transformation of the primary PGE mineral assemblage, probably, during serpentinization of ultramafic rocks.

The previous and new data on minerals and mineral assemblages of the Sugur copper deposit in the Sugur Mountains of South Urals are summarized. The ores of the deposit host abundant calcite, andradite, diopside, magnetite, chromite and serpentine together with pyrite, chalcopyrite, chalcocite, pyrrhotite, millerite, and rare baddeleyite. The genesis of the deposit was previously considered both contact-metasomatic and hydrothermal. The new data allow us to attribute the copper mineralization to a skarn-carbonatite system, which is locally recognized in the Karabash ultramafic massif.

The studies of minerals and melt inclusions in ultramafic rocks of the Kurtushiba ophiolite belt (Western Sayan) allowed us to establish the conditions of mineral-forming processes during their formation. The features of the compositions of minerals of ultramafic rocks of the Idzhim, Kalny and Ergaki massifs are considered. The presence of melt inclusions directly indicates a magmatic origin of some Cr-spinels from dunites of the Idzhim massif. A close assemblage of Cr-spinels from chromitites with igneous Cr-spinels is important because of the possible formation of chromite ore from melts. The study of melt inclusions revealed the evolution of melts (picrites – picrobasalts – basalts – basaltic andesites) during the consecutive crystallization of minerals from the Western Sayan ophiolites. Computational modeling based on the compositions of melt inclusions and olivine indicates that olivine of ultramafic rocks of the Kurtushiba ophiolites crystallized at a decreasing pressure from 9.2 to 6.2 kbar and a decreasing temperature from 1550 to 1430 °С from picritic melts, which evolved to picrobasaltic melts. With a decrease in pressure to 3.8–2.3 kbar and temperature of picrobasaltic melts (1240–1230 °C), the formation of olivine terminated following by the crystallization of clinopyroxene from basaltic magmas at 2.7–1.9 kbar and about 1215 °C. The postmagmatic stage was characterized by plastic deformations of the Western Sayan ultramafic rocks at a decreasing temperature from 850 to 640 °C, which resulted in a successive change from protogranular to mesogranular structures and then to porphyroclastic and porphyroleist structures.

The paper presents the results of the first LA-ICP-MS U-Pb geochronological studies of 60 perovskite grains from spurrite marbles of the Kochumdek contact aureole (East Siberia). Perovskite is a dominant accessory mineral of U- and Th-poor marbles, which formed at the contact with traps of the Kuz’movsky complex (southwestern margin of the Tunguska Syneclise). The chemically homogeneous perovskite is characterized by limited substitutions: Ti⁴⁺ → Zr⁴⁺ (up to 2.5 mol. % CaZrO₃) and Ca²⁺Ti⁴⁺ → REE³⁺Fe³⁺ (up to 1.8 mol. % REEFeO₃). Perovskite is the main host for Th (up to 1550 ppm), U (up to 450 ppm), Ti, Zr, and LREEs (La + Ce + Pr + Nd); the element incorporation ratios are K_Th = 516–870 and К_U = 374. The U-Pb isotopic age of perovskite of 248.0 ± 7.2 Ma is in agreement with isotopic age values of traps of the Kuz’movsky complex.