In this paper, we present the first detailed study on the chromitites and platinum-group element mineralization (PGM) of the Ulan-Sar’dag ophiolite (USO), located in the Central Asian Fold Belt (East Sayan). Three groups of chrome spinels, differing in their chemical features and physical–chemical parameters, under equilibrium conditions of the mantle mineral association, have been distinguished. The temperature and log oxygen fugacity values are, for the chrome spinels I, from 820 to 920 °C and from (−0.7) to (−1.5); for chrome spinels II, 891 to 1003 °C and (−1.1) to (−4.4); and for chrome spinels III, 738 to 846 °C and (−1.1) to (−4.4), respectively. Chrome spinels I were formed through the interaction of peridotites with mid-ocean ridge basalt (MORB)-type melts, and chrome spinels II were formed through the interaction of peridotites with boninite melts. Chrome spinels III were probably formed through the interaction of andesitic melts with rocks of an overlying mantle wedge. Chromitites demonstrate the fractionated form of the distribution of the platinum-group elements (PGE), which indicates a high degree of partial melting at 20–24% of the mantle source. Two assemblages of PGM have been distinguished: The primary PGE assemblage of Os-Ir-Ru alloys-I, (Os,Ru)S₂, and IrAsS, and the secondary PGM assemblage of Os-Ir-Ru alloys-II, Os⁰, Ru⁰, RuS₂, OsS₂, IrAsS, RhNiAs with Ni, Fe, and Cu sulfides. The formation of the secondary phases of PGE occurred upon exposure to a reduced fluid, with a temperature range of 300–700 °C, log sulfur fugacity of (−20), and pressure of 0.5 kbar. We have proposed a scheme for the sequence of the formation and transformation of the PGMs at various stages of the evolution of the Ulan-Sar’dag ophiolite.

Formation of noble metal nanoparticles is related to various geological processes in the supergene zone. Dispersed mineral phases appear during weathering of rocks with active participation of...

The platinum-group minerals (PGM) in placer deposits provide important information on the types of their primary source rocks and ores and formation and alteration conditions. Different characteristics of minerals can be determined by a set of conventional and modern in situ analytical techniques (scanning electron microscopy (SEM) and electron probe microanalysis (EPMA)). A study of PGM from placers of southern Siberia (Kuznetsk Alatau, Gornaya Shoria, and Salair Ridge) shows that the morphology and composition of PGM grains, the texture, morphology, and composition of silicate, oxide, and intermetallic microinclusions, and the type of mineral alteration can serve as efficient indicators of the primary sources of PGM. The widespread rock associations in the Kuznetsk Alatau, Gornaya Shoria, and Salair Ridge, the compositions of PGM and microinclusions in them, and the dominant mineral assemblages testify to several possible primary sources of PGE mineralization: (1) Uralian–Alaskan-type intrusions; (2) ophiolite associations, including those formed in a subduction zone; (3) ultramafic alkaline massifs; and, probably, (4) rocks of the picrite–basalt association. The preservation of poorly rounded and unrounded PGM grains in many of the studied placers of the Altai–Sayan Folded Area (ASFA) suggests a short transport from their primary source.

The platinum-group minerals (PGM) in placer deposits provide important information on the types of their primary source rocks and ores and formation and alteration conditions. The article shows for the first time the results of a study of placer platinum mineralization found in the upper reaches of the Kitoy River (the southeastern part of the Eastern Sayan (SEPES)). Using modern methods of analysis (scanning electron microscopy), the authors studied the microtextural features of platinum-group minerals (PGM), their composition, texture, morphology and composition of microinclusions, rims, and other types of changes. The PGM are Os-Ir-Ru alloys with a pronounced ruthenium trend. Many of the Os-Ir-Ru grains have porous, fractured, or altered rims that contain secondary PGE sulfides, arsenides, sulfarsenides, Ir-Ni-Fe alloys, and rarer selenides, arsenoselenides, and tellurides of the PGE. The data obtained made it possible to identify the root sources of PGM in the placer and to make assumptions about the stages of transformation of primary igneous Os-Ir-Ru alloys from bedrock to placer. We assume that there are several stages of alteration of high-temperature Os-Ir-Ru alloys. The late magmatic stage is associated with the effect of fluid-saturated residual melt enriched with S, As. The post-magmatic hydrothermal stage (under conditions of changing reducing conditions to oxidative ones) is associated with the formation of telluro-selenides and oxide phases of PGE. The preservation of poorly rounded and unrounded PGM grains in the placer suggests a short transport from their primary source. The source of the platinum-group minerals from the Kitoy River placer is the rocks of the Southern ophiolite branch of SEPES and, in particular, the southern plate of the Ospa-Kitoy ophiolite complex, and primarily chromitites.

Micrometric inclusions in platinum-group minerals (PGMs) from alluvial placers carry considerable information about types of primary rocks and ores, as well as conditions of their formation and alteration. In the present contribution, we attempt to show, with concrete examples, the significance of the data on the composition and morphology of micrometric inclusions to genetic interpretations. The PGM grains from alluvial placers of the Gornaya Shoria region (Siberia, Russia) were used as the subject of our investigation. In order to determine the chemical composition of such ultrafine inclusions, high-resolution analytical methods are needed. We compare the results acquired by wavelength-dispersive spectrometry (WDS; electron microprobe) and energy-dispersive spectrometry (EDS) and scanning electron microscopy (SEM) methods. The results obtained have good convergence. The EDS method is multi-elemental and more effective for mineral diagnostics in comparison with WDS, which is its certain advantage. The possible conditions for the formation of inclusions and layers of gold, sulfoarsenides and arsenides in Pt₃Fe grains, which have an original sub-graphic and layered texture pattern, are discussed. They are the result of solid solution and eutectic decompositions and are associated with the magmatic stages of grain transformation, including the result of the interaction of Pt₃Fe with a sulfide melt enriched with Te and As.

This study is concerned with the typochemical features of heavy-concentrate platinum from diamond placers of the Mayat-Vodorazdel’nyi site in the Anabar River basin. Platinum occurs there as Fe-Pt solid solutions (ferroan platinum of Ru-Rh specialization, ~ 90%), as well as minor Pd- and Ir-ferroplatinum. Among PGE minerals, Os-Ir-Ru alloys and chengdeite are revealed. Inclusions of PGE minerals are thoroughly studied, and the scales of their spread, the composition of mineral phases, and the relations between them are determined. Ruthenium-containing high-Rh ferroplatinum bears a lot of inclusions of Rh-S composition: Rh-containing PGE thiospinels and sulfides and arsenides of Rh, Ru, and Pt. In low-Rh platinum, mineral inclusions are much scarcer and usually form other parageneses. The mineral compositions in the system Rh-S-Ru indicate to the existence of a long isomorphous series Rh₃S₄-Ru₃S₄, with the content of Ru₃S₄ reaching 15 at.%.