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—On the territory of the Egor’evsk district, 3500 results of probe microanalyses of surface gold particles taken from 17 placers, weathering crusts, and orebodies were processed and summarized. High-fineness and mercury-containing gold is predominantly found within mother lodes and placers, and medium- and low-fineness gold is distributed in subordinate and dramatically subordinate quantities, especially in the placers. A unique feature of the gold composition, rarely occurring in other districts, is the constant and commonly simultaneous presence of mercury and copper impurities. Analysis of Ag, Hg, and Cu content variations has enabled us to identify five main grades of gold. The mother lodes of the predominant gold grades are metasomatites with beresite and listwaenite compositions, which are developed primarily after lower Cambrian volcanoterrigenous-carbonate rocks and ore-bearing mafic dikes. Mercury-containing gold is characteristic of beresites, but copper-bearing gold is typical of listwaenites. The relationship between corresponding grades of surface and ore gold is confirmed by the presence of microinclusions in the gold grains. Nonconformity between the content of gold from mother loads, weathering crusts, and placers is explained by the losses of Hg and Ag impurities by endogenous gold under subsurface hypergene conditions. Identification of mineral-geochemical properties of surface gold is of exceptional practical importance in ore-grade gold mineralization prediction.

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.

A microinclusion of colloform high-purity platinum in a grain of platinum-group minerals (PGM) from the alluvial gold-bearing placer deposit in the south of Western Siberia (Russia) was detected and characterized for the first time. It is different in composition, texture, and conditions of formation from high-purity platinum of other regions described in the literature. The main characteristics of investigated high-purity platinum are colloform-layered texture, admixture of Fe (0.37-0.78 wt.%), and paragenesis of Cu-rich isoferroplatinum, hongshiite, and rhodarsenide. The PGM grain with high-purity platinum is multiphase and heterogeneous in texture. It is a product of intensive metasomatic transformation of Cu-rich isoferroplatinum (Pt3 (Fe0.6Cu0.4)). The transformation was carried out in two stages: 1 -copper stage including three substages (Cu-rich isoferroplatinum, copper platinum and hongshiite); and 2 -arsenic (rhodarsenide). The formation of high-purity platinum was separated in time from the formation of isoferroplatinum and was carried out by precipitation from postmagmatic solutions.

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