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 Baranyevskoe Au-Ag epithermal deposit of low-sulfidation (LS) type is located on the Kamchatka Peninsula in the Neogene-Quaternary Central Kamchatka Volcanic Belt, where Au-bearing quartz veins are usually accompanied by veinlet stockworks. Two economic associations are typical of the Baranyevskoe deposit. The first corresponds to gold-pyrite-quartz association with low-grade native gold (521–738‱) intergrown with pyrite. Some accessory Au-Ag minerals within the early association were also identified: acanthite AgS2, hessite AgTe2, lenaite Ag(Fe,Cu)S2, petzite Ag3AuTe2, utenbogardite Ag3AuS2 and unnamed Ag-Sb-As sulfosalts. The former Au-Ag minerals were most likely formed in the temperature range of 320–330 °C based on the study of arsenopyrite thermometers and fluid inclusions. The second, a gold-sulfosalt-quartz association, includes high-grade native gold (883-941‱) in intergrowth with chalcopyrite. Cuprous phases (bornite, chalcocite, heerite, native copper, Cu-Zn solid solutions), Bi-rich sulfosalts (aikinite PbCuBiS3, emplectite CuBiS2, witticenite Cu3BiS3), stannoidite Cu8Fe3Sn2S12, mawsonite Cu6Fe2SnS8), Au-bearing galena, Te-free and Bi-rich tetrahedrite-tennantite represent this association. Fluid inclusions in gold-sulfosalt-quartz association are characterized by homogenization temperature ranging from 226 to 298 °C, and salinity from 0.4 to 1.2 wt. % NaCl eq.
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