We describe assemblages of platinum-group minerals (PGM) and associated PGE–Au phases found in alluvium along the River Bolshoy Khailyk, in the western Sayans, Russia. The river drains the Aktovrakskiy ophiolitic complex, part of the Kurtushibinskiy belt, as does the Zolotaya River ~15 km away, the site of other placer deposits. Three groups of alloy minerals are described: (1) Os–Ir–Ru compositions, which predominate, (2) Pt–Fe compositions of a Pt₃Fe stoichiometry, and (3) Pt–Au–Cu alloys, which likely crystallized in the sequence from Au–(Cu)-bearing platinum, Pt(Au,Cu), Pt(Cu,Au), and PtAuCu₂, to PtAu₄Cu₅. The general trends of crystallization of PGM appear to be: [Os–Ir–Ru alloys] → Pt₃Fe-type alloy (with inclusions of Ru-dominant alloy formed by exsolution or via replacement of the host Pt–Fe phase) → Pt–Au–Cu alloys. We infer that Rh and Co mutually substitute for Fe, not Ni, and are incorporated into the pentlandite structure via a coupled mechanism of substitution: [Rh³⁺ + Co³⁺ + □ → 3Fe²⁺]. Many of the Os–Ir–Ru and Pt–Fe grains have porous, fractured or altered rims that contain secondary PGE sulfide, arsenide, sulfarsenide, sulfoantimonide, gold, Pt–Ir–Ni-rich alloys, and rarer phases like Cu-rich bowieite and a Se-rich sulfarsenide of Pt. The accompanying pyroxene, chromian spinel and serpentine are highly magnesian, consistent with a primitive ultramafic source-rock. Whereas the alloy phases indicate a highly reducing environment, late assemblages indicate an oxygenated local environment leading to Fe-bearing Ru–Os oxide (zoned) and seleniferous accessory phases.

Native gold and its satellite minerals were studied throughout the 300 m section of oxidized ores of the Olympiada deposit (Eastern Siberia, Russia). Three zones are identified in the studied section: Upper Zone ~60 g/t Au; Middle Zone ~3 g/t Au; Lower Zone ~20 g/t Au. Supergene and hypogene native gold have been found in these zones. Supergene gold crystals (~1 μm), their aggregates and their globules (100 nm to 1 μm) predominate in the Upper and less in Middle Zone. Relic hypogene gold particles (flattened, fracture and irregular morphology) are sporadically distributed throughout the section. Spongiform gold occurs in the Lower Zone at the boundary with the bedrock, as well as in the bedrock. This gold formed in the process of oxidation of aurostibite, leaching of impurities and its further dissolution. Hypogene gold is commonly isolated but for supergene gold typically associated with ferric (hydr)oxides. New formation of gold occurred due to oxidation of sulfide ores and release of “invisible” gold, as well as dissolution, mobilization and re-deposition of metallic hypogene gold. A model for the formation of oxidized ores with the participation of meteoric and low-temperature hydrothermal waters has been proposed.

Highly atypical mineralization involving Pd-Pt, Au-Ag, REE, Y, Zr, U, Th, and Cl-F-enriched minerals is found in zones with base metal sulfides (BMS; ~5 vol.% to 20 vol.%) in the eastern portion of the Oktyabrsky deposit in the Norilsk complex (Russia). The overall variations in Mg# index, 100 Mg/(Mg + Fe²⁺ + Mn), in host-rock minerals are 79.8 → 74.1 in olivine, 77.7 → 65.3 in orthopyroxene, 79.9 → 9.2 in clinopyroxene, and An_79.0 → An_3.7. The span of clinopyroxene and plagioclase compositions reflects their protracted crystallization from early magmatic to late interstitial associations. The magnesian chromite (Mg# 43.9) trends towards Cr-bearing magnetite with progressive buildups in oxygen fugacity; ilmenite varies from early Mg-rich to late Mn-rich variants. The main BMS are chalcopyrite, pyrrhotite, troilite, and Co-bearing pentlandite, with less abundant cubanite (or isocubanite), rare bornite, Co-bearing pyrite, Cd-bearing sphalerite (or wurtzite), altaite, members of the galena-clausthalite series and nickeline. A full series of Au-Ag alloy compositions is found with minor hessite, acanthite and argentopentlandite. The uncommon assemblage includes monazite-(Ce), thorite-coffinite, thorianite, uraninite, zirconolite, baddeleyite, zircon, bastnäsite-(La), and an unnamed metamict Y-dominant zirconolite-related mineral. About 20 species of PGM (platinum group minerals) were analyzed, including Pd-Pt tellurides, bismuthotellurides, bismuthides and stannides, Pd antimonides and plumbides, a Pd-Ag telluride, a Pt arsenide, a Pd-Ni arsenide, and unnamed Pd stannide-arsenide, Pd germanide-arsenide and Pt-Cu arseno-oxysulfide. The atypical assemblages are associated with Cl-rich annite with up to 7.54 wt.% Cl, Cl-rich hastingsite with up 4.06 wt.% Cl, ferro-hornblende (2.53 wt.% Cl), chlorapatite (>6 wt.% Cl) and extensive solid solutions of chlorapatite, fluorapatite and hydroxylapatite, Cl-bearing members of the chlorite group (chamosite; up to 0.96 wt.% Cl), and a Cl-bearing serpentine (up to 0.79 wt.% Cl). A decoupling of Cl and F in the geochemically evolved system is evident. The complex assemblages formed late from Cl-enriched fluids under subsolidus conditions of crystallization following extensive magmatic differentiation in the ore-bearing sequences.

The Blagodatnoye deposit with 340 t gold reserves is one of the most productive mines in Russia. Modern methods of studying fluid inclusions were used to determine the properties of fluids that formed this deposit. A comprehensive study revealed that the Blagodatnoye gold deposit was formed between 120 and 350 °C and at 0.2–2.6 kbar, and from fluids with salinities ranging from 0.5 to 30 wt.% (NaCl–eq.). These fluids are: 1—water–carbon dioxide; 2—carbon dioxide–hydrocarbon; 3—highly saline aqueous. According to Raman spectroscopy and gas chromatography–mass spectrometry, ore–forming fluids contained H₂O, CO₂, hydrocarbons and oxygenated organic compounds, sulfonated, nitrogenated and halogenated compounds. Early oxidized water–carbon dioxide fluids formed barren associations of the deposit. Later reduced carbon dioxide–hydrocarbon fluids had a key role in the formation of gold-bearing quartz veins. The stable isotope data (δ³⁴S = 0.8 to 21.3‰, δ¹³C = −2.8 to −20.9‰, ³He/⁴He = 0.14 ± 0.3 × 10^–6) suggest the ore-forming fluids have a crustal source.

The Mössbauer spectroscopy method was used to study the ligand microstructure of natural arsenopyrite (31 specimens) from the ores of the major gold deposits of the Yenisei Ridge (Eastern Siberia, Russia). Arsenopyrite and native gold are paragenetic minerals in the ore; meanwhile, arsenopyrite is frequently a gold carrier. We detected iron positions with variable distribution of sulfur and arsenic anions at the vertexes of the coordination octahedron {6S}, {5S1As}, {4S2As}, {3S3As}, {2S4As}, {1S5As}, {6As} in the mineral structure. Iron atoms with reduced local symmetry in tetrahedral cavities, as well as iron in the high-spin condition with a high local symmetry of the first coordination sphere, were identified. The configuration {3S3As} typical for the stoichiometric arsenopyrite is the most occupied. The occupation degree of other configurations is not subordinated to the statistic distribution and varies within a wide range. The presence of configurations {6S}, {3S3As}, {6As} and their variable occupation degree indicate that natural arsenopyrites are solid pyrite {6S}, arsenopyrite {3S3As}, and loellingite {6As} solutions, with the thermodynamic preference to the formation of configurations in the arsenopyrite–pyrite–loellingite order. It is assumed that in the variations as part of the coordination octahedron, the iron output to the tetrahedral positions and the presence of high-spin Fe cations depend on the physical and chemical conditions of the mineral formation. It was identified that the increased gold concentrations are typical for arsenopyrites with an elevated content of sulfur or arsenic and correlate with the increase of the occupation degree of configurations {5S1As}, {4S2As}, {1S5As}, reduction of the share of {3S3As}, and the amount of iron in tetrahedral cavities.

—We present results of an investigation into the composition and parageneses of pyrrhotite at the Sovetskoe gold–quartz deposit (Yenisei Ridge, Russia). The variability of parameters (temperature T and sulfur fugacity fS2) during the stage crystallization of pyrrhotite-containing assemblages has been assessed from the composition of this mineral (Fe0.873±0.02S–Fe0.885±0.02S) and its parageneses. The compositions Fe0.873–0.875S close to Fe7S8 (Apy + Po + Rut + Qz), for which the estimated formation parameters are T = 486–465 °C and log fS2 = –4.71 to –5.28, are typical of early pyrrhotite in the form of microinclusions in arsenopyrite, associated with rutile and quartz. According to the composition of inclusions of pyrrhotite microcrystals (Fe0.873–0.881S) associated with pyrite in native gold (950‰) (Au + Po + Py), the formation parameters are T = 489–410 °C and log fS2 = –4.63 to –6.98. Coarse pyrrhotite grains containing microinclusions of relict arsenopyrite and galena, sometimes, in aggregate with siderite (Po + Apy + Ga + Sid), and pyrrhotite in aggregate with pyrite and siderite (Py + Po + Sid) have composition Fe0.874–0.878S and form at 479–443 °C and log fS2 = –4.9 to –5.9. The xenomorphic pyrrhotite microinclusions present together with galena and native gold (950‰) in pyrite crystals (Py + Po + Ga + Au) are characterized by higher contents of iron (Fe0.878–0.885S) and, correspondingly, lower temperatures of formation, 432–382 °C, and log fS2 = –6.27 to –7.95. The log fS2–T diagrams have been calculated for the systems Fe–S and Ag–Au–S in the temperature range 25–700 °C with regard for the stability fields of iron sulfides (pyrite FeS2, troilite FeS, and pyrrhotite Fe7S8), phases Fe11S12, Fe10S11, and Fe9S10, metallic iron, native sulfur, uytenbogaardtite, petrovskaite, and solid-solution phases Fe1–xS (0 < x < 0.125), Ag1–zAuz (z = 0, 0.25, 0.5, and 1), and Ag2–yAuyS (y = 0, 0.5, 1, and 2). The calculation results have demonstrated that there is a field of petrovskaite and uytenbogaardtite solid solutions and Au–Ag alloys (>670‰, Ag0.5Au0.5–Au) in the stability field of the pyrrhotite–pyrite parageneses of the Sovetskoe deposit. The gold and silver contents in iron sulfides of the Sovetskoe deposit show that the Au/Ag ratios in pyrrhotites (0.002–2.4) and pyrites (0.004–13) are lower than those in high fineness (950–980‰) gold (19–50). The difference in the Au/Ag ratios in these minerals and the results of thermodynamic calculations indicate the possible presence of Au–Ag sulfides and Au–Ag alloys of lower fineness in the pyrrhotite–pyrite ores of the studied deposit. The absence of visible mineral forms of gold sulfides from the ores suggests that these sulfides are present in finely dispersed or invisible microscopic forms. The pyrrhotite compositions in pyrite-containing parageneses as well as Au/Ag in pyrites, pyrrhotites, and visible native gold in sulfide ores of other gold and gold–silver deposits can be used to assess the possible presence of nanosized solid microinclusions of sulfide and other gold and silver forms.