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.

The Eldorado low-sulfide gold–quartz deposit, with gold reserves of more than 60 tons, is located in the damage zone of the Ishimba Fault in the Yenisei Ridge and is hosted by Riphean epidote–amphibolite metamorphic rocks (Sukhoi Pit Group). Orebodies occur in four roughly parallel heavily fractured zones where rocks were subject to metamorphism under stress and heat impacts. They consist of sulfide-bearing schists with veins of gray or milky-white quartz varieties. Gray quartz predominating in gold-bearing orebodies contains graphite and amorphous carbon identified by Raman spectroscopy; the contents of gold and amorphous carbon are in positive correlation. As inferred from thermobarometry, gas chromatography, gas chromatography–mass spectrometry, and Raman spectroscopy of fluid inclusions in sulfides, carbonates, and gray and white quartz, gold mineralization formed under the effect of reduced H2O–CO2–HC fluids with temperatures of 180 to 490 °C, salinity of 9 to 22 wt.% NaCl equiv, and pressures of 0.1 to 2.3 kbar. Judging by the presence of 11% mantle helium (3He) in fluid inclusions from quartz and the sulfur isotope composition (7.1–17.4‰ δ34S) of sulfides, ore-bearing fluids ascended from a mantle source along shear zones, where they “boiled”. While the fluids were ascending, the metalliferous S- and N-bearing hydrocarbon (HC) compounds they carried broke down to produce crystalline sulfides, gold, and disseminated graphite and amorphous carbon (the latter imparts the gray color to quartz). Barren veins of milky-white quartz formed from oxidized mainly aqueous fluids with a salinity of < 15 wt.% NaCl equiv at 150–350 °C. Chloride brines (> 30 wt.% NaCl equiv) at 150–260 °C impregnated the gold-bearing quartz veins and produced the lower strata of the hydrothermal-granitoid section. The gold mineralization (795–710 Ma) was roughly coeval to local high-temperature stress metamorphism (836–745 Ma) and intrusion of the Kalama multiphase complex (880–752 Ma).

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––The first results on the composition of fluids from native gold and associated pyrite and quartz have been obtained. Despite the small amount of analytical data, these results are of scientific and practical interest. The identified geochemical criteria can be used for the assessment of gold ore shoots and the substantiation of prospecting in the region. The one-act shock-destructive extraction of volatiles from fluid inclusions and their pyrolysis-free gas chromatography–mass spectrometry analysis made it possible to determine the composition of fluids in native gold and in associated pyrite and quartz. Based on these data, we have first shown that fluids in native gold, pyrite, and quartz are a mineral-forming multicomponent system. In addition to water and carbon dioxide, the studied fluid inclusions contain representatives of at least 11 homologous series of organic compounds, including oxygen-free aliphatic and cyclic hydrocarbons (paraffins, olefins, cyclic alkanes and alkenes, arenes, and polycyclic aromatic hydrocarbons), oxygenated hydrocarbons (alcohols, esters, furans, aldehydes, ketones, and carboxylic acids), and nitrogened, sulfonated, halogenated, and siliconorganic compounds. The portion of hydrocarbons together with S–N–Cl–F–Si compounds reaches 52.0 rel.% in fluid inclusions from native gold, 10.1 rel.% in fluid inclusions from pyrite, and 18.0 rel.% in fluid inclusions from quartz. Gold-transporting gas fluids have reducing properties. Pyrite and quartz contain oxidized water–carbon dioxide fluids with low contents of hydrocarbons and nitrogen–halogenated compounds.

—New thermobarogeochemical and isotope-geochemical data are presented, which show the intricate and long history of the formation of the unique Olimpiadinskoe gold deposit with predicted gold reserves of >1000 tons on the Yenisei Ridge. Metal-bearing oxidized water–carbon dioxide and reduced carbon dioxide–hydrocarbon fluids participated (at the same time or successively) in the formation of the deposit at 220–470 °C and 0.6–2.5 kbar. Fluids of gold-bearing mineral assemblages include CO2, hydrocarbons, and S-, N-, and halogen-containing compounds capable of transporting ore elements, including gold. Highly mobile carbon dioxide–hydrocarbon fluids were responsible for the appearance of disseminated gold mineralization in large bodies of quartz–carbonate–mica schists serving as geochemical barriers in the Olimpiadinskoe deposit. The deposit formed in the period from 817 to 660 Ma, which fits the time interval from crystallization to cooling (868–721 Ma) of the most proximal multiphase Chirimba granitoid pluton. The hydrothermal activity of the fluids that formed the Olimpiadinskoe deposit lasted at least 100–150 Myr year.

The Panimba gold deposit lies in the rocks of the epidote–amphibolite metamorphism facies and is confined to the exocontact zone of the Chirimba granitoid massif. Fluid inclusions in quartz and sulfides of two sites of the deposit, Mikhailovka and Zolotoi Brook, were studied by thermobarogeochemistry, gas chromatography, and chromatography-mass spectrometry. We have established that gold–quartz veins of the deposit were formed by metal-bearing Mg–Na–Cl-containing water–carbon dioxide–hydrocarbon fluids with salinity of 8–23 wt.% NaCl eq. at temperatures of 180 to 410 °C and pressures of 0.2 to 3.3 kbar. Hydrocarbons and nitrogen- and sulfur-containing compounds of the fluids can transport gold and might be positive indicators of the gold presence in quartz veins. Fluids with salinity of >30–40 wt.% and sulfur isotope values (δ34S) of 0.9 to 6.7‰ of sulfides are the result of the action of postmagmatic solutions of the nearby Chirimba granitoid massif. The age of hydrothermal gold–sulfide mineralization of the Panimba deposit is within 817.2 ± 5.3–744 ± 17 Ma and falls in the time interval of crystallization of the Chirimba intrusion, 868.9 ± 6.5 to 721.4 ± 1.6 Ma, but it is considerably younger than the age of the regional metamorphism (996.0 ± 32–889.0 ± 26 Ma).

—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.

At the Gerfed gold deposit, fluid inclusions were studied by thermobarometry, gas chromatography, Raman spectroscopy, and ICP MS in quartz samples of three types: quartzites, Au-poor (<1–2 ppm) feathering veins, and Au-rich (>2.8–10 ppm) feathering veins. It has been found that these three types were produced from fluids differing in composition and thermobarogeochemical parameters. The quartzites formed from low-salinity (<7.0 wt.% NaCl equiv) homogeneous fluids of essentially aqueous–chloride composition at 120–230 °C and 0.1–0.5 kbar. The gas phase in these fluids comprises H2O, CO2, CH4, and N2, with CO2/(CO2 + H2O) = 0.04–0.15 and CO2/CH4 = 2.2–3.8. The Au-poor feathering veins formed from homogeneous and heterogeneous fluids at 150–300 °C and 0.5–2.0 kbar. The salinity of the fluids increased to 10 wt.% NaCl equiv. The gas phase in them comprises H2O, CO2, N2, and CH4. Here, CO2/(CO2 + H2O) = 0.09–0.17 and CO2/CH4 = 2.2–2.3. The Au-rich feathering veins formed from heterogeneous and more saline (6.0–23.3 wt.% NaCl equiv) CO2–H2O fluids at higher temperatures (150–400 °C) and pressures (1.1–2.5 kbar). In this fluid CO2/(CO2 + H2O) = 0.18–0.27 and CO2/CH4 = 4.1–20.8. All three quartz types show negative Eu anomalies and a distinct predominance of LREE over HREE. Differently directed trends of REE and Eu/Sm in the quartzites and feathering veins suggest that the fluids were produced from different sources. The fluids of the gold-bearing quartz veins are enriched in K, Li, and Rb, and those of the Au-poor feathering veins, in Sr and Na. The quartzites have low Rb and Sr and similar Na and K contents. Areas with a high and bonanza gold content in feathering-vein stockworks formed when high-temperature saline H2O–CO2 fluids were superposed on the Au-poor quartzites and feathering veins.