—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.