Home – Home – Jornals – Russian Geology and Geophysics 2019 number 7

We present results of investigation into the composition and parageneses of pyrrhotite at the Sovetskoe gold-quartz deposit (Yenisei Ridge, Russia) have been studied. The variability of parameters (temperature T and sulfur fugacity f_S2) during the stage crystallization of pyrrhotite-containing assemblages has been assessed from the composition of this mineral (Fe_0.873±0.02S-Fe_0.885±0.02S) and its parageneses. The compositions Fe_0.873-0.875S close to Fe₇S₈ (Apy + Po + Rut + Qz), for which the estimated formation parameters are T = 486-465 ºС and lg f S2 = -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 (Fe_0.873-0.881S) associated with pyrite in native gold (950 ‰) (Au + Po + Py), the formation parameters are T = 489-410 ºС and lg f S2 = -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 Fe_0.874-0.878S and form at 479-443 ºС and lg f S2 = -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 (Fe_0.878-0.885S) and, correspondingly, lower temperatures of formation, 432-382 ºС, and lg f S2 = -6.27 to -7.95. The lg f S2-Т diagrams have been calculated for the systems Fe-S and Ag-Au-S in the temperature range 25-700 ºС 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 Fe_{1- x} S (0 < x < 0.125), Ag_1-zAu_z (z = 0, 0.25, 0.5, and 1), and Ag_{2- y} Au _y S (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 ‰, Ag_0.5Au_0.5-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 show 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.