Composition of Pyrrhotite as an Indicator of Gold Ore Formation Conditions at the Sovetskoe Deposit (Yenisei Ridge, Russia)

摘要

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 f(S2)) 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 degrees C and log 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 (Fe0.873-0.881S) associated with pyrite in native gold (950 parts per thousand) (Au + Po + Py), the formation parameters are T = 489-410 degrees C and log 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 Fe0.873-0.881S and form at 479-443 degrees C and log f(S2) = -4.9 to -5.9. The xenomorphic pyrrhotite microinclusions present together with galena and native gold (950 parts per thousand) in pyrite crystals (Py + Po + Ga + Au) are characterized by higher contents of iron (Fe0.873-0.881S) and, correspondingly, lower temperatures of formation, 432-382 degrees C, and log f(S2) = -6.27 to -7.95.