Bismuthite from Au–Bi and Post-Gold Sb Mineralizations within the Darasun Deposit, Eastern Transbaikalya

The Late Jurassic late-orogenic volcanogenic-plutonogenic Darasun deposit of the gold-sulphide-quartz formation holds Au–Bi and post-gold Sb mineralizations. Carbonate-quartz-sulfide veins in Western part of the deposit are surrounded by listvenite rims. Their golden ores were formed under conditions of low activity of S₂, they contain pyrrhotine, arsenopyrite, chalcopyrite, pyrite, bismuthate I (Bi_1.89–1.98Sb_0.11–0.02)₂S₃, galenobismuthite, nests of bismuth and ikunolite Bi₄S₃. There is observed exsolution of ikunolite mainly into the native bismuth (Bi_0.98–1Sb_0.02–0) and bismuthite-II; the composition of bismuthite-II in center of aggregates with the bismuth is (Bi_1.96–1.97Sb_0.04–0.03)₂S₃, whereas the composition on their periphery is an more antimonian one is (Bi_1.91–1.92Sb_0.09–0.08)₂S₃. While the high fineness gold (970–935 ‰) arose there by the action of gold-bearing hydrothermal solutions, the native bismuth has been partly replaced with maldonite. Jonassonite and Pb–Bi sulphosaults (mainly, cosalite Pb₂Bi₂S₅) were formed later in these ores. The overlaying Sb mineralization has given formation not of antimonite (stibnite), but of Pb–Sb sulphosaults (moeloite Pb₆Sb₆S₁₅, etc.), pseudomorphs of chalcostibite after chalcopyrite, as well as aurostibite AuSb₂ after minerals of gold. The replacement of maldonite by aurostibite was resulted in appearance of bismuthate III. The probable replacement reaction is: 2Au₂Bi + 6Sb solv. + + 3Sb₂S₃ solv. → 4AuSb₂ + Bi₂S₃. Bismuthite III (Bi_1.72–1.96Sb_0.29–0.94)₂(S_2.98–3Se_0–0.02)₃, containing 1–7 wt % of Sb, is a product of the maldonite replacement by aurostibite. Moeloite and stibian bismuthate III arose by the Sb mineralization overlaying ores with cosalite. The probable replacement reaction is: 3Pb₂Bi₂S₅ + 3Sb₂S₃ solv. → Pb₆Sb₆S₁₅ + 3Bi₂S₃. Stibian bismuthite-III contains 4–17 wt % of Sb in its composition (Bi_1.36–1.71Sb_0.64–0.29)₂S₃. Appearance of bismuthite with the Sb mineralization where it was developed over ores with native bismuth, maldonite and Pb–Bi sulphosaults is the evidence of key role of the mass action law in mineral-forming processes.