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

Abstract

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 (Bi1.89–1.98Sb0.11–0.02)2S3, galenobismuthite, nests of bismuth and ikunolite Bi4S3. There is observed exsolution of ikunolite mainly into the native bismuth (Bi0.98–1Sb0.02–0) and bismuthite-II; the composition of bismuthite-II in center of aggregates with the bismuth is (Bi1.96–1.97Sb0.04–0.03)2S3, whereas the composition on their periphery is an more antimonian one is (Bi1.91–1.92Sb0.09–0.08)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 (Bi1.72–1.96Sb0.29–0.94)2(S2.98–3Se0–0.02)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 (Bi1.36–1.71Sb0.64–0.29)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.