Maldonite and Products of Its Replacement—Pure Native Gold, Jonassonite, and Bismuthic Aurostibite—in Gold Ores of the Darasun Deposit (Eastern Transbaikalia)

Abstract The Late Jurassic orogenic volcanogenic–plutonogenic Darasun gold deposit of the beresite–listvenite gold–sulfide–quartz formation is situated in the Mesozoids of the Eastern Transbaikalian segment of the Mongol–Okhotsk folded belt. The deposit includes Au–Bi–Te and the postgold Sb mineralization. Carbonate–quartz–sulfide veins in the Western block of the deposit composed of gabbroids, gabbro amphibolites, and, to a lesser extent, ultramafic rocks are surrounded by listvenite aureoles. Au-rich ores were formed under conditions of the low activity of sulfide sulfur; they are enriched in pyrrhotite and contain bismuthine and/or Pb–Bi sulfosalts and, locally, rare nests of native bismuth and ikunolite Bi4S3. The high-fineness gold (970–925) associates with bismuthinite and Pb–Bi sulfosalts. Instead of native gold, nests of native bismuth and ikunolite include abundant maldonite Au2Bi, an Ag-free mineral. Under conditions of the higher activity of sulfide sulfur, maldonite in gold-bearing hydrotherms is replaced by intergrowths of pure native gold and bismuth, and bismuthinite. The subsequent significant increase in activity of sulfur in gold-bearing hydrotherms led to mass replacement of maldonite by jonassonite. Thus, the direct replacement of maldonite by jonassonite is observed in Darasun ores probably as result of the following reaction: Au2Bi + 5Bi + 2Bi2S3 + S2 sol → 2AuBi5S4. The composition of jonassonite varies from pure AuBi5S4, which is most abundant in Darasun ores, to Au(Bi, Pb)5S4 with 1–6 wt % Pb, in the case in which galena and bismutoplagionite occur among replaced minerals. Therefore, the jonassonite formula is AuBi5S4. Aurostibite pseudomorphs after gold minerals appeared in such ores due to superimposing Sb mineralization with native Sb. Apo-maldonite aurostibite pseudomorphs contain 4–10 wt % Bi. The probable aurostibite–maldonite replacement reaction is as follows: 2Au2Bi + 6Sb sol + Sb2S3 sol → 4Au (Sb, Bi)2 + (Bi, Sb)2S3. Aurostibite contains up to 18 wt % Bi within inner zones of pseudomorphs formed after maldonite decomposition products—pure native gold and bismuth.