Fülöppite Pb3Sb8S15 from the Darasun Volcanogenic–Plutonogenic Gold Deposit (Eastern Transbaikalia)

The Late Jurassic orogenic volcanogenic–plutonogenic Darasun gold deposit (the Eastern Transbaikalian segment of the Mongolia-Okhotsk Foldbelt) includes post-gold antimony mineralization. Aggregates of rice-like quartz, minerals of the jordanite–geocronite–schulzite series, sphalerite, galena, arsenopyrite, tennantite–tetrahedrite, calcite, and Mn–Mg siderite are its earlier formations; the later ones include rice-like quartz, low-iron sphalerite, Pb–Sb sulphosalts, antimonite, and berthierite. The Darasun trend of the sequence of Pb–Sb sulphosalts from bulangerite to fuloppite is typical of post-magmatic hydrothermal gold deposits and differs from that of telethermal ones. There are two fuloppite types in Darasun ores: fuloppite enriched in arsenic (up to 7.5 wt %), which is probably the product of replacement of geocronite and fahlerz mineral series, and arsenic-free fuloppite associated with antimonite and calcite. The composition of arsenic-bearing fuloppite is Pb2.90Ag0.06Cu0.05)3.01(Sb7.05As0.91Bi0.04)8.00S14.99 and the composition of arsenic-free fuloppite is (Pb2.83Cu0.18)3.01Sb7.98S15.01, which are close to stoichiometric. The Darasun fuloppite is characterized by a positive correlation between As, Bi, and Ag; arsenic-free fuloppite is enriched in copper.