Aurostibite, Silver-Bearing Gold, and Electrum As a Part of Post-Gold Antimony Mineralization in 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 and post-gold antimony mineralization. Carbonate–quartz–sulfide veins in the western part of the deposit, composed of gabbroids, gabbro amphibolites, and, to a lesser extent, ultramafic rocks are surrounded by listvenite aureoles. The Au-rich ores were formed under conditions of low activity of sulfide sulfur; they contain pyrrhotite, arsenopyrite, chalcopyrite, bismuthinite, and Pb–Bi sulfosalts associated with a high fineness gold (949–935‰). Aurostibite pseudomorphs after gold minerals have appeared there due to a superimposition of the antimony mineralization with native antimony on gold ores. This aurostibite contains 1.1–1.7 wt % Bi and 0.1–0.3 wt % As, isomorphically replacing Sb. Its composition is Au_{0.998–1.005}(Sb_{1.947–1.965}Bi_{0.024–0.036}As_{0.009–0.017})_{1.995–2.002}, and the average composition is Au_1.001(Sb_1.956Bi_0.031As_0.012)_1.999. Aurostibite does not contain silver. Silver released during the replacement of native gold with aurostibite occurs near its metacrystals in the composition of heterogeneous reaction rims of newly formed gold minerals. They are represented by silver-bearing native gold (fineness 922–712, mostly 919–911) and electrum (fineness 693–584, mostly 625–604). The distribution of the gold fineness in the newly formed minerals of the gold–silver series, as a part of the antimony mineralization, in the volcanogenic–plutonogenic Darasun deposit is very heterogeneous and “irregular.”