Temperature condition, source of bismuth and mechanism of Au-scavenging by Bi phases in the Atash-Anbar gold deposit (south Qazvin)

Based on thermodynamic modeling, chalcophile elements with low melting point such as bismuth play an important role in removing precious elements (such as gold, silver and platinum group elements) from the hydrothermal fluid, which known as the Liquid Bismuth Collector Model (LBCM). In this contribution, the chemical composition of freibergitesulfosalt, Bi-bearing sulfides and native gold particles (20 to 100 µm in size) in the quartz-sulfide breccia vein (as the highest grade of Au-Bi vein) from the Atash-Anbar gold deposit (south Qazvin) were investigated. The temperature was estimated by using fahloregeothermometer and calculation of Ag/(Ag+Cu) and Zn/(Zn+Fe) molar ratios in feribergite. Consequently temperature of ore forming was between 200-250 oC. The bismuth in the polymetallic veins of the Atash-Anbar deposit originated from plagioclase of the dacite porphyry host rock. Bismuth has been released from their composition. During alteration of plagioclase to the kaolinite±sericite, the oxidation-sulfidation reactions of hydrothermal fluid by ferroan dolomites gangue (as oxidizing agent and stable buffering phase) caused instability of Bi2S2(OH)2 and HBi2S4– complexes and converted Bi3+ to Bi0, based on the electrons released from this process Au+ was converted to (invisible gold) to Au0 (native gold) and absorbed by the bismuth phase.