Mineral paragenesis of the Anfangba Gold deposit, western Qinling Orogen, China: implication for coupled dissolution-reprecipitation reactions and the liquid bismuth collector model

Abstract The newly discovered Anfangba gold deposit is hosted in the Middle Triassic pumpellyite-actinolite facies metamorphosed turbidites in the western Qinling Orogen, Central China. It is characterized by a pyrite-quartz vein type, a medium-high temperature, and the development of visible gold and abundant bismuthides. It is the first reported Au-Bi-associated deposit in this area and remains poorly understood. In this study, the mineral paragenesis has been studied to further understand ore formation, including sulfides (pyrite, chalcopyrite, pyrrhotite, bornite, bismuthinite, tetrahedrite and arsenopyrite), Cu-Bi(Sb) sulfosalts (emplectite, wittichenite, and skinnerite), sulfoselenide (unnamed sulpho-tellurian nevskite), tetradymite group minerals (joseite-A, joseite-B and tetradymite), native bismuth and electrum. The physicochemical conditions of the ore-forming process can be estimated based on the mineral paragenetic sequence. The calculated logαH2S and logfO2 of the main mineralization stage are higher than -2.25 and lower than -37.96, respectively. EPMA mapping indicated that ore-forming fluid with low sulfur and oxygen fugacities may have played an important role in provoking the activity of the semimetals such as Bi, Sb, Se and Te. This may have result in the formation of accessory minerals, including native bismuth, skinnerite and an unnamed sulpho-tellurian nevskite via coupled dissolution-reprecipitation (CDR) reactions. Mineralography, EPMA and LA-ICP-MS analysis have revealed a close relationship between Au mineralization and Bi-blebs. It suggests that the “liquid bismuth collector model” contributed to the Au enrichment of the Anfangba deposit. As the coexistence of native bismuth and bismuthinite is commonly seen in different types of hydrothermal gold deposits, the presence of Bi-melts and the influence of the “liquid bismuth collector model” in these gold deposits may have been underestimated.