Revisiting the genesis of Bi-Te-Au-S associations: Implications for the ore-forming processes in gold deposits

Abstract

The enrichment of Bi in hydrothermal gold deposits is usually expressed by various Bi-Te-Au-S mineral assemblages, which are often used as mineralogical indicators for physicochemical environments and ore-forming processes. However, the genesis of these assemblages remains poorly understood, particularly the conditions under which S-bearing Bi minerals form and their potential to concentrate gold. The current prevailing proposal is that these S-bearing minerals represent secondary sulfide phases forming through hydrothermal sulfidation during interactions between late-stage, S-rich fluids and preexisting Bi minerals. Here, we conducted four annealing experiments over a wide temperature range (280 to 400 °C) and compositional analyses on natural Bi-rich gold ores from the large Wulong Au-Bi deposit in North China Craton to investigate the genesis of Bi-Te-Au-S associations. The mineral assemblages show a large variation in morphology, texture, and composition after the experiments. Notably, many euhedral bismuthinite, joséite-A and joséite-B mineral grains are observed, showing distinct differences in morphology and distribution from those pre-experiment mineral assemblages. These minerals commonly host many tiny grains of native gold and maldonite. Newly formed mineral phases after each run also include unnamed euhedral to anhedral unnamed Bi-S, Bi-Te-S mineral phases, and Bi-Ag sulfosalts, which are often found adhering to the surface of other Bi-bearing mineral phases. These characteristics show that Bi-Te-S and/or Bi-S minerals crystallized from Bi-Te-S melts at the experiment temperatures and these melts can act as agents for gold concentrations. Based on the results presented here combined with published data, we propose that the Bi-Te-Au-S metallic melts can be activated under the ore-forming conditions of structurally controlled gold deposits similar to the Wulong Au-Bi deposit. The different cooling rates of these melts may control the formation of various textures shown by Bi-Te-Au-S mineral assemblages. Our findings highlight that Bi-Te-Au-S mineral assemblages can form directly from metallic melts, rather than being exclusively linked to late-stage hydrothermal sulfidation during an overprinting event. The formation and crystallization of such melts represent a distinct mechanism for concentrating and cycling critical metals like Bi, Te, S, and Au within evolving ore-forming fluid systems.