Mineralogical characteristics of silver minerals from the Dongyang Gold deposit, China: Implications for the evolution of epithermal metallogenesis.

Abstract The Dongyang gold deposit is located in Fujian Province in southeastern China, mainly hosted by a NW–SE trending Late Jurassic porphyritic rhyolite, and classified as a low-sulfidation epithermal gold deposit. The units associated with the gold mineralization in this area are highly altered and have undergone sericitic and argillic alteration as well as silicification, with the latter two types of alteration especially associated with gold enrichment. The deposit contains pyrite, marcasite, and arsenopyrite, with lesser amounts of chalcopyrite, galena, sphalerite, and silver minerals. The sulfides and silver minerals within the deposit provide insight into the evolution of the ore-forming fluid and the processes that caused the gold enrichments to form in the study area. In addition, the geochemical similarity of gold and silver means these elements tend to migrate and precipitate simultaneously, in turn indicating that sulfides and silver minerals can provide insights into the processes that concentrate gold. Here, we present chemical compositional, and crystal structural data for sulfides and silver minerals within the Dongyang gold deposit by scanning electron microscopy (SEM), electron microprobe analyser (EMPA), X-ray diffraction (XRD) and laser Raman microprobe, and use these data to gain insights into the characteristics and evolution of the fluids that formed the deposit. We also outline key concepts for future gold deposit exploration in this region. Four types of pyrite in the deposit are recognized based on arsenic content and crystal morphology, evolving from early As-poor pyrite (Py1) to As-rich pyrite (Py2) and/or arsenopyrite, to non-cubic As-poor pyrite (Py3), and finally to cubic As-poor pyrite (Py4), with an inverse correlation between sulfur and arsenic concentrations. Pyrite crystal morphologies vary from earlier pentagonal dodecahedron forms to later cube forms, indicating a decrease in degree of supersaturation (as controlled by the activity of dissolved Fe and sulfide) and/or temperature conditions for pyrite growth. In addition, the presence of As-poor marcasite associated with early pyrite (Py1) suggests that the early ore-forming fluid was relatively acidic. Arsenopyrite is generally concentrated in the shallower parts of the deposit, indicating that later hydrothermal fluids were enriched in arsenic. This study also identified a number of key silver mineral phases that formed in a sequence from chalcopyrite, to a galena–fahlore–allargentum–dyscrasite–polybasite–electrum assemblage, to stephanite, and finally to chlorargyrite. Chalcopyrite replaced by fahlore suggests increased oxygen fugacity and decreased temperature. Fahlore Sb / (As + Sb) (>90%) and Ag / (Ag + Cu) ratios (6%–27%) are indicative of relatively high temperatures (>200 °C), and allargentum is thought to form in elevated pH conditions (pH > 7). Stephanite and chlorargyrite precipitated under relatively low temperature (<200 °C). Finally, the presence of chlorargyrite suggested that silver was transported as chloride complexes, indicating relatively high oxygen fugacity. The sequence of silver mineral precipitation suggests that the ore-forming fluids showed a temporal increase in pH and oxygen activity, and a decrease in temperature and sulfur fugacity. This indicates that the hydrothermal fluids that formed the deposit can be divided into early and late stage fluids, with the former being a relatively higher-temperature and higher‑sulfur fugacity fluid compared with the lower-temperature and higher‑oxygen-fugacity late-stage fluid. Finally, the presence of silver minerals with high gold contents (up to 2 wt%), as well as arsenian pyrite and arsenopyrite, may be useful indicators of prospective areas for gold exploration in this region. Highlights • Components and morphology of pyrite indicated decreased supersaturation and/or temperature. • Silver minerals indicated decreased temperature and sulfur fugacity, and increased pH and oxygen fugacity. • Ore-forming fluids are divided into early stage of reduced and high-temperature and late stage of oxidized and low-temperature. [ABSTRACT FROM AUTHOR]