Gold mineralization of the Huangjindong gold deposit in the Jiangnan Orogen, South China: Constraints from fluid inclusions and LA-ICP-MS analysis of pyrite and arsenopyrite

• EPMA determined extremely high gold fineness in native gold. • LA-ICP-MS revealed contents and distribution of trace elements in FI and sulfides. • Pyrite at stage I showed different geochemistry from other sulfides at stage Ⅱ. • Thermodynamic analysis indicated higher T and Au/Ag in the forming stage of gold. The Huangjindong gold deposit (∼80 t Au @ 5 g/t) is located in the middle segment of the Jiangnan Orogenic Belt, South China. Mineralization of the deposit is mainly controlled by the NE-trending faults and a series of EW-NWW-trending inverted folds, which are dominated by auriferous quartz veins, altered slates and breccias. As the gold-bearing minerals, pyrite and arsenopyrite are both classified into two generations (Py1, Py2, Apy-a and Apy-b). The mineralization comprises three stages, Py1 + quartz + sericite (stage I), Py2 + arsenopyrite + gold + galena + sphalerite + chalcopyrite + tetrahedrite + quartz (stage II), and quartz + chlorite + calcite (stage III). To understand the characteristics of gold mineralization and the ore-forming conditions, we conducted comprehensive studies of the fluid inclusions, EPMA, LA-ICP-MS and thermodynamic analyses. The microthermometric results reveal that the ore-forming fluids may have two temperature peaks at ∼180 °C and ∼280 °C. The EPMA results show that the fineness of the native gold ranges from 978 to 1000 (mean 995.9). The LA-ICP-MS analyses of fluid inclusions, pyrite and arsenopyrite formed at different stages reveal the distinct distributions of trace elements. Since the processed data from the LA-ICP-MS mapping analyses reveals that the Au/Ag values of Py2 (10–1000) and arsenopyrite (10–10,000) are higher than that of Py1 (0.1–100), we conducted thermodynamic calculations and plotted the 3D isopleth models for the gold solubility and the Au/Ag ratio. The results of this study suggest that gold mineralization mainly occurs in stage II with higher Au/Ag ratios and higher temperatures, and the decreased sulfur concentration might have caused large-scale gold precipitation.