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

The Huangjindong gold deposit (similar to 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 similar to 180 degrees C and similar to 280 degrees 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.