Fluid inclusion and stable (H-O-C) isotope studies of the giant Shuangjianzishan epithermal Ag-Pb-Zn deposit, Inner Mongolia, NE China

The Shuangjianzishan Ag-Pb-Zn deposit with a reserve of 145 Mt @128.5 g/t Ag (c. = 18,600 t or 600 Moz Ag) and 2.2 wt% @ Pb + Zn ( = 3.2 Mt) in the Great Hinggan Range (GHR) ore district in NE China is the largest known silver deposit in Asia. The Ag-Pb-Zn mineralized veins are primarily hosted by a Permian slate unit. Three primary paragenetic stages of early pyrite + quartz +/- K-feldspar (I), main ore sulfide + sulfo-salt + quartz + calcite + sericite + chlorite +/- epidote (II), and post-ore quartz (III) have been identified. The ore stage II was the dominant period for silver mineralization with canfieldite (Ag8SnS6), argentite (Ag2S) and freibergite [(Ag, Cu)(12)Sb4S13] the prevailing silver-bearing minerals. Fluid inclusion microthermometric studies revealed that the Ag-Pb-Zn vein formation occurred within a temperature range of 250 degrees to 210 degrees C and at a salinity of < 8.5 wt% NaCl equiv. The obtained D-O-C isotope compositions of the Shuangjianzishan Ag-Pb-Zn vein minerals suggested that the ore-forming fluids were initially sourced from a magmatic reservoir, and that the magmatic fluids subsequently mixed with a large incursion of meteoric waters. Although fluid boiling is considered a major depositional mechanism for epithermal deposits worldwide and evidence for such a process exists at the studied deposit, the boiling occurred prior to the main stage of ore deposition. Therefore, boiling could not have provided a viable mechanism for Ag-Pb-Zn deposition. By contrast, a combination of fluid-rock interactions and mixing between a magmatic fluid and meteoric waters leading to fluid cooling and reduction is advocated here as the important factors for ore deposition. In summary, our fluid inclusion and stable isotope analytical data are consistent with a classification of the giant Shuangjianzishan Ag-Pb-Zn deposit as an intermediate- to low-sulfidation epithermal system.