Fluid inclusions and H-O-C-S isotope constraints on fluid evolution and ore genesis of the Wangjiadashan Cu-Au deposit in Suizao area of the Tongbai-Dabie orogenic belt, central China

The Wangjiadashan Cu-Au deposit is a vein-type deposit controlled by the secondary faults of the Wushan Fault zone in the Suizao area of Hubei Province. Three hydrothermal stages are identified, namely, (I) barren quartz stage, (II) the quartz-sulphide stage, and (III) quartz-carbonate stage. Six types of fluid inclusions occur in quartz: (a) liquid-rich aqueous fluid inclusions (L + V), (b) vapour-rich aqueous-carbonic fluid inclusions (V + L), (c) water-rich aqueous-carbonic fluid inclusions (L + V + CO2), (d) CO2-rich aqueous-carbonic fluid inclusions (V + L + CO2), (e) pure carbonic fluid inclusions (pure CO2), and (f) daughter mineral-bearing multiphase inclusions (S-type) with three subclasses (S-1, S-2, and S-3 subtype). Two types of fluid inclusions occur in calcite: (a) liquid-rich aqueous fluid inclusions (L + V) and (b) pure carbonic fluid inclusions (pure CO2). The S-type fluid inclusions include halite (NaCl), calcite (CaCO3), and chalcopyrite (CuFeS2). The fluid system is composed of CO2-NaCl-H2O. The homogenization temperatures of the three stages are 295 degrees C to 390 degrees C, 245 degrees C to 280 degrees C, and 200 degrees C to 241 degrees C, with salinities of 7.3-19.8 wt% NaCl equiv. (except for several high values of 33.8 to 44.3 wt% NaCl equiv.), 7.7-13.7 wt% NaCl equiv., and 5.6-10.1 wt% NaCl equiv., respectively. Fluid immiscibility occurred in stage I that accounts for the high-salinity fluids, but the dominant ore precipitation occurred afterward during temperature drop. In situ sulphur isotope analysis indicates a delta S-34 value around 0 (+1.1% to +2.2%) for the chalcopyrite, suggesting that the sulphur likely derived from the deep-seated magma. Pyrites show delta S-34 values from -0.5 parts per thousand to +6.5 parts per thousand in stage II, from +0.7 parts per thousand to +6.6 parts per thousand in country rock of marble, but significantly higher from +12.8 parts per thousand to +21.7 parts per thousand in country rock of greenschist. These results indicate the sulphur for pyrite in stage II likely derived from the mixing of deep-seated magma and country rocks. From stages I to II, a slightly increase of delta D (from -86 parts per thousand and -73 parts per thousand to -59 parts per thousand and -56 parts per thousand) and a decrease of delta O-18 (from 8.9 parts per thousand and 7.1 parts per thousand to 1.3 parts per thousand and 3.4 parts per thousand) indicate that an increasing involvement of meteoric water. The carbon isotopes of fluid inclusions CO2 in quartz of stage I (-11.0 parts per thousand to -13.0 parts per thousand) indicate a major magmatic or metamorphic carbon source mixed with minor sedimentary sourced carbon, but a major organic carbon source (-32.0 parts per thousand to -35.0 parts per thousand) for stage II fluids. In conclusion, the Wangjiadashan Cu-Au deposit belongs to the orogenic type.