Calcite U-Pb, pyrite Re-Os geochronological and fluid inclusion and H-O isotope studies of the Dafang gold deposit, South China

Abstract

The Dafang gold deposit is one of the representative gold mineralization in southern Hunan province, South China. The gold orebodies mainly occur in the contact zones between late Jurassic granodiorite porphyry and late Paleozoic limestone as well as the fractures in them. Three primary mineralizing stages have been identified: pyrite + arsenopyrite ± chalcopyrite + quartz (stage I), pyrite + gold + calaverite + polymetallic sulfides + quartz (stage II), and pyrite + arsenopyrite + quartz + calcite (stage III). Pyrite samples separated from stage II yielded a Re-Os isochron age of 149 ± 16 Ma. Calcite samples from stage III demonstrated a U-Pb age of 157.7 ± 6.5 Ma. The above ages are similar to the zircon U-Pb ages of the local granodiorite porphyry although field evidences did not support a genetic link between them. The newly obtained mineralizing ages for Dafang indicate that this deposit is most likely related to deep or nearly late Jurassic intrusions (e.g., the Baoshan porphyry intrusion) and that this gold mineralization is synchronous with the large-scale W-Sn mineralization (160–150 Ma) for the Nanling Range in South China. The measured δDH2O (−103.5 to −81.4 ‰) and calculated δ18OH2O (−4.1 to + 4.9 ‰) isotope compositions for different stage vein quartz suggest that the Dafang ore-forming fluids were initially dominated by magmatic water, and were diluted by meteoric water in the late stage. Fluid inclusion studies indicate that the ore fluids belong to the H2O-NaCl-CO2-CH4 system, which cooled during its evolution (e.g., 230–337 °C, 150–246 °C, and 91–178 °C for stages I to III, respectively) with varied fluid salinity (4.7–9.2, 4.7–23.3, and 2.1–9.9 wt% NaCl eqv. for stages I to III, respectively). Fluid inclusions assemblages with different vapor and liquid phase ratios in a single quartz growth, and their similar homogenization temperatures with varying fluid salinities supported the fact that fluid boiling occurred in ore stage II. Combined with geological occurrences and the results from this study, we propose that gold was mainly transported in a reducing condition, and fluid boiling and host-rock sulfidation are the key factors for gold deposition at Dafang.