Fluid inclusion and H–O-S-Pb isotope constraints on the origin of carbonate-hosted Hg-Sb mineralization at Gongguan, central China

The Gongguan deposit in southern Qinling district, central China, is characterized by a Hg and Sb metal association, with proven reserves of 7,543 t Hg and 17,100 t Sb. The Hg-Sb mineralized veins are primarily hosted by local Lower Devonian dolomite. Three primary paragenetic stages have been identified, including (I) pyrite-dolomite-quartz, (II) stibnite-cinnabar-quartz-calcite and (III) post-ore calcite. Dominant mercury and antimony mineralization occurred in stage II. Fluid inclusion microthermometry of quartz and calcite reveal that the Hg-Sb mineralization is associated with a low temperature (142° to 240 °C) and low salinity (< 8.6 wt% NaCl equivalent) fluid, belonging to a H 2 O-NaCl ± CO 2 system. The H–O isotope compositions ( δ D H2O = −84 to −63‰, δ 18 O H2O = 1.3 to 12.7‰) of the hydrothermal fluid suggest a mixture of metamorphic fluid and meteoric water. In situ sulfur isotope results of cinnabar ( δ 34 S = 3.4 to 14.5‰), stibnite ( δ 34 S = 4.9 to 10.5‰) and pyrite ( δ 34 S = 3.8 to 5.4‰) indicate sulfur sources from decomposition of diagenetic pyrite and/or the thermochemical sulfate reduction in local sedimentary rocks. In situ obtained 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios (18.006 to 18.362, 15.445 to 15.610 and 37.871 to 38.288, respectively) of stibnite and pyrite substantially differ from those for the regional Carlin Au-Sb and SEDEX Pb–Zn ore deposits. The lead isotope results indicate that the underlying Paleozoic (Silurian to Cambrian) strata and Precambrian metamorphic basement supplied substantial metals for the studied Hg-Sb deposit, whereas the host Devonian carbonates did not contribute significant metals. In summary, we propose that fluid mixing between metamorphic fluid and meteoric water was the main cause for carbonate-hosted Hg-Sb deposition at Gongguan, which reduced the stibnite solubility, oxidized mercury (Hg 0 to Hg 2+ ), favoring the process of dedolomitization.