Constraints of C[sbnd]H[sbnd]O[sbnd]S[sbnd]Pb isotopes and fluid inclusions on the origin of the giant Zhugongtang carbonate-hosted Pb–Zn deposit in South China

Abstract

The Sichuan–Yunnan–Guizhou (SYG) metallogenic province on the western margin of the Yangtze Block has long been a research focus and is an important region of Pb–Zn mineralization in China, but its genesis is poorly constrained. This study examines the Zhugongtang carbonate-hosted Pb–Zn deposit (>3.0 Mt, 0.09–35.04 wt% Pb + Zn), the largest discovery made in the past decade in the southeastern part of the SYG metallogenic province. Fluid inclusions from sphalerite and calcite, CHO–isotopes from calcite, and S–Pb isotopes from sulfide minerals in the Zhugongtang Pb–Zn deposit were analyzed to understand the sources and genesis of the metals and hydrothermal fluids. Fluid inclusions in sphalerite and calcite from the largest ore body indicate that the fluids of the main mineralization stage had low temperatures (105 °C–210 °C) and moderate salinities (0.35–12.89 wt% NaCl equiv.). The sulfides are characterized by narrow ranges of sulfur isotope compositions (δ34Spyrite = +14.27 ‰ to +15.89 ‰, δ34Ssphalerite = +14.31 ‰ to +15.53 ‰, and δ34Sgalena = +10.64 ‰ to +12.97 ‰), indicating that reduced sulfur was derived from sedimentary sulfate in the Devonian to Permian carbonate host rocks by thermochemical sulfate reduction with the addition of organic matter. The δ13CPDB and δ18OSMOW values of calcite range from − 8.6 ‰ to +0.82 ‰ and +15.45 ‰ to +19.41 ‰, respectively, which are similar to values for mantle, marine carbonate rocks, and sedimentary organic matter. The negative correlation between δ13CPDB and δ18OSMOW values indicates that CO2 in the hydrothermal fluids was sourced from marine carbonate rocks and sedimentary organic matter. Calculated δ18OH2O and δDH2O values range from +3.5 ‰ to +7.5 ‰, and −93 ‰ to −52 ‰, respectively, indicating that the ore-forming fluids were dominantly metamorphic, but diluted by meteoric water that had reacted with organic matter. The galena samples yield 206Pb/204Pb values of 18.52 to 18.82, 207Pb/204Pb of 15.68 to 15.93, and 208Pb/204Pb values of 38.97 to 39.96. The μ and ω values of the sulfides range from 9.61 to 10.07 and 38.84 to 43.67, respectively. These data plot on the upper crust Pb evolution curve, generally in the field of Devonian to lower Permian sedimentary rocks, although the value for one sample falls in the field of Proterozoic metamorphic rocks, pointing to a complex origin for the ore-forming materials. Overall, the fluid inclusion and isotope data, together with geological evidence, suggest that the ore-forming fluids of the Pb–Zn deposit were sourced from deep metamorphic water with the features of low temperatures and moderate salinities, and the ore-forming metals were sourced from Devonian to lower Permian sedimentary rocks and partly from basement rocks. The genesis of the Zhugongtang ore is typical of SYG-type deposits.