Ore genesis of the Tianbaoshan carbonate-hosted Pb-Zn deposit, Southwest China: geologic and isotopic (C-H-O-S-Pb) evidence

The Tianbaoshan Pb-Zn deposit, part of the Sichuan-Yunnan-Guizhou (SYG) Pb-Zn metallogenic province, is located in the western Yangtze Block and contains 2.6 million tonnes of 10-15wt.% Pb+Zn metals. Ore bodies occur as vein or tubular types and are hosted in Sinian (late Proterozoic) carbonate rocks and are structurally controlled by the SN-trending Anninghe tectonic belt and NW-trending concealed fractures. The deposits are simple in mineralogy, with sphalerite, galena, pyrite, chalcopyrite, arsenopyrite, freibergite, and pyrargyrite as ore minerals and dolomite, calcite, and quartz as gangue minerals. These phases occur as massive, brecciated, veinlet, and dissemination in dolostone of the upper Sinian Dengying Formation. Hydrogen and oxygen isotope compositions of hydrothermal fluids range from -47.6 to -51.2 parts per thousand and -1.7 to +3.7 parts per thousand, respectively. These data suggest that H2O in hydrothermal fluids had a mixed origin of metamorphic and meteoric waters. Carbon and oxygen isotope compositions range from -6.5 to -4.9 parts per thousand and +19.3 to +20.2 parts per thousand, respectively. These compositions plot in the field between mantle and marine carbonate rocks with a negative correlation, suggesting that CO2 in the ore-forming fluids had multiple sources, including the Permian Emeishan flood basalts, Sinian-to-Permian marine carbonate rocks, and organic matters in Cambrian-to-Permian sedimentary rocks. Sulphur isotope compositions range from -0.4 to +9.6 parts per thousand, significantly lower than Cambrian-to-Permian seawater sulphate (+15 to +35 parts per thousand) and sulphate (+15 to +28 parts per thousand) from evaporates in Cambrian-to-Permian strata, implicating that the S was derived from host-strata evaporates by thermal-chemical sulphate reduction. Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 ratios range from 18.110 to 18.596, 15.514 to 15.878, and 38.032 to 39.221, respectively, which plot in field of the upper crust Pb evolution curve, unlike those of Proterozoic basement rocks, Sinian dolostone, Devonian-to-Permian carbonate rocks, and the Permian Emeishan flood basalts, implying complex derivation of Pb metal in the ore-forming fluids. Geological and isotopic studies of the Tianbaoshan Pb-Zn deposit reveal that constituents in the hydrothermal fluids were derived from multiple sources and that fluid mixing was a possible metallogenic mechanism. The studied deposit is not distal magmatic-hydrothermal, sedimentary exhalative (SEDEX), or Mississippi Valley (MVT) types, rather, it represents a unique ore deposit type, named in this article the SYG type.