Genesis of Pb-Zn deposits in northwestern Guizhou province of China: Constraints from the in situ analyses of fluid inclusions and sulfur isotopes

The northwestern Guizhou ore concentration area on the southwest margin of the Yangtze Block is an important part of the Sichuan-Yunnan-Guizhou Pb-Zn metallogenic province. Several large Pb-Zn deposits (e.g., Shanshulin, Shaojiwan, Yadu, and Tianqiao) are found in the area with similar mineral assemblages, ore-controlling structures, ore-hosting rocks, and genetic types. In this study, fluid inclusion microthermometry and in situ geochemical analysis were employed to determine the nature and compositions of the fluid inclusions trapped in calcite and sphalerite and the sulfur isotopic compositions of sphalerite and pyrite. Microthermometric results show that the ore-forming fluids are moderate to low temperature and salinity. The fluid inclusions in calcite and sphalerite share similar homogeneous temperatures (120 to 200 degrees C) and salinities (7-14 wt% NaCl equiv.). The in situ LA-ICP-MS analyses reveal that the fluid inclusions have high concentrations of elements such as Na, K, Mg, Ca, Rb, Sr, Ba, Cs, and Li. The ratios of K/Na, Rb/Na, Cs/Na, Ca/Na, and Ba/Na could be comparable to the basinal brines. The trend towards metamorphic water is possibly attributed to the involvement of basementderived fluids. The in situ delta 34S values of sphalerite and pyrite range from 11.3 to 20.2 %o and 12.0 to 18.9 %o, respectively, which indicate that the reduced sulfur is most likely related to the thermochemical reduction of sulfate with multiple sources either from the ore-hosting strata or from Precambrian-Cambrian basement rocks which have high sulfur isotopic compositions. We propose that the fluid mixing is the main mechanism for the formation of Pb-Zn deposits in NW Guizhou.