Fluid origin and evolution of the Taolin Pb-Zn deposit in northeastern Hunan Province, South China: Insights from fluid inclusions and H-O-He-Ar isotopes

Situated in northeastern Hunan Province, the vein-type Pb-Zn orebodies at Taolin are mainly hosted in NE-/NEE-trending faults between Dayunshan-Mufushan pluton and Neoproterozoic metasedimentary rocks of the Lengjiaxi Group. Hydrothermal mineralization can be divided into five stages: (1) coarse-grained quartz stage, (2) quartz-fluorite-base metal stage, (3) quartz-barite-fluorite-base metal stage, (4) pale pink and colorless quartz stage, and (5) fine-grained quartz stage. In this research, fluid inclusions as well as stable (H-O) and noble gas (He-Ar) isotope compositions were performed to uncover the nature, origin, and evolution of the ore-forming fluids, ore precipitation mechanisms, and mineralization process of the Taolin deposit. Four types of fluid inclusions, i.e., liquid-rich two-phase inclusions (LV-type), pure liquid phase inclusions (PL-type), vapor-rich two-phase inclusions (VL-type), and pure vapor phase inclusions (PV-type), were distinguished in sphalerite, quartz, and fluorite. Microthermometric analysis of fluid inclusion assemblages in sphalerite, quartz, and fluorite from different stages indicates that from the Stage 1 to Stage 5, the homogenization temperatures vary between 168 and 211 degrees C, between 151 and 198 degrees C, between 131 and 180 degrees C, between 132 and 164 degrees C, and between 118 and 138 degrees C, respectively, whereas the fluid salinities vary from 12.4 to 16.9 wt% NaCl equivalent, from 9.7 to 14.6 wt% NaCl equivalent, from 5.6 to 10.3 wt% NaCl equivalent, from 3.6 to 9.7 wt% NaCl equivalent, and from 0.9 to 3.8 wt% NaCl equivalent, respectively. The H-O isotope data of quartz and the He-Ar isotopic compositions of sulfide crystals suggest that the ore-forming fluids were a mixture of crust-derived magmatic hydrothermal fluid and meteoric water. Fluid mixing and cooling were likely the crucial mechanisms for ore precipitation.