The genesis of the Tianbaoshan Pb-Zn deposit in Sichuan, SW China: Insights from sphalerite and fluid inclusion compositions

The Tianbaoshan deposit, located in the Sichuan-Yunnan-Guizhou (SYG) metallogenetic belt, is one of famous PbZn deposits of Southwest China, with an established reserve of 2.6 Mt. Pb and Zn. According to the detailed field geological survey and microscopic petrography observation, three distinct ore fluid evolution stages can be divided. In this contribution, sphalerite and associated quartz from above diverse stages were carefully chosen for systematic fluid inclusion (FI) petrography, microthermometry, individual FI LA-ICP-MS analysis, Raman microspectroscopy, and in situ trace element investigation of sphalerite. The sphalerite only defines a two-phase aqueous FI type, while the FIs hosted in quartz exhibit diverse types, including two-phase aqueous FIs, vapor-rich FIs, and pure aqueous fluid inclusions. Our microthermometry results indicate that the homogenization temperature (Th) and salinity of FIs in sphalerite from the stage 1 are 109.6-182.2 degrees C and 12.51-21.61 wt% NaCl equiv., respectively. The Th of the sphalerite-hosted fluid inclusions in stage 2 ranges from 97 to 158.3 degrees C, with a salinity range of 14.46-19.76 wt% NaCl equiv. The FIs trapped in sphalerite exhibited medium-high salinity and low Th, and Raman spectrum analysis shows that vapor composition of the FI is mainly H2O, reflecting the typical characteristics of basin brine. Measurements on quartz-hosted FIs yield homogenization temperatures from 134.3 to 233.1 degrees C, with a salinity from 4.49 to 12.62 wt% NaCl equiv. The gas phase composition of these inclusions is complex and consists of several components, such as CH4, N-2, CO2, and H2O. The results indicate that the FIs present in quartz are dissimilar from those in sphalerite, and the sphalerite mineralization fluid has typical characteristics of basin brine and represents the ore-forming fluid. The significantly high Na and Ca of the mineralizing fluid is revealed, suggesting that the mineralizing fluid may be of the NaCa system. Combined with Ca/K, K/Na and Rb/Na ratio, typical basinal brines origin was constrained by fluid composition analysis. The gradual enrichment of Ca, Mg, and Sr in the mineralizing fluid emphasizes the positive effect of fluid-rock reactions on the precipitation of Zn. The trace element in sphalerite of the Tianbaoshan deposit is distinct from SEDEX-, skarn-, and volcanogenic massive sulfide (VMS)-type deposits and notably similar to MVT deposits. Combining with fluid inclusion and sphalerite trace elements feature, we define the Tianbaoshan deposit as a typical MVT PbZn deposit.