Ore-Forming Fluid Evolution and Its Implications for the Zedang Pb-Zn Deposit, Xizang

The Zedang Pb-Zn deposit is a typical Pb-Zn deposit in the North Himalayan metallogenic belt. The characteristics of its ore-forming fluids and the mechanism of ore mineral precipitation remain unclear. Based on hand specimen and microscopic observations, this study performed petrographic, microthermometric and Laser Raman compositional analyses on fluid inclusions in quartz from stage Ⅰ and stage Ⅱ hydrothermal mineralization and calcite from stage Ⅲ, aiming to investigate the characteristics and sources of ore-forming fluids and reconstruct the evolutionary process of ore-forming fluids. The results show that the ore-forming fluid in stage Ⅰ is a medium- to high-temperature (200–325 ℃, average 269 ℃), medium- to low-salinity (5–20 % NaCl_eqv, average 11 % NaCl_eqv) H₂O-CO₂-NaCl system with a density of 0.63–0.99 g/cm³, corresponding to ore-forming pressure of 7.5–40.2 MPa and depth of 1.0–2.6 km. The ore-forming fluid in stage Ⅱ is a medium- to high-temperature (225–365 ℃, average 307 ℃), medium- to low-salinity (10–23 % NaCl_eqv, average 15 % NaCl_eqv) H₂O-NaCl system with a density of 0.64–0.98 g/cm³, corresponding to ore-forming pressure of 12.3–51.5 MPa and depth of 1.3–3.4 km. The ore-forming fluid in stage Ⅲ is a low-temperature (100–200 ℃, average 159 ℃), medium- to low-salinity (2–17 % NaCl_eqv, average 9 % NaCl_eqv) H₂O-NaCl system with a density of 0.92–1.06 g/cm³, corresponding to ore-forming pressure of 7.0–23.3 MPa and depth of 0.5–1.3 km. Based on the variations in composition, properties and P-T conditions of fluid inclusions, this study concludes that the early ore-forming fluids of the Zedang Pb-Zn deposit experienced boiling, while fluid mixing occurred in the late stage. Fluid boiling and fluid mixing collectively resulted in mineral precipitation. The early ore-forming fluids were likely derived from metamorphic water and seawater, and gradually mixed with meteoric water in the late stage. The deposit is generally classified as a medium- to low-temperature epithermal deposit. The metallogenic mechanism of the Zedang Pb-Zn deposit is similar to that of the Zhaxikang polymetallic deposit, and it is inferred to have characteristics of hydrothermal exhalative sedimentation. These results provide a theoretical basis for establishing the metallogenic model of the Zedang Pb-Zn deposit, summarizing the Pb-Zn polymetallic metallogenic regularity in the North Himalayan metallogenic belt and conducting regional prospecting prediction.