Genesis of the Sinongduo Ag-Pb-Zn deposit: Insights from fluid inclusions, in-situ δ18O, and trace elements compositions of quartz

• The Sinongduo deposit is an epithermal Ag-Pb-Zn deposit. • Fluctuations in CL emission intensity correlate with variations in trace elements incorporation during quartz growth. • The ore-forming fluids in the Sinongduo deposit represent a NaCl – H 2 O ± CO 2 system. • Fluid mixing played a crucial role in ore deposition at the Sinongduo deposit. The Sinongduo Ag–Pb–Zn deposit is a classic example of an intermediate-sulfidation epithermal system within the vast Linzizong Group volcanic sequence of the Gangdese Orogen in Tibet. We obtained in situ trace element and O isotope data for hydrothermal quartz from this epithermal Ag–Pb–Zn deposit to better understand the origins of its ore-forming fluids and mineralization mechanisms. The Sinongduo deposit contains three distinct generations of quartz. Fluid inclusion microthermometry of quartz reveals the hydrothermal system had homogenization temperatures of 160–278 °C and salinities of 0.9–5.3 wt% NaCl eq . A gradual decrease in fluid inclusion homogenization temperatures and salinities, and quartz δ 18 O values and Al concentrations occurred from the initial to final stages of quartz crystallization. The evolution of the dominant fluid inclusion types from CO 2 -rich assemblages in stage II-2 to H 2 O-rich inclusions in stages II-3 and III indicates a progressive change in fluid composition. This trend, along with petrographic and geochemical evidence, suggest a gradual increase in pH resulted from CO 2 degassing, which then facilitated localized Ag–Pb–Zn precipitation. The quartz grains have a uniform texture and contain lithophile elements. Oxygen isotopic compositions of the three generations of quartz suggest the ore-forming magmatic fluids interacted gradually with meteoric waters. The Sinongduo deposit was formed by fluid mixing, which was the main control on metal precipitation. Based on our observations, fluid dilution is proposed to have played a key role in initiating Ag–Pb–Zn deposition. Our results contribute to a better understanding of the hydrothermal evolution in intermediate-sulfidation epithermal systems.