Lithium isotopes, chlorine isotopes, and Cl/Br ratios geochemistry of ore-forming fluids in the Yangla skarn Cu-Pb-Zn deposit, SW China

The elemental and isotopic compositions in ore forming fluids are controlled by fluid sources and migration, and are further influenced by mineralization during hydrothermal fluid activity associated with skarn formation and mineralization related to medium-acid intrusions. Tracking the sources and processes of ore-forming fluids by traditional geochemistry remains challenging. In this study, we investigate δ⁷Li, δ³⁷Cl values, and Cl/Br ratios of bulk fluid inclusions trapped in quartz from the Yangla skarn Cu-Pb-Zn deposit, SW China. The heavy Li isotopic feature is primarily controlled by the formation of hydro-silicate minerals during the skarn alteration. Extraneous fluids are rarely incorporated into studied mineralization system. Prominent negative tendency variation of Cl/Br ratios and δ³⁷Cl values is found, and some data show an affinity to magmatic fluids, which are responsible for metals mineralization. Based on previous experiments and simulations, we argue that halite precipitation and skarn alteration cannot be the principal factors for such variation in hydrothermal metallogenic system at the Yangla skarn deposit. Instead, the vapor–liquid phase separation in acidic ore-forming fluids produces lower Cl/Br ratios with heavy Cl isotopes in liquid-dominated phase. The systematic geochemical variation resulting from phase separation provides a new explanation for variations of δ³⁷Cl values in many deposits. Our findings also show that Li and Cl isotope geochemistry, combined with elemental fluid tracers (e.g., Cl/Br ratios), can effectively trace the source and evolution of fluids in magmatic-hydrothermal systems.