COPPER SULFIDE DEPOSITION TRIGGERED BY GROUNDWATER INCURSION IN MAGMATICALLY-DOMINATED HYDROTHERMAL SYSTEM (TONGCHANG PORPHYRY CU DEPOSIT, CHINA)

Porphyry Cu deposits, the major repositories for base and precious metals, carry significant fingerprints for understanding magmatic-hydrothermal processes in the upper crust. While abundant evidence for the presence of meteoric water has been reported, the relative timing of meteoric incursion with respect to the entire fluid history and its role in sulfide deposition remains hotly debated due to complex overprints.

The Tongchang porphyry copper deposit represents an extreme case where primary magmatic-hydrothermal mineral assemblages are intensely modified. Six quartz-depositing and two quartz-dissolving fluid events are recognized through detailed cathodoluminescence imaging. Thermobarometry (TitaniQ and fluid inclusion) and solubility analysis indicates that the fluid system underwent decompressional cooling (650 °C to 250°C) with intermittent pressurization and pressure fluctuation. Combined in-situ quartz O and anhydrite Sr isotopes revealed that the system was dominated by magmatic components in the early lithostatic environment, followed by major meteoric incursion (enriched in ¹⁸O and radiogenic Sr) in a complete hydrostatic environment. The fluid mixing induced the major Cu mineralization due to enhanced cooling and fluid dilution. Subsequently, the system was dominated by meteoric water, which experienced Rayleigh distillation to produce quartz with the highest δ¹⁸O (27‰) ever reported for porphyry systems.