Porphyry Cu deposits in collisional orogens are new targets for modern mineral exploration. A series of post-collisional (Miocene; 22–12 Ma) porphyry copper deposits with Cu reserves over 45 Mt have been discovered in the Gangdese magmatic belt, southern Tibet. These magmas were derived from the partial melting of sulfide-bearing Tibetan juvenile lower crust and were well primed for porphyry Cu deposits, having high oxidation states and being rich in volatiles such as water, S, and Cl. However, only some plutons ended up with economic ore concentrations. We measured the igneous zircon water contents of ore-related and barren high Sr/Y magmas in the Gangdese belt, southern Tibet, and found that magmas that produce giant porphyry Cu deposits have lower zircon water contents than barren and small-deposit related magmas. Combined with previous studies of fluid inclusions, magmatic breccias, apatite geochemistry, and thermal histories, which demonstrated higher initial water content in causative porphyries such as the giant deposits of Qulong and Jiama, we believe that the lower zircon water content may be attributed to these magmas experiencing faster cooling and sudden depressurization of a large, primed, volatile-saturated or supersaturated mid-to-upper crustal magma chamber, which led to rapid and voluminous volatile exsolution and fluid discharge. These processes caused intense hydrothermal alteration and large ore deposition. In contrast, magmas that experienced slow cooling and deep emplacement would undergo steady-state degassing and weak alteration. The variable intrusive and thermal histories across the Gangdese intrusions reflect the interplay between surface and lithospheric dynamics within the Himalaya-Tibet orogen. Our findings suggest that the differences in zircon water content reflect the emplacement rate and depth of each granitic body and, therefore, may serve as an indicator for tectonics and the potential for mineralization.

Article link: https://doi.org/10.2138/am-2024-9425