Formation of giant copper deposits driven by rapid uplift and sudden depressurization

Abstract

Abstract Porphyry Cu deposits (PCDs) in collisional orogens are new targets for modern mineral exploration. A series of post-collisional (Miocene; 22-12Ma) porphyry copper deposit (PCDs) with Cu reserve over 45 Mt have been discovered in the Gangdese magmatic belt, southern Tibet. These magmas derived from partial melting of sulfide-bearing Tibetan juvenile lower crust are fertile for porphyry Cu in many aspects, such as high oxidation state, and rich volatiles such as water, S, and Cl, but some of individual plutons ended up with economic concentrations. The fundamental question remains what accounts for the significant Cu endowments. 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 PCDs have lower zircon water contents than barren and small-deposit related magmas. Combined with previous fluid inclusions, magmatic breccias, apatite geochemistry, and thermal histories, which demonstrated higher initial water content in causative porphyries such as giant deposits of Qulong and Jiama, we believe that the lower zircon water content may be attributed to these magmas experiencing of faster cooling and sudden depressurization of a large, primed, volatile-saturated or supersaturated mid-upper crustal magma chamber, which could lead 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 Gangdese reflects interplay between surface and lithospheric dynamics within the Himalaya—Tibet orogen. Our findings suggest the difference of zircon water content reflects the emplacement rate and depth of granitic body, and therefore may serve as an indicator for tectonics and potential for mineralization.