Magmatic-hydrothermal evolution of the Yuanzhuding porphyry Cu-Mo deposit, South China: Insights from mica and quartz geochemistry

Abstract

The Yuanzhuding porphyry Cu-Mo deposit, located in the southern section of the Qinzhou-Hangzhou belt, is characterized by successive generations of quartz-bearing veins with complicated crosscutting relationships. Multiple analytical methods have been applied to different mica types and quartz-bearing veins to reveal the evolution history of the Yuanzhuding porphyry system.Four types of micas have been identified in the Yuanzhuding porphyry Cu-Mo deposit, including magmatic biotite (Bt-I), metamorphic biotite (Bt-II), hydrothermal biotite (Bt-III), and hydrothermal sericite (Ser), based on detailed petrological observations. All three types of biotite belong to the Mg-biotite group but can be compositionally distinguished, whereas Ser lies in the transition zone between Li-phengite and Li-muscovite. Generally decreasing crystallization temperatures with increasing distances from the center of the porphyry have been revealed in Bt-II and Bt-III based on their compositional variations, documenting the outward cooling trends of the Yuanzhuding porphyry system.Early quartz-bearing veins with potassic alteration are characterized by granular quartz with homogeneous CL textures, in combination with their wavy vein walls which indicate the host rock was ductile during vein formation, we infer that they were formed under dominantly lithostatic pressure. Late quartz-bearing veins with sericitic alteration are typically euhedral quartz with growth zones of oscillating CL intensity, and they are inferred to be formed under dominantly hydrostatic pressure. Intersection of fluid inclusion isochores with Ti-in-quartz isopleths revealed that early quartz veins with potassic alteration located close to the ore-forming porphyry were formed at up to ∼575 °C. Late quartz veins with sericitic alteration were formed at ∼310 °C.Formation depth of the Yuanzhuding porphyry deposit is inferred to be ∼5.3 km. Hydrogen and oxygen isotopes of Bt-III and quartz indicate that the ore-forming fluid was mainly magmatic fluid. The fluid pressure was high enough to remain stable in single phase through the entire fluid evolution history. All fluid inclusions are vapor-liquid inclusions. The fluid salinity and the Cl fugacity remained constantly low, which may have limited the efficiency of Cu extraction and transportation.The hornfels host significant amounts of Bt-II and may have served as a reservoir of ferrous iron to reduce the oxidized ore-forming fluid and generate reduced sulfur. Effective metal precipitation occurred in the Yuanzhuding porphyry system owing to the coexistence of the generation of reduced S2- and the drop of metal solubility caused by the decrease of fluid temperature.