Genesis of the South Pit Deposit in Jiama District, Tibet: Constraints from Geology, Geochronology and Amphibole Geochemistry

Abstract

The giant Jiama deposit is a post-collisional porphyry Cu-polymetallic system located in the Gangdese metallogenic belt of Tibet. It consists of three deposits: The Main deposit, the Zegulangbei deposit, and the South Pit deposit according to exploration and research. The South Pit deposit is a high-grade Cu-Pb-Zn deposit, but its genesis is unclear. To investigate its genesis, a detailed study was conducted on the deposit geology, geochronology and amphibole geochemistry. The results indicate that the weighted average ²⁰⁶Pb/²³⁸U age of the zircons from the granite porphyry in the South Pit is 15.38 ± 0.45 Ma, and the molybdenite from the mineralized skarn yield a Re-Os isochron age of 15.23 ± 0.22 Ma, in line with the age of the Main deposit (15.7−14.3 Ma). The amphiboles in the granite porphyry of the South Pit, magnesiohornblende and actinolite, are high in Mg and Ca and low in K. They crystallized at temperatures of 705–749 °C, pressures of 0.44–0.67 kbar, oxygen fugacity of −14.31− −13.69 (NNO), and depths of 1.7–2.5 km. Mapping of structure and alteration indicates that the South Pit skarn developed due to the metasomatism of marble of hornfels or carbonate in fold hinge dilation and an interlayer detachment zone by magmatic hydrothermal fluids. According to the age of magmatism and geological features, the South Pit deposit and the Main deposit have originated from the same Miocene magmatism, but the South Pit deposit was affected by the gliding nappe tectonic system. The amphibole geochemistry indicates that the ore-related magma of the South Pit has a high oxygen fugacity and is rich in water.

Keywords

geochronology / amphibole geochemistry / porphyry Cu-polymetallic system / Gangdese metallogenic belt / Tibet

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