Ore genesis of Axi post-collisional epithermal gold deposit, western Tianshan, NW China: Constraints from U–Pb dating, Hf isotopes, and pyrite in situ sulfur isotopes

The Axi gold deposit has long been regarded as a typical Paleozoic low sulfidation epithermal deposit, located in the western Tianshan, in the Central Asian Orogenic Belt (CAOB). SIMS zircon U–Pb dating of the ore-bearing andesite and dacite yielded concordia ages of 350.8 ± 2.7 Ma and 351.3 ± 3.1 Ma, respectively. In situ SIMS hydrothermal rutile U-Pb dating yielded two ²⁰⁷Pb corrected ages as 306.1 ± 16.9 Ma and 303.8 ± 14.6 Ma, respectively. These ages are considerably younger than the zircon SIMS U-Pb ages of ore-bearing volcanic rocks, which precludes a genetic link between the epithermal gold mineralization and the subduction-related Late Devonian-Early Carboniferous magmatism in the western Tianshan. Instead, the gold mineralization is interpreted to be genetically related to a buried ~300 Ma pluton that formed in a post-collisional environment. Combined with previously published zircon Hf isotopic compositions of ~300 Ma felsic intrusive rocks from western Tianshan, the results show that ~300 Ma felsic rocks have higher zircon ΕHf(t) values than those from ~351 Ma felsic rocks in the Axi deposit. These elevated ΕHf(t) values suggest that mantle material inputs of ~300 Ma Axi epithermal deposit in a post-collisional environment. Pyrite, the most common sulfide in the Axi ores, displays heterogeneous textures and has a large variation of As contents and sulfur isotopes, suggesting complex ore-forming processes in the Axi deposit. This study highlights the importance of in situ isotopes of hydrothermal minerals to decode the ore-forming histories of epithermal systems.
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