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 (CADS). 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 ruffle U-Pb dating yielded two Pb-207 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 similar to 300 Ma pluton that formed in a post-collisional environment. Combined with previously published zircon Hf isotopic compositions of similar to 300 Ma felsic intrusive rocks from western Tianshan, the results show that similar to 300 Ma felsic rocks have higher zircon epsilon(Hf)(t) values than those from similar to 351 Ma felsic rocks in the Axi deposit. These elevated epsilon(Hf)(t) values suggest that mantle material inputs of similar to 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.