Magma evolution and Cu-Au mineralization potential of the Upper Devonian-Lower Carboniferous Tulasu basin, Western Tianshan Orogen (NW China): Apatite U-Pb dating and geochemical perspectives

Abstract

The large Axi low-sulfidation epithermal gold deposit (reserve: 50 tonnes Au) is one of the most important gold deposits in the Tulasu basin (Western Tianshan Orogen, NW China), and is hosted by the Upper Devonian-Lower Carboniferous Dahalajunshan Formation volcanic rocks. Due to the close relationship between the genesis and spatial distribution of epithermal gold and porphyry copper deposits, whether the latter is buried beneath the Tulasu basin is highly debated.In this study, apatite and zircon grains from nine ore-hosting volcanic rocks were LA-ICPMS U-Pb dated and analyzed for their minor and trace element compositions. Zircon U-Pb dating suggests multiphase magmatism, with the main phase at ca. 370 and 360 Ma. Apatite U-Pb dating results are consistent with those of zircon U-Pb dating and previous zircon dating works (ca. 380 to 350 Ma), demonstrating the accuracy of apatite U-Pb dating. The apatite Cl content varies with age, which shows that it has the potential to record Cl replenishment from the subducting slab into the mantle wedge. Compared with the apatite Cl content from porphyry Cu deposits worldwide, the apatite Cl content from Axi is much (1–2 wt%) lower, suggesting low potential in forming porphyry copper deposits.