Origin of the subduction-related Carboniferous intrusions associated with the Yandong porphyry Cu deposit in eastern Tianshan, NW China: constraints from geology, geochronology, geochemistry, and Sr-Nd-Pb-Hf-O isotopes

The Yandong porphyry Cu deposit is located at the south margin of the Dananhu-Tousuquan arc belt in eastern Tianshan, northwest China. The Cu ores comprise mainly disseminations and vein zones in the potassic and phyllic alteration zones, and are predominantly hosted in diorite porphyry, tonalite, and quartz porphyry, which intruded into Carboniferous Qi'eshan Group volcanic rocks. The U-Pb ages indicate that four intrusions were emplaced between 338.6 +/- 2.9 and 326.1 +/- 2.6 Ma. Five molybdenite samples yield Re-Os model ages of 333.8-329.5 Ma with a weighted average age of 331.8 +/- 2.1 Ma. Fourteen pyrite samples have Pb-206/Pb-204 of 17.776-18.959, Pb-207/Pb-204 of 15.410-15.534, and Pb-208/Pb-204 of 37.323-38.127, similar to the age-corrected data of the Yandong tonalite. The tonalite shows adakite-like characteristics (e.g., high Sr/Y ratios and low Y contents), and has positive epsilon(Nd)(t) and epsilon(Hf)(t) values, and low zircon O isotopes (3.7-4.6 aEuro degrees), suggesting that the melt was derived from partial melting of a subducted oceanic slab followed by mantle peridotite interaction. The diorite porphyry exhibits high Mg-# and low Sr/Y values, slightly negative Eu anomalies, and positive epsilon(Hf)(t) values, indicating a lithospheric mantle source. The quartz porphyry, with stronger negative Eu anomalies, less evolved epsilon(Hf)(t) values, and low delta O-18 values (4.7-5.5 aEuro degrees), was probably derived from mantle melts that experienced mixing with lower crustal materials (melts or assimilation). The new data suggest that the Yandong intrusions formed in an arc setting. As the tonalite is genetically linked to the Cu mineralization, subduction-related slab melts must have played a key role in the formation of the Yandong deposit.