Zircon U–Pb–Hf–O and molybdenite Re–Os isotopic constraints on porphyry gold mineralization in the Bilihe deposit, NE China

Abstract

The Bilihe gold deposit in Inner Mongolia is situated in the Central Asian Metallogenic Domain. Its major orebody II is a porphyry-type body, spatially and temporally associated with granodiorite porphyry and granite aplite. In this study, the timing of gold mineralization is precisely constrained by using the zircon U–Pb dating for pre-mineralization intrusions and the molybdenite Re–Os dating for later molybdenite veins. Furthermore, zircon Hf–O isotope analyses have also been carried out to decipher the nature of primary magma. Zircon U–Pb dating shows that the granodiorite porphyry and the granite aplite were emplaced at 269 ± 2 Ma and 270 ± 2 Ma, respectively, indicating the gold mineralization occurring no earlier than 269 Ma. Meanwhile, molybdenite veins are developed within the fractures and commonly cut across the auriferous veins. Thus, combined with a molybdenite Re–Os isochron age of 268 ± 1 Ma, the gold mineralization in the Bilihe deposit can be precisely restricted to ca. 269 Ma. Zircon εHf (t) values are mostly positive (1.6–11.3), along with high δ18O values of 6.20–7.63‰, suggesting a mixed source between mantle materials and ancient continental crust (such as the Bainaimiao Group) for the Bilihe magma. It is also supported by the presence of the captured detrital zircons in these intrusions. Given a universally metallogenic environment for porphyry gold deposits, a thicken crustal setting related to the collisional intermission of the Paleo-Asian Ocean is favored to interpret the formation setting of the Bilihe deposit.