Zircon U-Pb-Hf Isotopes and Geochemistry of Mo-bearing Granite Porphyry in the Lower Urgen Mo Deposit: Implications for the Late Mesozoic Porphyry Mo and Cu Mineralization in the Northern and Central Great Xing’an Range, NE China

Abstract: The Lower Urgen deposit is a newly discovered porphyry Mo deposit in the northern and central Great Xing’an Range. Mineralization predominantly occurs within granite porphyry, yielding a zircon U-Pb age of 142.3 ± 1.5 Ma, thereby endorsing an Early Cretaceous Mo mineralization event. Zircon ε_Hf(T) values (5.5–7.7) and T(DM2-st) (707–844 Ma) suggest that the granite porphyry originated from the partial melting of the Neoproterozoic lower crust. These granite porphyries exhibit coherent geochemical signatures with regional Late Mesozoic Mo-causative granites. Classified as highly fractionated A-type granites, they are enriched in Rb, Th, U, and K, and depleted in Ba, Sr, P, Ti, and Eu. Notably, they possess higher Rb/Sr and Rb/Ba ratios, and lower (La/Yb)_N, Eu/Eu*, LREE/HREE, K/Rb, and Zr/Hf ratios than coeval Cu-causative granites, implying the extent of fractional crystallization plays a pivotal role in determining the mineralization styles (Mo- versus Cu-dominant). Two possible tectonic models are proposed. In one model, Late Jurassic Mo- and Cu-causative granites were formed in an intra-plate extensional setting and compressional setting induced by the flat-slab subduction of the Mongol-Okhotsk Ocean (MOO) plate, respectively, while Early Cretaceous Mo-causative granites were formed in a post-collision extensional setting following the final closure of the MOO. The post-orogenic lithospheric extension model related to the closure of the MOO provides another plausible explanation for the origin of the ore-causative granites. Early Cretaceous highly fractionated A-type granites and Late Jurassic low fractionated adakitic granites represent potential targets for future exploration of Mo- and Cu-dominant porphyry deposits, respectively.