Genesis and tectonic setting of the giant Diyanqin'amu porphyry Mo deposit in Great Hingan Range, NE China: Constraints from U–Pb and Re–Os geochronology and Hf isotopic geochemistry

Abstract

The Great Hingan Range is located in the eastern sector of the Central Asia Orogenic Belt, and is the second important Mo province in China, next to the Qinling–Dabie orogenic belt. Previous studies concluded that all the Mo deposits (Mo-only or Mo-dominated polymetallic) in the area were formed in the Mesozoic, and related either to the syn- to post-collisional setting following the closure of the Paleo-Asia Ocean, or to the westward subduction of the Pacific plate. However, the fluids forming the giant Diyanqin'amu porphyry Mo deposit in the area disagree with the features of collision-type Mo deposits, but accord with those of the subduction- or continental arc-type. The Mo mineralization is hosted in the Late Jurassic volcanic rocks and genetically related to the buried porphyritic granite and aplitic granite. Two porphyritic granite samples yield LA-ICP-MS zircon weighted mean 206Pb/238U ages of 158.7±0.8Ma (MSWD=0.64, 2σ) and 158.0±0.9Ma (MSWD=0.38, 2σ), respectively. The aplitic granite yields a zircon U–Pb weighted mean 206Pb/238U age of 156.9±1.1Ma (MSWD=0.48, 2σ), slightly younger than the porphyritic granite. Seven molybdenite samples from the ores yield Re–Os isotope ages of 156.1±2.2 to 158.1±4.3Ma. These ages constrain that the Diyanqin'amu Mo system was formed in the period of 159–156Ma. The granites at the deposit have high contents of SiO2, K2O and Al2O3, and low contents of TiO2, MgO and CaO, showing a peraluminous high-K calc-alkaline affinity. These granites and the host volcanic rocks are characterized by enrichment of K, Rb, U, Pb, Th and LREE, and depletion of Nb, Ta, Ti and HREE, with negative Eu anomaly, which are similar to magmatic rocks in the Andean arc. Zircon grains from the granites show positive εHf(t) values of 4.2–9.0, with TDM2(Hf) ages of 0.63–0.94Ga. These geochemical data imply that the granites at Diyanqin'amu are highly fractionated I-type, formed in a continental arc generated by the southeastward subduction of the Mongol–Okhotsk oceanic plate.