Discriminating characters of ore-forming intrusions in the super-large Chalukou porphyry Mo deposit, NE China

Abstract

The Chalukou porphyry Mo deposit, located in the Great Hinggan Range, is the largest Mo deposit in northeast China, although the age and genesis of the associated magmatic intrusions remain debated. Here we report zircon U–Pb ages and trace elements, whole rock geochemistry and Sr–Nd isotope data with a view to understand the relationship between the magmatism and molybdenum mineralization. Zircon U–Pb analysis yield an age of 475 Ma for rhyolite in the older strata, 168 Ma for the pre-mineralization monzogranite, and 154 Ma for the syn-mineralization granite porphyry. The granite porphyry and quartz porphyry are considered as the ore-forming intrusions. These rocks are peraluminous, alkali-calcic, and belong to high-K to shoshonitic series with a strong depletion of Eu. They also display characteristics of I-type granites. The rocks exhibit wide variations of (87Sr/86Sr)i in the range of 0.705426–0.707363, and εNd(t) of −3.7 to 0.93. Zircon REE distribution patterns show characteristics between crust and the mantle, implying magma genesis through crust-mantle interaction. The Fe2O3/FeO values (average 1) for the whole rock and EuN/EuN∗ values (average 0.45), Ce4+/Ce3+ values (average 301) for zircon grains from the granite porphyry are higher than those from other lithologies. These features suggest that the ore-forming intrusions (syn-mineralization porphyry) had higher oxygen fugacity conditions than those of the pre-mineralization and post-mineralization rocks. The Chalukou Mo deposit formed in relation to the southward subduction of the Mongol-Okhotsk Ocean. Our study suggests that the subduction-related setting, crust-mantle interaction, and the large-scale magmatic intrusion were favorable factors to generate the super-large Mo deposits in this area.