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

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 Sre 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 premineralization 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(87 Sr/86 Sr)iin the range of 0.705426 -0.707363, and ε_(Nd)(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 Fe_2O_3/FeO values(average 1) for the whole rock and EuN/Eu*Nvalues(average 0.45), Ce^(4+)/Ce^(3+) 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.

Age,Genesis,and Tectonic Setting of the Mo-W Mineralized Dongshanwan Granite Porphyry from the Xilamulun Metallogenic Belt,NE China

The Xilamulun molybdenum polymetallic metallogenic belt in eastern Inner Mongolia forms one of the most important Mo metallogenic belts in northeastern China. The Dongshanwan porphyry Mo-W polymetallic deposit, in the northeastern part of the Xilamulun metallogenic belt, occurs along the periphery of a granite porphyry and consists of Mo-W-Ag sulfide and oxide disseminations and veinlets in hydrothermal assemblages. LA-ICP-MS zircon U-Pb dating of the Dongshanwan granite porphyry yields a crystallization age of 142.15± 0.91 Ma, whereas molybdenite Re-Os isotopic dating model ages are of 139.9–141.5 Ma and an isochron age is of 140.5± 3.2 Ma（MSWD=1.2）. The age consistency indicates that the Dongshanwan deposit was a product of Early Cretaceous magmatism. The Dongshanwan granite porphyry is a high-alkali high-potassium intrusion and has high SiO_2（75.39 wt.%–76.15 wt.%）, low Al_2O_3（12 wt.%–13 wt.%）, Ba, Ti, P, and Sr contents, with negative Eu anomalies. The Y/Nb ratios are comparable to those of average continental crust and island arc basalts, corresponding to type-A2 granites. Our geochemical data indicate that the granite porphyry emplaced in an Early Cretaceous post-orogenic extensional environment following Mongol-Okhotsk oceanic subduction and subsequent continental collision.