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. 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, and molybdenite Re-Os isotopic dating model age 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 2 O 3 (12 wt.% - 13 wt.%), Ba, Ti, P, and Sr contents, with negative Eu anomalies. / Nb ratios are comp arable to those of average continental crust and island arc basalts, corresponding to type-A2 granites. 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.