Abstract: Dahu-Qinnan molybdenum deposit in Xiaoqinling is within the Xiong'ershan-Waifangshan metallogenic subzone, which is located at the margin of North China platform. Its mineralization types are divided into Mo-bearing secondary quartzite and vein disseminated associated with granitoid mineralization types. The textures are characterized by breccia,lump,porous,stockwork, and massive. The alterations are dominated by disseminated and stockwork of potassic + sillific + carbonated + argillic + anhydrited. The vein and stockwork with disseminated mineralization is closely associated with the granitoid,occasionally occurs within gneiss nearby the Mo-bearing secondary quartzite. The alterations in vein and stockwork ores are mainly characterized by feldspar + quartz + sericite,with minor pyrite + kaolin + carbonate. The Mo-bearing secondary quartzite ores usually contain granitoid breccia associated with veins. The field evidences show that the Mo-bearing granitoid is disintegrated near its contacted zone to small breccias,and finally disappeared inside of the Mo-bearing secondary quartzite; on the contrary, the Mo-bearing secondary quartzite thickens with the increase of intensive argillic and anhydrite alteration. The results suggest that the Mo-bearing secondary quartzite is derived from the Mo-bearing granitoids.12 samples of molybdenite were taken from the Mo-bearing secondary quartzite and the Mo-bearing granitoids in Dahu-Qinnan molybdenum deposit. The age of Re-Os molybdenite of Dahu-Qinnan molybdenum deposit is (223.6±4.1) Ma to (196.1±3.0) Ma and(197.8±3.2)Ma to(196.1±3.3)Ma. The Re-Os isochron age is (199 + 14/-25) Ma. This result suggests that the molybdenum mineralization of Dahu-Qinnan deposit took place in Indosinian, or early Yanshanian. The excellent correlation between the Re-Os ages from molybdenite and the ages of granitoids in the area indicate a direct genetic relation between the granitic porphyry and syenite porphyry. The similarity of the Mo-bearing granitoids in Dahu-Qinnan deposit and the classic porphyry deposit in mineralization and alteration pattern implies that this area has the porphyry molybdenum deposit potential for the exploration of porphyry molybdenum deposits. The Re content in molybdenite is 0.894×10^-6-2.964×10^-6,indicating that the molybdenum materials come from the crust. Alkaline rocks are dominated during Indosinian,suggesting that the molybdenum mineralization occurred in continental extension tectonics. It is suggested that exploration of molybdenum porphyry should be carried out in Indosinian granitoids.

Key words: Re-Os dating, Indosinian porphyries, Mo-bearing secondary quartzite, Dahu-Qinnan molybdenum deposit, Xiaoqinling