Geology, Geochemistry And Formation Mechanism Of The Huzhagaitu Porphyry Molybdenum Deposit In Inner Mongolia, NE China

The Huzhagaitu molybdenum deposit is a typical medium scale porphyry deposit in the middle segment of the Derbugan metallogenic belt in Inner Mongolia, Northeast China, which is located in the southeast side of the Mongolia-Okhotsk suture zone. The mineralization process is closely related to the Early Yanshanian granodiorite intrusion. The orebodies are hosted in the contact zone rock of the intrusion and the rhyolitic tuff wall rock. Molybdenite and pyrite mineralization mainly occurs in quartz-sericite alteration zone. In this paper, the author carried out a detailed research on petrography, rock geochemistry, isotopic geochronology, stable isotopes and fluid geochemistry, then finally discussed the mineralization mechanism of the Huzhagaitu Mo deposit.According to the rock geochemical analysis, the Ca O, Na O, Fe2O3 T and Mg O in mineralized rhyolitic tuff have been brought from the granodiorite. The rocks are characterized by relative richment of LREE and depletion of HREE, the fractionation between the LREE and HREE is apparent. Compared to the granodiorite, Eu negative anomaly in the mineralized rhyolitic tuff and rhyolite is more obvious. These rocks show pronounced enrichment in Rb, Th, U, K, La, Ce, and depletion in Nb, Ta, Ti, P.The zircon U–Pb ages of two pieces of volcanic-sedimentary rhyolitic tuff are 370 Ma and 372 Ma, respectively. It is consistent with the era of the Upper Devonian Daminshan Formation. Re–Os isotopic model age of molybdenite is 179.8±2.7 Ma, indicating that the deposit was formed in Early Jurassic.The fluid inclusions study results show that the temperature is mainly at 260oC~440oC, the salinity is concentrated in 8%~14% and 38%~52% at the major mineralization stage. Ore-forming fluid at the early stage belongs to H2O–CO2–Na Cl system. Along with the continuous development of mineralization, accompanied with a small amount of meteoric water. Ore-forming fluid change into low temperature and low salinity in the middle and late stages. The temperature is concentrated at 140oC~240oC, and the salinity is mainly in the 2%~6% in this period. The analysis of C-H-O-S stable isotopes indicate that the source of ore-forming materials in the Huzhagaitu Mo deposit is related to the magmatic activity of the granodiorite intrusion, mainly derived from the lower crust.From what has been mentioned above, the author can conclude that the Huzhagaitu Mo deposit occurred under the tectonic setting of collision or post collision after the Mongolia Okhotsk Ocean closure in Early Jurassic. Along with the occurrence of tectonic magmatic events, the volcanic rocks of the Daminshan Formation were intruded by molten magma. Ore-forming fluid was boiling in the main mineralization stage. Subsequently, the physical and chemical conditions of ore-forming fluid system changed, which lead to the molybdenite predication from the magmatic fluid.