Origins of ore-forming fluid and material of the quartz-vein type Mo deposits in the East Qinling-Dabie molybdenum belt: A case study of the Qianfanling Mo deposit

The Qianfanling quartz-vein type molybdenum deposit is located in the East Qinling-Dabie Mo belt. The Mo mineralization in the deposit is mainly present as structurally controlled Mo-bearing quartz veins. Based on chromatography analysis of the Qianfanling Mo deposit, the gas compositions of fluid inclusions in quartz dominantly contain H2O and relatively high concentrations of CO2 (11.019 to 18.554 mol%). Cations and anions of liquid compositions of fluid inclusions are mainly Na+ and Ca2+, and SO42- and Cl-, respectively. The characteristics of gas and liquid compositions of fluid inclusions in quartz indicate that the Qianfanling Mo deposit formed under conditions of high oxygen fugacity. Analytical results of He-Ar isotopes of fluid inclusions in pyrite from the Qianfanling Mo deposit show that He-3/He-4 ratios vary from 0.02438 to 0.0796 Ra (Ra is the He-3/He-4 ratio of air = 1.40 x 10(-6)), with (FHe)-He-4 values of 252,509-885,224. The calculated He-mantle values range from 0.18 to 0.87%, with an average of 0.39%. Ar-40/Ar-36 ratios are 694.4-1473.4, with the estimated Ar-40* (radiogenic Ar-40) proportion of 79.9%. These He-Ar isotopic data indicate that most of the He in the fluid inclusions was derived from the crust, with negligible atmospheric He and only a very small input from the mantle probably. Furthermore, the Ar in the ore-forming fluid was mainly derived from the crustal source, with minor atmospheric Ar involved. 17 Re-Os model ages of 15 molybdenite samples from the 4 quartz-vein type Mo deposits (Qianfanling, Daxigou, Maogou, and Zhifang) range from 215.0 to 248.2 Ma, with a mean value of 235.6 Ma, which is close to the age of peak collision between the North China and Yangtze Cratons. The Re contents in molybdenite from the 4 deposits range from 0.19 to 39.16 ppm, with an average of 17.41 ppm, indicating a crust-dominated source. Re contents in molybdenite from Mesozoic Mo deposits in different ages and types in the East Qinling-Dabie Mo belt show different characteristics of ore sources. Combined with gas and liquid compositions and He-Ar isotopic characteristics of fluid inclusions, as well as Re contents in molybdenite of this paper and previous studies, we suggest that the ore-forming fluid and material of the Qianfanling Mo deposit were mainly derived from crustal magmatic source. This magmatic source could have formed by partial melting of deep crustal rocks under a certain condition during the collision between the North China and Yangtze Cratons.