Geology, Geochemistry And Genesis Of Qianfanling Quartz-vein Mo Deposit In Songxian County, Western Henan Province

In recent years, the newly discovered type——quartz vein-type Mo deposits in the East Qinling area mainly located in the junction of Xiong’ershan and Waifangshan. 12 Quartz vein-type Mo deposits have been found in this area, which shows good prospect. Based on systematic field study of Qianfanling Mo deposit, using methods and means of fluid inclusion, stable isotopes and geochronology etc., we focused on the nature, evolution and origin of the ore-forming fluids, the Source of ore-forming materials, formation age and genesis of ore. Finally we made the following main results and understanding:1). The Mo orebodies mainly occur as Molybdenite-quartz veins and the minor orebodies occur as alteration wall rocks. The corresponding types of ore are quartz vein-type and altered rock-type. The main metallic minerals are Molybdenite, Pyrite, galena and so on. The main gangue minerals are quartz, barite, calcite, K-feldspar and so on. The main alteration types of wall rocks are silicification, K-felsparization, pyritization, carbonation, sericitization, epidotization and chloritization etc. According to the characteristics and combination of alterations, we can distinguish two bands of alteration: band of K-felsparization and band of propylitization. Based on mineral assemblages, ore structure and cross-cutting relationships of the different ore veins, we divided the hydrothermal mineralization period into 3 stages: pyrite-barite-quartz stage（I）; molybdenite-quartz stage（II）; quartz-calcite stage（）. The molybdenite- quartz stage is the main mineralization stage.2). The results of the temperature of fluid inclusions show that, the fluid inclusions mainly homogenize at temperature between 150℃⁴00℃, with the peak homogenization temperature at 250℃³50℃, and the temperature decrease from early stage to late stage. Fluid salinity rise from the early stage to late stage. There are a large number of daughter mineral-bearing inclusions, which is the result of fluid immiscibility. Laser Raman spectroscopy results show that the main component of fluid inclusions are H₂O, CO₂, N₂, without CH₄, H₂S and other reductive components. Ore-forming fluid is CO-2-H₂O-NaCl system, and it is a medium-high temperature, medium salinity and strong oxidizing fluid.3). Hydrogen, oxygen, carbon and sulfur isotopes results show that the quartz have a consistentδ¹⁸O value, between 9.8‰and 11.1‰. The calculatedδ¹⁸O水‰is between 1.3‰and 4.3‰.TheδD values of fluid range from -81‰to -64‰. Hydrogen and oxygen isotopes composition show that the ore-forming fluid have characteristics of magmatic water.δ¹3CV-PDB values of Calcite are -5.7‰^-1.8‰, the average values change from -4.7‰（stageⅡ） to -2.5‰（stageⅢ）, which shows the carbon in the fluid have a characteristics of source of igneous rocks. The sulfur isotope composition of Molybdenite, pyrite and barite shows bimodal characteristics. Molybdenite and pyrite are in a low negative value, and barites are in a high positive value, which is the result of sulfur isotope fractionation in the closed system. The calculated total sulfur isotope composition is 2‰, which shows deep sulfur source or magmatic sulfur source.4). In the study of ore-forming chronology, we have selected Qianfanling, Zhifang, Daxigou and Maogou using molybdenite Re-Os isotopic dating. Through the test, we got their model ages of （215.0±3.1）Ma^238.8±3.2Ma and isochron age of 237.0±1.9Ma, the middle Triassic. The result show that they have the same ore-forming age, and they formed in the same geodynamic background.5). The mineralization of Qianfanling Mo deposit occurred in the late stage of collision of North China and Yangtzi plate. Local shear and extension setting formed in a great regional collision. The strong collision caused deep magmatic activity. Carrying ore-forming material, the magma went up along the fracture channels and evolved ore-bearing fluid. Because of the fluid immiscibility and change of oxidation -reduction condition, the ore-bearing fluid unloaded the ore-forming materials. Ultimately, the fault-controlled quartz vein-type Mo deposits formed.