The Geochemical Formation Mechanism Of Shapinggou Molybdenum Deposit Inanhui Province

Shapinggou Mo deposit located in Jinzhai County, Anhui Province, China, is agiant porphyry Mo deposit formed in Early Cretaceous in the East Dabie metallogenicbelt. In this paper, we made the detailed petrographic, microthermometric, Raman andstable isotopic analysis of fluid inclusions of different stages in quartz veins, as wellas the regional geology and geological characteristics of the deposit, to obtain the ore-forming fluid features and evolution.Mo mineralization is associated with the Shapinggou granite porphyry andexplosive breccia. There are four types of wall-rock alteration: silicified，potassic,phyllic, argillic and propylitic alteration. According to the ore composition，mineralassemblage and the cross cutting relationships of veins, the mineralization can bedivided into four stages: Silication is mainly in the early mineralization stage，potassic alteration is in the early stage of main mineralization stage with pyrite phyllicalteration in the late main main mineralization stage，and chloritization is in the latemineralization. There are three big types of fluid inclusions in these quartz veins:CO2-rich two-phase or three-phase inclusions (C-type)，H2O-rich vapor-liquidinclusions (W-type) and halite-bearing multiphase inclusions (S-type). Quartz in theearly mineralization stage contains lots of C-type and W-type fluid inclusions as wellas rare S-type fluid inclusions，quartz in the main mineralization stage contains C-type, W-type and S-type fluid inclusions，but quartz in the late mineralizationcontains only W-type fluid inclusions.Fluid inclusion microthermometric results indicate that the homogenizationtemperatures and salinities of fluid inclusions gradually decrease from the earlymineralization stage to the late mineralization stage. In the main mineralization stage,some vapor-rich fluid inclusions coexist with some halite-bearing fluid inclusionswith similar homogenization temperatures, indicating that fluid boiling has ever takenplace. Raman analysis of fluid inclusions in the three mineralization stages show thatthe ore-forming fluids vary from CO2-rich fluids in the early mineralization stage toCO2-poor fluids in the late mineralization stage. Fluid boiling and late mixing are theimportant mechanism for mineral precipitation. The main mineralization depth ofShapinggou Mo deposit is about2.1-4.0km obtained from the geologicalreconstruction method, and the trapping pressures are estimated to be57.75–110MPa. The trapping temperatures obtained from the isochores of C-type fluid inclusions in the main mineralization stage are about270-372℃. H-O isotope resultsindicate that the ore-forming fluids come mainly from magma in sources, and Sisotope studies show that δ34S values of molybdenite and paragenetic pyrite are+2.96-+3.88‰suggesting that S may comes from magma. Therefore, the initialfluids are characterized by high temperature, high salinity, and rich CO2and daughterminerals. With the opening of system and mixing of meteoric waters, the ore-formingfluids turn into fluids of low temperature, low salinity, no daughter mineral and poorCO2.