The Relationship Between The Mantle Degassing And Uranium Metallogenetism

The Xiangshan uranium deposit , Jiangxi province, located in Xiangshan volcanic collapse basin of Mosezoic Gan-Hang volcanic rock belt, is the largest volcanic rock-hosted uranium deposit in China. The petrology, tectonics, fluid inclusions, isotope geochemistry, thermodynamics were used to determine the characteristics of uranium mineralization at the Xiangshan uranium deposit and discuss the process of mantle degassing and the relationship between mantle degassing and uranium mineralization. The main conclusions are listed as follow:1. The field observations suggest that the structure control is one of important factors associated with uranium mineralization.2. The thermodynamic calculation shows that 99.96% of total U in the solution is transported as UO2(CO3)22- and UO2(CO3)34- under alkalic condition (pH=9.5) in the early stage. Regional deep rift, mantel-derived magma movement, and alkali-metasomatic alteration are favorable condition and evidence that mantle-derived fluid, produced by degassing process, joined metallogenesis.3.The carbon and He isotopes indicate that the ore-forming fluid of Xiangshan uranium deposit is composed of CO2-3He-rich mantle-derived fluid and CC>2-poor meteoric fluid. CO2 of Xiangshan uranium deposit was provided by mantle or mantle-derived mafic dike-rock, which may be produced by mantle degassing process during crust pull-apart extension, then moved up and formed metallogenetic solution.4.The bubble of fluid inclusion is much bigger and the gas content is much higher at premetallogenetic period. Bubble of fluid inclusion is much smaller and a k of pure gas inclusion existing at metallogenetic and postmetallogenetic. A mass of pure gas inclusion existing indicates the inclusion formed during degassing by capturing gas released from metallogenetic solution. From the early stage to the late stage, the reduce of CO2 content also indicates degassing process existing during metallogentism.5.There were two stages degassing. Firstly, mantle degassing produced CO2 andtook it into metallogenetic solution; secondly, metallogenetic fluid degassing brought CO2 out of the metallogenetic solution. The mantle degassing produced the CO2-H2O-rich fluid, the fluid moved to crust shallow and mixed with meteoric water. So mineralizer-rich (CO2) metallogehetic solution could effectively extract U from wall rock (middle-acid volcanic rock and basement metamorphic rock). The first degassing produced CO2-H2O-rich mantle-derived fluid and provided the enough mineralizer (CO2) for metallogenesis, is the most base and important condition of metallogenesis.