下庄铀成矿古水热系统排泄区(减压区)铀成矿作用研究

A Study on Uranium Mineralization in the Discharge Area (Depressuring Area) of the Xiazhuang Uranium Ore_forming Fossil Hydrothermal System

形成于古水热系统排泄区 (减压区 )的下庄花岗岩型铀矿床是地下热水与岩石相互作用的产物。矿物流体包裹体水文地球化学分析表明 ,成矿期铀成矿古热水溶液气体成分主要为CO2 ,水化学类型为HCO3 _Ca·Na型、F_Ca型和HCO3 ·F_K型。地球化学模式和热力学计算证明 ,热水溶液中铀的存在形式为UO2 (CO3 ) 2 -2 ,UO2 F-3 和UO2 F2 -4 。热水溶液深循环过程中CO2 的加入可使溶液铀沉淀临界电位值 (EhC ,U)明显降低 ,从而保持水_铀比电位值 (ΔEhW ,U)为正值 (ΔEhW ,U=EhW -EhC ,U) ,使铀在深部相对还原的条件下仍能稳定迁移。当富铀成矿热液进入减压排泄区时 ,由于溶液物理_化学条件的改变 ,发生CO2 脱气作用和中和还原作用 ,导致ΔEhW ,U小于零 ,使铀沉淀、富集 。

The granite type uranium deposit in Xiazhuang, formed in the discharge area of fossil hydrothermal system, is a product of thermal water_rock interaction. Hydrogeochemical analysis of mineral inclusions indicates that gases in the solution are dominated by CO 2, and hydrothermal solutions are of HCO 3_Ca·Na, F_Ca and HCO 3·F_K types in the Xiazhuang uranium ore_forming fossil hydrothermal system. Geochemical model and thermodynamic calculations prove that modes of uranium occurrence in hydrothermal solutions are UO 2(CO 3) 2- 2, UO 2F - 3 and UO 2F 2- 4. In the course of deep circulation of water solution, the precipitation critical potential values of uranium ( E hC,U ) in the solution evidently decrease when CO 2 enters the solution, so that water uranium specific potentials (Δ E hW,U ) are usually of positive values (Δ E hW,U = E hW - E hC,U ). Therefore, uranium could easily migrate under the deep relative reduction condition. When uranium_rich hydrothermal ore solution entered the depressuring discharge area, carbon dioxide would experience degasification, and neutral reducing mineralization would occur due to alteration of physical_chemical conditions in solution, resulting in Δ E hW,U <0. In consequence, uranium would be precipitated and concentrated, and large_size granite type uranium deposit would be formed.