鄂尔多斯盆地东胜砂岩型铀矿床成矿水化学过程探讨

为了研究鄂尔多斯盆地东胜砂岩型铀矿成矿水化学过程,利用光薄片、电子探针、X射线衍射、扫描电镜和化学分析等方法对比分析了氧化带无矿化样品、氧化还原过渡带中低矿化及高铀样品的矿物学和地球化学特征。矿物学研究表明:①所有样品中斜长石均表现出强烈粘土化和绢云母化的特征;②铀矿物主要为铀石,呈胶状吸附在矿物颗粒(部分为炭屑)表面、粒间或裂隙中;③相对氧化带,氧化还原过渡带往往含有更多的炭屑和碳酸盐胶结物。稀土元素地球化学研究表明,氧化带无矿化样品和过渡带低矿化样品表现出较平坦的低分异的稀土配分模式;而过渡带高铀含炭屑样品表现出MREE富集的配分模式,高铀富碳酸盐胶结物的样品表现出轻稀土强烈左倾、重稀土平坦的配分模式。对比分析上述差异后认为,铀成矿与水化学作用密切相关,且成矿水溶液中无机络阴离子以CO32-为主,倾向于络合UO22+和HRE3+;而阳离子主要为斜长石的粘土化释放的Ca2+和SiO44-。当水溶液从盆地边缘向中心运移时,物化环境从氧化及酸性环境向还原及碱性环境转变,此时发生铀酰离子的还原并与SiO44-沉淀形成铀石、Ca2+与CO32-沉淀形成碳酸盐以及HREE的沉淀富集。

The Water-rock Interaction Process of the Dongsheng Sandstone-type Uranium Deposit, Ordos Basin

In order to investigate the relationship of the hydrochemical process to the uranium metallogenic mechanism of the Dongsheng uranium deposit in Ordos basin, the authors conducted contrastive mineralogical and REE geochemical analysis of non-mineralized samples in the oxidation zone and mineralized samples in the redox transitional zone by means of general slice, electron microprobe, clay mineral X-diffraction, SEM and chemical testing. Mineralogical studies show that argillic alteration and sericitization of plagioclase can be observed in all samples, and coffinite is the main uranium mineral and is always adsorbed by fragments or carbon dust. Another interesting phenomenon is that the mineralized samples in the redox transitional zone contain more carbon dust and carbonate than those in the oxidation zone. Geochemical studies indicate that non-mineralized samples in the oxidation zone and low mineralization samples in the redox transitional zone show flat REE patterns in PAAS-normalized rare earth element plots. However, samples with high uranium content in the redox transitional zone show two different REE patterns: samples with carbon dust show a MREE enrichment pattern and those with carbonate show a HREE enrichment pattern. Mineralogical and geochemical differences of samples from different zones indicate that aqueous chemical reaction is the main uranium mineralization rather than carbon absorption. In the aqueous chemical process, CO32- which prefers combining UO22+ and HRE3+ is the main inorganic complex anion in the mineralized hydrothermal water, and argillic alteration and sericitization of plagioclase with Ca2+ and SiO44- play an important role in the metallogenic process.