从沉积到成岩两种截然不同的地质环境——吐哈盆地砂岩型铀矿研究实例

Two Distinct Geological Environments from Sedimentary to Diagenesis Stages: Examples from Sandstone-Type Uranium Deposits, Turpan-Hami Basin

砂岩型铀矿床的形成既需要适当的沉积环境做准备 ,又与成岩环境关系密切 .通过宏观沉积学和泥岩元素地球化学分析探讨了吐哈盆地西南缘水西沟群的沉积介质环境 ,利用砂岩样品的流体包裹体及激光拉曼光谱分析阐明了成岩环境 .研究发现 ,从沉积期到成岩期研究区的水西沟群具有 2种截然不同的地质环境 :沉积期泥岩矿物中Sr/Ba比值小于 0 .3,B平均值为 4 6 .35× 10 -6,同时具有淡水动物化石组合 ,显示为温暖潮湿的贫氧、淡水介质环境 ,但到成岩期却演化为具有有机质参与的中低温 (15 6 .1℃ )、超盐度 (7.4 % )成岩环境 ,现今环境 (地下水盐度为 7.8% )恰好与成岩环境相似 .值得注意的是 ,研究区铀成矿的年龄 (10 4± 1)Ma、(2 4± 1)Ma、(7± 0 )Ma与成岩期基本吻合 ,表明具有有机质参与的中低温。

The formation of sandstone-type uranium deposits is not only related to sedimentary environment, but also related to diagenesis environment. This paper illustrates the original sedimentary environment of the Shuixigou Group using sedimentology methods and analysis results of element geochemistry in mudstone, and discusses the diagenesis environment by means of fluid inclusions and Raman spectrum analysis in sandstone in the southwestern margin of the Turpan-Hami basin. The results show that two distinct geological environments from sedimentary to diagenesis stages are observed in the Shuixigou Group. In sedimentary stage, mudstones have the Sr/Ba value of less than 0.3 and the average B content of 46.35 × 10-6, and the fresh water faunal fossils are observed, which indicates that in-situ sedimentary environment shows warm, wet, oxygen-deficient and fresh water. However, the diagenesis environment with middle-lower temperature (156.1^DDGC) and supersalinity (about 7.4%) as evidenced from fluid inclusions is the same as that of the present formation water with a salinity of 7.8%. The research results indicate that the mineralization ages of uranium are basically coincident with the main diagenesis stages, which are (104±1) Ma, (24±1) Ma and (7±0) Ma, respectively. So it is possible that supersaline hydro-carbon-bearing fluid with middle-lower temperature should be a favorable mineralization environment of uranium.