中文摘要：

鄂尔多斯盆地的东胜铀矿床是一个重要的大型砂岩型铀矿床,业已查明铀石是该铀矿床的主要含铀矿物,铀石的颗粒十分细小。大量的显微镜薄片和电子显微镜观察发现,铀石通常以不规则的集合体产出于蚀变黑云母裂隙和黄铁矿边缘。电子探针分析表明,铀石中w（UO2）变化较大,在50%-70%间。与铀石相关的黑云母已经强烈蚀变,转变为水黑云母、水白云母和绿泥石。黑云母在蚀变过程中K^＋逐渐流失,直到基本流失殆尽,与原始黑云母相比,水黑云母w（K2O）总体减少了6.54%[占原始蚀变黑云母中w（K2O）的74%],同时w（FeO）减少了7.40%[占原始蚀变黑云母中w（FeO）的29%];水白云母与原始黑云母相比,矿物成分也发生了很大的变化,其中,w（K2O）总体减少了7.87%[占原始蚀变黑云母中w（K2O）的89%],同时w（FeO）减少了19.22%[占原始蚀变黑云母中w（FeO）的76%],w（Al2O3）由16.64%增加至32.72%,增加了16.08%。根据铀石产出的特征和蚀变水黑云母和蚀变水白云母成分特征,探讨了铀酰离子（UO2^2＋）被还原成U^4＋和形成铀石[U（SiO4）1-x（OH）4x]的机理,指出古层间氧化带中铀石的形成与黑云母的蚀变以及黄铁矿的形成有着密切的联系。

英文摘要：

Dongsheng U-deposit in Ordos Basin is an important large scale sandstone type U-deposit in China. Coffinite, as the main U-bearing mineral in the U-deposit, occurs as a micro-grain. The SEM and EPMA study indicates that the formation of coffinite is closely related to biotite, pyrite and clay minerals in the ore and that w（UO2）in the coffinite varies from 50% to 70%. The biotite has strongly altered and changed into hydrobiotite, hydromuscovite and chlorite when the biotite altered and K^＋ is separated from the biotite. Compared with the original biotite, w（K2O）and w（FeO） reduced 6.54% and 7.40% respec tively in the hydrobiotite, reduced 7.87% and 19.22% separately in the hydromuscovite,while w（Al2O3） increased from 16.64% to 32. 72%. The article probes into the formation mechanisms of U^4＋ , which uranyl （UO2^2＋） was deoxidized into, and the formation mechanisms of coffinite （U（SiO4）1-x （OH）4x）. The occurrence of coffinite, pyrite and clay minerals reflects that the formation of coffinite was closely related to the decomposition of biotite and the formation of pyrite in ancient interlayer oxidizing zone.