鄂尔多斯盆地北部砂岩型铀矿铀矿物地质地球化学特征及其成因意义

Geological and Geochemical Characteristics of Uranium Minerals in the Sandstone-type Uranium Deposits in the North of Ordos Basin and Their Genetic Significance

鄂尔多斯盆地北部已发现系列砂岩型铀矿,由东往西依次有东胜铀矿、杭锦旗(纳岭沟)铀矿、大营铀矿等大型、特大型矿床,成为我国目前砂岩型铀矿发现规模最大、最具远景的地区。研究表明,它们具有相似的矿床地质特征和形成环境。在矿床成矿作用研究中,铀的存在形式及铀矿物特征对砂岩铀矿来说是一项重要的内容,其认识对铀的地浸开采可提供重要依据,也是了解砂岩型铀矿形成机制或矿床形成环境及成因的重要信息。本文主要从三个方面对鄂尔多斯盆地北部大营铀矿、纳岭沟铀矿等主要砂岩型铀矿中铀矿物地质地球化学特征和成因进行了研究和探讨。通过电子探针测试,高分辨率扫描电镜观察等手段,认为盆地北部铀矿的铀矿物类型主要为铀石,含少量的沥青铀矿、钛铀矿、水硅铀石、钍铀石等;铀矿物常与黄铁矿、有机质(煤屑)及方解石相伴生。采用逐级化学提取等方法定量地分析了铀矿物占矿石中铀配分的比例关系,认为其中铀矿物和吸附态约各占铀存在形式的50%。首次对砂岩型铀矿的铀矿物进行微区原位LA-ICP-MS稀土元素分析,认为ΣREE在铀矿物铀石中高度富集,是矿石中稀土元素的主要载体;稀土元素是铀矿石中可综合利用的有益组份,其标准化曲线表现为明显的右倾型,轻稀土富集,轻重稀土分馏强烈,δEu和δCe具较弱的负异常,说明总体上铀矿化形成于外生后成环境。从上述铀矿物特征,初步探讨了铀矿化形成的环境与成因,认为铀矿化形成经历了至少为低温流体作用的环境;铀矿化形成于浅部地壳即外生后成环境而与深源作用无关。

The north of Ordos Basin is found to have hosted a series of sandstone type uranium deposits, which from east to west are Dongsheng, Hangjinqi （Nalinggou）, and Daying uranium deposts, etc, suggesting it is China’s the largest and the most potential area for sandstone type uranium deposits. The research shows that these uranium deposits are of similar geological characteristics and formation environment. Existence form of uranium and mineral characteristics of U bearing minerals are the important contents in the study of mineralization process, because they provide not only the important basis for in situ leach mining of uranium but also key information on deposit formation mechanism or deposit formation environment of sandstone type uranium deposits. This study investigates mainly the geological and geochemical features of uranium minerals in sandstone type deposits （such as Daying and Nalinggou uranium deposits） and their genesis. By means of electron probe and high resolution scanning electron microscope, it can be concluded that the uranium mineral types of uranium deposits in the northern part of the basin are mainly coffinite, with minor brannerite, pitchblende, silicon water coffinite, thorium and uranium stone. Uranium minerals are often associated with pyrite and organic matter （soot） and calcite. The proportion of uranium minerals in the ores was quantitatively studied by sequential chemical extraction. The results show that the uranium minerals and adsorbed uranium account for about 50% of the existing forms of uranium. Uranium minerals of sandstone type uranium ores were analyzed using in situ LA-ICP-MS REE test for the first time. Our results suggest that ΣREE is highly enriched in coffinite, which is the main carrier for rare earth elements in ores. Rare earth elements are useful components in uranium ores and REE standard curves show obvious right dipping and LREE enrichment, LREE and HREE fractionation, weak negative δEu and δCe anomalies, indicating that the uranium mineralization formed in the exogenous environment. From the perspective of the above characteristics of uranium minerals, this study discusses the environment and genesis of uranium mineralization. Therefore, it can be concluded that uranium mineralization has experienced a low temperature fluid environment and the uranium deposits formed in the shallow crust, which is not related to the deep source, is the source of the deep crust.