巴音戈壁盆地南部塔木素铀矿床成因探讨

位于巴音戈壁盆地南部的塔木素铀矿床在矿化特征上明显有别于我国北方其他砂岩型铀矿床,对于该矿床的成因也存在较大的争议。通过岩石学、同位素地质学、扫描电子显微镜、阴极发光、径迹蚀刻等综合研究,结果显示塔木素铀矿床巴音戈壁组上段第三岩性段(K 1 b 2-3)含铀泥灰岩至少经历了4个阶段的成岩作用,铀矿物呈微粒(粒径<1μm)浸染状分布于最早期的角砾中,具有沉积成岩成因特征,成矿物质主要来自于沉积成岩期水体中溶解的铀并因蒸发浓缩作用而富集在特定的层位。巴音戈壁组上段第二岩性段(K 1 b 2-2)含铀砂岩在沉积成岩阶段盆地内封存的高矿化度地下水与碎屑物之间以及成岩后酸性地表水与碳酸盐胶结物之间共发生了两个阶段的水岩作用,每个阶段形成了表现特征不同的铀矿化。沥青铀矿U-Pb同位素测试结果显示,区内最早的铀矿化形成时间为111.6±8.1 Ma,与砂岩形成时间接近,而最新的铀矿化形成时间为2.5 Ma,具有明显后生成因特征,综合研究显示砂岩中的铀矿化具有沉积成岩及后期层间氧化叠加改造双重成因特征,沉积成岩期成矿物质主要来自封存的地下水,而层间氧化期成矿物质主要来自地表水带入的铀及富铀岩层。同时研究认为塔木素铀矿床存在后期热液活动,但暂未发现热液活动与铀成矿具有直接的成因联系。

Study on the genesis of Tamusu uranium deposit,south Bayingebi Basin

Tamusu uranium deposit has a different uranium mineralization features to the other sandstone type uranium deposits in North China,which result in fairly controversy about the genesis of this special uranium mineralization.The comprehensive studies,including petrology,isotope geology,scanning electron microscopy,cathodoluminescence,α-track etching et al,showed that there are at least four stages in the diagenesis of marl in the third lithologic segment of the upper member of Bayingebi Formation(K 1 b 2-3),microparticle disseminated pitchblende(<1μm)is found in the earliest breccias.The genesis of uranium mineralization in marl is sedimentation-diagenesis,and the ore-forming materials were derived from uranium dissolved in sedimentary diagenesis and were enriched in specific horizons due to evaporation and enrichment.Two stages of water-rock interaction have been found in the sandstone of the second lithologic segment of the upper member of section of Bayingebi Formation(K 1 b 2-2).The first stage of water-rock interaction between high salinity groundwater and plagioclases in sandstone took place in the sedimentation-diagenesis,driving pitchblende enrichment in specific geologic horizon.The second stage of water-rock interaction took place after diagenesis,the interlayer infiltration of the acidic surface water containing sulfate solution dissolved the carbonate cement in sandstone to increase the porosity,which provides space for further uranium migration,superposition and enrichment.The U-Pb isotopic age of pitchblende showed the oldest age is 111.6±8.1 Ma which is close to the age of sandstone,and the latest age is only 2.5 Ma.The genesis of uranium mineralization in sandstone is sedimentation-diagenesis and interlayer oxidation.The ore-forming materials in sedimentary diagenetic stage were derived from the sealed underground water while the uranium source in interlayer oxidation stage was derived from uranium brought in by surface water and uranium-rich rocks.Hydrothermal fluid was found in Tamusu uranium deposit,but there is no direct relationship between hydrothermal fluid and uranium mineralization.