Abstract

The black shale is considered one of the most important rock units in the lower part of Um Bogma Formation, where it contains the uranium, heavy metals and rare earth elements mineralization. The black shale samples were analyzed radiochemically by using alpha spectrometry technique. Most of uranium in the studied samples is authigenic and the Th/U ratio confirms the deposition of uranium in reducing environment. The activity ratios of the studied black shale samples were characterized by ²³⁴U/²³⁸U > 1 and ²³⁰Th/²³⁴U < 1, which showed relatively recent precipitation of uranium from water in reducing conditions. ²³⁴U/²³⁵U and ²³⁸U/²³⁵U activity ratio was relatively deviated from equilibrium due to the changes in the oxidation-reduction conditions. The disequilibrium of ²²⁸Th/²³²Th can be due to the co-precipitation of ²²⁸Ra and the migration of ²²⁸Th from the black shale into the percolating water. So, the water was percolated through the paleochannels and caves instead of the rocks causing uranium mobilization and the fractionation of uranium, forming the oxidation-reduction interface in the periods from <6 × 10⁴ to >3 × 10⁵ year.

摘要

黑色页岩被认为是 Um Bogma 组下部最重要的岩石单元之一，其中含有铀、重金属和稀土元素矿化。采用 α 光谱技术对黑色页岩样品进行放射化学分析。研究样品中的大部分铀是自生的，Th/U 比证实了铀在还原环境中的沉积。所研究的黑色页岩样品的活度比为²³⁴ U/ ²³⁸ U > 1 和²³⁰ Th/ ²³⁴ U < 1，这表明铀在还原条件下从水中沉淀得相对较新。²³⁴ U/ ²³⁵ U 和²³⁸ U/ ²³⁵由于氧化还原条件的变化，U 活性比相对偏离平衡。²²⁸ Th/ ²³² Th的不平衡可能是由于^{228 Ra 的共沉淀和228} Th 从黑色页岩迁移到渗滤水中。因此，水不是岩石而是通过古河道和洞穴渗透，引起铀的流动和铀的分馏，在<6×10 ⁴到>3×10 ⁵ 年期间形成氧化还原界面。