Uranium isotopic ratio of black shale and its role in detection of oxic-anoxic conditions of uranium depositions

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.

Keywords: Uranium isotopes; alpha spectrometry; black shale; oxic-anoxic conditions; thorium isotopes; uranium deposition.