Lead isotopes of the Tono uranium deposits; Uranium ore formation and radionuclides migration

Komuro, Kosei*; Yaguchi, Masashi*; Sasao, Eiji   

The Tono sandstone-type uranium deposits occur in the basal conglomerates of middle Miocene Toki lignite-bearing formation in the Mizunami group, which rest on the late Cretaceous granitic basement. In order to understand the age, origin, formation process, and radionuclides migration behavior of the deposits, lead isotopic compositions were measured for uranium ores and the associated sedimentary and granitic rocks. The Tono sandstone-type uranium deposits were formed by initial deposition caused by reduction of uranium in association with the decomposition of the plant materials at the time of the deposition of host conglomerates and sandstones in the Toki lignite-bearing formation, the uranium of which originated from the granitic basement. At the time of deposition of Seto group after the upheaval of this region, some parts of these ores dissolved by oxidative groundwater and the secondary ores were formed by the subsequent re-deposition by reduction.