LONG-TERM STABILITY OF URANIUM DEPOSITS IN ACTIVE ISLAND-ARC SYSTEM

In order to verify the safety assessment for geological disposal system of high-level radioactive waste in the Japanese Islands, many geological processes inherent in the tectonically active Island-Arc system should be also taken into consideration. The uranium deposits in Japan are subjected to many geological processes in the tectonically active Island-Arc system. Here, radiation damages by cathodoluminescence (CL) and lead isotopes were studied for the Miocene Tono sandstone-type uranium deposits.

Under the CL, radiation damage haloes of about 40 microns width by alpha-particles can be observed for quartz in contact with uranium phases. Their colors are known to change with uranium contents and retention time. The relationships between CL colors of haloes and the uranium contents of associated matrixes indicate that they were classified into three types from the viewpoint of uranium preservation: normal preservation, late or recent accumulation and leaching. The normal preservation type has a close correlation between CL colors and the associated uranium contents. The late or recent accumulation type has no or relatively weak haloes. The leaching type has distinct haloes with low uranium contents.

The 206Pb/204Pb,207Pb/204Pb,and 208Pb/204Pb of the ores are ranging in 18.41-1011.50, 15.57-115.61, and 38.23-38.94, respectively. There are two different isochrons (about 20 Ma and 5 Ma) in the 206Pb/204Pb-238U/204Pb diagram. The initial lead isotopic compositions of 20 Ma isochron are equal to the granitic rocks around the deposit. The enrichment had occurred initially at the sedimentation age of host sediments of 20 Ma. The 5 Ma isochron has radiogenic initial composition, suggesting the regional dissolution of preexisted uranium ores, homogenization of isotopic compositions, and re-accumulation of uranium. The hydrogeological processes of radionuclides migration and the preservation of uranium deposits in active Island-Arc system will be discussed.