Stability of radioactive minerals in an oxidizing hydrogeological environment; New results from an alluvial placer deposit, Naegi District, Central Japan.

Sasao, Eiji   ; Komuro, Kosei*; Nakata, Masataka*

Study of the stability and alteration of radioactive minerals in a placer deposit in the Naegi District, southeastern Gifu Prefecture, Central Japan contributes to understanding the modes of nuclide migration under various hydrogeological environments in the tectonically active Japanese Island Arc system. Under the optical and scanning microscopes, many grains of zircon have well-preserved crystal faces, whereas those of thorite were highly abraded and corroded. Most of monazite and fergusonite have partly abraded and corroded forms. This result indicates that uranium is strongly leached under oxidized hydrogeological environment but the behaviour is different for different minerals, being well harmony with the thermodynamic estimation. Leaching or release of uranium from radioactive minerals to surface geological environment is estimated to extend over long time under oxidized hydrogeological environment.