Abstract
Mizunami Group of Miocene sedimentary rocks in the Tono sand-stone-type uranium mine area, central Japan is composed of lacustrine lignite-bearing Toki Formation (20∼18 Ma) and marine Akeyo (16∼15 Ma) and Oidawara (5∼0.7 Ma) tuffaceous mudstone and siltstone Formations.
Pyrite in the Toki Formation is abundant, and euhedral-subhedral in form and has anomalously high δ34S values ranging from +4 to +57‰, whereas pyrite in the Akeyo is small in amounts, and framboidal in form and has low δ34S values (−28∼−8‰). The sulfur isotopic compositions of pyrite, and mineralogical and geological features of the Tono area are similar to the lake sediments of Morrison Formation of Western U.S.A. in which large sandstone-type uranium deposits occur. These similarities suggest that sedimentary and diagenetic processes occurred under alkaline conditions in middle Miocene age (18∼16 Ma) in the Mizunami district. Unusually high δ34S value of euhedral-subhedral pyrite in the Toki Formation is thought to have been caused by the extensive reduction of SO42- with high δ34S value. This high δ34S of SO42- was caused by the increase in δ34S value of seawater due to the mixing of seawater with fresh water accompanied by the precipitation of framboidal pyrite with low δ34S. The alkaline conditions are considered to have been caused by the change from fresh water environment to marine environment and from low temperature to high temperature-arid conditions at middle Miocene age when Japan Sea opened and a clockwise rotation of Southwest Japan occurred.
