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 δ
34S values ranging from +4 to +57‰, whereas pyrite in the Akeyo is small in amounts, and framboidal in form and has low δ
34S 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 δ
34S value of euhedral-subhedral pyrite in the Toki Formation is thought to have been caused by the extensive reduction of SO
42- with high δ
34S value. This high δ
34S of SO
42- was caused by the increase in δ
34S value of seawater due to the mixing of seawater with fresh water accompanied by the precipitation of framboidal pyrite with low δ
34S. 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.
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