Uranium minerals are widespread in Japan and are present in various geologic environments. Significant concentrations of the element, however, are limited to dissemination in Tertiary sedimentary rocks of small basins. These sedimentary rocks unconformably overlie Cretaceous to Paleogene granitic rocks.

The major sources of the uranium of the deposits are believed to be intergranular material and micas of the underlying granitic rocks. The average uranium content of the major granitic bodies in Japan ranges from 1.5 to 5.7 ppm, but in the vicinity of uranium concentration, can be as high as 11 ppm or more.

The uranium in the rocks probably was leached by circulating groundwater containing HCO₃- ion. Waters within granitic masses contain up to 8.5 ppb uranium whereas the content in average groundwater is less than 0.05 ppb. The element was transported as uranyl bicarbonate complex and deposited under reducing conditions or was adsorbed in clays and other material.

The primary ore minerals of the Tono mine are uraninite and coffinite, but the major form of uranium concentration here seems to be adsorption in montmorillonite and other materials. Recently, interesting and unique deposits of the element, such as a very close association with zeolite and concentration in traps between reverse faults and the basement granite, were found in this mine.

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