Uranium mineralization by ground water in sedimentary rocks, Japan

To solve the mechanism of uranium concentration in stratabound uranium deposits occurring in the basal part of Neogene sediments overlying granite basement, attention has been paid to uranium leaching from weathered granite by circulating carbonated fissure waters, to effective adsorbents for fixing uranium from uraniferous ground waters, to structural features controlling the ground-water circulation, and other relevant factors. The evidence for uranium transportation by hydothermal solutions, including hot spring waters, is hard to observe. Conclusions are summarized as follows:(1) Uranium in the deposits is supplied from surrounding source rocks, mostly from granite.(2) Uranium is transported by circulating ground-water solutions.(3) The uranium dissolved in ground water is fixed in minerals in various ways, the most important being adsorption by carbonaceous matter.(4) Ore-grade uranium concentrated from very dilute solutions occurs by multiple repetition of a leaching-and-fixation cycle between minerals or adsorbents and circulating uraniferous ground water.(5) Important factors for uranium mineralization are sufficient uranium, supplied mostly from granite, the existence of effective adsorbents such as carbonaceous matter in the host rocks, and favorable geological, geochemical, and geophysical environments. The last seem to require not only physical and chemical conditions but also correct flow and volume of ground water.