Abstract

Data characterizing the mineralogy, hydrochemistry and geomicrobiology of the Tono region of central Japan were used to interpret geochemical constraints on the origin and stability of the Tono Uranium Deposit. The derived constraints are compatible with models of deposit formation, which call for leaching of uranium from the upper weathered zone of the Toki Granite by relatively oxidizing groundwaters that are near-neutral to moderately alkaline and carboniferous. The oxidizing groundwaters then migrate into mudstones and sandstones of the overlying Toki Formation, where the uranyl species is reduced by water–rock–microbe interactions to uranous species, sorbed by various detrital and authigenic phases and eventually precipitated as uraninite, coffinite and the metastable, amorphous hydrous oxide, UO2(am).

Formation of the Tono deposit may have been more or less continuous up to the present time. The modern hydrochemical system, upon which the genetic model is based, began to evolve about 15 Ma when seawater was flushed out of the sedimentary cover and basement granite by fresh, meteoric waters during a period of uplift and erosion preceding Pliocene to Pleistocene sedimentation. Recharge with meteoric water continued to the present, which suggests that palaeohydrochemical conditions were probably similar to those observed in the region today when the Tono deposit began to form about 10 Ma.

Redox environments in the Tono region inferred from in-situ Eh measurements in deep boreholes and calculated potentials for the SO42−/HS redox couple appear to be controlled by heterogeneous reactions involving Fe(III)-oxyhydroxides. Metastable equilibria and particle-size effects associated with these reactions produce a range of possible redox environments that are equally compatible with both the relatively oxidizing and reducing groundwaters of the Toki Granite. This compatibility extends to sedimentary porewaters, where the redox environment is also controlled by microbially mediated sulphate reduction, oxidation of organic matter and precipitation of sulphide minerals. Redox conditions have been stable during at least the past several tens of thousands of years based on palaeoredox indicators interpreted from the trace element contents of fracture calcites. The pH and carbonate contents of palaeogroundwaters and modern groundwaters of the Tono region were, and are, controlled mainly by calcite equilibrium.

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