Geochemical micro-behavior of natural U-series nuclides in granitic conglomerate from the Tono mine, central Japan.

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  • Geochemical micro-behavior of natural U

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In order to understand uranium migration behavior through geological media, the granitic conglomerate from the Tono uranium mine, Gifu prefecture, central Japan, was studied by grain size division and selective chemical leaching technique. The bulk contents of U and Th in fine-grained samples are larger than those of coarse-grained ones, and have positive correlations with the abundance of detrital biotite grains. The total amount of U eluted by the leaching reagents was more than half in all divided samples. In particular, a large amount of U was leached by sodium acetate/acetic acid (pH 5.0), and hydroxylamine hydrochloride/acetic acid solutions. It is inferred that carbonate and iron oxide minerals play an important role in U ore genesis. The adsorption density of fine-grained samples is larger than that of the coarse-grained one, which suggests that the fine U minerals are abundant in fine-grained sample and/or the U concentration around the complexing site of iron oxyhydroxides may have become high. The bulk 234U/238U activity ratio is 0.95 ± 0.05, and the bulk 230Th/234U activity ratio is 1.36 ± 0.06. For most of the sieved samples, 234U/238U activity ratios are <1, and 230Th/234U activity ratios are >1, showing that U is partially removed from the sediments. The uranium nuclides in the matrix of conglomerate are inferred to be not in equilibrium within 300, 000 y. The ion exchangeable U, whose 234U/238U activity ratio was >1, is bound weakly and exchangeable with U in the surrounding water such as interstitial water and groundwater with 234U/238U activity ratios >1. The 234U/238U activity ratios of the residue fraction are less than 1 (finer sample) or more than 1 (coarser sample), indicating that the recoil ejection effect of U is advantageous for the fine-grained samples while injection effect is advantageous for the coarse-grained samples. The 234U/238U activity ratios of other fractions are less than 1, which is quite similar to the bulk ratio. The ratio decreases in the order of carbonate > iron oxide > organic/sulfide fractions. The 230Th/234U activity ratios is very low in ion exchangeable fraction and >1 in carbonate, iron oxide, and residue fractions. For the residue fraction, 234U/238U > 1 and 230Th/234U > 1 indicate the elution of U and enrichment of 234U relative to 238U by alpha recoil injection. The organic/sulfide complex fraction is depleted in 230Th compared to 238U and 234U. This fact suggests that U was secondarily enriched over Th. As U was enriched at the surface of pyrites, it is inferred that pyrites adsorbed a portion of the eluted U and helped the secondary fixation of U in modern times.

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