Hydrogeochemical and geochemical studies on uranium in Nakamaruke area, Niigata Prefecture

Hydrogeochemical study on uranium has been made for the river water samples and sedimentary rocks collected in Nakamaruke area. Sekikawa Village, Iwafune District, Niigata Prefecture. The qualities of river water samples in this area were the non-carbonate alkaline type. Uranium contents of the river water samples are less than 0.1 ppb, which is lower than those of other mineralized areas. The ratio of uranium in water to that in ore is less than 10-6. Since the contents of phosphate and fluoride ions in the river water samples were low, both ions were hardly adopted as the indices of uranium ore in this area, even though the fluoride ion content showed a good correlation with the uranium content. Therefore it is concluded that the uranium in stream water is the best indicator for uranium occurrence in this area. From the outcrop, the columnar specimens of sedimentary rocks were collected. Most of them were medium arkose sandstone, tuffaceous sandstone and coarse arkose sandstone. The highest uranium content in the medium arkose sandstone was 89 ppm. Among the specimens, uranium content shows a good correlation with the P2O5 content, which suggests that uranium has been concentrated in apatite. Because the coarse arkose sandstone collected from the lower part of the column shows a little enrichment of uranium, uranium may have moved downward during the period of diagenesis after sedimentation. The 234U/238U activity ratios in the samples from the upper part of the column were less than 1.0 while those from the lower part of the column were more than 1.0. This result also suggests that uranium moved downward from the upper horizon and reconcentrated again in the lower horizon. (author)