W- and M-type tetrad effects in REE patterns for water–rock systems in the Tono uranium deposit, central Japan

Abundances of rare earth elements plus Y (REE) were determined for granitic rocks, Tertiary sedimentary rocks, and related groundwater in the Tono area, central Japan. Tetrad effects, concave curves in REE patterns, were found for these rocks and groundwater samples. Conjugate M- and W-types of tetrad effects were simultaneously observed in these specimens which constitute the water–rock system; the granitic rocks show the M-type tetrad effects while the groundwater and the sedimentary rocks exhibit the W-type tetrad effects. Non-chondritic Y/Ho ratios were found especially in the groundwater; Y/Ho ratios showed larger values than the chondritic value for the samples showing W-type tetrad effects, and vice versa. These results suggest that: (1) preference of the groundwater for a W-type tetrad effect and larger Y/Ho ratio than chondrite produces an M-type tetrad effects and lower Y/Ho ratios in the granitic rocks during weathering processes; (2) the W-type tetrad effect and larger Y/Ho ratio for the sedimentary rocks is brought about by the fixation of groundwater REE onto the sedimentary rocks. The variation of the tetrad effect and Y/Ho ratio was confirmed by laboratory experiments on the leaching of REE from the granitic rocks. The W-type tetrad effect and large Y/Ho ratio in the U-mineralization zone in the sedimentary rocks were also found by SIMS analysis, which is consistent with the W-type tetrad effect and larger Y/Ho ratio in the whole rock abundances of the sedimentary rocks. Disequilibrium of the U series (234U/238U), analyzed by SIMS in the U-mineralization zone, suggests that the U-bearing minerals were formed through precipitation from groundwater as it became reducing. These results suggest that the REE in the sedimentary rocks were transported by the groundwater from the granitic rocks in the Tono area. Based on this study, it is implied that the degree of the tetrad effects and the variation of Y/Ho ratio can be valuable tools to investigate the migration of REE during water–rock interactions in natural systems.