抄録
87Sr/86Sr ratio as well as dissolved major elements in rainwater, surface water and groundwater around the Kamaishi mine were examined to understand in-situ water-rock interaction. Clay minerals exist in the soil covering the surface. However, they have little effect on the groundwater chemical composition. The groundwater does not change its Na/Ca, Rb/Sr and 87Sr/86Sr ratios in the depths shallower than 300 meters. It is considered that groundwater has been at steady state reaction condition with plagioclase in the surrounding granodiorite. Na/Ca, Rb/Sr and 87Sr/86Sr ratios increase with depth in the deeper aquifers than 300 meters, suggesting that the dissolution of K-feldspar and/or biotite rich in alkaline and high 87Sr/86Sr ratio is prompted. The concentration of Sr is lower in the deeper groundwater probably due to mineral precipitation and/or ion exchange reaction. Rb/Sr ratio of groundwater in the borehole, drilled 10 years ago, on the contrary, decreases with depth and becomes close to the value of bedrock at the deepest. It can be interpreted in two ways. One is a contribution of the deep groundwater and another is the influence of mineral dissolution at an early stage of water-rock interaction. Though the interpretations are seriously different, plagioclase must play an important role at the early stage of chemical weathering process.
