Alteration minerals in granitic rock at Ashio as radionuclide adsorption materials

Alteration minerals from a drill-core (maximum depth 2,002 m) in the granitic rock at Ashio, central Japan, were studied by optical microscopy, X-ray diffractometry and analytical scanning electron microscopy. In the host rock, biotite is altered to chlorite and plagioclase to illite. Calcite has precipitated in veinlets and grain boundaries. The host rock close to fractures is strongly altered, whereas the rocks distant from fractures are less altered. Quartz, illite, chlorite, laumontite and calcite have precipitated on fracture walls. The alteration minerals are estimated to have formed in the range 140--200 C, higher than the present temperature (13--96 C). The chemical composition of the ground water in the granitic rock at Ashio was estimated thermodynamically from the mineral assemblage. The alteration reaction of plagioclase and the precipitation of calcite may occur simultaneously. The alteration minerals formed in the host rock and in the fractures may adsorb radionuclides effectively, and thus may inhibit radionuclide transport to biosphere.