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ALTERATION MINERALS IN GRANITIC ROCK AT ASHIO AS RADIONUCLIDE ADSORPTION MATERIALS

S. HAMASAKI*, K. TSUKIMURA*, K. FUJIMOTO*, K. OMURA**, R. IKEDA** *Geological Survey of Japan, Higashi 1-1-3, Tsukuba 305, Japan. ** National Research Institute for Earth Science and Disaster Prevention, Tennodai 3-1, Tsukuba 305, Japan. ABSTRACT Alteration minerals from a drill-core (maximum depth 2002 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 palgioclase 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. INTRODUCTION High level nuclear wastes in the form of borosilicate glass may be buried at depths of 5001000 m in Japan. Although the borosilicate glass is sealed in engineering barriers, the radionuclides may be released to geologic environments after 1000 or 10000 years. Thus, we must select sites where the radionuclides in ground water hardly reach biosphere. The reaction of radionuclides with alteration minerals may inhibit transporting the radionuclides to the biosphere, because alteration minerals adsorb certain radionuclides effectively [1]. Thus, studying the alteration minerals in rocks is important. This study describes the alteration minerals formed in Cretaceous granitic rock at Ashio (Figure 1). Figure 2 shows the geology of the Ashio area [2]. In this area, several Cretaceous granitic

N

U Ashio area 2

0

1000 km

Fig. 1 Locality of Ashio area

il:i~:!Granodiorite"E~~i~i[" Quaternary andesite •~•::•!i•ii::::~:::•• 1Ji~:::::i!iii Neogene rhyolite

iiiiiiiiiiii i~iiiiii~iiiii~i!iiiiii~i ~ l~f Palo en geao , odionre

0

:::::::::::::::::::::::::::::::: ....EJ:::: Paleogene welded tuff Cretaceous granodioBite 10km !• E+ Paleozoic sediments

Fig. 2 Geological map of Ashio area

Mat. Res. Soc. Symp. Proc. Vol. 353 0 1995 Materials Research Society

1276

Table I List of samples Sample

Depth

Qtz

K-f

PI

Bio

Cal Lau

Chi

Ill

Alteration

Present0 temp.( C)

weak weak strong weak strong moderate

12.7 21.3 43.4 43.5 53.3 53.4

fresh

67.4

+

weak moderate

74.8 83

-

+

moderate

95

+

+

(m)

Host rock Ashio 1 Ashio2 Ashio 3 Ashio4 Ashio 5 Ashio6

23 243 850 851 1113 1115

(++) (4+) (++) (++) (++) (++)

(++) (++) (++) (++) (++)