Abstract
Andestic rocks intruded into or flowed over crystalline schists of the Sanbagawa metamorphic belt and sediments of the Paleogene Kuma Group in the Tobe district. The rocks were altered to pottery stone by hydrothermal solution in many places. The pottery stone with graphite occurs in Koboshi deposit and is composed of quartz and mica clay mineral (1M, polytype) and sometimes contains kaolinite. Graphite usually occurs in the middle horizon of the deposit.
Amygdaloidal graphite aggregate, diameter of which amounts up to 1cm, fills pores in the pottery stone usually associated with mica clay mineral. Some pores are filled by mica clay mineral (1M, polytype), tosudite or mixture of these minerals without graphite, and vacant pores are often observed. Constituent graphite crystals are tabular and 5-50 μm in size. The X-ray diffraction peaks are very sharp. The 13C/12C ratio of the graphite is 21.1‰ (PDB)
It is suggested that the graphite was formed by oxidation of hydrocarbon in ascending hydrothermal solution.
