総説
天草陶石鉱床の特徴と鉱床生成に関する考察
キーワード:
Amakusa pottery stone,
Characteristic,
ore genesis,
Mineral assemblage,
Texture,
Fluid inclusion,
Alteration type
2016 年 45 巻 2 号 p. 62-71
詳細
2016 年 45 巻 2 号 p. 62-71
Many pottery stone deposits are distributed in the western region of Amakusa-Shimoshima Island, Kumamoto Prefecture. They extend 30 km in the north-south direction and 5 km in the east-west direction. More than two million tons of pottery stone has been produced from these deposits and used as raw material for tableware and insulators. Nagasaki metamorphic rocks and Cretaceous and Paleogene sedimentary rocks that have been intruded by a Miocene granodiorite stock and rhyolite dikes are distributed in this area. The pottery stone deposits were formed by the alteration of the rhyolite dikes and they resemble a vein deposit in their form. Major deposits such as the Sarayama vein and the Kaigan vein extend 4-5 km. The width of the veins is generally about 10 m but reaches a maximum of 25 m. Previous studies on the pottery stones revealed the mineral assemblages, the geochemical features of major and minor elements, and the K-Ar ages. The mineral zonation at Denbeikoba quarry, the Sarayama vein, was also clarified. The remarkable characteristics of the Amakusa pottery stone deposits include their widespread distribution, long and narrow form of the ore bodies, and similarity in mode of alteration and mineral assemblage in the high-grade ore. The thermal history and thermal structure of the area surrounding the pottery stone deposits was proposed based on a close examination of the clay minerals in the pottery stone and the vitrinite reflectance of the carbonaceous matter in the surrounding sedimentary rocks. Recently, some data on the microthermometry of fluid inclusions in the pottery stone and radiometric dating by various methods have been reported. These results suggest that the genesis of the Amakusa pottery stone deposits has a close relation with the high geothermal gradient in this area. The genetic process of the Amakusa pottery stone deposits can be better understood by confirming the physicochemical properties, such as the temperature, salinity, and fugacity of carbon dioxide of the alteration fluids.
