Alteration and calcite formation in the granitic rocks, Asio area, central Japan

摘要

This paper has described the alteration minerals inhost rock and microfractures of granitic rocks, and the formationprocess of them. Samples were taken form the drill core,maximum depth 2002 m , in the granodiorite, Asio area, centralJapan, the host granodiorite has many microfractures with width ofless than 2 mm. The main alteration minerals in the host rock areillite, chlorite, calcite and fluorite. Plagioclase is altered to illiteand biotite to chlorite. The microfractures are filled with quartz,illite, chlorite, laumontite or calcite. There are rare correlationsbetween the alteration degree and the depth. The alteration degreeincreases toward the microfracture, so the hydrothermal solutionalong the fractures altered the nearby host rock. Although themeasured present temperature of 2000 m deep ground water is 91deg C, the alteration minerals formed at less than 100 deg C arenot observed. Therefore the alteration minerals have not beenformed by the present deep ground water.As the alteration proceeds, illite and chlorite are accompaniedwith much calcite. Based on the occurrence of calcite and chemicalcomposition of primary rock-forming minerals, the alteration ofAn-component plagioclase and precipitation of calcite haveoccurred intimately. It is considered that the pH and Ca~ (2+)concentration of the hydrothermal solution caused the accelerationof both reactions. The plagioclase-water interaction andprecipitation of calcite are very important in the alteration processof the granitic rock.