Alteration of chemical and physical properties of mudstone by intrusion of volcanic rocks

Massive mudstone of the Miocene Funakawa Formation in Akita prefecture that was altered by the andesite intrusion was studied. Mordenite, Opal-CT, Quartz, Albite, and Adularia were formed in it in the interval of 12 m from the intrusive contact. The change of physical properties of mudstone caused by the andesite intrusion was characterized by the behavior of the SiO2 dominant hydrothermal water. In case of hydrothermal water intrusion from the andesite to mudstone, permeability of mudstone decreased by the precipitation of amorphous silica in the pore space of mudstone. On the other hand, in case of no migration of water, the permeability increased with the gradual change of d-spacings of opal-CT from about 4.08 to 4.02 A. The transition boundary of albite /zeolite can be found within 2 m from the intrusive contact and its temperature is estimated as about 190degC. The opal-CT decreased and quartz increased in the interval from 2.2 m to 6.2 m from the intrusive contact and its temperature is estimated as 65degC-100degC. The behavior that happened around the intrusive rock can be thought to be analogous to that around the heat generated high-level waste repository. (author)