Hydrothermal Metamorphism in the Nigorikawa Caldera, Hokkaido, Japan

Abstract

The purpose of the investigation of hydrothermal metamorphism in the Mori geothermal field is to detect hot-water up-flow zones. This paper describes hydrothermal and its relationship to thermal structure before geothermal power generation. Hydrothermal metamorphism in this paper describes a type of metamorphism caused by the percolation of hot water through fiactures. Temperatures increase sharply near the caldera wall. Isograds of hydrothermal metamorphic minerals show a similar pattern of thermal structure. Further, distribution of these minerals demonstrates that they formed before and after caldera formation. Isograds of hydrothermal metamorphic Ca-silicates are concordant with isothermal contours : epidote corresponds to 200-250, garnet (grandite) to 210-250, actinolite and clinopyroxene to 210-260, We conclude, hydrothennal metamorphism in the caldera has been taking place where hot-water has risen along the caldera wall.