Hydrothermal Alteration in the Sunagohara Formation, Okuaizu Geothermal System, Japan

Abstract

Hydrothermal alteration of the Quaternary Sunagohara Formation was studied using bulk XRD techniques and thin sections. The Sunagohara Formation, which is composed of rhyolite lava domes and tuffaceous lacustrine sediments, is present at shallow depths in the Okuaizu geothermal system. The maximum depth of the base of the lake sediments is about 350 m and the maximum measured temperature in the Sunagohara Formation is about 120 0C. Five alteration zones can be recognized based on the distribution of clays and zeolites: the least altered (LA) zone is characterized by fresh volcanic glass and is found at the surface outside the Nishiyama spa and geothermal area. The smectite-mordenite (SM) zone is found at the surface around the Nishiyama spa and at shallow depths in the geothermal area. The interstratified clay mineral (IS) zone underlies the SM zone and is also found at the surface of the Nishiyama spa. These alteration zones are horizontaly zoned around the center of the geothermal system, with the IS zone in the center, followed outward by the SM and LA zones. The illite-chlorite (IC) zone is only found in drill hole samples from the geothermal area. The distribution of the SM, IS and IC zones at depth can be correlated with increasing temperature. The kaolinite (K) zone is present at the surface around the Nishiyama spa and geothermal area, and is thought to have been formed by interactions with near surface acid sulfate type water.