Mineralogical and geochemical investigations for subvolcanic hydrothermal alteration environments at an active volcano: An example of Hokkaido-Komagatake volcano, Japan

Abstract

At active volcanoes, hydrothermal alteration related to subvolcanic hydrothermal systems can lead to volcanic hazards, such as edifice collapses and phreatic eruptions. Although it is difficult to investigate subsurface hydrothermally altered zones on active volcanoes directly because their edifices are rarely eroded, lithic materials derived from edifice interiors, such as debris avalanche deposits and phreatic ejecta, can contain information on subvolcanic hydrothermal zones and provide a way to investigate this. To reveal the hydrothermal system active during the historical eruptive stage of Hokkaido-Komagatake volcano, this study investigates the characteristics of hydrothermally altered lithic products in volcanic deposits and evaluates the hydrothermal environments using geochemical modeling. The debris avalanche deposits originally comprising the uppermost part of the edifice mainly contain smectite, kaolinite, and cristobalite, suggesting a low-temperature (approximately 100 °C) and weakly acidic to neutral environment. The deposits also have jarosite with a light-δ34S value which is formed from the oxidation of pyrite in a supergene environment, and the acid alteration induced by pyrite oxidation might weaken the edifice. Chlorite and mixed-layer chlorite/smectite comprise the altered rocks derived from the deeper part of the edifice, indicative of a higher temperature (150 to >200 °C) and nearly neutral environment at depth. Strongly acidic alteration, represented by alunite precipitation, is restricted to a fumarolic environment around the volcanic conduit, and this hydrothermal environment developed during the historical eruptive stage. Because the edifice mostly consists of permeable Plinian pumice and weakly welded tuff, the primary thermal water derived by the condensation of magmatic gas in groundwater flows while interacting with the edifice rocks, allowing meteoric water infiltration. Consequently, a relatively low-temperature and nearly neutral environment prevails in the edifice. This is consistent with the absence of acidic, SO4-rich thermal water in the crater area, and low-temperature and nearly neutral pH thermal waters discharge at the foot of the volcano. One of these waters showed heating and increased chemical concentrations related to an increase in magmatic gas flux. This thermal water is assumed to be derived from magmatic gas without scrubbing by the hydrothermal system. As a result, changes in magmatic gas flux directly affect the thermal and chemical changes in the thermal water. Thus, this thermal water can be an efficient monitoring tool for understanding volcanic activity of the volcano.