Hydrothermal system at the Noboribetsu geothermal area, Kuttara volcano, southwestern Hokkaido, revealed by geochemical and mineralogical investigations

<p>In active geothermal areas, subsurface high-temperature thermal waters occasionally cause phreatic (hydrothermal) eruptions without any direct input of mass and energy from magma. So, understanding subsurface hydrothermal systems is critical to improving mitigation strategies for such hazards. The Noboribetsu geothermal area in Kuttara volcano, southwestern Hokkaido, has had repeated phreatic eruptions through the Holocene. In this study, to reveal the hydrothermal system beneath this geothermal area, we investigate (1) the chemical and isotopic compositions of thermal waters and fumarolic gases and (2) the characteristics of hydrothermally altered rocks in phreatic ejecta and around thermal water discharge areas. The chemical and isotopic features of the thermal waters indicate that the hydrothermal activity in this area is attributable to a deep thermal water with a Cl concentration of approximately 12,000 mg/L and a temperature>220 °C. The hydrothermally altered pyroclastic rocks in the phreatic ejecta often include vesicles filled with smectite, chlorite, and Ca-zeolite, implying that a low-permeability clay cap consisting of these minerals exists in the subsurface and impedes the ascent of the deep thermal water. The deep thermal water ascends partly to the shallow subsurface, causing separation of the vapor phase containing CO₂ and H₂S due to boiling, and the liquid phase discharges as neutral NaCl-type waters. In addition, absorption of the separated vapor phase by groundwater, with oxidation of H₂S, leads to the formation of steam-heated acid-sulfate waters, which cause acid leaching and alunite precipitation in the shallow subsurface. The Hiyoriyama fumaroles are derived from the vapor separated from the deep thermal water at 140 °C. Phreatic (hydrothermal) eruptions in the Noboribetsu geothermal area are assumed to have occurred due to rapid formation of a vapor phase caused by a sudden pressure drop of the deep thermal water. Because such eruptions are likely to occur in this area in the future, we should perform efficient monitoring using the constructed model of the hydrothermal system.</p>