2021 Volume 43 Issue 2 Pages 65-78
The Sanzugawa Caldera had formed through Neogene to Quaternary in south-east part of Akita Prefecture, Northeast Japan. The southern part of the caldera has been investigated for understanding geothermal energy potentiality. We carried out geological survey as a geothermal exploration, and behaviors of thermoluminescence (TL) of quartz were also applied to evaluate thermal effects. According to natural geothermal activities, TL intensity was decreased as a function of time and temperature.
Three types of hydrothermal alteration were identified in this area, which were neutral pH alteration consisted of montmorillonite, sericite and their mixed layered clay minerals, acid sulfate alteration composed of kaolinite and alunite, and alkaline alteration characterized by laumontite and wairakite. The latter two types of hydrothermal alterations were observed sporadically and correlated to low TL intensity.
Additionally, TL intensities of samples at high elevation showed relatively high and those of low elevation samples were decreased. Some samples including specific alteration minerals such as laumontite and wairakite showed relatively low TL intensity against general relation between TL intensity and elevation. On the other hand, neutral pH hydrothermal alteration distributed through whole area and showed no clear relation to TL behaviors.
Those facts strongly suggested that two stages of geothermal activity occurred in this area. First, a neutral pH alteration happened contemporaneous with the Torage-yama formation within the southern part of the Sanzugawa Caldera, and then both of alkaline and acid sulfate hydrothermal alterations were formed by later geothermal activities, showing low TL intensities.
General trends of TL behaviors which were related to elevation of surface suggest that potential heat source maybe widely distributed in subsurface, and sporadic hydrothermal alteration during later stage hydrothermal alteration associating with relatively low TL intensities was enhanced by upwelling flow through fractures. The latest geothermal activities were related to a discrete magma emplacement, which also indicates potential formation of deep-seated geothermal reservoirs.
