Fluid Inclusion Study of the Doroyu Geothermal Area, Akita

Abstract

More than 2, 000 fluid inclusions in 34 samples of calcite, quartz and wairakite collected from drill cores of the Doroyu geothermal area, Southern Akita, were studied in order to know the relation between the homogenization temperatures of fluid inclusions and measured well temperatures. Geology of the Doroyu area is mainly composed of the Pre-Tertiary basement, Miocene andesitic and dacitic formations, late Miocene lacustrine sediments and Quaternary volcanic formations. Geothermal fluids are found near the boundaries between the basement and Tertiary formations and along the NW-SE step-faults which separate the southern Oyasudake upheaval zone and northern Kijiyama subsided zone. The studied drill holes were selected to represent two vertical sections, the one of which cuts through the western low-temperature zone of the area, and the other runs through the eastern hightemperature zone. Calcite veinlets are abundant in the high-temperature zone and the homogenization temperatures of fluid inclusions in the calcites change concordantly with the present well temperatures. In the low temperature zone, secondary fluid inclusions in quartz grains of dacitic tuffs were mainly used for temperature determination owing to the scarcity of calcite veinlets. In general, ranges of homogenization temperatures in the lowtemperature zone are much higher than the present well temperatures. The present fluid inclusion study revealed that high temperature hydrothermal fluids were once active in the western zone but the activity is waning at present, and that the activity of the eastern zone is at the apex or on the way to increase.