Alteration type and thermoluminescence ages of altered rocks in the Mutsu-Hiuchidake volcanic field, Shimokita peninsula, NE Japan

Abstract

Mutsu Hiuchidake Volcano has an erosion caldera which shows a horseshoe like geomorphological feature toward east. Tertiary strata as a basement are distributed in the caldera. Tertialy strata and a part of pyroclastic deposits of the Mutsu Hiuchidake Volcano have altered strongly to moderately by hydrothermal activities related to the volcanism. This study is to clarify a progress history of the alteration by using a geological mapping, thermoluminescence (TL) dating, x ray diffraction analysis, an infrared reflection absorption analysis and a fluid inclusion study.Highly altered zone is recognized in the area of midstream to upstream along the Ohakagawa and the Koakagawa within erosion caldera. The argillic alteration zone surround a silicification zone in the highly altered area shows a circular distribution. The strongly altered areas are along NNW SSE to NNE SSW fractures. The alteration areas were divided into the smectite, kaolinite, alunite and pyrophyllite zones.The kaolinite and alunite zones give the TL ages of quartz 67 13 ka (KG 5), 88 18 ka (OG 4) and 91 23 ka (OG 1). The smectite zone within the argillic alteration zone of outside of collapse caldera, yield the ages 752 215 ka (SO 2) and 615 197 ka (KG 1). These TL ages suggest the hydrothermal activity end at 70 to 90 ka.The existence of pyrophyllite suggests that hydrothermal temperatures were 200 to 250 C in these area. This is supported from the homogenization temperatures of fluid inclusions in calcite, 242 C in average.

Geothermal Resource Exploration by Stream pH Mapping in Mutsu Hiuchi Dake Volcano, Japan

“…The highest recorded temperature of hot water was 228.5 • C at 1700 m depth within the well N59-SK-6 ( Figure 2) [17]. The area was selected for the present study because it shows several signs of geothermal activity, such as alteration zones and fumaroles relating to hydrothermal alteration [17,18], and is therefore considered suitable for geothermal development. The area within the erosion caldera is subsequently referred to as the Mutsu Hiuchi Dake geothermal field.…”

“…Mutsu Hiuchi Dake East Slope Fault is located closer to the summit-and hence closer to the center of the heat source-than are the geothermal wells drilled during previous surveys (Figures 2 and 3), which heightens the expectation that this location could be endowed with a geothermal resource of even higher temperature than those previously observed. Additionally, it is known that there are hydrothermal alteration zones relating to geothermal activities scattered across the Mutsu Hiuchi Dake geothermal field ( Figure 3) [17,18]. Tomiyama et al conducted detailed surveys of alteration zones in the Mutsu Hiuchi Dake geothermal field, and specified the distribution of silicified alteration zones ( Figure 3) [18].…”

“…Additionally, it is known that there are hydrothermal alteration zones relating to geothermal activities scattered across the Mutsu Hiuchi Dake geothermal field ( Figure 3) [17,18]. Tomiyama et al conducted detailed surveys of alteration zones in the Mutsu Hiuchi Dake geothermal field, and specified the distribution of silicified alteration zones ( Figure 3) [18]. The sections displaying the greatest silicified alterations were located across the Koaka forest road and midstream areas of the Ohaka and Koaka rivers, coinciding with the location of the Mutsu Hiuchi Dake East Slope Fault.…”

“…Based on these points, there is a high chance that the Mutsu Hiuchi Dake East Slope Fault has formed a geothermal reservoir, and thus might be an important fault for geothermal development. [18]. The red triangle shows the location of the summit of Mutsu Hiuchi Dake volcano.…”

“…Top: Map showing the location of the Mutsu Hiuchi Dake East Slope Fault. The yellow circle indicates the location of silicified alteration zones as indicated by Tomiyama et al[18]. The red triangle shows the location of the summit of Mutsu Hiuchi Dake volcano.…”