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[SVC30-P06] Hydrothermal alteration zone and acidic alteration by hot spring water of Unzen Jigoku in Nagasaki, Japan.
Keywords:Volcano, Hydrothermal alteration, Unzen, Geothermal
Unzen Jigoku is a geothermal manifestation area at the southwest of Unzen Volcano on the Nagasaki Prefecture. There are active fumaroles and hydrothermal alteration zones in the areas.
In this study, the occurrences of the alteration zones were observed in detail and classified to clarify the formation history of Unzen Jigoku. In addition, we analyzed chemical compositions of hot spring water and estimated the speciation of altered minerals to consider alteration reactions in current.
During the fieldwork, we observed the occurrence of altered rocks and collected some of the rocks. Hot spring water was measured about temperature, pH, ORP and EC, and collected samples for chemical analysis.
The microstructure of altered rock samples was observed by thin sections. In addition, the altered minerals were identified by X-ray diffraction analysis.
Hot spring water was also analyzed for chemically and considered the relation with the alteration minerals generation.
On the ground surface, the northern fumaroles were active, and the temperature of the hot spring water was also high.
All altered rocks contain quartz or cristobalite. Quartz is anhedral surrounded by other minerals in thin section, suggesting secondary crystallization due to hydrothermal alteration. Furthermore, the distributions of alunite and kaolinite were biased. Based on the distribution of these minerals a whole area of Unzen Jigoku was classified as the Silica zone, and the 'Silica zone' was further divided into the 'Acidic alteration zone' and the 'Silica residual zone' according to the presence or absence of alunite and kaolinite respect. The 'Acidic alteration zone' was subdivided into 'Alunite zone' and 'Kaolinite zone'.
The chemical compositions of the hot spring water were characterized by abundant SO4 (>300 ppm). In contrast, Cl was scarce at around 1-7 ppm. The chemical compositions show that all the hot spring water is steam-heated water, which is rainwater heated by volcanic gases.
The distribution of silica minerals at the Unzen Jigoku indicates that alteration rocks are commonly silicified. As the threshold for the quartz-cristobalite phase transition is around 100℃ at the shallow and low-temperature hydrothermal system, this Silica zone (with quartz) was formed underground.
The distribution of Acidic alteration zones generally corresponds to ground fumarolic activity. Mineral speciation estimated from chemical composition of hot spring water suggests that the dissolution of plagioclase and K-feldspar and the formation of kaolinite and alunite are proceeding in the SO4-type acidic hot spring water. This suggests that the Alunite zone and Kaolinite zone were formed recently during acidic alteration at the surface.
The alteration zone classification shows that the Quartz-Cristobalite boundary is crossed over the Alunite zone and Kaolinite zone. The occurrence shows the existence of an overlap of hydrothermal alterations (silicification and acidic alteration) in Unzen Jigoku.
In this study, the occurrences of the alteration zones were observed in detail and classified to clarify the formation history of Unzen Jigoku. In addition, we analyzed chemical compositions of hot spring water and estimated the speciation of altered minerals to consider alteration reactions in current.
During the fieldwork, we observed the occurrence of altered rocks and collected some of the rocks. Hot spring water was measured about temperature, pH, ORP and EC, and collected samples for chemical analysis.
The microstructure of altered rock samples was observed by thin sections. In addition, the altered minerals were identified by X-ray diffraction analysis.
Hot spring water was also analyzed for chemically and considered the relation with the alteration minerals generation.
On the ground surface, the northern fumaroles were active, and the temperature of the hot spring water was also high.
All altered rocks contain quartz or cristobalite. Quartz is anhedral surrounded by other minerals in thin section, suggesting secondary crystallization due to hydrothermal alteration. Furthermore, the distributions of alunite and kaolinite were biased. Based on the distribution of these minerals a whole area of Unzen Jigoku was classified as the Silica zone, and the 'Silica zone' was further divided into the 'Acidic alteration zone' and the 'Silica residual zone' according to the presence or absence of alunite and kaolinite respect. The 'Acidic alteration zone' was subdivided into 'Alunite zone' and 'Kaolinite zone'.
The chemical compositions of the hot spring water were characterized by abundant SO4 (>300 ppm). In contrast, Cl was scarce at around 1-7 ppm. The chemical compositions show that all the hot spring water is steam-heated water, which is rainwater heated by volcanic gases.
The distribution of silica minerals at the Unzen Jigoku indicates that alteration rocks are commonly silicified. As the threshold for the quartz-cristobalite phase transition is around 100℃ at the shallow and low-temperature hydrothermal system, this Silica zone (with quartz) was formed underground.
The distribution of Acidic alteration zones generally corresponds to ground fumarolic activity. Mineral speciation estimated from chemical composition of hot spring water suggests that the dissolution of plagioclase and K-feldspar and the formation of kaolinite and alunite are proceeding in the SO4-type acidic hot spring water. This suggests that the Alunite zone and Kaolinite zone were formed recently during acidic alteration at the surface.
The alteration zone classification shows that the Quartz-Cristobalite boundary is crossed over the Alunite zone and Kaolinite zone. The occurrence shows the existence of an overlap of hydrothermal alterations (silicification and acidic alteration) in Unzen Jigoku.