IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1P1_1E-O17

Room A3

Date/Time: July 21 15:30-15:45

Magma-related hydrothermal system in volcanic terrain: An example of the alteration inUnzen USDP-4 scientific drilling
Satoshi Hamasaki1 , Masaru Kurokawa2 , Hideo Hoshizumi1 , Setsuya Nakada21

Geological Survey of Japan, AIST, Japan, 2 Earthquake Research Institute, University of Tokyo, Japan
E-mail: hamasaki-st@aist.go.jp

Magmatic heat sources form hydrothermal systems in volcanic areas. Magmatic fluids transport various elements
and produce secondary minerals as a result of reactions with surrounding rocks. For describing the hydrothermalsystem inside volcanic terrain, this study examines the characteristics of the hydrothermal alteration undergone
by sample drilled cores and cuttings. The samples were taken from a site located about 840 m above sea levelon the northern side of Mt. Heisei-shinzan. The conduit drilling USDP-4, International Cooperative Research with
Scientific Drilling, began here in 2003. In July 2004, the drilling head reached the conduit zone corresponding to the
1990-1995 eruption (Nakada et al., 2005). The total length of the USDP-4 drilling hole is 1995.75 m. Cuttings weresampled every 2 m along the drilled hole, and sixteen cores named C1 to C16 were sampled at depths exceeding
1582 m. The drilled cores consist of hornblende-bearing andesite to dacite lavas or pyroclastic rocks.
According to the cutting samples, fresh volcanic rocks can be found at depths less than 200 m. Smectite and pyrite
occur at depths of more than 1100 m and 1400 m, respectively. Smectite disappears at a depth of 1600 m in both
cuttings and core samples. Chlorite appears at depths more than 1700 m in the core samples. The occurrence of
these alteration minerals shows generally good correlation with those expected at these depth. On the other hand,some minerals such as kaolinite occur along fractures or faults. Therefore, it is considered that a relatively hightemperature solution rose along the fractures or faults, which provided good pathways for hydrothermal fluids. Inaddition, hydrothermal breccia-bearing veins up to 1 m wide can be observed, indicating that some hydrothermalsolution rose explosively. Tuffisite veins are chlorite-altered with green color, which suggests that these veins madegood pathways for the migration of volcano-related fluids. Calcites occur as veinlets at depths of less than 1900 m inthe drilled cores. The calcite veins often cut through the tuffisite veins, suggesting that the calcite was precipitatedat the latest hydrothermal period.
Inside the Unzen volcanic terrain, the hydrothermal alteration is very similar to those of epithermal vein-type gold
deposits or geothermal fields. According to the alteration minerals, it can be estimated that the smectite zone
shallower than 1600 m was formed at 100 to 150°C, while the chlorite zone deeper than 1700 m was formed attemperatures higher than 190°C. These mineral assemblages correspond to II to IV zones (Izawa et al., 1990) in
the hydrothermal systems such as Hishikari, Kushikino gold deposits, and Ogiri geothermal field in Kyushu.

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