Bulletin of the Geological Survey of Japan Vol.52 No.11/12 (2001)
Surface and underground features in the Okuaizu geothermal system, Fukushima, Japan. Arsenic sulfide containing sub-ppm Au (orange in colour) precipitated in lacustrine sediments near present hot spring discharge (upper left), CO2 and H2S gas bubbles from river water along one of fracture zones (upper right), molten sulfur (amber in colour) recovered from geothermal fluid (lower left) and Anhydrite (Anh), sphalerite (Sp), pyrite (Py) and quartz (Qz) precipitated from geothermal fluid on surface of reservoir fracture (lower right).
(Photo and text by Yoji SEKI, Research Center for Deep Geological Environments, GSJ)
|Hydrothermal alteration of the Miocene Takizawagawa Formation in the Okuaizu geothermal area, Japan-Alteration mineralogy and geochemistry of the reservoir bedrock-.||Yoji SEKI （493-572）||52_11_01.pdf [5,512 KB]|
|Paleomagnetism and fission-track ages of the Mt. Wasso moonstone Rhyolitic welded tuff in the Ishikawa Prefecture, central Japan.||Yasuto ITOH, Shoji DOSHIDA, Kazuya KITADA and Tohru DANHARA （573-580）||52_11_02.pdf [806 KB]|
|Pyrophyllite mineralization of the Tanmai deposit, Quang Ninh Province, northern Vietnam -A result of reconnaissance survey-||Hideo HIRANO, Masahiro AOKI, Sadahisa SUDO and NGUYEN Van Quy (581-598)||52_11_03.pdf [2,356 KB]|
Hydrothermal alteration of the Miocene Takizawagawa Formation in the Okuaizu geothermal area, Japan-Alteration mineralogy and geochemistry of the reservoir bedrock-.
Alteration mineralogy and whole rock geochemistry of the Miocene Takizawagawa Formation distributed around the Okuaizu geothermal System were studied using bulk XRD techniques, bulk chemical analysis using XRF, NAA and ICP, and thin sections observation. This study aims to reveal the hydrothermal alteration zoning and to estimate gains and losses of major and minor elements in bedrocks around the reservoir depth of the Okuaizu geothermal system during the modern geothermal activity.
The Takizawagawa Formation, which is composed of Miocene rhyolitic to dacitic lavas and pyroclastic sediments, generally has undergone regional diagenetic alteration and overprinting local hydrothermal alteration both formed in Tertiary. In the Okuaizu geothermal system, the modern hydrothermal alteration overprints the Tertiary diagenetic and hydrothermal alteration.
Regional diagenetic and overprinting local hydrothermal alteration is mostly characterized by illite and chlorite (illite-chlorite (IC) zone).
In the reservoir depth of the modern geothermal system, following four alteration zones are recognized based on the distribution of clays, carbonates and sulfates.
1) Illite-chlorite+(IC+) zone
2) Illite (I) zone
3) Kaolinite (K) zone
4) Mixed-layered clay mineral (ML) zone
1) Illite-chlorite+(IC+) zone is widely distributed in the deeper zone (1000-2000m depth) with high temperature (200-350ºC), and is characterized by illite, chlorite, carbonate minerals (mostly dolomite and siderite with lesser amount of magnesite and rhodochrosite) and anhydrite.
2) Illite (I) zone is sporadically distributed around upper portion (300-1000m) of the reservoir fracture and is characterized by illite, carbonate minerals (dolomite and siderite with sporadic rhodochrosite).
3) Kaolinite (K) zone is distributed around shallower extension (shallower than 1000m) of the reservoir fracture. It forms irregular shape, and is characterized by kaolinite and abundant carbonate minerals (mainly dolomite and siderite).
4) Mixed-layered clay mineral (ML) zone is limitedly distributed adjacent to the kaolinite zone, and is characterized by illite-smectite mixed layered mineral.
On the assumption that Al2O3 is immobile during the modern geothermal system were compared. Gains and losses of chemical components in each alteration zone were calculated, with the result that gains of Ca, S, As, Cu and loss of Cs for the all alteration zones inside the system, in addition to gains of Mn, Au, Ag and loss of Na for the illite zone were recognized.
Judging from the result obtained in this study on hydrothermal Alteration, combined with the published geological, hydrological and geochemical data, the following genetical model is proposed for each alteration zone in the modern geothermal system.
1) Illite-chlorite+(IC+) zone is formed during large-scale infiltration of the deeper high temperature geothermal fluid with higer CO2 content.
2) Illite(I) zone is formed by local infiltration of relatively shallower geothermal fluid with boiling.
3) Kaolinite (K) zone is formed by relatively low pH shallower underground water generated by CO2 injection, which is brought by boiling of the deeper geothermal reservoir.
4) Mixed-layered clay mineral (ML) zone is formed as peripheral phase Of the Kaolinite zone.
Paleomagnetism and fission-track ages of the Mt. Wasso moonstone Rhyolitic welded tuff in the Ishikawa Prefecture, central Japan.
Yasuto ITOH, Shoji DOSHIDA, Kazuya KITADA and Tohru DANHARA
The Mt. Wasso Moonstone Rhyolitic Welded Tuff (hereafter referred to as Mt. Wasso Rhyolite) is assigned to the lowermost part of the Miocene Hokuriku Group in the eastern part of southwest Japan. In an effort to clarify the rotational movements of southwest Japan associated with opening of the Japan. Sea back-arc basin, we conducted paleomagnetic measurements and fission-track (FT) dating of the Mt. Wasso Rhyolite. Stable site-mean paleomagnetic directions and zircon FT ages were determined for five and three sites, respectively. The FT ages range between 20 Ma and 22 Ma, and accord with a previous age determined by Rb-Sr whole rock isochron method. The numerical ages around 20 Ma correspond to timing of a hydrothermal alteration event, which may have caused remagnetization of the Mt. Wasso Rhyolite. Tilt-corrected paleomagnetic directions are clustered with a large easterly deflection (ca.50º), indicating a clockwise rotation of the study area since 20 Ma. Compared with paleomagnetic directions of the Iozen Formation (15～16Ma) in adjoining hills, the present result implies that the study area did not significantly rotate during the easy Miocene.
Pyrophyllite mineralization of the Tanmai deposit, Quang Ninh Province, northern Vietnam -A result of reconnaissance survey-
Hideo HIRANO, Masahiro AOKI, Sadahisa SUDO and NGUYEN Van Quy
The Tanmai pyrophyllite deposit, Quang Ninh Province, was studied geologically and mineralogically. The deposit consisting of five pyrophyllite bodies, is classified by low iron content. The original rocks are volcanics of rhyodacitic composition such as tuff, volcanic breccia, lava and sheeted intrusive rocks. Siliceous zone originated from felsic intrusive rocks and massive lava.
The sequence of the alteration is estimated as follows: kaolin zone formed earlier under lower temperature, and pyrophyllite-and siliceous-zones followed at higher temperature. The contrast in the permeability of the original volcanic resulted in the difference of mineral composition; a pyrophyllite zone from high permeability and a siliceous zone for low permeability units. The diaspore zone has formed as fissure-filling veins selectively in the siliceous zone. This sequence of the formation probably progresses with temperature. Vien-filling diaspore + pyrophyllite in the siliceous zone has precipitated in a hydrothermal solution under decreasing PH2O caused by vertical fissure systems. The brittle nature of the siliceous zone has developed a vein-filling diaspore. The pyrophyllite zone being the majority of the Tanmai deposit has formed at a temperature of 260-290ºC, estimated by experimental date. Finally, alunite precipitated in the declining stage of hydrothermal activity.
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