|
|
|
|
| index | Vol. 51 | 1 | 2/3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | Japanese Index | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Hydrothermal alteration of the Sunagohara Formation in the Okuaizu geothermal area, Japan
Yoji SEKI(Mineral and Fuel Resources Department, GSJ)
2000
vol. 51 (7) P.329-367
24 figs, 7 tables, 1 appendix-tables
Keywords: Okuaizu geothermal system, Sunagohara Formation, Hydrothermal alteration, Geochemistry
Abstract : Hydrothermal alteration of the Quaternary Sunagohara Formation was studied using bulk XRD techniques, bulk chemical analysis using XRF, NAA and ICP, and thin sections, and SEM observation. The Sunagohara Formation, which is composed of rhyolite lava domes and tuffaceous lacustrine sediments, is present at shallow depths in the Okuaizu geothermal system. The maximum depth of the lake sediments' base is about 350 m and maximum measured temperature is about 120 oC in the Sunagohara Formation. Six alteration zones can be recognized based on the distribution of clays and zeolites: the least altered (LA) zone is characterized by fresh volcanic glass and found at the surface outside the Nishiyama spa and geothermal area. The smectite-zeolite (SZ) zone is found at the surface around the Nishiyama spa and shallow depths in the geothermal area. The SZ zone is characterized by smectite, clinoptilolite and mordenite. The mixed-layered clay mineral (ML) zone underlies the SZ zone and is also found at the surface of the Nishiyama spa. The ML zone is characterized by illite-smectite mixed-layered minerals and K-feldspar. The hydrothermal alteration is horizontally zoned around the center of the geothermal system, with the ML zone in the center, followed outward by the SZ and LA zones. The illite-chlorite (IC) zone is only found in drill holes of the geothermal area. The distributions of the SZ, ML and IC zones at depth can be correlated with increasing temperature. The kaolinite (K) zone is present at the surface around the northwestern end of Oisawa and several drill holes, and is characterized by abundant kaolinite. The K zone is thought to have been formed by interactions with near surface CO2-rich acid water. The alunite zone (A) occurs in a limited surface area near the northwestern end of Sarukurazawa and is characterized by abundant alunite. The A zone is thought to have been formed by near surface sulfate-rich acid water. The weight of the key components per unit volume for each alteration zone compared to the LA zone shows gains and losses for the following components. Gain of SiO2, CaO and S, and loss of MnO, MgO for the SZ zone, gain of SiO2, Fe2O3, K2O, S, Au, As, Hg and Sb, and loss of MnO, MgO, CaO and Na2O for the ML zone, gain of S, As and Sb, and loss of MnO, MgO, CaO, Na2O and K2O for the K zone. No significant changes of Al2O3 and TiO2 is recognized for all alteration zones. The behavior of these components can be correlated according to the following observation on minerals in this area: a) addition of hydrothermal quartz and argillization of mafic minerals in the SZ and ML zone, b) decomposition of plagioclase in the ML zone, c) replacement of volcanic glass to zeolite and K-feldspar in the SZ and ML zones, respectively, d) precipitation of pyrite and arsenic sulfide in the ML zone, and e) replacement of volcanic glass and plagioclase to kaolinite and alunite in the K and A zones, respectively.
K-Ar ages of the advanced argillic alteration in the Ugusu area, western Izu Peninsula, Japan
Satoshi HAMASAKI(Mineral and Fuel Resources Department, GSJ)
2000
vol. 51 (7) P. 369-372
1 fig, 1 table
Keywords: K-Ar age, Ugusu, advanced argillic altration, Fukata, Tanaba volcanic activity
Abstract : For the purpose of considering the age of advanced argillic alteration in the Ugusu area, western Izu Peninsula, K-Ar ages of separated sericite and alunite were measured. As a result, the ages of 1.57}0.28 Ma and 1.42}0.02 Ma from the Hakko deposit (sericite) and Saizu deposit (alunite), respectively were obtained. The ages of advanced argillic alteration in Ugusu is close to the younger ages of 1.20}0.25 and 1.21}0.13 Ma (MITI, 1987) from Fukata, 1.5 km far west of Ugusu. It is considered that the age of 1.5-1.2 Ma for acid hydrothermal activity in Ugusu and Fukata imply a genetical relationship with the Tanaba volcanic activity around 1.19-1.52 Ma.
Abstracts for Geological Survey Seminar, no. 268
Present state of the researches over MTL active faults system in the metropolitan area and the future prospect of the estimation of strong groundmotion
2000
vol. 51 (8), P. 369-372
