JOURNAL OF MINERALOGY, PETROLOGY AND ECONOMIC GEOLOGY
Online ISSN : 1881-3275
Print ISSN : 0914-9783
ISSN-L : 0914-9783
Volume 91, Issue 4
April
Displaying 1-3 of 3 articles from this issue
ORIGINAL ARTICLES
  • Yoshihiro HIRAOKA
    1996 Volume 91 Issue 4 Pages 123-132
    Published: 1996
    Released on J-STAGE: November 25, 2006
    JOURNAL FREE ACCESS
    The granitic rocks in the Hiei, Hira and Kurama areas of central Japan are characterized by their chemical compositions, especially the quantities of the alkali and the alkaline earth elements. The mineral analysis and the modal analysis were carried out to address how and which mineral species controled the difference in the chemical compositions of the whole granitic rocks.
         The modal analysis adopted in this study is same as the Ferry's method which was based on mass balances between chemical compositions of metamorphic minerals and those of whole rocks. However, the Ferry's method was modified to apply to the granitic rocks, that is, the numbers of mineral species and the equations were simplified, and a method of least squares was applied to solve the equations.
          The modal results were obtained secondarily using the results of chemical analysis of rocks and minerals, however this method is useful in checking up whether the analytical results were agreeable or not and in estimating the quantities of the main constituent minerals by weight percent.
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  • Fuxi JIN
    1996 Volume 91 Issue 4 Pages 133-144
    Published: 1996
    Released on J-STAGE: November 25, 2006
    JOURNAL FREE ACCESS
    A Pb-Zn orebody with high Ag concentration is situated in the Cretaceous rhyolites in Dalingkou, Tiantai County, Zhejiang Province, eastern China. The orebody occurs in brecciated and fissured zones. Unlike many other shallow-seated hydrothermal gold-silver ore deposits in North America and Japan, the Ag/Au ratio of the ore is very high and ranges from 97 to 60, 000 (average: 970). Main silver minerals are polybasite, tetrahedrite, argentite, and pyrargyrite. The gangue mineral assemblage is characterized by abundant rhodochrosite and the absence of adularia. Chemistry of fluid inclusion suggests ore-forming conditions, such as salinity 0.2-4.1 wt.%, pH=3.4-5.6 and pressure 98.3-186.8 bars, which are not much different from those of the other Ag-Au deposits. The ore body is an epithermal fracture-filling, Ag-Au-bearing, base metal ore deposits.
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  • Yoichi MURAMATSU, Ryo KOMATSU
    1996 Volume 91 Issue 4 Pages 145-161
    Published: 1996
    Released on J-STAGE: November 25, 2006
    JOURNAL FREE ACCESS
    The Kakkonda geothermal system, Iwate Prefecture, Northeastern Japan, consists of two reservoirs with different temperature and permeability: shallow and deep reservoirs. The shallow reservoir which is located at shallower than about 1, 500 m in depth, was developed for the Kakkonda geothermal power plant unit 1 (50 MWe) operated since 1978. Fluid inclusions in quartz, calcite, anhydrite and gypsum veinlets, and in quartz fragments of dacitic tuff from the shallow reservoir have been studied using microthermometric and crushing stages to clarify the fluid evolution in a natural-state shallow reservoir. Salinities of fluid inclusions were determined after correction of melting point of ice for CO2 content estimated semiquantitatively from the bubble behavior on the crushing stage.
         Four stages with different thermal and chemical properties of evolutional fluids with time was recognized based on the fluid inclusion data, combined with chemical analyses of the production fluids from the shallow reservoir: (1) stage I, polyphase inclusion-forming fluid; 280 to 350°C temperature, 31 to 35 wt.% salinity, (2) stage II, primary liquid inclusion-forming fluid; 220 to 310°C temperature, 0.0 wt.% and 0.4 to 2.0 wt.% CO2 contents, 0.0 to 1.2 wt.% (average 0.5 wt.%) salinity (NaCl equivalent), (3) stage III, secondary liquid inclusion-forming fluid; 220 to 250°C, 0.2 to 0.3 wt.%, 0.0 to 0.7wt.% (average 0.1 wt.%), (4) stage IV, geothermal fluid after the operation of the Kakkonda power plant unit 1; 215 to 261°C, 0.01 to 0.04 wt.%, 0.06 to 0.08 wt.%.
         Magmatic fluid at stage I probably caused by igneous activity related to Kakkonda granite intrusion (slightly younger than 0.7-1.0 Ma; Kanisawa et al., 1994), is assumed to represent the first step in the geothermal evolution of the Kakkonda system. The subsequent geothermal fluids at the shallow reservoir evolved generally to lower values of temperature, CO2 content and salinity with time. The precipitation of hydrothermal minerals such as quartz, anhydrite and K-feldspar (0.2±0.1 Ma; Koshiya et al., 1993) is related to stage II. Considerably decreasing CO2 content from stage III to IV indicate that degassing was caused by a production test at the early stages after the end of drilling.
         Distribution of temperature and polyphase inclusion in and around the field suggests that high temperature fluid ascends at the northwestern area in the field through all stages. Open fracture had gradually selfsealed with the hydrothermal minerals of stage II, but geothermal fluid of stage IV still flows through the mineral-unfilled open fractures.
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