火山岩関連金銀鉱脈ならびに銅鉛亜鉛鉱脈と地熱系との関係

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タイトル別名
  • Relation between Volcanic-Hosted Precious-and Base-Metal Deposits and Geothermal Systems
  • Relation between Volcanic-Hosted Precio

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Volcanic-hosted epithermal deposits which widely occur in "Green Tuff" regions in Japan, are classified into adularia-sericite type and acid-sulfate type based on ore and alteration mineral assemblages. Propylitization is only the precursor to the mineralization, and has no direct relation to ore deposition. The adularia-sericite type is represented by quartz-adularia Au-Ag veins and Cu-Pb-Zn veins in adularized or sericitized country rocks, while the acid-sulfate type is represented by Au deposits in silicified rocks with kaolinite and alunite, and locally with enargite and luzonite. The former suffered nearly neutral to alkaline alteration, while the latter suffered acidic one. The latter type deposits sporadically coexist with the former type in one area. The separated fluid by boiling from the original hydrothermal solution genetically related to adularia-sericite type deposits would be responsible for the mineralization of acid-sulfate type.<BR>Many geothermal systems are found in Quaternary volcanic regions composed mainly of andesitic rocks, nearly coincident with the "Green Tuff" regions. Most of these geothermal areas are characterized by hot-water system composed of NaCl-bearing hot-water with CO2 and H2S gases. In Sengan (Matsukawa) district, however, vapor-dominated fluid is found, associated with hot-water system. The fluid is also composed of H2O vapor with small amounts of CO2 and H2S. The alteration minerals which characterize these two types of geothermal areas are different. The former is associated with sericite, chlorite, albite, zeolite and kaolinite, while the latter area with kaolinite, alunite, pyrophyllite and sericite. The vapor-dominated geothermal fluids are considered to be separated from the hot-water systems through boiling processes.<BR>The geological and geochemical similarity between the hydrothermal deposits in "Green-Tuff" regions and the geothermal areas in Quaternary volcanic regions, allows to speculate that epithermal deposits of the adularia-sericite type were formed from hot-water geothermal fluids, while those of the acid-sulfate type were formed from vapor-dominated geothermal fluids. In acid-sulfate type gold deposits, gold exists (i) as substitutional solid solution in pyrite or aurostibite (AuSb2) molecule in pyrite, (ii) as native gold associated with enargite and luzonite, and (iii) in limonite associated with scorodite, Ba-pharmacosiderite, barite and tripuhyite. The occurrence of (iii) would be due to hypogene secondary (oxidation) processes.

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  • 資源地質

    資源地質 46 (256), 73-94, 1996

    資源地質学会

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