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 Relation between Volcanic-Hosted Precious-and Base-Metal Deposits and Geothermal Systems
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本邦第三紀中新世より鮮新世にわたる"緑色凝灰岩層"("Green Tuff Formation")中には,プロピライト(propylite)(緑泥石化,炭酸塩化を受けた安山岩)が発達する.プロピライト中の斜長石は曹長石化作用を受けていない.プロピライト中には浅熱水成含金・銀石英脈が存在し,HAYBAほかによるadularia-sericite type(氷長石―絹雲母型鉱床)に属する.これらの鉱脈の母岩は氷長石化作用または絹雲母化作用を蒙る.同様に浅熱水成銅・鉛・亜鉛鉱脈も氷長石―絹雲母型に属する,一方,カオリン化作用または明ばん石化作用を受けたプロピライト中の珪化岩が金・硫ひ鋼鉱ルソン銅鉱を含有する事がある.この型の鉱床に対しHAYBAほかはacid-sulfate type(酸性硫酸塩型)と名づけた.<BR>この二つの型の鉱床が一つの地域に共存する場合があり,筆者らは原鉱化流動体が沸騰・分離した結果両者を生成したと解釈する.酸性硫酸塩型は沸騰・分離した気相を起源とした酸性熱水に起因する.氷長石絹雲母型は沸騰・分離の際,残留した鉱化流動体または原鉱化流動体よりの熱水溶液より生成し,溶液はほぼ中性(弱酸性ないし弱アルカリ性)を呈する.<BR>本邦第四紀火山岩中に地熱地帯が存在し,蒸気卓越系(vapor-dominated hydrothermal system)と熱水系(hot-water system)とがある,仙岩地域には両者が共存する.前者は原流動体より沸騰・分離した蒸気であり,後者は原流動体または前者の分離後残留した流動体よりの熱水と解する.<BR>以上,"緑色凝灰岩層中の金・銀ならびに銅・鉛・亜鉛の氷長石絹雲母型鉱脈は地熱流動体の熱水系に相当し,硫ひ銅鉱・ルソン銅鉱などを含む酸性硫酸塩型金鉱床は蒸気卓越型に相当する.<BR>なお,酸性硫酸型金鉱床である赤石鉱床の最晩期の金(自然金)の鉱化作用には褐鉄鉱,スコロド石,重晶石,barium-phazmacosiderite,tripuhyiteを伴う.これは深成二次(hypogene secondary)(hypogene oxidation)の生成と考える.
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 CO<SUB>2</SUB> and H2S gases. In Sengan (Matsukawa) district, however, vapor-dominated fluid is found, associated with hot-water system. The fluid is also composed of H<SUB>2</SUB>O vapor with small amounts of CO<SUB>2</SUB> 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.
- Mining Geology
Mining Geology 46(2), 73-94, 1996-04-30
The Society of Resource Geology