後期中新世東北日本弧における鉱脈型銅鉱化作用に関連したマグマプロセス

• 資料種別: 博士論文
• タイトル: 後期中新世東北日本弧における鉱脈型銅鉱化作用に関連したマグマプロセス
• 著者・編者: 左部, 翔太
• 著者標目: 左部, 翔太
• 出版事項: 秋田大学
• 出版年月日等: 2022-03-22
• 出版年（W3CDTF）: 2022-03-22
• 並列タイトル等: Late Miocene magma processes associated with vein-type copper mineralization in the Northeast Japan anc
• 授与機関名: 秋田大学
• 授与年月日: 2022-03-22
• 授与年月日（W3CDTF）: 2022-03-22
• 報告番号: 甲第1407号
• 学位: 博士（理学）
• 博論授与番号: 甲第1407号
• 本文の言語コード: jpn
• 件名標目: Northeast Japan arc; Arakawa deposit; magmatic enclaves; redox state; ore-related magma; copper released from magma
• 対象利用者: 一般
• 一般注記: A large number of Neogene vein-type copper deposits are distributed in the Northeast Japan arc, which is a volcanic arc associated with subduction of the oceanic plate but lacks porphyry-type copper deposits. Those vein-type copper deposits consist of the largest copper district in Japan arc and associated with neighboring Miocene and Pliocene felsic intrusive rocks. Most of these intrusive rocks show granitic-dioritic compositions and fine-grained holocrystalline texture, with dark green to gray colored enclaves.The purpose of this study is to clarify the properties and generation process of magmas as a source of copper-bearing magmatic fluid, and to quantitatively clarify the amounts of copper and sulfur released from magma. Felsic intrusive rocks and included enclaves in the Arakawa area, which have been clarified to be temporally and spatially related to the copper mineralization in the Arakawa.The three felsic intrusive rocks in the Arakawa area consist of plagiorhyolite and amphibole micro-granite stocks. The enclaves contained in felsic intrusive rocks are basaltic to dacitic compositions and are mainly classified into three types: clinopyroxene-bearing hornblende gabbro, hornblende andesite and hornblende dacite. Some of these enclaves have a chilled margin or a chemical reaction halo formed by the reaction of two liquid magmas, and show characteristics of MME (mafic magmatic enclave/mafic microgranular enclave). Basaltic enclaves contain relatively large amounts of sulfide minerals. Bornite, chalcopyrite, and pyrite are included in late-crystallized amphibole in the enclaves and are often accompanied with magnetite.The whole rock chemical composition of the felsic intrusive rocks are classified to sub-alkaline rhyolite. Those felsic rocks were classified to magnetite-series granite and show a low Sr/Y ratio, which shows characteristic of rhyolitic magma accompanied with typical island arc magmatism derived from the wedge mantle. The whole-rock chemical compositions of basaltic enclaves show characteristics of sub-alkaline basalt and basaltic andesite accompanied with typical island arcs magmatism derived from wedge mantles.In addition, the anorthite (An) content of plagioclase shows the maximum of 89 %, indicating that it was formed from water-rich basaltic magma. All plagioclase in basaltic MME is lath-shape with An 36-89 % and Cu 2.8-35.7 ppm. The copper content of the whole rock and melt calculated using the partition coefficient of plagioclase and silicate melts are 65-106 ppm and 144.9 ± 47.6 ppm, respectively, and the minimum difference between the two is 0 ppm. Micro-granite mainly contain tabular-shaped plagioclase with An 11-38 % and Cu 0.5-5.1 ppm. The copper content of the whole rock and melt calculated using the partition coefficient are 1 ppm and 21.3 ± 2.8 ppm, respectively, and the minimum difference between the two is 17.5 ppm. Andesitic enclave mainly contains lath-shaped plagioclase (An 12-55 %, Cu 0.8-4.4 ppm) similar to the basaltic enclave. The copper content of the whole rock and melt calculated using the partition coefficient are 3.4 ppm and 18.8 ± 6.4 ppm, respectively, and the minimum difference between the two is 9.0 ppm. On the other hand, lath-shaped plagioclase (An 13-57 %, Cu 8.4-17.9 ppm) and tabular-shaped plagioclase (An 4-41 %, Cu 0.0-3.8 ppm) are coexisting in the dacitic enclave. In particular, the An and Cu contents of those lath-shaped plagioclase and plagioclase from the basaltic enclave show a continuous and linear compositional trend in which Cu content increases as An value decreases. It was suggested that dacitic magma was formed by mixing of andesitic magma differentiated from magma forming basaltic enclaves and rhyolitic magma according to the crystal habits and elemental compositions of plagioclase and mass balance calculation of whole rock chemical compositions. The copper content of the whole rock and melt calculated using the partition coefficient of plagioclase and silicate melts are 2 ppm and 59.0 ± 6.6 ppm, respectively, and the minimum difference between the two is 50.4 ppm. This value indicates that about 96 % of the copper contained in the magma was released with the fluid during crystallization process.The redox states of rhyolitic, dacitic and basaltic magmas were calculated by an amphibole geothermometer and oxybarometer. The redox states of rhyolitic and dacitic magma calculated to ΔNNO + 1-2, and basaltic magma was calculated to around NNO buffer. The former value shows a higher oxidation state than SSO (sulfide-sulfur oxide buffer), and the latter value shows a lower oxidation state than SSO. Those results suggested that copper was not precipitated in the magma as sulfides but separated and released together with the fluid in the crystallization process of oxidized dacitic magma, which has relatively high release amount and ratio of copper.The apatite contained in the micro-granite and dacitic enclave contains <0.77 and <0.92 wt.% of SO3, respectively. The sulfur content of the whole rock and rhyolitic melt calculated using the partition coefficient of apatite and melt are 22 ppm and <60 ppm, respectively, and the amount of sulfur released from the rhyolitic magma is around 40 ppm. The sulfur content of the whole rock and dacitic melt calculated using the partition coefficient of apatite and melt are 26 ppm and <120 ppm, respectively, and the amount of sulfur released from the dacitic magma is around 100 ppm.From the above results, the minimum volume of magma required to produce the Arakawa copper deposit was estimated when the copper and sulfur released amount from the dacitic magma are 50 ppm and 100 ppm, respectively. The minimum volume of magma required to produce the Arakawa deposit was estimated to be about 1.6 km3 from the released amount of copper and about 8.8 km3 from the separated amount of sulfur when the total amount of concentrated in the Arakawa deposit are set to 200,000 tons copper and 2,200,000 tons sulfur.According to these results, it is considered that the magma system that formed the Arakawa deposit was consists of the oxidized rhyolitic magma and the copper, sulfur, and water-rich basaltic magma associated with the island arc magmatism.In addition, it is suggested that the oxidized dacitic magma formed by mixing oxidized rhyolitic magma and andesitic magma (differentiation from the basaltic magma) efficiently releases copper and sulfur during crystallization and caused copper mineralization.
• DOI: 10.20569/00006023; https://doi.org/10.20569/00006023
• 国立国会図書館永続的識別子: info:ndljp/pid/12303951; https://dl.ndl.go.jp/pid/12303951
• コレクション（共通）: 障害者向け資料
• コレクション（障害者向け資料：レベル1）: テキストデータ
• コレクション（個別）: 国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文; https://dl.ndl.go.jp/collections/A00014
• 収集根拠: 博士論文（自動収集）
• 受理日（W3CDTF）: 2022-07-05T02:30:21+09:00
• 作成日（W3CDTF）: 2022-06-16
• 記録形式（IMT）: PDF
• オンライン閲覧公開範囲: 国立国会図書館内限定公開
• デジタル化資料送信: 図書館・個人送信対象外
• 遠隔複写可否（NDL）: 可
• 掲載誌（URI）: http://dx.doi.org/10.20569/00006023; http://hdl.handle.net/10295/00006023
• 連携機関・データベース: 国立国会図書館 : 国立国会図書館デジタルコレクション; https://dl.ndl.go.jp

目次

内容要旨及び審査結果要旨

本文