Mineral chemistry and sulfur isotope insights into magmatic and hydrothermal processes leading to porphyry Cu mineralization at Grasberg, Indonesia

• Material Type: 博士論文
• Title: Mineral chemistry and sulfur isotope insights into magmatic and hydrothermal processes leading to porphyry Cu mineralization at Grasberg, Indonesia
• Author/Editor: ADI, SULAKSONO
• Author Heading: ADI, SULAKSONO
• Publication, Distribution, etc.: 秋田大学
• Publication Date: 2021-03-22
• Publication Date (W3CDTF): 2021-03-22
• Alternative Title: 鉱物化学および硫黄同位体からみたインドネシア、グラスベルグ斑岩銅鉱床のマグマ―熱水過程
• Degree grantor/type: 秋田大学
• Date Granted: 2021-03-22
• Date Granted (W3CDTF): 2021-03-22
• Dissertation Number: 甲第1365号
• Degree Type: 博士（理学）
• Conferring No. (Dissertation): 甲第1365号
• Text Language Code: eng
• Target Audience: 一般
• Note (General): Preparation for sufficient amounts of Cl, S and metals during magma evolution is of paramount importance for the generation of porphyry Cu deposits. Reduction of oxidized S is essential for precipitation of sulfides in such deposits, in which mineralization predominantly comprises Cu sulfides, whereas their source magmas are oxidized with most of their S as SO42- and with exsolved fluids having SO2>>H2S. I explore how magma evolution and oxidation state affected the degassing of Cl- and S-bearing fluids from the source magma at Grasberg, based on igneous mineral paragenesis and amphibole, zircon and apatite chemistries. The process of S reduction required for sulfide mineralization during hydrothermal processes is also evaluated in this study on the basis of S isotope and biotite chemical composition. The low-Al amphiboles formed at approximately 730–700 °C, 1.1–0.7 kbar and FMQ+3.4 are contained in the syn-mineralization Main Grasberg Intrusion (MGI). The MGI magmatism was followed by the emplacement of the syn-mineralization Early South Kali Dike (ESKD) that contains amphibole with a disequilibrium texture of chemically distinct core and rim; the high-Al cores crystallized at approximately 900–850 °C, 4.3–3.3 kbar and FMQ+1, whereas the low-Al rims and associated low-Al amphibole phenocrysts formed at approximately 790–720 °C, 1.9–1.0 kbar and FMQ+2.7. The high-Al amphibole is anhydrite-, titanite-magnetite-quartz- and zircon-free, whereas the low-Al amphibole hosts inclusions of, and/or is associated with such minerals that indicate a high magma oxidation state. The low-Al amphiboles of both MGI and ESKD are characteristically low in REE+Y (particularly MREE) and V, and having large negative Eu, Ba, and Sr anomalies compared with the ESKD high-Al amphiboles. This confirms that the low-Al amphibole crystallized from a melt in equilibrium with plagioclase, K-feldspar, biotite, titanite (-magnetite-quartz), zircon, apatite and anhydrite, at lower pressure and temperature and higher fO2 than the conditions for the high-Al amphibole. Zircons from the MGI yield the elevated mean Ce4+/Ce3+ value of 519, whereas the ESKD zircons record a lower value of 248. Zircon Ce/Ce*, Ce/Nd and Eu/Eu* values also indicate that the MGI magma was more oxidized than the ESKD. The changes in melt Cl and S concentrations calculated from apatite chemistry indicate a degassing sequence which can be divided into: 1) degassing of Cl from the MGI magma after low-Al amphibole and plagioclase crystallizations, 2) degassing of S from the ESKD magma at ~800 °C and ~2 kbar, prior to the crystallizations of low-Al amphibole and plagioclase, 3) degassing of Cl from the ESKD magma after low-Al amphibole and plagioclase crystallizations. Initial hydrothermal events formed sulfide-free, anhydrite-rich K-feldspar and biotite alteration, followed by successive vein stages of 1) magnetite, 2) biotite, 3) quartz, 4) anhydrite-chalcopyrite, 5) chalcopyrite ± sericite selvages, and 6) pyrite-chalcopyrite- quartz + sericite selvages. Hydrothermal biotite within the potassic alteration zone is characterized by higher Mg# and lower Fe contents than igneous biotite. The ranges of δ34S values of S-bearing minerals hosted in the anhydrite- chalcopyrite veins are 1.2–4.2‰ for chalcopyrite (n=24; avg. 2.3‰), 1.1 and 2.1‰ for bornite (n=2) and 10.5–13.8‰ for anhydrite (n=26; avg. 12.2‰). Two chalcopyrite samples that occur along the centerlines of comb-textured quartz veins have δ34S values of 2.4 and 2.5‰ (n=2). One chalcopyrite vein that is accompanied by sericite alteration has δ34S = 2.1‰. The δ34S values for pyrite in a veinlet with sericite selvage are 2.8 to 4.8‰ (n=3; avg. 3.7‰), and those of anhydrite are 14.6–15.2‰ (n=2). A sample of the syn-mineralization ESKD MD yields a δ34S of 9.4‰. The combined igneous mineral paragenesis and geochemistry indicate that a less oxidized magma batch containing high-Al amphibole injected into a strongly oxidized MGI-related upper-crustal magma chamber, forming a hybrid magma from which the ESKD porphyry was derived. The degassing of Cl both from MGI and ESKD magmas is likely related to the crystallization of phenocryst in the upper crust, whereas the partitioning of S from the ESKD magma into fluids may require another process that occurs at higher pressure and temperature, prior to the crystallization of phenocrysts in the upper crustal magma chamber. I suggest that mixing of magmas of two distinct oxidation states generated SO2 by the reduction of sulfate from the oxidized magma via oxidation of Fe2+ derived from the other reduced or less oxidized magma: CaSO4 + 2FeO = CaO + SO2 + 2FeO1.5 Magmatic degassing that occurred intermittently at Grasberg implies that localized accumulation of ore-forming fluids before a sudden discharge is required for an efficient hydrothermal system. Ore-forming fluids may have been accumulated and stored in a stable fluid pocket beneath the cupola zone before a sudden discharge. Such a short-lived fluid discharge is in agreement with the fact that most porphyry deposits occur in a short period of time (<100 k.y.) compared to their associated magmatic systems. Just after the discharge, S is not ready for economic mineralization due to its unsuitable valences (SO2>>H2S). The δ34S values of sulfide-sulfate mineral pairs indicate SO2-derived SO42- and H2S in SO42-/H2S molar proportions of ~4:1 to ~3:1 at >550 °C. The hydrothermal fluid then likely followed a rock-buffered trajectory and became more reduced at <550 °C. Hydrothermal biotite that replaces igneous amphibole and biotite has a phlogopitic composition, suggesting that Fe2+ was liberated from igneous mafic minerals and oxidized by reaction with SO42- to form magnetite, resulting in sulfide formation by the simplified reaction: 12FeO + SO42- + 2H+ → 4Fe3O4 + H2S
• DOI: 10.20569/00005713; https://doi.org/10.20569/00005713
• Persistent ID (NDL): info:ndljp/pid/11679177; https://dl.ndl.go.jp/pid/11679177
• Collection: 障害者向け資料
• Collection (Materials For Handicapped People:1): テキストデータ
• Collection (particular): 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
• Acquisition Basis: 博士論文（自動収集）
• Date Accepted (W3CDTF): 2021-06-07T02:06:26+09:00
• Date Created (W3CDTF): 2021-06-03
• Format (IMT): PDF
• Access Restrictions: 国立国会図書館内限定公開
• Service for the Digitized Contents Transmission Service: 図書館・個人送信対象外
• Availability of remote photoduplication service: 可
• Periodical Title (URI): http://dx.doi.org/10.20569/00005713; http://hdl.handle.net/10295/00005713
• Data Provider (Database): 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