Recycling of Au during Serpentinization of Ultramafic Rocks: A Case Study from Neoproterozoic Forearc Ophiolites, Egypt

IF 2.2 4区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
Minerals Pub Date : 2024-09-06 DOI:10.3390/min14090916
Basem Zoheir, Astrid Holzheid, Aliaa Diab, Azza Ragab, Fatma Deshesh, Amr Abdelnasser
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引用次数: 0

Abstract

Gold, along with other highly siderophile elements, is hosted by Fe-Ni sulfide phases within peridotites and mantle melts. In this context, the lithospheric mantle emerges as a principal reservoir, providing materials crucial for the inception, augmentation, conveyance, and genesis of auriferous CO2-rich mantle fluids. EPMA and laser ablation ICP-MS data, integrated with petrographic and SEM studies, were used to assess the transfer of base and precious metals into the Earth’s crust, discerning between inputs from subduction-related processes and post-formation metasomatism. The study focuses on sulfide minerals in serpentinized peridotites of the Abu Dahr ophiolite in the Eastern Desert of Egypt. Originating in a supra-subduction setting during the Neoproterozoic era, the Abu Dahr peridotites underwent serpentinization and contain discrete sulfide minerals, including pentlandite, nickeloan pyrrhotite, millerite, chalcopyrite, and violarite. The uneven distribution of calcite ± magnesite ± serpentine veins throughout the host ophiolitic rocks reflects the intricate interplay of serpentinization and carbonation, as fO2 and fCO2 conditions fluctuated. Geochemical data of the host rocks reveal a progressive geochemical evolution marked by concurrent silicification and carbonate alteration, driven by the interaction of ultramafic rocks with hydrothermal fluids, ultimately leading to the extensive silicification and formation of birbirite. The ICP-MS data show that pentlandite contains up to 6.11 ppm of Au, pyrrhotite up to 0.41 ppm, millerite 0.34 ppm, and violarite 0.12 ppm. The gold concentration in pentlandite is significantly higher than in pyrrhotite, millerite, and violarite, which exhibit lower but detectable levels of Au. Desulfurization reactions of sulfide minerals during progressive serpentinization triggered the release and redistribution of Au as well as base metals and highly siderophile elements. Published thermodynamic modeling at temperatures below 300 °C and pressures of 50 MPa closely replicates the mineral assemblage observed in the Abu Dahr ophiolites, including sulfide assemblages and variations in major elements such as Mg and Fe. This suggests that the serpentinization process, along with associated hydrothermal fluids, played a crucial role in the mobilization and redistribution of gold, particularly affecting its incorporation into secondary sulfides. The mobilization of Au and other highly siderophile elements during serpentinization occurred in an environment marked by strong oxidation, as indicated by the presence of acicular antigorite, magnetite, millerite, and goethite intergrowths.
超基性岩蛇绿岩化过程中的金再循环:来自埃及新新生代前弧蛇绿岩的案例研究
金与其他高亲硒元素一起,被橄榄岩和地幔熔体中的铁-镍硫化物相所蕴藏。在这种情况下,岩石圈地幔成为主要的储层,为富含金的二氧化碳地幔流体的形成、增加、输送和成因提供了至关重要的材料。EPMA 和激光烧蚀 ICP-MS 数据与岩相学和扫描电子显微镜研究相结合,用于评估贱金属和贵金属向地壳的转移,区分来自俯冲相关过程的输入和形成后的变质作用。研究重点是埃及东部沙漠阿布达赫蛇绿岩中的硫化物矿物。Abu Dahr橄榄岩起源于新近新生代的超俯冲环境,经历了蛇纹石化过程,含有离散的硫化物矿物,包括五辉石、镍钴黄铁矿、千枚岩、黄铜矿和紫铁矿。方解石±菱镁矿±蛇纹石矿脉在整个主蛇绿岩中的不均匀分布,反映了蛇纹石化和碳化之间错综复杂的相互作用,以及 fO2 和 fCO2 条件的波动。主岩的地球化学数据显示,在超基性岩与热液相互作用的驱动下,地球化学演化以同时发生硅化和碳酸盐蚀变为特征,最终导致大面积硅化和桦皮岩的形成。ICP-MS 数据显示,五辉石中的金含量高达百万分之 6.11,黄铁矿高达百万分之 0.41,千枚岩为百万分之 0.34,长闪石为百万分之 0.12。五辉石中的金浓度明显高于黄铁矿、千枚岩和辉长岩,后者的金含量较低,但可检测到。在蛇纹石化过程中,硫化矿物的脱硫反应引发了金以及贱金属和高亲硒元素的释放和重新分布。已发表的温度低于 300 ℃、压力为 50 兆帕的热力学模型与在阿布达赫蛇绿岩中观察到的矿物组合,包括硫化物组合以及镁和铁等主要元素的变化密切相关。这表明蛇绿岩化过程以及相关的热液在金的移动和重新分布方面发挥了关键作用,特别是影响了金与次生硫化物的结合。蛇纹石化过程中金和其他高亲硒元素的移动是在强氧化环境中发生的,针状闪长岩、磁铁矿、千枚岩和网纹石互生体的存在表明了这一点。
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来源期刊
Minerals
Minerals MINERALOGY-MINING & MINERAL PROCESSING
CiteScore
4.10
自引率
20.00%
发文量
1351
审稿时长
19.04 days
期刊介绍: Minerals (ISSN 2075-163X) is an international open access journal that covers the broad field of mineralogy, economic mineral resources, mineral exploration, innovative mining techniques and advances in mineral processing. It publishes reviews, regular research papers and short notes. Our aim is to encourage scientists to publish their experimental and theoretical results in as much detail as possible. There is no restriction on the length of the papers. The full experimental details must be provided so that the results can be reproduced.
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