Combined effect of magmatic fluid–seawater mixing and host rock alteration on the formation of gold-enriched massive sulfides in the Forecast vent field, southern Mariana Trough

IF 3.2 2区 地球科学 Q1 GEOLOGY Ore Geology Reviews Pub Date : 2024-09-14 DOI:10.1016/j.oregeorev.2024.106235
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Abstract

Massive sulfides and sulfide-bearing breccia were collected from the Forecast vent field in the southern Mariana Trough. These samples exhibit varying degrees of hydrothermal alteration and/or mineralization, with the former type having significant enrichments in Au (up to 101 ppm). In this study, detailed analyses of mineralogy, geochemistry, S isotope, and fluid inclusion were conducted to understand the unusual Au-rich mineralization. Mineralogical investigations and geochemical analyses (LA–ICP–MS and EPMA) of dominant sulfide minerals indicate that electrums, the main phase of Au mineralization, are characterized by two different generations based on mineralogical relationships and fineness. Early-stage high fineness electrums (930–981 ‰) are associated with sphalerite under relatively high-temperature fluid conditions, whereas galena is main host mineral related to late-stage low fineness electrums (773–868 ‰). Sphalerite geothermometry (231–264 °C), the occurrence of colloform textured sulfides, and microthermometric characteristics of fluid inclusions (Th = 220–304 °C; Salinity = 1.4–6.6 wt% NaCl equivalent; a general trend of decreasing salinity with decreasing Th) indicate that significant decreases in H2S activity, attributed to seawater dilution and rapid precipitation of sulfide minerals, facilitated the efficient destabilization and deposition of Au transported as sulfide complexes from hydrothermal fluids. In particular, the lower 34S values (+0.2 ‰ to + 1.3 ‰) of sulfides compared to those of arc/back-arc lavas (δ34S = +5‰ to + 11 ‰), along with the prevalence of CO2 in the vapor phase of fluid inclusions, reflect that the significant contribution of magmatic volatile influx supplied most of the sulfur and metals (including Au) to the Forecast hydrothermal fluids. Additionally, the abundance of montmorillonite and varying proportions of sulfide and gangue minerals observed in hydrothermal samples suggest that the chemical buffering of fluids by associated substrates and/or sediments could be another significant factor in promoting Au-rich mineralization. Therefore we conclude that the combination of magmatic fluid–seawater mixing and host rock alteration plays an important role in the formation of Au-enriched massive sulfides in the Forecast vent field.

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岩浆流体-海水混合和母岩蚀变对马里亚纳海槽南部 Forecast 喷口区富含金的块状硫化物形成的综合影响
从马里亚纳海槽南部的 Forecast 喷口区采集了块状硫化物和含硫化物角砾岩。这些样本呈现出不同程度的热液蚀变和/或矿化现象,前一种类型的样本具有显著的金富集(高达 101 ppm)。本研究对矿物学、地球化学、S 同位素和流体包裹体进行了详细分析,以了解不寻常的富金矿化现象。对主要硫化物矿物进行的矿物学调查和地球化学分析(LA-ICP-MS 和 EPMA)表明,金矿化的主要阶段--电积物,根据矿物学关系和细度可分为两代。早期的高细度电晶石(930-981 ‰)与相对高温流体条件下的闪锌矿有关,而方铅矿则是与晚期低细度电晶石(773-868 ‰)有关的主要主矿物。闪锌矿的测地温度(231-264 °C)、胶质硫化物的出现以及流体包裹体的微测温特征(Th = 220-304 °C;盐度 = 1.4-6.6 wt% NaCl 当量;盐度随 Th 值降低而降低的总体趋势)表明,由于海水稀释和硫化物矿物的快速沉淀,H2S 活性显著降低,促进了热液中以硫化物复合物形式迁移的金的有效脱稳和沉积。特别是,与弧/背弧熔岩(δ34S = +5‰ 至 +11‰)相比,硫化物的 34S 值较低(+0.2‰ 至 +1.3‰),而且流体包裹体的气相中普遍含有二氧化碳,这反映出岩浆挥发物的大量涌入为预测热液提供了大部分硫和金属(包括金)。此外,在热液样本中观察到的大量蒙脱石以及不同比例的硫化物和煤矸石矿物表明,伴生基质和/或沉积物对流体的化学缓冲作用可能是促进富金成矿的另一个重要因素。因此,我们得出结论,岩浆流体-海水混合和母岩蚀变相结合,在预测喷口区富含金的块状硫化物的形成过程中发挥了重要作用。
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来源期刊
Ore Geology Reviews
Ore Geology Reviews 地学-地质学
CiteScore
6.50
自引率
27.30%
发文量
546
审稿时长
22.9 weeks
期刊介绍: Ore Geology Reviews aims to familiarize all earth scientists with recent advances in a number of interconnected disciplines related to the study of, and search for, ore deposits. The reviews range from brief to longer contributions, but the journal preferentially publishes manuscripts that fill the niche between the commonly shorter journal articles and the comprehensive book coverages, and thus has a special appeal to many authors and readers.
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