Supergene gold transformation: Secondary and nano-particulate gold from southern New Zealand
Supergene gold transformation: Secondary and nano-particulate gold from southern New Zealand
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DOI：
10.1016/j.chemgeo.2012.05.021
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发表时间：
2012-08
期刊：
影响因子：
3.9
通讯作者：
F. Reith;Lachlan Stewart;S. Wakelin
中科院分区：
文献类型：
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作者：
F. Reith;Lachlan Stewart;S. Wakelin
生物地球化学过程驱动地表环境中金（Au）的转化。在这项研究中，我们评估了金颗粒表面形态与表生转化过程之间的联系，重点研究了在温带环境下纳米颗粒金的形成。金粒是从新西兰南岛的六个地方收集的。在中等到非常高降水水平的地区，沉积物类型从淋积砂矿、冲积砂矿和海滩砂矿各不相同。采用光学显微镜（OM）、场发射扫描电镜（fg - sem）、聚焦离子束扫描电镜（FIB-SEM）结合x射线色散分析（EDXA）和电子探针分析（EPMA）对金颗粒进行了评估。在所有晶粒上都观察到表明Au和Ag溶解的形貌，例如晶界溶解，以及丰富的Au新生和聚集形貌。后者包括多种次级金形态，特别是纳米颗粒和μ晶形态以及细菌形态的金、片状金和多孔的支链金网络。在新西兰南岛西海岸受强地形降水影响，露头石英脉系统颗粒普遍溶蚀，风化石英脉和砂矿颗粒广泛形成纳米颗粒，表明这些气候条件促进了金颗粒的转化。在这些位置，金纳米颗粒在多晶层（即在转化金颗粒表面的生物膜、次生金、硅质和碳质材料的涂层）和与金颗粒相关的土壤物质中含量最高。在Orepuki海滩金颗粒的碳质（可能是外聚物）包覆层中，纳米颗粒金也非常丰富，表明海水中的Au溶解和微生物生物矿化是海滩砂矿床中Au蚀变的重要因素。总之，新西兰金颗粒的表面形态是当前环境下发生的表生转化的结果。纳米颗粒金的形成，以前被认为是由蒸发驱动的，在这些高降雨环境中可能是由于其他机制，如生物矿化。
Biogeochemical processes drive the transformation of gold (Au) in surface environments. In this study we assess the link between surface morphologies of Au grains and supergene transformation processes, with a focus on the formation of nano-particulate Au in temperate settings. Gold grains were collected from six localities across the South Island of New Zealand. Deposit styles vary from eluvial-, alluvial-, and beach placer deposits in areas of moderate to very high levels of precipitation. Gold grains were assessed using optical microscopy (OM), field emission scanning electron microscopy (FEG-SEM), focused ion beam-scanning electron microscopy (FIB-SEM) coupled with X-ray dispersive analysis (EDXA) and electron microprobe analyses (EPMA). Morphologies indicative of Au- and Ag dissolution, e.g., grain boundary dissolution, as well as abundant Au neoformation- and aggregation morphologies were observed on all grains. The latter include a variety of secondary Au morphotypes, in particular nano-particulate- and μ-crystalline forms as well as bacteriomorphic Au, sheet-Au and porous, branched Au networks. Pervasive dissolution features on grains from an outcropping quartz-vein system as well as extensive nano-particle formation on weathered quartz-vein- and placer grains from the west coast of New Zealand's South Island, which is subject to very heavy orographic precipitation, suggest that these climatic conditions enhance the transformation of Au grains. At these sites, Au nano-particles are most abundant in the polymorphic layer (i.e., a coating of biofilms, secondary Au, siliceous and carbonaceous materials on the surface of transforming Au grains), and in soil materials associated with the grains. Nano-particulate Au is also highly abundant in carbonaceous, likely exopolymeric, coatings on Au grains from Orepuki Beach, suggesting that Au dissolution in seawater and microbial biomineralization are important contributors to Au alteration in beach placer deposits. In conclusion, surface morphologies of Au grains from New Zealand are the result of supergene transformations occurring in current environments. The formation of nano-particulate Au, which was previously thought to be evaporation-driven, is in these high-rainfall environments likely due to other mechanisms, such as biomineralization.