Scanning electron microscope (SEM) imaging of high-grade gold ores from several middle Miocene bonanza epithermal Au-Ag deposits from northern NV shows that a significant amount of gold occurs as electrum nanoparticles and larger microparticles, the latter of which apparently formed by continued growth of nanoparticle precursors. The particles occur as discrete grains disseminated in gangue silicate minerals or as self-organized aggregates termed “fractal” dendrites. Common nanoparticle shapes observed include spheres, polyhedral crystals (dodecahedra, octahedra), and also triangular and hexagonal plates. Although gold nanoparticles locally occur in other types of hydrothermal gold deposits, it appears that evidence for nanoparticles is best preserved in epithermal ores due to the ubiquitous co-deposition of silica nanoparticles (as opal) in these ore-forming systems. It is possible that nanoparticle formation and aggregation could be intermediary steps in the formation of larger gold crystals under disequilibrium ore-forming conditions. Where and when the nanoparticles formed in the evolving ore-forming system are not well-constrained and warrants more study, two possibilities include, i.e., (1) extreme boiling (“flashing”) at shallow depth led to the in situ formation and aggregation of nanoparticles, or (2) electrum nanoparticles formed from a deep magmatic fluid and were physically transported up to the epithermal setting (or perhaps both processes operated in concert).

中文翻译：

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美国内华达州北部中中新世富矿低温热液矿中金（电）纳米颗粒的扫描电子显微镜成像

来自 NV 北部的几个中中新世富矿超热金银矿床的高品位金矿石的扫描电子显微镜 (SEM) 成像表明，大量的金以电子纳米粒子和较大的微粒形式出现，后者显然是由持续生长形成的纳米粒子前体。颗粒以散布在脉石硅酸盐矿物中的离散颗粒或称为“分形”枝晶的自组织聚集体的形式出现。观察到的常见纳米颗粒形状包括球体、多面体晶体（十二面体、八面体）以及三角形和六边形板。虽然金纳米粒子局部存在于其他类型的热液金矿床中，由于二氧化硅纳米粒子（如蛋白石）在这些成矿系统中无处不在的共沉积，纳米粒子的证据似乎在超热矿中保存得最好。在不平衡的成矿条件下，纳米颗粒的形成和聚集可能是形成较大金晶体的中间步骤。在不断变化的成矿系统中形成的纳米粒子在何时何地没有受到很好的约束并需要更多研究，两种可能性包括，即（1）在浅层深度沸腾（“闪蒸”）导致原位形成和聚集纳米粒子，或（2）由深岩浆流体形成的金纳米粒子，并被物理输送到超热环境（或者可能两个过程协同运行）。在不平衡的成矿条件下，纳米颗粒的形成和聚集可能是形成较大金晶体的中间步骤。在不断变化的成矿系统中形成的纳米粒子在何时何地没有受到很好的约束并需要更多研究，两种可能性包括，即（1）在浅层深度沸腾（“闪蒸”）导致原位形成和聚集纳米粒子，或（2）由深岩浆流体形成的金纳米粒子，并被物理输送到超热环境（或者可能两个过程协同运行）。在不平衡的成矿条件下，纳米颗粒的形成和聚集可能是形成较大金晶体的中间步骤。在不断变化的成矿系统中形成的纳米粒子在何时何地没有受到很好的约束并需要更多研究，两种可能性包括，即（1）在浅层深度沸腾（“闪蒸”）导致原位形成和聚集纳米粒子，或（2）由深岩浆流体形成的金纳米粒子，并被物理输送到超热环境（或者可能两个过程协同运行）。