Transport and Deposition of Metallic Nanoparticles as a Hydrothermal Ore-forming Process

纳米颗粒的尺寸通常在10-6到10-9米（1微米大小）之间，并且与大块矿物固体具有不同的化学和物理性质。我们现在知道它们是真正溶解的原子或分子和结晶固体之间的过渡，并且可能具有新的纳米技术可以利用的特殊性质。奥本大学先前的研究表明，美国西部的一些金属热液矿石是由金属纳米颗粒聚集而成的，特别是那些富含金银等贵重金属的矿石。因此，金、银和其他金属的固体纳米颗粒的运输似乎是一个新的但很少研究的成矿过程。以前，研究人员认为金属是在真正的溶液中被运送到它们最终沉积的地方的，而现在看来情况并非如此，至少在某些情况下是这样。这一过程的一个现代例子是从海底热液喷口冒出的黑烟，它主要是由夹带在超临界热液中的金属硫化物纳米粒子组成的。这项由美国国家科学基金会资助的研究将研究金属纳米颗粒在浅层金矿（浅成热液）和银脉矿床形成过程中的运输，这些金矿是19世纪美国西部通常开采的贵金属矿石。这些矿石不仅表现出由金属纳米颗粒聚集形成的壮观的矿物结构，而且初步的同位素（铜、铅和硫）研究表明，形成矿石矿物结构的纳米颗粒似乎来自地壳深处，而不是它们沉积的浅层环境。这项研究需要验证的假设是，玄武岩岩浆是由地幔的部分熔融形成的，可能会以一种富含硫的低密度阶段释放金属，类似于“蒸汽”，在某种程度的冷却后，会形成金属、类金属和硫的纳米颗粒。因此，纳米颗粒似乎锁定了与源岩浆房相似的高温和原始同位素组成，因此必须向上运输相当远的距离（几公里？）才能形成浅层矿石。如果内华达州和爱达荷州计划进行的新的矿石同位素调查能够证实这一点，这将是热液成矿过程的一个新方面，可以指导这些有价值的矿石在区域和地区范围内如何和在哪里出现。研究结果对更好地理解岩浆中的金属释放和运输具有重要意义，并可能揭示导致此类矿石形成的岩浆成分的来源。

Nanoparticles typically have sizes between 10-6 and 10-9 m (1 micron in size) and have different chemical and physical properties than the bulk mineral solids. We now understand that they are transitional between truly dissolved atoms or molecules and crystalline solids and may have special properties that can be exploited by new nanotechnologies. Previous research at Auburn University has shown that some metallic hydrothermal ores in the western United States formed by aggregation of metallic nanoparticles, particularly ones rich in such valuable metals as gold and silver. Thus the transport of solid nanoparticles of gold, silver and other metals appears to be a new but little-studied ore-formation process. Previously, researchers have assumed that metals were delivered to the site of their ultimate deposition in true solution, and this now appears not to be the case, at least in some situations. A present-day example of this process is the black smoke emanating from sea floor hydrothermal vents on the ocean floor, which is composed mainly of metal-sulfide nanoparticles entrained in supercritical hydrothermal fluids.This NSF-supported grant will look at transport of metallic nanoparticles involved in the formation of shallow bonanza ('epithermal') vein deposits of gold and silver, which were the type of precious-metal ores commonly mined in the western US in the 1800s. These ores not only exhibit spectacular mineral textures formed as a result of aggregation of metallic nanoparticles, but preliminary isotopic (copper, lead, and sulfur) investigations have suggested that the nanoparticles that formed the ore mineral textures, appear to have come from much deeper down in the crust than the shallow setting where they were deposited. The hypothesis to be tested with this study is that basaltic magmas, formed from partial melting in the underlying mantle, may release metals in a sulfur-rich, low density phase akin to a 'vapor', which forms nanoparticles of metals, metalloids, and sulfur upon some degree of cooling. Thus the nanoparticles appear to lock in high temperature and primitive isotopic compositions similar to the source magma chambers, and therefore must be transported upward a considerable distance (a few km?) to form the shallow ores. If this can be substantiated by the planned new isotopic investigations of ores from Nevada and Idaho, this will be a new aspect of hydrothermal ore-forming processes that can guide how and where such valuable ores occur, both at a regional and district scale. Results have implications for better understanding metal release and transport from magmas, and perhaps will shed light on what are the sources of magma composition that lead to formation of such ores.