Abstract
Here, we review the concept that nanoparticles and colloids may have played a significant role in forming some types of hydrothermal ores deposits, particularly epithermal. This concept was first proposed almost a century ago but the development of new analytical technologies, lab experiments, and the discovery of new epithermal deposits where nanoparticles are evident have added credence to the “gold colloid theory”. Nanoparticles are defined to have at least one dimension <10−7 m, and may have different chemical and physical properties than the bulk solids. Colloids are typically <10−6 m in diameter and have the added characteristic that they are dispersed in another medium. In epithermal ore-forming solutions, gold or electrum nanoparticles nucleate from supersaturated hydrothermal solutions, and thus this is a “far-from-equilibrium” process. In some cases, gold nanoparticles may simply play a transitory role of aggregating to form much coarser-grained crystals, where all of the evidence of nanoparticles precursor phases is not preserved. However, in some epithermal ores, silica nanoparticles also formed, and their co-deposition with gold (electrum) nanoparticles preserved the gold aggregation features as self-organized “fractal” dendrites. Here, we review existing the data on gold and electrum nanoparticles in epithermal ores, present images of electrum nanoparticles and their aggregates, and discuss the significance of gold nanoparticles formation and aggregation in helping to produce some of the highest-grade gold ores in the world.
Highlights
The term nanoparticle (NP) is generally defined in the physics and engineering literature as any solid particle having one or more dimension in the range of 1 to 100 nm (10−9 to 10−7 m)
The term “bonanza” implies a relatively rich ore of gold ± silver, and it has been proposed that metal nanoparticle formation and aggregation are important processes in producing such high-grade epithermal ores [12,13,14]
Gold/electrum nanoparticles are common in the LS epithermal ores we have studied in Nevada, USA, and there is no reason to believe the Nevada deposits are unique when compared to others elsewhere in the world
Summary
The term nanoparticle (NP) is generally defined in the physics and engineering literature as any solid particle having one or more dimension in the range of 1 to 100 nm (10−9 to 10−7 m). Perhaps it is more fitting to retain the term colloids for hydrothermal solutions that transport these small particles, but the recent economic geology and geochemistry literature uses both terms [1,2]. Given their size, colloids, and nanoparticles have very-high surface area to volume ratios, which can impart unique chemical properties that can differ from that of the bulk materials [1]. Boyle [11] summarized at Creede, Colorado,that andmore theyresearch attempted calculate on thethe thehydrothermal previous workore-forming on the “goldsolutions colloid theory”
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