Scanning-electron-microscope imaging of gold (electrum) nanoparticles in middle Miocene bonanza epithermal ores from northern Nevada, USA

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).