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

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Abstract

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).

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Acknowledgements

We are indebted to Newmont, Klondex, and Hecla mining companies, and the former companies Santa Fe Pacific and AMAX, for providing the access to their deposits (including ones described here). In particular, we would like to thank John Marma for his help with understanding the geology and context of samples collected at both Fire Creek and Midas. Note: Samples used for this study (and additional samples of NV-ID-WA-CA epithermal ores) are archived in the Gilles O. Allard Collection of ore-deposits samples housed by the Georgia Museum of Natural History at the University of Georgia.

Funding

The US National Science Foundation provided primary support for this research; support grants to Brueseke (EAR-0838139) and Saunders (EAR-91171713; EAR-0838208; EAR-1247857).

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Authors and Affiliations

  1. Auburn, AL, USA

    James A. Saunders

  2. Department of Geology and Environmental Earth Science, Miami University, 118 Shideler Hall,, Oxford, OH, 45056, USA

    Michelle Burke

  3. Department of Geology, Kansas State University, 108 Thompson Hall,, Manhattan, KS, 66506, USA

    Matthew E. Brueseke

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  1. James A. Saunders
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Saunders, J.A., Burke, M. & Brueseke, M.E. Scanning-electron-microscope imaging of gold (electrum) nanoparticles in middle Miocene bonanza epithermal ores from northern Nevada, USA. Miner Deposita 55, 389–398 (2020). https://doi.org/10.1007/s00126-019-00935-y

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