Initial Scanning Electron Microscopy of Macrocrystalline Gold from Eugene Mountains Nevada, Reveals Unexpected Complexities

Gold is an important resource, and its value has greatly increased over the last 25 years by approximately 1,000% with the most recent prices being well over $4,000 an ounce. Understanding the mechanisms of gold mineralization is critical to exploration and developing refined gold extraction techniques. Nevada is a major gold producer globally with several recognized gold deposits. However macrocrystalline gold is comparatively rare and its exact mechanisms are poorly understood. Samples of purchased macrocrystalline gold crystals from the Eugene Mountains, Humboldt County, Nevada were investigated with SEM to better understand the nature of crystallization using comparative criteria of Sunagawa and compare complexity of surface textures to other examples of macrocrystalline gold in Nevada. For example, Round Mountain macrocrystalline gold generally has well crystallized hopper and polyhedral surface textures that appear to align with classical growth models but are largely composed of nanoparticles in the interior. Initial SEM of the Eugene Mountain sample material indicates that surface textures are very diverse. Sample material seems to have a macroscopic millimeter scale hopper texture that has thick somewhat uniform gold growth over it, indicated by partially closed gaps and other textures. Also, weakly expressed finer hopper growth textures are evident in some areas. Damage to the crystal is evident in some minor areas and is likely due to past handling. Composition of the gold is likely close to electrum based on repeated well-defined particles and lines of silver. Calcite and Ca are evident in some areas of the sample material. Because the overall composition is similar to macrocrystalline gold investigated from Round Mountain, future work using TEM can further compare these similarities. Results also provide constraints on comparing quality of native gold crystals in general for collectors and distinguishing between natural growth textures and forgeries of large gold crystals.

Introduction

Nevada is one of the largest sources of gold in the world. It accounts for 78% of United States production and 5.0% of the world's production. The world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry, exceeding decorative value. Macrocrystalline gold refers to the distinct formation of native gold with visible crystals that are classically thought to have formed through the slow cooling of Au-rich solutions. These types of samples are comparatively rare, and details of their formation are poorly understood.

A topographical map of the state of Nevada (Geographic Names Information System. USGS United States Department of the Interior) with a star denoting the Eugene Mountains.

Gold crystal structure model retrieved from the Materials Project for Au (mp-81) from the database version v2025.09.25.

Illustration of different crystal textures, using growth rates (vertical axis) versus driving force (horizontal axis) relations in three types of growth mechanisms, smooth and rough interfaces. Curve (a) spiral growth, (b) two-dimensional nucleation growth mechanism, and (c) adhesive type growth mechanism (Sunagawa, 1987, 1999).

Methods

Eugene Mountain (Humboldt County, Nevada) macrocrystalline gold crystals were purchased. Scanning electron microscopy (SEM) characterization was performed at Miami University’s Center for Advanced Microscopy and Imaging (CAMI) using a variable-pressure Zeiss Supra 35VP FEG (field emission microscope featuring Gemini column technology) with nitrogen as the compensating gas. Samples were prepared for SEM by mounting individual macrocrystalline samples to aluminum stubs using carbon sticky tabs. SEM is a technique that utilizes a focused electron beam to scan across the surface of a sample to gain insight into surface texture and microtopography. This was used to investigate the sample’s crystallization and compare results with Burke et al. (2017) and criteria from Sunagawa (1999).

Scanning Electron Microscopy (SEM)

*Some images are omitted due to format.

a) Unknown particle with complex textures and likely mechanical abrasion texture in the background near the top. b) Loose particle with steep textures and background (hooper) pitted textures. c) Particle surrounded by a texture that is interpreted to be late-stage growth not abrasion wear.

a) Apparent hopper formation with some fine detail overgrowth. b) Small dispersed unidentified lower contrast inclusions (potentially quartz) with carbon tape in background. c) A distorted hopper crystal with inclusions and rough growth and large growths that are distinct from Round Mountain.

The crystals have complex growth that is observable in SEM imaging. There are apparent hopper textures indicating two-dimensional nucleation. This is the dominant growth mechanism. Across the surface are smaller pitted textures as well as what can be described as a late- stage/overgrowth layer. There are many lower contrast inclusions and unknown textures that will be analyzed in the future. Some imaging displays textures that show apparent mechanical abrasion that may be caused from previous handling. There are apparent euhedral and subhedral nanoparticles distributed over areas, similar to what is revealed in Burke’s TEM results but at a larger scale. Investigating these is a priority as it could be integral to a later stage growth model. Overall textures are far more complex than macrocrystalline gold from Round Mountain.

Burke et al. (2017)

SEM images of: A) Polygonized growth islands with a hexagonal morphology in Round Mountain gold. B) Rounded growth islands and depressions indicative of the growth roughening transition at higher supersaturation levels. C-D) Hopper surface also adjacent to areas of two-dimensional growth textures. Some of the hoppers have a tooth-like morphology indicating they began filling during growth. E) Gold exhibiting a granular texture (area in between white lines), with vein-like structures (indicated by arrow) adjacent to areas of two-dimensional growth textures. F) Stepped protrusions extending out from the surface (indicated by arrows) (Burke et al., 2017). Textures are more distinct than the Eugene Mountains Gold.

Transmission electron microscopy images of gold nanoparticles. A) Nanospherules in an adularia grain. B) Rutile crystals with gold nanoparticles. C) Aggregation of gold nanoparticles in a grain boundary between illite-sericite regions. D) High magnification images of nanospherules that have partially aggregated. Inset SAED pattern exhibits polycrystalline rings. Regions between nanoparticles are filled with amorphous silica (Burke et al., 2017).

Key Points

Overall, the samples show similarities with Round Mountain gold, both having an apparent hopper texture present commonly indicating dominant two-dimensional growth nucleation. However, the sample has larger and irregular late-stage surface textures lacking the clean distinct growth that was expected based on physical appearance.
This overgrowth is indicative of a second phase of crystal growth. The first is a phase that is apparent due to the well-developed hopper formations which seem to dominate before the secondary growth. Secondary growth timing and composition is an open question.
The late-stage overgrowth may be a dominant process and may occur within the sample material at depth as well. Growth history of the Eugene Mountain macrocrystalline gold may be more complex than the Round Mountain macrocrystalline gold.
These results provide constraints on comparing quality of native gold crystals in general for collectors and distinguishing between natural growth textures and forgeries of large gold crystals as well as assessing potential abrasion damage and impact damage.
These results provide insight into the details of gold growth and thus may contain indicators of gold ore concentration mechanisms with broader study, comparison and context.
These results may be useful for potentially explaining variations in cyanide leaching extraction efficiency generally and may enable the enhancement of gold recovery methods.
Although many questions remain this study opens many pathways to creating an understanding of the complete history of these macrocrystalline gold samples.

Future Work

Further investigation will try to determine if the formation correlates to a classical gold growth model or nanoparticle aggregation model. Transmission electron microscopy (TEM) is necessary to compare the sample’s overall composition with Round Mountain. Future work will be done using focused ion beam (FIB) laser ablation to image grain morphology and preparation for TEM. A next step would be to determine if nanoparticulate gold was the first stage of growth or if another process was more dominant. Detailed energy dispersive spectroscopy work will identify low contrast phases and solid solution compositions in gold.

The following is an image of poster presented at the 2026 Undergraduate Research Forum

Acknowledgements

We thank Miami University Department of Geology and Environmental Earth Science for supporting us to attend this meeting. K. Bensing was supported by a Miami University Honors College Fellowship and the Miami University Mineralogy & Petrology Career Development Fund. We thank Mr. Matt Duly of Miami University's Center for Advanced Microscopy and Imaging (CAMI) for support of the facility and general assistance and training. We thank Kinross USA for some sample materials used in Burke et al. (2017) and future comparative work.

References

Burke M. Rakovan, J., Krekeler M.P.S. (2017) A study by electron microscopy of gold and associated minerals from Round Mountain, Nevada. Ore Geology Reviews 91: 708-717
Sunagawa, I. (1999) Growth and Morphology of Crystals. FORMA, 14, 147–166.
Sunagawa, I. (1987) Morphology of minerals, Morphology of Crystals, Part B, ed. by I. Sunagawa, Terra Sci. Pub., Tokyo/ D. Reidel, Dordrecht, pp. 509–587.

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