Orogenic gold deposits, formed during mountain building events, are significant sources of gold globally, especially those in Archean terranes. These deposits are characterized by high-grade ore zones with coarse gold, which are challenging to explain using traditional models of gold transport in solution. Recent research has focused on the potential role of gold nanoparticles in enriching gold deposits, a mechanism previously demonstrated in epithermal, Carlin, and seafloor massive sulfide deposits. This study examines the Kenty gold deposit in the Abitibi greenstone belt, a well-preserved Archean terrane in Canada, known for its simple mineralization history and high-grade gold zones. Previous research has suggested that the late-stage coarse gold at Kenty resulted from remobilization from earlier auriferous pyrite through dissolution-reprecipitation processes. This study seeks to provide direct evidence of gold nanoparticles involved in this secondary remobilization process, extending the understanding of gold nanoparticle mechanisms to orogenic deposits and offering a potential explanation for the formation of ultra-high-grade ore zones.

The aqueous transport of gold nanoparticles has been established as a factor in high-grade gold ore zones of epithermal deposits. However, documentation of this mechanism in orogenic deposits through secondary processes has been lacking until now. Studies have shown the formation and aggregation of gold nanoparticles in epithermal ores and the potential for boiling-induced formation of colloidal gold in hydrothermal fluids. Research also exists on the thermal behavior of metal nanoparticles in geologic materials, highlighting the influence of temperature and host mineral on their stability and aggregation. Previous work on the Kenty deposit indicated gold remobilization from pyrite, based on textures suggesting dissolution-reprecipitation, elemental mapping showing gold loss from pyrite, and the presence of elements associated with gold in both pyrite and later coarse gold phases. While earlier work suggested the possible role of low-melting-point chalcophile element (LMCE)-rich melts in gold concentration, direct evidence of gold nanoparticles remained to be demonstrated.

The study focused on a mineralized sample from the Kenty deposit exhibiting evidence of gold remobilization. Focused ion beam-scanning electron microscopy (FIB-SEM) was used to extract sections from the sample, which were then thinned into foils for transmission electron microscopy (TEM) analysis. These foils were extracted from within a pyrite grain and across its interface with gold-hematite-albite-rutile grains. TEM examination, including high-angle annular dark-field (HAADF) imaging and scanning TEM-energy-dispersive X-ray spectroscopy (STEM-EDS), revealed the presence of gold nanoparticles. Selected area electron diffraction (SAED) was used to identify phases within iron oxides, and fast Fourier transform (FFT) patterns were employed to analyze crystal lattice orientations of the nanoparticles. The authors addressed potential artifacts from FIB milling by examining the presence of three-dimensional features, checking for changes during the thinning process, and confirming the presence of multiple Fe-oxide phases. The research incorporated previous work on dissolution-reprecipitation processes and LA-ICP-MS mapping.

TEM analysis revealed nano-size Au-Ag-Te-Pb domains within pyrite, suggesting a telluride phase. Sub-nanometer-size domains were also observed, indicating finely dispersed elemental Au, Ag, and Te throughout the pyrite. Importantly, the study found gold nanoparticles (1-5 nm) within iron oxides (hematite, goethite, magnetite/maghemite) and within rutile, adjacent to coarse gold. The iron oxides were present before and remained throughout the FIB thinning process. A thin layer of iron oxide containing gold nanoparticles with different lattice orientation compared to coarse gold was found between the gold and albite. Non-oriented attachment of nanoparticles to coarse gold was observed, suggesting coarsening via Ostwald ripening, a process of dissolution and monomer transfer to larger crystals. The observations suggest gold nanoparticles are trapped within the matrix before coarsening into larger gold domains. This provides evidence of gold nanoparticles in an Archean orogenic gold deposit associated with gold remobilization, supporting the idea that gold nanoparticles play a role in forming high-grade gold zones.

The findings provide direct evidence of gold nanoparticle transport and coarsening as a mechanism for upgrading orogenic gold deposits. Two models are proposed to explain the charge-dependent transport and coarsening of gold nanoparticles based on pH and counter-ion activity. One model suggests transport of positively charged gold nanoparticles at low pH, followed by aggregation near the point of zero charge (PZC) due to pH increase from mineral dissolution. The other model suggests transport of negatively charged nanoparticles at higher pH, with aggregation promoted by adsorption of counter-ions like Fe3+. The non-oriented attachment observed at Kenty supports Ostwald ripening as the primary coarsening mechanism, in contrast to oriented attachment observed in some epithermal deposits. The decreasing melting point of gold nanoparticles with decreasing size is considered, acknowledging the influence of host mineral on thermal stability.

This research provides compelling evidence for the role of gold nanoparticles in the formation of high-grade ore zones in Archean orogenic gold deposits, specifically the Kenty deposit. The findings demonstrate that gold nanoparticle transport and coarsening via Ostwald ripening can operate alone or in conjunction with fluid-mediated polymetallic melts to form high-grade ore. Further research should focus on understanding the role of silver in gold nanoparticle behavior, clarifying the relationship between gold nanoparticles, other elements, and host minerals, and determining the precise roles of temperature and particle size in gold nanoparticle formation and aggregation under ore-forming conditions. The implications of this research extend beyond geology, impacting fields such as material science, nanomedicine, and nanotechnology.

The study focused on a single sample from the Kenty deposit. While the meticulous methodology minimized artifacts from sample preparation, the generalizability of these findings requires further research on additional samples and deposits. The complex interactions of gold nanoparticles with various minerals and fluids are not fully understood and require further investigation. The models proposed for gold nanoparticle transport and coarsening are based on current understanding of nanoparticle behavior and may need refinement as our understanding improves.