As we’ve been reflecting on 2025, Simon and Aidan thought they would share one of their key takeaways from the SEG Conference in Brisbane and links to what we have seen in geochemical data.
Aidan attended a great talk that got us thinking about the mechanisms involved in the formation of high-grade (bonanza) gold deposits and a possible cause of the so-called “nugget effect.”
A talk by Duncan McLeish (and supplementary 2021 paper) identified that epithermal fluids with gold concentrations of 10-30 parts per billion (ppb) may be sufficient to explain veins reporting tens of grams per tonne. However, this is far too low a concentration to explain sporadic occurrences of bonanza mineralisation. McLeish and co-workers discuss how physical transport of gold in solid state as nanoparticles and flocculated aggregates could explain this paradox. Electron microscope imagery from Brucejack (British Columbia) revealed:
- Electrum nanoparticles (5–15 nm) in calcite nano-veinlets.
- Larger aggregates (30–150 nm) formed by nanoparticle clustering.
- At the nanoscale – flocculated Au along the edge of pyrite grains.
- Nanoparticles of Au become more “organised” in contact with pyrite.
- Pyrite with As can further induce Au flocculation (attracting negatively charged Au).
Flocculation of Au nanoparticles can be induced by processes such as boiling, cooling and fluid mixing. The study identifies a process where fluids with ppb-level gold may form bonanza deposits. The authors suggest colloidal transport may be widespread in hydrothermal gold systems in addition to conventional transportation models involving dissolved aqueous complexes (e.g. bisulphides). McLeish, et. al. argues that for bonanza grades to form, Au is physically transported as colloidal nanoparticles, not solely precipitating from solutions.
Insights from this talk prompted us to re-evaluate the mechanisms driving what we observe in many geochemical data. We often see very robust covariance between pyrite and gold, expressed as Au mineralised samples with Fe:S ratios consistent with pyrite (Figure 1). In these mineralised samples we also commonly observe a strong control between As and Au, especially at high gold grades (Figure 2).
Combining our observations and McLeish’s work with previous studies: arsenic substitution for sulphur in the pyrite crystal lattice creates semi-conductivity and “holes” generated in the lattice causes the pyrite grain surface to become positively charged (Möller & Kersten, 1994). Gold colloids in hydrothermal fluids are typically negatively charged, resulting in Au clumping and accumulation at pyrite grain boundaries. The strong As-Au covariance and Fe:S ratio specific to pyrite, especially at higher grade Au, suggests we are identifying the processes of colloidal flocculation which is described as a key driver for the formation of bonanza grades.
We believe that this work might be key in recognising that As isn’t just a pathfinder for Au, but essential in creating electrochemical conditions necessary for flocculation of gold colloids.
References
McLeish, D.F., Williams-Jones, A.E., Vasyukova, O.V., Clark, J.R. & Board, W.S. 2021. Colloidal transport and flocculation are the cause of the hyperenrichment of gold in nature. Proceedings of the National Academy of Sciences, 118, e2100689118. https://doi.org/10.1073/pnas.2100689118
Möller, P. & Kersten, G. 1994. Electrochemical accumulation of visible gold on pyrite and arsenopyrite surfaces. Mineralium Deposita, 29, 404–413.
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