In situ observations of gold deposition in a dense liquid layer at the pyrite–water interface

Pyrite-triggered precipitation of gold nanoparticles (AuNPs) is crucial for generating high-grade gold deposits, yet its dynamic process and mechanism at the pyrite-water interface remain unclear due to the lack of in situ observation. Here, utilizing in situ liquid cell transmission electron microscopy, we find a dense liquid layer mediated deposition of AuNPs at the pyrite-water interface in parts per billion-level gold-bearing solutions, a concentration that resembles crustal abundances. Real-time imaging reveals that a dense liquid layer forms at the pyrite-water interface, and it is proposed that AuNPs nucleate and grow in this layer. Results from in situ atomic force microscopy and ex situ transmission electron microscopy indicate that the growth kinetic process of AuNPs involves monomer-to-cluster aggregation, further enriching gold at the pyrite-water interface. Thermodynamic modeling demonstrates that precipitation of AuNPs is primarily driven by the oxygen fugacity decrease in the dense liquid layer due to pyrite dissolution. These findings reveal a localized gold concentration mechanism to interpret adsorption and nucleation of AuNPs on pyrite during its dissolution-precipitation cycles, which significantly enhances our understanding of the highly effective gold scavenging from fluid by pyrite. The mechanism of nanoparticle formation in the dense liquid layer at dissolving mineral-fluid interfaces represents a fundamental process that could be common in nature.