Migration and Transformation of Gold Nanoparticles in Synthetic Rock: A Study of Hydrothermal Ore Formation Processes

In this study, the behavior of gold nanoparticles (Au NPs) in a synthetic rock (SynRock) consisting of Au NPs sputtered on graphite powder and mechanically activated quartzite and pyrite crystals was investigated. This is a simplified but representative analog of specific natural hydrothermal gold deposits. Hydrothermal systems, where temperatures and pressures reach 450 °C and 1 kbar or more, facilitate the migration and precipitation of various types of native gold: metallic (Au0) micro- and nanoparticles, structurally bound or sorbed. These gold species play a key role in the formation of “invisible” gold present in various minerals, including pyrite and quartz. Nanoparticle behavior under these conditions is crucial for understanding natural ore formation processes, including mineralization, particle transport, and precipitation mechanisms. A comprehensive study was conducted using various analytical techniques, including atomic absorption analysis (AAA), atomic emission analysis (AEA), electron probe microanalysis (EPMA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The study yielded significant results. The AAA and AEA analysis results indicate that pyrite and quartzite are enriched in gold. Moreover, electron microscopy methods (EPMA and SEM) consistently yielded identical results. EPMA and SEM revealed the presence of spherical particles of varying sizes (micron and submicron gold) in pyrite and quartzite. XPS data showed that after hydrothermal treatment, gold on graphite existed in two forms: micro- (Au0) and Au NPs. The size of Au NPs increased due to the self-assembly mechanism. Under thermal gradient conditions, Au NPs are better “trapping” by pyrite crystals than quartz crystals of quartzite.