Adsorption and coarsening of gold nanoparticles in iron (oxyhydr)oxides lead to high-grade gold mineralization

Many hydrothermal gold systems within Earth’s crust develop high-grade ores that contain gold-rich iron (oxyhydr)oxides including hematite (α-Fe₂O₃) and goethite (α-FeOOH). The origin of these iron (oxyhydr)oxides and their role in gold enrichment, however, remain unclear. This study presents detailed micrometer- to nanometer-scale mineralogical and compositional studies of gold-rich iron (oxyhydr)oxides in a high-grade gold deposit at Jinying, China. Our results show that nanoparticles of goethite and gold formed from oxidizing fluids, which might be derived from interactions between magmatic-hydrothermal fluid and hematite-rich wall rocks and infiltration of oxygenated meteoric water. Goethite nanoparticles formed acicular crystals via aggregation and oriented attachment and then transformed into porous hematite via solid-state transformation. Gold nanoparticles that were initially adsorbed on the goethite surfaces via electrostatic interaction and surface adsorption were released during the transformation of goethite into hematite. The released gold was re-enriched within the small pore in hematite and grew into irregular micron-sized particles via aggregation and Ostwald ripening. These results highlight the previously undocumented impact of the formation and transformation of iron (oxyhydr)oxides on gold accumulation in forming some types of high-grade gold mineralization related to iron formation. This study provides significant new insights into the enrichment of gold nanoparticles in iron (oxyhydr)oxides that are common in a variety of oxidizing ore-forming environments.