Nanoparticle attachment promotes nugget effect of Au-rich metallic melts in hydrothermal ore deposits

The role of bismuth melts in scavenging Au from hydrothermal fluids has been increasingly recognized in the last decade, but the question of how the Au extracted by such melts transforms into nuggets to form high-grade ores remains obscure. Here, we have characterized the nanostructure of gold nanoparticles (AuNPs) in Bi-rich gold ores that precipitated from Bi-Au melts and propose a novel model to explain the genesis of gold nuggets. This model comprises three consecutive processes of Au crystallization in these melts into coarse grains: the initial formation of atomic clusters equivalent to Au nucleation, the coalescence of these clusters into low-crystalline AuNPs followed by their transformation into well-structured ones, and finally the preferential attachment of these NPs along the {111} lattice plane. This atomic crystallization pathway bridges the gap between Au scavenging by metallic melts and nugget formation, thus making the picture of the formation of high-grade gold ores in the context of melt-fluid interaction more complete.