Macro-to-nanoscale investigation unlocks gold and silver enrichment by lead-bismuth metallic melts in the Switchback epithermal deposit, southern Mexico

Abstract

Lead-bismuth (Pb-Bi) minerals of the galena (PbS)-matildite (AgBiS2) series and lillianite homologues (Pb3–2xAgxBi2+xS6) are intergrown with electrum (Au-Ag alloy) and chalcopyrite (CuFeS2) in specific bands within a colloform-banded vein at the Switchback epithermal deposit in southern Mexico. A macro-to-nano scale study revealed that these minerals fill small (<200 µm) cavities in the gangue minerals, showing curvilinear boundaries, bleb-like morphologies, and rounded nanoparticles (~100 nm). These observations are consistent with growth in a molten system from a precursor Pb-Bi melt containing Au, Ag, and Cu. Minerals in the galena-matildite series typically display Widmanstätten textures (i.e., octahedral-like or basket-weave matilditess lamellae in the galenass matrix), which have been traditionally linked to the decomposition of a high-temperature solid solution. However, galenass and matilditess show nano-scale sinuous reaction fronts and replacement relicts (“islands”) while maintaining the [011]Galena || [100]Matildite crystallographic orientation relationship. This suggests a topotaxial growth of matilditess mediated by coupled dissolution-reprecipitation reactions between galenass and the metallic melt upon cooling. A similar scenario is proposed for Pb-Bi sulfosalt intergrowths, which replace galena-matildite and electrum and grow topotaxially along (200)Galena. Collectively, these results suggest that Pb-Bi melts can exist in epithermal fluids, acting as precursors for the crystallization of ore minerals and able to sequester precious metals. This model explains abnormally high Au-Ag enrichments observed in some deposits that contain Pb-Bi ores.