Nano Cu-Pb-Bi melts as highly efficient agents for Au-Ag enrichment in epithermal ore systems

The role of bi- and poly-metallic melts (i.e., Pb, Bi, Se, Sb, Te, etc.) is increasingly gaining attraction as highly efficient carriers for the precious metals Au and Ag in ore-deposit genesis. These melts have been reported in several types of ore deposits (e.g., orogenic, intrusion-related, volcanogenic massive sulfide) at different temperature ranges. Nevertheless, their occurrence and evolution in low-temperature (<400 °C) hydrothermal ore deposits (e.g., epithermal) is poorly understood. In this communication, we report a micro-to-nanoscale investigation of electrum (Au-Ag alloy) from two epithermal deposits with a very similar ore mineralogy dominated by Cu-Pb-Bi minerals from. In specific bands within a colloform-banded vein at the Switchback epithermal deposit in southern Mexico, electrum (Au-Ag alloy) form intergrowths with chalcopyrite (CuFeS₂) and minerals of the galena (PbS)-matildite (AgBiS₂) solid solution (ss) and ourayite (Ag₃Pb₄Bi₅S₁₃). These minerals show a telescoping of their intergrowth and textures from the nano- to micron-sized scales, characterized by curvilinear boundaries, bleb-like morphologies, and rounded nanoparticles (~100 nm) within the gangue minerals (quartz, fluorite, and calcite) or pyrite. A similar telescoped transition from nano to micron-sized scales is observed in ore minerals of the epithermal gold-rich base metal prospect of Svishti Plaz in central Bulgaria. Here, electrum grains (~100 nm to 1 mm) are also intimately associated with Cu-Pb-Bi minerals, including: (1) chalcopyrite, bismuthite [Bi₂S₃]-pekoite (CuPbBi₁₁S₁₈), (2) aikinite (CuPbBiS₃)-friedrichite (Cu₅Pb₅Bi₇S₈), or (3) bismuthiferous galena hosting exslutions of berryite (Cu₃Ag₂Pb₃Bi₇S₁₆) and benjaminite (Ag₃Bi₇S₁₂). All these minerals also exhibit mutual curved outlines but are exclusively found filling cracks of high porosity zones of pyrite. Collectively, all these observations are consistent with transport in a molten state of a precursor Pb-Bi melt containing Au, Ag, and Cu. Our nanoscale study suggest that that nano Cu-Pb-Bi melts in epithermal fluids are highly efficient collectors of Au and Ag, while acting as  transient agents for the formation of larger pools of melt precursors for the  crystallization of ore minerals. The intervention of these melts in the hydrothermal fluids parental to epithermal ores satisfactorily explains the abnormally high Au-Ag enrichments (i.e., bonanzas) observed in the deposits targeted in this study and elsewhere.