Nanoscale constraints on Au-Ag mineralization in seafloor massive sulfides: Insights from the Deyin hydrothermal field, Mid-Atlantic Ridge

Although native gold and silver, as well as Au- and Ag-bearing minerals, are common in seafloor massive sulfide (SMS) deposits, nanoscale investigations remain limited, hindering our understanding of Au-Ag mineralization. The Deyin hydrothermal field (DHF) is a typical mafic-hosted SMS deposit with zoned sulfide chimneys where micron-sized native gold and silver, electrum, and acanthite were discovered in various zones and examined by high-resolution transmission electron microscopy. Native silver and acanthite occur in low-temperature pyrite or its interstices; both form aggregates of nanoparticles without apparent preferred orientation from particle-particle interactions. Nucleation of native silver and acanthite is attributed to seawater mixing within the chimney; the mixing caused cooling, a pH increase, and H2S consumption during pyrite precipitation, driving Ag+ oversaturation. Acanthite forms preferentially under higher H2S fugacity. Electrum is hosted in sphalerite and gordaite within low- to moderate-temperature assemblages. Unlike native silver and acanthite, electrum occurs as individual crystals with sharp boundaries, suggesting atom-by-atom growth, although seawater mixing also drove Au+ and Ag+ oversaturation during sphalerite precipitation. Native gold occurs in amorphous silica that crosscuts or surrounds sulfides from all zones. Like native silver and acanthite, native gold consists of sintered nanoparticle aggregates without apparent preferred orientation, confirming particle-mediated aggregation. However, the association of native gold with silica indicates that Au nanoparticles (negatively charged colloids) nucleated by boiling at depth. Gold colloids were likely transported as suspensions protected by silica gel; subsequent mixing with seawater triggered their flocculation and aggregation within chimney zones. Given the typical Au and Ag concentrations in the DHF, our findings demonstrate that nanoparticle-mediated processes, including particle aggregation and colloidal transport, can play a significant role in Au and Ag mineralization. Such mechanisms should be considered in genetic models for SMS deposits and other deposit types where similar processes may occur.