Source to sink overview of Au-rich SMS formation at Kolumbo volcano

Volcanic arcs host complex marine magmatic-hydrothermal systems where polymetallic Au-rich seafloor massive sulfides (SMS) may form. In such geological setting, reconstructing the mineral system leading to ore formation is challenging due to the diversity of potential metal sources, mobilizing mechanisms (i.e. hydrothermal leaching of country rocks, magmatic degassing), fluids (magmatic or seawater-derived hydrothermal fluids) and precipitation mechanisms on and below the seafloor. A case-study of the Au-rich SMS at Kolumbo volcano (Greece) provides new insights on the source to sink processes leading to epithermal metals enrichment in volcanic arcs. The favorable geological setting of Kolumbo allows for a representative sampling of the different potential metal sources of the magmatic-hydrothermal system. The country rocks forming the stratigraphy below Kolumbo were sampled on the neighboring islands of Ios, Anafi and Thera, while volcanic rocks of Kolumbo allowed to reconstruct its magmatic evolution and associated metal mobilizing processes. Whole rock geochemistry coupled with petrography and numerical modelling reveals that despite early sulfide saturation, the magma remains fertile until reaching magmatic degassing. Metals are transferred from the magma to the hydrothermal system as exsolving fluids leach metals, either from the melt or by oxidizing magmatic sulfides, as indicated by occurrence of sulfide-volatile compounds in the volcanic rocks. By comparing the Pb isotope signature of the SMS minerals with the potential source rocks, we show that hydrothermal leaching of rhyolite is associated with pyrite formation in hydrothermal chimneys while transient input of magmatic fluids provides Ag, As, Au, Cu, Hg, Pb, Sb and Sn, leading to formation of galena and Sb-Pb sulfosalts. A major outcome of this holistic study of the Kolumbo volcano relates to metal mobilization mechanisms in magmatic-hydrothermal system. We show that formation  and oxidation of sulfide-volatile compounds is leading to an efficient transfer of S and chalcophile metals from the magma to shallow hydrothermal systems. While this mechanism contributes to the formation of Au-rich SMS, it is likely also implicated in porphyry and epithermal deposits formation.