Unravelling metal mobilising processes in the Kolumbo volcano Au-rich SMS using Pb isotopes.

Gold-rich seafloor massive sulfides (SMS) form in various marine hydrothermal environments, including continental volcanic arcs, where magmatic fluids may contribute significantly to the metal budget of the hydrothermal system. To track the origin of metals and unravel the mineralising processes at play in Au-rich SMS formation (i.e. hydrothermal leaching of country rocks or magmatic degassing), we investigated the volcanic/basement rocks and Au-rich SMS of the Kolumbo submarine volcano (Greece). We combine in-situ Pb isotope analysis of the sulphide mineralisation and whole rock Pb isotope analysis of volcanic and basement rocks. During magmatic evolution, crustal material assimilation slightly shifts Pb isotope ratios toward more radiogenic values defining a differentiation trend from andesite to rhyolite. The mineralisation, dominated by pyrite with episodic galena, sphalerite, chalcopyrite and Sb-Pb sulfosalts formation, has Pb isotopes ratios pointing toward an igneous source. Galena and Sb-Pb sulphosalts have Pb isotopes ratios similar to volatile-saturated trachytic magma indicating Pb mobilisation during magmatic degassing, while Pb isotopes ratios in pyrite matches volatile-poor post-degassing rhyolite, suggesting Pb mobilisation by hydrothermal leaching of rhyolite. Lead isotope ratios reveals that Pb at Kolumbo is sourced from igneous rocks but the mechanisms for metal transfer vary from rhyolite leaching by hydrothermal fluid circulation to episodic input of magmatic fluid able to provide Ag, As, Au, Cu, Hg, Pb, Sb, Tl in addition to Pb. The magmatic evolution of Kolumbo highlights the diversity and complexity of metal mobilising magmatic-hydrothermal processes involved in Au-rich SMS formation.