The flotation of magmatic sulfides transfers Cu-Au from magmas to seafloor massive sulfide deposits

Abstract

Copper and gold-rich seafloor massive sulfide deposits formed in intra-oceanic subduction settings are typically associated with hydrous and oxidized magmas, but processes leading to their formation remain controversial. Sulfide-bubble interaction has been suggested to play an important role in metal transfer from magmas to seawater-derived hydrothermal fluids. Here we use textural observations of magmatic sulfides, geochemical numerical models of chalcophile element concentrations, and numerical models of magmatic sulfide growth within a mafic to felsic submarine magmatic suite (Fatu Kapa, SW Pacific) associated with copper-gold-rich seafloor massive sulfide deposits. We demonstrate that concomitant sulfide and aqueous fluid formation at the andesitic stage results in floating sulfide-bubble compound drops in magmas, which play a crucial role in the transfer of copper and gold toward the surface. We emphasize that late sulfide saturation in copper-gold-rich intra-oceanic subduction-derived felsic magmas favors upward sulfide transfer via flotation.