Boiling-induced extreme Cu isotope fractionation in sulfide minerals forming by active hydrothermal diffusers at the Aegean Kolumbo volcano: Evidence from in situ isotope analysis

We analyzed the first Cu isotopes in primary cupreous pyrite and orpiment, from modern CO2-degassing, seafloor massive sulfide diffuser vents ("KCO(2)Ds"), from the Kolumbo submarine volcano, Hellenic volcanic arc. Samples came from six KCO(2)Ds that are actively boiling. Pyrite comprises colloform pyrite-I and euhedral pyrite-II, which occur erratically distributed within the KCO(2)Ds and are contemporaneous with barite and spatially concurrent with the chalcopyrite that is lining narrow internal conduits, respectively. Orpiment occurs on the outer walls of the KCO(2)Ds with barite and stibnite. The delta Cu-65(pyrite-I) values show high variability, ranging from +2.93 parts per thousand to +6.38 parts per thousand, whereas the delta Cu-65(pyrite-II) and delta Cu-65(chalcopyrite) values vary from -0.94 parts per thousand to +0.25 parts per thousand and -0.45 parts per thousand to -0.09 parts per thousand, respectively. The range of delta Cu-65(orpiment) between +1.90 parts per thousand and +25.73 parts per thousand is the most extreme ever reported from any geological setting. Pyrite-I is concentrically layered, with a core comprising random crystallites, whereas the mantle crystallites have grain-size, shape, and orientation variability between layers. Pyrite-II forms aggregates of uniform euhedral pyrite crystals. Pyrite-I has higher concentrations of Cu (<= 21,960 ppm) compared to pyrite-II (<= 4963 ppm), and both have incompatible and volatile metal(loid)-rich composition and low Sb/Pb (<0.5) and Tl/Pb (<0.03) ratios. When combined with evidence for significant magmatic contributions at Kolumbo and geochemical and micro-textural evidence for recurrent intense boiling and/or flashing or gentle and/or non-boiling, the measured extreme delta Cu-65 values are consistent with transport of Cu by vapor that is preferentially enriched by heavy Cu-65 and controlled by continuous Rayleigh distillation-type Cu fractionation. Boiling-induced Cu vapor transport can generate extreme Cu isotope fractionation.