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 CO (sub 2) -degassing, seafloor massive sulfide diffuser vents ("Kappa CO (sub 2D) s"), from the Kolumbo submarine volcano, Hellenic volcanic arc. Samples came from six Kappa CO (sub 2) Ds that are actively boiling. Pyrite comprises colloform pyrite-I and euhedral pyrite-II, which occur erratically distributed within the Kappa CO (sub 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 (sub 2) Ds with barite and stibnite. The delta 65Cu (sub pyrite-I) values show high variability, ranging from +2.93 per mille0 to +6.38 per mille0;, whereas delta (super 65) Cu (sub pyrite-II) delta (super 65) Cu (sub chalcopyrite) values vary from -0.94 per mille to +0.25 per mille0 and -0.45 per mille0 to -0.09 per mille0, respectively. The range of delta (super 65) Cu (sub orpiment) between +1.90 per mille0 and +25.73 per mille 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 (< or =21,960 ppm) compare pyrite-II (< or =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 measure extreme delta (super 65) Cu values are consistent with transport of Cu by vapor that is preferentially enriched by heavy (super 65) Cu and controlled by continuous Rayleigh distillation-type Cu fractionation. Boiling-induced Cu vapor transport can generate extreme Cu isotope fractionation.