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 (“ΚCO₂Ds”), from the Kolumbo submarine volcano, Hellenic volcanic arc. Samples came from six ΚCO₂Ds that are actively boiling. Pyrite comprises colloform pyrite-I and euhedral pyrite-II, which occur erratically distributed within the ΚCO₂Ds 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 KCO₂Ds with barite and stibnite. The δ⁶⁵Cu_pyrite-I values show high variability, ranging from +2.93‰ to +6.38‰, whereas the δ⁶⁵Cu_pyrite-II and δ⁶⁵Cu_chalcopyrite values vary from −0.94‰ to +0.25‰ and −0.45‰ to −0.09‰, respectively. The range of δ⁶⁵Cu_orpiment between +1.90‰ and +25.73‰ 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 δ⁶⁵Cu values are consistent with transport of Cu by vapor that is preferentially enriched by heavy ⁶⁵Cu and controlled by continuous Rayleigh distillation−type Cu fractionation. Boiling-induced Cu vapor transport can generate extreme Cu isotope fractionation.