Isocubanite-chalcopyrite intergrowths in the Mid-Atlantic Ridge 26°S hydrothermal vent sulfides

Petrographic, SEM, and EPMA analyses are used to study the micro-textures and mineralogical composition of samples collected by a TV-grab from the 26°S SMAR (southern Mid-Atlantic Ridge) hydrothermal field. The investigated samples include the outermost chimney walls and sulfide debris. Isocubanite-chalcopyrite intergrowths are the major Cu-Fe sulfide phase in the chimney wall samples. These intergrowths include normal chalcopyrite, anomalous chalcopyrite (Cu-poor, Zn- and Fe-rich), normal isocubanite with Cu/Fe < 0.50, and Cu-rich isocubanite with Cu/Fe > 0.50. Anomalous chalcopyrite and Cu-rich isocubanite represent the intermediate phases between stoichiometric chalcopyrite and isocubanite in the Cu-Fe-S system. Anomalous chalcopyrite occurs as cores or thin rims bordering isocubanite, which associated with sphalerite. While Cu-rich isocubanite commonly associates pyrite. Based on textural relationships and microanalytical data of both phases, we interpret the abundant anomalous chalcopyrite and Cu-rich isocubanite as metastable or as high-temperature (~300 °C) rapidly precipitated hydrothermal sulfides. This interpretation advocates the SMAR 26°S hydrothermal field as an immature and short-living system.