Mineralogy and Geochemistry of High-Temperature Cu-rich Hydrothermal Chimneys from the Moytirra Field, Mid-Atlantic Ridge

Abstract

The Moytirra hydrothermal vent field is located on the Mid-Atlantic Ridge (MAR) at approximately 45°N, between the Azores and Iceland. This system was discovered in 2011 during an Irish-led scientific expedition aboard the RV Celtic Explorer, and was later extensively explored and sampled in 2019 by the Spanish EXPLOSEA 2 expedition aboard the RV Sarmiento de Gamboa, with the support of the remotely operated vehicle (ROV) Luso. It is the first known deep-sea hydrothermal vent field on the slow-spreading MAR north of the Azores. In June 2019, two ROV dives were conducted at a depth of approximately 2,900 meters to explore a fault scarp located on the eastern axial wall of MAR. The site featured ruins formed from talus and inactive, oxidized chimneys, alongside active sulphide-anhydrite chimneys emitting high-temperature black smokers. The studied active sulphide chimneys exhibit mineral concentric zonation around the hydrothermal fluid conduit and with lateral development of a 'Christmas tree' structure. They are composed essentially by primary pyrite (FeS₂), chalcopyrite (CuFeS₂) and anhydrite (CaSO₄); less abundant are pyrrhotite (Fe₁₋ₓS), cubanite-isocubanite (CuFe₂S₃), sphalerite (ZnS) and marcasite (FeS₂). Additionally, amorphous silica clays and barite (BaSO₄) are present, contributing to the chimneys' structure. Secondary minerals goethite (FeO(OH)) and atacamite (Cu₂Cl(OH)₃) appear in the surface of chimneys affected by incipient weathering. The mineralogical zonation reflects both active mineral precipitation and post-depositional alteration. Pyrite and chalcopyrite exhibit textures ranging from massive to euhedral, occasionally showing brecciated structures. Exsolution textures of cubanite–isocubanite within chalcopyrite are common, reflecting unmixing during cooling. Colloform textures in marcasite are also frequently observed, indicative of low-temperature precipitation. Locally, pyrite replaces anhydrite, suggesting late-stage sulfide mineralization. Overprinting textures are evident in pyrite, chalcopyrite, and sphalerite, indicating the occurrence of multiple mineralization pulses or a progressive evolution of hydrothermal fluids during the formation of the deposit. The chimneys are enriched in Fe (up to 45.6 wt%), Cu (up to 23.2 wt%) and Zn (up to 4.9 wt%). Other strategic and critical metals, including Co, Ba, As, and Ag, are also present in notable bulk concentrations within the Moytirra hydrothermal chimneys, with measured values of up to 2,562 µg/g, 2,542 µg/g, 502 µg/g, and 60 µg/g, respectively. These findings emphasize the metallogenic significance of the Moytirra Field and its potential role in deep-sea metal cycling beyond iron, copper, and zinc. Further research is required to clarify the extent of abiotic versus microbial influence in chimney formation.