Abstract
Fluid conditions of shallow marine hydrothermal vent sites (<200 mbsl) in island arcs resemble those of subaerial epithermal systems. This leads to a distinct mineralization-style compared to deeper arc/back-arc (>200 mbsl) and mid-ocean ridge-related environments (>2000 mbsl). At Calypso Vents in the Bay of Plenty and Paleochori Bay at the coast of Milos Island, fluids with temperatures <200°C are emitted through volcaniclastic sediments in water depths <200 mbsl. The hydrothermal mineralization from these fluids is dominated by pyrite and marcasite showing diverse textures, including colloform alternations, semi-massive occurrences surrounding detrital grains, vein-type pyrite, and disseminated fine-grained assemblages. Pyrite and marcasite from Calypso SE show elevated concentrations of volatile elements (e.g., As, Sb, Tl, Hg) implying a vapor-rich fluid phase. By contrast, elements like Zn, Ag, and Pb are enriched in hydrothermal pyrite and marcasite from Calypso SW, indicating a high-Cl liquid-dominated fluid discharge. Hence, vapor-liquid element fractionation induced by fluid boiling is preserved in the seafloor mineralization at Calypso Vents. Hydrothermal mineralization at very shallow vent sites (<10 mbsl), like Paleochori Bay, are affected by wave action causing a seasonal migration of the seawater-fluid interface in the sediment cover. The δ34S composition of native S crusts and crystalline S (0.7–6.7‰) is indicative for host rock leaching and thermochemical reduction of seawater sulphate. By contrast, the highly negative δ34S signature of native S globules in sediments (−7.6 to −9.1‰) is related to microbial sulphate reduction or a subordinate magmatic fluid influx. Alunite-jarosite alteration (Paleochori Bay) and a mineral assemblage consisting of orpiment, realgar, and native S (Calypso Vents) may also suggest a contribution by an oxidised (sulphate-rich) low pH fluid of potential magmatic origin. However, fluid boiling is pervasive at Calypso Vents and Paleochori Bay, and the condensation of vapor-rich fluids in a steam-heated environment may produce a similar alteration and mineralization assemblage without a significant magmatic fluid influx, as known from some subaerial epithermal systems.
Highlights
Submarine hydrothermal systems and associated sulphide occurrences are known from a wide range of tectonic settings including mid-ocean ridges, back-arc basins and island arc volcanoes (Fouquet et al, 1993; Hannington et al, 2005; de Ronde et al, 2011; German et al, 2016; Hannington et al, 2017)
Island arc and young back-arc hydrothermal systems occur at 2000 m water depth (Binns and Scott, 1993; Fouquet et al, 2010; Jenner et al, 2010; Keith et al, 2017; Patten et al, 2017)
Low temperature (2000 mbsl)
Summary
Submarine hydrothermal systems and associated sulphide occurrences are known from a wide range of tectonic settings including mid-ocean ridges, back-arc basins and island arc volcanoes (Fouquet et al, 1993; Hannington et al, 2005; de Ronde et al, 2011; German et al, 2016; Hannington et al, 2017). Submarine vent fields that occur at shallow water depths
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