We report on integrated geomorphological, mineralogical, geochemical and biological investigations of the hydrothermal vent field located on the floor of the density-stratified acidic (pH ~ 5) crater of the Kolumbo shallow-submarine arc-volcano, near Santorini. Kolumbo features rare geodynamic setting at convergent boundaries, where arc-volcanism and seafloor hydrothermal activity are occurring in thinned continental crust. Special focus is given to unique enrichments of polymetallic spires in Sb and Tl (±Hg, As, Au, Ag, Zn) indicating a new hybrid seafloor analogue of epithermal-to-volcanic-hosted-massive-sulphide deposits. Iron microbial-mat analyses reveal dominating ferrihydrite-type phases, and high-proportion of microbial sequences akin to "Nitrosopumilus maritimus", a mesophilic Thaumarchaeota strain capable of chemoautotrophic growth on hydrothermal ammonia and CO2. Our findings highlight that acidic shallow-submarine hydrothermal vents nourish marine ecosystems in which nitrifying Archaea are important and suggest ferrihydrite-type Fe3+-(hydrated)-oxyhydroxides in associated low-temperature iron mats are formed by anaerobic Fe2+-oxidation, dependent on microbially produced nitrate.

Debates on the formation of banded iron formations in ancient ferruginous oceans are dominated by a dichotomy between abiotic and biotic iron cycling. This is fuelled by difficulties in unravelling the exact processes involved in their formation. Here we provide fossil environmental evidence for anoxygenic photoferrotrophic deposition of analogue banded iron rocks in shallow marine waters associated with an Early Quaternary hydrothermal vent field on Milos Island, Greece. Trace metal, major and rare earth elemental compositions suggest that the deposited rocks closely resemble banded iron formations of Precambrian origin. Well-preserved microbial fossils in combination with chemical data imply that band formation was linked to periodic massive encrustation of anoxygenic phototrophic biofilms by iron oxyhydroxide alternating with abiotic silica precipitation. The data implicate cyclic anoxygenic photoferrotrophy and their fossilization mechanisms in the construction of microskeletal fabrics that result in the formation of characteristic banded iron formation bands of varying silica and iron oxide ratios.

(207 words) 11The paradigm for low-sulphidation (LS) volcanic-arc associated mineralization is the active 12 geothermal systems located along the Taupo Volcanic Zone (e.g. Broadlands). However, this 13 analogue is inapt where fluid salinities are consistently in excess of 3.5 wt % NaCl. 14 LS mineralization on Milos (Aegean arc) records high paleofluid-salinities. The δD and δ 18 O 15 data do not exemplify 18 O-shifted meteoric waters-typical of terrestrial geothermal systems. 16Nor is a submarine origin indicated-stable isotope data show mixing between meteoric, 17 seawater and volcanic-arc gases. Strontium isotope data are comparable to a nearby active 18 seawater-entrained geothermal system. These are features seen in hydrothermal systems 19 associated with emergent volcanoes. 20For the Milos LS mineralization, high-salinity fluids show it cannot be explained by a 21 Broadlands-type model. The absence of saliferous sequences and significant intrusive rocks 22 preclude these as salinity sources. The similarities between paleo and active systems in terms 23 of salinity, δD-δ 18 O and strontium isotope systematics strongly suggest that seawater is the 24 main source for Na and Cl. We suggest geothermal systems, containing seawater, associated 25 with emergent volcanoes are an alternative analogue for LS epithermal mineralization. 26 Furthermore, they bridge the gap between submarine, and large-scale terrestrial geothermal 27 systems-the modern analogues for VHMS and epithermal mineralisation in the scheme of 28 intrusion-centered hydrothermal mineralization. 29

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