Brøgger Seminar Series – Extant and extinct shallow submarine hydrothermal geobiology laboratories and ore-forming systems in the Aegean Volcanic Arc, Greece

by Stephanos Kilias 
Faculty of Geology and Geoenvironment, National Kapodistrian University of Athens – Greece

When? Thursday 16 April, 13h15
Where? Nordenskiöldsalen, Geovetenskapens hus

 
Abstract:
Modern and fossil geothermal systems associated with shallow submarine and emergent arc-volcanoes are diachronic hot spots for cryptic release of magmatic volatiles and metals. They constitute sources of seawater acidity, energy donors for marine microbial communities and, analogues and examples for ore-forming systems that have produced some of the most valuable metal deposits mined to date; these attributes result from a complex and dynamic interplay between geothermal, metallogenic, biological and volcanic processes. Here we present multi- and inter-disciplinary data derived from such systems associated with shallow submarine (<500 m) (Kolumbo, Santorini caldera) and submarine-to-subaerial (Milos) components of the Hellenic (Aegean) Volcanic Arc (HVA), southern Aegean Sea, Greece. The HVA is a young <5 Ma-to-present subduction-related volcanic arc where volcanism and hydrothermal activity occurs through thinned continental crust; the HVA consists of onshore and offshore volcanic cones, i.e. Methana, Poros, Milos, Santorini/Thira, Kos and Nisyros, and, i.e. Pausanias ( W. Saronikos Gulf), Santorini caldera, Kolumbo and several smaller cones NE of Santorini and offshore Nisyros Island, respectively. Natural hydrothermal CO2 releases either mixes into the seawater column causing density-stratified acidic (pH~5) seawater for ~10 m above the crater floor at Kolumbo submarine volcano, or forms previously unknown shallow (230–250 mbsl) acidic (pH~5.9) seafloor pools with highly concentrated CO2 within the Santorini volcanic caldera. The active Kolumbo and fossil and active Milos hydrothermal systems, respectively, share common mineralization and biogeochemical processes. Actively forming mineralization in the form of polymetallic sulfide-sulfate spires at Kolumbo crater floor, is enriched in “epithermal” metals (Sb, Tl, Hg, As, Au, Ag, Zn), whereas at Milos island this style of mineralization has been uplifted and preserved intact providing on-land analogue of hybrid epithermal-to-volcanic-hosted-massive-sulfide mineralization. Milos hosts the first identified ~2.0-million-year-old biogenic fossiliferous sedimentary iron formation (IF) comparable to Precambrian banded iron formations (BIFs), deposited in a restricted shallow marine basin at the foot of an andesite continental shelf. Microbially linked N and Fe cycling in the Milos IF has been analogous to that of Kolumbo’s active system where hydrothermal NH4+ is microbially oxidized to nitrate and employed in chemical oxidation of Fe2+ to Fe3+ in seafloor Fe mats. Ore-grade Mn-Ba beds, associated with the Milos IF, typify Microbially Induced Sedimentary Structures (MISS) formed due to interaction of littoral sedimentation, white smokers and active photosynthetic and/or chemotrophic microbial activity. The implications of the above for understanding ocean acidification and CO2 leakage and benthic accumulations from subsea carbon capture and storage sites, Fe- and Mn-biomineralization, submarine metallogenesis, volcanic hazard preparedness, and submerged metal and critical raw material resource potential are discussed.

Co-sponsored by the:
Bolin Centre for Climate Research
A cooperation between Stockholm University, SMHI and KTH 

 

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