Stable sulphur and oxygen isotopes as indicators of sulphide oxidation reaction pathways and historical environmental conditions in a Cu–W–F skarn tailings piles, south-central Sweden

Magnus Ivarsson 1,2, Stephanos P. Kilias 3, Curt Broman 4, Anna Neubeck 5, Henrik Drake 6 , Ernest Chi Fru 7 , Stefan Bengtson 2, Jonathan Naden 8, Kleopatra Detsi 3 and Martin J. Whitehouse 9

1 Department of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense, Denmark
2 Department of Paleobiology, Swedish Museum of Natural History, Box 50007, 10405 Stockholm, Sweden
3 Department of Economic Geology and Geochemistry, Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens, Panepistimiopolis, Zographou, 15784 Athens, Greece
4 Department of Geological Sciences, Stockholm University, SE-10691 Stockholm, Sweden
5 Department of Earth Sciences, Uppsala University, SE-75236 Uppsala, Sweden
6 Department of Biology and Environmental Science, Linnæus University, 392 31 Kalmar, Sweden
7 School of Earth and Ocean Sciences, Institute for Geobiology and Geochemistry, Cardiff University, Park Place, Cardiff CF10 3AT, UK
8 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
9 Department of Geosciences, Swedish Museum of Natural History, Box 50007, SE10405 Stockholm, Sweden

Abstract
The production of H2 in hydrothermal systems and subsurface settings is almost exclusively assumed a result of abiotic processes, particularly serpentinization of ultramafic rocks. The origin of H2 in environments not hosted in ultramafic rocks is, as a rule, unjustifiably linked to abiotic processes. Additionally, multiple microbiological processes among both prokaryotes and eukaryotes are known to involve H2-production, of which anaerobic fungi have been put forward as a potential source of H2 in subsurface environments, which is still unconfirmed. Here, we report fungal remains exceptionally preserved as fluid inclusions in hydrothermal quartz from feeder quartz-barite veins from the Cape Vani Fe-Ba-Mn ore on the Greek island of Milos. The inclusions possess filamentous or near-spheroidal morphologies interpreted as remains of fungal hyphae and spores, respectively. They were characterized by microthermometry, Raman spectroscopy, and staining of exposed inclusions with WGA-FITC under fluorescence microscopy. The spheroidal aqueous inclusions interpreted as fungal spores are unique by their coating of Mn-oxide birnessite, and gas phase H2. A biological origin of the H2 resulting from anaerobic fungal respiration is suggested. We propose that biologically produced H2 by micro-eukaryotes is an unrecognized source of H2 in hydrothermal systems that may support communities of H2-dependent prokaryotes

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