Inorganic and biogenic As-sulfide precipitation at seafloor hydrothermal fields

We investigated As-sulfides (orpiment, As₂S₃ and realgar, As₄S₄) from four seafloor hydrothermal fields: three from back-arc and one from mid-ocean ridge settings. Our studies suggest two different modes of As-sulfide precipitation at the seafloor hydrothermal sites: inorganic and biogenic. The Eh-pH modeling shows that the most stable As-sulfide at low-temperature (T<100°C) seafloor hydrothermal conditions is orpiment, whereas realgar appears as a stable As-sulfide phase at T=25°C and under a narrow range of Eh-pH values. Inorganically precipitated realgar forms euhedral crystals whereas the orpiment is mostly colloform. The biogenic type of orpiment (no realgar of this type was found) represents completely mineralized fungal hyphae. The proposed scenario of biogenic orpiment formation assumes that crack-dwelling fungal filaments in the sub-seafloor were killed by an As-rich hydrothermal fluid moving along the same crack network. The fungal organic matter served as a geochemical trap for hydrothermal As which reacted with S (leached from the basement or reduced from seawater sulfate) and mineralized the fungal hyphae as As₂S₃. This process of fungal orpiment precipitation is a mechanism of bio-sequestration of hydrothermal As and another mode of soft-tissue fossilization not described so far.