Abstract
Understanding microbial mediation in sediment-hosted Mn deposition has gained importance in low-temperature ore genesis research. Here we report Mn oxide ores dominated by todorokite, vernadite, hollandite, and manjiroite, which cement Quaternary microbially induced sedimentary structures (MISS) developed along bedding planes of shallow-marine to tidal-flat volcaniclastic sandstones/sandy tuffs, Cape Vani paleo-hydrothermal vent field, Milos, Greece. This work aims to decipher the link between biological Mn oxide formation, low-T hydrothermalism, and, growth and preservation of Mn-bearing MISS (MnMISS). Geobiological processes, identified by microtexture petrography, scanning and transmission electron microscopy, lipid biomarkers, bulk- and lipid-specific δ13Corganic composition, and field data, and, low-temperature hydrothermal venting of aqueous Mn2+ in sunlit shallow waters, cooperatively enabled microbially-mediated Mn (II) oxidation and biomineralization. The MnMISS biomarker content and δ13Corg signatures strongly resemble those of modern Mn-rich hydrothermal sediments, Milos coast. Biogenic and syngenetic Mn oxide precipitation established by electron paramagnetic resonance (EPR) spectroscopy and petrography, combined with hydrothermal fluid flow-induced pre-burial curing/diagenesis, may account for today’s crystalline Mn oxide resource. Our data suggests that MISS are not unique to cyanobacteria mats. Furthermore, microbial mats inhabited by aerobic methanotrophs may have contributed significantly to the formation of the MnMISS, thus widening the spectrum of environments responsible for marine Mn biometallogenesis.
Highlights
The link between geobiology and economic geology, i.e., the role of microbial activity on the formation of mineral ore deposits, is a relatively new topic [1,2,3]
The data suggest that unlike the patchy appearance of the contemporary mats, resulting from weak and diffuse hydrothermal activity, widespread mats on the Early Quaternary seafloor precipitated Mn released into seawater as a result of intense volcanic hydrothermal activity
The Mn-bearing MISS (MnMISS) have been developed along bedding planes of Upper Pliocene to Lower Pleistocene shallow-marine/tidal-flat volcaniclastic sandstones/sandy tuffs closely associated with the Cape
Summary
The link between geobiology and economic geology, i.e., the role of microbial activity on the formation of mineral ore deposits, is a relatively new topic [1,2,3]. One of the best examples is the microbial precipitation of Mn minerals and the formation of economic sediment-hosted Mn ore deposits [2,4,5,6]. The accepted pathway of concentration of Mn minerals in the marine sedimentary rock record involves direct processing by diverse Mn(II)-oxidising and Mn(III/IV)-reducing bacteria and fungi [4,5,7,14,15], or by the, indirect activities of microbial processing by phototrophic green algae, diatoms, and cyanobacteria [16].
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