Microbial Mat–Related Structures in the Quaternary Cape Vani Manganese-Oxide (-Barite) Deposit, NW Milos Island, Greece

Abstract

Microbial mat–related sedimentary structures are present in Lower Pleistocene mixed epiclastic-volcaniclastic sediments that host the Cape Vani manganese-oxide (-barite) deposit on NW Milos Island, Greece. Milos Island is a dormant and recently emergent 2 Ma volcano of the active Southern Aegean volcanic arc. The deposit occurs in a 1-km-long marine rift basin floored by a dacite dome. Basin fill is a .60-m-thick sequence of epiclastic glauconite-bearing sediments sandwiched between lower and upper mixed volcaniclastic sandy tuffs and epiclastic sandstones. Host siliciclastics consist of glass shards, lithic fragments, plagioclase, K-feldspar, biotite, pyroxene, and silica and clay cements, overprinted by a barite– silica–K-feldspar–illite assemblage. Manganese (IV)–oxide minerals include dominantly d-MnO2 (vernadite), hollandite group minerals, pyrolusite, ramsdellite, and nanocrystalline todorokite. Microbially induced structures occur in a specific lithofacies referred to as upper ‘‘ferruginous and white volcaniclastic sandy tuffs/sandstones’’ and are characterized by: (1) planar and herringbone cross-bedding, (2) small-scale, vertical fining-upward sequences, (3) flaser, wavy, and lenticular bedding, (4) marine trace fossils similar to Skolithos, and (5) beveling of ripple marks and desiccated silicified mudstone beds. These features, together with the microbially induced structures and the widespread presence of glauconite, reflect a littoral to tidal-flat paleoenvironment. The microbial mat–related sedimentary structures developed in the Mn-oxide ore formation are recognized as: (1) mat-layer structures, (2) growth bedding structures and nodules, (3) wrinkle structures and exfoliating sand laminae, (4) cracks with upturned and curled margins, (5) roll-up structures, (6) fossil gas domes, (7) mat fragments and chips, and (8) mat slump structures, suggesting photoautotrophic, possibly cyanobacterial, mats. The ubiquitous presence of barite, in the host sediments, in the mat-related structures, in feeder-vein and bedding conformable layers, and in the gravel unit that caps the Cape Vani sedimentary rocks, suggests that microbial mats were developed in association with white smokers acting as Mn(II) suppliers, in a sunlit shallow-water or tidal-flat paleogeothermal system. The intimate relationship of Mn(IV)-oxide ore mineralization with the microbial mat–related sedimentary structures, coupled with the presence of Mn mineralized microbial fossils in the ore, strongly suggests the possible role of bacterial photosynthesis in Mn(II) bio-oxidation and Mn(IV)-oxide biomineralization at Cape Vani. It is envisaged that most Mn(IV)oxide mineralization was synsedimentary and syngenetic and formed due to an interplay among shallow-marine/tidal-flat sedimentation, hydrothermal seafloor to subaerial hot spring activity, which provided Mn(II), and active, possibly photosynthetic, microbial activity. Chemotrophic influence on Mn(IV)-oxide biomineralization cannot be excluded.