A hot, hydrothermally-fed microbial tidal flat in the Paleoarchean Moodies Group, Barberton Greenstone Belt, South Africa?

Sandy alluvial-, deltaic-, and tidal-facies sediments of the Paleoarchean Moodies Group (ca. 3,220 Ma) are preserved several km thick in the central Barberton Greenstone Belt, interspersed with diverse units representing substantial mafic to intermediate (sub-)volcanism. Densely biolaminated sandstones feature common, up to 6 m high fluid-escape structures which fed small sand volcanoes during prolonged and/or recurring discharge of gases, liquids, and solids. The to-date highest documented concentration of fluid-escape structures occurs in a single, largely silicified unit of tidal-facies sandstones ca. 150 m thick, traceable along strike for ca. 14 km and located stratigraphically ca. 1 km above the Lomati River Sill, a 15 km long mafic sill of Moodies intrusive age. Fluid-escape conduits are filled by sand, sericitic clay and fine-grained organic matter. Semiquantitative XRF scanning of several slabbed fluid-escape structures indicates that conduits are enriched in Fe, Cr, Ti, and Mg in comparison to the mean composition of adjacent beds, suggesting that fluid-escape structures may not only have formed due to overpressure build-up from decaying microbial mats in the shallow subsurface but also resulting from release of hydrothermal fluids generated in a thermal aureole above the cooling sill. This inference is also supported by sediment textures characteristic of argillaceous and sericitic alteration, Raman temperatures ca. 50 - 100°C above the regional maximum metamorphic temperature of 320 - 370°C, and associated peperites nearby. Surficial hydrothermal activity in the tidal zone would have likely boosted microbial growth. Surficial pre-compaction carbonation and silicification greatly facilitated the preservation of delicate microbial mats.