Temperature of Moodies Group (3.2 Ga) fluid-escape structures: implications for microbial preservation

The mechanisms that preserved the microbial mats of the Paleoarchean Moodies Group sandstones (c. 3.22 Ga) remain unclear. These kerogenous, microbially laminated structures occur abundantly in medium- to coarse- grained tidal-flat sandstones. They are readily mappable along a 15 km-long ridge of silicified strata in the central Barberton Greenstone Belt, South Africa. Silicification is thought to be attributable to the Moodies-age Lomati River Sill (LRS), a large mafic laccolith, c. 1 km below. Tidal-flat sandstones overlying the LRS feature abundant, isolated or clustered fluid-escape structures (FES) up to 6 m high. µ-XRF-generated elemental maps of nine slabbed and polished FES samples show enrichment of Fe, Cr, Ti and V along their central conduits and margins. S-Fe maps show thick pyrite rinds overgrowing detrital grains, possibly attributable to sulfur-reducing bacterial processes, magma degassing or breakdown of minerals. We speculate that the FES represent discrete pathways of potentially hot and silica-rich fluids migrating within the halo of the LRS and erupting into sand volcanoes, in which sand grains, primarily of volcanic provenance, underwent local argillic alteration. Precompaction surficial silicification (or carbonatization) by mineralized fluids may have preserved composition and texture perfectly. Current challenges to our research include modifications by Archean aggressive weathering, regional greenschist-facies metamorphism, recent weathering, lack of recent analogs, and reset of the original isotopic systems, all of which blur detailed insights.