AN IRON SHUTTLE FOR DEEP-WATER SILICA IN LATE ARCHEAN IRON FORMATION

Iron formations (IF) are typically thinly bedded or laminated sedimentary rocks containing 15% or more Fe and a large proportion of SiO₂ (often >50%). In the ca. 2550-2450 Ma Campbellrand-Kuruman Complex, Northern Cape Province, South Africa, IF occurs as a sediment-starved deep-water facies distal to carbonates and shales. The silica primarily occurs as diagenetic chert (now microcrystalline quartz) replacing IF minerals (often siderite). Mechanisms proposed to explain precipitation of the Fe component of IF include photosynthetic oxygen production, anoxygenic photosynthesis, abiotic photochemistry, and chemoautotrophy using Fe(II) as an electron donor. The origin and mechanism of silica precipitation in these deep-water deposits however, has received less attention. Here we present a conceptual model of iron formation that offers some insight into the deposition of silica. The model hinges on the proclivity of hydrous silica (especially polymeric species) to become adsorbed onto Fe-oxyhydroxides. Fe²⁺_(aq) is oxidized in the surface ocean to form insoluble Fe-oxyhydroxides. Fe-oxyhydroxides bind silica and sink from the surface ocean along with organic matter, shuttling silica to basinal waters and sediments. Fe respiration in the sediments returns the majority of Fe to the water column, but some precipitates as ¹³C-depleted siderite with respiration-derived HCO₃^-. Silica liberated during Fe reduction becomes concentrated in pore fluids and is ultimately precipitated as a diagenetic mineral phase. The complex diagenetic history inherent in this model may explain why Late Archean IF sediments tend not to preserve microfossils, despite the likelihood of microbial participation in the Fe cycle. Furthermore, this deep-water silica sink may have have important consequences for the nature, timing, and fossil preservation potential of chert in Late Archean tidal flat carbonates.