Early precambrian banded iron formations: Biochemical precipitates from highly evaporated hydrothermal solutions of polar region lakes

• Autores: Zeev Lewy
• Localización: Carbonates and Evaporites, ISSN 0891-2556, Vol. 24, Nº. 1, 2009, págs. 1-15
• Idioma: inglés
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• Resumen

  • Banded iron-formations (BIFs), which provide the world's main source of iron, accumulated during a restricted Lower Precambrian interval. Their alleged precipitation from mid-ocean hydrothermal ferruginous solutions mixing with down-slope flowing oxygenated water is not possible. No free oxygen could have accumulated in water under the prevailing anoxic conditions. Newly discovered sedimentary structures corroborate very shallow depositional settings. Accordingly, BIFs precipitated in huge lakes of warm hydrothermal solutions undergoing intensive evaporation and mineral concentration in the freezing-cold Polar Regions. As a result of half a year of illumination, cyanobacteria oxygenic photosynthesis deposited iron oxides with silica (geyserite) followed by a lamina of silica only, forming recurrent annual varves. Diamictites on top of BIF successions in Western Australia and South Africa accumulated from melting glaciers when the plates shifted to lower latitudes, as corroborated by paleomagnetic high-latitude paleo-positions. Calcium carbonate precipitated under higher water levels and more diluted solutions. The chemistry of these BliFs and associated carbonates do not represent the Early Precambrian oceans, and the diamictites at the end of their sequence do not attest to global glaciation.