Abstract

A definitive interpretation of the genesis of banded iron-formations has long been sought. The explanation presented here involves the interpretation of an active continental margin as the depositional environment of the Minas Supergroup in Brazil, with the results that the same environment could explain the origin of the Caue Itabirite, a well known iron-formation. A model is proposed to cover the deposition of banded iron-formations in this setting, and also, in that of a passive con tinental margin, believed to be the two most important sites of precipitation of such ores.In active margins, during the powerful process of subduction, the ocean lithosphere is cut by faults and fractures through which seawater gains access and hydrothermally leaches the brittle yet hot lower layers. The process is analogous to that currently being studied near midocean ridges when iron and silica, among other compounds, are driven by thermal convection to the sea floor. It occurs in present subductions as it occurred in past ones, but in early Precambrian time, both hydrosphere and atmosphere were anoxic: ferrous iron was soluble in seawater and the earth's surrounding ozone layer had not yet been formed. A plume of FeO-SiO 2 -rich seawater was concentrated above and along the trench, covering the nearby cratonic border. During the sunny hours of the day, iron was oxidized into ferric hydroxide before precipitation. Most banded iron-formations laid down in early Precambrian time were subducted with their ocean plates and the only fraction preserved was that deposited above cratonic borders. The break in iron-formation deposition at the end of that era was due to seawater and atmosphere oxygenation which produced the ozone-filtering layer around the earth and also caused extinction of marine anaerobic microorganisms. Banded iron-formations deposited in active or passive continental margins may assist in the interpretation of the paleoenvironment of the regions where they occur. Attempts have been made to classify some well-known Precambrian iron-formations based on this assumption.

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