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Iron: Syngenetic Deposition Controlled by the Evolving Ocean-Atmosphere System

By
J. B. Maynard
J. B. Maynard
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Published:
January 01, 1991

Abstract

Iron is, after aluminum, the most abundant metallic element in the earth's crust, and is concentrated into ore grade accumulations in several sedimentary settings. These settings have changed with time in response to the evolution of the crust and of the ocean-atmosphere system. In the Archean, volcanic-associated deposits of the Algoma type dominate. The lower Proterozoic is marked by the appearance of the giant banded iron formations of the Lake Superior type, which are associated with carbonates and quartz-rich sandstones. The upper Proterozoic contains iron formations with a glacial association, referred to as Rapitan type. Finally, in the Phanerozoic we see the dominance of oolitic ironstones that are referred to either as Minette- type or Clinton-type deposits. To avoid confusion with the igneous rock minette, I will use the name Clinton to refer to these deposits.

Iron ore deposits with the characteristics of this type occur in rocks of a variety of ages, but are best developed in the Archean. The associated strata are volcanics and subordinate sandstones of graywacke type, a combination of lithologies that suggests deposition on the flanks of volcanic edifices with little sediment supplied by continental sources. The volcanic-sedimentary package in Archean examples follows a typical greenstone sequence. Ultramafic flows (komatiites) dominate the base of the section. These are followed upward by progressively more felsic volcanics, intercalated with varying thicknesses of graywacke turbidites derived from older volcanics. The iron formation can occur at various places in the section above the ultramafic unit, either in volcanics or

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Contents

Reviews in Economic Geology

Sedimentary and Diagenetic Mineral Deposits: A Basin Analysis Approach to Exploration

Eric R. Force
Eric R. Force
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J. James Eidel
J. James Eidel
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J. Barry Maynard
J. Barry Maynard
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Society of Economic Geologists
Volume
5
ISBN electronic:
9781629490120
Publication date:
January 01, 1991

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