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.