Iron has been mined from a variety of kinds of deposits, but production is now almost entirely from two types: banded iron-formations and oolitic ironstones. Banded iron-formations are rocks of mostly Precambrian age that are composed of interlaminated quartz and iron minerals. They can be subdivided into two varieties (Gross 1980): Algoma deposits, which are relatively small with an obvious volcanic association, and Lake Superior, which are much larger and have a shallow-shelf, orthoquartzite-carbonate association. The Algoma type is abundant in the Archean, but an Ordovician example is found at Bathurst, New Brunswick (see Chapter 8), and there is a possible analogue associated with the Carboniferous Pb-Zn deposit at Tynagh, Ireland (see Chapter 7). Lake Superior-type ores, by contrast, are confined to a particular time interval at around 2 billion years before the present. Oolitic ironstones have a more clastic association than the iron-formations, and are higher in Al. Instead of banding, their most prominent sedimentary structure is ooliths made up of hematite or chamosite. Chert is rare, and they are found in rocks of a variety of ages from Proterozoic to Pliocene.
KeywordsBlack Shale Iron Mineral Iron Silicate Vertical Sequence Detrital Clay
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