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iron formation Iron-rich sedimentary rocks, mostly of Precambrian age, containing at least 15% iron. The iron occurs as an oxide, silicate, carbonate, or sulphide, deposited as laminated, deep-water, shelf-sea, and lagoonal sediments, often associated with cherts (see also BANDED IRON FORMATION). Other iron formations contain iron-rich ooids, pellets, and intraclasts, representing deposits comparable to shallow marine limestones. The source of the iron in iron formations is the subject of considerable debate; origins from volcanic sources, biochemical precipitation, and the diagenetic (see DIAGENESIS) replacement of limestones are among the suggestions that have been made.