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.