Ancient iron formations - chemogenic sedimentary rocks rich in iron and silica of Precambrian age contain information about environmental conditions in the Archean (4.0–2.5 Ga) and Proterozoic (2.5–0.539 Ga) eons. Despite disagreements about the mechanisms of their formation, iron formations are evidence of the influence of the Precambrian biosphere on the chemical composition of the ancient ocean. Since many iron ore formations are chemogenic deposits reflecting the composition of the waters from which they precipitated, they can also serve as detailed geochemical archives for studying the temperature of ancient seas, redox states and the cycle of chemical elements.