The mean Mn/Fe ratio of 94 samples of Precambrian sedimentary Fe formations is 0.025 which is very close to the crustal average of 0.022 for this ratio. Post Precambrian ironstones, on the other hand, show a mean Mn/Fe ratio of 0.009 indicating a marked geochemical separation of these metals. The Mn/Fe ratios of Precambrian Fe formations exhibit a significant correlation with the degree of oxidation of the samples, whereas this correlation is absent in the younger ironstones. Ores formed on the Precambrian Fe formations in the Lake Superior region generally have abnormally low Mn/Fe ratios (0.009), as do the ores formed from the younger ironstones. The Mn/Fe ratio of a chemical sediment may be influenced by the source rocks, by selective dissolution at the source, by separation during transport, by the conditions of the depositional environment, and by diagenetic modifications. It is suggested that the present data and other chemical characteristics of Fe formations may best be accounted for by assuming a lower O content in the Precambrian atmosphere.