Bulk composition of Precambrian iron-formations is remarkably similar, despite differences in mineral facies. They differ from post-Precambrian iron formations in their low contents of alumina, titania, manganese, phosphorus, and lime. Uniformity of iron content, despite likelihood of variation due to sedimentary processes and depositional environment, suggests fundamental control at sources of materials. A model is proposed by which lateritic weathering in a reducing atmosphere yields solutions with Fe and Ca in approximately their 1:1 ratio of crustal abundance. These elements are deposited as banded calcite-siderite rocks. Different mineral facies are largely due to differing amounts of silica replacement, oxidation, etc., during diagenesis and metamorphism.