The deposition of iron-rich sediments in the dolomite of the Transvaal Supergroup some 2,300 million years ago was not a random process. Iron-formation precursor can be shown to be an end member in a series of well-defined chemical and sedimentary cycles. The stratigraphic column records a repeated chemical conditioning of the depositional environment, which resulted in the deposition of iron-rich carbonates, iron-formation precursor, and of banded iron-formation. Chemical conditioning can usually be related to carbonate depositional structures suggestive of the presence of shoals in the basin. It is envisaged that these banks or shoals of carbonate detritus caused a partial restriction between the iron-formation environment and the open ocean. The abnormal concentrations of iron, silicon, and other elements which developed in these restricted basins resulted in the precipitation of successively more iron-rich sediments and chert and culminated in deposition of banded iron-formation.