A reconnaissance study of six Precambrian cherty iron-formations revealed two dominant patterns of sedimentation. Three of the iron-formations consist largely or entirely of chemical mud (lutite), whereas chemical sands (arenite) are dominant in the other three. The sedimentary structures of the lutitic iron-formations (= classical banded iron-formation) indicate that they were deposited in basinal environments, whereas the arenitic iron-formations have sedimentary structures indicative of platformal sedimentation. Where contacts are conformable, the lutitic iron-formations are sandwiched between other basinal deposits, predominantly flysch sequences. In contrast, arenitic iron-formations are overlain by flysch-type units but underlain by high-energy shelf sandstones. When combined with mineralogical and textural data, these interpretations place certain constraints on the origins of iron-formation, including the following.
1. Iron-formations were deposited in marine environments that were highly variable but exclusively or largely subtidal.
2. Surface waters were sufficiently oxidizing to preclude their transporting any ferrous iron in true solution.
3. The sources of the chemical sediments that ultimately became iron-formation were located to basinward of the depositional environments.
4. Iron-formation deposition was superimposed on existing physical environments. Most iron-formations described in the literature appear to have characteristics that are compatible with these constraints, although some of the iron-formations of the Transvaal Basin of South Africa are notable exceptions. The model for the origin of iron-formations that is most easily reconciled with the sedimentological constraints proposed here is that iron-formations are primarily exhalative or hydrothermal in origin. This fact, combined with several independent lines of evidence, indicates that an exhalative origin far the large, early Proterozoic iron-formations is a hypothesis that deserves critical re-examination.