Precambrian iron-formations of Southern Africa

From existing information on the iron-formations of southern Africa the following conclusions are drawn: (1) The iron-formations occur in four different types of tectonic environment, namely greenstone belts, a metamorphic belt, four cratonic basins, and a mobile belt. (2) The iron-formations range in age from nearly the earliest geologic time recorded in southern Africa to Late Precambrian but show a strong tendency to be more prominently developed during the period prior to 2,000 m.y. (3) Based on their stratigraphic setting, the iron-formations can be classified as having volcanic, clastic, and chemical stratigraphic associations. The volcanic association is restricted to the Precambrian greenstone belts (> 2,700 m.y.), the clastic association dominates in the interval 2,300 m.y. to approximately 3,000 m.y., and the chemical association (shelf-carbonate and carbonate-manganese associations) is predominantly younger than 2,300 m.y. (4) The silica in the iron-formations of the volcanic association was derived from acid volcanism, whereas the iron in these deposits, as well as the iron and silica in most of the other associations, possibly originated from weathering of preexisting rocks. To account for this and to explain the restriction of iron-formation to early Precambrian depositional areas, an oxygen-deficient atmosphere must have existed. (5) The iron-formations are envisaged as having originated as gelatinous deposits, with the silica having been deposited as colloidal precipitates and the iron oxide minerals having been precipitated as crystallites from solution. Indications are that chemical deposition was aided by biological processes, that deposition took place in a marine environment, and that cold climates existed during deposition of certain of the iron-rich sedimentary rocks.