Regional Geology of Africa and Iron Ore Deposits

The African continent essentially consists of five ancient Precambrian cratons— Congo, West African, Kaapvaal, Zimbabwe, and Tanzania,—that were formed between about 3.6 and 2 billion years ago and that basically have been tectonically stable since that time; these cratons are bounded by younger fold belts formed between 2 billion and 300 million years ago. All these rocks have been extensively folded and metamorphosed. Precambrian rock outcrops appear on some 57 percent of the continent’s surface, while the rest of the surface consists of largely undeformed younger sediments and volcanic rocks.

The oldest rocks are of Archean age (about 4 to 2.5 billion years old) and are found in the so-called granite-gneiss-greenstone terrains of the Congo, West African, Kaapvaal and Zimbabwe cratons. They consist of gray, banded gneisses, various granitoids, and rather well-preserved volcanic rocks that show evidence of submarine extrusion and formation under high temperatures. The rock type komatiite is particularly diagnostic of these volcanic sequences and is almost exclusively restricted to the Archean Eon. The cratons were tectonically stabilized by voluminous granite intrusions toward the end of the Archean which contain most of the iron ore deposits and were then covered by clastic sediments, some of which contain economically important gold, copper and uranium deposits.

The Proterozoic is characterized by the formation of several mobile belts, which are long, narrow zones of strongly deformed and metamorphosed rocks that occur between the cratons and probably resulted from the collision between the cratons due to plate tectonic processes. The oldest mobile belts are found in Archean rocks, such as the Limpopo belt separating the Kaapvaal from the Zimbabwe craton. Younger belts were formed during a continent-wide thermotectonic event known as the Eburnian (2.2 to 1.8 billion years ago), which gave rise to the Birimian assemblage in western Africa, containing numerous deposits of iron ore, the Ubendian assemblage in east-central Africa, and large volumes of rocks in Angola. Still younger belts of the Kibaran thermotectonic event (1.2 billion to 950 million years ago) are found in eastern and Southern Africa.

The end of the Precambrian was marked by a major event of mobile-belt formation known as the Pan-African episode, which generated long fold belts, such as the Mozambique belt along the east coast of Africa, the Damara and Katanga belts extending from Namibia into the Democratic Republic of the Congo and Zambia, the West Congo belt between Angola and Gabon, the Dahomey-Ahaggar belt between Ghana and Algeria, and the Mauritanide belt from Senegal to Morocco.

A unique late Precambrian evolution is recorded in the so-called Arabian-Nubian Shield of northeastern Africa and Arabia. There, large volumes of volcanic and granitoid rocks were generated in an island-arc, marginal-basin setting—an environment similar to that of the present southwestern Pacific Ocean. Rocks were accreted onto the ancient African continent, the margin of which was then near the present Nile River, by subduction processes identical to those observed today.

The interiors of the ancient cratons were not affected by the above tectonic events, and intracratonic sedimentary and volcanic sequences accumulated in large basins. The most important of these are the Transvaal basin on the Kaapvaal craton that contains economically important iron ore deposits; the Congo basin; and the West African basin, with its thick late Proterozoic sediments including a prominent tillite horizon that marks a major glaciation event at the end of the Precambrian.

After the Precambrian, Africa’s geologic history is characterized by the following events: the formation of fold belts in the Paleozoic in South Africa (the Cape fold belt), Morocco (the Anti-Atlas belt), and Mauritania (the Mauritanide belt) bordering the older cratons; voluminous basaltic volcanism some 230 to 200 million years ago in South Africa, Namibia, and East Africa, known as the Karoo System, that was probably related to the beginning of the breakup of the Gondwana supercontinent; the formation of a young mountain belt in northwestern Africa some 100 to 40 million years ago as a result of collision between the African and European plates, together with the closure of the ancestral Mediterranean Sea (the Tethys Sea); and the development of the East African Rift System during the Cenozoic Era (i.e., roughly the past 65 million years), leading to the opening of the Red Sea, the northeast drift of the Arabian Plate, and the fracturing of the ancient crust of Africa along several long rift valleys, accompanied by extensive volcanism.