MAJOR EPOCHS OF PRECAMBRIAN FOLDING OF THE AFRICAN PLATFORM AND THE MAIN FEATURES OF THEIR METALLOGENY

V. I. Bazhenov, V. Ya. Medvedev

(Presented by the scientific seminar of the MPI department)

Among the numerous problems of the geology of the African Platform, one of the principal and, perhaps, least studied is the question of its tectonic structure. The solution of this problem is of great practical importance both for clarifying the nature of the geological history of the African Platform in the Precambrian and for establishing the spatial distribution of mineral deposits and their genetic connection with the enclosing structure. The latter is extremely important for assessing the prospects of prospecting work planned in various parts of the African continent.

The absence of regional schemes for the stratigraphy and tectonics of the African continent, which is explained by the recent colonial fragmentation of Africa, greatly hindered the generalization of the factual material accumulated by the middle of the twentieth century.

The first tectonic schemes of the African Platform, which laid the foundations of modern ideas about the structure of this ancient structure of the globe, were developed in the late 1950s and early 1960s by Z. Krenkel, R. Furon, A. Holmes, and others. In recent years a number of major works have been published that summarize the results of many years of research in the fields of stratigraphy, tectonics, and mineral resources of the African continent (M. V. Muratov, V. E. Fine, Yu. A. Shubert).

Among tectonic syntheses, it is necessary to mention the scheme of M. V. Muratov [4], who distinguished within the basement of the African Platform three large structural elements, different in the time of their consolidation, namely: the North African massif (1.8–2.5 billion years), the South African massif (2.5 billion years), and the Katanga–Arabian belt, with a basement age of 620 million years.

The present article gives a brief characterization of the main orogenic epochs of the Precambrian that occurred on the African continent, and of their influence on the spatial distribution and formation of endogenous deposits. The compilative nature of this work, on the one hand, and the incompleteness of the geological information, on the other, did not permit all individual propositions of this article to be argued with equal certainty. The conclusions presented below concerning the general sequence of major orogenic phases in the Precambrian geological history of the African Platform are based both on analysis of the spatial position of structural unconformity surfaces separating large metamorphic complexes and on

using numerous absolute-age data.

The principal evolutionary sequence of the major Precambrian structures of the African Platform indicates the leading role of mobile processes in its geological development. A characteristic feature of the latter is the unevenness of their manifestation at individual stages, which, in turn, led to a sliding of age boundaries and had a negative effect both on identifying the periodicity of tectogenesis and on correlating structural complexes developed in different parts of the continent.

Let us consider the factual material for individual regions of Africa. The most ancient part of the African Platform, the so-called South African massif, is situated south of the Zambezi River, encompassing the territory of Southern Rhodesia and the Republic of South Africa. The lower age limit of the metamorphic series of South Africa is older than 3500 million years. On the basis of numerous absolute-age data for the metamorphic sequences and their mutual relationships, it is possible, as a first approximation, to outline the succession of the principal folding epochs in the geological history of this territory \[2, 6\].

The metamorphic sequences of the basement of the South African massif, according to MacGregor’s data, are represented by three large metamorphic series. These are the Sebakwian, Bulawayan, and Shamvaian series. The lower, Sebakwian series, with an age of 3390 million years, is represented by ferruginous quartzites and magnesian rocks. The middle—Bulawayan—series, having an absolute age of 2850 million years, consists overwhelmingly of basic volcanic rocks, with a horizon of basal conglomerates at its base. The upper, Shamvaian series is represented by altered conglomerates with granite pebbles, sandstones (sometimes auriferous), and phyllites. The rocks of the named series form a single structure, although they are separated by unconformity surfaces. The age of the granites cutting the Shamvaian series is 2650 million years.

The main features of the structural plan and the absolute-age data for the metamorphic series considered above make it possible to speak of three most clearly manifested orogenic epochs, owing to which the basement of the southern African Platform was formed. The first, one of the most ancient, took place at the boundary of 3100–3000 million years, i.e., before the accumulation of the Bulawayan series. How widely this orogenic phase was distributed may be judged at least from the fact that folded systems of this age are known in Central and Northwestern Africa (the Western Nile, Dahomey, and Suggarian systems). The next orogeny is recorded at the boundary of 2900–2850 million years. It is possible that its manifestation is limited to the South African massif, since for the remaining part of the African Platform systems of similar age have not yet been established; those that show an age close to the indicated boundary, such as the crystalline schists of the Sula Mountains (Sierra Leone), the folded systems of the northern part of the Dodoma massif, apparently should be associated with an orogenic epoch having an age close to 3000 million years. The last of the three indicated orogenic phases—the post-Shamvaian—has an age of 2600–2500 million years. Unlike Western and Central Africa, where systems of analogous age are also known (the Atacorien, Simandou series, Kassai massif), for the south of the continent this folding epoch was the concluding one and ended with the complete consolidation of the basement of the South African massif.

All that has been said above makes it possible to regard the South African massif and the Central African shield as one of the most ancient structures of the African continent, sharply differing from structures formed—

...migrated in subsequent geosynclinal epochs both in the type of folding and in the character of magmatism. E. V. Pavlovsky proposed calling such structures protoplatforms, and the geological stage during which their formation took place the nuclear stage [5]. The differences were expressed not only in the type of folding and in the specific character of the magmatic complex of the early geological epoch of development of the Earth’s crust, but were also reflected, as will be shown below, in the character of the metallogeny of this epoch.

An analysis of the spatial distribution of endogenous deposits and of their genetic connection with magmatic complexes makes it possible to speak of the poverty in mineral resources of the indicated territories, if one does not take into account manifestations of endogenous mineralization genetically connected with younger magmatic activity of Proterozoic age, spatially superposed on the ancient Archean structures. The most characteristic type of ore deposits of Archean age are undoubtedly rare-metal pegmatites, genetically connected with the granite intrusions of the final stage in the formation of the South African massif. The absolute age of the pegmatites, determined from the lepidolite of the Bikita deposit (Southern Rhodesia), is 2850 million years [3]. The largest deposits are known in Southern Rhodesia and in the Republic of South Africa. Their characteristic feature is the predominance of lithium minerals, which constitute the principal industrial value of the pegmatite veins. Some quantities of beryl, tantalite, and pollucite are also recovered from them as by-products.

Concentrations of chromites in Selukwe, confined to the crystalline schists of the Bulawayan Series, formed as a result of deep regional metamorphism of ultrabasic rocks, are also of industrial importance.

A. Du Toit [2] also notes the presence of gold-ore deposits of vein and metasomatic type, containing silver, lead, and copper, genetically connected with ancient granites. The conclusion that the ancient granites are gold-bearing cannot be considered final, since there are indications [7] that the oldest gold-ore mineralization is connected with the younger post-Birimian granites.

Let us note certain characteristic features of the metallogeny of the platform stage of the South African massif, although this somewhat goes beyond the scope of the present article. This stage is characterized by repeated renewals of magmatic activity, which continued up to the Cenozoic. Without setting ourselves the task of considering all these stages, we shall dwell briefly on questions of the endogenous metallogeny only of the Precambrian epoch.

Of enormous importance for the South African massif, undoubtedly, is the metallogeny connected with the Bushveld magmatic complex. The emplacement of the enormous lopolith and its differentiation led to the formation of the world’s largest ore province, with a broad distribution of magmatic deposits of chromium, titanomagnetite, platinum, and also of liquation copper-nickel ores. Similar in composition and close in age to the Bushveld magmatic complex is the Great Dyke of Southern Rhodesia, extending for almost 500 km with a width of 4 to 10 km.

The largest deposits of chromites of the segregation type are connected with it. Somewhat earlier in age are concentrations of copper sulfide ores of the Namakwaland region, genetically connected with intrusions of basic composition. Finally, the youngest rocks of Upper Proterozoic or, possibly, Early Paleozoic age, developed in the cover of the South African massif, are...

young or Kaa granites, with which numerous occurrences of gold-ore mineralization are known (the Malmani and Pilgrims Rest deposits).

The formation of the structural pattern of the northwestern part of the African Platform, situated between the Gulf of Guinea and the Atlas, proceeded somewhat differently. Within this area, outcrops of the Precambrian basement are exposed along the coast of the Gulf of Guinea within the so-called Liberian, or otherwise Guinean–Liberian, Shield. In the north of the Sahara, metamorphic strata compose the Reguibat and Tuareg shields. The named structural elements are characterized by a similar structure and, what is especially important, by the same age of the final folding. Among the radiological measurements obtained for the rocks composing these structures, despite their sometimes contradictory values, three age categories are clearly distinguishable, apparently reflecting three major successive orogenic epochs.

The first, with an age of 3100–2900 million years, corresponds to the nuclear stage of the geological development of the African Platform. Within the Liberian Shield this is the Datomey metamorphic series, represented by gneisses, amphibolites, crystalline schists, and altered basic effusives. In the Reguibat and Tuareg shields, series of analogous composition and age are distinguished under the names Suttari and Amzaga.

The second group of rocks is represented by gneisses and ferruginous quartzites, with an age of 2700–2500 million years (the Simandou and Atacora series), the Man charnockites (Ivory Coast), and the crystalline schists of the Kassai massif (South Zaire). The absolute age of these series and their unconformable relationships with the ancient complex indicate that they belong to a later orogeny, which, apparently, should be identified with the terminal epoch of folding movements in South Africa. The third group of rocks is characterized by an absolute age of 2000–1790 million years. For northwestern Africa, the orogenic epoch of this age, which has received the name Post-Birimian in the literature, ended with the emplacement of large granitic intrusions that welded together individual blocks and finally fixed the stabilization of this part of the African Platform. In essence, the epoch under consideration is subdivided into two tectonomagmatic cycles: the Eburnean and the Mayombian.

The folded complexes of the Eburnean cycle fill narrow meridional troughs in the Archean basement. In the Liberian Shield these are the Birrimian and Marampa series. The most widespread and, perhaps, best-studied is the Birrimian series. Two parts are clearly distinguished within it: the lower, with a predominance of phyllites and metamorphosed sandstones, and the upper, composed mainly of greenstone rocks.

It may be considered established that the granites of western Africa (absolute age 2400–2200 million years), cutting the Simandou series, at least in the territory of Guinea have a cold contact with the Birrimian. The Birrimian shows cross-cutting relationships only with intrusions of the Early Proterozoic cycle. Concrete data on the upper age boundary of the Birrimian series are still lacking. As substantiation, data are cited for galena from quartz veins of the Ivory Coast that cut the Birrimian deposits (2000–1900 million years). The Marampa series, developed in the west of Sierra Leone, is represented by amphibolites and phyllites, sometimes containing commercial concentrations of iron.

Within the Reguibat and Tuareg shields, the upper structural-

the lower level of the platform basement is composed of chlorite-sericite schists and volcanic rocks, distinguished as the Farusi and Ietti series (1820–1730 million years) and assigned to the Mayombian tectonomagmatic cycle. This orogeny apparently should include the Ubendi–Ruzizi fold system, with an age of 2000–1800 million years, situated between the Dodoma Massif and Lake Tanganyika. Thus, the composition of the series considered, including their identical degree of regional metamorphism (greenschist facies) and identical structural position, confirmed by a comparatively narrow age interval (2000–1700 million years), indicates similar conditions of formation during a single geological epoch, which, incidentally, was reflected in a definite trend in the evolution of ore-forming processes.

The metallogenic features of the ancient complexes of West and Northwest Africa have as yet been studied insufficiently. This applies above all to the early, pre-Birrimian stages in the formation of these regions. Industrial ore deposits of this age are unknown here, unless one counts the rather widely distributed deposits of ferruginous quartzites, locally of industrial importance (Nimba, Fort-Gouraud, and others). This circumstance may be explained by the fact that a significant number of granite batholiths were formed by processes of palingensis, which had an unfavorable effect on the development of postmagmatic processes.

As for the later, Birrimian epoch, in comparison with the preceding stages in the formation of the basement of the African Platform, it proved to be considerably more productive with respect to ore deposits. Among them, vein gold deposits play a notable role, known within the Guinean–Liberian Shield (Siguiri, Banora in Guinea, Kogoro in Upper Volta, Obuasi in Ghana). A number of deposits of this type are known in the Republic of Zaire (Kilo-Moto, Kivu) and in Tanzania. In South Africa, according to R. Furon [11], gold mineralization, with the exception of the Witwatersrand, is also of Lower Proterozoic age, but here it is spatially superimposed on the more ancient structures of the South African proto-platform. Many of the noted gold deposits are of important industrial significance. The destruction of primary deposits gave rise to the formation of industrial alluvial placers (Gabon, Cameroon).

The question of the gold-bearing conglomerates of the Witwatersrand and Tarkwa deserves special attention. The former, as is known, are confined to the conglomerate horizons of the Witwatersrand system, which composes the ancient sedimentary cover of the South African Massif, whereas the latter belong to the Tarkwa system within the area of the Guinean–Liberian Shield, forming the base of the platform cover. Despite the different ages of the enclosing strata, the mineralization is distinguished by a remarkable similarity. However, the genesis of the mineralization remains debated to this day. A widespread view is that the gold-bearing conglomerates are ancient metamorphosed placers. R. Furon [11] believes that the source for the formation of such placers may have been gold-bearing quartz veins associated with post-Birrimian granites. If such an assumption is to some extent justified for Tarkwa, then for the Witwatersrand it appears unlikely, since, in the opinion of A. Du Toit [2], the known vein gold-ore mineralization there is genetically connected with the Bushveld magmatic complex. More acceptable is the hypothesis of a hydrothermal genesis for the gold mineralization, which C. A. Shcher [7] associates with the Bushveld complex.

The subsequent geological history of the development of the African continent is characterized by the predominance, over most of its territory, of platform conditions, signifying the cessation of magmatic and active tectonic activity. Only a narrow belt (the Katanga–Arabian belt) continued to retain the features of a mobile zone.

The process of final stabilization of the African Platform proceeded in two stages, which determined the presence of two folded systems of different ages in the tectonic structure of this zone. The structural elements of the first stage, developed in the southwestern part of the belt under consideration (Zaire, Tanzania, Zambia, Burundi) and in the Red Sea region, have a northwestern trend. In the southwest of the Katanga–Arabian belt this is the Kibara folded system (Zaire, Rwanda), represented by phyllites, quartzites, conglomerates and, more rarely, basic lavas, which are intruded by tin-bearing granites with ages of 1200–900 million years. The Urundi system (Tanzania, Uganda) is tectonically closely connected with the Kibara system. Developed in the northwestern part of Tanganyika and in Uganda, the Karagwe–Ankole folded system, composed of quartzites and conglomerates with an absolute age of 1150 million years, apparently represents the northwestern continuation of the Kibara–Urundi system. South of the Kibara system, directly along the northern margin of the South African massif, lies the Irumide system, composed of schists and quartzites intruded by Namaqualand granites with ages of 1170–980 million years. The absolute age of the folded systems considered is taken to be 1200–1100 million years, which also does not contradict their structural position between the Ubendian–Ruzizian system (2000–1800 million years) and the younger folded structures of Katanga–Damara, assigned to the second, younger stage of the Infracambrian (630–615 million years).

H. Schürmann’s indication of a Riphean age for the metamorphic sequences of the Red Sea region created favorable prerequisites for a possible tectonic connection of this region with the Mozambique zone, where Late Precambrian granites with an age of 600 million years have been identified [3, 4].

In terms of metallogeny, the Late Precambrian epoch proved to be exceptionally rich in useful minerals of various genetic types. Among them the chief place is undoubtedly occupied by the cupriferous sandstones of Zambia and Katanga (Zaire), which form the famous Copper Belt of Africa. Numerous deposits of this region are confined to rocks of the Katanga Group. In places, besides copper, the ores also contain cobalt, uranium, and nickel. The deposits are distinguished by enormous reserves. It is believed that they yield up to 20% of the world production of copper and contain about 2/3 of the world reserves of cobalt.

Rare-metal pegmatites genetically associated with granite intrusions are of great importance for the metallogeny of the folded zones of the Late Precambrian. Their absolute age varies from 1050 million years to 480 million years. They are known in the countries of Central and East Africa: Zaire, Uganda, Kenya, Mozambique, Somalia, the island of Madagascar, Rwanda, and the Republic of South Africa, where they serve as important sources of rare metals, the principal ones being beryllium, tantalum, tin, lithium, and niobium. Among the hydrothermal occurrences associated with magmatic activity of the Late Precambrian, mention should be made of small vein-type gold deposits in the ARE, located along the Red Sea coast, which were known and worked already in the time of the pharaohs; uranium mineralization in Gabon (the Mounana deposit) and Mozambique (the Mavuzi deposit). Important hydrothermal occurrences of this epoch also include lead-zinc-

Table 1

Correlation of fold systems of the African Platform

lead mineralization, known in the republics of Congo and Zaire. The ore mineralization is situated in the carbonate rocks of the Kundelungu series (Kipushi, M’Fuati deposits). Similar deposits are also known in Angola and in South-West Africa. Thus, the entire geological history of development of the basement of the African Platform represented an alternation of epochs of stabilization and activation. This alternation proceeded unevenly, in consequence of which each subsequent epoch differed from the preceding one not only in the character of activation of tectonic movements and magmatism, but, what is especially important, in its specific assemblage of useful minerals. Apparently, in the geological history of the African Platform the six orogenic phases noted above by no means exhaust the general geochronological list of Precambrian folding epochs in the development of this region. At the same time, only three of them, with which the stabilization of large blocks of the African Platform is connected, may be regarded as platform-forming (Table 1). The first (2700–2500 million years), the post-Limpopo epoch, caused the formation of the most ancient massifs of South and Central Africa. Fold systems of this age, formed at the boundary of the Archean and Proterozoic (2.5–2.6 billion years), are known on many continents of the globe. These are the Kuetin series in Canada, the Chara-Olekma massif on the Siberian Platform, and the Kalgoorlie and Pilbara in Australia. The second (2000–1750 million years) is the post-Eburnean epoch of stabilization (the northwestern part of the African Platform). Within the East European Platform, the Eburnean cycle corresponds to the Belomorides, while the structural complex of the Mayombian tectono-magmatic cycle apparently corresponds to the Upper Karelian.

The third orogenic phase, which welded the northern and southern parts of the continent into a single platform structure, is called Katangan in Africa. This orogenic epoch, corresponding to the Baikalides, was the most productive in terms of endogenous mineralization. A very important, favorable feature of this stage was, first, the presence of intrusive rocks and, second, the absence of a long hiatus between stabilization of the basement and the time of formation of the cover. It is precisely these factors that compel one to regard Late Precambrian orogeny as the most promising with respect to the search for diverse ore deposits in Africa.

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