DIAMONDS IN AFRICA
Based on the information provided by geologist Marcel Guillaume Bardet.
The Kasaï deposits of the region of Tshikapa in Congo continue far southwards in the NE territory of Angola named Lunda. Lunda alluvial deposits were exploited industrially since 50 years provided an important production of beautiful stones. To these can be added kimberlitic deposits, unknown in Tshikapa, recently discovered. Numerous other kimberlitic occurrences have been signaled in other regions of Angola. Notably in the center, north of Nova Lisboa, and the perspectives of this country concerning diamond are very encouraging.
According to B. REIS, to whom we owe the most recent clarifications (1972 and 71), the diamond have been known in Angola since the end of the 16th century, but its true discovery date of 1912, and it is in 1917 that was created the Companhia de Diamontes de Angola, the "Diamang", that monopolized the production. This production was solely from alluvial deposits (and sometimes from eluvial) drifting from a cretaceous sandstone formation, known as Calonda. These deposits look a lot like those of Tshikapa but they have many advantages.
First, the alluvial deposits seems even more extended and are better suitable to massive exploitations, besides one can treat some detrital deposits in place with acceptable grade (0.7 ct/m3) under overburden that is not always prohibitive as in the Kasaï. On the other hand the diamonds are bigger (5 p/ct) and of a better quality; in 1965 the percentage of gems reached the high number of 76.75% either 3 times more that in the Kasaï. These conditions permitted to exploit a grade two times weaker, about 0.4 ct/m3. The exploration, always very active, (43 teams at the end of 1967) had a tendency to find the reserves of gravel comparable to the volume extracted in the year.
In 1952, these favorable perspectives have been reinforced by the discovery of kimberlites in the District of Lunda of which some were economic. Since then more kimberlitic bodies were found.
For example, an old map made by STUTZER mentioned some close to Bimbe on the high central plateau. But the DIAMYNG that concentrated most of its activity in Lunda, the extreme northeast on the country, kept a lot of discretion about these other occurrences, and, except on the inshore strip, had insured the monopoly of the diamond in Angola. On the other hand, the government participated extensively in the earnings (40% then 50% since 1957) and benefited important loans at 2%. The DIAMYNG was one of the most important businesses of Portugal, and was owned between many shareholders. With its creation and development was associate the name of the Commander OF VILHENA. In May 14, 1971 the exclusive contract of the DIAMYNG arrived to its term and the rights of exploration in this immense concession were devolved to a Syndicate uniting the DIAMYNG and the DE BEERS. The exploration, that had started in 1970 by an important airborne geophysical campaign, are followed since more actively on the ground since the zones opened to prospecting progressively will be restricted, passing from about 800,000 km2 to 30,000 km2 only in 1977. The requirements by the Angolan government are stern enough and aim at a fast valorization of the possible discoveries. In the history of the diamond mines, Angola should be one of the stars during the next years.
Geological sketch of Angola
Imperfectly known in spite of a big recent progress, the geology of Angola is relatively complex, the formations being distributed parallel to the coast, NNW to the north, NNE to the south.
1° The Crystalline Basement; A big part of the country is formed by the Precambrian basement of the "Basis Complex", with twi very big batholiths of granite to the N. and to the S. of the Cuanza. The extreme NE is a part of the very old Kasaï Shield (2,500 MY or more); a zone theoretically favorable to the kimberlitic intrusions. In the whole remaining country we owe to F. MENDES numerous determinations of the age by atomic decay of Sr that show a big spatial variability and a picture of isochrones in "skin of leopard", again badly interpreted (F. MENDES C.R. Ac. Sc. Paris t. 258 1964, 1. 262 1966 and comm. pers. 1972). One sees rejuvenated zones surrounding small very old cores (2,692 MY to the S. of Nova Lisboa etc.), what is not without recalling what one observe in some regions of the central Brazil, for example close to Cérés (south Goïas), in the east of Minas Geraes (1).
2° The Oendolongo System (R. FURON): In this basement one can distinguish, especially to the south of the Cuanza, the Precambrian sedimentary formations that would correspond to the Mayumbe-Sansikwa System of the Low Congo (1,100 -1,950 MY) and would prolong it far southwards.
3° The Bembé System: These formations are equivalent of the System of Western Congo (Schists-limestone inferior, Schists-quartzites superior), of 450 to 750 MY, and draw a strip in NNW direction, crossing the NW of Angola toward Low Congo. One recovers only the tillite of base and schists-limestone in the south close to Namibia, and the equivalent of the Roan and the Kundelungu Systems to the extreme east on the High Zambezi. There is also some close to the coast.
4° System Karoo: a) The Set of Lutoé, Carbonaceous superior = Tillite of Dwyka) b) The Set of Cassanje, Permian - Triassic c) The Set of Lunda, superior Triassic which appears in ditches above the Formation of Luana that would be Precambrian and corresponds to the Schists-sandstone summit of the System of Western Congo.
5° The Formation of Calonda (Cretaceous) - It is the one that acts as reservoir to the diamonds, as in the Kasaï, and is assimilated in the Kwango. The diamantiferous conglomerate of the lower Kwango has sometimes of desert character with boulders very big and of the elements of the basement little displaced. It is surmounted by 30 to 50 m. of the purple sandstone with cross stratification, the beds of red argillites and other layers of sandstone and conglomerates that could take diamonds, etc.. (F. REAL, 1959 and DE ANDRADE 1953).
6° The Formation of the Kalahari (Tertiary): Polymorphous sandstone (Oligocene - Pleistocene) and sandy ocher silts. In the lower or middle Kalahari, under an arid climate, the materials deposited themselves without any order, in a discontinuous and irregular way along "dry rivers" at random of the flash floods, and the course of these temporary torrents would be without report with the structural lines etc., at the difference of present rivers that redistributed these materials and the diamonds.
7° The Cretaceous and Tertiary shores: All along the coast one finds a narrow fringe of Cretaceous, in part continental lagoons, but especially marine and fossilized, with a complete succession from the Aptien to the Campanien-Maestrichien. The Tertiary is represented from the Paleocene to the lower Miocene (Burdigalien).
8° The Marine Quaternary, represented by beaches raised from 5 to 135 m. that have the possibility, by their resemblance with those of Namibia, to contain the diamond. It is necessary to note that the whole oriental half of the country is covered by recent formations, and in particular by the sands of Kalahari that constitute a very bothersome mask for the prospecting, although of little thickness. Yet, curiously, sometimes one would recover, in the superficial cover, the traces of specific minerals of underlying kimberlites.
(1) A recent interpretation of the geological map of Arabia makes appear similar structures.
Rafal Swiecki, geological engineer email contact
This document is in the public domain.