DIAMONDS IN AFRICA
It should be noted an important post-Permian basic volcanism of dolérites with pigeonite.
"One must insist, especially says REIS, on the narrow relation, in the northeast (Lunda) between the deposit of the diamantiferous conglomerates and a major erosion cycle. This one, very slow, has been interrupted by movements in two phases, Carbonaceous and post-Triassic, associated with the formation of tectonic trenches". The tectonic fracturing was accompanied, at the end, by a volcanic episode.
The survey of gravels, heavy minerals and diamonds had made evident that, in some zones at least, existed some primary kimberlitic sources bound to this volcanism, and controlled by intense fracturing. The first kimberlite was found in 1952 on a small affluent of the River Chicapa by the prospector A. dos Santos CHAMPLON. Thereafter the discoveries multiplied in Lunda, and then in many other regions, as result of the growth of search for primary deposits whereas before the alluvial were especially prospected.
The kimberlites of Angola
The most prominent feature of geological structure in Angola is constituted by a structural zone of weakness crossing the entire country on more of 1,200 km from Lunda to the SW following a direction of 50 to 53°, to which are bound of the alkali ultrabasic intrusions going from kimberlites to carbonatites. On this same remarkable lineation are the pipes of Bakwanga in Congo, thus, its length passes 1,600 km.. The passage of this zone of fractures major NE-SW is complicated by accidents more or less orthogonal, marked in Lunda by the "Trench of Lucapa" that played an important role in the discovery of the first kimberlites. Toward the SW, on both sides of Nova Lisboa, already a number of alkali complexes and carbonatites were known. F. DE VRIES LAPlDO-LOUREIRO (1958) mentioned 18 of them, some on secondary alignments of 25° and 135°. The age of these intrusions is cretaceous: 112 MY in Chivira in a granite of 1,734 MY (F. MENDES). In Namibia in the Damaraland, 700 km more to the south, one recovers a "parallel alkali chain". The carbonatites are obvious or masked under silicified breccia or fenitised rocks (2).
F. REIS distinguishes 7 distinct provinces:
Besides some isolated occurrences are known in the east of the country. In all 94 pipes were known in 1971, a lot more in 1973.
The pipes generally appear in groups according to the network of local fractures (the dykes seems rare). They are of variable shape and dimension, some very big (up to 90 ha), and all varieties of kimberlites and breccias are represented, including the tufa types. To note that these last have been found at 150 m. depth in 2 bodies under a consolidated effusive-sedimentary material. This curious setting up mode, possibly due to a local subsidence, is known in Brazil east of Minas Geraes, also Mwadui and AK1 in Botswana. Some pipes are mineralized, sometimes having an economic grade.
I. THE FIELD OF THE NORTHEAST LUNDA
It presents a particular interest because it is the best known and linked to the exploited cretaceous and alluvial deposits. It consists of 41 pipes situated between Long. 7° 20' to 9° 30' E and Lat. 20° to 21° 30' S., and is distributed in 4 groups: 12 in Camutué, 19 in Calonda, 8 in Camatchias and 14 in Lôva. Generally, the biggest pipes are better mineralized - and their sizes appear relatively big.
This region, rising toward the WSW from 760 to 980 m. of altitude, is drained by a set of rivers flowing from south-north toward the Kasaï: Luembe, Luana, Chiumbe, Luachimo (Longatschimo) and Chicapa Rivers (from E to W) - the kimberlites not known on the first two most western. It is possible that the top of the pipes has been eroded and covered by Calonda sandstone then masked by of Kalahari sands.
It is marked by an important diastrophism with a NNE fractures, with the Trench of Cangoa (oldest), and ENE fractures that gave the Lucapa Graben baptized by DELVILLE (LUana-chiCAPA) that is marked by more or less aligned rests of Luana and Karoo Formations, on the Precambrian base of granite - gneiss (2,480 MY) that REIS says have been rejuvenated. This fracturing created a real grid. The pipes would especially be localized on secondary fractures with alignments of 350-355° or on 85-90° inflections the big ENE faults. One already finds elsewhere these privileged directions signaled in Africa, that mark here the contact of elevated and lowered zones. This tectonic control is very remarkable, and served, at the origin, in search for the kimberlites. In addition, there were basic intrusions of dolérites, gabbros... and even probably little older nepheline syenites. Such an association of the basic intrusions and kimberlites is classic, but here, as in Bakwanga, they are superimposed spatially, what facilitated the prospecting but antagonized the search with magnetometer.
Field of East - Lunda
This big diamantiferous field, the most oriental, presents a particular interest because there have been found the first and probably the richest kimberlites bound to trenches, from where derive cretaceous alluviums (conglomerate of Calonda) and present the biggest part of the diamond production of Angola.
The region forms a big sandy plateau rising toward the WSW of 760 to 980 m of altitude cut by the deep parallel valleys of the Luembé, Luana, Chiumbé, Luachimo (Longtatschimo) and Chicapa Rivers, all flowing south-north toward the Kasaï. The kimberlites are known currently only close to the two last rivers, the most western. The crystalline basement is very old, showing ages near of 2,500 MY, and locally maybe more, with however some rejuvenations to 1,700-2,000 MY, rarely 1,000 MY. Ii is part of a big Archean strip, in part charnockitic, that crosses east-west, from the heads of the Bushimaïes and Lubilashes River to Congo, and that one recovers on the Cuango River close to the border... and more to the west, N. of Salazar, near the coast, after a region covered with more recent formations. The seniority of these formations makes a preferential zone for the kimberlitic intrusions. Above this base one finds the Luana Formation of the System of the Western Congo and a little Karoo (Dwyka) that appear only in the trenches.
(2) Curious observation: in the adjustment Africa-Brazil, the extension of this extraordinary lineation passes a little to the SE of Tibagi in the old basement of Parana where should be the origin of the detrital diamonds of this deposit.
Rafal Swiecki, geological engineer email contact
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