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ANGOLA

Role of the trenches

This region is marked by an important diastrophism with two tectonic trench systems:
The 1st of NNW direction is post-Lutöe age (Dwyka) and affected the Inferior Karoo and evidently the previous formations.
The 2nd of 50-53° direction is more recent and post-Cassange (Permian - Triassic).
It has been baptized "Lucapa Graben" (of LUana-chiCAPA) by DELVILLE, and permitted the discovery of the kimberlites. It is long and narrow (130 km. X 9 km.) and constituted by a set of faults in echelon, and the whole draws a remarkable grid. The pipes are localized on crossings of secondary fractures with alignments at 350-355° and on 80-90° inflections of big ENE faults, and this tectonic control helped, at least in the beginning, to search for them.

Generally speaking, the kimberlites associated with trenches and often with the alkali complexes, don't have a good reputation since their mineralization is often poor. It is not the case in Lunda. To say truly, the role of these trenches was especially to preserve the top of the pipes in the lowered zone. The diamonds of the part eroded, relatively reduced, passed to the torrential cretaceous sediments of the Conglomerate of Calonda, of which one will see the essential role. On the other hand, the pipes localized in the elevated zones have been eroded more, and their diamonds dispersed themselves farther and didn't give as good concentrations as the ones in Calonda. They have been mobilized subsequently during Cenozoic and one recovers them in the "superior gravels" of the base of the Kalahari, more traveled, worn-out, and irregulars. These "high" pipes have been covered by the sands of the Kalahari following the Polymorphous Sandstone, and remain to be discovered under this more or less thick mask. The "low" pipes often don't show on the surface well and the quantity of diamonds that they send in the present drainage is rarely' in relation with their importance and their vast measurements. Most these come from the erosion of the Calonda. However, it seems that some detailed geological surveys permitted to localize the oriental extension of the Lucapa Graben, in a zone where precisely the Calonda is rich, but the kimberlites are still unknown.

The pipes

The main groupings of pipes are along the parallel valleys of the Luachimos River and Chicapas River distant of 30 km..

The Valley of the River Luachimo: The pipes are concentrated close to the Trench of Lucapa, the group of Sangamina being isolated more to the north. The main group of Camutué lies 5 km. along a big accident at 70-80°. Within 4 km. are gathered the pipes of: Sangombé, Camutué I and II, Caitondo, Muenza, Caixepa, Sachipito (between two big basic massifs) and Sacuango. The biggest, Camutué I, has 500m. X 200 m. in SE-NW elongation and is exploited. Hidden under 20 to 30 m of Kalahari sands, it only showed very little on the surface in a small valley of the Rio Camutué, and, without Calonda Intermediary, is covered by 100 m of kimberlitic sediments. Its grade is weak because the kimberlite has a lot of enclaves (often angular and from the basement etc.), but the diamonds are beautiful and unusually big - the stones of 80 carats are not rare. So, this pipe represents a "bonanza".

Ten kilometers to the north are the big pipe of Cariue of 400m. X 200 m., the big NNW axis is situated on an accident at 145° then bending to 160°. It signaled itself by a considerable concentration of heavy minerals, 100 to 250 kgs/m³. Finally, let mention the twin pipes, aligned NS, of Cambuaje I (550 m. X 250 m) and II (300 m. X 200 m.). The dimension of most these chimneys are impressive.

The Valley of the Chicapa: Three distinct groups space out themselves from the north to the south on 140 km between the cities of Verissimo Sarmiento and Henriques of Carvalho: Calonda, Camatchia and Lôva.

The Calonda group of 5 pipes is on an inflection at 90° to the big accident at 65-55°, at its intersection with a secondary NNW fault. A little to the west on a accident parallel to 170°, that follows the River Chicapa, one finds the extraordinary pipe of Camafuca-Camazambo, the biggest in the world, since it spreads its kimberlitic sediments in the valley on 3 km of long and on 1 km of large. These sediments are often so soiled with sandstone of the Klaroo that their true nature had not first been recognized and that one believed in the existence of two bodies. The pipe root itself would reach 1 km of diameter, and is installed on an accident putting in contact the basement and the Karoo. It is formed of a half of very dirty breccia and half of a clean basaltic kimberlite, with nodules of pyroxènite to garnet. The flats, on the Chicapa River, are big and well mineralized. Downstream, 8 km to the north, on the same accident at 350° is found the Cangoas pipe, with a dirty breccia but diamantiferous.

12 km to the south of the small mining city of Calonda one finds the Camuazanza pipe of (tufa and breccia con mica), Caindjamba in part breccia, the Caquele and Canzololo pipes of the effusive-sedimentary type and the Camaunbo pipe de tufa. Farther, at the confluent of the River Chicapa and the River Luo, is the Camatchia pipe that intrudes an E-W accident and reaches 600 m. X 500 m., with in part a very hard green bronze basaltic kimberlite or a charcoal gray kimberlite with ilmenite. Near enough is a Camagico pipe on a fault between Karoo and the basement, having 600 m of diameter, under a rich shred of Calonda Formation whose diamonds must come from another non identified pipe. The cover of kimberlitic sediments penetrates to 166 m of depth. Finally, in the most southern group close to H. of Carvalho, the big pipe of Lôve is rich and economically very important, although the diamonds are of less good quality that in Camutué. It is probable that a lot of other bodies remain to be found.

The conglomerate of Calonda and its role

The cretaceous basal conglomerate, said of Calonda, plays a very important role in Angola; the one of ore, representing a big exploitable reserve of diamond, and the one of indicator of the kimberlites.
It appears to correspond to what F. DE ANDRADE (1953) had described under the name of the "Inferior Formation" and that he believed to be of tertiary Kalahari and economically little interesting, at the difference of the more recent "Superior Formation" at the base of Kalahari that contains elements very rolled and classified. This last is a torrential alluvium that was deposited in a sub-desert climate under the effect of short and violent precipitations, creating a big display of the unclassified alluviums at the foot of steep mountains, as one observes it at the Sahara. This conglomerate contains fragments of the crystalline basement and of the Luana Formation (System of the Western Congo), some tender and more or less rolled, others angular, and sometimes of big boulders associated with small and very worn-out pebbles witnesses of transportation previous to the deposit.