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The Geomorphology
The sandstone layers forms an immense peneplane with a light Northwest slope that is covered by the sands of Kalahari whose erosion gave the argil-sandy cover of today.

The country is very flat, the hydrographical network not dense with a few big rivers and small effluents spaced from 4 to 5 km and the very small creeks of less than 2 km.

The sides of valleys are convex, the flats of small rivers are not very wide but they can be mineralized in all their length and reach up to 30 km.

The big rivers are very large up to 300 m. wide with big discontinuous flats that can pass to terraces or gravel of under bank and the mineralization can spread up to 20 km.

The lack of eluvium seems to indicate that the mineralization of the alluviums would come directly from the erosion of the conglomerates of the Kwango. The distribution of the diamonds In the Kasaï there are detrital secondary deposits only, and one doesn't know the primary deposits. The alluviums that are exploited are derived from the diamantiferous conglomerates of the Kwango Formation. These conglomerates are discontinuous, irregulars, in lentils, and represent typical continental deposits.

One distinguishes:
The "A" type - a thin conglomerates in lentils superimposed with levels sandstones inserted on 20 to 30 m of thickness, resting directly on the sandstones of Lualaba Formation.
The "B" type - thick conglomerates in pockets in depressions with the facies:
B1 - of small to middle size elements
B2 - of more recent big angular blocks of the floor derived of the B1 type.

The conglomerates A and B1, that constitutes the bottom part named Lugundi-Luganda of the Kwango Formation (Cretaceous superior) correspond to a dry enough climate, and are dispersed fanlike from the south to the north, with their diamonds, on the post-Lulumba peneplane. In the higher zone that is close to the border of Angola, the thicker deposits are in northward controlled channels. Then they spread in thin lentils toward the NW and the NE.

The "A" type forms the lentils less than 1 m thick, of 0.20 to 0.30 m on average with cross, or tilted and discontinuous stratifications, that would correspond to a deposit in shallow waters of delta fluvial - lacustrine (Lugundi facies).

The "B1" type is constituted by the river gravel of small pebbles of quartz, chalcedony or agate in a sandy matrix, and would be deposited in deep channels on the Lualaba sandstones or the Precambrian floor (Luganda facies).

The more recent conglomerates of "B2" type belong to the Kabemba layer and would reveal an arid climate, with rare and powerful downpours, and "dry river" and "flash floods" deposits. Resting often directly in the hollows of the Precambrian floor, they contain big blocks of this floor and tender argillite. The elements are often angular and unsorted, what is typical of a deposition in "fanglomerates". The mineralization is distributed uniformly without a concentration of bigger diamonds at the base,.

These conglomerates, which come from an overhaul of the "B" type deposits, are more recent and richer than those of the "A" and "B1" types. They only exist in the Kasaï at the southern limit, near d Angola, and show directions of irregular transportation drawn by the luck of the rises in the water level and don't correspond necessarily to the lines of former drainage.

Broadly speaking the conglomerates descend toward the northwest on a plan whose slope has 1.6 m/km. But in the detail this plan is very irregular of a vast delta.

During Oligocene a period of silicification corresponds to a very arid climate. In the Miocene rains appear and with them the "dry-rivers" and their deposits of non-classified angular fragments, enriched in ferriferous elements and in diamonds pulled from the previous phase. The climate becomes then more and more humid.

The diamantiferous conglomerates are generally red purplish, sometimes black. The matrix is often sandy-feldspathic is very crumbly, especially in the pockets of the "B2" facies, but maybe locally better cemented by the iron oxide or the calcite. These are polymictic conglomerates where very hard pebbles neighbour with blocks or boulders of metastable rocks more easily decomposed.

The conglomerates are especially rich in fine minerals (1.09 to 3.04 mm). The oxides of iron dominate: magnetite, martite, hematite, limonite (frequency 9), staurolite (frequency 4), ilmenite (frequency 2).

The garnet (frequency 5) is a constant and a typical component of the diamantiferous conglomerates. Next to a red with crimson reflections pyrope garnet, one enough often finds the brown or pink to brown reflections almandine garnets. The proportion of the red pyrope garnet dominates (60% to 80%). It is characteristic, with the chromiferous diopside, of kimberlitic origin of these minerals.

The other minerals are not as symptomatic and come from the erosion of the crystalline floor: disthene (frequency 1), andalousite, tourmaline (frequency 2) and of the traces of: gleam, chrysoberyl, zircon epidote, apatite, and zoisite. Excessively rare are: corundum, amphibole, biotite, and pyrolusite.

Description of the deposits
The Forminière (La Société Internationale Forestière et Minière du Congo created in 1906 by the Belgian king Léopold II with American groups represented by Thomas Fortune Ryan) distinguished three types of deposits:

The creek deposits, that means the affluents of the big rivers of the region that flow South-North, on sandstones, and rarely cuts migmatites and green rocks of the Precambrian floor. Their width is from 10 to 50 m, and the thickness of gravel is from 0.5 to 1.0 m.

The flat and terraces deposits on the big rivers that are several hundreds of meters large with thickness of gravel that can reach 3 to 4 m.

The basal conglomerate of the Kwango Formation is diamantiferous but in Kasaï it is too sporadic, and on the other hand it is concealed by thick overburden. It is not the case in Angola where the rising of the Precambrian floor entailed the erosion of the superior levels of the Kwango Formation.

The gravel is formed of rolled quartz, of quartzites, and of a very big quantity of agates (up to 50%). The concentrate of the diamantiferous gravel contains: garnet, staurotide, magnetite more ilmenite, amphibole, black tourmaline, spinelle, disthene, epidote, gleams, corundum, pyrites, and of the traces of chrysoberyl and zircon.

The stones of the Kasaï are of good quality but small. About 65% of the stones are of jewellery quality and the average would be of 10 stones per karat but can reach 32 stones per karat. The stones of 5 to 20 karats are rare, and above 20 karats are exceptional. The octahedral shapes with plane faces dominate.

Diamonds from Kasai District of Congo

Diamond Geology [ 1  India  3  4  5  6  7  8  Brazil  10  11  12  13  14  15  16  17  18  19  20  Borneo  22   South Africa  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  Venezuela, Guyana  42  Australia  44  Argyle  Congo  46  47  48  49  50  51  52  53  54  55  Angola  57  58  59  Guinea  ]

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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011