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DIAMONDS IN AFRICA

DEMOCRATIC REPUBLIC OF CONGO

The geological setting

The deposits of Bakwanga are on the tabular Katanga platform based on the "Kasaï Shield" that forms the south edge of the Congolese pan.

1° The Basement. The old basement of 2,700 to 2,100 MY old, is constituted by monzonitic granites, gneisses and migmatites, with basic rocks (gabbros diabases) and ultrabasic rocks (serpentines, pyroxenites, etc.) in part previous to the migmatisation that appears in a east-west strip between the parallels 6° 05' and 6° 20'. This basement has been broken and mylonitised, the general tectonic direction being also more or less east-west.

The cover of the platform is not very thick - 700 to 1,000 m. maximally (984 m, 10 km to the south of Bakwanga). It is comprised of:

2° the Bushimaïe Formation, superior Precambrian [1,120 MY (1)] formed of two sets, detrital and carbonated: it is the equivalent of the Roan Formation in Zambia. The detrital set at the base, baptized "set psammitique" by POLINARD or schisto-sandy is 270 m thick. It starts by conglomerates, then coarse sandstone and continues by marly sandstone and red argillites with thin white mica. It passes progressively to the carbonated set that includes the limy and dolomitic shales more or less clayey, the limestones and dolomites, the limestones constituted of stromatolithes. A slightly cupriferous level exists. This set, close to Bakwanga, is 660 m thick.
The deposit of the chalky schist were marked by episodes of subsidence, which has been followed by a long egress and an intense karstification, which plays the major role in the structure of diamond deposits in the region.

3° the important Basic flows coming in at a badly defined time, previous to the Mesozoic deposits, but that seems close enough to the beginning of these deposits. It corresponds to a rising, followed by erosion and detrital deposits.

4° on the limy schist rest discordantly the detrital Mesozoic deposits, starting in the Inferior Cretaceous, which are connected to the Lualaba Formation (and not to Karoo as it was once wrongly believed). The thickness of these sandstones goes from 50 to 250 m, and they start by a coarse conglomerate made of remnant fragments of the old basement, sometimes very big, and also of fragments of rocks of the Bushimaïe Formation and dolérites.

Before the existence of the kimberlites was suspected, it was believed that the diamonds were related to this conglomerate at the base (emplaced only little time previous to the arrivals of kimberlitic intrusions), and some exploration efforts seem have been once ill oriented, following a perception that was in short the one of the deposits of the Kasaï.

This Sandston Formations fill some karstic depressions in the dolomites, capable to form enormous pockets as the one of Disele oriented east-west in Bakwanga. The arrival of kimberlites occurred in part in these Loïa sandstones in part consolidated and was located therefore between the Wealdien and the Albien in the Lower Cretaceous.

5° the superficial Kalahari Formations (Eocene, etc.) subsist only in rare places on the high plateaus: polymorphous sandstone and especially sands and more recent silts.

Structure

The basement is cut up by a double game of diaclases oriented in the NNE and in the SSE. The surface dives mildly from 1 to 3° toward the NNE, with large very weak undulations whose axes are parallel to the diaclases. This observation would connect their origin to the set of the vertical tectonics that affects compartments of the basement separated by faults. The set of these blocks either collapsed or risen form a staircase that goes down toward the NNE in Bakwanga the pedestal is for example 3 to 4 times deeper that in Kalonji, massif kimberlitique situated 30 km in the South. One must notice the importance of the big east-west zone of weakness mentioned higher for its ultrabasic intrusions, that crosses the central Kasaï and of which the extension to the east passes very precisely by the swarm of the kimberlites of Bakwanga.

This zone of weakness would have replayed on four occasions at least, since one time previous to the migmatisation until the setting up of the kimberlites. The dominant direction of the ultrabasic massive is on average near 93°.

It is besides on the western extension of this zone that the FORMINIÈRE, had begun to search for other kimberlites in the Kasaï on the Luebo. The basic intrusions, close spatially to the kimberlites, are also close in time. Although their age is not defined well, one would probably be able to establish parallel with dolérites of the Karoo, dating from Lias to Medium Jurassic in South Africa.

One supposes a genetic relation between the kimberlites and the basic intrusions. One distinguishes:

1° the big dike of Tshimanga-Mérode 30 km to the south of Bakwanga, with orientation 80 to 95°, that was discovered by geophysics (first surface magnetism, then airborne and gravimetry). It would have more than 25 km of long on 400 to 2,000 m of thickness. A drilling has penetrated it under the kimberlite of Tshimanga: it is a hypersthene and augite gabbro.

2° the micro-dykes of the Katsha and the Movo, 30 km in the SW of Bakwanga is them controlled N 20° E and formed of a pyroxene diabase.

3° the dolérites of Tshala, 20 km NW of Bakwanga, are effusive con vacuoles (amethyst geodes).

The kimberlites

Particular conditions of their setting:
If the kimberlites of the region of Bakwanga are, by their composition, we will see, of the most classic type, their setting took place of a very particular way that confers to these intrusions originality and contributes, probably, to their extraordinary wealth.
Indeed, after a first explosive phase in depth, the kimberlites injected themselves under a plastic and muddy shape at low temperature while forming some sills in soft sandstone of which the top, that was not even consolidated, mixed itself with the intrusion. By the dislocations, accentuated by the intrusive pressure, circulated meteoric waters that encouraged the dissolution of the dolomitic limestone. Thus, it formed itself of the secondary breccias, of the calcite deposits in part re-dissolved, followed up by silica deposits (opal, chalcedony), etc.
This intimate mixture of sandstone, clay and kimberlite - that can contain 74% of SiO2 and 7% of Al2O3 - could introduce the error at the beginning and make believe in the absence of real kimberlites before these were discovered in depth by drilling. The conditions looked in short like those described by HARGER at Kruis River in South Africa or maybe at the famous "taua" of Agua Suja in Brazil (2).

(1) Dated by the common Pb.

(2) The recent research (in Angola, Brazil, etc.) shows that this type of "Somnital" deposit of kimberlitic sediments is actually very widespread.

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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011