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DIAMONDS IN AFRICA

DEMOCRATIC REPUBLIC OF CONGO

But in addition the muddy breccias-form flows "entered the neighbouring karstic pits, combining themselves with the mainly sandy semi-fluid mixture that would constitute their fill" (Mr. de STADELHOFEN). And from these heaps were able to form itself, by washing of, new deposits very enriched in the traps of this surface that topographically is relatively plane (550 m. - 650 m.) but hides a chaotic substratum pierced of pockets and funnels. One recovers, if one wants, the conditions that helped to create the rich "pots" of the Lichtenburg fields in South Africa (from the alluvium and no of the kimberlite herself). But it is at a scale tens of times bigger and at the immediate vicinity of the primary sources.

The mobility of the muddy kimberlite saturated by fluids, and the little resistance that offered, at the roof, the unconsolidated formations (and maybe again under water?) permitted to the diamantiferous intrusion to overflow, while spreading into a mushroom shape, bringing up an enormous quantity of diamonds... especially since the content of rock-mother was itself elevated.
The kimberlites themselves have been found in depth where their section is much smaller that the one of the mushroom. They form cracks of less than100 m large in the compact benches of the Bushimaïe Formation. The magma was getting in place pulsating up and down since xenoliths of cretaceous sandstone are found at 500 m. depth.

The actual massifs of Bakwangas (North zone)

In the North part of Bakwanga within few kilometres, 11 kimberlitic massifs, rudely aligned east-west, has been discovered up to this day, of which 4 or 5 are chimneys, and 6 of the replenishments of karstic losses.

"Situated in an active erosion zone in a relatively low altitude of 524 m, the massive VII and VIII... permit to observe the channel of arrival of the igneous flows. Of irregular ovoid and lenticular section these two chimneys are vertical and open in the straightforward limestone, without strangling or deviation. Their section seems to remain constant, about 15,000 m2 for each of the chimneys" (Mr. de STADELHOFFEN).

On the other hand, the massif of Disele spreads on the surface (elevation 610 m.) as a mushroom of 750 x 250 m, widening itself in the vestigial products of dissolution of the limestones and the upset sandstone.

Below the elevation 472 m it takes root in a chimney of less than 100 m of diameter. The superior altitude of sandstone at the time of the intrusion can be estimated, according to the shape of the mushroom, at 720-760 m, what only supposes a height of erosion of 100 to 150 m. Do other studies estimate the slice eroded to be 250 m thick?

The xenoliths are not laminated, nor crushed, and angular - except those pulled from the basement that are rounded - and go from the mm3 to several m3. Their proportion can reach 75%, whereas sometimes they are nearly completely lacking. All minerals of the kimberlitic breccia show the traces of a deep hydration and are extremely altered. As one thinks that the setting up took place under the shape of a disjointed heap of crystals completely penetrated by hydrothermal fluids that have serpentinised and chloritised the primary minerals.

On surface the breccia presents a pink yellow hue with green spots, on which comes out the red hue of the sandstone xenoliths. The mass underwent a strong folding and one sees many fold plans with kaolin coating. The top of the pipe is constituted, on some meters, by a red material with kaolin nodules probably resulting of the washing of the kimberlite, and enriched in diamond as far as frequently reaching the grade of 100 cts/m3. The origin of these "kaolin balls" is far from being clear: it can be from very altered feldspathic or clayey xenoliths, or rests of argillites deposited in funnels at the end of the volcanic activity.

It is certain that the hats of the pipes of Bakwanga remind the covers of "sedimentary kimberlites" of Mwadui, of AK 1 in Botswana or some Brazilian deposits, etc..

The decomposed breccia

On the first 10 to 30 meters, the breccia spread into a mushroom form has been decomposed completely in a crumbly red or green sand-clay mass speckled with white points of kaolin. The grains of sand dispersed constitute 40 to 80-90% of this material that looks a lot more like clayey sandstone then kimberlite. This decomposition made disappear all carbonates of Ca and Mg; also the enclaves of limestones or underlying dolomites are missing. There was a certain local silicification.

The cemented breccia

It essentially distinguishes itself of the previous, which surmounts it, by the presence of limestone-dolomite inclusions coming from the Bushimaïe Formation and reaching up to 90% of total mass. Its aspect is also the one of a detrital rock, intimate mixture of powdered sandstone and kimberlitic elements. This sandy breccia contains small nodules formed of kimberlite, and of intrusive kimberlitic breccia containing all sorts of inclusions: éc1ogites, granites, gneiss, limestone.

C. FIEREMANS gives the following analysis of a very calcareous kimberlitic nodule:

SiO2   31.70%
Na2O   0.10%
Al2O3   4.04%
K2O   traces
Fe2O3   4.64%
MnO   no measured
FeO   2.25%
TiO2   0.62%
MgO   16.63%
H2O (+ CO2?)   20.84%
CaO   18.33%
P2O5   (with Al2O3)

The advanced decomposition is probably responsible of a partial elimination of the magnesia, and of the alkalis and the invasion by the calcite. This analysis recalls the one of some carbonated kimberlites of Yakoutia - them also intruding through calcareous-dolomitic formations.

The analyzes of kimberlite in depth is quite normal:

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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011