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DIAMONDS IN AFRICA

DEMOCRATIC REPUBLIC OF CONGO

The massif "A" of Disele contains:

SiO2   40.39%
Na2O   0.83%
Al2O3   4.53%
K2O   0.63%
Fe2O3   4.14%
MnO   0.12%
FeO   1.72%
TiO2   0.83%
MgO   24.01%
CO2   2.0%
CaO   4.35%
P2O5   0.42%

Mineral "satellites"

The kimberlite contains more than 80 g/tonne of magnesian ilmenite of 11,7 to 12,40% MgO and 3,05 to 4,4% of Cr2O3 plus the traces of Ni.

The most often, ilmenite presents itself in rounded grains of less than 1 cm diameter. It is of a very constant composition and the absence of magnetite exsolution would indicate a fast cooling of the kimberlite below 600°. It is abnormally rich in Na2O (3,51%), and contains traces of Pb.

The garnet content varies from 0 to 60 g/tonne. It is a dark or pink red garnet with 19.7% of MgO and 3.5% of Cr2O3 (pyrope 72.2%, almandine 13.4, grossulaire 9.0) typical of peridotite with conchoïdal break.

The diopside, often altered into green-whitish, is present, but the perovskite is very rare. Zircon and disthene are found plus traces of corundum, fluorite, barytin, and vanadium. All primary minerals are very altered so as it often happens, the examination of the kimberlitic rock in thin section is very disappointing.

Mr. STADELHOFEN could not put in evidence of concordance between the diamond grade and the garnet grade. Could a certain proportion have meant that the diamantiferous mineralization came from the xenoliths of eclogite - besides very rare. This reasoning is not very truth because crystals of pyrope, existing in the kimberlitic cement, may not all be coming from eclogite.

The homogeneous distribution of diamonds in number (no in weight) in the kimberlite indicates that an intense mechanical mixing took place after crystallization. There was not a magmatic segregation in the kimberlitic chimney: for example, the content in ilmenite doesn't vary with depth.

To note finally that no trace of thermal metamorphism has been noted, even on the enclaves of most sensitive rocks, but only the aqueous alterations.

At no time, during its setting, the kimberlite was in a state of molten magma.

THE MASSIFS OF THE SOUTH ZONE

30 km to the south of Bakwanga, the rich alluviums were known to exist at the confluent of the Katsha River and the Bushimaïe River. The exploration carried out up the Katsha River had stopped at a bench of Mesozoic conglomerate, which, it was believed, was the source of the diamond, as it is the case in Western Kasaï.

It is only in 1958, while being guided by a strong magnetic anomaly discovered during airborne survey above the sources of Katsha River, that alluvial-eluvial exploration was taken upriver and uphill in search of "big ilmenite" that brought the discovery of the very important kimberlitic massif in Kalonji on the Tshébua River and three other smaller 30 km in the SW of Bakwanga.

In Kalonji the kimberlite again is extensively spread but more than in Disole. Indeed it shows on the surface at the elevation of 700 m. that wants to say higher than in Bakwanga. At this level the cretaceous sandstone, at the time of intrusion, was only little or not consolidated sands depositing under water. So the kimberlite is spread wide while forming a mushroom that, today, covers a surface of 32 hectares, two times more that in Disole, drawing an oval of 1,700 m. X 750 m..
In fact, at 300 m. of depth, where take root the effusion, the pipe has only 120 m. of diameter.

The massif of Kalonji contains a lot less xenoliths of sedimentary rocks. In particular, limestones and dolomites are nearly absent, what makes think that the carbonated formation, in the South, had to be eroded. On the other hand, there are more xenoliths of the basement: granite, sometimes in big enough blocks, gneiss, altered eclogite, etc., a lot of veins of calcite and heavy minerals. It is this abundance of "big ilmenite" that facilitated the discovery of the pipe. To note is the frequent presence of phlogopite, a marked difference of Bakwanga's kimberlites.
The diamond grade is probably high as well. One can wonder that a deposit as important situated very near of Bakwanga has be ignored during many years, but it is necessary to notice that the kimberlites of this region only mark themselves little or not at all on the topography, can't be seen on aerial photos and are often masked by thick silts, etc., at the surface of plateau. Also, they make the circular depressions analogous to the karstic funnels, which are very frequent and don't attract attention. The magnetic anomaly continues toward the River Lubi, to the west of the River Tshibua, so the exploration activity moved into this sector.

THE DETRITAL DEPOSITS

The detrital deposits posterior to the kimberlitic intrusions understands:
- the diamantiferous Kalahari sandstone.
- the gravely layers of plateaus at the base of the red clayey sands of plateaus.
- the eluviums of the sides of dry valleys, and the accumulations in the karstic pockets of limestones to the immediate vicinity of the primary sources.
- the alluviums of valley bottoms.

To should be noted with regard to these last, it is a very remarkable fact how small is the relative extension of mineralized alluvial deposits, in consideration of the enormity of primary sources.

The maximal extension is limited practically to 35 km, what is really very little if one compares it to that one can imagine in South Africa, or to observe in the Birim in Ghana, etc., but naturally some small stones could spill farther down-stream. The rich mineralized zone is on the hills at the confluent of the Kanshi and the Bushimaïe Rivers, and the only important placers on this big collector is in Lukelenge, only about ten kilometres down-stream. It is where have been found the first indications.

50 km lower, where the Bushimaïe River throws itself in the Lubilash River, very important river, which the mineralized contributions are probably excessively diluted. Maybe the mineralized flats do exist all the same, but no one searched for them. The B.C.K. who exploited Bakwonga had in actual fact before her so enormous reserves that this prospecting, far from Bakwanga, hardly presented an interest? A mission in the region of Luluabourg, 150 km to the west, didn't have any success.

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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011