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DIAMONDS IN AFRICA

DEMOCRATIC REPUBLIC OF CONGO

It seems, in all cases, that the traps existing in the chaotic surface of the plateau and on the slopes of the Bushimaïe-Kanshi Rivers junction captured the quasi-totality of the diamonds at the immediate vicinity of primary sources. Such a phenomenon is probably not unique, but nowhere else in the world is known another of a such size.

Distribution of the mineralization from the pipes

The erosion of the kimberlitic massive at the junction of the Kanshi-Bushimaïe Rivers created a rich gravel layer that got in place along the hill sides and in the valley on the right bank of the Kanshi River and the left bank of the Bushimaïe River.
From these alluvial flats, the exploitation went back up on the slopes in the richest zones and up on the plateau where the eluviums, more or less mineralized, spread around the pipes in a zone of 10 km. X 6 km..

The trails of enrichment often follow small dry valleys hardly perceptible where the accumulations increase thickness from the uphill toward the downstream while spreading considerably, until in the alluvial flats, and then become impoverished quickly enough at the end of few kilometres. The deposits of Lukulenge in the alluviums down-stream of Bushimaïe River are exhausted today, as well as the alluviums of Bushimaïe and Kanshi Rivers at the E and SE from the pipes.
These eluvial deposits between Kanshi-Bushimaïe Rivers are enormous as well as the grades that can reach 100 cts/m3 near the pipes, and go from 10 to 50 cts/m3 on the slopes of the Kanshi River side, with a thickness of gravel of 5 to 7 m of which the richest is at the summit.

In the alluvial flat of the Kanshi River, there was 3 m of gravel (on 5 to 7 m. in total) mineralized at 9 to 10 cts/m3, and this gravel is eluvial type, little or not rolled, containing a lot of polymorphous sandstone remnants. One passes in a continuous way from this flat deposit to the slope gravels on the right bank. The 8 m. thick eluvial gravel layer has the richest grade (9 to 10 cts/m3) in first 2 meters at the top, the content falling to 5 cts/m3 in the lower 6 m. based on the eroded dolomite.

Going back up the slope the thickness of gravel layer increases and reaches, for example, 30 m. on the pipe n° 5 at Disele.

The vertical cut of this most important layer at Disele, shows from the top to the bottom:
1° the superficial sandy humus,
2° the "sterile" fine clayey gravel, 20 to 40 m. thick,
3° the "paying" coarser gravel, but without big fragments, 8 to 9 m thick. At the base, a rich layer with the nodules of kaolin,
4° this exploited gravel rests either on sandstone, or on the altered breccia of the kimberlitic hat that is generally exploited.

The grade of out-flowing mineralized "runs" in the bedrock cracks or channels is very variable.

The exploitation is by open pit with enormous mechanical means. From the VII and VIII pipes a 1,5 km wide front moves back westward, and excavators exploit the V and VI pipes, under an overburden of 30 m..

Deposits in exploitations

Bakwanga, renamed M'Buji-Mayi (Lat. 6° 1' S, Long. 23° 45' E) has become, with the influx of the miners, a big city, in the Province of the East Kasaï. It is on the bank of Bushimaïe River, a big affluent of the River Sankuru that goes into the Rivier Kasaï, which throw itself into the Congo River.

Close to the city of Bakwanga, the set of the enormous yards forming on the surface biggest diamond mine in the world, is locked in a protective fence surrounding 25 km2,. The main sectors of exploitation are Diseles in the WNW, Dirale and Bakwanga-Center. Outside of this perimeter, in 1962, functioned other smaller exploitations in Katsha, Tshimanga and Kamisangi.

Besides, some marginal layers in the region were exploited clandestinely by African miners and could produce 2 to 3 millions karats per year. This diamonds passed toward Liberia and Rwanda, largely in transit through the purchasing offices in Brazzaville. The repression of this traffic is difficult, although it seems that lately the government could react more strongly and says to have seized enormous quantities of diamonds.

Diamonds

The diamonds of Bakwanga fundamentally are different from those of most of the other producing regions. Their quality is generally very mediocre, since there are only 2 to 3% of jewellery stones in octahedral transparent stones with rounded faces and 85% of crushing boart. The rest, 12 to 15%, is the industrial diamond of good quality.

A very large amount of stones are opaque or in crystals aggregate. The abundance of the cubic crystals is remarkable, always coloured brown, dark greenish etc., and a lot of diamonds are from two times of crystallization, with a transparent octahedral heart surrounded with an opaque crust in the shape of cube or dodecahedra. Some of these cubes are of very large.

Very remarkable is also the abundance of broken stones in the eluvium. In Disele, 60% of diamonds between 1 and 4 mm are broken, 52% only between 4 and 7 mm, and 46% between 7 and 10 mm, which is a lot.

This relative fragility (which evidently diminishes the quality of product) is explained by the effect of too fast pressure and temperature change letting the diamond in an unstable state.

There are three times less stones of 2-3 mm that of 1-2 mm, three times less than 3-4 mm that of 2-3 mm, etc. The big stones (+ 9 mm) represent (on the massive NW of Bakwanga, 0,6% in number and 25% in weight.

The average size is from 4 to 8 stones to the carat.

The cubes, the aggregations of crystals and the coloured stones are lighter than the octahedrons, and the transparent crystals.

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  ]


Related links: Diamonds: Large and Famous   Properties   Geology and Mining Diamond Cutting Gem Cutting Diamond Trade  Values of diamonds
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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011