DIAMONDS IN AFRICA

DEMOCRATIC REPUBLIC OF CONGO

Based on the information provided by geologist Marcel Guillaume Bardet.

Map of diamond fields in Kasai District of Congo

In quantity the RDC was the biggest producer of diamond in the world: 18,072,000 cts for the year 1961. This number has been achieved on the only one but enormous deposit of Bakwanga, in the region of Bushimai, which almost detained the monopoly of the "crushing boart". After the independence of 1st. July 1960 Congo dove in the socio-economic mess that stopped practically the organized statistics of production.

The RDC also possesses the alluvial fields of the Kasaï that were very rich, and of which the extension in Angola is in exploitation, mainly by artisanal mining, and achieves more than 1,000,000 cts per year.

Other occurrences of diamond have been signaled in this vast country, although it was not the subject of exploitation.

Fields of the Kasaï (Tshikapa)

The diamantiferous fields of Kasaï, the most formerly exploited in Central Africa, spreads between the parallels 8°30' and 5° South and the meridians 20° and 22°30' East. Its southern part is in Angola, and the Congolese sector stops about at the 7° parallel. Nevertheless, it covers more than 20,000 km2 around the mining centre of Tshikapa, that is 650 km to the ESE of Kinshasa, in a flat region at about 600 m of altitude.

The fields of the Kasaï are solely alluvial. The diamantiferous sedimentary layers date from the Cretaceous. The diamonds are found in the lower part of the Bokungu Formation.

The diamonds of the Kasaï are small but of a beautiful quality with up to 65% of jewelery stones.

Geological setting

The platform of the region of Tshikapa rests on the old very folded and metamorphic crystalline Kasaï Shield of 2 to 2.7 billions of years old, that hardly shows on the surface. It is only exposed at the bottom of river valleys.

The floor consists of the granites, gneiss and migmatites, syenites, of the metamorphic rocks: quartzites, amphibolites, itabirites, 2 micas schists, pegmatites and the old basic intrusions: uralitized dolerites and ophitic gabbros.

On this floor rests in discordance the following sub-horizontal formations:

The Lukuga formation (Permian) (Karoo inf.) (Equivalent to the Lutoë formation in Angola) A mudstone-sandstone formation of light brown to yellow colour composed from the bottom to the top:
- The fluvial-glacial conglomerate of angular and rolled elements derived from the Precambrian floor and quartzites of the Lula formation.
- The conglomeratic layers or lentils.
- The argillites.

This formation never is diamantiferous in Congo or in Angola. In Kasaï, one finds it as a rare outcrops in South in the depressions of Precambrian floor.

Mesozoic

Overlaying the Congolese Basin are Jurassic and Cretaceous sediments of the "Continental Inset". It is necessary to distinguish 3 layers currently in the formations known as Lualaba-Lubilash:
- The Stanleyville layer (Jurassic superior)
- The Loïa layer (Jurassic terminal and lower Cretaceous)
- The Kwango layer (lower and superior Cretaceous)

The Loïa layer in Kasaï is formed from the bottom up:
- The fluviatile conglomeratic rock, richer in elements of the floor that the Kwango formation and lacking agates, that often passes to granitic sand when the Lukuga formation is absent. The cement is sometimes calcareous.
- The arkose and coarse sandstone
- The thin intercalations of sandstone and argillites
- The sandstone with kidneys of silicified limestone (ostracode cyprus) This orange-brown layer has up to 80 m of thickness and sometime up to 200-300 m. It doesn't contain any diamonds.

The Kwango layer (Equivalent to the Lunda formation in Angola) made of sandstones and conglomerates is diamantiferous. Its purplish colour contrasts well with the bottom part of Lualaba-Lubilash formation while its thickness varies from 0 to 200m. In Kasaï, the Kwango layer starts as a conglomerate of deltaic origin. This conglomerate has the aspect of a lateritic crust often forming small irregular lentils with intercalations of thin layers of nodules of argillite. The agates are very frequent and there is a large assortment of heavy minerals.

Cenozoic The system of Kalahari rests on the applanation of the end of Cretaceous and is previous to the applanation of the superior Pliocene.

One distinguishes two sets of deposits:
The Palaeogene or a layer of the polymorphous sandstones (Eocene Oligocene)
The Mount Bunza near the border of Angola and the Kasaï offers an example of 100 m of different silicified sandstones, of ochre sands and gravels.
The Neocene or a layer of the ochre sands
This layer passes to the sands of the Pliocene and Pleistocene

Quaternary
It is very complex and includes up to 11 levels of terraces.

The doleritic intrusions
These intrusions are considered dating from the lower or middle Carbonaceous.

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