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DIAMONDS IN AFRICA

DEMOCRATIC REPUBLIC OF CONGO

Kimberlitic Pipes

The 24 known pipes are distributed in 2 groups:
- 14 to the west all on the plateau close to the cliff,
- 10 to the east, of which half at the bottom of the cliff.

These 2 groups are distributed each longitudinally with a predominance of alignments at 172°-175°. This same alignment, one recovers everywhere in Africa where kimberlitic pipes are found. Other kimberlitic pipes can be suspected to the NW (indications of Diamba-Malira-Maswala) and to the NE (indications of Luimanwe). The total surface of the 24 recognized pipes is of 150 ha, either 6,2 ha/pipe, an elevated number.

The reports (V. BRIEN, 1913 - Anonymous, 1926) conclude to the inexploitability because of the very weak content, of the smallness and especially of the bad quality of the diamond and the toughness of the kimberlite, expensive to treat. One also notices that a lot of samplings of pipes were massive and even exaggerated. It doesn't seem on the other hand that anyone was interested in the eluviums and the alluviums.

Kimberlites

The kimberlites of Kundelungu and their minerals have been studied well by J. VERHOOGEN (1938) and N. KORETZKY (1966) added number of complementary observations. The replenishment of the pipes is generally breccia very loaded with xenoliths pulled from sidewalls that can reach up to 50% of the total volume. The outcrops are often a hard layer (hardebank), otherwise the yellow muddy ground with heavy minerals predominates, the blue ground being rare. The experience seems to show that kimberlites hard on surface are often poor. There is no thermal metamorphism of enclaves, but there was a secondary feeding in quartz and microcline of sandstones and shales, probably posterior and hydrothermal. The temperature of the kimberlite at the time of intrusion had to be very low. KORETZKY notes the presence of inclusions of dolerite with oligoclase different from those of the Katanga, of mica-schists with magnetite that could belong to the Kibaras Formation that would be underlying, and of eclogites with sodium pyroxene that would have rather a metamorphic origin. Some kimberlites with mica exist, and others basaltic of brownish dark colours that constitute the essential of the pipes in the western group, with exception of Kashioba to the north that is isolated. The mineralogy of concentrates is simple and classic: ilmenite, garnet, olivine, diopside and magnetite. The pyrope is red dark (70% of the pyrope - 30% almandin), and the content made of garnet is very variable: 88,5% of the concentrates in Gondolo, 57,5% in Kashioba, 0,6% in Zefu, and without report with the diamond content. The olivine rich in forsterite is only frequent in Kashioba, very rare elsewhere. The green diopside only represents 1% of the concentrates.

Mr. STADELHOFFEN estimates that the kimberlites of the Kundulungu are more like alnoïtes that "true kimberlites".

To title of comparison one can give the following analysis: one will note the meaningful abundance of titanium.

Analysis of kimberlite of Katanga (Kundelungu)

SiO2   32.20%
Al2O3   3.55%
Fe2O3   10.75%
FeO   1.50%
TiO2   3.25%
MnO   0.20%
CaO   3.85%
MgO   29.25%
Na2O   <0,05%
K2O   0,05%
P2O5   2.45%
H2O   2.45%
H2O+   10.15%
Ni   820 ppm
Cr   1,470 ppm
Cu   151 ppm
Sc   17 ppm
Sr   315 ppm
Ba   404 ppm
Density:   2.45 g/c m3

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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011