DIAMONDS IN AFRICA
The bench of conglomerate has 1 to 3 m of thickness and rest on the basement or on the Karoo Formation. It contains a lot of kimberlitic mineral and diamonds, with a grade between 0,4 and 0,7 ct/m3. The economic limit is 0,2 ct/m3, naturally variable with the quality of the stones and the thickness of overburden. This last can be considerable: 40 m of the Kalahari sand at the Luaco Mine 6 where the grade of gravel being of 0,4 ct/m3 once excavated it falls to 0.01 ct/m3. But water monitoring, which requires 5 pumps of 200 Hp and 12 kgs/c m3 of pressure, is very economic, and the conglomerate is soft, easy to extract and to treat. It is a "fossil alluvium". The extension of the Calonda Formation is very big, and under the Kalahari sands it is systematically sought-after with small diameter drills and sampled with big diameter ones. Its part in the total production is certainly very big, the exploitation of the kimberlites being only at its beginning. It is important to notice that the feeding with diamonds of the recent alluviums directly from the kimberlite pipes is very subordinate to the one that has transited through the Calonda during Cretaceous.
The Calonda Formation, the indicator of kimberlites: The Calonda reveals a weak transportation, especially if it contains fragments of tender rocks (sometimes of kimberlites) or the chromiferous diopside that doesn't travel beyond 3,5 km according to B. REIS (the fine ilmenite: 25 km, the pyrope: 16 km, maximal distances). It can provide therefore in some cases the indication of the proximity of pipes, masked under the sands. For example,, the pipe Camagico whose summit is formed of poorly mineralized kimberlitic sediments are surmounted by a rich Calonda... a witness of the proximity of another unknown and rich pipe. It is therefore obvious that the presence of kimberlitic minerals (especially little traveled) in the conglomerate that can be discovered by drilling can encourage the search of pipes in the neighborhood. This mode of approach to the prospecting is probably only at its beginnings, but will be able to develop itself with the improvement of the fashions of detection of the kimberlites under the mask of the Kalahari, by multiplication of polls of small diameter and employment of various geophysical processes including the seismic that can give the relief of the sands-conglomerates contact, etc. and also the interpretation of geological surveys. For example in the oriental zone of the East Lunda, between Luana and Luembé etc., the alluviums are rich in diamonds coming well from a mineralized Calonda... and one doesn't know any kimberlite up there. But one can think that it exist under the Kalahari sands.
II. FIELD OF THE HIGH HAUT-CUANGO and the Cucumbis, Cacuilo, Lufulé Rivers,
Situated 250 km SW of the Field of East Lunda, this field extends 300 km in NNW (325°) in the high basin of Cuango River, where exist diamantiferous alluviums well farther down-stream in Congo. It is necessary to distinguish two distinct zones there:
The valley of the Cuango River herself.
The very old basement (of 2,627 MY toward the N to 1,981 MY in the S) shows on the surface in part, but can be covered by the succession: the System of the Western Congo (terminal Precambrian), the Karoo (Jurassic and beginning of the Cretaceous), and the Calonda (Cretaceous). The Kalahari is thin or missing. This cover especially appears in the hollows of the valleys. In this region of the high Cuango kimberlites are not known. Only alluvial deposits and the Calonda, rich and thick that nourished them in diamonds, are exploited. The valley is deep, cut by big falls and zones of rapids that keep a very diamantiferous gravel in pots on the Precambrian sandstone: 1,93 ct/m3, for example, in the Quimangoa Mine, that recalls those of the Oubangui. In Cafunfo, the 2 to 3 m thick Calonda is exploited with useful grade of 0,60 ct/m3. Along the Cuango 5 mines with modern recovery plants are working. The beautiful fall of Cafunfo is going to be equipped with a hydroelectric plant.
Region of the Cucumbi, Cacuilo, Lufulé Rivers.
Toward the SSE, this region drained by these right-hand affluents of the head of Cuango River presents itself far differently and is on the contrary very rich in kimberlitic intrusions, with rather poor alluviums. These intrusions are excessively numerous, probably hundreds of generally small pipes (but there are some big), and very visible, easy to discover from soil level or air. The pipes that show on the surface in the resistant Inferior Karoo (Dwyka), form some hollows deprived of vegetation, killed by the excess of Cr, that presents themselves as a very remarkable "fairy's rings". In the Continental Inset, on the other hand, the kimberlitic bodies appear in relief, and are often very covered by trees because the hard kimberlite (hardebank) retains soluble chemical elements harmful to vegetation. Thus some "kimberlite mounds" reach more than 50 m high, what is surprising enough! However, one of the biggest pipes, Xangando Leamba (800 m. X 600 m.) was discovered by loaming from satellite minerals found in the thin Kalahari sands. It is at present, it is difficult to have an idea of the value of this zone, because the rhythm of the sampling cannot follow the fast discoveries of kimberlitic bodies.
III. FIELD OF THE CUANZA
The Cuanza River come of the 14° parallel, bends toward the NW, and throws himself in the ocean little to the south of Luanda. The structural control of the arrivals kimberlitic intrusions, according to B. RETS, is linked to the hinge of the contact of Precambrian basement and the schists-sandstone of the Western Congo System on border geosynclinal of terminal Precambrian. Between 1l ° and 12° parallel a group of 14 pipes are known at 1,200-1,700 m of altitude, older than the dolérites and intrusives in the schists-sandstones. These kimberlites seems to be sterile or very poor, although the alluviums are diamantiferous, and the ilmenite predominates there distinctly over the pyrope. This field is 250 km. SW of the precedent on the same alignment.
IV. FIELD OF NOVA-LISBOA
While pursuing 250 km in the same SW direction one comes into a new field where predominate the centered alkali complexes and carbonatites, with some subordinate kimberlites. This elevated region, close to 2,000 m, form a water tower from where leave southwards the Cunéné River that turns westward and becomes a border with Namibia, to the westward Catumbela River, and to NW the Queve River. The control of the alkali ultrabasic intrusions are mainly a set of faults in directions 50-55° and 135°, and these intrusions distribute themselves in a strip of 55 to 85 km wide and 400 km long. To the south of Nova Lisboa and on the meridian of this city one finds other kimberlitic occurrences more or less in Cassinga and Artur of Païva etc. The features of these bodies are not known.
V. FIELD OF THE LONGA
This detached field is 250 km in the NW of the one of Cuanza, where the Longa River bends from NNW to the west, while following a flexure marking the contact of the crystalline plateau in the East and the sub-horizontal Western Congo System of in the west. The intrusions, few, are at the crossing of NNW accidents and E-W faults. One can notice that, in the same direction of NNW tectonic control one recovers at 200 km to the west of Salazar and 500 km more to the north other groups of alkali complexes.
Rafal Swiecki, geological engineer email contact
This document is in the public domain.