International Geologiical Congress - Oslo 2008

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MRD-12 Fluvial palaeo-systems: Evolution and mineral deposits

 

Mesozoic palaeochannels as diamond source of alluvial placers

 

Igor Shmakov, Moscow State University (Russian Federation)
 

 

The Kasai Occidental diamondiferous province of Democratic Republic of Congo is one of the richest producers of alluvial diamonds. These diamonds are extracting from river alluvials and, to a lesser extent, from the Cretaceous conglomerates. The geological position of alluvial province determinates by gentle slope of Kasai shield northwestward to Congo River basin.
The geology comprises of Karoo Supergroup sediments unconformably overlying the basement. The basement is represented by Precambrian gneisses and granites. The Karoo Supergroup consists of the Permo-Carboniferous tillites (Lukuga Series) and the Early Cretaceous sandstones (Loia and Bakungu Series). These formations are not diamondiferous.
These ones are overlain by the diamondiferous Kwango Series (Late Cretaceous) which is equal Calonda Formation of Angola. This formation is comprised of poorly sorted conglomerate, which is consists of agate and laterite fragments. These layers have a fluvial detritic origin.
The Kwango Series is covered by Kalahari aeolian sands.
Loose diamondiferous sediments are mostly composed by alluvial sediments. The terraces complex of main rivers consists of sediments of Cenozoic terraces and flats.
The majority of researchers believe that primary sources of this diamonds are known Angolan kimberlite pipes (110+/10My). The main diamond input into studied area has generated as result of alluvial transportation during of Late Cretaceous period. Main directions of diamond transportation were north-west and north.
Terrace complex and recent river channels of main inputs of Kasai River system comprise basal gravel with well reworked Kwango Series material. The rich terrace gravels have been found in close proximity to the Kwango Series conglomerate outcrops.
Characteristics of alluvial gravel are very depending of the nature of bedrock. The Precambrian basement usually is intensively fractured and intruded by dikes with different competency. These structures form of rapids, channels and trapsites, composed of coarse gravel, suitable for diamond enrichment. These trapsites are limited and not significant as diamond reserves, while the vast rivers plain, underlined by soft sandstone bedrock, forms a smooth surface which constitutes gravel layers with significant volume and low diamond concentrations. Kasai Occidental diamonds shows tendency to change a grade, size and other characteristics as result of transportation factors. Alluvial processes have sorted diamond population along province from south up north. This kind of sorting is typical for distant alluvial or aeolian placers.
Natural irradiation of diamonds by alpha-particles leads to a green color, limited to a thin zone or spots at the surface of the stone. These features are witnesses of ancient origin for small proportion of diamonds (2-10%).
Thus, the most of diamonds of Kasai Occidental province show the distant origin from remote Angolan kimberlites and, to a lesser extent, from ancient controversial sources.

 

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