Diamond-Bearing Gravels Along the Lower Kwango River DRC - Geology and Resource Potential of the Congo Basin - page 335

Fig. 16.5 Mid-Pleistocene hand axes made from silcrete or gr ` s polymorph in K33 Hilltop terrace ( Right )

however, some physical characteristics of a previous aeolian
transportation phase.
Mineralogical evidence of kaolinite-iron oxyhydrate clay
mineral associations, present in both the Upper and Lower
Kalahari sands, indicate deep weathering of the underlying
formations, which formed under hot and humid
palaeoclimates of the Late Cretaceous and Mid Tertiary
pedogenic stages (De Ploery et al. 1968).
Raised terrace deposits and lower level flats are found
adjacent to the modern river system. Just below the
Francois-Joseph Falls hill-top gravels have been mapped at
roughly 40 m above the present river level and these contain
artifacts (hand-axes) mostly made from

generally contain higher diamond grades. The migmatite base-
ment forms an irregular floor characterised by gravel-filled
troughs and potholes (Fig. 16.6 ), exhibiting extremely high
diamond trapping potential. Extensive indurated, closely-
packed diamondiferous gravel was observed in these potholes
and gullies.
Between and directly below the Guilaume Falls near
Tembo and the Francois-Joseph Falls near Nzasi-Muadi
(Fig. 16.7 ), exposures of Palaeozoic fluvial-glacial sedi-
ments have been mapped forming a short but thin package
and are overlain by Cretaceous aeolian sediments of the
Kwango Group (Linol et al. 2011 ). These red aeolian sand-
stones form the footwall of the remainder of the Kwango
River downstream of the Francois-Joseph Falls.
Where the Kwango Group sediments form the base to the
gravels, the River is wider and has a gentler gradient with
more extensive floodplains or flats and terraces. The terraces
contain widespread basal gravel layers representing broader,
shallower palaeo-channels, dominated by silcrete and
calcrete clasts derived from the now eroded Upper Kwango
Group sediments. Over the relatively flat Mesozoic base the
trapping of diamonds appears to have been controlled by
these boulders of silicified sandstone and calcrete. The
gravels are generally thin (

gr ` s polymorph

'
silcrete (Fig. 16.4 ). These resemble the Mid Pleistocene
tools described by Clark ( 1966 ) and are characteristic of
the Lumpembian industry (Fig. 16.5 ).
This provides a maximum age for the Kwango terraces at
approximately 300,000-500,000 years. Similar tools are found
in the terrace gravels farther downstream deposited on
Kwango sandstones. The terrain models of some
areas highlight the presence of several terraces, of which the
most elevated contain the Middle Pleistocene tools. Most of
the artisanal activity is however confined to the lower terraces.

'

30 cm), but appear to be thicker
upstream near the basement contact. In all areas the gravels
are overlain by younger fluvial quartz sand/silt sediment that
is between 1 and 15 m thick. The youngest deposit types are
those within the current river bed and these are being
exploited for diamonds around Nzasi-Muadi and just down-
stream from Kitangu.
Based on the geomorphological studies, using air-
photography, Landsat imagery, helicopter surveys and field
mapping, 24 targets were identified for further investigation.
These include 11 terraces (including a hilltop remnant), 11
flats and two splay deposits (Fig. 16.8 ).
The theoretical influence of the river geomorphology on
trap site development, gravel deposition and diamond grades
is schematically presented in Fig. 16.9 . Geomorphological
the river is confined and controlled by bedrock and bedrock

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16.4 Geomorphology

In Angola the Cuango River rises in Kalahari sands and Upper
Cretaceous sediments before it reaches the sandstone and
shales of the Karoo Supergroup in the Cassanje Graben, a
depression in north-central to north-western Angola. From
there the Cuango flows onwards to Luremo where it cuts into
basement rocks. It is here that some of the most important
diamond concentrations occur within the Kwango/Cuango
Valley.
In the DRC crystalline basement also forms the footwall
where the Kwango River passes between a narrow, steep-sided
valley upstream of Tembo (Fig. 16.6 ). In these narrow
channels, river-bank deposition is limited, but the gravels