Geology Reference
In-Depth Information
650
Conglomerate
Guilaume Falls
Granite/Gneiss
600
Siltstone
Mawangu Quarry
MIDAMINES
Sandstone
550
SRTM DEM
Diff GPS Survey 2007
500
Kwango River Targets
450
Francois-Joseph Falls:
Mesozoic conglomerate
and siltstone exposures
confined to bottom half of
nick-point
400
Kwango Group Red sandstones
350
9,095,000
9,125,000
9,155,000
9,185,000
9,215,000
9,245,000
Northing (m)
Fig. 16.6
Abandoned channel at
Midamines
Kakolo Mine. Note straight channels following local basement joint patterns (photo Ward)
'
structures between targets K24 (just north of Tembo) and
K33 (Figs.
16.8
and
16.9
). Here the gravel volumes are low,
but conversely the grades are high. At K33 downstream from
Tembo the first splay develops and it is here where the river
enters the Upper Kwango aeolian sediments, depositing its
coarse bedload. Basal gravel thicknesses remain limited and
although volumes have increased due to lateral expansion,
grades have now lowered. Downstream of the first splay the
Kwango River Valley widens between the K33 and K48
targets resulting in the formation of wide terraces and flats
and deposition of excessive amounts of sand (Flats 3 to 11).
Here the gravels are thin and grades are low, virtually no
artisanal diamond activity is observed suggesting an absence
of any significant deposits. Between K48 and K56 the valley
narrows on the soft Kwango Group footwall and narrow
terraces have developed (Figs.
16.8
and
16.9
). Gravel
deposits, dominated by silcrete and calcrete clasts, have
increased but the grades remain low to moderate.
16.5 Geophysics
16.5.1 Ground Geophysics
Ground surveys using electromagnetic, gravity and magnetic
methods were tested to detect sediment-filled palaeo-
channels and/or bedrock irregularities, and whether an air-
borne equivalent could be used for this purpose. Five areas
were surveyed, (K24, K33) near Tembo and (K48, K52 and
K56) around Kitangu (Fig.
16.10
), where the extent of the
gravels could be extrapolated with reasonable confidence
from the artisanal workings. The results suggested that an
airborne electromagnetic (EM) method would be most effec-
tive in locating gravel-filled palaeo-channels, particularly in
terraces away from the river.
The electromagnetic profiles across the targets best demon-
strate the presence of buried channel features, particular in the
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