Geology Reference
In-Depth Information
diamond size 3.9 cts/stn), compared to 6.93 % in diamond
screen size no. 15 and 1.21 % in diamond screen size no. 21
for the lower Kwango area (Fig. 16.19 ). Both the BRC and
Midamines areas are represented by well-sorted diamond
populations but the Midamines area showing a distinctly
coarser trend compared to those derived from the BRC
project area. There are three reasons for this. Firstly, the
BRC study area is located between 30 km and 130 km
downstream from the Midamines area. This represents the
most distal part of the advancing
of the Kwango
diamond trail from Angola and will therefore be finer
grained. Secondly and more importantly, the footwall of
the Midamines area is composed of competent basement
rock with very favourable concentration sites in the form
of gullies and potholes which are able to trap and retain
larger diamonds. In contrast, the footwall farther down-
stream of the Kwango below the Nwazi-Muadi knick-point
is composed of relatively soft Cretaceous sediments. This
results in a smoother channel floor with only silcrete
boulders forming the traps to retain heavy minerals, and is
far less effective as a concentrating mechanism.
Finally the diamonds from the BRC project area contain a
high percentage of fragments and broken stones (42 %), while
30 % of the diamonds show abrasion features (Table 16.3 ).
Since these are all recovered downstream of the two waterfalls
many of the bigger diamonds would have been broken in the
process of being transported over the Falls, which has a 150 m
or more drop over a relatively short distance (Fig. 16.7 )on
competent basement rock, subjecting the stones to an aggres-
sive process of mechanical abrasion.
As mentioned above the small and well-sorted stone size
distribution within the BRC areas supports the premise that
these diamonds represent the
front
'
'
Fig. 16.18 Percentage diamond weight per sieve class for diamonds
from lower Kwango River
of a diamond trail
originating from Angola, and moving down the Kwango
River. Although the Falls are effective in lowering the
stone size over a fairly short distance due to mechanical
abrasion, the decrease in size continues gradually below
the Falls with diamonds reduced to 0.05 cts/stn at
Popokabaka, a distance of almost 300 km. The ages of the
terraces indicate that this post-Cretaceous distribution is
relatively recent, dating from the Early to Mid-Pleistocene.
More significantly it also suggests the coarser stone size
population has not reached the lower reaches of the Kwango
River.
The diamonds which were screened and analysed from
the nine targets within the Study Area feature a high propor-
tion of gem quality diamonds, with colourless stones
representing between 53 to 68 % of the population
(Fig. 16.20 ). The percentage of brown and yellow diamonds
only varies from 10 to 17 %.
head
Fig. 16.19 Comparison of the diamond populations between
Midamine and Lower Kwango areas
'
'
river splays of Group 1 and one from the wide valley section
typical of Group 3 (Fig. 16.9 ).
Bigger diamonds, with an average stone size of 0.25
cts/stn, have been recovered upstream of Tembo at the
Midamines alluvial diamond mining operations (De Decker
2006 ) where the Kwango River passes over the basement
footwall. In this area Midamines were recovering diamonds
from Kakolo, an abandoned channel (Fig. 16.18 ) and from
dredging operations within the Kwango River near the
Lufutu tributary, some 30 km upstream of the Francois-
Joseph Falls (Fig. 16.2 ).
The diamond population from these areas are also well-
sorted with some 24.18 % in the diamond screen size no. 15
(approximate critical diamond size of 1.2 cts/stn) and 0.88 %
in the diamond screen size no. 21 (approximate critical
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