Diamond-Bearing Gravels Along the Lower Kwango River DRC - Geology and Resource Potential of the Congo Basin - page 332

other major exploration and mining activities took place
until the mid-1950s.
In Angola diamonds were first found in 1912 in Lunda
Norte Province also by Formini`re . Later diamonds were
also found along the Cuango River but like in the DRC,
where exploitation of the high-grade Tshikapa and Mbuji
Mayi deposits took preference, mining along the Cuango
River only started in the 1950s as it was far to the west of
the rich diamond deposits of the Lunda Provinces in north-
east Angola.
In 1954 Companhia de Diamantes de Angola (Diamang)
in Angola undertook an investigation along the western bank
of the Kwango River in the vicinity of the Francois-Joseph
Falls (Fieremans 1977 ). Prospecting focussed on the natural
traps formed by the many gullies, fractures and potholes in
the granites around the falls near Tembo where washing
gravels, some 25 km upstream of the Francois-Joseph
Falls, returned 81 diamonds weighing 9.42 cts. This led to
the exploration of the entire upper Cuango Valley. The
Portuguese during their early prospecting work believed
that local diamonds were derived from basal conglomerates
of the Kwango Group. Diamang started a systematic sam-
pling program of the river gravels using diversions. In
places, where the gullies and potholes were cleaned out,
subsequent investigations showed renewed mineralization
in these localities. The most important concentrations
occurred where the river cut the basement and rocks of the
overlying Bembe Group, and in particular where the contact
between the two is exposed. By 1958 the prospecting had
progressed to about 130-150 km south of the DRC border
and important concentrations of diamonds were found in the
vicinity of the bridge over the Cuango River near Luzamba.
Further downstream, beyond the confluence of the Lulo-
Cuango, diamonds of 0.5-1 cts were found. In 1958 river
diversions were used around Luzamba, Cafunfo and Luremo
and by late 1980s production reached 100,000 cts/month
from the Cuango River. Potholes and water falls (e.g Tazua
falls) were targeted with record recoveries of up to 50,000
cts from one pothole. By 1983 29 % of Diamang ' s produc-
tion came from Cuango Division and by 1991 as much as
80 % of Angola

potholes and gullies. However, diamonds along the Kwango
River in the DRC are smaller typically ranging between
0.07cts and 0.15 cts, whereas in Angola they range between
0.1 and 1 cts (Fieremans 1996 ).
Finally, the activities along the Kwango on the DRC side
declined after independence in 1960 and although low-level
artisanal activities did continue, it was not as intensive as
that in Tshikapa and Mbuji Mayi. It was not until 2003, with
the introduction of the new mining legislation, that the
international Kwango was covered by prospecting licences
awarded to Le Grand Congo sprl (later to be renamed Acacia
sprl ) with Midamines (De Decker 2006 ) securing ground
directly upstream of the Falls. Canadian junior BRC Pty
Ltd signed an option agreement with Acacia in 2005 and
started a systematic exploration program of the Kwango
alluvial deposits almost immediately. The following

summarizes some of the BRC

' s work and results along the

river.

16.3 Geology

The Kwango River lies in a broad valley cut into mainly
Cretaceous sediments and the river has exhumed sections of
Precambrian granites and gneisses notably around Tembo in
the south-western corner of the DRC (Fig. 16.3 ). Similar
basement rocks have been exposed in the southern Wamba
Valley some 60 km to the east.
De Carvalho et al. ( 2000 ) refer to the basement in the
Cuango Valley as the Cuango Shield, which is composed
of undifferentiated granite-gneiss-migmatites and gabbro-
norite-charnockite complexes. The migmatites have been
dated between 2680-2590 Ma (Delhal et al. 1975 ), and the
charnockisation event has been dated at 2822 Ma and
2820 Ma by Delhal et al. ( 1976 ) and Cahen et al. ( 1984 )
respectively.
In Angola, granite-gneiss basement has been mapped
along the Lui River, near the confluence with the Cuango.
Neoproterozoic rocks of the West Congolian Belt (Bembe
Group) have been described by Forward and Jeffcock ( 2003 )
along the Cuango north of Luremo. Interestingly around
the town of Cuango in Angola the hills are composed of
gneiss and charnokite, whilst the valleys have been devel-
oped in the softer

s production came from the Cuango (PAC
2004 ). The increase in diamond sizes upstream in the
Cuango Valley is further illustrated from workings in the
Caculio tributary. Here large diamonds (131.5 and 38.3 carat
stones) have recently been recovered in alluvial operations
(Mining Weekly 2012 ) that have reported average stone
sizes of between 1.35 cts/stn and 2.84 cts/stn (Lonhro
Mining 2011 ).
Based on these results the DRC government ordered
another investigation of the region between the Guiliame
and Francois-Joseph Falls in 1961. The results confirmed
earlier work from 1959 and demonstrated that significant
local concentrations of diamonds were to be found in

'

meta-sediments of the
Neoproterozoic Bembe Group (Delhal and Fieremans 1964 ).
Basement rocks from the Francois-Joseph Falls area were
giving a minimum age of 2600 Ma by Mendes (Cahen et al.
1984 ) based on geochronological results which plot close to
the Dibayan Complex gneisses

' schisto-calcaire '

isochron in the Kasai,
approximately 300 km to the east. The Dibayan complex
was later dated at 2650 Ma (Delhal 1991 ). Fieremans ( 1977 )
interprets conglomerates and schists that occur close to the
Francois-Joseph Falls as being part of the Bembe Group.

'