Minerals Report - International Seabed Authority

formation of a large accretion coarse gravel barrier system some 4 km long
and 0.5 km wide [20]. In contrast to the linear beaches further north, the
barrier sequence directly overlies fluvial sediments of the palaeo-Orange
River. The erosive contact is commonly identified by a low-angle,
seaward-dipping surface on which cobbles and boulders (termed locally
“water-melon gravel”) form a transgressive gravel sheet. The internal
geometry of the accreted barrier complex above this contact is complex,
with gravel spit, sub-tidal, inter-tidal and back-barrier facies preserved. In
the absence of Precambrian footwall, the processes of transport and
deposition characterising the different facies have controlled diamond
distribution and concentration.
NSS
L
NDB
B
PB
Figure 4. Spatial variability in littoral ore body architecture based on applied research by
NAMDEB. AB = accretionary coarse gravel barrier complex, LB = linear beach complex,
PB = pocket beach. O = Oranjemund, C = Chameis, B = Bogenfels, L = Luderitz.
The reduced supply of coarse gravel to the north of the
accretionary barrier complex appears to have lead to the development of
narrower linear beaches that are underlain by late Proterozoic bedrock.
This predominantly schistose bedrock has been sculpted by the highenergy
nearshore and shore face processes to form extensive wave-cut
platforms with magnificent examples of large-scale gullies and potholes,
some of which can be several metres deep (Figure 5). These features form
stable (fixed) trap sites in which diamond concentration has taken place
over substantial periods of time, significantly increasing the diamond
grade with respect to those found in the accreted gravel barrier complex to
the south. For this reason, the linear beaches have played an extremely
important role as the main source of diamonds from mining operations at
NAMDEB throughout the past sixty years.
C
LB
100 Km
INTERNATIONAL SEABED AUTHORITY 578
O
AB