Minerals Report - International Seabed Authority

The Orange River, one of Africa’s great rivers, has largely been
responsible for the introduction of diamonds to the southwestern
continental margin of Namibia. The erosion of kimberlite pipes within the
interior of southern Africa has released large quantities of diamonds for
transport to the continental margin, with the most recent pipes being
intruded into the Kimberley region of the KaapVaal craton from 120 to 90
Ma 5 . Diamonds have also been derived through the erosion of older
sedimentary sequences ranging in age from 2700 My (Archaean) to 70 My
(Cretaceous) within southern Africa [2,3]. Since establishment in its
current position some 80 Ma, the Orange River has supplied clastic
sediment to the post-Gondwana continental margin, making a major
contribution to the development of the continental shelf [4,5,6]. Changes in
the base level of the system driven by eustatic and tectonic changes from
the Orange River mouth upstream have promoted placer formation within
onshore reaches of the palaeo-Orange system. At the same time this has
presented opportunities for the river to supply coarse clastic gravel to the
sub-aerially exposed continental shelf, with the result that the deposition
of extensive fan-delta complexes have provided enormous quantities of
material for subsequent shore faces to rework. Fluvial ore bodies are
preserved within palaeo-valleys [7,8,9] that were operative from 19 to 17
Ma and 5 to 3 Ma [10,11,12,13]. Strong evidence also exists for diamond
introduction during the Eocene (50 to 35 Ma) based upon the presence of
diamondiferous palaeoshorelines up to 160 masl [14,15].
Repeated marine regression(s) and transgression(s) spanning some
60 My, which became more frequent during the Pleistocene
(approximately the last 2.5 My), have been instrumental in the formation
of both the onshore and offshore marine diamond deposits. In both
instances, the destruction of the clastic beaches by arid zone processes has
released diamonds for further transport – principally by the aggressive
sand-laden southerly winds. Subsequently, river systems within the
deflation basin have periodically reworked the aeolian placers [14,17,18].
Exoreic systems reintroduce the diamondiferous sediments onto the
continental shelf, where reworking by long shore wave transport
reconcentrated the diamonds in well-developed trap sites sculpted by
nearshore and shore face processes into the Precambrian rocks flooring the
inner continental shelf [18].
5 Millions of years.
INTERNATIONAL SEABED AUTHORITY 576