Deposits Formed by Sedimentary and Surficial Processes - Metals and Society: An Introduction to Economic Geology - page 124

Fig. 5.3 Location of three different types of diamond deposit in southern Africa: primary
kimberlitic diamond; alluvial deposits in present and past river beds, and alluvial deposits in
offshore gravels (From Lynn et al. ( 1998 ))
5.2.3 Alluvial Diamonds
The first diamonds discovered in South Africa were in gravels of the Orange River
and its tributaries. Tracing these rivers back to their sources led first to the discovery
of the primary diamond sources in kimberlites around the town of Kimberley in the
centre of South Africa, and then to enormous beach placers at the western coast of
the continent in South Africa and Namibia (Fig. 5.3 ).
Placer deposits are the source of about 34% of global diamond production. By
far the largest fields are those along the western coast of southern Africa, from Cape
Town to the Congo, but placer diamonds have been mined in India throughout
history and also in large quantities in Russia and Brazil. Placer deposits normally
contain a high proportion of high-quality gem diamond (up to 97%) because large
flawless single crystals survive the rigours of sediment transport and deposition
better than smaller flawed crystals.
Alluvial diamonds in the Orange River and tributaries (Fig. 5.4 ) are mined at
scales from artisanal to medium-sized commercial operations. Recovery of
diamonds in offshore gravels, which are present at water depths up to several
100 m, requires the heavy equipment - a diverse flotilla of barges connected to
large tubes that suck the diamonds from the sea floor (Fig. 5.5 ).