Diamonds in southern Africa

Southern Africa currently produces more than 50% of world’s total rough diamond
market now valued at greater US$8 billion in 2004. Botswana leads the world in
the production from Debswana’s two large mine at Jwaneng and Orapa. Diamonds
form and are stable under high pressure in the world’s thickened lithospheric keels.
Haggerty (1986) first proposed a model of how diamonds can form and be stored
within these old lithospheric zones within the diamond temperature and pressure stability
field at 160 km depth and 1325°C if a typical shield geothermal gradient is
assumed. These lithospheric keels can be imaged by deep seismic tomography studies
(Karason and van der Hilst, 2000), and they are particularly evident in southern
Africa beneath the Kalahari and Congo cratons.
Kimberlites are hybrid, volatile-rich (carbon dioxide) potassic ultramafic rocks.
Diamonds occur as mantle xenocrysts entrained by kimberlite magma intruding
through these deep lithospheric keels or diamonds reservoirs. Kimberlites range in
age from Proterozoic to Tertiary. In southern Africa, age dating of all the kimberlites
has not been systematic. However, there appears to be four main age clusters for most
kimberlite magmatism, in the mid-Mesozoic (235–156 Ma), Cretaceous (136–113 Ma
and 95–78 Ma), and finally in the Late Cretaceous-Tertiary (66–54 Ma).
Africa was breaking apart from the large Pangaea supercontinent during this
time period, when a number of intracratonic rifts, large igneous provinces and dike
swarms formed due to a number of hypothesized mantle plumes. The association of
kimberlites with mantle plumes remains controversial, but they could provide the
necessary small volume decompression partial melting within the mantle to form the
mafic alkaline magmatism of the kimberlite and carbonatite clan (Bell, 2001).
Exploration for diamond-rich kimberlites involves extensive regional till,
stream, and sediment geochemical surveys targeting diamond indicator minerals.
Detailed magnetic and electromagnetic geophysical surveys help define drill targets.
BHPBilliton’s proprietary Falcon airborne gravity gradiometer system is another
advanced geophysical tool for delineating new kimberlite pipes in covered, magnetically
noisy, or conductive terrains.