The Source of Our Diamonds

 

A vast resource of 90-95% gem-quality diamonds exists off the west coast of Southern Africa.

 

The diamonds occurring in marine gravels along the West Coast originated from kimberlites intruded into the interior of South Africa during the Cretaceous period, between 80 and 120 million years ago. Since the Late Cretaceous, erosion resulting from continental uplift has removed an estimated 1400m of the kimberlite pipes and surrounding country rocks. The bulk of the erosion took place during the Late Cretaceous and little denudation of the southern African interior occurred during the Tertiary.

 

The gradual change from a warm humid climate during the Late Cretaceous to arid conditions in the Late Tertiary resulted in a dramatic reduction in the volume of sediment supplied to the West Coast in the Cenozoic. The implication of this is that by the end of the Cretaceous most of the diamonds released by erosion were made available to the West Coast. During this period the main drainage system from the diamondiferous interior to the West Coast was along the so called Karoo River which debouched onto the continental shelf via the present day Olifants River.

 

Tectonic uplift, differential erosion rates and climatic aridification resulted in the so called Kalahari River (proto Orange River) capturing the headwaters of the Karoo and by the end of the Miocene the modern Orange River drainage system was established and would have acted as the principal transport route for diamonds to reach the coast. Secondary river courses, such as the Buffels and Swartlintjies Rivers, which were far more active during the past, have cut back into the old interior land surfaces and reworked fossil gravels.

 

This latitudinal change in the palaeo-drainage pattern of major river systems flowing to the west coast during the Cretaceous and the Tertiary together with powerful wave action and longshore drift, have produced diamondiferous deposits along the length of the west coast from south of the Olifants River to the present day Orange River as well as along the Namibian coast at least as far north as Conception Bay.

 

The size and quantity of wave-transported diamonds decreases away from palaeo-river mouths which have acted as point sources for the gems. The tailing off is much more gradual northward from the river mouths than southward, this being attributed to strong northward directed longshore drift.

 

Exploration for marine alluvial diamonds both onshore and in the surf zone, shows that there are preferential localities in which marine sedimentary deposits have higher probabilities of containing diamonds. These include gullies, potholes and bedrock depressions, all of which are associated with marine wave-cut terraces. Palaeo drainage channels, the southern side of headlands and wave abraided terraces all have higher probabilities of containing diamonds.

 

Diamonds have a higher specific gravity than most common minerals. Consequently, they are generally found close to the bedrock and are deposited in high-energy environment sediments containing pebbles, cobbles and boulders. These sediments commonly owe their existence to storm beach deposits along the base lines of low cliffs that back wave cut terraces.

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