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Indicator minerals for diamond include, in order of decreasing significance: garnet, chromite, ilmenite, clinopyroxene, olivine, and zircon. But the order of persistence in streams is zircon, ilmenite, chromite, garnet, chromian diopside, and olivine. Diamond itself is obviously a most important indicator.

The First Kimberlites

The first diamond to be discovered in South Africa was found in 1866 on a farm near Hopetown, south of Kimberley. This was followed by the finding of kimberlite bearing pipes and further discoveries of alluvial diamond fields in the drainage system of the Vaal River in Lichtenburg and Namaqualand, and eventually even on the shores of the Atlantic Ocean off the west coast.

Between 1870 and 1891 no less than six kimberlite pipes were discovered in Kimberley, while the Koffiefontein pipe, 130 kilometres south of Kimberley, was discovered in 1880. The Premier, the biggest pipe in South Africa and, geologically, by far the oldest, was discovered at Cullinan near Pretoria in 1902. The world's largest gem diamond, the 3 106 carat Cullinan, was found here in 1905. Several other small but payable deposits were discovered elsewhere in the country in the early years of the century, though most of these have been worked out and abandoned.

At Kleinzee, south of Port Nolloth in Namaqualand, diamonds were found in marine terraces in 1925. These diamonds were brought down by the Buffels River system in an ancient bed a few kitometres from its present course. A year later, another deposit was found at Alexander Bay on the South African side of the Orange River mouth to the north of which, in Namibia, Consolidated Diamond Mines (now Namdeb Diamond Corporation) was mining diamond bearing marine terraces along the whole coastline to Lüderitz.

Diamond deposits were found in 1927 around Lichtenburg in the North West Province, though today these are virtually exhausted.

A more recent discovery was made in 1961 when Allister Fincham discovered the Finsch pipe, which proved to be one of the largest on the continent, Finsch, in the Northern Cape, came into production in October 1965.

In the early '80s a diamond bearing pipe was discovered at Venetia in the far north of the country close to the Limpopo River. In late 1989, De Beers Consolidated Mines Limited announced the go-ahead for the Venetia diamond mine at an estimated cost of more than R1 billion. The company is developing an opencast mine to treat 3,3 million tons of ore and produce 4 million carats of medium quality diamonds a year, The mine will have an estimated economic life of about 20 years.

Two Kimberlites in Upper Mantle

Relatively high pressures and temperatures are necessary to cause the formation of diamonds, rather than producing the more abundant graphitic form of carbon. Diamonds are formed deep below the earth's surface, typically at depths between 150 and 200 km in a region known as the "upper mantle". Some diamonds may actually have formed much deeper, perhaps at depths in excess of 700 km. Diamonds are generally of two types: p-type and e-type. P-type or "peridotitic" diamonds are derived from peridotitic source rocks in the upper mantle. These rocks are typically composed of olivine, ortho- and clinopyroxene, chromite and pyrope garnet. E-type or "eclogitic" diamonds are derived from eclogitic source rocks in the mantle. These eclogites are essentially biminerallic rocks composed of pyrope-almandine garnet and omphacitic clinopyroxene. In addition to supplying diamond, these peridotitic and eclogitic rocks are also the source of the indicator minerals that are typically found in kimberlites.

(Source: Ashton)

Old Archons Best for
Diamondiferous Kimberlites

Commercial diamond deposits are generally found in two rock types - kimberlite and lamproite, and less commonly, ecologite. Secondary (alluvial) deposits known as placers derived from the erosion of these rocks, may also be important sources of diamonds.

Diamondiferous kimberlite and lamproite are restricted to cratonic environments (or very old continental cores). Cratons represent ancient, stable, continental cores that respond as rigid blocks to deformation and possess gentle, cool, geothermal gradients necessary for the generation of mantle-derived magmas from the diamond stability field more than 90 miles below the earth’s surface. Wyoming, which is underlain by very old rocks, is part of a larger craton known as the Wyoming craton, which extends from Wyoming under Montana, and small parts of Idaho, Utah, and Colorado.

Kimberlite and lamproite are two of the rarest rock types found on the earth. Thus the discovery of a new kimberlite or lamproite, is considered a significant event. Diamondiferous kimberlite and lamproite are even much rarer. Statistics show that only 1 in 10 kimberlites are diamond-bearing, and 1 in 100 contain commercial mineralization. Diamondiferous lamproites are even rarer.

Where found, diamondiferous kimberlite and lamproite are essentially restricted to cratons. Janse (1994) suggested that cratons should be separated into terranes of favorability to facilitate the exploration of diamonds. Based on this concept, cratons are divided into stabilized Archean basement complexes (highly favorable); cratonized mid- to early-Proterozoic basement (moderately favorable); and stabilized late-Proterozoic basement (low favorability).

Of the three terranes, Archons are considered to have high potential for discovery of diamondiferous kimberlite. Protons are less favorable for diamondiferous kimberlite, but are considered to have the greatest potential for the discovery of diamondiferous lamproite. Tectons are considered to have very low potential for the discovery of diamond deposits associated with kimberlite or lamproite. Geological terranes younger than Tectons, are considered to have no potential for diamondiferous kimberlite and lamproite.