South African Coastal Diamonds - Namaqualand Mines : Koingnaas, Buffels Inland and Marine, AlexKor

Northern Cape, South Africa

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Coastal diamondiferous accumulations are spread over an approximately 280 km interval of the Alantic Coast of South Africa, from Alexander Bay near the mouth of the Orange River in the north, to south of Kleinzee. This coastline is characterised by a series of wave-cut terraces in a bedrock of Precambrian schist, phyllite and gneiss on which are deposited diamondiferous sediments.

Diamonds were discovered in 1925 by a prospector south of Port Nolloth, and mining commenced in 1927. Diamond distribution correlates with the nature of the bedrock and sediment. Diamonds are predominantly concentrated in the basal gravel unit of the terrace sediments which forms the main ore zone in most areas, although they can occur virtually anywhere throughout the stratigraphic sequence.

The distribution of diamonds reflects the regional interaction between the northward transport of diamonds along the coast by littoral drift and the entrapment of diamonds by various features such as coastal embayments. The bedrock morphology is also a major control on diamond accumulation, with stones being preferentially concentrated in bedrock trap sites. The littoral (or long-shore) drift in the surf zone along the coast, results from the interaction between long period south-westerly swells and powerful south-westerly winds that prevail through much of the year. The main diamond distribution spreads northwards from the mouth of the Buffels River, with a corresponding northward decrease in average stone size, from 0.85 carats per stone immediately north of the Buffels River, to 0.37 carats per stone at Oubeep, 30 kilometres further north.

There is also a noticeable variation in average diamond concentration between the various raised beaches. The most important is the 45 metre beach terrace, suggesting that this level corresponded to a period when the Buffels River was supplying diamonds to the surf zone at a relatively high rate.

There are four main mining areas as follows:

Koingnaas Complex - between Mitchells and Somnaas Bays. (De Beers Namaqualand Mines)
Two types of deposits are exploited at Koingnaas, a lower non-marine "Channel Clay" deposit, which are overlain by younger marine sediments and preserved in broad bedrock depressions. The "Channel Clay" deposit comprises a series of steep gradient, contributory channel systems filled with texturally immature, compositionally mature basal gravels. During the many post-Miocene transgressive and regressive cycles, some of the channel deposits were reworked by marine processes and the diamonds concentrated in the basal marine gravel in bedrock depressions. Such diamondiferous marine packages are recorded at 90, 50 and 30 metre levels, while recent emergent terraces have beaches at 3, 6 and 10 metres above sea level.

Buffels Inland Complex - on terraces of the Buffels River inland of Kleinzee (De Beers Namaqualand Mines).
Diamondiferous gravel deposits are found along the lower parts of the Buffels River below the Great Escarpment. The best preserved examples occur as basal indurated gravels of the terraces, below a series of sandstones, siltstones and clays that fill bedrock channels. Typically they comprise well-rounded, clasts with grey, milky quartz clasts dominating.

Buffels Marine Complex - on the raised beaches north of the mouth of the Buffels River. (De Beers Namaqualand Mines)
This complex of deposits extends for some 40 kilometres north from the mouth of the Buffels River and is related to the bedrock competency which influences the diamond distribution. Deep potholes and gullies were developed on the gneisses, in which diamonds have concentrated. In contrast, on the less competent, foliated or calcreted lithologies, a more subdued bedrock profile developed without good trap-sites. A number of raised beaches of Pliocene to Pleistocene age occur within the Complex, locally divided into three terraces, the Upper (75-95 masl), Middle (30-65 masl) and Lower (10-30 masl). Lithologically, the terraces have a basal transgressive unit of diamondiferous gravel overlain by sediments of nearshore, upper shoreface and foreshore facies. The fluvial channel and marine sediments are overlain by aeolian sands up to 25 metres in thickness.

The total combined reserves and resources of the three De Beers Namaqualand mines, namely Buffels Inland, Buffels Marine and Koingnaas in 2000 were 51.7 million sq. metres @ 0.25 carats/sq. metre, for 12.9 million carats. These figures are based on a 2 mm stone cut-off and yielded an average value of USD 159 per carat.
Production from the three De Beers Namaqualand mines in 2000 totalled 0.810 million carats of diamonds at an average grade of 13.2 cpht.

Port Nolloth to Alexander Bay - from Port Nolloth to the mouth of the Orange River - Alexander Bay Development Corporation (AlexKor)
Presently day operations exploit both both beach deposits and shallow marine accumulations. Production in 1999 totalled 0.097 million carats.

The most recent source geological information used to prepare this summary was dated: 2003.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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