Introduction

The west coast of Southern Africa has been the greatest source of gem quality diamonds for over one hundred years.

However, since the turn of the century there has been a significant shift in the exploration and mining of the diamondiferous gravel deposits.

The relatively accessible beach and nearshore deposits have been significantly depleted, and a number of the most bountiful ore deposits have now been completely mined out.

With the depletion of onshore reserves, the mining companies have moved the focus of exploration and mining activity to the potential vast reserves of diamonds located on the continental shelf up to 120m below modern sea levels.

This paper briefly reviews the history of diamond mining on the west coast of Southern Africa, including the journey from their primary source, the characteristic coastal distribution patterns, and the techniques used to extract the diamonds.

The paper focuses on the Namaqualand mining complex, situated along a 250km stretch of the Atlantic coast of South Africa and until recently owned by De Beers Consolidated Mines. Some 31 million carats of gem quality stones were extracted from this area by the turn of the century.

Although De Beers has managed to continue its mining operations in the area by tapping into the vast diamond reserves offshore, this paper also explores the social, economic and environmental consequences of the paradigm shift from onshore to marine mining activity.

This onshore mining legacy is explored through the demise of the once prosperous mining towns of Kleinzee and Koignaas in Namaqualand, and the recent sale of Namaqualand Mines by De Beers to Trans Hex Corporation.

History

The discovery of the 21.25 ct ‘Eureka’ diamond in late 1866 and then the 83.50 ct ‘Star of South Africa’ in 1869 on the banks of the Orange river triggered the South African diamond rush.

However, it was not until 1925 that the first diamond was discovered in Namaqualand, on the Port Nolloth Reserve (see fig 1).

Figure 1 – Key rivers and towns on the
west coast of Southern Africa

Following the discovery of rich diamond deposits at Alexander Bay in 1926, thousands of diggers rushed to Namaqualand to seek their fortunes.

However, this diamond ‘free for all’ was short-lived, and in the following year the South African government issued laws to restrict any further prospecting in the area.

By 1928, the diamond mining rights on the Namaqualand coastline were starting to take shape. The area from Port Nolloth to Alexander Bay is owned by the South African government, who started mining operations through a company known as Alexkor.

The other main mining areas are centred around the mining towns of Kleinzee and Koignaas, and were consolidated by De Beers into De Beers Namaqualand Mines.

Until recently, De Beers not only owned, controlled and operated the Namaqualand mining areas, but owned and controlled most of the coastal strip of farming land extending south from Port Nolloth for about two hundred miles to Lamberts Bay.

The first offshore diamond prospecting was started in 1954, and geological exploration and mining on the continental shelf gained momentum towards the turn of the century.

With diamond land deposits expected to be exhausted in as little as 15 years, De Beers is building up a naval fleet to mine marine diamonds off the coast of South Africa and Namibia.

Namdeb, De Beers’ 50:50 operations with the Namibian government, estimates that 95 per cent of its diamonds will in future come from the sea bed.

Primary Source of the west coast diamonds

The discovery of diamonds on the west coast of Southern Africa inevitably led to a search for their primary origin. There is now overwhelming evidence to suggest that the diamonds originated from kimberlite pipes intruded into the interior of South Africa during the cretaceous period, between 80 and 120 million years ago.

Since their formation, many of these kimberlite pipes have been extensively eroded and their diamonds released for transportation into secondary deposits. Figure 2 shows the present Orange river drainage basin, including the present position of the Orange and Vaal rivers.

Also shown are the locations of kimberlites with and without diamonds as well as the Gibeon and Namaqualand kimberlite fields, both of which are barren of diamonds.

Alluvial diamonds have been found throughout the drainage basin, but the vast majority have been transported westward towards the Atlantic ocean.

Figure 2 – Kimberlites within the Orange River drainage basin

The journey to the sea

The dominant drainage in Southern Africa since the kimberlite intrusions has been westward.

For at least the last 80 million years, the Orange river has transported sediments from the continental interior to the Atlantic ocean through two main courses, which have led to the deposition of diamondiferous sediments at different positions along the coastline (figure 3).

It is likely that, for 45 million years (from 20-65 million years ago) the mouth of the Orange river was located about 400km south of its current location, in the area that now forms the mouth of the Olifants river (Dingle and Hendry,1984).

Diamonds have also been transported to the sea along secondary river courses, such as the Buffels river, which is believed to be one of the principal transport routes for diamonds found in the area from Kleinzee to Port Nolloth. This area has been extensively mined by De Beers Namaqualand Mines.

Figure 3 – The orange river drainage system has migrated over time. The possible
ancient course is shown in purple and the current course is shown in blue.

Distribution patterns

Wave action plays an important role in the distribution of diamonds along the west coast. The waves are generated in the South Atlantic and attack the coastline from the southwest, reinforced by the prevailing south westerly wind. This results in a strong littoral drift of sediments northwards from the mouths of the rivers.

As diamonds are chemically inert and hard they are only minimally affected by abrasion or weathering during transportation along the coast.

Poorly shaped and strongly fractured stones that survived river transport to reach the ocean are preferentially destroyed in the high energy wave environment.

The destruction of poor quality stones is reflected in the diamond population. Well over 90% of diamonds recovered from the coast are of gem quality.

Another effect of littoral drift is the distribution in stone size. Near the mouths of the major rivers, the average stone size is relatively large and the stone size decreases northwards from the river mouths. At the mouth of the Orange river, for example, the average diamond size is 1.5 ct whereas at Luderitz, some 200km to the north, the average stone size is 0.1-0.2 ct. In Namaqualand, the average stone size just north of the Buffels river near Kleinzee is 0.85 ct. The stone size decreases northwards along the coast to about 0.3 ct at Port Nolloth.

Another property of diamonds, their high specific gravity, increases the propensity of diamonds to be concentrated in gullies, potholes and bedrock depressions. When extracting diamonds from beach gravels, these bedrock features have uncovered some of the richest diamond deposits along the coastline.

Diamond recovery

Until the turn of the century, the vast majority of diamonds on the west coast were recovered from onshore and foreshore beach deposits. With the gradual decline of these reserves, focus has now turned to the exploration and recovery of diamonds deposited on the continental shelf.

Onshore and Foreshore deposits

Onshore beach diamondiferous gravels were deposited up to 200m beyond the current high water mark. The gravels are often covered by up to 35 metres of overburden, which initially needs to be removed by giant scrapers or rotary bucket excavators.

The overburden is either back-dumped into mined out areas or, in the case of foreshore mining, dumped onto the lower beach to build a retaining wall or dyke to keep out the sea.

The exposed diamondiferous gravel is then removed and taken to a treatment plant for further processing. As mentioned earlier, rich diamond deposits are often found in potholes, gullies and cracks in the bedrock, and great care is taken to remove the denser material that has accumulated in these areas using large suction pumps – see figure 4.

Historically, the bedrock was swept clean by a team of ‘bedrock cleaners’ to ensure that every last grain of ore was removed.

Figure 4 – Giant vacuum cleaners are used to remove the diamond rich gravels from crevices and potholes in the                  bedrock at the Kleinzee mine.

At the treatment plant the pebbles and boulders are first removed by passing the gravel through revolving cylindrical sieves or trommels.

The remaining ore then passes through a dense media separation process, where the high-density diamonds and heavy metals (the concentrate) settle out.

The next stage of the process uses important properties of diamond to allow it to be separated from the concentrate. Historically, grease tables were used to separate diamonds from the concentrate, taking advantage of the fact that diamonds do not wet in water and are attracted to grease. A more common way of separating the diamonds nowadays is via an X-ray separator, a technique that utilises the fact that diamonds fluoresce under X-rays. The final stage of the process requires skilled sorting by hand.

Marine deposits

Since the discovery of diamonds along this coastline, there has been speculation about diamonds existing on the adjacent seabed.

In terms of the potential size of the deposits, it is postulated that of all the diamonds that have been released into the Orange drainage basin, less than 10% are on land and the great majority have been deposited across the now submerged continental shelf. Marine diamond deposits on this coastline have been estimated to far exceed 1.5 billion carats.

DebMarine Namibia, a partnership between De Beers and the Namibian government, now have a fleet of five specially-adapted ships fitted with giant tractors, drills and suction pumps. The fleet now mine more than one million carats a year.

Figure 5 – One of a fleet of Dredgers owned by DebMarine Namibia

The ships dredge the ocean floor using a windscreen-wiping arm, sucking 60 tonnes of sediment each hour up to the ship through a giant hosepipe.

On-board, the sediment is washed and sifted into increasingly smaller stones using a series of vibrating racks and rotating drums. What is no longer needed is then flushed back into the water.

The diamond-bearing gravel is further processed on-board before being helicoptered, three times a week, to vaults onshore.