DIAMOND SPECIAL — Mining for diamonds off the southern

The discovery of diamonds on the west coast of southern Africa inevitably led to a search for their origin in the immediate hinterland. Only one reputable geologist — ironically the highly respected H. Merensky, who is credited with discovering the major South African platinum deposits — ever seriously believed that the primary origin of these diamonds was submarine kimberlites in the Atlantic Ocean.

All others, particularly consulting geologist E. Reuning, postulated a primary origin somewhere in the continental interior from which, following erosional processes, the diamonds were transported to the sea by such rivers as the Orange, Buffels and tributaries to the Olifants. (The literature on this subject is voluminous, but a comprehensive review can be found in Williams, 1932.)

It has long been known that the primary sources of most diamonds are kimberlite pipes intruded into older parts of the continental interior, that is, cratons (for a review of this subject, see Kirkley et al., 1991). Most of the diamondiferous kimberlites in southern Africa are between 80 and 120 million years old. In the interval between their formation and the present, many of these pipes have been extensively eroded and their diamonds released for transportation into secondary (alluvial, beach, etc.) deposits. In some cases, such as around Kimberley, as much as 1,400 metres of the original depth of the numerous pipes and surrounding country rocks has been eroded (Kirkley et al., 1991). If we consider only the Kimberley mine (“Big Hole”) as an example, and take into account its shape (cone), dimensions (depth of mining, surface area), and amount of erosion since emplacement, calculations show that about 34 times the volume of rock mined has actually been eroded. The volume of rock mined yielded about 14.5 million carats of diamonds before mining ceased in 1914.

Assuming that the pipe had a uniform content of diamonds throughout (a conservative assumption because diamond grades tend to increase, and pipes tend to flare out, toward the top), then about 500 million carats were eroded away from this one pipe alone and released into the drainage basin. There are an estimated 3,000 kimberlite pipes and dikes in southern Africa and, although not all contain diamonds, erosion of their combined original contents (by even the most conservative estimates) is sufficient to far exceed the 1.5 billion carats of diamonds postulated for the marine deposits. This last figure allows for the destruction of many flawed, heavily included, lower-clarity stones en route to the sea.

The dominant drainage in southern Africa has been westward since the emplacement of most of the known kimberlites as long ago as 100 million years (Dingle and Hendry, 1984). Currently, sediment transportation from the kimberlites in the interior of southern Africa is confined to the Orange River drainage system. However, over time, changes in climate and geomorphology have had dramatic effects on river courses, rates of flow, volumes of runoff, rates of erosion, etc.

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

Diamonds have also been transported to the sea along shorter river courses, such as the Buffels, which have cut back into the old interior land surfaces and reworked fossil gravels. Other geologic situations — for example, where small rivers have reworked old exposed beaches to concentrate diamonds into new deposits — are also known but are beyond the scope of this report. From what has been discussed to this point, it should be clear that alluvial diamonds can be found anywhere along the extensive Orange River drainage basin between the primary kimberlite sources and the point at which the diamonds entered the sea. In fact, inland alluvial diggings have been important in South Africa since the discovery of the primary deposits. Nevertheless, of all the gem diamonds that have been released into the drainage basin and have survived the erosional processes, we believe that less than 10% are on land; the great majority have traveled to the sea. Wave action is a powerful agent for transporting material, particularly on the west coasts of South Africa and Namibia, where the winter months are characterized by wild and stormy seas. The waves are generated in the South Atlantic and attack the coastline from the southwest, reinforced by the prevailing southwesterly wind. This results in a strong northerly littoral (that is, along the shore) drift of sediments.

This wave and wind regime has existed along the west coast of southern Africa for millions of years. Thus, littoral drift has played a major role in distributing diamonds along the coast. Coarse sediment (sand and gravel plus diamonds) transported to the sea by rivers is steadily moved northward from the mouths of those rivers.

As diamonds are chemically inert and hard, they are only minimally subject to mechanical abrasion or weathering during transportation along the coast. On the other hand, poorly shaped and strongly fractured stones that survived river transport to reach the ocean are preferentially destroyed in the high-energy wave environment. This destruction of poor-quality stones is reflected in the marine diamond population: well over 90% of marine diamonds recovered are of gem quality, whereas most diamonds mined from kimberlites are industrial.

Another effect of littoral drift is that the process is more efficient for smaller stones, which are transported further than are larger stones. This can be seen along the coast: near the mouths of major rivers, the average stone size is relatively large; at recovery sites progressively farther north of a river mouth, stone sizes are proportionately smaller.

At the mouth of the Orange River, for example, the average diamond size is 1.5 ct, whereas at Luderitz, some 200 km to the north, the average stone size is 0.1-0.2 ct (Sutherland, 1982). Large stones found at the mouths of the Olifants, Groen, and Buffels Rivers are similar in size to those found at the mouth of the Orange River. Thus, the diamonds are sorted and sized during, and as a result of, marine transportation subsequent to their initial deposition into the ocean.

Diamonds have a higher specific gravity (3.52) than do most common minerals (for example, quartz at 2.66) and rock pebbles. Consequently, they tend to gravitate, along with other relatively heavy minerals, to the base of trap sites such as gullies, potholes, south-facing bays and old beach levels. In some instances, spectacular grades occur where the sea has concentrated thousands of carats of diamonds in very small areas.

In general, the smaller the average size of the diamonds, the more evenly the stones are distributed over a beach level. Bigger diamonds are sometimes found in “jackpot” trap sites — usually small, very specific features with only a few cubic meters of gravel. Although the smaller diamonds are less valuable, they are more abundant.

Sea levels have fluctuated widely in the last 100 million years or so, from more than 500 metres below present levels to 300 metres above present levels (Siesser and Dingle, 1981). During times when the sea level was significantly lower, rivers flowed across the now-submerged continental shelf off South Africa and Namibia, and diamonds were transported to the then-prevailing beaches. About 25 million years ago, when the sea level was about 500 metres lower than it is now, some of these beaches were as much as several hundred kilometers into the Atlantic Ocean compared to the position of the present shoreline, because much of the continental shelf was exposed. Littoral drift processes similar to those that operate today distributed diamonds along the ancient coastline. Where the sea level remained constant for some time, wave-cut cliffs and terraces formed, as did sites in which diamonds could be trapped. Today, there are at least eight different levels — ranging from 20 to 120 metres — below modern sea level, in which persistent wave-cut terraces can be traced over much of the length of the west coast of southern Africa; all potentially hold diamonds. Beaches that formed when sea levels were higher than today are currently exploited for their diamonds. In Namibia, for example, CDM is currently mining (or has mined) at least four exposed beach levels that extend to 90 metres above sea level.

In South Africa, in the region between the Orange River and Port Nolloth, diamonds are present in raised beaches at seven elevations ranging from 9 to 84 metres (Geological Department, De Beers Consolidated Mines, 1976, p.27). It was in this coastal area that a 211.3-ct diamond was recovered. The modern beach level and associated terrace is also well mineralized with diamonds, and both surface and underwater mining operations occur from south of the mouth of the Olifants River to north of Luderitz.

Thus have sea-level variations and littoral drift distributed diamonds over the continental shelf off the west coast of southern Africa, making it in all probability the greatest resource of gem diamonds in the world. — From the publication “Gems and Gemology,” by John Gurney, Alfred Levinson and H. Stuart Smith.