To All:
The following is an article on Marine Mining in Africa, It clearly illustrates the enormous potential for the discovery of world class gem quality diamond deposits. Keep in mind that it was published in 1991, since then the technology for mining the deep waters has progressed significantly.
Read it carefully and as always your questions or comments are welcomed.
Regards,
John
Marine Mining of Diamonds off the West Coast Of Southern Africa
By John J. Gurney, Alfred A. Levinson and H. Stuart Smith
(Gems & Gemology, winter 1991)
A vast resource of gem quality diamonds exists off the west coast of Southern Africa. Over the course of millions of years, many diamond bearing kimberlite pipes in the Orange River drainage basin have been extensively eroded to the west coast. Raised marine deposits now on land have yielded almost 100 million carats of predominantly gem diamonds; similar marine deposits and feeder channels are now known to exist offshore. Techniques for exploiting the offshore resources have been proved on a small scale in shallow (<15m) waters. New technological developments in underwater mining have progressed to the point where mining has commenced in deep (about 100m) Namibian waters. It is anticipated that production of diamonds from the sea will increase substantially in the future.
Because diamonds are the heart of the jewelry trade, the continued supply of fine diamonds from the mines into the marketplace is of critical importance to this industry. According to the Central Selling Organisation, about one-eighth (approximately 13 million carats) of the diamonds now mined annually eventually end up in jewelry. Yet for the largest producer of diamonds in 1990, the Argyle mine in Western Australia (36 million carats), fine gem quality diamonds represented only about 5% of the total yield. In addition, older deposits of gem quality diamonds are gradually being exhausted. For example, the total production at the Kimberly pool of mines was 1,173,042 ct in 1980 but only 574,188 ct 1990 (DeBeers Consolidated Mines Ltd., 1981, 1991).
In the future, the steady supply of gem diamonds to the jewelry industry will depend on the discovery of new deposits and the engineering expertise to extract the diamonds economically. Because the search for new diamond reserves in conventional primary (e.g., kimberlite or lamproite) or secondary (e.g., alluvial) deposits is very expensive and generally has a low probability of success, mining concerns are looking to the extraction of diamonds from known, if unconventional, sources, such as the undersea deposits off the west coasts of South Africa and Namibia.
These exceptional deposits of gem quality diamonds have been known for some time, but they have not been exploited fully because of the technological difficulties of recovery. Estimates of the amount of diamonds range upward from a conservative 1.5 billion carats, of which approximately 90%-95% are gem quality (Wilson, 1972, Meyer,1991). Thus, the marine deposits off southwestern Africa apparently contain at least 100 times as many gem diamonds (by weight) as are presently used annually in jewelry. In Addition, this source contains a high percentage of rough suitable to cut the small (0.25-0.75 ct) gems that are very important in the jewelry industry.
The economic and technological climate now permits mining of these deposits. Although the problems of recovery are major, as will become clear from the discussion below, considerable progress has been made in recent years to establish a viable extraction industry. The financial risks continue to be significant, but the vast reserves hold extraordinary promise.
History The first discovery of diamonds related to marine deposition in Southern Africa was on land in 1908 near Luderitz, Namibia, the history of this discovery is described in detail by Levinson (1983). This led to the subsequent discovery of rich diamond fields along the west coast of then German South West Africa, and the development, within a few years, of a huge industry in this arid, inhospitable region. Later, diamonds were also discovered and mined elsewhere along the vast coastline from south of the Olifants River in South Africa to north of Hottentot Bay in Namibia, a distance of about 1,000 km. Although the great majority have been mined on what is now land (on beaches and raised terraces), the diamonds were originally deposited under water, having been carried into the sea by rivers at a time when the oceans were at a higher level.
Further details on the historic aspects of marine coastal diamonds off Southern Africa may be found in the articles by the Geological Department, De Beers Consolidated Mines (1976), Van Wyk and Pienaar (1986), and Meyer (1991), and in books by Wilson (1982), Levinson (1983), and Joyce and Scannell (1988).
FORMATION OF THE MARINE DIAMOND DEPOSITS
Source of the Diamonds. 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 Dr. H. Merensky, who is credited with discovering the major South African platinum deposits, ever seriously believed that the primary origin for these diamonds was submarine Kimberlites in the Atlantic Ocean. All others, particularly consulting geologist Dr. E Reuning, postulated a primary origin somewhere in the continental interior from which, following erosion 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 Kimberlites 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, may 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 1,400m 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 Kimberlites 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 coasting. 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 Kimberlites 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.
Marine Distribution. 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 (i.e., 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 diamonds 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 further north from 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 200km 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 (e.g., 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 m below present levels to 300 m 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 500m 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 120m – 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 presently mining (or has mined) at least four exposed beach levels that extend to 90m 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 84m (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.
EXPLORATION, RECOVERY, AND NEW TECHNOLOGY
The captivating questions that follow on the realization that there are potentially huge diamond deposits in the sea are: (1) where exactly are the diamonds located and (2) can they be recovered economically.
The marine deposits can be conveniently divided into two zones on the basis of their depth beneath the water (these zones should not be confused with the A, B and C zones used in connection with leasing concessions). Those in water shallower than 15m are being very actively reworked by waves and currents. Those in deeper water are today preserved as fossilized horizons (stable locations unaffected by waves and currents). The practical reason for recognizing these two zones is that divers can operate for extended periods of time in shallow water without the need for sophisticated equipment, whereas in deeper water such is not the case.
Since the late 1970s, independent divers have been recovering small volumes of gravel from favored trap sites in shallow water all along the western seaboard of Southern Africa. In the process, they have demonstrated the presence of rich concentrations of diamonds from the mouth of the Olifants River in the south to Hottentot Bay North of Luderitz. The divers have recovered these gravels using suction-pump equipment mounted on tractors on beaches and rocky promontories, or on small boats. On several occasions over the past 15 years, individual pump sites have yielded over 1,000 ct of diamonds from less than 10 cubic meters of gravels.
This level of activity in the shallow zone (<15m), however, is not likely to flood the world market with diamonds. Even in a record year such as 1990, only 127,000 ct of diamonds were recovered by these methods, and the prospects for improving on this figure are limited.
The real potential lies in the deeper water, where it is possible to explore systematically for the hidden gemstones. For practical purposes, these deeper waters can be divided into two zones: (a) water depths of 15 to 40 m, in which exploration activities, including sampling, can be carried out with existing technology, and (b) water depths in excess of 40 m, in which more advanced equipment is needed.
Exploration in these two zones initially takes the form of a geophysical survey that uses sophisticated position-fixing and data gathering equipment to produce the equivalent of an aerial photograph of the sea floor on which can be superimposed a number of remote sensing measurements. Such surveys are costly ship-borne operations that are best carried out in good sea conditions. The amount of detail that can be recorded and interpreted has escalated by leaps and bounds in recent years through computerization of data bases, satellite navigation, and marked improvement in the sensitivity of geophysical equipment. Not only can submerged river channels and deltas, cliff lines, and storm beaches be readily identified, but individual gullies, basin shaped deposits, and even fossil ripple marks in gravels-the types of features that could contain diamonds-can also be recognized and located again easily.
The next step in the exploration for deep-water deposits is to sample these features to establish whether or not they are mineralized. Devices such as airlifts, underwater robots, or jet pumps can be custom designed for the job. Following exploration and sampling in the deeper-water areas, mining must follow the high-tech route. Ocean going vessels that can stay at sea for extended periods must be used. Robotics of some sort are mandatory, some of the vessels even contain full facilities to separate and sort the diamonds.
Conclusions A unique combination of geologic and geographic (climatic and geomorphologic) factors has resulted in the concentration of an estimated 1.5 billion carats of gem quality diamonds in the sea off the west coasts of Namibia and South Africa. These factors include: (a) the occurrence of many diamond bearing kimberlite pipes in the present Orange River drainage basin; (b) the extensive erosion of these pipes over the last 100 million years; and (c) the consistent drainage of the present and ancestral Orange River westward into the Atlantic Ocean. Wave and wind action, and littoral drift to the north, have resulted in a predictable distribution of diamonds with respect to size. These marine deposits are probably the largest known resource of gem quality diamonds in the world. However, large scale recovery of the diamonds from beneath the sea poses major engineering and mining problems. Nevertheless, the prognosis is good for the economic success of the venture, despite the technological challenges, thus ensuring a significant component of the world's future requirements for gem diamonds. |
