EXPLORATION ’94 — The lure of Africa’s offshore diamonds

The diamond deposits off the western coast of Southern Africa are luring a growing number of companies interested in exploiting them.

Although De Beers continues to dominate the offshore lease areas of both South Africa and Namibia, several new players have arrived on the scene, attracted by immense deposits estimated at more than 1.5 billion carats, with upwards of 90% classified as “gem quality.”

These diamonds are widely accepted to have come from the diamondiferous kimberlites of southern Africa, situated in the Orange River drainage basin. The pipes are extensively eroded and the diamonds have been transported to the sea by the Orange and other rivers.

It has been estimated that for every diamond mined from kimberlites in the Kimberley district, 30 times as many have been eroded away and transported westward to the Atlantic Ocean.

The marine diamond deposits have been shaped by forces other than erosion, in particular the wild and stormy seas that often batter the southern coast during winter. The high-velocity winds and waves are believed to have existed for millions of years and played a major role in distributing diamonds along the coast.

Experts say the high percentage of gem stones found in offshore deposits reflects the fact that poorly shaped and flawed stones are usually destroyed, either in the process of being transported by the river to the ocean or by the waves.

The stormy conditions also have the effect of sorting and sizing the diamonds, once they are deposited in the ocean. Near the mouths of current and past riverbeds, recovered diamonds can be relatively large. The more abundant, smaller stones are transported to recovery sites farther along the coast, resulting in a general decrease in size with distance. Companies interested in these transported diamonds concentrate not just on the current beaches, but along ancient coastlines as well. Diamonds have been mined along as many as seven raised beaches or terraces above the modern beach level.

Attempts were made to mine marine concentrations of diamonds several decades ago. By today’s standards, the recovery techniques used were primitive, with work carried out by divers and dredgers operating in shallow (but turbulent) waters.

A Texan entrepreneur, Samuel Collins, is reported to have recovered more than 380,000 carats between 1969 and 1970 by dredging diamonds off the ocean floor, north of Luderitz, Namibia. Other entrepreneurs followed suit, but because recovery techniques were so inefficient, their cottage-industry efforts caused barely a ripple in world diamond markets.

Marine diamond mining today is largely based on technology developed by De Beers’ marine division and is, for the most part, focused on relatively shallow waters.

Mechanized recovery typically involves either suction pumps or large-diameter, rotating drills, as well as seabed crawler systems. Large-scale recovery, particularly in deeper waters, is the challenge of the 1990s as De Beers’ marine division, newcomer BHP Minerals and other groups seek to exploit a diamond resource widely considered to be the largest in the world.

Thus, considerable work is being directed toward the technical problems of mining underwater by mechanical means. Innovative thinking is the order of the day, as competing groups seek to adapt existing technology (such as robotics, airlifts, and the like) for mining purposes.

Exploration techniques to define marine resources appear to be more advanced. Geophysical work involving position-fixing and data-gathering equipment can today produce the equivalent of a photograph of the sea floor, complete with remote-sensing measurements. Such techniques can identify old river channels and storm beaches, as well as trap sites for diamonds, such as gullies and basins.