Marine Diamonds

For thousands of years, diamonds have been swept away by the Orange River to the Atlantic Ocean, only to be picked up by wind-driven ocean currents and deposited on ancient beaches.  Many of these diamonds also ended up on the ocean floor.

Diamonds have a higher specific gravity than most common minerals.  Consequently, they are generally found close to the bedrock and are deposited in high-energy environment sediments containing pebbles, cobbles and boulders.  

It was not until 1954 that the first offshore diamond prospecting started and between 1961 and 1965, a Texan entrepreneur - through the Marine Diamond Corporation (now De Beers Marine) - gave the impetus needed for geological exploration on the continental shelf by exploiting diamonds offshore along the coastline of South West Africa (Namibia today).  This has manifested itself into a growing industry with diamonds being discovered and mined in the nearshore and offshore environments along the coastline from south of the Olifants River in South Africa to some 120km north of Hottentots Bay in Namibia.

Mining the ocean bed is a daunting task and finding diamonds in this dark, deep world is like searching for a needle in a haystack with your eyes closed.  The first step to mining is to map the mountains and valleys of the ocean floor and the different layers within it.  Offshore diamonds are known to concentrate in gullies, potholes, offshore ridges and ancient submerged bays.  Sophisticated bathymetric (echo-sounding, side scan) sonar and seismic surveys are used to identify these features.  Tiny fossil shells of animals known as foraminifera (informally called forams) provide clues where to search for diamond bearing rocks.  Each sediment layer contains distinctive species of forams, which may be warm- or cold-water species and so convey information about ancient climates and sea levels.

In the shallow inshore zones, to depths of 30m, mining is by diver-operated air-lift dredging.  This entails divers using a suction hose to vacuum up alluvial gravel which is sorted and screened at the surface.  Divers working in gullies close to the shore operate from a shore-based unit, while those in deeper water work from a small vessel.  At depths of 30-80m, most mining is by dredges operated directly from larger vessels.

Deep sea mining, currently limited to waters less than 130m deep, makes use of two techniques.  Vertical mining involves using 7m- diameter drill bits that maintain a force of up to 20 tons.  The mining ships work according to a planned grid, covering the seafloor in overlapping circles to maximise yield.  The other technique, known as horizontal mining, uses a remotely operated vehicle called a crawler.  The crawler advances down specific lanes according to a mapped grid, mining gravel and mechanically transporting it onto a screen.  Oversized material is discarded, and the rest is transported as a slurry via flexible hoses to the ship.  The slurry is then processed on board, being passed through various sorting screens.  The oversize material is returned overboard, while the fluorescent material containing diamonds is separated out automatically, using sophisticated X-ray and robotics equipment, and is then hermetically sealed in cans.  No-one ever sees the diamond treasure in these “Neptune cans” until it arrives at the diamond sorting house back on shore.

In response to concerns that the inshore diamond-mining industry was negatively impacting the West Coast rock lobster fishery, an environmental assessment was conducted by the consulting firm OLRAC.  The study revealed that diver-operated air-lift dredging resulted in only a localised and short-term impact.  The biological community quickly recovered and within a year no difference between mined and unmined sites could be detected.  The consultants nevertheless recommended that the diamond divers should minimise kelp-cutting and avoid revisiting mining sites. 

An environmental impact assessment was also commissioned by De Beers Marine, when the company began mining in 1990.  There were concerns that the ocean floor communities would be irreversibly altered by the disturbance, and that the sediment cast overboard would smother them as it settled.  The study conducted by a team of scientist from the University of Cape Town, revealed that it takes a number of years for the seafloor community to recover.  However, since only a few square kilometers are worked each year, the overall impact is minimal.  The effects of the plume of sediment are limited to a small area around the ship.