Diamond Origin | From Kimberlites to Sea Diamonds | Waves and Ocean Currents Redistribute the Diamonds | History of Mining on the Namibian Coast | Modern Marine Mining Techniques
History of Marine Diamonds
All diamonds are created deep in the earth's crust at very high temperatures and under incredibly high pressure. As magma works its way to the surface through deep fractures it sometimes traps diamonds within it. The magma is very low in silica and after it erupts and cools, it forms Kimberlite or lamprolite rocks. The pipe or dyke represents the conduit that brought the diamonds to the surface. These eruptions were short, but many times more powerful than volcanic eruptions that happen today. The magma in these type of eruptions originated at depths of at least 150 km, three times deeper than the magma source for volcanoes like Mount St. Helens.
Kimberlite pipes are only found in Archean-aged Cratons - areas of rock that are at least 2.5 billion years old. The first kimberlite pipe was discovered on the Kaapvaal Archean-aged Craton near the town of Kimberly, South Africa, hence the name.
Numerous kimberlite pipes have since been found on the Kaapvaal Craton, which extends through parts of South Africa, Botswana and Zimbabwe. The majority of these pipes were formed during two episodes in the Cretaceous Period. The first occurred between 125 and 115 million years ago, the second between 90 and 80 million years ago. The majority of the commercially economic diamond deposits are from the younger period known as the Late Creataceous.
|Figure 1: This is a drawing of an idealized kimberlite pipe, the result of a kimberlitic eruption. It illustrates the relative erosional level of three kimberlite provinces in southern Africa. The Kimberley area includes kimberlites emplaced in the Late Creataceous and contain the majority of commercially economic deposits.|
From Kimberlites to Sea Diamonds
Research has shown that over the past 90 million years most of the drainage basins covering the Kaapvaal Craton have flowed from east to west and emptied into the Atlantic Ocean. This means that weathering, mostly from rain, has eroded diamonds out of kimberlite pipes and swept them into river systems. The diamonds were then carried out onto the delta system when the river meets the ocean. Initially the diamonds were concentrated in small tidal channels on the river delta and in beaches near the delta. They were later redistributed up along the coast of Namibia.
Animation illustrating the origin of marine diamond deposits.
(Flash Animation, 1 MB)
Waves and Ocean Currents Redistribute the Diamonds
|Figure 3: This picture shows an alluvial mine of an ancient onshore beach terrace. Notice the depth of the sediment in which the diamonds are contained. Also notice the grooves in the bedrock that act as trap sites where diamonds can potentially accumulate.|
All of the sediment in the delta, including the diamonds, has been reworked and redistributed through wave action and ocean currents. Over time, wave action moved the diamonds northwards, depositing them in seabed trap sites and beaches along the coast. Since diamonds are heavier then most minerals found in sand and gravel, the continual re-distribution also led to diamond concentration. Because of the weight difference, the diamonds accumulated in low lying depressions while the lighter sand was moved onwards.
Over the past 90 million years the sea level has gone up and down many times, adding another level of complexity to the process. These transgressive and regressive cycles resulted in the migration of shorelines of Namibia in a range of up to 100 km
As the sea level goes up, the wave action re-mobilizes the diamonds into new depressions and trap sites and beaches further inland. When the sea level dropped, raised beaches were formed containing diamonds.
Wind Also Moved the Diamonds
Strong wind currents, originating from the south, also played a role in redistributing diamonds in the beach terraces. Small diamonds, in particular, are affected by eolian or wind erosion as they were swept up distinct channels inshore of the Namibian coast.
These wind and water transport mechanisms are still active today, moving diamonds from one resting place to another, creating and destroying deposits, raising and lowering concentration levels. However, since the processes occur over a long time period, it has little affect on the current deposits.
Only the Strongest Diamonds Survive
Diamonds are the hardest substance known to exist, but they are brittle when fractures and inclusions exist in their structure. All of the erosional and weathering processes the diamonds go through cause a great deal of stress on the structure. As a result, many of the imperfect stones are destroyed during their journey from the kimberlite to the ocean. Subsequently, marine diamonds have a remarkable high ratio of gem quality diamonds - as much as 95%. This ratio is much greater than that of land based kimberlite mines, and in general, leads to higher per carat prices obtained from diamonds recovered from the sea.
History of Mining on the Namibian Coast
Mining of Onshore Beach Terraces
Diamond mining in Namibia dates back to 1908 when a rail worker accidentally discovered a diamond in an ancient beach terrace at Kolmanskop station near Lüderitz. This discovery sparked a diamond rush and by 1913 the onshore Namibian diamond fields accounted for 20% of world diamond production. By 1920 many of the companies mining in the area were amalgamated by DeBeers into a corporation called Consolidated Diamond Mines (CDM). This company performed most of the alluvial diamond mining in Namibia over the past century.
The deposits onshore near Luderitz were soon depleted, but ancient beach terraces existed north of the Orange River and in 1928 diamonds were discovered and mining resumed in that area and exists to this day.
It is estimated that a total of 100 million carats of 90% gem quality diamonds have been produced from the on shore deposits to date. Mining is still occurring and new deposits are being discovered and put into production in South Africa and Namibia.
Marine Diamond Mining
Starting as early as 1958, people have been exploring the seabed for alluvial diamonds. At first the operations consisted of diver operated sediment excavation in water shallower than 35 meters. It wasn't until the 1980's when technology was developed for deeper water large scale marine diamond mining and not until the 1990's when the industry began to take shape.
Modern Marine Mining Techniques
DeBeers Marine Ltd was the first to employ the technology using either vertical or horizontal type technology. The vertical technology involves lowering a large area drill device that digs in to the loose sediment. While the drill is boring into the sea bed, the sediment is pumped up to the vessel for processing where the diamonds are recovered.
|Figure 4: This is a vertical technique of diamond recovery. A large drill like structure is lowered to the seabed floor and bores into the sediment. The sediment is then transported to the boat for diamond recovery.|
The horizontal technology utilizes a crawler like technology. A tracked crawler is lowered to the sea bed floor. With a large dredge pump mounted on a articulated arm, the tracked vehicle travels on the ocean floor sucking up the sediment containing the diamonds. That slurry is transported up to the vessel and diamonds are recovered by an onboard sorting plant.
|Figure 5: This is a general configuration of a horizontal technique for recovering marine diamonds. The crawler excavates the sediment and transports it to the vessel for diamond recovery.|
This is the method that is currently used by Samicor's ship the MV Kovambo on Diamond Fields' concession near Luderitz in Namibia.
There is a third method that has been successfully deployed for marine diamond mining and that is the airlift system. This is a method by which a vacuum is created using compressed air in order to suction the sediment to an onboard separation plant. This method has been used successfully in the past by Diamond Fields.
|Figure 6: This is the airlift system. Compressed air is pumped into the pipes near the seabed floor creating a vacuum which transports the sediment onto the boat for diamond recovery.|