Elsevier

Environmental Development

Volume 36, December 2020, 100579
Environmental Development

Marine diamond mining in the Benguela Current Large Marine Ecosystem: The case of Namibia

https://doi.org/10.1016/j.envdev.2020.100579Get rights and content

Abstract

The first diamonds from the sea floor were recovered in shallow waters off the Namibian coast some 110 years ago. However, it took 50 more years, before some systematic sea floor diamond mining with purpose-built barges occurred in Namibian waters in the early 1960s. This was followed by a period of detailed exploration and mining tool development, which saw Namibia emerge as the leading nation in marine diamond mining in the late 1980s. Today, a fleet of seven modern mining and exploration vessels is involved in the recovery of more than three quarters of Namibia's diamond production. As the process involves modification of the sea bed, careful monitoring and impact mitigation is carried out according to international best standards.

Introduction

Despite their sparkle, the treasures of the Namibian coast remained hidden for a long time compared to other mineral resources. By the beginning of the 1900s, Namibia had developed a number of mines that had been producing a range of metals for many years, but diamond mining had remained a dream. This all changed one day in April 1908, when Zacharias Lewala discovered the first diamond. Since then, the stretch between Lüderitz in the north and Oranjemund in the south has developed into one of the most important diamond-producing areas in the world. Since diamonds occur not only onshore, but also in submarine deposits, a unique high-tech offshore diamond mining industry has developed in Namibia, making the country the World technological leader in marine diamond mining. The Namibian coast is therefore sometimes also very aptly referred to as the Diamond Coast.

The Kaapvaal Craton, a geologically stable and ancient region more than 2.5 billion years old, is the source of southern Africa's prolific diamond production. The majority of economic diamondiferous kimberlites known worldwide today occur here. During the Cretaceous period, some 135 to 65 million years ago, they penetrated the Earth's crust in substantial numbers across what is southern Africa today, carrying with them a precious cargo. At the same time, the very humid climate then prevailing in southern Africa meant erosion was at its peak, with numerous rivers transporting vast amounts of eroded material into the young South Atlantic Ocean, formed by the break-up of Gondwanaland and the resulting drifting apart of the continents of Africa and South America (Hawthorne, 1975). The kimberlites of southern Africa were eroded along with the other rock formations, and diamond-bearing detritus was transported downstream towards the newly established south-western coast of Africa. Taking the rate of erosion into account, it has been estimated that some 10 000 million carats of diamonds must have been liberated from kimberlites and sent on a westward journey. However, it is also estimated that some 7000 million carats were destroyed during the transport process; nonetheless, leaving Namibia with a bounty of 3000 million carats.

After being separated from their kimberlite hosts, the alluvial diamonds were subjected to a process of intense abrasion as the sediments were reworked again and again on their 1000-km journey to the Atlantic coast, while travelling further northwards along the shore, and eventually by howling, sand-laden winds on their way inland again. Only the best gems without inclusions, flaws or other imperfections in their crystal lattice were able to survive this ordeal of a voyage. For this reason, the Namibian diamond deposits today contain some of the best and most highly priced gems in the world. As a consequence, 95% of the Namibian diamond production is of gem quality, while the gemstone grade of an average kimberlite pipe is only 25–35%.

In the Early Cretaceous period, the predecessor of today's Orange River was a slow-flowing, meandering river that dumped millions of tonnes of fine sediment into the sea. However, the energy of its waters was not sufficient to transport larger pebbles and the heavy diamonds far from their source. This changed when southern Africa underwent major uplift some 60 million years ago, which led to the incision of the riverbeds and faster-flowing rivers capable of transporting diamonds together with considerable amounts of sand and gravel. In addition, during the last 3 million years, glacial periods locking up huge amounts of sea water in the polar caps caused sea levels to drop, and further promoted the incision of rivers, including the Orange River. Millions of carats of diamonds were carried to the Atlantic coast, and, upon reaching the sea, were distributed along the southern Atlantic coastline by north-flowing longshore currents. The southern Atlantic is well known for its high-energy regime, and storm events with swells several metres in height are not uncommon. This energy constantly reworks the sediments supplied by the Orange River, and is responsible for the distribution of the diamonds along the coastline. Transgressions of the sea resulted in the cutting of new beach terraces, while regressions formed surf-zone deposits.

Onshore production to date in Namibia and South Africa amounts to approximately 205 million carats. This leaves a balance of some 2800 million unrecovered carats on the basis of an original resource of 3000 million carats. Since the onshore mines are nearing the end of their lives as the deposits become more and more depleted, many more diamonds than those mined from these onshore deposits must have been transported by the Orange River, and these carats must therefore lie on the floor of the sea. Just like the dramatic changes in sea level during the past 60 million years have influenced the transport and deposition of diamonds along the banks of the Orange River and along the beaches of the Atlantic shoreline, they are also responsible for the formation of beaches that were later drowned by marine transgressions. During times when the sea level was considerably lower than today as a consequence of glaciation locking up sea water in polar ice caps, processes in an intertidal environment could form diamond placers in areas that are today located at the bottom of the sea. With the enormous potential resulting from the calculation above, the marine diamond deposits constitute the future of Namibian diamond mining (Schneider, 2008).

As early as during the German colonial days, the possibility of the presence of diamonds on the sea floor was suggested. According to colonial government geologist Reuning, some diamonds were even dredged from the Lüderitzbucht harbour by the cutter Stella. But an imperial decree, vesting all rights in diamonds on the sea floor in the colonial treasury was issued in 1910, and the crew of the Stella went to prison, while the 33 carats they had recovered were confiscated by the authorities. Another attempt to look for diamonds, this time on the offshore islands which belonged to the Union of South Africa at the time, was made in 1910. Prompted by reports from guano workers and the rich recoveries made by the Germans on the mainland, the Union Government sent an expedition to visit Halifax, Ichaboe, Penguin, Pomona, Possession and Seal Islands. Diamonds were indeed found on Possession, and immediately, washing machines, sieves, picks, dynamite and spades were sent to the island. A trench 275 m long, 6 m wide and 1.5 m deep was dug, and some 14 m³ of gravel were treated, which produced 232.5 carats of diamonds valued at 511 pounds 10 shillings in total. However, not a single diamond was found on the other islands. As the cost of the expedition was 825 pounds 7 shillings and 7 pence, it was wisely terminated (Wagner, 1914; Williams, 1996).

In the late 1950s, a certain Johann Vivier acquired a concession for the exploration of diamonds off the coast of Namibia, in an area west of a line between the high- and low-water marks, and bordering the onshore diamond concession. Vivier's company, SWA Prospekteerders, worked the shallow parts of the concession by scooping up gravels and sorting them onshore. In 1958, 39 diamonds weighing 21.5 carats in total were recovered this way. These were the first-ever marine diamonds to be mined! SWA Prospekteerders subsequently acquired the fishing boat Karibib, and equipped it with pumps, suction pipes, and other diamond recovery equipment. By 1961 conclusive evidence of diamonds on the seabed had been produced. Vivier was the first person to succeed in sucking diamonds from the seabed – albeit in very small amounts – when, some 20 km south of Chamais, at a depth of 23.7 m underwater, diamonds were found. It was then that Vivier was introduced to Samuel Collins. Collins, who was a world authority on submarine pipelines, bought the license from Viviers in 1961. Between 1961 and 1965, he played a pioneering role in mining diamonds from the sea in Namibian waters and, indeed, in the world, since this was the first attempt to tackle seabed mining systematically and on a large scale (Williams, 1996).

Collins extended the licence to comprise a huge area bounded by the Orange River mouth in the south, Diaz Point to the north, the high-water mark in the east, and the 200-m isobath in the west. He acquired the steam tug Emerson K, and equipped it with an airlift plant, vibrating screens and a Plietz jig. Towards the end of 1961, the airlift pump went down almost 15 m and penetrated well into the seabed. It sucked up gravel, and soon the jigs recovered 38 diamonds. From then on, exploration was done in a systematic way. The Emerson K worked her way down in shallow water from Diaz Point, and took samples every 500 m. At Wolf Bay, on November 15, 1961, the first diamonds in quantity were found, and then the extensive beds of diamondiferous gravels of the Chameis and Baker's Bays were discovered. By the end of March 1962, the Emerson K had taken samples on a regional scale along the entire coastline of the licence area, and an additional 44 diamonds weighing a combined 11.7 carats had been recovered. Particularly promising results were recorded on April 16, 1962, when 38 diamonds weighing 14 carats altogether were recovered from the area between Chameis Head and North Rock. By the end of June 1962, all operations in the licence area had yielded 179 stones weighing a total of 58.81 carats. The discoveries in the Chameis area were able to sustain the operations, and allowed for further exploration in the other areas (Collins, 1962; Debmarine Namibia, 2020).

In 1962, Collins adapted a barge formerly used in his pipeline operations in the Mexican and Persian Gulfs to become the forerunner of a fleet of floating mining operations. The vessel, Barge 77, was fitted with a 30-t-per-hour recovery plant and accommodation for 53 people (Fig. 1). While Barge 77 had no engines of its own, it was able to move over an area of 1 million m2 within its own anchor spread, with the help of specialised anchor winches. Mining was done by dredging using a 30-cm airlift able to suck up silt, sand, gravel and large boulders together with tons of seawater, which was then delivered to the purpose-built recovery plant, capable of treating 30 t per hour. Barge 77 went into position near Chameis in August 1962, and during the first eleven months of operation, 51 000 carats of diamonds were recovered. The existence of diamonds in economic quantities on the seabed had now not only been proven, but the foundation for an entirely new type of mining operation had been laid.

When Barge 77 was swept onshore by a freak wave and irreparably damaged in 1963, Collins commissioned a new and larger barge. Mining equipment from Barge 77 was salvaged and installed on the new vessel named Barge 111, which had more anchors and two airlifts, each with its own screen circuit, plus a heavy-media plant. It was commissioned at Chameis in November 1963. In addition, two more vessels were converted into mining units in Cape Town. The Colpontoon was a barge with four times the capacity of Barge 111, while a US Navy ship was converted into the world's largest seagoing diamond mining plant and renamed the Diamantkus. The Diamantkus had six airlifts and two independent recovery circuits, and together with the Colpontoon handled 300 t of gravel per hour. The Colpontoon had two dredge pumps that could be replaced by airlifts, and, like the Diamantkus, two independent recovery circuits. Both mining units started their work near Chameis in January 1964. Diamond production consequently rose to unprecedented heights, and the combined production for one day were sometimes as high as 1500 carats.

The plants typically consisted of large dewatering cyclones, which removed about 66% of the sea water. From there, the remainder of the mixture passed on to scalping screens, which removed all materials over 25 mm and under 1.5 mm in diameter. This material was dumped overboard at the rear end of the vessel in areas that had already been mined out. The remaining material was passed over washing screens, where high-pressure water jets washed out the remaining fines. From there the material was transferred to storage bins; next, it was carried by belts to a mixing cone, where a ferro-silicon suspension medium was added to it. The mixture was then pumped into a cyclone where the light material overflowed onto a tailings screen; these tailings were then dumped overboard. The heavy fraction, containing the diamonds, was discharged onto a concentrate screen, which removed all particles over 10 mm. These particles were fed into a ball mill, where they were reduced to −10 mm in size and passed on to the sorting house. There, the reduced material joined the −10 mm material from the concentrate screen, and together they were fed into Plietz jigs. Final recovery was done by hand-sorting.

However, the Diamantkus experienced major problems with its six airlifts, which made the vessel uneconomic. The resulting accumulating financial difficulties left Collins no option but to look for a partner. De Beers, having witnessed Collins' company, the Marine Diamond Corporation (MDC), demonstrating that there were in fact diamonds in recoverable and economic quantities on the seabed, made an offer and Collins opted to sell De Beers a majority share in MDC. The MDC's production over the years was as follows:

Having seen the success of the Marine Diamond Corporation, other groups applied for concessions along the coast, and by mid-1963 concessions covered the entire coastline from the Orange to the Kunene River. Ivan Prinsep, a financier from Switzerland, had been contemplating the idea of seabed diamond mining since the early 1970s, after having discussed such a marine enterprise with the famous oceanographer Dr Jacques Piccard. In 1978, Prinsep entered into an agreement with a fishing boat owner from Port Nolloth who had installed a pump on his boat, the Wakkerstroom. They shipped it into the vicinity of De Beers' operations at Kleinzee, and when a diver with a mere hosepipe made the first attempt at mining the seabed, they immediately found some diamonds. In the first month of production, they produced 337 carats, with an average stone size of 0.87 carats. By 1980, Princep's company Dawn Diamonds had made an annual profit in excess of 2 million US$. Princep furthermore obtained the right to explore and mine diamonds from the territorial waters of 12 South African islands off the Namibian coast. On the basis of these rights, Prinsep, in 1983, founded Ocean Diamond Mining (ODM). The company soon developed its own expertise and technology, using Collins-type methods, and by November 1986 had collected a total of 6051 carats of diamonds. In anticipation of Namibia's Independence in 1990, ODM Namibia was registered in Windhoek in 1989, and ODM partnered with Benguela Concessions (Benco). Together, ODM and Benco concentrated on mining some South African offshore concessions they had been awarded, and proved very successful. Benco managed to get one of the world's biggest mining companies, Australian-based Broken Hill Proprietary (BHP) as a partner, while Prinsep attracted the interest of the South African insurer Sanlam, which invested heavily in ODM. This paved the way for the success of the company after Namibia's Independence (Schneider, 2008).

As early as 1963, Sam Collins had approached De Beers, who in turn engaged the American company Ocean Science and Engineering to carry out systematic mapping and sampling to evaluate the Namibian offshore diamond potential as quickly as possible. A small coaster was re-equipped as an exploration and sampling vessel. It started operations in 1964, and recoveries were made along the entire coastline probed, and some encouraging results were obtained from Hottentot's Bay. A marine geological office was opened at Oranjemund, and the interaction of the geologists, including those working on the onshore deposits, would eventually lead to the early recognition of terrace gravels in the marine deposits.

Early in 1965, the Ocean Science and Engineering reconnaissance sampling programme had been completed, and a reserve of some 9 million carats of diamonds was calculated. It was then decided to phase out production gradually and rather develop better mining methods in order to enhance recovery at a later stage. De Beers started to develop a new and better mining vessel, the Pomona, utilising the experience gained by the MDC, which was commissioned in June 1967. At the time, the Pomona was the biggest, heaviest and most complex vessel ever built in South Africa. It had two 450-mm primary digging heads, one 450-mm secondary head, four 45-cm dredge pumps, and a highly efficient recovery plant with four 2-m dewatering cyclones, a Cleveland radial jig, vibrating screens, and heavy-media cyclones.

Between 1968 and 1970, two smaller vessels carried out geophysical research work on the bedrock gullies, while another one continued sampling. Integration and analysis of the information gathered showed that there were preferential elevations for diamond deposition, and that the major deposits occurred at 22, 26 and 33 m below sea level. Formal ore reserves could now be calculated, compiled and managed from the grade contour plans, and continued mining by the Pomona assisted in the verification of the exploration results.

In November 1969, the Pomona was moved to Baker's Bay to mine residual patches of ore, as previous mining in the area had been incomplete. But recovery was far too low, and it was established that a strong seabed surge had dispersed the diamonds from the residual patches into the mined-out areas, diluting the grade and rendering the remaining deposits unpayable. It became clear that more efficient and systematic mining methods needed to be developed to prevent a situation where payable ore was left behind. In addition, the Pomona had been at sea for an uninterrupted period of three years and was due for a major overhaul. By the end of March 1971, mining operations came to a standstill.

While the end of the Pomona operations presented a major setback in the marine diamond mining, it provided the geological team with an opportunity to undertake research. Before, the need to establish reserves to sustain the Pomona had not allowed this. It had become increasingly clear that thorough exploration was necessary for long-term planning in such a remote mining venture, and De Beers now embarked upon an extensive exploration programme. This exploration programme would last many years, and included the development of the next generation of mining vessels and tools. Marine diamond mining itself did not resume until the late 1980s, but when it did it was with renewed vigour.

In July 1975, the deep-water exploration programme commenced. By early 1977, a total of 3297 samples had been taken in water depths of up to 90 m. The diamond resource had been calculated, but, while impressive, was shown not to be big enough to justify major investments into marine mining technology. Between 1977 and 1982, altogether 9228 samples were taken in water depths between 100 and 135 m in an area between the Orange River and Diaz Point. The samples not only revealed the presence of diamonds, but they also enabled the geologists to develop a complete understanding of the geological nature of the deposit, including conclusions drawn with reference to changes in sea level and palaeo-climate, so important for the diamond transport and deposition mechanisms.

By 1980, it had been realised that the offshore deposit was so large that the sampling ship Spencer Bay would not be able to carry out all the necessary sampling, and another sampling ship, the Shearwater Bay was acquired. It was equipped with two Megadrills and a dense-media separation plant. The sampling programme continued to be accompanied by a geophysical programme. This was carried out by the Spencer Bay in addition to her sampling duties. However, as the importance of geophysical data became increasingly apparent, a full-time geophysical vessel, the Meob Bay, was equipped to handle a deep-water tow fish for geophysical equipment, and would survey 54 580 line km between 1979 and 1991. Sampling and geophysical work in deep waters in order to delineate the deposit and gain insight as to the overall size of the potential resource continued into the early 1980s. In 1983, the programme moved into an evaluation and ore reserve generation phase Schneider, 2008.

Meanwhile, south of the Orange River mouth, De Beers Consolidated Mines was awarded two areas in 1983, and the newly formed company, De Beers Marine, was engaged to undertake operations on their behalf. This took some of the focus of the operations away from the Namibian operations, and the Spencer Bay and the Meob Bay now worked in South African waters. An additional sampling vessel, the Douglas Bay, was acquired in 1986, but initially also only operated in South African waters. But luckily, as results in Namibian waters had been much more encouraging, work in South African waters was reduced considerably from 1989 onwards, and exploration once again concentrated on the much more viable projects north of the Orange River mouth (Foster, 1996). The production of MDC as a subsidiary of De Beers was as follows:

By 1983, it had become clear that a world-class deposit of marine diamonds had been found off the coast of Namibia. However, this deposit occurred at up to more than 100 m below sea level, and required massive investment to develop appropriate marine mining technology. Now that a mineable resource had been established, developing technology for mining in deep water became of paramount importance. A Mining and Research Division was therefore formed in 1984 in Cape Town. Meanwhile, ore reserves needed to be improved, and target areas were sampled in fine detail. This way, possible, probable and proven ore reserves were generated.

The concept of utilising a bottom crawler carrying mining equipment had been actively pursued since the inception of the Mining and Research Division. In 1985, a mini-crawler, 1 m wide and 1.5 m long, was deployed in target areas. The crawler survey also proved that the topography of the seabed was quite variable, so a video tow fish system was brought in 1987. In 1988, a Minirover Remotely Operated Vehicle (ROV) was hired to conduct geological work in support of mining design. The small swimming vehicle was used to observe the performance of the first mining equipment being tested and to make geological observations.

The first approaches towards developing mining equipment for deep waters were made in 1986. The first concepts aimed at mining with a large device based on a megadrill. But it was clear that this would only be the start, and sophisticated equipment such as seabed crawlers would have to be developed. A suitable vessel was also needed to act as a platform for the development work, and possibly as a prototype mining vessel later. After extensive modification, including the installation of a large dense-media separation plant, drier and X-ray equipment, handling equipment and swell compensators, the mining research vessel Louis G Murray was commissioned in December 1987 (Fig. 2).

The first mining device to be built and tested on the Louis G Murray was a scaled-up megadrill with twin 200-mm airlifts. However, the Louis G Murray was later converted to handle a crawler. The concept design of a crawler had begun in 1986, and the conversion of the Louis G Murray included the installation of a large A-frame to deploy the crawler, and large transformers to provide the crawler with power. The mining equipment installed on the crawler comprised of a large cutter wheel with an axial suction port, it was fitted on a slewing boom, and basically represented a small version of a bucket-wheel excavator. The mined material was airlifted to the surface through two 200-mm flexible hoses. The Louis G Murray left Cape Town in December 1989, marking the end of a decade of enormous developments in marine mining technology, and the beginning of another decade of marine diamond mining, which would see the newly independent Namibia emerging as a world leader in seabed mining. Production figures for De Beers Marine before Namibian Independence were as follows:

Following Namibia's Independence, De Beers Marine continued exploration in the concession areas held under the previous dispensation. In 1990, the crawler of the Louis G Murray was fitted with a cutter wheel with an axial suction port. The crawler advanced slowly across the seabed, with the cutter wheel moving from side to side, thereby systematically excavating the mining blocks. After the introduction of suction boxes, recovery increased dramatically, in fact, recovery was so good that 1990 was later officially declared the year when De Beers Marine moved from exploration to production.

Meanwhile, as development of the crawler system continued, research was undertaken to develop alternative concepts. A huge drill bit attached to a bolted steel riser pipe 0.5 m in diameter, lowered to the seabed via a moon pool in the middle of a vessel was one such concept, the design was considered revolutionary at the time. It combined the technology of large-diameter drilling on land with the control technologies of offshore oil drilling. Diamond-bearing sediment loosened by the drill bit would be airlifted via the pipe to the vessel, where it would be treated on board in a plant, with the over- and under-sized material being returned to the seabed. The oil drilling vessel Coral Sea was purchased and converted into the first large-diameter drill mining vessel. She received a treatment plant with screens, a drier and X-ray sorting facility, and sailed on her first production voyage to Namibian waters in April 1991. The commissioning of the Coral Sea boosted production in 1991 to more than five times that of the previous year. A sister ship to the Coral Sea, the Grand Banks, was also purchased, converted and commissioned as a second large-diameter drill mining vessel in October 1992. Later modifications allowed it to be operated remotely in a completely hands-free process, which today can be found on all De Beers' ships. Production in 1993 was double that of 1991 due to the commissioning of the Grand Banks.

To achieve the best mining results, careful exploration work was undertaken along with the mining effort. A geological classification of the seabed in the licence area was undertaken through major geological surveying programmes by the Spencer Bay. This included geophysical surveys as well as geological mapping and sampling. The success of the exploration work resulted in the search for more vessels suitable for conversion. Two vessels later renamed Debmar Pacific and Debmar Atlantic arrived in Cape Town in 1992 and 1993 respectively. Both ships were considerably larger than the Coral Sea and the Grand Banks, and most importantly had larger moon pools which would allow the use of larger drill bits. Consequently, the two ships could be fitted with 6.8-m diameter drill bits, able to excavate 36 m2 of seabed at once.

The Namdeb Agreement, signed on November 24, 1994, rearranged the licence holding situation. The Namibian government granted a large offshore licence area named Atlantic 1 for a period of 25 years in accordance with the new Minerals (Prospecting and Mining) Act, 1992 (No. 33 of 1992). In addition, 20 exclusive prospecting licences were issued for the offshore area (Fig. 3). By 1995, Atlantic 1 had contributed one third of Namibia's revenue from diamonds, and had therefore become a significant diamond producer within the Benguela Current Large Marine Ecosystem.

By the mid-1990s, technology had been enhanced yet again, allowing mining vessels to operate comfortably in a 7- to 9-m swell, while the depth of operation had reached some 130 m. The final section of a reconnaissance geophysical survey of the coast from the Orange River to Lüderitz and out to the 5.5-km limit was completed and interpreted, while the Advanced Seabed Mapping Project started in 1996. The crane ship Voyager was converted in 1997 and 1998, renamed !Gariep, it started mining in April 1999. But the !Gariep experienced problems with the new remote crawler mining tool commissioned in 1999. As a result, in May 2001, the !Gariep was removed from operations and the mine plan was changed; and in 2002, the !Gariep was revamped as a drill ship (Schneider, 2008).

In the meantime, and as a result of the five-yearly review of the De Beers Marine/Namdeb Service Agreement in accordance with the contract between the Government of the Republic of Namibia and De Beers, it was announced in 2000 that De Beers Marine Namibia would be established. The company was registered in Namibia in 2002, and the subsequent relocation of De Beers Marine staff from Cape Town to Windhoek was completed in November 2003.

As early as mid-2003, De Beers Marine Namibia chartered the mining vessel YaToivo. In addition, De Beers Marine Namibia bought the seabed crawler unit deployed from the Ya Toivo. The commissioning of the Ya Toivo and the revamped !Gariep represented an expansion in operations of over 40%. In 2004, the Debmar Atlantic broke every record in terms of the area mined and the number of carats produced by a single vessel in the Atlantic 1 concession: it had mined 1.1 km2 to produce over 210 000 carats, following a plant upgrade of N$ 73.7 million.

In 2005, history was made when De Beers Marine Namibia's marine production overtook Namdeb's land production for the first time. As a result, De Beers Marine Namibia became the leading diamond miner in the country, producing 922 000 carats in 2005 alone. The following year marine production exceeded 1 million carats for the first time (Schneider, 2008).

In 2011, the government of the Republic of Namibia and De Beers signed new shareholder agreements, and De Beers Marine Namibia became a joint venture marine diamond prospecting and mining company owned in equal shares by the Government of Namibia and De Beers. The trading name of the company became Debmarine Namibia.

A record production of 1.1 million carats was achieved in 2012, and Debmarine Namibia purchased the mining vessel Peace in Africa, the world's largest marine diamond mining vessel, from De Beers Consolidated Mines that same year. The vessel was modified to mine the Atlantic 1 deposit and contributed 381 000 carats in 2012, a new production record for a single vessel. The Peace In Africa was upgraded in 2013, and renamed the Mafuta. Production increased to over 500 000 carats per annum from 2013 onwards. The equipment aboard the Mafuta includes a 240-t crawler, connected to the ship by a 655 mm internal diameter rubber hose, and a diamond recovery treatment plant. The dredge of the crawler has a suction capacity of about 10 000 m3 of water and gravel per hour, resulting in about 250 t of material to be processed. The anticipated yield is approximately 240 000 carats per annum.

In 2015 the construction of a new deep water exploration and sampling vessel, the SS Nujoma, commenced in Norway, and the ship was successfully launched a year later (Fig. 4). It subsequently undertook its maiden voyage to Cape Town in July 2016, and arrived in Namibian waters in December. The ship represents an investment of BP 100 million, and is the World's largest and most advanced diamond exploration and sampling vessel. It is equipped with a subsea sampling system and sample processing plant. It is also fitted with a large drilling system capable of collecting 50 seabed samples a day. The processing plant aboard the ship employs screening, scrubbing and other methods to recover diamonds from the samples. Resource mapping is performed using side-scan sonar, chirp sub bottom and airgun profilers, multi-beam echo-sounders, and autonomous underwater vehicles. The SS Nujoma carries out exploration activities on the ocean floor at water depths between 90 m and 150 m (Debmarine Namibia, 2020).

At present Debmarine Namibia owns, operates and maintains five production vessels, namely the Debmar Atlantic, the Debmar Pacific, the Grand Banks, the !Gariep, and the Mafuta. The sixth member of the Debmarine Namibia fleet, the SS Nujoma undertakes exploration and sampling. Debmarine Namibia also charters the Coral Sea for additional production.

Knowing the mineral resource of a mining licence is critical to ensure sustainable mining. Before mining, resource development is therefore carried out by scanning the seafloor using geophysical mapping, followed by sampling to determine the reserve inventory. The inventory, together with other parameters, is then processed into a mine plan, which ensures the most sustainable and economic use of the resource in the long-term.

The production vessels mine diamonds off the sea floor using highly advanced technology, supported by sophisticated tracking, positioning and surveying equipment. Two mining technologies are currently deployed to suite different ground conditions, namely the airlift-drill technology and the crawler technology. The airlift-drill technology uses 6.8–7.2 m in diameter drill bits working in overlapping circles on the sea floor. The crawler technology uses a 280 tonne track-mounted crawler dredging on the sea floor. Five of the production vessels use the airlift-drill technology, whilst only one vessel, the Mafuta uses the crawler technology. Up to 60 t of sediment are lifted each hour to the Mafuta through a giant pipe attached to the crawler. Mining takes place on the sea floor at water depths of up to 150 m.

On board, the sediment is washed and sifted into increasingly smaller particles using a series of vibrating racks and rotating drums that crush consolidated sediment. Sediment that does not contain diamond-bearing material is passed back to the ocean, while the natural eco-system is closely monitored. Eventually, a diamond concentrate is produced that way, while no human hands touch the diamonds as they follow an automated process before being sealed into small containers. These are flown from the vessels to shore by helicopter three times a week. All Debmarine Namibia production is handed to Namdeb Holdings, which in turn sells it onwards to the Namibia Diamond Trading Company, in terms of its sales and marketing agreement with that company.

In 2019, Debmarine Namibia announced that it will commission the construction of the World's first ever custom-built diamond recovery vessel. The new vessel is expected to cost some US$ 470 million, and represents the largest ever single investment in the marine diamond industry. It will incorporate the latest marine technologies that will drive improved safety performance while optimising efficiency and utilisation rates. The vessel will become the seventh vessel in the Debmarine Namibia fleet and is scheduled to commence operations in 2022. On completion, it is expected to add 500 000 carats annually to Debmarine Namibia's production, an increase of approximately 35% on current production (Debmarine Namibia, 2020).

Debmarine Namibia has been able to grow its production output substantially since it commenced operations. With the current size of operations, the expansion under way, and the expertise developed, Debmarine Namibia represents the future of Namibian diamond production. Production from the marine licence since Independence, with 2018 so far the year with the highest production, has been as follows:

At Namibia's Independence, Ocean Diamond Mining (ODM) held mining licence ML 36, covering the area of three nautical miles around the South-African-owned Albatross, Halifax, Hollamsbird, Ichaboe, Long, Mercury, Penguin, Plumpudding, Pomona, Possession, Seal, and Sinclair's Islands, and measuring 374 km2 in size, and mining in ML 36 took place between 1991 and 1993. When in February 1994 the port of Walvis Bay and the offshore islands were reintegrated into Namibia, the Namibian government honoured pre-existing rights, and ODM applied for the extension of its licence, which was granted, effectively increasing it about fifteenfold. Exploration was carried out, and followed by successful production by both, ODM vessels and contractors. In 1999, ODM was taken over by Namibian Minerals Corporation (Namco).

As a new entity in marine diamond mining, Namco was registered on April 15, 1992, and was issued with an exclusive prospecting licence in the Hottentot's Bay area in 1993. In December 1993, Namco commissioned the ex-Royal Navy hydrographical vessel Fox to carry out a comprehensive 6560-line-km geophysical survey of its licence area, encompassing bathymetry, Side Scan Sonar, seismic profiling, vibrocoring and grab sampling. Interpretation of the results led to the calculation of a potential resource of some 70 million carats at the time. In preparation for production, the ManSSol mining tool, a highly efficient seabed crawler, which was considered one of the most sophisticated production systems, was developed. This tool was subsequently deployed from the mining vessel Kovambo. The vessel was equipped with a fully integrated floating production plant capable of processing 1 million m³ of material per annum. Namco started commercial operations on April 13, 1998. In an area where the average stone weighed 0.36 carats, they recovered a 10.24-carat stone within the first month of production! In 1999, the second full year of production, Namco more than doubled its output. Namco was also awarded ISO 14001 accreditation in 1999 in recognition of its excellent environmental programmes. However, at the end of 2000, an unfortunate operational accident occurred, involving the NamSSol unit on the Kovambo. Due to a collapse of the underwater mining face, the NamSSol was buried. Extensive recovery operations only managed to recover the upper structure, but it proved to be impossible to pull out the undercarriage. As manufacturing of a new undercarriage would take some months, production could obviously not take place during that time, resulting in considerable losses. As a result of the cessation of mining activities and subsequent loss of revenue, exacerbated by ongoing financial commitments, Namco was placed under provisional liquidation in 2001. New investment into the company by the world's largest diamond manufacturer, Lev Leviev's diamond mining and processing agglomerate based in Tel Aviv, Israel, resulted in a takeover of all assets belonging to Namco. The Leviev Group assumed a controlling interest together with exclusive marketing rights to Namco's production. After the completion of the takeover on May 15, 2001, the mining vessel Ya Toivo resumed mining operations with the NamSSol II seabed crawler mining tool. In April 2003, major investment from the controlling shareholder resulted in the establishment of a new company called Sakawe Mining Corporation (Samicor) which continued mining the Namco license areas. However, the international economic crisis of 2008 put an end to the operations of Samicor.

In the late 1980s Namibian West Coast Diamonds was founded to utilise certain concession areas off the coast of Lüderitz and west of the low-water mark. The company initially operated with subcontracted divers from the beach and later also subcontracted small boats. A limited geophysical survey was also undertaken. In June 1993, Canadian company Diamond Fields Resources, later Diamond Fields International (DFI), purchased Namibian West Coast Diamonds. In 1999, some 27 million Namibia dollars were spent for a scoping study to evaluate the near-shore Marshall Fork project. Initial results were extremely encouraging, and DFI embarked upon a programme of acquiring its own mining vessel, but adversities led to the realisation of this in 2004 only. In the meantime, DFI contracted Samicor to mine in DFI's license area. Following successful exploration in 2006, which resulted in an estimated indicated resource of 63 000 carats covering 315 000 m2 of DFI's mining licence area, production continued from 2007 onwards. But as with Samicor, the international economic crisis of 2008 did not allow for the continuation of production (Schneider, 2008).

Marine diamond mining in Namibia occurs within the Atlantic Ocean's Benguela upwelling system, a biologically productive system that supports a high abundance of marine organisms; therefore, any effects on the ecosystem must be carefully considered. Any operator must comply with the provisions of the Environmental Management Act, Act No 7 of 2007.

Between 1990 and 1996, a multidisciplinary team of scientists from the University of Cape Town undertook an environmental impact assessment for the Atlantic 1 mining licence area. The De Beers Group, including Debmarine Namibia, has been studying the impacts of marine diamond recovery and the subsequent recovery of the seabed, since 1994. The extensive environmental monitoring programme includes the gathering and analysis of sediment samples, benthic macrofauna, geophysical survey data and photographic records (G Ngaisiue, pers. comm., 2020). It is carried out on an annual basis to achieve the following objectives:

  • Obtain pre-mining baseline information on the seabed habitat and macrofaunal communities living in the top 30 cm of the seabed. These animals are usually sessile, with a long generation time, which means that the benthic community structure reflects the environmental conditions in a particular area, integrated over a period of time.

  • Investigate the relationship of benthic community structure with water depth, sediment type, geographic position, and other factors.

  • Assess and monitor the rate of recovery of seabed habitat and macrofaunal communities following mining disturbance. Current monitoring studies indicate that seabed recovery occurs naturally at a rate dependent on sediment supply from adjacent areas. The rate can vary from 2 to 3 years in areas of abundant sediment supply, such as close to the Orange River mouth, while in areas of slower sediment infill, recovery can take between 3 and 10 years. In rocky terrain, where sediment supply is reduced, recovery can take more than 10 years. There is no direct overlap between commercial fishing grounds, or known fish spawning, feeding or nursing areas and the Atlantic 1 license area.

There are currently 93 environmental monitoring stations across the Atlantic 1 mining licence area, which are sampled on every year as part of Debmarine Namibia's annual benthic sampling campaign that usually takes place between October and November each year.

The environmental monitoring programme forms part of Debmarine Namibia's approved Environmental Management Program, the Biodiversity Action Plan, and the Closure Plan for the Atlantic 1 mining licence area (G Ngaisiue, pers. comm., 2020). The company's overarching closure and rehabilitation objective is to leave a post-mining environment that has returned to a state where ecological function has returned to a state equivalent to comparable undisturbed sites. This is defined as being at least 80% similar in terms of species composition, abundance and biomass measured over a period of at least 3 consecutive years.

In 1996, the first-ever manned dives off the southern Namibian coast were undertaken using the submersible Jago, a two-person submarine owned by the Max Planck Institute in Germany (Fig. 5). Sustaining marine production requires an in-depth knowledge of seabed geology and an understanding of the behaviour of the mining tool. So far, geologists and engineers had to rely on remote information generated by a geo-survey vessel, and were not able to observe the geology of the seabed directly. Many new observations were made about the nature of the geology and the way in which it influences the efficiency of the sampling and mining processes, as well as about the natural environmental conditions on the Namibian continental shelf (Schneider, 2008).

The Jago is now returning to the area at regular intervals for exploration purposes and further research to monitor mining efficiency, and the impact of deep sea mining on the environment. The Jago research allows obtaining greater knowledge of the natural variability of the marine environment in the dynamic Benguela Current environment, to understand the consequences of marine mining, and to monitor changes over time. The monitoring surveys are conducted by independent scientists, and the results are published in an annual report which is peer-reviewed by internationally recognised marine scientists (G Ngaisiue, pers. comm., 2020).

Considering the size of the Benguela Current Large Marine Ecosystem (BCLME), mining impacts a very small area of the ecosystem and is deemed to be of low significance at a regional scale, as the mining license covers only 1.2% of the entire BCLME. The ecosystem has a relatively high abundance of organisms but a low diversity of species. Natural variability is very high in both space and time and can have as significant an impact as mining or greater at times. The seabed organisms are hardy and well adapted to this high natural variability and thus recover comparatively rapidly after being disturbed. Fortunately, there is no direct overlap between commercial fishing grounds, or known fish spawning, feeding or nursing areas and the mining license. 99% of the sediment mined, which is on average 0.5 m thick, is discharged and settles back to the seabed.

Debmarine Namibia has a complement of five full-time environmental staff, and compliance to environmental objectives is managed within the framework of an ISO14001 certified Environmental Management System. Over N$ 15 million is spent annually on the environmental monitoring programme, which is conducted by independent marine scientists and evaluated by an independent advisory committee of academics and scientists in the region. The success of the environmental monitoring programme is measured by the sampling rates and quality achieved during the sampling campaigns, the expansion of biodiversity knowledge, and demonstration of recovery against the rehabilitation objective. Following analysis, Debmarine Namibia's benthic samples are shared with other local entities such as the University of Namibia and the Ministry of Fisheries and Marine Resources, to facilitate education, capacity-building and the training and development of local marine researchers within Namibia.

In 2012, Debmarine Namibia established a Marine Scientific Advisory Committee (MSAC), comprising of recognised marine scientists from academia and industry and other key stakeholders. The committee advises on the monitoring of design and research techniques, reviews the results of the environmental monitoring programme to determine its effectiveness, and provides recommendations to Debmarine Namibia on marine environmental science related aspects.

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Conclusion

Diamonds are Namibia's most important commodity, contributing almost 10% to GDP, and making valuable provisions to the state coffers in terms of taxes, royalties and export earnings. It is well known that mining has the potential to contribute positively to all 17 UN Sustainable Development Goals (SDGs). Indeed Namibian marine diamond mining promotes the eradication of poverty through local procurements and inclusive employment (SDG 1); supports good health through occupational health and

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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