Terrestrial concessions, including beach mining concessions, are mined between the Orange River mouth and an area slightly south of the Olifants River. Two major companies, Alexkor (a parastatal organisation which is currently being privatised) and De Beers Namaqualand (Pty) Ltd dominate diamond production along the shores of Namaqualand and the Northern Cape. Alexkor operates from the Orange River mouth to just south of Port Nolloth, while De Beers operates from Alexkor’s border to slightly north of the Olifants River (Figure 1). Around these major concessions are a number of smaller concessions operated by companies like Trans Hex Investments (Pty) Ltd.
3.1.2 Offshore Mining
In South Africa the offshore concessions stretch from the border with Namibia off the Orange River mouth, to an area just south of Saldanha Bay (Figure 2). Each concession area is further split into four sub-areas in an offshore direction (Figure 3). The ‘a’ concession extends from low water to 31.5 m offshore, while the ‘b’ concession extends from the western boundary of the ‘a’ concession to a co-ordinated boundary approximately 5 km offshore (less from concession number 15-20). The ‘c’ concession runs from the western boundary of ‘b’ concession to the 200m isobath and the ‘d’ concession runs from the western boundary of ‘c’ concession to the 500m isobath.
The major players, including concession owners and concession operators in the offshore diamond industry of South Africa are listed in Table 1. Three companies dominate the offshore diamond industry: De Beers Consolidated Mines (mainly mid- to deep-water concessions), Alexkor (mainly shallow-water concessions) and Trans Hex (shallow and deep-water concessions). Other significant players are BHP-Benguela Nominees, Ocean Diamond Mining, De Beers Marine (undertakes all De Beers offshore operations), Namagroen prospecting, Benguela Concessions and Marine 17 Mining.
3.2.1 Onshore mining
Namdeb Diamond Company (Pty) Ltd (formally Consolidated Diamond mines of SWA) controls mining of the ‘Sperrgebiet’ under the Namdeb Agreement (1994). They have exclusive rights to prospect for and mine diamonds within the Sperrgebiet until the year 2010. The Sperrgebiet stretches from the Orange River in the south to latitude 26ºS in the north, extending 100 km inland (Figure. 4). Namdeb is a 50/50 partnership between De Beers and the Namibian government and is responsible for the majority of onshore production in the southern parts of the Sperrgebiet (immediately north of Oranjemund), in their beach mining operations. Some smaller contractors also operate onshore on the Namibian coast, but they mine predominantly in the northern license areas.
3.2.2 Offshore mining
Namibian offshore concessions cover the full length of counties coastline, extending from the Orange River to the Kunene (Figure. 5). Concession boundaries are not regulated by depth as in South Africa. Onshore mining licenses extend 3 km offshore, and Namdeb thus controls most of the shallow water mining activities (with the exception of the island concessions, which are operated by ODM). Offshore licenses are issued after specific application to government, which must include the co-ordinates of the intended mining area.
he major companies involved in the Namibian offshore diamond industry are Namdeb, Ocean Diamond Mining (ODM)(of which Eiland Diamante is a subsidiary), the Namibian West Coast Mining Company - Diamond Fields International (DFI) group, the Namibian Minerals Corporation (NAMCO) - Arena Mining group, and Tidal Diamonds (an associate company of Namdeb). Numerous other companies like De Beers Marine, Yam Diamonds (who also own their own concession) and Windvogel Diamonds mine mainly on contract to the larger players.
No coastal or offshore mining currently occurs in Angola. Prospecting and mining activities are continuing inland but the Angolan government has issued no licenses for coastal areas, preferring to concentrate their efforts on terrestrial operations.
Figure 2. Map of the offshore concession areas of the Republic of South Africa. A key to the concession holders and operators is provided in Table 1 (provided by De Beers Marine).
Table 1. Diamond mining concession holders and operators in the Republic of South Africa.
Figure 5. Major coastal and offshore diamond mining concession areas of Namibia. (Original coverage provided by B. Beuthin, Geological Survey, Ministry of Mines and Energy, Namibia).
4. Mining Techniques and Operational Practices
Diamonds in the southern African coastal region are concentrated in the vicinity of paleo drainages, in bedrock gullies, potholes and depressions, along offshore ridges and south facing paleo bays, headlands and aeolian transport corridors. The greatest concentrations of diamonds usually occur near or on the bedrock. In order to retrieve diamonds, the overlying marine and terrestrial deposits (overburden) first has to be removed, followed by the collection of the diamondiferous gravel below. The techniques used to remove the overburden and to collect the gravel varies considerably according to where the gravel is located, the richness of the ore and the thickness of the overlying deposits. Basic distinctions can be drawn between operations conducted on land, and in shallow-, mid- and deep-water areas. These are elaborated separately below.
4.1 Terrestrial and Beach Mining
Onshore mining activities immediately adjacent to the coast are undertaken mostly using heavy-duty earth moving equipment, operational in large-scale open cast mines. Five fairly distinct phases are involved in the mining operation, including prospecting, overburden stripping, excavation of terrace gravels, mineral processing and sorting. The precise methods used in each of these phases has been developed and improved upon over the years and varies somewhat between areas and from company to company. What follows here is a generic description of the methods currently in use.
The major form of prospecting involves the drilling of small diameter percussion holes, followed in some cases with large diameter auger drilling and/or the excavation of trenches of varying depth and length. Overburden material from the trenches is generally deposited in overburden dumps on the surface next to the trenches and the underlying gravel is removed for processing. These three methods of prospecting are used to locate, intercept and sample the terrace gravels. As terraces have eroded erratically and diamond occurrence is sporadic, prospecting is generally undertaken in a closely spaced pattern in order to delineate the ore bodies accurately.
The next phase, overburden stripping, is done using a variety of earth moving machines including bowl scrapers, bulldozers, mass excavators and in some areas bucket-wheel excavators. Excavation is generally undertaken on a block-by–block basis and the overburden removed is dumped into trenches from which the underlying gravel has been removed; piled into overburden dumps; or used to create a seawall, extending the shoreline several hundred meters out to sea.
By constructing protective seawalls, gravels up to 20m below sea level and several hundred meters beyond the present shoreline can be recovered using terrestrial operations. Once the overburden has been removed, the bulk of the diamond bearing gravel is excavated mechanically. Deposits remaining in gullies and potholes on top of the bedrock are then swept and collected manually using pick, broom and shovel teams or using suction equipment. All the ore is transported to treatment plants where the gravel is crushed, washed, sieved and concentrated in a solution of seawater and ferrosilicone using heavy media cyclones. The gravel concentrate is then dried, sorted by means of x-ray machines and finally the diamonds picked out by hand. Gravel, from which the concentrate has been extracted, is disposed on tailings dumps, while water containing sand and fine sediment is discharged into slimes dams or pumped directly into the sea. Most of the ferrosilicone is recovered before it leaves the plants and is recycled in the process.
4.2 Shallow-water Mining
Shallow-water mining operations are conducted using small-scale, diver assisted suction equipment. Shore-based operators generally operate in the intertidal down to a depth of around 10 m, while the boat-based operators usually work in the 10-30 m depth range. This delineation is not strict, however, with the boat-based miners moving inshore in areas where access to the shoreline is difficult (where sea-cliffs abut directly onto the shore or in the offshore islands concessions).
The techniques used for shore- and boat-based operations are very similar, expect that boat-based operations generally employ larger equipment and more divers. A shore-based operation typically consists of 2-3 divers, their assistants and a tractor modified to drive a rotary classifier and centripetal pump to which an eight inch suction hose is attached. The divers, operating on surface-supplied diving equipment, guide the terminal end of the hose into the gravel deposits, which are sucked up and delivered directly to the classifier. Concentrate is bagged and brought to central sorting houses onshore. Large rocks are often moved by the divers (or pulled up onto the shore using a tractor) to allow the pump nozzle to reach deep layers of gravel where the heavy diamonds settle. Coarse material is allowed to build up on the shore, while fine material is returned to the sea. In some instances, kelp may be cut to facilitate access to shallow inshore ore bodies.
A typical boat-based operation consists of a 10-15m vessels with a 5-8 man crew, of which 2-3 are divers. The vessels are equipped with 1-2 hoses per boat, with the duration of their activities limited to daylight hours for 3-10 diving days per month. Some 20-22m vessels, offering surface decompression facilities and with 8-11 man crews, are also operational. These larger boats are able to work on a 24h basis for up to 21 days per month. However, due to the water depths involved, diving in nearshore mining operations necessitates strictly enforced decompression routines thereby limiting bottom working time. The diamondiferous concentrate is bagged onboard, and brought ashore for final processing in shore-based jig plants and sorting houses. On vessels operating further afield, the initial jigging may be conducted on board. Oversize tailings are either returned directly overboard to the mined area or transported further offshore in inflatable boats and dumped. The fines are washed overboard. In the mining process large rocks may either be exposed, or removed by divers to allow the suction nozzle to reach deeper gravel layers. The rocks are sometimes accumulated by the divers into rock piles.
Shallow-water mining is opportunistic in nature and highly dependent on weather and sea conditions. Due to the difficulty of employing modern geophysical survey techniques and large-boat sampling in shallow water nearshore areas, exploration and investigation in water depths <30m, is generally limited to irregular side-scan sonar mapping and prospecting dives from the shore or from small boats. These surveys are often based on historical recoveries. Small-scale sampling is undertaken by diver-assisted dredging, the gravel being bagged and processed ashore. Many of the nearshore operations are currently being enhanced by more sophisticated tracking and positioning systems to help focus efforts on the more productive areas. Both shore- and boat-based miners generally only operate in exposed rocky shore areas where gravel is pumped from deeper gullies, or on the edges of sandy bays where the layer of overburden is relatively thin. Mining off sandy beaches is generally unprofitable for these small-scale operators due to the large volumes of overburden that have to be removed before it is possible to gain access to the heavier gravel. However, the use of underwater ‘blowers’ to shift overlying fine sediment, initially used only when operating in shallow water (<8 m depth), is becoming more widespread as it allows for the exploitation of gravel deposits which were previously uneconomic to recover.
4.3 Mid-Water and Deep-Water Mining
A variety of methods are used to mine marine diamond resources in water depths >30 m, which may be split into mid-water operations (down to a depth of 75 m) and deep-water operations (down to a depth of 200 m). The geophysical survey and prospecting methods in use are similar for both regions, and include high resolution side-scan sonar, shallow reflection seismic profiling, video, vibrocoring, rock drilling and grab sampling. The resulting data are used to produce maps of the seabed geomorphology, sediment and bedrock distribution, bathymetry and sediment type and thickness profiles. From these maps, areas of unconsolidated sediment suitable for sampling are identified, and a sampling grid is positioned over the area. Surveying activities are usually ongoing in order to develop geological models encompassing all the concession areas held by a company. Precise sediment sampling using penetrating tools is subsequently carried out on the grid.
Once a mineable ore reserve has been identified, bulk sampling is conducted in the sampling grid. The bulk sampling process is done in a scattered grid pattern and is similar to mining but on a smaller scale. Mineable marine ore reserves are divided into rectangular blocks of 50x50 m which are then systematically and contiguously dredged. While some block groups may only be a few 100 m long, others can stretch 1-2 km in length. Commonly, Wirth Drill or seabed crawlers are used to clear 50 m2 areas of sea floor to bedrock level. Seabed crawlers, equipped with anterior articulated cutting and/or sucking devices are lowered onto the seabed on a hoist rope, with power and signal umbilical cable attached and controlled remotely from a surface support vessel. The vehicle mines by systematically advancing along a specific ‘lane’ achieving precise coverage of the area to be mined. This mining tool is especially suitable on flat areas with few boulders and is capable of mining sediment thicknesses of up to 5 m in water depth of up to 150 m. The Wirth Drill is a vertically mounted, larger diameter drill-head used to recover diamond bearing gravel in a systematic pattern of overlapping circles over the mining block. The drill is capable of drilling through more than 5 m of sediment and penetrating rock in water depths to 150 m.
Mining using the above techniques involves the removal of only the unconsolidated superficial sediments. The dredged sediment-slurry is airlifted to the surface, discharged into a slotted circular tower and dropped onto a multi-decked screen, which separates the oversize and undersize fractions. These are immediately discarded overboard, care being taken to prevent covering unmined areas. Of the material airlifted to the surface, 99.9% is returned directly to the sea. Re-mining of an area occurs only when the initial coverage of a block by the mining tool was insufficient.
The fraction of interest (plantfeed) is fed through a ball mill to fragment the shell and clay components, before being mixed with ferrosilicone and pumped under pressure into a dense medium separation plant. Low-density materials (floats) are separated from the concentrated plantfeed and discarded overboard. The remaining high-density fraction is dried and passed through an x-ray sorting machine to separate the diamonds. Non-fluorescent material is discarded overboard and the fluorescent fraction is automatically sealed in cans. On some of the smaller offshore vessels the high-density fraction is still hand sorted for diamonds.
The prospecting and mining vessels currently employed in offshore diamond recovery in Namibian and South African waters are semi-mobile platforms on a dynamic positioning system, or self-mooring systems comprising three to four anchors. These ships, which range in size from 50 m (26 crew) prospecting vessels to 140 m (90 crew) mining vessels, are fully self contained mining units operating on a 24 h basis through 11 months of the year.
Several new techniques are planned for the future that will reduce mining costs and allow for the exploitation of lower grade ores in the mid- and deep-water zones. These include jack-up rig platforms and bulk dredging operations. Jack-up rigs will be semi-mobile platforms housing the separation and recovery plants, which are fed slurried gravel via flexible hoses attached to multiple dredge crawlers. These platforms will be moved as required (probably not more than once or twice per year) and will be serviced by ship or helicopter from the nearest logistical base.
Massive sheet gravels occur in some mid- and deep-water areas that are uneconomic to exploit using currently available methodology. Plans are afoot, however, to begin exploiting this low-grade ore using high volume suction hopper dredges. These plans call for the use of large dredges on which under- and over-size material will be screened to separate hoppers, while plant feed material is transferred to processing plants at sea or on the shore. Tailings disposal would be back to the sea in designated tailings areas.
5. Environmental policy and Legislation