Political issues associated with efforts to establish an ocean-mining regime are another deterrent to proceeding with deep-seabed mining. The less-developed countries are determined to obtain a greater share of the wealth that might be provided by deep-seabed mineral resources. Numerous industrial states are equally determined that the welfare of their populace and national economy will not be jeopardized. With offshore technology increasing using better equipment, and with more understanding of what resources are available, and fifteen years of work on Law of the Sea treaties, marine mining may become competitive with onshore mining.
In the U.S., there is little or no marine mining of sand and gravel, metallic minerals, and phosphorites. The U.S. has used up the majority of its land-based mineral resources. Therefore, the potential for minerals from offshore is great for the United States and even though the market is depressed, the reduction of mineral imports would reduce the national debt.
Diamonds have been mined off the South African coast and barite off the Alaskan coast, tin off the coasts of Thailand and Indonesia, oyster shells off the Gulf Coast of the United States, and sand and gravel off the coast of the United Kingdom.
The chief tool for underwater mineral exploration is the practicing marine geologist using seismic and magnetic profiling, dredging and coring tools, depth sounding, laboratory analyses, and geological and bathymetric mapping. At the present, less than 5% of the coastal seafloor of the world has been scientifically surveyed. In the search of the deep sea for minerals, if all of the dredge samples taken to date were averaged over the deep sea floor, this would give three dredge hauls per million square kilometers --three samples to evaluate the state of Texas.
Japan is the world's largest producer of offshore aggregates. Japan's onshore aggregate industry is experiencing increasingly stringent environmental regulations which makes offshore extraction of sand and gravel appealing. Between 20 and 25 per cent of Japan's supplies of natural aggregate comes from marine sources. Canada's offshore sand and gravel production in late 1984 was limited to the Arctic, where it was being dredged to build artificial islands for oil and gas drilling in the Beaufort Sea. Collectively Canada's coastal provinces have good offshore aggregate potential. Canada's offshore predicted sand and gravel production in the year 2000 might range between 1.8 million and 46 million tons.
On the east coast, Newfoundland-Laborador and Nova Scotia are projected to be the leading sand and gravel producers. Approximately 50 percent of all aggregates produced from the east coast provinces are consumed there. Great Briton has a major offshore aggregate industry. By the mid-1970's, production from offshore accounted for 12 percent of the national total (15.6 million tons). In 1986, 27 percent of the total aggregates used in southeastern England came from offshore showing a continued rise in offshore production.
Because of diminishing onshore reserves, an increasing public concern for the environment and a likely growth in consumption, offshore dredgers will need to produce additional tonage. The demand for construction sand and gravel in the U.S. is expected to grow at an annual rate of about 2.9 percent between 1982 and the year 2000. by the year 2000 estimated requirements are 8.1 x 108m3 for the US -rebuilding major segments of the interstate highway system, deep draft port facilities, airports and beach replenishment. A rough approximate of east coast United States sand resources are 5 x 1012m3 of sand. obviously not all of the 8.1 x 108m3 annual need will come from marine resources; but demand may initiate more use of offshore resources.
Exploration tools for sand resources are seismic profiling and side scan sonar; followed by vibrocoring. recovery can be by a variety of dredges. Sand and gravel mining is done with two systems: clamshell type buckets and sand pumps -- a hydraulic system. the hydraulic hopper dredge seems to be a universal type suited for open marine waters but it causes a higher degree of turbidity and environmental problems with ensuing regulation of operation. Besides turbidity, a possible problem with sand extraction is a change in bottom bathymetry, which could cause a change in wave regime, in turn a problem of coastal erosion. In Japan, this is eliminated by restricting extraction to water depths greater than 20 meters.
Industrial sand is another most widely used non-metallic commodity. Pure silica sands are dredged for the glass and chemical industry off Japan, northern New Zealand, and in the Baltic. Of the nearly 25 million tons of industrial sand produced in the U.S. during 1983, glass manufacturers used 36 percent, foundries 26 percent and abrasives producers 8 percent.
Aragonite sand occurs in shallow waters off Andros, Bimini, and Eleuthera in the Bahamas as oolite shoals. Marcona Ocean Industries uses a suction dredge to extract aragonite to a barge which then carries it to a stockpile on a combination of natural cays and interfilled area. It is moved from this storage for shipment to US and Caribbean markets. It is used for beach fill , acid neutralizing plants, and in agricultural and industrial chemical processes.
Marine shell material has been used as an alternate to sand and gravel and has been used as a source of lime for cement. Because of Iceland's lack of sedimentary rock, the government has developed a state-owned cement-works and an agricultural lime establishment that use seashells as their raw material. Brazil began producing lime from seashells in the 1950's. Several areas in the U.S. produce, or have produced seashells, including California, Texas, Louisiana, Mississippi, Alabama, Florida, Virginia, and Maryland. In the Gulf Coast, local road-builders surface roads with shells, where good construction sand, gravel, and stone are not available locally. Currently, Louisiana has an important shell-dredging industry. The state has no major commercial limestone or other calcium carbonate resources. Shells help fill this gap in Louisiana's resource base, and the industry contributes significant revenues to the state's coffers.
Controversy has centered on the environmental impact of shell dredging. Three complete Environmental Impact Studies (Mobile Bay, Tampa Bay, and San Antonio Bay) show that shell dredging causes only minor damage. Control of allowable dredging sites has prevented kills of live oysters; nearly all sediments settle out within about 120m of the dredge -actually less disturbance than a shrimp trawl. Dredging in Texas and Florida has stopped because of: depleted shell supplies, competition from lower cost crushed limestones and other limestone sources, economic recession and increased costs for environmental protection.
Deterrents to marine mining have been aesthetic and environmental concerns, development of technology, cost-benefit factors, and the lack of clear government regulations. The removal of the sand can cause severe erosion and loss of marine habitat. Research has shown that dredging operations can cause impacts that range from detrimental to beneficial. The impacts are predictable and careful planning can minimize undesirable ecological effects.
Another problem is purely economical, the cost of transportation to the market. The maximum transport distance for aggregate to be competitive with traditional onshore sources is about 150 km. Silica sand and carbonate aggregates have a higher unit value and can be transported farther distances. In the case of aggregates, port facilities and storage space are also important cost factors. Local markets and favorable geology are important for aggregate mining. Placer minerals which are concentrated at the site and which have much higher unit value are less sensitive to transportation.
The concentration of minerals depends on the source rock, weathering, transportation and trapping by subaerial processes and subsequent reworking and modification by marine processes. Locating potential marine placer resources is often less difficult than determining their exploitability, because varying littoral drift rates and directions, altering wave-energy distributions, and changing water levels contribute to regional and local variations in placer distributions.
Many mineral-bearing offshore and beach placers (diamond, gold, platinum, tin, chromite, iron sand, zircon, ilmenite, rutile, and monazite) are now mined or have been mined in the past. The major placer materials in the U.S. are tin, titanium, gold, platinum, and chromium.
Diamonds are the only gemstone presently mined offshore. World production of diamonds has grown to 100 million carats in the nineties but offshore mining produced less than 100 thousand carats in 1995. Diamond mining from the beaches and nearshore in Southwest Africa was done from 10 to 40 meters water depth with suction and airlift dredges, but the loss of three dredges caused a halt in 1971. In 1978, mining began again with SCUBA divers and suction tubes, with recoveries of about 26,000 carats of gem quality stones per year. Dredging for diamonds in Brazilian waters has been reported. Demand for gemstone diamonds is likely to expand as personal incomes increase in the US and other industrialized states.
The noble elements such as gold and platinum yield very high returns and can be mined from beaches and the nearshore off Alaska, British Columbia, Nova Scotia, Philippines, and Russia. More than $500 billion dollars of gold placers are estimated in Alaska. Presently, no platinum placers are mined in the world's oceans, but at one time, dredgers worked them in waters near the villages of Platinum and Goodnews Bay, Alaska. The western world's estimated demand for platinum is greater than the supply and Alaska still has a good potential for offshore platinum mining. The US consumes about one-third of the world's total annual platinum mine production, and it's probable consumption is projected to increase by 63 percent from 1983 to the year 2000 so there should be an interest in the offshore potential. A continuing vulnerability of the US's platinum supply, and an anticipated growth in demand for the metal, make it important that a greater effort be made to locate and develop domestic sources.
The strategic element titanium, derived mainly from ilmenite and rutile ores, is an important placer mineral. About 5 percent of the world's annual production are consumed in making metal alloys that need special strength and corrosion resistance. Titanium placers occur in West Africa, along the coast of Mauritania, on the continent's opposite coast, near the south-eastern shore of Madagascar. The titanium mining industry, in the past, experienceed "boom and bust" periods. Its fortunes are tied to the amount of activity in the military and commercial aircraft industries. With future growth likely in the worldwide aerospace industry, titanium will continue to play an important role in world markets. Total titanium metal consumption is expected to increase annually in the U.S. by 5.5 per cent between 1983 and 2000; the probable annual rate for the rest of the world will be 26.2 percent.
The best known and most important marine metallic placer is tin which is found in the mineral cassiterite. Today tin goes into the fabrication of containers, solders, engine bearings, and air-cleaner and oil-filter cartridges. The world's most important area for offshore tin production is in waters surrounding the Malay Peninsula and those lying between Sumatra and Kalimanan. About 7% of the global production of tin in mined offshore. Malaysia, the world's most important tin producer, had 31 percent of its total tin output come from the offshore in 1984.
The offshore now accounts for 50 percent of Thailand's total annual tin production. The Indonesian Government expected to produce nearly half the country's annual total output of 27,000 tons in 1986 from the offshore. Offshore or beach tin placers also occur in the UK, USSR, US (Alaska) Burma, the Peoples Republic of China, and the Philippines.
The world market for offshore production of tin is estimated to be from $1-2 million to $100 million, but the industry has been in a deep depression. The U.S. alone has potential tin placers off the coast of Alaska worth approximately $39 billion dollars, which is not being mined presently. The surplus problem is made worse because not all tin-producing states are members of the trade cartel and Bolivia and Brazil, the world's fourth and fifth largest market-economy producers continue to produce without quotas. The tin market is in oversupply, and the outlook for offshore tin mining, in the near future is grim. The tin industry should recover, although many of the less efficient producers may have been eliminated.
Because placers are available in marine beaches and nearshore environments, they are more accessible than the deep-sea minerals. Overall, the physical and economic problems confronting placer producers should be less severe than those faced by deep seabed miners, and more importantly, most placers lie within the continental shelf, which is legally controlled by the adjacent nation. Marine placer extraction should form an increasingly significant part of the offshore mining industry.
Different operating conditions, seabed characteristics and economic demands have contributed to the development of a variety of dredges for exploitation of placer deposits. Shallow-water dredges may be broadly classified into hydraulic and mechanical types. Hydraulic units siphon (by pipe) the desired material onboard as an ore and water slurry, whereas mechanical units cut into the ore and lift the fragments onboard by a dragline, a dipper, a clam shell or an endless-chain bucket system. Mechanical dredges can operate in waters of unlimited depth, but ordinary hydraulic dredges are limited to about 60m, with most operating at depths of no more than 30m. Seabed dredging creates sediment plumes that, after settling out, may affect some sessile and mobile benthic biotic communities.
Phosphorite deposits occur on several continental shelves -Atlantic waters of the U.S., Morocco, Gabon, Congo, Namibia, and South Africa and in the Pacific Basin in the US, Mexico, Ecuador, Peru, and New Zealand. They usually are in less than 1000m of water in relatively tropical regions where they are linked to zones of coastal upwelling, divergence and biological productivity. Most deposits are of Miocene age, reflecting good environmental conditions for phosphorite deposition at that time.
Specific areas in the United States include the Blake Plateau , offshore areas of California's central and southern coasts. Phosphorites are in a sediment sequence up to 10m thick on the Blake Plateau off Florida and economic studies of several shelf deposits indicate that if thick deposits exist in relatively shallow water and near markets they would be potentially economic today.
Sulfur is used in manufacturing and agriculture. Native sulfur is associated with salt domes in the offshore of the US Gulf Coast in salt dome cap rock . Most is produced onshore, but one offshore mine is in shallow water off central Louisiana. The sulfur is extracted by the Frasch system, which uses the injection of superheated water through boreholes to melt the sulfur which is forced to the surface by compressed air. The operation is minimal because of glutted sulfur markets stemming from by-product sulfur extraction in environmental control systems and petroleum refining which accounted for 55% of total world production.
Although metallic and precious minerals are less bulky themselves, large amounts of material must be removed to recover the minerals. This not only parallels the removal problems of bulk mineral mining, but adds return of the waste material as fill.