Several types of valuable mineral deposits exist under the oceans and other large bodies of water. Where water is shallow, placer deposits can be recovered by large dredges (eg, tin minerals off the coasts of Java and Borneo). Where deposits lie in shallow water near the shore, dams may be built around sections of the shoreline and the areas pumped out so that shovels or scrapers can be used to excavate the material. Diamond-bearing gravels are mined in this way along the coast of southwestern Africa.
Conventional Underground Mining
Conventional underground mining methods may be used to follow mineral deposits extending beyond shorelines if the rock strata above the ore bodies or coal seams are thick, strong and not porous or fractured. For example, coal is mined beneath the Atlantic Ocean near Cape Breton Island. The seams were first found on land, and mine buildings, shaft and slope entrances were established on land. Mining by the room-and-pillar and then longwall methods has followed the seams as they dip beneath the ocean. One mine, now closed, advanced more than 8 km beyond the shoreline. Although rock strata above the seams subside into the excavated areas after they are mined, no water has entered these mines. Iron ore mining also extended 5.5 km beyond the shore at Wabana, Nfld. The empty chambers have remained dry and are available for storage purposes.
Deep-ocean exploration has discovered jets and springs of hot water (to 350° C) rising from fissures and vents in ocean floors. These emissions appear to occur mainly along major ocean ridges and along fractures of the Earth's crust, such as in the Red Sea and off the BC coast. They may carry various metals and other elements in solution which, on encountering cold ocean water, precipitate as muds, layers, crusts or chimneys. Sulphides of copper, zinc, iron and other metals have been found in such deposits. These discoveries provide new information to geologists about the origin of hydrothermal mineral deposits. They may also provide future sources of economically recoverable minerals, if deposits of suitable size, grade and location are found.
The recovery of such deposits and of other mineral deposits known to lie at some depth below the ocean floor and at considerable distances from the nearest land will be difficult. Where the distance is too great to warrant driving long tunnels from shore-based shafts, it may be necessary to adapt techniques for offshore petroleum recovery. The difficulties are further complicated by the need to establish national and international legal controls. The recovery of metal-bearing nodules - for example, those occurring on the ocean floor in the mid-Pacific at depths of up to 5 km - is a challenge for the future. These fist-sized nodules contain manganese, copper, nickel and cobalt. Recovery will depend on the development of suitable technology and on the establishment of international agreements to regulate the work. Several multinational groups have investigated methods of recovery, including use of continuous lines of buckets, drag scrapers and suction devices. Research is also needed to develop suitable methods for processing nodules on boats or barges at sea.