Background (Encyclopedia of Global Resources)
Placer deposits are found throughout the world wherever the mechanisms of concentration, water and wind, have been active and the resulting concentrates have not been redispersed by later processes. The best known types of placers occur in river channels and in beach sediments.
The weathering and erosion of rocks release particles of varying size, shape, and density. Soluble materials are dissolved and removed in surface water or groundwater. Some minerals, such as feldspars, are hydrated and converted into clay minerals, which, being soft, small, and of low density, are relatively readily removed in suspension. Quartz (SiO2), common in many kinds of rocks, generally weathers out as roughly equant grains that, because of their hardness and insolubility, wash into streams and rivers, where they are moved by rolling, bouncing, and pushing along toward the oceans. The abundance of quartz and its resistance to mechanical and chemical weathering results in its being the most abundant placer mineral and the principal constituent of temperate and cold climate beaches throughout the world.
During the weathering and erosional processes, other minor or trace minerals, which are resistant to breakdown, are also transported along with the quartz grains and pebbles in river channels to the ocean margins. If the mineral particles possess high densities, they may be selectively concentrated as the transporting agent (usually water) more...
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Economically Important Placer Minerals (Encyclopedia of Global Resources)
Many types of minerals and rock materials can occur in placer deposits; among the most important are gold, titanium minerals, zircon, tin oxide, diamonds, platinum, and sand and gravel. Throughout history, gold has no doubt been the most important placer mineral. Gold is soft and malleable but is otherwise nearly inert in the weathering realm. Hence, once gold is weathered out of the lode deposits where it initially formed, the grains may survive transport in streams and rivers over long distances. It was the discovery of such gold grains that led to nearly all of the world’s major gold rushes, including the California gold rush in 1849. Gold placers have formed throughout geologic time; the world’s largest gold reserves in South Africa occur in placers formed 2 billion years ago.
The titanium minerals, ilmenite and rutile, occur in minor amounts as small grains in many types of igneous and metamorphic rocks. These minerals are highly resistant to weathering and hence are liberated intact from their host rocks. Although their densities are less than twice that of quartz, they are quite effectively concentrated by flowing water in rivers and by the agitation of waves along beaches as the lower density quartz grains are winnowed out. The zirconium silicate zircon is a common accessory mineral in alkaline igneous rocks. It weathers out as the titanium minerals do and is generally found with those...
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Types of Placer Deposits (Encyclopedia of Global Resources)
Placer deposits have been classified into several different types on the basis of location of formation. The major types include residual placers, eluvial placers, stream or river placers, riverbank and flood placers, eolian placers, and beach placers. Residual placers are occurrences of minerals at or near their point of release from the original source rocks. There has been some degree of enrichment of the placer minerals as the result of the removal of other portions of the host rocks by weathering. Eluvial placers are transitional placers in which concentrations of placer minerals occur downslope from the source rocks but where the valued minerals have not yet washed into streams and rivers that would transport them for long distances.
Stream or river placers are the best known placers and are the types responsible for most famous gold discoveries. The movement of the running water, especially where there is turbulence, is effective in sorting rock fragments and mineral grains according to size and density. Because of their higher densities, gold grains and several other placer minerals settle out. They are readily trapped in crevices and irregularities on the stream bed or among larger boulders, as the lower density materials are more easily washed away. This type of placer sometimes grades into deltaic beds where a river drains into a lake or the ocean.
Riverbank and flood placers are deposits adjacent...
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Mining of Placer Deposits (Encyclopedia of Global Resources)
Gold panning is probably the best known method of exploiting placer deposits. A circular motion of water in a pan containing gold along with other sediments effectively separates the minerals on the basis of their densities. The same general principle is used in sluices, channel-like boxes with barriers to create turbulence in the water so that sorting can take place. On a large scale, modern placers are mined by the scooping up of the unconsolidated materials and the use of either spiral classifiers or heavy media to separate the heavy materials from the light materials. The differences in the densities of the minerals allow for effective separation.
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Further Reading (Encyclopedia of Global Resources)
Bell, Fred J., and Laurance J. Donnelly. “Placer Deposits and Mining.” In Mining and Its Impact on the Environment. New York: Taylor & Francis, 2006.
Boggs, Sam, Jr. Principles of Sedimentology and Stratigraphy. 4th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006.
Dixon, Colin J. Atlas of Economic Mineral Deposits. Ithaca, N.Y.: Cornell University Press, 1979.
Guilbert, John M., and Charles F. Park, Jr. The Geology of Ore Deposits. Long Grove, Ill.: Waveland Press, 2007.
Hartman, Howard L., and Jan M. Mutmansky. Introductory Mining Engineering. 2d ed. Hoboken, N.J.: J. Wiley, 2002.
McCulloch, Robin, et al. Applied Gold Placer Exploration and Evaluation Techniques. Butte: Montana Bureau of Mines and Geology, 2003.
Macdonald, Eoin H. Alluvial Mining: The Geology, Technology, and Economics of Placers. New York: Chapman and Hall, 1983.
Valentine, David. “Chinese Placer Mining in the United States: An Example from American Canyon, Nevada.” In The Chinese in America: A History from Gold Mountain to the New Millennium, edited by Susie Lan Cassel. Walnut Creek, Calif.: AltaMira Press, 2002.
Wells, John H. Placer Examination: Principles and Practice. Washington, D.C.: U.S. Department of the Interior, Bureau of Land Management, Phoenix Training Center, 1989.
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