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Places containing a sufficient amount ofminerals, which may be used to obtain metals, are called ore deposits, and minerals containing metals are called ore minerals. They are chemical compounds of metal elements combined with sulphur or oxygen. Ore in pure form is found very seldom, especially in larger pieces, they are combined generally with rock minerals, which are also important for us. Some precious or heavy metals, for example gold, occur naturally in the form of elements.
A valuable raw material is obtained by separating the ore from the host rock. A deposit is economically viable only if a certain element occurs in a substantially higher concentration than in the average rock of theearth's crust.
Deposits are formed by various geological processes. There are sedimentary, igneous, and hydrothermal deposits.
Sedimentary deposits are among the most important raw material resources on earth. Through chemical andphysical action during the sedimentation process, many economically important minerals were separated. The most important factor in this case is the dissolving property of water. An example of this process are the silica sands. The water currents and waves change sands to such a degree, that only pure quartz is left behind. Quartz is an important raw material in the glass industry and for the telecommunications technology (fibre optics).
Copper,iron, and other metals are also important sedimentary ore deposits. The most important ores are found in the Precambrian sedimentary rock. There are large deposits of this type in Brazil, India, Australia, and North America.
Ore deposits with a high content of gold or other heavy metals, for example titanium minerals, are called placer deposits.
A well-known method of obtaining gold is the panning. Byswirling a pan filled with fluvial deposits, one first separates the lighter minerals, because the heavy gold settles on the bottom of the pan.
Diamond, one of the most valuable minerals, comes to the earth's surface through thin, long tubes. Diamonds form in the depth of the upper earth's mantle, because only there thepressures are such as to allow the formation of this mineral.
Igneous deposits often occur in layers. They form, when minerals crystallise from the moltenmagma, sink, and are enriched on the bottom of the igneous chamber. In southern Africa, important deposits of platinum and chromium were find in this type of deposits.
Hydrothermal deposits form thanks to the action of hot solutions during magma melting processes, or in the hot underground water.
Minerals are separated by the hot liquids from the rock and when they are transported to cooler rocks, they settle in the cracks and cavities. This is how iron sulphides, zinc, and copper form.
If the hydrothermal solutions reach the earth's surface, it gives rise tohot springs and geysers containing mercury and zinc. Large hydrothermal deposits are found for example in the Mississippi river basin in Kansas and Oklahoma.
On the boundaries ofcolliding plates and in subduction zones, we find other deposits, of both the hydrothermal and igneous origin. This is an indication that there must be a link between the movement of the tectonic plates and the formation of the deposits.
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