Geology. Козлова Е.П. - 20 стр.

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                             Types of mineral deposits

       Mineral deposits are generally classified on the basis of the geological
processes responsible for their formation. They may be magmatic, contact,
metasomatic, pegmatitic, hydrothermal, sedimentary, residual and regional
metamorfie deposits.
       Magmatic deposits originated by cooling and the crystallization of magma and
the concentrated minerals form part of the body of the igneous rocks. Magmatic
deposits are relatively simple in mineral composition and few in number.
       Pegmatites, for example, are relatively coarse-grained rocks found in igneous
and metamorphic regions. The great majority of them consist of feldspar and quartz,
often accompanied by mica, but complex pegmatites contain unusual minerals and
rare elements. Many pegmatites are regular tabular bodies; others are highly irregular.
       At the earth’s surface the action of the atmosphere and hydrosphere alters
minerals and forms new ones that are more stable under the existing conditions.
Sedimentary deposits are bedded deposits derived from pre-existing material by
weathering, erosion, transportation, deposition and consolidation. The mineral
deposits that are formed as a results of sedimentary and weathering processing are
commonly grouped as follows: 1) sedimentary deposits, not including products of
evaporations, 2) chemical evaporites, 3) placer deposits, 4) residual deposits, and 5)
organic deposits.
       Sedimentary deposits are the extensive coal beds of the world, the great
petroleum resources, clay deposits, limestone and dolomite beds, sulphur deposits,
etc.
       Chemical evaporites consist of soluble salts formed by evaporation, in closed
or partly closed shallow basins. Deposits of gypsum and common salt (halite) are
found in many countries.
       Placers are the result of mechanical concentration whereby, heavy, chemically
resistant, tough minerals are separated from light, friable minerals. The most
important economic placer deposits are those formed by stream action. Stream and
beach placers widespread in occurrence and include the famous gold placers of the
world as well as deposits of magnetite zircon and garnet. Placer deposits of diamond
and platinum ale less common.
       Large residual deposits of clay bauxite, from which aluminum is extracted,
phosphate, iron and manganese have been worked in many parts of the world.
Smaller deposits of nickel and other minerals have also been worked.
       Organic deposits are formed by the remains of animals and plants. Plants and
animals collect and use various inorganic substances in their live processes and the
concentration of certain of these substances upon the death of the organisms may
result in the formation a mineral deposit. Coal and peat are formed from plant
remains and represent concentrations of carbon from the carbon dioxide of the
atmosphere .

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