Wednesday, May 13, 2015

Biogenic Sedimentary Ore Deposits

Biogenic Sedimentary Ore Deposits
Biogenic Sedimentary Ore Deposits
Limestone is an extremely common rock formed as shell beds on a shallow sea floor. Purity depends on the environment of deposition and the subsequent mineralogical and tectonic history that may include metamorphism to marble. Limestone is exploited for uses ranging from construction aggregates and railroad ballast to cement and lime manufacturing, and glassmaking to GCC used as functional fillers in paper, plastics, and paint. The relatively modest price even for the high-calcium and high-brightness grades of GCC (less than $200/t) means that consumption is generally close to the point of production (i.e., a local or regional market). In the United States, for example, crushed limestone is produced in all states except for Louisiana (which does produce shell), includes more than 2,500 quarries, and accounts for two thirds of the nation’s crushed stone output. High-quality, filler-grade GCC produced in Vermont, Massachusetts, and Ontario in Canada serves many parts of the United States, including the Northeast, Maryland, the mid- Atlantic states, Georgia, Alabama, Illinois, Texas, California, and Washington. In Western Europe, chalk is important in the United Kingdom, France, and Belgium, whereas crushed marble is often used in Italy and Greece.

Has many of the uses outlined for limestone, plus several others, including in refractories, in seawater magnesia and magnesium metal production, and as a dimension stone. Although less common than limestone, dolomite production, particularly for aggregates, is extremely widespread. In the United States, nonaggregate production is concentrated in California, Ohio, Michigan, Alabama, Texas, Connecticut, and Pennsylvania. Europe has an active dolomite industry where it is used extensively as a raw material for refractories and seawater magnesia production. The main producers are Spain, the United Kingdom, Belgium, France, Germany, Norway, Sweden, and Finland.

Diatomite deposits are formed through the accumulation of the frustules of diatoms, small animals that thrived after the Cretaceous period. Diatoms require marine or freshwater rich in nutrients like phosphates, nitrates, and silica, and relatively free of sediment. In many cases, the diatomite is associated with volcanic activity, which may be the source of silica. Important diatomite production sites include the western United States—especially California, Nevada, Washington, and Oregon. Almost three quarters of the world’s production comes from the United States, China, Japan, and Denmark (although the last produces mainly moler, an impure diatomite product). Other producers are Spain, Germany, Italy, Iceland, and Korea.

Phosphate Rock
The bulk of commercial phosphorus-based compounds are derived from marine sedimentary phosphate rock deposits, with much of the rest derived from igneous deposits. Sedimentary phosphate deposits are concentrated in two main belts, the trade-wind belt that is aligned north–south from the equator to 50° latitude, and the equatorial belt, oriented west to east in low latitudes. In the tradewind belt, deposits are located in the southeastern and northwestern United States (Florida, North Carolina, and Idaho); Baja California, Mexico; Sechura Desert, Peru; and the Caribbean Sea. In the equatorial belt, deposits are found in North Africa (Morocco, Tunisia, Algeria, Togo); the Middle East (Jordan, Israel, Syria, Senegal, Iran); the former U.S.S.R.; Venezuela; and Colombia.
More than 80% of the world’s phosphate rock production comes from the United States, Morocco, China, Russia, Tunisia, and Jordan. Morocco is particularly important because it the leading exporter. A series of middle-range producers include Brazil, Israel, Togo, and South Africa. The production in Russia, Brazil, and South Africa is largely derived from igneous deposits that may also yield rare earths, vermiculite, or copper.

The discretionary extraction of sulfur has declined as more and more production is based on nondiscretionary by-product sulfur extracted to comply with environmental regulations at nonferrous smelters and roasters, natural gas and crude petroleum processing plants, coking plants, and the like. Native sulfur associated with the cap rock of salt domes and in sedimentary deposits is still mined by the Frasch hot-water method (mainly in Poland but no longer in the United States), and the mining of pyrite as a source of sulfur is restricted to China.
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