Wednesday, May 13, 2015

Chemical (Evapourite) Sedimentary Ore Deposits

Chemical (Evapourite) Sedimentary Ore Deposits
Chemical (Evapourite) Sedimentary Ore Deposits
Certain minerals are found dissolved in seawater and various other brines; evaporate deposits form when the concentration reaches saturation and precipitation occurs. This process requires favorable conditions such as a barred basin or broad shelf environment, plus a hot, dry climate that encourages evaporation. Once formed, the fragile deposit needs to be preserved in subsequent geological events such as burial. Halite, gypsum, and anhydrite, often interbedded with limestone and dolomite, are the most common minerals present in marine evaporates, which often extend over hundreds of square kilometers and attain a thickness greater than several thousand meters. In rarer cases, other evaporate minerals are present, such as potassium minerals, borates, and strontium minerals. 

Common salt lives up to its name by being ubiquitous in most brines and many evaporate deposits. It is exploited commercially from seawater where the modern climate allows evaporation (e.g., in Western Australia, Mexico, Bahamas, Netherlands Antilles, Chile, India, Brazil, Italy, and Spain). Large, buried rock salt and salt dome deposits are important in the northeastern, southern, and midwestern areas of North America; central and northern Europe; parts of the former U.S.S.R.; and the Middle East. Saline lakes are worked in the western United States, for example, the Great Salt Lake in Utah (which yields or has yielded sodium sulfate, potassium minerals, magnesium chloride, bromine, and lithium as well as salt) and Searles Lake in California (sodium sulfate and borates). Despite the widespread production of salt, three quarters of the world’s production is accounted for by the United States, China, Germany, India, Canada, Mexico, Australia, France, Brazil, Chile, and the United Kingdom.

Potassium Minerals
In some cases, potassium minerals are found associated with salt. During the Permian period in Europe, for example, the Zechstein Basin extended from northern Britain through the Netherlands, Denmark, and Germany to Poland. This was a shallow stable sea that allowed thick evaporate sequences to accumulate, resulting in the large potash and salt deposits that are exploited today. To the east, three basins contain vast reserves of potash—Stebnik/Kalush in Ukraine near the border with Poland, Soligorsk near Minsk in Belarus, and Solikamsk/Berezniki west of the Urals. In western Canada during the Silurian to mid-Devonian period, the land subsided to form a large basin stretching southward into the United States. This allowed vast thicknesses of evaporates to be formed, including the potash resources of Saskatchewan, Canada. A large potash/salt-rich basin identified in Thailand has potential for commercial production. Despite its strategic importance as a fertilizer, large-scale potash production is fairly restricted with just 10 countries—Canada, Russia, Germany, Belarus, Israel, the United States, Jordan, the United Kingdom, Spain, and Brazil—accounting for 97% of world production.

Strontium is recovered from two strontium minerals, strontianite (strontium carbonate) and celestite (strontium sulfate). Virtually all the world’s celestite is mined in just six countries— Mexico, Spain, Turkey, Argentina, Iran, and Pakistan. In northern Mexico, the output from several mines accounts for half of the world’s production; Spain contributes an additional 40%.
Most strontium (76% of the strontium consumed each year) is used to make compounds that are applied to the glass picture tubes on color television sets. This compound blocks the X-rays created by the picture tube.Some strontium (10%) is used to make special magnets called ferrite ceramic magnets. Strontium is the element that gives road flares and fireworks a bright red color. Pyrotechnics and flares account for 5% of the annual strontium consumption.

As mentioned previously, barite can be associated with evaporate minerals such as celestite and gypsum. In addition, barite is also found as a hydrothermal vein filling associated with stratiform massive sulfide deposits and as a residual deposit. China has emerged as the world leader in barite production, accounting for more than 50% of the world total. In contrast, barite production in the United States has declined to 6% of world production because of the availability of imports from China and, to a lesser extent, Mexico and Morocco. Other suppliers are India, Iran, and Turkey.

About 60% of the world’s borate production is controlled by the United States and Turkey, with Argentina, Chile, and Russia supplying most of the balance. Part of the production is based on brines and encrustations in Searles Lake, California, and from small concentrations along a stretch of the Andes Mountains encompassing Argentina, Bolivia, Chile, and Peru. The major source of borate minerals is a buried mass of sodium borate at Boron in the Mojave Desert of California. Other buried and surface borate deposits occur in Death Valley, including the Billie colemanite deposit, which was mined until the mid-1980s. In Turkey, borates are concentrated in six areas—the most important being the Emet and Kirka areas (both sodium borate) and the Bigadiç area (calcium borates).

Natural nitrate production is now confined to an area of northern Chile that has an annual rainfall of less than 1 cm. In addition, iodine is co-produced from the caliche ore and accounts for more than 50% of world production. Elsewhere, commercial iodine production is based on brines largely found in Japan (35% of world production), the United States (Oklahoma), China, Azerbaijan, Russia, and Turkmenistan. Bromine is also extracted from well brines in Arkansas and Michigan in the United States, the Dead Sea in Israel, potash brines in Germany and France, and seawater in the United Kingdom, France, Spain, and Japan. The United States and Israel combine to account for 75% of world production. 
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