Sunday, May 3, 2015

METAMORPHIC Ore Deposits

Metamorphic Ore Deposits
Metamorphic Ore Deposits
Asbestos
Asbestos is found in several metamorphic environments usually associated with ultramafic rocks and serpentinization.
Asbestos minerals were once used in a wide variety of products. However, due to adverse health effects, the use of asbestos in the U.S. has been significantly decreased. In 2013, for example, the total amount used was only 950 tons, all of which was chrysotile, and was mined in Brazil. Many other countries still mine and use asbestos in insulation products due to less stringent health and safety regulations.
Large deposits are exploited in Quebec, eastern Canada; in the Transvaal and Cape Province of South Africa and in Swaziland and Zimbabwe in Africa; Russia, Italy, and Greece in Europe; New South Wales in Australia;; Kazakhstan; and India. 
World Resources:

The world has 200 million tons of identified resources of asbestos. U.S. resources are large but are composed mostly of short-fiber asbestos, for which use in asbestos-based products is more limited than long-fiber asbestos.
Asbestos Mining:
Chrysotile mining developed through a succession of mining practices and equipment. Originally, the simplest hand methods were employed in shallow open-pit workings. Later, when pits reached considerable depth, overhead cableway derricks were used. Now, power shovels and heavy-duty trucks have replaced other loading and transporting equipment for open-pit quarry methods. 
In its early history, most chrysotile was mined in Canada using underground methods, which included glory holes, shrinkage and sublevel stoping, and block caving. Now open-pit mining prevails. Surface mining offers advantages in recovery, grade control, economy, and safety. The shift to open-pit mining resulted mainly from the introduction of large-capacity power shovels, trucks with higher payload capacities, improved rock-drilling equipment, and new blasting agents and techniques.
In Africa, chrysotile is still mined underground. Most ore bodies are tabular in shape with a pronounced dip so that the economic limit for quarry mining is reached at a comparatively early stage. Ore widths in the larger mines commonly range from 20 to 60 m and as much as 120 m. Some ore bodies, notably in the Shabani District of Zimbabwe, are long. In one case, development extended for 5 km along the strike and was being developed or diamond drilled to more than 300 m in depth.
Several underground methods have been used. Sublevel stoping and caving may be initiated by blasting holes drilled upward from sublevel crosscuts, starting first on the hanging-wall side and retreating over a considerable width toward the footwall. Development and retreat also may be along the strike of the ore. In some cases, high pressures tend to develop from an arching effect. These pressures can be released by cutting a vertical slot that may extend to the surface. In the sublevel stoping method, a slot also may be opened across the center of the ore body. The holes that are fanned out from the sublevel drifts are blasted toward the slot, and mining proceeds as a systematic retreat in two directions away from the opening.



Talc
Import Sources (2009–12): China, 35%; Canada, 31%; Pakistan, 18%; Japan, 4%; and other, 12%.The United States and Canada account for about 12% of world talc production with output from Vermont, upstate New York, Montana, Texas, and California in the United States and Quebec and Ontario in Canada. In Australasia, China is by far the largest producer, followed by India, North and South Korea, Japan, and Australia. In Europe, significant talc producers include France, Italy, Austria, Finland, and Norway, along with Russia. In South America, Brazil is the largest producer. 
Ground talc is used as an ingredient in ceramics, paper, paint, roofing, plastics, cosmetics, talcum and baby powders, and a variety of other assorted uses such as making rubber and plastics.
World Resources: The United States is self-sufficient in most grades of talc and related minerals. Domestic and world resources are estimated to be approximately five times the quantity of reserves.

Substitutes: Substitutes for talc include bentonite, chlorite, kaolin, and pyrophyllite in ceramics; chlorite, kaolin, and mica in paint; calcium carbonate and kaolin in paper; bentonite, kaolin, mica, and wollastonite in plastics; and kaolin and mica in rubber.
The total estimated use of talc in the United States, including imported talc, was plastics, 27%; ceramics, 18%; paint, 16%; paper, 15%; roofing, 6%; cosmetics, 5%; rubber, 3%; and other, 10%. One company in North Carolina mined pyrophyllite.


Wollastonite
Wollastonite is formed through the metamorphism of rocks containing silica and calcium. Major producing areas of high-quality wollastonite include the Adirondack Mountains of upstate New York in the United States, southeastern Finland, several provinces in China and India, and Russia.

Wollastonite serves as a flux for welding, a source for calcium oxide, a slag conditioner, and to protect the surface of molten metal during the continuous casting of steel. As an additive in paint, it improves the durability of the paint film, acts as a pH buffer, improves its resistance to weathering, reduces gloss, reduces pigment consumption, and acts as a flatting and suspending agent. In plastics, wollastonite improves tensile and flexural strength, reduces resin consumption, and improves thermal and dimensional stability at elevated temperatures.

Garnet
Garnet is associated with some wollastonite deposits, including one at Willsboro, New York. In the same area of the state, a large-scale hard-rock garnet mine is in operation. Elsewhere in the United States, although there are hard-rock garnet deposits in Maine and Nevada, the most important commercially are placer deposits in Idaho. Garnet is also produced as a by-product of mineral sand operations in Western Australia, India, and Sri Lanka.

Mining methods for the extraction of garnet vary depending on the geologic environments responsible for the host rock. At hard-rock locations, such as the Barton mine in northern New York, open-pit methods have been employed for decades. In China, hard-rock mining may consist of more primitive methods including hand mining.
Garnets are extracted and processed more easily from alluvial deposits. For example, at the Emerald Creek mine in Idaho, garnet is recovered from stream gravels from slots cut by backhoes or small draglines. These gravels are passed through a trammel to reject the oversize, and garnet is concentrated on large wet-jigging tables. The garnet is then shipped to the mill for final processing and packaging.
 Beach deposits, such as those mined in Western Australia and in southern India, lend themselves to low-cost earth-moving techniques using scrapers and bulldozers to cut and excavate benches parallel to the trend of the beach and/or bar deposit. Because manual labor remains central to the operator’s community responsibility in India, mechanized mining provides only a portion of the mine activity there.
In general, processing involves separation of the heavier garnet from lighter gangue minerals and the further separation of garnet into discrete size classes designed to meet the needs of specific markets. Following crushing (which is determined by the nature of the specific deposit or the market served), ores typically are washed to segregate material by specific gravity. Most operators employ traditional spiral classifiers for this step, some in conjunction with hydrosizers. At least one company uses flotation methods for the separation of garnet from heavy nonmagnetic fractions. Concentrates are dried and then sorted by both high-intensity magnetic and electrostatic separators.

One producer that markets very fine grades for precision grinding and polishing markets also relies on wet-separation techniques for quality control. Final separation yields a product exceeding 95% garnet minerals, which usually contains less than 0.5% quartz by weight. The final production step is dry screening and classifying into marketable sizes, then packaging for sales and distribution.



Kyanite Group
The kyanite group of minerals occurs in aluminous metamorphic rocks and their weathered derivatives. Production is restricted to a handful of countries, including South Africa, Russia, the United States, France, India, Sweden, Spain, China, and Zimbabwe. Like garnet, some are found associated with placer mineral sand deposits, particularly in India.
Kyanite Mining:
The kyanite quartzite is drilled and blasted; secondary breaking is sometimes done with a hydraulic hammer. The ore is picked up with diesel-powered shovels, loaded into trucks, and hauled to the primary crusher. At the primary crusher, the ore is reduced to –4 cm and passed by conveyor belt to the rod mill, which is in a closed circuit with a classifier, to grind the ore to –20 mesh.
Water is added that creates a slurry from which the –200 mesh is removed (deslimed). The new slurry is conditioned with several ingredients and passed through a series of flotation cells that remove the pyrite and micaceous contaminants from the slurry. Tailings from the pyrite circuit are again deslimed and conditioned with other reagents and passed through a section of rougher flotation cells. The rougher concentrate goes to a two-stage recleaning circuit; tails of the rougher circuit go to waste.
The ore goes on to further processing in floatation. Then it is dried and magnetically separated until the final product which is a raw -35 mesh kyanite product. 




Pyrophyllite
Weathering may form sericite or pyrophyllite, the hydrous aluminum silicate. The main pyrophyllite deposits, however, are formed through the hydrothermal alteration of acidic volcanic rocks. This is particularly well developed in areas of Japan and the Republic of Korea—accounting for 85% of world production between them. Smaller producers include Canada, the United States, India, China, Thailand, Australia, Brazil, and Argentina. Corundum Natural corundum is another alumina-rich mineral formed through metamorphism. The main producers are Zimbabwe and South Africa, the former U.S.S.R., and India. Production of the impure form, emery, is restricted to Turkey and Greece.
Ground pyrophyllite is used in the production of ceramics, heat-resistant products called fractories, and paint.Soapstone was once used to make chemical-resistant sinks and countertops for laboratories. Before the days of furnaces, blocks of soapstone were heated on stoves and used as bed warmers.

Graphite

It forms as veins and disseminations in metamorphic rocks as the result of the metamorphism of organic material included in limestone deposits. It is the only non-metal element that is a good conductor of electricity.  Natural graphite is used mostly in what are called refractory applications.  One example of this use is in the crucibles used in the steel industry. Such refractory applications account for the majority of the usage of graphite. It is also used to make brake linings, lubricants, and molds in foundries. A variety of other industrial uses account for the remaining graphite consumed each year.
world production is concentrated in fewer than 20 countries, more than 60% is produced in Asia—China, the Republic of Korea, Sri Lanka, and India. In the Americas, Mexico and Brazil are well-established producers, and Canada is emerging as a major supplier. The main producers are Germany, Austria, the Czech Republic, Norway, Romania, Turkey, and Russia in Europe, and Zimbabwe and Madagascar in Africa.

Source of information ....
Economic geology book  2011 by Prof. W.L.Pohl
World Distribution of Industrial Minerals Deposits by Prof. Peter W. Harben
MINERAL COMMODITY SUMMARIES 2014. USGS
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