Placer


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placer

[′plās·ər]
(geology)
A mineral deposit at or near the surface of the earth, formed by mechanical concentration of mineral particles from weathered debris. Also known as ore of sedimentation.

Placer

 

(placer deposit), a surficial accumulation of small fragments of rocks or minerals that result from the destruction of primary deposits of useful minerals or rocks. Placers contain inclusions of valuable minerals in amounts that permit economically profitable extraction.

Placers are usually composed only of minerals that are resistant to the processes of transportation and weathering, minerals such as diamond, gold, platinum, cassiterite, monazite, ruby, sapphire, ilmenite, zircon, and amber. They are formed wherever rubble, gravel, sand, and other loose material are distributed, from place of origin to the seacoast, where the products of the destruction of the continents are ultimately carried. A number of sequential stages in the formation of placers are distinguished: formation of a placer at the point of origin (eluvial placers), on the slopes or at the foot of elevations (diluvial, solifluction, colluvial, landslide, proluvial placers), in river valleys (alluvial placers, placers of subaerial deltas and broad alluvial plains), and on the shores of seas and lakes (placers of underwater deltas, beach and shelf placers, lacustrine placers). Unrelated to these stages, glacial deposits and eolian placers occur in regions of glaciation and in arid regions. The order of formation of particular genetic types of placers is not always maintained, and sometimes the process ends in one of the early stages. Placers related to different geomorphological elements of river valleys are shown in Figure 1.

Figure 1. Distribution of placers in a cross section of a river valley

According to time of formation, placers are classified as ancient (Mesozoic, Paleozoic, and Precambrian), which are usually fossil placers; Quaternary (Anthropogenic); and modern. In addition, they are classified by mode of occurrence as surface and buried (beneath the sheets of effusive rocks, moraines, landslides, and the like). By composition they are classified as monomineralic and polymineralic; by structure, as single-layered and multilayered; and by shape, as mantle, sheet, lens, ribbon, or pocket. Placers may be several hundred meters and even several kilometers long, from a few meters to several tens of meters wide, and from a few centimeters to tens of meters thick. Proceeding from top to bottom, the following basic parts are distinguished in the structure of the most common types of placers: peat, usually with few, if any, valuable rock components; sand, productive beds containing valuable components in industrial quantities; and bedrock, the surface on which the placer is located.

The best-known placers are the alluvial gold-bearing placers in the Kolyma and Chukchi regions of the USSR and in Alaska and California; the diamond placers in the Republic of South Africa and in Namibia; the beach placers of cassiterite in Malaysia, Indonesia, Thailand, and Burma; and the placers of titanium minerals in the Ukraine, New South Wales (Australia), India, and Florida.

Placers produce roughly 50 percent of the world total of diamonds, titanium, gold, and tin, about 10 percent of the world total of platinum, and 100 percent of the world total of titanium minerals and amber.

REFERENCES

Bilibin, Iu. A. Osnovy geologii rossypei, 3rd ed. Moscow, 1956.
Osnovypoiskov rossypei. Leningrad, 1961.
Trofimov, V. S. “Stadii obrazovaniia rossypei i ikh fatsii.” In the collection Voprosy geologii Kol’skogo poluostrova. Moscow-Leningrad, 1962.
Geologiia rossypei: Sb. st. Moscow, 1965.

V. S. TROFIMOV

Working. Placers are worked by the underground and open-cut methods. The underground method is usually used to work permafrost placers more than 10–15 m deep and unfrozen placers more than 15–40 m deep.

The most common method of working placers is mechanized opencut mining, which makes it possible to successfully work placers 15–18 m thick—up to 60 m thick if dredges are used. Rock containing sand is fed to gravitational concentrating devices, where the rock is washed in a stream of water to extract gold, platinum, and crystals (diamonds, optical quartz) and concentrates of nonferrous metals (tin, tungsten, tantalum).

Placer working involves preliminary, opening, preparatory, and extraction work. Preliminary work entails drying or flooding the placer region, in the latter case making use of amphibious mining machines. Opening work creates access to the placer horizons, sets extraction machines on the deposit, and provides transportation links. In preparatory work, the surface is cleaned, the peat is removed, the operating blocks are cut, and seasonally frozen and permafrost ground are thawed. Extraction work involves excavating and the washing out of sand. The technology of extraction work is determined primarily by the method of excavating productive rocks and the system used for working, that is, the order and procedure followed in extraction and cutting.

In the USSR, opencut placer mining makes use of multi-bucket dredges that hold up to 600 liters per bucket, bulldozers tractor-mounted with power ratings up to 283 kilowatts (385 hp), power-driven wheeled scrapers with scoops that hold up to 34 cu m, and powerful dragline excavators.

REFERENCES

Leshkov, V. G. Sovremennaia tekhnika i tekhnologiia drazhnykh rabot. Moscow, 1971.
Shorokhov, S. M. Tekhnologiia i kompleksnaia mekhanizatsiia razrabotki rossypnykh mestorozhdenii, 2nd ed. Moscow, 1973.
Sulin, G. A. Tekhnika i tekhnologiia razrabotki rossypei otkrytym sposobom. Moscow, 1974.

V. G. LESHKOV

References in periodicals archive ?
Ladbrokes, however, continue to accept the Placer, but as with any other bet they offer, will impose restrictions based on business-orientated customer limits, a spokesman saying: "As things stand, we accept the Placer, but like all our bets it is under continuous review.
Sin este proceso de constante replecion no puede haber placer ni vida de placer.
Meanwhile, the price of gold, which had been at $400 per ounce in 1996, dropped to $250, and in July 1999 Placer suspended work on the mine.
Drill samples are collected in buckets and processed in a small Long Tom sluice box, which accurately simulates conventional placer mining equipment.
Based on the positive results of the preliminary sampling and the observation that large amounts of un-mined placer material remain within the drainage, Keener recommends a Phase One placer examination in 2007.
There are some significant synergies to be captured in the Porcupine camp," said Placer Dome president and CEO Jay Taylor in a press release.
Reynolds' placer claims could not be established and were found invalid.
With a scoping study for potential mine development in progress, this strategic mineral rights acquisition advances prospects for the Indian River Placer Project considerably", says Hans Boge, P.
In emphasizing the new importance of the Indian River Placer Project to Klondike Star, Hans Boge, P.
PHOTO : Placer Dome's Dona Lake Mine, located south of Pickle Lake in northwestern Ontario, started production in February of 1989 and is expected to produce 40,000 ounces of gold annually.
The Indian River placer gold project covers over 60 miles of placer claims and leases along the southern edge of the Klondike placer mining region.
Martis Valley lies between Truckee and North Lake Tahoe, with 25,000 acres falling under the jurisdiction of Placer County.