Placer deposit

Placer deposit

In geology, a placer deposit or placer is an accumulation of valuable minerals formed by deposition of dense mineral phases in a trap site. The name is from the Spanish word "placer", meaning "alluvial sand". Types of placer deposits include alluvium, eluvium, beach placers, and paleoplacers.

Alluvial placers

Typical locations for alluvial placer deposits are on the inside bends of rivers and creeks, in natural hollows, at the break of slope on a stream, the base of an escarpment, waterfall or other barrier, within sand dunes, beach profiles or in gravel beds.

Alluvial placers are formed by the deposition of dense particles at a site where water velocity remains below that required to transport them further. To form a placer deposit, the particles sought after must show a marked density contrast with the gangue material, which is able transported away from the trap site. Only if the deposit is winnowed in this way can the minerals be concentrated to economic levels.

Placer materials must be both dense, and resistant to weathering processes. Placer environments typically contain black sand, a conspicuous shiny black mixture of iron oxides, mostly magnetite with variable amounts of ilmenite and hematite. Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite.

Exceptionally dense substances such as gold and the platinum group members will accumulate in placers, when they are present. Placer mining is an important source of gold, and was the main technique used in the early years of the California Gold Rush.

Substances commercially mined from placer deposits:
* Gold
* Platinum group metals
* Tin
* Diamonds
* Thorium
* Uranium (from paleoplacers)

See also

* Heavy mineral sands ore deposits
* Black sand
* Placer mining
* Gold rush
* Sedimentology

External links

* [ The Blue Ribbon Mine Alaska]
* [ Goldplacer]
* [ Arizona Gold Prospectors]

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Look at other dictionaries:

  • placer deposit — Natural concentration of heavy minerals caused by the effect of gravity on moving particles. When heavy, stable minerals are freed from their matrix by weathering processes, they are slowly washed downslope into streams that quickly winnow the… …   Universalium

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  • deposit — /di poz it/, v.t. 1. to place for safekeeping or in trust, esp. in a bank account: He deposited his paycheck every Friday. 2. to give as security or in part payment. 3. to deliver and leave (an item): Please deposit your returned books with the… …   Universalium

  • placer — placer1 /plas euhr/, n. Mining. 1. a surficial mineral deposit formed by the concentration of small particles of heavy minerals, as gold, rutile, or platinum, in gravel or small sands. 2. the site of a form of mining (placer mining) in which a… …   Universalium

  • placer — I plac•er [[t]ˈplæs ər[/t]] n. 1) min mir a natural concentration of heavy metal particles, as gold or platinum, in sand or gravel deposited by rivers or glaciers 2) min the site of a form of mining(plac′er min ing)in which a placer deposit is… …   From formal English to slang

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  • placer — ► NOUN ▪ a deposit of sand or gravel in the bed of a river or lake, containing particles of valuable minerals. ORIGIN Latin American Spanish, deposit, shoal …   English terms dictionary

  • Placer — Plac er, n. [Sp.] A deposit of earth, sand, or gravel, containing valuable mineral in particles, especially by the side of a river, or in the bed of a mountain torrent. [U.S.] [1913 Webster] …   The Collaborative International Dictionary of English