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Placer mining

From Academic Kids

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Placermine.jpg
A sluice box used in placer mining

Placer mining (pronounced "plass-er") is a open-pit or open-cast form of mining by which certain valuable minerals are extracted from the earth without tunneling. Excavation is accomplished using water pressure (hydraulic mining) or surface excavating equipment.

The name derives from Spanish, placer, meaning "sand bank" and refers to precious metal deposits (particularly gold and gem stones) found in alluvial deposits—deposits of sand and gravel representing stream beds. The containing material is generally too loose to safely tunnel into and the metal or gemstones, having been moved by stream flow from an original source such as a vein, is typically only a minuscule portion of the deposit. Where water is available, hydrostatic pressure is used to mine, move, and separate the precious material from the deposit.

Placers supplied most of the gold for a large part of the ancient world. (Inclusions of platinum-group metals in a very large proportion of gold items indicate that the gold was largely derived from placer or alluvial deposits. Platinum-group metals are seldom, if ever, found with gold in reef or vein deposits.) In the United States, placer mining was famous in the context of several gold rushes, particularly the California gold rush. It is a source to this day of gems in Myanmar and Sri Lanka, and of gold in the Klondike.

The simplest technique of placer gold mining is panning. In panning, some sediment is placed in a large metal pan, combined with a generous amount of water, and agitated so that the sand flows over the side. Any gold particles contained in the sand, due to the higher density of gold, will tend to remain on the bottom of the pan after all of the sand and mud have been removed. The same principle may be employed on a larger scale by constructing a short sluice box, with barriers along the bottom to slow the movement of gold particles. This method better suits excavation with shovels or similar implements to feed sediment into the device.

Environmental activists describe placer mining as very destructive, because of the large amounts of silt that it adds to previously clear running streams.

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