The Placer mining reference article from the English Wikipedia on 24-Apr-2004
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Placer mining

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Placer mining (pronounced "plass-er") is a technique of mining by which valuable materials are extracted with little or no excavation. The name placer, derives from Spanish. Ore extraction usually requires excavation, but precious deposits such as gold are so highly prized in even minuscule amounts that placer mining becomes feasible. In the United States, placer mining is famous in the context of several gold rushes, particularly the California gold rush. It is used to this day in Myanmar and Sri Lanka for the recovery of gems.

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 will, due to their much higher density, tend to remain in 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 length of artificial streambed, with barriers to halt the movement of gold particles. This method requires excavation with shovels or similar implements to feed sediment into the device.