Placer mining

Technique of mining stream bed deposits for minerals / From Wikipedia, the free encyclopedia

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Placer mining (/ˈplæsər/)[1] is the mining of stream bed (alluvial) deposits for minerals.[2] This may be done by open-pit (also called open-cast mining) or by various surface excavating equipment or tunneling equipment.

19th-century miner pouring water into a rocker box which, when rocked back and forth, will help separate gold dust from the alluvium

Placer mining is frequently used for precious metal deposits (particularly gold) and gemstones, both of which are often found in alluvial deposits—deposits of sand and gravel in modern or ancient stream beds, or occasionally glacial deposits. The metal or gemstones, having been moved by stream flow from an original source such as a vein, are typically only a minuscule portion of the total deposit. Since gems and heavy metals like gold are considerably denser than sand, they tend to accumulate at the base of placer deposits.

Placer deposits can be as young as a few years old, such as the Canadian Queen Charlotte beach gold placer deposits, or billions of years old like the Elliot Lake uranium paleoplacer within the Huronian Supergroup in Canada.[3]

The containing material in an alluvial placer mine may be too loose to safely mine by tunnelling, though it is possible where the ground is permanently frozen. Where water under pressure is available, it may be used to mine, move, and separate the precious material from the deposit, a method known as hydraulic mining, hydraulic sluicing or hydraulicking.

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