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In geology, a placer deposit or placer is an accumulation of valuable minerals formed by gravity separation from a specific source rock during sedimentary processes. [1] The name is from the Spanish word placer , meaning " alluvial sand".
But the placer deposits worked in the early years were quickly exhausted, and production crashed. Hardrock mining (in California called quartz mining) began in 1849, and placer mining by hydraulic mining began in 1852. Despite the new mining methods, by 1865 production was 867,000 troy ounces (27,000 kg), less than one-quarter of peak production.
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 ...
Placer mining is a method of extracting gold from alluvial deposits such as sand, gravel, and sediment. [42] [43] These are known as placer deposits which are typically found in riverbeds, stream beds, and floodplains. [44] These deposits typically contain minerals that are resistant to weathering and eroision like gold, platinum, diamonds, and ...
Sedimentary placer deposits are known as 'leads' or 'deep leads'. Placer deposits are often worked by fossicking, and panning for gold is a popular pastime. Laterite gold deposits are formed from pre-existing gold deposits (including some placer deposits) during prolonged weathering of the bedrock. Gold is deposited within iron oxides in the ...
In the most complex placer mining, groups of prospectors would divert the water from an entire river into a sluice alongside the river, and then dig for gold in the newly exposed river bottom. [15] Modern estimates by the U.S. Geological Survey are that some 12 million ounces [ 16 ] (370 t ) of gold were removed in the first five years of the ...
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Heavy mineral sands are placer deposits formed most usually in beach environments by concentration due to the specific gravity of the mineral grains. It is equally likely that some concentrations of heavy minerals (aside from the usual gold placers) exist within streambeds, but most are of a low grade and are relatively small.