Editing Placer mining (section)

{{Short description|Technique of mining stream bed deposits for minerals}}
{{More citations needed|date=September 2010}}
[[File:Henry Sandham - The Cradle.jpg|thumb|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''' ({{IPAc-en|ˈ|p|l|æ|s|ər}})<ref>{{cite web|url=http://www.collinsdictionary.com/dictionary/american/placer-mining |title=Placer mining (US)|publisher=American English Dictionary|access-date=January 9, 2015}}</ref> is the mining of stream bed deposits for [[mineral]]s.<ref name="The Century illustrated monthly magazine; 1883 Jan., p. 325">{{Cite web|url=https://www.britannica.com/technology/placer-mining|title=Placer mining &#124; Techniques, Processes & Equipment &#124; Britannica|website=www.britannica.com}}</ref> This may be done by [[open-pit mining]] or by various surface excavating equipment or tunneling equipment.

Placer mining is frequently used for [[precious metal]] deposits (particularly [[gold]]) and [[gemstone]]s, both of which are often found in [[Alluvium|alluvial]] deposits—deposits of sand and gravel in modern or ancient [[stream bed]]s, 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.<ref>{{Cite journal|last=McCracken, A.D., Macey, E., Monro Gray, J.M., and Nowlan, G.S.|date=March 1, 2018|title=Placer Gold|url=https://www.gac.ca/PopularGeoscience/factsheets/PlacerGold_e.pdf|journal=Popular Geoscience |publisher=Natural Resources Canada}}</ref>

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 [[permafrost|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 [[Sluice#Placer mining applications|sluicing]] or [[hydraulicking]].