Placer deposit

thumb|[[Heavy minerals (black) forming placers along ripple marks]]

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. The name is from the Spanish word placer, meaning "alluvial sand". Placer mining is an important source of gold, and was the main technique used in the early years of many gold rushes, including the California Gold Rush. Types of placer deposits include alluvium, eluvium, beach placers, aeolian placers and paleo-placers.

Placer materials must be both dense and resistant to weathering processes. To accumulate in placers, mineral particles must have a specific gravity above 2.58. Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite.

Eluvial placers are typically not large enough to support large scale mining, however in one case in Nevada there has been large scale gold mining operations based on placer deposits at Round Mountain. Wind erosion leaves a cement like substance in which material like gold are found. Wind action blows sand and dirt away leaving the deposits close to the surface. Historically, aeolian placers have been mined by hand with simple tools by miners due to the proximity to the surface, and small concentration. These deposits are typically very far underground in ancient riverbeds, beaches, or slopes. The Witwatersrand Basin in Africa mentioned above is a recognized alluvial diamond deposit.

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