Placer deposit

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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". 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.^[2]

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.^[1]

Placer environments typically contain black sand, a conspicuous shiny black mixture of iron oxides, mostly magnetite with variable amounts of ilmenite and hematite.^[3] Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite.^[3] Early mining operations were probably a result of placer deposits as they were easily accessible and potential size.^[4] The events known as gold/diamond rushes were caused by placer deposits and have proved to be plentiful.^[4]