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Concept
Placer deposit
Summary
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.
Placer environments typically contain black sand, a conspicuous shiny black mixture of iron oxides, mostly magnetite with variable amounts of ilmenite and hematite. Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite. Early mining operations were likely a result of placer deposits as they were easily accessible and potential size. The events known as gold/diamonds rushes were caused by placer deposits and have proved to be plentiful.
Placer minerals are defined as having a specific gravity above 2.58. The separation of the valuable minerals from the most common non-economic mineral, quartz, depends on the difference in specific gravity/density.The weathering process allows for the accumulation of placer minerals, while less dense material such as quartz are swept away.
Hardness allows a placer minerals to resist mechanical breakdown and loss of mass from abrasion during transport. A hardness greater than quartz is desired, however substances such as gold typically deform and create irregularly shaped nuggets when subject to mechanical stress.
Chemical stability is important for a placer mineral to resist chemical breakdown such as oxidation, Minerals lacking chemical stability will form less desirable alteration when subject to chemical breakdown.
Alluvial placers are those formed in river or stream sediments.
Official source
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