In geology, a placer deposit is a deposit of earth, sand, or gravel, containing valuable minerals in particles, especially by the side of a river, or in the bed of a mountain stream. Placers may also be found at the oceanside, deposited by the action of waves.
Placer mining is an important source of gold, and was the main technique used in the early years of the California gold rush. The name is from the Spanish word placer, meaning "pleasure", because placer mining is much easier than pick-and-shovel mining.
Placer deposits form due to the differential settling of the denser, heavier components of moving sand and gravel. Placer materials must be both dense, and resistant to weathering processes.
Notably, placer environments typically contain black sand, a conspicuous shiny black mixture of iron oxides, mostly magnetite with variable ilmenite and hematite components. Accessory minerals often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite.
There are heavy silicates, such amphibole and pyroxene that will collect in extensive placers where they comprise the country rock. Of course, exceptionally dense substances like gold and the platinum group members will accumulate in placers, when they are present.
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