Typical locations for alluvial placer deposits are on the inside bends of rivers and creeks, in natural hollows, at the break of slope on a stream, the base of an escarpment, waterfall or other barrier, within sand dunes, beach profiles or in gravel beds.
Alluvial placers are formed by the deposition of dense particles at a site where water velocity remains below that required to transport them further. To form a placer deposit, the particles sought after must show a marked density contrast with the gangue material, which is able transported away from the trap site. Only if the deposit is winnowed in this way can the minerals be concentrated to economic levels.
Placer materials must be both dense, and resistant to weathering processes. 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.
Exceptionally dense substances such as gold and the platinum group members will accumulate in placers, when they are present. Placer mining is an important source of gold, and was the main technique used in the early years of the California Gold Rush.
Substances commercially mined from placer deposits: