Placer deposits are collections of some mineral existing in discrete particles, mixed with sand, gravel, and other forms of eroded rock. Some of the minerals most commonly found in placer deposits are diamond, gold, platinum, magnetite, rutile, monazite, and cassiterite. These deposits are formed by the action of wind, water, and chemical changes on more massive beds of the mineral. Placer deposits can be classified according to the method by which they are produced. Some examples include stream placers, eolian placers (formed by the action of winds), beach placers, and moraine placers (formed by glacial action).

Placer deposits are important sources of some minerals that are more dense than the sand or gravel around them. For example, the gold in a placer deposit tends to settle out and accumulate in a stream as surrounding materials wash away.

With few exceptions, the mining of placer deposits is...