A placer, defined here as being of past or present economic importance, is a detrital deposit of Phanerozoic age within a geomorphological feature by which it may be classified. Its constituent, higher density detrital minerals are concentrated by the action of a transport medium, almost invariably water. The historic importance of placers for gold production has now been replaced by hard-rock mining, whereas placers continue to be a significant source of platinum, cassiterite, and diamonds. The emphasis of placer production has now changed to mineral sands (ilmenite, rutile, and zircon) that exist as higher volume and higher grade deposits. Placers occur worldwide, at all elevations, and at most latitudes. Existing placers formed predominantly in the Cenozoic, from Mesozoic and earlier primary mineralization, disseminations, regional background levels, and intermediate, lithified sources.

Placer development is initiated by a combination of tectonic activity and variations in climate and sea level. Physical and chemical weathering facilitates all forms of erosion and release of minerals into sediments that are transported, sorted, deposited, and commonly reworked. Placer deposits occupy a very large range of geomorphological features, by which they are more easily classified. Mineral sands, and in places diamonds, favor coastal environments, whereas the other placer minerals are concentrated most commonly in fluvial deposits.

Changes in energy levels and the relative physical characteristics of the sediments and minerals determine the style of placer formed. Lag placers result from an increase of energy that winnows out the hydraulically lighter particles (lights) near surface. Accumulation placers are formed by selective settling of hydraulically heavier particles (heavies) due to a reduction in energy. Both styles are found in all placer types. Repetition of supply, multiphase accumulation, and reworking are common and usually beneficial. Minerals are preferentially concentrated and depleted at various scales, especially in a fluvial environment, by external physical conditions that create localized energy change. Beach deposits of mineral sands are a form of lag deposit.

Depending on the nature of its source, several situations must coincide and events occur to form a significant placer. The most important economic attributes are size and grade (from parts per billion for diamonds to wt percent for mineral sands), together with sediment particle sizes, bedrock qualities, and mineral size and morphology. Postdepositional changes, especially the upgrading of ilmenite by alteration, are important. The maximum distance separating a placer from its source varies widely; from less than a kilometer to hundreds of kilometers. Diamond, rutile and zircon placers have formed over a thousand kilometers from their sources. Transport distance is increased by high-energy flow in constricted fluvial channels. An understanding of how placers form, combined with the study of gold particle morphology and composition, assists in the search for primary deposits.