placer

A placer is a naturally occurring concentration of valuable minerals formed by gravity separation within loose sediments. Placer deposits commonly contain metals such as gold, platinum-group metals, tin, tungsten, and gemstones like diamonds and sapphires. The term is used for deposits created by mechanical sorting and concentration rather than chemical precipitation.

Formation occurs when ore-bearing rocks are weathered and eroded. The resulting sediment is transported by water, wind, or ice, and the heaviest minerals tend to settle out first as energy decreases. Over time these grains accumulate in stream bends, behind obstructions, on gravel bars, or in ancient river channels preserved in the landscape. Environments include alluvial placers in active river systems, beach placers along coastlines, and glacial or outwash placers in former ice-margin areas.

Mining and processing rely on gravity separation and mechanical concentration. Methods include panning, sluicing, dredging, and hydraulic mining, followed by further concentration with devices such as sluice boxes, shaking tables, or spirals. In the past, mercury amalgamation and cyanidation were used in some operations; modern practices emphasize environmental safeguards and reclamation.

Geology and economics: placers are contrasted with lode (primary) deposits, where minerals occur in hard rock veins. Placer mining can yield relatively rapid, large-scale recoveries when rich deposits are accessible, but production is often episodic and sensitive to market prices and regulatory controls.