placers

Placers are concentrations of heavy minerals formed by gravity in alluvial deposits along rivers, beaches, or ancient shorelines. They result from the weathering of primary ore bodies and the subsequent transport of sediments by water; as energy decreases, dense minerals settle while lighter material is carried farther. Over time, repeated aggradation and reworking concentrate dense minerals into pay streaks within bars, river bends, floodplains, or coastal deposits. Placers are distinguished from lode deposits, which are primary ore bodies in rock.

Formation and properties: Dense minerals are released from primary sources and transported by currents. In slow-moving parts of a stream, behind obstructions, or at bends, heavier minerals settle and accumulate. Over geological time, these accumulations can become sizable placer deposits. The mineral assemblage is governed by density, grain size, and transport history. Common placer minerals include gold, cassiterite (tin oxide), platinum-group metals, diamonds, zircon, and magnetite or ilmenite.

Occurrence and environments: Placers occur in modern river systems, on beaches and dunes, in deltaic settings, and in buried palaeo-placers within older sediments. They can range from shallow surface concentrations to thick, consolidated gravels or sands.

Mining and processing methods: Prospecting uses sampling and gravity-based separation to estimate content. Traditional methods include panning and sluicing; more intensive techniques involve dredging, hydraulic mining, or mechanical placer mining with excavators, trommels, jigs, and concentrating tables. Gravity separation is central to recovery, with further upgrading by shaking tables or spiral separators.

Environmental and economic context: Placer mining can disturb streams, increase turbidity, and affect aquatic habitats, leading to regulatory permitting and reclamation requirements. Economic viability depends on ore grades, deposit accessibility, and metal prices.