What are placer deposits and why are they important sources of platinum?

A placer deposit is a natural concentration of heavy, chemically resistant, and durable minerals formed by the action of gravity on moving particles. As weathering releases these minerals from their original matrix, they are transported by water, wind, or wave action. Because these minerals have a high specific gravity, they settle and accumulate in areas where the velocity of the transporting medium decreases, such as in stream beds or along seashores.12

Placer deposits are important sources of platinum because they provide naturally concentrated ore that is often easier and more economical to mine than primary deposits. Historically, stream (alluvial) placers have been particularly significant; for example, the platinum placers of the Ural Mountains have served as notable sources of the metal. Their relative richness and accessibility have made them a primary target for mining operations throughout history.12