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In geology, a placer deposit or placer is an accumulation of valuable minerals formed by gravity separation during sedimentary processes. The name is from the Spanish word ''placer'', meaning "alluvial sand". Placer mining is an important source of gold, and was the main technique used in the early years of many gold rushes, including the California Gold Rush. Types of placer deposits include alluvium, eluvium, beach placers, and paleoplacers. Placer materials must be both dense and resistant to weathering processes. To accumulate in placers, mineral particles must be significantly denser than quartz (whose specific gravity is 2.65), as quartz is usually the largest component of sand or gravel. Placer environments typically contain black sand, a conspicuous shiny black mixture of iron oxides, mostly magnetite with variable amounts of ilmenite and hematite. Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite. == Substances mined == Substances commercially mined from placer deposits include: * Diamonds * Gold * Garnet * Iron, from ironsands containing high concentrations of magnetite * Platinum group metals * Quartz Monazite Sands * Rare earth elements, from the mineral monazite * Ruby * Sapphire * Thorium, from the mineral monazite * Tin, in the mineral cassiterite * Titanium, from the minerals ilmenite and rutile * Uranium, from Precambrian paleoplacers * Zirconium, from the mineral zircon 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Placer deposit」の詳細全文を読む スポンサード リンク
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