Placer mining

Placer mining /ˈplæsər/ is the mining of stream bed (alluvial) deposits for minerals. This may be done by open-pit (also called open-cast mining) or by various surface excavating equipment or tunnelling equipment. Placer mining /ˈplæsər/ is the mining of stream bed (alluvial) deposits for minerals. This may be done by open-pit (also called open-cast mining) or by various surface excavating equipment or tunnelling equipment. Placer mining is frequently used for precious metal deposits (particularly gold) and gemstones, both of which are often found in alluvial deposits—deposits of sand and gravel in modern or ancient stream beds, or occasionally glacial deposits. The metal or gemstones, having been moved by stream flow from an original source such as a vein, are typically only a minuscule portion of the total deposit. Since gems and heavy metals like gold are considerably denser than sand, they tend to accumulate at the base of placer deposits. It is important to note that placer deposits can be as young as a few years old, such as the Canadian Queen Charlotte beach gold placer deposits, or billions of years old like the Elliott Lake uranium paleoplacer within the Huronian Supergroup in Canada. The containing material in an alluvial placer mine may be too loose to safely mine by tunnelling, though it is possible where the ground is permanently frozen. Where water under pressure is available, it may be used to mine, move, and separate the precious material from the deposit, a method known as hydraulic mining, hydraulic sluicing or hydraulicking. The word placer derives from the Spanish placer, meaning shoal or alluvial/sand deposit, from Catalan placer, (shoal), from plassa, (place) from Medieval Latin placea (place) the origin word for 'place' and 'plaza' in English. The word in Spanish is thus ultimately derived from placea and refers directly to an alluvial or glacial deposit of sand or gravel. Placers supplied most of the gold for a large part of the ancient world. Hydraulic mining methods such as hushing were used widely by the Romans across their empire, but especially in the gold fields of northern Spain after its conquest by Augustus in 25 BC. One of the largest sites was at Las Médulas, where seven 30 mile long aqueducts were used to work the alluvial gold deposits through the first century AD. (Inclusions of platinum-group metals in a very large proportion of gold items indicate that the gold was largely derived from placer or alluvial deposits. Platinum group metals are seldom found with gold in hardrock reef or vein deposits.) In North America, placer mining was famous in the context of several gold rushes, particularly the California Gold Rush and the Colorado Gold Rush, the Fraser Canyon Gold Rush and the Klondike Gold Rush. Placer mining continues in many areas of the world as a source of diamonds, industrial minerals and metals, gems (in Myanmar and Sri Lanka), platinum, and of gold (in Yukon, Alaska and British Columbia).