Placer miningFrom Wikipedia, the free encyclopediaThis article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (September 2010)19th-century miner pouring material from a stream bed into a rocker box, which when rocked back and forth will help to separate gold dust from the sand and gravelPlacer mining (/ˈplæsər/ or /ˈpleɪsər/)[1] is the mining of alluvial deposits for minerals. This may be done by open-pit (also called open-cast mining) or by various surface excavating equipment or tunneling 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, is typically only a minuscule portion of the total deposit. Since gems and heavy metals like gold are considerably more dense than sand, they tend to accumulate at the base of placer deposits.The containing material may be too loose to safely mine by tunneling, 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.Contents1 Etymology2 History3 Deposits3.1 Residual3.2 Alluvial3.3 Bench4 Methods4.1 Panning4.2 Rocker4.3 Sluice box4.4 Dry washing4.5 Trommel4.6 Underground mining5 Environmental effects6 See also7 References8 External linksEtymologyThe 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.[2] The word in Spanish is thus ultimately derived from placea and refers directly to an alluvial or glacial deposit of sand or gravel.HistoryPlate depicting placer mining from the 1556 book De re metallicaPlacers 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).DepositsMain article: Placer depositA well protected area from the flow of water is a great location to find gold. Gold is very heavy and is often found in a stream bed. Many different gold deposits are dealt with in different ways. Placer deposits attract many prospectors because their costs are very low. There are many different places gold could be placed, such as a residual, alluvial, and a bench deposit.ResidualResidual deposits are more common where there has been weathering on rocks and where there hasn’t been water. They are deposits which have not been washed away yet or been moved. The residual usually lies at the site of the lode. This type of deposit undergoes rock weathering ("Types of Placers," ).AlluvialAlluvial
Read more…
Comments