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A gold rush is an interval of feverish migration of workers to an area that has had a dramatic discovery of gold deposits. Major gold rushes took place in the 19th century in Australia, New Zealand, Brazil, Canada, South Africa, and the United States, while smaller gold rushes took place elsewhere. There were six major gold rushes. The permanent wealth that resulted was distributed widely because of reduced migration costs and low barriers to entry. While gold mining itself was unprofitable for most diggers and mine owners, some people made large fortunes, and the merchants and transportation facilities made large profits. The resulting increase in the world's gold supply stimulated global trade and investment. Historians have written extensively about the migration, trade, colonization, and environmental history associated with gold rushes. Gold rushes were typically marked by a general buoyant feeling of a "free for all" in income mobility, in which any single individual might become abundantly wealthy almost instantly, as expressed in the California Dream. Gold rushes helped spur a huge immigration that often led to permanent settlement of new regions and define a significant part of the culture of the Australian and North American frontiers. As well, at a time when the world's money supply was based on gold, the newly mined gold provided economic stimulus far beyond the gold fields. Gold rushes presumably extend back as far as gold mining, to the Roman Empire, whose gold mining was described by Diodorus Siculus and Pliny the Elder, and probably further back to Ancient Egypt. ==Life cycle of a gold rush== Within each mining rush there is typically a transition through progressively higher capital expenditures, larger organizations, and more specialized knowledge. They may also progress from high-unit value to lower unit value minerals (from gold to silver to base metals). A rush typically begins with the discovery of placer gold made by an individual. At first the gold may be washed from the sand and gravel by individual miners with little training, using a gold pan or similar simple instrument. Once it is clear that the volume of gold-bearing sediment is larger than a few cubic metres, the placer miners will build rockers or sluice boxes, with which a small group can wash gold from the sediment many times faster than using gold pans. Winning the gold in this manner requires almost no capital investment, only a simple pan or equipment that may be built on the spot, and only simple organization. The low investment, the high value per unit weight of gold, and the ability of gold dust and gold nuggets to serve as a medium of exchange, allow placer gold rushes to occur even in remote locations. After the sluice-box stage, placer mining may become increasingly large scale, requiring larger organizations, and higher capital expenditures. Small claims owned and mined by individuals may need to be merged into larger tracts. Difficult-to-reach placer deposits may be mined by tunnels. Water may be diverted by dams and canals to placer mine active river beds or to deliver water needed to wash dry placers. The more advanced techniques of ground sluicing, hydraulic mining, and dredging may be used. Typically the heyday of a placer gold rush would last only a few years. The free gold supply in stream beds would become depleted somewhat quickly, and the initial phase would be followed by prospecting for veins of lode gold that were the original source of the placer gold. Hard rock mining, like placer mining, may evolve from low capital investment and simple technology to progressively higher capital and technology. The surface outcrop of a gold-bearing vein may be oxidized, so that the gold occurs as native gold, and the ore needs only to be crushed and washed (free milling ore). The first miners may at first build a simple arrastra to crush their ore; later, they may build stamp mills to crush ore more quickly. As the miners dig down, they may find that the deeper part of vein contains gold locked in sulfide or telluride minerals, which will require smelting. If the ore is still sufficiently rich, it may be worth shipping to a distant smelter (direct shipping ore). Lower-grade ore may require on-site treatment to either recover the gold or to produce a concentrate sufficiently rich for transport to the smelter. As the district turns to lower-grade ore, the mining may change from underground mining to large open-pit mining. Many silver rushes followed upon gold rushes. As transportation and infrastructure improve, the focus may change progressively from gold to silver to base metals. In this way, Leadville, Colorado started as a placer gold discovery, achieved fame as a silver-mining district, then relied on lead and zinc in its later days. Butte, Montana began mining placer gold, then became a silver-mining district, then became for a time the world’s largest copper producer. 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「Gold rush」の詳細全文を読む スポンサード リンク
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