PLACER MINING

oldest method of recovering gold from alluvial depositsi.e., gold-bearing sands and gravel that settle out from rapidly moving streams and rivers at points where they slow down. Placer mining takes advantage of gold's high density, which causes it to sink more rapidly from moving water than the lighter siliceous materials with which it is found. Though the basic principles of placer mining have not altered since early times, methods have improved considerably. Panning, used by miners during the great gold strikes of the 19th century, employed a pan or a batea (a pan or basin with radial corrugations) in which a few handfuls of the gold-bearing soil or gravel and a large amount of water were placed. By swirling the contents of the pan, the miner washed the siliceous material over the side, leaving the gold and heavy materials behind. An improvement over the pan and batea was the cradle, named for its resemblance to a child's cradle. As it was rocked, it sifted large quantities of ore. Gravel was shovelled onto a perforated iron plate, and water was poured over it, causing the finer material to drop onto the apron that distributed it across the riffles, pieces of wood or iron perpendicular to the bottom and sides of the cradle. As the material moved through the cradle, the gold was caught on the riffles, to be removed later. Dredging became the most important placer mining method in the early 20th century and remains so today. Used worldwide is the bucketladder dredge, characterized by a continuous chain of buckets rotating around a rigid adjustable frame called the ladder. Paddock dredging, a later development, allows mining of placer deposits even if they are not in or near a riverbed. The dredge floats in its own pond that is continuously extended by digging at one end and simultaneously filled at the other end with waste, or tailings. In sluicing or hydraulicking methods, a slightly sloping wooden trough called a box sluice, or a ditch cut in hard gravel or rock called a ground sluice, is used as a channel along which gold-bearing gravel is carried by a stream of water. Riffles placed transversely along the bottom of the sluice cause the water to eddy into small basins, retarding the current so that gold may settle and be trapped.