Although surface and underground mining are the most common techniques, there are a number of other mining methods. In solution mining the valuable mineral is brought into a liquid solution by some chemical or bacteria. The resultant liquid is pumped to the surface, where the mineral or metal is taken out of solution by precipitation or by ion exchange (e.g., the Frasch process). In glory-hole mining a steep-sided, funnel-shaped surface excavation is connected to tunnels below it. Rocks blasted off the sides of the excavation fall into the tunnels, from which they are then removed. Gopher mining is an old-fashioned method still used in very small mines. Narrow, small holes are driven in order to extract the ore (e.g., gold) as cheaply as possible. In placer mining no excavation is involved; instead, gravel, sand, or talus (rock debris) is removed from deposits by hand, hydraulic nozzles, or dredging. The ore is separated from the waste by panning or sluicing.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.