Mining through shafts, declines and tunnels, reaching ore too deep or too narrow to strip from surface.
Mining method used to extract ore from deposits located deep beneath the surface. Underground mining is more expensive than open-pit but is necessary for deposits that are too deep or have too much overburden for surface mining.
Underground costs far more per tonne but moves almost no waste, so it suits narrow high-grade systems that would be uneconomic in a pit. Most orogenic gold and epithermal vein deposits end up here.
Underground mining introduces dilution from the walls of a narrow stope. A 10 g/t vein mined at two metres against a one-metre true width delivers about half the grade to the mill.