Ore grade, metal production, and energy

 Recent resource estimates have been stimulated by national concern over present and pending shortages of energy and mineral resources. Although some believe that the resource base of a commodity is the total amount in the crust, the energy consumption for mining and milling under present technology suggests that grades of the metallic ores have a lower limit when production is assumed to be for common usages. The tonnage required to mine and mill ores to obtain one unit of metal is a hyperbolic function of the grade, and as the tonnage increases hyperbolically, so does the energy consumed. For copper, the hyperbolic relation suggests that deposits with grades below about 0.20-0.25 percent Cu will not be mined to produce metal for common electrical and construction uses. Although the energy used to mine and mill a unit of metal differs from one metal to another, all show the hyperbolic increase in energy consumption as grade decreases.