Stephen E. Kesler Abstract
Most geologists view Earth's stock of mineral resources as finite. Even economists, many of whom view the stock as more flexible, recognize that some limit will eventually be reached. Recycling and substitution postpone the problem of exhaustion, but do not resolve it. The outlook for stocks depends, in part, on the energy that is required to mine and process mineral resources into a useable form. In general, processing requires more energy than mining, so resources used in mineral and rock forms that do not require processing have larger stocks. The best quantitative estimate for stocks of known (discovered) conventional mineral deposits is the reserve-base estimate of the U.S. Geological Survey. Data for 2007 show that the global reserve base can supply current consumption for periods ranging from a low of about 15 years for diamond to a high of about 4400 years for perlite, and an arithmetic mean of about 350 years. Most estimates of undiscovered stocks of conventional mineral deposits focus on the uppermost kilometer or so of Earth's crust and use either geological or production data. Only one estimate, for copper in conventional deposits, has been made for the entire crust, and it indicates that current consumption could be supplied for about 5000 years from deposits within 3.3 km of the present surface (a likely future depth limit for exploration and mining), but only if sufficient energy and water are available, if deep mining technology develops rapidly, and if access to land for exploration and exploitation can be assured. The assumption that other elements are in deposits with crustal behavior similar to that of copper would indicate that resources in conventional deposits to similar depths could supply present consumption for periods ranging from about 2000 to 200,000 years. The nature of unconventional resources, and especially the relation between elements in ore minerals and substituting in silicate minerals, is not sufficiently well known to estimate their magnitude. From an immediate standpoint, the greatest opportunity for mineral sustainability is to mine as much as possible of each deposit rather than focusing on ores of the highest quality.
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