Copper in rocks in crust
contain about 3 x 1011 tons of copper (assuming that tectonic diffusion paths of other copper deposits are similar to those of porphyry copper deposits). This copper is sufficient to supply about 19,000 years of present world production. Not all of this copper will be available to society, however, because deposits deep in the crust will be difficult to find by exploration and even more difficult to mine. Applying a likely depth limit of about 3.3 km yields a more realistic resource of about 8.4 x 1010 tons of copper, amounting to about 5400 years of current production (Kesler and Wilkinson 2008). Thus, the ultimate (potentially mineable) global resource of conventional copper deposits is about 90 times larger than the reserve base and about 10 times larger than the near-surface resource.
Similar estimates have not been made for other metals, largely because of the limited amount of information on the ages of their deposits. However, most mineral deposits form at depth in the crust and probably follow tectonic-diffusion paths similar to that of porphyry copper deposits. (The only metals not likely to follow tectonic-diffusion paths of this type are those of Al, Fe, and Ti, which form largely as sedimentary deposits at and near Earth's surface.) If this simple assumption is valid, ultimate resources of conventional deposits for most metals in the upper 3.3 km of the crust should show the same approximate relation to reserve base that was estimated for copper deposits. Results for most metals, which are shown in Figure 7.3 as the number of years of current supply that they might provide, range from about 2,000-200,000 years, with arsenic, antimony, gold, silver, and tin least abundant and chromium, cobalt, lithium, and vanadium most abundant. If deposits formed intermittently rather than continuously through geologic time, ultimate resources would differ. Similar constraints probably apply to some mineral resources used in mineral form, particularly asbestos, barite, clays, and diamonds. Other mineral
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