This table from the Concise Encyclopedia of Economics is pretty amazing.

TABLE 1
World Reserves and Cumulative Production of Selected Minerals: 1950-1980 (millions of metric tons of metal content)
Mineral	1950 Reserves	Production 1950-1980	1980 Reserves
Aluminum	1,400	1,346	5,200
Copper	100	156	494
Iron	19,000	11,040	93,466
Lead	40	85	127
SOURCE: Repetto, p. 23.

So that's what "known reserves" means, huh? That we have no clue how much aluminum, copper, iron and lead really exists? I wonder about oil?