18

U.S. per capita consumption of manufacturing metals, 1940
-1970

19

U.S. per capita consumption of transport and manufacturing nonmetallic minerals, 1940-1970

64

FORECASTING DEPLETION

Earl Cook

Texas A&M University

Illustrations of depletion

To miners and oilmen, depletion is a physical fact as well as a

controversial tax allowance.

Every mine and every oil well has a life determined by (1) the amount of ore or oil that can be extracted from it at a profit and (2) the rate of extraction. Mineral deposits and oil fields are relatively small and rare within the earth's crust; they are geological concentrations of substances useful to man, and the processes of concentration are so very slow that these mineral resources are nonrenewable; they will not grow again, as a crop does, and the exploitation of them leads inevitably to depletion and exhaustion.

In almost no case does the total amount of valuable material produced equal the amount originally in the ground. For every barrel of oil produced in the United States, more than two have had to be left in the ground; most petroleum reservoirs lack sufficient permeability to allow all the contained oil to be driven or sucked through its minute interstices to the development wells, and oil clings tenaciously to the sand grains which have formed its home for millions of years. Most ore deposits have to be abandoned before all the metallic mineral material has been extracted, because the mineralized rock becomes too lean or too deep to be extracted profitably. Not even deposits of coal and bedded

rock salt can be mined out completely, since pillars need to be left for support and thin beds may not be minable.

2.

For each valuable mineral deposit, there is then an intrinsic limit of exploitation, determined by its geologic boundaries, and an economic limit of exploitation, determined mainly by ore grade, value of the product at the mine mouth or wellhead, and the rate and cost of extraction. Although the intrinsic limit does not change with time, the economic limit may. New technology may lower mining costs or improve extraction efficiencies; it may create new uses and greater demand for the material being mined and thus augment its value. On the other hand, opening of other, lower-cost, mines or oil fields, or the introduction of a substitute material, may depress the market price obtainable and shrink the economic boundaries of a mine or oil field.

The production history of a mine or oil well is a unique event, one that cannot be repeated. The production history of the Comstock Lode (Fig. 1), a rich silver lode in Nevada, shows three stages of exploitation. In the first stage, during which by far the greatest amount of the total value of the Lode was extracted, the high-grade ore was mined at a fast rate. In the second stage, lower-grade ore, bypassed in the first stage, was recovered at a lower rate. In the final phase, reworking of waste rock by improved technology extended the life of the Lode but added little to the value already produced. The stages seen in the Comstock history may be likened to youth, maturity, and old age.

As with a single lode, so with a mining district. After a long history of great contribution to the industrialization of the United States, the rich "direct shipping" iron ores of the Lake Superior district (Fig. 2) are now virtually exhausted. In the mature stage of development, beneficiation of low-grade material into shippable concentrates added considerably to the total production. In this

YIELD, S

FIGURE 1

CYANIDIZATION)
OF TAILINGS

4.