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18

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

62

19

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

64

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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.

ineral deposits and oil fields are relatively

small and rare within the earth's crust; they are geological concentra

tions 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 exploita

tion.

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

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

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35 700

PRODUCTION OF THE COMSTOCK LODE, NEVADA, 1860-1920 SHOWING THREE STAGES OF DEPLETION OF A MINE.

YIELD, $

30 600

* Data for 1860-1881, from Eliot Lord;" Comstock Mining & Miners,"

1883 p.416 Data for 1882-1920, from Grant H Smith, "The History of the Comstock Lode, 1850-1920," 1943, 297.

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