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U.S. per capita consumption of manufacturing metals, 1940
U.S. per capita consumption of transport and manufacturing nonmetallic minerals, 1940-1970
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.
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.
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
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
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
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.
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
PRODUCTION OF THE COMSTOCK LODE, NEVADA, 1860-1920 SHOWING THREE STAGES OF DEPLETION OF A MINE.
* 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.