During the 1950s, with the soft, rich iron
deposits of the Mesabi running low, a new method of
concentration gave the Mesabi renewed life. Some of its
very hard ore, called taconite, was magnetic, and after
the ore was finely crushed, the iron content could be
extracted magnetically and remolded into pellets suitable
for use in blast furnaces. But by 1969 still more of the
smaller mines of Minnesota and Michigan had been closed.
Mining activities, which had been called upon to
meet rapidly increasing domestic needs over the past
hundred years, reduced initially "inexhaustible" iron
and copper resources to amounts which appear not only
limited but quite capable of exhaustion within a very
short time. A definitive knowledge of the amounts of
iron and copper ore of relatively high concentration
still available in the United States is difficult to
determine, first because of difficulties associated
with exploration and proving of ore reserves, and
second because a great deal of secrecy is practiced
by mining concerns regarding exploratory operations and
15
their results. But the days of shipments of mass
copper from Michigan and high-grade copper from Arizona
15
Data on copper and iron reserves are presented
in Table 2.