Copper and Molybdenum Deposits in the United States


Copper and molybdenum resources were not recognized as valuable
commodities until economic needs demanded the collection and processing of these
minerals in large amounts. The most expansive deposits of copper and molybdenum
occur in massive low grade ores and are found in intrusive porphyry formations,
although many smaller sized but higher grade ores are located in non-porphyry
areas. The nation has abundant domestic copper ore reserves but because of many
detrimental economic factors much of the copper used by the U.S. industry is
imported. Molybdenum ore is profuse and exports of it are high to fulfill the
needs of foreign demand.
Copper was first used by people around 4000 B.C. in the manufacture of
tools because of its malleability and later became an important additive in
harder, more useful metals such as bronze (copper+tin; 2500 B.C.) and brass
(copper+zinc; 0 A.D.). The growth of copper production in the United States has
been a relatively recent occurrence. North American French explorers knew of
sources of native copper in the region of Lake Superior and the area natives had
copper jewelry and ornamentation. Earnest copper mining began in Simsbury,
Connecticut about 1709 and copper was actually exported to England after a
source was discovered in New Jersey around 1719. In later times domestic copper
resources did not satisfy national needs until the discovery of gold in
California shifted the focus of mineral exploration westward and strikes of rich
copper ores occurred in Tennessee and the Cordilleran base regions. The Civil
War caused copper demand to increase greatly in order to manufacture cartridges
and canned goods, this resulted in the openings of numerous copper mines of
which more than 90% were in the Lake Superior area giving an important advantage
to the Union armies. Major copper production districts then shifted to Montana
and Arizona in the early 1890\'s. Production increased to reach peak levels of
900,000 tons a year during World War I and in 1970 1,600,000 tons of copper were
produced but recent levels are lower, fluctuating between 1-1.5 million tons a
year. Technology has aided in increasing production efficiency wich resulted in
spectacular resource development in the U.S. and around the world.
Molybdenum has been a major mineral since 1898 when it was discovered to
harden steel as an additive and useful in compounding chemicals and dyes.
Substantial mining began in 1900 in the southwest but the demand was so low that
activity ceased in 1900. In 1906 the molybdenum industry boomed and with the
dawn of WWI the need for quality steel further increased the necessity for this
important additive. The highest production levels occurred during the early
1980\'s when 68,000 tons were mined, current levels are lower mirroring the
copper production curve because more than half of the molybdenum produced is a
by-product of the copper industry.
There are many different types of copper and molybdenum deposits in the
world all containing different categories of ores. The classes are divided into
two main groups, porphyry and non-porphyry intrusives, which in turn branch off
into several sub-groups. Both copper and molybdenum can be classified using the
two main groups but each mineral has unique sub-groups.
The first of the porphyry copper lodes is the type from which the group
takes its name, the copper porphyry. San Manuel, Arizona is the location of the
first copper porphyry, a stockwork of veinlets in hydrothermally altered
intrusives with closely spaced phenocrysts in a microaplitic quartz-feldspar.
The intrusive ranges in age from the Mesozoic to the Cenozoic and in composition
from tonalite to granite. Ore is found in stockwork veinlets and random grains
in the intrusive and surrounding fractures. The ore includes chalcopyrite,
pyrite, and sometimes molybdenite, magnetite, and gold. Green and blue copper
carbonates and silicates developed into weathered outcrops overlying enriched
zones containing chalcocite and other sulfides. There are 31 U.S. porphyry
copper locations with an average grade of .54% copper ranging from a low of .31%
to a high of .94%.
Another type of porphyry is the copper-gold porphyry in Dos Pobres,
Arizona composed of a stockwork of chalcopyrite, bornite, and magnetite veinlets
in porphyritic intrusions. The igneous associations of the copper-gold porphyry
around the world include tonalite, monzogranite, coeval dacites, andesite flows,
and tuffs of ages from the Triassic in British Columbia to the Quaternary in the
South Pacific. The ore zone in Arizona is bell shaped and localized at the top
of a volcanic intrusive center with the highest ore grades located in the upward
branching stock. Ore minerals include a network of veinlets, scattered grains
of bornite, chalcopyrite, and traces of native gold, electrum, sylvite, and
hessite bordering altered