Copper and Molybdenum Deposits in the United States

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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 which 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...
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