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 wallrock of inner quartz and an outer propylitic zone. Dos Pobres is the only copper-gold deposit in the U.S. out of the forty located worldwide with median grades of .5% Cu, .38 g/t Au and 1.0 g/t Ag with small amounts of molybdenite.
A third sub-group of the porphyry type of copper deposit is the copper-molybdenum porphyry characterized by the site at Sierrita, Arizona. The location is a stockwork of veinlets and erratic grains of chalcopyrite in native rocks near a porphyritic disturbance. The porphyry is of an age from the Mesozoic to the Tertiary, ranging in consistency from a tonalite to monzogranite and developed as dikes, stocks and breccia pipes containing sparse phenocrysts. The ore minerals consist of chalcopyrite, pyrite and molybdenite. Ore grade is metered by the close spacing of veinlets and the ore zone is sometimes the site of a magnetic low because of the displacement of magnetite. Surface rocks are profoundly leached creating a layer of supergene copper below the leached zone. There are six copper-molybdenum sites in the U.S. and 10 others in the world. The median size is 500 million tons with the average grade being .42% Cu, .016% Mo, .02 ppm Au and 1.2 ppm Ag.
There are some considerable districts which are unique and contain geological features of several deposit types, such as the site in Bingham, Utah. The area contains stockwork veinlets and scattered ore minerals in an altered igneous rock. The intrusives are of an early Tertiary age and occur as stocks and dikes in a highly faulted and folded carbonate, as well as a hydrothermally altered craton shelf. Peripheral copper-gold bearing skarns are located in metamorphosed carbonates along the porphyry contact zone. The ores contain sphalerite, galena, silver, manganese, pyritic copper and native gold. Median tonnage for the jumbled arrays of minerals vary greatly from site to site around the world but the production levels in Bingham can give some idea of the productivity of these areas. Production through 1972 is as fallows; 11,856,600 t Cu, 504,700 kg Au, 2,473,000 t Pb, 1,038,000 t Zn and 8,421,000 kg Ag.
The first of the porphyry molybdenum deposits is a site in Climax, Colorado. The granite- high F porphyry is an umbrella-shaped stockwork of molybdenite, quartz, and fluorite in a Tertiary aged granite porphyry composed of 75% SiO2 cut by dikes and breccias. Molybdenite, quartz, fluorite, and sometimes K-feldspar, pyrite, wolframite, casserite, and topaz compose the ores of the porphyry and occur mainly in fractures or scattered grains. Due to glacial erosion there is little sedimentary or metamorphic rock cover at Climax. From nine sites worldwide a grade and tonnage level can be drawn up with a median size of 200 million t and an average grade of .19% Mo. Climax itself has produced over 430 million tons of ore with a recovery of 832,000 t of Mo, over 38% of the worlds total, with a projected reserve of about 1 million t of Molybdenum.
The second type of porphyry molybdenum deposit is a calc-alkaline-low F porphyry location in Buckingham, Nevada. The intrusive ranges in age from the Mesozoic through the Tertiary and is composed of porphyritic tonalite, granodiorite, or monzogranite with deposits of quartz-molybdenite veinlets. The ore minerals found in Buckingham are molybdenite, pyrite, and occasionally scheelite, chalcopyrite, and argentian tetrahedrite controlled by close-spaced fractures. When weathered the site produces yellow ferrimolybdenite and secondary copper minerals. A median size for this type is 94 million t and a median grade of .085% Mo.
Non-porphyry systems account for about 1/3 of the world's copper supply. The minerals are mainly found as strata-bound ores in sedimentary rocks, volcanogenic massive sulfides, and as Ni-Cu ores in mafic intrusives. Keweenaw, Michigan is the location of the first type of non- porphyry copper deposit, a volcanogenic-sedimentary red bed. In the overlying clastic sediments are copper sulfides and below in thick basalts there are native copper and copper sulfide locations within host rocks ranging from shallow marine interlayered basalt flows to interbedded red bed sandstones. The most common ore horizons are fragmentary and porous amygdular layers, flow- top breccias, and faults in the basalts and overlying carbonates containing deposits formed in the Proterozoic, Triassic, Jurassic or Tertiary ages along a continental rift zone near a marine interface of a former equatorial position. These deposits include native copper and some silver in the flows and Cu2S minerals along the fractures. Copper distribution was regulated by the host rock permeability and fracturing of basalt flows and sedimentary beds. Some copper nuggets are found in stream beds due to weathering of the site. Michigan copper districts produced more than 5.95 million t of copper with an average grade of 1.48%, Kennecott accounted for about 618,000 t of this total.
Skarn deposits occur in Carr Fork, Utah and Copper Canyon, Nevada. The former is a porphyry bordering the Bingham, Utah site while the latter is associated with barren stock. Copper Canyon is a skarn bordering a weakly mineralized granitic and breccia pipe intrusive, which invades carbonate strata, containing chalcopyrite, pyrite, and some hematite, magnetite, bornite, pyrrhotite, molybdenite and many other minor minerals. Alteration of the wallrock resulted in the formation of diopside and andradite in the central section, wollastone and tremolite in the outer, and marble in the peripheral zone. Ores are found in irregular or tabular bodies in the clastic rocks near the intrusion and breccia pipes that cut the skarn which are weathered to form copper carbonates, silicates, and an iron-rich gossan. This type of deposit has a median size of .56 million t and an average grade of 1.7% Cu.
The next type of non-porphyry copper deposit is a vein located in Butte, Montana. Over 8 million t of copper has been produced at this mine in addition to large amounts of silver, gold, zinc, manganese, and lead. Vein deposits are associated with replacement deposits and with other sites that are peripheral to some porphyry copper deposits. The polymetallic deposits at Butte are found in a quartz monzonite stock as wells as in pegmatite and quartz porphyry dikes. The stock was introduced along a continental rift during the Cretaceous orogeny. The veins have an average width of 6-9 m but can increase in size up to 30 m wide in the central zone. From 1880 to 1972 9 million t of Cu, 245,000 t of Zn, 1.9 million t of Mn, 43,000 t of Pb, 23 million kg of Ag, and 102,000 kg of Au have been recovered from Butte.
The fourth example of a non-porphyry occurs in Superior, Arizona. In addition to copper there are also deposits of gold, silver, sphalerite, and galena. The ores are found in a series of disconnected shoots in host rocks of shattered carbonate, quartzite, and diabase which lye in two shear zones. The main ore minerals are pyrite, bornite, chalcopyrite, and enargite. Gold and silver are associated with malachite and chrysocolla in an iron oxide gangue. Approximately 311,000 t of copper with a median grade of 6.3% was produced from 1911 - 1943.
Another unique type of site is the massive sulfide labeled kuroko-type with locations in West Shasta, California and Crandon, Wisconsin. The rocks are marine volcanogenic of felsic to intermediate composition and include copper- and zinc-bearing massive sulfides. The ages of the deposits stretch from the Archean to the Cenozoic and consist of marine rhyolite, dacite, lesser basalt, along with mudstones and shales. Mineralization occurred along a marine volcanic-derived hot spring along island-arc belts indicated by greenstones of ancient accreted marine terrains. The deposits are distinguished by an upper stratified (black ore) zone, a lower stratiform (yellow ore) zone, and an underlying dispersed stockwork feeder zone. The black ore is pyrite, sphalerite, chalcopyrite, pyrrhotite, and occasionally galena, barite, tetrahedrite, and bornite. Yellow ore is composed of pyrite and chalcopyrite, with occasional sphalerite, pyrrhotite, and magnetite. The stockwork veinlets are pyrite, chalcopyrite, gold, and silver. Massive ore is found in a center of felsic volcanics near local fracturing associated with hot-springs, organic mudstones, pyritic siliceous shale, sulfide clasts, and breccia fragments. A median deposit size is 1.5 million t with an average grade of 1.3% CU, 2.0% Zn, .16% g/t Au, and 13 g/t Ag, based on 432 deposits worldwide.
In White Pine, Michigan there is a sediment-hosted dissemination which is located in shales, carbonates, sandstone/quartzite, and red beds. Deposits of copper-bearing shales, siltstones, sandstones, carbonates, evaporates, conglomerates, and dolomites formed along the boundaries of shallow marine basins. They range in age from the Proterozoic to the Mesozoic and occur along intercontinental rifts and passive continental margins. The ore minerals are chalcocite and other Cu2S minerals which replaced pyrite, bornite, and silver. The ores were controlled by a low pH environment, an abundance of sulfur, sediments, and petroleum. Reserves plus production at White Pine is approximated at 8 million t of Cu with a median grade of 1.2%. In other locations there is strong association with thick evaporate beds.
The final example of a non-porphyry copper deposit are the magmatic segregations or disseminations in mafic rocks located in Duluth, Minnesota and Stillwater, Montana. The Duluth mine is characterized by a Cu-Ni-PGE type which includes erratically distributed sulfides associated with the basal portions of layered intrusions in a cratonal rift zone. Ore minerals include pyrrhotite, pentlandite, chalcopyrite, cubanite, and platinum group minerals (PGE). Ages of the deposits go from the Precambrian to the Tertiary. Stillwater is of a Ni-Cu type in a large mafic to ultramafic intrusive containing nickel and copper sulfides. These deposits also range from the Precambrian to the Tertiary. They are located in cratonal shield terrains and include ore minerals of pyrrhotite, chalcopyrite, pentlandite, cobalt sulfide, and PGE.
The non-porphyry molybdenum deposits account for less than 5% of the total Mo mined in the United States. The first type is the vein deposit in Questa, New Mexico. It is a small but very rich molybdenum-quartz vein formed along fractures and contact zones of porphyritic aplite dikes. The deposits have a biotite granite pluton underneath them, a shallower aplite intrusive, and porphyry dikes which disrupt a Tertiary volcanic field, the molybdenum deposit is found in the aplite intrusive. The vein consists mainly of molybdenite and quartz with the central part of the vein containing fluorite, rhodochrosite, quartz, and calcite from being reopened locally. The ores were controlled by the three intersecting shear systems forming and reopening during intrusive surges. Oxidation has occurred on the surface and ferrimolybdenite and molybdenum-bearing limonite is joined by manganese oxides. The median grade is about 5% with 9,072 t of molybdenum having been produced.
The second type of non-porphyry molybdenum deposit is the skarn found in Pine Creek, California. Molybdenum and copper are produced here from a scheelite-bearing skarn which was formed by pyrometasomatic replacement of calcareous sedimentary rocks to marble and skarn assemblages along an intrusive granite. The median grade of the ore ranges from .6% to 1% Mo. The ore was controlled by the geometry of the contact between the intrusive rocks and the marble layers. The scheelite was formed during the early contact metamorphism with sulfides being produced by the subsequent hydrothermal alteration.
The final type is the pegmatite and sediment-hosted molybdenum deposits which occur in pegmatites and stratabound sedimentary rocks of little value. The bodies contain erratically distributed crystalline rosettes and flakes of molybdenum but are rarely ore grade. Some areas around the country may contain concentrations of significant amounts.