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.
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