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Essay/Term paper: Copper and molybdenum deposits in the united states

Essay, term paper, research paper:  World History

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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 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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