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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 coppermolybdenum 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 copperbearing 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.
Keywords:
copper molybdenum resources were recognized valuable commodities until economic
needs demanded collection processing these minerals large amounts most expansive
deposits copper molybdenum occur massive grade ores found intrusive porphyry
formations although many smaller sized higher grade ores located porphyry areas nation
abundant domestic copper reserves because many detrimental economic factors much
used industry imported molybdenum profuse exports high fulfill needs foreign demand
first used people around manufacture tools because malleability later became important
additive harder more useful metals such bronze brass zinc growth production united
states been relatively recent occurrence north american french explorers knew sources
native region lake superior area natives jewelry ornamentation earnest mining began
simsbury connecticut about actually exported england after source discovered jersey
around later times domestic resources satisfy national needs until discovery gold
california shifted focus mineral exploration westward strikes rich ores occurred tennessee
cordilleran base regions civil caused demand increase greatly order manufacture
cartridges canned goods this resulted openings numerous mines which more than were
lake superior area giving important advantage union armies major production districts
then shifted montana arizona early production increased reach peak levels tons year
during world tons were produced recent levels lower fluctuating between million tons
year technology aided increasing efficiency wich resulted spectacular resource
development around world been major mineral since when discovered harden steel
additive useful compounding chemicals dyes substantial mining began southwest demand
that activity ceased industry boomed with dawn need quality steel further increased
necessity this important additive highest levels occurred during early when mined current
lower mirroring curve because more than half produced product industry there many
different types deposits world containing different categories classes divided into main
groups porphyry intrusives which turn branch into several groups both classified using
main groups each mineral unique first lodes type from which group takes name manuel
arizona location first stockwork veinlets hydrothermally altered intrusives with closely
spaced phenocrysts microaplitic quartz feldspar intrusive ranges from mesozoic cenozoic
composition from tonalite granite found stockwork veinlets random grains intrusive
surrounding fractures includes chalcopyrite pyrite sometimes molybdenite magnetite gold
green blue carbonates silicates developed into weathered outcrops overlying enriched
zones containing chalcocite other sulfides there locations with average grade ranging
high another type gold pobres arizona composed stockwork chalcopyrite bornite
magnetite veinlets porphyritic intrusions igneous associations include tonalite
monzogranite coeval dacites andesite flows tuffs ages triassic british columbia quaternary
south pacific zone bell shaped localized volcanic center highest grades located upward
branching stock minerals include network scattered grains bornite chalcopyrite traces
native electrum sylvite hessite bordering altered wallrock inner quartz outer propylitic
zone pobres only deposit forty located worldwide median grades small amounts
molybdenite third group type deposit characterized site sierrita location erratic grains
native rocks near porphyritic disturbance mesozoic tertiary ranging consistency tonalite
monzogranite developed dikes stocks breccia pipes containing sparse phenocrysts
minerals consist pyrite molybdenite metered close spacing zone sometimes site magnetic
displacement magnetite surface rocks profoundly leached creating layer supergene below
leached there sites others median size million average being some considerable districts
unique contain geological features several deposit types such site bingham utah area
contains scattered altered igneous rock intrusives early tertiary occur stocks dikes highly
faulted folded carbonate well hydrothermally craton shelf peripheral bearing skarns
metamorphosed carbonates along contact contain sphalerite galena silver manganese
pyritic median tonnage jumbled arrays vary greatly bingham give some idea productivity
these areas through fallows deposits climax colorado granite high umbrella shaped quartz
fluorite tertiary aged granite composed dikes breccias fluorite sometimes feldspar pyrite
wolframite casserite topaz compose occur mainly fractures scattered glacial erosion little
sedimentary metamorphic rock cover climax nine sites worldwide tonnage level drawn
size million average climax itself produced over recovery over worlds total projected
reserve about second calc alkaline location buckingham nevada ranges mesozoic through
composed porphyritic granodiorite monzogranite found buckingham occasionally
scheelite argentian tetrahedrite controlled close spaced fractures when weathered
produces yellow ferrimolybdenite secondary size this systems account about supply
mainly strata bound sedimentary rocks volcanogenic massive sulfides mafic keweenaw
michigan volcanogenic sedimentary overlying clastic sediments sulfides below thick
basalts sulfide locations within host ranging shallow marine interlayered basalt flows
interbedded sandstones most common horizons fragmentary porous amygdular layers
flow breccias faults basalts overlying carbonates formed proterozoic triassic jurassic ages
along continental rift near marine interface former equatorial position these include some
silver flows along distribution regulated host rock permeability fracturing basalt beds
nuggets stream beds weathering michigan districts than kennecott accounted total skarn
carr fork utah canyon nevada former bordering bingham utah while latter associated
barren stock canyon skarn bordering weakly mineralized granitic breccia pipe invades
carbonate strata hematite bornite pyrrhotite other minor alteration wallrock resulted
formation diopside andradite central section wollastone tremolite outer marble peripheral
irregular tabular bodies clastic near intrusion breccia pipes that skarn weathered form
silicates iron rich gossan next vein butte montana over been mine addition large amounts
silver zinc manganese lead vein associated replacement other sites that peripheral
polymetallic butte monzonite stock wells pegmatite introduced continental rift during
cretaceous orogeny veins have width increase wide central have recovered butte fourth
example occurs superior addition also sphalerite galena series disconnected shoots host
shattered carbonate quartzite diabase shear zones main enargite associated malachite
chrysocolla iron oxide gangue approximately another unique massive sulfide labeled
kuroko locations west shasta california crandon wisconsin marine volcanogenic felsic
intermediate composition zinc bearing ages stretch archean cenozoic consist rhyolite
dacite lesser basalt mudstones shales mineralization occurred volcanic derived spring
island belts indicated greenstones ancient accreted terrains distinguished upper stratified
black lower stratiform yellow underlying dispersed feeder black sphalerite pyrrhotite
occasionally galena barite tetrahedrite yellow occasional pyrrhotite center felsic volcanics
local fracturing springs organic mudstones pyritic siliceous shale sulfide clasts fragments
based worldwide white pine michigan sediment hosted dissemination shales sandstone
quartzite beds bearing shales siltstones sandstones evaporates conglomerates dolomites
formed boundaries shallow basins they range proterozoic intercontinental rifts passive
continental margins chalcocite replaced controlled environment abundance sulfur
sediments petroleum reserves plus white pine approximated strong association thick
evaporate final example magmatic segregations disseminations mafic duluth minnesota
stillwater montana duluth mine characterized includes erratically distributed basal
portions layered intrusions cratonal rift pentlandite cubanite platinum group precambrian
stillwater large mafic ultramafic nickel also range precambrian they cratonal shield
terrains pentlandite cobalt account less total mined united states vein questa mexico small
very rich formed contact zones aplite have biotite pluton underneath them shallower
aplite disrupt volcanic field aplite consists mainly central part fluorite rhodochrosite
calcite being reopened locally controlled three intersecting shear systems forming
reopening surges oxidation surface ferrimolybdenite limonite joined manganese oxides
having second pine creek california here scheelite pyrometasomatic replacement
calcareous marble assemblages ranges geometry contact between marble layers scheelite
metamorphism being subsequent hydrothermal alteration final pegmatite sediment hosted
pegmatites stratabound little value bodies contain erratically distributed crystalline
rosettes flakes rarely areas country concentrations significant
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