Aesthetic 25mm specimen of native Copper with some micro Cuprite crystals

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Aesthetic 25mm specimen of native Copper with
some micro Cuprite crystals
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Trio of Enargites. specimen is 1.8 cm tall.
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Large cluster of lustrous pyrite crystals
overgrown on milky quartz (visible on the bottom
of the specimen. Size of individual crystals 1-2
cm.
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A cluster of sprays of quartz crystals is covered
with balls of shiny tetrahedrite, there is also
some minor pyrite dotted about.
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Cuprite is commonly found as an oxidation product
of copper sulphides in the upper zones of veins.
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Brilliant, lustrous sphalerite crystals. 6x4x3 cm
Locality Elmwood mine, Carthage, Central
Tennessee Ba-F-Pb-Zn District, Smith Co.,
Tennessee, USA
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Large cubic crystals of gray metallic galena on
limestone matrix. Locality Baxter Springs,
Picher Field, Tri-State District, Cherokee Co.,
Kansas, USA
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Porphyry copper deposit Porphyry copper deposits
are copper ore bodies which are associated with
porphyritic intrusive rocks. The ore occurs as
disseminations along hairline fractures as well
as within larger veins, which often form a
stockwork. The orebodies typically contain
between 0.4 and 1 copper with smaller amounts
of other metals such as molybdenum, silver and
gold. They are formed when large quantities of
hydrothermal solutions carrying small quantities
of metals pass through fractured rock within and
around the intrusive and deposit the
metals. Porphyry copper deposits are the largest
source of copper, and are found in North and
South America, Europe, Asia, and Pacific islands.
None are documented in Africa. The largest
examples are found in the Andes in South America.
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Typical porphyry from an economic copper deposit.
Quartz monzonite porphyry associated with
economic copper mineralization at the Robinson
Mining District, Nevada. The porphyritic rock
contains coarse crystals of orthoclase feldspar,
plagioclase feldspar, and quartz set in a finely
crystalline ground mass generated during
decompression and quenching of the melt. The
largest orthoclase crystal (white) is 3.0 cm in
diameter.
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• Characteristics of porphyry copper deposits
  include
• The orebodies are associated with multiple
  intrusions and dikes of diorite to quartz
  monzonite composition with porphyritic textures.
• Breccia zones with angular or locally rounded
  fragments are commonly associated with the
  intrusives. The sulfide mineralization typically
  occurs between or within fragments.
• The deposits typically have an outer epidote -
  chlorite mineral alteration zone.
• A quartz - serite alteration zone typically
  occurs closer to the center and may overprint.
• A central potassic zone of secondary biotite and
  orthoclase alteration is commonly associated with
  most of the ore.
• Fractures are often filled or coated by
  sulfides, or by quartz veins with sulfides.
  Closely spaced fractures of several orientations
  are usually associated with the highest grade
  ore.
• Porphyry copper deposits are typically mined by
  open-pit methods.

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MINERALIZATION  Original sulphide minerals in
these deposits are pyrite, chalcopyrite, bornite
and molybdenite. Gold is often in native found as
tiny blobs along borders of sulphide crystals.
Most of the sulphides occur in veins or plastered
on fractures most are intergrown with quartz or
sericite. In many cases, the deposits have a
central very low grade zone enclosed by 'shells'
dominated by bornite, then chalcopyrite, and
finally pyrite, which may be up to 15 of the
rock. Molybdenite distribution is variable,
Radial fracture zones outside the pyrite halo may
contain lead-zinc veins with gold and silver
values.
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• Examples of porphyry copper deposits
• La Caridat, Sonora, Mexico
• Ok Tedi, Papua New Guinea
• Dizon, Philippines
• Chile
• Chuquicamata
• El Teniente
• United States
• Ajo, Arizona
• Bagdad, Arizona
• Lavender Pit, Bisbee, Arizona
• Morenci, Arizona
• San Manuel, Arizona
• Sierrtita, Arizona
• El China, Santa Rita, New Mexico
• Ely, Nevada
• Bingham Canyon mine, Utah

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DISTRIBUTION AND AGE  Porphyry copper provinces
seem to coincide, worldwide, with orogenic belts.
This remarkable association is clearest in
Circum-Pacific Mesozoic to Cenozoic deposits but
is also apparent in North American, Australian
and Soviet Paleozoic deposits within the orogenic
belts. Porphyry deposits occur in two main
settings within the orogenic belts in island
arcs and at continental margins. Deposits of
Cenozoic and, to a lesser extent, Mesozoic age
predominate. Those of Paleozoic age are uncommon
and only a few Precambrian deposits with
characteristics similar to porphyry coppers have
been described. Deformation and metamorphism of
the older deposits commonly obscured primary
features, hence they are difficult to recognize.
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Distribution of porphyry copper deposits
world-wide
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Granitic rocks (Cretaceous)
Basaltic rocks (Cenozoic)
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Porphyry-type ore deposits for metals other than
copper Copper is not the only metal that occurs
in porphyry deposits. There are also porphyry ore
deposits mined primarily for molybdenum, many of
which contain very little copper. Examples of
porphyry molybdenum deposits are the Climax,
Urad, and Henderson deposits in central Colorado,
and the Questa deposit in northern New
Mexico. The US Geological Survey has classed the
Chorolque and Catavi tin deposits in Bolivia as
porphyry tin deposits. Some porphyry copper
deposits in oceanic crust environments, such as
those in the Phillippines, Indonesia, and Papua
New Guinea, are sufficiently rich in gold that
they are called copper-gold porphyry deposits.
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Chuquicamata, or "Chuqui," as it is commonly
called, is one of the largest open pit copper
mine in the world. It was named after a small
city in the north-west of Chile. It began copper
production on May 18, 1915. The Bingham Cayon
Mine in the U.S. state of Utah vies with
Chuquicamata for the title of world's largest
open pit copper mine. Chuquicamata is located 15
km north of the city of Calama in the region of
Antofagasta. The mine is elliptical in form, with
a surface of almost 8,000,000 m2, and it is 900 m
deep.
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Supply of Cu (Mo)
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Figure 1. Vertical cross section showing a
porphyry copper deposit as it occurs deep within
the earth. (Modified from Evans, 1980)
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Figure 2. Geologic map showing the aerial view of
a porphyry copper deposit
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Geochemical Exploration Such a map may look like
Figure 3. The circles along the streams are
locations at which gold (Au) has been panned. The
numbers refer to the number of gold grains that
were found at these locations. The arrows point
in the direction that the stream is flowing (the
"v's" formed by the joining streams always point
downstream). After studying the map, geologists
would predict that gold deposits may occur
upstream from the two highest gold values (35 and
21). Unusually high concentrations such as these
are termed geochemical anomalies by exploration
geologists. The number of grains decreases
downstream from these anomalous values. Upstream
from the predicted gold deposit, there are few
gold grains in the sediments. This is because the
stream can only carry the gold grains downstream
from the deposit, not upstream.
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Figure 3. Map showing a stream and sediment
survey
Black dots the place where Au has been panned
and shown with gold value
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Another commonly used geochemical exploration
technique is soil geochemistry. Geologists
establish a sampling grid over an area of
interest. Figure 3 shows such a grid. It is
defined by the letters A through D on the
north-south axis, and the numbers 1 through 5 are
on the east-west axis. Geologists analyze soil
samples at each node of the grid (where the lines
cross). They then construct a map showing the
concentration of gold at each location. On this
map, the highest value of gold (4.3 ppm) occurs
at node B3. Node B4 has a lower gold value than
B3 (0.53 ppm), but higher than all of the other
soil samples in the area. Geologists could use
these anomalous values, together with the
anomalous stream sediment values to predict that
an ore body was present below the soil somewhere
in the blackened area.
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Porphyry Deposits-summary   A large body of
rock, typically a porphyry of granitic to
dioritic composition, that has been fractured on
a fine scale and through which chalcopyrite and
other copper minerals are disseminated. Porphyry
copper deposits commonly contain hundreds of
millions of metric tons of ore that averages a
fraction of 1 percent copper by weight although
they are low-grade. The major products from
porphyry copper deposits are copper and
molybdenum or copper and gold. The term
porphyry copper now includes engineering as well
as geological considerations It refers to large,
relatively low grade, epigenetic,
intrusion-related deposits that can be mined
using mass mining techniques.
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Geologically, the deposits occur close to or in
granitic intrusive rocks that are porphyritic in
texture. There are usually several episodes of
intrusive activity, so expect swarms of dykes and
intrusive breccias. The country rocks can be any
kind of rock, and often there are wide zones of
closely fractured and altered rock surrounding
the intrusions. This country rock alteration is
distinctive and changes as you approach
mineralization. Where sulphide mineralization
occurs, surface weathering often produces
rusty-stained bleached zones from which the
metals have been leached if conditions are
right, these may redeposit near the water table
to form an enriched zone of secondary
mineralization.
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PORPHYRY COPPER CLASSIFICATION Porphyry copper
deposits comprise three broad types plutonic,
volcanic, and "classic".                          
                            Plutonic porphyry
copper deposits occur in batholithic settings
with mineralization principally occurring in one
or more phases of plutonic host rock. Volcanic
types occur in the roots of volcanoes, with
mineralization both in the volcanic rocks and in
associated comagmatic plutons. Classic types
occur with high-level, post-orogenic stocks That
intrude unrelated host rocks mineralization may
occur entirely within the stock entirely in the
country rock, or in both. The earliest mined
deposits, as well as the majority of Cenozoic
porphyry copper deposits, are of the classic
type.
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Intrusions Associated with Porphyry Copper
Deposits Intrusions associated with porphyry
copper deposits are diverse but generally felsic
and differentiated. Those in island arc settings
have primitive strontium isotopic ratios
(87Sr/86Sr of 0.702 to 0.705) and, therefore, are
derived either from upper mantle material or
recycled oceanic crust. In contrast, ratios from
intrusions associated with deposits in
continental settings are generally higher.
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WHAT TO LOOK FOR IN THE FIELD 1. Dykes and
granitic rocks with porphyritic textures. 2.
Breccia zones with angular or locally rounded
fragments look for sulphides between fragments
or in fragments. 3. Epidote and chlorite
alteration. 4. Quartz and sericite alteration.
5. Secondary biotite alteration - especially if
partly bleached and altered. 6. Fractures
coated by sulphides, or quartz veins with
sulphides. To make ore, fractures must be closely
spaced generally grades are better where there
are several orientations (directions).  
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STRUCTURAL FEATURES  Mineralization in porphyry
deposits is mostly on fractures or in alteration
zones adjacent to fractures, so ground
preparation or development of a 'plumbing system'
is vitally important and grades are best where
the rocks are closely fractured. Porphyry-type
mineral deposits result when large amounts of hot
water that carry small amounts of metals pass
through permeable rocks and deposit the metals.
Strong alteration zones develop in and around
granitic rocks with related porphyry deposits.
Often there is early development of a wide area
of secondary biotite that gives the rock a
distinctive brownish colour. Ideally, mineralized
zones will have a central area with secondary
biotite or potassium feldspar and outward
'shells' of cream or green quartz and sericite
(phyllic), then greenish chlorite, epidote, sodic
plagioclase and carbonate (propylitic)
alteration. In some cases white, chalky clay
(argillic) alteration occurs.
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THEORY  The spectrum of characteristics of a
porphyry copper deposit reflects the various
influences of four main and many transient stages
in the evolution of the porphyry hydrothermal
system. Not all stages develop fully, nor are all
the stages of equal importance.          Various
factors, such as magma type, volatile content,
the number, size, timing and depth of emplacement
of mineralizing porphyry plutons, variations in
country rock composition and fracturing, all
combine to ensure a wide variety of detail. As
well, the rate of fluid mixing, density contrasts
in the fluids, and pressure and temperature
gradients influence the end result. Different
depths of erosion alone can produce a wide range
in appearances even in the same deposit.  No
single model can adequately portray the
alteration and mineralization processes that have
produced the wide variety of porphyry copper
deposits. However, volatile-enriched magmas
emplaced in highly permeable rock are ore-forming
processes that can be described in a series of
models that represent successive stages in an
evolving process.
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End-member models of hydrothermal regimes attempt
to show contrasting conditions for systems
dominated by magmatic (waters derived from molten
rock) and meteoric waters (usually groundwater),
respectively. Both end-members are depicted
after enough time has elapsed following
emplacement for water convection cells to become
established in the country rock in response to
the magmatic heat source. The convecting fluids
transfer metals and other elements, and heat from
the magma into the country rock and redistribute
elements in the convective system.               
 The two models represent end-members of a
continuum. The fundamental difference between
them is the source and flow path of the
hydrothermal fluids.
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CONCLUSION The search for porphyry copper
deposits, especially buried ones, must be founded
on detailed knowledge of their tectonic setting,
geology, alteration patterns, and geochemistry.
Sophisticated genetic models incorporating these
features will be used to design and control
future exploration
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Example Bingham Canyon Mine The Bingham Canyon
Mine is is an open-pit mine extracting a large
porphyry copper deposit southwest of Salt Lake
City, USA, in the Oquirrh Mountains. The mine has
been in production since 1906, and has resulted
in the creation of a pit over 0.75 miles deep,
2.5 miles wide, and covering 1,900 acres -- the
world's largest man-made excavation.
Bingham Canyon Mine, April 2005.
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Over its life, Bingham Canyon has proven to be
one of the world's most productive mines. As of
2004, ore from the mine has yielded more than
than 17 million tons of copper, 23 million ounces
of gold, 190 million ounces of silver, and 850
million pounds of molybdenum. Cumulatively,
Bingham Canyon has produced more copper than any
other mine in the world, although mines in Chile,
Arizona, and New Mexico now exceed Bingham
Canyon's annual production rate. Rising
molybdenum prices in 2005 made the molybdenum
produced at Bingham Canyon in that year worth
even more than the copper. The mine is regarded
as one of the most up-to-date integrated copper
operations in the world, employing 1,400 people.
The smelting and refining facilities are
recognised as being among the world's best for
environmental protection practice and
achievement.
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The Bingham Canyon open pit stretches for 2.5
miles across the rim and is the largest manmade
excavation on Earth
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Bingham Canyon Mine To begin with a few
production statistics, the Bingham Canyon Mine
has produced more copper than any other mine in
history--about 14.5 million tons of the metal.
Bingham Canyon is primarily a copper mine, but it
has also yielded a bonanza in byproduct metals.
These include 18.5 million troy ounces (about 620
tons) of gold, 157 million troy ounces (nearly
5,000 tons) of silver, 610 million pounds of
molybdenum and significant amounts of platinum
and palladium. The cumulative value of Bingham
Canyon metals far exceeds the total worth of the
Comstock Lode and the California and Klondike
gold rushes combined. With production statistics
like that, it's no wonder that the Bingham Canyon
Mine has been nicknamed "the Richest Hole on
Earth."
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Supplement 1
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High Sulfidation and Low Sulfidation Porphyry
Copper/Skarn Systems Characteristics, Continua,
and Causes Marco T. Einaudi, Stanford University
Stanford, California, U.S.A. In the past
decade, multi-disciplinary research on active and
fossil hydrothermal systems in volcano-plutonic
arcs has resulted in important new information on
the physical and chemical evolution of
hydrothermal fluids of diverse origin, on the
sources of metals, sulfur and other dissolved
constituents, and on the possible genetic
transitions between different ore-forming
environments. Among the most studied magma-
hydrothermal systems are those linked to felsic
magmatism, whose products extend from plutonic
porphyry-Cu deposits to volcanic epithermal-Au
deposits.
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In-between these end members can be skarns or
massive sulfide replacement bodies and veins of
both base- and precious-metals. Here I focus on
transitions between ore types in porphyry copper
systems and use the the sulfidation state of
hydrothermal fluids as a framework. I adopt the
sulfidation state as a means of classification of
ore-forming environments because this variable
spans all deposit types and is independent of
host rocks, metals contained, and textures
exhibited (in contrast with terms such as
"acid-sulfate" which is restricted to
quartzo-feldspathic host rocks, or "epithermal"
which is restricted to one class of deposit).
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Sulfidation State. McKinstry (1959, 1963) and
Barton (1970) applied the terms "sulfur content"
and "sulfidation state", respectively, to denote
the relative values of the chemical potential of
sulfur implied by sulfide mineral assemblages in
ore deposits. Both authors noted the general
tendency for sulfidation state to increase as
unbuffered hydrothermal solutions evolve from
high to low temperatures in base-metal veins
associated with felsic igneous rocks. The concept
of sulfidation state was put on a firm
theoretical and experimental base by Skinner and
Barton (1967, 1979) and applied to base-metal
veins by Meyer and Hemley (1967).
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The sulfidation state of hydrothermal fluids can
be classified on a continuous scale on the basis
of key sulfidation reactions based on Skinner and
Barton (1979). In bold letters are minerals or
assemblages that span only two defined
sulfidation states in italic bold are minerals
or mineral assemblages that occupy only one
defined state. Only a few minerals or mineral
assemblages are diagnostic of a given sulfidation
state. For example, although covellite is
diagnostic of very high sulfidation states,
enargite is less diagnostic, being stable from
upper intermediate, through high, and very high
sulfidation states. Because of the chemical links
between sulfidation, oxidation, and ionization
states of hydrothermal fluids (Meyer and Hemley,
1967), covellite would be expected to be
associated with acid-sulfate fluids and advanced
argillic alteration containing alunite. Enargite,
on the other hand, could be deposited from
hydrothermal fluids of intermediate
oxidation-sulfidation state and be associated
with less advanced degrees of base-cation
leaching of wall rocks (e.g., sericitic
alteration).
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Porphyry Copper and Related Deposits 1940-1980.
The first detailed studies of advanced argillic
(including acid-sulfate) and sericitic alteration
associated with relatively high sulfidation state
sulfide assemblages were focussed on
enargite-bearing veins associated with felsic
igneous rocks at Cerro de Pasco, Peru, and Butte,
Montana (Graton and Bowditch, 1936 Sales and
Meyer, 1948 1949). Field documentation that
enargite-bearing veins with acid-sulfate
alteration commonly were superimposed on the
upper portions of porphyry copper deposits (Meyer
and Hemley, 1967 Meyer et al., 1968 Taylor,
1933), systematization of naturally occurring
sulfide mineral assemblages as a function of
"sulfur content" (McKinstry, 19xx, 19xx), and
experimental definition of mineral equilibria as
a function of temperature and fluid compositions
(Barton et al., 1963 Hemley and Jones, 1964
Hemley et al., 1969), led to increased
understanding of the geologic and geochemical
factors that control the formation of very high-
to high- sulfidation enargite-covellite ores
versus low-sulfidation (magnetite-bornite) to
intermediate-sulfidation (chalcopyrite-pyrite)
ores in porphyry-related systems (Hemley and
Jones, 1964 Meyer and Hemley, 1967 Hemley et
al. 1969 Gustafson and Hunt, 1975 Einaudi,
1977 Knight, 1977 Brimhall, 1977, 1979).
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At the same time, there was increasing field
evidence that some epithermal high-sulfidation
deposits are somehow linked to deeper porphyry
systems (Sillitoe, 1973 Wallace, 1979). Combined
with an enlarging base of descriptive models of
porphyry copper deposits (Titley and Hicks, 1966
Lowell and Guilbert, 1970 Rose, 1970 Guilbert
and Lowell, 1974 Sutherland-Brown, 1976 Titley,
1975), data on temperature-salinity (Roedder,
1971 Moore and Nash, 1974 Eastoe, 1978) and
sources of water in hydrothermal fluids (Sheppard
et al., 1969, 1971 Sheppard and Taylor, 1974
Taylor, 1974), and models of the physical and
chemical nature of the magma-hydrothermal
transition and of overlying vapor-dominated
systems (Burnham, 1967, 1979 White et al., 1971
Holland, 1972 Phillips, 1973 Whitney, 1975
Henley and McNabb, 1978), an evolutionary theme
for porphyry copper and closely related deposits
emerged. Although this evolutionary model was
briefly swayed from its magmatic roots (Norton,
1972 Norton and Cathles, 1976), by 1980 the
magmatists prevailed.
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Porphyry Copper Systems Characteristics The
evolutionary framework for porphyry- related
deposits formed at depths of 2 to 4 km,
established by workers cited above and further
refined in the early 1980's (Brimhall, 1980
Burnham and Ohmoto, 1980 Titley and Beane, 1981
Einaudi, 1981, 1982 Eastoe, 1982 Sillitoe,
1983a, 1983b) can be cast in terms of the
observed space-time distribution of two
ore-forming environments, as illustrated in
Figure 1 (A C) and summarized in Table 2 .
major at Ely (Robinson), Nevada).
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Low- to intermediate sulfidation environment (A)
Early and/or deep stages are characterized by
potassic alteration and anhydrous skarn with
disseminated/veinlet chalcopyrite-bornite-(magneti
te) related to refluxing magmatic brines (saline,
and hypersaline if a vapor plume is released) at
600-400 C, lithostatic pressure, and intermediate
sulfidation-oxidation states (arrow 2, Fig. 1).
This early stage is succeeded by late,
superimposed and high-level sericitic alteration
of porphyry and retrograde alteration of skarn,
accompanied by pyrite-chalcopyrite- (hematite),
in through-going veins. Late fluids are
dominantly meteoric, boiling at 350-250 C under
hydrostatic pressures, and are characterized by
moderate acididity, low-salinity, and high
sulfidation-oxidation states (arrows 5, Fig. 1).
The degree of development of sericitic alteration
varies significantly at present levels of
exposure in porphyry copper districts (e.g.,
minor at Bingham, Utah
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High- to very high-sulfidation environment (C)
Some porphyry deposits contain very late,
high-level advanced argillic alteration (encased
in sericitic) with pyrite-alunite, in some cases
accompanied by digenite, covellite, and/or
enargite (e.g., Butte, Montana Chuquicamata,
Chile), in other cases barren of copper (e.g., El
Salvador, Chile). Acid-sulate alteration is
localized in faults, hydrothermal breccias and
around pebble dikes. Acid-sulfate fluids are of
meteoric water (arrow 7) and/or magmatic-vapor
plume origin (arrow 6), at 350-200 C,
near-hydrostatic pressure, and high to very high
sulfidation-oxidation states (arrow 8). In skarn
or carbonate wall-rocks, these fluids generate
silica-pyrite Cu- (Au) fissures and replacement
bodies (e.g., Bisbee, Arizona Yauricocha, Peru).
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Continua and Causes Variations on the degree of
development of low sulfidation (A) versus
high-sulfidation (C) fluids in porphyry-related
copper deposits, as exhibited by localities
summarized in Table 2, are controlled by local
tectonic, magmatic, and hydrodynamic conditions.
Formation of a "classic" low sulfidation porphyry
copper deposit (environment A, Fig. 1) would be
favored by relatively deep emplacement of
multiple, non-venting, intrusions into anhydrous,
unfractured rocks in a relatively stable tectonic
environment. In contrast, formation of a
high-sulfidation "Cordilleran lode" deposit
(environment C, Fig. 1) consisting of massive
pyritic copper ores encased in advanced argillic
and sericitic alteration would be favored by
relatively shallow subvolcanic emplacement of
isolated stocks and plugs into fractured rocks
saturated with meteoric water in an active
tectonic environment.
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Abrupt superposition of high-sulidation veins
(environment C) on to low-sulfidation
disseminated ores (environment A) could result
from "tectonic quenching", such as pressure
release and incursion of meteoric water due to
large-scale crustal faulting(Gustafson and Hunt,
1975 Einaudi, 1977 Brimhall, 1980 Einaudi,
1982), or by removal of overlying rocks by
erosion during rapid uplift or mass-wasting of
volcanic edifices (Sillitoe and Gappe, 1984). In
some cases, tectonic quenching could effectively
suppress the development of disseminated porphyry
copper deposits at (A), resulting in a lode
deposit without porphyry roots (Einaudi, 1977,
1982).
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Porphyry Copper - Epithermal Gold Systems The
View from Above, 1990 With increased interest in
precious metals during the 1980s, research in ore
deposits shifted to gold- rich porphyry copper
deposits, epithermal systems and other
environments of precious-metal deposition. The
result is important new information on (1)
geologic settings of high-sulfidation epithermal
deposits and their links to deeper
magma-hydrothermal systems (Sillitoe, 1988, 1989,
1992 Heald et al., 1987 White, 1991) and (2)
case studies of high-sulfidation epithermal
districts that integrate geology, geochemistry,
fluid inclusions, and light stable isotopes
(Bethke, 1984 Stoffregen, 1987 Bove, 1988
Arribas et al., 1989 Deen, 1990 Bove et al.,
1990 Muntean et al., 1990 Rye et al., 1992
Vennemann et al., 1993 Hedenquist et al, 1994).
These studies have lent further support to the
idea that high-sulfidation epithermal deposits
have a magmatic fingerprint and that some are
closely linked to deeper porphyry systems
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The present conceptual model that ties porphyries
with high-sulfidation epithermal systems (in
contrast with high-sulfidation copper lodes),
based on the studies cited above, is paraphrased
here from Sillitoe (1989, Fig. 9) and Rye (1993,
Figs. 1 33). The porphyry copper environment
that occupies a position between the water- rich
carapace of the magma and the overlying
transition from plastic to brittle rock, also may
be of critical importance to epithermal deposits.
This volume, characterized by the presence of
saline magmatic water, quasiplastic behaviour,
and low waterrock ratios, and by the absence of
long-lived fractures and of meteoric water, may
be the reservoir for evolved magmatic fluids that
generate epithermal ores.
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At the ductile-brittle transition, saline
magmatic fluids encounter open fractures and
hydrostatic pressures boiling of these fluids
results in a hypersaline brine that remains at
the ductile-brittle transition (arrow 2, Fig. 1)
and a vapor plume (arrow 1, Fig.1 ) that rises to
high levels where it generates barren
acid-sulfate alteration following condensation
(arrow 3, Fig. 1). As the ductile-brittle
transition withdraws to deeper levels with time,
metal-bearing saline and hypersaline liquid-phase
fluids that have been refluxing within the stock
(arrow 2) are tapped (arrow 4) and may ascend
rapidly to high levels. These are the epithermal
ore fluids (environment B, Fig. 1). If the deep
environment (A, Fig. 1) fails to evolve to
environment (C), then the high-sulfidation
deposit (B) is separated from its roots (A) by a
rock volume with little or no signs of
hydrothermal activity.
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The present challenge to students of porphyry
systems is to distinguish, within individual
districts, between the model end member processes
that generate acid-sulfate fluids (vapor plume or
liquid-phase mixing?), the causes of mineralized
versus barren acid-sulfate zones, and the
potential continuum between copper-rich and gold-
rich high-sulfidation deposits.
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Supplement 2
69
PORPHYRY COPPER MINERALISATION OF WESTERN
USA Allan J.R. White (VIEPS, The University of
Melbourne, Victoria 3010, Australia) Porphyry
copper deposits of western USA are very large low
grade deposits dominated by disseminated Cu
mineralisation but commonly with appreciable Mo
and Au. Many deposits began as gold camps.
Mineralisation is centred on, and mostly within,
near surface quartz monzonite intrusions in which
there is inner and deeper concentric Mo-rich
shells. Mo shells are followed by Cu-rich shells,
then pyrite and there may be an outermost
Pb-Zn-Ag zone as in the country rock skarns of
the Bingham deposit. Cu-Mo mineralisation
occurs within a potassic alteration zone
characterised by secondary biotite. Extensive
outer sericitic (phyllic), argillic and
propylitic alteration zones do not necessarily
conform to the concentric pattern. Large scale
bulk mining of a porphyry deposit was first
carried out at Bingham.
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There is a belt of economic deposits extending
from Butte Montana, through Bingham Utah, to
Arizona where deposits are most abundant, and New
Mexico. This review is based on visits to many
deposits along the whole length of the belt and
various petrological observations at Butte,
Bingham, Bagdad, Miami-Globe and Sierrita. Most
deposits are Laramide (approx. 70 Ma), a notable
exception being Bingham (40 Ma). The Laramide
belt is inboard up to 1200 km from the Pacific
coast of the US where there are Recent to
Mesozoic subduction related rocks. It is
suggested that the Laramide igneous rocks were
not formed as a result of subduction but as a
result of rifting within the Precambrian
basement.
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At Butte Montana (references in Miller 1978)
where the rich main veins (e.g. Anaconda Vein)
cut typical shells of the porphyry system, the
host rock is quartz monzonite. Two suites of
associated rocks have been recognised within the
Boulder Batholith. Rocks of the high-K,
mineralised suite range from mafic monzonites or
high-K diorites through quartz monzonites to low
quartz granites. Associated volcanic rocks
include voluminous latites. Latites are high- K,
acid to intermediate SiO2 rocks in which there
are virtually no quartz phenocrysts. The original
quartz monzonite was described from Walkerville
(a northern suburb of Butte) more than 100 years
ago, but the significance of this rock has been
lost, mainly because many later petrologists
referred to the rock type as granite or
adamellite (now monzogranite) probably because
the Butte sample is on the borderline between
quartz monzonite and granite. A sample, collected
near the type locality, is described. It consists
of plagioclase, K-feldspar, quartz, biotite,
hornblende (commonly with pyroxene cores),
relatively abundant magnetite and very small
amounts of titanite and apatite. Quartz is lower
than in typical granite and magnetite is more
abundant. Magnetite is commonly seen as
aggregates along with apatite, suggestive of
crystallisation from an immiscible Fe-P melt
phase!
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Monzonites very low in quartz are very common at
Bingham. At Miami-Globe (Peterson 1962) there are
typical quartz monzonites and very low quartz
granites, and at Sierrita (Anthony Titley 1988)
rocks range from high-K diorites (and high-K
andesites) through quartz monzonites (and
latites) to low quartz granites (and rhyolites).
Bagdad (Anderson) is a high-K diorite lower in
K2O than most other igneous rocks of the Laramide
belt.
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The quartz monzonite host rocks for the US
deposits reviewed here have high K2O Na2O. As
in host rocks of other economic porphyry Cu
deposits, K2O is high but normally less than
Na2O. Gerel (1995) says Cu-Au porphyry systems
have K2O/Na2O 0.7 to 1.3 whereas the porphyry
Cu-Mo deposits are associated with rocks with
K2O/Na2O 0.3 0.7. Bingham and Butte both have
K2O/Na2O 1.2 to 1.3 but have produced about
1000 and 100 tonnes of Au respectively. Bingham
has produced appreciable Mo. Higher alkalis in
the quartz monzonite magma produced higher
feldspar in the rock and consequently lower
quartz. Another characteristic geochemical
feature is high Ba commonly amounting to 1000 ppm
or more. Oscillatory zoning within K-feldspar
seen in the field and in thin section is
indicative of high Ba. Oxygen fugacity is
extremely high. Values of ?NNO gt 2 can be
calculated from more reliable rock analyses in
which FeO and Fe2O3 have been recorded, and from
biotite compositions (e.g. Anthony Titley 1988).
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The following features indicate that host rocks
were intruded close to the surface at pressures
near 50 MPa (500 bars) 1. There are associated
volcanic rocks. 2 Some intrusions are porphyries
that are pressure-quenched rocks. 3 There are
acid rocks with miarolitic cavities (crystals of
copper and molybdenum sulfide have been reported
in some cavities). ? 4 Granophyric intergrowths
of quartz and alkali feldspar are seen in thin
section. 5 Occurrence of hydrothermal breccias. 6
Quartz monzonites have closely spaced vertical
joints.
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It is concluded that economic porphyry copper
systems of the western USA are associated with,
and probably derived from, high temperature
monzonitic suites in which the variety of rock
types are the result of fractional
crystallisation. Mineralised rocks are
near-surface quartz monzonites similar to I-type
granites but with lower quartz contents. Many are
on the borderline between quartz monzonite and
granite. Some deposits are only economic if
there has been secondary enrichment.
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References ANDERSON C.A. 1950. Alteration and
metallization in the Bagdad porphyry copper
deposit, Arizona. Economic Geology 45,
609-628. ANTHONY E.Y. TITLEY, S.R. 1988.
Progressive mixing of isotopic reservoirs during
magma genesis at the Sierrita porphyry copper
deposit, Arizona inverse solutions. Geochemica
et Cosmochimica Acta 52, 2235-2249. GEREL O.
1995. Mineral resources of the western part of
the Mongol-Okhotsk Foldbelt. In Ishihara S.
Czamanske G.K. eds. Resource Geology Special
Issue 18, 151-157. JOHN E.C. 1978. Mineral zones
in the Utah copper orebody. Economic Geology 73,
1250-1259. MILLER R.N. 1978. Butte Field Meeting
Guidebook. Anaconda Company, Butte Montana
(second printing). PETERSON N.P. 1962. Geology
and ore deposits of the Globe - Miami district,
Arizona. United States Geological Survey
Professional Paper 342.
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