Download Porphyry Copper

Aesthetic 25mm specimen of native Copper with some micro
Cuprite crystals
Trio of Enargites. specimen is 1.8 cm tall.
Large cluster of lustrous pyrite crystals overgrown on milky
quartz (visible on the bottom of the specimen. Size of individual
crystals: 1-2 cm.
A cluster of sprays of quartz crystals is covered with balls
of shiny tetrahedrite, there is also some minor pyrite
dotted about.
Cuprite is commonly found as an oxidation product of copper
sulphides in the upper zones of veins.
Brilliant, lustrous sphalerite crystals. 6x4x3 cm
Locality: Elmwood mine, Carthage, Central Tennessee Ba-F-PbZn District, Smith Co., Tennessee, USA
Large cubic crystals of gray metallic galena on limestone matrix.
Locality: Baxter Springs, Picher Field, Tri-State District,
Cherokee Co., Kansas, USA
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.
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.
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.
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 leadzinc veins with gold and silver values.
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
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.
Distribution of porphyry copper deposits world-wide
Granitic rocks
(Cretaceous)
Basaltic rocks
(Cenozoic)
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 coppergold porphyry deposits.
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.
Supply of Cu (Mo)
Figure 1. Vertical cross section showing a porphyry copper deposit
as it occurs deep within the earth. (Modified from Evans, 1980)
Figure 2. Geologic map showing the aerial view of a porphyry copper deposit
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.
Figure 3. Map showing a stream and sediment survey
Black dots: the place where Au has been panned and shown with gold value
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.
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.
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.
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.
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.
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).
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.
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.
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.
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
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.
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.
The Bingham Canyon open pit stretches for 2.5 miles across
the rim and is the largest manmade excavation on Earth
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."
Supplement 1
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.
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).
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).
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).
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 acidsulfate 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).
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 magmahydrothermal 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.
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 spacetime distribution of two ore-forming environments, as illustrated in
Figure 1 (A & C) and summarized in Table 2 .
major at Ely (Robinson), Nevada).
Low- to intermediate sulfidation environment (A): Early and/or
deep stages are characterized by potassic alteration and
anhydrous skarn with disseminated/veinlet chalcopyrite-bornite(magnetite) 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;
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. Acidsulfate fluids are of meteoric water (arrow 7) and/or magmaticvapor 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 silicapyrite Cu- (Au) fissures and replacement bodies (e.g., Bisbee,
Arizona; Yauricocha, Peru).
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, nonventing, 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.
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).
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 highsulfidation epithermal deposits have a magmatic fingerprint and
that some are closely linked to deeper porphyry systems
The present conceptual model that ties porphyries with highsulfidation 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
water:rock 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.
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 liquidphase 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.
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 highsulfidation deposits.
Supplement 2
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 Curich 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.
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.
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 highK, 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!
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.
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 Kfeldspar seen in the field and in thin section is indicative of
high Ba. Oxygen fugacity is extremely high. Values of ΔNNO >
+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).
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.
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.
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.