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

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-Pb-Zn 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 lead-zinc 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 copper-gold 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 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).

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.

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).

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. 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).

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.

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 high-sulfidation epithermal deposits have a magmatic fingerprint and that some are closely linked to deeper porphyry systems

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 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 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.

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.

Supplement 2

PORPHYRY COPPER MINERALISATION OF WESTERN USAAllan 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.

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 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!

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 K-feldspar 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

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