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Sierrita - Esperanza

Arizona, USA

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The Esperanza and Sierrita open pits are located in the Pima District of southern Arizona, some 12 km to the SSW of the Mission Mine Complex orebodies and 50 km WSW of Tucson (#Location: 31° 52' 18", 111° 8' 17"W).

They lie within the Arizona-New Mexico Basin and Range Province. West and Aiken (1982) quote Titley as suggesting the Sierrita Range is transected, immediately to the north-east of the orebodies, by a major structural feature, the NW-SE trending Sawmill Canyon discontinuity, which he regards as a 'fundamental crustal lineament' that has been periodically active from the Proterozoic to the Tertiary.

The total combined metal production at Sierrita and Twin Buttes in 1992 was 131 000 t of copper and 6800 t of molybdenum. The Sierrita mine exploits both oxide and secondary sulphide mineralisation, as well as primary sulphide ores. The Sierrita mill treated around 97 000 tpd of sulphide ore at a head grade of around 0.3% Cu, 0.35% Mo in 1993 (American Mines Handbook, 1994). In 2012, the Sierrita operation included a 102 000 tpd concentrator producing copper and molybdenum concentrates, a run-of-mine oxide-leaching system and a copper sulphate crystal plant (Freeport-McMoRan, 2012).

The first claims in the Sierrita mine area were recorded in 1895. Mineralisation was first worked as an underground mine in 1907.

Pre-mine stripping of the Esperanza supergene blanket deposit was commenced in 1957 and production began in 1959. The production grade from the start to the end of operations in 1977 averaged 0.5% Cu and 0.028% Mo. Pre-mine stripping at the Sierrita deposit was begun in 1968, with the first ore production in mid-1970. The ore mined between 1970 and 1977 averaged 0.39% Cu and 0.028% Mo (West & Aiken, 1982).

Sierrita was one of the mines that continued profitable operation throughout the 1980's while maintaining a head grade of around 0.3% Cu and 0.035% Mo. The Mo grade and associated Ag, which may be up to 3 g/t, enhance the economics (S Titley, pers. comm., 1994).

In December 2009, Freeport-McMoRan purchased the adjacent Twin Buttes copper mine, which ceased operations in 1994.

Geology

Mineralisation at Sierrita and Esperanza is associated with the early Eocene (57 Ma) Ruby Star Quartz-Monzonite (adamellite) Porphyry Stock, developed on the southern margin of the Palaeocene (62 to 58 Ma) Ruby Star Granodiorite. The geological setting of the Pima District is outlined in the section covering the Mission Complex orebodies above.

The main ore hosts in the mine areas are as follows, from oldest to youngest:

Ox Frame Volcanics, dated at around 220 Ma - within the mine area these volcanics are present both as andesite and rhyolite units with isolated pods and irregular masses of quartzite. The andesite is a dark grey to greenish-black with 15 to 35% of 1 to 5 mm micro-phenocrysts of albite in a groundmass of albite, actinolite and magnetite and interstitial crypto-crystalline "felsite". The andesite was an important host at Esperanza, with groundmass biotite and hornblende grains often being a good site for supergene and hypogene sulphide deposition. The rhyolite occurs as a welded tuff in the upper parts of the Ox Frame Volcanics, including vitric, fragmental, siliceous-aphanitic tuffs, rhyolite tuff breccia, rhyolite crystal tuff and quartz-latite crystal tuff. The rhyolite was not a good host as it contained no reactive minerals, and is only poorly fractured (West & Aiken, 1982).
Harris Ranch Quartz-Monzonite (adamellite), dated at 190 to 210 Ma - characterised by medium grained, purple-grey, subhedral perthitic orthoclase (35%), with oligoclase-andesine (24%), quartz (23%) and biotite rock. It outcrops in the Sierrita pit, but not in the Esperanza Mine and is not generally a good host to ore. However, in the west-central part of the Sierrita Pit where it is well broken and forms an extensive, strongly altered crackle zone, it hosts large tonnages of high grade ore (West & Aiken, 1982).
Quartz Latite Porphyry, un-dated - which intrudes the Ox Frame Volcanics. It is a light coloured porphyry with phenocrysts of perthite and albite in a groundmass of quartz, albite, orthoclase and chlorite. It was a favourable host to primary and supergene Cu and Mo mineralisation in the Esperanza pit, but due to its limited size did not contain a great tonnage (West & Aiken, 1982).
Biotite-Quartz-Diorite, dated at 67 Ma - which represents the remnants of a NW trending Cretaceous pluton that passes into the mine area. It is variably black to dark green, reflecting the groundmass of secondary biotite, amphiboles and chlorite, with minor phenocrysts of variably biotitised clusters of hornblende or hornblende after pyroxene, and plagioclase. It is a good host for hypogene Cu-Mo mineralisation, and is moderately shattered, probably associated with the quartz-monzonite intrusion. To the west it grades into a biotite-hornblende diorite (West & Aiken, 1982).
Ruby Star Granodiorite, dated at 62 to 58 Ma - this composite batholith is largely developed to the north-west of the mine area, and has a central porphyritic core surrounded by an equi-granular border phase. It largely comprises an equi-granular, light grey, medium grained, holocrystalline biotite granodiorite, with the gradational porphyritic core phase having up to 25% pink to white potash phenocrysts (West & Aiken, 1982).
Ruby Star Quartz-Monzonite (adamellite) Porphyry, dated at 57 Ma - This is taken to be a late differentiate of the granodiorite batholith, occurring as a stock with several associated plugs. The contact of the stock with the granodiorite batholith to the north is largely gradational. It appears to be intimately associated with the mineralisation and alteration at both Esperanza and Sierrita. Five principal types of this porphyry are recognised, each generally having a sharp contact with its neighbour. These comprise,
a). Aphanitic porphyry with up to 50% andesine and orthoclase phenocrysts;
b). Phaneritic porphyry with 1 to 7 mm phenocrysts of oligoclase, quartz eyes, biotite books and orthoclase;
c). Aplite which frequently occurs near the top of the system and often has gradational borders with other phases;
d). Dacite porphyry, which is dark grey in colour with up to 20% phenocrysts of zoned plagioclase and of orthoclase, and may be a late stage phase;
e). Quartz-latite porphyry, occurring as dykes cutting all rocks, including the Ruby Creek intrusives, although they are mineralised, but post-date major alteration (West & Aiken, 1982).
Breccia, dated at 57 Ma - several irregular breccias occur in the eastern Sierrita pit. These are believed to be remnants of a much larger, roughly east-west trending body. Clasts of the Ox Frame Volcanics, Harris Ranch Quartz-Monzonite, Biotite-Quartz-Diorite and Ruby Star Quartz-Monzonite Porphyry are common. The matrix is mainly fine grained biotite, silica, rock flour and minor local magnetite. Near the edge of the breccia zone, the breccia resembles a crackle zone, passing into a sub-angular mosaic breccia and to intervals where the clasts are rounded. The breccia zone is 760 m across and 180 to 300 m wide in the upper sections of the mine, but narrows with depth. Cu-Mo mineralisation is good in the upper levels, generally increasing relative to the surrounding rocks, but diminishes with depth (West & Aiken, 1982).

Mineralisation & Alteration

Hypogene sulphide mineralisation in the Sierrita and Esperanza deposits consists of pyrite, chalcopyrite, molybdenite and lesser pyrrhotite with comparatively minor amounts of galena, sphalerite, magnetite, tennantite-tetrahedrite and rare marcasite, cubanite and bornite. Within the ore zones the total sulphide content is of the order of 2 to 3% or less and seldom exceeds 4%, while the pyrite:chalcopyrite ratio is generally between 1:2 and 1:3. This ratio increases to 20:1 in the propylitic zone with 2 to 4% total sulphides (West & Aiken, 1982; S Titley, pers. comm., 1994).

The Sierrita deposit is a low total sulphide, low pyrite and low grade orebody. Alteration is only poorly developed, although the fracture density is relatively high. Several different porphyries are found in the mine. All are mineralised and fractured, as are all other rocks within the mine. It is consequently difficult to determine which (if any) of the porphyries is related to the mineralisation. The ore assays 0.32% Cu and 0.45% Mo, while the recovered grade is 0.28% Cu and 0.035% Mo. The metal levels within the surrounding rocks drops off outside of the pit to a background of around 0.15 to 0.18% Cu over large areas. The main difference between the ore grade and background mineralisation is the density of fracturing (S Titley, pers. comm., 1994).

The mineralised system occurs within a zone of three major regional trends, ENE, WNW and NNW. The ENE fabric dominates in the mine area. The main ore trends are parallel to a NW trend, although in detail a NE pattern is also discernible. The NW trend is represented by the 60 to 80° south-west dipping Sierrita Zone, and the steeply north-east dipping Amargosa Zone. These are connected by the NE trending Cross Zone in the Sierrita pit, with a barren core with <0.2% Cu equivalent. This barren core has a low fracture density, weak alteration and a low density of quartz-orthoclase veining (West & Aiken, 1982). Titley (pers. comm., 1994) has indicated however, that recent presentations by Cyprus staff has not shown the pattern of ore zones described above, implying that subsequent work and mining has not supported the interpreted pattern of the 1970's.

Fracture studies carried out by Titley, et al., (1986) have shown that the Sierrita-Esperanza hydrothermal system covers an area of 60 km2 within which fractures related to porphyry copper mineralisation are found. Fractures in the system occur on all scales, from hairline healed cracks to veins with selvages of alteration many centimetres in width. Fracturing and alteration are most intense within the pits, where volumes of rock up to many cubic metres have been pervasively altered to orthoclase, or more rarely biotite. This style of more intense fracture related alteration has not been observed in outcrop beyond known economic mineralisation.

Three main fracture types are recognised at Sierrita (Titley, et al., 1986). They comprise:

J 1, which are smooth surfaced, planar and continuous fractures. The walls of the fracture are always parallel, even where flanked by selvages of up to a centimetre in thickness. Most of the J1 fractures are vertical to sub-vertical at Sierrita, with <10% being horizontal or sub-horizontal. No rotation or transport is indicated within the sheeted volume of rock.

J 3, which is the most abundant. It comprises irregular, discontinuous hairline fractures of short length ranging down to micro-fractures. In general they are less than a few centimetres in length with associated alteration products in fillings or selvages less than 1 mm wide. This variety provide the sites for more than 60% of the copper at Sierrita.

J 2, which constitutes the balance of fracturing at Sierrita. J 1 and J 3 together account for 95% of the total fracturing. J 2 fractures are generally curvi-linear, discontinuous joints which have rough surfaces and are not altered. As they are un-altered and generally displace the other two fracture types they are regarded as being post-mineralisation. Below the depth of weathering in the orebodies they are commonly filled with a thin layer of gypsum.

Titley, et al., (1986) conclude that the evolution of the 60 sq. km fracture system, and the 43.5 sq. km of the main ore-related fracture set at Sierrita is centred on the quartz-monzonite porphyry bodies which are restricted to an exposed area of 1.3 sq. km within the open pits. Overall the fracture densities increase towards the quartz-monzonite porphyry stocks within the Sierrita pit. In addition the area of fracturing collapses (decreases) towards the porphyry stocks with time (with each fracture set), while fluid inclusions indicate that the temperature of formation also decreases with each repeated pulse of veining. In most of the area studied fractures showed a preference for an east-north-east trend, as is common in the porphyry deposits of Arizona.

Within the orebody area distinctive fracture types, as recognised by fracture style and alteration assemblages, occur in a consistent paragenetic succession. This fracturing is episodic. It was also noted that younger vein sets may cross older sets without offset or deviation. The same succession of veins/fractures has been recognised in the areas peripheral to the pits with variations for each alteration type as influenced by the wall rock mineralogies. No open space fracture filling is indicated (Titley, et al., 1986).

Fracture density maps of the nine different sets recognised the following alteration types and distribution of fractures:
Set 1 - centred to the west of the pit and probably pre-ore in age within the Harris Ranch Quartz-Monzonite. Alteration is epidote-quartz with some tourmaline and orthoclase-epidote selvages.
Set 2 - characterised by quartz-epidote-sulphide assemblages is also restricted to the area to the west of set 1 and of the pit, and is also believed to be prior to the introduction of the main quartz-monzonite porphyries.
Set 3 - is centred on the porphyries and comprises veins of quartz and orthoclase. This set is widespread and overprints both of the previous sets. It may represent an early alteration and fracturing front accompanying the earliest stages of emplacement of the porphyries.
Set 4 - this set is more closely centred on the exposed porphyry stocks and is characterised by quartz-sulphide fillings. This ore-related stage of fracturing is the most intense and persists for several kilometres away from the orebodies.
Set 5 - is the latest which is consistently recognised as being distinctly related to the hypogene porphyry ore deposits. It over-prints sections of the set 3 and 4 fractures and is characterised by quartz-epidote-chlorite-sulphide within the Harris Ranch Quartz-Monzonite.
Set 6 - has widely spaced quartz-sericite alteration and associated pyrite and orthoclase in different rock types within the pit.
Set 7 - is mainly quartz-chlorite and/or epidote.
Set 8 - has late stage calcite fillings, while
Set 9 - are gypsum lined within the pit. In summary this succession represents a sequence of vein alteration, commencing with quartz-orthoclase alteration, passing through precipitation of quartz and sulphides, and ending with the formation of epidote-orthoclase-chlorite (Titley, et al., 1986).

The mineralogy of the individual Hydrothermal alteration types is variable, dependant upon the composition of the particular hosts. These patterns and the succession of alteration listed below from West & Aiken, (1982) do not precisely coincide with those of Titley, et al., (1986) as described above.

The low total sulphide ores at Sierrita are associated with a complex of alternating and over-printing potassic and propylitic alteration assemblages in which biotite, orthoclase, epidote and chlorite, together with quartz, iron oxides, and copper and molybdenum sulphides are present. The hydrothermal system is anomalous in some respects in comparison to other centres of porphyry copper alteration in Arizona in the lack of profound and overwhelming fracture related and pervasive quartz-sericite-pyrite alteration which is texturally destructive. The absence of this alteration has been important in two respects, namely 1). the older potassic and propylitic alteration suites have been preserved, and the original mineralisation is preserved and 2). the lack of strong pyrite development and the associated destructive weathering has meant that supergene enrichment has not been well represented (Titley, et al., 1986).

The following stages and types of alteration are recognised by West & Aiken, (1982),

Potassic - Early Stage, which was both the first phase of hydrothermal alteration, and the main mineralising event. It developed widespread potassic alteration, principally in the form of quartz-orthoclase-chalcopyrite-pyrite-molybdenite-(pyrrhotite) vein-selvage assemblages, with both groundmass and veinlet biotitisation. Chlorite, epidote, sericite, albite, calcite and purple anhydrite occur as prominent accessory minerals with this veining, and magnetite occurs with biotite. The occurrence of epidote with quartz-orthoclase is rare in Arizona, reflecting the interaction of the local host rocks. Variable degrees of alteration and of alteration products are common, reflecting the irregular penetration of a wide range of mixed composition hosts that have been strongly fractured. This phase was formed at 400 to 350°C.
Phyllic - Middle Stage, characterised by quartz veins with accompanying sulphides, usually chalcopyrite-pyrite±molybdenite, typically sheathed in a quartz-sericite envelope. The alteration was apparently formed at between 300 and 190°C, with the sulphides being emplaced at the lower end of the range. This alteration, as is the next listed, is far less intensely developed than is usual for phyllic stage alteration in porphyry copper deposits.
Pervasive "Green" Phyllic Alteration, which is a late stage phyllic alteration developed in the Ruby Star Quartz Monzonite Porphyry and to a lesser degree in the Harris Ranch Quartz Monzonite. This style is found in the heart of the deposit and persists to the margins and to depth. This is a low temperature alteration formed at around 200°C.
Propylitic Stage, represented by epidote veining, commonly associated with granular pyrite, calcite, quartz and chlorite. It is widespread in the upper sections of the mine, particularly in the Harris Ranch Quartz Monzonite and Quartz Diorite. A regional epidote-pyrite aureole extends eastward towards the Twin Buttes deposit. Epidote±pyrite, calcite, chlorite and quartz penetrated down into the potassic zone at depth in the orebodies, being poorly developed in some quartz-monzonitic rocks, but common in mafic lithologies.
Late Stage, hydrothermal veining with diverse vein fillings, including quartz-sphalerite-galena, molybdenite paint and gypsum veins, often with stibnite.
Crackle Zone Alteration, developed mainly in the Harris Ranch and Ruby Star Quartz Monzonites, with chlorite, sericite and abundant calcite, lesser montmorillonite and hematite accompanying residual rock orthoclase and quartz.

Despite the alteration types described above, the overall alteration at Sierrita is weak, being predominantly reflected in the development of biotite and chlorite. The biotite-diorite of the pit which has 90% of the mafics as biotite, apparently grades outwards into a hornblende-biotite diorite to the west. An apparently characteristic low grade ore pile observed during the traverse was made up of weakly altered quartz-monzonite 'porphyry' with disseminated and vein sulphides. The porphyry was more properly described as a porphyritic rock, as it was made up of medium grained crystals of plagioclase, orthoclase, quartz and chloritised biotite set in an aplitic matrix which accounted for about 10% of the rock. The biotite, which is apparently secondary, could be seen to have been locally chloritised under high magnification while the feldspars were largely un-altered. The ore was cut by 2 to 5 mm quartz veins with little sulphide, and by thin sulphide veinlets with little quartz. The latter sulphide veinlets cut the quartz veins. In samples sighted the quartz veins were only mineralised where cut by the younger sulphide veinlets. Molybdenite was developed on the fracture surfaces. Selvages were only minimally developed adjacent to the sulphide veining (S Titley, pers. comm., 1994; Pers. observ.).

The ore and surrounding mineralised country rocks have a high magnetic susceptibility, around 1 to 2000 SI units, strong enough to attract a hand magnet. Due to the low level of alteration and pyrite development, the magnetite common to the Laramide intrusives is not substantially destroyed. It is possible that the main biotite/K-silicate alteration of the system actually enhances the magnetite content (S Titley, pers. comm., 1994).

In the upper western bench of the Sierrita pit where the grade is of the order of 0.2% Cu, there is well developed fracturing with goethite coatings and smeared out chlorite. Sparse neotocite occurs as sporadic, irregular coatings on the fractures (Pers. observ.).

Some 1 km from the edge, and 2.5 km from the centre of the pit, oxidised outcrop of low grade outer mineralisation was observed. The significance of the outcrop that would attract interest was the presence of slight iron staining on the well developed fracturing and the presence of an altered selvage to those iron stained fractures. The fracturing was in two principal directions, namely 80° (or ENE) and an associated orthogonal set. The main trend had a fracture density of one fracture approximately every 5 to 50 mm. There were two types of selvage developed, one that was resistant (quartz-orthoclase-sulphide and quartz-sulphide) and the other recessive (orthoclase-sulphide). Occasional float samples in the creek contained patches of malachite staining from 5 to 10 mm across on one or two faces of a 200 x 100 mm floater. The density of fracturing in the area was also readily reflected in the size of the rubble fragments on the hillside (S Titley, pers. comm., 1994; Pers. observ.).

In areas of increased fracturing, where the density is from 0.25 to 0.4 per cm, the sulphide content increases from the background of <2% to the 2% more characteristic of the ore. In such area, after a rain pools are rimmed by alum formed from the breakdown of feldspars by pyrite. In these zones of increased fracture density the magnetic susceptibility decreases, predominantly expressed as a lowering over the vein selvages to 0 to 200 SI units, rather than between the veins (S Titley, pers. comm., 1994).

The mineralisation observed on the upper bench and during the traverse represented the oxidised, but not leached cap to the low grade periphery of the Sierrita deposit. The mineralisation is not leached due to the low sulphide and low pyrite content of the original mineralisation (S Titley, pers. comm., 1994).

The supergene enrichment blanket at Esperanza varied from 9 to 45 m in thickness, but averaged around 30 m, and was roughly conformable with the pre-mine topography. It comprised 47.5 Mt @ 0.62% Cu and 0.025% Mo. A well developed set of east-west joints dipping at 45 to 50°N were an important localiser of enrichment. In the capping the extent of sulphide decomposition locally ranged from light oxidation to total disintegration. Replacement of pyrite and chalcopyrite by chalcocite in the enriched zone was generally incomplete, with the majority of chalcocite ore showing remnant pyrite centres. Covellite is a common mineral in both the oxide and supergene zones where it coats pyrite, chalcopyrite and chalcocite. Mafic units, particularly the andesite, were highly receptive to the deposition of chalcocite, because it contained more primary sulphides, was well fractured and contained reactive minerals such as calcite (West & Aiken, 1982).

The leached capping at Esperanza was also variable, ranging from 3 to 55 m in thickness. Some rocks have been more susceptible to leaching, such as the Ox Frame Rhyolites, while others have been more receptive, such as the Ox Frame Andesites, with much of the limonite on joint surfaces being transported. Leaching and enrichment apparently took place in two stages, and resulted in a factor of upgrading of 1.5 to 2 overall, but locally achieved a 5-fold increase. The primary sulphide content of the Esperanza ore was 3 to 4%, with a pyrite:chalcopyrite ratio of 1:1 to 2:1, which is low and inhibited acid formation. The main iron oxides over the chalcocite orebody were reddish-orange-brown in volcanics and orange-brown in quartz-monzonite. Hematite was the dominant iron oxide, followed by goethite and jarosite. Some indigenous relief limonite after chalcocite and covellite were reported (West & Aiken, 1982).

Only a weak oxidised cap ranging from 0 to 23 m, averaging 15 m, developed over the Sierrita orebody, with azurite, malachite, tenorite and melaconite, localised in highly shattered rocks and fault zones. Little or no supergene enrichment was reported. Ferrimolybdite and primary sulphides were observed in outcrop.

Published production and reserve figures for the Sierrita and Esperanza orebodies include the following:

Sierrita - Production, 1968-78 - 200 Mt @ 0.27% Cu, 0.022% Mo, 1.03 g/t Ag (Titley, 1989).
      Reserve, 1978 - 387 Mt @ 0.30% Cu, 0.035% Mo (USBM).
      Reserve, Reserve, 1989 - 510 Mt @ 0.34% Cu (Titley, 1992).
Esperanza - Production, 1959-78 - 86 Mt @ 0.4% Cu, 0.021% Mo (Titley et al., 1989).
      Reserve, 1982 - 55 Mt @ 0.27% Cu, 0.033% Mo (West & Aiken, 1982).
      Reserve, 1978 - 26 Mt @ 0.38% Cu (Titley, 1982).
      Reserve, 1989 - 43 Mt @ 0.27% Cu, 0.034% Mo (Titley, 1992).
Sierrita - Twin Buttes
      Probable reserve, Dec 2002 - 1.067 Gt @ 0.27% Cu, 0.03% Mo (Infomine, website, 2006).
Sierrita - Remaining proved + probable reserves - at December 31, 2011 (Freeport-McMoRan, 2012):
    mill ore - 2766 Mt @ 0.23% Cu, 0.025% Mo, 1.39 g/t Ag (Cu - 83.0%; Mo - 80.7%; Ag - 49.3% recovery);
    ROM leach ore - 11 Mt @ 0.12% Cu (52.7% recovery).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1998.     Record last updated: 26/8/2013
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Haynes F M, Titley S R  1980 - The evolution of fracture-related permeability within the Ruby Star granodiorite, Sierrita porphyry copper deposit, Pima County, Arizona: in    Econ. Geol.   v75 pp 673-683
Lynch D W  1966 - The economic geology of the Esperanza Mine and vicinity: in Titley S R, Hicks C L 1966 Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson    pp 267-279
Preece R K, Beane R E  1982 - Contrasting evolutions of hydrothermal alteration in quartz monzonite and quartz diorite wall rocks at the Sierrita porphyry copper deposit, Arizona: in    Econ. Geol.   v77 pp1621-1641
Stavast W J A, Butler R F, Seedorff E, Barton M D and Ferguson C A,  2008 - Tertiary Tilting and Dismemberment of the Laramide Arc and Related Hydrothermal Systems, Sierrita Mountains, Arizona: in    Econ. Geol.   v.103 pp. 629-636
Titley S R, Thompson R C, Haynes F M, Manske S L, Robison L C, White J L  1986 - Evolution of fractures and alteration in the Sierrita-Esperanza hydrothermal system, Pima County, Arizona: in    Econ. Geol.   v81 pp 343-370
West R J, Aiken D M  1982 - Geology of the Sierrita-Esperanza deposit, Pima mining district, Pima County, Arizona: in Titley S R 1983 Advances in Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson    pp 433-465


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