magmatic processes in the development of porphyry-type ore systems

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IntroductionPORPHYRY and related deposits of copper, tungsten, tin,molybdenum, gold, and silver are associated spatially andtemporally with granitic (s.l.) rocks (Seedorff et al., 2005).These igneous rocks are, generally, low- to high-K, metalumi-nous to peraluminous arc magmas, and range from diorite totrue granite in composition. These associations lead to the hy-pothesis that intermediate to felsic magmas may be thesource for much or all of the ore material in granite-relateddeposits. Indeed, many studies have shown that the chemical,isotopic, and thermodynamic characteristics of these “granite-related” ores are consistent with this hypothesis and that or-thomagmatic-hydrothermal processes have played a centralrole in ore genesis. Therefore, it is reasonable to propose thatthe genesis of magmatic-hydrothermal ore deposits, such asthe porphyry class of ores, is a by-product of the irreversibletransfer of heat, by magma flow, from Earth’s interior towardits surface.

Many factors affect magmatic-hydrothermal ore genesis,including magma composition, magmatic oxidation and sulfi-dation state (for a discussion of the sulfidation state of por-phyry and related ores, see Einaudi et al., 2003), the relative

timing of crystallization, magma ascent, magmatic volatile-phase exsolution, magma-chamber geometry, local hydrologi-cal and rock mechanical properties, and the depth of magmaemplacement (see Cerny et al., 2005, for more discussion).Some relatively shallow granitic bodies (i.e., those emplacedat depths of ca. 8 km or less) represent the crystallized re-mains of magmas that were related to subvolcanic, ore-gener-ative, hydrothermal systems. Further, these shallow plutonscan have associated volcanic rocks. Studies of the chemicaland isotopic composition, texture, and field relations of theseigneous rocks can elucidate the processes that converge togenerate porphyry, skarn, epithermal and related high-tem-perature veins, and massive sulfide deposits (Franklin et al.,2005; Meinert et al., 2005; Seedorff et al., 2005; Simmons etal., 2005).

Over 65 percent of all copper mined comes from depositsassociated with igneous rocks (i.e., porphyry, skarn, replace-ment, and massive sulfide deposits), with just over half com-ing from porphyry-type ores (Singer, 1995). Singer also calcu-lated that ~20 percent of mined gold comes fromigneous-related systems, and about one-third of igneous-re-lated gold comes from epithermal deposits. Molybdenum ismore strongly associated with igneous rocks than is copper,with over 99 percent of the world’s molybdenum coming fromporphyry-type deposits (Kirkham and Sinclair, 1996).

Magmatic Processes in the Development of Porphyry-Type Ore Systems

PHILIP A. CANDELA† AND PHILIP M. PICCOLI

Laboratory for Mineral Deposits Research, Department of Geology, University of Maryland, College Park, Maryland 20742

AbstractThe close spatial and temporal association between intermediate to felsic igneous intrusions and large ton-

nage, low-grade porphyry-type mineral deposits in arc environments is consistent with the hypothesis that mag-mas were the dominant source of the ore metals. In this paper, we review some aspects of the origin and em-placement of porphyry ore-related magmas, the controls on the magmatic concentration of ore metals, water,chlorine, sulfur, and related elements, and the factors that affect the partitioning of metals in magmatic-hy-drothermal systems.

The intermediate to felsic igneous rocks associated with ore are the end product of magmatic evolution thatbegins with the generation of mantle-derived arc magmas. High-alumina basalt is generated by the complex in-teraction of fluids released from subducting slabs and the overlying mantle wedge. Fluids released early fromthe slab may be higher in chlorine, as well as in related volatile and fluid-soluble elements. Fugacities of oxy-gen and sulfurous gases in arc magma systems may be controlled in part by sulfate-oxide-sulfide assemblagesin the subducting plate. Water, chlorine, and sulfur may be sourced partially from seawater by way of sub-ducted oceanic lithosphere. Ore metals in arc magmas probably have diverse origins, including the mantlewedge, the lower continental crust, and the subducted lithosphere.

Dilational tectonic features may accommodate some high-level plutons, as well as their associated cupolasand apophyses. The large-scale through-going fractures that host these local zones of dilation can extend tolower crustal depths and thereby facilitate the movement of magma from depth. The structures that representzones of crustal weakness below the magma chamber and that promote magma ascent also provide regions ofweakness above the chamber and promote the formation of cupolas, apophyses, and zones of high vein den-sity. During the growth of plutonic complexes, active magma chambers may be smaller than the developingplutonic complex at any given time. Significant devolatilization may occur upon magma rise, with ore zones lo-cated above the root-feeder zone of the chamber.

Volatile-melt-crystal interactions are important at all structural levels in the crust, and may be quite impor-tant in controlling not only the water, sulfur, and chlorine concentrations but also the metal concentrations inthe epizonal magmas that generate porphyry ore deposits. Generally, the formation of magmatic-hydrothermaldeposits of chalcophile metals is favored by magmatic characteristics such as high Cl/H2O ratio, and earlyvolatile exsolution relative to crystallization progress. The oxidation state of the magma is probably importantin producing variations in ore-metal ratios in magmatic-hydrothermal ore deposits.

† Corresponding author: e-mail, [email protected]

©2005 Society of Economic Geologists, Inc.Economic Geology 100th Anniversary Volumepp. 25–37

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Porphyry deposits are the archetype of granite (s.l.)-relateddeposits. They exhibit a strong spatial and temporal relation-ship between shallow, porphyritic or variably-textured inter-mediate to felsic igneous rocks, and usually steep-walled,crudely cylindrical to bell-shaped sulfide-rich orebodies (See-dorff et al., 2005). The high-level igneous stocks associatedwith porphyry and related deposits typically form the roofzones of shallow plutons where vertically elongated, high-as-pect ratio cupolas (Sutherland-Brown, 1976) are the locus ofupward surges of magma and associated magmatic volatiles.Porphyry copper ore occurs as disseminations or stockworkveins yielding grades of copper on the order of 0.4 to 1 wtpercent, with subordinate molybdenum and gold. Porphyrymolybdenum deposits have grades of a few tenths of a per-cent MoS2, and porphyry gold deposits contain on the orderof 1 ppm gold. Characteristic alteration styles represent thecombined effects of cooling of the aqueous ore-formingphase and the mixing of magmatic and meteoric waters(Hedenquist and Richards, 1998).

In this paper, we describe some of the characteristics ofporphyry ore systems that yield critical information on theformation and development of magmatic-hydrothermal oredeposits; we will not focus on issues that have been describedelsewhere in the literature, and this volume, such as detaileddescriptions of ore zones, alteration, or the mineralogy or pet-rography of the associated igneous rocks. Rather, we focus thearticle around a series of relevant issues that arise naturallyfrom the first-order observations of these deposits, includingthe following: their occurrence in arc environments; their as-sociation with shallow, generally porphyritic, oxidized, inter-mediate-to-felsic intrusions; the importance of cupolas andrelated structures; the sulfur-rich nature of the ore; the dom-inant role of fracturing in magma emplacement and ore lo-calization; the saline nature of the ore-forming fluids; and thedistinctive suite of elements commonly found in the deposits.

Subduction: Stoking the Magmatic HearthMany hydrothermal ore deposits are associated closely with

volcano-plutonic complexes in present-day convergent platemargins or their older equivalents. Given the close relation-ship between magmatism and subduction in arc systems, weexamine the varying roles that subduction may play in oregenesis. Subduction is a flux of serpentinized mantle, alteredbasalt and gabbro, ocean water, and sediment into the man-tle. A portion of this mantle-directed flux, in the form ofbuoyant, slab-derived fluid, is released into the overlyingmantle wedge where hydrous arc magmas are subsequentlyformed.

Here we will explore selected issues related to magmas andsubduction, in order to elucidate how subduction affects oregenesis. Previous reviews on this subject include Sillitoe(1972, 1987) and Richards (2003). As a first step, we discussthe generation of arc magmas. High-alumina basalt, charac-terized by Al2O3 concentrations greater than 16 wt percent, isthe preferred parent of arc magma systems (e.g., Berndt etal., 2005). Basaltic magmas begin crystallizing Fe-Ti oxidesearly at high water contents and high oxygen fugacities(Berndt et al., 2005). Arc magmas contain up to 6 wt percentH2O, compared to generally <0.4 wt percent H2O for mid-ocean-ridge basalts (MORB) and <1.0 wt percent H2O for hot

spot tholeiites such as Kilauea, United States (Johnson et al.,1994). Associated lavas can include the full range of the basalt-andesite-dacite-rhyolite compositional range. Questions ariseas to the extent to which the more felsic components of thesuite owe their origin to fractionation and/or contamination ofhigh-alumina basalt by, for example, assimilation-fractionalcrystallization processes (DePaolo, 1981). Partial melting of amixture of lower crust and previously crystallized basalt mayyield granulite + dacitic magma, with the dacite fractionatingor being contaminated by more primitive magmas to yield thefull range of arc magmas (Hildreth and Moorbath, 1988).Richards (2003) provides a detailed discussion of theprocesses involved in arc magma genesis with specific refer-ence to the origin of porphyry-type deposits.

Most models for the generation of arc basalts suggest that amobile, high-temperature aqueous phase (>50 wt % water) isreleased from the subducting plate over a range of appropri-ate pressures and temperatures (Wyllie, 1979; Grove et al.,2002; see Fig. 1). According to this paradigm, the fluids orfluid mixtures rise into the overlying asthenosphere by virtueof their buoyancy, and react with and metasomatize the man-tle, acting as a flux to cause melting of the wedge, thus gen-erating a volcanic arc as its surface expression. The volcanicarc forms on the surface between 90 and 150 km above thedevolatilizing slab. The width of volcanic arcs is typically ≤100km (d’Ars et al., 1995). According to Grove et al. (2002), themajor elements in a slab-derived fluid are H2O (~55–68 wt%), Na2O (~25–33 wt %), and K2O (~5–13 wt %). Further-more, Grove et al. (2002) suggest that this composition mayrepresent a mixture of fluids (a higher density “melt”-likefluid and a lower density aqueous fluid) or a supercriticalequivalent derived by dehydration of the descending andheating slab. Given the salinities suggested by Kent et al.(2002), this slab-derived fluid possesses Cl/H2O ratios on thesame order as the salinity of seawater (1/50). Melting of theasthenospheric mantle wedge probably involves the interac-tion of slab-derived fluid, depleted mantle, and possibly meltfrom subducted sediments (McDade et al., 2003). Thesecomponents combine to produce a modified wedge peridotitethat can melt to yield mafic arc magma. In summary, complex,alkali- and chloride-bearing fluids with a significant silicatecomponent are released over a rather wide interval from thesubducting plate, acting as a flux to cause melting of the man-tle wedge, and yielding arc basalt.

Whence the Ore Substance?Ore substance is used here in a broad sense to mean ore

metals, ligands, and other components that either constitutethe target ore-metal anomaly or are ultimately responsible fordelivering these constituents via an aqueous solution to themineralized volume of rock. The magmatic inventory of oremetals and ligands may have widely diverse origins. Becausethe inventory of any given element in any igneous rock massmight come, in part, from the asthenospheric or lithosphericmantle, the lower or upper crust, or a subducted plate, oresubstances can also be expected to originate from a mixedparentage. A wide variety of isotopic and trace- and major-element data suggest that both the slab and the mantle wedgecontribute materially to primary arc magmas, and thereforeore metals can be expected to come from both sources (Noll

26 CANDELA AND PICCOLI

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et al., 1996; McInnes et al., 1999). Furthermore, the lowercrust can also be an important reservoir for the constituentsof arc magmas, especially in continental settings (Hildrethand Moorbath, 1988; Barra et al., 2002). Ore substance cancome from any of these sources, and it is probably best tothink of the ore metal inventory of magma as derived fromsome mixture of these deeper reservoirs.

The slab reservoir comprises the subducting oceanic lithos-phere. Hydrothermal alteration on the sea floor before sub-duction alters the composition of oceanic crust, changing itsoxidation state, and adding sulfur and other elements. Philip-pot et al. (1998) suggested that sea-floor alteration by oceanwater is water-dominated at the shallow levels in the oceaniccrust that comprise relatively permeable basalt and sheeteddikes, and is rock-dominated at deeper structural levels thatcomprise gabbro. Kerrick and Connolly (2001) also point outthat alteration is most intense in the upper kilometer of the

ocean crust. A significant proportion of sheared and perme-able ultramafic rocks is also present (Stern, 2002). During al-teration, water, chlorine, sulfur, and boron from seawater areadded (in addition to sodium and potassium at the expense ofmagnesium and calcium) to the oceanic crust (Noll et al.,1996). Copper, lead, zinc, manganese, cobalt, arsenic, anti-mony, thallium, selenium, tellurium, cadmium, gold, silver,mercury, and bismuth are among the ore elements of interestthat are probably redistributed by hydrothermal processes,with many of these elements ultimately residing in sulfides inthe upper levels of the altered oceanic crust. This complexpackage of altered ocean lithosphere forms the raw materialfor the reactions that later occur in subduction zones.

Chlorine

Chlorine is stored in the oceanic crust in silicates, apatite,chlorides, and as pore and inclusion fluids. Although fluid

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FIG. 1. Possible scenarios for the subduction of young, warm lithosphere (A), and old, cold lithosphere (B) (after Leemanet al., 1994). Volatiles are released over a range of temperatures. Cooler fluids result in serpentinization of the mantle wedge.The magmatic front lies 124 ± 38 km above the inclined seismic zone, and this relationship does not vary systematically withany subduction variable, such as convergent rate, or age of lithosphere being subducted (Stern, 2002). See text for details.

inclusions in eclogites can be quite saline, indicating thatchlorine can be carried to great depths in subduction zones(Kent et al., 2002), one might hypothesize that early fluids re-leased from the down-going slab are enriched in chlorine rel-ative to later subduction-derived fluids. To examine this pos-sibility, Walker et al. (2003) analyzed olivine-hosted meltinclusions in volcanic rocks from across the width of the Cen-tral American volcanic arc. We have calculated Cl/H2O ratiosfrom their data, and plotted the results versus distance be-hind the volcanic front (Fig. 2). These data suggest that theCl/H2O ratio decreases with distance behind the volcanicfront, from 0.1 within 20 km of the front to 0.025 at 80 km be-hind the front. Kent et al. (2002) demonstrated that theCl/H2O ratio can be relatively high in primitive back-arc basinbasalts that are erupted proximal to the arc front, with theratio decreasing rapidly with increasing distance from the arcfront. For basalts of the Lau basin, Scotia Sea, and MarianaTrough, Kent et al. (2002) reported Cl/H2O mass ratios thatvary from 0.12 less than 200 km from the arc front, to valuesnear 0.01 at locations in excess of 400 km from the arc front.These values span the range of Cl/H2O ratios commonly re-ported for arc magmas (Candela and Piccoli, 1995). Kent etal. (2002) also concluded that the slab fluid component has ahigher Cl/H2O ratio than that expected from simple meltingof MORB-like mantle sources. Sisson and Bronto (1998) re-port Cl/H2O as high as 0.3 in undegassed frontal arc basaltsfrom Galunggung, Indonesia, supporting the notion that chlo-rine is distilled early from the subducting slab.

Piccoli and Candela (1994) estimated model liquidus chlo-rine concentrations for the idealized magma for each unit ofthe zoned Tuolumne Intrusive Suite of the Sierra Nevadabatholith, United States. The granitic rocks of this suite rangefrom the granodiorite-diorite of Kuna Crest (~59 wt % SiO2)to the Johnson Granite (s.s.) Porphyry (~74 wt % SiO2) (Bate-man et al., 1983). The more melanocratic units, which alsohave lower bulk-rock SiO2 concentrations, have higher modelinitial chlorine concentrations, with values in the Kuna Crest

granodiorite of ~400 ppm, and ~50 ppm in the Johnson Gran-ite Porphyry. The inferred initial chlorine concentration ofthe magmas correlates inversely with initial whole-rock87Sr/86Sr. This result is consistent with the hypothesis thatchlorine was added to the crust via arc magmas, ultimatelyfrom a subcrustal fluid component derived from a subductedslab. This study is consistent with the hypothesis that chlo-rine, an important complexing agent for many metals includ-ing copper and gold, may attain higher concentrations inmagmas that have higher ratios of subcrustal versus continen-tal crustal components.

Taken together, these studies suggest that chlorine is con-tributed significantly from subcontinental sources, and thatthe Cl/H2O ratio is a complex function of seawater composi-tion, sea-floor alteration, and dewatering of the subductingoceanic lithosphere, as well as interactions that occur in thelower crust.

Pathfinder elements

Elements such as boron, lead, arsenic, and antimony, aswell as strontium, the large ion lithophiles, and other ele-ments, appear to be mobilized efficiently from the slab. Vol-canic arc basalts are enriched in boron, and their B/Zr ratiosare generally 10 to 100 times higher than in intraplate basalts(Leeman and Carr, 1995), which can be attributed to additionof boron to arc magma source regions by slab-derived fluids.In an analysis of representative transitional tholeiitic to calc-alkaline lavas from seven different arcs, including rocks rang-ing from subalkalic basalts to dacites, Noll et al. (1996) pre-sented data consistent with the efficient mobilization of lead,arsenic, antimony, and possibly thallium into arc magmasource regions by hydrothermal transport from the slab,based on the similarity of their behavior to boron. Conversely,tin, molybdenum, and tungsten show little such co-enrich-ment. As evidence that slab-derived fluid mobility, ratherthan crystal fractionation or a related process, is responsiblefor the zonation of the volatile elements they observed, Nollet al. (1996) pointed out that arsenic, antimony, lead, andboron concentrations decrease dramatically relative to thelight rare earth elements (LREE) with distance from the vol-canic front toward the back-arc basin in Japan and theKuriles, mirroring the behavior of chlorine. They explainedthe decrease in these elements across the arc by the decreas-ing fluid flux as the slab progressively dewaters with depth,converting to eclogite, and creating compositional gradientsin the mantle wedge related to early distillation of more fluid-compatible components such as arsenic and antimony, whichthen reside in the wedge (for a discussion, see Hattori andGuillot, 2003). Because bismuth tends to follow antimony, atleast in near-surface hydrothermal settings, bismuth may alsofollow this trend. Hattori et al. (2002) summarized studiessuggesting that the relative preferential removal of thechalcogen elements from the mantle follows the sequence, S> Se > Te. Hattori et al. concluded that partial melting can re-sult in the fractionation of sulfur, selenium, and tellurium inthe mantle and in progressively derived arc magmas. Theseconsiderations have implications for the modification of ele-mental ratios in ore magmas as a function of the amount ofslab devolatilization that occurs prior to fluxing of melting inthe hotter parts of the mantle. Clearly, devolatilization occurs


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FIG. 2. Plot of Cl/H2O (wt % basis) in melt inclusions across the CentralAmerican subduction zone in Guatemala. Data are from table 1 of Walker etal. (2003). Values are averages of the melt inclusion populations analyzed ateach distance behind the volcanic front (measured orthogonal to the front).The error bar represents the 1σ standard deviation of the mean.

over a range of depths (see fig. 4 of Hattori and Guillot,2003), and beneath mantle of various temperatures. If a sig-nificant amount of devolatilization occurs from a hot plate be-neath cold mantle, the more volatile elements, including ar-senic and antimony, may be added to cooler mantle byRayleigh distillation from the slab, and may not be directly in-volved in magma generation (Leeman, 2001). The extent towhich sediments participate in devolatilization at the depthsappropriate for arc magma generation may also be a majorfactor in determining the volatile element inventory of an arcmagma system (Rupke et al., 2002). Arsenic concentrationscan command penalties in ore concentrates from porphyrycopper deposits. Are porphyry copper deposits related tomagmas formed from early slab fluid release higher in arsenicthan deposits related to magmas that formed from later slab-derived fluids?

Of course, other variables can come into play. Boron can bestored in the lower crust or the mantle wedge as tourmaline,and may be mobilized during later events. Also, boron can bemobilized by the melting of sediments in the crust, unrelatedto position in the arc. Bismuth and sulfur may be stored in thelower crust or the mantle wedge, whereas chlorine may havea much lower residence time in the lower crust. Elementsstored in the mantle wedge may be taken to greater depths,being released later to feed magmatism that is realized land-ward toward the cratonic margins (Thompson et al., 1999).Indeed, Hattori and Guillot (2003) suggest that the break-down of serpentinites, which can act as a sink for water andfluid-soluble elements released from the underlying slab inthe mantle wedge, can lead to arc magmatism at volcanicfronts. The downward movement of the serpentinite layer bymantle flow transports these fluid-soluble elements todeeper, hotter levels in the mantle.

Metals

Isotopic and melt inclusion studies suggest that ore met-als in porphyry copper systems are sourced from a numberof different reservoirs. Based on melt inclusion data fromvolcanic rocks from Galunggung volcano, Indonesia, deHoog et al. (2001) suggested that arc magmas are slightlyenriched in copper relative to MORB, perhaps due to theenrichment of the subarc mantle by slab-derived copper.They also inferred that the primary Galunggung melts con-tained on the order of 290 ppm Ni, 60 ppm Co, 190 ppmCu, and 3 ppm Pb. Lead could have been added by the slab-derived fluids, whereas cobalt and nickel concentrationswere largely controlled by the pre-subduction mantle. How-ever, it is not at all clear that these processes are significantin affecting ore genesis in arcs. Isotopic data from El Te-niente, located in the central Chilean Andes and the world’slargest porphyry copper deposit, tell a different story. There,mineralization occurs with magmatic-hydrothermal biotite,anhydrite, and tourmaline breccias hosted in Miocene maficintrusive rocks. Skewes et al. (2002) presented lead and os-mium isotopic data and suggested copper was derived frommantle-derived magmas such as tholeiitic and calc-alkalinebasalts and basaltic andesites. According to their model, thedevelopment of ore at El Teniente involved volatile-richmantle-derived mafic magmas mixing within an open-sys-tem magma chamber. A different story is suggested based

on isotopes of minerals in the Bagdad porphyry Cu-Mo de-posit, United States. Barra et al. (2002) examined the iso-topic composition of rhenium and osmium in sulfide miner-als using osmium as a tracer for the source of ore metals inthe ore system. Molybdenite, chalcopyrite, and pyrite frompotassically altered quartz monzonite and porphyritic quartzmonzonite were analyzed. Their results suggest that a sig-nificant portion of the metals and magmas have a crustalsource, as has been suggested for other copper deposits anddistricts in Arizona. These two studies suggest that metalsand other elements in porphyry deposits may have diverseorigins, with crustal sources perhaps important in thesmaller deposits. However, it should be noted that it is diffi-cult to infer confidently the origin of ore metals based onthe behavior of isotopes of another element.

Tin, molybdenum, and tungsten behave more like titanium,niobium, and tantalum in their arc systematics, suggestingthey are associated with dominantly crust derived igneousrocks. The ratio of these lithophile elements to the morevolatile chlorine, boron, arsenic, antimony and bismuth, etc.,and by extension, to the chloride-complexed metals such ascopper, may be determined in part by the relative importanceof subduction fluids in the generation of the host magmas.However, higher slab contributions to ore-generative magmasmay be indicated by higher concentrations of boron, arsenic,bismuth, and antimony in granites and their associated ores.If chlorine correlates with these elements, then we might alsoexpect higher concentrations of Cl-complexed metals such ascopper. Generally, however, metals and source regions areprobably not limiting factors in the development of porphyry-type ores.

Sulfur (and magma oxidation state)

Sulfur in naturally occurring silicate melts can exist as bothreduced (sulfide) and oxidized (sulfate) species. Carroll andRutherford (1988) showed that the sulfate/sulfide ratio in sil-icate melts generally increases with increasing temperature,pressure, and oxygen fugacity. Sulfur concentrations arelower in melts in lower fO2 systems saturated with sulfides(e.g., pyrrhotite at fO2 values less than two orders of magni-tude above the quartz-fayalite-magnetite buffer, referred toas <QFM + 2), compared to higher fO2 systems where sulfateminerals are stable (Jugo et al., 2005). Further, the volatile-magma partitioning of sulfur will be enhanced at higher oxy-gen fugacity for pyrrhotite-saturated magma (Candela andBlevin, 1995a). De Hoog et al. (2001) studied mafic melt in-clusions (with sulfur contents in the range 350–2,900 ppm)hosted in olivine in high-Mg basalts of the Galunggung vol-cano, Java, Indonesia. The authors showed that the sulfur iso-tope compositions are consistent with a slab-derived origin,and proposed that the mantle beneath Galunggung is signifi-cantly enriched in sulfur relative to the MORB source man-tle. de Hoog et al. suggested that large-scale transfer of sulfurinto the mantle wedge occurred during slab dehydration, andthat the sulfur isotopic data are consistent with a slab-derivedorigin. In addition, Sasaki and Ishihara (1979) suggested thatenrichment of isotopically heavy sulfur in hydrous magmas inarc settings is common, and also concluded that seawater-de-rived sulfur was added to the arc magma system via the sub-ducted oceanic lithosphere.


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In our view, the high oxygen fugacity of the oceanic lithos-phere is transmitted to the mantle wedge by the hydrogenand sulfur species in the slab-derived fluids. The higher oxy-gen fugacity of the subducted crust relative to pristine MORB(oxygen fugacity ~ QFM) results from the prior interaction ofoceanic lithosphere with seawater. As the down-going slab re-leases fluid in a subduction zone, the ratios of SO2/H2S andH2O/H2 will reflect the range of oxygen fugacities present inthe dehydrating slab. These slab-derived fluids will interactwith, and will almost certainly oxidize the overlying mantlewedge, with the fluid becoming reduced in the process. De-tails of these interactions determine the ultimate range of ox-idation states of arc magmas. The subducting oceanic lithos-phere will not have a unique oxygen fugacity at any giventemperature and pressure, since the slab is a mosaic of rocktypes and compositions generated, in part, from variable de-grees of alteration. The proportion of subducted basalt andperidotite altered at higher sea water/rock ratios (and there-fore producing more oxidized alteration products) will deter-mine the aggregate oxygen fugacity of the fluid(s) releasedfrom the slab.

If the evolving, slab-derived fluids derived from differentregions within the devolatilizing slab do mix and react upondehydration, and approach some equilibrium within alteredoceanic crust at any given temperature and pressure, equilib-rium may be approximated by assemblages such as quartz +hedenbergitic pyroxene + anhydrite + magnetite + pyrrhotite,which can buffer the fugacities of oxygen and sulfur at anygiven pressure, temperature, and pyroxene composition.Ludden et al. (1999) have shown that even the oldest oceaniccrust contains intervals of minimally altered basalt, so theratio of oxidized to reduced rocks is likely to be related to fac-tors such as the gross fracture permeability of ocean floor thatis subsequently subducted. The oxygen fugacity of the uppermantle varies by several orders of magnitude (de Hoog et al.,2004), and the titration of slab-derived fluid into the mantlewedge probably increases both the oxygen and sulfur fugaci-ties of the subarc mantle. Blatter and Carmichael (1998)showed that some oxidized, subarc mantle peridotites canhave oxygen fugacities as high as QFM + 2.4. Jugo et al.(2005) suggest that sulfides will not be stable in these morehighly oxidized subarc mantle regions; therefore, chalcophilemetals such as gold and copper may not be retained in thecrystalline residue in the arc magma source, yielding meltswith higher concentrations of chalcophile ore metals.

In summary, water, sulfur, carbon, and chlorine are impor-tant constituents of the slab-derived fluid. Other elements,especially the pathfinder elements boron, bismuth, arsenic,and antimony, as well as some ore metals, also appear to becontributed to the site of melt generation by fluids derivedfrom subducted oceanic lithosphere. Because of its interac-tion with ocean water, the oxygen fugacity of the slab will beelevated compared to the initial oxidation state of pristineMORB. However, as has been stated before (e.g., Candelaand Blevin, 1995a; Richards, 2003), discussions of the“source” of ore metals are fraught with difficulty. Copper,gold, and other ore metals can all be supplied by the sub-ducting plate, by the mantle wedge, or by the lower crust. Oremetals may be stored in lower crustal mafic rocks, to betapped during later orogenic events. Chlorine partitioning

from a basaltic magma into juxtaposed felsic magma is possi-ble, but complex lower crustal interactions are probably in-volved in the mixing of chlorine into felsic magmas from sub-crustal sources. Sulfur is more easily stored in the lower crustthan is chlorine, and is thus more likely to be added to thesubarc lithosphere and possibly brought up to more shallowlevels in gas bubbles contained within the magma (Varekampet al., 1984; Lowenstern, 2001).

Questions Regarding the Special Role of Mafic Magmas in the Upper Crust

The role of upper crustal mafic magmas in magmatic hy-drothermal ore genesis has been debated in the recent litera-ture (e.g., Hattori and Keith, 2001). Certainly, mafic magmasin arc environments are mantle derived, and can contributeheat and matter to magmatic-hydrothermal ore systems. Fur-thermore, mafic magmas represent the primitive arc magma,and are clearly important in generating more felsic arc mag-mas associated with porphyry-type deposits. If it is agreedthat these mafic magmas are in part the source of elevatedchlorine and sulfur in more evolved magmas, then the onlyquestion is where and when are mantle-derived mafic mag-mas involved in the parentage of ore-generative arc magmas.

Exactly how mafic, mantle-derived components mix withcrust-derived components to form hybrid magmas that crys-tallize to yield upper crustal plutons is far from clear. Hildrethand Moorbath (1988) proposed in their MASH (melting, as-similation, storage, and homogenization) hypothesis that pro-gressive gabbroic diking in the deep crust, overlapping inspace and time, is in part responsible for the variable compo-sitions that we commonly refer to as “mixed magmas”. Theproduction of zoned intrusions, as well as the progression ofmagmas in volcano-plutonic systems, may be controlled bythe deep crustal diking of mantle-derived gabbroic magmainto lower crustal mafic to intermediate rocks. Richards(2003) expressed similar ideas in his recent review of the ori-gin of porphyry-related arc magmas.

Klepeis et al. (2003) studied exposures of early Mesozoicarc crust in western New Zealand to elucidate magma gener-ation and transport processes. The rocks include migmatites,granulite-facies mineral assemblages, and layered mafic-in-termediate intrusions that formed in the lower and middlecrust of the arc at paleodepths of 25 to 50 km during the EarlyCretaceous. Klepeis et al. (2003) suggested that the lowercrust had accumulated at least a 10-km thickness of mafic-in-termediate magma. The first phases were dominantly gab-broic whereas later phases were dominantly diorite (now theWestern Fiordland Orthogneiss). These magmas have beeninterpreted to have added sufficient heat to the lower crust topartially melt the vertically stratified mafic-intermediate hostgneisses, themselves the product of similar but earlier events.Compositions of the earlier melts were granitic due to biotitedehydration melting (for a discussion of the timing of biotitedehydration melting, see Bingen and Stein, 2003), but be-came granodioritic to tonalitic with increasing temperature asthe main reaction shifted to the fluid-absent melting of horn-blende ± clinozoisite. Clearly, the mixing of rocks of diverseorigins by diking of mantle-derived magma for long periodscould yield fertile source rocks for the extraction of “hybrid”melts (Hildreth and Moorbath, 1988).


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Forging the Magmatic Hearth: The Localization of Upper Crustal Magmatism

Broadly, M- (mantle source) and I-type (lower crust-uppermantle igneous source) magmas are associated with conti-nental margins, and are characteristic of arc systems. How-ever, the distribution of magmatism in arcs is complex and iscontrolled by many factors. Plutonism and volcanism in arcterranes are episodic, and appear to be related to periods ofcrustal deformation (Barton et al., 1988; Richards, 2003).Furthermore, in his study of granitic rocks in the westernUnited States, Glazner (1991) suggested that the obliquity ofsubduction affects the ratio of plutonism to volcanism at anygiven time. Obliquity can generate strike-slip motion that, to-gether with existing faults that are oriented at high angles tothe subduction zone, can result in local zones of extension incompressional settings. The resultant dilational features canaccommodate shallow-level plutons, cupolas, and apophyses.The large-scale through-going fractures that host these localzones of dilation can extend to lower crustal depths andthereby facilitate the ascent of deep magma.

Many studies have demonstrated the importance of thesestructures in creating space for ore-generative magmas. Forexample, according to Hill et al. (2002), from which the fol-lowing account is taken, the Grasberg porphyry copper-golddeposit, Irian Jaya, is localized in the hanging wall of a large-scale (~10 km) basement-involved thrust fault. The NewGuinea margin of the Indo-Australian plate underwent riftingoblique to the structural fabric in the crust, which resulted inextensional faulting in the Jurassic. Extension was followed inthe Oligocene by the collision of the Philippine-Caroline arcwith New Guinea. Toward the end of the middle Miocene,compression was initiated in New Guinea. Few magmas wereemplaced except in areas of local dilation. As deformation mi-grated to the south, it reactivated preexisting extensionalfaults. Local dilation occurred at the intersection of thesefaults with north-northeast–trending fracture zones, allowingemplacement of ore-generative magmas from the mantle orlower crust.

Basement-involved faults have also been postulated to existin the Ok Tedi district and at Porgera. The Porgera gold de-posit is spatially associated with the regional-scale Porgeratransfer structure (Corbett and Leach, 1998), which is athrough-going, arc-normal wrench fault that spans most ofPapua New Guinea. Hill et al. (2002) suggested that transferstructures may localize mineralization by providing magmaand/or ore fluid conduits for magma-ore fluid ascent, partic-ularly at intersections with oblique structures.

In central British Columbia, Late Cretaceous to Eocenegranitic plutons and associated porphyry copper depositsoccur in a broad northeast-trending belt. Babine Lake, Canada,is a porphyry copper district near the northeast terminus ofthe belt, and mineralization is associated with high-levelbiotite-feldspar porphyry. The andesites of the Newman vol-canic rocks are the extrusive equivalents of these intrusions.In the Babine district, localization of magmatic and hydro-thermal activity as well as porphyry copper ores was con-trolled by zones of extension (pull-apart basins) between dex-tral strike-slip faults (MacIntyre and Villeneuve, 2000). Theseauthors point out that Laramide magmatism is attributed to

rapid oblique convergence and shallow eastward subductionof oceanic lithosphere under the North American plate. Inthe Tertiary of North America, the Laramide was quite pro-ductive in generating porphyry, skarn, and associated hy-drothermal deposits.

At Escondida, Chile, pluton emplacement was localizedwithin a broad zone formed by the intersection between theWest Fissure zone and a regionally extensive northwest-trend-ing structural corridor (the Archibarca lineament; Richards etal., 2001). These authors suggested that this structural geom-etry was conducive to the formation of transtensional pull-apart structures during relaxation or reversal of dextral shearon the West Fissure zone. The dilational structures focusedboth the ascent and the emplacement of magma in the uppercrust and maximized the potential for formation of magmatic-hydrothermal ore deposits. These histories exemplify the roleplayed by local, dilational structures in otherwise compressivesettings, and by preexisting structures in localizing the mag-matism that focuses high-temperature magmatic hydrother-mal mineralization.

These studies suggest that some of the magmatism inEarth’s upper crust, as well as associated ores, is associatedwith large, through-going fracture systems that are controlledby cumulative tectonic history. However, some authors (e.g.,Paterson and Schmidt, 1999) have noted that many graniticplutons have no apparent relationship to specific structures.In part, this may be a matter of perspective. The smallapophyses and cupolas, commonly localized by fracture sys-tems and associated with magmatic hydrothermal ores de-posits, may occur in the roof zone of a larger underlying plu-ton. When exposed at a somewhat deeper level, these larger,horizontally oriented, tabular- or funnel-shaped igneous bod-ies may show no clear evidence of structural control. Indeed,root zones and cupolas may be controlled by local, verticallyoriented dilational structures that form by a variety of mech-anisms, including extension associated with dilational jogs incompressional settings, whereas the larger flat-lying magmachambers could grow laterally by chamber-floor depressionand roof lifting (Tosdal and Richards, 2001).

The Chamber Hearth: Magma Chambers and the Localization of Associated Mineralization

Magmatic-hydrothermal ores, as well as their associated ig-neous rocks, are the result of time-integrated processes (See-dorff et al., 2005). Upper crustal magma chambers are opensystems, gaining magma from below, and losing magma todiking and volcanism. Ponding of the increasingly viscousmagma may be aided by tectonic factors. Magmas may beemplaced either in concentric fashion or adjacent to previousintrusions (Vigneresse and Bouchez, 1997; Seedorff et al.,2005). Whether a magma-hydrothermal system is producedas a result of a single pulse or multiple pulses of magma, andwhether the multiple pulses of magma produce a single plu-ton or multiple intrusive phases will be a complex function ofthe rate of magma generation, upper crustal strain rate, andthe time-integrated physical properties of the magma (Han-son and Glazner, 1985). Plutons are the product of crystal-lization within a magma chamber over time, but plutonsshould not be equated with frozen magma chambers. Thevolume of magma at any time is almost certainly less than the


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volume of the existing pluton, and there are still outstand-ing questions concerning how upper crustal magma cham-bers grow, and how chamber evolution is related to the gen-erally episodic nature of magmatic hydrothermal oreformation. Consequently, the geometry of volatile egress inrelation to the present-day geometry of a given pluton maynot be clear.

The ascending magma in a developing and growing cham-ber may undergo volatile exsolution and crystallization thatyields variably textured porphyritic stocks (Candela andBlevin, 1995b). The structures that represent zones of crustalweakness below the magma chamber and that promotemagma ascent can also provide regions of weakness above thechamber and promote the formation of cupolas or otherapophyses. As a result, these structures can lead to the devel-opment of high densities of hydrothermal veins (Seedorff etal., 2005).

As a magma chamber grows by any process, new magmainput at the bottom of the chamber may rise through the ex-isting magma if a significant bubble fraction is present. Theprobability of a high bubble fraction in the melt entering thechamber is favored by low total (lithostatic) pressures in thechamber (i.e., proximity to the surface), high volatile concen-trations, high ratios of (CO2 + SO2)/H2O, and crystallizationof the input magma. Magma may reach a growing, upper levelmagma chamber already saturated with respect to an H2O-CO2-H2S-SO2 volatile phase (Lowenstern, 2001). In this case,the entering magma may devolatilize over the root zone (e.g.,Roy and Clowes, 2000) of a growing chamber. With decreas-ing pressure and increasing crystallization progress in the hostmagma, the exsolving volatile phase may become progres-sively richer in water as less soluble carbonic and sulfurouscomponents are preferentially removed in the earlier fluids(Lowenstern, 2001).

Candela and Blevin (1995a) pointed out that plutons withany composition can occur without associated mineralizationor alteration, suggesting that physical magmatic parametersare important in determining whether or not the end stagesof magma evolution will include the generation of associatedhydrothermal mineral deposits. There have been few modelsof magmatic volatile phase egress from magmatic systems. Bymagmatic volatile phase, we mean the water and/or salt-richphases in, or issuing from, the magma, and that may variablybe defined as vapor, brine, or supercritical gas. We definethese phases in the following way. A supercritical gas (a gas isa state of matter that fills its container, and therefore all su-percritical fluids are gases) is such that changes in composi-tion alone will yield neither condensation (formation of ahigh-salinity brine) nor vaporization (formation of a low-salin-ity vapor). In other words, there is no dew point or bubblepoint at the temperature and pressure of interest. Here, wedefine vapor as a low-density gas that can be brought to itsdew point by changes in composition alone, at the tempera-ture and pressure of interest, and we define brine as a salt-rich aqueous liquid that can be brought to its bubble point bychanges in composition alone, at the temperature and pres-sure of interest. However, the reader should note that, be-cause we are attempting to categorize elements of a contin-uum, definitions of phases near critical points are commonlyless than satisfying.

The magmatic system may be saturated with one volatilephase, or two volatile phases (i.e., vapor and brine). Models ofmagmatic volatile phase exsolution from the melt are cen-tered on the percolation of the volatile phase or phasesthrough pore space in shallow intrusions, and convection ofbubble-laden foam. Candela (1991) and Candela and Blevin(1995b) suggested that plumes of bubble-laden magmaand/or volatile phase permeability through interconnectedvolumes of the magmatic volatile phase act to transportvolatiles toward the top of a chamber. As an alternative to themodel of Candela (1991), Shinohara et al. (1995) suggestedthat bubble-rich magma undergoes open-system degassing atshallow depths by convective transport of bubble-rich magmato upper regions where it loses volatiles, then sinks back intothe chamber.

Melt inclusions from the Bishop Tuff exhibit a range ofH2O and CO2 concentrations in the glass that correlate withtrace-element concentration gradients, as well as calculatedFe-Ti oxide temperatures, inferred to reflect depth in themagma column (Wallace et al., 1999; Anderson et al., 2000).These data suggest the formation of a stagnant evolved cap ofdifferentiated magma in the upper reaches of the magmabody. Some plumes might rise into the upper level of thechamber, but the cap is clearly not well mixed, as one mightexpect if vigorous convection occurred (T. Sisson, pers. com-mun, 2005).

We suggest that buoyant, volatile phase-saturated and bub-ble-bearing magma may ascend into a magma chamber, thensubsequently rise to the top of the chamber where it can de-volatilize. As the chamber grows, the partially devolatilizedmagma may move laterally, and more volatile-charged magmamay be brought into the chamber from below. Alternatively,periods of crystallization and devolatilization in a stratifiedmagma, accompanied by permeable flow of the magmaticvolatile phase and formation of a highly crystalline mush, maybe punctuated periodically by the incremental addition ofmagma during tectonic dilation. This process may continueover a protracted period of time. This model obviates theneed to propose that large volumes of magmatic volatiles bescavenged from a large magma chamber and focused into asmall apical region. The cupola would sit over the root zoneof the chamber, with the root and the cupola-apophyses oc-cupying the same structural weakness (Fig. 3). This modelwould produce a horizontally elongated pluton, with distalportions of the pluton forming largely from the partly de-gassed magma. This model is consistent with the structures,determined by geophysical methods, for the Guichon Creekbatholith, Canada (Roy and Clowes, 2000).

Granite textures in the porphyry ore environment

Many of the igneous rocks associated with magmatic hy-drothermal ores are porphyritic in character. However, thestudy of granitic textures, especially of those rocks associatedwith ores, is problematic. A comparison of the textures foundin pristine felsic volcanic rocks and in typical granitic-gran-odioritic rocks of closely similar composition reveals somesimilarities, yet some important differences as well. Earlyformed, calcium-rich minerals in volcanic and plutonic rockstend to exhibit shared textural elements. Zoning and euhe-dral crystal morphology in plagioclase and titanite are good


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examples. Apatite in both volcanic and plutonic rocks canhave a similar range of morphologies, with some crystalsequant and others acicular. On the other hand, plutonic alkalifeldspar and magnetite both exhibit compositions that deviatesignificantly from their volcanic counterparts, owing to thefact that these minerals reequilibrate and react with alteration(Piccoli et al., 2000). Quartz in typical hypidiomorphic granu-lar granite is anhedral and interstitial, whereas in volcanicrocks and some hypabyssal porphyries, it is commonly phe-nocrystic. In fact, many of the shallow-level porphyries asso-ciated with magmatic hydrothermal ores are called “quartzporphyries.” Usually, these rocks have phenocryst assem-blages comprising quartz, feldspars, and other minerals; thequartz phenocrysts remain prominent even after extensive al-teration. However, locally quartz may form more rapidly thanfeldspar phenocrysts, in magmas cosaturated with both quartzand feldspar, due to kinetic factors. Candela (1997) and Pic-coli et al. (2000) suggested that quartz nucleates more easily

than feldspar, explaining the existence of quartz eyes in mod-erately super cooled granite dikes. This may also be a factorin the case of granitic rocks associated with hydrothermal oredeposits.

An important distinction must be made between isobaricdevolatilization (i.e., crystallization-driven devolatilization at astatic level in the crust) which can be a near-equilibriumprocess, and devolatilization due to depressurization, whichcommonly occurs under moderately to strongly irreversibleconditions. Most likely, the porphyritic texture of ore-relatedplutons is due to the latter, and is therefore probably indica-tive of decompression of a partially crystallized magma. De-compression may occur due to an upward surge of magmainto a cupola or dike, or may be due to the lowering of themagma chamber pressure from lithostatic to a lower pressureby the breeching of the barrier that separates a shallowmagma chamber from the hydrostatic regime that surroundsit (Fournier, 1999). The magma may be already saturated


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FIG. 3. One possible scenario for the growth of a magma chamber, showing (A) devolatilization focused above the feederof the growing chamber. In this hypothesis, magma rises from the lower crust by fracture flow. Volatiles partially exsolve uponascent and accumulate near the top of the chamber by plume or bubble rise, or by flow in permeable channels (Candela,1991). (B) Magma spreads laterally into the growing chamber, creating space for new batches of magma. (C) Successivebatches of volatile-charged magma form a composite cupola, each batch representing bubble-laden froth related to cham-ber-recharge events.

A)

B)

C)

with respect to a volatile-phase, or decompression may initi-ate volatile-phase saturation. In either case, the concentrationof water and other volatiles in the melt may be lowered nearlyisothermally. Supersaturation of the melt phase with respectto a number of minerals can occur, with a degree of supersat-uration causing a high nucleation rate, leading to the produc-tion of a larger number of crystals per unit volume, and there-fore smaller crystals.

Candela (1997) suggested that plumes of rising, volatile-saturated melts lead to the formation of miarolitic cavities.However, it should also be noted that most miarolitic granitesare rather leucocratic. This association could result from sub-tle kinetic factors that may only be speculated on here (seeCerny et al., 2005, for more discussion). The high concentra-tion of iron and magnesium in hotter, less felsic melts (e.g.,granodiorites) may poison the nucleation of quartz andfeldspar to such an extent that their integrated nucleationrates are lower in the less felsic melts (Swanson and Fenn,1986). In more felsic melts, nucleation is more rapid, andcrystal sizes are smaller. For a granite to be miarolitic, thecharacteristic crystal size must be on the same order as thecharacteristic bubble diameter, or smaller. If the crystals areon the same size as the bubbles, or larger, then the volatilephase will form an inter-granular film, the geometry of whichis more difficult to preserve in the rock record.

The Chemistry of Volatile Phase ExsolutionMagma rising in the crust may be saturated or undersatu-

rated with respect to a volatile phase; saturation will occurwhen the total confining pressure is reduced to the totalvapor pressure of the magma. Rising magma may reach thesurface and erupt; alternatively, magma ascent may be slowedby external structural factors or by an increase in viscosity dueto crystallization or bubble production. As magma ascentslows, some crystallization will occur due to irreversible lossof heat to the cooler surroundings of a magma, and volatileexsolution will be driven, increasingly, by crystallization ratherthan by decompression. Depending upon the Cl/H2O ratio inthe melt and pressure-temperature conditions, the magmamay saturate with respect to a supercritical gas, a vapor +brine, or brine alone (see Webster et al., 1999; Webster, 2004).Magmatic brines and vapors are thought to be critical agentsin ore formation because of their high fluidity and buoyancy,and because of the affinity of many ore metals for volatilephases relative to rock-forming minerals (cf. Candela and Pic-coli, 1995). The data presented by Lowenstern (2001), for ex-ample, show that silicate melts are commonly saturated withrespect to volatiles in the Earth’s mid- to upper crust.

Lowenstern (1994) studied inclusions in the Pine Groveporphyry molybdenum deposit, and found quartz-hostedmelt inclusions with up to 8 wt percent water and almost 0.1wt percent CO2. With these high volatile contents, the meltswould be saturated at depths of nearly 16 km. It is hard toimagine that magma with a long history of volatile exsolutionor fractionation could yield such high CO2 concentrationswithout late inputs of CO2 from either crustal or subcrustalsources. In fact, CO2 may be a marker for recharge of a felsicmagma chamber, or lack of significant prior loss of volatiles.Because the sulfurous gases behave similarly to CO2 duringvolatile exsolution, these considerations may also be critical in

the formation of sulfur-rich magmatic hydrothermal ores. Onewould expect that sulfur would only be present in low concen-tration in highly differentiated granites such as magmas asso-ciated with Climax-type porphyry molybdenum deposits (Can-dela and Holland, 1986). Yet, even the highly fractionatedmagmas that give rise to porphyry Mo deposits contain sulfur.This suggests that the convergence of physical and chemicaleffects plays a role in the formation of such ore systems, withthe whole being greater than the sum of the parts.

The basic premise of the magmatic hydrothermal model forthe origin of porphyry-type mineral deposits involves the par-titioning of ore metals from a mixture of melt ± crystals intoexsolving and evolving magmatic volatile phase(s) (Candelaand Piccoli, 1995, 1998). Crystal separation has been recog-nized as a first-order process of magmatic differentiationsince at least the time of Darwin (1844), and crystal fraction-ation processes are likely to be a first-order control on the orepotential of any given magmatic system. Crystal fractionationis seen commonly as a force that promotes formation of in-compatible element-rich ore deposits. Whereas fractionationdoes increase the concentration of incompatible trace ele-ments (with 1 > D > 0, where D is the bulk crystal/melt par-tition coefficient for the element in question) in the remain-ing melt phase, thereby increasing the (intensive) chemicalpotential of the ore substance, the total extensive amount ofany given metal in the melt phase decreases, and this in-evitably decreases the total tonnage available for ore forma-tion. However, higher concentrations of metals in the meltlead to higher concentrations in a magmatic-hydrothermalore fluid, and this can increase the probability of forming anore deposit, ceteris paribus.

A necessary step in the development of our understandingof fractionation is to measure the partitioning of elements be-tween silicate melts and volatile phases on one hand, and be-tween silicate melts and minerals on the other. Partition coef-ficients have been measured for the distribution of copper,gold, zinc, molybdenum, and other constituents between amelt and a supercritical gas, or in some cases, subcritical va-pors and brines. The partitioning of copper, gold, molybde-num, tungsten, and other metals has also been studied be-tween melts and various mineral phases. Many of thesepartition coefficients have been collected and reported inother reviews (e.g., Candela and Piccoli, 1995), and are notreproduced here. Furthermore, since many other papershave discussed the chemistry of magmatic devolatilization(e.g., Candela and Piccoli, 1995; Ulrich et al., 1999; Frank etal. 2002; Simon et al., 2005), only the chemical magmaticprocesses leading up to devolatilization will be covered here.

Modeling (Candela, 1989; Candela and Piccoli, 1995) hassuggested a number of factors may be important in magmatichydrothermal ore genesis. Blevin and Chappell (1992) showedconvincingly that the type of mineralization that forms in amagmatic hydrothermal setting is a function of oxygen fugac-ity. Decreasing magmatic oxygen fugacity increases the parti-tioning of molybdenum relative to tungsten into crystallinephases, thus decreasing the probability of Mo (relative to W)partitioning into an evolving ore fluid in reduced systems(Candela and Bouton, 1990), consistent with the observationssummarized by Burnham and Ohmoto (1980) and Thompsonet al. (1999).


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As the ratio of the initial water content of the melt to thesaturation water content of the melt decreases, the efficiencyof compatible-element removal from the magma into an ex-solving magmatic volatile phase decreases. For example, formagmas of a given initial water, copper, and molybdenumconcentration, the integrated Cu/(Mo+W) ratio of a (poten-tial) ore fluid decreases with increasing depth in the crust.The general observation of shallow copper and deeper molyb-denum and tungsten porphyry deposits supports this mag-matic hydrothermal theory, and this also follows from thequantitative model (Candela, 1992). Thus, melts at deeperlevels can fractionate further, before volatile saturation, rela-tive to melts that crystallize at a shallow level. In this model,compatible elements are sequestered in the crystallizationproducts, relative to more incompatible elements.

Partitioning data (Lynton et al., 1993; Jugo et al., 1999)show that copper partitions strongly into pyrrhotite from sil-icate melts. Gold also can be accommodated in pyrrhotite, al-though it partitions more strongly into magmatic Cu-Fe sul-fides (Jugo et al., 2000). A study by Simon et al. (2003)suggested a partition coefficient for gold between magnetiteand melt on the order of 4. Early volatile exsolution or con-ditions that minimize magnetite and/or Cu-Fe sulfide crys-tallization favor the partitioning of copper and gold intovolatile phases relative to crystalline phases at the magmaticstage.

Scaillet and Evans (1999) confirm that pyrrhotite is stablein magmas up to an oxygen fugacity of QFM + 2, a result thatis broadly consistent with the recent results by Jugo et al.(2005). Below this oxygen fugacity, pyrrhotite can be stable,and the tonnage of copper available to the hydrothermal sys-tem may be diminished if pyrrhotite is left behind, se-questered in the crystalline residuum that may be distributedthroughout the crust as the result of the protracted evolutionof magmas during their repeated chambering and upwardmovement. Magmatic sulfides that occur in the variably tex-tured plutons proximal to the sites of ore deposition in por-phyry and related deposits may be the mere vestige of the sul-fides that have been removed from magmas during theirtranscrustal evolution (Simon et al., 2003). Volatile saturationat an oxygen fugacity greater than approximately QFM canresult in the destabilization of magmatic pyrrhotite, thus re-leasing chalcophile metals, therefore making the timing ofvolatile saturation critical. If a significant amount ofpyrrhotite crystallization occurs before volatile saturation,then a significant amount of available copper can be lost. AtfO2 < QFM, the consumption of sulfur by the magmaticvolatile phase is not as great as at high oxygen fugacities, andpyrrhotite may persist during volatile-saturated crystalliza-tion, thus further reducing the availability of chalcophile oremetals to the volatile phase.

Concluding RemarksStudies of minor and trace ore metals in peridotites and

serpentinites, in melt inclusions, in glasses in arc lavas, etc.,together with our developing ideas of processes in subductionzones, are beginning to shed light on the control on con-centrations of chlorine, sulfur, water, and incompatiblepathfinder elements in arc systems. The interactions of slab-derived fluids with the mantle wedge, as well as interactions

of mafic arc magmas with both lower crust rocks and morefelsic magmas, control the magmatic concentrations of car-bon dioxide, sulfur, water, and chlorine. Whether or not mag-mas are sulfide-saturated deep in the crust exerts a strongcontrol on the amount of gold, copper, and other ore metalsand pathfinder elements that are available for partitioninginto exsolving magmatic volatile phase at shallower levels.Whereas extensive pyrrhotite formation and sequestering ofcopper due to fractionation can inhibit the development of amagmatic hydrothermal copper deposit, it has less of an ef-fect on gold owing to its lower partitioning into pyrrhotite rel-ative to copper; however, fractionation of even small amountsof Cu-Fe sulfides in the magma can deplete a melt in gold. Bycontrast, a high oxygen fugacity can preclude the formation ofmagmatic sulfides. Candela and Blevin (1995a) argued thatore formation in the magmatic hydrothermal environment isprobabilistic in nature, with a number of factors combining toproduce ore deposits, and rarely, giant ore deposits; if too fewof these factors operate, then subeconomic deposits may re-sult. We concur with Clark (1993), Tosdal and Richards(2001), and others who have suggested that porphyry Cu-Mo-Au deposits require the coincidence and positive interactionof a series of individually commonplace geological processessuch as convergent margin magmatism, and that they reflectthe dynamic interplay between magmatic, hydrothermal, andtectonic processes.

AcknowledgmentsWe thank Jeff Hedenquist and John Thompson for the in-

vitation to write this paper. Jake Lowenstern is thanked for hisearly contributions to the paper, and for his continuing en-couragement during the writing process. Comments by MarkCloos, Steve Kesler, Tom Sisson, Jeremy Richards, and JeffHedenquist are greatly appreciated. This work was sup-ported, in part, by the National Science Foundation (EAR-0125805 and EAR-0309967). We would like to thank CallanBentley for his contributions to the drafting of figures for thispaper.

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