Magmatic to Hydrothermal Metal Fluxes in Convergent and Collided Margins

Ore Geology Reviews 40 (2011) 1 –26

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Ore Geology Reviews j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o r e g e o r ev

Review

Magmatic to hydrothermal metal �uxes in convergent and collided margins Jeremy P. Richards ⁎ Dept. of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3

a r t i c l e

i n f o

Article history:

Received 1 December 2010 Received in revised form 18 May 2011 Accepted 19 May 2011 Available online 27 May 2011 Keywords:

Porphyry deposit Epithermal deposit Subduction Post-subduction Magmatic–hydrothermal � uid Ore formation

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a b s t r a c t

Metals such as Cu, Mo, Au, Sn, and W in porphyry and related epithermal mineral deposits are derived predominan predom inantly tly from fromthe the asso associated ciatedmagma magmas, s, via magma magmatic tic–hydrothermal �uids exsol exsolved ved upon empla emplacement cementinto into themid- to upp upper er cru crust.Four st.Four mai main n sou source rcess exis existt formagm formagmas,and as,and the theref reforemetal oremetals, s, in con conver vergen gentt andcolli andcollidedplate dedplate margins: the subducting oceanic plate basaltic crust, subducted sea �oor sediments, the asthenospheric mantle wedge wed ge bet betweenthe weenthe sub subduct ducting ing and ove overri rridin dingg pla plates tes,, and the upp upper er pla plate te lit lithos hosphe phere. re. Thi Thiss pap paper er �rstly rstlyexamin examines es the source of normal arc magmas, and concludes that they are predominantly derived from partial melting of the metasomati metas omatized zed mantl mantlee wedge, with poss possible ible mino minorr contr contributio ibutions ns from subducted sediments. Although some metals may be transferred from the subducting slab slab via dehydration �uids, the bulk of the metals in the resultant magmas are probably derived from the asthenospheric mantle. The most important contributions from the slab froma metal metallogen logenic ic perspec perspectiveare tiveare H2O,S,andCl,aswellasoxidants.Partialmeltingofthesubductedoceaniccrust and/or sediments may occur under some restricted conditions, but is unlikely to be a widespread process (in Phanerozoic arcs), and does not signi �cantly differ metallogenically from slab-dehydration processes. Primary, mantle-d mantle-derived erived arc magmas are basaltic basaltic,, but differ from mid-ocean ridge basalt in having higher water contents (~10× higher), oxidatio oxidation n states states (~2 log f O2 higher), and concentration concentrationss of incompat incompatible ible elements O2 units higher), and other volatiles (e.g., S and Cl). Concentrations Concentrations of chalcophil chalcophilee and siderophi siderophile le metals in these partial melts depend critically on the presence and abundance of residual sul �de phases in the mantle source. At relatively high abundances of sul �des thought to be typical of active arcs where f S2 S2 and f O2 O2 are high (magma/sul �de ratio=10 2–105), sparse, highly siderophile elements such as Au and PGE will be retained in the source, but magmas may be relatively undepleted undepleted in abundant abundant,, moderately chalcophile elements such as Cu (and perhaps Mo).. Suc Mo) Such h ma magma gmass ha have ve th thee pot potent entia iall to fo form rm por porphy phyry ry Cu±Mo dep depos ositsuponempl itsuponemplace acemen mentt inthe upp upper er cru crust st.. Gold-rich porphyry deposits would only form where residual sul �de abundance was very low (magma/sul�de ratio N 105), perhaps due to unusually high mantle wedge oxidatio oxidation n states. In contrast, some porphyry Mo and all porphyry Sn–W deposits are associated with felsic granitoids, derived primarily from melting of continental crust during intra-plate rifting events. Nevertheless, mantle-derived magmas may have a role to play as a heat source for anatexis and possibly as a source of volatiles and metals. In pos post-su t-subduct bduction ion tect tectonic onic sett setting ingss Tulloch Tulloch and Kimb Kimbroug rough, h, 2003 2003,, such as subd subducti uction on rever reversal sal or migratio migration, n, arc collision, collisio n, continent–continent collision, and post-collisional rifting, a subducting slab source no longer exists, and magm magmas as are pred predomi ominant nantly ly deriv derived ed fro from m part partialmeltingof ialmeltingof the upperplate lith lithosph osphere.This ere.This lith lithosph osphere ere will have undergone signi�cant modi�cation during the previous subduction cycle, most importantly with the introduc intr oduction tion of larg largee volu volumes mes of hydr hydrous, ous, ma �c (amp (amphibo hibolit litic) ic) cumu cumulat lates es resi residual dual from low lower er crus crustal tal differen diff erentiat tiation ion of arc basa basalts.Small lts.Small amo amountsof untsof chal chalcoph cophile ile and sid sideroph erophile ile elem elementent-rich rich sul�de dess ma mayy al alsobe sobe le left ft in these cumulates. Partial melting of these subduction-modi �ed sources due to post-subduction thermal readjustments readjustme nts or astheno asthenospheric spheric melt invasion will generate small volumes of calc-alkaline to mildly alkaline magmas, which may redissolve residual sul�des. Such magmas have the potential to form Au-rich as well as norma nor mall Cu± Mo po porph rphyryand yryand epi epith therm ermalAu alAu sys system tems, s, dep depend endingon ingon th thee am amoun ountsof tsof sul�de pre presen sentt in th thee lo lower wer crustal crust al sour source. ce. Alk Alkalic alic-typ -typee epit epitherm hermal al Au depo deposit sitss are an extr extreme eme endend-memb member er of thi thiss rang rangee of post post-sub -subduct duction ion deposits, formed from subduction subduction-modi -modi�ed mantle sources in extensio extensional nal or transtens transtensional ional environments. environments. Ore formation in porphyry and related epithermal environments environments is critical critically ly dependent on the partitioning partitioning of metals from the magma into an exsolving magmatic –hydrothermal �uid phase. This process occurs most ef �ciently at depths greater than ~ 6 km, within large mid- to upper crustal batholithic batholithic complexes fed by arc or post-subd postsubductio uction n magma magmas. s. Unde Underr such condi condition tions, s, meta metals ls will partition partition ef �cient ciently ly into a singl single-pha e-phase, se, supercritical supercritic al aqueous � uid (~2–13 wt.% NaCl equivalent), which may exist as a separate volatile plume or as bubbles entrained in buoyant magma. Focusing of upward � ow of bubbly magma and/or � uid into the apical regions of the batholithic complex forms cupolas, which represent high mass- and heat- �ux channelways

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J.P. Richards / Ore Geology Reviews 40 (2011) 1 – 26

towards the surface. Cupolas may be self-organizing to the extent that once formed, further magma and � uid �ow will be enhanced along the weakened and heated axes. Cupolas may form initially as breccia pipes by volatile phase (rather than magma) reaming-out of extensional structures in the brittle cover rocks, to be followed follow ed immediately by magma injection to form c ylindrica ylindricall plugs or dikes. Cupola Cupo la zone zoness mayextend to surf surface,where ace,where magm magmas as and �uidsvent as volc volcanicproduct anicproductss and fuma fumarole roles. s. Betw Between een the surf surface ace and the unde underlyi rlying ng magm magmaa cham chamber, ber, a very ste steep ep the thermalgradien rmalgradientt exis exists ts (700 (700°°–800 °C °C over b 5 km), whichis wh ichis theprim theprimar aryy cau cause se ofverti ofverticalfocu calfocusin singg of or oree min minera erall dep depos osit itio ion. n. Th Thee bu bulk lk of me meta tals ls (Cu±Mo± Au)that formss por form porphyryore phyryore bodi bodies es areprecipit areprecipitatedover atedover a narr narrowtemperat owtemperatureinterva ureintervall betw between~ een~ 425°and 320 °C,where isotherms in the cupola zone rise to within ~2 km of the surface. Over this temperature range, four important physical and physicochemical factors act to maximize ore mineral deposition: (1) silicate rocks transition from ductile to brittle behavior, thereby greatly enhancing fracture permeability and enabling a threefold pressure drop; (2) silica shows retrograde solubility, thereby further enhancing permeability and porosity for ore deposition; depositi on; (3) Cu solubili solubility ty dramatically decreases; and (4) SO 2 dissol dissolved ved in the magm magmatic atic–hydrothermal �uid phase disproportionates disproportionates to H 2S and H2SO4, leading to sul �de and sulfate mineral deposition and the onset of increasingly acidic alteration. The bulk of the metal � ux into the porphyry environment may be carried by moderately saline supercritical vapors, rs, wit with h a volu volumetr metrical ically ly less lesser er amo amount unt by sal saline ine liqu liquid id cond condensa ensates.However tes.However,, thes thesee vapo vapors rs rapi rapidly dly �uidsor vapo become dilute at lower temperatures and pressures, such that they lose their capacity to transport metals as chloride complexes. They retain signi�cant concentrations of sulfur species, however, and bisul �de complexing of Cu and Au may enable their continued transport into the epithermal regime. In the high-sul �dation epithermal environment, intense acidic (advanced-argillic) alteration is caused by the �ux of highly acidic magmaticvolatil magm aticvolatiles es (H2SO4, HCl HCl)) in thi thiss vap vapor or pha phase.Ore se.Ore fo forma rmatio tion, n, ho howev wever,is er,is par parage agene netic tical ally ly la late,and te,and ma mayy be located loca ted in thes thesee extr extremel emelyy alt altered ered and leac leached hed cap rock rockss lar largely gely beca because use of thei theirr highpermeab highpermeabilit ilityy and poro porosit sity, y, rather than there being any direct genetic connection. Ore-forming �uids, where observed, are low- to moderate-salinity liquids, and are thought to represent later-stage magmatic –hydrothermal � uids that have ascended along shallow shallower er (cooler) geotherma geothermall gradients gradients that either do not, or barely, intersect the liquid–vapor solvus. Such �uids “ contract” from the original supercritical � uid or vapor to the liquid phase. Brief intersect intersection ion of the liquid–vapor solvus may be importan importantt in shedding excess chloride and chloride-complexed metals (such as Fe), so that bisul�de-complexe de-complexedd metals remain in solution. Such a restrict restrictive ive pressure–temperatur temperaturee path is likely like ly to occu occurr onl onlyy tran transie sientlyduring ntlyduring the evol evolutio ution n of a magm magmatic atic–hydro hydrother thermalsystem,which malsystem,which may expl explain ain the rarity rari ty of high high-sul -sul�dati dation on Cu–Au ore depo deposit sits, s, desp despite ite the ubiq ubiquito uitous us occu occurren rrence ce of adva advancednced-argi argilli llicc alte alterati ration on in the lithocaps above porphyry-t porphyry-type ype systems. © 2011 Elsevi Elsevier er B.V. All rights reserved.

Contents

1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magma Mag ma gen genera erati tion on in con conver vergen gentt an andd co colli llided ded mar margin gins: s: geo geoche chemi mical cal cha charac racter teris istic ticss an andd par parti titio tioni ning ng of me meta tals ls 2.1. Slab deh ehyd ydrration and asthen enoospheric melting in subduc ucttion zones . . . . . . . . . . . . . . . . 2.1.1. Behavior of metals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Sediment dehydration and/or melting in subduction zones . . . . . . . . . . . . . . . . . . . . 2.2.1. Behavior of metals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Oceanic slab melting in subduction zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Behavior of metals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Supr praa-subd bduuction zone lithospheri ricc mel eltting: the MA MASH SH proces esss . . . . . . . . . . . . . . . . . 2.4.1. Behavior of metals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2. Sources of Mo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Lithospheric melting during post-subduction events . . . . . . . . . . . . . . . . . . . . . . . 2.5.1. Behavi Behavior or of metal metalss in subduct subduction-m ion-modi odi�ed sources . . . . . . . . . . . . . . . . . . . 2.6. Crustal melting during post-co colllisional stress relaxation . . . . . . . . . . . . . . . . . . . . . 2.6.1. Sources of metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Behavior of metals during magma fraction fractionation ation and � uid exsolution in the upper crust . . . . . . . . . . . 3.1. Parti Partitioni tioning ng of metal metalss from from magma magma into exsol exsolving ving hydrothe hydrothermal rmal � uid . . . . . . . . . . . . . . . 4. Ma Magm gmat atic ic–hydrothermal ore formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Porphyry Cu ore formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. 4. 2. Ep Epit ithe herm rmal al Cu–Au ore formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 4. 2.1.. Hi High gh-s -sul ul�dation epithermal Cu–Au deposits. . . . . . . . . . . . . . . . . . . . . . . 4.2.2 4. 2.2.. Lo Loww-su sull�da dati tioon ep epit ithe herm rmal al Au de depo posi sits ts (i (inc nclu ludi ding ng al alka kali licc-ty type pe de depo posi sits ts)) . . . . . . . . . . 5. Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Sources of magmas and metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Porphyry and epithermal ore formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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J.P. Richards / Ore Geology Reviews 40 (2011) 1– 26

1. Introduction

The question of the source of various elements in convergent and collided margin magmashas challenged geologistsfor decades. Igneous petrologists seek to understand the petrogenesis of such magmas through geochemicaland isotopic tracing, whereas economic geologists are generally more interested in the source of potentially valuable elementssuchasCu,Mo,Sn,W,Au,andplatinumgroupelements(PGE), which may ultimately be found in intrusion-related hydrothermal deposits. Igneous petrologistsare broadly in agreement that arcmagmas are primarily derived from hydrous melting of the asthenospheric mantle wedge above subducting plates, but melts from thesubducted oceanic crust (including sediments) and the upper plate lithosphere may also be involved to varying degrees. Economic geologistsare also broadly in agreement that ore-forming elements are partitioned from such magmas into an exsolving volatile phase upon emplacement in the upper crust, and may then be precipitated from these �uids during cooling, �uidmixing, andwallrock reaction processes in porphyry-type and related epithermal mineral deposits. However, these process theories do not address where the metalsoriginally came from, nor why porphyry deposits vary so widely in their metal contents (from Au-rich, through Cu±Mo±Au, to Moonly deposits, with Sn –W deposits forming a distinct variant). In addition to subduction-related calc-alkaline magmas, a diverse suite of calc-alkaline to alkaline magmas is generated in postsubduction and collisional tectonic settings, and these magmatic systems may also generate porphyry and epithermal ore deposits. Such systems raise an additional set of petrogenetic and metallogenic questions. It is the intent of this paper to merge these different geological perspectives on magmagenesis and metallogeny in order to discuss primary metal �uxes in convergent and collisional margins in terms of igneous petrogenetic and magmatic–hydrothermal processes. The ultimate metal inventory and metal ratios in any given porphyry or related deposit is secondarily controlled by late-stage magmatic and shallow crustalprocesses. These processes are examined,closing with a review of � uid and metal sources and behavior in related epithermal environments. 2. Magma generation in convergent and collided margins: geochemical characteristics and partitioning of metals

Most magmaserupted through or emplaced within the Earth's crust are not primary magmas (in the sense of being chemically unmodi �ed since extraction from their source), and most are not even primitive (in the sense of being relatively unevolved; Hildreth and Moorbath, 1988; Leeman, 1983; Neuendorfet al., 2005; Smith et al., 2010; Thirlwallet al., 1996). Exceptfor magmasproduced anderupted inextensional tectonic regimes (where rapid ascent to the surface is facilitated by normal faulting), most deeply-derived magmas undergo some degree of fractionation and crustal contamination during their passage towards the surface. It is therefore challenging to isolate geochemical and isotopic characteristicsof magma sourceregions from theeffects of later processes (Davidson, 1996). Magmas erupted through mature continental crust are the most dif �cult to � ngerprint uniquely in terms of source characteristics because wallrock assimilation and fractional crystallization (AFC; DePaolo, 1981) are ubiquitous and commonly extensiveprocessesthatwillsigni�cantlymodify bulkrockgeochemical and isotopic compositions; and yet, these are also the magmas that are most commonly associated with porphyry- and epithermal-type mineral deposits. The dif �culty in constraining source characteristics in such magmas is perhaps responsible for the plethora of theories that have been proposed forthe originof ore-forming magmasin convergent margin settings, ranging from the melting of subducting oceanic crust and/or sea�oor sediments, through melting of subduction metasoma-

3

tized asthenospheric or lithospheric mantle, to melting of underplated or primitive lower crustal rocks, and even melting of evolved crustal rocks in the case of some felsic porphyry Mo and Sn –W magmas. Therefore, rather than start by trying to identify a unique source for the typical intermediate-to-felsic calc-alkaline magmas that are associated with ore deposits in mature convergent margins, I begin this review by focusing on the much better constrained primitive island arc environment, where the effects of fractionation and crustal contamination, particularly by continentally derived materials, are minimized, and processes in mantle source regions can be more clearly de�ned. 2.1. Slab dehydration and asthenospheric melting in subduction zones

There is a general consensus that, with the exception of young oceanic lithosphere ( b 25 m.y.-old; Defant and Drummond, 1990) or plate edges (Yogodzinski et al., 2001), basaltic oceanic crust undergoes low-temperature, high pressure metamorphism upon subduction, whichreleases �uidsthrough a seriesof prograde dehydration reactions to form anhydrous eclogite (Fig. 1). Water and other volatile components and solutes (including S and Cl) were originally incorporated into oceanic crustal and upper mantle rocks during oxidizing sea�ooralteration,generatinghydrousmineralssuchasserpentine,talc, amphibole, micas, chlorite, zoisite, chloritoid, and lawsonite. Various experimental studies have shown that these minerals undergo dehydration reactions over a depth range extending to ~100 km, corresponding to the blueschist –eclogite transition in crustal rocks; serpentine (antigorite) and the 10 Å equivalent of chlorite may extend this rangeto 200 km (e.g.,Dvir etal., 2011;FornerisandHolloway, 2003; Fumagalli and Poli, 2005; Poli and Schmidt, 2002; Schmidt and Poli, 1998; Ulmer and Trommsdorff, 1995). Below these depths, the anhydrous eclogitic crust is essentially infusible, and the dense slab continues its descent into the mantle without melting. See reviews of this subject by Richards (2003, 2005, and references therein) . Numerous studies have explored the character of the �uids that are released from thedehydrating slab, because they arethought to account for the unique geochemical character of subduction-related magmas during later partial melting in the metasomatized asthenospheric mantle wedge (located between the downgoing slab and the upper plate). Slab-derived �uids are thought to be water-rich at these depths, and to carry signi�cant amounts of other volatile components such as Cl and S. For example, salinities in the range of 4 –10 wt.% NaCl have been inferred from primitive basalt or melt inclusion studies ( de Hoog et al., 2001; Kent et al., 2002; Portnyagin et al., 2007; Wallace, 2005; Wysoczanski et al., 2006), and salinities of 0.4 –2 wt.% NaCl equivalent were measured in �uid inclusions from high-pressure rocks thought to representsubductedoceanic mantle (Scambelluri et al.,2004). At higher pressures (~6 GPa) and greater depths (~175 km) there may no longer be a physical distinction between solute-rich aqueous liquids and hydrous silicate melts, and the �uid may be supercritical in nature (e.g., Kessel et al., 2005a,b), but the role of such deeply released �uids in subduction zone magmatism is unclear (see discussion in Richards and Kerrich, 2007). In addition to volatiles, water-soluble large ion lithophile elements (LILE:K,Rb,Cs,Ca,Sr,andBa,andU),andB,Pb,As,andSb(Breedinget al., 2004; Hattori et al., 2005; Hattori and Guillot, 2003; Kogiso et al., 1997; Manning,2004; Tatsumiet al.,1986) arethoughtto bemobilized into the forearc mantle wedge by dehydration �uids, which may then be convected by corner-�ow into the sub-arc melting zone (Fig. 1). These �uid–mobile components are also characteristically enriched in arc magmas (e.g., typical ranges of: 1 –3 wt.% H2O, 500–2000 ppm Cl, 900– 2500 ppm S; Davidson, 1996; Gill, 1981; Noll et al., 1996; Sobolev and Chaussidon, 1996; Wallace,2005; Portnyagin et al.,2007),whichis taken as evidence of aqueous �uidmetasomatism of themantle wedgemagma source. Silica may also be signi �cantly mobilized in these slab �uids (Aerts et al., 2010; Manning,2004),aswellasTlandCu(Noll et al., 1996;

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Volcanic arc Sea level Oceanic crust Chlorite + e l t serpentine e n r a e Talc h m p Chlorite c i s n o a h t e i c l O

Asthenosphere

0 km

Sediment

Oceanic crust Mantle lithosphere 6 00 ° C

A m p

D h b o e i h l e y d r a l t i i t o h o n s o Z p h f o e o c i s i r e e t a n e 6 0 i c C 0 °

C

1 0 0 0 C °

10 0 0° C

M a n tl e c or ne r

f lo w

Partial melting Metasomatized asthenosphere

C h t d l o t + r i s e e – 1 r 1 p e 0 Å 0 n t p h 0 i n e a s E 0 C e c l o g i t S e e r p e n t i n e °

1 4 0 0 ° C

100 km

1 4 0 0 C °

200 km Fig. 1. Structure and processes beneath an oceanic island arc (sources: Tatsumi and Eggins, 1995; S chmidt and Poli, 1998; Winter, 2001 ; Poli and Schmidt, 2002; Fumagalli and Poli,

2005). Primary hydrous basaltic arc magmas are derived from partial melting of the metasomatized asthenospheric mantle wedge. Mineral zones shown in the subducting plate indicate lower limits of stability of hydrous phases in the basaltic oceanic crust and peridotitic mantle lithosphere. Abbreviation: Ctd=chloritoid.

Stolper and Newman, 1994). The normally relatively incompatible high �eld strength elements (HFSE) Ti, Nb, and Ta are not signi �cantly � uid soluble under subduction zone conditions, and are retained in minerals such as rutile either in the slab or the mantle wedge ( Audétat and Keppler, 2005;Brenanet al., 1994;GreenandAdam, 2003). Arcmagmas derived from these sources therefore show characteristic negative anomalies for these three elements on mantle-normalized spider diagrams, but display enrichments in most other incompatible elements (Foley et al., 2000; Gill, 1981; Klemme et al., 2005; Ryerson and Watson, 1987; Schmidt et al., 2004; Schmidt et al., 2009 ). Hydrous metasomatism of the mid-ocean-ridge basalt (MORB)depleted asthenospheric mantle wedge causes partial melting by lowering the solidus of peridotite (Arculus, 1994; Kushiro et al., 1968; StolperandNewman,1994).Thisoccurseither throughdirect in�ltration metasomatism by slab-derived �uids percolating into thehotinner zone ofthemantlewedge(Bourdonetal.,2003;Groveetal.,2006;Kelleyetal., 2010; Peacock, 1993), or by convective corner- �ow mixing of metasomatized peridotite into these hotter central regions ( Fig. 1; Schmidt and Poli, 1998; Tatsumi, 1986; Wysoczanski et al., 2006). Partial melting of hydrated peridotite under these conditions in the mantle wedge generates high-Mg basalts (Greene et al., 2006; Pichavant et al., 2002; Smith et al., 2010 ). Such arc basalts are distinguished from MORB by higher contents of incompatible elements (as noted above) and water (up to 6 wt.% H 2O; Cervantes and Wallace, 2003; Grove et al., 2003; Pichavant et al., 2002; Sobolev and Chaussidon, 1996). Critically, Hamada and Fujii (2008) and Zimmer et al. (2010) report that a water content of 2 wt.% separates “dry” tholeiitic (olivine+ plagioclase/orthopyroxene) from “ wet” calc-alkaline (clinopyroxene+ magnetite) magmatic fractionation trends. Arc basalts are also characterized by distinctly higher oxidation states than MORB (up to 2 log units above the fayalite –magnetite– quartz buffer: Δ FMQ+2; Ballhaus, 1993; Brandon and Draper, 1996;

Parkinson and Arculus, 1999; Rowe et al., 2009). The relatively high oxidation state of arc magmas is a critical factor in their subsequent metallogeny, and originates from oxidative sea �oor alteration of the oceanic plate (Staudigelet al., 1996), transmitted into themantlewedge by the metasomatic � uid � ux (Brandon and Draper, 1996; Kelley and Cottrell, 2009; Malaspina et al., 2009). 2.1.1. Behavior of metals

Most base and precious metals would be expected to have at least moderate solubilities in the hot, relatively oxidized, saline aqueous �uids exsolved from the downgoing slab. In particular, as noted in Section 2.1, Pb,As, and Sb are strongly mobilized by such �uids,possibly along with Tl and Cu (Noll et al., 1996). The behavior of highly siderophileelements(HSE)suchasAuandPGEislesswellknownunder these conditions, but studies of metasomatized mantle xenoliths from island arc lavas suggest that Au, Re, and the Pd-group elements (including Pt) are mobilized into the mantle wedge during subduction metasomatism (Dale et al., 2009; Kepezhinskas et al., 2002; McInnes et al., 1999; Sun et al., 2004a; Widom et al., 2003). The volumetric extent and ef �ciency of mobilization of metals into themantlewedgebythisprocessareunknown,but �uid metasomatism clearly represents one viable mechanism for metal transfer into arc magma sources. The behavior of chalcophile and siderophile metals during subsequent partial melting of the metasomatized mantle wedge depends critically on oxidation state (f O2) and sulfur fugacity(f S2), because these parameters control the stability and abundance of sul �de phases. Gold and PGE partition strongly into sul �de phases relative to silicate melts, but Cu to a somewhat lesser degree (Campbell and Naldrett, 1979; Peach et al., 1990), so if sul�des are abundant in the magma source region, partial melts will be depleted in Au and PGE relative to Cu (Fig. 2). Under the high f O2 and f S2 conditions of the supra-subduction zonemantlewedge,thebulkofthesulfur �uxwilllikelyconsistofSO 2 or


sulfate, dissolved �rst in slab �uids and then magma (Carroll and Rutherford, 1985; Jenner et al., 2010; Jugo et al., 2005a ). However, because of theequilibria between varioussulfur species,at high f S2 some condensed sul�de phases will likely also be present ( McInnes et al., 2001). Consequently, Richards (2009) suggested that normal arc magmas will be minimally depleted in Cu (due to its higher abundance and moderate chalcophile nature) relative to sparse, highly siderophile elementssuch as Au andPGE, whichwill be strongly retained in residual sul�de phases in the source region (e.g., Hamlyn et al., 1985; Mitchell andKeays,1981;Peachetal.,1990). This mayexplain theCu-rich nature of typical arc-related porphyry deposits (relative to Au and PGE; Richards, 2005). In contrast, Au-rich porphyry deposits may require atypical subduction-related or collisional tectonic settings and petrogenetic processes, which act to destabilize residual sul �de phases and render Au incompatible (e.g., Jégo et al., 2010; Richards, 1995, 2009; Sillitoe, 2000; Solomon, 1990; Wyborn and Sun, 1994; see Section 2.5). The low abundances of PGE in many arc-related ore deposits suggest a further separation of these elements from Au and Cu, perhaps through theformation of residual platinoidalloy phases(e.g.,Barneset al., 1985; Borisov and Palme, 1997; Kepezhinskas et al., 2002; Peach et al., 1990 ) or Cr-spinels (into which Ir-group PGE strongly partition; Hattori et al., 2010; Righter et al., 2004). 2.2. Sediment dehydration and/or melting in subduction zones

Sea�oor sediments on the surface of the downgoing slab are another potential source of metasomatic contributions to the mantle wedge. Much of this sedimentary material will be scraped off at the trench to form an accretionary prism, but varying amounts may also be subducted, depending on the degree of coupling between the upper andlower plates, andalso the sediment input load (Fig. 1). Such sediments will be water-rich and pelitic in bulk composition, and thus are more likely to undergo partial melting under subduction zone conditions than basaltic oceanic crust (Hermann and Spandler, 2008).

Porphyry Cu potential arc magmas

106

i n e d c h id e n r i l s u l f e A u i d u a r e s

105 104 e f d l i s u i n u C e f d l i u n s i A u

103 102

) ) b m p p 10 p p ( ( u u 1 A C

a g m m a n i C u

0.1

10–2 10

10–4

50 ppm Cu

m a Au maximized in a g m magma (R ≥ 10 6 ) d i n e t e l p Sulfide/silicate melt partition d e coefficients: A u

1

10

5 ppb Au

D(Cu) = 103 D(Au) = 105

Metal concentrations in magma in absence of sulfide: Cu = 50 ppm Au = 5 ppb

10–5 102

103

104

105

106

107

Nevertheless, Aizawa et al. (1999); Dreyer et al. (2010); and Duggen et al. (2007) have suggested that dehydration is the principal process affecting sediments down to depths of ~100 km (i.e., to below the volcanic arc), with melting only occurring signi �cantly at greater depths when temperatures exceed ~800 °C (possibly re �ected in the geochemistry of some back-arc magmas). Sediment contributions to the source of arc magmashave been the subject of numerous studies, with the least ambiguous evidence coming from island arcs (e.g., MacDonald et al., 2000; Thirlwall et al., 1996; Wysoczanski et al., 2006 ). In continental arcs, it can be dif �cult to distinguish between chemical and isotopic signatures from subducted continent-derived sediment versus crustal contamination during magma ascent (e.g., Hildreth and Moorbath, 1988; Kemp et al., 2007): both sources will contribute incompatible elements and crustal isotopic values to primary mantle-derived arc magmas (Breeding et al., 2004). Trace elements commonly used as indicators of sediment contributionstoarcmagmasareBa,B,Be,Th,andPb( Dreyeret al., 2010; Johnson andPlank, 1999),andBa/LaandTh/Laratioscanbeusedasameasureof sediment versusmantlesourcecomponents (Plank,2005; Walkeret al., 2001). In particular, the cosmogenic radioisotope 10Be can be used as a tracer of recent ( b 10 Ma) introduction of sediments into arc magma sources (Dreyer et al., 2010; Morris et al., 1990). However, although a clear sediment-derived isotopic signature can be observed in many island arc systems, the volumetric contribution of sediments to island arc magmas seems to be relatively minor (Hawkesworth et al., 1994; Kilian and Behrmann, 2003; Poli and Schmidt, 2002; Stern et al., 2006). One additional element that may be added to the mantle wedge from subducted sediments is sulfur, as suggested by the positive δ 34S compositions of arc magmas (de Hoog et al., 2001), which are similar to those of sea �oor sediments (Alt et al., 1993). Analyses of glass inclusions in olivine from primitivearc magmas reveal concentrations of up to 2900 ppm S (de Hoog et al., 2001), and Jugo et al. (2005b) measured experimental concentrations of up to 1.5 wt.% S in oxidized arc basalts. These high sulfur contents have great signi �cance for the behavior of chalcophile and siderophile metals (see Sections 2.1.1, 2.5.1, and 3). 2.2.1. Behavior of metals

Porphyry Cu-Au potential magmas

Cu maximized in magma (R ≥ 10 3 )

a g m m a n i A u

–3

5

108

R = (mass of silicate melt)/(mass of sulfide) Fig. 2. Concentrations of Cu and Au in silicate magma and coexisting sul �de liquid as a

function of R= [mass of silicate melt]/[mass of sul �de melt] (Campbell and Naldrett, 1979; diagram modi �ed from Richards, 2005, 2009 ). At R-factors below ~10 2, magmas will be depleted in both Cu and Au. At R-factors between ~10 2–105, magmas will be depleted in Au but essentially undepleted in Cu (porphyry Cu-potential magmas). At Rfactors N 105, magmas will be undepleted in Au and Cu (porphyry Cu –Au–potential magmas). Acorollary of thisdiagram is that arc magmatism will leave small amounts of relatively Au-rich sul�de in the mantle source or lithosphere during fractionation, which can be remelted during post-subduction tectonomagmatic processes, to form small-volume, alkaline, porphyry Au-potential magmas.

Lead, which is signi �cantly enriched in the continental crust (and crustally-derived sediments) relative to themantle, is theonly metal for which a clear sedimentary source can be inferred in some island arc magmas, because it can be readily identi �ed by its radiogenic isotopic composition compared with depleted mantle sources. However, in continental arcs, distinguishing a subducted sediment source of radiogenic Pb from crustal contamination during magma ascent is very dif �cult (e.g., Barreiro, 1984; Chiaradia et al., 2004; Kontak et al., 1990). This has led to diverging opinions: for example, Aitcheson et al. (1995);Hildrethand Moorbath (1988); James(1982);Kay et al.(1999); and Tilton et al. (1981) concluded that the bulk of the radiogenic Pb in central Andean magmas comes from upper plate crustal sources, whereas Macfarlane (1999); McNutt et al. (1979); Mukasa et al. (1990); and Sillitoe and Hart (1984) preferred a subducted sediment source for Pb in some Andean igneous rocks and ore deposits. Inferringa sea�oor sedimentsource for othermetallic components in arc magmas (and related ore deposits) is much more speculative, and generally involves the subduction of metal-rich manganese nodules or even massive sul �de deposits. The latter, however, likely oxidize and disperse geologically rapidly after formation on the sea�oor (e.g., Edwards, 2004; Herzig et al., 1991) unless quickly buriedby lava; they might thus only be expected to be subductedwith very young oceanic crust. Few studies have speci �cally invoked a subducted sediment source for ore metals other than a component of Pb, and the majority of authors have concluded that such a source is either unnecessary or unproven (e.g., Burnham, 1981; Chiaradia et al., 2004; de Hoog et al., 2001; Fontboté et al., 1990 ).

6


2.3. Oceanic slab melting in subduction zones

The question of whether the downgoing oceanic slab, or more speci�cally the basaltic oceanic crust, melts during subduction has prompted lively debate recently, not only in the petrology literature (e.g., Conrey, 2002; Defant and Kepezhinskas, 2001, 2002; Garrison and Davidson, 2003), but also amongst economic geologists because of the suggestionthat “slabmelts” mightinsomewaybeuniquelyfertileforlater porphyry ore formation (e.g., Mungall, 2002; Oyarzun et al., 2001, 2002; Sajona and Maury, 1998; Thiéblemont et al., 1997; for contrary opinions, see: Rabbia et al., 2002; Richards, 2002; Richards and Kerrich, 2007 ). The idea that the hydrated basaltic ocean crust might melt during subduction was an early assumption of the plate tectonic revolution, because it seemed conveniently to explain the relatively felsic composition of arc magmas (as opposed to basalts formed by melting of peridotitic mantle). The theory was given credence by the experiments of Rapp et al. (1991) and Rapp and Watson (1995), who showed that melting of amphibolite under upper mantle conditions (1025 °C and 0.8–1.6 GPa) could generate an intermediatecomposition tonalite–trondhjemitemelt,not dissimilar to anarcandesite. Defant and Drummond (1990) termed the products of subducted slab melting “adakites”, after a single anomalous lava �ow on Adak Island in the Aleutians described by Kay (1978). Because garnet should be present in the eclogitic source of these magmas, Defant and Drummond (1990) argued that such melts could be distinguished by anomalously low concentrations of heavy rare earth elements (HREE) and Y (which are compatible with garnet) relative to lightrare earth elements(LREE), and high concentrations of Sr (because of the absence of plagioclase at such depths). Thus, slab melts, or adakites, could be distinguished on Sr/Y versus Y, or La/Yb versus Yb diagrams from normal arc magmas formed in the absence of garnet. However, despite the theoretical possibility of melting subducted oceanic crust, most thermal models of subduction zones indicate that temperatures in the slab do not normally reach melting conditions (N 800 °C) prior to dehydration and eclogitization (Fig. 1), which would render the slab infusible (e.g., Davies and Stevenson, 1992; Peacock, 1996; Poli and Schmidt, 2002). Thus, Defant and Drummond (1990) proposed that slab melting might be restricted to the subduction of young (≤ 25 m.y. old) and therefore still hot oceanic crust, and Peacock et al. (1994) were even more restrictive ( b 5 m.y. old). Other relatively uncommon scenarios that might result in slab melting include shallow or stalled subduction (whereby the slab has more time to heat up at shallow depths; Gutscher et al., 2000; Peacock et al., 1994), ridge subduction (Guivel et al., 2003; Kay et al., 1993 ), or edge-melting of detached slabs or slab windows (Haschke and Ben-Avraham, 2005; Thorkelson and Breitsprecher, 2005; Yogodzinski et al., 2001 ). A further complication is added by the fact that most adakites described in the petrology literature arenot in fact primary slab melts, but are substantially evolved, having reacted or hybridized with the asthenosphere during ascent (and likely also the upper plate lithosphere). This modi �cation to the adakite slab-melting model is required to explain the high contents of MgO, Ni, and Cr present in some adakites relative to expected levels for hydrated basalt partial melts (Defant and Kepezhinskas, 2001; Drummond et al., 1996; Martin, 1999; Martin et al., 2005; Yogodzinski et al., 1995 ). Direct evidence for slabmeltingis lacking,but supra-subductionzone xenoliths from the Tabar-Lihir-Tanga-Feni (Papua New Guinea), Philippine, and Patagonian arcs preserve hydrous, silica-rich glass inclusions that are thought to represent migrating slab melts (respectively: Kilian and Stern, 2002; McInnes and Cameron, 1994; Schiano et al., 1995 ). The glass inclusions characterized by Schiano et al. (1995) were calc-alkaline in composition, with high incompatible element and low Ti, Nb, and Y contents, high LREE/HREE ratios, and homogenization temperatures of ~920 °C. They thus compositionally resemble melts that would be predictedto form from slab melting,and appearto provide evidence that this process occurs at least locally where conditions permit.

The chemical difference between slab dehydration and slab melting would seem to be rather small, given that both media would be enriched in volatiles, incompatible elements, and silica. Indeed, as noted in Section 2.1, there may well be a continuum between silica-rich aqueous �uids and aqueous silicate melts at greater depths in subduction zones (Kawamoto, 2006; Kessel et al., 2005a,b; Manning, 2004; Portnyagin et al., 2007). This likely explains why the debate between slab melting and dehydration is somewhat intractable, and mostly hinges on subtle trace element characteristics. 2.3.1. Behavior of metals

Slab melts are predicted to be volatile-rich (including H 2O, S, and Cl) and oxidized, and thus, like hydrous slab �uids, would be expected to be able to transport base and precious metals at least to some degree. However, analyses of such metals (except iron) are not reported in most melt inclusion studies (e.g., Kilian and Stern, 2002; McInnes and Cameron, 1994; Schiano et al., 1995 ), so there are no direct constraints on the capacity of such melts to transfer chalcophile and siderophile metals from the slab to the mantle wedge. In a study of metasomatized mantle xenoliths from a submarine volcano near Lihir Island, Papua New Guinea, McInnes et al. (1999) concluded that enrichmentsin Cu,Au,and PGEwere caused by slab �uid metasomatism, rather thanmelts. In contrast,Kepezhinskaset al. (2002) measured the concentrations of Au and PGEs in mantle xenoliths from the Kamchatka arc, and suggested that a � uid-transported component could be distinguished froma slabmelt component by co-enrichments in PGE and high �eld strength elements (HFSE) in thelatter, because of the low capacity of aqueous �uids to carry HFSE. Intriguingly, they noted no suchcorrelationbetweenHFSE-enrichments andAu,andconcluded that, whereas PGEmight be transportedinto themantle wedgeby both �uids and melts, Au was likely only carried by hydrous �uids. Two theoretical studies have proposed that slab melts should be unusually effective as metal-transporting and ore-forming agents. Oyarzun et al. (2001) argued that slab melts should be unusually oxidized and rich in H 2O and SO2 (relative to normal arc magmas derived by asthenospheric partial melting), although no evidence was given for this assertion. Such magmas, they argued, should be particularly suitable for the formation of magmatic–hydrothermal porphyry copper deposits upon emplacement in the upper crust. Mungall (2002) presented a theoretical model for oxidation of the mantle wedge by Fe3+-rich slab melts to the point of complete sul�de destruction, thereby rendering chalcophile and siderophile elements incompatible in mantle phases, and free to partition into silicate melts. He argued that ferric iron is a much stronger oxidant than slabderived water, and that slab melts should be richin Fe3+ generatedby oxidative sea�oor alteration. Thus, slab melts might be uniquely favorable for the subsequent generation of metal-rich, and particularly Au-rich, magmas derived from the mantle wedge. Mungall's(2002)model mayhave applicability forless commonAurich porphyry depositsformed in atypical subduction zonesettings that might cause slab melting, but does not seem well suited to explain regulararc porphyry Cu deposits. In either case, metalsare envisagedto be sourced from the mantle wedge, not the slab. In contrast, Oyarzun et al.'s (2001) model does not address the source of metals, and is at root based on the assumption that slab melts are uniquely more H 2O- and SO2-rich, and more oxidized than normal arc magmas, leading to speci�c oredepositionalprocessesrather thansource processes. It is not clear that these assumptions are justi �ed, and some have argued that slabmelts might in fact be relatively reducing because of the additional presence of organic-rich sediment melts (Wang et al., 2007a). 2.4. Supra-subduction zone lithospheric melting: the MASH process

Hydrous basaltic magmasgenerated in themantle wedge will have temperatures in excess of 1000 °C (Eggins, 1993; Grove et al., 2006; MacDonald et al., 2000), and perhaps as highas 1350 °C (Schmidt and

7


Poli, 1998; Tatsumi, 2003). Because their densities will be lower than the mantle but not the crust ( Herzberg et al., 1983), they will tend to risefromtheirasthenospheric source region and penetratethe mantle lithosphere, but pool at the crust/mantle density barrier ( “level of neutral buoyancy”: Fig. 3; Fyfe, 1992; Hildreth, 1981). Here, if the �ux of magma and heat is maintained and supplemented by the latent heat of crystallization as the magma begins to crystallize, high temperatures can be brought to bear on lower crustal rocks that will cause partial melting (Annen et al., 2006; Bergantz and Dawes, 1994; Huppert and Sparks, 1988; Klepeis et al., 2003; Petford and Gallagher, 2001; Rushmer, 1993). Hildreth and Moorbath (1988) suggested that it is the interaction between this hot, hydrous basalt �ux from the subduction zone and felsic crustal partial melts that gives rise to the uniform composition of andesites in continental volcanic arcs, by a process they dubbed melting –assimilation–storage–homogenization (MASH). In a re�nement of this model, Annen et al. (2006) referred to the region of magma –crust interaction as a “hot zone” (Fig. 3). Because garnet is a product of such lower crustal fractionation and partial melting processes (Alonso-Perez et al., 2009; Berger et al., 2009; Dufek and Bergantz, 2005; Garrido et al., 2006; Hansen et al., 2002; Klepeis et al., 2003; Rushmer, 1993; Wolf and Wyllie, 1994 ), derivative calc-alkaline magmas may display trace element compositions that resemble adakites (see Section 2.3) but which are unrelated to slab melting (Klepeis et al., 2003; Macpherson et al.,

2006; Richards and Kerrich, 2007; Tulloch and Kimbrough, 2003 ). Such common processes, affecting batches of magma crystallizing and fractionating at different crustal depths (e.g., Annen et al., 2006) are entirely consistent with petrological observations in arc volcanic systems where “adakite-like” (i.e., high-Sr/Y) andesitic lavas may be interlayered with “normal” andesites in a single volcano, and do not require a fundamental change in magma source 100 km below the volcano (e.g., Feeley and Davidson, 1994; Grunder et al., 2008; Richards et al., 2006a). Once these hybrid magmas reach basaltic andesitic to andesitic compositions, their densities are low enough to allow them to rise through the lower continental crust (Herzberg et al., 1983), but they tend to stall again at a second density barrier in the middle to upper crustbelowlight supracrustalrocks. This isthelevel(5 –10 km) at which large arc batholiths will form if the � ux of magma is sustained, and is also thelevelat whichevolved felsicmeltsand volatilesare accumulated (Fig. 3;see Richards,2003, andreferences therein).Thesevolatilesdrive buoyant, bubbly, evolved magma upwards into the cover rocks to form subvolcanic stocks and dikes, or explosive volcanic eruptions if they reach surface. The volatiles may also separate from the magma � ux to form a separate �uid plume, which ultimately vents at the surface (fumaroles) but mayalso formporphyry- and epithermal-type deposits in the hypabyssal and near-surface environment (see discussion of these processes in Sections 3 and 4).

Volcanic arc

Epithermal deposits Porphyry deposits

Hydrothermal alteration Sea level

0 km

Mid- to upper crustal batholithic complex

Feeder dikes Continental crust

Lower crustal MASH or “hot zone”

6 00 ° C

50 km

10 0 0° C

D e h y d r a t i n g o c O e a c n i l i t h m a e a n c o s n t i c 6 c r u p h l e 0 s t 0 e r e C °

M a n t l e c o r n e r

Subcontinental mantle lithosphere f l o w

Partial melting 1 0 0 0 ° C

Metasomatized asthenosphere

1 4 0 0 C °

1 4 0 0 ° C

Asthenosphere 100 km

Fig. 3. Schematic section through a continental arc, showing the development of a MASH or “ hot zone” at the base of the crust where basaltic arc magmas pool at their level of neutral

buoyancy, differentiate, and interact with crustal rocks and melts. Evolved, less dense, andesitic magmas rise into the mid-to-upper crust where they pool at their new level of neutral buoyancy to form batholithic complexes. Along with volcanic structures, porphyry and epithermal deposits may form at shallower levels above these batholithic complexes where exsolved magmatic �uidsascend,cool, andinteract withnear-surface upper crustal rocks. Modi�ed from Richards(2003, 2005); sources:Hildrethand Moorbath (1988), Winter (2001), Annen et al. (2006), and Sillitoe (2010).

8


2.4.1. Behavior of metals

Some of the clearest evidence for the involvement of crustalrocks in continental arcmagmas comesfrom Pb isotopic data(e.g.,Wörneret al., 1992), although as noted in Section 2.2, there may be ambiguity between lower crustal melting and themeltingof subducted continentderived sediments. Lead and uranium are much more abundant in the bulk continental crust (11 ppm Pb, 1.3 ppm U; Rudnick and Gao, 2003) or lower continental crust (4 ppm Pb, 0.2 ppm U; Rudnick and Gao, 2003) than the primitive mantle ( b 0.2 ppm Pb, b 0.02 ppm U; Taylor and McLennan, 1985; Sun and McDonough, 1989), MORB (0.3 ppm Pb, 0.05 ppm U; Sun and McDonough, 1989), or typical low-K ma�c arc andesites (b 1.8 ppm Pb, b 0.2 ppm U; Gill, 1981), so it only requires small amounts of contamination by radiogenic crustal lead to signi�cantly modify a magma's Pb isotopic composition. Thus, it is not clear that a particularly large amount of Pb in arc magmas is sourced from crustal rocks versus the subduction zone, and Macfarlane et al. (1990) have argued that the crustal contribution is minimal in Central Andean magmas and ores. Moreover, porphyry-type systems are typically not Pb-rich, except in late-stage skarns and distal veins where someof the Pb may have beenderived fromlocal host rocks (e.g., Mukasa et al., 1990). Most researchers have assumed that, because of the higher concentrations of Cu in primitive andesites (145 ppm; Gill, 1981) compared with the bulk continental crust (27 ppm; Rudnick and Gao, 2003), the bulk of Cu in porphyry-type deposits is mantle-derived. A similar assumption is made for Au, although the primitive mantle and continental crust actually have comparable concentrations (1 –3 ppb Au; Rudnick and Gao, 2003; Taylor and McLennan, 1985 ). Moreover, porphyry Cu–(Au) deposits are found in association with mantlederived arc magmas worldwide, regardless of crustal type (oceanic or continental) or thickness (Kesler, 1973), so a crustal heritage for these metals does not appear to be critical. Nevertheless, a lower crustal source, perhaps hybridized with mantle-derived magmas, has been proposed by Bouse et al. (1999) for both magmas and metals in the Laramide porphyry systems of Arizona. Moreover, Titley (1987, 2001) has speci�cally suggested that Au and Ag are crustally derived in a range of ore deposits including porphyries in southwestern USA, because the ratios of these elements correlate closely with two distinct basement domains in this region. Given that Au is not especially enriched in the mantle (see above), and that Ag is in fact more abundant in the crust than the mantle (80 ppb in the bulk continental crust, versus b 19 ppb in the primitive mantle; Taylor and McLennan, 1985), a crustal source for at least some proportion of these minor metals, and especially Ag, in arc magma-related systems may be reasonable. However, it seemsunlikely that this argument can be extended to copper, except perhaps on the margins. 2.4.2. Sources of Mo

Molybdenum occurs in varying amounts in porphyry-type deposits, rangingfrom trace levels(b 0.01 wt.%Mo) in porphyry Cu–(Mo)deposits, where it maynot evenbe recovered asa byproduct, to being the main ore component (up to 0.3 wt.% Mo) in porphyry Mo deposits(Seedorff et al., 2005; WestraandKeith, 1981).At the Mo-rich end of thespectrum,there aretwoclearlydifferenttectonomagmaticassociations, onlyone of which is directly related to subduction: calc-alkaline porphyry Mo deposits are generally relatively low grade (0.1–0.02 wt.% Mo; Carten et al., 1993), whereas intra-cratonic rift-related deposits associated with high-silica, �uorine-rich, peraluminous granitoids are relatively high grade (0.1 – 0.3 wt.% Mo; e.g., “Climax-type” deposits; Cartenetal.,1993;Kirkhamand Sinclair, 1996; Sinclair, 2007; Stein, 1988; White et al., 1981 ). Kesler (1973) noted a general association (with exceptions) of porphyry Cu–Au deposits in island arcs, and porphyry Cu –Mo deposits in continental arcs, and it is clear that the peraluminous felsic rocks associated with rift-related Climax-type porphyry Mo deposits are primarily of continental crustal origin (Farmer and DePaolo, 1984; Stein, 1988). This has led to one view that Mo might be predominantly

derived from continental crustal sources ( Farmer and DePaolo, 1984; Stein, 1988; Klemm et al., 2008; White et al., 1981). On the other hand, minor amounts of Mo do occur in some island arc-related porphyry deposits where no continental crustal sources are inferred (Westra and Keith,1981), so a mantle (subduction zone) sourcefor at least someMo cannot be excluded. Moreover, Blevin and Chappell (1992) and Blevin et al. (1996) have demonstrated a continuum from Cu –Au deposits associated with unevolved, ma�c I-type granitoids to W –Mo deposits associated with cogenetic, evolved granites in eastern Australia, suggesting a common, magmatic source for all of these elements. A complication is introduced in the Climax-type deposits, because although the immediate source of the Mo-bearing �uids is felsic magma of clear crustal origin, many deposits also show a close genetic association with ma�c alkaline magmas, which may have introduced volatiles, S, and possibly Mo into the evolved felsic magma chamber (Audétat, 2010; Carten et al., 1993; Keith et al., 1986, 1998 ). Keith et al. (1997), Hattori and Keith (2001), and Maughan et al. (2002) have also suggested that injections of ma �c alkaline magmas into the evolving Bingham Canyon magmatic system may have given rise to the unusually large size and high grades of this porphyry Cu –Mo–Au deposit. Along the same lines, Pettke et al. (2010) have proposed that the unusual Cu–Mo–Au endowment of the southwestern USA (e.g., the giant Bingham, Butte, Climax, Henderson, and Questa porphyry Cu–Mo–Au and porphyry–Mo deposits) re �ects Cenozoic remobilization of Proterozoic subduction-metasomatized subcontinental mantle lithosphere (see Section 2.5). Thus, at this time there is no consensus regarding the crustal versus mantle origin of molybdenumin porphyry deposits,although it is clear that the highest grade porphyry Mo deposits are formed in intra-plate continental settings, and if a mantle source is important in these cases, it is not directly related to subduction activity but rather to rifting or reactivation of previously subduction-enriched lithospheric sources. 2.5. Lithospheric melting during post-subduction events

A number of mineral deposits with broad similarities to those formed by subduction-related processes are also found in postsubduction tectonic settings, such as subduction reversal or migration, arc collision, continent –continent collision, and post-collisional rifting. They include porphyry Sn–W, Mo, Cu–Mo, and Cu–Au deposits and epithermal Au deposits, and in many cases are only known not to be directly related to subduction because of precise geochronology and plate tectonic reconstructions that place their formation after subduction has demonstrably ceased. Associated magmas are typically calcalkaline, but tend towards somewhat more alkaline compositions compared with normal arc magmas (Richards, 2009). In complex accretionary arcs, it can be very dif �cult to ascribe any given pluton (and any associated mineral deposits) to a particular subduction or collisional event, because subduction commonly continues after collision, albeit normally with a shift in the locus of magmatism. However, in continent –continent collision zones or where arccollisionterminates subduction, there canbe greater certainty about the timing of cessation of subduction magmatism. Consequently, it is in collisional orogens such as theNeo-Tethyan belt of southeasternEurope andsouthern Asia that some of theclearest examples ofpost-subduction magmatism and mineralization are found. These include, from east to west,the Miocene Gangdeseporphyry Cu–Mo belt in theTibetan orogen (Houetal.,2006,2009;Yangetal.,2009 ), theMioceneKermanporphyry Cu–Mo belt in southeastern Iran (Sha�ei et al., 2009), the Miocene Sari Gunay epithermal Au deposit in northwestern Iran (Richards et al., 2006b), theEocene Çöpler epithermal Au deposit insoutheastern Turkey (Keskin et al., 2008; Kuscu et al., 2010 ), the Pliocene Kisladag porphyry Au deposit in western Turkey, the Miocene Skouries porphyry Cu –Au– PGEdepositinGreece(Economou-EliopoulosandEliopoulos,2000),and


the Roşia Montană epithermal Au deposit in Romania (Manske et al., 2006; Neubauer et al., 2005). Similarly, in the southwest Paci �c ocean, accurate plate tectonic reconstructions permit the identi�cation of a number of postsubduction porphyry and epithermal deposits, such as the Grasberg porphyry Cu–Au deposit in Papua, Indonesia (Cloos et al., 2005; Paterson and Cloos, 2005), the Ok Tedi porphyry Cu –Au deposit (van Dongenet al., 2010) and the Porgeraalkalic-typeepithermal Au deposit in mainland Papua New Guinea (Richards et al., 1990; Richards and Kerrich, 1993), the Lihir alkalic-type epithermal Au deposit on Lihir Island,PapuaNewGuinea(Carman, 2003; Kennedy et al., 1990),andthe Emperor alkalic-type epithermal Au deposit in Fiji (Gill and Whelan, 1989; Setter�eld et al., 1992). (For reviews of alkalic-type epithermal deposits, see Jensen and Barton, 2000 and Richards, 1995). Because subduction has ceased in these regions, a fresh supply of �uids, volatiles, and other slab-derived components to the mantle wedge no longer exists. Nevertheless, the broad geochemical similarity of many of these magmas to normal arc magmas, including their hydrous and generally oxidized nature, suggests some link to subduction metasomatism. Consequently, many researchers have implicated upper-plate lithospheric sources, modi�ed by earlier subduction-related �uids and/or hydrous melts (e.g., Clemens et al., 2009; Cloos et al., 2005; Guo et al., 2007; Harris et al., 1986; Johnson et al., 1978; Pearce et al., 1990; Pettke et al., 2010; Richards, 2009 ). Previously subduction-modi�ed asthenosphere is unlikely to be a viable source except for a short period after subduction has ceased (e.g., Richards et al., 1990; Solomon, 1990), because such material will be quickly dispersed by mantle convection. The key to all of these models is subduction-derived water, which is most likely stored in amphibolitic cumulates, residual from the earlier arcmagma �ux andlocated in thedeep crust or mantlelithosphere(e.g., Claeson and Meurer, 2004; Davidson et al., 2007; DeBari and Coleman, 1989; Jagoutz et al., 2009; Larocque and Canil, 2010; Müntener and Ulmer, 2006; Tiepolo and Tribuzio, 2008). Water lowers the solidus of silicate assemblages, and will lead to the formation of hydrous partial melts during pro-grade metamorphism or ma �c melt invasion (Beard and Lofgren, 1991; Rushmer, 1991; Wolf and Wyllie, 1994 ). Thermal rebound in thickened orogenic crust, delamination of sub-continental mantle lithosphere, or post-collisional rifting (with ingress of asthenospheric melts into the lower crust in the last two cases) can all cause small-volume partial melting of arc-metasomatized lithosphere and/or hydrous lower crustal cumulates (Fig. 4; Brown,2010; Clemens et al., 2009; Harriset al., 1986; Richards,2009). Such melts, being derived from subduction-modi �ed sources, will share many of the characteristic geochemical features of arc magmas, including their relatively high water contents and oxidation states, and potentially metalcontents (seeSection 2.5.1). Thesmallervolume of partial melting to be expected in such tectonic settings will give these magmas a somewhat more alkaline composition than arc magmas (e.g., Clemens et al., 2009), and will also mean that large batholithic complexes are unlikely to be formed, consistent with the generally smaller and more isolated occurrence of such postsubduction magmatic systems (compared with arc-related Cordilleran batholiths, or collisional S-type batholiths; Pitcher, 1997). Because these post-subduction magmas are derived from amphibolitic sources in which garnet (±titanite) is likely also present, and because their hydrous nature will suppress plagioclase fractionation (similar to other hydrous arc magmas), they may be characterized by elevated Sr/Y and La/Yb ratios; that is, they may display adakite-like trace element characteristics. 2.5.1. Behavior of metals in subduction-modi �ed sources

As discussed in Section 2.1, arc magmas are characterized by high f O2 and f S2 relative to normal melts from MORB-depleted asthenosphere. Consequently, such magmas may be sul �de-saturated (at oxidation states up to ΔFMQ+2.3; Jugo, 2009) despite sulfur being

9

predominantly present in the melt as sulfate or SO 2 (Carroll and Rutherford, 1985). Xenoliths from supra-subduction zone mantle (McInnes et al., 1999) and samples of ma �c cumulates from lower crustal arc roots (Fig. 5; Canil et al., 2010; Greene et al., 2006; Jagoutz et al., 2007) reveal the common presence of small amounts of sul �de, typically trapped as inclusions in silicate phases (suggesting a primary magmatic rather than secondary hydrothermal origin). Hamlyn et al. (1985), Richards (1995, 2009), Solomon (1990), and Wyborn and Sun (1994) have explored the role of residual sul �de phases on the metal content of fractionating magmas, and also of partial melts formed during later, post-subduction melting events. The high partition coef �cients for chalcophile and highly siderophile elements (HSE) between sul �de phases and silicate melt mean that such metals should be strongly partitioned into any coexisting sul �de phases (Campbell and Naldrett, 1979; Peach et al., 1990). As shown in Fig. 2, at high abundances of sul �de relative to silicate melt (low Rfactor; Campbell and Naldrett, 1979), the melts will be depleted in all of these chalcophile and siderophile elements. In contrast, at intermediate abundances of sul�de (intermediate R-factor), only originally sparse HSE will show signi�cant depletions. This led Richards (2005, 2009) to propose that small amounts of sul �de left behind as residual phases from fractionation of arc magmas in the deep lithosphere (or asthenosphere) will not signi �cantly deplete those magmas in relatively abundant chalcophile elements such as Cu and Mo, but might signi�cantly deplete them in highly siderophile elements such as Au. This would give rise to magmas with relatively high Cu/Au ratios (which might form Cu-rich porphyry deposits), but would leave a residue of potentially HSE-rich sul �des in the mantle and/or lower crustal amphibolitic cumulate arc roots. AsnotedinSection 2.5, subduction-modi�ed asthenosphericsources will be rapidly convected away when subduction ceases, and so could only contribute to immediately post-subduction magmatism. In contrast, deep crustal amphibolites are preserved in the lithosphere, andwillbesusceptibletopartialmeltingatanylatertimeduetothermal reboundor reheatingby invading asthenospheric melts.Under lowerf S2 post-subduction conditions (a �ux of S from the subduction zone is no longer present), any residual sul �de phases would likely dissolve into theS-undersaturated silicate melt, carrying their metal loads with them (e.g., Ackerman et al., 2009). Richards (2009) proposed that this might explain the occurrence of Au-rich porphyry and related epithermal systems in some post-subduction settings, such as the alkalic-type epithermal Au deposits of theSW Paci�c, and various post-collisionalAu deposits in the Balkans –Turkey–Iran Neotethyan belt. Pettke et al. (2010) have proposed a similar model for giant porphyry Cu –Mo–Au deposits in the southwestern USA. This model can also explain the occurrence of Au –poor porphyry Cu–(Mo) deposits in post-subduction settings, such as those in Tibet and Iran (Hou et al., 2009; Sha�ei et al., 2009; Wang et al., 2007b), the only difference being that in this case larger proportions of sul �de may have fractionated out from the original arc magmas in the deep crust. Such sul �des would have retained signi �cant amounts of the subduction �ux of Cu and Mo, but HSE would be diluted to low concentrations by the greater volume of sul �de (low R-factor; Fig. 2). Second-stage melts from such cumulate sources would therefore be Cu–(Mo)-rich, but not necessarily Au-rich. A control on these two scenarios (abundant Cu-rich residual sul �de versus sparse but HSE-rich residual sul �de, or low versus high R-factor) mightbe theaverageoxidation state andsulfur fugacityof thegenerative subduction system.In moreoxidizedor S-poor systems, smaller volumes of HSE-rich sul�de would exsolve from the silicate melt (high R-factor; Campbelland Naldrett, 1979), whereasin lessoxidized or S-rich systems, larger volumes of Cu-rich but HSE-poor sul �de would exsolve (low Rfactor; Fig. 2). In particular, the proportion of sul�des exsolving from arc magmas may be very sensitive to small changes in their oxidation state, because of the rapid change from sul �de to sulfate dominance in magmatic systems between Δ FMQ+1 and +2 ( Jugo et al., 2010). The

10


Fig. 4. Post-subduction tectonic environments conducive to the formation of porphyry and epithermal deposits by remobilization of previously subduction-modi �ed lithosphere

(modi�edfrom Richards, 2009). (a)PorphyryCu± Mo deposits formedin normalarc settings;a continental arcis shown, butsimilarprocessescan occur in matureisland arcs. (b–d) During post-subduction tectonic processes, previously subduction-modi �ed sub-continental lithospheric mantle (SCLM) or lower crustal hydrous cumulate zones residual from previous arc magmatism (black layer) may undergo small-volume partial melting. Such magmas may remobilize Au as well as Cu±Mo left behind in residual sul �de phases by arc magmatism, leading to the potential formation of porphyry Cu±Au±Mo and alkalic-type epithermal Au deposits. Magmas may be characterized by high Sr/Y and La/Yb ratios due to the presence of hornblende (±garnet, titanite) in the amphibolitic lower crustal source rocks. See text for discussion.

oxidation state of the mantle wedge will depend on the character of the �ux from the subducting slab (e.g., a higher proportion of subducted organic-rich sediment would lead to lower oxidationstates; Wang et al., 2007a); this property is therefore likely to have a characteristic average value along any given arc at any particular period of time. Variations in oxidation state over typical ranges for arc magmas (ΔFMQ=0 to +2; Ballhaus, 1993; Brandon and Draper, 1996; Blatter and Carmichael, 1998; Malaspina et al., 2009; Parkinson and Arculus, 1999; Rowe et al., 2009) will not greatly affect thepotential to form synsubduction porphyry Cu–(Mo) deposits, but might control the Cu/HSE ratio in later magmas formed by post-subduction melting of these sul�de-bearing residues. Speci�cally, Au-rich (low Cu/Au) post-subduction porphyries might form in settings where previous arc magmatism was relatively oxidized (sparse but HSE-rich sul �de residue), whereas Au-poor (high Cu/Au) post-subduction porphyries might form where previousarcmagmatism was relatively reduced(more abundantCu-rich sul�de residue). Such a mechanism might also explain why coeval belts of porphyry deposits tend to have characteristic Cu/Au ratios. Finally, partial melting of predominantly reduced,sul�de-rich crustal rocks in orogenic settings may lead to chalcophile and siderophile element-depleted, but potentially lithophile element-rich S-type magmas (see Section 2.6). 2.6. Crustal melting during post-collisional stress relaxation

Collisional orogens commonly undergo crustal thickening followed by extensional or transpressional collapse. Bimodal magmatism is

characteristic of suchtectonic settings, resulting from partialmelting of pelitic protoliths in the deep crust triggered by the heatfrom upwelling asthenospheric melts (Hildreth, 1981). Peraluminous S-type granites (Chappell and White, 1974) are subsequently emplaced as large batholith complexes in the mid- to upper orogenic crust (e.g., the Hercynian peraluminous granites of Europe; Barbarin, 1996; Clemens, 2003; Darbyshire and Shepherd, 1994; Harris et al., 1986; Wyllie et al., 1976). These granites tend to be enriched in lithophile rather than chalcophile elements, re �ecting their crustal origins, and may generate magmatic–hydrothermaldepositscontainingSn,W,U,Mo,REE,Li,Be,B, and F. 2.6.1. Sources of metals

Tin and especially tungsten commonly accompany molybdenum in porphyry deposits as trace metals and byproducts, but they also form a class of porphyries on their own, associated with S-type granites in continental orogens (Hart et al., 2005; Ishihara, 1981; Ishihara and Murakami, 2006; Kerrich and Beckinsale, 1988; Kirkham and Sinclair, 1996; Lehmann, 1982). The Hercynian tin granites of Europe, and the Bolivian and SE Asian tin belts are examples of such deposits, with mineralization occurring in skarns and greisens around the granite intrusions, and to a lesser extent as internal stockworks and disseminations (e.g., Černý et al., 2005; Meinert et al., 2005). As with the source magmas, metals in these deposits appear to be predominantly of crustal origin (Hedenquist and Lowenstern, 1994). For example, in a recent assessment of the source of Sn in the Cornubian batholith of SW England, Williamson et al. (2010) concluded that all of

J.P. Richards / Ore Geology Reviews 40 (2011) 1– 1 – 26

(a)

Chalcopyrite

(b)

Pyrite

Chalcopyrite Pyrrhotite

Fig. 5. Re �ected light photomicrographs of sul �de inclusions in amphibole-rich lower

crustal arc cumulates crustal cumulates from from:: (a) the Talke Talkeetna etna arc, Alaska; (b) the Bonanza arc, Vancouver Island, Canada (samples courtesy A. Greene and D. Canil, respectively).

the Sn could have been extracted from the crustally-derived granites. Uraniu Ura nium m is als alsoo sig signi ni�can cantlyenric tlyenriched hedin in cru crusta stall roc rocks ks ver versus sus the theman mantle tle (0.91 ppm versus ~ 0.02 ppm, respec respectively; tively; Taylor Taylor and McLenn McLennan, an, 1985), 1985 ), and so is unlikely to have a mantle source in such deposits. However, Dietrich et al. (1999) have suggested a possible role for However, mantle-derived magmas in triggering triggering volatile (and metal) release from evolve evo lved, d, fel felsic sic mag magmas mas in the Boli Bolivia vian n tin bel belt, t, and Wal Walshe she et al. (20 (2011) 11) have identi�ed a mantle Nd isotopic signature in tin granites from eastern Australia. In contrast, in the case of W skarns associated with Mo mineralization in calc-alkaline I-type magmas, a shared mantle origin with Mo might mig htbe be ind indica icated(e.g., ted(e.g.,Newbe Newberry rryand and Swans Swanson, on, 1986 1986), ), consis consistent tentwith with the similar siderophile tendencies of these two elements, and their position in the periodic table (group VIB). 3. Beh Behavi avior or of met metals als dur during ing mag magma ma fra fracti ctiona onatio tion n and exsolution exsolu tion in the upper crust

�uid

Key to the for format mation ion of mag magmat matic ic–hydro hydrother thermal mal depos deposits its of chalcophile and siderophile elements elements in the upper crust is the lack of signi�cant saturation with and loss of sul �de phases prior to aqueous volatile exsolution from a cooling magma (Candela, ( Candela, 1989b, 1992; Candela Cande la and Holla Holland, nd, 1986;Candela and Picc Piccoli, oli, 2005 2005;; Rich Richards ards,, 1995 1995;; Richards and Kerrich, 1993; Spooner, 1993). 1993 ). As discussed in Sections in Sections 2.1.1 and 2.5.1, 2.5.1, chalcophile chalcophile and siderop siderophile hile elemen elements ts partition

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strongly into sul �de phases exsolving or crystallizing from silicate melts. melt s. Thus Thus,, if exte extensiv nsivee frac fractiona tionation tion and remo removal val of magm magmatic atic sul�de phases were to occur, the remaining silicate melt would be strongly depleted in these elements ( Jugo ( Jugo et al., 1999; Lynton et al., 1993). 1993 ). This is in essence the model for formation of orthomagmatic sul�de deposits from relatively reduced ma �c magmas (e.g., Naldrett, (e.g., Naldrett, 1989), 1989 ), and perhaps explains the de �ciency of chalcophile elements such as Cu and Au in more reduced, evolved, Sn –W-bearing S-type magmas (Blevin (Blevin and Chappell, 1992; Hedenquist and Lowenstern, 1994). 1994 ). In more oxidized subduction-re subduction-related lated or post-subduction magmas, although sul�de saturation likely occurs at some point during their evolution (as indicated by the common presence of sparse sul �de inclusions in phenocrysts in arc volcanic volcanic rocks as well as lower crustal crustal arc cumulates; e.g., Burnham, e.g., Burnham, 1979; Halter et al., 2002; Hattori, 1997; Keith et al., 1997; Stavast et al., 2006; 2006 ; Fig. 5), 5), it may not occur to the extent that sul�des physically separate from the magma, or at least not in lar large ge amo amount unts. s. Ins Instea tead, d, sm small all sul�de dro drople plets ts or cr cryst ystals als maybe entrained in magma ascending buoyantly through the crust (e.g., Bockra Boc krath th et al. al.,, 200 2004; 4; Tom Tomkin kinss and Ma Mavro vrogen genes es,, 200 20033) or as inc inclus lusion ionss in silicate phenocrysts, and will not be substantially lost to the overall magma �ux. Indeed, several authors have argued that pre-concentration of ore metals in magmatic sul�de phases may be an important step in porphyry metallogenesis (e.g., Jenner et al., 2010). 2010). In these models, sul�de phases subsequently break down due to changes in oxidation state and sulfur fugacity in response to volatile exsolution and magnetite crystallization upon emplacement in the upper crust, thereby rendering metals available for redissolution in the volatile phasee (e.g. phas (e.g.,, Cyg Cygan an and Can Candel dela, a, 199 1995; 5; Halter Halter et al., 2002, 2002, 200 2005; 5; Jug Jugoo et al., 1999; Keith et al., 1997; Stavast et al., 2006 ). Other authors have argued that this process, while it may occur, is not critical to metal behavior in magmatic–hydrothermal systems, and that direct partitioning from the silicate melt to the hydrothermal � uid phase is the dominant mechanism (e.g., Audétat (e.g., Audétat and Pettke, 2006; Lynton et al., 1993; Simon et al., 2008; Sun et al., 2004b 2004b). ). As noted in Section 2.5.1 Section 2.5.1,, Richards Rich ards (2009 (2009)) sug sugges gested ted tha thatt sep separa aratio tion n of sm small all amo amount untss of sul�de from fro m arcmagma arcmagmass at dep depth th in low lower er cr crust ustal al MAS MASH H cum cumula ulate te zon zones es may provide a source of metals (especially HSE) for later post-subduction magmas, but that this process may not substantially affect the Cu content of the original arc magmas. Regardless of the exact role of magmatic sul �des, the ultimate relationship is that of partitioning of metals between silicate melts and exsolving hydrothermal �uids, with sul�des as a possible intermediary step. Our current understanding of these partitioning processes is now quiteadvanc quite advanced ed follow following ing seve several ral decade decadess of hydro hydrotherma thermall exper experiments iments (e.g., Candela (e.g., Candela and Piccoli, 1995) 1995 ) and more recently the advance of quantitative quant itativesingle single �uid and andmel meltt inc inclus lusion ion ana analys lysis is (e. (e.g., g., Hei Heinri nrich ch et al., 2003a,b), 2003a,b ), which has enabled direct measurement of metal contents in ore-forming �uid uidss an andd me melt lts. s. In thefoll thefollow owin ingg se sect ctio ions ns,, I re revi view ew so some me of the key factors factors in met metal al sol solvat vation ion and tra transp nsport ort in mag magmat matic ic – hydrothermal �uids. 3.1. Partitioning of metals from magma into exsolving hydrothermal �uid � uid

Subduction-re Subducti on-relatedmagmas latedmagmas comm commonly onlycont contain ain at least4 wt.% H2O during dur ing cru crusta stall asc ascent ent,, as evi eviden denced ced by the pre prese senceof nceof hor hornbl nblen ende de and biotite phenocrysts in many andesitic volcanic rocks and arc plutons (Burnham, 1979; Naney, 1983; Rutherford and Devine, 1988 ). In response to the decreasing solubility of water in silicate melts as pressure pres sure decr decrease eases, s, such hydrous magma magmass inev inevitabl itablyy exso exsolve lve an aqueous aque ous volat volatile ile phase upon emplacem emplacement ent at shal shallow low crus crustal tal levelsor levels or on eruption (Burnham, (Burnham, 1979, 1997; Eichelberger, 1995). 1995 ). This process has in the past rather confusingly been called � rst and second boiling (although neither process is technically “boiling”), the �rst event occurring during ascent and depressurization of the magma, and the

12


second occurring after emplacement as crystallization progressively increases the concentration of volatiles in the residual melt (Candela, 1989a). In reality, volatile exsolution is probably a more-or-less continuous process during arc magma ascent and cooling, starting at depth with the exsolution of relatively insoluble CO 2 (Blundy et al., 2010; Holloway, 1976; Lowenstern, 2001; Wallace, 2005 ). However, the bulk of the magmatic water content likely exsolves relatively rapidly as the magma approaches its solvus, at depths of ~5 –10 km, depending on magma composition and water content ( Burnham, 1979). The composition of this exsolved magmatic volatile phase is dominantly aqueous, containing sulfur species (predominantly SO 2 in oxidized systems, but also some reduced S species), CO 2, NaCl, KCl, HCl, andmetal chlorides. Theexact composition depends on many variables, including the depth of exsolution and the magma composition (especially the initial magmaticCl/H2O ratio andalkali content; Candela, 1989c; Candela and Piccoli, 2005; Cline and Bodnar, 1991; Webster, 1992), but typical estimates for a single-phase supercritical �uid exsolved at depths below the H2O–NaCl solvus are ~2 –13 wt.% NaCl equivalent (average 5 wt.% NaCl equivalent) with minor CO 2 (Audétat and Pettke, 2003; Audétat et al., 2008; Burnham, 1979; Candela, 1989c; Hedenquist et al., 1998; John, 1991; Redmond et al., 2004), and up to 1.3 wt.% Cu and 0.3 wt.% Fe (Klemm et al., 2007; Rusk et al., 2004, 2008; Sawkins and Scherkenbach, 1981). These high observed metal solubilities are consistent with or exceed experimental observations and theoretical predictions based on chloride complexing alone (e.g., Candela and Holland, 1984, 1986), and suggest that other volatile ligands such as sul�de species mayenhance thesolubility of chalcophile elements such as Cu and Au in high temperature aqueous � uids (e.g., Heinrich et al., 1992; Pokrovski et al.,2005,2008; Seoet al.,2009;Simon et al., 2006; Zajacz et al., 2008, 2011). Shallowly emplaced magmas will exsolve �uids under pressure – temperature (P–T) conditions that lie within the two-phase liquid – vapor �eld for the bulk �uid composition, resulting in immediate formation of an immiscible low salinity vapor and high salinity brine (Fig. 6a). The critical point in the H 2O–NaCl system (which is commonly used as a proxy for magmatic � uids) occurs between ~1.0 and 1.4 kb for �uid temperatures between 600° and 800 °C (the typical temperature range for �uids exsolved from intermediate to felsic magmas) (Pitzer and Pabalan, 1986; Sourirajan and Kennedy, 1962 ), which is equivalent to depths of between ~3 and 5 km at lithostatic pressures. Most porphyry deposits and their host plutons are emplaced at depths between 1 and 6 km (Seedorff et al., 2005), so �uids exsolving directly from magmas at these depths will typically form immiscible liquid and vapor plumes (e.g., Henley and McNabb, 1978; Nash, 1976). However, the bulk of the �uids and metals in porphyry deposits are likely initially sourced at deeper levels (5 – 10 km, as noted above) from larger volumes of magma in mid- to uppercrustal batholithic complexes (Candela and Piccoli, 2005; Cloos, 2001; Damon, 1986; John, 1991; Richards, 2003, 2005; Shinohara and Hedenquist, 1997), at which depths the �uids will be supercritical. As these deep � uids rise into shallow cupola zones extending above the main batholith, they will likely intersect the solvus on the vapor side, and will begin to condense a dense saline brine ( Ahmad and Rose, 1980; Figs. 6a and 7). Supercritical �uids arehighly mobile(e.g., Coumouet al., 2008;Dunn and Hardee, 1981; Norton and Dutrow, 2001) and behave differently in terms of magma–�uid partitioning compared to �uids exsolved at shallower depths in the two-phase �eld. In particular, Henley and McNabb (1978) suggested that the higher density and viscosity of saline brine condensates might restrict their �ow, leaving them as a dense residual liquid in the deeper parts of evolving magmatic hydrothermal systems(seealso: Lewis and Lowell, 2009; Whiteet al., 1971). Thelower density vapor or supercritical �uid would be expected to be highly upwardlymobile, aswellaslarger intermsof both volumeandmass than

(a) 2000

Deep supercritical magmatic fluid exsolution (10 wt.% NaCl)

1

V-L isotherm (vapour) V-L isotherm (liquid) Critical / boiling curve H2 O critical point Halite saturation Salinity isopleth

Supercritical fluid intersects 1500 ) 2-phase surface s r a b ( P

Supercritical fluid Liquid path Vapour path Coexisting vapour+liquid

V+L

0 100

500

0

Liquid phase progressively separates from vapour

e t i l a H + L

Shallow magmatic fluid exsolution

2 0 0 1 0 8 l 0 C 6 a 0 N 4

r y h y p e r t i l P o a

H + V

0 0 9

0 0 0 8 0 7

0 0 6

0 0 5

T ( C ) °

% .

0 0 4

0 0 3

0 t 2 W 0 0 2

0 0 1

0

0

(b) Deep supercritical magmatic fluid exsolution (10 wt.% NaCl)

2000

1500

V+L 4

) s r a b ( P

Critical / boiling curve H2O critical point Halite saturation

3

0 0 9

Supercritical fluid Liquid path Vapour path Contraction path

Liquid phase progressively separates from vapour

0 0 6

0 0 5

0 0 4

T ( C ) °

e t i l a H + L

Low to moderate salinity liquids

e t i l a H + V

0 0 0 8 0 7

Salinity isopleth

Coexisting vapour+liquid

Vapour phase departs from 2-phase surface

500

0

V-L isotherm (liquid)

Supercritical fluid never intersects 2-phase surface

Supercritical fluid intersects 2-phase surface

0 100

V-L isotherm (vapour)

0 0 1 0 8 l 0 C 6 a

al m r e it h Ep

0 0 3

0 0 2

0 0 1

0

0 N 4

% .

0 t 2 W 0

Fig. 6. P–T–XNaCl phase diagram, modi�ed fromDriesnerand Heinrich(2007), illustrating �uidpathways fora

magmatic–hydrothermal �uidexsolved with an initialbulk salinity of 10 wt.% NaCl. (a) Early, high thermal gradient � uids exsolved from deeply (path 1) and shallowly (path 2) emplaced magmas (porphyry environment). (b) Late, low thermal gradient �uids exsolved from deeply emplaced magma (high-sul �dation epithermal environment). Path 3 illustrates the supercritical �uid contraction path proposed by Hedenquist et al. (1998), whereas path 4 illustrates a slightly steeper thermal gradient with brief intersection of the 2-phase (L –V) � eld, as proposed by Heinrich et al. (2004). Notethatinthetwo-phase �eld,thedensesalineliquidphaseprogressively separatesfrom the vapour phase, and the two-phase pathways shown do not represent a closed system. See text for discussion.

the brine phase (assuming an initial bulk salinity below ~ 20 wt.% NaCl), andsohasmuchgreaterpotentialasanef �cient transportingmediumfor ore components. However, until fairly recently, it was assumed that low salinityvapors would nothave the capacityto dissolve large quantities of basemetalsas chloridecomplexes,andthebrine phase wastherefore the favored ore-forming medium (e.g., Bodnar and Beane, 1980; Cline and Bodnar, 1991; Eastoe, 1982; Hedenquist and Richards, 1998; Moore and Nash, 1974; Nash, 1976; Shinohara, 1994; Williams et al., 1995). Observations of chalcopyrite crystals trapped in some vapor-rich �uid inclusions (e.g., Bodnar and Beane, 1980) were explained by some as products of heterogeneous trapping (see discussion in Mavrogenes and


13

Fig. 7. Schematic cross-section through a typical coupled arc batholith –cupola–volcanic system, with associated porphyry Cu±Au and linked high sul �dation Cu–Au epithermal

deposits. Also shown are the thermal structure, � uid � ow pathways and characteristics during the main stage of hydrothermal activity, and overlapping hydrothermal alteration zones. Propylitic alteration by circulating heated groundwaters can be assumed to affect all the supracrustal rocks in the � eld of view, with greatest intensity (epidote, actinolite) close to the intrusions, fading to background distally. Modi �ed from Richards (2005); sources: Sillitoe (1973, 2010), Dilles (1987), Shinohara and Hedenquist (1997), Hedenquist et al. (1998), and Fournier (1999) .

Bodnar, 1994). The advent of quantitative analysis of single �uid inclusions by synchrotron X-ray microprobe, proton-induced X-ray emission spectroscopy (PIXE), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) revealed that, contrary to these earlierassumptions,considerableamountsof metal, including Cu andAu, wereindeed consistentlypresent in somevapor-rich inclusions (Audétat et al., 2000; Harris et al., 2003; Heinrich et al., 1992, 1999; Klemm et al., 2007; Lowenstern et al., 1991; Simon et al., 2005, 2006; Ulrich et al., 2001), and subsequent work hasshown that much of this metal content is transported as sul �de complexes rather than as (or in addition to) chloride complexes (Cauzid et al., 2007; Heinrich et al., 1992, 1999; Pokrovski etal.,2005,2008; Seoet al., 2009;Simon etal.,2006;Zajacz and Halter, 2009; Zajacz et al., 2010). Early resistance to these ideas was, I believe, at least in part due to confusions of terminology: most papers dealing with this subject have referred to metal solubility andtransport in a vapor phase, suggestingto theunwary a lowdensity, lowsalinity gas, whereas forthe most part the �uids in question were either single-phase supercritical �uids, or relatively high density vaporsjustbelowtheircriticalpoints. Undersuch high P–T conditions, vapors are almost as saline as the initial singlephase � uid from which they evolved, and therefore they still contain plenty of chloride for base metal complexing. But perhaps more importantly, as noted above, these vapors will also contain a high concentration of volatile sulfur species, which now appear to be essential for the ef �cient solvation of chalcophile elements under high P–T conditions.When combined with thehighermass proportion of the vapor phase(versus brinecondensate) andits high mobility,a modelfor transport of the bulk of the metal �ux in porphyries by relatively dense vapors or supercritical �uids is now well established (Klemm et al., 2007; Landtwing et al., 2010; Williams-Jones and Heinrich, 2005 ). The rapid reduction in the ef �ciency of transport of metals as the vapor plume rises, cools, and becomes less dense by brine condensation,

may explain precipitation of the bulk of Cu, Mo, and some Au over a relatively narrow temperatureintervalbetween425°–320 °C (Hemley et al., 1992;Klemm etal., 2007;Landtwingetal., 2005). At shallower depths and lower temperatures, the vapor phase may become too dilute to transport signi�cant amounts of base metals as chloride complexes, but may continue to carry some metals such as Au, Cu, As, and Sb as sul �de complexes (Deditius et al., 2009; Simon et al., 2006, 2007 ), eventually either venting them to the surface in high-temperature fumaroles (e.g., Chaplygin et al., 2007; Hedenquist et al., 1994a; Symonds et al., 1987; Tarana et al., 1995; Tessalina et al., 2008 ) or precipitating them in the near-surfacehigh-sul�dationepithermalenvironment (see Section4.2.1; Deditius et al., 2009; Hedenquist et al., 1993,1994b; Heinrich et al., 1999, 2004; Larocque et al., 2008; Murakami et al., 2010; Pudack et al., 2009; Williams-Jones and Heinrich, 2005). 4. Magmatic–hydrothermal ore formation

The focus of this paper is on the �ux of metals in subductionrelated magmatic systems, but this would be of little practical interest if that �ux did not ultimately lead to ore formation. Thus far, we have focused on the importance of � rstly not losing signi �cant amounts of metal to a fractionating or residual sul�de phase, and then ef �ciently partitioning those metals into a highly mobile aqueous �uid phase. What subsequently happens to that �uid phase dictates whether economic concentrations of metals are precipitated (grade), whereas the scale of the magmatic and derivative hydrothermal system controls the total amount of metals precipitated (tonnage). 4.1. Porphyry Cu ore formation

In a landmark paper, Cline and Bodnar (1991) presented a model for the evolution of magmatic-hydrothermal systems from initial

14


aqueous � uid exsolution to cooling and mineral precipitation. A key �nding of this work was that, although there are many variables that can affect the speci�c evolutionary path of a given system, an economic porphyry Cu deposit can potentially be formed from quite small volumes of typical andesitic arc magma. For example, the authors showed that 15 –90 km 3 of andesitic magma initially containing 50 ppm Cu, 2.5 wt.% H 2O, and with a Cl/H2O ratio=1, is suf �cient to generate a 250 Mt deposit with a grade of 0.75 wt.% Cu. Therangeof required magma volumes re �ects variables such as the depth of magma emplacement and the compatibility of Cu with fractionating mineral phases (larger volumes are required if Cu is compatible with early fractionating silicate, oxide, or sul �de minerals). Details of the model, later updated in Cline (1995), show that magmas emplaced at moderate depths (1.0 to 2.0 kb, equivalent to depths of ~4 –8 km) most ef �ciently partition Cu into early saturating saline �uids, whereas Cu and Cl are only released from shallowly emplaced (0.5 kb, or ~2 km) magmas during late stages of crystallization (see also Candela, 1989b). (Note that these depths re �ect the locus of �uid and metal exsolution from the magma under lithostatic pressure conditions, as opposed to the depth of subsequent hydrothermal metal deposition, which will be at shallower levels and likely under hydrostatic pressure conditions; Fournier, 1999.) In large measure, these differences re �ect the change in properties of the magmatic – hydrothermal �uid, which will separate initially as a moderately saline supercritical �uid in deeper systems (Fig. 6a, path 1), but as immiscible vapor and brine at shallower levels ( Fig. 6a, path 2). Cline (1995) concluded that optimum conditions for porphyry Cu ore formation are obtained where magma containing ~4 wt.% H 2O and with a high initial Cl/H 2O (typical of most arc magmas) is emplaced at moderate crustal depths, thereby maximizing the ef �ciency of Cu partitioning into an early saline � uid phase. The results of these studies indicate that no special conditions or magmatic metal enrichment arerequired to form even large porphyry Cu deposits. Instead, the question is rather one of process ef �ciency: where (what depth) and how are metalliferous �uids released and channeled? This �nding is of fundamental importance from an exploration perspective, because it shifts the focus from seeking anomalously metal-rich magmas (which have proven elusive; Jenner et al., 2010) to searching for optimal ore depositional settings in otherwise normal tectonomagmatic environments (e.g., Tosdal and Richards, 2001). Cloos (2001), Shinohara et al.(1995), andShinohara andHedenquist (1997) considered the question of process ef �ciency from the perspective of physical separation and focusing of volatile release from the magma. Cloos (2001) suggested that the classic cupola shape (Norton, 1982) of porphyry systems above larger batholithic magma chambers re�ects convective circulation of bubbly magma into the shallowapicalpartsofthesesystems(at1 –3 km depth), wherevolatiles physically separate from the melt and coalesce to form a discrete �uid�lled cupola. The now-dense, volatile-depleted magma forms a downward return � ow to complete the convective cycle. In contrast, Shinohara and Hedenquist (1997) envisaged � uids physically separating from the magma at greater depth within the underlying magma chamber, and rising as a discrete plume up apical channelways formed by fractures and dikes in the brittle carapace ( Fig. 7). A keyaspect of both of these modelsis that volatile separation, and Cu partitioning, occurs from a much larger volume of magma (emplaced at deeper levels) than that preserved and commonly visible within the shallow-level ore body. In Cloos's (2001) model, volatile-rich, bubbly magma rising from the underlying batholith convects through the cupola zone where it releases its �uids, whereas in Shinohara and Hedenquist's (1997) model, vesiculation and convective circulation occur in the underlying magma chamber itself, and �uids are released as a plume into the base of the apical dike system. Combining these models with ( Cline, 1995; Cline and Bodnar's, 1991) calculations suggests that maximum ore-forming

ef �ciency in porphyry systems is likely achieved where volatile saturation occurs in large ( ≥ 100 km 3) mid- to upper crustal magma chambers at depths ≥ 6 km, containing moderately hydrous ( N 4 wt.% H2O) and Cl-rich magmas. These �uids either rise as bubbly magma or as a separate volatile plume into the apical parts of the system where decreasing pressure and temperature cause deposition of Cu and Mo± Au (Candela, 1989b; Shinohara et al., 1995). Focusing of magma ascent and �uid �ow into narrow apical regions, or cupolas, is likely to be a function of structure in the brittle rocks overlying the batholithic system ( Tosdal and Richards, 2001, and references therein). Shallow crustal magma emplacement will cause extensional doming in the cover rocks, with dilational fault zones providing high-permeability pathways for �uid and magma ascent (Burnham, 1979). Evidence from the dike emplacement literature suggests that such fractures may �rst be opened and propagated by volatile pressure, and only later � lled by more viscous magma (Burnham, 1979; Carrigan et al., 1992; Rubin, 1995). This raises the intriguing possibility that the cylindrical shapes of many porphyry stocks may have arisen �rst as breccia pipes or diatremes bored out by rapidly escaping volatiles, only later to be back- �lled with porphyritic magma (Fig. 8; Norton and Cathles, 1973; see also Fig. 2 in Anderson et al., 2009, and Fig. 8 in Sillitoe, 2010, and Fig. 17 in Vry et al., 2010). To some extent, focusing of magma and �uid � ow along narrow conduits may be self-organizational, because once initial channel ways have developed, they will represent high-permeability pathways and are likely to thermally weaken the wall rocks and promote further fracturing and channeling. Narrow focusing of apical fracturing and subsequent �uid �ow are likely to be critical to the formation of high grade porphyry deposits (e.g., El Teniente, Chile; Vry et al., 2010), while multiple breccia/ intrusive events can potentially increase tonnage (provided that later events do not destroy earlier mineralization). Given suitable �uid �ow focusing, three further factors combine to cause maximum ef �ciency of metal deposition within relatively small volumes in porphyry Cu±Mo±Au deposits. All three effects are related to the steep temperature gradient in the cupola zone (Fig. 7), and they therefore control the vertical range of ore deposition. On the otherhand, �uid focusingcontrols the lateral extent of mineralization. In combination, the highest grades will occur where ore deposition is both focused laterally and restricted vertically. The �rst factor is that Cu solubility (as chloride species) decreases dramatically as � uids cool through the temperature interval ~400° to 300 °C (Crerar and Barnes, 1976; Hemley et al., 1992; Klemm et al., 2007; Landtwing et al., 2005; Xiao et al., 1998 ). Given the very sharp temperature gradient implied by near-surface emplacement of magma (Fig. 7), this temperature interval will correspond to a narrow depth range, likely with 1 or 2 km of the surface (although it may extend to greater depths with time as the magmatic–hydrothermal system begins to cool). This depth range is typical for ore formation in many porphyry systems. The second factor is that SO2 dissolved in the magmatic– hydrothermal �uid phase progressively disproportionates to H2S and H2SO4 as the � uid cools below ~400 °C (Holland, 1965; Kusakabe et al., 2000; Reeves et al., 2010; Sakai and Matsubaya, 1977 ): 4SO2 + 4H 2 O ⇔ H2 S + 3H2 SO4 :

ð1Þ

This reaction generates both hydrogen sul�de, which initiates abundant precipitation of sul �de minerals (i.e., chalcopyrite, pyrite, molybdenite), and also sulfuric acid, which causes early deposition of large volumes of anhydrite in the potassic alteration zone, and progressively increasing degrees of hydrolytic alteration (an initial shift from feldspar-stable potassic alteration, to muscovite/sericitestable phyllic alteration). Consequently, the bulk of Cu-sul �de mineralization occurs at the low-temperature, late-stage end of the potassic

15

J.P. Richards / Ore Geology Reviews 40 (2011) 1– 1 – 26

(b)

(a) Mixed magmatichydrothermal breccia Porphyry intrusion

Porphyry intruding comagmatic breccia

Cuspate intrusive border

Porphyry dike Hydrothermal quartz filling cavity ~20 cm

(c) Limestone wallrock clast

Cuspate porphyry clast

Mixed magmatichydrothermal breccia

Fig. 8. Photographs of porphyritic magma invading co-magmatic hydrothermal breccia pipe (a, c) and breccia vein/dike (b) from the Pachapaqui Ag

–Cu–Pb–Zn deposit, Péru. The breccia consists of fragments of porphyry magma and country rock; note scalloped, cuspate margins of the porphyry body in (a, b) and porphyry clasts in (c), indicating that the magmaa was molte magm molten n at the time of brecc breccia ia form formationand ationand subse subsequent quent intru intrusion sion into the brecc breccias. ias. In (b), large vuggy space spacess are part partially ially �lled with hydrotherma hydrothermall quartz, re�ecting the role of � � uids in breccia formation. Scale in (c) is in centimeters.

alteratio altera tion n phas phase, e, jus justt pri prior or to the ons onset et of phy phylli llicc alte alterati ration on (i. (i.e., e., correspond corre sponding ing to the “ore oreshe shellll” of the theclas classic sic Low Lowell elland andGui Guilbe lbert, rt, 197 19700, porphyry Cu model). The third factor is the increased permeability over this temperature range caused by a combination of the transition in silicate rocks from ductile to brittle behavior at temperatures (between ~400° – 350 °C; i.e i.e.,., the bri brittle ttle–ducti ductile le trans transition; ition; Fig. Fig. 7; Cathles, Cathles, 1991; Fournier, 1999; Landtwing et al., 2005), 2005 ), and a window of retrograde silica solubility (between ~550 –350 °C; Fournier, °C; Fournier, 1985). 1985 ). Not only do these processes result in the formation of open-standing brittle veins and por porosit osity, y, ther thereby eby faci facilita litating ting rap rapid id upw upward ard �uid �ow and wallrock wall rock perm permeati eation on (e.g. (e.g.,, the crac crackle kle brec breccias cias,, stock stockwork works, s, and disseminatedd mineralization textures so characteristi disseminate characteristicc of porphyry Cu deposits), but this transition also represents the boundary between lithostatic and hydrostatic �uid pressures (a pressure differential of � uids across this boundary can be ~3×). Sudden depressurization of � expected to have major effects on �uid properties, including phase separation (Fig. (Fig. 6) 6) and consequent changes in metal solubility (e.g., Landtwing et al., 2005, 2010; Murakami et al., 2010 ). In com combi binati nation, on,the these se fou fourr fac factor tors, s, (1)spati (1)spatial al foc focusi using ngof of �uid �owin narrow narr ow cupol cupolas, as, (2) reduc reduction tion of meta metall solub solubilit ility, y, (3) incr increase easedd dissolvedsul�de deact activi ivity,and ty,and (4)perme (4)permeabi abilit lityy inc increa reasedue sedue to totran transiti sition on from ductile to brittle fracturing and retrograde silica solubility (with a

large pressure drop), serve to narrowly focus Cu-sul �de mineralization both laterally and vertically within cupola zones above large mid- to upperr crus uppe crustal tal bath batholith olithic ic com complex plexes. es. The mos mostt like likely ly reas reasons ons for otherwise prospective porphyry systems to be unproductive will be either a failure to focus � uid �ow, or simply insuf �cient �uid supply (likely due to an insuf �ciently large underlying magmatic system). 4.2. Epithermal Cu– Cu– Au ore formation 4.2.1. High-sul �dation �dation epithermal Cu– Cu– Au deposits

As no note tedd in Secti Section on 3.1 3.1,, al alth thoug ough h th thee bu bulk lk of Cu (a (and nd Mo Mo)) ap appe pear arss to be preci precipitate pitatedd over overthe thetemper temperature atureinterv interval al 425°–320 °C, °C,som somee Cu and other metals such as Au, Sb, and As may remain in solution as sul �de complexes, to be carried into the shallow epithermal regime. Observations from rare �uid inclus inclusions ions in high-s high-sul ul�dation epithermal epithermal Cu–Au deposits deposi ts sugge suggest st that the ore-fo ore-forming rming � uid was a low- to moderatesalini sal inity ty liq liquid uid (0. (0.22 to 4.5 wt.%NaCl equ equiva ivalen lent; t; Hed Hedenq enquistet uistet al.,1994b al.,1994b;; Mancano Manc ano and Campb Campbell, ell, 1995), 1995), whic which h parag paragenet enetical ically ly post post-date -datess advanced argillic alteration formed by highly acidic magmatic gasses (Arr Arribas ibas,, 1995 1995;; Hede Hedenqu nquist ist et al., 1994 1994b; b; Stof Stoffre fregen, gen, 1987 1987). ). Thi Thiss apparent appa rent inco inconsis nsistenc tencyy has been expl explained ained by Hedenqui Hedenquist st et al. (1998), (199 8), Hein Heinrich rich et al. (20 (2004) 04),, and Hein Heinric rich h (200 (2005) 5) in terms terms of contraction contr action of a moder moderate ate salini salinity ty superc supercritica riticall magmat magmatic ic �uid or

16


vapor by rapid cooling during ascent such that it does not touch, or barely touches, the two-phase solvus (Figs. 6b and 9, paths 3 and 4, respectively).Because of thecurvature ofthesolvuscrest (critical curve) to lower salinities at low temperatures and pressures, this moderately saline � uid will lie on the liquid side of the solvus at shallow depths, although it may have contracted from what was originally a vapor or supercritical �uid phase at higher temperatures and depths. Heinrich et al. (2004) speci�cally suggested that brief intersection with thesolvusat high pressures andtemperatures(Figs. 6band 9,path 4) might be an important wayto shed chloride-complexed components suchasFefromthevaporinabrinecondensate,leavingbisul �de ligands free to bond with Cu and Au in the residual vapor and transport it to shallower(epithermal)levels. This model requires that thevapor leaves thetwo-phasesurfaceagainbycooling at pressure,thereby passingover the crest of the solvus (the critical curve; ( Figs. 6b and 9, path 4); upon subsequent ascent and depressurization, this �uid will be a liquid, as described above. Heinrich et al. (2004) argued that the Fe-rich brine condensation step is essential for retention of Au (and Cu) in the vapor phase, because otherwise Fe will tend to precipitate as Cu –Fe-sul�de minerals and strip the �uid of bisul�de ligands, thus causing Au to coprecipitate at depth (a possible mechanism for the formation of porphyry Cu–Au deposits). However, this condensation step must then be followed by cooling while still at depth, in order to “lift” the vaporphaseoffthetwo-phasesurface,andallowittocontracttoaliquid (Figs. 6b and 9, path 4). 0

100

200

300

400

A more detailed analysis of the phase diagram shown in Fig. 6 reveals that a typical magmatic �uid of 2–13 wt.% NaCl would have to �rst intersect the solvus at temperatures above ~400° –500 °C for this model to work optimally, otherwise it would fall on the liquid side of the two-phase � eld, and would become more saline by boiling off a dilute vapor (Figs. 9 and 10). Assuming that this did indeed happen, then the smaller (by mass) vapor component would have to leave the two-phase surface again at pressure before cooling below ~ 400 °C, otherwise its salinity rapidly falls to sub-weight percent levels at lower temperatures and lower pressures (Figs. 6 and 10), which are inconsistent with �uid inclusionevidencefor lowto moderately saline liquids in high-sul �dation epithermal ore formation (Hedenquist et al., 1994b;Mancano andCampbell, 1995). Thus, the P–T trajectory ofa �uid that satis �es Heinrich et al.'s (2004) model for high-sul �dation epithermal Cu–Au mineralization (Figs. 6band 9, path 4) is somewhat unique, and may occur only rarely or �eetingly during the waning stages of a cooling magmatic–hydrothermal system. A more normal, or early ascent pathway for a magmatic– hydrothermal �uid would be for it to rise more-or-less is enthalpically or quasi-adiabatically along a steep P –T gradient (e.g., Hemley and Hunt, 1992; Henley and Hughes, 2000; Wood and Spera, 1984 ), and therefore to dive deeply into the two-phase � eld and separate into an increasingly dilute vapor phase and a saline brine (Figs. 6 and 9, path 1), or even to boil dry to halite plus vapor ( Figs. 6 and 9, path 2). Such �uid pathways might be consistent with the widespread and intense

500

600

V-L solvus (vapour) V-L solvus (liquid)

2000

) c i t a e t l t s t i r o r B d y h (

1500

700

800

W W e e t t g g r a ar n n i t o e d i o s o r i Deep l d i te u s magmatic s o fluid dl i exsolution u s

( l i D t h o u s c t a t i l e t i c )

900

1000 8 (lith)

% . t w 0 2

7 (lith)

. t % w 0 1

6 (lith) 15 (hyd) t. % 5 w

14 (hyd) 5 (lith) 13 (hyd)

Deep singlephase fluid

) s r a b (

P1000

12 (hyd) 11 (hyd) 4 (lith) 10 (hyd)

3 4

9 (hyd)

1 r m e h t o e g m k / C 0 3

500

D e p t h ( k m )

1 wt.%

Early porphyry veins (2-phase fluid)

Shallow magmatic fluid exsolution

m C / k 3 0 0 o t h e r m g e °

7 (hyd) 6 (hyd) 2 (lith) 5 (hyd)

2

°

8 (hyd) 3 (lith)

4 (hyd) Main-stage brittle porphyry veins

V+Halite

Epithermal

0

0

100

200

300

400

500

600

3 (hyd) 1 (lith) 2 (hyd)

V / L + H a l i t e

700

800

1 (hyd)

900

0

1000

T (°C) Fig. 9. Pressure (depth) –temperature section through the H 2O–NaCl phase diagram, with vapour –liquid (V –L) solvi drawn for 1, 5, 10, and 20 wt.% NaCl (data from Driesner, 2007;

Driesner and Heinrich, 2007). Red curves indicate that the solvus phase is a vapor, blue curves that it is a liquid; the transition point corresponds to the critical point for that composition. Also shown are the wet granite and granodiorite solidi ( Burnham, 1979), and average crustal (30 °C/km) and high (300 °C/km, near active volcanism) geothermal gradients (Barbier, 2002; Goff et al., 1992; Noorollahi et al., 2007 ). Depths are indicated for hydrostatic (hyd) and lithostatic (lith) pressure conditions. Typical temperature –depth rangesfor supercritical magmatic �uid exsolution,early high-temperature porphyry veins,later main-stagebrittle porphyry veins,and epithermalmineralization are indicated.Four �uid P–T paths are shown corresponding to: (1) a typical porphyry-forming � uid path; (2) a shallow high-temperature path boiling to dryness (V+Halite � eld); (3) the deep contraction path of Hedenquist et al. (1998); and (4) the contraction with minor brine condensation path of Heinrich et al. (2004).

17


-8 -7 -6 -5 -4 -3 -2 -1 0 1300

10

20

Single phase V-L solvus (vapour) magmatic V-H solvus (vapour) fluid

V-L solvus (liquid)

1200

30

40

50

60

70

80

90

100 1300

700 C °

e rv u c l a ic rit C

1100

1200

1100

V+L (700 C) °

1000

1000 600 C °

900

900 V

800

800 V+L (600 C) °

) 700 s r a b ( P600

700

600

500 C °

500

V+L (500 C)

) ) C ° C ° 0 0 0 0 5 5 ( ( H L + L

°

400 300 200

) V+H ) C ° C ° 0 0 0 4 0 ( 4 ( H L + L

400 C °

V+L (400 C)

Critical P of H2O

°

) ) C C ° ° 0 0 0 0 6 ( 6 ( H L + L

500

V+H (600 C) °

(500 C) °

) ) C C ° ° 0 0 0 0 7 7 ( ( H L + L

400

300

V+H (700 C) °

200

V+H (400 C) °

100

100

0 -8 -7 -6 -5 -4 -3 -2 -1 0/1

log (wt.% NaCl)

10

20

30

40

50

60

70

80

90

0 100

wt.% NaCl

scale change Fig.10. Pressure–XNaCl section through theH 2O–NaCl phasediagram,with vapor–liquid(V –L)solvi drawn atvarious temperatures(data from Driesner, 2007;Driesner and Heinrich,

2007). Red curves indicate that the solvus phase is a vapor, blue curves that it is a liquid; below the vapor –halite (V –H) solvus, vapor curves are shown in orange. Note the scale change on the salinity axis at 1 wt.% NaCl, in order to illustrate the extremely low salinity of low-temperature vapor phases. The range of salinity for typical deeply exsolved singlephase (supercritical) magmatic �uids (2–13 wt.% NaCl) is shown in gray. Fluidsthat intersectthe V –L solvusabove~ 400°–500 °C will be moderate-density vapors and will condense a small amount of dense, saline liquid; � uids that intersect the solvus below this temperature will be liquids and will boil off a dilute, low-density vapor phase.

acidic (advancedargillic) alterationcommonly foundat shallow levels above porphyry systems, which is caused by acidic gasses (H 2SO4, HCl) condensing from a low-density vapor plume. As Heinrich et al. (2004) noted,such acidicvapors do not appearto transportAu (or Cu) effectively because bisul �de (HS–) ligands are hydrolized to H 2S. This may explain why advanced argillic alteration caps are commonly barren (in terms of Au and Cu) and merely generate permeability that potentially focuses later ore-forming � uid � ow. Thus, mineralization may only occur where later moderate salinity liquids have followed a higher pressure, rather specialized cooling path, as described above (Heinrich et al., 2004). 4.2.2. Low-sul �dation epithermal Au deposits (including alkalic-type deposits)

Although low-sul�dation epithermal Au deposits are commonly found in volcanic terrains, their link to magmatism is more tenuous than high-sul�dation deposits, and there is commonly evidence for a greater involvement of meteoric groundwater in their formation than magmatic �uids (e.g., Faure et al., 2002; Field and Fifarek, 1985; Heald et al., 1987). Nevertheless, alkalic-type epithermal Au deposits, which are mineralogically similar to low-sul �dation deposits (adularia and sericite — or roscoelite [vanadium mica] — are stable), do show a strong temporal and genetic relationship to alkalic magmas, typically in relatively small and isolated intrusive complexes located in backarc or post-subduction settings (e.g., Jensen and Barton, 2000; Kelley

et al., 1998; Müller and Groves, 1993; Mutschler et al., 1985; Richards, 1995; Thompson et al., 1985). Fluids in these alkalic-type deposits are commonly low- to moderate salinity (0–10 wt.% NaClequiv.), low temperature (typically ≤ 250 °C) liquids, with evidence for decompressional boiling or � uid mixing as the prime ore depositional mechanism in high grade breccias and veins ( Jensen and Barton, 2000; Richards, 1995). Stable isotopic compositions of these �uids are generally ambiguous, and permit interpretations of the involvement of either isotopically exchanged meteoric waters or magmatic �uids, or both (Ahmad et al., 1987; Carman, 2003; Richards, 1995; Richards and Kerrich, 1993; Ronacher et al., 2004; Scherbarth and Spry, 2006; Zhang and Spry, 1994). Similar stableisotopicdata from other low-sul�dation deposits arecommonlyinterpretedto re�ect a meteoric �uid sourcebecause of the absence of clearly coeval magmatism (as noted above). However, the close link with magmatism in alkalic-type systems suggests that a magmatic � uid source is more likely (e.g., Carman, 2003; Scherbarth and Spry, 2006; Simmons and Brown, 2007), and the vapor contraction model described in Section 4.2.1 could explain the observed characteristics of these ore � uids (i.e., direct contraction to a moderate salinity liquid from a high temperature magmatic � uid at depth). Because of the association of these deposits with relatively small intrusive complexes, and therefore a smaller crustal thermal anomaly, a shallower �uid P–T path with cooling at depth is more likely, consistent with a model of vapor contraction.

18


5. Summary and conclusions 5.1. Sources of magmas and metals

Magmatic–hydrothermal porphyryCu ±Mo±Au, Au,Mo, andSn–W deposits (and related epithermal Au deposits), derive their metals from their associated magmas. Withthe exception of porphyrySn–W deposits that are associated with crustally derived S-type granites, most other deposits in this grouping are formed by calc-alkaline to mildly alkaline I-type granitoids directly or indirectly related to subduction. Sources of these magmas include subduction-metasomatized asthenospheric mantle wedge, basaltic oceanic crust and/or sea �oor sediments, and, in post-subduction settings, subduction-modi�ed upper plate lithosphere. The majority of normal arc porphyry systems are generated from hydrous mantle wedge melts that have interacted to varying degrees with the upper plate lithosphere during passage towards the surface. Assimilation and fractional crystallization processes fundamentally change the composition of the primary basaltic arc magmas to intermediatecalc-alkaline compositions, withfractionatedtrace element patterns and evolved (crustal) isotopic signatures. Sea�oor sediments and basaltic oceanic crust only melt under unusually hot subduction zone conditions or locally at plate edges, and despite widespread claims of the identi �cation of slab melts (adakites) in the literature based on high Sr/Y and La/Yb ratios in some evolved granitoids, this process is unlikely to be a major contributor to arc magmatism and metallogeny. Rather, these subtle trace element ratios can be readily explained by fractionation of hornblende±titanite and residual garnet, and suppression of plagioclase fractionation from water-rich mantle wedge basaltic magmas. These trace element characteristics are therefore an indicator of high magmatic water content rather than being a sourcesignature, andthis likelyexplains the common association of high-Sr/Y (i.e., hydrous) magmas with magmatic–hydrothermal ore deposits — without magmatic water, such deposits cannot form. Potential sources of metals in arc magmas include the oceanic crust and sediments (via dehydration � uids or melting), the mantle wedge, andtheupperplatecrust.Fluid–mobileelementssuchasK,Rb,Cs,Ca,Sr, Ba, U, B, Pb, As, Sb, Tl, and possibly Cu, Au, Re, and the Pd-group elements,alongwith large amounts of H2O,Cl,andS,are �uxed from the dehydrating subducting slab into the mantle wedge, causing metasomatismand partial melting by lowering theperidotitesolidus. Although there is some evidence for slab-derived � uid contributions of chalcophile and highly siderophile elements (Cu, Au, PGE) to the mantle wedge,it is notclear that this isa necessarymetallogenic step, theupper mantlealready containingsigni�cant amounts of theseelements.Likely, a more important control on the metal content of subsequent partial melts is the abundance and stability of residual sul �de phases in the asthenospheric mantle source. Under the high f O2 and f S2 conditions of arcmagmatism, sulfurwill be dominantly present as sulfate andsulfate, but saturation in small amounts of sul �de phases is also likely. These sul�de phases will tend to deplete the magma in highly siderophile elements (Au and PGE), but will not be present in suf �cient volume to signi�cantlydepletethemagmainmoreabundantchalcophileelements such as Cu and Mo. Such magmas therefore have the potential subsequently to formporphyry Cu–Mo deposits. Thus, Cu (and perhaps Mo) are thought to be predominantly derived from the mantle, plus or minuscontributionsfrom thesubducting slab. Gold andsilverpresentin minor amounts in such deposits may also be derived from subduction sources, although there is some evidence for additional contributions from the upper plate crust (especially for Ag, and also perhaps Mo). Arc-like magmas and related porphyry and epithermal ore deposits also occur in post-subduction tectonic settings, such as subduction reversal or migration, arc collision, continent –continent collision, and post-collisional rifting. Theyare distinguished from normalsubductionrelated suites by slightly higher magmatic alkali (K 2O and Na2O) contents, and by the occurrence of Au-rich deposits (although normal

porphyryCu–Modepositscanalso occur). Such magmatic–metallogenic systems are thought to form by remelting of previously subductionmodi�ed upper plate lithosphere, and in particular the lower crustal amphibolitic cumulate roots of former arc magmatic complexes. Remelting can be triggered by crustal thickening and thermal rebound following arc or continent collision, delamination of sub-continental mantle lithosphere causing direct exposure of the lower crust to asthenospheric temperatures and melts, and asthenospheric upwelling during rifting of former arc crust. Sparse sul �de phases in these arc cumulates, residual from fractionation of previous arc magmas, will likely be rich in chalcophile and highly siderophile elements. During low-volume meltingunder relatively low f S2 conditions (in the absence of a � ux of S from active subduction), these sul �de phases will likely redissolve in the mildly alkaline partial melt, and may provide a source for Au-rich (±PGE) post-subduction porphyry Cu –Au and epithermal Au systems: examples include the Roşia Montană, Skouries, Kisladag, Çöpler, and Sari Gunay porphyry Cu –Au and epithermal Au deposits in the Neo-Tethyan belt of Romania, Greece, Turkey, and Iran, and the Grasberg, Ok Tedi, Porgera, Lihir, and Emperor porphyry Cu –Au and epithermal Au deposits in the southwest Paci�c. However, gold enrichments may not occur where more abundant sul�des were present in the former arc complex, leading to more “ normal” porphyry Cu±Mo systems: examples include the Kerman porphyry Cu belt of central Iran, and the Gangdese porphyry Cu belt of Tibet. Porphyry Mo and Sn–W deposits associated with felsic magmas in continental interiors are thought to form mainly by partial melting of continental crust during rifting to form S-type, lithophile element-rich granitic magmas. A role for ma�c, mantle-derived magmas is suggested by the common association with such rocks, but their role may be predominantly as a heat source for crustal melting and a trigger for volatile saturation and eruption, rather than as a unique source of metals. 5.2. Porphyry and epithermal ore formation

In the porphyry and epithermal ore depositional environment, a critical role is played by aqueous �uids exsolving from hydrous magmasemplaced in the mid- to upper crust. The P–T–X properties of magmatic hydrothermal �uids, approximated by the H 2O–NaCl system, combinedwith volatile solubility in intermediate composition magmas, suggest that � uid saturation occurs at depths of 5–10 km in the batholithic roots of arcmagmaticsystems.Volatileexsolution may lead to the upward propagation of buoyant bubbly magma as dikes and stocks intruded into the overlying shallow crust (with or without subsequent eruption at surface), and/or the rapid ascent of a separate volatile plume. Structural focusing and chanelling of these evolved magmas and �uids creates a cupola zone characterized by high thermal gradients and �uid �ux. Metals (Cu, Mo, Au), which partition strongly into the saline (2–13 wt.% NaCl equivalent) and S-rich magmatic hydrothermal phase at high P and T in the underlying batholithic magma chamber, experience rapid reduction in solubility as these �uids ascend, depressurize, and cool, with the bulk of Cu and Mo being precipitated over a temperature range of 425°–320 °C at 1 – 6 km (commonly ≤ 2 km) depth. This depth–temperature interval is critical because it also represents: (1) the upward transition from ductile to brittle behavior in the cover rocks (~400°–350 °C), which facilitates fracturing and rapid �uid depressurization; (2) a window of retrograde silica solubility (~550° – 350 °C), which enhances permeability and porosity for ore deposition; and (3) the temperature range ( b 400 °C) over which SO 2 in the � uid phase begins to disproportionate to H 2S and H2SO4, which causes precipitation of sul�de minerals (chalcopyrite, pyrite, molybdenite, rare bornite). This reaction also generates increasingly acidic �uids, leading to the characteristic progression from feldspar-stable alteration assemblages (potassic), through muscovite/sericite-stable assemblages


(phyllic), to clay- (argillic) and alunite-stable assemblages (advanced argillic). Depending on thedepth of exsolution, theinitialmagmatic �uid will either be a supercritical �uid (below ~6 km depth) orwillexistas a twophase moderate salinity vapor and high density brine. As the plume ascends, it will intersect its solvus (in the case of a deeply exsolved supercritical �uid), and the vapor phase will become progressively less saline through brine condensation. At the level of porphyry ore formation, both the brine and vapor phase may contribute to metal transport and deposition, although there is increasing evidence for the importance of the vapor phase as a large-volume, highly upwardly mobiletransportation medium.However, as this vapor phase continues to ascend, it will rapidly decrease in salinity, such that chloridecomplexed metals are unlikely to be transported by such �uids to shallow epithermal levels. It will also increase in acidity, thereby reducing the solubility of bisul �de-complexed metals such as Au (and perhaps also Cu)by protonation of HS– to H2S. This acidicvapor phase is responsible for the extreme acid leaching in advanced argillic lithocaps above porphyry systems. High-sul�dation epithermal Cu–Au deposits are hosted by these highly permeableadvanced argillic alteration zones, but appear to have been formed by later, moderately saline (0.2 to 4.5 wt.% NaCl equivalent), less acidic liquids. Two models have been proposed to explain the origin of these paragenetically late mineralizing �uids in terms of contraction of a single phase (supercritical) magmatic �uid, with (Heinrich et al., 2004) or without (Hedenquist et al., 1998) brief intersection of the solvus. Because of the topology of the P –T–XNaCl phase diagram, such � uids contract to a liquid phase upon cooling at pressure. Thus, these authors propose that high-sul �dation epithermal Cu–Au deposits may be formed directly from late-stage magmatic hydrothermal �uids. Although low sul�dation epithermal Au deposits have not been discussed in detail in this paper, because most such deposits are not directly related to porphyry-type magmatic –hydrothermal systems, the vapor contraction mechanism might have applicability to alkalictype epithermal gold deposits, which do show a close genetic relationship to post-subduction alkalic magmas. Acknowledgments

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