Chemical Mobility of Gold in the Porphyry-Epithermal Environment

Economic Geolo•oy Vol 92, 1997,pp. 45-59

ChemicalMobilityof Goldin the Porphyry-Epithermal Environment C. H. GAMMONS = ANDA. E. X'VILLIAMS-JONES Department •f EarthandPlanetar•j Sciences, McGillUnit'e•s'ity. 3450University Street,31ontreal, Quebec, CanadaH3A 2A7 Abstract

Usingrecently published experimental data,xvehavecalculated thesolubility ofgoldforsimplified magmatic fluidsthatcoolbeP, veen500øand300øC.The starting fluidhasthefollowing characteristics: P = 1 kbar,ZC1 = 2.0 m, ZKC1/ZNaC1 = 0.25,pH fixedby muscovite + K feldspar+ quartz, jSL,fixedby SO2/H2S, and att2s fixedby magnetite + pyrite.Parallelcalculations wereperformed asstuning nodropin pressure during cooling(isobaric model)or an instantaneous dropin pressure to 500bars,resulting in separation of a dense brineanda low-salinity vapor(boilingmodel).Theisobaric modelapplies to magmas emplaeed at hypozonal or mesozonal depths, whereas theboilingmodelis moreappropriate for shallroy porphywdeposits. In the isobaric model,goldsolubility is initiallydominated by AuCIj at 500øC,1 kbar.If H2Slevelsare high(pyritestable),the dominant complex shiftsto Au(HS)2uponcoolingbelow•450øC,andsolubilities remainelevated (> 100ppb)overtheentiretemperature range.If H2Slevelsarelow(magnetite stable), gold solubility decreases steadily to 300øC,withAuCI• thedominant complex throughout. Thus,golddissolved in H2S-riehfluidswill tendto be carriedaway&orethe parentmagma,whereas goldin H2S-poor fluids•411 tendto precipitate closerto the source.At 500øC,goldsolubility asAuCI] is highestfor fluidsthat are oxidized(SO2/H2S> l), acidic.highlysaline,and potassium rich. Gold mayprecipitatein response to a numberof mechanisms, including cooling, pH increase, anddilution. Magmatie fluidsthatevolvefi'omshallow porphy U,bodiesareaptto boilshortly afterleaving themelt,at whichpointmostof the dissolved goldwill partitionalongwith chlorideintothe brinephase.Thismetalrichfluid,because of its highdensity,will tendto sinkor refluxnearthe parentintrusion, possibly forming an Au-riehporphyryCu deposit.Massbalancecalculations suggest thatmagmatie brineswill initiallybe undersaturated witbrespect to metallic gold,although themetalmaystillprecipitate asAu-riehcoppersulfide minerals (iss,bornitc).Duringboiling,mostof theH20 andH2Swillpartitionintothecoexisting vaporphase. Asthisvaporcools, it mayreeondense intoa low-salini•,H2S-rieh xvater of mixedmagmatie-meteoric heritage thathasa highpotential for dissolving andremobilizing significant quantities of goldasAu(HS)•.Migration of thisfluidto shallower levelsmayeventually formepithermal deposits of lmv-or high-sulfidation affinity,

depending onthepH-buffering capacity ofthewallrocks, andtheextent ofdirectmagmatie involvement.

Lackofcontact withretrograde, HzS-rieh magmatie-meteoric xvaters maybea prerequisite {brthepreservation of earlyAu-riehporphyry-style mineralization andmayalsoexplain the observed association bet•veen gold andhypogene ironoxidealteration in manyporphyry deposits. Whetherornotthereisadirecttemporal linkbetxveen ore-{brming processes in theporphyry andepithermal regimes, an intrusive eventmaybe important asa meansof introducing a largequantib, of loxv-grade gold whichisthenavailable forlaterremobilization andconcentration bycirculating fluidsof nonmagmatie origin. Moreover, an earlyporphyry eventmaycausex•4despread sulfidation of surrounding rocks.Laterfluidsof meteoric origincirculating throughthispyrite-rieh wallrockwillhaveHzSconcentrations thatremainelevated duringcooling andascent, increasing thechances of forminga large,high-grade epithermal golddeposit. Introduction

casethe mineis moreprecisely referredto asa "porphyry' gold" deposit (e.g., Marte, Chile: Vila et al.,1991).Fe•vsuch WHEREAS thetransport anddeposition of goldin epithermal deposits have been discovered to date (Vila and Sillitoe, deposits hasbeenextensively studied, relatively littleatten- 1991). tionhasbeenpaidto the hydrothermal geochemistry of this of gold-richporphyrycopper metalin porphyry environments. Nonetheless, it iswellestab- The generalcharacteristics orebodies have been reviewed by SillRoe(1979, 1989, lishedthat porphyry copperdeposits are an importantre1995a,b), Singer and Cox (1986), Lowell(1989),Vila and source of gold.In mostorebodies of thistype,average gold (1991),Langetal.(1995),andThompson et al.(1995). concentrations are ratherlow (typically700øC),andstableisotopedatathatindicatea mag• Present address: Department of Geological Engineering, Montana Tech cases, matic fluid contribution.

of the Universi•of Montana,Butte,Montana59701-8997. 0361-0128/97/1903/45-1556.00

45

Some of these same features are

46

GAMMONS ANDWILLIAMS-JONES

evidentin a fewdeposits thatarenonporphyritic in stylebut atedwith porphyrycopperdeposits typicallyformfromIwhichare inferredto haveformedthroughorthomagmatictypemagmas thataremoreoxidized(•SOs-HsSboundary). processes, e.g.,thebreccia pipe-hosted Kidston deposit, Aus- At temperatures belowapproximately 400øC,SOsdisproportralia (Baker and Andrew, 1991).

tionatesto a 3:1 mixtureof HsSO4and HaS (Burnham,1979).

Economic geologists havelongdebatedthe link between Consequently, the oxidation stateof SOs-bearing magmatic theevolution andmigration of late-stage magmatic fluidsand fluidswasapproximated bytheaqueous sulfate-sulfide isoacat T < 400øC(seealsoGiggenbaeh, 1992). theformation oflowertemperature hydrothermal golddepos- tivityboundary its (seeSillitoe,1989;Giggenbach, 1992;Richards andKerThepH of themodelsolutions wasconstrained by coexisrich, 1993; Spooner,1993;Hedenquistand Lowenstern,tenceof K feldspar, muscovite, andquartz.Thisassumption 1994;Richards, 1995;Carman,1996;Spryet al., 1996;and maybe invalidat theveryhightemperatures corresponding Losada-Calder6nand MePhail, 1997, for recent discussions). topotassic alteration (typically, biotite+ K feldspar _ magneIt is widelybelievedthat so-called "highsulfidation"-style tite,withmuscovite rareor absent), or at lowertemperatures epithermal golddeposits formin partby interaction of mag- wherephyllicandargillicalterationmayresultin the commaticvolatiles withcoolerground waters(Haybaet al.,1985; pletedestruction of feldspar to muscovite orclay.Therefore, Hedenquist et al., 1994).However,for the majorityof epi- alternative acidityvs.temperature pathswere alsoconsidthermalandmesothermal golddeposits, therelationship be- ered. tweenigneous activityandgolddeposition is moretenuous. TheHaSfugacity wasinitiallyfixedfor anygiventemperaWhereasthereis universal agreement thatintrusions are an tureandHaS/SOs ratiobythecoexistence ofpyrite+ magneimportant vehicle totransport heatintotheshallow crust(and tite (reaction10, Table 1). Thesevalueswere then converted HaSconcentrations assuming a Henry'slawcontherefore to drivelarge-scale hydrothermal convection cells), to aqueous a far morecontroversial issueinvolves theroleof magmas as stantof 103bars/molefractionHaS.The lattervalueis based a sourceof goldandotherore-forming components. on an extrapolation of the trendsin the dataof Suleimenov Thepurpose ofthispaperistoderiveanddiscuss athermo- andKrupp(1994)andmaybe in errorby asmuchasone are dynamic modelforthehydrothermal transport anddepositionlogunit at 500øC.Althoughbothpyriteandmagnetite of goldunderconditions thatapproximate thelaterstages of common inAu-richporphyry copper deposits (Sillitoe, 1979), coolingmagmaticfluids.Althoughmanyof the ideasad- we recognize thatthetwominerals donotnecessarily reprevancedin thispaperare not new,pastworkershavenot sentanequilibrium assemblage. Forthisreason, HaS(g) concalculated by reaction10 (Table1) shouldbe attempted a rigorous thermodynamic analysis. Recentexperi- centrations mentaldataon the solubility of goldin HaS-andchloride- considered maximumestimates at very high temperature isoftenabundant butpyriterareor absent) richhydrothermal fluidsat temperatures up to 500øCand (wheremagnetite at lowertemperature (wherepyrite pressures to 1.5 kbars(Zotovet al., 1985,1991;Gammons andminimumestimates andWilliams-Jones, 1995a;BenningandSeward,1996)now isubiquitous andmagnetite typically absent). providethisopportunity. Theories regarding thebehavior of goldin theporphyry-epithermal environment arereevaluatedChoiceof P-T trajectories beginwith a 5:C1= 2.0 m fluidat in lightof thesedata,andpotential areasforfurtherresearch All of ourcalculations are identified. 500øCand 1 kbar.In one scenario, the magmatic fluidis simplycooledto 300øCat a constant pressure of 1 kbar.In Calculation Procedures a second setof calculations, fluidimmiseibility is simulated by dropping the fluid pressure at 500øC to 0.5 kbars. These Selection of chemicalconstraints two contrasting scenarios are referredto as the "isobaric and In orderto performthe goldsolubility calculations pre- model"andthe "boilingmodel"in furtherdiscussions, in the P-X diagramof Figure1. Although sentedin thispaper,a numberof simplifying assumptionsare summarized hadto be made.To beginwith,we chosea valueof 2.0 m we recognize thatotherP-T pathsmaybe moreappropriate (10wt % NaC1equiv)for the•;C1concentration of thefluid forsome porphyry systems, thechoice inthisease waslimited at the time of its exsolution fromthe parentmelt (second bytheavailability of experimental solubility data.Theboiling boiling),in agreementwith the calculations of Burnham modelis probably morerealistic for porphyry deposits era(1979).Because thesalinity of magmatic fluidscanbeconsid- placedat shallow depthwherea dropin pressure islikelyto of the associated magerablygreaterthanthis(ClineandBodnar,1994),the effect occurat somestagein the evolution system. In contrast, the isobaric model of increasing the •;C1wasalsotested.Chloridewasparti- matiehydrothermal tioned between NaC1, KC1, and HC1. In most of the calcula- maybe moreapplicable for magmas eraplaced at greater tions,a KC1/(KC1+ NaC1)ratio of 0.25 was assumed,which

depth(e.g.,in themesozonal or hypozonal realm). In theisobaric model,it is assumed thatthe salinityof the is a typicalvaluefor a magmatic fluidin equilibrium with unchanged at •:C1= g.0m fromthe moment two feldspars (Burnham,1979).The effectof changing this fluidremains quantitywasalsoexamined. of separation fromthemeltto thepointatwhichit coolsto The oxidation statewassetto relativelyoxidizedor rela- 300øC.In theboilingmodel,the0.5-kbarpressure decrease of the initialmagmatie fluid tivelyreducedconditions by the gaspairsSOs-HaS or COs- at 500øCresultsin separation CH4,respectively. BurnhamandOhmoto(1980)proposedinto a higherdensitybrineanda lowerdensityvapor.We of 5.0 m (roughly 30 wt % NaC1equiv)and thatfluidsofporphyry tin affinityevolvefromS-typemagmas chosesalinities and haveoxidationstatesbufferednear the COs-CH4bound- 0.2 m (roughly 1 wt % NaC1equiv)for thetwoimmiseible withthedataof Sourirajan andKennedy ary.In contrast, according to the sameauthors, fluidsassoci- fluids,in agreement

CHEMICAL MOBILITY OF Au

47

1200

1000

800 600 400

200

0

0.01

0.05

0.1

0.5

1

5

10

50 100

NaC1,wt. percent F•o. 1. NaC1-H20phasediagram,showing the locationof the critical curve(dashed) andthetwo-phase solvus (thinsolidcurves) at 350øto 700øC. Thepointlabeled"a"corresponds to a fluidwith 10wt percentNaC1equiv at500øC,1kbar,i.e.,theinitialboundaw conditions usedin ourcalculations. If thisfluidcoolsisobarically to 300øC,it xvillneverintersect the two-phase solvus andboilingwill not occur.In contrast, if fluidpressure at 500øCis suddenly droppedto 500bars,twoimmiscible fluidsxvillformwithcompositionsb and b'. Thesefluidscorrespond to the vaporand brine phases, respectively, in the boilingmodelof thispaper.Theoretically, if the system isclosed, subsequent cooling at500barswillcause bothphases to rehmnogenize. However,thisis not possible if the brineand vaporare physically separated at the momentthatboilingoccurs, asdiscussed in the text.This diagramis a modification of figure14-5 of Roedder(1979)andis mainly basedonthe experiments of Sourirajan andKennedy (1962).

(196'2;seeFig. 1). In thiscase,37.5wt percentof the total waterin theinitialmagmatie aqueous-phase fractionares into the brineduringphaseseparation, and62.5wt percentinto thevapor.Thus,thevaporphaseis moreabundant thanthe brine,bothby massandbv volrune.

Thecomposition andd•nsity of immiseible fluids in the ß

H.20-NaC1 system arehighlydependent onthe temperature at whichboilingoccurs. At anygivenpressure, an increase in thetemperature ofphaseseparation causes a muchgreater contrast betweenthe densitvandsalinityof the twophases

(Fig.1).Ourestimate ofEC•= 5.0m•brthebrine side ofthe

H•O-NaC1solvus istherefore a conservative value,bearing in mindthathypersaline fluidinclusions in porphyry Cu deposits canattainsalinities in excess of 40 m NaC1equiv(•70 wt % NaC1equiv;Roedder,1984).We deliberately choseless extremeend-member compositions in ourexamples to avoid extrapolation farbeyond therangeof fluidcompositions used in the experimental solubili•studies. For theboilingmodel,we madetheadditional simplifying assumptions thatthetwoimmiseible fluidswerenotin contact with eachotherafterthe momentof phaseseparation, and thatthesalinity of thetwofluidsdidnotchange duringcooling.Thefirstassumption issupported bythet•aet thata brine, owingto its highdensity, wouldmostlikelysinkor remain closeto the pointof phaseseparation, whereas a lessdense phasewouldtend to rise as a vaporplume(Henleyand MeNabb,1978).The secondassumption followsfrom the firstbut maybe invalidif, for example, ascending fluidsmix withwatersof nonmagmatie origin.

48

GAMMONS AND •VILLIAMS-JONES

Speciation calculations: Brinephase approximately isoeoulombie and the pressureeffectsare small. The standard state for all of the data in Table 1 is Followingthe assumptions outlinedabove,a distribution ideal1-msolution at T andP (aqueous of aqueous species wascalculated at eachtemperature using (1) the hypothetical species), (9.) the ideal gas at T and 1 bar (allgases, including theprograin EQBRM(Anderson andCrerar,1993).Thecalwater vapor), and (3) the pure phase at T and P (minerals culations wereperformedin the temperature range300ø to and water). 500øC,for a constantpressureof either0.5 or 1.0 kbars. Sources of datafor all relevantreactions and their log K Speciation calculations: Vaporphase valuesare listedin Table 1. Fugaeitycoefficients for the calculations are gasesH2S,SO2,H20, and O2 weretakenfrom Ryzhenko For the boilingmodel,ourgoldsolubility limited by the fact that thermodynamic data are lacking for andVolkov(1971).Activitycoefficients for neutralaqueous the partitioning of gold between brine and vapor at the temspecies weresetto unity.Activitycoefficients for all charged andpressures of interest. However,a growing body aqueous species (including goldcomplexes) werecalculated peratures of empirical evidence from direct sampling of volcanic gases usingthe Daviesversionof the Debye-Hiiekelequation, andindirectanalyses of fluidinclusions suggests thatvaporwhichis builtintothe EQBRM program: phasetransport of goldmaybe significant (Heinrichet al., 1999.;Goffet al., 1994;Hedenquist, 1995).Therefore,it is desirable at leastto consider thispossibility in the present paper.Asa firstapproximation, we havemadethe following the equilibrium constants for the variousgold whereZi is the chargeof the ion and I is the true ionic assumptions: reactions maybeapplied withoutcorrection toboth strength.Valuesfor the Debye-Hfiekel A parameter were solubility the brine and vapor phases; and the activity coefficients for obtained fromthefollowing equation (Helgeson et al.,1981): all chargedand uncharged speciesin the vaporphaseare 1.8248'106' uni,t 7. Someexperimental dataexistto suggest thatthe first ^ = assumption is notentirelywithoutjustification. For example, (•. T) Hemleyet al. (1999.) foundthatapproximately 10percentof Fe, Zn, andPb in theirsolubility experiValuesofp, thedensity ofwater,weretakenfromSUPCRT92 the totaldissolved (Johnson et al., 1992).Valuesof •, the dielectric constant of ments(500øC,0.5 kbars,EC1= 1.0 m) waspresentin the water, were taken from table C2 of Shocket al. (199'2).The vaporphase. Moreimportantly, whennormalized tochloride, calculated activitycoefficients for singlycharged species fell the Fe/C1,Zn/C1,andPb/C1ratioswerenearlythe samein Williams in the range0.09 to 0.56, depending on the temperature, bothbrineandvapor(seetheirtable7). Similarly, constants pressure, andionicstrength. To testthe sensitivity of our et al. (1995)foundthat the apparentequilibrium results to theestimation of theactivitycoefficients, thecalcu- forCu-Naexchange betweenbrineandvaporin equilibrium lationswererepeated assuming 3' = 1.0for all aqueous spe- with rhyolitemelt at 800øC(1.0 kbar)wereverycloseto cies.For mostspecies,the resultswith or withoutactivity unity.Theseresultsimplythat the solubility quotients for aschloridecomplexes in coexisting brine coefficients wereverysimilar,although thesolubility of gold metalstransported andvaporaresimilar.However,theexactstoichiometries of ascharged complexes increased slightly for 3' < 1.0. in thesestudieswerenot deterOncethedistribution of aqueous species wasobtained, the the volatilemetalspecies thedielectric constant of vaporislowerthan solubilityof goldwascalculated for eachof the species mined.Because AuCi•, AuHSø,Au(HS)•,andAuOHø (seeTable1 for equi- that of a coexisting brine,it is possible that volatilegold librium constantsand sourcesof data). We did not consider specieswill existas unchargedmolecules(e.g., AuOHø, ø,AuHSø,NaAuCI•,NaAu(HS)•),in whicheaseapplicathe HAu(HS)•species(HayashiandOhmoto,1991),asthe AuC1 for chargedspecies suchas morerecentworkof Benningand Seward(1996)suggeststion of equilibriumconstants thatAuHSøis the dominant bisulfide species of goldat low AuCi.•andAu(HS)• to the vaporphasemaygiveerroneous pH. It wasnecessary to extrapolate the dataof Benning and results. Seward (1996) for AuHSø and Au(HS)• at T > 400øC. This We realizethat our calculations of goldsolubility in the reliability.However, wasaccomplished by fittingtheirlogK valuesfor reactionsvaporphaseat 500øCare of dubious 12 and13 at 200øto 400øC(Table1) to a simplepolynomial. belowa temperature of roughly 475øC,thebulkcomposition Likewise, for AuCI•, the data of Gammons and Williams- of the vaporphasein our boilingmodelpasses abovethe solvus for the H20-NaC1system(seeFig. 1). UnJones(1995a)at 300øC,andZotovet al. (1991)at 450ø and two-phase der theseconditions, the vaporis transformed into a con500øC,werefit to a simplepolynomial. low-salinity liquidandwilltherefore havebulkphysFromTable1, it isseenthattheequilibrium constants for densed, properties similar tothoseoftheaqueous media certainreactions are strongly pressure dependent, whereas ico-chemieal experiments. Thus,below475øC,our othersareindependent of pressure (ornearlyso).Thelatter usedin the solubility calculations fortheboiledvaporphaseareonmuch phenomenon wasshown byGuet al.(1994)tobe a character- solubility istic feature of well-balanced isoeoulombiereactions.Thus, firmerground. theextentto whichtheequilibrium constants varywithpresResults sureislargely a reflection ofwhether thereactions (aswritten) Theresults of ourspeciation calculations aresummarized havebalanced likecharges. The dissociation reactions 1 to 6 in theconcentration arestrongly nonisoeoulombie andalsoshowthelargest pres- in Figures9.to 8. In Figure9.,changes aqueous species areshownasa function suredependence. In contrast, all of the otherreactions are of someimportant

log %= i + +o.o,^z --

(1)

CHEMICAL MOBILITY OF Au

49

sureof 1kbar.Golddissolves mainlyasAuC1]athightemperature,whereasAu(HS)• predominates below450øC.The neutral complexes AuOH ø and AuHS ø are lessimportantat --total chloride all temperatures, and do not have predominance areas.The . NaCl(aq) ' 0 t•voboldcurvesshowtotalgoldsolubilities for t•vodifferent :----I•-==--•:-C-I-==-=----:---_•__ --_ KCl(aq• geochemical situations. Thetopcurve(labeled A) represents thesumof thecontributions fromall4 aqueous goldspecies, assuming thatH2Sconcentration isbufferedto relatively high valuesby the coexistence of pyriteand magnetite. In this ease,totalgoldsolubility decreases slightly froma highof •2 /14Cl{,aø• ppmat 500øC,passes througha shallow minimumat 380øC, -4O reaches alocalmaximum at360øC, andthendecreases slightly againto a low of -0.3 ppm at 300øC.The fact that the calculated solubilities aregreaterthan0.3ppmovertheentire -6 , , , , , , , -60 temperature interval suggests that unboiled,H.•S-rieh fluids 300 350 400 450 500 arecapableof mobilizing significant quantities of goldaway Toc fromthe parentmagmaintothe surrounding environment. the trendin totalgold FIO.2. Changes intheconcentration ofsome important aqueous species The bottomcurve(labeledB) shows for a fluidwhoseH.•Sconcentration liesfar below withtemperature. Thecalculations assume a constant pressure of 1 kbar,a solubility totalC1concentration of2.0m (10vet% NaC1equiv),anda constant ZNaC]/ the stabilityfield of pyrite.In this ease,gold solubility is ZKC1ratio= 4.0.Valuesof logf o_,arealsoshown (seerighty axis),for dominated by AuCI.•,anddecreases steadily withcoolingto oxidation statefixedbythecondition fs%/fHzs = 1. H2S/,,q/ concentration is valueof •20 ppbat 300øC.Thus,goldin H2Sfixedby coexistence of magnetite + pyrite,andpH is fixedby coexistencea minimum poor magmatic fluidswill tendto depositcloseto the porof K feldspar+ muscovite + quartz.The concentrations of all charged andnoremobilization of goldasbisulfide comspedes(dashed curves)decrease with an increase in temperature at the phyrysource, expense of neutralspecies andionpairs(nodash). plexeswill occurduringcooling. 2O

P=1kbar

_•_•----•

In the above calculations,oxidation state was fixed at rela-

of temperature for a solution containing •C1 = 2.0m, Na/K = 4, andP = 1 kbar.Severalsignificant trendsshouldbe noted.For example, all of the charged species become less abundant withanincrease in temperature relativeto theion pairsHC1,NaC1,andKC1.A similar phenomenon occurs with a decrease in pressure (notshown). Ion association islargely

tivelyhighvaluesby the condition fo., = fH2s.The effectof changing oxidation stateon goldsolubility aschloridecomplexesis illustrated in Figure5. At anygiventemperature, goldsolubility increases withincrease in oxidation state.However,it is importantto notethatthecalculated goldsolubili-

due to a decreasein the dielectricconstantof water, which

decreases itsabilityto solvate charged species. Thisisalsothe mainreason thatpH increases withincrease in temperature, despite thedownward trendin theequilibrium constants for thefeldspar hydrolysis reaction (reaction 11,Table1). Equilibriumbet•veen magnetite andpyritewasusedto constrain the H.2Sconcentrationof the solution,which increasesstead-

ily from --0.005m at 300øCto -0.2 m at 500øC.Thefo2 valuesalsoincrease steadily withtemperature, although the relativeoxidationstate, here definedby the condition fsoJfH2s = 1, remainsconstant. The aqueous speciation of goldasa functionof ac•-and aH2sat 500øC,i kbar,is summarized in Figure3. Three important trendsshould benoted:(1) athighac•/amsratios, AuCI.•is the dominantspecies; (2) at highaH•s/aclconditions,Au(HS).•isthedominant species; and(3)atlowconcentrationsof both ligands,gold solubilityis controlledby AuOHø. The latterspecies impartsa baselineor minimum goldsolubility foranygivenvalueoffo•.In thechosen exam-

ple,thisminimum valueisapproximately 10-6"5 m,orroughly

-2

AuOH(aq)

Au(HS)2-

500øC, 1 kbar

pH = 5.0

logfo2 = -18.8 -4

I

I

-3

-2

I

-1

'

I

0

'

•

1

activityof H2S(aq)

60 ppb,a quantitythatis by no meanstrivial.However,at FIO.3. Theaqueous speciation of goldat 500øC,1 kbar.Coldsolubility aregivenby thindashed linesandarein logactivityunits.The the highligandconcentrations typicalof magmatic brines, contours assumes a pH of 5.0 andan oxidation statebufferedat the SO2even highergold solubilities are possibleas AuCI.5-and diagram Au(HS).•, as will be demonstratedbelow.

HsSisofugacity boundaw. An increase in pH wouldenlargethe predominanceareaof Au(HS)=7 relative to AuCI•.A change in oxidation statewould

'alterthe absolutesolubilities,but would not effect the relative stabilitiesof Goldsolubility: isobaric model thecomplexes shown. AuHS/•, cxvas included inthecalculations, butdoes not Figure4 summarizes thesolubility of goldfora ZC1= 2.0 possess a predominance areaat500øC,1 kbar,based onourthermodynamic m solution thatcoolsfrom500øto 300øCat a constant pres- database.

50

GAMMONS ANDWILLIAMS-JONES

isobaric Au(HS) 2. • [• dominant AuCl2. model dominant I

•

lppmAu •

•

0.1ppm Au •

///•

I

• •

•t•-

'•

AuOH•

• -7[/1 I/

-7

•

/

•.

I

350

4•

450

I s_o_2m2s = !.o

II

2

4

6

8

10

total chloride, molal

500

FIG. 6.

TøC

The effect of the •C1 concentrationand the •KC1/(•NaC1 +

•KC1) ratioonthe solubility of goldtransported asAuC]• at T = 500øC,P

F•G.4. Goldso]ubili•fora magmatic fluidcont•ning2.0m EC1•vhich coolsfrom500ø to 300øCat a constant pressure of 1 kbar(the isob•ic model).O•dationstateis •ed by the con•tionfso:•:s = 1. The pH is •ed by the coe•stence of K feldspar, musco•te,andquaRz,assuming a constant ENaC•KC1

-........... I oooc,

•10ppbAu 0

3•

100ppb)in thereduced example atT ->500øC, illustrating Theeffectof salinity a500øCsolution ati kbarisshown inFigure6. Goldsolubility decreaseswith a decreasein total chloride concentration,and increases with an increase in the K/Na ratio of the ore fluid.

Thelattereffectisdueto a shiftin theequilibrium boundary of reaction11 (Table1) to lowerpH valuesasY•Kincreases. Thus,highlysalineore fluidsthat are potassium rich are especially goodcandidates fordissolving highconcentrations of gold(e.g.,> 10ppm),provided thatequilibrium is mainrainedbetweenquartz,muscovite, andK feldspar.Figure 6 alsoillustrates that dilutionis a veryeffectivemeansof precipitating gold.Thestoichiometry oftheAuCI,•solubility

-5

reaction(no. 14, Table 1) demandsa hundred-folddecrease

-10

300

350

400

450

500

in dissolved goldfor a ten-folddecrease in Y•C1. Thus,if a magmatic brineis diluted10-foldwith meteoricfluidcontaining negligible chloride, roughly 90percent ofitscontained goldwillbeprecipitated, assuming thebrineisinitiallysaturatedwithgold.Thiseffectwouldbeamplified if thediluting fluid is cooler than the brine.

ToC

In contrast to cooling, pressure changes aloneareunlikely to havemucheffecton the solubility of goldaschlorideor portedasAuClg.Theoxidation stateisbuffered to relatively highvalues by bisulfide complexes in porphyry environments, unless fluid FIG. 5. The effectof the oxidation stateon the solubility of goldtrans-

the coexistence of SO2andHgS(topcurves) or to relatively lmvvalues by COg and CH 4 (bottom curves).In each case,the solubilitieshave been

contoured for gasratiosof 100:1,1:1,and 1:100.Seecaptionto Figure4 formoreinformation regarding thephysico-chemical conditions ofthemodel fluid.

immiscibility occurs (seebelow).For example, Zotovet al. (1991)measured nearlyidentical goldsolubilities asAuCI,• in parallelexperiments conducted at 450øCandP = 500, 1,000,and1,500bars.Thepressure dependence of thesolu-

CHEMICAL MOBILITY OF Au

bilityof goldasbisulfidecomplexes wasalsoshownto be essentially negligible (lessthan0.3logunitsdifference in the range500-1,500bars;BenningandSeward,1996).

•

51

I vapor oiled dominant Au(HS)2'

Coldsolubility: Boilingmodel Figure7 sumlnarizes the calculated solubility' of goldin a brine(Y,C1 = 5.0 m) whichseparates froma homogeneous magmatie fluidat 500øC,0.5 kbars,andthencoolsisobariely ' to 300øC.The overalltopology of Figure7 is similarto that ofFigure4, butdiffersintwoimportant aspects: goldsolubilitiesasAuCI:•aremuchhigher,especially athightemperature (>100 ppmat 500øC);andthe temperature dependence of goldsolubility is steeper.Thus,coolingis a moreeffective depositional mechanism for the degassed brinethanfor the unboiledfluid(seeabove).Again,twototalsolubility curves are drawn,labeledA andB, whichimplythe presence or -8 • 350 4• 450 500 absence of significant quantities of H2S.PathB is probably morelikelyforaboiledbrine,asmostoftheHzSwillpartition TøC irreversibly intothevaporduringphaseseparation. Following pathB, >99.9 percentof thetotalgoldprecipitates by the FK;. 8. The solubility of goldin a low-salini•vaporphase(ZC1= 0.2

• -7'.

time the brine cools to 350øC. This assumes that the ore

m) thatcoolsfkom500øto 300øCat a constant pressure of 0.5 kbars.The

usedtodrawthisdiagram assume thattheequilibrium constants fluidis initiallysaturated with goldandthat equilibriumis calculations Forall aqueous reactions mavbe usedx•thoutmodification for boththe maintained between muscovite, K feldspar, andquartz.Loss brineandvaporphases. Because thisassumption is untested experimentally of acidicvolatiles (HC1,CO=,, SO2)duringboilingcoulddrive (andisunlikelytobecorrect), thediagram should betreated x•th skepticism thefigureillustrates in a qualitative waythepossibilfluidpH values abovethemuseovite-K feldspar boundary, in (seetext).Nonetheless, andupwardtransport of goldby an H•S-riehvapor whichease,the solubility of goldasAuCI:jwoulddecrease ity of remobilization phase.TheboldeumelabeledA assumes thatdl fourgoldspedeshavethe at an evensharperrate. capacity to tkaetionate intothevaporphase.The boldetm'elabeledB asThe solubility of goldin the vaporphaseof the boiling sumesthatonlythe neutralgoldcomplexes havean appreciable volatili•,. modelis shownin Figure8. Again,it is stressed that the Thecontrols onpH, o•dationstate,andHsSconcentration arethestoneas calculations atT > 475øCrestonassumptions whichmaybe in Figure4. invalid(see"Speeiation calculations: Vaporphase"), andare thereforeprovisional in nature.The curvelabeledA shows the totalgoldsolubility assuming that AuC12andAu(HS):j arethe dominant species andthatequilibrium constants for thesecomplexes maybe appliedwithoutcorrection to the vaporphase. ThecurvelabeledB assumes thatonlytheneutral spedesAuOHø andAuHSø havea significant volatility. brine Data for AuC1 ø are lacking,but are unlikelyto changethe generaltrendsshown.Threemainpointsare indicatedby solubilities of goldin the vapor Au(HS)2• • AuCl 2- v;.,,C•,glOppmAu Figure8: (1) the calculated phaseat 500øC,0.5 kbarsare greaterthen threeordersof magnitude lessthanthosein the coexisting brine;(2) at a constant pressure of 0.5 kbars,the solubility of goldin the low-salinity vaporpasses througha minimumandthenincreases slightly with a decrease in temperature; and(3) the solubility of goldin the condensed vaporat 300øto 350øCis actuallygreaterthanthat in the boiledbrine.All of these conclusions arevalidwhetheronechooses to includeor ignore the contributions from the chargedAuCI• and Au(HS)•-species. The aboveresultssuggest thatgoldwill partitionstrongly • I [• • . I . -8 into the saline brine upon phase separation at500øC,and0.5 300 350 4• 450 500 kbars.However,whereasgoldsolubilityin the brine deTøC ereases sharply witha decrease intemperature, goldsolubility increases withcooling. Because thevapor F•G.7. The solubility of goldin a salinebrinewithZC1 = 5.0 m that in thevaporactually separates œrom a vaporphaseat 500øC,and0.5kbars,andthencoolsvdth phase usedin ourmodelwillrecondense at •475øC,pathA no furtherpressure decrease to 300øC.Seecaptionto Figure4 lbr the in Figure8 shouldbe validbelowthistemperature. Thus, controlson pH, oxidation state,and H2S concentration. The bold curves watersof low salinity formed by condensation of maginatic labeled A andB showtotalgoldsolubilities thatinclude or exclude, respechavea highpotential forremobilizing goldasbisulfide tively,thecontributions œrom bisulfide complexes. Asdiscussed in thetext, steam provided thatthe pH is bufferedto near-neutral irreversible lossoœH2Sto thevaporphasemayoccurduringboiling, in complexes, whicheasecurveB is a morelikelyscenario. values by equilibrium betweenK feldspar andmuscovite. As

boiled I' ' '

ß

døminantA• •1 vvm Au

59,

GAMMONS ANDWILLIAMS-JONES

discussed in a latersection of thispaper,thesameconclusionderivedfromthesemagmas, basedontheabovemass balance applies if H2S-rieh vapors of magmatie originarecondensedconstraints, wouldbe onthe orderof 1 or 2 ppm. intooverlying watersof meteoricorigin. According to our previous equilibrium calculations, gold solubilities in the range1 to > 100ppmcanbe expected as Discussion AuClgfor magmatic fluidsat T (500øC,EC1_>2.0 m, and SO2/H2S-->1, with the highestsolubilities occurring in the The calculations presented aboveemphasize thehighmodegassed brine (see Figs. 3-6). This range is considerably bilityofgoldin magmatie fluids.Goldtransport maybedomiattainable gold natedby eitherchloridecomplexes or bisulfidecomplexes,higherthanour estimateof the maximum concentrationsbasedon massbalanceconstraints.Therefore,

depending ontemperature, pressure, andsolution chemistry. mostmagmatic fluidswill be undersaturated with respectto Aswell,the solubility of goldasthe simplehydroxy complex metallicgold at the time of exsolution from the parent is by no meansnegligible (atfso2= fH.2S, > 10 ppb for the magma.Goldprecipitation will thereforebe delayeduntil entiretemperature rangeof interest; seealsoRyabehikov et somechange in physieo-ehemieal conditions allows saturation al., 1985).The followingdiscussion concernsthe ultimate tooccur(however, seesection below"Copreeipitation ofgold fateof thismagmatie gold.First,someideasarepresented withcopper"). A similarconclusion hasbeenreached in the regarding themaximum goldconcentrations thatmaybeconeaseof coppertransport in porphyry environments (Bodnar, sideredreasonable for magmatie fluids,considering thevery low natural abundance of this element.

Are magmatic fluidssaturated withgold?

1992).

Massbalance calculations alsoplaceconstraints onthevolumeof a magma neededto forma large(> 1 MozAu)goldrichporphyry deposit. Forexample, a smallintrusion measur-

It is possibleto placea crudeupperlimit on the gold ing 1km acontains rou•,hly 3'10 •'5 gofrock (talcing anaverage contents of magmatic fluidsusingsimplemassbalancecon- rock density of 3 g/em ). Assuming the •m6agma originally constraints. Burnham(1979)estimated thatgraniticmeltscon- tained2 ppbAu,thistranslates to 6- 10 g of gold,or about the 10 Moz or soof goldcontained tainapproximately 0.1 wt percentchloridebeforethe onset 200,000oz.To produce deposit, Utah(Grimour, 1982),theintrusion of crystallization. Because of theverylargepartitioncoeffi- in the Bingham quartzmonzocientof C1- betweenaqueous fluid andmelt,almostall of wouldneedtobe-50 km3in size.Thealtered thisligandentersthe fluidphaseonceit forms(Burnham, nite porphyrythat formsthe centerof the orebodyat Bin1979).The chlorideconcentration of thelattercanrangeup ghamhasanaerialextentof lessthani km'• (Lanieret al., mustapply:(1) metal-rich to 50 wt percent(80 wt % NaC1equiv;Roedder,1984)if 1978).Thus,oneof the following lossof steamto the vaporphaseoccurs.This represents a magmatie fluidsthat formedthe Binghamorebodies were upwardfrom a muchlargerstockor batholithat C1-enrichment factorof roughly 500x fromtheparentmelt. focused A similarenrichment factorwouldbe expected for otherele- depth,(2)themagma hadanunusually high(> 10ppb)initial is of nonmentsthatpartition strongly intothebrinephase. Forexam- goldcontent,or (3) muchof the goldat Bingham ple,copperhasoneof thehighest fluid-meltpartitioncoeffi- magmatieorigin. cients ofanybasemetal(Candela andHolland,1984).Assumof goldwith copper inganaverage Cucontent of 12ppmfora granite(Wedepohl, Coprecipitation 1969), a 500x enrichmentfactor would lead to a concentraAlthough saturation withnativegoldmaynotoccuratneartemperatures, goldmobilitycouldnonetheless be tionof 6,000ppmin thesalinecondensate, assuming thatno magmatic byincorporation of thiselementasa traceimpucopperis lostto the vaporphase.Thisis closeto the upper constrained Anomalous Au contents havebeenrelimitincopper contents reported fromanalyses ofhypersalinerity in otherphases. fluidinclusions in porphyry deposits (Roedder,1984).If we portedin Cu-Fesulfideminerals fromgold-rich porphyry Cu takea goldconcentration of2 ppbfora typicalgranite(Wede- deposits (300-400ppmAuinbornitcfromPanguna; Baldwin (1995)confirmed thisexperipohl,1969),a similar500x enrichment factorwouldleadto etal.,1978).CyganandCandela findingup to 3,000ppmAu in chalcopyrite (intera brinethatcontains i ppmgold,againassuming thatmost mentally, or iss)grownat gold-saturated condiofthegoldwillpartition intothebrinevs.thevapor.Although mediatesolidsolution, haveverified nomelt-fluid partitioncoefficients areavailable forgold,the tionsat 600øto 700øC.Morerecentexperiments totalenrichment factorcannotbe greaterthan500x, because thatgoldenterstheissstructure asa Cu(I)-Au(I)solidsoluexsolves upon coolingto form inthisfigurealreadyassumes quantitative transferof goldinto tion, but subsequently the brine. tergrowths of nativegoldandchalcopyrite (P.Candela, pets. Oneproblem withcalculations of thesortin thepreceding commun., 1996).By analogy, an extensive solidsolution is paragraph is thatthe metalconcentrations of igneous rocks knownto occurbetweenAu(I) andAg(I) in argentitc,even aslow as300øC(Barton,1980).Gammons are not necessarily representative of the metalcontents of at temperatures (1995b)suggested that auriferous Ag2S the magmas fromwhichtheyformed.Thus,Connors et al. andWilliams-Jones couldbeanimportant solubility-limiting phase forgoldinAg(1993)arguedthattheverylowaverage goldconcentrations fluids.Likewise, CyganandCandela(1995) of felsicvolcanic rocks(-0.2 ppb),couldreflectscavengingrichepithermal thattheresidence of goldasa tracecomponent in of goldfromthe melt by orthomagmatic fluids.However, proposed minerals couldexplainthe positive correlagiventhe low naturalabundance of thiselement,it seems copper-bearing in manyCu-Audeposits of magunlikely thatanaverage magma offelsictointermediate com- tionbetweentheseelements position couldcontainmuchmorethana fewppbAu. If this maticaffinity. poristrue,themaximum goldconcentrations in magmatic fluids Asshownin Figure9, theAu/Curatiosof gold-rich

CHEMICAL MOBILITY Au/Cu mass ratio

10-5

10-4

10-3

10-2

0.1

OF Au

53

et al., 1990;Hoosain andBaker,1996).In general, sulfide saturation issuppressed if theES2- concentration ofthecooling magmais relatively low,aswouldbe expected for an oxidized magma in whichSO2>>H2S.A number of previous workers havediscussed thisproblem in greater detail(Carroll andRutherford, 1985;WybornandSun,1994;Cyganand Candela, 1995).

Depositional mechanisms

According to ourcalculations, if an ascending magmatic fluidseparates intotwoimmiscible phases, goldwillfractionateintothebrineasAuCI•.Goldmayprecipitate withcopper 0.00001 0.0001 0.001 0.01 0.10 1.0 due to cooling near the core of the porphyry system or it mole fraction Au maymigratelaterally intoadjacent rocksif thetemperature is small.Thefirstscenario is probably a fittingdeFIG.9. Diagram shmving therange in theAu/Cumass ratioofporphyry gradient copper deposits incomparison totheAu/Curatioofgold-saturated chalcopy-scription for manyporphyry Cu-Audeposits in whichgold rite(iss)at 600øC(based ontheexperiments of Cygan andCandela, 1995). shows a close spatial association withhigh-temperature potasTheshaded region ontheleftsideof thediagram shows therangeof Au/ sicalteration and hypogene copper mineralization (Sillitoe, Curatios thatcouldbeexplained bydeposition ofgoldasa traceimpurity scenario islesswelldocumented, butmay inchalcopyrite. Incorporation ofgoldasa solid solution inchalcopyrite also 1989).Thesecond applyto gold-rich veindeposits thatappearto haveformed lowers thesolubility ofmetallic goldandcould beanimportant alepositional mechanism, asexplained in thetext.A largemiscibility gapexists (unshadedfromfluids ofmagmatic originbutarenotspatially associated region) between gold-saturated chalcopyrite andcuprian gold.TheCucon- with anyobvious porphyry deposit. The unusually Au-rich centration of goldin equilibrium •vithchalcopyrite at thistemperature is veins(up to i oz/t)foundnearthe MountEstellepluton, not known, and is therefore dashed. Alaska, maybe anexample of sucha setting (Croweet al., 1991).

magmatic goldtransported phyrydeposits arelowerthantheAu/Curatios ofexperimen- Twootherwaysto precipitate includea decrease in C1- concentratallygrown, gold-saturated ehaleopyrite at 600øC. Bysimple aschloridecomplexes in pH. Dilutionis the mosteffective mass balance, it follows thatmuchofthegoldintheorebodiestion,andan increase of decreasing Y•C1 andcanoccurin porphyry environin question couldhaveresided asgold-rich ehaleopyrite or means withconvecting waters ofnonmagmatic oriissatthetemperature oforeformation. If goldenters a solid mentsbymixing wouldprobably alsoresultincooling, further solution in another phase, its thermodynamic activity and, gin.Thisprocess theprecipitation ofgold.Anincrease in pH could byconsequence, itssolubility maybe greatly reduced. The promoting fluid,since eopreeipitation hypothesis is therefore compelling for two occurduringboilingof a primaryorthomagmatic (e.g.,HC1,H2S,SO2)are known reasons: it helpsto explain thedosecorrelation betweenCu mostacidiccomponents intothe vaporphaserelativeto theirbasic andAuin manyporphyry deposits; andit provides a means to fractionate (KC1,NaC1,HS-; Drummondand Ohmoto, to deposit goldfromhigh-temperature magmatie fluidsthat counterparts in pH wouldalsobeexpected if therocks areundersaturated withthepuremetal.Thesecond pointis 1985).Anincrease theparent magma iseraplaced contain asignificant important in lightof ourcalculations whichindicatethatthe intowhich component. Sillitoe (1995a) notedthatmanyofthe solubility of metallic goldisextremely high(> 100ppmfor carbonate ß ß example) ß Au-richporphyry deposits (based ontotalcontained th e boiled bnne at500oC. Theexperiments ofCan- largest bycarbonate rock(e.g.,Bingham, Utah;Ok delaandcoworkers showthatgold,originally dissolved in gold)arehosted Indonesia). Although high-temperature iss,quickly exsolves uponcooling toambi- Tedi,PapuaNew Guinea;Grasberg, explanation for this enttemperature. Thisreaction ismitigated bytheiss-ehaleo-Sillitoefavoreda structural-mechanical it isalsoplausible thatpH changes during fluidpyritelatticeinversion (P. Candela, pers.eommun., 1996). observation, in some waypromoted deposition ofgold.It We inferthatit maybe difficult or impossible to finddirect rockinteraction couldforma evidence forhigh-temperature Au-Cusolidsolutions in natu- is naturalto presumethat sucha mechanism skarndeposit, butfieldevidence suggests thatmost ralehaleopyrite specimens, sincetheexsolved goldwillhave gold-rich mostlikelymigrated by diffusion to formits owndiscrete largegoldskarns are associated with retrograde, H2S-rich grains. Moreresearch isneeded to determine whether gold fluids(Meinert, 1989). dissolved inotherCu-bearing minerals (e.g.,bornite) iseasier Withoutmoredetailed fieldinformation onthespatial and to quench. temporal distribution ofgoldin specific deposits, it isdifficult The propensity of sulfideto sequester goldwill havea to ranktherelative importance ofcooling, boiling, fluidmixdistinctly negative impact onthehydrothermal mobility of ing,andwater-rock interaction asa means toprecipitate magthismetalif theparent meltcondenses a sulfide phase before maticgold.Ouranalysis suggests thateachof thesemechaanyaqueous fluidis evolved.Sulfidesaturation couldoccur nisms couldplayanimportant roleindividually, or perhaps either in the form of an immiseiblesulfide melt or as sulfide in concert. Coprecipitation with chalcopyrite is alsoa viable

minerals crystallizing atthesolidus (e.g.,pyrrhotite). In either means ofprecipitating goldfromsolutions thatareundersatuease, available dataindicate thatgoldis strongly partitionedratedwiththenativemetal.Production of H,2S(e.g,bythe fromthesilicate meltintothecoexisting sulfide phase (Stone disproportionation of SO2)couldtriggerdeposition ofchalco-

54

GAMMONS ANDWILLIAMS-JONES

pyritewhichin turn couldincorporate tracebut significantWesternAustralia). Sucha linkhaslongbeenadvocated by quantities of gold. manyauthors, although thereisstillwidespread disagreement Asa finalcomment, it isinteresting tonotethatthesolubili- regarding theprecise roleof magmas in theformation of the tiesof the platinum-group elements are alsoquitehighas deposits in question. We arenotin a position to takea stand chloridecomplexes in oxidized magmatic fluidsthatare ex- in thisdebate,eventhoughourcalculations doindicatethat ceptionally rich in chloride(Gammonset al., 1992).Some significant contributions of goldfrommagmatie fluidsto the Au-richporphyry deposits areindeedenriched in palladium mesothermal environment aretheoretically possible. andplatinum (Werleetal.,1984;Petrunov andDragov,1993; Directtransferofgoldvia thegaseous phase: Conventional TarkianandKoopmann, 1995). wisdom wouldmaintainthatwhena magmatie fluidboilsat some point after exsolution from the parent magma, most Themagmatic-epithermal (-mesothennal) transition of the goldwill fractionate into the salinebrineaschloride We willnowexplore in greaterdetailthepotential of mag- complexes. Our provisional calculations supportthis view masto supplygoldto theepithermal andmesothermal envi- (compare Figs.7 and8), although it is stressed thatexperironments. Fivescenarios areconsidered, arranged in anorder mental work is needed to make a more reliable assessment. of decreasing levelof involvement of magmatic fluids: Thisis especially truein lightof the fluidinclusion studyof Heinrich et al. (1992), which indicates that copper may parti1. Goldistransported to surrounding watersviathemagtionstrongly intoH.2S-rich vapors trappedatmoderately high maticbrinephase. (a similarconclusion hasbeenreachedindepen2. Goldandothervolatilecomponents aretransported in pressure 1996).Evenif most the gaseous statewheretheyare subsequently condenseddentlyby R. J. Bodnar,pers.commun., of the gold partitions into a brine, the low quantities of gold intooverlying meteoricwatersor released directlyintothe remaining in the vapormaystillbe significant, especially atmosphere. the muchlargermobilityandmassflux 3. Magmatic vaporstransport H.2S(butnot gold)that is whenoneconsiders relative to thebrine.Measureable quanticondensed intooverlying meteoric waters, therebyincreasingofthevaporphase ties of gold have been detected in fumarole gases of active theabilityof thelatterto dissolve andredistribute gold. (Hedenquist et al., 1993;Goffet al., 1994;Heden4. No significant exchange of matteroccursbetweenthe volcanoes that magmatic and epithetrealregimes,althoughheatfrom the quist,1995),andthe sameauthorshavedemonstrated quantities of goldcouldaccumulate in zonesof cooling intrusion drivesconvection of surrounding meteoric appreciable water. hydrothermal alteration overlying magmachambers thatdeperiodof time.Althoughintriguing, a 5. No significant golddeposits areformedduringthecool- gasoveran extended evaluation of the roleof vapor-phase transport of inghistoryof theinitialintrusion, although convecting mete- thorough andtheoretical studies. oricwaterfroma laterhydrothermal eventremobilizes low- goldmustawaitfutureexperimental grademagmaticgoldthat wasintroducedduringthe first Condensation ofmagmatic H2SandS02intometeoric waevent. ters:Here it is assumed thatH•S andothervolatilespecies (e.g.,H20,CO2,CO, SO2,HC1)partition intothevaporphase Thesescenarios are discussed separately below,although during boiling but that gold does not. It is alsoassumed that it isrecognized thattwoor moremayapplyat thesametime a large percentage of these volatiles are recondensed into and/orin sequence duringtheformation of a givendeposit. overlying meteoric water. Under these conditions, one can Directtransferof goldvia thebrinephase:Following the imagine the condensation process as a chemical titration in isobaric model(Fig.4) wecalculate thatapproximately 10-• which the concentration of H.2S and other species in the m gold(•0.2 ppm)willremainin solution afteranorthomagis steadilyincreased. maticfluidcoolsto 300øC.In the boilingmodel(Fig.7) the meteoricwaterreservoir Figure 10 summarizes somepossible reactionpathsthat amount ofdissolved goldleftaftercooling to300øCisroughly 10x less,assuming thatmostof the magmatic H,2Sis parti- maydevelopfor a dilute(F•C1= 0.1 m) meteoricwaterat of muscovite + tionedintothevaporphase duringtheboilingevent.In addi- 300øCwhosepH is bufferedby coexistence K feldspar + quartz, and whose oxidation state is buffered tion,the solubility gradientis muchsteeperfor a cooling sulfide-sulfate isoaetivity boundary. Initially, brinethathasdegassed H•S andsteam(compare the slope at the aqueous of themeteoric fluidincreases steadily of the totalgoldsolubility curvesin Figs.4 and 7). Thus, theH2Sconcentration of magmatie H.2S), whereas the pH of magmatie waters thateo•lwithout boiling havea greater(dueto condensation nearneutralowingto the conversion of K potentialto delivera significant fractionof theirinitialdis- the fluidremains feldspar to muscovite (i.e., phyllie alteration zone). As mn•s solvedgoldto shallower levels.It is unlikelythat magmatie the solubilityof goldas Au(HS)• alsoincreases fluidsbornat depthslessthana few kilometers couldpass increases, eventually reaching values ashighas10ppmor more directlyintoa eonveeting meteoricsystem withoutboiling, steadily, region).Sucha fluidwouldbe a primecandidate to because of the verylargepressure differential betweenthe (shaded golddeposit,especially if the meteoric environment of the e17stallizing magma(•lithostatie)and forman epithermal the meteoricfluid (•hydrostatie).Therefore,the isobaric systemis eonveetingat a vigorousrate. modelofthispaperismainlyapplicable to magmas emplaeed If the H,2S-enriehedmeteoricwater remainsrock-buffered to pH, it mayeventually forma lowsulfidationat greaterdepths(>5 km). Thisraisesthe questionof the withrespect golddeposit.However,at somepointthe involvement of magmatie fluidsin the genesis of so-called styleepithermal ofacidicvolatilespecies (SO•,HC1,CO2) "mesothermal" golddeposits (e.g.,mostof the deposits in rateofintroduction the Abitibisubprovince of Canadaandthe Yilgarnblockof mayeventually exceedthe capacity of the rockto bufferpH

CHEMICAL MOBILITY OF Au

55

Regardless of the exactpathwayfollowed,aeidifieation of log {Au} = -7

SO4 2.\

-6

\

-5

\ \

-4

\

H2S-rieh meteoric waters results in a dramatic decreasein

-3

\

\

\

goldsolubility (uptothreeordersofmagnitude). Thisprocess couldleadto the deposition of goldin a "high-sulfidation" environment, asoriginally proposed by Stoffregen (1987).In somehigh-sulfidation epithermalgolddeposits (e.g.,Lepanto,Philippines), thereis texturalandisotopicevidence thatacidicalteration occurred at anearlystagefromoxidized magmatie fluidsandthatgoldwasintroduced shortly thereafterbymorereduced fluidsofpresumed meteoric origin(Arri-

--•-----

muscovite

5

AuOH(aq)

• \\•U(5• )2\\\ \

\

\

\

pH X\

\

bas, 1995;Arribaset al., 1995). If this is the case,the trends

muscovite

in goldsolubility depictedin Figure10 maystill apply.A near-neutral, H2S-riehfluid maybecomeacidicsimplyby interacting with a preexisting alterationassemblage con300øC, 0.5 kbar tainingminerals suchaskaolinite,alunite,nativesulfur,and/ m•;Cl = 0.1 or pyrophylite. Aeidifieation \vouldbe augmented if mixing Na/K = 10 occurs (byadvection or diffusion) between ingressing neutral fO2 = HSOn-/H2S pH andegressing lowpH porewaters.In thisrespect, the 2 vuggy silicacoreof suchdeposits wouldhaveanexceptionally -5 -4 -3 -2 -1 0 highground-water storativity, andporewatersthat areinitiallystrongly acidicwouldhaveto be flushedwith several logall2S volumes of dilutegroundwaterbeforetheaddisneutralized. F•;. 10. Diagramillustrating the changein goldsolubility fbr a dilute Continued fieldstudiesshouldhelpresolvethe question of meteoric waterat300øCand0..5kbars,asa function ofpH andHaSconcen- therelative timingofacidalteration andprecious metaldepotration.The oxidation statehasbeensetto the HSO4/H2S(,•q, isoaetM• of thistype. boundary, xvhich itselfis a function of pH. The fieldof liquidsulfi•rgives sitionin deposits The mobilization of goldby H•S-riehmeteoric waterscan an upperlimitto the HaSconcentration at anygivenpH. The boldsolid asa multistage process, asillustrated in Figure linesshowthestability fieldsofthealteration minerals K feldspar, musemite, be envisioned kaolinitc, andalunite.Thebolddashed linesshowthepredominance areas 11aandb. In the firststage,magmatic goldis partitioned of theaqueous goldspecies, whereas thindashed linesgivegoldsolubility intoanimmiseible brinepoolandprecipitates (along \vithCucontours (logmolalunits).The shadedregionshowsthatportionof the Fe sulfide minerals) in the potassie zone due to the combined diagram overwhichthesolubili•of goldexceeds 10ppm(mg/kg). Seetext effects ofcooling, dilution, and/orpH changes (seepreceding for a discussion of the reaction pathslabeledA to E. section).At the sametime, in the overlying phylliezone, ascending H.2S-rieh vapors mixwitheonveeting meteoric wairon-bearing silicateand oxidemineralsto (e.g.,'allfeldspar converted to muscovite). The pH of the ters,converting activity,the meteoric waterwillthendecrease rapidly(pathB).Important pyrite.In the waningstagesof hydrothermal reactions thatcouldgenerate add includethe hydrolysis of upperlevelsof the maglnachambersolidifyandthe watermelt(i.e.,the source of neworthomagmatie fluids) magmatie SO2(reaction 3, below),theoxidation of magmatie saturated H.•Sto sulfate(reaction4), andthe dissociation of HC1 (reac- retreatsto greaterdepth(Burnham,1979).As thisoccurs, tion 5): eonveeting, H.2S-rieh meteoric waterscannoxv leachgoldthat waspreviously deposited in thepotassie zone,andtransport 4SO2 + 4HsO = H.2S+ 3HSOj + 3H +, (3) thismetaltoshallower levels. Thisprocess couldbeenhanced by rapid erosion of overlying rocks in zones of tectonic uplift H2St.,p + 202