Au Solubility in Oxidized and Reduced Water-saturated Mafic Melt

7/30/2019 Au Solubility in Oxidized and Reduced Water-saturated Mafic Melt

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Gold solubility in oxidized and reduced, water-saturatedmafic melt

Aaron S. Bell a,, Adam Simon a, Marcel Guillong b

a Department of Geoscience, High Pressure Science and Engineering Center, University of Nevada Las Vegas, 4505 Maryland Pkwy, Las Vegas,

NV 89154-4010, USAb ETH Institute for Isotopengeologie/Mineral.Rohstoffe, Clausiusstrasse 25, Zurich, CH 9082, Switzerland

Received 31 August 2010; accepted in revised form 2 December 2010; available online 4 January 2011

Abstract

We have performed experiments to evaluate Au solubility in natural, water-saturated basaltic melts as a function of oxygenfugacity. Experiments were carried out at 1000 C and 200 MPa, and oxygen fugacity was controlled at the fayalite-magnetite-quartz (FMQ) oxygen fugacity buffer and FMQ + 4. All experiments were saturated with a metal-chloride aqueous solutionloaded initially as a 10 wt% NaCl eq. fluid. The stable phase assemblage at FMQ consists of basalt melt, olivine, clinopyrox-ene, a single-phase aqueous fluid, and metallic Au. The stable phase assemblage at FMQ + 4 consists of basalt melt, clino-pyroxene, magnetite-spinel solid solution, a single-phase aqueous fluid, and metallic Au. Silicate glasses (i.e., quenchedmelt) and their contained crystalline material were analyzed by using both electron probe microanalysis (EPMA) and laserablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Measured Au concentrations in the quenched meltrange from 4.8 lg g1 to 0.64 lg g1 at FMQ + 4, and 0.54 lg g1 to 0.1 lg g1 at FMQ. The measured solubility of Auin olivine and clinopyroxene was consistently below the LA-ICP-MS limit of detection (i.e., 0.1 lg g1). These melt solubilitydata place important limitations on the dissolved Au content of water-saturated, Cl- and S-bearing basaltic liquids at geolog-ically relevant fO2 values. The new data are compared to published, experimentally-determined values for Au solubility in dryand hydrous silicate liquids spanning the compositional range from basalt to rhyolite, and the effects of melt composition,oxygen fugacity, pressure and temperature are discussed. 2011 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

The geochemical behavior of Au as a trace componentdissolved in silicate liquids, common magmatic silicate min-erals, and oxide minerals has implications for processesranging from the genesis of magmatichydrothermal ore

deposits (i.e., porphyry-, high-sulfidation epithermal-,andlayered-mafic intrusion-hosted deposits) (Naldrett andDuke, 1980; Hedenquist et al., 1994; Audetat et al., 1998;Frank et al., 2002; Simon et al., 2005; Bell et al., 2009) to

core formation and the chemical differentiation of the earlybulk earth (ONeill et al., 1995; Ringwood, 1966; Cottrelland Walker, 2006; Righter et al., 2008). Three proposed ori-gins for metallically-fertile, siderophile-element-enrichedsilicate liquids are: (1) the low degree partial melting of oxi-dized, sulfide-free asthenospheric mantle (Mungall, 2002);

(2) the non-mutually exclusive process of siderophile ele-ment enrichment in residual liquids, driven by the crystalli-zation and differentiation of sulfide-free basaltic magmas(Mustard et al., 2006); and (3) the large degree of partialmelting of asthenospheric mantle that fully incorporatesthe stable mantle sulfide phase(s) into a sulfide undersatu-rated silicate liquid. Mantle derived silicate liquids arewidely accepted as the primary agent of chemical masstransfer for Au and other siderophile elements from mantleto crustal reservoirs. Therefore, it is essential to understand

0016-7037/$ - see front matter 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.gca.2010.12.022

Corresponding author. Address: Dept. of Earth and PlanetaryScience, American Museum of Natural History, USA. Tel.: +1212-769-5459.

E-mail address: [email protected] (A.S. Bell).

www.elsevier.com/locate/gca

Available online at www.sciencedirect.com

Geochimica et Cosmochimica Acta 75 (2011) 17181732
http://dx.doi.org/10.1016/j.gca.2010.12.022mailto:[email protected]://dx.doi.org/016/j.gca.2010.12.022http://dx.doi.org/016/j.gca.2010.12.022mailto:[email protected]://dx.doi.org/10.1016/j.gca.2010.12.022


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the thermodynamic and compositional variables that exertinfluence on the capacity of mantle derived basaltic liquidsto dissolve and potentially fractionate Au and other sidero-phile elements during open system degassing and crystalli-zation processes. Solubility data defining the maximumAu capacity of silicate liquids in equilibrium with metallicalloys or sulfide phases are essential for the purpose of

modeling Au delivery into crustal magmatic systems thatsource ore forming magmatichydrothermal systems.

Extant experimental studies have demonstrated theimportance of oxygen fugacity (fO2) and to a more limitedextent, the effects of pressure and bulk composition, on thesolubility of many siderophile elements in silicate liquid.Several experimental studies have concluded that the varia-tion of the fO2 imposed on select silicate liquids alters, byorders of magnitude, the solubility of many siderophile ele-ments in the silicate liquid (Borisov and Palme, 1996, 2000;Ertel et al., 1999; Blaine et al., 2005; Brenan et al., 2005).Experiments with anorthitediopside eutectic liquids dem-onstrate that the solubility of most siderophile elements de-

creases linearly with the imposed decreasing oxidation stateof the liquid, following a slope defined by the oxidationstate of the stable metallic oxide species in the melt. Addi-tionally, the small, yet variable, effect of pressure on themeasured solubility of Ni, Pt, and Pd in silicate melt is alsoan important factor (Ertel et al., 2006; Righter et al., 2008).Botcharnikov et al. (2010) experimentally quantified the ef-fects of dissolved Cl and S on Au solubility in andesitic andrhyodacitic liquids at fO2 $ NNO, 1050 C, and 200 MPa.Their data indicate that the addition of Cl or S to the meltappear to be correlated with an increase in the solubility ofAu in the silicate liquid. Jego et al. (2010) quantified therelationship between Au solubility in intermediate silicatemelt (i.e., dacitic; $65 wt.% SiO2) and report that increas-

ing fO2, at 1000 C and 400 MPa in a S free assemblage,is correlated with an increase in the solubility of Au inthe silicate melt. Jego et al. (2010) also report data from alimited number of S-bearing experiments that are consistentwith the results of Botcharnikov et al. (2010) suggestingthat the addition of S to intermediate to felsic silicate liq-uids is correlated with an increase in the measured Au sol-ubility values in these melt compositions.

The aforementioned studies have advanced significantlyour collective understanding of the solubility behavior ofselect siderophile metals in silicate melt. However, thereare relatively few experimental data that constrain thesolubility behavior of siderophile metals in natural H2O-,Cl-, and S-bearing mafic liquids saturated with crystallinesilicate and oxide phases. Further, there are few data thatconstrain the Au-sequestering potential of modally-abun-dant silicate phases which may exert a significant effect onthe budget of siderophile elements during the evolution ofmafic magmatic systems. In this study, we performed exper-iments to elucidate the influence of fO2 on Au solubility inH2O-saturated, Cl- and S-bearing basaltic silicate liquids, inequilibrium with clinopyroxene and olivine at FMQ andwith clinopyroxene and magnetite-spinel solid solution atFMQ + 4, to quantify both the solubility of Au in the sili-cate liquid and the solubility of Au in clinopyroxene andolivine in equilibrium with the experimental melt to better

understand the effects of crystal fractionation on the Aucontent of basaltic melts during crystallization along liquidlines of decent.

2. EXPERIMENTAL DESIGN AND RATIONALE

2.1. Experimental techniques and PT conditions

Experiments were performed in near-horizontal (i.e., thevessel and furnace are both at a positive angle of 10 rela-tive to the horizontal lab bench), rapid quench titaniumzirconium molybdenum (TZM) pressure vessels and aShaw-membrane-equipped, internally heated gas pressurevessel (IHPV). All experiments were carried out in eitherAr H2 or ArCH4H2 mixed gas pressure media at1000 C and pressures ranging from 195 to 200 MPa. Mea-sured temperature gradients imply that the temperatureuncertainty in the TZM runs at any position within thecharge is 12 C from the reported run temperature. Tem-peratures in the IHPV experiments have been measured to

be 5

C from the reported run temperatures. Pressure wasmonitored by using a bourdon tube strain gauge with a pre-cision of 5 MPa in both the IHPV and TZM experiments.

2.2. Starting materials

Starting materials consisted of a dolerite powder (tung-sten carbide milled;


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to trap the aqueous phase as synthetic fluid inclusions. Asdiscussed below, the long-duration experiments, i.e., thosethat best represent steady-state conditions, did not containquartz.

2.3. Capsule configuration

The experimental capsule configuration (Fig. 1) con-sisted of a small length of Au tubing (1 mm ID, 1.2 mmOD 10 mm length) welded at the base and loaded withapproximately 1013 mg of powdered dolerite which wascompressed into the bottom of the tube. The top of thiscapsule was mechanically crimped, but not welded. Thissmaller capsule containing the dolerite was then loaded intoa larger Au capsule (4.8 mm ID with a 5 mm OD 20 mmlength) along with 30 lL of starting aqueous solution whichwas pipetted into the base of the outer capsule. This capsuleconfiguration was designed to allow us to recover thesilicate melt as a cylindrical aliquot of glass (i.e., quenchedmelt) + crystalline material that preserves the geometry ofthe fluidmelt interface. The outer capsule was triplecrimped and welded shut. Masses of the capsules weremonitored both before and after welding to ensure thatthe solution was not lost during welding. The volume ofsolution loaded was carefully selected to ensure that the

calculated volume of the aqueous phase at the PT condi-tions of the experiment (approximated using the densitypredicted by the algorithm of Driesner et al., 2007) wouldnot exceed the total available volume of the experimental

capsules. Based on the analog phase relations for theNaClH2O system (Driesner et al., 2007), the aqueous fluidin the experiments is a single phase, supercritical fluid at runconditions.

2.4. Control and estimation of experimental fO2 values

Oxygen fugacity of the experiments was fixed at either$FMQ + 4 or $FMQ. Low fO2 values were achievedthrough the addition of CH4 and H2 to the Ar pressure med-ium in theTZM andIHPV experiments,respectively. Oxidiz-ing experiments were run at the intrinsicfO2 oftheTZMandIHPV pressure vessels. Several experimental studies have uti-lized pure-Ar as a pressure medium in TZM and IHPV appa-ratuses along with NiPd or hydrogen redox sensors tomeasurefO2 (Popp et al., 1984; Matthews et al., 2003; Berndtet al., 2005; Schuessler et al., 2008). All published experimen-talstudies that have measured theintrinsicfO2 of an Ar-pres-sure medium TZM and IHPV vessel consistently documentthat the fO2 of these vessels, accounting for subtle variationin metal content of the vessel, ranges from FMQ + 3.5 toFMQ + 4.5. The fO2 of the IHPV utilized in the currentstudy has been measured as FMQ + 3.5. Based on the avail-able published fO2 measurements and that measured for theIHPV used in the current study, we assumed an oxidationstate of FMQ + 4 0.5 for the oxidizing runs. Reducingexperiments in the TZM vessels were performed with a fixed

partial pressure of 6.5 bars of CH4 that was pre-charged intothe pressure vessel at ambient conditions. Using the equilib-rium constant for the CH4 dissociation reaction and a tabu-lated hydrogen fugacity (fH2) coefficient, it is possible toestimate thefH2 of thepressure mediumallowing us to calcu-late the approximatefO2 of theexperiment at runconditions.Calculated fO2 values of the reducing experiments areapproximately FMQ 0.5 log units. Hydrogen fugacity inthe IHPV experiments was fixed by using a Shaw membraneand continuously monitored with a pressure transducer.

All experimental charges were saturated with a super-critical aqueous phase in the MeClnH2O (where n isproportional to the charge of the metal) system. In order

Table 2Run conditions for all experiments and quantified Au solubility of the quenched melts. Au concentrations are presented as the mean of 713unique LA-ICP-MS analyses per quenched experimental glass; the 1r standard deviation follows the mean Au solubility to the right. The 2rerror reported is the LA-ICP-MS analytical error derived from counting statistics

Run Duration (H) fO2 Pressure Temperature C Au 1r (lg g1) 2r Analytical error SiO2 added

TZM 11 7 FMQ + 4.0 0.5 195 MPa 10 990 12 1.46 0.11 0.22 YesTZM 20 18.5 FMQ + 4.0 0.5 195 MPa 10 990 12 1.10 0.35 0.12 Yes

TZM 12 24 FMQ + 4.0 0.5 195 MPa 10 990 12 4.90 0.21 0.36 YesTZM 21 25 FMQ + 4.0 0.5 195 MPa 10 990 12 2.90 0.69 0.24 YesIHPV 1-2 144 FMQ + 4.0 0.5 200 MPa 5 1000 5 1.05 0.03 0.16 NoIHPV 1-1 172 FMQ + 4.0 0.5 200 MPa 5 1000 5 0.64 0.03 0.1 NoTZM 23 6 FMQ 0.5 195 MPa 10 990 12 0.35 0.12 0.1 YesTZM 30 12 FMQ 0.5 195 MPa 10 990 12 0.09 0.01 0.04 YesTZM 31 24 FMQ 0.5 195 MPa 10 990 12 0.26 0.10 0.08 YesIHPV 1 192 FMQ 0.1 200 MPa 5 1000 5 0.49 0.05 0.12 No

Fig. 1. Schematic diagram of the experimental capsuleconfiguration.

1720 A.S. Bell et al. / Geochimica et Cosmochimica Acta 75 (2011) 17181732


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to explore the potential for negative deviation the aH2Odue to non-ideal mixing in the fluid phase, the aH2O ofour experimental system was calculated for the analogH2ONaCl system by using the equation of Aranovichand Newton (1996). Calculated aH2O values are 0.96 forthe aqueous phase at experimental PT conditions. This va-lue is in good agreement with observations that aH2O tends

toward unity due to non-ideal mixing in low salinity fluidsnear the critical point (Webster and Mandeville, 2007) Theeffect of aH2O on the calculated fO2 values of an experi-ment is shown in Fig. 2.

3. ANALYTICAL METHODS

3.1. Electron microprobe analysis

The glass cylinders and glass-hosted crystals (i.e., cpxand ol at FMQ, and cpx and mt-sp at FMQ + 4) recoveredfrom each run were mounted in epoxy and polished forelectron probe microanalysis (EPMA). Phase identification

and textural relationships were documented by using energydispersive spectrometry (EDS) and back-scattered electron(BSE) imaging. Silicate glass, clinopyroxene, and olivinewere analyzed for major elements by using a four spectrom-eter JEOL 8900 SuperProbe at the University of Nevada,Las Vegas to perform wavelength dispersive spectrometry(WDS). Analysis of the silicate glasses employed a 15 kVaccelerating potential and a 3 nA beam current used inconjunction with a de-focused 10 lm beam in order to min-imize alkali diffusion (Morgan and London, 2005). Analysisof pyroxene and olivine utilized a 10 nA beam current witha 2 lm beam diameter. A ZAF matrix correction algorithmwas applied to calculate element concentrations. Natural

glass and crystal standards were used for all analyzedelements. Standards for glass analyses included: VG-568rhyolite glass for Si, Na, and K; sillimanite for Al; horn-blende for Ca; VG-A99 basaltic glass for Fe and Ti; kaersu-tite for Mg; scapolite for Cl; and rhodonite for Mn. Thebasaltic glass standards VG-A99 and VG-2 were analyzedperiodically as blind standards to monitor the accuracy of

the glass analytical routine. The concentration of H2O inthe glass was determined by EPMA difference and shouldbe considered a semi-quantitative estimate.

3.2. LA-ICP-MS analysis

Laser ablation inductively coupled plasma mass spec-trometry (LA-ICP-MS) analyses were performed on the glassand crystalline silicate phases at the Institute for Isotopenge-ologie and Mineral Rohstoffe at the ETH in Zurich, Switzer-land. All analyses utilized a homogenized Eximer ArF laser(k = 193 nm) coupled with a Perkin-Elmer 6100 DRC quad-rupole ICP mass spectrometer. A carrier gas mixture consist-

ing of He (1.15 L min

1

) and H2 (5 ml min

1

) was employedto increase sensitivity and to lower the limits of detection forall elements analyzed (Guillong and Heinrich, 2007). Abla-tion crater size was varied from 40 lm for glass to 20 lmfor clinopyroxene crystals. The repetition rate for the abla-tion was fixed at 10 Hz with energy densities ranging from10 to 14 J cm1. The SILLS software package (Guillonget al., 2008) and NBS-610 glass standard were utilized forall data processing and reduction. Internal standards of Aland Si (quantified by using EPMA as described above) wereutilized in data reduction for homogenous glasses (i.e., longduration experiments). Compositionally, inhomogeneousshort duration experiments (discussed below) were reduced

Fig. 2. Plot defines the magnitude of the displacement of position of the FMQ redox buffer as a function of water activity in the experimentalsystem. Actual experimental water activities and oxygen fugacity values are marked with the heavy black rectangle. Potential positivedeviations ofaH2O from unity (not contoured on the diagram) will lead to higher calculated experimental fO2 values (e.g. aH2O = 1.3 yieldsfO2 = DFMQ + 0.3).

Au solubility oxidized and reduced melts 1721


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188 via normalization of the major oxide components of themelt to the anhydrous major element analytical totals deter-mined by EPMA. Data reduction by both methods yieldssta-tistically identical concentrations of Au and other tracecomponents. The basaltic glass standard VG-A99 was ana-lyzed intermittently as a monitor of analytical accuracy.Additionally, the Au/Ca count-rate ratios for NIST-610

were carefully evaluated for each standard analysis in orderto ensure that a potentially inhomogeneous distribution ofAu in theNIST-610glass standard or sensitivity drift with re-spect to Au was manifested as variability in thecalculated Auconcentrations of the experimental glasses. No systematic er-ror caused by either inhomogeneous Au distribution in theNIST610 or sensitivity drift were observed in any of the ana-lytical blocks; thus, the apparent variability in the analyzedAu concentrations, discussed in detail below, representsaccurately the actual, subtle variability in the Au content ofthe experimental glasses.

4. RESULTS AND DATA

4.1. Spatial relationships: analytical transects and phase

distributions

Recovered cylinders of glass including the glass-hostedcrystalline material, were mounted horizontally in epoxywafers such that the full length of the glass cylinder was ex-posed and available for analysis. This orientation preservedthe original fluidmelt interface geometry and allowed us toperform EPMA and LA-ICP-MS analytical transects alongthe full length of the original melt column. Crystallinematerial was homogenously distributed throughout the vol-ume of quenched melt in the short duration experiments.

The majority of crystalline material in some longer durationruns was confined to the portion of the liquid column oppo-site the fluidmelt interface. Fig. 3 is a back-scattered elec-tron (BSE) image of a recovered glass (i.e., quenched melt)cylinder and illustrates differences in the spatial distributionof crystalline material. We interpret the observed texture torepresent density-driven crystal settling in long durationruns. Clinopyroxene and olivine crystals were analyzedthroughout the entire volume of melt when possible.

4.2. Stable phase assemblages

The stable phase assemblage varied with the fO2 imposedon the run. Phase assemblages from experiments performed

at FMQ + 4 contained silicate liquid, clinopyroxene, magne-tite-spinel solid solution, and a single-phase aqueous fluid.Phase assemblages recovered from experiments performedat FMQ contained silicate liquid, clinopyroxene, olivine,and a single-phase aqueous fluid. The modal abundance ofcrystalline material was estimated visually to be 3035%,with clinopyroxene being the most abundant crystallinephase at both oxidizing and reducing conditions. Theobserved clinopyroxene crystals are generally less than25 lm maximum dimension. Olivine abundance in reducingexperiments is low relative to thetotal abundance of clinopy-roxene, with ratios of clinopyroxene/olivine that are

approximately 20:1. The grain size of the crystalline materialwas observed to coarsen with increasing run duration.

4.3. Major element compositions of the quenched melt

Table 3 presents the melt composition data for allEPMA analytical transects from a given experiment. The

glasses recovered from the long-duration runs are chemi-cally homogeneous (Table 4). EPMA data from an analyt-ical transect of the compositionally homogenous melt fromIHPV1-2 are presented in Table 4. We observe some vari-ability in the chemical composition of the silicate liquid inshort duration experiments in both oxidizing and reducingexperiments (Table 5). Compositional data from an EPMAanalytical transect, set orthogonal to the fluidmelt inter-face in short-duration run TZM 21, are presented in Table5. These data illustrate the tenor and magnitude of the ob-served compositional variability in the short durationexperiments. Concentrations of Na, K and Cl in the meltrecovered from quartz-bearing short-duration runs gener-ally increase monotonically approaching the exchangeinterface from the base of the melt column. The SiO2 con-tent of the melt also increases approaching the fluid meltinterface, however this increase is generally more restrictedand confined to the 100200 lm of melt immediately adja-cent to the fluidmelt interface. The increasing concentra-tions of Na, K, Cl, and Si in the melt are interpreted toreflect the diffusion of Na, K, Cl, and Si into the portionsof the liquid that are under-saturated (i.e., reflective ofdisequilibrium with the aqueous phase) with respect tothese components. The volumetric extent to which thesefluid components have interacted with the melt is controlledby the relative differences in the diffusivities of each

Fig. 3. Backscattered electron image of experimental glass cylinderrecovered from IHPV1-1. The arrow represents the typical EPMA

transect from the base of the melt column to the fluidmeltinterface, the latter is not visible in this photomicrograph. Thesmall bright spots represent the class of large metallic nuggetsdiscussed in the text (excluding the large piece of experimentalcapsule visible in the upper left portion of the image).



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component; thus, un-surprisingly we observed that the net-work modifying cations Na1+ and K1+ have diffused intothe melt to a much greater volumetric extent than has thenetwork forming cation IVSi4+. The measured concentra-tions of Al, Mg, Ca, and Fe decrease from the base of themelt column to the fluid melt interface only in the shortduration runs. The measured elemental concentration gra-

dients in these runs may reflect simple dilution of the meltfrom the addition of 58 wt.% SiO2 via introduction as asoluble component from the aqueous phase. The depletionof Al, Mg, Ca, and Fe near the fluid melt interface mayalternatively reflect the leaching of these components intothe chloride bearing aqueous solution. The exact natureand origin of the compositional gradient in the melt arenot specifically relevant to Au solubility and will be dis-cussed in detail in a separate publication. We highlight herethat the measured Au concentrations along the entireheight of the melt column remain relatively consistent ex-cept for two short-durations.

4.4. Solubility of Au in quenched melt

The measured Au concentrations in silicate glass are pre-sented in Table 2 and plotted as a function offO2 and exper-imental run duration in Figs. 4 and 5, respectively. Themeasured concentrations of Au in the silicate liquid are con-sistent between runs of thesame oxidationstate. Theconcen-trations of Au in the melt from experiments at $FMQ + 4range from a high of 4.9 lg g1 to a low of 0.64 lg g1, witha mean (1r) of 1.98 lg g1 1.45 lg g1. The concentra-tions of Au in the melt from experiments at $FMQ have amuch more restricted range from 0.54 lg g1 to 0.1 lg g1,with a mean (1r) of 0.35 lg g1 0.18 lg g1. The re-ported solubility values are the average of 713 separate

LA-ICP-MS analyses along the entire length and width ofthe recovered glass cylinder.

4.5. Solubility of Au in clinopyroxene and olivine

Quantifying the concentration of Au, by using LA-ICP-MS, in the equilibrium crystalline phases proved to be ex-tremely difficult owing to the relatively small size (i.e., 535 lm) of clinopyroxene and olivine crystals recoveredfrom most experiments. Furthermore, analyses of smalldiameter crystalline material would inadvertently includesome volume of quenched liquid, yielding a mixed LA-ICP-MS signal that required de-convolution of the contri-

bution from the crystal and the quenched melt. An addi-tional analytical complication was that clinopyroxene andolivine crystals were often intergrown with Au particles,as displayed in Fig. 6, rendering an analysis contaminatedwith a large mass of metallic Au. Making use of the factthat Sc concentration in the clinopyroxene was a factor of45 greater than that of the melt, the count rate 45Sc wasmonitored in order to delimit the crystal only portion ofthe signal from the mixed crystal-melt portions of the sig-nals. The study of Brenan et al. (2005) indicates that crys-talline olivine may be incredibly sluggish, even at hightemperatures, to equilibrate with the siderophile elementcontent of silicate melts. In light of this observation, it isT

able3

Averagecompositionofthequenchedmelt.The1r

standarddeviationiscontaine

dwithinparenthesestotherightoftheaveragevalue.The#analysesindicatesindividualpointanalysesof

quenchedglassperexperimentalcharge

RunID

SiO2

Al2O3

TiO2

FeO

MgO

CaO

K2O

Na2O

Cl

Total

H2O

#Analyses

TZM

11

55.4

(2.0

7)

15.8

2(0.7

2)

0.6

5(0.0

8)

4.2

8(0.5

9)

3.7

4(0.4

9)

7.5

3(1.0

)

2.1

6(0.7

0)

2.45(0.1

4)

1.5

0(0.1

7)

92.8

8(0.2

9)

7.1

2

14

TZM

12

53.8

8(0.8

0)

15.3

0(1.1

0)

0.4

9(0.0

8)

3.6

1(0.2

2)

3.7

0(0.2

4)

7.6

3(0.7

1)

2.5

0(0.2

1)

2.20(0.1

0)

1.7

8(0.0

8)

91.9

0(0.6

4)

8.1

10

TZM

20

53.8

5(2.2

4)

15.5

7(0.6

7)

0.6

4(0.1

0)

5.2

5(1.0

4)

4.0

3(0.4

3)

7.3

8(0.8

6)

2.3

2(0.4

8)

2.56(0.2

1)

1.2

0(0.0

9)

92.6

5(0.3

0)

7.3

5

13

TZM

21

54.6

5(0.6

8)

15.8

0(0.5

6)

0.6

4(0.0

8)

4.1

2(0.0

9)

3.9

4(0.1

6)

7.9

0(0.5

6)

2.0

1(0.3

5)

2.47(0.1

3)

1.3

3(0.1

8)

92.7

6(0.4

2)

7.2

4

19

TZM

23

54.9

5(3.7

0)

14.7

7(0.7

7)

0.6

2(0.0

9)

5.7

0(2.3

9)

3.9

3(0.5

7)

7.5

9(1.5

9)

2.1

9(0.9

8)

2.42(0.2

6)

1.2

7(0.1

5)

93.3

2(0.6

0)

6.6

2

13

TZM

30

54.1

3(0.6

1)

16.5

4(0.1

7)

0.7

7(0.1

1)

5.3

2(0.5

0)

3.1

6(0.1

1)

7.1

0(0.2

6)

2.5

0(0.1

8)

2.34(0.0

8)

1.2

9(0.0

9)

92.9

8(0.4

8)

7.0

2

14

TZM

31

55.3

0(4.3

3)

14.6

5(0.9

9)

0.6

5(0.1

3)

5.1

6(2.3

6)

3.7

0(0.7

1)

7.3

5(1.7

3)

2.3

4(0.8

9)

2.34(0.3

3)

1.2

4(0.2

4)

92.7

1(0.5

3)

7.2

9

13

IHPV1

51.6

4(0.3

3)

17.3

6(0.1

3)

0.6

8(0.0

5)

3.7

0(0.1

2)

3.9

1(0.1

0)

8.3

2(0.1

1)

2.6

6(0.0

8)

2.86(0.1

2)

1.9

3(0.0

7)

92.8

1(0.5

1)

7.1

9

16

IHPV1-1

51.1

0(0.2

7)

17.3

0(0.1

5)

0.6

7(0.0

8)

4.4

1(0.1

1)

3.9

9(0.1

2)

8.4

4(0.0

8)

2.6

4(0.0

5)

2.65(0.0

9)

2.1

2(0.1

0)

93.5

8(0.3

9)

6.4

2

14

IHPV1-2

51.5

2(0.2

2)

17.0

4(0.1

0)

0.6

6(0.1

1)

4.2

7(0.1

1)

4.1

0(0.0

9)

8.5

0(0.1

3)

2.5

5(0.0

5)

2.51(0.1

0)

1.9

8(0.0

9)

92.9

2(0.3

0)

7.0

8

15



7/15

Table 4EPMA analytical transect for IHPV1-2. The run time was 142 h.

SiO2 Al2O3 TiO2 FeO MgO CaO K2O Na2O Cl Total

51.64 17.08 0.77 4.40 3.96 8.52 2.68 2.73 1.93 93.3351.48 17.10 0.85 4.40 4.13 8.41 2.60 2.80 1.93 93.3551.86 17.25 0.59 4.42 4.13 8.23 2.68 2.59 1.81 93.3851.27 17.03 0.64 4.44 4.23 8.32 2.67 2.85 2.02 93.13

51.34 17.03 0.66 4.25 4.29 8.58 2.66 2.63 2.02 93.1651.61 16.99 0.71 4.25 4.14 8.55 2.57 2.59 1.96 93.1451.14 16.88 0.77 4.25 4.15 8.78 2.60 2.56 1.88 92.6351.48 17.12 0.65 4.06 4.05 8.41 2.56 2.64 2.12 92.7651.73 17.10 0.65 4.23 3.94 8.61 2.53 2.57 2.03 93.0351.43 17.11 0.61 4.09 4.10 8.60 2.52 2.70 2.04 92.9051.40 16.93 0.66 4.27 4.16 8.51 2.64 2.49 1.97 92.74

51.49 17.06 0.69 4.28 4.12 8.50 2.61 2.65 1.97 93.05

0.21 0.10 0.08 0.13 0.10 0.15 0.06 0.11 0.09 0.26

Table 5EPMA analytical transect for TZM 21. The run time was 25 h.

SiO2 Al2O3 TiO2 FeO MgO CaO K2O Na2O Cl Total

53.62 16.01 0.75 4.65 4.03 8.16 1.58 2.44 1.49 92.5454.02 16.23 0.55 4.41 4.08 8.07 1.73 2.39 1.48 92.8553.40 16.56 0.49 4.73 4.13 8.43 1.60 2.26 1.41 92.8953.86 16.29 0.61 4.58 4.11 7.72 2.14 2.40 1.59 93.1153.36 16.45 0.57 4.67 4.02 8.19 1.67 2.47 1.58 92.7653.32 16.64 0.75 4.53 3.93 8.31 1.70 2.30 1.57 92.8756.31 15.03 0.67 4.21 3.39 6.76 2.63 2.56 1.15 92.6057.22 15.42 0.61 3.66 3.24 6.47 3.00 2.52 1.32 93.2657.59 15.17 0.69 3.68 3.20 6.19 3.04 2.43 1.49 93.1558.51 14.57 0.62 2.76 2.78 5.65 3.53 2.62 1.45 92.2059.32 14.19 0.59 2.45 2.91 5.39 3.58 2.74 1.47 92.37

55.50 15.69 0.63 4.03 3.62 7.21 2.38 2.47 1.45 92.78

2.32 0.85 0.08 0.80 0.52 1.14 0.80 0.14 0.13 0.33

Fig. 4. Au concentration plotted as a function of oxygen fugacity. The dashed line is the linear regression including all of data in the currentstudy; the dotted line is the linear regression of only the long duration experiments. The filled circles are all of the Au solubility data fromBotcharnikov et al. (2010), who report that the increasing Au concentration in their study corresponds to increasing chlorine and sulfuractivities in the melt. Error bars on the plot represent the 1r standard deviation based on the mean of each datum.



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possible that the Au concentrations measured in crystallinematerial from the short duration runs may be out of equi-librium with the Au content of the early silicate liquids. De-spite our best attempts to reduce the LA-ICP-MS limits ofdetection for Au by employing H2 in the carrier gas (cf.,Guillong and Heinrich, 2007) and utilizing the largest pos-sible beam diameter for analyzing the crystalline material,the measured Au concentrations were consistently belowthe limit of detection. The limits of detection for the

analyses of both olivine and clinopyroxene ranged from0.1 to 0.05 lg g1. We suggest that these data constrainthe Au solubility of both clinopyroxene and olivine to100 h run times) indicate thatthe recovered glasses are compositionally homogeneous withrespect to all elements, including Au. While this does notexplicitly demonstrate equilibrium, it is a necessary conditionfor equilibrium. Demonstrating absolutely the attainment ofequilibrium with respect to Au in the silicate liquids is chal-lenged by the fact that our experiments approached equilib-rium by saturating thesilicate liquid with Au from thelow Auconcentration side. To our knowledge, the demonstration ofequilibrium with respect to siderophile element solubility hasto date not been demonstrated through experimentalreversals (i.e., approaching the equilibrium concentration

from both sides of the reaction). Similarly, no thermody-namic data for dissolved AuO species in silicate liquidsare available such that equilibrium Au solubilities can be cal-culated from a purelytheoretical approach.Thus,in order toevaluate the potential time lengths required to reach equilib-rium with respect to Au solubility in our experiments, we cal-culated the length of time required for Au to fully diffusethroughout the entire volume of silicate liquid in a givenexperiment; in this application of this calculation to ourexperimental system, we have assumed that Au diffusion offrom Au-saturated to Au-undesaturated portions of the li-quid represents the largest kinetic barrier to attainment ofequilibrium Au concentration in the melt. There are no data

Fig. 5. Au concentration as a function of run duration. High Au concentrations in the short duration experiments may reflect the higher Ausolubility in relatively oxidizing conditions in the first few hours of the experiments run at the intrinsic oxidation state (i.e., FMQ + 4.5). The

xs represent reducing runs and the circles represent oxidizing runs. Error bars represent the 1r standard deviations; where no error barsare visible the standard deviations are smaller than the symbols.

Fig. 6. Backscattered electron image of a LA-ICP-MS crater fromthe analysis of a clinopyroxene crystal. The crater contains a large

metallic Au particle; a second glass-hosted gold nugget at theinterface between melt and another crystal is denoted by the arrow.



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in the literature which explicitly constrain the diffusion of Auin hydrous silicate melts; however, following the method ofMungall (2002) we have estimated a diffusion coefficientusing an effective ionic radius of 1.54 A for Au1+ coefficientfor Au of 1.7 109 m2 s1 for the our experimental meltat 1000 C. Using a very conservative diffusion length scalefor our experimental configuration of 5 mm, we calculate

that Au should be homogenous distributed in the melt inapproximately 4 h.. However, as discussed below, the mea-sured variability in Au solubility in short-duration runs(i.e., runs of less than 24 h in duration) indicates that ahomogenous Au distribution was notachieved. We also notethat the long-duration experiments in the current study haverun times exceeding 100 h. These run times are consistentwith a plethora of published experimental studies that docu-ment that run times >100 h at the experimental conditionsused in the current study are sufficient for silicate liquidsto fully equilibrate with the experimental metal capsule.(Candela and Holland, 1984; Webster, 1992; Frank et al.,2002; Simon et al., 2004, 2005, 2006, 2007; Botcharnikov

et al., 2010; Jego et al., 2010).

5. DISCUSSION

5.1. Insights from LA-ICP-MS signal processing and

equilibrium metallic micronuggets

LA-ICP-MS analysis of quenched silicate melt affordsunique insights into the spatial distribution of Au in themelt, beyond facilitating the simple exclusion of Au parti-cles and micronuggets from signals. The presence, andtreatment during signal processing, of metal micronuggetsin silicate glass has been discussed in a number of studies(e.g., Cottrell and Walker, 2006, Simon et al., 2008; Jego

et al., 2010) with no consensus on whether the observedmicronuggets should be included or filtered (i.e., removed)from LA-ICP-MS transient signals during signal process-ing. In the current study, we observed the presence of bothlarge gold nuggets, i.e., visible in BSE images (Fig. 6), andsmall scale inhomogeneties in the LA-ICP-MS signal whichare not correlated with Au particles visible in BSE images.We interpret the BSE-observable Au nuggets as equilibriummetallic phases that coexisted with the melt and not as me-tal nuggets which nucleated and grew during quench. Theseobserved large Au particles, often associated with vesiclesor crystalline material were generally visible and easilyavoided during LA-ICP-MS analysis.

A second type of cryptic nugget was identified only dur-ing LA-ICP-MS signal processing of experimental silicateglasses with low Au concentrations that generally displayedinhomogeneous Au count rates (1r ranges from 30% to50% relative) even in the areas of melt where we observedno visible metallic particles. The results from previous stud-ies of the solubility of metals in silicate melts suggest thatsimilar heterogeneities observed in nominally nugget-freeportions of LA-ICP-MS signals may represent nano-nug-gets that cannot be spatially resolved and, thus, cannot beremoved during signal processing (Ertel et al., 1999).Such cryptic nano-particles contaminate the signal withexcess metallic Au, yielding concentrations that are not

representative of the total dissolved metal content of thequenched liquid. To explore whether or not the small-scaleAu inhomogeneties (


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5.2. Au solubility in silicate melts

Previous studies of Au solubility in hydrous and drybasaltic liquids over a range of pressures and temperatureshave reported Au solubility values ranging from


11/15

suggest that AuO0.5 is the most prominent Au species inchloride-bearing silicate melts at the high oxidation statesprevalent over the first several hours at run temperature.Thus, fO2 is the primary variable controlling Au solubilityin the early experimental liquids. As fO2 values inside thecharge adjust to the imposed fO2, the dominant Au specia-tion may shift to either AuCl or AuS complexes, muting

the effect offO2 while emphasizing the effects of the chlorineand sulfur activities of the melt. This finding is consistentwith the results from Botcharnikov et al. (2010).

Interestingly, no consistent relationship between thecomposition of the melt and Au content is observed. Inexperiments TZM 20 and TZM 21 the concentration ofAu increases subtly by a factor of 1.52.0 (i.e., an absoluteincrease of 1 lg g1 to 1.8 lg g1) in the volume of meltadjacent the fluidmelt interface. The origin of this Au con-centration gradient appears to be correlated with the totalalkali, Cl, and Si content of the liquid such that portionsof the melt richer in alkalis, Cl, and Si contain higher con-centrations of Au. However, this relationship is not ob-

served in all experiments and awaits future study.The observed temporal evolution of the Au contents ofthe melts is significant for the experiments run atFMQ + 4. The temporal variation of Au solubility in themelt is plotted in Fig. 5. The highest measured Au concentra-tions in silicate glass arefor theexperimentsof 24 hduration.The measured concentrations of Au in silicate glass subse-quently decrease with increasing run time. We suggest thatthe temporally increasing Au concentrations are caused bythe initially highly oxidizing conditions of the charge presentbefore internal redox equilibrium is attained. In contrast,experiments at reducing conditions have high fH2 valuessuch that theosmotic equilibrium of H2 between the pressurevessel and the charge attains redox equilibrium more quickly

relative to theexperiments at intrinsicoxygenfugacity values(cf., Gaillard et al., 2002). Temporal variability in the mea-sured Au content of the silicate liquids is not observed inthe experiments at $FMQ. Inadvertent, subtle variationsin the ratio of atmospheric gas to aqueous solution loadedinto the charges may also affect the initial oxidation stateof the charge, causing Au to dissolve readily into the meltas an oxide species during the initial hours of the experiment.

5.3. Au concentration in clinopyroxene and olivine

Data from the long duration experiments for Au solubil-ity in olivine are consistent with data from Brenan et al.(2005). They report maximum Au concentrations in olivineranging from 0.1 lg g1 to 0.005 lg g1. The highly incom-patible nature of Au in the lattice of both clinopyroxene andolivine phases is not unexpected considering the relativelylarge ionic radius and the probable 1+ oxidation state ofAu. As a monovalent cation, the diadochy of Au in the oliv-ine lattice appears to be extremely limited. Data from thecurrent study suggest that Au compatibility on the clinopy-roxene lattice appears similar in magnitude to that of oliv-ine, but the relatively high LA-ICP-MS limits of detectionprohibit making more than semi-quantitative comparisons.

Potential coupled substitution mechanisms involvingAu1+ in the M2 site accompanied by Fe3+ or Al in the M1

site, or alternatively the presence of IVAl may enhance Aucompatibility in clinopyroxene. It seems unlikely that cou-pled substitutions involving Au1+ would greatly enhancethe capacity of clinopyroxene to sequester significant quanti-ties of Au. The extremely low Au activities endemic to natu-ral silicate liquids are likely the dominant factor dictating theAu content of the crystalline material of, even considering

potential coupled substitution mechanisms. However, com-positionally controlled enhancement of partition coefficientvalues for Au between pyroxenes and melt cannot beexcluded from further consideration and may have impor-tant implications for the evolution of the Au budgets ofhighly fractionated silicate liquids.

5.4. A global comparison of Au solubility data in melts with

relevance to ore-forming magmatic systems

In Fig. 8 we present the new data from the current studyand published Au solubility data that allow us to explorethe effects offO2, pressure, temperature, melt composition,

and melt volatile content on the solubility of Au in a silicateliquid saturated with metallic Au. The fO2 values for alldata plotted in Fig. 8 have been recast relative to theFMQ buffer in order to normalize for the effects of temper-ature. We include only data from Au-saturated experimen-tal assemblages. Several experimental studies of Ausolubility in silicate liquids have been performed in hy-drous, Cl-, S-, bearing haplogranite assemblages. Bellet al. (2009) quantified Au solubility in hydrous S- andCl-bearing haplogranite; the silicate melts were saturatedwith intermediate solid solution and pyrrhotite, or a CuFe-sulfide melt. They used LA-ICP-MS to quantify Au sol-ubility, and report values that range from 0.35 lg g1 to0.019 lg g1 at 800 C and 150 MPa. Simon et al. (2007) re-

port a Au solubility value of 0.60 lg g1

, quantified byusing LA-ICP-MS, in pyrrhotite-saturated haplogranite at800 C and 120 MPa. Studies by Simon et al. (2003) and Si-mon et al. (2005) report Au solubility values ranging from0.15 lg g1 to 0.79 lg g1, quantified by using LA-ICP-MS, in hydrous Cl-bearing haplogranite at 800 C overpressures ranging from 100 MPa to 145 MPa. Frank et al.(2002) quantified Au solubility in a hydrous Cl-bearinghaplogranite at 150 MPa and 800 C by using secondaryion mass spectrometry; they report a mean Au solubilityof 1.0 lg g1. Jugo et al. (1999) report a mean Au solubilityof 4.0 lg g1 in hydrous, Cl-bearing haplogranite, saturatedwith intermediate solid solution and pyrrhotite at 850 Cand 100 MPa. It is tempting to associate the increased mea-sured Au solubility in the silicate liquid in the Jugo et al.(1999) study with the presence of sulfur, but we highlightthat the study by Bell et al. (2009) included the identical sul-fide assemblage and the measured Au solubility values inthe latter study are approximately one order of magnitudeless than those reported by Jugo et al. (1999). We suggestthat perhaps the measured Au concentrations in the meltin the Jugo et al. (1999) study may reflect the use of the bulktechnique of instrumental neutron activation analysis(INAA) which may include Au micronuggets in theanalysis; hence, artificially increasing the measured Auconcentration.



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The solubility of Au in intermediate composition meltshas been the focus of several recent studies by Botcharnikovet al. (2010) Jego et al. (2010) and Zajacz et al. (2010). Botch-arnikov et al. (2010) determined Au solubility in water-sat-urated andesitic and rhyodacitic melts as a function of Cland S concentrations at fO2 $ NNO, 1000 C, and200 MPa. LA-ICP-MS determined Au concentrations intheir study range from 0.32 lg g1 to 0.77 lg g1 in andes-itic melt and from 0.33 lg g1 to 0.83 lg g1 in dacitic melt.The lowest values in the quoted concentration range repre-sent the S- and Cl-free liquids, whereas the larger values rep-resent a measured increase in Au concentration of thesilicate liquid that is presumed to be associated with increas-ing S and Cl activity in the silicate liquid. Jego et al. (2010)report concentrations of Au ranging from 0.03 lg g1 to0.25 lg g1 in S-free, water-saturated melt of dacitic andandesitic composition at 1000 C, 400 MPa, and fO2 fixedat $NNO 1, $NNO +1.5 and $NNO +3. Jego et al.(2010) reported that the solubility of Au in the silicate liquidis correlated with increasing fO2, in a manner consistentwith the discussion above. Jego et al. (2010) also report datafrom one series of S bearing, hydrous dacite melts that areconsistent with the results of Botcharnikov et al. (2010)wherein a positive correlation between Au solubility and S

abundance is observed. Jego et al. (2010) report that Au sol-ubility increases from $0.1 lg g1 to between $1 and$4 lg g1 with the addition of S to dacite melt held at an

fO2 of$NNO-1. Zajacz et al. (2010) report Au solubilityvalues, determined by using LA-ICP-MS, in water-satu-rated andesite at 1000 C and 200 MPa, fO2 of NNO-0.4,of 80 ng g1 in a S-free assemblage and a maximum of0.65 lg g1 in the S bearing assemblage.

Gold solubility has also been quantified in anhydrousS- and Cl-free magmatic systems over a wide range of fO2values. Borisov and Palme (1996) report values ranging from0.30 lg g1 to 1.0 lg g1 in anorthitediopside melt at 13001400 C and atmospheric pressure. Brenan et al (2005) have

quantified Au solubility in natural, anhydrous, S-free basal-tic liquids by using LA-ICP-MS. They reported values rangefrom 12.0 lg g1 to 0.86 lg g1. Oxygen fugacity values inboth of these studies were variedover multiple ordersof mag-nitude (Fig. 8); thus, the large range in reported values isattributed to the large range of imposed fO2 values. Bothof these studies convincingly demonstrate that Au solubilityin silicate liquid, in theabsence of volatiles (i.e.,H2O,Cl,S)isa function offO2. Jugo et al. (2005) quantified Au solubilityin a sulfide- and sulfate-saturated anhydrous basalt (lherzo-litic coexisting assemblage of olivine, orthopyroxene,clinopyroxene and spinel) at 1300 C, 1 GPa and fO2 ofFMQ 1 (sulfide saturated) and FMQ + 1.9 (sulfate satu-rated). They used LA-ICP-MS to measure Au concentrationin the quenched silicate glass and report that Au solubilityincreased from 0.52 0.08 lg g1 in the reduced, sulfidesaturated assemblage, to 4.7 0.9 lg g1 in the oxidized,sulfate-saturated assemblage. The authors do not speculateas to whether themeasuredincrease in Au solubility is strictlya function offO2, the presence and speciation of S in the sil-icate liquid (i.e., sulfide vs. sulfate), or both. We note that theresults ofJugo et al. (2005) are consistent with observationsin thecurrent study as well as those reported in Botcharnikovet al.(2010) andJego et al.(2010) suggesting a symbiotic rela-

tionship between Au solubility and dissolved sulfur in silicateliquids that are basaltic to intermediate in composition (i.e.,contrast these results with experimental data from rhyoliteassemblages). The new and published Au solubility data(Fig. 8) indicate that Au solubility in silicate liquids, acrossthe range of geologically plausible pressure, temperature,bulk composition, including potential Au complexingvolatiles S and Cl, and oxidation state documented for theevolution of magmatichydrothermal systems (i.e., from$FMQ 1 to $FMQ + 3), varies roughly one order ofmagnitude from$0.05 ug g1 to $1 ug g1. The Au solubil-ity data for volatile-free basaltic liquids (i.e., Borisov andPalme, 1996; Brenan et al., 2005) indicate a positive

Fig. 8. Plot contains all of the published Au solubility values that are compared and discussed in Section 5.4 of the text.



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correlation between Au solubility and fO2 (see Section 5.2).The reported Au solubility values obtained for volatile-freebasaltic liquid are elevated slightly, albeit less than one orderof magnitude, with respect to the Au solubility values ob-tained from water-saturated, Cl- and S-bearing basalt (i.e.,compare data from the current study with those fromBorisov and Palme (1996) and Brenan et al. (2005) in

Fig. 8). We suggest that this may reflect the higher tempera-tures of thedry mafic melts followingthe general relationshipobserved by Borisov and Palme (2000) for Au concentrationand temperature in dry haplobasaltic melts.

There are no data with which to evaluate the relationshipbetween the solubility of Au in rhyolite melts and the im-posed fO2 of the system; i.e., all experiments to quantifyAu solubility in rhyolite melts have been performed atapproximately NNO 0.5 (Fig. 8). There is no reported ob-served systematic relationship between Au solubility and theactivity of S in the experimentaldata from rhyolite melt com-positions (i.e., Bell et al. (2009) experiments were sulfide sat-urated; Frank et al. (2002) experiments were sulfide free;

Simon et al. (2003) experiments were sulfide free; Simonet al. (2005) experiments were sulfide free; Simon et al.(2007) experiments were sulfide saturated; Jugo et al.(1999) experiments were sulfide saturated). The range of sul-fur concentrations in the sulfide-saturated rhyolite meltsspans the range observed for natural sulfide-saturated rhyo-lite melts; i.e., $50 lg g1 to$300 lg g1. We also note thatthe Cl concentrationin theseexperimental meltsvaries acrosstherangeobservedfor natural felsicliquids andthereis seem-ingly no systematic relationship between Au solubility andthe Cl content of these experimental melts.

The available data for intermediate melt compositions(i.e., dacites from Jego et al. (2010), rhyodacites and ande-sites from Botcharnikov et al. (2010), and andesites from

Zajacz et al. (2010), respectively) suggest that increasingthe activity of S in an intermediate to andesitic melt is cor-related with an increase in the measured Au solubility in thesilicate liquid. The absolute magnitude of the increase in Ausolubility, over the range of experimentally imposed sulfuractivities, is approximately one-half of one order of magni-tude in the Botcharnikov et al. (2010) study, approximatelyone order of magnitude in the Jego et al. (2010) study, andapproximately one order of magnitude in the Zajacz et al.(2010) study. These data clearly illustrate that the presenceof S has a measurable effect on the solubility of Au in inter-mediate to mafic silicate liquids. However, the availabledata do not conclusively identify the speciation of Au inthese intermediate to mafic liquids, and, thus, we feel thatit is premature to advocate for a dominant Ausulfur spe-cies in the silicate liquid. Further, the lack of any observedcorrelation between Au solubility and the S content of rhy-olite melts, over a wide range of S concentrations, suggeststhat perhaps melt structure plays a role in controlling Ausolubility in evolved silicate liquids. Clearly, additionalexperiments are required to fully address the speciative roleof S, and Cl, on the behavior of gold, and other siderophileelements, in mafic to intermediate silicate liquids.

The Au solubility data presented and discussed abovehave important implications for the ore-potential of mag-matic systems. In order to apply these experimental data

to natural systems it is necessary to consider the differencebetween the concentration of Au in a silicate liquid that isin equilibrium with metallic Au and the concentrations ofAu in natural silicate melts. Frank et al. (2002) report thatthe concentration of gold in natural silicate melts is approx-imately three orders of magnitude below that required formetallic Au saturation (aAu = unity). Thus, it is reasonable

to expect that most silicate liquids, across a wide range ofcompositions at geologically reasonable pressures, tempera-tures, fO2, and fS2, will remain Au-undersaturated duringtheir evolution. The available experimental data indicatethat increasing concentrations of volatiles, such as S andCl, will increase the absolute concentration of Au in maficto intermediate melts; however, the measured magnitude ofthis increase (i.e., from $0.1 to $1 lg g1), assuming thatthe positive effect of S and Cl on the dissolved concentra-tion of Au in these liquids scales linearly from Au-satura-tion to the lower Au concentrations in natural melts,cannot be solely responsible for the ore-generative abilityof a magmatic system. Gold-undersaturated melts inherit

a finite Au budget from melting the ultimate source mate-rial (perhaps a few ng g1), thus the absolute Au concentra-tion of the melt is dependent on the initial Au input fromthe source as well as the partitioning behavior of Au duringascent and fractionation. The data presented and discussedabove suggest that the absolute abundance of Au-complex-ing volatiles does not modify significantly the source-inher-ited metal budget in Au undersaturated magmas but, ratherwill alter Au partitioning behavior among the melt, aque-ous, and crystalline phases. Certainly, more work is re-quired to constrain comprehensively the effect of the roleof dissolved volatiles on Au solubility in melts across thecompositional range from basalt to rhyolite; however, inlight of the available data (Fig. 8), we suggest that while

magmatic variables such as fO2 and dissolved sulfur cer-tainly play a deterministic role in moderating the absoluteabundance of Au in mafic to intermediate silicate liquids,the critical controls on the ore potential of any magmaticsystem are the timing and the nature of the volatile phase(s)that is exsolved from the silicate melt (cf. Burnham, 1979;Candela and Piccoli, 2005). The timing of volatile phasesaturation is critical owing to the requirement, in terms ofore potential, that the volatile phase must percolatethrough the magma and vent to the overlying country rock(Candela, 1991). A silicate melt that saturates with a vola-tile phase after significant sulfide crystallization clearly hasa low ore potential, irrespective of the original Au contentof the melt. The composition of the volatile phase, specifi-cally the abundance of metal-scavenging ligands (i.e., Sand Cl), controls the mass transfer of Au, and other metals,from the melt to the potentially ore-formative volatilephase. Thus, it is not necessarily as important that higherconcentrations of S and Cl in the silicate melt increase theAu-carrying capacity of the melt, but rather that higherconcentrations of S and Cl, under the optimal redox condi-tions where the majority of the S is present as S2, lead tosignificantly greater mass transfer of the Au from the meltto the volatile phase and this is potentially the most impor-tant factor in the development of a magmatichydrother-mal ore deposit.



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6. CONCLUSIONS

The new experimentally determined Au solubility dataplace important constraints on the maximum Au capacityof water-saturated mafic silicate liquids, over the range of

fO2 documented for ore-generative magmatichydrother-mal systems, containing geologically realistic (with respect

to ore forming systems) concentrations of chlorine and sul-fur. A comparison of the new data with published Au sol-ubility data indicates that the precise nature of siderophileelement speciation in silicate liquids at the fO2 values ex-pected for most magmatichydrothermal systems remainsenigmatic. There is clearly a positive relationship betweenAu solubility and the fO2 of the system; however, data fromthe current study and available literature indicate that theaddition of magmatic volatiles such as Cl and S may actas complexing anions that enhance the overall Au solubilityin mafic to intermediate silicate melts, as well as altering Auspeciation at low, geologically relevant fO2 values. Thepresence of water in silicate liquids seems to have little effect

on the Au content of silicate melts, based on comparisonsof Au solubility data in both hydrous and anhydrous stud-ies. The existence of prominent Au species in silicate meltsother than simple oxides has not yet been confirmed byspectroscopic techniques and must be inferred throughexperimentally produced solubility data wherein allvariables are fixed except for the studied volatile. Perhapssuitable analog melts containing appropriately high quan-tities of Au may be used in the future to quantitatively mea-sure Au speciation. Furthermore, the effect of pressure onthe Au capacity of Cl- and S-bearing melts has not yet beenexpanded to the conditions through which natural mantlederived arc magmas ascend through the upper mantle,crust, and eventually into the near surface. High pressures

may further promote the formation of Au-chloride and/orAu-sulfide species in the melt.

The compatibility of Au as lattice bound components inclinopyroxene and olivine, common silicate phases presentin basaltic magmatic systems and their upper mantlesources, was constrained to be


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Associate editor: Edward M. Ripley


Au Solubility in Oxidized and Reduced Water-saturated Mafic Melt

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