Chemical Mobility of Gold in the Porphyry-Epithermal Environment

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

Chemical Mobility of Gold in the Porphyry-Epithermal Environment C. H. GAMMONS = AND A. E. X'VILLIAMS-JONES

Department •f Earth and Planetar•j Sciences, McGill Unit'e•s'ity. 3450 University Street, 31ontreal, Quebec, Canada H3A 2A7

Abstract

Using recently published experimental data, xve have calculated the solubility of gold for simplified magmatic fluids that cool beP, veen 500 ø and 300øC. The starting fluid has the following characteristics: P = 1 kbar, ZC1 = 2.0 m, ZKC1/ZNaC1 = 0.25, pH fixed by muscovite + K feldspar + quartz, jSL, fixed by SO2/H2S, and att2s fixed by magnetite + pyrite. Parallel calculations were performed asstuning no drop in pressure during cooling (isobaric model) or an instantaneous drop in pressure to 500 bars, resulting in separation of a dense brine and a low-salinity vapor (boiling model). The isobaric model applies to magmas emplaeed at hypozonal or mesozonal depths, whereas the boiling model is more appropriate for shallroy porphyw deposits.

In the isobaric model, gold solubility is initially dominated by AuCIj at 500øC, 1 kbar. If H2S levels are high (pyrite stable), the dominant complex shifts to Au(HS)2 upon cooling below •450øC, and solubilities remain elevated (> 100 ppb) over the entire temperature range. If H2S levels are low (magnetite stable), gold solubility decreases steadily to 300øC, with AuCI• the dominant complex throughout. Thus, gold dissolved in H2S-rieh fluids will tend to be carried away &ore the parent magma, whereas gold in H2S-poor fluids •411 tend to precipitate closer to the source. At 500øC, gold solubility as AuCI] is highest for fluids that are oxidized (SO2/H2S > l), acidic. highly saline, and potassium rich. Gold may precipitate in response to a number of mechanisms, including cooling, pH increase, and dilution.

Magmatie fluids that evolve fi'om shallow porphy U, bodies are apt to boil shortly after leaving the melt, at which point most of the dissolved gold will partition along with chloride into the brine phase. This metal- rich fluid, because of its high density, will tend to sink or reflux near the parent intrusion, possibly forming an Au-rieh porphyry Cu deposit. Mass balance calculations suggest that magmatie brines will initially be undersaturated witb respect to metallic gold, although the metal may still precipitate as Au-rieh copper sulfide minerals (iss, bornitc). During boiling, most of the H20 and H2S will partition into the coexisting vapor phase. As this vapor cools, it may reeondense into a low-salini•, H2S-rieh xvater of mixed magmatie-meteoric heritage that has a high potential for dissolving and remobilizing significant quantities of gold as Au(HS)•. Migration of this fluid to shallower levels may eventually form epithermal deposits of lmv- or high-sulfidation affinity, depending on the pH-buffering capacity of the wall rocks, and the extent of direct magmatie involvement. Lack of contact with retrograde, HzS-rieh magmatie-meteoric xvaters may be a prerequisite {br the preservation of early Au-rieh porphyry-style mineralization and may also explain the observed association bet•veen gold and hypogene iron oxide alteration in many porphyry deposits.

Whether or not there is a direct temporal link betxveen ore-{brming processes in the porphyry and epithermal regimes, an intrusive event may be important as a means of introducing a large quantib, of loxv-grade gold which is then available for later remobilization and concentration by circulating fluids of nonmagmatie origin. Moreover, an early porphyry event may cause x•4despread sulfidation of surrounding rocks. Later fluids of meteoric origin circulating through this pyrite-rieh wall rock will have HzS concentrations that remain elevated during cooling and ascent, increasing the chances of forming a large, high-grade epithermal gold deposit.

Introduction

WHEREAS the transport and deposition of gold in epithermal deposits has been extensively studied, relatively little atten- tion has been paid to the hydrothermal geochemistry of this metal in porphyry environments. Nonetheless, it is well estab- lished that porphyry copper deposits are an important re- source of gold. In most orebodies of this type, average gold concentrations are rather low (typically <1 g/t), although their huge tonnage allows for the recovery of large quantities of gold as a by-product of copper benefieiation. Outstanding examples include Panguna (Papua New Guinea), Ok Tedi (Papua New Guinea), and Lepanto (Philippines), which to- gether contain or have produced roughly 1,400 metric tons (t) of gold (Sillitoe, 1989). The economic value of the contained gold can sometimes exceed that of copper, in which

• Present address: Department of Geological Engineering, Montana Tech of the Universi• of Montana, Butte, Montana 59701-8997.

case the mine is more precisely referred to as a "porphyry' gold" deposit (e.g., Marte, Chile: Vila et al., 1991). Fe•v such deposits have been discovered to date (Vila and Sillitoe, 1991).

The general characteristics of gold-rich porphyry copper orebodies have been reviewed by SillRoe (1979, 1989, 1995a,b), Singer and Cox (1986), Lowell (1989), Vila and SillRoe (1991), Langet al. (1995), and Thompson et al. (1995). Among these authors there is a general consensus that magmas play a key role in ore genesis as a source of energy (e.g., heat, P-V work), as well as a souce of fluids and metals (Cu, Au). Evidence for the latter includes the presence of gold in stockwork vein systems near or within the host intrusions, a close association with early potassic and/or magnetite-rich alteration, a strong positive correlation between gold and copper grades, the presence of hypersaline inclusion fluids with homogenization temperatures exceeding 500øC (in some cases, >700øC), and stable isotope data that indicate a magmatic fluid contribution. Some of these same features are

0361-0128/97/1903/45-1556.00 45

46 GAMMONS AND WILLIAMS-JONES

evident in a few deposits that are nonporphyritic in style but which are inferred to have formed through orthomagmatic processes, e.g., the breccia pipe-hosted Kidston deposit, Aus- tralia (Baker and Andrew, 1991).

Economic geologists have long debated the link between the evolution and migration of late-stage magmatic fluids and the formation of lower temperature hydrothermal gold deposits (see Sillitoe, 1989; Giggenbach, 1992; Richards and Ker- rich, 1993; Spooner, 1993; Hedenquist and Lowenstern, 1994; Richards, 1995; Carman, 1996; Spry et al., 1996; and Losada-Calder6n and MePhail, 1997, for recent discussions). It is widely believed that so-called "high sulfidation"-style epithermal gold deposits form in part by interaction of magmatic volatiles with cooler ground waters (Hayba et al., 1985; Hedenquist et al., 1994). However, for the majority of epithermal and mesothermal gold deposits, the relationship between igneous activity and gold deposition is more tenuous. Whereas there is universal agreement that intrusions are an important vehicle to transport heat into the shallow crust (and therefore to drive large-scale hydrothermal convection cells), a far more controversial issue involves the role of magmas as a source of gold and other ore-forming components.

The purpose of this paper is to derive and discuss a thermodynamic model for the hydrothermal transport and deposition of gold under conditions that approximate the later stages of cooling magmatic fluids. Although many of the ideas advanced in this paper are not new, past workers have not attempted a rigorous thermodynamic analysis. Recent experimental data on the solubility of gold in HaS- and chloride- rich hydrothermal fluids at temperatures up to 500øC and pressures to 1.5 kbars (Zotov et al., 1985, 1991; Gammons and Williams-Jones, 1995a; Benning and Seward, 1996) now provide this opportunity. Theories regarding the behavior of gold in the porphyry-epithermal environment are reevaluated in light of these data, and potential areas for further research are identified.

Calculation Procedures

Selection of chemical constraints

In order to perform the gold solubility calculations presented in this paper, a number of simplifying assumptions had to be made. To begin with, we chose a value of 2.0 m (10 wt % NaC1 equiv) for the •;C1 concentration of the fluid at the time of its exsolution from the parent melt (second boiling), in agreement with the calculations of Burnham (1979). Because the salinity of magmatic fluids can be considerably greater than this (Cline and Bodnar, 1994), the effect of increasing the •;C1 was also tested. Chloride was partitioned between NaC1, KC1, and HC1. In most of the calculations, a KC1/(KC1 + NaC1) ratio of 0.25 was assumed, which is a typical value for a magmatic fluid in equilibrium with two feldspars (Burnham, 1979). The effect of changing this quantity was also examined.

The oxidation state was set to relatively oxidized or relatively reduced conditions by the gas pairs SOs-HaS or COs- CH4, respectively. Burnham and Ohmoto (1980) proposed that fluids of porphyry tin affinity evolve from S-type magmas and have oxidation states buffered near the COs-CH4 boundary. In contrast, according to the same authors, fluids associ-

ated with porphyry copper deposits typically form from I- type magmas that are more oxidized (•SOs-HsS boundary). At temperatures below approximately 400øC, SOs dispropor- tionates to a 3:1 mixture of HsSO4 and HaS (Burnham, 1979). Consequently, the oxidation state of SOs-bearing magmatic fluids was approximated by the aqueous sulfate-sulfide isoac- tivity boundary at T < 400øC (see also Giggenbaeh, 1992).

The pH of the model solutions was constrained by coexistence of K feldspar, muscovite, and quartz. This assumption may be invalid at the very high temperatures corresponding to potassic alteration (typically, biotite + K feldspar _ magnetite, with muscovite rare or absent), or at lower temperatures where phyllic and argillic alteration may result in the com- plete destruction of feldspar to muscovite or clay. Therefore, alternative acidity vs. temperature paths were also considered.

The HaS fugacity was initially fixed for any given temperature and HaS/SOs ratio by the coexistence of pyrite + magnetite (reaction 10, Table 1). These values were then converted to aqueous HaS concentrations assuming a Henry's law constant of 103 bars/mole fraction HaS. The latter value is based on an extrapolation of the trends in the data of Suleimenov and Krupp (1994) and may be in error by as much as one log unit at 500øC. Although both pyrite and magnetite are common in Au-rich porphyry copper deposits (Sillitoe, 1979), we recognize that the two minerals do not necessarily repre- sent an equilibrium assemblage. For this reason, HaS(g) concentrations calculated by reaction 10 (Table 1) should be considered maximum estimates at very high temperature (where magnetite is often abundant but pyrite rare or absent) and minimum estimates at lower temperature (where pyrite is ubiquitous and magnetite typically absent).

Choice of P-T trajectories All of our calculations begin with a 5:C1 = 2.0 m fluid at

500øC and 1 kbar. In one scenario, the magmatic fluid is simply cooled to 300øC at a constant pressure of 1 kbar. In a second set of calculations, fluid immiseibility is simulated by dropping the fluid pressure at 500øC to 0.5 kbars. These two contrasting scenarios are referred to as the "isobaric model" and the "boiling model" in further discussions, and are summarized in the P-X diagram of Figure 1. Although we recognize that other P-T paths may be more appropriate for some porphyry systems, the choice in this ease was limited by the availability of experimental solubility data. The boiling model is probably more realistic for porphyry deposits eraplaced at shallow depth where a drop in pressure is likely to occur at some stage in the evolution of the associated magmatie hydrothermal system. In contrast, the isobaric model may be more applicable for magmas eraplaced at greater depth (e.g., in the mesozonal or hypozonal realm).

In the isobaric model, it is assumed that the salinity of the fluid remains unchanged at •:C1 = g.0 m from the moment of separation from the melt to the point at which it cools to 300øC. In the boiling model, the 0.5-kbar pressure decrease at 500øC results in separation of the initial magmatie fluid into a higher density brine and a lower density vapor. We chose salinities of 5.0 m (roughly 30 wt % NaC1 equiv) and 0.2 m (roughly 1 wt % NaC1 equiv) for the two immiseible fluids, in agreement with the data of Sourirajan and Kennedy

CHEMICAL MOBILITY OF Au 47

ß

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-H20 phase diagram, showing the location of the critical curve (dashed) and the two-phase solvus (thin solid curves) at 350 ø to 700øC. The point labeled "a" corresponds to a fluid with 10 wt percent NaC1 equiv at 500øC, 1 kbar, i.e., the initial boundaw conditions used in our calculations. If this fluid cools isobarically to 300øC, it xvill never intersect the two-phase solvus and boiling will not occur. In contrast, if fluid pressure at 500øC is suddenly dropped to 500 bars, two immiscible fluids xvill form with compositions b and b'. These fluids correspond to the vapor and brine phases, respectively, in the boiling model of this paper. Theoretically, if the system is closed, subsequent cooling at 500 bars will cause both phases to rehmnoge- nize. However, this is not possible if the brine and vapor are physically separated at the moment that boiling occurs, as discussed in the text. This diagram is a modification of figure 14-5 of Roedder (1979) and is mainly based on the experiments of Sourirajan and Kennedy (1962).

(196'2; see Fig. 1). In this case, 37.5 wt percent of the total water in the initial magmatie aqueous-phase fractionares into the brine during phase separation, and 62.5 wt percent into the vapor. Thus, the vapor phase is more abundant than the brine, both by mass and bv volrune.

The composition and d•nsity of immiseible fluids in the H.20-NaC1 system are highly dependent on the temperature at which boiling occurs. At any given pressure, an increase in the temperature of phase separation causes a much greater contrast between the densitv and salinity of the two phases (Fig. 1). Our estimate ofEC• = 5.0 m •br the brine side of the H•O-NaC1 solvus is therefore a conservative value, bearing in mind that hypersaline fluid inclusions in porphyry Cu deposits can attain salinities in excess of 40 m NaC1 equiv (•70 wt % NaC1 equiv; Roedder, 1984). We deliberately chose less extreme end-member compositions in our examples to avoid extrapolation far beyond the range of fluid compositions used in the experimental solubili• studies.

For the boiling model, we made the additional simplifying assumptions that the two immiseible fluids were not in contact with each other after the moment of phase separation, and that the salinity of the two fluids did not change during cooling. The first assumption is supported by the t•aet that a brine, owing to its high density, would most likely sink or remain close to the point of phase separation, whereas a less dense phase would tend to rise as a vapor plume (Henley and MeNabb, 1978). The second assumption follows from the first but may be invalid if, for example, ascending fluids mix with waters of nonmagmatie origin.

48 GAMMONS AND •VILLIAMS-JONES

Speciation calculations: Brine phase

Following the assumptions outlined above, a distribution of aqueous species was calculated at each temperature using the prograin EQBRM (Anderson and Crerar, 1993). The calculations were performed in the temperature range 300 ø to 500øC, for a constant pressure of either 0.5 or 1.0 kbars. Sources of data for all relevant reactions and their log K values are listed in Table 1. Fugaeity coefficients for the gases H2S, SO2, H20, and O2 were taken from Ryzhenko and Volkov (1971). Activity coefficients for neutral aqueous species were set to unity. Activity coefficients for all charged aqueous species (including gold complexes) were calculated using the Davies version of the Debye-Hiiekel equation, which is built into the EQBRM program:

--

log % = i + + o.o,^z (1) where Zi is the charge of the ion and I is the true ionic strength. Values for the Debye-Hfiekel A parameter were obtained from the following equation (Helgeson et al., 1981):

1.8248' 10 6' ^ =

(•. T)

Values of p, the density of water, were taken from SUPCRT92 (Johnson et al., 1992). Values of •, the dielectric constant of water, were taken from table C2 of Shock et al. (199'2). The calculated activity coefficients for singly charged species fell in the range 0.09 to 0.56, depending on the temperature, pressure, and ionic strength. To test the sensitivity of our results to the estimation of the activity coefficients, the calculations were repeated assuming 3' = 1.0 for all aqueous species. For most species, the results with or without activity coefficients were very similar, although the solubility of gold as charged complexes increased slightly for 3' < 1.0.

Once the distribution of aqueous species was obtained, the solubility of gold was calculated for each of the species AuCi•, AuHS ø, Au(HS)•, and AuOH ø (see Table 1 for equilibrium constants and sources of data). We did not consider the HAu(HS)• species (Hayashi and Ohmoto, 1991), as the more recent work of Benning and Seward (1996) suggests that AuHS ø is the dominant bisulfide species of gold at low pH. It was necessary to extrapolate the data of Benning and Seward (1996) for AuHS ø and Au(HS)• at T > 400øC. This was accomplished by fitting their log K values for reactions 12 and 13 at 200 ø to 400øC (Table 1) to a simple polynomial. Likewise, for AuCI•, the data of Gammons and Williams- Jones (1995a) at 300øC, and Zotov et al. (1991) at 450 ø and 500øC, were fit to a simple polynomial.

From Table 1, it is seen that the equilibrium constants for certain reactions are strongly pressure dependent, whereas others are independent of pressure (or nearly so). The latter phenomenon was shown by Gu et al. (1994) to be a character- istic feature of well-balanced isoeoulombie reactions. Thus, the extent to which the equilibrium constants vary with pressure is largely a reflection of whether the reactions (as written) have balanced like charges. The dissociation reactions 1 to 6 are strongly nonisoeoulombie and also show the largest pressure dependence. In contrast, all of the other reactions are

approximately isoeoulombie and the pressure effects are small. The standard state for all of the data in Table 1 is

(1) the hypothetical ideal 1-m solution at T and P (aqueous species), (9.) the ideal gas at T and 1 bar (all gases, including water vapor), and (3) the pure phase at T and P (minerals and water).

Speciation calculations: Vapor phase For the boiling model, our gold solubility calculations are

limited by the fact that thermodynamic data are lacking for the partitioning of gold between brine and vapor at the temperatures and pressures of interest. However, a growing body of empirical evidence from direct sampling of volcanic gases and indirect analyses of fluid inclusions suggests that vapor- phase transport of gold may be significant (Heinrich et al., 1999.; Goff et al., 1994; Hedenquist, 1995). Therefore, it is desirable at least to consider this possibility in the present paper. As a first approximation, we have made the following assumptions: the equilibrium constants for the various gold solubility reactions may be applied without correction to both the brine and vapor phases; and the activity coefficients for all charged and uncharged species in the vapor phase are uni,t 7. Some experimental data exist to suggest that the first assumption is not entirely without justification. For example, Hemley et al. (1999.) found that approximately 10 percent of the total dissolved Fe, Zn, and Pb in their solubility experiments (500øC, 0.5 kbars, EC1 = 1.0 m) was present in the vapor phase. More importantly, when normalized to chloride, the Fe/C1, Zn/C1, and Pb/C1 ratios were nearly the same in both brine and vapor (see their table 7). Similarly, Williams et al. (1995) found that the apparent equilibrium constants for Cu-Na exchange between brine and vapor in equilibrium with rhyolite melt at 800øC (1.0 kbar) were very close to unity. These results imply that the solubility quotients for metals transported as chloride complexes in coexisting brine and vapor are similar. However, the exact stoichiometries of the volatile metal species in these studies were not deter- mined. Because the dielectric constant of vapor is lower than that of a coexisting brine, it is possible that volatile gold species will exist as uncharged molecules (e.g., AuOH ø, AuC1 ø, AuHS ø, NaAuCI•, NaAu(HS)•), in which ease applica- tion of equilibrium constants for charged species such as AuCi.• and Au(HS)• to the vapor phase may give erroneous results.

We realize that our calculations of gold solubility in the vapor phase at 500øC are of dubious reliability. However, below a temperature of roughly 475øC, the bulk composition of the vapor phase in our boiling model passes above the two-phase solvus for the H20-NaC1 system (see Fig. 1). Un- der these conditions, the vapor is transformed into a condensed, low-salinity liquid and will therefore have bulk physico-chemieal properties similar to those of the aqueous media used in the solubility experiments. Thus, below 475øC, our solubility calculations for the boiled vapor phase are on much firmer ground.

Results

The results of our speciation calculations are summarized in Figures 9. to 8. In Figure 9., changes in the concentration of some important aqueous species are shown as a function


2O

P = 1 kbar --total chloride . NaCl(aq) ' 0

:----I•-==--•:-C-I-==-=----:---_•__ --_ KCl(aq•

_•_•----•

/14Cl{,aø• -4O

-6 , , , , , , , -60 300 350 400 450 500

Toc

FIO. 2. Changes in the concentration of some important aqueous species with temperature. The calculations assume a constant pressure of 1 kbar, a total C1 concentration of 2.0 m (10 vet % NaC1 equiv), and a constant ZNaC]/ ZKC1 ratio = 4.0. Values of log f o_, are also shown (see right y axis), for oxidation state fixed by the condition fs%/fHzs = 1. H2S/,,q/ concentration is fixed by coexistence of magnetite + pyrite, and pH is fixed by coexistence of K feldspar + muscovite + quartz. The concentrations of all charged spedes (dashed curves) decrease with an increase in temperature at the expense of neutral species and ion pairs (no dash).

of temperature for a solution containing •C1 = 2.0 m, Na/K = 4, and P = 1 kbar. Several significant trends should be noted. For example, all of the charged species become less abundant with an increase in temperature relative to the ion pairs HC1, NaC1, and KC1. A similar phenomenon occurs with a decrease in pressure (not shown). Ion association is largely due to a decrease in the dielectric constant of water, which decreases its ability to solvate charged species. This is also the main reason that pH increases with increase in temperature, despite the downward trend in the equilibrium constants for the feldspar hydrolysis reaction (reaction 11, Table 1). Equi- librium bet•veen magnetite and pyrite was used to constrain the H.2S concentration of the solution, which increases steadily from --0.005 m at 300øC to -0.2 m at 500øC. The fo2 values also increase steadily with temperature, although the relative oxidation state, here defined by the condition fsoJfH2s = 1, remains constant.

The aqueous speciation of gold as a function of ac•- and aH2s at 500øC, i kbar, is summarized in Figure 3. Three important trends should be noted: (1) at high ac•/ams ratios, AuCI.• is the dominant species; (2) at high aH•s/acl conditions, Au(HS).• is the dominant species; and (3) at low concentrations of both ligands, gold solubility is controlled by AuOH ø. The latter species imparts a baseline or minimum gold solubility for any given value offo•. In the chosen example, this minimum value is approximately 10 -6"5 m, or roughly 60 ppb, a quantity that is by no means trivial. However, at the high ligand concentrations typical of magmatic brines, even higher gold solubilities are possible as AuCI.5- and Au(HS).•, as will be demonstrated below.

Gold solubility: isobaric model Figure 4 summarizes the solubility of gold for a ZC1 = 2.0

m solution that cools from 500 ø to 300øC at a constant pres-

sure of 1 kbar. Gold dissolves mainly as AuC1] at high temperature, whereas Au(HS)• predominates below 450øC. The neutral complexes AuOH ø and AuHS ø are less important at all temperatures, and do not have predominance areas. The t•vo bold curves show total gold solubilities for t•vo different geochemical situations. The top curve (labeled A) represents the sum of the contributions from all 4 aqueous gold species, assuming that H2S concentration is buffered to relatively high values by the coexistence of pyrite and magnetite. In this ease, total gold solubility decreases slightly from a high of •2 ppm at 500øC, passes through a shallow minimum at 380øC, reaches a local maximum at 360øC, and then decreases slightly again to a low of -0.3 ppm at 300øC. The fact that the calculated solubilities are greater than 0.3 ppm over the entire temperature interval suggests that unboiled, H.•S-rieh fluids are capable of mobilizing significant quantities of gold away from the parent magma into the surrounding environment. The bottom curve (labeled B) shows the trend in total gold solubility for a fluid whose H.•S concentration lies far below the stability field of pyrite. In this ease, gold solubility is dominated by AuCI.•, and decreases steadily with cooling to a minimum value of •20 ppb at 300øC. Thus, gold in H2S- poor magmatic fluids will tend to deposit close to the porphyry source, and no remobilization of gold as bisulfide complexes will occur during cooling.

In the above calculations, oxidation state was fixed at relatively high values by the condition fo., = fH2s. The effect of changing oxidation state on gold solubility as chloride complexes is illustrated in Figure 5. At any given temperature, gold solubility increases with increase in oxidation state. How- ever, it is important to note that the calculated gold solubili-

-2

-4

AuOH(aq)

500øC, 1 kbar

pH = 5.0

log fo2 = -18.8

Au(HS) 2-

I I I ' I ' •

-3 -2 -1 0 1

activity of H2S(aq)

FIO. 3. The aqueous speciation of gold at 500øC, 1 kbar. Cold solubility contours are given by thin dashed lines and are in log activity units. The diagram assumes a pH of 5.0 and an oxidation state buffered at the SO2- HsS isofugacity boundaw. An increase in pH would enlarge the predominance area of Au(HS)=7 relative to AuCI•. A change in oxidation state would 'alter the absolute solubilities, but would not effect the relative stabilities of the complexes shown. AuHS/•, c xvas included in the calculations, but does not possess a predominance area at 500øC, 1 kbar, based on our thermodynamic database.


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

• lppmAu •

• 0.1 ppm A u • ///• I

• AuOH•

• -7

• / •. I •10 ppb Au

3• 350 4• 450 500

TøC

F•G. 4. Gold so]ubili• for a magmatic fluid cont•ning 2.0 m EC1 •vhich cools from 500 ø to 300øC at a constant pressure of 1 kbar (the isob•ic model). O•dation state is •ed by the con•tion fso:•:s = 1. The pH is •ed by the coe•stence of K feldspar, musco•te, and quaRz, assuming a constant ENaC•KC1 ratio of 4.0. Solubilities are sho• for each of the

in•dual complexes AuCI•, AuOH ø, AuHS ø, and Au(HS)• (thin cu•es). Tot• solubili• cu•es are •ven by the bold cu•es (labeled A and B). Cu•e A assumes that •1 four aqueous gold species are present and that the H•S concentration is •ed by the coehstence of ma•etite and p•te. The temperature range in •vhich each complex is predominant is indicated by the vertic• dashed lines. Cu•e B assumes that the concentration of the bisulfide

complexes is negligible, as may be the case for magmatic fluids •th a ve• ]mv H:S concentration.

ties are only • 1.1 log units apart for a strongly oxidized fluid (SO.•/H.•S = 100) vs. a strongly reduced fluid (CO2/CH4 = 0.01) at 500øC. Gold concentrations are still quite high (i.e., > 100 ppb) in the reduced example at T -> 500øC, illustrating

-5

-10 300 350 400 450 500

ToC

FIG. 5. The effect of the oxidation state on the solubility of gold transported as AuClg. The oxidation state is buffered to relatively high values by the coexistence of SO2 and HgS (top curves) or to relatively lmv values by COg and CH 4 (bottom curves). In each case, the solubilities have been contoured for gas ratios of 100:1, 1:1, and 1:100. See caption to Figure 4 for more information regarding the physico-chemical conditions of the model fluid.

•t•- <lppm '• -5

- ........... • -7[/1 I oooc, I I/ I s_o_2m2s = !.o I

0 2 4 6 8 10

total chloride, molal

FIG. 6. The effect of the •C1 concentration and the •KC1/(•NaC1 + •KC1) ratio on the solubility of gold transported as AuC]• at T = 500øC, P = 1 kbar, andfsoff•:s = 1.0. In all cases, the pH is fixed by the equilibrium between muscovite, K feldspar, and quartz. The solubility of gold as AuOH ø is also shown (dashed line). This species provides a lmver limit to the solubility of gold that is independent of salinity and pH.

that the potential of orthomagmatic fluids to transport significant quantities of gold is by no means limited to oxidized magmas. In the case of gold bisulfide complexes, the effect of oxidation state on gold solubility is the same as that for AuCI,•, as long as H,•S or HS- are the dominant forms of aqueous sulfur. However, at high fo• values, H,•S is oxidized to SO,• and/or aqueous sulfate, which sharply decreases gold mobility as bisulfide complexes. Thus, strongly oxidized conditions will favor transport of gold as chloride complexes (or AuOH ø) vs. bisulfide complexes at all temperatures.

The effect of salinity and K/Na ratio on gold solubility for a 500øC solution at i kbar is shown in Figure 6. Gold solubility decreases with a decrease in total chloride concentration, and increases with an increase in the K/Na ratio of the ore fluid.

The latter effect is due to a shift in the equilibrium boundary of reaction 11 (Table 1) to lower pH values as Y•K increases. Thus, highly saline ore fluids that are potassium rich are especially good candidates for dissolving high concentrations of gold (e.g., > 10 ppm), provided that equilibrium is main- rained between quartz, muscovite, and K feldspar. Figure 6 also illustrates that dilution is a very effective means of precipitating gold. The stoichiometry of the AuCI,• solubility reaction (no. 14, Table 1) demands a hundred-fold decrease in dissolved gold for a ten-fold decrease in Y•C1. Thus, if a magmatic brine is diluted 10-fold with meteoric fluid containing negligible chloride, roughly 90 percent of its contained gold will be precipitated, assuming the brine is initially saturated with gold. This effect would be amplified if the diluting fluid is cooler than the brine.

In contrast to cooling, pressure changes alone are unlikely to have much effect on the solubility of gold as chloride or bisulfide complexes in porphyry environments, unless fluid immiscibility occurs (see below). For example, Zotov et al. (1991) measured nearly identical gold solubilities as AuCI,• in parallel experiments conducted at 450øC and P = 500, 1,000, and 1,500 bars. The pressure dependence of the solu-


bility of gold as bisulfide complexes was also shown to be essentially negligible (less than 0.3 log units difference in the range 500-1,500 bars; Benning and Seward, 1996).

Cold solubility: Boiling model Figure 7 sumlnarizes the calculated solubility' of gold in a

brine (Y, C1 = 5.0 m) which separates from a homogeneous magmatie fluid at 500øC, 0.5 kbars, and then cools isobariely to 300øC. The overall topology of Figure 7 is similar to that of Figure 4, but differs in two important aspects: gold solubilities as AuCI:• are much higher, especially at high temperature (>100 ppm at 500øC); and the temperature dependence of gold solubility is steeper. Thus, cooling is a more effective depositional mechanism for the degassed brine than for the unboiled fluid (see above). Again, two total solubility curves are drawn, labeled A and B, which imply the presence or absence of significant quantities of H2S. Path B is probably more likely for a boiled brine, as most of the HzS will partition irreversibly into the vapor during phase separation. Following path B, >99.9 percent of the total gold precipitates by the time the brine cools to 350øC. This assumes that the ore

fluid is initially saturated with gold and that equilibrium is maintained between muscovite, K feldspar, and quartz. Loss of acidic volatiles (HC1, CO=,, SO2) during boiling could drive fluid pH values above the museovite-K feldspar boundary, in which ease, the solubility of gold as AuCI:j would decrease at an even sharper rate.

The solubility of gold in the vapor phase of the boiling model is shown in Figure 8. Again, it is stressed that the calculations at T > 475øC rest on assumptions which may be invalid (see "Speeiation calculations: Vapor phase"), and are therefore provisional in nature. The curve labeled A shows the total gold solubility assuming that AuC12 and Au(HS):j

-8 300

boiled I' ' ' brine

ß

Au(HS)2- • • AuCl 2- v;.,,C•, glOppmAu

døminantA• •1 vvm Au • I [ • • . I .

350 4• 450 500

TøC

F•G. 7. The solubility of gold in a saline brine with ZC1 = 5.0 m that separates œrom a vapor phase at 500øC, and 0.5 kbars, and then cools vdth no further pressure decrease to 300øC. See caption to Figure 4 lbr the controls on pH, oxidation state, and H2S concentration. The bold curves labeled A and B show total gold solubilities that include or exclude, respectively, the contributions œrom bisulfide complexes. As discussed in the text, irreversible loss oœ H2S to the vapor phase may occur during boiling, in which ease curve B is a more likely scenario.

I oiled vapor • Au(HS)2' dominant

'

• -7' . -8 • 350 4• 450 500

TøC

FK;. 8. The solubility of gold in a low-salini• vapor phase (ZC1 = 0.2 m) that cools fkom 500 ø to 300øC at a constant pressure of 0.5 kbars. The calculations used to draw this diagram assume that the equilibrium constants For all aqueous reactions mav be used x•thout modification for both the brine and vapor phases. Because this assumption is untested experimentally (and is unlikely to be correct), the diagram should be treated x•th skepticism (see text). Nonetheless, the figure illustrates in a qualitative way the possibility of remobilization and upward transport of gold by an H•S-rieh vapor phase. The bold eume labeled A assumes that dl four gold spedes have the capacity to tkaetionate into the vapor phase. The bold etm'e labeled B assumes that only the neutral gold complexes have an appreciable volatili•,. The controls on pH, o•dation state, and HsS concentration are the stone as in Figure 4.

are the dominant species and that equilibrium constants for these complexes may be applied without correction to the vapor phase. The curve labeled B assumes that only the neutral spedes AuOH ø and AuHS ø have a significant volatility. Data for AuC1 ø are lacking, but are unlikely to change the general trends shown. Three main points are indicated by Figure 8: (1) the calculated solubilities of gold in the vapor phase at 500øC, 0.5 kbars are greater then three orders of magnitude less than those in the coexisting brine; (2) at a constant pressure of 0.5 kbars, the solubility of gold in the low-salinity vapor passes through a minimum and then increases slightly with a decrease in temperature; and (3) the solubility of gold in the condensed vapor at 300 ø to 350øC is actually greater than that in the boiled brine. All of these conclusions are valid whether one chooses to include or ig- nore the contributions from the charged AuCI• and Au(HS)•- species.

The above results suggest that gold will partition strongly into the saline brine upon phase separation at 500øC, and 0.5 kbars. However, whereas gold solubility in the brine de- ereases sharply with a decrease in temperature, gold solubility in the vapor actually increases with cooling. Because the vapor phase used in our model will recondense at •475øC, path A in Figure 8 should be valid below this temperature. Thus, waters of low salinity formed by condensation of maginatic steam have a high potential for remobilizing gold as bisulfide complexes, provided that the pH is buffered to near-neutral values by equilibrium between K feldspar and muscovite. As

59, GAMMONS AND WILLIAMS-JONES

discussed in a later section of this paper, the same conclusion applies if H2S-rieh vapors of magmatie origin are condensed into overlying waters of meteoric origin.

Discussion

The calculations presented above emphasize the high mobility of gold in magmatie fluids. Gold transport may be dominated by either chloride complexes or bisulfide complexes, depending on temperature, pressure, and solution chemistry. As well, the solubility of gold as the simple hydroxy complex is by no means negligible (at fso2 = fH.2S, > 10 ppb for the entire temperature range of interest; see also Ryabehikov et al., 1985). The following discussion concerns the ultimate fate of this magmatie gold. First, some ideas are presented regarding the maximum gold concentrations that may be considered reasonable for magmatie fluids, considering the very low natural abundance of this element.

Are magmatic fluids saturated with gold?

It is possible to place a crude upper limit on the gold contents of magmatic fluids using simple mass balance constraints. Burnham (1979) estimated that granitic melts contain approximately 0.1 wt percent chloride before the onset of crystallization. Because of the very large partition coeffi- cient of C1- between aqueous fluid and melt, almost all of this ligand enters the fluid phase once it forms (Burnham, 1979). The chloride concentration of the latter can range up to 50 wt percent (80 wt % NaC1 equiv; Roedder, 1984) if loss of steam to the vapor phase occurs. This represents a C1- enrichment factor of roughly 500x from the parent melt. A similar enrichment factor would be expected for other elements that partition strongly into the brine phase. For example, copper has one of the highest fluid-melt partition coefficients of any base metal (Candela and Holland, 1984). Assum- ing an average Cu content of 12 ppm for a granite (Wedepohl, 1969), a 500x enrichment factor would lead to a concentration of 6,000 ppm in the saline condensate, assuming that no copper is lost to the vapor phase. This is close to the upper limit in copper contents reported from analyses of hypersaline fluid inclusions in porphyry deposits (Roedder, 1984). If we take a gold concentration of 2 ppb for a typical granite (Wede- pohl, 1969), a similar 500x enrichment factor would lead to a brine that contains i ppm gold, again assuming that most of the gold will partition into the brine vs. the vapor. Although no melt-fluid partition coefficients are available for gold, the total enrichment factor cannot be greater than 500x, because this figure already assumes quantitative transfer of gold into the brine.

One problem with calculations of the sort in the preceding paragraph is that the metal concentrations of igneous rocks are not necessarily representative of the metal contents of the magmas from which they formed. Thus, Connors et al. (1993) argued that the very low average gold concentrations of felsic volcanic rocks (-0.2 ppb), could reflect scavenging of gold from the melt by orthomagmatic fluids. However, given the low natural abundance of this element, it seems unlikely that an average magma of felsic to intermediate composition could contain much more than a few ppb Au. If this is true, the maximum gold concentrations in magmatic fluids

derived from these magmas, based on the above mass balance constraints, would be on the order of 1 or 2 ppm.

According to our previous equilibrium calculations, gold solubilities in the range 1 to > 100 ppm can be expected as AuClg for magmatic fluids at T (500øC, EC1 _> 2.0 m, and SO2/H2S --> 1, with the highest solubilities occurring in the degassed brine (see Figs. 3-6). This range is considerably higher than our estimate of the maximum attainable gold concentrations based on mass balance constraints. Therefore, most magmatic fluids will be undersaturated with respect to metallic gold at the time of exsolution from the parent magma. Gold precipitation will therefore be delayed until some change in physieo-ehemieal conditions allows saturation to occur (however, see section below "Copreeipitation of gold with copper"). A similar conclusion has been reached in the ease of copper transport in porphyry environments (Bodnar, 1992).

Mass balance calculations also place constraints on the vol- ume of a magma needed to form a large (> 1 Moz Au) gold- rich porphyry deposit. For example, a small intrusion measur- ing 1 km a contains rou•,hly 3' 10 •'5 g of rock (talcing an average rock density of 3 g/em ). Assuming the •m6agma originally contained 2 ppb Au, this translates to 6- 10 g of gold, or about 200,000 oz. To produce the 10 Moz or so of gold contained in the Bingham deposit, Utah (Grimour, 1982), the intrusion would need to be -50 km 3 in size. The altered quartz monzo- nite porphyry that forms the center of the orebody at Bin- gham has an aerial extent of less than i km '• (Lanier et al., 1978). Thus, one of the following must apply: (1) metal-rich magmatie fluids that formed the Bingham orebodies were focused upward from a much larger stock or batholith at depth, (2) the magma had an unusually high (> 10 ppb) initial gold content, or (3) much of the gold at Bingham is of nonmagmatie origin.

Coprecipitation of gold with copper Although saturation with native gold may not occur at near-

magmatic temperatures, gold mobility could nonetheless be constrained by incorporation of this element as a trace impurity in other phases. Anomalous Au contents have been reported in Cu-Fe sulfide minerals from gold-rich porphyry Cu deposits (300-400 ppm Au in bornitc from Panguna; Baldwin et al., 1978). Cygan and Candela (1995) confirmed this experimentally, finding up to 3,000 ppm Au in chalcopyrite (intermediate solid solution, or iss) grown at gold-saturated conditions at 600 ø to 700øC. More recent experiments have verified that gold enters the iss structure as a Cu(I)-Au(I) solid solution, but subsequently exsolves upon cooling to form in- tergrowths of native gold and chalcopyrite (P. Candela, pets. commun., 1996). By analogy, an extensive solid solution is known to occur between Au(I) and Ag(I) in argentitc, even at temperatures as low as 300øC (Barton, 1980). Gammons and Williams-Jones (1995b) suggested that auriferous Ag2S could be an important solubility-limiting phase for gold in Ag- rich epithermal fluids. Likewise, Cygan and Candela (1995) proposed that the residence of gold as a trace component in copper-bearing minerals could explain the positive correlation between these elements in many Cu-Au deposits of magmatic affinity.

As shown in Figure 9, the Au/Cu ratios of gold-rich por-


Au/Cu mass ratio

10 -5 10 -4 10 -3 10 -2 0.1

0.00001 0.0001 0.001 0.01 0.10 1.0

mole fraction Au

FIG. 9. Diagram shmving the range in the Au/Cu mass ratio of porphyry copper deposits in comparison to the Au/Cu ratio of gold-saturated chalcopyrite (iss) at 600øC (based on the experiments of Cygan and Candela, 1995). The shaded region on the left side of the diagram shows the range of Au/ Cu ratios that could be explained by deposition of gold as a trace impurity in chalcopyrite. Incorporation of gold as a solid solution in chalcopyrite also lowers the solubility of metallic gold and could be an important alepositional mechanism, as explained in the text. A large miscibility gap exists (unshaded region) between gold-saturated chalcopyrite and cuprian gold. The Cu concentration of gold in equilibrium •vith chalcopyrite at this temperature is not known, and is therefore dashed.

phyry deposits are lower than the Au/Cu ratios of experimentally grown, gold-saturated ehaleopyrite at 600øC. By simple mass balance, it follows that much of the gold in the orebodies in question could have resided as gold-rich ehaleopyrite or iss at the temperature of ore formation. If gold enters a solid solution in another phase, its thermodynamic activity and, by consequence, its solubility may be greatly reduced. The eopreeipitation hypothesis is therefore compelling for two reasons: it helps to explain the dose correlation between Cu and Au in many porphyry deposits; and it provides a means to deposit gold from high-temperature magmatie fluids that are undersaturated with the pure metal. The second point is important in light of our calculations which indicate that the solubility of metallic gold is extremely high (> 100 ppm for h ß ß o ß t e boiled bnne example) at 500 C. The experiments of Can- dela and coworkers show that gold, originally dissolved in high-temperature iss, quickly exsolves upon cooling to ambi- ent temperature. This reaction is mitigated by the iss-ehaleopyrite lattice inversion (P. Candela, pers. eommun., 1996). We infer that it may be difficult or impossible to find direct evidence for high-temperature Au-Cu solid solutions in natural ehaleopyrite specimens, since the exsolved gold will have most likely migrated by diffusion to form its own discrete grains. More research is needed to determine whether gold dissolved in other Cu-bearing minerals (e.g., bornite) is easier to quench.

The propensity of sulfide to sequester gold will have a distinctly negative impact on the hydrothermal mobility of this metal if the parent melt condenses a sulfide phase before any aqueous fluid is evolved. Sulfide saturation could occur either in the form of an immiseible sulfide melt or as sulfide minerals crystallizing at the solidus (e.g., pyrrhotite). In either ease, available data indicate that gold is strongly partitioned from the silicate melt into the coexisting sulfide phase (Stone

et al., 1990; Hoosain and Baker, 1996). In general, sulfide saturation is suppressed if the ES 2- concentration of the cooling magma is relatively low, as would be expected for an oxidized magma in which SO2 >> H2S. A number of previous workers have discussed this problem in greater detail (Carroll and Rutherford, 1985; Wyborn and Sun, 1994; Cygan and Candela, 1995).

Depositional mechanisms

According to our calculations, if an ascending magmatic fluid separates into two immiscible phases, gold will fractionate into the brine as AuCI•. Gold may precipitate with copper due to cooling near the core of the porphyry system or it may migrate laterally into adjacent rocks if the temperature gradient is small. The first scenario is probably a fitting de- scription for many porphyry Cu-Au deposits in which gold shows a close spatial association with high-temperature potassic alteration and hypogene copper mineralization (Sillitoe, 1989). The second scenario is less well documented, but may apply to gold-rich vein deposits that appear to have formed from fluids of magmatic origin but are not spatially associated with any obvious porphyry deposit. The unusually Au-rich veins (up to i oz/t) found near the Mount Estelle pluton, Alaska, may be an example of such a setting (Crowe et al., 1991).

Two other ways to precipitate magmatic gold transported as chloride complexes include a decrease in C1- concentration, and an increase in pH. Dilution is the most effective means of decreasing Y•C1 and can occur in porphyry environments by mixing with convecting waters of nonmagmatic origin. This process would probably also result in cooling, further promoting the precipitation of gold. An increase in pH could occur during boiling of a primary orthomagmatic fluid, since most acidic components (e.g., HC1, H2S, SO2) are known to fractionate into the vapor phase relative to their basic counterparts (KC1, NaC1, HS-; Drummond and Ohmoto, 1985). An increase in pH would also be expected if the rocks into which the parent magma is eraplaced contain a significant carbonate component. Sillitoe (1995a) noted that many of the largest Au-rich porphyry deposits (based on total contained gold) are hosted by carbonate rock (e.g., Bingham, Utah; Ok Tedi, Papua New Guinea; Grasberg, Indonesia). Although Sillitoe favored a structural-mechanical explanation for this observation, it is also plausible that pH changes during fluid- rock interaction in some way promoted deposition of gold. It is natural to presume that such a mechanism could form a gold-rich skarn deposit, but field evidence suggests that most large gold skarns are associated with retrograde, H2S-rich fluids (Meinert, 1989).

Without more detailed field information on the spatial and temporal distribution of gold in specific deposits, it is difficult to rank the relative importance of cooling, boiling, fluid mixing, and water-rock interaction as a means to precipitate magmatic gold. Our analysis suggests that each of these mechanisms could play an important role individually, or perhaps in concert. Coprecipitation with chalcopyrite is also a viable means of precipitating gold from solutions that are undersaturated with the native metal. Production of H,2S (e.g, by the disproportionation of SO2) could trigger deposition of chalco-


pyrite which in turn could incorporate trace but significant quantities of gold.

As a final comment, it is interesting to note that the solubilities of the platinum-group elements are also quite high as chloride complexes in oxidized magmatic fluids that are exceptionally rich in chloride (Gammons et al., 1992). Some Au-rich porphyry deposits are indeed enriched in palladium and platinum (Werle et al., 1984; Petrunov and Dragov, 1993; Tarkian and Koopmann, 1995).

The magmatic-epithermal (-mesothennal) transition We will now explore in greater detail the potential of mag-

mas to supply gold to the epithermal and mesothermal environments. Five scenarios are considered, arranged in an order of decreasing level of involvement of magmatic fluids:

1. Gold is transported to surrounding waters via the magmatic brine phase.

2. Gold and other volatile components are transported in the gaseous state where they are subsequently condensed into overlying meteoric waters or released directly into the atmosphere.

3. Magmatic vapors transport H.2S (but not gold) that is condensed into overlying meteoric waters, thereby increasing the ability of the latter to dissolve and redistribute gold.

4. No significant exchange of matter occurs between the magmatic and epithetreal regimes, although heat from the cooling intrusion drives convection of surrounding meteoric water.

5. No significant gold deposits are formed during the cooling history of the initial intrusion, although convecting meteoric water from a later hydrothermal event remobilizes low- grade magmatic gold that was introduced during the first event.

These scenarios are discussed separately below, although it is recognized that two or more may apply at the same time and/or in sequence during the formation of a given deposit.

Direct transfer of gold via the brine phase: Following the isobaric model (Fig. 4) we calculate that approximately 10 -• m gold (•0.2 ppm) will remain in solution after an orthomagmatic fluid cools to 300øC. In the boiling model (Fig. 7) the amount of dissolved gold left after cooling to 300øC is roughly 10x less, assuming that most of the magmatic H,2S is partitioned into the vapor phase during the boiling event. In addition, the solubility gradient is much steeper for a cooling brine that has degassed H•S and steam (compare the slope of the total gold solubility curves in Figs. 4 and 7). Thus, magmatie waters that eo•l without boiling have a greater potential to deliver a significant fraction of their initial dissolved gold to shallower levels. It is unlikely that magmatie fluids born at depths less than a few kilometers could pass directly into a eonveeting meteoric system without boiling, because of the very large pressure differential between the environment of the e17stallizing magma (•lithostatie) and the meteoric fluid (•hydrostatie). Therefore, the isobaric model of this paper is mainly applicable to magmas emplaeed at greater depths (>5 km). This raises the question of the involvement of magmatie fluids in the genesis of so-called "mesothermal" gold deposits (e.g., most of the deposits in the Abitibi subprovince of Canada and the Yilgarn block of

Western Australia). Such a link has long been advocated by many authors, although there is still widespread disagreement regarding the precise role of magmas in the formation of the deposits in question. We are not in a position to take a stand in this debate, even though our calculations do indicate that significant contributions of gold from magmatie fluids to the mesothermal environment are theoretically possible.

Direct transfer of gold via the gaseous phase: Conventional wisdom would maintain that when a magmatie fluid boils at some point after exsolution from the parent magma, most of the gold will fractionate into the saline brine as chloride complexes. Our provisional calculations support this view (compare Figs. 7 and 8), although it is stressed that experimental work is needed to make a more reliable assessment.

This is especially true in light of the fluid inclusion study of Heinrich et al. (1992), which indicates that copper may partition strongly into H.2S-rich vapors trapped at moderately high pressure (a similar conclusion has been reached indepen- dently by R. J. Bodnar, pers. commun., 1996). Even if most of the gold partitions into a brine, the low quantities of gold remaining in the vapor may still be significant, especially when one considers the much larger mobility and mass flux of the vapor phase relative to the brine. Measureable quantities of gold have been detected in fumarole gases of active volcanoes (Hedenquist et al., 1993; Goff et al., 1994; Heden- quist, 1995), and the same authors have demonstrated that appreciable quantities of gold could accumulate in zones of hydrothermal alteration overlying magma chambers that de- gas over an extended period of time. Although intriguing, a thorough evaluation of the role of vapor-phase transport of gold must await future experimental and theoretical studies.

Condensation of magmatic H2S and S02 into meteoric waters: Here it is assumed that H•S and other volatile species (e.g., H20, CO2, CO, SO2, HC1) partition into the vapor phase during boiling but that gold does not. It is also assumed that a large percentage of these volatiles are recondensed into overlying meteoric water. Under these conditions, one can imagine the condensation process as a chemical titration in which the concentration of H.2S and other species in the meteoric water reservoir is steadily increased.

Figure 10 summarizes some possible reaction paths that may develop for a dilute (F•C1 = 0.1 m) meteoric water at 300øC whose pH is buffered by coexistence of muscovite + K feldspar + quartz, and whose oxidation state is buffered at the aqueous sulfide-sulfate isoaetivity boundary. Initially, the H2S concentration of the meteoric fluid increases steadily (due to condensation of magmatie H.2S), whereas the pH of the fluid remains near neutral owing to the conversion of K feldspar to muscovite (i.e., phyllie alteration zone). As mn•s increases, the solubility of gold as Au(HS)• also increases steadily, eventually reaching values as high as 10 ppm or more (shaded region). Such a fluid would be a prime candidate to form an epithermal gold deposit, especially if the meteoric system is eonveeting at a vigorous rate.

If the H,2S-enriehed meteoric water remains rock-buffered with respect to pH, it may eventually form a low sulfidation- style epithermal gold deposit. However, at some point the rate of introduction of acidic volatile species (SO•, HC1, CO2) may eventually exceed the capacity of the rock to buffer pH


5

pH

log {Au} = -7 -6 -5 -4 -3

SO4 2. \ \ \ \ \ \ \ \ --•-----

muscovite

AuOH(aq)

muscovite

kaolinite

• \\•U(5• )2 \\\ \ \ \ \

X\ \

300øC, 0.5 kbar

m•;Cl = 0.1 Na/K = 10

fO 2 = HSOn-/H2S

2 -5 -4 -3 -2 -1 0

log all2 S

F•;. 10. Diagram illustrating the change in gold solubility fbr a dilute meteoric water at 300øC and 0..5 kbars, as a function ofpH and HaS concentration. The oxidation state has been set to the HSO4/H2S(,•q, isoaetM• boundary, xvhich itself is a function of pH. The field of liquid sulfi•r gives an upper limit to the HaS concentration at any given pH. The bold solid lines show the stability fields of the alteration minerals K feldspar, musemite, kaolinitc, and alunite. The bold dashed lines show the predominance areas of the aqueous gold species, whereas thin dashed lines give gold solubility contours (log molal units). The shaded region shows that portion of the diagram over which the solubili• of gold exceeds 10 ppm (mg/kg). See text for a discussion of the reaction paths labeled A to E.

(e.g., 'all feldspar converted to muscovite). The pH of the meteoric water will then decrease rapidly (path B). Important reactions that could generate add include the hydrolysis of magmatie SO2 (reaction 3, below), the oxidation of magmatie H.•S to sulfate (reaction 4), and the dissociation of HC1 (reaction 5):

4SO2 + 4HsO = H.2S + 3HSOj + 3H +, (3)

H2St.,p + 202<g/ = HSO;- + H +, (4) and

HCI(.q) = H + + C1-. (5)

As shown in Figure 10, a number of different reaction paths are possible after the onset of aeidifieation, depending on the sequence of alteration minerals formed and whether the production of acid is accompanied by oxidation of H,2S. In path C, pH decreases steadily with no change in H.2S concentration until the field of liquid sulfur is reached (pH • 3). In path D, H2S and pH are buffered by the production of alunite at the expense of muscovite or kaolinite. In path E, the H•,S concentration steadily decreases (e.g., by oxidation of HsS to sulfate) as muscovite is converted to kaolinite, but the buffering capacity of the rock is sufficient to maintain fluid pH around 4.0. At very- low pH, aluminum in kaolinite and other phyllosilieates will eventually dissolve, leaving a vuggy silica deposit behind.

Regardless of the exact pathway followed, aeidifieation of H2S-rieh meteoric waters results in a dramatic decrease in gold solubility (up to three orders of magnitude). This process could lead to the deposition of gold in a "high-sulfidation" environment, as originally proposed by Stoffregen (1987). In some high-sulfidation epithermal gold deposits (e.g., Le- panto, Philippines), there is textural and isotopic evidence that acidic alteration occurred at an early stage from oxidized magmatie fluids and that gold was introduced shortly thereaf- ter by more reduced fluids of presumed meteoric origin (Arri- bas, 1995; Arribas et al., 1995). If this is the case, the trends in gold solubility depicted in Figure 10 may still apply. A near-neutral, H2S-rieh fluid may become acidic simply by interacting with a preexisting alteration assemblage containing minerals such as kaolinite, alunite, native sulfur, and/ or pyrophylite. Aeidifieation \vould be augmented if mixing occurs (by advection or diffusion) between ingressing neutral pH and egressing low pH pore waters. In this respect, the vuggy silica core of such deposits would have an exceptionally high ground-water storativity, and pore waters that are initially strongly acidic would have to be flushed with several volumes of dilute ground water before the add is neutralized. Continued field studies should help resolve the question of the relative timing of acid alteration and precious metal deposition in deposits of this type.

The mobilization of gold by H•S-rieh meteoric waters can be envisioned as a multistage process, as illustrated in Figure 11a and b. In the first stage, magmatic gold is partitioned into an immiseible brine pool and precipitates (along \vith Cu- Fe sulfide minerals) in the potassie zone due to the combined effects of cooling, dilution, and/or pH changes (see preceding section). At the same time, in the overlying phyllie zone, ascending H.2S-rieh vapors mix with eonveeting meteoric waters, converting iron-bearing silicate and oxide minerals to pyrite. In the waning stages of hydrothermal activity, the upper levels of the maglna chamber solidify and the water- saturated melt (i.e., the source of new orthomagmatie fluids) retreats to greater depth (Burnham, 1979). As this occurs, eonveeting, H.2S-rieh meteoric waters can noxv leach gold that was previously deposited in the potassie zone, and transport this metal to shallower levels. This process could be enhanced by rapid erosion of overlying rocks in zones of tectonic uplift (Cooke and Berry, 1996) or explosive volcanic eruptions (Sil- litoe, 1994).

Indirect magmatic involvement: Scenarios 4 and 5 imply an indirect relationship between magmatism and epithermal mineralization, either in space (4), or in time (5). The importance of magmas as a source of heat to drive convection of surrounding waters (scenario 4) is obvious and needs no further discussion. Scenario (5) is also intuitive, 'although a few points are worth stressing.

An early porphyry event may play an important role as a means of producing a large, low-grade gold deposit that can later be upgraded by eonveeting meteoric waters to a higher grade epithermal or mesothermal deposit (i.e., a "protore", in the sense used by Brimhall, 1979). Furthermore, fracturing and breeeiation related to porphyry-style mineralization en- hances the permeability of overlying rock units, increasing the likelihood of later penetration of meteoric waters. In addition, conversion of ferrous iron to pyrite in the phyllie, argillie, and


k, meteoric/ • •: :•......• advanced

LS i ..'•:. i i'i•.. Sargillic

•rateeCrnC / !!h/.•,:S•O2::/:!i• water

• potassic FIG. 11. Schematic diagrams summarizing two stages in the evolution of a porphyry-epithermal system. In (a), fresh

magma has intruded to a shallo;v depth, causing fumarolic activity at the surface and intermittent volcanic eruptions. Orthomagmatic fluids exsolve from the crystallizing melt and make their ;vay up;yard until the zone of boiling and potassic alteration is reached. Gold partitions into the dense, saline brine as AuCI• and precipitates in the potassic zone due to cooling, dilution, and/or pH changes accompanying boiling. Meanwhile, a rising plume of HaS-rich steam causes phyllic alteration of the overlying rocks, as ;yell as sulfidation of ferrous iron to pyrite. At shallroy depths, mixing with cool meteoric ;vaters causes the vapor plume to condense along its margins, with local advanced argillic alteration. In (b), the ;vater- saturated melt has retreated to deeper levels, allowing invasion of heated meteoric ;vater and widespread phyllic overprinting of earlier potassic alteration. HaS and other magmatic volatiles are no longer vented to the surface, but are condensed into overlying meteoric ;vaters, increasing the capacity of the latter to remobilize gold that ;vas previously deposited in the potassic zone (a). Migration of metal- and HaS-enriched meteoric ;vaters may ultimately form a lo;v sulfidation (LS) or high sulfidation (HS) deposit, depending on whether or not the fluids and surrounding country rocks are capable of neutralizing acidic volatiles such as HC1 and SOs.

propylitic zones of porphyry systems creates a wall rock that is conducive to the remobilization of gold as bisulfide complexes by later meteoric waters, as explained in the following paragraph.

Barring any contributions from magmatic sources, the H2S content of most deep geothermal waters will be fixed by reactions with Fe sulfide, oxide, and/or silicate minerals. Anal- yses from explored geothermal systems show a general increase in H2S concentration with increase in temperature, and fall within the approximate bounds of the pyrite + hematite + magnetite, and pyrite + pyrrhotite + magnetite buffer assemblages (Fig. 12). Thus, convecting meteoric waters that are heated up tend to dissolve H•S from the rock, whereas cooling fluids tend to lose H•S by conversion of ferrous minerals to pyrite. However, if all of the ferrous iron in a given body of rock has already been converted to pyrite (e.g., from an early porphyry-style event), the rocks have no capacity to buffer H•S, and this quantity will remain high during cooling and ascent (Fig. 12). Because the solubility of gold in a meteoric fluid increases with HsS concentration (Benning and Seward, 1996), it follows that gold solubilities will also remain high. Such a situation is favorable for leaching of gold out of a porphyry protore and transporting it to shallower crustal levels, where processes such as boiling or fluid mixing may eventually form a large, high-grade epithermal deposit.

It is possible that secondary remobilization of gold by H2S- rich fluids of late-stage magmatic or meteoric origin is a widespread phenomenon in porphyry systems. If so, then the preservation of Au-rich porphyry deposits may be attributed to

the absence of such fluids. This may occur if the system in question is S deficient or is highly oxidized (SOz stable). Low HzS concentrations in any porphyry system will favor deposition of magnetite vs. pyrite, especially at very high temperature. Indeed, a general association between gold mineralization and magnetite (or hematite) alteration has been noted for gold-rich porphyry Cu deposits (Sillitoe, 1979; Vila et al., 1991; Clark and Arancibia, 1995).

Conclusions

This paper has applied thermodynamic calculations to de- scribe the fate of magmatic gold mobilized by fluids of varying chemistry. We recogmze that the calculations have a significant degree of uncertainty due to the number of inherent assumptions, as well as the uncertainties in the thermodynamic database. However, we believe our results are accurate enough to draw a number of firm conclusions. These include the following:

1. Magmatic brines have the potential to transport significant quantities of dissolved gold (>0.1 ppm). This is true regardless of the oxidation state of the system in question. Depending on the physico-chemical conditions attending fluid evolution, this gold may ultimately precipitate close to the magma source (e.g., as a gold-rich porphyry deposit) or be flushed into surrounding ground waters.

2. At very high temperature, gold is dissolved mainly as AuCI•, and solubilities decrease steadily with cooling. At lower temperature, the dominant complex eventually


150

2OO

I I

shallow ore fluid

TøC 250

1. Hveragerdi, Iceland X 'fi' • 2. Wairakei, N.Z. \

3. Reykjanes, Iceland ] 300 4. Broadlands, N. Z.

I 5. Rotokawa, N.Z. py-po-mt

hem-mt-py

350 I I -6 -5 -4 -3 -2 -1

H2S concentration (log molal)

Fw.. 12. Diagram showing the concentration of HaS as a function of temperature for cooling hydrothermal fluids of meteoric origin. Data from five geothermal waters are shown to plot between the pyrite-pyrrhotite- magnetite (py-po-mt) and hematite-magnetite-pyrite (hm-mt-py) buffer curves. The latter curve assumes a constant pH orS.0; the former is independent of pH. Meteoric waters heated to T > 300øC will contain 10 -3 to > 10 o• m H2S and will be capable of dissolving significant quantities of gold ("deep ore fluid"). As these fluids rise and cool, they will follow either the buffbred paths, if ferrous iron is available, or the unbuffered path, if all ferrous iron has been converted to pyrite in a previous alteration episode (e.g., a porphyry event). As discussed in the text, the unbuffered path is more likely to lead to a large, high-grade epithermal gold deposit ("shallow ore fluid"). Geothermal well data are from Ellis (1979) and Krupp and Seward (1987). Data needed to construct the mineral buffer curves are from Barton and Skinner (1979).

switches to Au(HS),j, at which point gold solubility may increase with cooling. Thus, gold in H2S-rieh fluids will tend to be flushed away from the parent intrusion, whereas gold in H2S-poor fluids will tend to deposit dose to the parent melt. The temperature of the ehloride-bisulfide transition depends on the pH and H2S/Ci ratio of the original fluid, and whether or not boiling occurs.

3. The highest gold solubilities (> 100 ppm) are achieved as chloride complexes by fluids that are hot (T = 500øC), oxidized, highly saline, and acidic, and that have a high KC1/ NaC1 ratio. It is likely that such fluids will be highly undersaturated with respect to gold when they first exsolve from the parent melt. Gold deposition from chloride complexes could be triggered by cooling, dilution, or acid neutralization. In addition, gold may precipitate from undersaturated solutions by incorporation as trace impurities in other phases, in partic- ular, Cu-Fe sulfide minerals (iss, bornite, ehaleopyrite).

4. If fluid immiseibility occurs at high temperature (>400øC), our calculations indicate that gold will follow chloride into the brine phase as AuCi,j. In contrast, water, H2S, and acidic volatile species (HC1, SO.•, CO.2) will selectively partition into the vapor. As the vapor rises and cools, it may reeondense, forming a sulfur-rich, chloride-poor solution capable of dissolving gold and transporting it to shallower

crustal levels. This gold may ultimately reside in a low- or high-sulfidation style epithermal gold deposit, depending on the capacity of the country rocks to neutralize acidic vapors.

5. Downward collapse of an evoMng porphyry system may lead to overprinting of earlier gold-rich magnetite and potassic alteration zones by retrograde, pyrite-rieh, phyllie alteration. If so, late fluids may remobilize significant quantities of previously deposited gold as bisulfide complexes.

6. Even if no direct temporal link exists between early porphyry style mineralization and later epithermal gold mineralization, the porphyry event may play an important role as a means of (a) introducing a large quantity of low-grade gold protore, (b) increasing the secondary permeability of overlying wall rocks through fracturing, breeeiation, and/or chemical dissolution, and (e) eausing widespread pyritization of overlying wall rocks. All of these factors increase the likelihood that later geothermal activity may lead to the deposition of a large, high-grade epithermal orebody.

Because this is one of the first attempts to quantify gold transport and deposition in magmatie fluids, a number of questions remain. A major problem at the current time is the lack of experimental data on the partitioning of gold and other metals between aqueous liquid (i.e., brine) and vapor over the range of temperature, pressure, and salinity of porphyry- epithermal systems. In addition, it is by no means dear why some porphyry deposits are unusually gold rich, whereas others are gold poor. This question is particularly vexing in light of our calculations, which strongly suggest that virtually all aqueous magmatie fluids should be capable of transporting significant quantities of gold. The question of Au-rieh vs. Au- poor deposits may be linked to the concentration of gold and other components in the soume area of the magmas from which the hydrothermal solutions are derived. It may also depend on whether or not gold is sequestered from the magma into crystallizing minerals or immiseible sulfide melts before a separate H20-rieh aqueous phase is evolved. Finally, gold-rich porphyry deposits may owe their unusual metallo- geny to the lack of significant retrograde events which would otherwise tend to remobilize gold into the epithermal environment. The relative importance of these and other pro- eesses will no doubt be the focus of continued research.

Acknowledgments We thank Liane Benning and Terry Seward (ETH, Ztirieh)

for the free exchange of ideas and results during our respec- tive experimental projects on gold solubility. The manuscript was substantially modified and improved after the reviews of Economic Geology referees and the suggestions of Liane Benning and David Cooke. Funding for this study was made possible through Natural Sciences and Engineering Research Council (Canada) grants to A.E.W.-J.

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Chemical Mobility of Gold in the Porphyry-Epithermal Environment

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