GEOLOGY, October 2008 815

INTRODUCTIONThere is now signifi cant evidence of a genetic role for polymetallic

melts in metamorphosed ore deposits, where they concentrate metals as melt components by diffusion and dissolution during melt mobilization (e.g., Tomkins et al., 2007). In hydrothermal systems affected by lower grade metamorphism, this can occur by partitioning of aqueous fl uid com-ponents into coexisting metallic melts (e.g., Douglas et al., 2000). Bismuth is a major component of low melting point assemblages, particularly in variations of the Au-Bi-Te-S system, common in magmatic-hydrothermal gold deposits. For example, an Au-Bi correlation is distinctive in intrusion-related gold (IRG) systems with correlation coeffi cients of 0.70.9 com-monly reported (e.g., Baker et al., 2005). Au skarns also display strong Au-Bi correlations (e.g., Meinert, 2000), as do some orogenic systems (e.g., Hattu Schist Belt, Finland; Nurmi and Sorjonen-Ward, 1993).

Native bismuth (melting point 271 C) and Bi-rich polymetallic assem-blages (e.g., Au-Bi with a eutectic of 241 C; Fig. 1) are molten at tempera-tures that overlap with the formation conditions of a large range of gold deposits. The implications of this were illustrated by Douglas et al. (2000), who presented preliminary experiments in which Au was scavenged from hydrothermal solutions at temperature, T ~300 C by Bi melt, a process they termed the liquid bismuth collector model. This model has been con-sidered for interpreting Au-Bi deposits across the magmatic-hydrothermal spectrum, including: (1) IRG veins at Pogo, Fort Knox (Tintina Belt, Alaska; McCoy, 2000); (2) epithermal-porphyry transition at Larga (South Apuseni Mountains, Romania; Cook and Ciobanu, 2004); (3) Au skarns as in the Ortosa and El Valle in the Rio Narcea Gold Belt (Asturias, Spain; Cepedal et al., 2006); and (4) recent volcanic massive sulfi de (VMS) system in the Escanaba Trough (Southern Gorda Ridge; Toermanen and Koski, 2005).

The phase relationships of the Au-Bi binary system have been well described in metallurgical literature (see the GSA Data Repository1).

The Au-Bi binary phase diagram illustrates the potential of Bi melt as a gold scavenger (Fig. 1). An Au-Bi melt has a eutectic with ~19 wt% Au at 241 C, and thus is able to incorporate much higher Au concentra-tions than aqueous fl uids at this and higher temperatures. This solubil-ity difference means that Au is expected to partition into Bi melts even from undersaturated aqueous solutions. However, this model has not been thermo dynamically assessed due to a lack of capability of most geochemi-cal thermodynamic modeling programs to handle complex melts.

In this paper we assess Au scavenging by the Bi melt using equi-librium thermodynamic modeling involving an aqueous electrolyte solu-tion, Au-Bi minerals, and a non-ideal Au-Bi melt. This model is used to address two important questions related to the genetic role of melts in hydro thermal systems: under which conditions a fl uid and melt coexist,

Geology, October 2008; v. 36; no. 10; p. 815818; doi: 10.1130/G25093A.1; 4 fi gures; Data Repository item 2008209. 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.

1GSA Data Repository item 2008209, thermodynamic model and properties for the Au-Bi melt under hydrothermal conditions, is available online at www.geosociety.org/pubs/ft2008.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

Modeling of gold scavenging by bismuth melts coexisting with hydrothermal fl uids

Blake Tooth1, Jol Brugger1,2, Cristiana Ciobanu1,2, Weihua Liu31School of Earth and Environmental Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia

2South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia3CSIRO Exploration & Mining, School of Geosciences, Monash University, Clayton, Victoria 3168, Australia

ABSTRACTThe effect of gold scavenging by bismuth melts is investigated using equilibrium ther-

modynamic modeling of an aqueous solutionmineralmelt system. The calculations for the Au-Bi-Na-Cl-S-H-O system, performed at temperatures between 300 and 450 C, demonstrate that Au concentrations in the melt are several orders of magnitude higher than in the coexisting fl uid, indicating the possible formation of economic gold deposits from undersaturated aqueous fl uids, in which mineralization would not be expected in the absence of a bismuth melt. The model applies to any deposit where a Bi melt is stable and coexists with a hydrothermal fl uid; examples of such deposits are known from skarn, intrusion-related, orogenic, and volcanogenic massive sulfi de (VMS) gold systems. In sulfur-poor systems the partitioning curves presented here can be used directly to correlate the gold concentration in the fl uid and the Au grade in the ore (e.g., Escanaba Trough VMS deposit). These results also illustrate important principles generally applicable to understanding magmatic-hydrothermal and metamorphic deposits that may have contained signifi cant volumes of more complex polymetallic melts.

Keywords: hydrothermal fl uids, melts, thermodynamics, numerical modeling, gold, bismuth, ore deposits.

Au BiMole % Bi

Gold + Bismuth

Maldonite + Bismuth

Maldonite + Melt

Gold + Melt

Melt

Bismuth + Melt

Maldonite + Gold

10 20 30 40 50 60 70 80 90

400

450

350

300

250

200

150

100

50

Tem

pera

ture

(C)

Figure 1. Phase diagram for Au-Bi system at 1 bar, calculated using HCh thermodynamic model developed in this study (see Data Repos-itory [see footnote 1]). Experimental data points on liquidus compo-sition by Nathans and Leider (1962) are shown as solid circles.

816 GEOLOGY, October 2008

and what are the resultant equilibrium concentrations of Au in the melt and fl uid? The modeling will help produce accurate predictions of the physico-chemical conditions under which the Bi collector may occur and of the ore grades arising from such a mechanism.

COMPARATIVE GEOCHEMISTRY OF Au AND BiThere is broad agreement about Au speciation under conditions most

relevant to deposit formation: Au occurs as Au+ aqueous complexes, of which AuHS(aq), Au(HS)2, and AuCl2 are of particular importance depend-ing on the concentration of chloride and sulfur in the fl uids (Figs. 2A and 2C) (e.g., Stefansson and Seward, 2003, 2004). For example, Au bisulfi de complexes predominate in the low-salinity fl uids with moderate sulfur contents, typical of orogenic Au systems (e.g., Mikucki, 1998), and Au chloride complexes are important in the sulfur-poor, saline to hypersaline fl uids typical of gold skarn deposits (Meinert, 2000).

In contrast, limited information is available about the geochemistry of Bi at temperatures relevant to ore formation. The most important oxi-dation state of Bi in aqueous solutions is Bi3+, but only two experimental studies are dedicated to the hydrothermal transport of Bi up to 300 C, close to the minimum temperature relevant for scavenging by Bi melts (Kolonin and Laptev, 1982; Wood et al., 1987). Bismuth complexes with sulfi de ligands may be important, but the only available properties have been extrapolated based on comparison with the other metalloids, Sb and As, and remain to be experimentally confi rmed (Skirrow and Walshe, 2002; Wood et al., 1987). The speciation model outlined by Skirrow and Walshe (2002) is the most complete to date and forms the basis of the speciation for Bi used in the present study (see the Data Repository). However, the lack of experimental studies on Bi hydrothermal geochem-istry means that the predicted Bi mineral solubilities must be considered as semiquantitative only.

The diagrams in Figure 2 show the solubility and aqueous speciation for both Au and Bi at temperatures relevant for the formation of the main types of Au deposits (e.g., Mikucki, 1998; Cooke and Simmons, 2000; Meinert, 2000; Baker et al., 2005) but also above the melting point of native bismuth (271 C). At 300 C, the fi elds of Au aqueous bisulfi de complexes and solid bismuthinite (melting point 775 C; Lin et al., 1996) overlap, thus no Bi melt is predicted to coincide with high Au solubility in the fl uid (Figs. 2A and 2B). However, if the hydrothermal system is reduced, i.e., pyrrhotite stability or CH4(g) rich, Bi melts can coexist with Au-carrying fl uids. At 450 C the bismuthinite stability fi eld shrinks, while at the same temperature Au chloride complexes become important for acidic to near neutral fl uids (Fig. 2B). Therefore, at higher temperatures there is more scope for coexistence of a Bi melt and Au-bearing fl uids.

THERMODYNAMIC MODELING OF AU SCAVENGINGEquilibrium among the minerals + melt + aqueous fl uid system was

computed using the Gibbs free energy minimization algorithm employed in the HCh package (Shvarov and Bastrakov, 1999). The Bi-Au melt was introduced as a distinct phase in the model, and its Gibbs free energy described using the non-random two liquids (NRTL) equation (Renon and Prausnitz, 1968). The three empirical NRTL parameters were fi tted to the Au-Bi binary diagram. The modeled system was Au-Bi-Na-Cl-S-H-O, and the calculations included the minerals maldonite, native bismuth, bis-muthinite, and native gold. To avoid the additional chemical and numerical complexity related to the addition of extra components, mineral buffering reactions likely to control pH [e.g., CO2 (g), silicates] and f O2(g) and f S2(g) (e.g., magnetite-pyrite-pyrrhotite) are excluded. The pH was therefore adjusted using individual components such as NaOH and HCl, and the effect of redox was calculated at different f O2(g) values. For full details of the thermodynamic model and properties used, see the Data Repository.

The central problem, i.e., how much Au exists in a fl uid at equilib-rium with an Au-Bi melt, was explored by equilibrium calculations for a closed system consisting initially of a hydrothermal fl uid and liquid

bismuth , to which additional small amounts of Au are added. The initial melt composition was 100% Bi, and fi nal melt composition was that of the Au-Bi solvus at the chosen temperature, and is the point where Bi melt is saturated with Au, i.e., it coexists with an Au mineral (either maldonite or native gold). The resulting Au partitioning curves between fl uid and melt are shown in Figure 3 for several fl uid compositions at 450 C and for a specifi c example at 300 C. These conditions are chosen to illustrate the effect of various parameters on the partitioning of Au between the melt and aqueous phases.

The process of Au incorporation into Bi melt can be illustrated by reactions with fl uids that carry the predominant types of Au aqueous com-plexes in the hydrothermal systems discussed in the previous section. The most important variables can be summarized by the following equilib-ria for the case of chloride- and sulfur-rich fl uids, representing the pre-dominant melt partitioning processes in skarn and orogenic Au deposits, respectively:

4AuCl2 + 2H2O = 4Au0 (in Bi melt) + 8Cl + 4H+ + O2(g), (1)

Cl-bearing fl uid (e.g., skarn);

4Au(HS)2 + 4H+ + 2H2O = 4Au0 (in Bi melt) + 8H2S(aq) + O2(g), (2)

S-bearing fl uid (e.g., orogenic Au).The equilibrium gold concentration is infl uenced by the redox poten-

tial of the fl uid [ f O2(g)], pH, and the concentrations of chloride and bisul-fi de ions in solution. For equation 1, low pH, high salinity, and f O2(g) favor Au solubility in the fl uid. The main difference between equilibria 1 and 2 is in the effect of pH in controlling HS concentration in a reduced envi-ronment, and consequently Au solubility, which is at a maximum where pH is equal to the fi rst dissociation constant (pK1) of H2S(aq).

Along curves a and b in Figure 3, the fl uid has the same alkaline pH (7.6) but 3 units difference in log f O2(g). Taking an arbitrary Au weight

0 2 4 6 8 10 12 14

40

35

30

25

20

15

10

log

a O

2(aq)

AuCl2

Gold

Gold

AuOH(aq)

Au(O

H)2

AuCl 4

300 C

AuCl 4

Au(O

H)2

300 C, 500 baraAu = 103

aS = 101

~5% NaCl

0 2 4 6 8 10 12 14

40

35

30

25

20

15

10

pH

BiCl63

Bi-melt

Bism

Bi(OH)3(aq)

300 C

Py

Po

Hm

Mt

300 C, 500 baraBi = 104.3

aS = 101

~5% NaCl

0 2 4 6 8 10 12 14

30

25

20

15

10

pH

log

a O

2(aq)

AuCl2

AuH

S(aq

)

Au(HS)2 Gold

AuOH(aq)

450 C

450 C, 500 baraAu = 103

aS = 101

~5% NaCl

450 C0 2 4 6 8 10 12 14

30

25

20

15

10450 C,1000 baraBi = 104.3

aS = 101

~5% NaCl

Hm

Mt

Po

PyBsm

Bi-melt

BiCl6

Bi(OH)3(aq)

A

pH 6.0

B

C D

c b

a

d

CH /CO4 2

KMQ

buf

fer

KMQ

buf

fer

CH /CO4 2

AuHS

(aq)

Au(HS)2AuHS(a

q)

Au(HS)2

Figure 2. fO2(aq) versus pH diagrams showing stability fi elds of Au-Bi minerals and aqueous complexes for 300 and 450 C. For reference, predominance fi elds for common Fe minerals and the CH4(g) /CO2(g) [fCO2(g) = fCH4(g)] boundary are shown with pH values for the KMQ (K-feldsparmuscovitequartz) buffer (aK+ = 0.1 and 0.01). Circled letters on diagrams correspond to conditions for which correspond-ing partitioning curves are drawn in Figure 3.

GEOLOGY, October 2008 817

percent value in the Au-Bi melt (e.g., 20 wt%), the Au concentration between the two fl uids is nearly an order of magnitude higher for the more oxidized case (curve a). This means that a Bi melt can incorporate the same amount of gold from a less concentrated fl uid at more reducing conditions, e.g., pyrrhotite stability or at the CO2/CH4 buffer (curve b) versus pyrite stability (curve a) (compare Figs. 2C and 2D). The same melt composition will result from a fl uid with two orders of magnitude less Au if a similar reduced fl uid as in curve b is considered without sulfur (curve e). The effect of the change in pH in the partitioning of Au between an S-bearing fl uid and the melt is illustrated by curves c, b, and d, of Figure 3, which defi ne a profi le across Figures 2C and 2D from acidic to highly alkaline conditions, at the same reduced f O2(g) value (note that this comparison is qualitative only because of different sulfur activities between the curves and the activity diagrams). As the condi-tion where pH = pK1 of H2S(aq) is approached (curve b in Fig. 3), Au solubility in the fl uid is highest, close to an order of magnitude higher than curve c and two orders of magnitude higher than curve d. We have deliberately excluded consideration of the very acidic case, because as can be observed in Figure 2, bismuth solubility as chloride complexes is large at low pH, destabilizing Bi melt under these conditions.

Another important result of these calculations is that a fl uid that is undersaturated with respect to native gold will coexist with a bismuth melt that contains large amounts of Au. For example, at saturation a hydro-thermal fl uid corresponding to curve b (Fig. 3) contains ~20 ppb Au and coexists with native gold and a melt with 42 wt% Au. Under such condi-tions, a fl uid undersaturated by an order of magnitude (2 ppb Au) would coexist with a Bi melt that still contains ~18 wt% Au. Even at 0.2 ppb, this fl uid coexists with a melt containing ~5 wt% Au. This means that a deposit containing only 100 ppm Bi deposited as Bi melt in the presence of an excess of such a fl uid would display Au grades of 5 ppm. These

values are well in the range found in many deposits displaying the Au-Bi association: IRG deposits commonly display Bi grades >100 ppm (in many cases >1000 ppm; Thompson et al., 1999). The most important consequence of the melt model is that economic deposits can form even when Au is undersaturated by several orders of magnitude in the fl uid.

DISCUSSIONA Bi-rich melt may coexist with an aqueous fl uid at temperatures

higher than the melting point of native bismuth. The precipitation of a Bi-rich melt instead of bismuthinite is favored mainly by high tempera-tures (400 C) or by reducing conditions ( f CH4/f CO2 > 1; pyrrhotite stable). These conditions correspond to the reported occurrences of Au scavenging by Bi melts. The Au-Bi signature recognized in deposits of magmatic affi nity such as skarns (Meinert, 2000) and some of the IRG deposits (Baker et al., 2005) may suggest that they are best suited for application of the model presented here. However, given the geochemical affi nity between Au and Bi (e.g., Spooner, 1993), such an Au-Bi signature may simply refl ect the partitioning of Au and Bi at comparable high val-ues from the magmatic source. Thus, understanding of phase relationships among minerals is essential for assuming a melt scenario.

The Escanaba Trough VMS system is one such example best suited for application of the Bi-collector model (Toermanen and Koski, 2005). On the basis of well-preserved mineral assemblages, Bi melts were inter-preted to form during the circulation of hydrothermal fl uids through pyrrhotite-rich massive sulfi de lenses within organic-matter rich sedi-ments near an active ocean ridge system. The conditions of mineraliza-tion [T ~300 C; CH4(g)-rich fl uids equilibrating with Po] correspond to conditions in Figures 2A and 2B, in which Au-bearing fl uids coexist with Bi melt. If we assume the Au in the Escanaba Trough was enriched solely though the Bi-melt collector, model predictions can be tested by compar-ing Au and Bi grades. The average Bi concentration is 65 ppm and Au concentrations range from 1.4 to 10.1 ppm (Toermanen and Koski, 2005). Assuming that the bulk of both Au and Bi precipitated as Au-Bi melt, the calculated proportion of Au in these melts is in the range 213 wt% Au. Under the conditions of formation of this deposit (pyrrhotite stable, pH = 56, T = 275325 C), Au solubility as AuHS2 is up to 1 ppb, and the liquidus composition of the Au-Bi melt is ~2024 wt% Au. Hence, the hydrothermal fl uid at Escanaba may have been undersaturated by ~12 orders of magnitude with respect to maldonite (Fig. 3 inset), and in the absence of the Bi collector, no Au mineralization would have occurred.

The Bi-mineral associations observed in Au deposits are often more complex than those considered here, usually including S- and/or Te-bearing minerals. Primary phase relationships and textures may also be obliterated due to interaction with aqueous fl uids postdating melt precipitation. One common case of overlapping mineralizing events is illustrated by metamorphic terranes where magmatic intrusions predate or postdate an orogenic event, e.g., at Maldon (Victoria, Australia). The typical ore shows the coexistence of maldonite with native Bi (Fig. 4) representing the eutectic association formed at 241 C in the Au-Bi sys-tem. Maldonite partially decomposes into symplectites of native Au and Bi during cooling below ~116 C. However, the same ores include Bi-tellurides and/or sulfotellurides (e.g., joseite B, Fig. 4), jonassonite, and bismuthinite (Ciobanu et al., 2007). Of these, only bismuthinite shows clear overprint of former assemblages. The others may be interpreted either as (1) the result of interaction between later, S- and Tebearing fl uids and minerals from the Au-Bi association; or (2) crystallization from Au-Bi-Te-S melts formed from either orogenic or magmatically derived fl uids. The latter implies that the applicability of the thermo-dynamic model to natural systems and the accuracy of the predictions can be improved by considering additional components. However, the more complex a melt composition, e.g., 45 components such as Au-Bi-(Pb)-Te-S, the more diffi cult it is to recognize equilibrium associations (eutectics), given the diffi culty in obtaining them experimentally.

103

102

0.1

1

0 2 4 6 8 1010

1913 20

pH 6

pH 5

105

104

103

102

0.1

1

10

10010 20 30 400

c b

e d

a

Au

in fl

uid

(ppb

)

Au in melt (wt%)

Figure 3. Calculation of Au partitioning between Bi-rich melt and aqueous fl uid. Plot of log concentration of Au (ppb) in hydro thermal fl uid versus Au in coexisting melt (wt%) for system Au-Bi-Na-Cl-S-H-O at 450 C (main diagram) and 300 C (inset). Concentration of chloride is 1 molal and total sulfur concentration is 0.001 molal, unless otherwise stated. Other conditions for each curve are as fol-lows. apH = 7.6, fO2(g) = 25; bpH = 7.6, fO2(g) = 28; cpH = 5.6, fO2(g) = 28; dpH = 7.6, fO2(g) = 28, no S; epH = 10.5, fO2(g) = 28. Circles represent saturation with native gold, stars represent satura-tion with maldonite. Inset: partitioning curves drawn for conditions of Escanaba Trough fl uids; T = 300 C, fO2(g) = 40, concentration of H2S = 0.01 molal, concentration of Cl = 0.5 molal, and pH values = 5 and 6. Range of observed melt compositions for given range of fl uid characteristics is between these curves.

818 GEOLOGY, October 2008

A more diverse melt composition has important effects, including melting point depression and changing stability of the melt with respect to important physico-chemical parameters. For example, the addition of Te would likely increase the f O2(g) range for Bi-containing melts coexisting with Au-bearing fl uids. Native tellurium stability is coincident with hematite (e.g., McPhail, 1995), and observations of natural assemblages support this hypothesis (e.g., Ciobanu et al., 2005). This will extend the application of a Bi-melt collector model to epithermal systems that are otherwise excluded given their rather oxidized nature (e.g., Cooke and Simmons, 2000).

In general, the model is useful for understanding the formation of any deposit where a Bi melt coexists with a hydrothermal fl uid, if equi-librium can be assumed. Because aqueous fl uids are ubiquitous in many environments that may also contain low-temperature polymetallic melts (200400 C), the interaction between such melts and fl uids is probably a more signifi cant mechanism of ore enrichment and geochemical exchange than currently recognized.

ACKNOWLEDGMENTSWe are grateful to Bill Birch for providing the Maldon specimen (see Fig. 4),

to John Mavrogenes and Dick England for valuable insight into previous work, and to Andy Tomkins and Adam Simon for thorough reviews. This work was enabled by Australian Research Council fellowships DP0208323 to Brugger and DP0560001 to Ciobanu.

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Manuscript received 25 April 2008Revised manuscript received 1 July 2008Manuscript accepted 2 July 2008

Printed in USA

Figure 4. Microphotograph (oil immersion) of typical gold ore at Maldon (Victoria, Australia) illustrating association of maldonite with native bismuth that represents eutectic assemblage at 241 C in Au-Bi system. Note areas with symplectites showing decomposi-tion of maldonite into native gold and bismuth and grain of joseite B at left side margin of this patch, suggesting melt composition in the Au-Bi-Te-S system.