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Solubility of gold in hydrothermal chloride solutions
Chemical Geology, 11 (1973) 73-87 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
SOLUBILITY
OF GOLD IN HYDROTHERMAL
CHLORIDE
SOLUTIONS
R.W. HENLEY Department of Geology, University of Otago, Dunedin {New Zealand)
(Accepted for publication November 20, 1972)
ABSTRACT Henley, R.W., 1973. Solubility of gold in hydrothermal chloride solutions. Chem. Geol., 11 : 73- 87. The solubility of gold has been determined in chloride solutions in the temperature range 300500°C corresponding to the inferred range for the formation of "hypothermal" gold deposits. The solutions were buffered with respect to HC1 by a K-feldspar-muscovite-quartz assemblage, and to oxygen by the assemblage haematite-magnetite. Solubilities increased rapidly with temperature from about 10 p.p.m, at 300°C, to 500 and 1000 p.p.m, at 500°C at 1000 and 2000 bar, respectively. These results are discussed in terms of possible solution species in this high-temperature region where molecular behaviour predominates in the solution equilibria. It is suggested that gold and other metals may be transported to the site of ore-deposition in undersaturated high-temperature solutions. Ore deposition may take place at lower temperatures where ionic gold chloride or sulfide species dominate the chemistry of the ore solutions. INTRODUCTION Metallic gold occurs with base-metal sulfides in hydrothermal vein and replacementtype ore-bodies, which are found most commonly in Archaean greenschist terrains such as at Morro Velho, Brazil (Gait, 1962) and Yellowknife, Canada (Boyle, 1959) and in areas of T e r t i a r y - R e c e n t volcanic activity (e.g., in the circum-Pacific and Alpine belts). Solubility information is a prerequisite to the understanding of the genesis of these orebodies. In the Archaean group of deposits wallrock alteration assemblages, developed adjacent to the ore, are often indistinguishable from those produced by greenschist facies metamorphism, while in the Tertiary group of ores, propylitic wallrock assemblages are often developed which are similar in character to greenschist or a l b i t e - e p i d o t e - h o r n blende hornfels facies assemblages. This suggests that, although ore-deposition may have occurred at lower temperatures, the ore-transporting solutions at some stage in their evolution were at temperatures appropriate to the development of these facies. Thus, for a study of the chemistry o f gold-transport a broad P-T field is outlined coincident with that for the development o f the greenschist facies, with temperatures in the range 3 0 0 500°C and pressures up to 10000 bar (Turner, 1968). Current evidence (White, 1968) suggests that ore-bearing solutions are, in general, brines with total chloride concentrations up to 5 M. Similar but less concentrated
74
R.W. HENLEY
solutions are well-known in active geothermal areas (e.g., Taupo, New Zealand), but these have low sulphur contents, and it is uncertain whether their low sulphur concentration is also representative of that in ore-forming systems. In the Salton Sea geothermal system (White, 1968) the total molal metal content is well in excess of the sulphur molality suggesting that the metals are dissolved as chloride rather than sulfide complexes. Some authors (e.g., Barnes, 1967) suggest, however, that the sulphur content of ore-forming solutions was higher than that of the geothermal systems, so that total base metals equalled total sulphur content; metal-sulphur complexes could then account for ore transport. Above 300°C the self-ionization of water is about one-thousand times greater than at room temperature (Quist, 1970) but due to the breakdown of the liquid structure at the higher temperature, the dielectric constant is only equivalent to that of organic solvents at room temperature (Quist and Marshall, 1965). For this reason the ionization constants for strong electrolytes, such as sodium chloride, decrease rapidly with increasing temperature so that undissociated solute molecules become more and more abundant and "molecular" rather than ionic equilibria predominate in the hydrothermal solution. Because of this behaviour, for example, chloride occurs largely as the undissociated HCI, KCI, NaC1 monomeric or dimeric molecules while sulphur occurs as molecular H2S, SO2, or as undissociated species such as NariS. Solution species will therefore be quite different in their chemical nature and structure than the species which account for solubility at lower temperatures. In deference to the few studies carried out in this region, it would be unwise to apply the genetic term "ion-pair" to the high temperature species; the writer prefers at this stage to use the term "molecular complex" to describe the undissociated solution species, which may have little or no ionic behaviour. Weissberg (1970) and Seward (1972) have studied the solubility equilibria of gold up to 300°C in sulphur and sulphur-chloride solutions, and Seward (1972) concluded that complex ionic species such as [Au (HS)] o, [Au (HS)2]- or [Au2 (HS)2S]-2 (depending on pH) allow gold solubilities in hydrothermal solutions much higher than those computed for gold-chloride species by Helgeson and Garrels (1968). At higher temperatures, however, metal complexing with the very abundant and strongly-bonding chloride species may be favoured more than with sulphur species. This paper reports the results of an experimental study of the solubility of gold in alkali chloride solutions at temperatures in excess of 300°C and up to 2000 bar pressure. The dissolution reaction of a metal involves both oxidation and complex formation, and for gold a formal reaction may be written: Au + aHC1 + bO2 ~ ~Aucomplex] + c H 2 0
(1)
In this study an attempt was made to determine the stoichiometry of such a reaction and to determine the variation of the equilibrium constant, K, over a range of temperatures and pressures. In order to experimentally determine solubility equilibria it is convenient to buffer the activities of both oxygen and ligands by reaction with solid assemblages. Hemley (1959) has calibrated the useful HC1/KCI buffer assemblage quartz-
SOLUBILITYOF GOLD IN CHLORIDE SOLUTIONS
75
muscovite-K-feldspar while Eugster and Wones (1962) give data for the useful redox buffer, haematite-magnetite. Both these assemblages occur in the earth's crust and limit the activities of 02 and HC1, although the haematite-magnetite pair may represent an upper limit to the range of possible oxygen activities in ore-forming systems. The activities in these buffered systems may represent extremes in the ore-forming environment, but the use of other more realistic assemblages is prohibited by the addition of extra components, kinetics of the buffer reactions, and scarcity of well-calibrated assemblages in the temperature range considered here. The use of the buffer systems described above allows the determination of the equilibrium constant (K) for the dissolution reaction so that for other conditions gold solubilities can be calculated thus: log a(Au species) = log KTp + a lOg aHC1 + b log f o 2 - - c logfH~O
(2)
EXPERIMENTAL APPROACH The solubility of gold in buffered chloride solutions was measured directly by analysis of an extracted solution sample, using a technique especially developed for this study. The solid buffer assemblages and solution (about 0.5 g) are weighed into a 0.5 cm diameter gold capsule (5 cm long) together with an open-ended tube, 2 cm long, consisting of Teflon, silica glass or alumina, depending upon the temperature of the run. On temperature quenching at constant pressure, about 0.1 g of solution becomes trapped in the tube as the gold capsule collapses around it. The tube is then removed, wrapped in aluminium foil, weighed, carefully dried and reweighed to determine the mass of trapped solution (by evaporative weight loss). The runs were carried out in large stainless steel pressure vessels (O.D. = 5 cm, I.D. = 1.25 cm), so that two or three capsules could be run simultaneously. Temperatures were measured using an external thermocouple and, where possible, with an internal sheathed thermocouple in contact with the capsule. The quartz-muscovite-K-feldspar solid buffer assemblage was prepared by finegrinding (to < 200 mesh) natural materials (crystalline quartz, adularia and muscovite). In some runs synthetic assemblages were used, crystallized from gels (either of muscovite or K-feldspar composition of a 1 : 1 : 1 quartz-muscovite-K-feldspar composition) prepared by the method of Hamilton and Henderson (1968). Potassium chloride solutions were used (2.0, 1.0 and 0.5 M at N.T.P.), and in some cases a solution was used in which the mole ratio KCI: HC1 was initially approximately equal to that of the buffered solution. ANALYSIS The small solution volume and low solubilities limited total gold content and necessitated the use of neutron activation analysis for the determination of gold in the aluminium wrapped tube. The analyses were performed by the Joint Manchester-Liverpool Universities Research Reactor Analysis Service, using a Bendix Multi-channel X-ray
R.W. HENLEY
76 TABLE I Experimental results for gold solubility at 1 kbar A. 2 MKC1 Run No.
Temperature (° C)
Solubility (p.p.m.)
Run No.
Temperature ( o C)
Solubility (p.p.m.)
E 50 E51 17 20 39 41 63 64 65 61 68 69 66 67 81 84 85 86 87 88 89
305 305 310-325 305-308 313 313 507 507 507 485-509 509 509 495-504 495-504 500 513 513 513 513" 513" 513"
38 31 18 87 7 46 936 114 328 507 2113 484 196 68 65 263 133 592 2987 87 818
42 43 49 50 46 27 96 97 98 99 100 101 130 132 133 134 135 136 137 151 162 163 164 165
266-308 266-308 295-340 295-340 311-342 402 512 512 512 499 499 499 504* 517 489-508 502-506* 487 504 503 456* 400 H 504 H 508 H 450 H
67 18 23 19 21 61 298 348 83 14 30 3 133 414 60 159 68 18 26 67 42 17 38 20
P P S S S S
B. 1MKC1 Run No.
Temperature ( oC)
Solubility (p.p.m.)
14 15 18 25 28 62 70 156 157
295 295 310-325 430-454 400 485-500 485 - 5 0 0 493-499 H 556
2.9 6.9 60 22 63 260 2968 1778 15
S S S S S
TABLE I (continued) C. 0.5 M KCI Run No.
Temperature ( ° C)
Solubility (p.p.m.)
16 19 29 26 79 159 160 161
295 310-325 420-450 400 507 556 H 461H 508*
5 35 11 81 795 40 31 76
Notes: Duration of runs 1 0 - 2 0 days. Collection thimble was alumina except where indicated: P = PTFE (Teflon); S = silica glass. Temperature measurement: cold seal b o m b s with external thermocouple except where marked: * internal sheathed thermocouple; H = hot-seal bomb. The thermocouple temperature was measured by potentiometer to -+ 2 C. Mixture was synthetic quartz, muscovite and K-feldspar. Mixture of natural silicates was used for runs 96 et seq. Consecutive run No. at the same temperature indicate that these runs were held together in the same pressure vessel. 60C
/
1000
800
500
~" ,~i,iI
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i
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Fig.1. Some experimental data for gold solubility in the range 3 0 0 - 5 0 0 ° C at 2000 bar total pressure in 2 M (at N.T.P.) potassium chloride solutions, buffered by the assemblages q u a r t z - m u s c o v i t e - K feldspar and haematite-magnetite. Fig.2. Experimental data for gold solubility in 2 M (at N.T.P.) potassium chloride solutions at 1000 bar total pressure at 4 8 0 - 5 2 0 ° C. Q u a r t z - m u s c o v i t e - K - f e l d s p a r and h a e m a t i t e - m a g n e t i t e buffer assemblages present. Errors in temperature measurement are indicated by the bar length.
78
R.W. HENLEY
spectrometer. Radiochemical separations were unnecessary. The standard'deviation of these analyses was very much less than the variance of the gold solubilities determined in these experiments. ERRORS Precipitation of solute during the quench is always a danger in solution studies, but in these experiments no evidence was seen for crystallization of gold on the vessel walls. In any case, provided no precipitated gold is washed into the tube, quench precipitation should have no effect upon the results. To test the efficacy of the method the solubility of sphalerite was determined using both weight loss on crystal fragments and tube analysis by X-ray fluorescence, for each run. The chloride solutions were buffered by the quartz-muscovite--K-feldspar assemblage and the results were in good agreement with similar solubility runs by Hemley et al. (1967), and the difference between the mean weight loss and X.R.F. results indicated a deviation from the mean of 16%. Neutron activation analysis may involve slightly greater variance, so that for the gold solubility runs, variance of 16-30% was expected. A further source of error may lie in the slow attainment of equilibrium by the buffer assemblages. Hemley (1959) indicates that the quartz-muscovite--K-feldspar assemblage attains equilibrium at 300-500°C, within two weeks, while the haematite-magnetite assemblage may be very slow to react below 500°C (Eugster and Wones, 1962). Run times of between 10 and 20 days were therefore employed. Henley (1971) showed that gold + HC1 reactions went to completion within a few hours in this temperature range. The reaction rates of the buffer assemblages were thus the major factor controlling the attainment of solubility equilibrium. RESULTS Results for gold solubility in buffered potassium chloride solutions are given in Tables I and II for the range 300-500°C, 1000-2000 bar total pressure. These are seen to be extremely variable within small temperature ranges and much of the time involved in this study was devoted to trying various experimental modifications in an attempt to improve the reproducibility of the results. In Fig.1 results at 2000 bar total pressure are seen to have an almost exponential trend with solubilities increasing rapidly near to 500°C. Fig.2 shows solubility results at 1000 bar in the narrow temperature range 480-520°C, and emphasizes the asymptotic form suggested in Fig.1. Small errors in temperature measurement and control on the very steep part of the curve would involve marked differences in the solubility measured at the apparent m e a n temperature.
SOLUBILITY OF GOLD IN CHLORIDE SOLUTIONS
79
TABLE II Results at 2 kbar for 2 M KC1 solutions Run No.
Temperature (° C)
Gold solubility (p.p.m.)
Means of collection
F 52 F 53 21 ] 22 f 24 77-~ 78J
295 293
18 18 17 21 112 886 1070
Teflon collection thimbles Teflon collection thimbles silica thimbles silica thimbles silica thimbles corundum thimbles corundum thimbles
302 420-450 507
DISCUSSION OF RESULTS In Fig.3 the variation of the molality o f the silicate-buffered species HC1 with temperature is seen to have the same form as the solubility curve obtained for gold in similarly buffered solutions. This parallelism suggests that HC1 may be an important species in the gold solubility equilibrium. Fugacities of oxygen buffered by h a e m a t i t e - m a g n e t i t e also increase rapidly with temperature and contribute to the steepness of the gold solubility curve particularly above 450°C. As shown above (eq.2) the solubility equilibrium can be written as a function of the activities o f the reaction species. For simple gold species such as Au C1, Au C13 etc., the term a is-always greater than 1, while b is < 1. The solubility 20
U o
×1o
//
~,
5
360
460 Tempe rat. u re °C
Fig.3. Variation of the molality of H + and HC1 in potassium chloride solutions buffered by the assemblage quartz-muscovite-K-feldspar (calculated from the data of Hemley, 1959).
80
R.W. HENLEY
contribution of the HC1 term is therefore considerably greater than that of 02. The decrease in the hydrogen ion molality for the buffered solution shown in Fig.3 indicates that this ionic species may take little, if any, part in the gold-solubility equilibrium. In Fig.4 the gold-solubility results are compared with solubility data for sphalerite in buffered chloride solutions (Hemley et al., 1967). These show similar and parallel trends, which are also, above 400°C, parallel to the molalities of HC1 in buffered solutions. Below 400°C ionic equilibria become more significant and account for the change of slope of the solubility curves. For comparison, the solubility of quartz at 1000-2000 bar is shown,
-2
0
E
o _J
5(~)0
460
360 T*C
Fig.4. Experimental solubility data for gold and sphalerite in quartz-muscovite-K-feldspar buffered solutions at 300,400 and 500 ° C, together with the molality of HC1 in chloride solutions (calculated from data of Hemley, 1959) and the solubility of quartz. Data sources: sphalerite - Hemley et al. (1967); quartz - Weill and Fyfe (1964); gold - this study.
since quartz solubility is scarcely affected by the presence of HC1 in the solvent (Anderson and Burnham, 1967). The poor reproducibility of the solubility results did not allow direct determination of the equilibrium constant or the stoichiometry of the reaction; however, the important contribution of the HC1 species was demonstrated and is further discussed below. Barnes and Ernst (1963) showed that at high temperatures (about 500°C) hydrothermal solutions tend toward the behaviour of ideal mixtures. This suggests that the solution species can be considered as molecular species which have minimum interaction with one another.
SOLUBILITY OF GOLD IN CHLORIDE SOLUTIONS
8l
Using this as a model system, equilibrium solubilities can be calculated for gold on the basis of known gaseous gold chlorides, Au C1, Au2 C12, Au C13, Au2 C16. High-temperature thermodynamic data (compiled in Henley, 1971) show that the dimeric species, Au2 C16, is likely to be the most stable in a high temperature gas phase, as was demonstrated experimentally by Landsberg and Hoatson (1970) in vapor pressure studies. Solubility reactions for each of these species can be written in the form of eq.1 and equilibrium constants calculated. Insertion then of data for a HCI~,fo~, fH20 into solubility functions like eq.2 allows the calculation of an equilibrium solubility for gold. Thus at 500°C, with data for the buffers used in the experiments, the solubility of gold in this model system is 10 -6 p.p.m, at 1 kbar rising to 10 -4's p.p.m, at 5 kbar for the species Au2CI6, while the contributions of the other gold species are insignificant. The writer is aware of the dangers of this approach; however, until quantitative data is available for solution chemistry in this very high-temperature field, no better method of calculation is feasible. The trend of the calculated solubility curves is also similar to those obtained experimentally and demonstrates that very large solubility variations are to be expected over small temperature ranges in the vicinity of 500°C. The experimental work demonstrated gold solubilities up to 1000 p.p.m, in the buffered system at 2 kbar 500°C, while the calculated solubility for the same conditions is less than 10 -4 p.p.m. Anderson and Burnham (1967) also noted high experimental gold solubilities up to 1000 p.p.m, in unbuffered HCI and KCI solutions at 600°C, 3 kbar and Burnham (from H.L. Barnes, personal communication, 1972) has also found solubilities up to 3000 p.p.m, in 0.58 M HCI in contact with "granite" at 600°C up to 4 kbar. The very large discrepancy between calculated and experimental solubilities for gold may offer some clue to the nature of the solution species. In the thermodynamic calculations above, no account was taken of solvation of the gold chloride solution species. Water is still a polar solvent at these high temperatures and pressures, so that solvated chloride species will certainly occur. Copeland et al. (1953) noted very high molar volumes for sodium chloride in dilute solutions, above the critical point, and they ascribed this observation to extensive hydration of the solution species, and Crerar and Anderson (1971) have studied the hydration of silica species in high-temperature solutions. In order to account for the difference between the observed and calculated solubilities two possibilities are apparent: (1) The assumption that the high-temperature species is based on the planar Au2 El6 molecule may not be valid. Thermodynamic data favours the assumption of auric species, but monomeric auric species, or perhaps higher order polymers may occur (Brewer, 1958). (2) Extensive hydration of the simple gold chloride species may involve sufficient solvation free energy to account for the higher experimental solubilities. The asymmetric bond polarity in gold chlorides favours weak co-ordinate bond formation between water molecules and the complexed gold, and similarly hydration of the complexed chloride atoms would occur, by hydrogen bonding. Furthermore, in this high-temperature region where ionic equilibria are insignificant, the availability of undissociated HC1 and alkali chloride molecules suggests that these too may take part in solvation equilibria. The gold
82
R.W. HENLEY
solubility equilibria considered in this temperature region take place in a complex mixed solvent consisting of reactive dipolar species, i.e., H20, HC1, KC1, (KCI)2 all of which can take part in solvation equilibria with the gold species. Marshall and Quist (1967) have stressed that such ion-ion-pair-solvent equilibria play a major role in solution chemistry, and must be taken into account in determining complete solubility equilibria constants. These authors suggest that ion-pair-solvent equilibria involve dipole-dipole interactions, but it would be unwise at this stage to speculate too much on the precise nature of the gold species-solvent bonding in the high temperature and pressure region under consideration. If gold chloride-HCl solvation does occur, then substitutional solvation equilibria will be involved in gold solubility, of the form: [Au2CI 6 (H~O)a (Hfl)b] + cH20 ~ [Au2C16 (H20)a+c(Hfl)b-c] + cHCI
Up to twelve possible "inner-sphere" co-ordination positions would be available to HC1 for direct coordination, but the actual solvation numbers of HC1 and H20 would be expected to be sensitive to both temperature and pressure. Further experimental work (Henley, 1971) in the system A u - H 2 0 - H C I - 0 2 has suggested that at 500°C, 1000 bar, the solvation number for HCI is six, while at 2000 bar this decreases to two. Similar solvation equilibria can be written for the dipolar KC1 and other species in oreforming solutions. SOLUBILITYOF GOLD IN ORE-FORMINGENVIRONMENTS The primary object of a study of this kind is to follow the solution behaviour of metal species in order to attempt to understand the physical and chemical controls which cause the deposition of the metal in an ore-body. The experimental work and its interpretation demonstrate that gold is readily soluble in hydrothermal solutions and that about 1000 p.p.m, of the metal may be transported as chloride species at 500°C, 2 kbar under the conditions of the haematite-magnetite buffer. In buffered systems at temperatures where the solution approaches an ideal mixing model, the presence of other components can have little effect upon the solubility of gold. The solubility could be decreased, however, if other species are present which also involve solvation by chloride ligands (HC1, KC1, NaC1... ), and this includes most of the oremetals. The ZnS solubility (Hemley et al., 1967) shown in Fig.4 demonstrates that [ZnCI2 (HCI)n ] complexes may occur, and similarly pyrite solubility may involve a complex such as [Fe2C16(HC1)n] in this temperature field. The variation in the type of solution complexes and their stoichiometry makes the draughting of solubility curves for gold over wide temperature ranges extremely difficult. In Fig.5 a generalized solubility curve is shown for chloride complexes only, calculated from an equilibrium constant obtained from the experimental data at 500°C, 2 kbar, assuming the presence of the species [Au2C16 (HC1)2 (H20)n] • Three fields are indicated, characterized by the dominance of ionic or molecular behaviour in the solutions.
SOLUBILITY OF GOLD IN CHLORIDE SOLUTIONS
83
3 Ionic -~-
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TemperotureOC Fig.5. General solubility curve for gold in 3 M potassium chloride solution, The curve beiow 300°C is that of Helgeson and Garrels (1968) for solutions in contact with quartz and pyrite. The curve for T > 300°C, P = 1 kbar is calculated from solubility data (this study) for solutions where mHC 1 is buffered by the quartz-muscovite-K-feldspar assemblage. Oxygen fugacity is buffered at values intermediate between those on the quartz-fayalite-magnetite and haematite-magnetite buffers,
Helgeson and Garrels (1968) have computed the "sub-critical" solubility of gold for a 3 M sodium chloride solution in equilibrium with quartz, pyrite and gold. Their data showed that a u r o u s chloride complexes [Au C12] are most important up to about 300°C, although gold-sulphur species were not considered. Their chloride solubilities are shown in Fig.5, region A, where it is evident that falling temperature rapidly reduces the equilibrium solubility of gold. Seward (1972) has demonstrated that, in the temperature range below 300°C pHdependant thiocomplexes of gold exist in solution and that the resulting solubilities are considerably higher than those calculated by Helgeson and Garrels (1968) for chloride complexes. The HS- stability field may exist well above 300°C (Ellis and Giggenbach, (197 l)so that gold thiocomplexes (terminology used by other workers, e.g., Seward (1972)), may be stable in this region. The rapidly increasing oxygen fugacity buffered by natural assemblages may,however,oxidize sulphur in solution to sulphate species, less suitable for gold complexing. The temperature region B (Fig.5) is highlighted, where three important phenomena occur: (1) Redox transitions between aurous and auric chloride complexes favoured by the higher buffered fo2 in this region. (2) Transition between low temperature ionic thiocomplexes and high-temperature molecular chloride complexes. (3) General trend from ionic to molecular solution chemistry.
84
R W. HENLEY
The general higher temperature solubility (region C) is shown for a gold chloride complex, e.g., [Au2 C16(HC1)2], at 2000 bar in a 3 M chloride solution with fo2, buffered by an assemblage between quartz-fayalite-magnetite and haematite-magnetite. The solution considered is buffered with respect to HC1 by a quartz-feldspar-muscovite assemblage, so that the solubility curve (C) corresponds to that for a hydrothermal solution passing through a granite (e.g., at the Mazoe gold mine, Rhodesia (Henley, 1971)). The solubility curve shows that gold may be rapidly deposited from a saturated solution due to falling temperature. Similar curves will apply to solutions in contact with other HCI or O2-buffer assemblages (e.g., chlorite-biotite, amphibole-chlorite, etc.). For the Tertiary-Recent gold deposits (of "epithermal" character) lower fluid pressures prevailed during ore-deposition. Low pressure (< 1 kbar) favours molecular species, so that the solubility curve may have a pronounced inflection in region B (Fig.5). For these deposits the higher acidity (as HC1) due to wallrock assemblages such as kaolinite-prophyllite, may allow higher gold solubilities than indicated in the diagram, but the very low unbuffered oxygen content of solutions at shallow depths in the crust, may reverse this trend. The inflection in the curve would allow two stages of ore-deposition. The first deposition occurs at about 450°C, and "late" gold precipitates below 300°C. Some of the early gold may be redissolved as cooler solutions pass, particularly if gold thiocomplexes can form. At higher pressures (> 1 kbar) ionic equilibria are important up to higher temperatures, so that the solubility curve has a less pronounced inflection, and this curve would apply to the Archaean group of deposits. Gold deposition in response to falling temperature occurs only below 300°C, so that only "late" gold would be observed in these ores. With falling temperature gold, pyrite, quartz, etc., will coprecipitate, so that for a quickly cooling solution, rapidly deposited fine-grained pyrite would correspond to high gold values as is the case at the Mazoe mine, Rhodesia (Henley, 1971). An intermediate colloidal phase may occur during rapid deposition, and the distribution of gold in quartz veins has often been discussed in terms of colloidal transport. Replacement relationships showed sometimes by gold (e.g., gold commonly replaces pyrrhotite) may be accounted for by the competition between gold and the displaced metal for HC1 ligands or complimentary redox reactions between the two metals if the solution is not effectively buffered. In the above discussion the possibility was mentioned that thiocomplexes allow higher gold solubilities in the subcritical ionic region A. Incorporation of this possibility into solubility schemes for gold must, however, be made with caution because, although chlorine-containing species are known to be abundant, it is by no means certain that the species of sulphur are equally so. The stoichiometry of the reported sulphur-metal complexes indicate that for a hydrothermal solution the total molality of metals must be equal to the total sulphur molality, so that this condition must be fulfilled before consideration of hydrothermal thio-complexes can be made. Furthermore in low sulphurhigh metal solutions, the metals must compete for the available sulphur ligands and this will result in substantial modification of the equilibrium solubilities of metals in this region. Seward (1972) further discusses this problem and has indicated that thiocomplexes
SOLUBILITYOF GOLD IN CHLORIDE SOLUTIONS
85
can account for the gold content of present-day geothermal solutions. Tile transition region B, between dominantly ionic solutions and the higher temperature molecular solutions is stressed here because of the lack of quantitative understanding of solution behaviour within it and its obvious importance in ore-metal transport and deposition. It is important to obtain solubility information in this region (300-450°C) and to correlate this with a thermodynamic study, possibly combining an extension of Helgeson's (1964) methods and incorporating a correction for the transition to molecular behaviour. In the high-temperature region C (Fig.5) very high equilibrium solubilities are to be expected for most ore-metals, on account of the ideal mixing behaviour of the solution species, where species interaction is at a minimum and therefore does not limit solubilities. With high equilibrium solubilities, it is unlikely that hydrothermal solutions above 400°C are ever saturated with respect to the metals. If the hydrothermal solutions responsible for mineralization in the Morro Velho gold mine, Brazil (described by Gair, 1962), for example, were derived by metamorphic dehydration reactions in the P-T range of the greenschist-amphibolite facies transition, and all the trace gold in the country rock affected by these reactions was leached out, a solution with a maximum gold content of 0.1 p.p.m, would result. A similar estimate of the geological "solubility" was made by Helgeson and Garrels (1968). These estimates are well below the experimentally determined equilibrium solubilities of gold at temperatures above 350-400°C. This suggests that an efficient transport and concentration mechanism can be envisaged for gold to account for the formation of ore-bodies (where the metal content is commonly well in excess of 10 p.p.m.). Gold would be readily transported as chloride species in hightemperature solutions derived from a widely dispersed source (such as a metamorphic pile) to the ore-zone, in undersaturated solutions and then efficiently concentrated to form an ore-body by deposition as the solution cooled below 300°C. Particularly important in ore deposition are the relative rates of change of the solution parameters, fo2, fHC1 etc., in determining the paragenesis of the ore-minerals and their disposition through the ore-body. These aspects of ore-formation have been discussed by Helgeson (1970a, b) who provides a powerful model for base-metal ore-deposition. At this time, however, there is insufficient information available concerning the solubility of the other ore-components, let alone the kinetics of their deposition, to allow the development of a quantitative geochemical model for the formation of a gold deposit. ACKNOWLEDGEMENTS The experimental work reported in this paper was carried out at the Geology Department, University of Manchester, during the tenure of an N.E.R.C. Research Studentship. I am grateful to Professor W.S. Fyfe for suggesting and encouraging this field of research. Mr. B.L. Goodwin (Joint Universities Research Reactor, Risley) carried out the neutron activation analyses rapidly and efficiently. I am also grateful to Professor Fyfe, Professor D.S. Coombs, Dr. T.M. Seward and the many others at the Chemistry Division, D.S.I.R.,
86
R.W. ttENLEY
Wellington and the Chemistry Department, University of Otago, for critical reading and discussion o f the manuscript. The author accepts the entire responsibility for any deficiencies o f this paper.
REFERENCES Adams, D.M. and Churchill, R.G., 1968. Vibrational spectra of halogen-bridged systems, II: Au2C16, A12Br2, A1216, In216. J. Chem. Soc. A (1968): 2141-2144. Anderson, G.M. and Burnham, C.W., 1967. Reactions of quartz and corundum with aqueous chloride and hydroxide solutions at high temperatures and pressures. Am. J. Sci., 265: 12-27. Barnes, H.L. (Editor), 1967. Geochemistry o f Hydrothermal Ore Deposits. Holt, Rinehart and Winston, New York, N.Y., 670 pp. Barnes, H.L. and Czamanske, G.K., 1967. Solubilities and transport of ore minerals. In: H.L. Barnes (Editor), Geochemistry of Hydrothermal Ore Deposits. Holt, Rinehart and Winston, New York, N.Y., pp. 334-381. Barnes, H.L. and Ernst, W.G., 1963. Ideality and ionization in hydrothermal fluids: the system MgOH20-NaOH. Am. J. Sci., 261:' 129-150. Beattie, I.R., Gilson, T.R. and Ozin, G.A., 1968. Vibrational spectra of Au2CI6 etc. J. Chem. Soc. A (1968): 2765-2771. Boyle, R.W., 1959. Geochem, origin, role of CO2, H20, S, B, at Yellowknife. Econ. Geol., 54: 1506. Brewer, L., 1958. Undiscovered compounds. J. Chem. Educ., 35: 153-156. Copeland, C.S., Silverman, J. and Benson, S.W., 1953. System NaCI-tt20 at supercritical temperatures and pressures. J. Chem. Phys., 21: 12. Crerar, D.A. and Anderson, G.M., 1971. Solubility and solvation reactions of quartz in dilute hydrothermal solutions. Chem. Geol., 8: 107-122. Ellis, A.J., 1969. Present day hydrothermal systems and mineral deposition. Commonw. Min. Metall. Cong., 9th, Min. Pet. GeoL Sect., London, 1969, Pap., 7 (presented paper). Ellis, A.J. and Giggenbach, W., 1971. Hydrogen sulfide ionization and sulfur hydrolysis in high temperature solution. Geochim. Cosmochim. Acta, 35 : 247-260. Eugster, H.P. and Wones, D.R., 1962. Stability relations of the ferruginous biotite, annite. J. Petrol., 3: 82-125. Gair, J.E., 1962. Geology and ore deposits of the Nova Lima and Rio Acima quadrangies, Minas Gerais, Brazil. U.S. Geol. Soc. Prof. Pap. 341A: 67 pp. Hamilton, D.L. and Henderson, C.M.B., 1968. The proparation of silicate compositions by a gelling method. Mineral. Mag., 36: 832-838. Helgeson, H.C., 1964. Complexing and Hydrothermal Ore Deposition. Pergamon, New York, N.Y., 128 pp, Helgeson, H.C., 1970a. Reaction rates in hydrothermal flow systems. Econ. Geol., 65: 299-303. Helgeson, H.C., 1970b. A chemical and thermodynamic model of ore deposition in hydrothermal systems. MineraL Soc. Am. Spec. Pap. 3: 155-186. Helgeson, H.C. and Garrels, R.M., 1968. Hydrothermal transport and deposition of gold. Econ. Geol., 63: 622-635. Hemley, J.J., 1959. Some mineralogical equilibria in the system K20-AI203-SiO2-H20. Am. J. Sci., 257: 241-270. Hemley, J.J., Meyer, C., Hodgson, C.J. and Thatcher, A.B., 1967. Sulfide solubilities in alteration controlled systems. Science,t158: 1580-1582. Henley, R.W., 1971. Geochemistry and Genesis of Precambrian Gold Deposits. Thesis, University of Manchester, 177 pp. Landsberg, A. and Hoatson, C.L., 1970. Kinetics and equilibria of the Au-CI system. J. Less Common Metals, 22: 327-339. Marshall, W.L. and Quist, A.S., 1967. A representation of isothermal ion-ion-pair-solvent equilibria independent of changes in dielectric constant. Proc. Nat. Acad. Sci., 58:901-906. Quist, A.S., 1970. Ionization constant of water. J. Phys. Chem., 74: 3396-3402.
SOLUBILITY OF GOLD IN CHLORIDE SOLUTIONS
87
Quist, A.S. and Marshall, W.L., 1965. Estimation of the dielectric constant of water to 800 ° C. Z Phys. Chem., 69: 3165-3167. Seward, T.M., 1972. Thio-complexes of gold and the transport of gold in hydrothermal ore solutions. Geochim. Cosmochim. Acta (in press). Turner, F.J., 1968. Metamorphic Petrology. McGraw-Hill, New York, N.Y., 403 pp. Weill, D.F. and Fyfe, W.S., 1964. Solubility of quartz in water in the range 1000-4000 bars and 400 O 550 C. Geochim. Cosmochim. Acta, 28: 1243-1255. Weissberg, B.C., 1970. Solubility of gold in hydrothermal alkaline sulfide solutions. Econ. Geol., 65: 551-556. White, D.E., 1968. Environments of generation of base-metal ore deposits. Econ. Geol., 63: 301 335.
SOLUBILITY
OF GOLD IN HYDROTHERMAL
CHLORIDE
SOLUTIONS
R.W. HENLEY Department of Geology, University of Otago, Dunedin {New Zealand)
(Accepted for publication November 20, 1972)
ABSTRACT Henley, R.W., 1973. Solubility of gold in hydrothermal chloride solutions. Chem. Geol., 11 : 73- 87. The solubility of gold has been determined in chloride solutions in the temperature range 300500°C corresponding to the inferred range for the formation of "hypothermal" gold deposits. The solutions were buffered with respect to HC1 by a K-feldspar-muscovite-quartz assemblage, and to oxygen by the assemblage haematite-magnetite. Solubilities increased rapidly with temperature from about 10 p.p.m, at 300°C, to 500 and 1000 p.p.m, at 500°C at 1000 and 2000 bar, respectively. These results are discussed in terms of possible solution species in this high-temperature region where molecular behaviour predominates in the solution equilibria. It is suggested that gold and other metals may be transported to the site of ore-deposition in undersaturated high-temperature solutions. Ore deposition may take place at lower temperatures where ionic gold chloride or sulfide species dominate the chemistry of the ore solutions. INTRODUCTION Metallic gold occurs with base-metal sulfides in hydrothermal vein and replacementtype ore-bodies, which are found most commonly in Archaean greenschist terrains such as at Morro Velho, Brazil (Gait, 1962) and Yellowknife, Canada (Boyle, 1959) and in areas of T e r t i a r y - R e c e n t volcanic activity (e.g., in the circum-Pacific and Alpine belts). Solubility information is a prerequisite to the understanding of the genesis of these orebodies. In the Archaean group of deposits wallrock alteration assemblages, developed adjacent to the ore, are often indistinguishable from those produced by greenschist facies metamorphism, while in the Tertiary group of ores, propylitic wallrock assemblages are often developed which are similar in character to greenschist or a l b i t e - e p i d o t e - h o r n blende hornfels facies assemblages. This suggests that, although ore-deposition may have occurred at lower temperatures, the ore-transporting solutions at some stage in their evolution were at temperatures appropriate to the development of these facies. Thus, for a study of the chemistry o f gold-transport a broad P-T field is outlined coincident with that for the development o f the greenschist facies, with temperatures in the range 3 0 0 500°C and pressures up to 10000 bar (Turner, 1968). Current evidence (White, 1968) suggests that ore-bearing solutions are, in general, brines with total chloride concentrations up to 5 M. Similar but less concentrated
74
R.W. HENLEY
solutions are well-known in active geothermal areas (e.g., Taupo, New Zealand), but these have low sulphur contents, and it is uncertain whether their low sulphur concentration is also representative of that in ore-forming systems. In the Salton Sea geothermal system (White, 1968) the total molal metal content is well in excess of the sulphur molality suggesting that the metals are dissolved as chloride rather than sulfide complexes. Some authors (e.g., Barnes, 1967) suggest, however, that the sulphur content of ore-forming solutions was higher than that of the geothermal systems, so that total base metals equalled total sulphur content; metal-sulphur complexes could then account for ore transport. Above 300°C the self-ionization of water is about one-thousand times greater than at room temperature (Quist, 1970) but due to the breakdown of the liquid structure at the higher temperature, the dielectric constant is only equivalent to that of organic solvents at room temperature (Quist and Marshall, 1965). For this reason the ionization constants for strong electrolytes, such as sodium chloride, decrease rapidly with increasing temperature so that undissociated solute molecules become more and more abundant and "molecular" rather than ionic equilibria predominate in the hydrothermal solution. Because of this behaviour, for example, chloride occurs largely as the undissociated HCI, KCI, NaC1 monomeric or dimeric molecules while sulphur occurs as molecular H2S, SO2, or as undissociated species such as NariS. Solution species will therefore be quite different in their chemical nature and structure than the species which account for solubility at lower temperatures. In deference to the few studies carried out in this region, it would be unwise to apply the genetic term "ion-pair" to the high temperature species; the writer prefers at this stage to use the term "molecular complex" to describe the undissociated solution species, which may have little or no ionic behaviour. Weissberg (1970) and Seward (1972) have studied the solubility equilibria of gold up to 300°C in sulphur and sulphur-chloride solutions, and Seward (1972) concluded that complex ionic species such as [Au (HS)] o, [Au (HS)2]- or [Au2 (HS)2S]-2 (depending on pH) allow gold solubilities in hydrothermal solutions much higher than those computed for gold-chloride species by Helgeson and Garrels (1968). At higher temperatures, however, metal complexing with the very abundant and strongly-bonding chloride species may be favoured more than with sulphur species. This paper reports the results of an experimental study of the solubility of gold in alkali chloride solutions at temperatures in excess of 300°C and up to 2000 bar pressure. The dissolution reaction of a metal involves both oxidation and complex formation, and for gold a formal reaction may be written: Au + aHC1 + bO2 ~ ~Aucomplex] + c H 2 0
(1)
In this study an attempt was made to determine the stoichiometry of such a reaction and to determine the variation of the equilibrium constant, K, over a range of temperatures and pressures. In order to experimentally determine solubility equilibria it is convenient to buffer the activities of both oxygen and ligands by reaction with solid assemblages. Hemley (1959) has calibrated the useful HC1/KCI buffer assemblage quartz-
SOLUBILITYOF GOLD IN CHLORIDE SOLUTIONS
75
muscovite-K-feldspar while Eugster and Wones (1962) give data for the useful redox buffer, haematite-magnetite. Both these assemblages occur in the earth's crust and limit the activities of 02 and HC1, although the haematite-magnetite pair may represent an upper limit to the range of possible oxygen activities in ore-forming systems. The activities in these buffered systems may represent extremes in the ore-forming environment, but the use of other more realistic assemblages is prohibited by the addition of extra components, kinetics of the buffer reactions, and scarcity of well-calibrated assemblages in the temperature range considered here. The use of the buffer systems described above allows the determination of the equilibrium constant (K) for the dissolution reaction so that for other conditions gold solubilities can be calculated thus: log a(Au species) = log KTp + a lOg aHC1 + b log f o 2 - - c logfH~O
(2)
EXPERIMENTAL APPROACH The solubility of gold in buffered chloride solutions was measured directly by analysis of an extracted solution sample, using a technique especially developed for this study. The solid buffer assemblages and solution (about 0.5 g) are weighed into a 0.5 cm diameter gold capsule (5 cm long) together with an open-ended tube, 2 cm long, consisting of Teflon, silica glass or alumina, depending upon the temperature of the run. On temperature quenching at constant pressure, about 0.1 g of solution becomes trapped in the tube as the gold capsule collapses around it. The tube is then removed, wrapped in aluminium foil, weighed, carefully dried and reweighed to determine the mass of trapped solution (by evaporative weight loss). The runs were carried out in large stainless steel pressure vessels (O.D. = 5 cm, I.D. = 1.25 cm), so that two or three capsules could be run simultaneously. Temperatures were measured using an external thermocouple and, where possible, with an internal sheathed thermocouple in contact with the capsule. The quartz-muscovite-K-feldspar solid buffer assemblage was prepared by finegrinding (to < 200 mesh) natural materials (crystalline quartz, adularia and muscovite). In some runs synthetic assemblages were used, crystallized from gels (either of muscovite or K-feldspar composition of a 1 : 1 : 1 quartz-muscovite-K-feldspar composition) prepared by the method of Hamilton and Henderson (1968). Potassium chloride solutions were used (2.0, 1.0 and 0.5 M at N.T.P.), and in some cases a solution was used in which the mole ratio KCI: HC1 was initially approximately equal to that of the buffered solution. ANALYSIS The small solution volume and low solubilities limited total gold content and necessitated the use of neutron activation analysis for the determination of gold in the aluminium wrapped tube. The analyses were performed by the Joint Manchester-Liverpool Universities Research Reactor Analysis Service, using a Bendix Multi-channel X-ray
R.W. HENLEY
76 TABLE I Experimental results for gold solubility at 1 kbar A. 2 MKC1 Run No.
Temperature (° C)
Solubility (p.p.m.)
Run No.
Temperature ( o C)
Solubility (p.p.m.)
E 50 E51 17 20 39 41 63 64 65 61 68 69 66 67 81 84 85 86 87 88 89
305 305 310-325 305-308 313 313 507 507 507 485-509 509 509 495-504 495-504 500 513 513 513 513" 513" 513"
38 31 18 87 7 46 936 114 328 507 2113 484 196 68 65 263 133 592 2987 87 818
42 43 49 50 46 27 96 97 98 99 100 101 130 132 133 134 135 136 137 151 162 163 164 165
266-308 266-308 295-340 295-340 311-342 402 512 512 512 499 499 499 504* 517 489-508 502-506* 487 504 503 456* 400 H 504 H 508 H 450 H
67 18 23 19 21 61 298 348 83 14 30 3 133 414 60 159 68 18 26 67 42 17 38 20
P P S S S S
B. 1MKC1 Run No.
Temperature ( oC)
Solubility (p.p.m.)
14 15 18 25 28 62 70 156 157
295 295 310-325 430-454 400 485-500 485 - 5 0 0 493-499 H 556
2.9 6.9 60 22 63 260 2968 1778 15
S S S S S
TABLE I (continued) C. 0.5 M KCI Run No.
Temperature ( ° C)
Solubility (p.p.m.)
16 19 29 26 79 159 160 161
295 310-325 420-450 400 507 556 H 461H 508*
5 35 11 81 795 40 31 76
Notes: Duration of runs 1 0 - 2 0 days. Collection thimble was alumina except where indicated: P = PTFE (Teflon); S = silica glass. Temperature measurement: cold seal b o m b s with external thermocouple except where marked: * internal sheathed thermocouple; H = hot-seal bomb. The thermocouple temperature was measured by potentiometer to -+ 2 C. Mixture was synthetic quartz, muscovite and K-feldspar. Mixture of natural silicates was used for runs 96 et seq. Consecutive run No. at the same temperature indicate that these runs were held together in the same pressure vessel. 60C
/
1000
800
500
~" ,~i,iI
40C
i ii !i¸¸ i ¸¸ ?
i
i
ill~i!ii ~ i
E c~
~ ? i~?~ii~i !i¸
v
3OC r~
~5 6O0
2 kbar
03
~3
20(2
o v1
"1 r
40C
10£
0
_i ~ / ~ 300
"/I 400 500 Temperature*C
c
480
500 520 Tern perQt u re *C
Fig.1. Some experimental data for gold solubility in the range 3 0 0 - 5 0 0 ° C at 2000 bar total pressure in 2 M (at N.T.P.) potassium chloride solutions, buffered by the assemblages q u a r t z - m u s c o v i t e - K feldspar and haematite-magnetite. Fig.2. Experimental data for gold solubility in 2 M (at N.T.P.) potassium chloride solutions at 1000 bar total pressure at 4 8 0 - 5 2 0 ° C. Q u a r t z - m u s c o v i t e - K - f e l d s p a r and h a e m a t i t e - m a g n e t i t e buffer assemblages present. Errors in temperature measurement are indicated by the bar length.
78
R.W. HENLEY
spectrometer. Radiochemical separations were unnecessary. The standard'deviation of these analyses was very much less than the variance of the gold solubilities determined in these experiments. ERRORS Precipitation of solute during the quench is always a danger in solution studies, but in these experiments no evidence was seen for crystallization of gold on the vessel walls. In any case, provided no precipitated gold is washed into the tube, quench precipitation should have no effect upon the results. To test the efficacy of the method the solubility of sphalerite was determined using both weight loss on crystal fragments and tube analysis by X-ray fluorescence, for each run. The chloride solutions were buffered by the quartz-muscovite--K-feldspar assemblage and the results were in good agreement with similar solubility runs by Hemley et al. (1967), and the difference between the mean weight loss and X.R.F. results indicated a deviation from the mean of 16%. Neutron activation analysis may involve slightly greater variance, so that for the gold solubility runs, variance of 16-30% was expected. A further source of error may lie in the slow attainment of equilibrium by the buffer assemblages. Hemley (1959) indicates that the quartz-muscovite--K-feldspar assemblage attains equilibrium at 300-500°C, within two weeks, while the haematite-magnetite assemblage may be very slow to react below 500°C (Eugster and Wones, 1962). Run times of between 10 and 20 days were therefore employed. Henley (1971) showed that gold + HC1 reactions went to completion within a few hours in this temperature range. The reaction rates of the buffer assemblages were thus the major factor controlling the attainment of solubility equilibrium. RESULTS Results for gold solubility in buffered potassium chloride solutions are given in Tables I and II for the range 300-500°C, 1000-2000 bar total pressure. These are seen to be extremely variable within small temperature ranges and much of the time involved in this study was devoted to trying various experimental modifications in an attempt to improve the reproducibility of the results. In Fig.1 results at 2000 bar total pressure are seen to have an almost exponential trend with solubilities increasing rapidly near to 500°C. Fig.2 shows solubility results at 1000 bar in the narrow temperature range 480-520°C, and emphasizes the asymptotic form suggested in Fig.1. Small errors in temperature measurement and control on the very steep part of the curve would involve marked differences in the solubility measured at the apparent m e a n temperature.
SOLUBILITY OF GOLD IN CHLORIDE SOLUTIONS
79
TABLE II Results at 2 kbar for 2 M KC1 solutions Run No.
Temperature (° C)
Gold solubility (p.p.m.)
Means of collection
F 52 F 53 21 ] 22 f 24 77-~ 78J
295 293
18 18 17 21 112 886 1070
Teflon collection thimbles Teflon collection thimbles silica thimbles silica thimbles silica thimbles corundum thimbles corundum thimbles
302 420-450 507
DISCUSSION OF RESULTS In Fig.3 the variation of the molality o f the silicate-buffered species HC1 with temperature is seen to have the same form as the solubility curve obtained for gold in similarly buffered solutions. This parallelism suggests that HC1 may be an important species in the gold solubility equilibrium. Fugacities of oxygen buffered by h a e m a t i t e - m a g n e t i t e also increase rapidly with temperature and contribute to the steepness of the gold solubility curve particularly above 450°C. As shown above (eq.2) the solubility equilibrium can be written as a function of the activities o f the reaction species. For simple gold species such as Au C1, Au C13 etc., the term a is-always greater than 1, while b is < 1. The solubility 20
U o
×1o
//
~,
5
360
460 Tempe rat. u re °C
Fig.3. Variation of the molality of H + and HC1 in potassium chloride solutions buffered by the assemblage quartz-muscovite-K-feldspar (calculated from the data of Hemley, 1959).
80
R.W. HENLEY
contribution of the HC1 term is therefore considerably greater than that of 02. The decrease in the hydrogen ion molality for the buffered solution shown in Fig.3 indicates that this ionic species may take little, if any, part in the gold-solubility equilibrium. In Fig.4 the gold-solubility results are compared with solubility data for sphalerite in buffered chloride solutions (Hemley et al., 1967). These show similar and parallel trends, which are also, above 400°C, parallel to the molalities of HC1 in buffered solutions. Below 400°C ionic equilibria become more significant and account for the change of slope of the solubility curves. For comparison, the solubility of quartz at 1000-2000 bar is shown,
-2
0
E
o _J
5(~)0
460
360 T*C
Fig.4. Experimental solubility data for gold and sphalerite in quartz-muscovite-K-feldspar buffered solutions at 300,400 and 500 ° C, together with the molality of HC1 in chloride solutions (calculated from data of Hemley, 1959) and the solubility of quartz. Data sources: sphalerite - Hemley et al. (1967); quartz - Weill and Fyfe (1964); gold - this study.
since quartz solubility is scarcely affected by the presence of HC1 in the solvent (Anderson and Burnham, 1967). The poor reproducibility of the solubility results did not allow direct determination of the equilibrium constant or the stoichiometry of the reaction; however, the important contribution of the HC1 species was demonstrated and is further discussed below. Barnes and Ernst (1963) showed that at high temperatures (about 500°C) hydrothermal solutions tend toward the behaviour of ideal mixtures. This suggests that the solution species can be considered as molecular species which have minimum interaction with one another.
SOLUBILITY OF GOLD IN CHLORIDE SOLUTIONS
8l
Using this as a model system, equilibrium solubilities can be calculated for gold on the basis of known gaseous gold chlorides, Au C1, Au2 C12, Au C13, Au2 C16. High-temperature thermodynamic data (compiled in Henley, 1971) show that the dimeric species, Au2 C16, is likely to be the most stable in a high temperature gas phase, as was demonstrated experimentally by Landsberg and Hoatson (1970) in vapor pressure studies. Solubility reactions for each of these species can be written in the form of eq.1 and equilibrium constants calculated. Insertion then of data for a HCI~,fo~, fH20 into solubility functions like eq.2 allows the calculation of an equilibrium solubility for gold. Thus at 500°C, with data for the buffers used in the experiments, the solubility of gold in this model system is 10 -6 p.p.m, at 1 kbar rising to 10 -4's p.p.m, at 5 kbar for the species Au2CI6, while the contributions of the other gold species are insignificant. The writer is aware of the dangers of this approach; however, until quantitative data is available for solution chemistry in this very high-temperature field, no better method of calculation is feasible. The trend of the calculated solubility curves is also similar to those obtained experimentally and demonstrates that very large solubility variations are to be expected over small temperature ranges in the vicinity of 500°C. The experimental work demonstrated gold solubilities up to 1000 p.p.m, in the buffered system at 2 kbar 500°C, while the calculated solubility for the same conditions is less than 10 -4 p.p.m. Anderson and Burnham (1967) also noted high experimental gold solubilities up to 1000 p.p.m, in unbuffered HCI and KCI solutions at 600°C, 3 kbar and Burnham (from H.L. Barnes, personal communication, 1972) has also found solubilities up to 3000 p.p.m, in 0.58 M HCI in contact with "granite" at 600°C up to 4 kbar. The very large discrepancy between calculated and experimental solubilities for gold may offer some clue to the nature of the solution species. In the thermodynamic calculations above, no account was taken of solvation of the gold chloride solution species. Water is still a polar solvent at these high temperatures and pressures, so that solvated chloride species will certainly occur. Copeland et al. (1953) noted very high molar volumes for sodium chloride in dilute solutions, above the critical point, and they ascribed this observation to extensive hydration of the solution species, and Crerar and Anderson (1971) have studied the hydration of silica species in high-temperature solutions. In order to account for the difference between the observed and calculated solubilities two possibilities are apparent: (1) The assumption that the high-temperature species is based on the planar Au2 El6 molecule may not be valid. Thermodynamic data favours the assumption of auric species, but monomeric auric species, or perhaps higher order polymers may occur (Brewer, 1958). (2) Extensive hydration of the simple gold chloride species may involve sufficient solvation free energy to account for the higher experimental solubilities. The asymmetric bond polarity in gold chlorides favours weak co-ordinate bond formation between water molecules and the complexed gold, and similarly hydration of the complexed chloride atoms would occur, by hydrogen bonding. Furthermore, in this high-temperature region where ionic equilibria are insignificant, the availability of undissociated HC1 and alkali chloride molecules suggests that these too may take part in solvation equilibria. The gold
82
R.W. HENLEY
solubility equilibria considered in this temperature region take place in a complex mixed solvent consisting of reactive dipolar species, i.e., H20, HC1, KC1, (KCI)2 all of which can take part in solvation equilibria with the gold species. Marshall and Quist (1967) have stressed that such ion-ion-pair-solvent equilibria play a major role in solution chemistry, and must be taken into account in determining complete solubility equilibria constants. These authors suggest that ion-pair-solvent equilibria involve dipole-dipole interactions, but it would be unwise at this stage to speculate too much on the precise nature of the gold species-solvent bonding in the high temperature and pressure region under consideration. If gold chloride-HCl solvation does occur, then substitutional solvation equilibria will be involved in gold solubility, of the form: [Au2CI 6 (H~O)a (Hfl)b] + cH20 ~ [Au2C16 (H20)a+c(Hfl)b-c] + cHCI
Up to twelve possible "inner-sphere" co-ordination positions would be available to HC1 for direct coordination, but the actual solvation numbers of HC1 and H20 would be expected to be sensitive to both temperature and pressure. Further experimental work (Henley, 1971) in the system A u - H 2 0 - H C I - 0 2 has suggested that at 500°C, 1000 bar, the solvation number for HCI is six, while at 2000 bar this decreases to two. Similar solvation equilibria can be written for the dipolar KC1 and other species in oreforming solutions. SOLUBILITYOF GOLD IN ORE-FORMINGENVIRONMENTS The primary object of a study of this kind is to follow the solution behaviour of metal species in order to attempt to understand the physical and chemical controls which cause the deposition of the metal in an ore-body. The experimental work and its interpretation demonstrate that gold is readily soluble in hydrothermal solutions and that about 1000 p.p.m, of the metal may be transported as chloride species at 500°C, 2 kbar under the conditions of the haematite-magnetite buffer. In buffered systems at temperatures where the solution approaches an ideal mixing model, the presence of other components can have little effect upon the solubility of gold. The solubility could be decreased, however, if other species are present which also involve solvation by chloride ligands (HC1, KC1, NaC1... ), and this includes most of the oremetals. The ZnS solubility (Hemley et al., 1967) shown in Fig.4 demonstrates that [ZnCI2 (HCI)n ] complexes may occur, and similarly pyrite solubility may involve a complex such as [Fe2C16(HC1)n] in this temperature field. The variation in the type of solution complexes and their stoichiometry makes the draughting of solubility curves for gold over wide temperature ranges extremely difficult. In Fig.5 a generalized solubility curve is shown for chloride complexes only, calculated from an equilibrium constant obtained from the experimental data at 500°C, 2 kbar, assuming the presence of the species [Au2C16 (HC1)2 (H20)n] • Three fields are indicated, characterized by the dominance of ionic or molecular behaviour in the solutions.
SOLUBILITY OF GOLD IN CHLORIDE SOLUTIONS
83
3 Ionic -~-
2 7
1
+~
C Brines derived
____ -2 0
P~I kbo~/ /
Wo~roke~!r?"?"~.................. -e~tT°~'~'h-!~-~ C ~t°W_rm~J _b_"_'_~'?_
__1
--~ M o l e c u l o ~
.
.
.
.
J. . . . . . . . . . . . . . . . . . . . . . . . . . .
-,4
6
~6o
26o
36o
46o
56o
6&
TemperotureOC Fig.5. General solubility curve for gold in 3 M potassium chloride solution, The curve beiow 300°C is that of Helgeson and Garrels (1968) for solutions in contact with quartz and pyrite. The curve for T > 300°C, P = 1 kbar is calculated from solubility data (this study) for solutions where mHC 1 is buffered by the quartz-muscovite-K-feldspar assemblage. Oxygen fugacity is buffered at values intermediate between those on the quartz-fayalite-magnetite and haematite-magnetite buffers,
Helgeson and Garrels (1968) have computed the "sub-critical" solubility of gold for a 3 M sodium chloride solution in equilibrium with quartz, pyrite and gold. Their data showed that a u r o u s chloride complexes [Au C12] are most important up to about 300°C, although gold-sulphur species were not considered. Their chloride solubilities are shown in Fig.5, region A, where it is evident that falling temperature rapidly reduces the equilibrium solubility of gold. Seward (1972) has demonstrated that, in the temperature range below 300°C pHdependant thiocomplexes of gold exist in solution and that the resulting solubilities are considerably higher than those calculated by Helgeson and Garrels (1968) for chloride complexes. The HS- stability field may exist well above 300°C (Ellis and Giggenbach, (197 l)so that gold thiocomplexes (terminology used by other workers, e.g., Seward (1972)), may be stable in this region. The rapidly increasing oxygen fugacity buffered by natural assemblages may,however,oxidize sulphur in solution to sulphate species, less suitable for gold complexing. The temperature region B (Fig.5) is highlighted, where three important phenomena occur: (1) Redox transitions between aurous and auric chloride complexes favoured by the higher buffered fo2 in this region. (2) Transition between low temperature ionic thiocomplexes and high-temperature molecular chloride complexes. (3) General trend from ionic to molecular solution chemistry.
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R W. HENLEY
The general higher temperature solubility (region C) is shown for a gold chloride complex, e.g., [Au2 C16(HC1)2], at 2000 bar in a 3 M chloride solution with fo2, buffered by an assemblage between quartz-fayalite-magnetite and haematite-magnetite. The solution considered is buffered with respect to HC1 by a quartz-feldspar-muscovite assemblage, so that the solubility curve (C) corresponds to that for a hydrothermal solution passing through a granite (e.g., at the Mazoe gold mine, Rhodesia (Henley, 1971)). The solubility curve shows that gold may be rapidly deposited from a saturated solution due to falling temperature. Similar curves will apply to solutions in contact with other HCI or O2-buffer assemblages (e.g., chlorite-biotite, amphibole-chlorite, etc.). For the Tertiary-Recent gold deposits (of "epithermal" character) lower fluid pressures prevailed during ore-deposition. Low pressure (< 1 kbar) favours molecular species, so that the solubility curve may have a pronounced inflection in region B (Fig.5). For these deposits the higher acidity (as HC1) due to wallrock assemblages such as kaolinite-prophyllite, may allow higher gold solubilities than indicated in the diagram, but the very low unbuffered oxygen content of solutions at shallow depths in the crust, may reverse this trend. The inflection in the curve would allow two stages of ore-deposition. The first deposition occurs at about 450°C, and "late" gold precipitates below 300°C. Some of the early gold may be redissolved as cooler solutions pass, particularly if gold thiocomplexes can form. At higher pressures (> 1 kbar) ionic equilibria are important up to higher temperatures, so that the solubility curve has a less pronounced inflection, and this curve would apply to the Archaean group of deposits. Gold deposition in response to falling temperature occurs only below 300°C, so that only "late" gold would be observed in these ores. With falling temperature gold, pyrite, quartz, etc., will coprecipitate, so that for a quickly cooling solution, rapidly deposited fine-grained pyrite would correspond to high gold values as is the case at the Mazoe mine, Rhodesia (Henley, 1971). An intermediate colloidal phase may occur during rapid deposition, and the distribution of gold in quartz veins has often been discussed in terms of colloidal transport. Replacement relationships showed sometimes by gold (e.g., gold commonly replaces pyrrhotite) may be accounted for by the competition between gold and the displaced metal for HC1 ligands or complimentary redox reactions between the two metals if the solution is not effectively buffered. In the above discussion the possibility was mentioned that thiocomplexes allow higher gold solubilities in the subcritical ionic region A. Incorporation of this possibility into solubility schemes for gold must, however, be made with caution because, although chlorine-containing species are known to be abundant, it is by no means certain that the species of sulphur are equally so. The stoichiometry of the reported sulphur-metal complexes indicate that for a hydrothermal solution the total molality of metals must be equal to the total sulphur molality, so that this condition must be fulfilled before consideration of hydrothermal thio-complexes can be made. Furthermore in low sulphurhigh metal solutions, the metals must compete for the available sulphur ligands and this will result in substantial modification of the equilibrium solubilities of metals in this region. Seward (1972) further discusses this problem and has indicated that thiocomplexes
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can account for the gold content of present-day geothermal solutions. Tile transition region B, between dominantly ionic solutions and the higher temperature molecular solutions is stressed here because of the lack of quantitative understanding of solution behaviour within it and its obvious importance in ore-metal transport and deposition. It is important to obtain solubility information in this region (300-450°C) and to correlate this with a thermodynamic study, possibly combining an extension of Helgeson's (1964) methods and incorporating a correction for the transition to molecular behaviour. In the high-temperature region C (Fig.5) very high equilibrium solubilities are to be expected for most ore-metals, on account of the ideal mixing behaviour of the solution species, where species interaction is at a minimum and therefore does not limit solubilities. With high equilibrium solubilities, it is unlikely that hydrothermal solutions above 400°C are ever saturated with respect to the metals. If the hydrothermal solutions responsible for mineralization in the Morro Velho gold mine, Brazil (described by Gair, 1962), for example, were derived by metamorphic dehydration reactions in the P-T range of the greenschist-amphibolite facies transition, and all the trace gold in the country rock affected by these reactions was leached out, a solution with a maximum gold content of 0.1 p.p.m, would result. A similar estimate of the geological "solubility" was made by Helgeson and Garrels (1968). These estimates are well below the experimentally determined equilibrium solubilities of gold at temperatures above 350-400°C. This suggests that an efficient transport and concentration mechanism can be envisaged for gold to account for the formation of ore-bodies (where the metal content is commonly well in excess of 10 p.p.m.). Gold would be readily transported as chloride species in hightemperature solutions derived from a widely dispersed source (such as a metamorphic pile) to the ore-zone, in undersaturated solutions and then efficiently concentrated to form an ore-body by deposition as the solution cooled below 300°C. Particularly important in ore deposition are the relative rates of change of the solution parameters, fo2, fHC1 etc., in determining the paragenesis of the ore-minerals and their disposition through the ore-body. These aspects of ore-formation have been discussed by Helgeson (1970a, b) who provides a powerful model for base-metal ore-deposition. At this time, however, there is insufficient information available concerning the solubility of the other ore-components, let alone the kinetics of their deposition, to allow the development of a quantitative geochemical model for the formation of a gold deposit. ACKNOWLEDGEMENTS The experimental work reported in this paper was carried out at the Geology Department, University of Manchester, during the tenure of an N.E.R.C. Research Studentship. I am grateful to Professor W.S. Fyfe for suggesting and encouraging this field of research. Mr. B.L. Goodwin (Joint Universities Research Reactor, Risley) carried out the neutron activation analyses rapidly and efficiently. I am also grateful to Professor Fyfe, Professor D.S. Coombs, Dr. T.M. Seward and the many others at the Chemistry Division, D.S.I.R.,
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Wellington and the Chemistry Department, University of Otago, for critical reading and discussion o f the manuscript. The author accepts the entire responsibility for any deficiencies o f this paper.
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Quist, A.S. and Marshall, W.L., 1965. Estimation of the dielectric constant of water to 800 ° C. Z Phys. Chem., 69: 3165-3167. Seward, T.M., 1972. Thio-complexes of gold and the transport of gold in hydrothermal ore solutions. Geochim. Cosmochim. Acta (in press). Turner, F.J., 1968. Metamorphic Petrology. McGraw-Hill, New York, N.Y., 403 pp. Weill, D.F. and Fyfe, W.S., 1964. Solubility of quartz in water in the range 1000-4000 bars and 400 O 550 C. Geochim. Cosmochim. Acta, 28: 1243-1255. Weissberg, B.C., 1970. Solubility of gold in hydrothermal alkaline sulfide solutions. Econ. Geol., 65: 551-556. White, D.E., 1968. Environments of generation of base-metal ore deposits. Econ. Geol., 63: 301 335.