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Economic Geology; January 2005; v. 100; no. 1; p. 157-164; DOI: 10.2113/100.1.0157
© 2005 Society of Economic Geologists
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Scientific Communications

HYDROUS SULFIDE MELTING: EXPERIMENTAL EVIDENCE FOR THE SOLUBILITY OF H2O IN SULFIDE MELTS

Jeremy L. Wykes{dagger},*

Department of Earth and Marine Sciences, Australian National University, Canberra, ACT 0200, Australia

John A. Mavrogenes

Department of Earth and Marine Sciences, and Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

{dagger} Corresponding author: e-mail, jeremy.wykes{at}ems.anu.edu.au


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
The effect of H2O on sulfide melting temperatures has been investigated in the FeS-PbS-ZnS system at 1.5 GPa, revealing that the addition of H2O results in a 35°C drop in melting temperature from 900° to 865°C. In addition to the melting point depression, the solubility of H2O is confirmed by the presence of vesicles in the quenched melt. No oxide phases were present in any of the run products, ruling out oxidation as a cause of the melting point depression. Confirmation of the solubility of H2O in sulfide melts is consistent with the recent suggestion by Mungall and Brenan (2003) of a magmatic origin for halogen-rich alteration associated with magmatic sulfide ore deposits, as the hydrous component of the alteration may similarly originate in the fractionating sulfide melt. Anatectic sulfide melts could be expected to contain more H2O than magmatic sulfide melts, owing to the lack of a parental silicate melt that buffers the H2O content of magmatic sulfide melts. Fluids expelled from the cooling anatectic melts, such as that present during granulite facies metamorphism of sulfide deposits at Broken Hill, Australia, may have been responsible for associated retrograde hydrothermal alteration.


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
The compositions of magmatic and anatectic sulfide melts, particularly volatile contents, are difficult to determine from the resultant crystalline products. The original volatile content of a sulfide melt is obscured by the inability of major sulfide phases to incorporate anions other than sulfur and the extensive low-temperature textural reequilibration of sulfide minerals. The dissolution of volatiles in sulfide melts is a potentially important phenomenon, in terms of lowering the solidus of sulfide melts, changing their physical properties (density, viscosity), and the formation of metasomatic alteration halos following the exsolution of volatiles. The recent experimental study of Mungall and Brenan (2003) has demonstrated the solubility of halogens in sulfide melts and thus provides a plausible explanation for the coincidence of elevated halogen contents in the rocks surrounding many magmatic massive sulfide deposits. Kress (1997) investigated the solubility of O in Fe-S-O liquids and suggested that the presence of C and H may alter the surface tension of silicate liquid-sulfide liquid interfaces and the addition of H2O to a silicate melt containing immiscible sulfide liquid may enhance separation and coalescence of the sulfide liquid.

Anatectic sulfide melting, or partial melting of existing sulfide occurrences during high-temperature metamorphism, has been suggested on the basis of phase relationships (Brett and Kullerud, 1967; Mavrogenes et al., 2001; Frost et al., 2002) and field evidence (Lawrence, 1967; Hofmann, 1994; Hofmann and Knill, 1996; Sparks and Mavrogenes, 2003; Tomkins and Mavrogenes, 2002, 2003; Tomkins et al., 2004). As a process, anatectic sulfide melting of a hydrothermally derived sulfide protolith containing low melting point chalcophile elements such as Sb and As is capable of producing sulfide liquids at significantly lower temperatures than those at which conventional magmatic Fe-Ni-Cu-S-(O) melts form. Typically, magmatic sulfide melts form through saturation of a parental silicate melt in an immiscible sulfide liquid. As such, the composition of the sulfide liquid is dictated by partitioning between the sulfide liquid and the much larger volume of silicate melt and the fO2 it imposes. As demonstrated by Mungall and Brenan (2003), the halogen contents of a sulfide melt are lower that that of the silicate melt with which it is in equilibrium. Anatectic sulfide melts may form through direct melting of sulfide minerals in metamorphic environments where fluids are common. Therefore, anatectic sulfide melts have the potential to contain significant volatiles and will be subject to the associated effects, such as a lowered solidus temperature and decreased density and/or viscosity. This paper reports experiments conducted to determine the effect of H2O on sulfide melting.

The limitations of experimental techniques have hampered experimental investigations into hydrous sulfide melts, rendering results contradictory and inconclusive. The investigation of Naldrett and Richardson (1967) into the effect of H2O on FeS-Fe3O4 at 2 kbars encountered many experimental difficulties, outlined in some detail by the authors. Loss of iron from the sample to the gold capsule and the unknown effect of gold and pressure on the melting point of FeS-Fe3O4 were some of the problems encountered. To overcome the effect of gold on melting, hydrous and anhydrous FeS-Fe3O4 assemblages were run concurrently. However, the H2O-free sample melted, whereas the H2O-bearing sample did not, which Naldrett and Richardson (1967) ascribed to imprecise temperature control in their internally heated pressure vessel. They concluded that H2O has little influence on the melting point of FeS-Fe3O4 and oxide-free Fe-bearing sulfide melts. Konnikov (1997) investigated the effect of H2O on the melting of pyrrhotite at 1 kbar and reported a maximum melting point depression of 80° to 90°C at 10 wt percent H2O. The results of Konnikov (1997) were reinterpreted by Mungall and Brenan (2003) as reflecting oxidation through H2 loss and subsequent melting on the FeS-Fe3O4 cotectic, an interpretation favored here.

Thus, poor characterization of the pressure dependence of sulfide (and sulfide-oxide) melting temperatures, interaction between sulfide melt and noble metal capsules, and H2 diffusion are all factors that have limited the ability of previous studies to investigate the fluxing effect of H2O. In addition, it is not possible to directly measure the H2O content of a quenched sulfide melt due to the inability of sulfides to quench to glass.

We have reinvestigated the solubility of H2O in sulfide melts and here report experiments designed to measure the effect of H2O on the temperature of melting of a FeS-PbS-ZnS assemblage at 1.5 GPa. A depression of the melting point and textural features are presented as evidence for the solubility of H2O. The temperature and composition of the ternary eutectic in the H2O-free FeS-PbS-ZnS system at 1.5 GPa was also determined. The pressure of 1.5 GPa was chosen for this study to maximize the melting point depression due to the addition of water, based on the assumption that the magnitude of such a melting point depression increases with pressure.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
Capsule method
Noble metals (Ag, Au, Pd, Pt) commonly employed as capsule materials are highly chalcophile and dissolve readily into a sulfide melt, making such capsules unsuitable for sulfide melting experiments. Large quantities of sulfide melt are also difficult to contain in other materials, such as graphite or boron nitride (BN). Thus, a novel capsule technique was devised for this investigation. Capsules were manufactured by cold pressing ~0.25 g of coarse galena (PbS) powder (natural galena from the West Fork mine, Viburnum Trend, Missouri) in a 4-mm-diameter pellet press, to produce a bucket-shaped capsule, and ~0.10 g to form a lid. The advantage of such a capsule is that it does not add an additional component to the system but acts as a ready supply of reagent. A backscattered electron image of a typical galena capsule is shown in Figure 1.


Figure 015801
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  		FIG. 1. Backscattered electron image of a polished section from a typical galena capsule experiment (JW02-34). The galena capsule has been loaded with a pellet of sphalerite (sp) and troilite (tr).

 
Polycrystalline, cold-pressed capsules are an unproven technique, and their ability to contain water was a prerequisite for their use in this investigation. To test this, a series of experiments involving a silicate starting composition of known H2O content were conducted. The H2O content of the resulting glass was then quantified using Fourier transform infrared (FTIR) spectroscopy, to evaluate the performance of galena and troilite capsules. The silicate composition was also run in a welded 2.3-mm (outside diam) Pt capsule for comparison.

Piston-cylinder technique
All experiments were conducted at 1.5 GPa, utilizing a 12.7-mm end-loaded piston cylinder apparatus located at the Research School of Earth Sciences, Australian National University. Galena capsules were placed in BN sleeves with BN disks at the top and bottom and were positioned inside the graphite heater with MgO pieces. An MgO sleeve and top disk were employed for the Pt capsule. A low-friction pyrex-NaCl-teflon foil pressure assembly was utilized. Temperature was monitored using a type B, Pt94Rh6-Pt70Rh30, thermocouple, housed in 2-bore mullite tubing. Temperature measurements were accurate to within ±10°C; precision was estimated to be less than ±5°C. No pressure correction was applied to the electromotive force output of the thermocouple. The piston-out technique was employed in all experiments, as cold pressurization was required to press the lid onto the capsule. Microscopic inspection at the completion of successful experiments confirmed annealing of the lid to the capsule. In a typical 1.5 GPa experiment, 1.0 GPa was applied to the sample before the furnace was connected, and the sample was heated at 150°C/min. The final 0.5 GPa was applied once the temperature reached 400°C. The sample was always overpressured by ~0.1 GPa (determined by experience) to account for settling of the assembly. Following the experiment, capsules were extracted from the assembly and mounted in epoxy before polishing and examination under reflected light. Epoxy mounts of silicate melting experiments were ground to 300- to 1,200-µm thickness and doubly polished. Thicknesses were measured by a micrometer and are accurate to ±10 µm.

Starting materials
Starting compositions for sulfide melting experiments were mixtures of FeS, ZnS (Aldrich 99.9%), and natural PbS. The galena was the same as that used to construct the capsules and contains very low amounts of Ag and Sb (confirmed by EMP (electron microprobe analysis) and LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry; Wykes, 2003) FeS was synthesized by heating stoichiometric proportions of high-purity S and Fe metal in an evacuated silica tube at 800°C overnight. Stoichiometry was confirmed via EMP analysis. This troilite also was used to construct capsules for the silicate melting experiments. Experimental mixtures were homogenized by grinding in an agate mortar and pestle. The compositions of the different starting mixtures used in this study are listed in Table 1. The terms binary and ternary refer to the number phases present in the particular mixture; e.g., a binary mixture contains only FeS and ZnS, whereas a ternary mixture contains FeS, ZnS, and PbS. However, experiments utilizing binary mixtures involve FeS, ZnS, and PbS, as galena (PbS) is provided by the capsule. Capsules were loaded with a 2.5-mm pressed pellet of starting material. Initial experiments were conducted using the FeS-ZnS mixtures, because at a temperature below the FeS-ZnS binary eutectic, but above the FeS-PbS-ZnS ternary eutectic, melt can only form where all three phases (galena, troilite and sphalerite) are in contact (e.g., at the contact between the FeS-ZnS pellet and the PbS capsule). This method was employed to aid in the identification of small degrees of melting, as melt is restricted to the edge of the FeS-PbS sample. Once the approximate temperature of the ternary eutectic was identified, an FeS-PbS-ZnS composition was used, which was based on the measured composition of quenched melt from earlier experiments. By proceeding in this manner it was hoped that the eutectic could be dramatically demonstrated by the entire sample melting over a finite temperature interval.


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  		TABLE 1 TABLE 1. Starting Compositions for Sulfide Experiments

 
A haplogranite mixture (listed in Table 2) was prepared from fired high-purity carbonates and oxides, to which Al(OH)3 was added, before grinding under acetone. The H2O content of the Al(OH)3 was determined by mass difference after heating an aliquot of Al(OH)3 at 900°C for 1 h.


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  		TABLE 2 TABLE 2. Haplogranite Composition

 
Electron microscopy
All samples were examined using backscattered electron (BSE) imaging with a JEOL JSM6400 scanning electron microscope (SEM) at the Electron Microscopy Unit, Research School of Biological Sciences, Australian National University. Quantitative analyses of mineral phases and quenched melts were obtained using the JSM6400 Oxford Link ISIS EDS (energy dispersive spectrometry) accessory. An accelerating voltage of 15 kV and a beam current of 1 nA were used and the counting time was 120 s. Mineral phases (pyrrhotite, galena, and sphalerite) were analyzed, using a focused 1-µm spot, whereas melt compositions were obtained from area scans, ranging from 10 to 500 µm across, to accommodate the heterogeneity of the sulfide melt quench intergrowth.

Infrared spectroscopy
The H2O contents of hydrous glasses were calculated from IR spectra collected using a Bruker IFS28 infrared spectrometer equipped with a Bruker A590 infrared microscope. The instrument comprises a Globar light source, KBr beamsplitter, and an MCT (HgCdTe) detector. Absorption spectra were collected between 600 and 5,500 cm–1 at a spectral resolution of 2 cm–1. Spot sizes ranged from 50 to 200 µm and 32 scans were accumulated for each spectrum. Background spectra were measured frequently. Between four and six spectra were recorded for each sample, and attempts were made to analyze only homogeneous and transparent portions of the glass. However, this was not always possible due to the presence of heterogeneities. Spectra showing obvious attenuation were discarded. Baseline correction of the spectra was performed by fitting a flexicurve baseline (Sowerby and Keppler, 1999) . Calculations of peak area by fitting different baselines and peak boundaries to a single spectrum showed the repeatability of the area determinations to be ±1.3 percent. The density (2,330 g • L–1) and integrated molar extinction coefficients (278 ± 28 L • mol–1 • cm–2 for the 4,500-cm–1 peak and 2,480 ± 10 L • mol–1 • cm–2 for the 5,200-cm–1 peak) determined by Withers and Behrens (1999) for a synthetic Ab37Or29Qz34 rhyolite were used for quantification of the H2O content.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
Silicate melt experiments
Table 3 contains run conditions, calculated H2O contents, sample thickness, and the standard deviation of the water contents. Uncertainty is associated with the choice of density and molar extinction coefficients, although these parameters are not listed in Table 3 as this study is concerned with the relative differences in H2O content between experiments rather than the absolute H2O content. Vesicle-free glass was present in all experiments, although transmitted and/or reflected light microscopy and SEM imaging revealed that all experiments contained sulfide inclusions and crystalline material, presumably unmelted starting material, although the degree of melting was estimated to be >80 percent in all experiments. All glasses contained quenched kyanite crystals.


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  		TABLE 3 TABLE 3. Fourier Transform Infrared Analyses of Hydrous Glasses from Silicate Melting Experiments

 
Experiment JW02-36 (PbS capsule) was the first silicate melt experiment performed, and the product had a significantly lower H2O content than that of the other PbS-encapsulated experiment (JW02-41). Experiment JW02-36 was loaded with an insufficient amount of oxide mix, which resulted in cracking of the capsule during pressurization, and the capsule presumably lost most of its H2O. Comparison between the H2O contents of the successful experiments in pyrrhotite and galena capsules with that of the Pt capsule reveals surprisingly good agreement. There is a maximum relative difference of only 12.5 percent between the H2O contents of the experiments in the sulfide and Pt capsules. These results demonstrate that sulfide capsules are capable of containing H2O to produce hydrous melts, approximating the efficiency of noble metal capsules. Additional experiments in the PbS capsule were performed in which free H2O was added to an Al(OH)3-bearing oxide mix. These runs produced vesicle-rich H2O-oversaturated glasses, demonstrating that the PbS capsule also can seal free H2O. Thus cold-pressed polycrystalline capsules are suitable for the investigation of hydrous sulfide melting.

Anhydrous experiments
The run conditions and results of H2O-free experiments are given in Table 4. The onset of melting in the FeS-PbS-ZnS system was observed between 895° and 900°C at 1.5 GPa, using an FeS-ZnS sample, and was conclusively demonstrated at 900°C, using an FeS-PbS-ZnS mix. Complete melting of the sample was observed between 900° and 905°C. BSE images of sample textures from runs over the temperature range 895° to 905°C are presented in Figure 2. Almost complete melting of the sample over a 10°C temperature interval is proof of a eutectic melting relationship. The 900° ± 10°C temperature of the FeS-PbS-ZnS eutectic at 1.5 GPa is in excellent agreement with the 60°C per GPa pressure dependence of the eutectic determined by Mavrogenes et al. (2001) .


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  		TABLE 4 TABLE 4. Run Conditions and Results of Anhydrous FeS-PbS-ZnS Melting Experiments

 

Figure 016102
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  		FIG. 2. Backscattered electron images from H2O-free melting experiments in the FeS-PbS-ZnS system. Abbreviations: gn = galena, sp = sphalerite, tr = troilite. A. Textures from experiment JW02-34 performed at 895°C, below the FeS-PbS-ZnS eutectic. B. Textures from experiment JW02-31 performed at 900°C. The onset of melting is indicated by the pockets of quenched melt formed between the troilite + sphalerite sample and the wall of the galena capsule. Small amounts of melt are present along tr-sp grain boundaries. C. Textures from experiment JW02-47 loaded with a tr-gn-sp sample pellet, as opposed to the tr-gn pellet of experiment JW02-31. This experiment dramatically demonstrated the presence of a small-temperature gradient within the capsule. Originally loaded with a sample geometry identical to that in Figure 1, the lower half of the sample, at temperatures high enough to melt, has migrated uptemperature toward the bottom of the capsule. D. Textures from experiment JW02-66, performed at 905°C. The 5°C increase in temperature results in almost complete melting of the sample.

 
Unusual textures were observed in experiment JW02-47 (Fig. 2C), in which the melt patch was separated from the unmelted portion of the sample by a band of galena containing small amounts of interstitial melt. This texture is interpreted to be the result of a small vertical temperature gradient within the capsule. The upper portion of the sample was below the eutectic temperature, whereas the lower portion was above the eutectic temperature. In response to this thermal gradient the melt migrated up temperature, dissolving galena on the hotter side of the melt patch (in this instance, the lower boundary) and precipitating galena on the cooler side. Small amounts of melt were trapped between galena grains during this process. Thermal migration of this kind has been observed in salt-hosted fluid inclusions (Roedder, 1984) and in experiments simulating sulfide migration in iron meteorites (Buchwald et al., 1985). Melts are typically very sensitive to small temperature gradients (Wark and Watson, 2002), and the magnitude of the temperature gradient is also likely to be small as its effects are only visible in experiments within 5°C of the eutectic. As all experiments utilized a sample assembly of the same dimensions, the temperature gradient is assumed to have been similar in all cases.

The composition of the eutectic melt obtained from EDS area scans is listed in Table 5, with the 1-bar eutectic from Mavrogenes et al. (2001) for comparison. Melt compositions are reported in terms of monosulfides as the sulfur content of the melt from experiment JW02-47 was 50.03 mol percent (1{sigma}: 0.37) and similar for all other experiments. Increasing pressure from 1 bar to 15 kbars reduced the ZnS content of the melt by ~50 percent and increased the PbS content.


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  		TABLE 5 TABLE 5. Melt Compositions from Hydrous and Anhydrous Melting Experiments

 
Hydrous experiments
The results and experimental conditions of the H2O-bearing experiments are listed in Table 6. Large amounts of H2O were added to allow for the expected migration of an unknown amount of H2O into the capsule walls over the course of the experiment. Figure 3A- C presents BSE images of textures from experiment JW02-56. The addition of H2O produced a 35°C depression in the eutectic temperature and resulted in the development of vesicles, typically rimmed by galena, in samples containing quenched melt. Fluid pores were present in the sample and capsule walls of all experiments, melt-bearing or otherwise. Despite extensive searching, using BSE imaging and X-ray mapping, no oxide phases were identified in any experiment. Isolated occurrences of small, (<5-µm) round blebs of a Pb-rich phase (presumably a quenched Pb melt) were identified within the quenched intergrowth of the sulfide melt in experiment JW02-56 (Fig. 3D).


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  		TABLE 6 TABLE 6. Run Conditions and Results of Hydrous FeS-PbS-ZnS Melting Experiments

 

Figure 016203
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  		FIG. 3. Backscattered electron images from H2O-bearing melting experiments in the FeS-PbS-ZnS system. A. and B. Experiment JW02-56 at 865°C. The addition of H2O depresses the melting temperature in the FeS-PbS-ZnS system and produces patches of quenched melt with large vesicles, partially rimmed with galena. Much of the troilite and sphalerite has been recrystallized to textures similar to those in Figure 2C. Intergranular fluid pores are present throughout the capsule, confirming that the experiments were fluid saturated. C. Detail from (B), showing typical quench textures and small blobs of Pb-rich material within the quenched melt. D. Experiment JW02-50 at 875°C. This experiment contained two distinct types of quench textures. In the lower right corner of the sample there is a pocket of regular quench, whereas the rest of the sample consists of a coarse skeletal intergrowth of galena, sphalerite, and troilite.

 
The presence of fluid pores is taken as evidence that H2O was present within the capsule for the duration of the experiment. In experiments in which melting occurred, the depression in the melting point relative to the anhydrous eutectic and the occurrence of vesicles in melt patches is interpreted as evidence for the solubility of H2O in the sulfide melt. As sulfide melts quench to a fine intergrowth of sulfide phases (troilite, sphalerite, and galena) that do not accommodate H2O, H2O is thus exsolved on quenching. The absence of oxide phases and the presence of very minor amounts (<0.01 vol %) of Pb melt suggest that insignificant S was lost from the melt to form H-O-S volatiles. In experiments in which melting was not observed, neither PbS nor ZnS was observed outside the sample area, even though fluid pores were well developed in the capsule walls. This suggests that fluid transport of PbS and ZnS was insignificant.

EDS analyses of quenched melt from experiment JW02-56 (S content 49.44 mol %, 1{sigma}: 0.44; Table 5) demonstrate that the addition of H2O did not significantly alter the relative proportions of FeS, PbS, and ZnS in the eutectic melt (identical within 2{sigma}).

The results of experiment JW02-53 (870°C, no melting) are not consistent with the results of experiments JW02-52 and JW02-56 (865°C, melting). Fluid-filled pores were present in the sample and capsule walls of experiment JW02-53, confirming the presence of a fluid phase. Given that melting was observed in two experiments at 865°C, the results of experiment JW02-53 are considered anomalous.

The addition of H2O appears to have changed the physical properties of the sulfide melts, such that the H2O-bearing melts were more mobile within the capsule, in some cases migrating through the capsule wall and ponding against the BN sleeve (experiment JW02-50). In experiments JW02-49 and JW02-50, separate domains of very coarse and fine quenched products, respectively, were present in the sample (Fig. 3D). Dissolved H2O may have altered the structure of the sulfide melt, resulting in the different quenching behavior.


   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
The results of this experimental study clearly demonstrate that the addition of H2O to a sulfide assemblage results in a melting point depression, which is interpreted as evidence for the solubility of H2O in the melt. These findings encourage further experimentation as well as a search for supporting evidence in natural systems.

There are reports in the literature supporting the solubility of H2O in sulfide melts from the Copper zone of the Fraser mine (formerly the Strathcona Deep Copper zone), Sudbury Igneous Complex, Canada, where highly fractionated sulfide magma was injected into the footwall country rocks, isolating it from the parent silicate magma (Li et al., 1992). Li et al. (1992), Farrow and Watkinson (1992), and Li and Naldrett (1993) described veinlets that splay off the Cu sulfide stringers. These veinlets are zoned from proximal sulfide rich to distal quartz rich and are surrounded by concentric halos of epidote and chlorite alteration. Li et al. (1992) suggested that the veinlets and surrounding alteration were formed by fluids exsolved from the cooling sulfide magma. Several workers (Jago et al., 1994; Molnar et al., 2001; McCormick et al., 2002; Hanley and Mungall, 2003; Mungall and Brenan, 2003) have identified elevated halogen concentrations and high Cl/Br and Cl/F ratios in alteration halos associated with magmatic sulfide deposits, including in the Fraser mine. Through experiments, Mungall and Brenan (2003) confirmed that the halogen contents of a sulfide melt would increase during fractionation, leading to the exsolution of a saline fluid. However, on the basis of the existing experimental studies of Naldrett and Richardson (1967) and Konnikov (1997), Mungall and Brenan (2003) concluded that the exsolved fluid would most likely be anhydrous. The results of the present study confirm the possibility that a hydrosaline fluid phase derived from a sulfide melt could be an end-member fluid responsible for high Cl/Br hydrous alteration associated with magmatic sulfide deposits. The controls on halogen partitioning between sulfide and silicate melts outlined by Mungall and Brenan (2003) are likely also applicable to H2O, and we suspect that, like Cl, H2O will partition preferentially into a silicate melt rather than a sulfide melt. Thus, significant H2O contents will only be observed in magmatic sulfide melts that have undergone fractionation isolated from the parent silicate melt.

The solubility of other nonchalcophile components (e.g., SiO2) in natural sulfide melts may be more extensive than currently recognized. The overwhelming majority of experiments on sulfide phase relationships have been conducted using evacuated silica tubes, and little evidence exists for the solubility of SiO2 in (slowly quenched, low-pressure) sulfide melts. However, Kalinowski (2002) reported the growth of euhedral Mn silicates in experiments on the melting Mn-bearing sulfides, and Mungall and Brenan (2003) observed an unidentified Fe-Si-Cl phase in the quenched products of Cl-bearing sulfide melts. In these cases we might speculate that the presence of small amounts of one nonchalcophile component (e.g., Mn or Cl) may result in increased solubility of another nonchalcophile component (e.g., SiO2), similar to the influence of Cl on sulfur solubility in silicate melts (Botcharnikov et al., 2004).

Field relationships at the Noril’sk-Talnakh deposits suggest that the sulfide melt was mobilized and emplaced in its current location after solidification of the parent silicate melt (Kunilov, 1994; Yakubchuk and Nikishin, 2004). The role of volatiles in this process is unknown, although the basal contact between the massive sulfides and the underlying Devonian evaporite-bearing sequences would have provided ample opportunity for incorporation of volatiles (SO4, Cl) into the sulfide melt. A full understanding of the effect of dissolved volatiles on solidus temperatures, density, and viscosity, and the effects of their subsequent exsolution on the observed relationships between sulfide and silicate rocks at Norils’k-Talnakh and other magmatic sulfide deposits awaits further experimental and field investigation.

Recently identified partial melting of sulfides at Broken Hill (NSW, Australia; Mavrogenes et al., 2001; Sparks and Mavrogenes, 2003) provides an explanation for features such as Ag-rich "droppers" (Sparks and Mavrogenes 2004) and the localized presence of Ag-rich galena in fold hinges (Plimer, 1986). We speculate that during prograde metamorphism the incipient sulfide melt at Broken Hill may have incorporated significant H2O due to the lack of a parental silicate melt to buffer the H2O content at a low level. Crystallization of anhydrous sulfide phases during retrograde cooling also would have increased the H2O content to saturation, possibly causing hydrothermal alteration of the host silicate rocks. Thus, we suggest that some of the retrograde alteration associated with the Broken Hill ore may have been the result of fluids expelled from a cooling sulfide melt. Unfortunately, the amount of H2O that can be dissolved in a sulfide melt remains unknown, so the importance of sulfide melts as a source for the hydrous component in alteration zones is still somewhat of an open question. Hopefully, the results of this study will provoke further research into the solubility of H2O in sulfide melts and the geologic significance of hydrous sulfide melts.


   Acknowledgments
 
This work comprises the bulk of JLW’s B.Sc.(Honours) thesis, which was partly supported by a Society of Economic Geologists student research grant. Frank Brink and Nick Ware are thanked for their assistance with SEM and EMP analyses. Andrew Berry is thanked for guidance using FTIR spectroscopy. The invaluable help and knowledge offered by Bill Hibberson and Dean Scott eased JLW’s introduction to experimental petrology. This paper was much improved following informal reviews by Bear McPhail, Steve Beresford, and benefited from comments by Economic Geology reviewers Jim Mungall, James Brenan, and Mark Hannington.

May 28, November 8, 2004


   Footnotes
 
* Current address: Department of Earth and Space Sciences, University of California at Los Angeles, Los Angeles, California 90095-1567. Back

May 28, 2004; November 8, 2004


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 

Botcharnikov, R.E., Behrens, H., Holtz, F., Koepke J., and Sato, H., 2004, Sulfur and chlorine solubility in Mt. Unzen rhyodacitic melt at 850°C and 200 MPa: Chemical Geology, v. 113, p. 207–225.

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