Reaction path modelling in the As–S system: a case study for geothermal As transport

Applied Geochemistry 18 (2003) 1325–1345 www.elsevier.com/locate/apgeochem

Reaction path modelling in the As–S system: a case study for geothermal As transport James S. Cleverleya,*,1, Liane G. Benninga, Bruce W. Mountainb a

School of Earth Sciences, University of Leeds, Leeds, LS2 9JT, UK Institute of Geological and Nuclear Sciences, Wairakei Research Centre, Private Bag 2000, Taupo, New Zealand

b

Abstract Geochemical speciation and reaction path modelling with the Geochemists Workbench (GWB) software was used to investigate zonal As sulphide mineral precipitation and As transport in an active geothermal field, the Uzon Caldera, Kamchatka. A new compilation and critical review of published experimental and theoretical thermodynamic data for As phases was used to modify and update a SUPCRT92 database with important missing phases. The equilibrium constants for these As phases were then added or modified in the current GWB database. Speciation calculations predict that the sampled fluids are undersaturated with respect to As phases and aqueous As is dominantly transported as 2
the complex H3AsO3(aq) and to a lesser extent the As sulphide complexes As2S3(aq), HAs2S
4 and As2S4 . Modelling the changes in concentration of dissolved As between the samples (0.2–8.6 mg/kg) indicates a strong dependence on redox (log O2(g) from
53 to
60) and temperature (95–65  C), and illustrates the importance of mixing between the hydrothermal fluid and an oxygenated fluid. Reaction path models that follow the cooling of a H2S(aq) dominated, As enriched (15 mg/kg) fluids from 125 to 25  C, with sliding redox from log O2(g)
55 to
60, predict the mineral paragenesis: (native As+pyrite)
(realgar+pyrite)
(orpiment+pyrite)
(pyrite). This mineral sequence closely resembles the natural layering observed in the Uzon Caldera. Although field measurements contain a lower reduced S concentration than the model, the error margin in the measured sulphide inherent from in situ oxidation during sampling is enough to account for the discrepancy. Despite assumptions in fluid parameters and modelling approaches, as well as deficiencies in the thermodynamic data and an equilibrium approach, this study has shown that acceptable and useful analogues for natural As-rich systems can be developed. # 2003 Elsevier Science Ltd. All rights reserved.

1. Introduction The deposition of metal sulphide phases in active ore-forming environments is strongly controlled by the metal speciation in the hydrothermal fluids as well as the solubility of the precipitating sulphides. Rapid changes in pH, temperature and redox potential, greatly affect the solubility and speciation and thus the character of an ore deposit. The active precipitation of As sulphide phases has been described from a variety of geothermal systems around the circum-pacific rim * Corresponding author. E-mail address: [email protected] (J. S. Cleverley). 1 Now at Economic Geology Research Unit, School of Earth Science, James Cook University, Townsville, Queensland, 4811 Australia.

(e.g., Kamchatka, Japan, Philippines, New Zealand, Argentina, USA; White et al., 1963; Karpov and Pavlov, 1982; Spycher and Reed, 1989; Welch et al., 2000; Seki, 2000). Detailed studies of As geochemistry included experimental data (Mironova and Zotov, 1980; Mironova et al., 1984, 1990; Webster, 1990; Pokrovski et al., 1996; Zakaznova-Iakovleva et al., 2000), compilations of thermodynamic data (Spycher and Reed, 1989; Zotov et al., 1994; Pokrovski et al., 1996; Shock et al., 1997; Nordstrom and Archer, 2002) and theoretical modelling (Spycher and Reed, 1989; Helz et al., 1995; Tossel, 1996, 1997) provide a basis for understanding the reactions in the As–S–O system. However, the distribution of the aqueous and solid As–S phases, their relative stability and stochiometry in relation to physical and chemical changes in the precipitating fluids are poorly understood.

0883-2927/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0883-2927(03)00054-4

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Here the results are presented of a speciation and reactive path modelling study, in which the chemical characteristics of 4 representative fluids sampled from the active As–Sb sulphide forming environment in the Eastern geothermal field (Uzon Caldera, Kamchatka) were used as input data. Difficulties in modelling the metal speciation and reaction paths arise from missing or poorly constrained thermodynamic data for many aqueous As species and minerals. Data for the S species are better constrained than the As species with published data (Shock et al., 1989, 1997) for many of the impor2
tant complexes (e.g. H2S(aq), SO2
4 , S2O3 ). In addition, the situation is complicated by the fact that in such a system, particularly at lower temperatures, kinetic effects will control the reactions. However, with certain assumptions (discussed below) the reaction path modelling shows that the precipitation patterns observed in the field can be reproduced with reasonable consistency.

2. Geology and fluid geochemistry The Kamchatka peninsula is located at the far-northeastern end of Russia and constitutes the most-NW segment of the western circumpacific arc. Kamchatka has a complex geological history with multiple cycles of subduction, terrane accretion and arc volcanism and it is currently one of the world’s most active volcanic zones (Kepezhinskas et al., 1997 and references therein). One of the most striking active geothermal areas in Kamchatka, the Uzon Caldera geothermal field, is located in the Eastern Volcanic zone, about 150 km NE of Petropavlovsk Kamchatsky, the capital of the peninsula. The Uzon Caldera was formed in the Pleistocene as an integral part of the larger Uzon-Geyser Valley geothermal system. The caldera is defined by a steeply dipping network of ring faults, and is filled with pumice and ignimbrite sequences that were subsequently reworked by a lacustrine river system. The geothermal activity is driven by the heat supply from shallow intrusions beneath the Uzon -Geyser Valley system. At depth, the hydrothermal fluids and the cold meteoric waters react with the wall rocks causing alteration and mineralization (Karpov and Pavlov, 1982; Alekhin et al., 1987; Naboko and Glavatskih, 1970), and at the surface these waters are responsible for areas of intense hydrothermal activity (hot pools, mud pots, etc.). During ascent these fluids change their chemistry and physical properties and the precipitation of a series of mineral zones is observed. The most prominent feature in the Uzon Caldera is the presence of the actively forming As– Sb–Hg sulphide rich mineralisation which is localised at depths between 0.05 and 1 m and which exhibits a zonal character, that is believed to correspond to sharp redox and temperature changes (e.g. Karpov and Pavlov, 1982; Benning and Mountain, 1996; Migdisov and Bychkov,

1998). From top to bottom, the following sulphide layers can be observed: S+amorphous As2S3 ) orpiment (As2S3) ) orpiment+realgar (AsS) ) realgar ) realgar+pyrite (FeS2) ) pyrite. Locally, uzonite (As4S5), alacranite (As8S9) and stibnite (Sb2S3) are found, and recently native As (As(s)) was observed within wall-rock precipitates (Gorringe, 2000). All these studies have concluded that these sharp changes in mineralogy and composition can in part be correlated to changes in temperature, pH and redox which in turn strongly affect As speciation and the As–S chemistry of the geothermal fluid from which As minerals precipitate. Mygdisov and Bychkov (1998) suggest that these redox changes are in part caused by increasing H2S(aq) concentration in the fluids as a result of condensation of H2S(g) from the boiling zone as the fluids cool. 2.1. Sampling methodology In a previous study, a 42 m traverse over the eastern Geothermal field (Benning and Mountain, 1996) was investigated with the goal of understanding the processes leading to the formation of the observed mineral zones. Solid and fluid samples were collected at regular intervals along this traverse. The solids were sampled in holes between 5 and 65 cm deep that provide a representative cross-section of the layering (Fig. 1). Once the solids were removed, the pH was measured in the solution entering the hole at the bottom. The temperature was determined at the various layer transitions and a fluid sample was collected from the fresh solution. The total reduced S was determined via an iodometric tritration method on site after filtering in a closed system through a 0.45 mm filter. Note however, that the fluids were already partly oxidised and thus the fluid that is in equilibrium with the solids before disturbance induced by the sampling will have a H2S(aq) content that is possibly orders of magnitudes higher than the values measured. For further chemical analyses, the fluids were immediately filtered and acidified. Total As was determined by atomic absorption spectroscopy and the total SO2
concentration was measured by ion chromato4 graphy. Chemical characteristics of 4 representative fluid samples (temperature, pH, total As, total reduced S and total SO2
4 ) were used as input data for speciation and reactive path modelling analyses in this study (Table 1). It has to be noted, however, that the fluid composition measured in each sample represents the average over the entire depth of the sampling hole and variations with depth (i.e., change in pH, temperature and redox) could not be taken into account. 2.2. Fluid chemistry The chemical analyses (Table 1) show that the fluids are NaCl brines containing reasonable concentrations of

J.S. Cleverley et al. / Applied Geochemistry 18 (2003) 1325–1345

1327

Fig. 1. Details of the layer distribution and mineralogy for the sample holes (UZ18, 15, 11 and 4) from Benning and Mountain (1996) described and used in this study. Relevant fluid temperature and pH are given for each column, along with the position on the 42 m SW–NE sampling profile.

K, Ca and SO2
4 , B, As, and Fe with As concentrations ranging between 0.2 and 8.6 ppm. The highest aqueous As concentration was found at the south-western end of the profile where the temperature at the surface reaches 98  C (Benning and Mountain, 1996).

3. Thermodynamic data 3.1. Modelling applications and databases Geochemical speciation and reaction path modelling was conducted using The Geochemists Workbench software package (version 3.2.1, GWB; Bethke, 2001). Three software components of GWB were implemented in this study: ACT2 and TACT were used for activity-activity and temperature-activity diagrams, and REACT was used for both fluid speciation and fluid-rock reaction path modelling. Additional thermodynamic data was added to or modified in the GWB database by generating equilibrium constants utilising SUPCRT92 (Johnson et al., 1992) and the slop98 database (GEOPIG, 1998 compilation of data from Shock et al., 1997). In order to conduct these modelling runs it is necessary to create an internally consistent database which

incorporates the best available experimental and theoretical data for aqueous As species and solid As minerals. Numerous data compilations incorporate a variety of parameters related to the thermodynamics of As species. However, as will be shown below it is not uncommon to find for the same species and the same parameter differences of up to several orders of magnitude. The aim of this modelling study was to compile a set of thermodynamic data for aqueous As and solid phases that could be used to simulate the distinctive As mineral zonation observed in field studies of the Uzon caldera (Benning and Mountain, 1996; Mydisov and Bychkov, 1998). 3.2. Modifying the GWB database The original GWB database (Lawrence Livermore National Laboratory release thermo.com.R6.V8, see Bethke, 2001) contained few As-sulphide species and minerals (HAsSo2, orpiment and realgar), and therefore, all thermodynamic data for aqueous As species and solid As sulphide phases available in the literature were compiled into a new database, Thermo2000 (Cleverley et al., 2001 and this study, Table 2). The data included in the new database were derived from published experimental log K values (Pokrovski et al., 1996, Zakaznova-Iaklovleva et

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J.S. Cleverley et al. / Applied Geochemistry 18 (2003) 1325–1345

Table 1 Chemical analysis of selected Uzon Caldera fluids used in this study (Benning pers. com.) *H2S(aq) is on site measured total reduced S in mol UZ4 Position Temp ( C) PH Al+3 As+3 B+3 Ca+2 Cu+2 Fe+2 K+ Mg+2 Mn+2 Na+ Si+4 F
Cl
Br
NO
3 SO2
4 H2Sa(aq) S2O2
3 I
PO2
4

UZ8

UZ15

UZ18

38.0 34.0 20.5 17.5 80 35 95 68 2.7 3.1 6.2 5.3 mg/l 0.12 0.33 0.05 0.07 8.6 0.17 2.0 0.88 38.7 92.5 65.1 66.7 13.9 47.1 38.1 38.6 0.002 0.009 0.008 0.009 0.089 0.2 0.11 0.09 70 139 85 88 2.64 4.88 4.4 1.84 0.11 0.28 0.182 0.132 546 1436 794 916 103 109 106 128 1.2 3.1 1.3 1.5 1143 3293 1433 1662 5 9.5 5.4 4.1 1.7 1.2 1.4 0.5 98.4 152 115 79.3 1.110
4 3.010
4 4.310
4 2.010
4 0.35 0.09 0.05 0.13 0.38 0.76 0.55 0.64 0.058 0.38 0.12 0.12

Reactions 1, 2 and 4 as published in Helz et al., (1995) and reaction 3 from Thermo2000 database in this study.

al., 2000) or from recalculated data (Zotov et al., 1994; Pokrovski et al., 1996; Shock et al., 1997). The published values for
Go,
Ho, So, Vo and the Maier–Kelly Cp fitting coefficients (minerals) or the HKF parameters (aqueous species) were added to an existing HKF-type database (SUPCRT92, Johnson et al., 1992; updated database Slop98.dat, GEOPIG, 1998), and the new results were cross checked with the original experimental data source where possible. Data tabulated in Zotov et al. (1994) did not contain values for enthalpy of formation for the aqueous species and therefore, these were calculated from the element data of Robie et al. (1979). All data were compiled and added to the SUPCRT92 slop98 database. In a second step, revised equilibrium constants for As species were generated for temperatures between 0.01 and 300  C and pressures on the water liquid–vapour curve (Table 2) and these values were incorporated into the revised GWB database. As phases added or modified in the SUPCRT92 data2
base include As2S3(aq), HAs2S
4 and As2S4 (from Zotov
et al., 1994), HAsO2(aq) and H2AsO4 (from Shock et al., 1997), and mineral data for claudetite (As2O3,mon), arsenolite (As2O3,cub), orpiment (As2S3) and realgar (AsS) (from Pokrovski et al., 1996). Equilibrium constants for

reactions between the modified As phases and the modified GWB As basis species (H2AsO
4 ) from 0.01 to 300  C were calculated with SUPCRT92 and added to Thermo2000. For As2S3(am) the experimental log K data of Eary (1992) at 25–90  C were fitted to a temperature dependent polynomial (log K vs 1/T) and after extrapolating to 100  C they were added directly to the Thermo2000 database. For H3AsO3(aq), the experimental data for the first dissociation constant (pK1) between 25 and 300  C were added directly to the database as log K (Zakaznova-Iaklovleva et al., 2000). 3.3. Critique of the thermodynamic data Equilibrium constants for secondary reactions (i.e. not involving the basis species H2AsO
4 ) between phases in the GWB database are calculated by addition or subtraction of the relevant basis reactions. The equilibrium constants can be composites of data from many data sources and types (e.g. extrapolated or interpolated versus direct experimental data). All data used in this study are tabulated as a function of temperature with the data sources in Table 2. The following section compares the modified data for As phases from GWB to published compilations or original experimental values. As–O: experimental data on orpiment solubility indicate that H3AsO3(aq) is the most stable complex between pH 0 and 8, temperatures up to 300  C and solutions with <1 molar As (Pokrovski et al., 1996; ZakaznovaIaklovleva et al., 2000) and this observation is corroborated by theoretical molecular orbital modelling by Tossell (1997) who showed that the trimeric H3AsO3(aq) is the dominant As species in fluids undersaturated with respect to As-sulphides. The experimentally determined dissociation of H3AsO3(aq) (Pokrovski et al., 1996; Zakaznova-Iaklovleva et al., 2000) and the derived equilibrium constants (Pokrovski et al., 1996, Spycher and Reed, 1989) are plotted in Fig. 2. While at low temperature the data are in reasonable agreement, at high temperatures discrepancies of up to > 2 log units are observed. A polynomial fit (stars and dashed line in Fig. 2) to the experimental spectrophotometric data of Zakaznova-Iaklovleva et al., (2000) was used to generate equilibrium constants for input into the modified Thermo2000 database. As a demonstration of the compounding effects resulting from the differences in the data, the equilibrium constants for the hydrolysis of orpiment from three different data sources, SUPCRT92 (Pokrovski et al., 1996), Thermo2000 (mixed Pokrovski et al., 1996 and Zakaznova-Iaklovleva et al., 2000) and the compilation of Spycher and Reed (1989) are plotted in Fig. 3. Again the discrepancy is small within the temperature range of this study (  0.5 log units at 100  C), but more significant at high temperatures (up to 4.5 log units at 300  C). The exact nature of the difference between data

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J.S. Cleverley et al. / Applied Geochemistry 18 (2003) 1325–1345 Table 2 Equilibrium constants for As species added to or modified in the Thermo2000 database GWB log K grid (italics from S&R,89) ( C)

Reaction

25

60

Native arsenic+1.5H2O+0.375SO2
4 +0.375H+=H3AsO3(aq)+0.375HS


0.22 (0.14

0.91 0.54

1.40 0.98

1.75 1.52

1.93 2.05

2.01 2.59

Arsenolite(mono)+3H2O=2H3AsO3(aq)


1.56


1.24


1.24


1.53


2.00


2.60


3.40

96Pok

Arsenolite(cubic)+3H2O=2H3AsO3(aq)


1.29


0.89


0.77


0.80


1.21


1.68


2.34

96Pok

Claudetite+3H2O=2H3AsO3(aq)


1.23


0.87


0.80


1.00


1.38


1.92


2.67

96Pok

Orpiment+6H2O=3H +3HS +2H3AsO3(aq)


46.41 (
47.19


41.76
42.44


38.05
38.42


35.02
34.85


33.24
32.45


32.46
30.99


32.74
30.58)

96Pok

Realgar+0.5SO2
4 +10H2O=4H3AsO3(aq) +4.5HS
+3.5H+


64.90 (
64.59


57.82
57.43


52.19
51.20


47.59
45.47


44.87
41.37


43.60
38.56


43.86
39.09)

96Pok

As2S3(am)++ 6H2O=3H++3HS
+2H3AsO3(aq)


44.83


40.37


36.74


33.69


31.82


30.87


30.84

92Ey

As2S3(aq)+6H2O=3H++3HS
+2H3AsO3(aq)


40.92


36.52


33.03


30.23


28.63


27.00


28.40

94Zot

+
As2S2
4 +6H2O=2H +4HS +2H3AsO3(aq)


37.63


33.80


30.76


28.28


26.82


26.17


26.38

94Zot


45.90


41.26


37.59


34.65


32.98


32.34


32.80

94Zot

9.27 (9.24

8.74 8.79

8.25 8.58

7.77 8.62

7.43 8.88

7.21 9.30


13.31


12.24


11.23


10.21


9.40


8.77


8.27

131.33

115.49

100.55

85.29

72.72

62.00

52.45

6.99 (6.99

6.65 6.74

6.48 6.52

6.50 6.53

6.68 6.74

7.02 7.11


2.90


3.06


3.12


3.04


2.87


2.65


2.35

40.68

35.84

31.77

28.17

25.75

24.21

23.47

+

HAs2S
4+

+







6H2O=3H +4HS +2H3AsO3(aq)

+ H2AsO
3 +H =H3AsO3(aq)



H3AsO3(aq)+0.25SO2
4 =0.25HS +H2AsO4 +0.75H+

H2S(aq)+2O2(aq)=2H++SO2
4


+

HS +H

=H2S(aq)

O2(g)=O2(aq) + 4H2(g)+SO2
4 +2H =H2S(aq)+4H2O

from Pokrovski et al. (1996) and Zakaznova-Iaklovleva et al. (2000) is unknown, and a recent study of arsenenopyrite solubility (Pokrovski et al., 2002a) using the calculated H3AsO3(aq) value of Pokrovski et al. (1996) produces results that are consistent with measured values in natural fluids. Although the experimental determined values for H3AsO3(aq) of Zakaznova-Iaklovleva et al. (2000) are used in this study the compounding differences are typically small ( < 1 log unit) at temperatures < 120  C and their effect on this modelling study is minimal. However, it is worth pointing out that at higher temperatures the choice of data source might introduce extreme variations in any modelling attempt and this has to be considered carefully. Using the literature values for HAsO2(aq) and H3AsO3(aq) of Pokrovski et al. (1996) and ZakaznovaIaklovleva et al. (2000) leads to a configuration where HAsO2(aq) dominates over H3AsO3(aq) at temperatures > 50  C and pH < 9. This relationship is, however, inconsistent with experimental results of Zakaznova-Iaklovleva et al. (2000) who did not detect the presence of HAsO2(aq)

100

150

Refs.

200

250

300 1.97 3.20)

7.09 9.85)

92G/F

88S/H 00Z
I 00Z
I 88S/H

7.55 7.62)

in their UV–vis spectrophotometric measurements at any temperature. In addition, it is inconsistent with the Raman evidence of Pokrovski et al. (1996) who suggested that polymeric H3n
2mAsnO3n
m compounds may only be present in concentrated (As > 1 molal) fluids and at > 125  C, which is beyond the present range of interest. Therefore, in this study where As concentrations are low ( 10
4 mol) and temperature is limited to less than 125  C, in order to overcome this inconsistency, HAsO2(aq) was suppressed in all calculations. As–S: the As-sulphide species within Thermo2000 are all dimeric (As–As) complexes (Table 2). However, molecular orbital modelling of EXAFS data for saturated As2S3 systems, and recalculation of pre-existing solubility data (Helz et al., 1995) suggest that dimer As–S complexes might be less important and that either
monomeric (H2AsS
2 ) or trimeric (H2As3S6 ) complexes dominate depending on the aAs2S3 (Helz et al., 1995; Tossell, 1996). Helz et al. (1995) recalculated experimental orpiment solubility measurements of Mironova et al. (1990), and these were used to compare the changes in

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J.S. Cleverley et al. / Applied Geochemistry 18 (2003) 1325–1345

Fig. 2. Log K values for the dissociation of H3AsO3(aq) from a variety of data sources. Only data from Zakaznova-Iakovleva et al. (2000) are pure experimental values, all other values are calculated from base thermodynamic values (i.e. Go, Ho, HKF parameters etc.), previous experimental data (e.g. Pokrovski et al., 1996) or are compiled from various sources by previous authors. SUPCRT92— Data for H3AsO3(aq) calculated by Pokrovski et al. (1996) and H2AsO
3 from SUPCRT92 database (compiled in Shock and Helgeson, 1988). Spycher and Reed (1989)—compiled data for As species as reported in their Appendix. Pokrovski—H3AsO3(aq) data from Pokrovski et al. (1996) and H2AsO
3 data from Pokrovski et al. (2002b). GWB—the polynomial fit used to generate log K data in the GWB database where H3AsO3(aq) is added directly from the experimental data of Zakaznova-Iakovleva et al. (2000).

log K with different complex models for the solubility of orpiment. Reactions for which estimated or experimental log K for the first ionisation reaction for orpiment (at 25  C) with monomeric, dimeric or trimeric thioarsenite complexes were derived and are presented in Table 3 with their associated log K (25  C). These orpiment solubility data span a range of  1 log unit when using the different aqueous complexes. In the compilation of data used here (Thermo2000) the values of Pokrovski et al. (1996) were used for orpiment and those of Zotov et al. (1994) for HAs2S
4 with the reaction stoichiometry being directly comparable with the earlier data of Mironova et al. (1990). There is approximately 1 order of magnitude difference in the equilibrium constant at 25  C between the data used in this study (reaction 3, Table 3) and the refitted data of Mironova et al. (1990) by Heltz et al. (1995) (reaction 2, Table 3). Although there is a variation in the data between the different modelled As–S species of Heltz et al. (1995) and that used here, this small variability will not effect

the general results or conclusions of the models presented. The exact nature (stochiometry and structure) of As–S complex types is however, still broadly unknown. A comparison of the log Ks for the dissolution of both As2S3(am) and orpiment with respect to H3AsO3(aq), [the predominant aqueous As species reported by Eary (1992) and Zakaznova-Iaklovleva et al. (2000)], is plotted against 1/T in Fig. 4. Amorphous As2S3 is not stable at high temperatures (> 100  C), and it has been shown that while the kinetics of the As2S3(am) to crystalline orpiment transformation are slow at 75  C, they are very quick at 120  C ( 2 h; Benning, 2002; Benning and Cleverley, 2002). The solubility lines for both phases never cross, and over the temperature range 25–100  C As2S3(am) is more soluble by 1–1.5 log units relative to orpiment. However, in natural settings at temperatures below 100  C, amorphous-As2S3 is observed in (metastable) equilibrium with orpiment, realgar and other As and Sb sulphide minerals. Therefore, in this study in

J.S. Cleverley et al. / Applied Geochemistry 18 (2003) 1325–1345

1331

Fig. 3. Comparison of the log K of reaction for orpiment solubility with respect to H3AsO3(aq) for different datasets. GWB (Thermo2000) is used during modelling and contains data for orpiment from Pokrovski et al. (1996) and data for H3AsO3(aq) from Zakaznova-Iakovleva et al. (2000); the Spycher and Reed (1989) compilation in their appendix, and updated SUPCRT92 database where orpiment and H3AsO3(aq) data are from Pokrovski et al. (1996). See text for further explanation.

order to exemplify the stability field for As2S3(am), the activity diagram calculations (Figs. 7,8 and 14) were carried out by suppressing the precipitation of orpiment. S: the first ionisation constants for H2S(aq) as tabulated in the original GWB database (compiled by Shock et al., 1989 from data of Barbero et al., 1982 and Wagman et al., 1982) deviate at high temperatures substantially from the new spectrophotometric measurements by Suleimenov and Seward (1997). However, at the temperatures of this study (25–125  C) the effect is small (e.g.  0.01 log units at 100  C) and can therefore be neglected. There are a complex array of other possible S species (e.g. H2S2(aq), S2O2
and So(aq)) available in 3 geothermal fluids (i.e., see Migdisov and Bychkov, 1998), but in the calculations presented here they were neglected because no thermodynamic data were available. Results for S speciation in the sampled fluids (Table 4) are calculated to be dominated by SO2
4 and H2S(aq), compared to the estimates of Mygdisov and Bychkov (1998) that indicate that components such as H2S2(aq) and S2O2
3 may account for up to 50% S(aq). Note also that the total H2S(aq) measured in these fluids is an absolute minimum (Section 2.1).

4. Modelling results and discussion 4.1. Estimating the redox state of the fluids The redox state of the system was estimated from the equilibrium between measured SO2
4 and total sulphide via the reactions, þ H2 SðaqÞ þ 2O2ðgÞ () SO2
4 þ 2H

ð1Þ

þ H2 SðaqÞ þ 4H2 O () SO2
4 þ 2H þ 4H2ðgÞ

ð2Þ

+ where the concentration of Sreduced, SO2
4 and H (pH) were measured (Tables 2 and 4). In all fluids, H2S(aq) (  10
4 mol) was assumed to be the most dominant reduced S species at the measured pH (2.7 to 6.2). The measured value for SO2
4 (76 to 115 mg/kg) may include partially oxidised reduced S species (Morse et al., 1987) but in small quantities. Therefore it is assumed that the total S shown in Table 4 represents the maximum limit. It follows that the calculated redox potential as log f O2(g) (range:
59 to
53) also represents an upper limit. The

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J.S. Cleverley et al. / Applied Geochemistry 18 (2003) 1325–1345

Table 3 Equilibrium constants at 25  C for solubility of orpiment with respect to different As–S complexes Reaction

Log K25

Refs.

+ (1) 0.5As2S3(s)+1.5H2S(aq)=H2AsS
3 +H + (2) As2S3(s)+H2S(aq)=HAs2S
4 +H + (3) As2S3(s)+H2S(aq)=HAs2S
4 +H + (4) 1.5As2S3(s)+1.5H2S(aq)=H2As3S
6 +H


7.34
8.36
7.49
7.82

Helz et al., 1995 Mironova et al., 1990 refit by Helz et al., 1995 Thermo2000, see references earlier Helz et al., 1995

Reaction for HAsO2(aq) is for comparison with H3AsO3(aq) (see text). Refs: 92Ey—Eary (1992); 92G/F—Grenthe et al., (1992); 96Pok—Pokrovski et al. (1996); 88S/H—Shock and Helgeson (1988); 00Z-I—Zakaznova-Iakovleva et al. (2000). References for the As data sources explained fully in the text.

Table 4 Key fluid data from trench holes used in this study Sample

UZ18

UZ15

UZ11

UZ4

T ( C) (meas) pH (meas) TDS (mg/kg) Sreduced (mg/kg) (meas) SO2
4 (mg/kg) (meas) S (mg/kg) As (mg/kg) (meas) Log fO2(g) Log fH2(g) Calculated species distribution (molality) SO2
4 H2S(aq) HS
HSO
4

68 5.3 3058 6.4 79.4 32.9 0.88
58
6.2

95 6.2 2951 13.8 115.0 52.1 2.0
54
5.0

65 5.4 3023 9.0 76.9 34.6 0.24
59
6.1

80 2.7 2164 3.5 98.4 36.3 8.6
53
7.2

H3AsO3(aq) H2AsO
3 As2S3(aq) HAs2S
4 As2S2
4

7.910
4 1.710
4 1.110
5 7.410
7

1.110
3 2.510
4 1.610
4 3.010
7

4.410
4 2.710
4 2.010
5 5.110
7

6.510
4 8.110
5 1.510
8 3.710
4

6.610
6 – 1.710
6 8.410
7 2.110
8

1.910
5 1.810
7 4.310
7 2.310
6 1.210
6

1.310
6 – 4.810
7 4.510
7 –

1.210
4 – 3.510
6 – –

Data measured in the field or in the laboratory (meas); all other data were calculated using GWB and the composition of the analysed fluid. Note that S is recalculated as elemental S.

redox potential is also reported as log f H2(g) in Table 4 [see Eq. (2)]. The redox potential estimates (Table 4) broadly fall within the range of the potentiometric measurements reported by Migdisov and Bvychkov (1998), recalculated from Eh and shown in Fig. 5 as log f O2(g) from
60 to
42 (open squares). However, the calculated values from this study, which represent an average of the entire depth of the sample hole, are more reducing at higher temperatures than the values of Mydisov and Bychkov (1998) (Fig. 5). This discrepancy is in part related to the fact that the temperatures measured at the bottom of the sampling hole represent minimum while, pH, SO2
4 and reduced S are measured as an average over the whole water column. The measured total reduced S (as H2S(aq) in Table 4) is an absolute minimum value because of partial oxidation during sampling.

4.2. Fluid speciation The fluids were speciated using the REACT sub-program of GWB at the measured temperature, pH conditions and a redox state set by equilibrium between the measured SO2
4 /H2S(aq) (see Section 4.1). In all the samples the calculated ionic strength for the fluids ( < 0.045) is well within the limitations of the extended Debye–Hu¨ckle (B-dot) method (e.g. Helgeson et al., 1981). Aqueous As species with concentrations > 10
7 molal were noted, and the distribution of these species as a function of As in the 4 fluids are illustrated in Fig. 6. In all cases the H3AsO3(aq) complex (40–90% As(aq)) dominates over the aqueous sulphide complexes 2
(As2S3(aq), HAs2S
4 and As2S4 ). Although aqueous As sulphide complexes in all samples are dominated by

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Fig. 4. Trend of log K with 1/T for dissolution of orpiment and As2S3(am) with respect to H3AsO3(aq) from data in the GWB database (0–300  C). Reaction is given on the graph, where * is either As2S3(am) or orpiment, and is based on that proposed by Eary (1992). The data for As2S3(am) is experimental (open triangles; Eary, 1992) and is linearly extrapolated in the database using log K=
4.875(T
11000) + 4.417 (R2=0.99), where T is in Kelvin. The data for As2S3(am) is not extrapolated beyond 100  C for kinetic reasons (see text). The H3AsO3(aq) data is from Zakaznova-Iakovnova et al. (2000), and orpiment was recalculated from the original data to use this new H3AsO3(aq) value. The data indicates that As2S3(am) is always more soluble at Psat.

As2S3(aq) and HAs2S
4 , in UZ15 15% of the total As is predicted to be speciated as As2S2
4 . This is not surprising as UZ15 happens to be the fluid with the highest maximum temperature (95  C), and pH (6.2). 4.3. Phase diagrams for the As–S system Phase diagrams for the system As–S–H2O were calculated using the ACT2 sub-program of GWB and the Thermo2000 database. As previously discussed from the available thermodynamic data orpiment is always more stable relative to As2S3(am), However, in order to illustrate the field where amorphous As2S3 would be located a ‘metastable’ phase boundary is plotted onto the diagrams by suppressing orpiment (Figs. 7 and 8), which is more stable at the diagram conditions. Fig. 7 illustrates variable activities of S (as H2S(aq)) and As (as H3AsO3(aq)), at fixed redox (log fO2(g)=
58) and pH. Conditions were selected to be similar to those derived from speciating the fluid in UZ18 (Table 4). It can be seen that by tracing the boundary between the solid and aqueous phases with decreasing aH2S(aq) the As solubility decreases to a minimum plateaux at approximately log aH2S(aq)=
3.5 to
3, before

increasing again with further reduction in H2S(aq) activity. The range of sulphide activities calculated from the fluid samples, lie close to the minimum values measured (see Table 4). The phase boundary for As2S3(am) is about 1.5 log units above the orpiment minimum solubility and under these conditions As would have to be oversaturated by about 60 mg/kg with respect to orpiment in order to form the amorphous-As2S3 phase. The redox (log fH2(g)) – pH relationships in the As–S system are illustrated in Fig. 8. The diagram was calculated for total S activity of 10
3.5, which lies within the range of values measured. However, the aH3AsO3(aq) (10
3.5) is significantly increased to enable the calculation of boundaries for the solid As–S phases that were undersaturated in the natural fluids (see Section 4). At 70  C the calculated concentration of H3AsO3(aq) is approximately equivalent to 60 mg/kg As. Again, the As2S3(am) boundary is metastable relative to other As–S phases. The boundaries have all been drawn for the different S sub-species, which are shown as dashed lines. At lower activities of H3AsO3(aq) the size of the orpiment-realgar stability field shrinks until species such as 2
predominate in the As2S3(aq), HAs2S
4 and As2S4
H2S(aq)/HS field. The plotted phase boundaries for

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J.S. Cleverley et al. / Applied Geochemistry 18 (2003) 1325–1345

Fig. 5. Distribution of calculated log fO2(g) versus temperature for Uzon Caldera fluids. The log fO2(g) values were calculated from measured total reduced S/SO2
4 ratios (data of Benning and Mountain, 1996) or recalculated from published Eh potentials (Mygdisov and Bychkov, 1998). Fluids used during the modelling in this study are labelled UZ11, UZ18, UZ4 and UZ15.

these species were calculated by suppressing orpiment and realgar (1 and 2 in Fig. 8), and are shown as fields delineated by dotted lines. It is also interesting to note that the solubility boundary for As2S3(am) and orpiment is almost coincidental in the SO
4 field, so that kinetic effects will have an important role in the precipitation order of orpiment versus As2S3(am) at these conditions. 4.4. Solubility of As In the geothermal fluids collected in the Uzon Caldera (60–100  C, reducing, S bearing) 4 dominant solid As phases are close to saturation: native As, realgar, orpiment and amorphous-As2S3. In the solid samples collected from the profiles alacranite and uzonite have also been found in small quantities. Experimental evidence indicates that alacranite only forms under very reducing conditions (Benning and Cleverley, 2002) and it is believed that such conditions exist in small micro-niches in the geothermal field. However, the lack of thermodynamic data prevents the incorporation of these phases in any modelling and thus alacranite and uzonite were neglected. The calculation of As solubilities was carried out for a fluid with chemistry similar to UZ18 but over a range of

temperatures (130–60  C) and for two different redox states (log fO2(g)=
58 and
55) to encompass the range of conditions known from the studied samples. The solubility of As was computed with respect to each phase (native As, orpiment and realgar) as a function of temperature at fixed fO2(g) and pH. At conditions far beyond the thermodynamic stability for crystalline As-sulphides ( <50  C) at the fixed redoxpH conditions the model did not converge in < 400 Newton-Raphson iterations and the calculation was abandoned. Fig. 9 is the result of the solubility modelling with respect to orpiment, realgar and native As in the UZ18 fluids at a range of temperatures. As discussed earlier (Section 3.1) As2S3(am) has a higher solubility than the crystalline As–S phases, but is metastable and thus is not plotted onto this diagram. At log fO2(g)=
58 (Fig. 9A), there is a gradual change from a native As to a realgar and to an orpiment dominated stability field and a generally increasing As solubility between 120 and 65  C. At log fO2(g)=
55 (Fig. 9B) the same general phase relationships exist but shifted to higher temperatures. The maximum calculated concentration of As in the natural fluids (using the REACT program), with respect

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Fig. 6. Calculated aqueous As species distribution in the 4 samples. The distribution is shown as%As(aq) (molal) for all species with concentrations >10
7 mol (legend is at the top of the graph). Relevant fluid temperature, pH, redox and measured As are given below each chart, along with the position on the 42 m SW–NE sampling profile. Details for the layering of the solid As–S phases is shown in Fig. 1.

to orpiment was 36.2 mg/kg (UZ4), 47.5 mg/kg (UZ15), 3.0 mg/kg (UZ11) and 4.7 mg/kg (UZ18). These values indicate that at the time of sampling the fluids were undersaturated with respect to As–S solid phases.

different problems: (1) to investigate the high As concentration in UZ4 fluid and its relationship to the much lower As concentrations in the other studied fluids, and (2) model the reaction paths that replicate the As–S mineral zonation observed in the sampling holes.

5. Reaction path models

5.1. Relationships to UZ4

Reaction path modelling was used to predict the effect of changing physical parameters (e.g. pH, redox and As concentration) in the fluid on the chemistry of both solid and aqueous As phases. Combinations of different models were attempted (heating, cooling and changing redox) and estimations of the physico-chemical parameters that affect the transport and precipitation were derived. Reaction path modelling was applied to two

UZ4 is anomalous amongst the other samples chosen in this study for both low pH (2.7), high temperature and higher than average As(aq) concentration (8.6 mg/ kg), and appears to be an acid-sulphate-type fluid, possibly derived from the mixing of shallow oxygenated waters or atmospheric O2 (see Fig. 5). In addition, at this site no distinct As-sulphide mineral zonation was observed (Fig. 1) and only a thin amorphous As2S3 and S rich layer

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J.S. Cleverley et al. / Applied Geochemistry 18 (2003) 1325–1345

Fig. 7. Diagram to illustrate the phase relationships in the system As–S–H2O for variable activities of S (as H2S(aq)) and As (as H3AsO3(aq)) and at a fixed pH (5.3), temperature (70  C) and redox (log fO2(g)=
58) approximating the composition of sample UZ18. Solid lines separate As minerals and aqueous species. The dotted line is the metastable dominance boundary for As2S3(am) calculated by suppressing orpiment.

was present. The estimated redox (log fO2(g)=
53) is also less reducing than other samples (see Table 4). A simple reaction path model that investigates if reduction (decreasing log fO2(g)) and cooling can predict the variation in As contents between the UZ4 fluid and other fluids in the study area was derived with the constraints chosen to overlap with the ranges observed in the field. In the model the redox state was changed from log fO2(g)=
53 to
60 (log fH2(g)=
3.8 to
7.3), temperature was varied from 80 to 60  C, pH was freely variable and the solid phases were allowed to precipitate, but remain part of the reacting system (closed system behaviour). The constraints were chosen to overlap with the ranges observed in the field. Fig. 10 shows the results of this model with the predicted As content of the solution and the amount of precipitated minerals. The actual As concentrations and the calculated log fO2(g) for the sampled fluids are shown as black circles. For this reaction path the predicted As matches closely total As concentrations found in all samples. The discrepancy in pH ( 2 log units) between the low temperature part of the reaction path (pH 3.4 at 60  C) and the actual fluids (pH 5.3, UZ18) does not appear to effect the predictions significantly. Orpiment and

pyrite are the stable phases, and orpiment controls As solubility under these conditions. This model appears to indicate that UZ4 is indeed subject to mixing with an oxygenated source (atmosphere or water), and that this is enough to suppress the formation of As sulphides and increase the solubility of As relative to the other sampled fluids. However, the fluid is only marginally undersaturated with respect to orpiment (under the calculated redox state), as a small shift to more negative log fO2(g) values initiates orpiment precipitation (Fig. 10). 5.2. Modelling the As-mineral zonation Reaction path modelling was also used to investigate the mineral zonation patterns observed with depth in the sample trenches (Benning and Mountain, 1996). The zonation, pyrite–realgar–orpiment–As2S3(am), broadly follows decreasing fluid temperature with decreasing depth. The transition between the different mineral zones is gradational and intermediate zones that contain mixed mineral assemblages (e.g. realgar + orpiment) were reported, however, these transitions occur over relatively small distances (cm-scale). When the fluid analysis from Benning and Mountain (1996) and Mygdisov and

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Fig. 8. Redox-pH phase relationships in the As–S–H2O system. The redox is given as log fH2(g) (left axis) and log fO2(g) (right axis). Solid lines represent the stability limits for As–S solids and dominant aqueous As species while dashed lines are the boundaries between predominant S species. The dotted lines are metastable boundaries (with orpiment and realgar suppressed) for As2S3(am), and
 (1) HAs2S
4 and (2) As2S4 . Note also that the field for As2S3(am) overlies the field for As2S3(aq). Conditions: T=70 C, log aH3AsOo3 =
3.5 and log aSO2
=
3.5. The activity of As is higher than in the measured fluids to enable solid As-sulphide phase 4 stability (e.g. orpiment and realgar) which are undersaturated in the sampled fluids.

Bychkov (1998) are recalculated to log fO2(g) it is clear that the cooler fluids have a lower log fO2(g) (Fig. 5), while measured SO2
4 /H2S(aq) is between 0.15 and 0.25 (Table 4). From the natural fluid data, the most primary fluid is UZ15 (i.e. hottest, highest pH and log fO2(g)) however, UZ15 has enriched SO2
4 /H2S(aq) relative to the other fluid analysis, and higher reported SO2
4 than previously reported (Mygdisov and Bychkov, 1998). Therefore, for modelling, the UZ18 fluid (Tables 1 and 4) was chosen as having typical As–S relationships, and reported SO2
4 and H2S(aq) within the range of values reported by Mygdisov and Bychkov (1998). The value of log fO2(g) is used to control the redox state of the fluid during modelling as a simpler alternative to changing or fixing activities of multiple S species, which are all in redox equilibrium. It was assumed that the measured aqueous As concentration in all fluids represents a minimum concentration related to the precipitation of As minerals with the fluids cooling during migration to the surface.

This warrants the assumption that prior to As–S phase precipitation, the As concentration was much higher, and the total As in the model fluids was increased to replicate this. The measured SO2
4 /H2S(aq), which was used to fix the redox state of the fluid, is too SO2
4 enriched to be able to precipitate As sulphides (e.g. SO2
4 /H2S(aq) is positive and log fO2(g) >
50 at 125  C). However, a change of an order of magnitude in the measured reduced S is enough to reverse this relationship and make SO2
4 /H2S(aq) negative, which is the desired starting condition. The measured total reduced S is  10
4 mol and this is a minimum value as it is dependent on the oxidation of the fluids prior to analysis (see sampling details). Therefore, one order of magnitude higher total reduced S is reasonable, as the oxidation of H2S/HS
is fast at these temperatures (Morse et al., 1987). If the UZ18 fluid is heated to 125  C (starting temperature for the cooling models) without redox and pH constraints, the SO2
4 /H2S(aq) stays close to constant, while the log fO2(g) changes from
58 to
48.

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Fig. 9. Saturation (or solubility) curves for native As, realgar and orpiment at pH=5.3, fluid chemistry of UZ18 over variable temperature (130–60  C) and at two different reduced redox conditions, (A) log fO2(g)=
58 and (B)
55.

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Fig. 10. Reaction path model for the UZ4 fluid showing the changes in As(aq) in solution and the precipitation of pyrite and orpiment. pH is a free constraint and changes from
2.7 to
3.4, temperature is varied from 80 to 60  C and log fO2(g) slides from
53 to
60 (see text for details).

By increasing the log fO2(g) to
55 during heating to 125  C the fluid becomes H2S(aq) dominant. A 3 stage approach was used in the model: firstly heating the UZ18 fluid to 125  C and sliding log fO2(g) to
55, followed by increasing the As concentration to 15 mg/kg. The third and main modelling step required cooling the fluid to 25  C with changing log fO2(g) (Figs. 11 and 12). The 15 mg/kg As starting concentration was higher than the maximum As concentration measured in UZ4 (8.6 mg/kg) but was warranted by the fact that in UZ 18 most As–S phases had precipitated prior to sampling. The cooling reaction path model was accomplished by linearly decreasing temperature (125– 25  C) while changing log fO2(g) from
55 to
60, which covers the range calculated for the natural fluid samples (Table 4 and Fig. 5). The results of the reaction path are illustrated in Figs. 11 and 12, with the concentration (mg/kg solution) of both solid phases and aqueous As complexes in equilibrium during cooling. Precipitated minerals were allowed to back-react with the fluid so that the continuous flushing of As-charged fluid was simulated, and the total concentration of As in the system at any temperature was maintained at 15 mg/kg. This model predicts that an ‘up-hole’ or cooling trend would be responsible for the paragenetic sequence (native As+pyrite)
(realgar+pyrite)
(orpiment+ pyr-

ite)
(pyrite) which is reproduces quite accurately the observed field sequence (Benning and Mountain, 1996). Native As has only locally been observed (Gorringe, 2000), yet its stability is predicted at high temperatures and reducing conditions. The precipitation curves in Fig. 11A show that realgar is stable over a very short temperature interval (115–95  C) while orpiment (95– 50  C) spans a larger temperature range. The transition from orpiment to realgar causes an increase ( 4 mg/kg) in As solubility around the temperature range 90–95  C, which is illustrated in the total aqueous As curve (Fig. 11B). The results for this reaction path model predict that with cooling As is transported as a variety of As-complexes. At (a) > 100  C As is transported as H3AsO3(aq), (b) from 100 to 75  C as As2S3(aq) > HAs2S
4 >
 H3AsO3(aq) > AsS2S2
4 , (c) 75–55 C as HAs2S4 , and (d) <55  C As transport is dominated by H3AsO3(aq) and solid phases are absent The pH remains close to constant ( 5) between 125 and 85  C, decreases rapidly ( <3) at temperatures between 85 and 60  C, then stays constant at low temperatures. Conversely Eh increases over the reaction path from
250 to 300 mV (Fig. 12A) so that although log fO2(g) decreases the redox potential increases. The system becomes SO2
dominated at temperatures 4 below 55–65  C for this reaction path (Fig. 12B), which

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Fig. 11. Reaction path modelling results for the cooling of As-rich fluids (15 mg/kg) while linearly changing log fO2(g) from
55 to
60. A) As solid phase and aqueous species distribution. Solid phases (labelled and in bold) are similar to the observed ‘up-hole’ sequence in field samples. Dominant aqueous species are (i) thin solid line is As–O species (1) H3AsO3(aq), (ii) Thin dashed lines are
As–S species with (2) As2S3(aq), (3) HAs2S
4 and (4) As2S4 . (B) Change in the concentration (mg/kg) of total As and S in the fluid during the same reaction path. The pH is allowed to vary freely in the model.

J.S. Cleverley et al. / Applied Geochemistry 18 (2003) 1325–1345

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Fig. 12. Eh–pH and S species relationships from reaction path cooling model. (A) pH (right hand axis and dotted line) and Eh (mV, left hand axis and solid line) plotted against temperature. (B) Distribution of S species as a function of temperature.

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Fig. 13. Mineral precipitated during the second reaction path model with 30 mg/kg As and orpiment suppressed to allow the precipitation of As2S3(am). All other parameters remain close or equal to those in Figs. 11 and 12.

coincides approximately with the limit of As sulphide precipitation. Suppressing the precipitation of orpiment in the reaction path did not cause As2S3(am) precipitation until the concentration of As in the fluid was increased to 30 mg/kg (double the amount in the initial model). Fig. 13 is the predicted mineral precipitation sequence for this second model. The As2S3(am) has a much smaller stability range than orpiment under these conditions which increases realgar stability to lower temperatures ( 75  C). A number of permutations and reaction paths were tested by varying the end point log fO2(g) value from
30 to
65, and a model with fixed SO2
4 /H2S(aq) (i.e. no addition of O2(g)) where log fO2(g) is
70 at 25  C. In all of these models As sulphides precipitate in the order native As-realgar-orpiment, however the point at which orpiment is not stable shifts to lower temperatures as the end point log fO2(g) value decreases. At log fO2(g) <
65 orpiment is stable at 25  C, while at log fO2(g) of
30 orpiment is only stable above 103  C. 5.3. Discussion The temperature relationships for the As–S–H2O system are calculated for variable aH2S(aq)/aSO2
4 (Fig. 14), with the aH2S(aq)/aSO2
ratio used to calculate the 4

redox state of the sampled fluids. The diagram is plotted for equal activities of H2S(aq) and H3AsO3(aq), and pH 6. Decreasing the activity of H3AsO3(aq) will shrink the stability fields of all the solid phases (orpiment, realgar and native As). The relationships in Fig. 14 show that in order to precipitate the As sulphide minerals in the observed order with cooling, the fluid has to shift from H2S(aq) dominant to SO2
4 dominant. The natural fluid data in this study has an almost constant aH2S(aq)/SO2
4 (Table 2), which is why calculated log fO2(g) decreases with decreasing temperature [i.e. equilibrium of Eq. (1)]. Alternatively Mygdisov and Bychkov (1998) suggest that the ratio of H2S(aq)/SO2
4 increases with cooling because of condensation of H2S from a gas phase. However, both these observations are incompatible with the relationships in Fig. 14, given the observed mineral zonation, and the principal reaction path modelling results that require a starting fluid that was H2S(aq) dominant. The different reaction paths with variable log fO(g) end values can be illustrated on Fig. 14 as lines starting at a constant point but having different slopes, and as such crossing the mineral boundaries at different positions. Note however, that specifically at lower temperatures, kinetic effects will play an important role yet these effects are not incorporated in any of the thermodynamic models presented here.

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Fig. 14. Relationships between log aH2S(aq)/aSO2
and temperature for the As–S system. Log aH3AsO3(aq)=
3.5, log 4 aH2S(aq)=
3.5 and pH=6. The redox of the system is a direct function of the H2S(aq)/SO2
4 ratio and is reducing to the right and more oxidising to the left. The approximate reaction path (large hatched circles and thick arrow) is for the cooling of As-enriched fluid with log fO2(g) changing from
55 to
60 described in this study. The hatched circles represent the starting and end-points of the reaction path with the associated log fO2(g) value. Mineral abbreviations: rg – realgar; orp – orpiment.

6. Conclusions The distribution and relative importance of aqueous As species and the precipitation paths for As sulphide minerals as observed in an active geothermal As–Sb precipitating field (Uzon Caldera, Kamchatka), have been simulated using the chemical information derived from a series of fluid samples. In order to carry out this study a new compilation of the best available thermodynamic data for existing aqueous and solid As phases was created (Thermo2000) because in many of the existing thermodynamic databases, crucial As phases were missing. The results of the speciation and solubility calculations show that the fluid samples are undersaturated with respect to solid As phases, and that the As complex H3AsOo3 dominates relative to the As sulphide com2
plexes, As2S3(aq), HAs2S
4 and As2S4 . Variation in the total aqueous As measured in each sample is related to variation in the measured temperature and calculated redox (Table 4).

Reaction path modelling of Uzon geothermal fluid has shown that cooling of As-charged (15 mg/kg As), H2S(aq) dominant fluids from 125 to 25  C with sliding log fO2(g) to
60 and variable SO2
4 /H2S(aq) predicts a bottom to top mineral sequence of (native As+pyrite)– (realgar+pyrite)–(orpiment+pyrite)–(pyrite) (Fig. 11A). The model predicts that at temperatures above 115  C all As is fixed as native As, while between 115 and 50  C As sulphide complexes are dominant. At temperatures below 50  C, under these conditions, 15 mg/ kg As is completely soluble as H3AsOo3 , compatible with both experimental and theoretical observations. As2S(am) was only seen in models where orpiment is suppressed and the concentration of As is increased to 30 mg/kg. The lower limit of As sulphide stability occurs at higher temperatures where the model fluids contain more O2 at 25  C (i.e. higher log fO2(g)), and this indicates that the depth of the mineral zones are dependent on the proportion of mixing of an oxygenated source.

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J.S. Cleverley et al. / Applied Geochemistry 18 (2003) 1325–1345

Although molecular theory predicts that some important As and S species are missing from this model (Section 3), changes in the log K for important reactions are small and probably within the error of this type of calculation. Co-existing orpiment and As2S3(am) observed in the natural system, and missing in this model, is related to inconsistencies in thermodynamic data and equilibrium modelling. However, despite these assumptions and difficulties the predicted mineral sequence closely matches the observed natural mineral paragenesis. The order of As mineral zonation observed in the Uzon Caldera can only be replicated by modelling if the measured aSO2
4 /aH2S(aq) relationships in the fluid that actively precipitates these phases is initially H2S(aq) dominated. This relationship can be achieved by only a moderate increase in the measured sulphide content (  1 order of magnitude) which might be expected from sampling errors associated with in situ oxidation. While highlighting deficiencies in data, and problems with an equilibrium approach, this study illustrates the usefulness of basic geochemical modelling to the understanding of As mineral paragenesis and speciation in natural geothermal systems.

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Acknowledgements The field study of the Uzon Caldera (LGB, BWM) was financed by the Kommission fur Reisenstipenden der Scweizerischen Akademic der Naturwissenschaften. We thank D.K. Nordstrom and N.F. Spycher for their comprehensive and thoughtful reviews that greatly benefited this manuscript. References Alekhin, Y.V., Dadze, T.P., Zotov, A.V., Karpov, G.A., Mironova, G.D., Sorokin, V.I., 1987. Condition of modern Hg–Sb–As ore-formation of caldera Uzon (Kamchatka). Vulkanologiya i Seysmologya, Moscow 2, 34. Barbero, J.A., McCurdy, K.G., Tremaine, P.R., 1982. Apparent molal heat capacities and volumes of aqueous hydrogen sulfide and sodium hydrogen sulfide near 25  C: The temperature dependence of H2O ionization. Can. J. Chem. 60, 1872–1880. Benning, L.G., 2002. Arsenic sulphides: nucleation and growth from aqueous solution. In: Geochemistry of the Earth’s Surface, Hawaii, pp. 294–298. Benning, L.G., Cleverley, J.S., 2002. Arsenic sulphides: nucleation and growth from aqueous solution. GAC–MAC Abstracts with Programs, 27. Saskatoon, 27–29 May.

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