0016-7037/88/$3.00 +.00
Geochimtca et Cosmochmnca 4L'ta Vol 52. pp 813-820
Copynght © 1988 Pergamon Press pic Pnnted 10 USA
The solubility of sphalerite and galena in 1-5 m NaCI solutions to 300°C
T. J. BARRETT 1 and G. M. ANDERSONz
'Mineral Exploration Research Institute, Department of Geological Sciences, McGill University, Montreal, Quebec, Canada H3A 2A7
zDepartment of Geology, University of Toronto, Toronto, Ontario, Canada M5S lAI
(Received October 21. 1986; accepted in revised form January 13, 1988)
Abstract-Sphalerite and galena solubilities have been experimentally determined under HzS-saturated conditions
over the 3-5 molal (=m) NaCi range and for temperatures up to 95°C. Both ZnS and PbS are about 5 times more
soluble in 5 m than in 3 m NaCi brines. ZnS is more soluble than PbS by factors of 30-100 over the experimental
conditions investigated. Some representative molal solubilities at pH = 2, based on the best-fit solubility isotherms,
are: 3 m NaC!, 80°C: [Zn] = 10-325 , [Pb] = 1O~5 10; 4 m NaC!, 80°C: [Zn] = 1O~2 86, [Pb] = 10-4 .75; 5 m NaC!, 80°C:
[Zn] = IO- Z 55, [Pb] = 10-4 46 • An increase in temperature of only 15°C produces a solubility increase comparable to
or greater than that produced by the 3 to 5 m NaCI increase: 5 m NaC!, 95°C: [Zn] = 10- 1 93, [Pb] = 10-3 71• The
difference in the solubilities of ZnS and PbS decreases with increasing temperature.
Using recent literature values for the stability constants of the chloride complexes of Zn and Pb up to 300°C, we
have calculated ZnS and PbS solubilities over the 25-300°C and 1-5 m NaCI range. ZnS is consistently more soluble
than PbS over this range of conditions, with the difference in molal solubilities ranging from at least two log units at
25°C to at least one log unit at 300°C. For temperatures below 100°C, the resultscan be compared with the experimental
ZnS and PbS solubilities obtained under HzS-saturated conditions. Agreement is very good at 60° to 95°C and all
NaCI molalities; at 25°C the calculated solubilities are low by up to one log unit.
Field data from various geothermal systems have been used to calculate equilibrium solubilities of sphalerite and
galena in these systems. High-salinity brines appear to range from supersaturated (Salton Sea, high-temperature) to
strongly supersaturated (Red Sea, low temperature) with respect to these sulphides. By contrast, high-temperature
seawater-salinity solutions at sediment-free spreading axes are grossly undersaturated in sphalerite and galena. The
latter situation is of interest in that massive sulphide deposits are nevertheless forming from such solutions. On the
other hand, vent fluids depositing sulphides at the sediment-covered axis in the Guaymas Basin appear to be near
saturation in sphalerite and galena. This is probably related to the higher pH of these fluids, and the higher metal
contents of the underlying sediments relative to basalts. Calculated solubilities for on-land geothermal systems (high
temperature, seawater to near-meteoric salinity) yield values in reasonable agreement «0.2 log units) in two of
three cases.
INTRODUCTION
NaCI solutions up to 300°C for geologically relevant conditions (undersaturated HzS contents and slightly acidic pH
values). Finally, the results are compared with chemical data
on solutions from natural systems such as geothermal reservoirs and deep-sea hydrothermal vents.
THIS PAPER PRESENTS new experimental data on sphalerite
and galena solubilities in HzS-saturated solutions at temperatures ranging from 25-95°C and NaCl concentrations of
3-5 m. As in our previous solubility study for 1-3 m NaCI
brines (BARRETTand ANDERSON, 1982), we use such singlesalt brines to simulate naturally occurring hot brines found
in a variety ofgeological environments. fluid inclusion studies on ore minerals and direct observations of geothermal
systems on land indicate that hydrothermal brines are responsible for the transport and deposition of base metals in
metal sulphide deposits ranging from syngenetic to stratabound Mississippi Valley-type to epigenetic. Geothermal and
oil field brines have salinities up to 6 molal in extreme cases
(WHITE, 1981), while fluid inclusions in sphalerite from Mississippi Valley-type deposits consistently indicate mCI values
of about 1-3 molal, with occasional values up to 4 molal
(HALL and FRIEDMAN, 1963; ROEDDER, 1967; RICKARD et
al., 1979; NORMAN et al.. 1985).
Using recent literature data on the stability constants of
the Pb and Zn chlorocomplexes at temperatures of up to
300°C, we present new calculated sphalerite and galena solubilities for two sets of solution conditions. First, the experimentally determined HzS values from BARRETT et at. (1988)
are used to calculate ZnS and PbS solubilities under very
acidic HzS-saturated conditions for comparison with our experimental ZnS and PbS solubilities in 3-5 m NaCl solutions
up to 95°C. Secondly, solubilities are calculated for 1':'5 m
EXPERIMENTAL CONDITIONS
Experimental runs
Sphaleriteand galena were equilibrated simultaneouslyin the same
solution flasks, and therefore under identical experimentalconditions,
at =25°, 60°, 80° and 95°C, and in 3, 4 and 5 m NaCI solutions.
The solutions were kept strongly acidic (pH < 2) in order to ensure
that metal chloride complexeswerethe only complexesof importance
(i.e. sulphide and hydroxide complexes unstable), and to yield sufficiently high metal concentrations for accurate analysis. All experimental runs were carried out in 2 litre multi-necked glassflasksfitted
with Glass-Col electrical heating mantles, and filled to two-thirds
capacity with the desired NaC! solution. Solutions were stirred constantly using a rotating magnetic stirring bar, even during sample
taking. Experimental runs typically lasted two days, although equilibrium was apparently achieved within hours, judging by the rate at
which the pH of a solution achieved a constant value following its
adjustment. In general, 10-20 solubility points were obtained at each
set of temperature-mNaCI conditions by varying the pH from one
run to the next; such a set of runs constitutes a "run series." Two
data points generally resulted from each run; these represent two
samples taken independently of each other (at the end of a run) and
usually at a fractionally different pH. The pH was varied within a
run series by the addition of a few ml of HCI or NaOH, resulting in
the respective dissolution or precipitation of ZnS and PbS. Within a
run series, data points produced well-defined isotherms, regardless
of whether equilibrium was approached from above or below (su813
814
T. J. Barrett and G. M. Anderson
persaturated and undersaturated starting solutions, respectively). This
indicates that equilibrium wasattainedduringthe experimental runs.
All data were obtained usingvery fine-grained ZnS and PbS previously produced by bubbling H2S through a solution containing
dissolved ZnCh and PbC1 2.Thesesolidsulphidephases, whichprecipitate as sludges almost immediately upon contact of the solutions
with the H2S, were collected by filtering, and dried and stored for
future use. As starting materials, they are advantageous relative to
natural mineralsin that the time required to achieve equilibration
in the solubility runs is much more rapid.
Run temperatures were maintained within ± I °C of the desired
temperature by thermistor proportional control thermoregulators
(except at "",25°C, where the runs were at room temperature). pH
readings weremade using a combination electrode coupled with an
automatictemperature control(ATC) sensor, and a digital pH meter.
Immediately priorto insertionin the experimental solution,the electrode was calibrated in two buffers of known pH (usually pHs of
1.679 and lOO) to ensurethat it wasresponding accurately overthat
range. A constant pH reading was usually obtained within a few
minutes. Upon return of the electrode to the buffers, pH values within
0.01 pH unitsof the buffers were generally recorded. Remeasurement
of the experimental solutionalsoyielded pH values within0.01 units
of the initiallymeasuredvalue.
Samplingand analysis
Allsamples weretaken using eithera 100ml glass pipetteto transfer
the experimental solution into a receptacle syringe, or directlywith
the syringe via a 10 em length of Tygon tubing. The transferred
solution, which had a volumeof"",10-20ml, wasimmediately filtered
through a 0.20 or 0.45 /.L filterinto a flask and weighed, then diluted
with distilled water until in the appropriate concentration range for
analysis. It is estimated that solutions cooled no more than a few
degrees relative to the run temperature during filtering; all samples
wereperfectly clear following the process. Samples wereanalysed as
dilutedaqueoussolutions using atomicabsorption spectrophotometry
on a Perkin-Elmer model 3400instrument. Concentrationsweredetermined using either calibration against normal standards, or the
method of standard additions (whichminimizes matrix effects). No
systematic differences in metal concentrationsof the unknownswere
observed between these two methods. The H2S concentrations of
metal-free brinesweredetermined in separate solubility experiments,
as reported in BARRETT et al. (1988).
EXPERIMENTAL RESULTS BELOW 100°C
The new data on ZnS and PbS solubilities are given in
Table I. Figure I expresses solubilities as a function of temperature for 3 and 5 m NaCI solutions. In Fig. 2, solubilities
are shown and as a function of NaCI content at two representative temperatures (60° and 95°C). Table 2 gives the
best-fit equations for all solubility isotherms, together with
statistical data which provide an indication of the precision
of the results. For acidic solutions under H 2S-saturation, the
slope of the solubility isotherms (m) can be shown theoretically to be -2. Thus the values of m in Table 2, which are
generally in the range -2.0 ± 0.1, indicate that the run series
data have consistently correct trends.
At 3 m NaCl, ZnS is more soluble than PbS by factors
ranging from ~2.3 log units at 25°C to ~ 1.8 log units at
95°C. At 5 m NaCl, the factors range from ~2.0 log units
at 25°C to ~ lAlog units at 95°C. These data also indicate
that the difference between ZnS and PbS solubilities decreases
fairly systematically with increasing temperature at a given
NaCI molality.
In order to compare with our earlier results for 1-3 m
NaCl solutions (BARRETI and ANDERSON, 1982), we repeated
the 3 m NaCI solubility runs. The data sets agree closely at
~25°C and 60°C, but the new results are slightly lower at
80°C and 95°C. We believe the new data to be more accurate,
given the improved statistics and the fact that most isotherm
slopes are very close to -2. This may reflect the greater number of individual runs per isotherm in the present study, together with slightly improved pH and metal concentration
measurements.
At higher salt contents of 4 and 5 m NaCl, both ZnS and
PbS solubilities continue to increase relative to 3 m solutions.
Table I.
Experimental galena and sphalerite solubilities in 3-5m NaCi solutions
at25°-95°C and H2S-saturation
EO
RH
25.2
25.2
252
25.2
24.7
247
24.8
24.8
248
24.8
24.8
248
247
24.7
25.0
250
25.0
1.18
1.18
0.82
0.82
0.71
0.71
1.11
1.11
0.97
0.97
0.60
0.60
0.53
0.53
0.46
0.46
101
61.5
61.5
610
61.0
61.0
610
615
61.5
61.0
61.0
615
610
610
61.0
61.5
615
610
61.0
610
610
81.0
81.0
81.0
810
81.0
102
1.02
0.89
089
083
083
090
0.90
1.03
103
113
148
083
083
098
098
1.09
109
1.31
1.31
I 11
111
102
0.95
0.95
reo
RH
25.9
25.9
25.7
25.7
23.5
24.5
245
24.5
24.6
23.5
23.5
27.0
25.6
615
61.5
60.5
60.5
605
60.5
60.4
60.4
60.4
60.4
603
60.3
60.3
60.3
604
60.4
60.1
601
61.4
614
80.2
80.2
1.03
103
0.96
0.96
0.78
0.96
0.91
0.91
1.21
0.97
0.97
180
1.15
2.20
220
1.27
127
1.20
1.20
1.03
1.03
0.98
0.98
1.15
1.15
1.44
1.44
109
1.09
1.29
1.29
1.44
1.44
1.44
144
3m NaCI
3m NaCl
I.&&m£!l I.&&mZn
-5.00
-4.95
-4.79
-4.88
-5.47
-5.57
-5.25
-5.25
-466
-4.65
-4.54
-4.50
-438
-4.33
-4.17
-415
-4.07
-4.19
-377
-385
-3.83
-394
-4.25
-419
-434
-3.68
-406
-424
-439
-444
-356
-354
·3.24
-308
-3.75
-3.73
-2.78
·2.77
-2.74
-2.76
-3.66
-3.67
-3.44
·3.44
-256
-2.55
-243
-2.43
-2.30
-2.29
-349
-2.31
-2.31
-2.07
-207
-1.96
-198
-2.04
.207
-2.21
-225
.2.47
.305
-1.66
.167
.2.08
.2.08
-254
-253
.2.87
·2.95
·1.67
·1.67
·1.51
·1.37
.136
RH
81.5
81.5
81.0
81.0
SO.O
80.0
80.0
SO.O
790
79.0
81.5
81.5
80.0
SOO
81.0
810
95.0
95.0
95.0
95.0
95.0
950
95.0
95.0
950
95.0
950
95.0
95.0
95.0
950
95.0
95.0
95.0
95.0
950
94.5
94.5
95.0
950
940
94.0
95.0
950
1.04
1.04
1.09
1.09
1.97
1.97
1.55
1.55
1.08
108
1.45
145
140
1.40
1.68
1.68
183
184
1.52
152
151
1.51
129
129
1.27
127
1.21
1.21
145
145
2.34
234
1.39
139
173
1.73
129
1.29
1.54
154
1.23
123
138
1.38
reo
RH
SOl
80.1
S02
SOl
80.1
BOO
SOO
80.1
SO.1
800
80.0
SOO
80.0
SO.l
80.1
SOl
80.1
96.2
961
96.3
96.3
95.7
95.7
956
95.6
96.0
96.0
93.7
93.7
93.9
93.9
949
94.9
93.5
93.5
1.50
1.50
1.34
134
1.28
1.28
1.18
1.18
1.18
1.18
1.11
1.11
116
1.16
1.04
1.04
098
0.98
1.33
1.33
1.50
150
1.85
1.85
1.74
1.74
1.67
167
1.54
1.54
154
1.54
1.49
1.49
1.38
1.38
4mNaCI
-416
-4.11
-4.08
-4.07
-371
-3.73
-3.57
-3.56
-4.01
-400
-4.53
-446
-3.81
-387
-410
-410
-457
-4.56
-3.67
-362
-3.29
-344
-3.36
-3.50
-445
-439
-3.26
-3.41
-4.09
-411
-400
-4.06
-4.13
-3.93
-3.34
-3.32
-3.30
-330
-301
-2.85
·3.10
-3.02
-2.62
-2.67
-3.34
-3.37
-4.88
-4.95
-3.26
·344
·3.77
·3.83
-3.21
-323
-333
·3.49
·2.97
-3.05
·2.94
-300
-146
-1.47
-153
-1.54
-3.48
-348
·2.79
-2.78
-1.47
-149
-214
-2.14
-2.30
-230
-2.65
·2.65
-2.34
-2.23
-1.56
-1.56
-160
·159
-1.23
-1.18
-134
-1.36
'{)97
-0.96
-1.69
-1.69
-3.36
-344
·157
·1.58
·2.09
-2.15
-147
-1.47
·1.76
·1.82
-1.34
-134
-138
-139
4mNaCI
wm£!l I.&&mZn
-5.01
-493
-4.97
-457
-4.81
-4.85
-471
-489
-479
-6.32
-5.18
wm£!l w!llZn
EO
·3.45
-331
·3.31
·3.31
·330
·3.71
·3.13
-312
-3.67
-433
-4.36
-2.45
-244
·238
-2.38
-205
-205
-1.93
-193
-229
-2.29
-2.87
-2.85
-2.17
-2.20
-2.42
-2.43
-2.88
-2.89
-2.03
-2.00
BO.2
wm£!l w mZn
·3.71
·373
-3.52
-348
·3.33
·3.30
·3.01
·3.03
·3.09
-3.06
-2.94
·2.94
·299
·3.08
-2.80
-2.78
-269
-2.71
-237
-2.39
-2.71
-2.72
-3.45
-3.44
-3.27
-3.24
-3.03
·301
-2.80
-2.82
-2.82
-2.84
-2.70
-2.69
-2.52
-251
-210
.2.12
-1.90
·1.90
-173
-173
·1.52
·1.51
·1.51
·1.51
·1.36
-136
-1.23
.1.23
-113
-1.13
.{).96
.{).95
-1.10
·109
-190
-193
-1.74
-174
-1.55
-153
-1.36
-136
-1.35
-1.35
-1.19
-1.20
-1.06
-1.05
815
Sphalerite and galena solubility in brine
Table I. (Continued)
5mNaCl
5mNaCl
IT'Q
Iili
IT'Q
l&&!!lfll l&&~
iili
-,
l&&!!lfll l&&~
e
....
~
N
E
259
259
262
262
260
26.0
258
258
257
260
260
257
260
604
604
605
60.5
604
60.4
606
606
615
61.5
605
60.5
598
598
051
051
076
0.76
0.98
0.98
061
061
065
088
088
0.82
051
-360
-359
-398
-399
163
163
1.24
1.24
112
1.12
108
1.08
137
137
100
1.00
128
128
-444
-446
-364
-365
-344
-343
-3.29
-329
-3.75
-370
-390
-4.27
-429
-419
-358
-314
-317
-3.62
-360
-172
-171
-227
-229
-2.71
-272
-2.01
-257
-258
-229
-276
-275
-1.94
-194
-176
-1.74
-152
-1.53
-196
-1.97
-142
-1.42
-191
-191
801
801
800
80.0
80 I
801
800
800
801
801
800
800
794
794
1.57
157
1.44
144
138
138
128
128
119
1 19
115
115
121
121
-354
-354
-323
-329
-3.05
-305
-285
-2.86
-271
-2.70
-2.60
-262
-275
-275
-192
-192
-1.63
-166
-1.48
-1.48
-129
-128
-115
-1.15
-1.05
-104
-121
-1.20
954
954
951
951
95.7
951
951
938
938
958
95.8
946
946
157
157
246
2.46
196
186
186
174
174
160
1.60
153
153
-250
-253
-445
-447
-324
-300
-298
-280
-281
-261
-2.63
-251
-249
-116
-114
-298
-298
-177
-153
-154
-133
-134
-115
-114
-101
-101
N
..
E
-a
-e
0
.J
.J
-.
-a
-.
-,
°
pH
PH
-,
-a
L09m Pb
9~'C
."..
."..
E
E
go
.
.J
.J
0
-.
-, +--..,.---,--JL.,Ll-----!
10
pH
The only exception to this trend is at "",25°C, where 3 and
4 m isotherms are almost identical. However, at these low
temperatures, fluctuations in equilibrium H 2S content are
significant if the temperature varies even I-2°C, as was commonly the case in these runs. Such fluctuations directly affect
the metal contents of the solutions, and may explain this
apparently aberrant solubility trend.
CALCULAnON METHODS
.0
"
FiG. 2. Experimental ZnS and PbS solubilities under H 2S-saturated
conditions as a function of NaCl concentration. Results at 60· and
95°C are shown. Slopes of best-fit isotherms in each plot are close
to -2 (Table 3a).
ified. The total lead or zinc content of a NaCl solution saturated with galena or sphalerite is given by:
S = K MS' Q> • (aw?/(mH2S • "YH2S' "YM2+),
General
4
Calculations of galena and sphalerite solubilities were carried out in essentially the same manner as in BARRETT and
ANDERSON (1982), although all parameters have been mod-
where
Q>
= I + L:(.B~·m&-).
MS refers to either PbS or ZnS, and M2+
or Zn 2 +.
1S
either Pb2+
Table 2 Regression coefficients and statisticsfor the best-fitsolubility
isotherms to the data 10Table 1 (log molality = a pH + b)
~
N
E
~
-,
Sphalerite
.
N
E -,
~
~
o
.J
.J
-,
-e
pH
pH
-,
-,
-a
IeQ
E
;
go
!l
1lIl·
12
·212
-127
-018
070
120
17
20
2t
27
0.97
092
097
096
010
0.14
0.09
008
3m
3m
3m
3m
25
60
95
-204
-209
-193
4m
4m
4m
4m
25
60
80
95
-202
-197
-197
-203
-129
0.00
0.80
128
9
20
18
18
082
1.00
099
0.96
035
0,03
004
009
5m
5m
5m
5m
25
60
-218
-200
-203
-208
-0.61
060
128
221
10
14
t4
13
099
095
099
099
009
o t3
005
008
80
80
95
Galena
-3
~
o
.J
.J
3m
3m
3m
3m
-e
pH
S!&l.mQ[
a
"...
"...
pH
FiG. I. Experimental ZnS and PbS solubilities under H 2S-saturated
conditions as a function of temperature. Results at 3 and 5 m NaCl
are shown.
(I)
1
4m
4m
4m
4m
5m
5m
5m
5m
IeQ
a
!l
1lIl·
12
S!&l.=
25
60
-1.73
-2.04
-2.05
-181
-359
-213
-118
-067
14
16
16
27
099
079
0.97
094
005
028
010
009
-172
-196
-209
-203
-322
-1.68
-0.62
032
11
18
20
18
098
097
098
099
0,07
0.09
006
004
-263
11
12
14
13
0.98
0.99
099
099
0.10
0.08
006
008
80
95
25
60
80
95
80
-185
-203
-2.18
95
-212
25
60
-i
n
-009
083
T. J. Barrett and G. M. Anderson
816
{3~
Table 3 Geochemical parameters used in calculations.
is defined as:
=
{3i
= {3MClz = ({3MClz • 'YM2+ ' 'YCl-Z)/'YMClz
{33
{3MCI+
=
'YMZ+'YCl- )/'YMCl+ = mMCl+!(mMz+' mCi-)
{3'1
({3MCI+'
Table 3a. Log K values for hydrolySIs of metal sulphides
T ('C)
= (3MCI)" = ((3MCI)", 'YM2+' 'YCl-3)/'YMCI,
= {3MCl~- =
({3Mcl-' 'YM2+ '
'YCl-')/'YMCI~-
·441
-790
-411
·6.95
·549
·367
·330
-296
·261
·2.31
Calculation ofactivitycoefficients
Calculated solubilities are, of course, quite sensitive to the
activity coefficients used for the solute species. At present,
there is in fact no way of calculating meaningful values of
the coefficients in concentrated brines at elevated temperatures. However, HELGESON (1969) and HELGESON et al.
(1981) have suggested procedures for estimating activity coefficients which mayor may not be applicable to trace quantities
of complex species in brines. We first utilized Eq. (298) in
HELGESON et al. (1981), using effective ionic radius (re,J) values for each complex. These were calculated from the entropy
correlations shown in Fig. 1 of HELGESON et al. (1981), and
given in more detail in HELGESON and KIRKHAM (1974).
The entropies of the complex species at 25°C were obtained
from the ~S values of SEWARD (1984) and RUAYA and SEWARD (1986); ionic entropies were from HELGESON et al.
(1981). The re, } values were used in the w-term of the Born
parameter of the extended Debye-Huckel equation, while d
was given a constant value of 3.72· 10- 8 , the value for NaCl
from HELGESON et al. (1981). This procedure gives seemingly
reasonable values for singly-charged ions but very small values
«0.0 I) for doubly-charged metal chloride ions. Because the
latter ions dominated the solubility calculations at higher
NaCl concentrations, the resulting solubilities were much too
high.
We therefore reverted to a modification of the B-dot
method (HELGESON, 1969), where the B-dot values (now
termed b..,) for NaCl were taken from Table 26 of HELGESON
et al. (1981) and d was made equal to re, l for each complex
species. True ionic strength and activity coefficient values are
listed in Tables 3b and 3c, respectively. For Pb 2+ and Zn 2+,
d values were taken from GARRELS and CHRIST (1965), al-
-436
·341
.258
-187
Table3b. True Ionic strengthvaluesat saturationvapourpressure
= mMCI~-!(mMz+. mb-)
where {3n is the overall stability constant for the formation of
the nth Pb or Zn chloride complex as given by SEWARD (1984)
and RUAYA and SEWARD (1986), respectively. The chloride
ion molality, mCi-, is equal to the "true ionic strength" of
the solution; it was calculated from Equation 180 of HELGESON et al. (1981) using KN aCI and l' ±,NaCI values calculated
from data therein. K MS is the equilibrium constant for the
hydrolysis of the metal sulphides MS, i.e. MS + 2H+ = M 2+
+ H 2S. Values calculated from BOWERS et al. (1985) are given
in Table 3a.
Use of recently published values for the stability constants
of the zinc chloride complexes (BoURCIER and BARNES, 1987)
leads to calculated ZnS solubilities which differ by less than
0.5 log units from solubilities based on RUAYA and SEWARD'S
(1986) values, although the relationship between the data
sets is complex (see Fig. 8 in BOUROER and BARNES, 1987).
logK(2)
log K (I)
25
50
100
150
200
250
300
= mMcJ3!(mMz+' mtl-)
{34
(2) PbS + 2H+ = Pb++ + HzS
(I) ZnS + 2W = Zn++ + HzS
10 NaCI
lIT)
Q.l
25
50
100
150
200
250
300
0.099
0.099
0099
0099
0.099
0.096
0091
1
~
~
.i
257
2.62
262
2.55
243
2.29
223
3.43
329
330
321
306
291
2.76
380
3.87
388
3.78
361
349
3.71
Z
0.95 1.81
096 184
0.96 184
0.95 180
092 1.72
086 161
079 1.51
Table 3c. Representative calculated activity coefficient (1) values to 300'C
T ('C)
Pb 2+, ImNICI
25
50
100
150
200
250
300
PbCI+, 3mN.C1
017
016
013
0.10
0.068
0.040
0017
ZnC142.,
060
0.61
056
046
0.35
0.24
013
Activity coefficients were calculated usmg.
5mNaCI
0.27
0.28
024
0.17
010
0.051
0056
-logy, =
-A-rZz JO 5
+ fy + byl
I +4By105
where Ay, By, and by are from Helgeson et a!. (1981),1 IS the true 100lc strength,
and
= -Iog( I + 0.018015310· where m" is the total molality of all
roy
species in solution.
The following ;\ values were used:
CI-
Znz+
ZnCI+
ZnCI"
ZnCl.z-
Pb 2•
18
60
1.8
25
6.4
45
PbCI+
23
PbCI3'
46
PbC42.
5.6
though this is an unimportant factor as metal solubilities in
brines are insensitive to the activity coefficients of Pb 2+
and Zn 2+.
Activity coefficients of neutral species in brines is another
difficult problem. Activity coefficients for H 2S for our range
of experimental conditions are available in BARRElT et al.
(1988); we used these also for PbCI2(oq> and ZnCh(..,). For temperatures up to 300°C, activity coefficients for H 2S were calculated from the model of BARTA and BRADLEY (1985). Activity coefficients for CO 2 or Si02(oq> could also be used for
these complexes and would make very little difference to the
results, but H 2S seems appropriate as it is a polar species, as
probably are PbCh(oq) and ZnCh(oq> (D. A. CRERAR, pers.
commun., 1987).
RESULTS AND DISCUSSION OF CALCULATIONS
H2S-saturated conditions up to 95°C
Calculated ZnS and PbS solubilities, expressed both in ppm
and metal molalities, are given in Table 4 for 1-5 m NaCl
solutions at temperatures of 25°-95°C under conditions of
H 2S-saturation. The effect of changes in these parameters on
the calculated ZnS and PbS solubilities can be assessed from
data in the tables. As evident from the general solubility Eqn,
(1), a decrease in pH of one unit causes an increase in metal
sulphide solubilities of two log units, whereas a decrease in
mS(rl of one log unit leads to an increase of one log unit in
solubilities.
Sphalerite and galena solubility in brine
817
Table 4 Calculatedgalena and sphalente solubrhuesunder H2S-saturation.
for 25-95'C and pH = 2
3m NoCI
(using measured H2Svalues from Barrett et al , this Issue)
~
flQS
~
~
Log ppm Ph
Log ppm Zn
Logm Pb
Log m Zn
..
E
-381
-172
-911
-651
-193
-040
-723
-519
-091
040
-621
-439
-008
108
-538
-371
-325
-140
-852
-616
-132
019
-659
-458
-031
101
-558
-376
050
168
-477
-308
5m NoCI
e
N
lmNaCl
LOQ m In
LOQ m Zn
c
-,
N
E
oJ
-.
-a
-.
2m..!'iaO
Log ppm Ph
Log ppm Zn
Log m Ph
Log m Zn
pH
Log m Pb
Log m Zn
Log ppm Ph
Log m Ph
Log m Zn
-282
-106
-806
-581
-085
069
-610
-406
014
150
-510
-3.25
094
217
-431
-258
..
.
.3
-.
-050
106
-573
-367
048
156
-475
-286
125
251
-398
-221
Log ppm Zn
Logm Pb
-213
-045
-734
-515
-023
135
-543
-335
075
215
-446
-255
150
278
-371
-193
Log m Zn
The calculated solubilities for ZnS under H 2S-saturation
agree well with the experimental results for temperatures of
60°,80° and 95°C. Selected comparisons are shown in Fig.
3 for temperatures of 25° and 95°C, and NaC! contents of
3 and 5 m. For ZnS, agreement is within ±0.25 log units,
except for the results at 80°C and 4 m NaC! (calculations
higher than the experimental values by almost 0.6 log units),
and at 25°C and 3 m (lower by OAlog units). Calculated PbS
solubilities agree with the measured values to within 0.3 log
units, except for the results at 25°C, which are ",,0.7 log units
lower for 3, 4 and 5 m NaC!. Overall, calculations match
experimental isotherms to within ±0.1O log units in 10 of
the 24 cases investigated (Table 2). The calculations also indicate that the relative difference in ZnS and PbS solubilities
(under the same solution conditions) decreases with increasing
temperature, as was observed experimentally over the 25°95°C range.
The calculations on the whole, therefore, are an improvement over the model of BARRETT and ANDERSON (1982). In
addition to employing a different method for estimating activity coefficients, the calculations in the latter paper utilized
HELGESON's (1969) stability constants for the Pb chlorocomplexes, and those extrapolated by ANDERSON (l973) from
FEDEROV et al. (1970) for the Zn chlorocomplexes.
Despite the satisfying level of agreement between measured
and calculated solubilities reported here, there is still some
reason to doubt that we know the true solubilities of sphalerite
and galena in these solutions. On the experimental side, the
main problem is the possibility of significant error in the
measured pH values due to variations in the liquid junction
potential in the concentrated solutions. Despite considerable
discussion in the literature (e.g. BATES, 1964; HARVIE et al.,
1984), there is still no reliable method of estimating the size
of this error, so that whatever the reliability of the metal
concentration measurements there is still doubt as to the
concentration/pfl relations. On the calculation side, there
are two major problems. One is of course the difficulty in
••
5m NoCI
-.
..
.. -.
.3
0.
0.
E
-245
-075
-768
-547
.
'
LOQ m Pb
3m NoCI
E
-.
-.
-.
~
Logppm Ph
pH
LOQ m Pb
4mNaCl
Logppm Zn
"
00
-,
-a
~
Log ppm Ph
Log ppm Zn
-e
go
0
oJ
pH
°
pH
FIG. 3. Comparison of calculated and experimental ZnS and PbS
solubility results for selected temperatures of 25° and 95°C,in 3 and
5 m NaCl solutions. Calculated solubility isotherms have the ideal
-2 slope.
calculating meaningful activity coefficients for trace levels of
complex species in brines. The other is doubt as to the solubility products of the metal sulphides. For example, UHLER
and HELZ (1984) have found the solubility product of galena
at 25°C to be almost two orders of magnitude greater than
the values we used from BOWERS etal. (1985), although there
is no obvious flaw in their procedure. Indeed, it would seem
to be the best value available, except for the fact that it would
result in calculated solubilities much greater than our measured ones (other parts of the calculation model being the
same).
Table 5. Calculatedgalenaand sphalente solubihnesto 3OO'C. for pH = 4
and total reduced sulphur = 0.001 m
25'C
50'C
ioo-c
150'C
2OO'C
250'C
3OO'C
-5.88
-379
-11 20
-858
-475
-310
-10.00
-789
-299
-187
-828
-666
-151
-054
-680
-533
003
078
-532
-401
156
2.46
-3.73
-233
325
3.88
-2.04
-091
-5.49
-353
-1070
-830
-4.19
2.58
-946
-7.34
-2.43
1.29
-770
-605
-1.06
005
-633
-482
0.27
111
-500
-366
181
2.83
-346
-194
341
4.00
-185
-076
-501
-325
-10 30
-8.00
-378
-214
-9.02
-6.89
-203
-084
-727
-5.58
-076
033
-599
-4.42
043
134
-482
-3.40
192
3.04
-3.33
-171
345
405
-1.80
-070
-4.69
-299
-992
-771
-3.47
-181
-8.69
-6.54
-1.73
-0.50
-695
-5.23
-050
062
-572
-4.11
0.54
153
-4.68
-3.19
198
318
-324
-154
3.44
4.07
-1.78
-065
-4.42
-274
-9.63
-744
-323
-156
-8.44
-627
-150
-025
-6.70
-496
-029
0.85
-5.49
-385
064
1.70
-4.56
-301
204
329
-317
·],41
343
4.11
-177
-0.60
1m NaCI
Log ppm Ph
Log ppm Zn
Log m Ph
Log m Zn
2m.&Cl
Log ppm Pb
Log ppm Zn
Log m Ph
Log m Zn
2Jnl!llQ
Log ppm Ph
Log ppm Zn
Logm Pb
Log mZn
~
Logppm Pb
Logppm Zn
Log m Pb
Log m Zn
~
Logppm Pb
LogppmZn
Logm Pb
Log m Zn
Note: The equationfor calculanngmolal solubninesat othervaluesofpH and
m~r) IS. log mPb.Zn - (no. in table) - (3 + log m~r») + 2(4 - pH)
818
T.1. Barrett and G. M. Anderson
SPhalerite solubilities in 1-5mNaCI brines
-2
C
-4
N
E
'"
..J
0
-6
-8
(a)
-10
0
100
200
300
TIOC)
Galena solubilities an 1-5 m No CI brines
0
-2
-4
J>
11.
E
-6
0
'"
..J
-8
-10
-12
0
100
200
300
nOC)
FlG. 4. a) Calculated ZnS solubilities at 25° to 300°C, 1-5 m
NaC!. b) Calculated PbS solubilities at 25° to 300°C, 1-5 m NaCI.
All calculations for pH = 4 and mS(r) = .001.
General conditions up to 300°C
Calculated ZnS and PbS solubilities,expressed both in ppm
and metal molalities, are given in Table 5 and plotted in Fig.
4 for 1-5 m NaCI solutions at temperatures of25-300°C for
the specific set of conditions: pH = 4.0 and mS(r) = 0.001.
In order to test the general applicability of these calculations, we can compare the results with natural geothermal
systems provided the necessary chemical data are available.
In addition to the Zn and/or Pb concentrations in the solution, the temperature, pH, and total chloride and reduced
sulfur concentrations must be known. There are relatively
few recent analyses which provide such information. Nevertheless,we can examine three main geological environments
in which high-temperature metal-bearing solutions are found.
I) Geothermal brines: a) Iceland/New Zealand type: nearneutral pH, seawater to meteoric water salinities. The Reykjanes brine at depth has the following characteristics: mCI
= 0.54, pH = 5.0, T = 270°C, log mH 2S = -3.0 and log
mPb"", -5.3 (KRISTMANNSDOTIIR, 1983). Our calculations,
when extrapolated to this mCl, yield a log mPb value of -5.46.
In the Broadlands region of New Zealand, the geothermal
waters are at a similar temperature of 265°C, but are very
low in chloride (mCl "'" 0.04). Log mPb and Zn values are
"'" -7.8 and ss -7.6, respectively, at a pH of6.1 and log
mH 2S = -2.7 (HENLEY and BROWN, 1985). We obtain calculated log mPb and Zn solubilitiesof -8.78 and -7.55 values
after extrapolation to mCl = 0.04. Extrapolations to other
temperatures were made using log mw 2 versus l/T(OK) plot
at 0.1, 2, 3, 4 and 5 m NaCI. Resulting values were fit with
a polynomial to obtain mW2 for a specific NaCI molality.
b) Salton Sea type: near-neutral pH, very saline brines. In
the Salton Sea reservoir, brines have the following characteristic: T"", 300°C, mCl = 5.9, pH = 5.4, log mH 2S = -3.2
(McKIBBEN and ELDERS, 1985). Our calculations indicate
log m values for Zn and Pb of - 3.17 and -4.36, in comparison with measured values of -1.9 and -3.3, respectively. If
our values are approximately correct, this would imply that
the brines are supersaturated with respect to galena and
sphalerite. McKIBBEN and ELDERS (1985) also calculated Zn
and Pb solubilities for this system: their Zn and Pb values
are respectively "",3.1 and se 1.5 log units higher than ours.
In the Cheleken and Red Sea systems, brines are of similar
high salinity, but surface temperatures are much lower, 79°C
and 56°C at pHs of 6.0 and 5.3, respectively. Using the data
summarized in Table I of WHITE (1981), our calculations
indicate Zn and Pb concentrations for these brines of some
three orders of magnitude less than the measured values. We
suspect non-equilibrium in such brines; they may have cooled
quickly near the surface and become extremely supersaturated
in metals. In the other systems discussed, the data used in
the calculations are for the deep brines at their sources. An
alternative explanation is that metal anion complexes other
than chloride complexes are stable and contribute significantly to the solubility of the metals.
2) Oilfield brines: Data on oil-field brines from Mississippi
(CARPENTER et al., 1974) and Texas (KHARAKA et al., 1980)
have recently been summarized by SVERJENSKY (1984).
Temperatures are in the 130-l50°C range, but the Mississippi
brines are much more saline than the Texas brines, and also
carry up to two orders of magnitude more dissolved Zn and
Pb. SVERJENSKY (1984) has calculated the pH values of the
Mississippi and Texas brines as "",4.3and <5.7, respectively;
however, mS(r) is known only from the Texas brine: log mH 2S
"'" -4.5. Using the maximum pH value of 5.7 for a solution
approximating the Texas brine (2 m NaCl, 150°C), our calculations yield log mZn and Pb solubilities of -6. 7 and -8.2,
respectively. These concentrations are well below the measured log m values of"", -4.8 and se -5.3, which is probably
a consequence of the maximum pH value we used.
3) Hydrothermal solutions at spreading axes: a) Sedimentfree ridge: high-temperature, acid pH, seawater salinity. Hydrothermal solutions discharging at vent sites on seafloor
spreading ridges are very enriched in metals and H 2S relative
to unmodified seawater. The 350°C endmember solutions
at 21ON on the East Pacific Rise (EPR) have log mZn and
Pb values in the ranges of -4.0 to -4.4 and -6.4 to -6.7,
respectively, with high log mH 2S values of"", -2.1, and a
pH of 3.5 (VON DAMM et al., 1985a). The measured Zn and
Pb values are respectively about four and six orders of magnitude less than the calculated values. Measured values at
high-temperature vents on the Explorer Ridge are even lower
(TUNNICLIFFE et al., 1986). Therefore, these solutions appear
Sphalerite and galena solubility in brine
to be very undersaturated with respect to galena and sphalerite, although it must be emphasized that the extrapolations
to 350°C make the calculations uncertain, particularly since
near-critical conditions are approached.
b) Sediment-buried ridge: high-temperature, neutral pH,
seawater salinity. In the Guaymas Basin, Gulf of California,
hydrothermal solutions pass through, and react with, a few
hundred metres of sediment cover before reaching the vent
sites. The 315°C endmember solutions have Zn, Pb and
mH 2S values similar to those on the EPR, but the fluid pH
has been increased to 5.9 as a result of interaction with the
underlying sediments (VON DAMM et al., 1985b).Calculated
log m solubilities are -5.5 for Zn and -6.7 for Pb; measured
values for Zn are in the range -5.7 to -4.4, and for Pb in
the range -6.2 to -6.7. Therefore, hydrothermal solutions
in the Guaymas appear either close to saturation or are
somewhat supersaturated with respect to sphalerite and galena. This may reflect the higher pH of the Guaymas solutions, and the fact that the sediments through which they
passed have higher concentrations of Zn and Pb than midocean ridgebasalts (VON DAMM et al., 1985b). It is of interest
that potential Cu-Zn ore deposits on both types of spreading
axes are forming from solutions whose Zn contents are about
100 times less than the minimum values of ~ 1 ppm considered necessaryfor formation ofMVT deposits. Two possible
reasons for why these axial solutions are still capable of forming ore-grade deposits are: i) the extremely high rates of discharge at such ridge vent sites (MACDONALD et al., 1980)
partly compensate for low metal concentrations in the solutions; and ii) the excessof reduced sulphur over base metals
(VON DAMM et al., 1985a,b) means that the metals should
be efficiently precipitated as sulphides upon discharge, rather
than released in dissolved form to bottom waters.
CONCLUSIONS
Experimental determination of sphalerite and galena solubilities under H 2S-saturated conditions has been carried out
over the 3-5 m NaCl range and for temperatures of 25° to
95°C. Under these conditions, ZnS is more soluble than PbS
by factors of 30-100. Both ZnS and PbS are about 5 times
more soluble in 5 m than in 3 m NaCl brines. An increase
in temperature of only 15°C produces a solubility increase
comparable to or greater than that produced by the 3 to 5 m
NaCl increase.
Using recent literature values for the stability constants of
the chloride complexes of Zn and Pb up to 300°C, we have
calculated ZnS and PbS solubilities over the 25-300°C and
1-5 m NaCl range. ZnS is consistently more soluble than
PbS over this range of conditions, with the difference in molal
solubilities ranging from at least two log units at 25°C to at
least one log unit at 300°C. Agreement with experimental
ZnS and PbS solubilities obtained under H2S-saturated conditions is very good at 60° to 95°C and all NaCI molalities;
at 25°C the calculated solubilities are low by up to one log
unit.
Field data from variousgeothermal systemshave been used
to calculate equilibrium solubilities of sphalerite and galena
in these systems. The results suggestthat high-salinity brines
range from supersaturated (Salton Sea, high-temperature) to
819
strongly supersaturated (Red Sea, low temperature) with respect to these sulphides. By contrast, high-temperature seawater-salinity solutions at sediment-free spreading axes are
grossly undersaturated in sphalerite and galena. The latter
situation is of interest in that massive sulphide deposits are
nevertheless forming from such solutions. On the other hand,
vent fluids depositing sulphides at the sediment-covered axis
in the Guaymas Basin appear to be more or less saturated in
sphalerite and galena. Calculated solubilities for on-land geothermal systems(hightemperature, seawaterto near-meteoric
salinity)yield values in reasonable agreement «0.2 log units)
in two of three cases.
Acknowledgements-We are particularly thankful to Joanna Lugowski forher skilled analytical assistance throughout the course of
thisstudy. Dr. J. C. van Loon kindly allowed access to various laboratory facilities at theUniversity ofToronto. Some of'the calculations
were performed at the Laboratoire de Geochemie et de Pedalogie at
the Universite PaulSabatier, C.N.R.S., in Toulouse, France. GMA
thanks Jacques Schott, JeanLouis Dandurand, Christophe Monnin
and other members of the lab for their interest and support. We
thank N. Z. Boctor and T. M.Seward fortheirhelpful reviews, and
S. Wood for hiscomments on an earlier version of the paper. This
research was supported in part by the Natural Sciences and Engineering Research Council of Canada.
Editorial handling: T. Paces
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