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Effects of mineral precipitation on the sulfur isotope composition of hydrothermal solutions
Chemical Geology, 105 ( 1993 ) 259-269 Elsevier Science Publishers B.V., Amsterdam
259
[MB]
Effects of mineral precipitation on the sulfur isotope composition of hydrothermal solutions Yong-Fei Zheng and Jochen Hoefs Geochemisches Institut, Universitiit G$ttingen, Goldschmidtstrasse 1, W-3400 Gi~ttingen, Germany (Received May 6, 1992; revised and accepted October 1, 1992)
ABSTRACT Zheng, Y.-F. and Hoefs, J., 1993. Effects of mineral precipitation on the sulfur isotope composition of hydrothermal solutions. Chem. Geol., 105: 259-269. The effect of precipitation of mineral phases on the sulfur isotopic composition of hydrothermal minerals is quantitatively evaluated in terms of a Rayleigh distillation process. The precipitation of either sulfate or sulfide minerals is considered as the removal mechanism for dissolved sulfur from a finite solution reservoir. The isotopic composition of sulfur in hydrothermal minerals is strongly controlled by the fraction of sulfur remaining in the solution as well as by the relative proportions of oxidized to reduced sulfur compounds. When sulfate is precipitated from a SO~--dominant solution with minor amounts of HzS, its j34S-value can change, with time, from slightly greater than that of the original solution in early stages to significantlylower than that of the original solution in late stages. Conversely, when sulfide minerals are deposited from a H2S-dominantsolution with minor amounts of SO~4-, their JMS-values may resemble the J34S-value of the original solution in early stages, but may change to values remarkably higher than that of the original solution in late stages. Sulfur and oxygen isotopic data for barite from the Bad Grund Pb--Zn deposit in the Harz Mountains, Germany, illustrate the application of the reservoir effect, and a few barites with low j34S-values are interpreted to represent the product of precipitation of the gangne mineral in late stages.
I. Introduction
The precipitation of sulfate and/or sulfide minerals is an important mechanism for the removal of sulfur from hydrothermal solution reservoirs. The influence of precipitation of mineral phases on the sulfur isotopic composition of solutions was first mentioned by Ohmoto (1972) in dealing with the effect of pH and fo2 on the sulfur isotope composition of hydrothermal fluids. A precipitation model was presented by Ding and Rees (1984) for describing the reservoir effect as a function of physico-chemical conditions in an ore-formCorrespondence to: Yong-Fei Zheng, Institute for Mineralogy, Petrology and Geochemistry, University of Tiibingen, Wilhelmstrasse 56, W-7400 Tiibingen 1, Germany.
ing fluid, with the particular application to the observed pattern of sulfur isotopic abundance variation in the Taolin Pb-Zn deposit, China. Although the precipitation model used by Ding and Rees (1984) is quite sophisticated, it fails to give the general pictures of sulfur isotopic variation as a function of dissolved sulfate/ sulfide ratio and precipitation fraction. Ohmoto (1986) pointed out that the removal of a significant fraction of reduced sulfur during galena precipitation may cause the $34S-value of the remaining H2S to change significantly because of the large sulfur isotope fractionation between H2S and PbS, particularly at low temperatures. The effect of sulfide precipitation from a pure H2S-bearing solution reservoir on sulfur isotope composition has been quantitatively modeled by Zheng and
0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
260
V.-F. ZHENG AND J. HOEFS
Hoefs (1993) on the basis of a Rayleigh distillation process. In this paper, we extend the theoretical perspective to a solution system which contains both dissolved sulfate and sulfide in varying proportions. The results corroborate the importance of considering the reservoir effect in interpreting sulfur isotope data from hydrothermal ore deposits. An example of how the reservoir effect can be applied to a specific mineral deposit is presented for the barite deposition of the Bad Grund Pb-Zn deposit in the Harz Mountains, Germany. Although speciation and incremental reaction calculations can be a more appropriate tool for determining reservoir effects in hydrothermal systems, simple Rayleigh calculations provide a first approximation to solution of the same questions.
tions of sulfur through the Rayleigh-type mineral precipitation are quantitatively assessable in the light of the isotopic fractionation relationship between mineral and solution under equilibrium conditions: 103 In
rumineral ~ 34 34 t~solution ~ ~ S m i n e r a I - - ¢~ S s o l u t i o n
( 3 )
which is a function of temperature and oxygen fugacity (reflected by dissolved sulfate/sulfide ratio) at the time of mineral precipitation. Considering that a hydrothermal solution contains both dissolved sulfate (i.e. SO~- ) and sulfide (i.e. H2S) and the other dissolved sulfur species such as HS- and S2- are relatively negligible in it, then the t~saS-value of the total sulfur in the solution is given by: 6
34
S s o l u t i o n ~..~-X 6 3 4 5 H 2 S - - ( 1
--x) 834Ss0]-
(4) 2. Model for mineral precipitation Precipitation of minerals from a finite reservoir can be described by the Rayleigh (fractional) equilibrium process. The expression governing the sulfur isotopic variation of a solution system is: ~34~f __ , ~ 3 4 ~ i Osolution -- ~ Osolution
!/~fl/~/.~,mineral 1 ) In F "4F l v v v ~ Otsolution - -
( 1)
where superscripts f and i denote the final and initial isotope composition, respectively, with respect to mineral precipitation; F is the fraction of total sulfur remaining in the solution; and a is the sulfur isotopic fractionation factor. Apparently, eq. 1 can be valid even if the fractionation factor a changes during mineral precipitation (i.e. F varying from < 1 to > 0). Following the approximation by Zheng (1990) for the Rayleigh distillation equation, eq. 1 becomes: t~34~f ~ .$ 3 4 ~ i Osolution ~ v Osolution
+ 103 In rt-tsolution u m i n e r a l In F
(2)
Consequently, the J34S-values of a hydrothermal solution which has lost varying propor-
where x is the mole fraction of H2S in the solution. Introducing eq. 4 into eq. 3 and rearranging it, we obtain: 10 3 In rumineral 34 34 tJtsolution -~. IJ S m i n e r a I - ¢~ 8s024 so 2 +X 10 3 In aH~s
(5)
In order to perform specific calculations we take into account two extreme cases where either sulfate or sulfide is precipitated from the solution. As done by Janecky and Shanks (1988) in computation of chemical reaction pathways, sulfate and sulfide aqueou s species and minerals are treated either as being in total equilibrium, or as two independent sets of sulfide and sulfate chemical components. Although the presence of isotopic equilibrium between the oxidized and reduced sulfur species has been questioned by Ohmoto and Lasaga (1982) for hydrothermal systems, mixing of different fluids can be considered as an important cause for isotopic disequilibrium (Zheng, 1991a). Note that x in eq. 5 changes with F during mineral deposition, but the fractionation factors, 103 In a, are only a function of temperature.
261
SULFUR ISOTOPE COMPOSITION OF HYDROTHERMAL SOLUTIONS
2.1. Sulfate precipitation
In this case, the relationship between the parameters x and F is given by:
If only sulfate is progressively precipitated from the solution, the sulfur isotopic composition of the final solution can be described by:
x=F/(F+R)
j34~f .g34~i Osolution ~ ~ Osolution .
SO 2 -
+ x In F 103 In aH2S
(6)
assuming that the sulfate mineral has the same fractionation behavior as aqueous SO 2(Ohmoto and Rye, 1979 ). Considering an instantaneous equilibrium fractionation between the removed sulfate and the total sulfur remaining in the solution, the resulting sulfate mineral has the following isotopic composition: 34
.$34~i Ssulfate ~ ~ Osolution so42-
+ x( l + ln F) l O31n aH2S
(7)
Because the mole fraction of H2S in the solution changes with precipitation of the sulfate mineral, the parameter x changes with the parameter F. Suppose that the initial ratio of dissolved sulfate/sulfide in the solution is defined by R, the relationship between x and F is transformed to:
x = I / ( I + FR)
(8)
This relationship will be introduced into the model calculations based on eqns. 6 and 7.
2. 2. Sulfide precipitation If the deposited mineral is only sulfide, the resulting isotopic composition of sulfur remaining in the solution is given by: ~34~f Osolution :
.g34~i tl O s o l u t i o n "]- [ 1 0
3 In ,~sumde t-tH2S SO 2- -
--(1--x)lO31nc~H2s I l n F
(9)
Similarly, for the sulfide mineral: ~34~ ~34~i O s u l f i d e ~ - ¢P O s o l u t i o n
+ ( 1 + In F) [ 103 In ~sumde UtH2S SO42 -
-- (1 --X) 103 In CtH2S ]
(10)
( 11 )
If R = 0, eq. 10 reduces to one given by Zheng and Hoefs (1993). By assuming sulfur isotopic fractionation factors between sulfide mineral and H2S (Ohmoto and Rye, 1979) and between dissolved sulfate and sulfide (Ohmoto and Lasaga, 1982), and by assuming the sulfur isotopic composition of the initial solution (j34S~s) and the initial ratio of dissolved sulfate/sulfide (R) in the solution, the equations derived above enable the theoretical perspective on the changes in the sulfur isotopic composition of both residual solution and deposited minerals as a function of progressively changing fraction of sulfur remaining in solution with different initial ratios of dissolved sulfate/sulfide. Calculations in the present case assume a constant temperature during mineral deposition. If a progressive decrease in temperature is coupled with the removal of sulfur from the solution, the resultant magnitude of j34S variations in later stages would be considerably larger than that from the constant-temperature effect. 3. Results and discussion
An illustration of how the sulfur isotopic composition of a hydrothermal solution is affected by the progressive precipitation of sulfate mineral is shown in Fig. I. Suppose the sulfur in the solution is distributed between dissolved sulfate and sulfide, the sulfur isotope composition of the initial solution system (j34S~s) is 0%o (relative to CDT), T=250°C, and the initial ratio of SO 2-/H2S varies from 100 to 0.1. Obviously the removal from the solution of isotopically heavy sulfur, as precipitated sulfate, can lead to a significant decrease in the t~34S-value of the total sulfur remaining in the solution. As a result, the decrease in the t~34S-values of the solution sulfur leads to a
262
V.-F.ZHENGANDJ. HOEFS RI.Z.IF ,,,,
'
I;~
--
La"(;e
' ' ' ' 2,o.'c' t ,L
\.\
A
,j r4
~
0-
i
-5-
o m
-10 -
-15-20 I
0.8
0.6
0.4
0.2
0
F
0.8
0.6
O.t~
0.2
0
F
Fig. 1. Effect of sulfate precipitation on the sulfur isotopic composition of a hydrothermal solution during Rayleigh distillation process at 250°C with varying proportions of dissolved sulfate/sulfide (R). ~34Sks is taken to be zero.
Fig. 2. Influence of the fraction of sulfur remaining in hydrothermal solution on the sulfur isotopic composition of sulfate mineral during Rayleigh-type precipitation at 250°C with varying ratios of SOl-/H2S (R). ~34Sks is taken to be 0%o.
progressive decrease in the fi34S-values of precipitated sulfate mineral, as illustrated in Fig. 2. As shown in Fig. 2, the magnitude of the decrease in the sulfate 634S-values is dependent on the initial ratio of dissolved sulfate/sulfide (R) and the fraction of sulfur remaining in the solution (F). In an extreme case where the solution contains only dissolved sulfate with no sulfide, the sulfate precipitated would show a constant ~34S-value identical to that of the initial solution from an earlier phase to a later phase. With increasing mole fraction of H2S in the solution, the reservoir effect becomes gradually significant and the resulting sulfate mineral can have quite different t~34S-values from the initial solution. According to the calculations, hydrothermal sulfate mineral can show slightly higher ~34S-values ( + 1 to + 5°/00) than
the initial solution in earlier phases but significantly lower ~345-values ( - 5 t o - 1 5 ° / 0 o ) in late phases within geologically reasonable ranges of SO42-/H2S ratio when ~345k- S = 00/00. In the environment of ore deposition different sulfides may precipitate, depending on the relative abundance of different metal ions (e.g., Pb 2+, Zn 2+, Fe 2+, Cu 2÷ ). Fig. 3 shows an example of the effect of pyrite precipitation on the sulfur isotopic composition of solution at 250°C with a34Sks=0%0 and the initial SO42-/H2S ratios from 0.05 to 10. The ~345value of sulfur remaining in the solution increases progressively with precipitating pyrite. And so does the ~34S-value of pyrite deposited, as depicted in Fig. 4. Initially negative ~34Svalues for pyrite are yielded due to the significant presence of dissolved sulfate. The ~34Svalues of other sulfide minerals (e.g., galena,
SULFURISOTOPECOMPOSITIONOF HYDROTHERMALSOLUTIONS ltm'lF
t.tam
--
263
I~te
20
~5
10
A
g
~t
~4 0
4.0 0
-L
-5
-10
-15
1
0.8
0.6
0.4
0.2
0
P
Fig. 3. Effect of pyrite precipitation on the sulfur isotopic composition of hydrothermal solution during Rayleigh distillation process at 250°C with different proportions of dissolved sulfate/sulfide (R). J34S~s is taken to be zero.
sphalerite, chalcopyrite) could be drawn by using the appropriate fractionation factors. Because galena is most depleted in 34S relative to the other c o m m o n sulfide minerals, it can be expected that the progressive precipitation of galena can cause the solution to be enriched in 34S to the greatest extent relative to the precipitation of the other sulfides under the same conditions. In an extreme case where the solution contains only dissolved sulfide with no sulfate, the j34S-value of the deposited pyrite might be slightly higher than that of the initial solution, depending on temperature, at the beginning of deposition, but progressively decrease with precipitation and even significantly lower than that of the initial solution in late phases, as shown in Fig. 5A. Conversely, the ~34S-value
-20 1
0.8
0.6
0.4
0.2
0
P
Fig. 4. Influenceof the fraction of sulfur remainingin hydrothermal solution on the sulfur isotope composition of pyrite during Rayleigh-typeprecipitation at 250°S with different ratios of SO2-/H2S (R). ~34S~s is taken to be 0%0.A, B and C represent three simple paths which a solution can followduring sulfideprecipitation (see text for discussion). of galena would gradually increase with its progressive precipitation from the pure H2Sbearing solution. However, the pattern of the g34S variation of pyrite would significantly change when the solution contains a minor a m o u n t of dissolved sulfate besides dominant H2S, as illustrated in Fig. 5B for the case where the initial SO 2 - / H 2 S ratio is 0.01. From Fig. 4 it can be seen that the d34S-value of the resulting pyrite is significantly influenced by the relative proportion of dissolved sulfate to sulfide in the solution. According to the theoretical results, hydrothermal sulfides can have tSa4S-values ( + 1%o) similar to the initial solution in early deposition phases, but significantly higher 334S-values ( + 5 to
264
~-F. ZHENG AND J. HOEFS
Early
,
'
%~ale
I
'
I
~_
v
I
'
1
,
La~i !
,
I
,
25o' c
(A) R=0
t~
:
o
-2
]
I
t
0.8
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I
0.6
0.4
0.2
F ~ly
time
~
ia%,e
8
6-
O-
-2t -4.
-
,
I
0.8
,
1
,
0.6
I
0.~
,
I
,
0.2
F
Fig. 5. Influenceof the fraction of sulfur remainingin hydrothermal solution on the sulfur isotopiccompositionof sulfidesduring Rayleigh-typeprecipitation at 250°C with ~345~s =0% from." ( A ) a pure H2S-bearingsolution; and (B) a H2S-dominantsolution with a minor amount of SO24- (where R=SO]-/H2S=0.01 ).
+ 15%o) in late phases within geologically reasonable ranges of S O ] - / H 2 S ratio when ~34S~s--":'-00/O0. This conclusion is essentially
identical to that previously drawn by Ding and Rees (1984). Field et al. ( 1984 ) modeled the isotopic effect of pyrite formation from a finite reservoir of H2S that is progressively oxidized to SO~at 300°C in a hydrothermal system, assuming that the aqueous SO42- is continually removed from the system, either by deposition of sulfate minerals or by loss to the ascending hydrothermal fluids, after equilibration with the residual H2S. As a consequence of isotopic fractionation between H2S and evolved SO 2-. and subsequent removal of this sulfate from the reservoir, the residual H2S becomes increasingly depleted in 3 4 5 and less abundant as the fraction of unoxidized H2S diminishes. Using a modified Rayleigh distillation equation, Field et al. (1984) calculated the isotope composition trend of the precipitated pyrite. The sulfur isotopic ratios of the sequentially evolved pyrite may range from + 1.2°0 initially (100% H2S) to - 48.2%o near the final stages ( 10% H2S) of oxidation. This result is identical to that from the present modeling on the effect of sulfate precipitation on the sulfur isotope composition of hydrothermal solution (Fig. 1 ). It was implicit in their modeling that the formation of pyrite is contemporaneous with both H2S oxidation and sulfate removal. However. it is unclear whether such physico-chemical conditions are favorable for the precipitation of pyrite or not. It is well known that the major factors which control the sulfur isotopic composition of hydrothermal minerals are: ( 1 ) temperature. which determines the magnitude of fractionations between sulfur-bearing species; (2) (~34512"S, which is controlled by the source of sulfur; (3) the proportion of oxidized to reduced sulfur species in fluids; and (4) relative amount of the mineral deposited from the fluids. The effect of the oxidized and reduced sulfur species in fluids on the sulfur isotope composition of minerals has been evaluated comprehensively by Sakai ( 1968 ) and Ohmoto (1972) in terms of pH and fo2 of hydrother-
SULFURISOTOPECOMPOSITIONOF HYDROTHERMALSOLUTIONS
mal fluids. The present calculations on the reservoir effect can be very valuable in the interpretation of sulfur isotopic variations in hydrothermal mineral deposits with respect to time. The model equations developed here are for a system from which there is no loss of sulfur except by precipitation and for which there is no input of sulfur from sources other than the original one. Such a closed-system assumption is essentially similar to that previously taken by Sakai ( 1968 ) and Ohmoto (1972) in constructing pH-fo2 diagrams. The significance of the time-dependent variations in mineral t~34S-valuesis an unresolved question in the interpretation of sulfur isotope data from hydrothermal ore deposits. Rye and Ohmoto (1974) generalized the trends of sulfide t~34S-values in time for various occurrences. As noted by the authors, the ~ 3 4 S - v a l ues can either increase with time (e.g., the Echo Bay deposit, Northwest Territories, Canada) or decrease with time (e.g., Darwin, California, U.S.A., and Mogul, Ireland, deposits). Ohmoto (1972) and Rye and Ohmoto (1974) attempted to apply the pH-fo2 diagrams to the interpretation of the temporal (and spatial) variations in the sulfide c~34S-valuesfor some ore deposits. Because the Rayleigh-type precipitation model has direct bearing on the temporal sequence of mineral deposition, the present results can be used to reappraise sulfur isotope data for which the relationship to mineralizing time is known. For example, the large variation in the c~348-valuesof sulfide minerals from ~ --22%0 in the early stage to ~ + 25%0 in the later stage of mineralization at the U N i - C o - A g - C u deposits of the Echo Bay mine can be explained if the fluid had the initial SO2-/H2S ratio of 5-10 with ~ 3 4 S ~ - ~ - 0 % 0 (see Fig. 4). This explanation requires that there would exist a large amount of dissolved sulfate in the fluid to equilibrate with the dissolved sulfide. Although the mineralogy of the ore and gangue minerals changed with time in the deposits (Robinson and Ohmoto, 1973), it has been not reported that there is sulfate
265
mineral to coexist with the sulfides. In view of mass conservation during isotopic partitioning between sulfate and sulfide, the present explanation can be valid only when the dissolved sulfate did exist during the precipitation of sulfides but was not able to deposit due to the restriction of physico-chemical conditions (e.g., no available metal cations such as Ba 2+ for formation of sulfate mineral). The same assumption must be taken in the previous interpretation of applying the PH-f02 diagrams to the sulfur isotope data. The chemical state of hydrothermal fluids during mineral deposition is controlled by the initial state and by subsequent changes caused by reaction with wall rocks and by the removal of previously deposited minerals. The last effect has been taken into account in modelling the reservoir effect. Fig. 4 illustrates three different paths for the chemical evolution of fluids. The first path, A, is toward the right, which may be applied to a case where the chemical state of the fluid changes only with the precipitation of pyrite. As a result, the t534Svalues of the resulting pyrite change with time, a typical case for mineral precipitation from a closed system. The second path, B, is downward to the right, which is a combination of path A with an increase in SO 2-/H2S ratio during deposition of pyrite. The increase in the SO 2-/H2S ratio can be caused in various ways, such as by incorporating sulfate into the solution from host rocks. In ore deposits formed from this solution, a trend of decreasing t~3asvalues toward the later stages of mineralization could be expected in hydrothermal sulfides. The third path, C, is upward to the right, which is a combination of path A with a decrease in SO42-/H2S ratio during sulfide precipitation. The decrease in the SO 2-/H2S ratio can be caused by incorporating sulfide into the solution from wall rocks. In ore deposits formed from this solution the c534S-values of sulfide might show a trend of rapid increase in the early stages of mineralization. Under natural conditions, the changes in the chemical
266 state of hydrothermal fluids could have been more complex. The S O ] - / H 2 S ratio may stay unchanged iffo2 is kept constant due to fluidrock interactions. It must be pointed out that the closed-system behavior has been assumed in the present modelling on reservoir effect. For many of the cases presented, significant sulfur isotope variations do not occur until about half the sulfur is precipitated. The question then arises as to how well the sulfide- or sulfate-depositing systems in nature fit the closed-system model. Perhaps the closed-system assumption is not necessarily valid in practical applications without constraints on fluid transport directions and fluid-rock exchange characteristics. Nevertheless, the present calculations can at least provide a useful approximation to estimate of possible changes in the sulfur isotope composition of hydrothermal minerals. A treatment of coupled reaction and isotope fractionation driven by one-dimensional infiltration may be probably more promising, as illustrated by Bickle and Baker (1990) for metamorphic rocks. 4. Application to the Bad Grund barite deposition The Bad Grund P b - Z n deposit occurs at the intersection of several fault zones with the Lower Carboniferous greywackes and shales in the Upper Harz, Germany. In the ore veins, galena and sphalerite are the main ore minerals, accompanied by minor amounts of chalcopyrite and pyrite. Quartz, calcite, siderite and barite are the main gangue minerals in most of the mineralized locations. The sulfides are mainly associated with the calcite and occur in the main mineralization phases (IIb, IIc, lid, Ilia and IIIb after Sperling, 1973; Stedingk and Schnorrer-Krhler, 1988). The barite occurs in a later phase (Illc) in the mineral paragenesis and is associated only with minor amounts of the sulfides. The detailed geological and geochemical characteristics of the Bad
Y-F. ZHENGANDJ. HOEFS
Grund deposit have been described by Sperling (1973), Mohr (1978) and Walther (1986). Sulfur isotopic investigations of ore sulfides have been carried out by Sperling and Nielsen ( 1973 ), R u m p ( 1978 ) and Zheng and Hoefs (1993). Sulfur and oxygen isotopic investigations have been conducted for ~ 60 samples of hydrothermal barites, which form a part of the stable isotopic studies of hydrothermal mineralizations in the Harz Mountains. Sulfur isotopic analysis was carried out by preparing H2S through reaction with Kiba ® solution at 350 °C (Hoefs, 1987 ) and the results are reported relative to CDT with a precision of + 0.2%0. Oxygen isotopic analysis of the same samples proceeded by preparing CO2 through reaction with spectral-pure graphite at 1000°C and the resuits are reported relative to SMOW with a precision of + 0.4%o (Zheng and Hoefs, 1993 ). The detailed description of the samples and a complete data set is presented in Zheng (1991b). As shown in the ~345 vs. ~ 1 8 0 diagram of Fig. 6, the ~345 range of the barites is from - 1 to +18°/0o with a mode of + 13 to + 15%o and the ~180 range is from + 1 2 to + 17%owith a m o d e o f + 14to + 16%o. Zechstein evaporites in this region have the ~34S-values of about + 10 to + 13%o with a mode of + 11%o (Nielsen and Ricke, 1964: Pilot et al., 1972). These values are close to, but slightly lower than, the mode ~345-values of + 13 to + 15%o for the Bad Grund barites. Because the barites occur in the late phase (IIIc) of the hydrothermal mineralization and are thus younger than the main P b - Z n ore deposition [ < 180 Ma after Haack ( 1990], the Zechstein evaporites are the most likely candidate for the source of barite-sulfur. However, questions remain as to how the low (~345values from - 1 to + 8%o can result for a few barites while most have ~34S-values higher than + 1 0 t o +13%o. It has been generally believed in sulfur isotope geochemistry that hypogene sulfate, equilibrated with sulfide, should be distinctly en-
SULFUR ISOTOPECOMPOSITIONOF HYDROTHERMALSOLUTIONS
20--
18-
16-
12-
\'.
10it .o
8
~_e -2 10
I
I
12
1b,
16
I
I
18
20
8' * o ( ~ )
Fig. 6. Graph of the ~34S- vs. ~180-values of barites from the Bad Grund P b - Z n deposit in the Harz Mountains, Germany. The ~'sO results are interpreted to indicate that the barites were precipitated from a low-temperature fluid of subsurface origin with a ~'80-value ~ 0%0 (for details see Zheng and Hoefs, 1993 ).
riched in 348 relative to sulfide and to any supergene sulfate derived from quantitative unidirectional oxidation of sulfide-sulfur (e.g., Field, 1966 ). Further, supergene sulfate should show tj34S-values identical to those of the initial sulfide (Ohmoto and Rye, 1979). In this regard, the isotopically light barites in the Bad Grund deposit could have derived their sulfur through oxidation and redeposition of sulfidesulfur in hydrothermal systems. However, de-
267 termination of the oxygen isotopic composition of barites indicates that the tjlSO-values for all of barites (except one) vary uniformly from + 12 to + 17%0. Only one sample has a 8'SO-value of + 1.7%o and a 834S-value of - 4 . 5 ° / 0 0 (not shown in Fig. 6 ). Obviously, we can only attribute this barite to a supergene origin. The barites which have low ~34S-values from - 1 to +8%0 show 8taO-values from + 14 to + 17%o, suggesting the same hypogene origin as the other barites with ~34S-values from + 10 to + 18%0. Thus the decrease in the 834Svalues of a few barites is due to the effect of precipitation of a sulfate mineral phase (i.e. barite) on the sulfur isotopic composition of hydrothermal fluid. According to a systematic investigation of sulfur and oxygen isotopes in the sulfides and barite from the Bad Grund deposit, Zheng and Hoefs (1993) conclude that the ore sulfides were deposited from a high-temperature fluid of deep-seated origin whereas the barite was precipitated from a low-temperature fluid of subsurface origin. A temperature below 120 ° C has been inferred for barite precipitation in this deposit according to sulfur isotopic geothermometry for sphalerite and galena pairs in phase Ilia (Zheng and Hoefs, 1993 ). Microthermometric investigation of fluid inclusions in barites gave a temperature range mainly below 100°C, some even below 70°C (Adeyemi, 1982). If we take an average value of 100°C as the temperature of barite precipitation, a theoretical diagram can be constructed with 834S~rs = + 11%00 for interpreting the observed 834S variation. As illustrated in Fig. 7, the reservoir effect can be applied to the pattern of sulfur isotopic variation in the Bad Grund barites, assuming the fluid was dominated by SO 2- with minor amounts of H2S ( S O l - / H2S~ 10-20) as suggested by the mineral paragenesis. Most of the barite samples have the 834S-values significantly higher than + 1 1%0 and thus should be deposited in early stages. Whereas the other barites which have the 834S-values significantly lower than + 1 1%o
~ .-F. ZHENGAND J. HOEFS
268 20
tAae
F,a z . l y ......
Late
18
16
8
4
-2
,
I 0.8
,
1
J
0.6
I 0.~
~ ~1~ ~ 0.2
F
Fig. 7. Influence of the fraction of sulfur remaining in hydrothermal fluid on the sulfur isotope composition of barite during Rayleigh-type precipitation of isotopicalty heavy barite at 100°C with ~345~s ~ + 1 1%0. Lined pattern indicates the predominant variation in the ~34S-values of barite from the Bad Grund Pb-Zn deposit with progress of precipitation. R is the initial ratio of dissolved sulfate/ sulfide prior to barite precipitation.
are considered as the product of precipitation in late stages. With time the change in the fluid t~34S-value with precipitation of the isotopically heavy barite can be coupled with a decrease in temperature which governs the magnitude of isotopic partitioning between dissolved sulfate and sulfide. As a result, the magnitude of ~345 variation in precipitated barites can be considerably greater than that expected in the illustration of Fig. 7.
5. Conclusions Theoretical modelling has demonstrated how Rayleigh-type distillation process during precipitation of mineral phases can significantly change the isotopic composition of sulfurbearing solutions. The reservoir effect is particularly significant when a hydrothermal solution contains not only dissolved sulfate but also dissolved sulfide. The precipitation of sulfate mineral from a closed-solution system can
lead to a significant decrease in the t~34S-value of sulfur remaining in the solution with time, whereas the precipitation of sulfide minerals can cause the solution to be progressively enriched in 345. Consequently, sulfate precipitated from a S O ] - - d o m i n a n t solution with small amounts ofH2S (1-10%) can have ~345values slightly greater than the t~34S-value of the initial solution in early stages, but remarkably lower ci34S-values in late stages. Conversely, sulfide deposited from a H2S-dominant solution with minor amounts of SOl- (1-5%) can have t~34S-values similar to the t~34S-value of the initial solution in early phases but significantly higher ~34S-values in late phases. The theoretical results have important bearing on the interpretation of sulfur isotopic variations with respect to the time sequence of mineral deposition in the life of a hydrothermal system. Sulfur and oxygen isotopic variations of barite in the Bad Grund deposit can be interpreted to represent the precipitation of barite from a solution reservoir with ~345~S ~ "~- 1 1%o at temperature about 100°C. Those barites which show the low ~34S-values from - 1 to + 8%o are considered as the product of deposition in late stages. The minor amounts of dissolved sulfide are suggested to coexist with dominant SO 2- in the low-temperature fluid.
Acknowledgements This study has benefited from the support by the German Science Foundation ( D F G ) within the framework of the project "Intraformational Mineralizations". We are grateful to Dr. K. Stedingk for providing some samples. Comments on this manuscript by Drs. M.J. Bickle, C.W. Field and H. Sakai as well as two anonymous reviewers help to clarify some ambiguities.
References Adeyemi, A., 1982. Vergleichende mikrothermometrische Untersuchungen an Fliissigkeitseinscbliissenaus
SULFUR ISOTOPE COMPOSITION OF HYDROTHERMAL SOLUTIONS
hydrothermalen Mineralisationen der Rhenoherzynischen Zone. Dissertation, University of G6ttingen, G~Sttingen, 123 pp. Bickle, M. and Baker, J., 1990. Migration of reaction and isotopic fronts in infiltration zones: assessments of fluid flux in metamorphic terrains. Earth Planet. Sci. Lett., 98: 1-13. Ding, T. and Rees, C.E., 1984. The sulphur isotope systematics of the Taolin lead-zinc ore deposit, China. Geochim. Cosmochim. Acta, 48:2381-2392. Field, C.W., 1966. Sulfur isotopic method for discriminating between sulfates of hypogene and supergene origin. Econ. Geol., 61: 1428-1435. Field, C.W., Sakai, H. and Ueda, A., 1984. Isotopic constraints on the origin of sulfur in oceanic igneous rocks. In: A. Wauschkuhn, C. Kluth and R.A. Zimmerman (Editors), Syngenesis and Epigenesis in the Formation of Mineral Deposits. Springer, Berlin, pp. 573589. Haack, U., 1990. Datierung mit Rb/Sr-Mischungslinie? Ber. Dtsch. Mineral. Ges., 2:86 (abstract). Hoefs, J., 1987. Stable Isotope Geochemistry. Springer, Berlin, 3rd ed., 241 pp. Janecky, D.R. and Shanks III, W.C., 1988. Computational modeling of chemical and sulfur isotopic reaction processes in seafloor hydrothermal systems: Chimneys, massive sulfides, and subjacent alteration zones. Can. Mineral., 26: 805-825. Mohr, K., 1978. Geologie und Minerallagerstiitten des Harzes. E. Schweizerbart (Niigle & Obermiller), Stuttgart, 387 pp. Nielsen, H. and Ricke, W., 1964. Schwefel-Isotopenverhiiltnisse von Evaporiten aus Deutschland - - Ein Beitrag zur Kenntnis von 34S im Meerwasser-Sulfat. Geochim. Cosmochim. Acta, 28: 577-591. Ohmoto, H., 1972. Systematics of sulfur and carbon isotopes in hydrothermal ore deposits. Econ. Geol., 67: 551-579. Ohmoto, H., 1986. Stable isotope geochemistry of ore deposits. In: J.W. Valley, H.P. Taylor, Jr. and J.R. O'Neil (Editors), Stable Isotopes in High Temperature Geological Processes, Ch. 14. Mineral. Soc. Am., Rev. Mineral., 16: 491-559. Ohmoto, H. and Lasaga, A.C., 1982. Kinetics of reactions between aqueous sulfates and sulfides in hydrothermal systems. Geochim. Cosmochim. Acta, 46: 17271745. Ohmoto, H. and Rye, R.O., 1979. Isotopes of sulfur and carbon. In: H.L. Barnes (Editor), Geochemistry of Hydrothermal Ore Deposits, Ch. 10. Wiley, New York, N.Y., 2nd ed., pp. 509-567. Pilot, J., R/Ssler, H.V. and Miiller, P., 1972. Zur geochem-
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ischen Entwicklung des Meerwassers u n d mariner Sedimente im Phanerozoikum mittels Untersuchungen yon S-, O- und C-Isotopen. Neue Bergbautech., 2: 161-168. Robinson, B.W. and Ohmoto, H., 1973. Mineralogy, fluid inclusions, and stable isotopes of the Echo Bay U - N i Co-Ag-Cu deposits, Northwest Territories, Canada. Econ. Geol., 68: 635-656. Rump, F., 1978. Geochemische Untersuchungen zu den Entstehungsbedingungen der hydrothermalen BleiZink-Erzlagerst~itte Grund im Oberharz. Dissertation, University of Gfttingen, G6ttingen, 69 pp. Rye, R.O. and Ohmoto, H., 1974. Sulfur and carbon isotopes and ore genesis: A review. Econ. Geol., 69: 826842. Sakai, H., 1968. Isotopic properties of sulfur compounds in hydrothermal processes. Geochim. J., 2: 29-49. Sperling, H., 1973. Die Erzg~nge des Erzbergwerks Grund. Geol. Jahrb., D2: 1-205. Sperling, H. and Nielsen, H., 1973. Schwefel-Isotopenuntersuchungen an der Blei-Zink Erzlagerstiitte Grund (Westharz, Bundesrepublik Deutschland). Miner. Deposita, 8: 64-72. Stedingk, K. and Schnorrer-KShler, G., 1988. Ein neues Vorkommen yon totem Glaskopf, Antimonit und Stibiconit am Erzbergwerk Grund (Oberharz). Aufschluss, 39: 282-288. Walther, H.W., 1986. Mineral deposits in Federal Republic of Germany. In: F.W. Dunning and A.M. Evans (Editors), Mineral Deposits of Europe, Vol. 3. Central Europe. Inst. Min. Metall.-Mineral. Soc., London, pp. 230-237. Zheng, Y.-F., 1990. The effect of Rayleigh degassing of magma on sulphur isotope composition: A quantitative evaluation. Terra Nova, 2: 74-78. Zheng, Y.-F., 1991a. Sulphur isotopic fractionation between sulphate and sulphide in hydrothermal ore deposits: disequilibrium vs. equilibrium processes. Terra Nova, 3: 510-516. Zheng, Y.-F., 199 lb. C-, O- und S-Isotopengeochemische Untersuchungen an Hydrothermalen Lagerst~itten des Harzes. Dissertation, University ofG6ttingen, G5ttingen, 130 pp. Zheng, Y.-F. and Hoefs, J., 1993. Stable isotope geochemistry of hydrothermal mineralizations in the Harz Mountains, II. Sulfur and oxygen isotopes of sulfides and sulfate and constraints on metallogenetic models. In: P. Mfller (Editor), Formation of Pb-Zn, Barite and Fluorite Vein Mineralizations in the Harz Mountains/ Germany. Monograph Series on Mineral Deposits. Borntraeger, Berlin (in press).
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Effects of mineral precipitation on the sulfur isotope composition of hydrothermal solutions Yong-Fei Zheng and Jochen Hoefs Geochemisches Institut, Universitiit G$ttingen, Goldschmidtstrasse 1, W-3400 Gi~ttingen, Germany (Received May 6, 1992; revised and accepted October 1, 1992)
ABSTRACT Zheng, Y.-F. and Hoefs, J., 1993. Effects of mineral precipitation on the sulfur isotope composition of hydrothermal solutions. Chem. Geol., 105: 259-269. The effect of precipitation of mineral phases on the sulfur isotopic composition of hydrothermal minerals is quantitatively evaluated in terms of a Rayleigh distillation process. The precipitation of either sulfate or sulfide minerals is considered as the removal mechanism for dissolved sulfur from a finite solution reservoir. The isotopic composition of sulfur in hydrothermal minerals is strongly controlled by the fraction of sulfur remaining in the solution as well as by the relative proportions of oxidized to reduced sulfur compounds. When sulfate is precipitated from a SO~--dominant solution with minor amounts of HzS, its j34S-value can change, with time, from slightly greater than that of the original solution in early stages to significantlylower than that of the original solution in late stages. Conversely, when sulfide minerals are deposited from a H2S-dominantsolution with minor amounts of SO~4-, their JMS-values may resemble the J34S-value of the original solution in early stages, but may change to values remarkably higher than that of the original solution in late stages. Sulfur and oxygen isotopic data for barite from the Bad Grund Pb--Zn deposit in the Harz Mountains, Germany, illustrate the application of the reservoir effect, and a few barites with low j34S-values are interpreted to represent the product of precipitation of the gangne mineral in late stages.
I. Introduction
The precipitation of sulfate and/or sulfide minerals is an important mechanism for the removal of sulfur from hydrothermal solution reservoirs. The influence of precipitation of mineral phases on the sulfur isotopic composition of solutions was first mentioned by Ohmoto (1972) in dealing with the effect of pH and fo2 on the sulfur isotope composition of hydrothermal fluids. A precipitation model was presented by Ding and Rees (1984) for describing the reservoir effect as a function of physico-chemical conditions in an ore-formCorrespondence to: Yong-Fei Zheng, Institute for Mineralogy, Petrology and Geochemistry, University of Tiibingen, Wilhelmstrasse 56, W-7400 Tiibingen 1, Germany.
ing fluid, with the particular application to the observed pattern of sulfur isotopic abundance variation in the Taolin Pb-Zn deposit, China. Although the precipitation model used by Ding and Rees (1984) is quite sophisticated, it fails to give the general pictures of sulfur isotopic variation as a function of dissolved sulfate/ sulfide ratio and precipitation fraction. Ohmoto (1986) pointed out that the removal of a significant fraction of reduced sulfur during galena precipitation may cause the $34S-value of the remaining H2S to change significantly because of the large sulfur isotope fractionation between H2S and PbS, particularly at low temperatures. The effect of sulfide precipitation from a pure H2S-bearing solution reservoir on sulfur isotope composition has been quantitatively modeled by Zheng and
0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
260
V.-F. ZHENG AND J. HOEFS
Hoefs (1993) on the basis of a Rayleigh distillation process. In this paper, we extend the theoretical perspective to a solution system which contains both dissolved sulfate and sulfide in varying proportions. The results corroborate the importance of considering the reservoir effect in interpreting sulfur isotope data from hydrothermal ore deposits. An example of how the reservoir effect can be applied to a specific mineral deposit is presented for the barite deposition of the Bad Grund Pb-Zn deposit in the Harz Mountains, Germany. Although speciation and incremental reaction calculations can be a more appropriate tool for determining reservoir effects in hydrothermal systems, simple Rayleigh calculations provide a first approximation to solution of the same questions.
tions of sulfur through the Rayleigh-type mineral precipitation are quantitatively assessable in the light of the isotopic fractionation relationship between mineral and solution under equilibrium conditions: 103 In
rumineral ~ 34 34 t~solution ~ ~ S m i n e r a I - - ¢~ S s o l u t i o n
( 3 )
which is a function of temperature and oxygen fugacity (reflected by dissolved sulfate/sulfide ratio) at the time of mineral precipitation. Considering that a hydrothermal solution contains both dissolved sulfate (i.e. SO~- ) and sulfide (i.e. H2S) and the other dissolved sulfur species such as HS- and S2- are relatively negligible in it, then the t~saS-value of the total sulfur in the solution is given by: 6
34
S s o l u t i o n ~..~-X 6 3 4 5 H 2 S - - ( 1
--x) 834Ss0]-
(4) 2. Model for mineral precipitation Precipitation of minerals from a finite reservoir can be described by the Rayleigh (fractional) equilibrium process. The expression governing the sulfur isotopic variation of a solution system is: ~34~f __ , ~ 3 4 ~ i Osolution -- ~ Osolution
!/~fl/~/.~,mineral 1 ) In F "4F l v v v ~ Otsolution - -
( 1)
where superscripts f and i denote the final and initial isotope composition, respectively, with respect to mineral precipitation; F is the fraction of total sulfur remaining in the solution; and a is the sulfur isotopic fractionation factor. Apparently, eq. 1 can be valid even if the fractionation factor a changes during mineral precipitation (i.e. F varying from < 1 to > 0). Following the approximation by Zheng (1990) for the Rayleigh distillation equation, eq. 1 becomes: t~34~f ~ .$ 3 4 ~ i Osolution ~ v Osolution
+ 103 In rt-tsolution u m i n e r a l In F
(2)
Consequently, the J34S-values of a hydrothermal solution which has lost varying propor-
where x is the mole fraction of H2S in the solution. Introducing eq. 4 into eq. 3 and rearranging it, we obtain: 10 3 In rumineral 34 34 tJtsolution -~. IJ S m i n e r a I - ¢~ 8s024 so 2 +X 10 3 In aH~s
(5)
In order to perform specific calculations we take into account two extreme cases where either sulfate or sulfide is precipitated from the solution. As done by Janecky and Shanks (1988) in computation of chemical reaction pathways, sulfate and sulfide aqueou s species and minerals are treated either as being in total equilibrium, or as two independent sets of sulfide and sulfate chemical components. Although the presence of isotopic equilibrium between the oxidized and reduced sulfur species has been questioned by Ohmoto and Lasaga (1982) for hydrothermal systems, mixing of different fluids can be considered as an important cause for isotopic disequilibrium (Zheng, 1991a). Note that x in eq. 5 changes with F during mineral deposition, but the fractionation factors, 103 In a, are only a function of temperature.
261
SULFUR ISOTOPE COMPOSITION OF HYDROTHERMAL SOLUTIONS
2.1. Sulfate precipitation
In this case, the relationship between the parameters x and F is given by:
If only sulfate is progressively precipitated from the solution, the sulfur isotopic composition of the final solution can be described by:
x=F/(F+R)
j34~f .g34~i Osolution ~ ~ Osolution .
SO 2 -
+ x In F 103 In aH2S
(6)
assuming that the sulfate mineral has the same fractionation behavior as aqueous SO 2(Ohmoto and Rye, 1979 ). Considering an instantaneous equilibrium fractionation between the removed sulfate and the total sulfur remaining in the solution, the resulting sulfate mineral has the following isotopic composition: 34
.$34~i Ssulfate ~ ~ Osolution so42-
+ x( l + ln F) l O31n aH2S
(7)
Because the mole fraction of H2S in the solution changes with precipitation of the sulfate mineral, the parameter x changes with the parameter F. Suppose that the initial ratio of dissolved sulfate/sulfide in the solution is defined by R, the relationship between x and F is transformed to:
x = I / ( I + FR)
(8)
This relationship will be introduced into the model calculations based on eqns. 6 and 7.
2. 2. Sulfide precipitation If the deposited mineral is only sulfide, the resulting isotopic composition of sulfur remaining in the solution is given by: ~34~f Osolution :
.g34~i tl O s o l u t i o n "]- [ 1 0
3 In ,~sumde t-tH2S SO 2- -
--(1--x)lO31nc~H2s I l n F
(9)
Similarly, for the sulfide mineral: ~34~ ~34~i O s u l f i d e ~ - ¢P O s o l u t i o n
+ ( 1 + In F) [ 103 In ~sumde UtH2S SO42 -
-- (1 --X) 103 In CtH2S ]
(10)
( 11 )
If R = 0, eq. 10 reduces to one given by Zheng and Hoefs (1993). By assuming sulfur isotopic fractionation factors between sulfide mineral and H2S (Ohmoto and Rye, 1979) and between dissolved sulfate and sulfide (Ohmoto and Lasaga, 1982), and by assuming the sulfur isotopic composition of the initial solution (j34S~s) and the initial ratio of dissolved sulfate/sulfide (R) in the solution, the equations derived above enable the theoretical perspective on the changes in the sulfur isotopic composition of both residual solution and deposited minerals as a function of progressively changing fraction of sulfur remaining in solution with different initial ratios of dissolved sulfate/sulfide. Calculations in the present case assume a constant temperature during mineral deposition. If a progressive decrease in temperature is coupled with the removal of sulfur from the solution, the resultant magnitude of j34S variations in later stages would be considerably larger than that from the constant-temperature effect. 3. Results and discussion
An illustration of how the sulfur isotopic composition of a hydrothermal solution is affected by the progressive precipitation of sulfate mineral is shown in Fig. I. Suppose the sulfur in the solution is distributed between dissolved sulfate and sulfide, the sulfur isotope composition of the initial solution system (j34S~s) is 0%o (relative to CDT), T=250°C, and the initial ratio of SO 2-/H2S varies from 100 to 0.1. Obviously the removal from the solution of isotopically heavy sulfur, as precipitated sulfate, can lead to a significant decrease in the t~34S-value of the total sulfur remaining in the solution. As a result, the decrease in the t~34S-values of the solution sulfur leads to a
262
V.-F.ZHENGANDJ. HOEFS RI.Z.IF ,,,,
'
I;~
--
La"(;e
' ' ' ' 2,o.'c' t ,L
\.\
A
,j r4
~
0-
i
-5-
o m
-10 -
-15-20 I
0.8
0.6
0.4
0.2
0
F
0.8
0.6
O.t~
0.2
0
F
Fig. 1. Effect of sulfate precipitation on the sulfur isotopic composition of a hydrothermal solution during Rayleigh distillation process at 250°C with varying proportions of dissolved sulfate/sulfide (R). ~34Sks is taken to be zero.
Fig. 2. Influence of the fraction of sulfur remaining in hydrothermal solution on the sulfur isotopic composition of sulfate mineral during Rayleigh-type precipitation at 250°C with varying ratios of SOl-/H2S (R). ~34Sks is taken to be 0%o.
progressive decrease in the fi34S-values of precipitated sulfate mineral, as illustrated in Fig. 2. As shown in Fig. 2, the magnitude of the decrease in the sulfate 634S-values is dependent on the initial ratio of dissolved sulfate/sulfide (R) and the fraction of sulfur remaining in the solution (F). In an extreme case where the solution contains only dissolved sulfate with no sulfide, the sulfate precipitated would show a constant ~34S-value identical to that of the initial solution from an earlier phase to a later phase. With increasing mole fraction of H2S in the solution, the reservoir effect becomes gradually significant and the resulting sulfate mineral can have quite different t~34S-values from the initial solution. According to the calculations, hydrothermal sulfate mineral can show slightly higher ~34S-values ( + 1 to + 5°/00) than
the initial solution in earlier phases but significantly lower ~345-values ( - 5 t o - 1 5 ° / 0 o ) in late phases within geologically reasonable ranges of SO42-/H2S ratio when ~345k- S = 00/00. In the environment of ore deposition different sulfides may precipitate, depending on the relative abundance of different metal ions (e.g., Pb 2+, Zn 2+, Fe 2+, Cu 2÷ ). Fig. 3 shows an example of the effect of pyrite precipitation on the sulfur isotopic composition of solution at 250°C with a34Sks=0%0 and the initial SO42-/H2S ratios from 0.05 to 10. The ~345value of sulfur remaining in the solution increases progressively with precipitating pyrite. And so does the ~34S-value of pyrite deposited, as depicted in Fig. 4. Initially negative ~34Svalues for pyrite are yielded due to the significant presence of dissolved sulfate. The ~34Svalues of other sulfide minerals (e.g., galena,
SULFURISOTOPECOMPOSITIONOF HYDROTHERMALSOLUTIONS ltm'lF
t.tam
--
263
I~te
20
~5
10
A
g
~t
~4 0
4.0 0
-L
-5
-10
-15
1
0.8
0.6
0.4
0.2
0
P
Fig. 3. Effect of pyrite precipitation on the sulfur isotopic composition of hydrothermal solution during Rayleigh distillation process at 250°C with different proportions of dissolved sulfate/sulfide (R). J34S~s is taken to be zero.
sphalerite, chalcopyrite) could be drawn by using the appropriate fractionation factors. Because galena is most depleted in 34S relative to the other c o m m o n sulfide minerals, it can be expected that the progressive precipitation of galena can cause the solution to be enriched in 34S to the greatest extent relative to the precipitation of the other sulfides under the same conditions. In an extreme case where the solution contains only dissolved sulfide with no sulfate, the j34S-value of the deposited pyrite might be slightly higher than that of the initial solution, depending on temperature, at the beginning of deposition, but progressively decrease with precipitation and even significantly lower than that of the initial solution in late phases, as shown in Fig. 5A. Conversely, the ~34S-value
-20 1
0.8
0.6
0.4
0.2
0
P
Fig. 4. Influenceof the fraction of sulfur remainingin hydrothermal solution on the sulfur isotope composition of pyrite during Rayleigh-typeprecipitation at 250°S with different ratios of SO2-/H2S (R). ~34S~s is taken to be 0%0.A, B and C represent three simple paths which a solution can followduring sulfideprecipitation (see text for discussion). of galena would gradually increase with its progressive precipitation from the pure H2Sbearing solution. However, the pattern of the g34S variation of pyrite would significantly change when the solution contains a minor a m o u n t of dissolved sulfate besides dominant H2S, as illustrated in Fig. 5B for the case where the initial SO 2 - / H 2 S ratio is 0.01. From Fig. 4 it can be seen that the d34S-value of the resulting pyrite is significantly influenced by the relative proportion of dissolved sulfate to sulfide in the solution. According to the theoretical results, hydrothermal sulfides can have tSa4S-values ( + 1%o) similar to the initial solution in early deposition phases, but significantly higher 334S-values ( + 5 to
264
~-F. ZHENG AND J. HOEFS
Early
,
'
%~ale
I
'
I
~_
v
I
'
1
,
La~i !
,
I
,
25o' c
(A) R=0
t~
:
o
-2
]
I
t
0.8
I
I
0.6
0.4
0.2
F ~ly
time
~
ia%,e
8
6-
O-
-2t -4.
-
,
I
0.8
,
1
,
0.6
I
0.~
,
I
,
0.2
F
Fig. 5. Influenceof the fraction of sulfur remainingin hydrothermal solution on the sulfur isotopiccompositionof sulfidesduring Rayleigh-typeprecipitation at 250°C with ~345~s =0% from." ( A ) a pure H2S-bearingsolution; and (B) a H2S-dominantsolution with a minor amount of SO24- (where R=SO]-/H2S=0.01 ).
+ 15%o) in late phases within geologically reasonable ranges of S O ] - / H 2 S ratio when ~34S~s--":'-00/O0. This conclusion is essentially
identical to that previously drawn by Ding and Rees (1984). Field et al. ( 1984 ) modeled the isotopic effect of pyrite formation from a finite reservoir of H2S that is progressively oxidized to SO~at 300°C in a hydrothermal system, assuming that the aqueous SO42- is continually removed from the system, either by deposition of sulfate minerals or by loss to the ascending hydrothermal fluids, after equilibration with the residual H2S. As a consequence of isotopic fractionation between H2S and evolved SO 2-. and subsequent removal of this sulfate from the reservoir, the residual H2S becomes increasingly depleted in 3 4 5 and less abundant as the fraction of unoxidized H2S diminishes. Using a modified Rayleigh distillation equation, Field et al. (1984) calculated the isotope composition trend of the precipitated pyrite. The sulfur isotopic ratios of the sequentially evolved pyrite may range from + 1.2°0 initially (100% H2S) to - 48.2%o near the final stages ( 10% H2S) of oxidation. This result is identical to that from the present modeling on the effect of sulfate precipitation on the sulfur isotope composition of hydrothermal solution (Fig. 1 ). It was implicit in their modeling that the formation of pyrite is contemporaneous with both H2S oxidation and sulfate removal. However. it is unclear whether such physico-chemical conditions are favorable for the precipitation of pyrite or not. It is well known that the major factors which control the sulfur isotopic composition of hydrothermal minerals are: ( 1 ) temperature. which determines the magnitude of fractionations between sulfur-bearing species; (2) (~34512"S, which is controlled by the source of sulfur; (3) the proportion of oxidized to reduced sulfur species in fluids; and (4) relative amount of the mineral deposited from the fluids. The effect of the oxidized and reduced sulfur species in fluids on the sulfur isotope composition of minerals has been evaluated comprehensively by Sakai ( 1968 ) and Ohmoto (1972) in terms of pH and fo2 of hydrother-
SULFURISOTOPECOMPOSITIONOF HYDROTHERMALSOLUTIONS
mal fluids. The present calculations on the reservoir effect can be very valuable in the interpretation of sulfur isotopic variations in hydrothermal mineral deposits with respect to time. The model equations developed here are for a system from which there is no loss of sulfur except by precipitation and for which there is no input of sulfur from sources other than the original one. Such a closed-system assumption is essentially similar to that previously taken by Sakai ( 1968 ) and Ohmoto (1972) in constructing pH-fo2 diagrams. The significance of the time-dependent variations in mineral t~34S-valuesis an unresolved question in the interpretation of sulfur isotope data from hydrothermal ore deposits. Rye and Ohmoto (1974) generalized the trends of sulfide t~34S-values in time for various occurrences. As noted by the authors, the ~ 3 4 S - v a l ues can either increase with time (e.g., the Echo Bay deposit, Northwest Territories, Canada) or decrease with time (e.g., Darwin, California, U.S.A., and Mogul, Ireland, deposits). Ohmoto (1972) and Rye and Ohmoto (1974) attempted to apply the pH-fo2 diagrams to the interpretation of the temporal (and spatial) variations in the sulfide c~34S-valuesfor some ore deposits. Because the Rayleigh-type precipitation model has direct bearing on the temporal sequence of mineral deposition, the present results can be used to reappraise sulfur isotope data for which the relationship to mineralizing time is known. For example, the large variation in the c~348-valuesof sulfide minerals from ~ --22%0 in the early stage to ~ + 25%0 in the later stage of mineralization at the U N i - C o - A g - C u deposits of the Echo Bay mine can be explained if the fluid had the initial SO2-/H2S ratio of 5-10 with ~ 3 4 S ~ - ~ - 0 % 0 (see Fig. 4). This explanation requires that there would exist a large amount of dissolved sulfate in the fluid to equilibrate with the dissolved sulfide. Although the mineralogy of the ore and gangue minerals changed with time in the deposits (Robinson and Ohmoto, 1973), it has been not reported that there is sulfate
265
mineral to coexist with the sulfides. In view of mass conservation during isotopic partitioning between sulfate and sulfide, the present explanation can be valid only when the dissolved sulfate did exist during the precipitation of sulfides but was not able to deposit due to the restriction of physico-chemical conditions (e.g., no available metal cations such as Ba 2+ for formation of sulfate mineral). The same assumption must be taken in the previous interpretation of applying the PH-f02 diagrams to the sulfur isotope data. The chemical state of hydrothermal fluids during mineral deposition is controlled by the initial state and by subsequent changes caused by reaction with wall rocks and by the removal of previously deposited minerals. The last effect has been taken into account in modelling the reservoir effect. Fig. 4 illustrates three different paths for the chemical evolution of fluids. The first path, A, is toward the right, which may be applied to a case where the chemical state of the fluid changes only with the precipitation of pyrite. As a result, the t534Svalues of the resulting pyrite change with time, a typical case for mineral precipitation from a closed system. The second path, B, is downward to the right, which is a combination of path A with an increase in SO 2-/H2S ratio during deposition of pyrite. The increase in the SO 2-/H2S ratio can be caused in various ways, such as by incorporating sulfate into the solution from host rocks. In ore deposits formed from this solution, a trend of decreasing t~3asvalues toward the later stages of mineralization could be expected in hydrothermal sulfides. The third path, C, is upward to the right, which is a combination of path A with a decrease in SO42-/H2S ratio during sulfide precipitation. The decrease in the SO 2-/H2S ratio can be caused by incorporating sulfide into the solution from wall rocks. In ore deposits formed from this solution the c534S-values of sulfide might show a trend of rapid increase in the early stages of mineralization. Under natural conditions, the changes in the chemical
266 state of hydrothermal fluids could have been more complex. The S O ] - / H 2 S ratio may stay unchanged iffo2 is kept constant due to fluidrock interactions. It must be pointed out that the closed-system behavior has been assumed in the present modelling on reservoir effect. For many of the cases presented, significant sulfur isotope variations do not occur until about half the sulfur is precipitated. The question then arises as to how well the sulfide- or sulfate-depositing systems in nature fit the closed-system model. Perhaps the closed-system assumption is not necessarily valid in practical applications without constraints on fluid transport directions and fluid-rock exchange characteristics. Nevertheless, the present calculations can at least provide a useful approximation to estimate of possible changes in the sulfur isotope composition of hydrothermal minerals. A treatment of coupled reaction and isotope fractionation driven by one-dimensional infiltration may be probably more promising, as illustrated by Bickle and Baker (1990) for metamorphic rocks. 4. Application to the Bad Grund barite deposition The Bad Grund P b - Z n deposit occurs at the intersection of several fault zones with the Lower Carboniferous greywackes and shales in the Upper Harz, Germany. In the ore veins, galena and sphalerite are the main ore minerals, accompanied by minor amounts of chalcopyrite and pyrite. Quartz, calcite, siderite and barite are the main gangue minerals in most of the mineralized locations. The sulfides are mainly associated with the calcite and occur in the main mineralization phases (IIb, IIc, lid, Ilia and IIIb after Sperling, 1973; Stedingk and Schnorrer-Krhler, 1988). The barite occurs in a later phase (Illc) in the mineral paragenesis and is associated only with minor amounts of the sulfides. The detailed geological and geochemical characteristics of the Bad
Y-F. ZHENGANDJ. HOEFS
Grund deposit have been described by Sperling (1973), Mohr (1978) and Walther (1986). Sulfur isotopic investigations of ore sulfides have been carried out by Sperling and Nielsen ( 1973 ), R u m p ( 1978 ) and Zheng and Hoefs (1993). Sulfur and oxygen isotopic investigations have been conducted for ~ 60 samples of hydrothermal barites, which form a part of the stable isotopic studies of hydrothermal mineralizations in the Harz Mountains. Sulfur isotopic analysis was carried out by preparing H2S through reaction with Kiba ® solution at 350 °C (Hoefs, 1987 ) and the results are reported relative to CDT with a precision of + 0.2%0. Oxygen isotopic analysis of the same samples proceeded by preparing CO2 through reaction with spectral-pure graphite at 1000°C and the resuits are reported relative to SMOW with a precision of + 0.4%o (Zheng and Hoefs, 1993 ). The detailed description of the samples and a complete data set is presented in Zheng (1991b). As shown in the ~345 vs. ~ 1 8 0 diagram of Fig. 6, the ~345 range of the barites is from - 1 to +18°/0o with a mode of + 13 to + 15%o and the ~180 range is from + 1 2 to + 17%owith a m o d e o f + 14to + 16%o. Zechstein evaporites in this region have the ~34S-values of about + 10 to + 13%o with a mode of + 11%o (Nielsen and Ricke, 1964: Pilot et al., 1972). These values are close to, but slightly lower than, the mode ~345-values of + 13 to + 15%o for the Bad Grund barites. Because the barites occur in the late phase (IIIc) of the hydrothermal mineralization and are thus younger than the main P b - Z n ore deposition [ < 180 Ma after Haack ( 1990], the Zechstein evaporites are the most likely candidate for the source of barite-sulfur. However, questions remain as to how the low (~345values from - 1 to + 8%o can result for a few barites while most have ~34S-values higher than + 1 0 t o +13%o. It has been generally believed in sulfur isotope geochemistry that hypogene sulfate, equilibrated with sulfide, should be distinctly en-
SULFUR ISOTOPECOMPOSITIONOF HYDROTHERMALSOLUTIONS
20--
18-
16-
12-
\'.
10it .o
8
~_e -2 10
I
I
12
1b,
16
I
I
18
20
8' * o ( ~ )
Fig. 6. Graph of the ~34S- vs. ~180-values of barites from the Bad Grund P b - Z n deposit in the Harz Mountains, Germany. The ~'sO results are interpreted to indicate that the barites were precipitated from a low-temperature fluid of subsurface origin with a ~'80-value ~ 0%0 (for details see Zheng and Hoefs, 1993 ).
riched in 348 relative to sulfide and to any supergene sulfate derived from quantitative unidirectional oxidation of sulfide-sulfur (e.g., Field, 1966 ). Further, supergene sulfate should show tj34S-values identical to those of the initial sulfide (Ohmoto and Rye, 1979). In this regard, the isotopically light barites in the Bad Grund deposit could have derived their sulfur through oxidation and redeposition of sulfidesulfur in hydrothermal systems. However, de-
267 termination of the oxygen isotopic composition of barites indicates that the tjlSO-values for all of barites (except one) vary uniformly from + 12 to + 17%0. Only one sample has a 8'SO-value of + 1.7%o and a 834S-value of - 4 . 5 ° / 0 0 (not shown in Fig. 6 ). Obviously, we can only attribute this barite to a supergene origin. The barites which have low ~34S-values from - 1 to +8%0 show 8taO-values from + 14 to + 17%o, suggesting the same hypogene origin as the other barites with ~34S-values from + 10 to + 18%0. Thus the decrease in the 834Svalues of a few barites is due to the effect of precipitation of a sulfate mineral phase (i.e. barite) on the sulfur isotopic composition of hydrothermal fluid. According to a systematic investigation of sulfur and oxygen isotopes in the sulfides and barite from the Bad Grund deposit, Zheng and Hoefs (1993) conclude that the ore sulfides were deposited from a high-temperature fluid of deep-seated origin whereas the barite was precipitated from a low-temperature fluid of subsurface origin. A temperature below 120 ° C has been inferred for barite precipitation in this deposit according to sulfur isotopic geothermometry for sphalerite and galena pairs in phase Ilia (Zheng and Hoefs, 1993 ). Microthermometric investigation of fluid inclusions in barites gave a temperature range mainly below 100°C, some even below 70°C (Adeyemi, 1982). If we take an average value of 100°C as the temperature of barite precipitation, a theoretical diagram can be constructed with 834S~rs = + 11%00 for interpreting the observed 834S variation. As illustrated in Fig. 7, the reservoir effect can be applied to the pattern of sulfur isotopic variation in the Bad Grund barites, assuming the fluid was dominated by SO 2- with minor amounts of H2S ( S O l - / H2S~ 10-20) as suggested by the mineral paragenesis. Most of the barite samples have the 834S-values significantly higher than + 1 1%0 and thus should be deposited in early stages. Whereas the other barites which have the 834S-values significantly lower than + 1 1%o
~ .-F. ZHENGAND J. HOEFS
268 20
tAae
F,a z . l y ......
Late
18
16
8
4
-2
,
I 0.8
,
1
J
0.6
I 0.~
~ ~1~ ~ 0.2
F
Fig. 7. Influence of the fraction of sulfur remaining in hydrothermal fluid on the sulfur isotope composition of barite during Rayleigh-type precipitation of isotopicalty heavy barite at 100°C with ~345~s ~ + 1 1%0. Lined pattern indicates the predominant variation in the ~34S-values of barite from the Bad Grund Pb-Zn deposit with progress of precipitation. R is the initial ratio of dissolved sulfate/ sulfide prior to barite precipitation.
are considered as the product of precipitation in late stages. With time the change in the fluid t~34S-value with precipitation of the isotopically heavy barite can be coupled with a decrease in temperature which governs the magnitude of isotopic partitioning between dissolved sulfate and sulfide. As a result, the magnitude of ~345 variation in precipitated barites can be considerably greater than that expected in the illustration of Fig. 7.
5. Conclusions Theoretical modelling has demonstrated how Rayleigh-type distillation process during precipitation of mineral phases can significantly change the isotopic composition of sulfurbearing solutions. The reservoir effect is particularly significant when a hydrothermal solution contains not only dissolved sulfate but also dissolved sulfide. The precipitation of sulfate mineral from a closed-solution system can
lead to a significant decrease in the t~34S-value of sulfur remaining in the solution with time, whereas the precipitation of sulfide minerals can cause the solution to be progressively enriched in 345. Consequently, sulfate precipitated from a S O ] - - d o m i n a n t solution with small amounts ofH2S (1-10%) can have ~345values slightly greater than the t~34S-value of the initial solution in early stages, but remarkably lower ci34S-values in late stages. Conversely, sulfide deposited from a H2S-dominant solution with minor amounts of SOl- (1-5%) can have t~34S-values similar to the t~34S-value of the initial solution in early phases but significantly higher ~34S-values in late phases. The theoretical results have important bearing on the interpretation of sulfur isotopic variations with respect to the time sequence of mineral deposition in the life of a hydrothermal system. Sulfur and oxygen isotopic variations of barite in the Bad Grund deposit can be interpreted to represent the precipitation of barite from a solution reservoir with ~345~S ~ "~- 1 1%o at temperature about 100°C. Those barites which show the low ~34S-values from - 1 to + 8%o are considered as the product of deposition in late stages. The minor amounts of dissolved sulfide are suggested to coexist with dominant SO 2- in the low-temperature fluid.
Acknowledgements This study has benefited from the support by the German Science Foundation ( D F G ) within the framework of the project "Intraformational Mineralizations". We are grateful to Dr. K. Stedingk for providing some samples. Comments on this manuscript by Drs. M.J. Bickle, C.W. Field and H. Sakai as well as two anonymous reviewers help to clarify some ambiguities.
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SULFUR ISOTOPE COMPOSITION OF HYDROTHERMAL SOLUTIONS
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