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Fluorite deposition in hydrothermal systems
0016.7037:79/0801-13?75o2.oo'o
Goochimice er Cosnwchima Arm Vol. 43. pp 1327 10 1335 0 Pergamon Press Ltd. 1979. Prmred m Great Bntam
Fluorite deposition in hydrothermal systems CATHERINE K. RICHARDSON* and H. D. HOLLAND Department
of Geological Sciences. Harvard University, Cambridge, MA 02138, U.S.A. fReceioed
1 September
1978; accepted
in revised form
4 April 1979)
Abstract-During the formation of fluorite deposits fluorite is precipitated either as a consequence of changes in temperature and pressure along the Aow path of hydrothermal solutions or due to fluid mixing or as the result of the interaction of hydrothermal solutions with wall rocks. A decrease in temperature in the flow direction is the most appealing, though still unproven, mechanism of fluorite deposition in Mississippi Valley fluorite deposits. Mixing can produce solutions which are either undersaturated or supersaturated with respect to fluorite. The most important parameters are the temperature, the salinity, and the calcium and fluoride concentration of the fluids prior to mixing. A variety of wall rock reactions can lead to fluorite precipitation. Among these reactions which increase the pH of initially rather acid (pH 5 3) hydrothermal solutions are apt to be particularly important.
INTRODU~ION
FLUORITEOCCURSin a wide range of ore deposits both as an ore and as a gangue mineral. It is a common and sometimes abundant primary mineral in some of the stratiform galena-sphalerite-fluorite-barite (‘Mississippi Valley’ type) deposits and in many hydrothermal replacement and fissure-filling deposits. It also occurs commonly in tin greisen deposits, hydrothermal gold-telluride deposits, tin-tungsten deposits, and hydrothermal uranium deposits. In addition, fluorite occurs as an accessory mineral in some volcanic rocks, fluorine-rich pegmatiteq and skarns. Where fluorite is the major ore mineral, an understanding of the probable mechanism(s) by which it is deposited may well be useful in locating new deposits or ore horizons. Where ffuorite is a gangue or accessory mineral, the mechanism of fluorite deposition may be an important clue to the processes which controlled the deposition of the ore minerals. Recent solubility data (RICHARDSON and HOLLAND, 1979) can be applied directly to the study of fluorite ore deposits which formed between 100” and 260°C and in which the major ions present in the fluoritedepositing solutions were restricted to Ca”, Na’, K’, Mg’+, Cl-, HCO;. CO:-, SO:-, H2S, H’ and F-. If the concentration of other ions such as Fe3+, Sn’+, Sn’+, U6+, U4’, or B3’ was unusually high in the hydrothermal fluids, the stability constants for complexes involving these ions must be known before the solubility of fluorite can be calculated. Since most of these constants are not known. it is likely that the data in this study are not directly applicable to the study of fluorite deposition in hydrothermal uranium deposits,‘ tin greisens, or hydrothermal tintungsten deposits. * Present address: Department of Earth Sciences, Iowa State Uriiversity of Science and Technology, Ames, IA 50011, U.S.A.
Sources of data solutions
on the composition
of &rite-depositing
In order to apply the available solubility data for fluorite to specific ore deposits, a good deal must be known about the composition of the solutions from which the fluorite was deposited. The most informative sources of such knowledge’ are leach analyses of the major elements in inclusion fluids in fluorite. Salinity measurements of the inclusion fluids in fluorite are also quite useful. The data available on the composition of inclusion fluids in fluorite are summarized in Table 1. A secondary source of data for the composition of ore forming fluids from which fluorite was deposited is the mineralogy and zoning of ore deposits. The carbonate mineralogy and the presence or absence of feldspars and their a&ration min&ls can sometimes place limits on parameters such as the Ca/Mp, ratio, the Na/K ratio, and ihe pH of the fluids. Unfortun%ely, in many kuorite deposits, most of the minerals that are particularly useful in defining the composition of the ore-forming fluids are absent. However, the simple presence of limestone as a wall rock in many duorspar deposits provides an upper limit for the Mg/Ca ratio of the fluids and a lower limit for their pH. The composition of deepseated brines produced from drill holes is another source of information regarding the composition of the ore fluids. Many of the Mississippi Valley deposits are found near commercial oil fields. Analyses of these fluids give the present day composition of connate brines in the producing horizon. Two lines of evidence suggest that the fluids responsible for Mississippi Valley type fluorite deposits may have been predominantly connate water : (1) in many fluorite deposits organic matter both as asphaltum and as oil droplets is present in fluid inclusions. (2) the composition of inclusion fluids is similar to that of brines in nearby sedimentary basins. At Cave-in-Rock. Illinois, for instance, the composition of the primary component of the inclusion fluids is similar to that of brines in the Illinois basin (HALL and FRIEDMAN, 19631 Data for the composition of oil field brines are available for most of the regions of the United States where Mississippi Valley type deposits occur (see for instance CARPENTER et al.. 1974). Most of these brines are saline to very saline and are rich in NaCl and CaC12 (I = l.O6.Of, Na/Ca = 2-5).
1327
C. K. RICHARDSQN and H. D. HOLLAND
I328
Table 1. Summary of data available on the composition of inclusion fluids in fluorite Results
Deposit
References
I. Quantitative analyses of fluid inclusions in fluorite (T < 260°C) Southern Illinois-Kentucky I = 1.5-4.7f; tn&cII = 0.1-0.3_f; m~gc-,I= 0.04-0.3f deposits
I = 0.9-1.8; mc,clz = 0.1-0.31’; %gci2 = @O.l2f II. Qualitative and Partial analyses of fluid inclusions in fluorite (T < 260°C) rich in NaCl and CaCl,; salinities Northern Coahuila, Mexico are 8-40 equiv. NaCl wt.% rich in NaCl and CaC12 Hansonburg, New Mexico Aurakhmat fluorite deposit, Central Asia
III. Salinities of inclusion fluids in fluorite (T < 260°C) 26-50 equiv. NaCl wt.% Jamestown, Colorado Mex-Tex, Hansonburg, New Mex. 10.5-17 equiv. NaCl wt.“; Northern Pennine orefield, 20equiv. NaCl wt.“:, Derbyshire, England 5-15 equiv. NaCl wt.?; Fluorite polymetallic S. Gissar
FLUORIDE
TRANSPORT
Compositional data available from these various sources suggest that the solubility data reported by RICHARDSON and HOLLAND(1979) can be used to calculate the solubility of fluorite in the solutions which produced many low temperature fluorspar deposits. Complexing played an important role in fluoride transport in these solutions. In brines containing what appear to be normal concentrations of NaCl, CaCl, and MgCl* nearly all of the fluoride is present as a constituent of CaF+, MgFc and NaF” complexes. In acid solutions of pH 5 3 more than half of the Buoride is apt to be complexed as HF”. Thus the concentration of HF” can be sienificant in fluids from which alteration minerals such as-&unite are formed. However, the available mineralogical and chemical data for Mississippi Valley type deposits imply a pH between 4 and 7 for the ore-forming solutions responsible for these deposits (BANA~ZAK,1975; HELGESON,1964); in this pH range transport of fluoride as a constituent of HF” complexes is negligible.
FLUORITE
DEPOSITION
Fluorite can be precipitated from hydrothermal solutions by a variety of mechanisms. The most likely among these are (1) deposition due to changes in the temperature and pressure of the ore fluid; (2) deposition by mixing two or more fluids of different chemi-
Table 2. Composition
HALLand FRIEDMAN,1963 (see Table 2) PINCKNEYand HAFFTY,1970 PINCKNEY,personal communication GRUsntct~, 1958
KESLER,1977 AMES,1958 NASH and CUNNINGHAM,1963 ROEDDERet al., 1968 SAWKIN~,1966 RAKHMANOV. 1968
cal composition; (3) deposition due to the interaction of ore-forming fluids with wall rocks. In the foilowing section each of these precipitation mechanisms will be considered in some detail. The discussion will focus primarily on the quantity of fluorite which can be precipitated by the operation of each mechanism. The fluorite deposits in the Cave-in-Rock district (Illinois) will be used as a specific example for testing the efficacy of the several depositional mechanisms. Temperature changes Precipitation of fluorite due to the cooling of hydrothermal fluids has been proposed as a possible fluorite depositional mechanism by FREAS (1961) BENEWVA et al. (1969), BANASZAK (1975) and PINCKNEY(unpublished manuscript). The solubility of fluorite in most aqueous solutions decreases with decreasing temperature; fluorite therefore tends to precipitate with decreasing temperature (ANIKIN and SHWHKANOV, 1963; STRUBEL, 1965; STRUJJELand SCHAEFER, 1975; RICHARDSON and HOLLAND, 1979). Dilute (< 1.0 M) NaCl or KC1 solutions are an exception to this rule. In such dilute solutions, the solubility of fluorite goes through a maximum of about 100°C and decreases with increasing temperature up to ca.
of fluid inclusions from the Cave-in-Rock District, Illinois (HALL and FRIEDMAN,1963)
Mineral
Na
K
Yellow fluorite Yellow fluorite Early white fluorite Blue fluorite Blue fluorite Early purple fluorite Late purple fluorite Late purple fluorite Late purple fluorite Late purple fluorite
1.96 2.13 1.00 1.74 2.04 1.78 1.88 2.62 1.88 2.20
0.06 0.07 0.04 0.07 0.08 0.08 0.05 0.13 0.10 0.10
Concentration (mol/kg solution) Ca Cl Mg 0.15 0.19 0.12 0.30 0.21 0.11 0.15 0.20 0.17 0.10
0.09 0.05 0.04 0.27 0.23 0.25 0.04 0.45 0.25 0.16
2.37 2.65 1.33 2.65 2.77 2.20 2.45 3.78 2.62 2.82
SO,
Total
Ionic strength
0.03 0.02 0.01 0.05 0.04 0.06 0.01 0.07 0.01 0.02
4.66 5.11 2.54 5.08 5.37 4.48 4.58 7.25 5.03 5.40
2.73 2.94 1.52 3.47 3.41 2.87 2.63 4.70 3.16 3.12
Fluorite deposition in hydrothermal
systems
90 NoCl
80
0
I
NoCl -
Hz0
60
I=4
CeC12
- ~~0
,’
/i
1329
/I
OM ,‘4-
f”
0 M Nacl
10 60
c----g
0 3M coc12 3 l M NoCl
‘---xx
260
200
I50
100
0 2 M COC12 3.4M NoCI
50
25
TnC
260
200
150
100
50 25
T’ C Fig. 1. The quantity of fluorite precipitated by cooling NaCl solutions of varying ionic strengths from 260’ to 25°C. Negative amounts of fluorite deposition indicate fluorite dissolution. 360°C. A series of curves have been constructed
in Fig. 1 defining the quantity of fluorite that precipitates from NaCl solutions of various concentrations as they are cooled from 260” to 25°C (see Appendix for method of calculation of the 25°C data). These 100
90
NoCI-
0
CaCl2-
Hz0
N
‘,
70
I=
2,ow
: u”
60
:
;
50
0 0 c ::
40
0,”
30
‘;:
20
0.9 0.4M M NoCl coc12 0,3 M C0C12 I. I M NaCl 0.2 M CDCl2 1.4M NPCI 0. I M cac12 1.7M NaCl co
z G
0.02 I.94
M cac12 M NoCl
IO
0 260
200
I50
IO0 TO
50
25
c
Fig. 2. The quantity of fluorite precipitated by cooling NaCI-CaCI,-Hz0 solutions with an ionic strength of 2.0 M from 260’ to 25°C. Dashed part of the curves go through points at temperatures that are below those at which solubility measurements were made in this study (see Appendix 1 for method of calculation).
Fig. 3. The quantity of fluorite precipitated by cooling NaCI-CaQ-Hz0 solutions with an ionic strength of 4.0M from 260’ to 25°C. Dashed part of the curves go through points at temperatures that are below those at which solubility measurements were made in this study (see Appendix 1 for method of calculation).
curves show that the quantity of fluorite precipitated increases with the salinity of the fluids, that at all ionic strengths the solubility of fluorite changes most rapidly with temperature at lower temperaturesespecially between 50’ and 25°C; and that as 0.5 M and 0.1 M NaCl solutions are cooled from 260°C they first. become undersaturated with respect to fluorite and do not begin to precipitate fluorite until they have cooled to temperatures below 100°C. A similar set of curves was calculated for solutions containing CaCl, or (CaCl, + MgCl,) in addition to NaCl (Figs 2-5). In NaCl solutions precipitation of fluorite with decreasing temperature is the result of changes in the value of the solubility product K, of fluorite with temperature and of the strength of the complex NaF”. The corresponding curves for fluorite deposition from NaCl-CaCl,-MgC1, solutions are influenced in addition by changes in the stability of CaF’ and MgF+ with decreasing temperature. As temperature decreases, the complexes MgF+, CaF’ and NaF” all become less stable (RICHARDSONand HOLLAND. 1979): on cooling they release free fluoride ions which can combine with calcium to precipitate fluorite. The curves in Figs 2-5 indicate that the amount of fluorite precipitated per kilogram of solution increases with the Ca (or Ca + Mg) concentration and with increasing ionic strength except in solutions containing very little CaCl,. The solubility surface which defines the quantity of fluorite deposited as a function of (Q,~,, + m,&,,) and temperature goes through a minimum between rncaC,>= 0 This miniand mcacl, = 0.02 M at all temperatures. mum separates the region of solution compositions
K.
C.
1330
NaCl
90
and H. D.
RICHARDSON
- Cm2
- MgCl2-
I=
2.0
Ca/Mp
=
H20
60
0
9/l
r”
HOLLAND
0 44M
e---PO-66M
Jr-
e-40.33M
I.01
,---oO22M
I
34M
0
I
L .RO
200
260
150
50
100 7’
M
cac12 NaCl
t
CaCl2 NaCl
t
M9C12
C0C12 NaCl
t
M9Cl2
I
I M cac12 67M NaCl
O-022 I 936
M9C12
t
M9Cl2
M CaCI2 M NaCl
t
MQC12
25
C
Fig. 4. The quantity of fluorite precipitated by cooling NaCl-CaCls-MgCl,-Hz0 solutions with a Ca/Mg ratio of 9/l and an ionic strength of 2.0 M from 260” to 25°C. Dashed parts of the curves go through points at temperatures below those at which solubility measurements were made (see Appendix 1 for method of calculation).
occurs between 260” and 200°C. As the temperature
in which the volubility of fluorite is controlled largely by fluoride complexing from the region where the solubility of fluorite is controlled largely by the com-
decreases
mon ion effect. Figures 2-5 show that the largest change in solubility with temperature (dSol/dT) I IO
NaCl
- CaCl2
- MgCl2
-
.Q Ca/Mg*
9/l
b / m. I
---. 90 / o I” 0
:
P z
loo”C,
dSol/dT
gradually
decreases.
H20 ---_
100
to
Between 100°C and 25’C in 2.0 M solutions, for instance, the amount of fluorite that can be precipitated by cooling either NaCl + CaCl* solutions or
0 44 2.66
M M
cac12 NoCl
t
MpCl2
0.33 M 3.0 I M
CaCl2 NaCl
t
MgC12
0.22 334M
M
CaCl2 NaCl
t MqCl2
0 I I M I ev..
CaCl2 .,_.-I
t
0.022M 393M
COCl2 NaCl
6 0 ‘s 7
6
ur
----IQ
&
/--r
M9C12
t
MiJC12
OYI
260
200
I50
too T*
50
25
c
Fig. 5. The quantity of fluorite precipitated by cooling NaCl-CaCl,-MgCl,-H,O solutions with a Ca/Mg ratio of 9/l and an ionic strength of 4.0 M from 260” to 25°C. The dashed parts of the curves go through points at temperatures below those at which solubility measurements were made (see Appendix 1 for method of calculation).
Fluorite deposition in hydrothermal 110
11 pH = 2
100 90
NaGI- HCI - H20 I = 2.0
I" 0
60
1
260
i
200
150
100 T’
50
25
C
Fig. 6. The quantity of fluorite precipitated by cooling NaCI solutions at varying pH values from 260” to 25°C.
NaCl + CaCl, + MgClz solutions is only 3-6 mg CaF,/kg Hz0 (rather than 6-43mg CaFdkg Hz0 in equivalent 75°C temperature intervals at higher temperatures). The equivalent curves for ionic strength 4.0M are quite similar to those in 2.0M solutions; however, there may be a small reversal in the precipitation curves below 100°C. If this reversal turns out to be real, dissolution of fluorite in these solutions can take place as temperatures fall below 50°C. The curves in Fig. 6 show the quantity of fluorite deposited due to cooling from 2.0 M NaCl solutions containing small concentrations of HCl. As much as a hundred mg CaF,/kg Hz0 can be precipitated by cooling a 2.0 M NaCl-HCl solution with a pH of 2 from 260’ to 200°C. The ore-forming fluids which produced fluorite deposits of the Mississippi Valley type could not, however, have had such a low pH. The potential importance of cooling as a depositional mechanism in a particular fluorite deposit depends on the amount of cooling which occurred during fluorite deposition. Little is known about temperature gradients within ore deposits during mineral deposition. Fluorites in some deposits, such as those in the Cave-in-Rock district, show a fine succession of delicate growth bands. The same pattern of growth banding has been observed in fluorite crystals throughout individual deposits as well as in mines throughout the district (PINCKNEYand HUGHES,unpublished manuscript). Careful measurements of the filling temperature of fluid inclusions in individual growth bands in samples of fluorite from all parts of these deposits should therefore yield information on the temperature distribution during fluorite depo-
systems
1331
sition. A thorough study of this type has not been done. Filling temperature differences of 40-50°C for fluid inclusions within the same growth band in fluorite crystals from different parts of the same deposit would certainly have been noticed in previous studies (GROGANand SHRODE.1952; FREAS, 1961: ROEDDER. 1967): differences of lO-20°C could have been overlooked. It is therefore unlikely that the ore fluids cooled as much as 40-50” as they passed through a single deposit in the Cave-in-Rock district, but a temperature change of lO-20°C during a single stage of fluorite deposition is not ruled out by the data available at the present time. Vertical temperature gradients in the deeper parts of bore holes in active geothermal systems are frequently in the range of 16”-40”C/km (WHITE, 1968: McNrrr, 1970; MUFFLER,1975). It seems likely that temperature gradients during the formation of hydrothermal ore deposits are similar in magnitude. If so. the temperature differential across Mississippi Valley type fluorite ore deposits, many of which are ca. 500m in length, may well have been on the order of 8’-20°C. The quantity of fluorite which can precipitate from a variety of fluids during cooling by 10°C in the applicable range of temperatures falls between 0 and 25 mg CaFz/kg H20, except for solutions which have a pH 5 4 (see Figs l-5). Quantities of fluorite at the high end of this range are considerably greater than the amount proposed by HOLLAND (1967) prior to the discovery of the importance of CaF+ and MgF+ for fluoride transport in hydrothermal solutions, and are consistent with reasonable volumes and fiow rates of the fluids which produced, for instance, the Cave-in-Rock deposits in Illinois. Fluorite precipitation as a consequence of simple cooling is also consistent with the delicate color banding of fluorite throughout this and related deposits. However, proof that cooling was the major cause of fluorite deposition still awaits convincing data that define the temperature gradient in these ore deposits during fluorite precipitation. Precipitation qfjhmite
due to changes in pressure
As solutions flow towards the surface, there is normally a decrease in confining pressure. The solubility of most gangue minerals (calcite, barite, dolomite. anhydrite and qUartZ--HOLLAND, 1967) decreases as pressure is decreased. Solubility studies by STR~JBEL (1965), RUMYANTW and RUMYANTSEVA (1969), and MACDONALDand NORTH (1974) have shown that fluorite is no exception to this rule. At 255°C the solubility of fluorite in water decreases from 16.2 to 13.0 mg CaF,/kg as the pressure is dropped from 1642 atm to 43.3 atm. Most huorite deposits span a vertical range of 10&5OOm, which corresponds to a pressure decrease of only 10-50 atm (hydrostatic gradient). The amount of fluorite precipitated from dilute aqueous solutions as a consequence of a pressure decrease is therefore probably about an order of magnitude less than the quantity of fluorite precipitated
C. K. RICHARDKIN and H. D. HOLLAND
1332
0
0
,og 25 \p 20
T=
NW
T = ZOO’C
$2
I. - 25 2.r" 'No 20 u-:: I5 :'o IO F 5 -----_Q
NaCl-H20
;'; "0.
I5 IO 5 E" __--_O i --_-_-_--___---__-
NoCl
200'C
- CoCl2 - Ii20
CaCl2
5 IO 15 20 25 30
F
0
2
4
L
vol. vol
0
,2 vol. vol.
.4
,6
original
original
.8
I.0
Sol’n. sol’n.
t
vol.
water
6
I.0
6
OrIgInalsoih
original
sol’n.
+
YOI
waG
Fig. 8. The quantity of fluorite dissolved or precipitated per kg Ha0 during the dilution of NaCI-CaC12-Hz0 solutions at ionic strengths of 2.0 M and 4.0 M and 200°C with water at the same temperature.
Fig. 7. The quantity of fluorite dissolved per kg H,O dur-
ing the dilution of NaCl solutions at 200°C with water at the same temperature. from brines due to the accompanying decrease in temperature. The effect of pressure on the solubility of fluorite in concentrated NaCl-CaCI, solutions has not been measured, and since the molar volume of CaF’ is not known, the AV for the reaction CaF, + Ca’+ P 2CaF+ cannot be calculated. It seems likely, however, that the change in fluorite solubility with pressure in hydrothermal solutions along most flow paths in hydrothermal systems is small compared to the change in solubility due to the accompanying change in temperature in concentrated brines as well as in dilute solutions. Mixing of jluids
to fluorite results in undersaturation with respect to fluorite. If 4 M NaCl solutions saturated with respect to fluorite at 200°C are mixed with NaCl-free solutions also saturated with respect to fluorite at 200°C. the mixtures are all undersaturated with respect to fluorite. The degree of undersaturation can be estimated from Fig. 7 by anchoring all of the curves at the point mg CaFz/kg Hz0 dissolved = 0, volume original solution/volume original solution + volume water = 0. The same calculations were performed for brine solutions which contained CaCl, or CaCl, + MgCl, . These calculations are summarized in Figs 8 and 9. The curves in these figures show that mixing pure water in any proportion with 2.0 M NaCl + CaCl, or NaCl + CaCl* + MgClz *solutions causes the mixture to become undersaturated with respect to fluorite. In 4.0 M NaCl + CaCl* + MgClz solutions, however, the addition of a small amount of pure water causes the mixture to become supersaturated
Mixing of two or more fluids of different chemical composition has been proposed as a possible mechanism for fluorite deposition by HALL and FRIEDMAN T = 200'C (1963), DUNHAM(1966), SAWKMS(1966), HEYL et al. NaCI- CaC12 - MgC12 -H20 (1974), BANASZAK(1975), and KESLER(1977). Mixing 0 C0/Mg 8 9/l N can occur in several different ways and can involve =3 25 a variety of solutions. Meteoric water can mix with :"E 20 t I a concentrated brine at the same temperature (dilution at constant temperature); meteoric water can mix 0.44 M COCl2 + MqC12 with a concentrated brine at the same temperature (dilution at constant temperature); meteoric water can mix with a concentrated brine at a different temperature (dilution plus cooling); and two brines of different chemical composition can mix isothermally or non-isothermally. A series of calculations were performed in which ,2 .4 ,6 .6 I.0 brine solutions initially saturated with respect to vol. original rol’n. fluorite at 200°C were mixed with varying proporvol. original sol’n. t vol. wobr tions of pure water, also at 200°C. The results of these calculations for NaCl brines are summarized in Fig. 7. Fig. 9. The quantity of fluorite dissolved or precipitated per kg Ha0 during the dilution of NaCl-CaCl,-MgCl,The curves in this figure show that dilution of 0.5 Ha0 solutions at ionic strengths of 2.0 M and 4.0 M and 4.0M NaCl solutions initially saturated with respect 200°C with water at the same temperature.
Fluorite deposition in hydrothermal systems with respect to fluorite by O.l-1.2mg CaFl/kg H20. As more water is mixed with these brines, they become undersaturated with respect to fluorite. These calculations show that mixing brine solutions with pure water is a mechanism primarily for dissolving rather than precipitating fluorite; from mixtures that do become supersaturated with respect to fluorite. only ca. 0.1-l mg CaFl/kg HZ0 can be deposited. This quantity is considerably smaller than that probably deposited by the simple cooling of most ore fluids. If the brines in Figs 8 and 9 are mixed with dilute NaCl free solutions saturated with respect to fluorite (mg CaF,/kg dissolved = 0 and vol original solution/v01 original solution + water = 0). a larger quantity of fluorite would be precipitated. This quantity, however. is still about the same as or less than the amount deposited by cooling similar solutions by 10°C. If a hot, concentrated brine moving up a fracture mixes with cooler meteoric water, a change in temperature accompanies dilution. Calculations were performed in which NaCl or NaCl + CaCl, -t MgCI, solutions at 200°C mix with water at 25’C (AH of mixing assumed = 0). This temperature difference was chosen to define the maximum probable effect of dilution accompanied by cooling. For NaCl solutions, dilution plus cooling results in mixtures that are undersaturated with respect to fluorite at all ionic strengths. The dilution of a NaCl-CaCl,--MgCl, solution with I = 4.0 M. m,, = 0.4 M, and mug = 0.04 M at 200°C with water at 25°C is followed in Fig. 10. This figure indicates that dilution and cooling of such a brine results in supersaturation with respect to huorite until the water: brine ratio is approximately 9:l. Up to 15 mg CaF,/kg HZ0 can be precipitated during mixing of two such solutions. If the cold dilute solution is already saturated with respect to fluorite. the amount of precipitation of fluorite upon mixing the solutions would be even greater than that shown in Fig. 10. In the Cave-in-Rock district the temperature decrease during fluorite deposition within a given ore 0.4M 0
CaC12, 2.66
N
I-0 ,” H ._o
20
MgCI2,
I
= 4-OM
L
I
__ ______________________
IO 15
\;
20
IAT?*; s
25
E”
M
5
0
2;
0.04 NaCI,
25
-______o I”
M
u 0
.2
.4
.6
v-31 original vol. original
.6
I.0
sol’n
sol’n. + vol water
Fig. 10. The quantity of fluorite precipitated per kg Hz0 during the dilution of a solption with I = 4.0 M. mrrC,, = 0.4 M and mHeri2= 0.4M at 2CO”Cwith water at 25’C.
1333
deposit was apparently less than or equal to 20-C. This corresponds to the dilution of a 200°C brine by less than or equal to 109; of its mass of pure water at 2O’C. Figure 10 shows that the amount of fluorite precipitated by such dilution is comparable to the amount of fluorite which would be precipitated by simple cooling of the brine. The mixing of two brines. one rich in calcium. the other rich in fluoride, has often been proposed as a mechanism for depositing fluorite. Most proponents of this mechanism have advocated mixing calciumbearing connate waters with deep solutions carrying fluoride of volcanic origin (HALL and FRIEDMAN. 1963: DUNHAM.1966: SAWKINS.1966). The changes in the ionic strength and in the calcium, magnesium and fluoride concentration that occur as the solutions are mixed determine whether and how much fluorite is precipitated or could be dissolved. The amount of fluorite which might be precipitated per unit mass of mixture is considerably larger than the quantity that is apt to be precipitated by cooling alone. and. the total volume of water that would be necessary to form a given deposit would be smaller than that predicted for a simple cooling model. interaction
with wall-rocks
Fluorite can be precipitated as a result of the interaction of ore-forming solutions with host rocks. Fluorite deposition can follow, for instance. pH changes and changes in the calcium and magnesium concentration of hydrothermal fluids. Fluorite is quite soluble in acid solutions (RICHARDSON and HOLLAND. 1979) (Figs 3 a-c). and a pH change from acid to near neutral can lower the solubility of fluorite by as much as 100 mg CaF,/kg HzO. At 2OO’C the solubility of fluorite in a 2.0 M NaCl solution at a pH of 5 is 63mg CaFJkg HzO, whereas its solubility at 200°C in a 2.0 M NaCl solution at a pH of 3 is 104 mg CaF,/kg HZO. The pH of hydrothermal solutions is generally > 3 at 200°C; the pH of ore-formmg fluids of Mississippi Valley type deposits was probably in the range of 4-5 at 200°C (BANASZAK.1975); it is therefore unlikely that pH changes contributed significantly to the precipitation of fluorite in these deposits. However. in the formation of fluorite deposits associated with volcanics. rather acid solutions may well have been involved, and pH changes due to interaction with wall rocks may well have contributed significantly to the precipitation of fluorite. If a hydrothermal fluid entering a limestone terrain dissolves calcite. the calcium concentration of the fluid is increased. If the initial solution is very close to a pure NaCl solution. such an increase in the calcium concentration can lead to the precipitation of fluorite (see Fig. 5 in RICHARDSONand HOLLAND, 1979). However, most hydrothermal fluids contain rather large concentrations of calcium, and it is likely that this mechanism for precipitating fluorite is not important. In fluids that contain large amounts of calcium (0.1-0.5 m) such as those at Cave-in-Rock.
C. K. RICHARDSON and H. D. HOLLAND
1334
increasing the calcium concentration of the fluid increases, rather than decreases, the solubility of fluorite and is apt to produce fluorite dissolution. The solubility of fluorite increases with increasing magnesium concentration as well as with increasing calcium concentration in solution (RICHARDSONand HOLLAND, 1979, Fig. 7). Thus, a decrease in the calcium or in the magnesium concentration by precipitation of calcium and/or magnesium minerals may lead to fluorite deposition. Dolomitization of limestone may also lead to fluorite deposition. Since MgF’ is more stable than CaF+, a reduction of the magnesium concentration, even if it is accompanied by an equivalent increase in the calcium concentration, can cause precipitation of fluorite. If a solution at 200°C that has an ionic strength of 3.0M, an mc, of 0.3 m, and an mr,,, of 0.08 m exchanges Mg for Ca by dolomitizing a limestone until mc, = 0.35 m and m,,s = 0.03 m, the solubility of fluorite changes from 45.4 to 36.0mg CaFJkg HsO. As much as 9.4 mg CaFZ/kg Hz0 could therefore precipitate from this fluid. This quantity of CaF2 is comparable to that potentially precipitated by cooling the initial fluid from 200” to 180°C. However, in order to change the magnesium concentration of the fluid by 0.05 mol/kg, 9.2 grams of dolomite must be produced by each kg of solution. This in turn implies that the ratio of dolomite/fluorite in a deposit formed by the proposed mechanism is ca. 980/l. Some dolomitization of the limestone in the Cave-in-Rock district has been reported (PINCKNEY and RYE, 1972), but the ratio of dolomite to fluorite is orders of magnitude smaller than 980/l. Other deposits, such as the MexTex deposits in New Mexico and the fluorspar deposits of Northern Coahuila, Mexico, occur in virtually undolomitized limestone. It is therefore unlikely that dolomitization of limestone host rock has been a major factor in the deposition of fluorite in any of these fluorspar deposits.
CONCLUSIONS
This study of the deposition of fluorite ores suggests that the most likely mechanisms for fluorite deposition are (1) simple cooling of ore fluids; (2) dilution of ore fluids with cool meteoric water; (3) mixing of two brines of different chemical composition, and (4) an increase in the pH of acid ore fluids. For the fluorite deposits in the Cave-in-Rock district, Illinois, the most appealing mechanism of fluorite deposition appears to be simple cooling of the ore fluids. However, the evidence in favor of this mechanism is still inconclusive. Temperature gradients during ore deposition within the district that are consistent with this interpretation are permitted but not demonstrated by the available fluid inclusion data; additional fluid inclusion filling temperature measurements are needed to establish the magnitude of temperature gradients within the ore bodies during fluorite deposition. Other precipitation mechanisms
are currently unattractive. Mechanisms which involve the mixing of solutions are unattractive, because they are difficult to reconcile with the consistent patterns of color banding in fluorite that persist throughout the Cave-in-Rock district. Wall-rock reactions can be ruled out as a major cause of ore deposition because neither the probable changes in pH nor the permitted decrease in Mg/Ca ratio in the fluids can produce the deposition
of significant
quantities
of fluorite.
Acknowledgements-The content of this paper has profited from discussions with U. PETERSEN,D. M. PINCKNEYand U. FEHN. Financial support for the experiments on which this study was based was provided by the Committee on Experimental Geology at Harvard University. Financial support for the preparation of this manuscript was from NSF grant No. EAR 78-15190 (to CKR). REFERENCES AMESL. L. (1958) Chemical analyses of the fluid inclusions in a group of New Mexico minerals. Econ. Geol. 53, 473-480. ANIKINI. N. and SHUSHKANOV A. D. (1963) The solubility of fluorite in aqueous solutions of electrolytes. KristallogruJiya 8, 128-130. BANASZAKK. J. (1975) Genesis of the Mississippi ValleyType lead-zinc ores. Unpublished Ph.D. Thesis, &&western Univers$y. BENE~OVA Z. and CADEKJ. (1969) Temperature of homogenization of inclusions in fluorite deposits of Czechoslovakia. (abst.) in Proceedings of COFFI 2, 13-14. CADEK J., VE~ELLJ. and SULCEKZ. (1971) Bildung von Fluoridkomplexen mit Erdalkalimetallen. Colfection Czechosloo. Chem. Commun. 36, 3377-3381. CARPENTER A. B., TROUTM. L. and PICKETTE. E. (1974) Preliminary report on the origin and chemical evolution of lead and zinc rich oil field brines in central Mississippi. Econ. Geol. 69, 1191-1206. DUNHAMK. C. (1966) Role of juvenile solutions, connate waters and evaporitic brines in the genesis of lead-zincfluorite-barite deposits. I.M.M. Trans. 75, B226229. ELQUI~TB. and WEDBORGM. A. (1978) Stability constants of NaS04-MgSO,, MgF+, MgCl’ ion pairs at the ionic strength of seawater by potentiometry. Mar. Chem. 6, 243-252.
FREAS D. H. (1961) Temperatures of mineralization by liquid inclusions, Cave-in-Rock fluorspar district, Illinois. &on. Geol. 56, 542-556. GR~GAN R. M. and SHRODER. S. (1952) Formation temperatures of Southern Illinois bedded fluorite as determined from fluid inclusions. Am. Miner& 37, 555-566. GRUSHKING. G. (1958) Physicochemical factors affecting equilibrium during mineralization of the Aurakhmat fluorite deposit (Central Asia). Vses. Nauchnolssled. Inst. P’ezooprichesk. Mineral. Syr’ya Trudy 2, 81-92.
HALL W. E. and FRIEDMANI. (1963) Comoosition of fluid inclusions, Cave-in-Rock fluorite distri’ct, Illinois and Upper Mississippi Valley zinc-lead district. Econ. Geol. 58, 886911. HELGE~~NH. C. (1964) Complexing and Hydrothermal Ore Deposition, Pergamon Press. HEYLA. V., LANDL~G. P. and ZARTMANR. E. (1974) Isotopic evidence for the origin of Mississippi Valley-type mineral deposits: A Review. &on. Geol. 69, 992-1006. HOLLANDH. D. (1967) Gangue minerals in hydrothermal deposits in Geochemistry of Hydrothermal Solutions (ed. _~ H. L. Barnes), Holt, Rinehart & Winston. KESLER S. E. (1977) Geochemistry of Manto Fluorite deposits, Northern Coahuila, Mexico. Icon. Geol. 72, 204-218.
Fluorite deposition in hydrothermal MACDONALDR. W. and NORTH N. A. (1974) The effect of pressure on the solubility of CaCO,, CaF,, and SrSO, in water. Can. J. Chem. 52, 3181-3186. MCNITT J. R. (1970) The geologic environment of geothermal fields as a guide to exploration. Proceedings of the United Nations Symposium on the Development and Utilization of Geothermal Resources, Piss, 1970. Geothermics Special Issue 2, v. 1. 24-30.
R. and KESTER D. R. (1976) Sodium fluoride ion-pairs in seawater. Mar. Chem. 4, 67-82. MUFFLERL. J. P. (1975) Summary of Section II geology, hydrology, and geothermal systems. Proceedings of the
systems
1335
Thermodynamic parameters of the alkaline earth monofluorides. J. lnorg. Nucl. Chem. 30, 2067-2070. WHITE D. E. (1968) Hydrology, activity, and heat flow of the Steamboat Springs thermal system, Washow County, Nevada. U.S.‘Geol. Survey Prof. Paper 458-C, l-109. WHITE D. E., ANDERSON E. T. and GRUE~BS D. K. (1963) Geothermal brine well. Science 139. 919-922.
MILLER G.
Second United Nations Svmnosium on the Develonment and Use of Geothermal Resources. San Francisco,‘l975,
1, xLv-Lii. NASH J. T. and CUNNINGHAM C. G. (1973) Fluid-inclusion studies of the fluorspar and gold deposits, Jamestown district, Colorado. Econ. Geol. 68, 1247-1262. PINCKNEYD. M. and HAFF~ J. (1970) Content of zinc and copper in some fluid inclusions from the Cave-inRock District. Southern Illinois. Econ. Geol. 65, 451-458. PINCKNEYD. M. and RYE R. 0. (1972) Variation of o’s10’6. Ci3/C’2, texture and mineralogy in altered limestone in the Hill Mine, Cave-in-Rock District, Illinois. Econ. Geof. 67, 1-18. RAKHMANOV M. A. (1968) Physico-chemical conditions of formation of the Takob fluorite-polymetallic deposit (Tadzhikstan, South Gissar) in Abstracts of Reports of 3rd all-Union conference on Mineralogical Thermometry and Geochemistry of Deep-Seated MineralForming Solutions. Sept. 9-15, 1968, Moscow, 61-62. RICHARDSON C. K. and HOLLANDH. D. (1979) The solubility of fluorite in hydrothermal solutions-an experimental study. Geochim. Cosmochim. Acta 43, 1313-i325. ROBINSONR. A. and STOKESR. H. (1959) Efectrolvte I . .Solutions. Butterworths. ROEDDERE. (1967) Environment of deposition of stratiform (Mississippi Valley Type) ore deposits from studies of fluid inclusions. In Genesis of Stratijorm Lead-ZincFluorite-Barite Deposits (ed. J. S. Brown). Econ. Geol. Monograph 3, 349-362.
ROEDDERE.. HEYL A. V. and CREEK J. P. (1968) Environment of ore deposition at the Mex-Tex deposits, Hansonburg district, New Mexico, from studies of fluid inclusions. &on. Geol. 63, 336348. RUMYANTSEV V. N. and RUMYANTSEVA G. V. (1969) Solubility of slightly soluble compounds in water at higher temperatures and pressures. Russ. J. Inorg. Chem. 14, 855-859. SAWKINSF. J. (1966) Ore genesis in the North Pennine orefield, in the light of fluid inclusion studies. Econ. Geol. 61, 385-401. S’IRDBELG. (1965) Quantitative Untersuchungen iiber die hydrothermale Loslichkeit von Flusspat (CaF,). Neues Jahrb. Min. Mh. H. 3, 83-95. STRDEL
G. and SCHAEFFERB. (1975) Experimentelle Untersuchungen zur hydrothermalen Lijslichkeit von Fluorite im system CaF,-NaCl-H,O. In Geochemie der GDMB-DMG Lagersttittenbildung und-prospektion, Symposium, 21-23 February, 1974, Karlsruhe. TANNERS. P., WALKERJ. B. and CHOPPIN G. R. (1968)
G.C.A. 43/K--I
APPENDIX Description of calculations for Figs 1-6
Fig. I. The 25°C data points for 0.1-2.0 M NaCl solutions were obtained from measurements in SYRUBEL(1965). The point at 25°C and 4.0 M NaCl was calculated using the fluorite solubility in pure water measured by STRCJBEL (1965) to calculate K, and then converting K,to Q,. using the Debye-Htickel expression (log y k = - S ,/I/ (1 + AJf)) for y*C,F2 (A = 1.73). Figs 2-7. The data in these figures at 260”, 200” and 100” were obtained using the equilibrium constants, activity coefficient expressions, and solubility equations derived in RICWARD~~N and HOLLAND(1979). The 25°C data points were’calculated from data available in the literature. The 25°C data for I = 1.0 M are quite reliable, since QymF, QGF3 QN~Fand Q, values measured in 1.0 M sol&&s are available in the literature (STRUDEL.1965: TANNERet al., 1968; CADEKet al., 1971; M~LER and I&ma, 1976; and ELGQUIST and WEDBORG, 1978). For this ionic strength, activity coefficients did not have to be calculated-the values for these constants were used directly in eqns (19) and (21) of RICHARDSONand HOLLAND(1979). For ionic strengths of 2.0 M and 4.0 M, the values for Q&r, Q MrFand QNnFin 1.0 M solutions were converted to thermodynamic equilibrium concentration quotients (Q’s) in 2.0 M and 4.0 M solutions. The activity coefficients used in this process were obtained from measured mean activity coefficients. yC, and yx were obtained from y*kCl in 2.0 and 4.0 M KCI SOhltiOttS YF Was MlCUlatcd from y’,‘r using the values of yx obtained from y*xCi. yC, and 7”s were calculated from y~C.C,I and ytMICll using previous values of yC, and yAwa. y,.,, was calculated from Y*,,,.~,. All of the mean activity coefficient data for yhKF. y,N,C,, yAKa, Y*GCl, and Y’M,cI, were obtained from ROBINSONand STOKES(1959). The approximation used by RICHARDSON and HOLLAND(1979): yF = yGF = ykllF was also used here. The value of yco, was used for the y of uncharged species (y,&. Since the measured 25°C data for 2.0 M and 4.0 M solutions had to be converted from concentration quotients to activity quotients and back, the calculated fluorite solubilities in these solutions are not as reliable as those in solutions of ionic strength 1.OM. Since activity coefficients calculated by the mean salt method are most reliable for solutions of ionic strength less than 1.0 M, the activity coefficients calculated by this method at high ionic strengths are rather uncertain. Furthermore, activity coefficients increase rather steeply with increasing salt concentration at ionic strengths 2 4.0 M; thus, small errors in the position of these curves can create large differences in the values of the mean activity coefficients.
Goochimice er Cosnwchima Arm Vol. 43. pp 1327 10 1335 0 Pergamon Press Ltd. 1979. Prmred m Great Bntam
Fluorite deposition in hydrothermal systems CATHERINE K. RICHARDSON* and H. D. HOLLAND Department
of Geological Sciences. Harvard University, Cambridge, MA 02138, U.S.A. fReceioed
1 September
1978; accepted
in revised form
4 April 1979)
Abstract-During the formation of fluorite deposits fluorite is precipitated either as a consequence of changes in temperature and pressure along the Aow path of hydrothermal solutions or due to fluid mixing or as the result of the interaction of hydrothermal solutions with wall rocks. A decrease in temperature in the flow direction is the most appealing, though still unproven, mechanism of fluorite deposition in Mississippi Valley fluorite deposits. Mixing can produce solutions which are either undersaturated or supersaturated with respect to fluorite. The most important parameters are the temperature, the salinity, and the calcium and fluoride concentration of the fluids prior to mixing. A variety of wall rock reactions can lead to fluorite precipitation. Among these reactions which increase the pH of initially rather acid (pH 5 3) hydrothermal solutions are apt to be particularly important.
INTRODU~ION
FLUORITEOCCURSin a wide range of ore deposits both as an ore and as a gangue mineral. It is a common and sometimes abundant primary mineral in some of the stratiform galena-sphalerite-fluorite-barite (‘Mississippi Valley’ type) deposits and in many hydrothermal replacement and fissure-filling deposits. It also occurs commonly in tin greisen deposits, hydrothermal gold-telluride deposits, tin-tungsten deposits, and hydrothermal uranium deposits. In addition, fluorite occurs as an accessory mineral in some volcanic rocks, fluorine-rich pegmatiteq and skarns. Where fluorite is the major ore mineral, an understanding of the probable mechanism(s) by which it is deposited may well be useful in locating new deposits or ore horizons. Where ffuorite is a gangue or accessory mineral, the mechanism of fluorite deposition may be an important clue to the processes which controlled the deposition of the ore minerals. Recent solubility data (RICHARDSON and HOLLAND, 1979) can be applied directly to the study of fluorite ore deposits which formed between 100” and 260°C and in which the major ions present in the fluoritedepositing solutions were restricted to Ca”, Na’, K’, Mg’+, Cl-, HCO;. CO:-, SO:-, H2S, H’ and F-. If the concentration of other ions such as Fe3+, Sn’+, Sn’+, U6+, U4’, or B3’ was unusually high in the hydrothermal fluids, the stability constants for complexes involving these ions must be known before the solubility of fluorite can be calculated. Since most of these constants are not known. it is likely that the data in this study are not directly applicable to the study of fluorite deposition in hydrothermal uranium deposits,‘ tin greisens, or hydrothermal tintungsten deposits. * Present address: Department of Earth Sciences, Iowa State Uriiversity of Science and Technology, Ames, IA 50011, U.S.A.
Sources of data solutions
on the composition
of &rite-depositing
In order to apply the available solubility data for fluorite to specific ore deposits, a good deal must be known about the composition of the solutions from which the fluorite was deposited. The most informative sources of such knowledge’ are leach analyses of the major elements in inclusion fluids in fluorite. Salinity measurements of the inclusion fluids in fluorite are also quite useful. The data available on the composition of inclusion fluids in fluorite are summarized in Table 1. A secondary source of data for the composition of ore forming fluids from which fluorite was deposited is the mineralogy and zoning of ore deposits. The carbonate mineralogy and the presence or absence of feldspars and their a&ration min&ls can sometimes place limits on parameters such as the Ca/Mp, ratio, the Na/K ratio, and ihe pH of the fluids. Unfortun%ely, in many kuorite deposits, most of the minerals that are particularly useful in defining the composition of the ore-forming fluids are absent. However, the simple presence of limestone as a wall rock in many duorspar deposits provides an upper limit for the Mg/Ca ratio of the fluids and a lower limit for their pH. The composition of deepseated brines produced from drill holes is another source of information regarding the composition of the ore fluids. Many of the Mississippi Valley deposits are found near commercial oil fields. Analyses of these fluids give the present day composition of connate brines in the producing horizon. Two lines of evidence suggest that the fluids responsible for Mississippi Valley type fluorite deposits may have been predominantly connate water : (1) in many fluorite deposits organic matter both as asphaltum and as oil droplets is present in fluid inclusions. (2) the composition of inclusion fluids is similar to that of brines in nearby sedimentary basins. At Cave-in-Rock. Illinois, for instance, the composition of the primary component of the inclusion fluids is similar to that of brines in the Illinois basin (HALL and FRIEDMAN, 19631 Data for the composition of oil field brines are available for most of the regions of the United States where Mississippi Valley type deposits occur (see for instance CARPENTER et al.. 1974). Most of these brines are saline to very saline and are rich in NaCl and CaC12 (I = l.O6.Of, Na/Ca = 2-5).
1327
C. K. RICHARDSQN and H. D. HOLLAND
I328
Table 1. Summary of data available on the composition of inclusion fluids in fluorite Results
Deposit
References
I. Quantitative analyses of fluid inclusions in fluorite (T < 260°C) Southern Illinois-Kentucky I = 1.5-4.7f; tn&cII = 0.1-0.3_f; m~gc-,I= 0.04-0.3f deposits
I = 0.9-1.8; mc,clz = 0.1-0.31’; %gci2 = @O.l2f II. Qualitative and Partial analyses of fluid inclusions in fluorite (T < 260°C) rich in NaCl and CaCl,; salinities Northern Coahuila, Mexico are 8-40 equiv. NaCl wt.% rich in NaCl and CaC12 Hansonburg, New Mexico Aurakhmat fluorite deposit, Central Asia
III. Salinities of inclusion fluids in fluorite (T < 260°C) 26-50 equiv. NaCl wt.% Jamestown, Colorado Mex-Tex, Hansonburg, New Mex. 10.5-17 equiv. NaCl wt.“; Northern Pennine orefield, 20equiv. NaCl wt.“:, Derbyshire, England 5-15 equiv. NaCl wt.?; Fluorite polymetallic S. Gissar
FLUORIDE
TRANSPORT
Compositional data available from these various sources suggest that the solubility data reported by RICHARDSON and HOLLAND(1979) can be used to calculate the solubility of fluorite in the solutions which produced many low temperature fluorspar deposits. Complexing played an important role in fluoride transport in these solutions. In brines containing what appear to be normal concentrations of NaCl, CaCl, and MgCl* nearly all of the fluoride is present as a constituent of CaF+, MgFc and NaF” complexes. In acid solutions of pH 5 3 more than half of the Buoride is apt to be complexed as HF”. Thus the concentration of HF” can be sienificant in fluids from which alteration minerals such as-&unite are formed. However, the available mineralogical and chemical data for Mississippi Valley type deposits imply a pH between 4 and 7 for the ore-forming solutions responsible for these deposits (BANA~ZAK,1975; HELGESON,1964); in this pH range transport of fluoride as a constituent of HF” complexes is negligible.
FLUORITE
DEPOSITION
Fluorite can be precipitated from hydrothermal solutions by a variety of mechanisms. The most likely among these are (1) deposition due to changes in the temperature and pressure of the ore fluid; (2) deposition by mixing two or more fluids of different chemi-
Table 2. Composition
HALLand FRIEDMAN,1963 (see Table 2) PINCKNEYand HAFFTY,1970 PINCKNEY,personal communication GRUsntct~, 1958
KESLER,1977 AMES,1958 NASH and CUNNINGHAM,1963 ROEDDERet al., 1968 SAWKIN~,1966 RAKHMANOV. 1968
cal composition; (3) deposition due to the interaction of ore-forming fluids with wall rocks. In the foilowing section each of these precipitation mechanisms will be considered in some detail. The discussion will focus primarily on the quantity of fluorite which can be precipitated by the operation of each mechanism. The fluorite deposits in the Cave-in-Rock district (Illinois) will be used as a specific example for testing the efficacy of the several depositional mechanisms. Temperature changes Precipitation of fluorite due to the cooling of hydrothermal fluids has been proposed as a possible fluorite depositional mechanism by FREAS (1961) BENEWVA et al. (1969), BANASZAK (1975) and PINCKNEY(unpublished manuscript). The solubility of fluorite in most aqueous solutions decreases with decreasing temperature; fluorite therefore tends to precipitate with decreasing temperature (ANIKIN and SHWHKANOV, 1963; STRUBEL, 1965; STRUJJELand SCHAEFER, 1975; RICHARDSON and HOLLAND, 1979). Dilute (< 1.0 M) NaCl or KC1 solutions are an exception to this rule. In such dilute solutions, the solubility of fluorite goes through a maximum of about 100°C and decreases with increasing temperature up to ca.
of fluid inclusions from the Cave-in-Rock District, Illinois (HALL and FRIEDMAN,1963)
Mineral
Na
K
Yellow fluorite Yellow fluorite Early white fluorite Blue fluorite Blue fluorite Early purple fluorite Late purple fluorite Late purple fluorite Late purple fluorite Late purple fluorite
1.96 2.13 1.00 1.74 2.04 1.78 1.88 2.62 1.88 2.20
0.06 0.07 0.04 0.07 0.08 0.08 0.05 0.13 0.10 0.10
Concentration (mol/kg solution) Ca Cl Mg 0.15 0.19 0.12 0.30 0.21 0.11 0.15 0.20 0.17 0.10
0.09 0.05 0.04 0.27 0.23 0.25 0.04 0.45 0.25 0.16
2.37 2.65 1.33 2.65 2.77 2.20 2.45 3.78 2.62 2.82
SO,
Total
Ionic strength
0.03 0.02 0.01 0.05 0.04 0.06 0.01 0.07 0.01 0.02
4.66 5.11 2.54 5.08 5.37 4.48 4.58 7.25 5.03 5.40
2.73 2.94 1.52 3.47 3.41 2.87 2.63 4.70 3.16 3.12
Fluorite deposition in hydrothermal
systems
90 NoCl
80
0
I
NoCl -
Hz0
60
I=4
CeC12
- ~~0
,’
/i
1329
/I
OM ,‘4-
f”
0 M Nacl
10 60
c----g
0 3M coc12 3 l M NoCl
‘---xx
260
200
I50
100
0 2 M COC12 3.4M NoCI
50
25
TnC
260
200
150
100
50 25
T’ C Fig. 1. The quantity of fluorite precipitated by cooling NaCl solutions of varying ionic strengths from 260’ to 25°C. Negative amounts of fluorite deposition indicate fluorite dissolution. 360°C. A series of curves have been constructed
in Fig. 1 defining the quantity of fluorite that precipitates from NaCl solutions of various concentrations as they are cooled from 260” to 25°C (see Appendix for method of calculation of the 25°C data). These 100
90
NoCI-
0
CaCl2-
Hz0
N
‘,
70
I=
2,ow
: u”
60
:
;
50
0 0 c ::
40
0,”
30
‘;:
20
0.9 0.4M M NoCl coc12 0,3 M C0C12 I. I M NaCl 0.2 M CDCl2 1.4M NPCI 0. I M cac12 1.7M NaCl co
z G
0.02 I.94
M cac12 M NoCl
IO
0 260
200
I50
IO0 TO
50
25
c
Fig. 2. The quantity of fluorite precipitated by cooling NaCI-CaCI,-Hz0 solutions with an ionic strength of 2.0 M from 260’ to 25°C. Dashed part of the curves go through points at temperatures that are below those at which solubility measurements were made in this study (see Appendix 1 for method of calculation).
Fig. 3. The quantity of fluorite precipitated by cooling NaCI-CaQ-Hz0 solutions with an ionic strength of 4.0M from 260’ to 25°C. Dashed part of the curves go through points at temperatures that are below those at which solubility measurements were made in this study (see Appendix 1 for method of calculation).
curves show that the quantity of fluorite precipitated increases with the salinity of the fluids, that at all ionic strengths the solubility of fluorite changes most rapidly with temperature at lower temperaturesespecially between 50’ and 25°C; and that as 0.5 M and 0.1 M NaCl solutions are cooled from 260°C they first. become undersaturated with respect to fluorite and do not begin to precipitate fluorite until they have cooled to temperatures below 100°C. A similar set of curves was calculated for solutions containing CaCl, or (CaCl, + MgCl,) in addition to NaCl (Figs 2-5). In NaCl solutions precipitation of fluorite with decreasing temperature is the result of changes in the value of the solubility product K, of fluorite with temperature and of the strength of the complex NaF”. The corresponding curves for fluorite deposition from NaCl-CaCl,-MgC1, solutions are influenced in addition by changes in the stability of CaF’ and MgF+ with decreasing temperature. As temperature decreases, the complexes MgF+, CaF’ and NaF” all become less stable (RICHARDSONand HOLLAND. 1979): on cooling they release free fluoride ions which can combine with calcium to precipitate fluorite. The curves in Figs 2-5 indicate that the amount of fluorite precipitated per kilogram of solution increases with the Ca (or Ca + Mg) concentration and with increasing ionic strength except in solutions containing very little CaCl,. The solubility surface which defines the quantity of fluorite deposited as a function of (Q,~,, + m,&,,) and temperature goes through a minimum between rncaC,>= 0 This miniand mcacl, = 0.02 M at all temperatures. mum separates the region of solution compositions
K.
C.
1330
NaCl
90
and H. D.
RICHARDSON
- Cm2
- MgCl2-
I=
2.0
Ca/Mp
=
H20
60
0
9/l
r”
HOLLAND
0 44M
e---PO-66M
Jr-
e-40.33M
I.01
,---oO22M
I
34M
0
I
L .RO
200
260
150
50
100 7’
M
cac12 NaCl
t
CaCl2 NaCl
t
M9C12
C0C12 NaCl
t
M9Cl2
I
I M cac12 67M NaCl
O-022 I 936
M9C12
t
M9Cl2
M CaCI2 M NaCl
t
MQC12
25
C
Fig. 4. The quantity of fluorite precipitated by cooling NaCl-CaCls-MgCl,-Hz0 solutions with a Ca/Mg ratio of 9/l and an ionic strength of 2.0 M from 260” to 25°C. Dashed parts of the curves go through points at temperatures below those at which solubility measurements were made (see Appendix 1 for method of calculation).
occurs between 260” and 200°C. As the temperature
in which the volubility of fluorite is controlled largely by fluoride complexing from the region where the solubility of fluorite is controlled largely by the com-
decreases
mon ion effect. Figures 2-5 show that the largest change in solubility with temperature (dSol/dT) I IO
NaCl
- CaCl2
- MgCl2
-
.Q Ca/Mg*
9/l
b / m. I
---. 90 / o I” 0
:
P z
loo”C,
dSol/dT
gradually
decreases.
H20 ---_
100
to
Between 100°C and 25’C in 2.0 M solutions, for instance, the amount of fluorite that can be precipitated by cooling either NaCl + CaCl* solutions or
0 44 2.66
M M
cac12 NoCl
t
MpCl2
0.33 M 3.0 I M
CaCl2 NaCl
t
MgC12
0.22 334M
M
CaCl2 NaCl
t MqCl2
0 I I M I ev..
CaCl2 .,_.-I
t
0.022M 393M
COCl2 NaCl
6 0 ‘s 7
6
ur
----IQ
&
/--r
M9C12
t
MiJC12
OYI
260
200
I50
too T*
50
25
c
Fig. 5. The quantity of fluorite precipitated by cooling NaCl-CaCl,-MgCl,-H,O solutions with a Ca/Mg ratio of 9/l and an ionic strength of 4.0 M from 260” to 25°C. The dashed parts of the curves go through points at temperatures below those at which solubility measurements were made (see Appendix 1 for method of calculation).
Fluorite deposition in hydrothermal 110
11 pH = 2
100 90
NaGI- HCI - H20 I = 2.0
I" 0
60
1
260
i
200
150
100 T’
50
25
C
Fig. 6. The quantity of fluorite precipitated by cooling NaCI solutions at varying pH values from 260” to 25°C.
NaCl + CaCl, + MgClz solutions is only 3-6 mg CaF,/kg Hz0 (rather than 6-43mg CaFdkg Hz0 in equivalent 75°C temperature intervals at higher temperatures). The equivalent curves for ionic strength 4.0M are quite similar to those in 2.0M solutions; however, there may be a small reversal in the precipitation curves below 100°C. If this reversal turns out to be real, dissolution of fluorite in these solutions can take place as temperatures fall below 50°C. The curves in Fig. 6 show the quantity of fluorite deposited due to cooling from 2.0 M NaCl solutions containing small concentrations of HCl. As much as a hundred mg CaF,/kg Hz0 can be precipitated by cooling a 2.0 M NaCl-HCl solution with a pH of 2 from 260’ to 200°C. The ore-forming fluids which produced fluorite deposits of the Mississippi Valley type could not, however, have had such a low pH. The potential importance of cooling as a depositional mechanism in a particular fluorite deposit depends on the amount of cooling which occurred during fluorite deposition. Little is known about temperature gradients within ore deposits during mineral deposition. Fluorites in some deposits, such as those in the Cave-in-Rock district, show a fine succession of delicate growth bands. The same pattern of growth banding has been observed in fluorite crystals throughout individual deposits as well as in mines throughout the district (PINCKNEYand HUGHES,unpublished manuscript). Careful measurements of the filling temperature of fluid inclusions in individual growth bands in samples of fluorite from all parts of these deposits should therefore yield information on the temperature distribution during fluorite depo-
systems
1331
sition. A thorough study of this type has not been done. Filling temperature differences of 40-50°C for fluid inclusions within the same growth band in fluorite crystals from different parts of the same deposit would certainly have been noticed in previous studies (GROGANand SHRODE.1952; FREAS, 1961: ROEDDER. 1967): differences of lO-20°C could have been overlooked. It is therefore unlikely that the ore fluids cooled as much as 40-50” as they passed through a single deposit in the Cave-in-Rock district, but a temperature change of lO-20°C during a single stage of fluorite deposition is not ruled out by the data available at the present time. Vertical temperature gradients in the deeper parts of bore holes in active geothermal systems are frequently in the range of 16”-40”C/km (WHITE, 1968: McNrrr, 1970; MUFFLER,1975). It seems likely that temperature gradients during the formation of hydrothermal ore deposits are similar in magnitude. If so. the temperature differential across Mississippi Valley type fluorite ore deposits, many of which are ca. 500m in length, may well have been on the order of 8’-20°C. The quantity of fluorite which can precipitate from a variety of fluids during cooling by 10°C in the applicable range of temperatures falls between 0 and 25 mg CaFz/kg H20, except for solutions which have a pH 5 4 (see Figs l-5). Quantities of fluorite at the high end of this range are considerably greater than the amount proposed by HOLLAND (1967) prior to the discovery of the importance of CaF+ and MgF+ for fluoride transport in hydrothermal solutions, and are consistent with reasonable volumes and fiow rates of the fluids which produced, for instance, the Cave-in-Rock deposits in Illinois. Fluorite precipitation as a consequence of simple cooling is also consistent with the delicate color banding of fluorite throughout this and related deposits. However, proof that cooling was the major cause of fluorite deposition still awaits convincing data that define the temperature gradient in these ore deposits during fluorite precipitation. Precipitation qfjhmite
due to changes in pressure
As solutions flow towards the surface, there is normally a decrease in confining pressure. The solubility of most gangue minerals (calcite, barite, dolomite. anhydrite and qUartZ--HOLLAND, 1967) decreases as pressure is decreased. Solubility studies by STR~JBEL (1965), RUMYANTW and RUMYANTSEVA (1969), and MACDONALDand NORTH (1974) have shown that fluorite is no exception to this rule. At 255°C the solubility of fluorite in water decreases from 16.2 to 13.0 mg CaF,/kg as the pressure is dropped from 1642 atm to 43.3 atm. Most huorite deposits span a vertical range of 10&5OOm, which corresponds to a pressure decrease of only 10-50 atm (hydrostatic gradient). The amount of fluorite precipitated from dilute aqueous solutions as a consequence of a pressure decrease is therefore probably about an order of magnitude less than the quantity of fluorite precipitated
C. K. RICHARDKIN and H. D. HOLLAND
1332
0
0
,og 25 \p 20
T=
NW
T = ZOO’C
$2
I. - 25 2.r" 'No 20 u-:: I5 :'o IO F 5 -----_Q
NaCl-H20
;'; "0.
I5 IO 5 E" __--_O i --_-_-_--___---__-
NoCl
200'C
- CoCl2 - Ii20
CaCl2
5 IO 15 20 25 30
F
0
2
4
L
vol. vol
0
,2 vol. vol.
.4
,6
original
original
.8
I.0
Sol’n. sol’n.
t
vol.
water
6
I.0
6
OrIgInalsoih
original
sol’n.
+
YOI
waG
Fig. 8. The quantity of fluorite dissolved or precipitated per kg Ha0 during the dilution of NaCI-CaC12-Hz0 solutions at ionic strengths of 2.0 M and 4.0 M and 200°C with water at the same temperature.
Fig. 7. The quantity of fluorite dissolved per kg H,O dur-
ing the dilution of NaCl solutions at 200°C with water at the same temperature. from brines due to the accompanying decrease in temperature. The effect of pressure on the solubility of fluorite in concentrated NaCl-CaCI, solutions has not been measured, and since the molar volume of CaF’ is not known, the AV for the reaction CaF, + Ca’+ P 2CaF+ cannot be calculated. It seems likely, however, that the change in fluorite solubility with pressure in hydrothermal solutions along most flow paths in hydrothermal systems is small compared to the change in solubility due to the accompanying change in temperature in concentrated brines as well as in dilute solutions. Mixing of jluids
to fluorite results in undersaturation with respect to fluorite. If 4 M NaCl solutions saturated with respect to fluorite at 200°C are mixed with NaCl-free solutions also saturated with respect to fluorite at 200°C. the mixtures are all undersaturated with respect to fluorite. The degree of undersaturation can be estimated from Fig. 7 by anchoring all of the curves at the point mg CaFz/kg Hz0 dissolved = 0, volume original solution/volume original solution + volume water = 0. The same calculations were performed for brine solutions which contained CaCl, or CaCl, + MgCl, . These calculations are summarized in Figs 8 and 9. The curves in these figures show that mixing pure water in any proportion with 2.0 M NaCl + CaCl, or NaCl + CaCl* + MgClz *solutions causes the mixture to become undersaturated with respect to fluorite. In 4.0 M NaCl + CaCl* + MgClz solutions, however, the addition of a small amount of pure water causes the mixture to become supersaturated
Mixing of two or more fluids of different chemical composition has been proposed as a possible mechanism for fluorite deposition by HALL and FRIEDMAN T = 200'C (1963), DUNHAM(1966), SAWKMS(1966), HEYL et al. NaCI- CaC12 - MgC12 -H20 (1974), BANASZAK(1975), and KESLER(1977). Mixing 0 C0/Mg 8 9/l N can occur in several different ways and can involve =3 25 a variety of solutions. Meteoric water can mix with :"E 20 t I a concentrated brine at the same temperature (dilution at constant temperature); meteoric water can mix 0.44 M COCl2 + MqC12 with a concentrated brine at the same temperature (dilution at constant temperature); meteoric water can mix with a concentrated brine at a different temperature (dilution plus cooling); and two brines of different chemical composition can mix isothermally or non-isothermally. A series of calculations were performed in which ,2 .4 ,6 .6 I.0 brine solutions initially saturated with respect to vol. original rol’n. fluorite at 200°C were mixed with varying proporvol. original sol’n. t vol. wobr tions of pure water, also at 200°C. The results of these calculations for NaCl brines are summarized in Fig. 7. Fig. 9. The quantity of fluorite dissolved or precipitated per kg Ha0 during the dilution of NaCl-CaCl,-MgCl,The curves in this figure show that dilution of 0.5 Ha0 solutions at ionic strengths of 2.0 M and 4.0 M and 4.0M NaCl solutions initially saturated with respect 200°C with water at the same temperature.
Fluorite deposition in hydrothermal systems with respect to fluorite by O.l-1.2mg CaFl/kg H20. As more water is mixed with these brines, they become undersaturated with respect to fluorite. These calculations show that mixing brine solutions with pure water is a mechanism primarily for dissolving rather than precipitating fluorite; from mixtures that do become supersaturated with respect to fluorite. only ca. 0.1-l mg CaFl/kg HZ0 can be deposited. This quantity is considerably smaller than that probably deposited by the simple cooling of most ore fluids. If the brines in Figs 8 and 9 are mixed with dilute NaCl free solutions saturated with respect to fluorite (mg CaF,/kg dissolved = 0 and vol original solution/v01 original solution + water = 0). a larger quantity of fluorite would be precipitated. This quantity, however. is still about the same as or less than the amount deposited by cooling similar solutions by 10°C. If a hot, concentrated brine moving up a fracture mixes with cooler meteoric water, a change in temperature accompanies dilution. Calculations were performed in which NaCl or NaCl + CaCl, -t MgCI, solutions at 200°C mix with water at 25’C (AH of mixing assumed = 0). This temperature difference was chosen to define the maximum probable effect of dilution accompanied by cooling. For NaCl solutions, dilution plus cooling results in mixtures that are undersaturated with respect to fluorite at all ionic strengths. The dilution of a NaCl-CaCl,--MgCl, solution with I = 4.0 M. m,, = 0.4 M, and mug = 0.04 M at 200°C with water at 25°C is followed in Fig. 10. This figure indicates that dilution and cooling of such a brine results in supersaturation with respect to huorite until the water: brine ratio is approximately 9:l. Up to 15 mg CaF,/kg HZ0 can be precipitated during mixing of two such solutions. If the cold dilute solution is already saturated with respect to fluorite. the amount of precipitation of fluorite upon mixing the solutions would be even greater than that shown in Fig. 10. In the Cave-in-Rock district the temperature decrease during fluorite deposition within a given ore 0.4M 0
CaC12, 2.66
N
I-0 ,” H ._o
20
MgCI2,
I
= 4-OM
L
I
__ ______________________
IO 15
\;
20
IAT?*; s
25
E”
M
5
0
2;
0.04 NaCI,
25
-______o I”
M
u 0
.2
.4
.6
v-31 original vol. original
.6
I.0
sol’n
sol’n. + vol water
Fig. 10. The quantity of fluorite precipitated per kg Hz0 during the dilution of a solption with I = 4.0 M. mrrC,, = 0.4 M and mHeri2= 0.4M at 2CO”Cwith water at 25’C.
1333
deposit was apparently less than or equal to 20-C. This corresponds to the dilution of a 200°C brine by less than or equal to 109; of its mass of pure water at 2O’C. Figure 10 shows that the amount of fluorite precipitated by such dilution is comparable to the amount of fluorite which would be precipitated by simple cooling of the brine. The mixing of two brines. one rich in calcium. the other rich in fluoride, has often been proposed as a mechanism for depositing fluorite. Most proponents of this mechanism have advocated mixing calciumbearing connate waters with deep solutions carrying fluoride of volcanic origin (HALL and FRIEDMAN. 1963: DUNHAM.1966: SAWKINS.1966). The changes in the ionic strength and in the calcium, magnesium and fluoride concentration that occur as the solutions are mixed determine whether and how much fluorite is precipitated or could be dissolved. The amount of fluorite which might be precipitated per unit mass of mixture is considerably larger than the quantity that is apt to be precipitated by cooling alone. and. the total volume of water that would be necessary to form a given deposit would be smaller than that predicted for a simple cooling model. interaction
with wall-rocks
Fluorite can be precipitated as a result of the interaction of ore-forming solutions with host rocks. Fluorite deposition can follow, for instance. pH changes and changes in the calcium and magnesium concentration of hydrothermal fluids. Fluorite is quite soluble in acid solutions (RICHARDSON and HOLLAND. 1979) (Figs 3 a-c). and a pH change from acid to near neutral can lower the solubility of fluorite by as much as 100 mg CaF,/kg HzO. At 2OO’C the solubility of fluorite in a 2.0 M NaCl solution at a pH of 5 is 63mg CaFJkg HzO, whereas its solubility at 200°C in a 2.0 M NaCl solution at a pH of 3 is 104 mg CaF,/kg HZO. The pH of hydrothermal solutions is generally > 3 at 200°C; the pH of ore-formmg fluids of Mississippi Valley type deposits was probably in the range of 4-5 at 200°C (BANASZAK.1975); it is therefore unlikely that pH changes contributed significantly to the precipitation of fluorite in these deposits. However. in the formation of fluorite deposits associated with volcanics. rather acid solutions may well have been involved, and pH changes due to interaction with wall rocks may well have contributed significantly to the precipitation of fluorite. If a hydrothermal fluid entering a limestone terrain dissolves calcite. the calcium concentration of the fluid is increased. If the initial solution is very close to a pure NaCl solution. such an increase in the calcium concentration can lead to the precipitation of fluorite (see Fig. 5 in RICHARDSONand HOLLAND, 1979). However, most hydrothermal fluids contain rather large concentrations of calcium, and it is likely that this mechanism for precipitating fluorite is not important. In fluids that contain large amounts of calcium (0.1-0.5 m) such as those at Cave-in-Rock.
C. K. RICHARDSON and H. D. HOLLAND
1334
increasing the calcium concentration of the fluid increases, rather than decreases, the solubility of fluorite and is apt to produce fluorite dissolution. The solubility of fluorite increases with increasing magnesium concentration as well as with increasing calcium concentration in solution (RICHARDSONand HOLLAND, 1979, Fig. 7). Thus, a decrease in the calcium or in the magnesium concentration by precipitation of calcium and/or magnesium minerals may lead to fluorite deposition. Dolomitization of limestone may also lead to fluorite deposition. Since MgF’ is more stable than CaF+, a reduction of the magnesium concentration, even if it is accompanied by an equivalent increase in the calcium concentration, can cause precipitation of fluorite. If a solution at 200°C that has an ionic strength of 3.0M, an mc, of 0.3 m, and an mr,,, of 0.08 m exchanges Mg for Ca by dolomitizing a limestone until mc, = 0.35 m and m,,s = 0.03 m, the solubility of fluorite changes from 45.4 to 36.0mg CaFJkg HsO. As much as 9.4 mg CaFZ/kg Hz0 could therefore precipitate from this fluid. This quantity of CaF2 is comparable to that potentially precipitated by cooling the initial fluid from 200” to 180°C. However, in order to change the magnesium concentration of the fluid by 0.05 mol/kg, 9.2 grams of dolomite must be produced by each kg of solution. This in turn implies that the ratio of dolomite/fluorite in a deposit formed by the proposed mechanism is ca. 980/l. Some dolomitization of the limestone in the Cave-in-Rock district has been reported (PINCKNEY and RYE, 1972), but the ratio of dolomite to fluorite is orders of magnitude smaller than 980/l. Other deposits, such as the MexTex deposits in New Mexico and the fluorspar deposits of Northern Coahuila, Mexico, occur in virtually undolomitized limestone. It is therefore unlikely that dolomitization of limestone host rock has been a major factor in the deposition of fluorite in any of these fluorspar deposits.
CONCLUSIONS
This study of the deposition of fluorite ores suggests that the most likely mechanisms for fluorite deposition are (1) simple cooling of ore fluids; (2) dilution of ore fluids with cool meteoric water; (3) mixing of two brines of different chemical composition, and (4) an increase in the pH of acid ore fluids. For the fluorite deposits in the Cave-in-Rock district, Illinois, the most appealing mechanism of fluorite deposition appears to be simple cooling of the ore fluids. However, the evidence in favor of this mechanism is still inconclusive. Temperature gradients during ore deposition within the district that are consistent with this interpretation are permitted but not demonstrated by the available fluid inclusion data; additional fluid inclusion filling temperature measurements are needed to establish the magnitude of temperature gradients within the ore bodies during fluorite deposition. Other precipitation mechanisms
are currently unattractive. Mechanisms which involve the mixing of solutions are unattractive, because they are difficult to reconcile with the consistent patterns of color banding in fluorite that persist throughout the Cave-in-Rock district. Wall-rock reactions can be ruled out as a major cause of ore deposition because neither the probable changes in pH nor the permitted decrease in Mg/Ca ratio in the fluids can produce the deposition
of significant
quantities
of fluorite.
Acknowledgements-The content of this paper has profited from discussions with U. PETERSEN,D. M. PINCKNEYand U. FEHN. Financial support for the experiments on which this study was based was provided by the Committee on Experimental Geology at Harvard University. Financial support for the preparation of this manuscript was from NSF grant No. EAR 78-15190 (to CKR). REFERENCES AMESL. L. (1958) Chemical analyses of the fluid inclusions in a group of New Mexico minerals. Econ. Geol. 53, 473-480. ANIKINI. N. and SHUSHKANOV A. D. (1963) The solubility of fluorite in aqueous solutions of electrolytes. KristallogruJiya 8, 128-130. BANASZAKK. J. (1975) Genesis of the Mississippi ValleyType lead-zinc ores. Unpublished Ph.D. Thesis, &&western Univers$y. BENE~OVA Z. and CADEKJ. (1969) Temperature of homogenization of inclusions in fluorite deposits of Czechoslovakia. (abst.) in Proceedings of COFFI 2, 13-14. CADEK J., VE~ELLJ. and SULCEKZ. (1971) Bildung von Fluoridkomplexen mit Erdalkalimetallen. Colfection Czechosloo. Chem. Commun. 36, 3377-3381. CARPENTER A. B., TROUTM. L. and PICKETTE. E. (1974) Preliminary report on the origin and chemical evolution of lead and zinc rich oil field brines in central Mississippi. Econ. Geol. 69, 1191-1206. DUNHAMK. C. (1966) Role of juvenile solutions, connate waters and evaporitic brines in the genesis of lead-zincfluorite-barite deposits. I.M.M. Trans. 75, B226229. ELQUI~TB. and WEDBORGM. A. (1978) Stability constants of NaS04-MgSO,, MgF+, MgCl’ ion pairs at the ionic strength of seawater by potentiometry. Mar. Chem. 6, 243-252.
FREAS D. H. (1961) Temperatures of mineralization by liquid inclusions, Cave-in-Rock fluorspar district, Illinois. &on. Geol. 56, 542-556. GR~GAN R. M. and SHRODER. S. (1952) Formation temperatures of Southern Illinois bedded fluorite as determined from fluid inclusions. Am. Miner& 37, 555-566. GRUSHKING. G. (1958) Physicochemical factors affecting equilibrium during mineralization of the Aurakhmat fluorite deposit (Central Asia). Vses. Nauchnolssled. Inst. P’ezooprichesk. Mineral. Syr’ya Trudy 2, 81-92.
HALL W. E. and FRIEDMANI. (1963) Comoosition of fluid inclusions, Cave-in-Rock fluorite distri’ct, Illinois and Upper Mississippi Valley zinc-lead district. Econ. Geol. 58, 886911. HELGE~~NH. C. (1964) Complexing and Hydrothermal Ore Deposition, Pergamon Press. HEYLA. V., LANDL~G. P. and ZARTMANR. E. (1974) Isotopic evidence for the origin of Mississippi Valley-type mineral deposits: A Review. &on. Geol. 69, 992-1006. HOLLANDH. D. (1967) Gangue minerals in hydrothermal deposits in Geochemistry of Hydrothermal Solutions (ed. _~ H. L. Barnes), Holt, Rinehart & Winston. KESLER S. E. (1977) Geochemistry of Manto Fluorite deposits, Northern Coahuila, Mexico. Icon. Geol. 72, 204-218.
Fluorite deposition in hydrothermal MACDONALDR. W. and NORTH N. A. (1974) The effect of pressure on the solubility of CaCO,, CaF,, and SrSO, in water. Can. J. Chem. 52, 3181-3186. MCNITT J. R. (1970) The geologic environment of geothermal fields as a guide to exploration. Proceedings of the United Nations Symposium on the Development and Utilization of Geothermal Resources, Piss, 1970. Geothermics Special Issue 2, v. 1. 24-30.
R. and KESTER D. R. (1976) Sodium fluoride ion-pairs in seawater. Mar. Chem. 4, 67-82. MUFFLERL. J. P. (1975) Summary of Section II geology, hydrology, and geothermal systems. Proceedings of the
systems
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Thermodynamic parameters of the alkaline earth monofluorides. J. lnorg. Nucl. Chem. 30, 2067-2070. WHITE D. E. (1968) Hydrology, activity, and heat flow of the Steamboat Springs thermal system, Washow County, Nevada. U.S.‘Geol. Survey Prof. Paper 458-C, l-109. WHITE D. E., ANDERSON E. T. and GRUE~BS D. K. (1963) Geothermal brine well. Science 139. 919-922.
MILLER G.
Second United Nations Svmnosium on the Develonment and Use of Geothermal Resources. San Francisco,‘l975,
1, xLv-Lii. NASH J. T. and CUNNINGHAM C. G. (1973) Fluid-inclusion studies of the fluorspar and gold deposits, Jamestown district, Colorado. Econ. Geol. 68, 1247-1262. PINCKNEYD. M. and HAFF~ J. (1970) Content of zinc and copper in some fluid inclusions from the Cave-inRock District. Southern Illinois. Econ. Geol. 65, 451-458. PINCKNEYD. M. and RYE R. 0. (1972) Variation of o’s10’6. Ci3/C’2, texture and mineralogy in altered limestone in the Hill Mine, Cave-in-Rock District, Illinois. Econ. Geof. 67, 1-18. RAKHMANOV M. A. (1968) Physico-chemical conditions of formation of the Takob fluorite-polymetallic deposit (Tadzhikstan, South Gissar) in Abstracts of Reports of 3rd all-Union conference on Mineralogical Thermometry and Geochemistry of Deep-Seated MineralForming Solutions. Sept. 9-15, 1968, Moscow, 61-62. RICHARDSON C. K. and HOLLANDH. D. (1979) The solubility of fluorite in hydrothermal solutions-an experimental study. Geochim. Cosmochim. Acta 43, 1313-i325. ROBINSONR. A. and STOKESR. H. (1959) Efectrolvte I . .Solutions. Butterworths. ROEDDERE. (1967) Environment of deposition of stratiform (Mississippi Valley Type) ore deposits from studies of fluid inclusions. In Genesis of Stratijorm Lead-ZincFluorite-Barite Deposits (ed. J. S. Brown). Econ. Geol. Monograph 3, 349-362.
ROEDDERE.. HEYL A. V. and CREEK J. P. (1968) Environment of ore deposition at the Mex-Tex deposits, Hansonburg district, New Mexico, from studies of fluid inclusions. &on. Geol. 63, 336348. RUMYANTSEV V. N. and RUMYANTSEVA G. V. (1969) Solubility of slightly soluble compounds in water at higher temperatures and pressures. Russ. J. Inorg. Chem. 14, 855-859. SAWKINSF. J. (1966) Ore genesis in the North Pennine orefield, in the light of fluid inclusion studies. Econ. Geol. 61, 385-401. S’IRDBELG. (1965) Quantitative Untersuchungen iiber die hydrothermale Loslichkeit von Flusspat (CaF,). Neues Jahrb. Min. Mh. H. 3, 83-95. STRDEL
G. and SCHAEFFERB. (1975) Experimentelle Untersuchungen zur hydrothermalen Lijslichkeit von Fluorite im system CaF,-NaCl-H,O. In Geochemie der GDMB-DMG Lagersttittenbildung und-prospektion, Symposium, 21-23 February, 1974, Karlsruhe. TANNERS. P., WALKERJ. B. and CHOPPIN G. R. (1968)
G.C.A. 43/K--I
APPENDIX Description of calculations for Figs 1-6
Fig. I. The 25°C data points for 0.1-2.0 M NaCl solutions were obtained from measurements in SYRUBEL(1965). The point at 25°C and 4.0 M NaCl was calculated using the fluorite solubility in pure water measured by STRCJBEL (1965) to calculate K, and then converting K,to Q,. using the Debye-Htickel expression (log y k = - S ,/I/ (1 + AJf)) for y*C,F2 (A = 1.73). Figs 2-7. The data in these figures at 260”, 200” and 100” were obtained using the equilibrium constants, activity coefficient expressions, and solubility equations derived in RICWARD~~N and HOLLAND(1979). The 25°C data points were’calculated from data available in the literature. The 25°C data for I = 1.0 M are quite reliable, since QymF, QGF3 QN~Fand Q, values measured in 1.0 M sol&&s are available in the literature (STRUDEL.1965: TANNERet al., 1968; CADEKet al., 1971; M~LER and I&ma, 1976; and ELGQUIST and WEDBORG, 1978). For this ionic strength, activity coefficients did not have to be calculated-the values for these constants were used directly in eqns (19) and (21) of RICHARDSONand HOLLAND(1979). For ionic strengths of 2.0 M and 4.0 M, the values for Q&r, Q MrFand QNnFin 1.0 M solutions were converted to thermodynamic equilibrium concentration quotients (Q’s) in 2.0 M and 4.0 M solutions. The activity coefficients used in this process were obtained from measured mean activity coefficients. yC, and yx were obtained from y*kCl in 2.0 and 4.0 M KCI SOhltiOttS YF Was MlCUlatcd from y’,‘r using the values of yx obtained from y*xCi. yC, and 7”s were calculated from y~C.C,I and ytMICll using previous values of yC, and yAwa. y,.,, was calculated from Y*,,,.~,. All of the mean activity coefficient data for yhKF. y,N,C,, yAKa, Y*GCl, and Y’M,cI, were obtained from ROBINSONand STOKES(1959). The approximation used by RICHARDSON and HOLLAND(1979): yF = yGF = ykllF was also used here. The value of yco, was used for the y of uncharged species (y,&. Since the measured 25°C data for 2.0 M and 4.0 M solutions had to be converted from concentration quotients to activity quotients and back, the calculated fluorite solubilities in these solutions are not as reliable as those in solutions of ionic strength 1.OM. Since activity coefficients calculated by the mean salt method are most reliable for solutions of ionic strength less than 1.0 M, the activity coefficients calculated by this method at high ionic strengths are rather uncertain. Furthermore, activity coefficients increase rather steeply with increasing salt concentration at ionic strengths 2 4.0 M; thus, small errors in the position of these curves can create large differences in the values of the mean activity coefficients.