New geothermometers for carbonate—evaporite geothermal reservoirs

Geothermics, Vol. 15, No. 1, pp. 77- 86, 1986.

0375 - 6505/86 $3.00 + 0.00 Pergamon Press Ltd. © 1986CNR.

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NEW GEOTHERMOMETERS FOR CARBONATE-EVAPORITE GEOTHERMAL RESERVOIRS L. M A R I N I , * G. C H I O D I N I * and R. C I O N I t *Geotermica Italiana, Lungarno Mediceo 16, 56100Pisa, Italy and tlstituto di Geocronologia e Geoehimica Isotopica, C.N.R., Via C. Maffi 36, 56100 Pisa, Italy (Received April 1985; accepted for publication July 1985)

new Ca/Mg and SO,/F geothermometers specific for carbonate- evaporite geothermal reservoirs are proposed. General considerations on waters interacting with such rocks suggest that Ca2., Mg2÷, COl-, SOb, F- and SiO2are compatible components of the pertinent thermodynamicsystem and, therefore, their activities are fixed by five solid phases at equilibrium conditions. Geothermometric elaboration is based on the assumption that the five solid phases are represented by pure calcite, dolomite, anhydrite, fluorite and an SiO2 mineral, e.g. quartz or chalcedony or amorphous silica. Pressure and ion association effects are not taken into consideration. Preliminary applications both to thermal waters and geothermal wells are promising. These new geothermometers could be widely used in the geothermal exploration of areas with carbonate-evaporite reservoirs, such as the two main geothermal fields of Italy, Larderello and Mt. Amiata. Further calibration by experimental studies and additional data from geothermal boreholes is needed, however, to test the practical reliability of the new geothermometers. Abstract--Two

INTRODUCTION Geochemical geothermometry has been used intensively in geothermal exploration in the last 15 years. The most commonly used empirical geothermometers, the N a / K (White, 1965; Ellis and Mahon, 1967; Ellis, 1970; Truesdell, 1975) and the N a - K - C a (Fournier and Truesdell, 1973), or the corresponding temperature functions proposed by Tonani (1980), are based on the theoretical assumption of equilibrium between water and a feldspathic (hydrothermal or primary) paragenesis. They therefore give inconsistent results when applied to waters coming from carbonate aquifers. Carbonate rocks are often very permeable and represent the main target of geothermal exploration in many areas: for example Larderello and Amiata, the two main geothermal systems of Italy, have their reservoirs within fractured Mesozoic c a r b o n a t e - e v a p o r i t e rocks. Research was therefore initiated in order to implement geothermometric techniques specific to carbonate geothermal reservoirs through the study of the system involving water and the main mineralogical constituents of these rocks (calcite, dolomite and anhydrite). This paper summarizes the results of a theoretical study of the equilibrium conditions of that system, together with some preliminary applications of the new geothermometers obtained. G E N E R A L C H E M I C A L C H A R A C T E R I S T I C S OF W A T E R S C O M I N G FROM C A R B O N A T I C RESERVOIRS The numerous thermal springs of Tuscany (Central Italy) offer good examples of the different kinds of water that have interacted with c a r b o n a t e - evaporite rocks. Hydrogeochemical knowledge of these waters has increased in recent years thanks to the works of many authors (Tonani, 1957; Francalanci, 1959; Fancelli and Nuti, 1975; Bencini et al., 1977; Panichi et al., 1977; D ' A m o r e et al., 1980; Panichi, 1982). Considering the main dissolved constituents (Ca 2÷, Mg 2÷, Na ÷, K ÷, H C O ; + COl-, SO,2- and CI-), by means of the 77

78

l r Marini, G. ("hiodini a n d R. Ciont

Piper diagrams and other relevant correlation diagrams these thermal springs can be classified into three chemical types: -a l k a l i n e - e a r t h s u l p h a t e , which was generally related by previous authors to MesozuJ¢ anhydrite-bearing formations and sometimes also to Tertiary evaporite units, a l k a l i n e - e a r t h b i c a r b o n a t e , which groups waters circulating into Mesozoic carbonate and c a r b o n a t e - a n h y d r i t e formations, -a l k a l i n e chloride, which is related to Neogenic evaporite sequences or to the crystalline basement. Sulphate and bicarbonate waters are by far the most frequent chemical types and the only ones pertinent to the forthcoming discussion. These two chemical families only will be considered from now on. Since their temperature is generally in the range from 30 to 40°C, and they appear to be saturated with respect to calcium sulphate, Trevisan (1951), Tonani (1957) and Bencini et al. (1977) suggested that these waters heated up as a consequence of the hydration of anhydrite to form gypsum. This would, of course, be valid for temperatures no greater than the equilibrium point, which should be at 57°C, 1 atm in water, according to Hardie's (1967) data. The N a / K , C a / K and C a / N a geothermometers, applied as proposed by Tonani (1980), give, of course, inconsistent geochemical temperatures because the mineral assemblages of the relevant aquifers are different from those used for the calibration of these geothermometers. T H E T H E R M O D Y N A M I C SYSTEM FOR T H E A L K A L I N E - E A R T H S U L P H A T E AND T H E A L K A L I N E - E A R T H B I C A R B O N A T E T H E R M A L WATERS In order to study the chemical equilibrium among some minerals and an aqueous solution it is necessary to distinguish between: - - m o b i l e c o m p o n e n t s , i.e. soluble chemical species, whose concentrations increase in aqueous solution, not being limited by the precipitation of solid phases and - - c o m p a t i b l e c o m p o n e n t s . These chemical species participate in at least one mineral - solution equilibrium, which limits their solubility. This distinction, originally proposed by T h o m p s o n (1955), Korzhinskii (1959) and Zen (1963), has been rediscussed recently by many authors (e.g. Barton and Skinner, 1979). Considering the compatible components, the phase rule can be written as: V=C+2-P w h e r e V is the number of independent variables needed to define the system, C is the number of compatible components and P is the number of phases. At equilibrium at fixed pressure and temperature the number of solid phases will be equal to the number of compatible components minus one (the aqueous solution). Ca 2÷, Mg 2÷, CO32-, SOl-, F- and SiO2 can be considered as compatible components in the specific case of the a l k a l i n e - e a r t h bicarbonate and the a l k a l i n e - earth sulphate thermal waters. Therefore five minerals are expected to be present at equilibrium conditions; considering the lithology of the aquifers involved, the predicted solid paragenesis should include calcite, dolomite, anhydrite, fluorite and quartz, provided that sufficient amounts of the chemical species required to form such solid phases are available. This is very likely the case for calcite and anhydrite, which belong to both the primary and hydrothermal paragenesis of studied c a r b o n a t e - evaporite reservoirs: Larderello (Cavarretta el al., 1980) and Latera (Cavarretta et al., 1983). Dolomite is a primary constituent of these rocks. Quartz is extensively present as an authigenic mineral (Cavarretta et al., 1980, 1982, 1983). Fluorite occurs as dispersed mineralization in the c a r b o n a t e - e v a p o r i t e series; in fact saturation with respect to calcium fluoride is normally achieved during the evaporation step of calcium sulphate precipitation (Bencini et al., 1977). Authigenic fluorite occurs in Latera wells

G e o t h e r m o m e t e r s f o r C a r b o n a t e - E v a p o r i t e G e o t h e r m a l Reservoirs

79

(Cavarretta et al., 1983); on the other hand, neither fluorite nor any other F-bearing mineral appears in the hydrothermal paragenesis characterizing the Calcare cavernoso (cavernous limestone) which is the present day productive formation in the L a r d e r e l l o - T r a v a l e geothermal region (Cavarretta et al., 1980). In such cases primary fluorite probably controls the dissolved fluoride, since it is very unlikely that fluoride behaves as a mobile component in solutions interacting with such rocks. Similar ideas were expressed by Tonani (1957), following another line of reasoning. The activities of the compatible components should, therefore, be controlled bY the following equilibria: K c a I -~- a c a 2 + " a(2o32 K D o I -~ a c a 2 +

(1)

• aMg2+ • ( a c 0 2 - )

2

(2)

KA,, = aca2+ " aso42

(3)

K H = aca2+

(4)

• (aF-) 2

Ksio2 = asio2

(5)

Equation (5) is just one of the different silica geothermometers. The other four temperaturedependent equations could be utilized in this shape [as previously proposed by other authors, i.e. Tonani (1970) and Ellis and Mahon (1977)] but we prefer to rearrange them so as to obtain relationships more suited to geothermometric applications, removing the activities of some chemical species. We have omitted the calcite - anhydrite - aqueous solution equilibrium because the change in carbonate content during the ascent of the thermal waters makes it unsuitable as a geothermometer. CALCITE - DOLOMITE - AQUEOUS SOLUTION EQUILIBRIUM

-

From equations (1) and (2) we obtain the thermodynamic constant of the calcite - dolomite aqueous solution equilibrium: Kcu =

K~al/KDol =

mea2+

" "/Ca 2+ /mM,g2~ " 7Mg2+

(6)

where m is molality and 7 the activity coefficients. Assuming that 7c,2+ ~Mt,.2÷ we have =

g e l ) = mca2+ /mM~,2~

Changes of log (mca2+/mMg2,) VS temperature are shown in Fig. 1, where the following curves are plotted. Curve A. We considered the high-temperature experimental data of Rosenberg and Holland (1964) and of Rosenberg et al. (1967) in 1 and 2 M solutions, together with the calcite and dolomite solubility products at 25°C. For dolomite, a log value of - 17.02, as indicated by many authors (see discussion in Helgeson et al., 1978, p. 107), was considered. For calcite, the log value of - 8.37 given by Heigeson (1969) was chosen. Curve B. Shows Helgeson et al. (1978) data referred to disordered dolomite and pure calcite. Curve C. Presents Helgeson et al. (1978) data referred to ordered dolomite and pure calcite. Curve A better fits data of geothermal wells, as shown in the following discussion and was,

1.. /~4Ut'im, G. ('hiodini and R. Cioni

80

,c ^-mca Iv ~ m--'77-E, -

\\

Mg~

1.5-

\\

A\,\B'L

\ \\..

\\

",, \\

\\

•

Xx\\\

\~katera

\~3~

0.5-

• +

\

Tor r e Alfina

\\

S. Sisto \\ \

+ ++

\

\ +

Fig. 1. Plot of Ca/Mg ratio vs temperature. Curve A refers to the proposed geothermometric function, whereas curves B and C refer to disordered dolomite/calcite and to ordered dolomite/calcite equilibria, respectively, considering Helgeson et al. (1978) data. Data from Latera (solid circles), Torre Alfina, Kizildere (crosses) and S. Sisto (Viterbo area) geothermal wells are plotted. The scatter of Kizildere data is presumably due to Ca-mineral precipitation (see text): the same process occurs in the Latera well with the lowest Ca/Mg ratio. therefore, chosen as r e p r e s e n t a t i v e o f the c a l c i t e - d o l o m i t e e q u i l i b r i u m c o n s t a n t at d i f f e r e n t temperatures. ANHYDRITE-

FLUORITE-

AQUEOUS SOLUTION EQUILIBRIUM

The a n h y d r i t e - f l u o r i t e - a q u e o u s s o l u t i o n e q u i l i b r i u m is described by

Kx~ = as,,~ /(a, )2 = nT~(,j • Y~(,j / ( m l

• yl )2

(7)

derived f r o m e q u a t i o n s (3) a n d (4). E q u a t i o n (7) can be rewritten as: log (ms<,] / ( m I )2) = log K,~i - log (7so] /(7t

)2)

Values o f K x~have been c o m p u t e d c o n s i d e r i n g the d a t a o f K h a r a k a a n d Barnes (1973) for a n h y d r i t e a n d the d a t a revised by N o r d s t r o m a n d J e n n e (1977) for fluorite. In this case two ions o f d i f f e r e n t c h a r g e are involved; the values o f ~, have thus been calculated by m e a n s o f the D e b y e - Ht~ckel e q u a t i o n (where 1 is the ionic strength a n d z~ the ion charge):

log y+ = -

A zi2 x/l 1 + a, B~I

+ CI

using the a c o n s t a n t r e p o r t e d by Klotz (i 964) a n d the p a r a m e t e r s A , B a n d C given by Helgeson et al. (1971). D i f f e r e n t r e l a t i o n s h i p s o f log (rnso ~ / ( m j )2) vs t e m p e r a t u r e hold at d i f f e r e n t ionic strength (see Fig. 2); all o f them have been o b t a i n e d t h r o u g h N e w t o n ' s i n t e r p o l a t i o n o f existing data.

Geothermometers f o r C a r b o n a t e - Evaporite Geothermal Reservoirs

81

APPLICATIONS In order to test the theoretically derived geothermometers, data from geothermal wells producing from carbonate a n d / o r c a r b o n a t e - a n h y d r i t e reservoirs have been considered. Unfortunately few data on geothermal fields of this kind were found in the literature: Kizildere (Turkey) in Kurtman and ~&milgil (1975), Alpan (1975), Ellis and Mahon (1977) and Ellis (1979), - - Torre Alfina (Italy) in Barelli et al. (1978), Latera (Italy) in Cavarretta et al. (1983). Unpublished data on the Latera geothermal field (Northern Latium) were kindly provided by G. C. Ferrara and R. Beltrami (ENEL - Italian Electricity Board). No geochemical information sufficiently reliable to serve as a geothermometry test is available from the producing Tuscan fields (Larderello, Amiata), but these data should be obtained soon, thanks to the cooperation of ENEL. Data from Latera and Torre Alfina (and S. Sisto, see following discussion) are consistent with the C a / M g theoretical equation (curve A in Fig. 1). In the Kizildere field calcium scale minerals are precipitated rapidly from solutions rising to the surface, so that the spread of results in Fig. 1 is probably due to variable mineral precipitation occurring during equally variable procedures of sampling and discharging wells (A. J. Ellis, personal communication). Sulphate and fluoride data from Kizildere, Latera and S. Sisto fit quite well with the thermodynamic equations (see Fig. 2); no F data are available for Torre Alfina. -

-

-

-

7"

I=1

[ocjmS04 -(mF) 2

I=10

6"

,.Sisto

5-

Lateraq

IO /T(K) 2

21o

zi5

3:0

31s

Fig. 2. Plot of SO,/F ratio vs temperature for different ionic strengths (see text). Data are from Latera 2, Kizildere (crosses) and S. Sisto (Viterbo area) geothermal wells.

L. Marini, G. Chiodini and R. Cioni

82

These geothermometers have also been applied to many springs of Tuscany, using the data published by Bencini e l al. (1977). The C a / M g a n d SO4/F g e o t h e r m o m e t e r s show some discrepancy a n d t e m p e r a t u r e estimates are generally between 50 a n d 100°C. For a better u n d e r s t a n d i n g of this p r o b l e m , a geochemical field survey was carried out on two groups of hot springs (Table 1), near R a p o l a n o (Siena Province, T u s c a n y ) a n d near Viterbo (Latium), i n c l u d i n g the a b o v e - m e n t i o n e d artesian well S. Sisto.

Experimenlal Raw, filtered (0.45 Ixm) and filtered acidified (with HCI 1 : 1) aliquots were collected in polyethylene bottles, with temperature, pH, silica and bicarbonate determined directly in the field. Ca 2÷, Mg 2÷, Na ÷, K ÷, Li ÷, HCOL SOl-, C1- and F- were analysed in the laboratory. Analytical methods and results (as milliequivalents/litre) are listed in Table 2. Table 1. List of thermal waters from Rapolano and Viterbo Sample

Water point

Rapolano area

1 3 6 7 9 10 I1 12 13 15 18

Terme di S. Giovanni spring Terme della Querciolaia spring Bagni Freddi spring Artesian well near sample 6 Le Ficaiole spring Cave Paradiso pond Noceto spring Acqua Passante spring Bagni di Montalceto spring Casalino cold spring Le Rombole spring

Viterbo area

24 25 26 27 30 31 33 35 36 37

Bullicame ovest spring Bullicame spring La Zitella spring La Zitella spring L'Asinello spring L'Asinello spring S. Sisto artesian well Bagnaccio spring Bagnaccio spring Bagnaccio spring

Discussion o f results Geochemical temperatures were evaluated both by the new S O , / F and C a / M g geothermometers and the "classical" ones (Table 3): the S O , / F and C a / M g geothermometers provide coherent results for most waters with higher salt content (which are likely to be pure thermal waters), while less coherent estimates are observed for the mixtures between thermal waters and groundwaters. Some differences in the set of data from Viterbo hot springs could be attributed to Ca-minerals precipitation. The discrepancy between the C a / M g and S O , / F temperature estimates using literature data on Tuscan thermal springs could derive from the analytical techniques used. The fluoride concentration in our acidified samples was, in fact, one order of magnitude greater than in untreated aliquots, where precipitation of solid phases (mainly CaCO3) was observed, -

-

Geothermometers for Carbonate- Evaporite Geothermal Reservoirs Table

2.

Chemical

Sample

Ca 2÷

analysis o f Rapolano (Tuscany) and Viterbo (Latium) hot springs. milliequivalents/litre for ionic species and in millimoles/litre for SiO, Mg 2"

Na ÷

K÷

Li*

HCO; + CO]-

SO~-

Rapolano area 1 48 3 36 6 32 7 38 10 28 11 31 12 22 13 29 15 11 18 32

19 14 13 17 11 12 7.6 11 2.8 13

18 9.7 12 17 6.0 6.8 3.6 5.8 1.5 7.3

1.3 0.71 0.84 1.1 0.48 0.51 0.29 0.48 0.097 0.58

0.20 0.10 0.12 0.17 0.064 0.074 0.035 0.063 0.0096 0.080

50 37 35 41 30 31 23 28 10 36

26 15 17 23 12 16 8.8 15 4.1 13

Viterbo area 24 27 25 27 26 29 27 29 30 34 31 35 33 35 35 29 36 29 37 27

11 10 12 12 14 13 13 12 13 13

0.88 0.86 0.86 0.86 0.84 0.83 0.85 0.82 0.85 0.85

0.034 0.033 0.032 0.034 0.037 0.037 0.033 0.033 0.032 0.034

16 16 16 16 17 17 17 16 17 13

24 23 27 27 33 33 33 27 27 28

1.5 1.4 1.4 1.4 1.5 1.5 1.5 1.5 1.6 1.5

CI-

83

Results are

in

F-

SiO2

T(°C)

8.9 5.4 6.7 9.5 3.4 3.7 2.1 3.2 1.0 4.5

0.15 0.13 0.13 0.15 0.096 0.079 0.056 0.11 0.042 0.12

0.35 0.35 0.25 0.32 0.23 0.32 0.27 0.27 0.18 0.23

38 40 26 28 12 35 23 30 13 22

0.44 0.46 0.43 0.43 0.46 0.42 0.42 0.42 0.45 0.45

0.22 0.20 0.20 0.20 0.19 0.19 0.19 0.20 0.20 0.23

0.99 0.94 0.88 0.86 0.85 0.83 0.80 0.85 0.86 0.86

57 57 63 63 45 54 59 65 65 46

Analytical methods: Ca, Mg, Na, K and Li, atomic absorption spectrophotometry; H C O . titration against HCI with methyl orange as indicator; SO,, turbidimetry with BaCI2; C1 colorimetry with the ferricyanide method; SiO2, colorimetry with the molybdate method; F, ion selective electrode.

Table 3. Geochemical temperatures (°C) of Rapolano and Viterbo hot springs Sample

/Na/K

Rapolano area 1 220 3 221 6 216 7 206 10 233 I1 224 12 234 13 237 18 232 Viterbo area 24 25 26 27 30 31 33 35 36 37

793 821 821 821 766 759 773 753 737 773

ICa/K

TCa/Na

tqz

/cha

ICa/Mg

ISO4'1.

102 88 95 100 80 80 70 80 84

44 26 36 47 14 17 2 13 19

72 72 61 69 58 69 63 63 58

37 37 25 34 22 34 27 27 22

60 64 58 45 64 64 78 66 58

64 69 66 68 57 44 40 59 68

99 98 97 97 94 93 94 95 96 98

- 24 - 26 - 27 - 27 -27 - 28 - 28 - 25 - 24 - 24

111 109 106 105 105 104 102 105 105 105

81 79 76 75 74 73 71 74 75 75

58 69 56 56 56 69 69 56 46 36

85 81 78 78 71 71 71 78 78 84

L. Marini, G. Chiodini and R. Cioni

84

the N a / K , C a / K and C a / N a geothermometers (Tonani, 1980), give inconsistent estimates, probably because the mineral assemblages involved are different from those used for the calibration of these geothermometers, the quartz solubility function (Truesdell, 1975) gives results consistent with those of the S O , / F and C a / M g geothermometers for Rapolano hot springs, whereas chalcedony seems to be the silica mineral controlling dissolved silica concentration in the Viterbo hot springs (considering the temperature function proposed by Arnorsson et al., 1983). It should be noted that the estimated temperatures for Viterbo hot springs and S. Sisto well agree with those measured in nearby geothermal wells drilled by Terni and ENEL. These temperatures range from 60 to 78°C (Borghetti et al., 1984). Mixing model Mixing between thermal waters and groundwaters might explain the difference between the S O , / F and C a / M g temperatures observed in low salinity waters of Rapolano area. It is possible to eliminate this problem through simple calculations. If no precipitation takes place, the mass balance for SOl-, F-, Ca 2÷ and Mg 2÷ is: C,v'x=

G,M-

C,~'(l

- X)

where C,.T is the concentration of the ith species and X is the fraction of the thermal component in the mixture. The subscripts T, M and G refer to the thermal component, to the mixture and to the groundwater component, respectively. Rearranging we obtain: Cso4.T

(CF,T) 2

x

• (Cso4.M -- Cso,.~ " (1 -- X ) )

(CF,M -- Cv,o " (1 - ) 0 ) 2

and

(1

Cca,y

Cca,M --

CMg,T

CMg.M- CMg,o " (1

Cca,G

"

-

X)

- X)

Knowing the concentrations for the groundwater component and for the mixture, the temperature of the pure thermal component can be evaluated by tentatively changing X until the S O , / F and the C a / M g ratios for the thermal component give the same temperature. For example, considering sample 12 as a mixture between groundwater (represented by sample 15) and a thermal component, we obtain for X = 0.20: S O , / F temperature = 52°C C a / M g temperature = 55°C ionic strength = 0.17 moles/liter. CONCLUSIONS Theoretical geothermometers based on the equilibria a m o n g an aqueous solution and two pairs of pure solid phases (calcite - dolomite and anhydrite - fluorite) are proposed for thermal waters that originated from the interaction with carbonate a n d / o r c a r b o n a t e - a n h y d r i t e formations. The fact that the proposed equations are specific for the considered mineral assemblage is of positive rather than negative value; in this respect the proposed correlations strongly differ from some of the conclusions reached by Fournier and Potter (1979). Preliminary applications

Geothermometers f o r Carbonate - Evaporite Geothermal Reservoirs

85

to both thermal springs and geothermal wells have given promising results. The SO,/F temperature function seems to provide the best results, as the Ca/Mg geothermometer is sometimes affected by Ca-mineral precipitation; on the other hand information about scaling tendencies can be obtained in such cases. These geothermometers must, however, be verified and calibrated by means of additional data from geothermal wells and the results of experimental research on specific water/rock interactions. Acknowledgements--The authors gratefully acknowledge Prof. F. Barberi for his critical review of the manuscript. Thanks are also due to Drs G. C. Ferrara and R. Beltrami of the Italian Electricity Board (ENEL) and to Dr. G. Verdiani of AGIP for their cooperation.

REFERENCES Alpan, S. (1975) Geothermal energy exploration in Turkey. In Proc. Second U.N. Syrup. on theDevelopmem and Use of Geothermal Resources, San Francisco, Vol. 1, pp. 25 - 28. Arnorsson, S., Gunnlaugsson, E. and Svavarsson, H. (1983) The chemistry of geothermal waters in Iceland - - Ill. Chemical geothermometry in geothermal investigations. Geochim cosmochim. Acta 47, 567- 577. Barelli, A., Celati, R., Manetti, G. and Corsi, R. (1978) Reservoir engineering study in Alfina water-dominated field. In Italian Days o f Science, Budapest, pp. l - 18. Barton, P. B. Jr. and Skinner, B. J. (1979) Sulfide mineral stabilities. In Geochemistry o f Hydrothermal Ore Deposits, 2nd edn (Edited by Barnes H. L.), Chapter 7, pp. 278-403. Wiley. Bencini, A., Duchi, V. and Martini, M. (1977) Geochemistry of thermal springs of Tuscany (Italy). Chem. Geol. 19, 229 - 252. Borghetti, G., La Torre, P., Sbrana, A. and Sollevanti, F. (1984) Geothermal exploration in Monti Cimini permit (North Latium, haly). AGIP S.p.A., internal report. Cavarretta, G., Gianelli, G. and Puxeddu, M. (1980) Hydrothermal metamorphism in the Larderello geothermal field. Geothermics 9, 297 - 314. Cavarretta, G., Gianelli, G. and Puxeddu, M. (1982) Formation of authigenic minerals and their use as indicators of the physicochemical parameters of the fluid in the Larderello-Travale geothermal field. Econ. Geol. 77, 1071 - 1084. Cavarretta, G., Gianelli, G., Scandiffio, G. and Tecce, F. (1983) Evolution of the Latera geothermal system--ll. Metamorphic hydrothermal mineral assemblage and fluid chemistry. Workshop on Water-Rock Interaction, Tokyo. D'Amore, F., Squarci, P. and Panichi, C. (1980) Hydrogeology and geochemistry of the thermal springs of south-west Tuscany. In Adv. European Geother. Res. Proc. Second Int. Syrup., Strasbourg, pp. 315-329. Ellis, A. J. (1970) Quantitative interpretation of chemical characteristics of hydrothermal systems. Geothermics Special Issue 2, 492-515. Ellis, A. J. (1979) Explored geothermal systems. In Geochemistry o f Hydrothermal Ore Deposits, 2nd edn (Edited by Barnes, H. L.), Chapter 13, pp. 632-683. Ellis, A. J. and Mahon, W. A. J. (1967) Natural hydrothermal systems and experimental hot w a t e r - rock interactions (Part 11). Geochim. cosmochim. Acta 31,519-538. Ellis, A. J. and Mahon, W. A. J. (1977) Chemistry and Geothermal Systems. Academic Press, New York. Fancelli, R. and Nuti, S. (1975) Studio sulle acque termali e minerali della parte orientale della Provincia di Siena. Boll. Soc. Geol. h. 94, 135-155. Fournier, R. O. and Potter I1, R. W. (1979) Magnesium correction to the N a - K - C a chemical geothermometer. Geochim. cosmochim. Acta 43, 1543 - 1550. Fournier, R. O. and Truesdell, A. H. (1973) An empirical N a - K - C a geothermometer for natural waters. Geochim. cosmochim. Acta 37, 1255- 1275. Francalanci, G. P. (1959) Contributo per la conoscenza delle manifestazioni idrotermali della Toscana. Atti Soc. Tosc. Sci. Nat. 65, 372- 432. Hardie, L. A. (1967) Gypsum- anhydrite equilibrium at one atmosphere pressure. Ant. Mineral. 52, 171 -200. Helgeson, H. C. (1969) Thermodynamics of hydrothermal systems at elevated temperatures and pressures. Am. J. Sci. 267. 729 - 804. Helgeson, H. C., Delany, J. M., Nesbitt, H. W. and Bird, D. K. (1978) Summary and critique of the thermodynamic properties of rock forming minerals. Am. J. Sci. 278A, 1 -229. Helgeson, H. C., Jones, J. A., Mundt, T., Brown, T. H., Nigrini, R. H., Leeper, R. H. and Kirkham, D. H. (1971) Path I and data bank PATDAT, program No. 1000. Comp. library, H. C. Helgeson, U. C. Berkeley, U.S.A. Kharaka, Y. K. and Barnes, I. (1973) SOLMNEQ: solution-mineral equilbrium computation. U.S. Department of the Interior, Geol. Surv. Computer Contribution, Report No. USGS-WRD-73-002. Korzhinskii, D. S. (1959) Physicochemical basis of the analysis of the paragenesis of minerals. Consultants Bureau, New York.

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1. Marini. G. Chiodini and R. Cioni

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