Geochemical Journal, Vol. 10, pp. 155 to 161, 1976 155

Subsuperficial changes in chemical composition of the thermomineral waters of Vichy basin. Geothermal implications

GIL MICHARD, ANTON STETTLER*, CHRISTIAN FOUILLAC, GERALDOUZOUNIAN and DOMINIQUEMANDEVILLE Laboratoire de Geochimieet Cosmochimie(LA 196) UniversiteParis 7, France

(Received July 23, 1976)

The chemical analyses of the mineral waters issuing in the basin of Vichy St Yorre provide the means to decipher different subsuperficialmodifications superposed on the fluid originatingat great depths: (1) Increasingamounts of dissolvedCa, Sr, Mg and sometimesK, by gradual attack of shallowhost rocks. (2) Precipitation of silica and associatedaluminium. (3) Mixing with fresh surface waters. The comprehensive study of these alterations allows to shed light on the initial composition of deep water. From the silica content, the Na/K ratio and the calcite saturation, a temperature of 135°C is inferred for this water.

INTRODUCTION tinguish the following geographically separated The chemistry of numerous springs issuing groups (cf. Table 1): in the thermal field of Vichy (France) has been -the group of Vichy on the right bank of Allier thoroughly investigated. The principal aim was -the group of Cusset , east of Vichy to derive the temperature of the deep seated -the group of Abrest-Bellerive , SW of Vichy, hot reservoir, using quantitative geothermo includes the hottest waters meters: silica (FOURNIER and ROWE, 1966), and -the groups of Hauterive , St Sylvestre and St sodium-potassium (WHITE, 1965; ELLIS, 1970). Priest, on the left bank of the Allier However, one had to expect the original water -the group of St Yorre , NE of the preceding composition to be considerably altered during ones on the right bank of the Allier. the penetration into subsurface sedimentary Before discharging at the surface, the thermal rocks. Presuming a variable contribution from fluids are captured in aquifers embedded in the these rocks to each individual spring of the field, series of Stampian marls. we analysed a great variety of different springs. Considering the lateral distribution of the In fact, this method yielded strong evidences springs, their temperatures and the dip of the for the chemical composition of the common marl strata, ARMAND (1933) concludes that the thermal reservoir. fracture which furnishes the springs, is situated somewhat in the west. This author adopts the SAMPLING LOCATIONS occurrence of a dislocation denoted "Macaux fault" which supplies the springs in the entire All samples reported in the present work basin. The waters issuing from the fault hence have been collected in the active thermal area follow the cracks in the series of marls to reach of the Vichy basin (France), situated in the large directly the surface or to accumulate in the Stampian graben of the Limagne. A compre shallow aquifers. We have analysed samples hensive study of the region is given by DECROS from each of the different groups, but focused (1963) and THUIZAT (1973). A very large our interest onto the hottest springs, namely number of springs are issuing and one can dis those from the groups of Vichy and Abrest

* Present address: Physikalisches Institut, Universitdt Bern, Switzerland. 156 G. MICHARD et al.

Table 1. List of the studied springs in the Vichy area ratio, and simultaneously the Rb and Sr con tents, the isotopic dilution technic thoroughly le Samy Basin City Spring described by BIRCK and ALLEGRE(1973, 1976)

Vichy Vichy Chomel F 22 has been applied. Procedure blanks turned out du Parc F 23 to be 0.010ng for Rb and 0.035 ng for Sr, with Lucas F 24 a reproducibility of ± 20%. A mass spectro Grande Grille F 25 meter reproducibility of ± 0.00005 (2a) for Hopital F 26 the 87Sr/86Sr ratio has been inferred from peri Lardy F 27 Gdndreuse F 28 odical runs of the Sr standard NBS 987. The 3 Etoiles F 29 results are compiled in table 4. Larbaud F 30 Celestins F 31 Table 3. Chemical analyses of some Vichy springs Bellerive Boussanges F 33 (trace elements)

Abrest Dome F 34 Fe Mn Sr Al Rb F Lys F 35 n° 10-5 10-6 10-5 10-6 10-6 10-3 Corndlie F 36 F 1 2.75 3.0 1.93 46 10 0.49 St Yorre Hauterive Denise F 2 F 7 3.99 3.9 1.46 46 10 0.48 St Ange F 3 F12 5.79 3.4 1.90 46 10 0.46 F22 0.38 2.8 2.02 1.8 8 0.42 St Yorre L. Armand F 7 F24 0.77 4.9 2.06 4.6 8 0.41 L. Blanquet F 8 F25 0.39 2.8 2.00 1.15 7.5 0.44 St Sylvestre Agrdable III F 1 F30 1.90 8 Berthommier Larbaud F 9 F31 0.34 2.5 1.34 5 5.5 0.30 Moina F 10 F34 0.36 0.7 1.76 0.7 8 0.48 Fderique F 11 F35 0.34 0.8 1.8 1.5 8 0.48

St Priest Jade F 12 Cusset Cusset Tracy F 4 Table 4. Rubidium and strontium data determined by Lafayette F 5 isotopic dilution of mineral watersfrom Vichy. Errors St Denis F 6 Mesdames F 32 for elemental concentration are ± 1 %. The strontium isotopic ratios are normalised to 86Sr/88Sr= 0.1194 and errors are quoted in terms of 10-5. Bellerive. Rb Sr 87Sr/86Sr n° Rb/Sr 10-5 10-5 EXPERIMENTAL TECHNICS AND RESULTS F24 0.783 1.963 0.399 0.71412 ± 7 The analyses have been performed applying F25 0.743 1.971 0.377 0.71406 ± 12 F30 0.754 1.806 0.417 0.71408 ± 4 the methods described by CAILLEAUX et al. F31 0.561 1.352 0.415 0.71402 ± 6 (1976). At least, four samples' were collected F34 0.795 1.700 0.468 0.71438 ± 4 at each spring: one sample unaltered (analyses of anions and some cations: Mg", K', Li+) DISCUSSION one sample diluted by a factor of ten with distilled water immediately after collection (ana The groups of Vichy, Bellerive and Abrest lyses of Na+, Ca", HC03 and SiO2 ) Twelve of the 14 sampled springs show one acidified to pH 2 with hydrochloric practically constant concentrations of Cl (9.47 acid (Suprapur) (trace elements, Sr isotopes) to 9.70X 10-3M/1), Na+ (79.8 to 81 X 10-3 one unaltered (isotopic analyses) M/1), Li (0.75 to 0.80 X 10-3M/1) and Rb+ The first three samples were stored in poly (7.6 to 8 x 10-IM/1). Only Lardy and Celestins ethylene bottles, the fourth in a tight glass diverge from these values: Lardy is slightly more bottle. Both pH and silica have been measured concentrated and Celestins distinctly more dilut on the field. ed. Considering the four elements mentioned Table 2 comprises the concentrations of the above, the dilution factor turns out to be 1.41 major constituents and table 3 those of trace ± 0.02. In contrast Cd" concentration is highly elements. variable from 1.00 X 10-3 M/ 1 (Genereuse) to For the determination of the Sr isotopic 4.28 X 10-3 M/ 1 (Lucas). To illustrate the

n Subsuperficial changes in chemical composition 157

to O to M N 0\• O O\ \0 to to N O 00 d lq' N to to to t M Q O r+ O o M M .--I N O -1 0 0 0 0 .--1 O C> CD 0 -( CD C,-] C,4 -4 CD

N' 0 M N 0\ \O N M O O d' O v'1 N 0\ N N l r, O 0\ IN M M 00 O 0\ 0\ all M M N M In . V M M M M M I t O\ t "' i C\ el to 00 M N M M 0 0 00 O O C) O O 0 0 0 0 0 .-i O O 0 0 0 - 0 0 .--~ -q

M O\ C M N 00 N O\ N N 00 N N 00 N O\ CA M M \p M 00 00 M It 0 U M N 00 00 to 00 to d' M d' N O\ O N O M d' M O 00 d' M try N N l \0 to 00 \O tN \O N t,0 N N 00 N 00 00 N N N to N N N N t` x

I 00 to O '.O to dt O N N 00 t \O O \O N r+ M O\ N O .- d' M C- I. ) +y M \O N to I tr) 00 en V) to 00 O 00 O\ N[t It t0 O\ lw O\ N N" t` N N t 0 00 . N .--i -4 .--i . -1 q . ~--i N - - --i 1--4 r., .-i r-i 1-4 N 1-4 .--r .-1

I ON M Id, W) N \0 00 v \O N O to to N Q\ \O .-) N N O et tt to N M N M O\ tr^ N C't '.O M N to to t to t% en t^ t ^ to '.O O ON0; 0\ tri 00 00 O\ 00 00 N 0; o, o\ o\ 0: o\ O 0\ O1 0\ \0 0\ O\ 0\ 0\

N N 0\ 00 0\ O\ 00 00 0\ O O\ \0 O\ O\ O\ N N 00 to t b0 \O N N to O\ r%, 00 \O N l0 N N N N 00 N 00 N N N to N 00 t1N N O O O O O O O O O O O O O O O O O O O O O O O O O O O

\O I'! I'! \O O eh 00 'O 00 00 \O O O N O N It N \O 00 \O N d' O N V cd O~ Oi Oi O~ \O N .~ 00 \O N O~ r+ O O ti ti N O 0\ 0\ \D N O 0\ 0\ 0\ z t~ t~ to 00 00 \O t~ \O 00 00 00 00 00 00 00 t t- to to 00 t t- t

00 a\ .4 00 M N 00 M 00 O S N r m to \O 00 ep N M t S t/) 00 t% 0c I' O O O\ N N O to v) \O It \O N \O 00 00 to \0 to \p N N N N N M M M N M M M N N N N N N N N N -4 N N N N N

rn PIII d' N e 00 to to d' N v \O \O 5 O M O\ .-4 O\ ~--I . -. M O\ t M N to rs +00 et ~t et \O V It It It I to to t d; O\ M "t , , tD dt tfi rzs O O O O O _q O O O O O O O O O O O O O O O O O O O O O Q'i tl QO N .-e O W) O N N t~ M 00 N 00 M 00 00 to O \D O to •-. et 0 •-1 d N 0\ N 00 00 \0 .--4 M \0 N M .-i M N N N .-i N to 0\ 1.0 O C-. M \p 5 N N N N 0 . .-+ N et M et N N N N N N N d ~t N M eF .4 •-+ ~-+ N M en -4 .--1 N v

N E 00 00 ' t- 00 00 00 00 M O N N 00 00 N O \O O 00 N N 00 N O\ -}' O \0 \O t~ to 00 M \p 0 ~ O'. SO to tlN V) N O\ 00 M It to to I It 00 O i a\ ts x tZ \O \G \O t0 '.O ~ %0 %0 '0 \0 %0 \0 %0 1~0 1.0 %0 \0 t0 \O N N

o

0 H to M 00 00 t, to to M to O 00 O O N O N O O O O to O O to to to- M.-4 S.--1 N.--4 N --4 M1-4 .-a kn-4 C-4-. en .--1 -4 C4-4 M ~ONen C) M-4 N C'4 N CnN "N en.-~ d \O \O of o ,o t7

a Q z x ¢ ¢ a a w Az as ~ a In w w a a a w

w w w H cn ] w w ~, a ¢ wz ~ ~. ~. ¢ o z ~' w wW a z~H Q w .o Q H ¢ a o x w w Q ~o a Q" u¢ w z A~ o ~'Q g F w z z~ o W ¢ `~ w zx C a= w ti v a a 0 a a H a u M Q > c U 0)

. N M d' to \0 l 00 0\ O .--i N N M tt to \O t 00 0\ O rl N M d to '0 .--i .--~ .--~ N N N N N N N N cM to M M M M M

G=. fxt w f1. w 40 w rJ., f1. P. [s4 fJ w fJ (J. Gi fJ w W k, 4. W "4 " " V. Ll. ~C 4

OU 158 G. MICHARD et al.

0 0 Nsl :61 K N a i5 (x 'M

CI 10 W 4 0

•

3 5

0.5 1 G G G B G (xioo) N a MY1x4)

3 4 Ca 1 2 Fig. 3. Relation K/Na versus Mg/Na • springs of Vichy-Abrest group Fig. 1. Concentrationsof dissolved main species in the O springs of St Yorre group Vichy group springs versus Cd" concentrations. All concentrations are in 10-3M/1. Mg++ exhibits a positive but non-linear cor behaviour of the different elements Ei, we relation (Fig. 1) with a stronger gradient for choose therefore a plot Ei = f (Ca"), (Fig. 1). the water richest in Ca. A similar pattern is The constancy of C 1-might be explained by observed for K+, and Fig. 3 illustrates the result two reasons: ing correlation between K+ and Mg++. The due to their low C1 content, the sedimentary trace elements Sr and Mn are closely connected rocks passed by the rising fluids cannot supply to Ca, whereas Fe correlates rather with Mg; important quantities of C1 into solution, (Fig. 2). Generally Si decreases as a function of once in solution, Cl does not react with the increasing Ca but the inverse correlation is not surrounding rocks; the only process which could very pronounced. therefore change considerably the Cl content of a thermal water is a mixing with surface The groups of St Yorre, St Sylvestre, Hauterive water. Apart from the spring Celestins, -all Mixing with surface water has been re springs from Vichy are evidently uncontaminat ported to be important for all the small springs ed by surface waters. The lack of variation of St Yorre (DECROS,1963). The C1 concentra found for Na+, Li' and Rb+ shows that these tions of the 9 springs analysed are slightly more elements are not subjected to chemical exchange variable than those of Vichy-Abrest, indicating between solution and sedimentary rocks. contributions from surface water. Since we focused our interest onto the wells pierced most recently, it is obvious that mixing phenomena

m are less apparent. Na+/Cl and Li'/Cl ratios 0 both retrain constant, showing that again Na 08 0

a a and Li are not exchanged between rocks and 1 0 0 Si

0 a solution. Calcium is less variable than in Vichy, 0 having concentrations corresponding to the average value of the springs of Vichy. Mg/Na ratio behaves similarly to Vichy, whereas K/Na

0 LI 050) ratio is more dispersed (Fig. 3). However, like

Sr (:L5 in Vichy the variations of K/Na are linearly cor related with those of Mg/Na, but the slope of 1 Mn (x Z) the correlation line is distinctly increased. For

F. all the springs of the group, the silica concentra t tion is low, varying between 0.3 and 0.4 X 10 -3

I 2 4 C. M/1. The trace elements were determined only in Fig. 2. Concentrations of silica and some trace elements three cases; we observe an unusual high Al con in the Vichy group springs, versus Ca++concentrations. tent. Subsuperficial changes in chemical composition 159

The waters from Cusset The springs from observed relation is consequently attached to a Cusset yield a definitely different composition. subsuperficial alteration. Fig. 3 suggests strongly With the exception. of one sample, all others are an evolution of two groups of springs derived distinctly more diluted. Na/Cl and Li/Cl ratios from one common source having the composi are not constant, and Na/K ratio is the lowest tion of Genereuse. The temperature of this among the springs from all groups. Finally it deep seated common source as derived from the has to be mentioned that two of the four ana Na/K ratio (=32.4) turns out to be 135°C. lyses carried out in Cusset are somewhat less precise. Evolution of silica The solubility of all The chemical behaviour of the waters from silica compounds is positively related to the Cusset cannot easily be included in the schematic temperature. The overall modification of the framework elaborated in the next paragraph. water on its way to the surface in fact cor The subsurface modifications of the four sam responds to a convectional cooling. During this ples analysed are far too complex to be inter process Si concentrations may even decrease. preted unambiguously. Geological studies, Hence, the Si content of the deep water is at however, indicate that these springs might be least equal to the highest silica content of 1.59 branches from the powerful springs of Vichy. X 10-'M/1, observed in the water from Gene (DECROS, 1963). reuse. The corresponding temperature, inferred from the quartz-water equilibrium, is 135°C. INTERPRETATION The thermal waters issuing in the northern part of the Massif Central generally show re General Our present interpretation is based equilibration effects with amorphous silica at on the evolution of the elemental concentrations surface temperature (FOUILLACet al., 1975) given in Tables 2 and 3 and on the application leading to a relation between SiO2 and tem of both silica and sodium-potassium geothermo perature. In contrast, the Vichy springs show meters. We omit Na.K.Ca geothermometer a remarkable correlation between SiO2 and (FOURNIER and TRUESDELL, 1973), since it ap aluminium (cf. Fig. 4), which can be interpreted pears to be inapplicable to C02-rich systems in terms of clay mineral formation. The high (MICHARD and FOUILLAC, in prep.). The two amounts of fluoride found could well provoke former geothermometers correspond to the an enhancement of the Al concentration (HEM, chemical equilibria quartz-water, and low albite 1968) and therefore a decrease of the silica adularia-water. content. The geological setting (all fluids originating from the Macaux fault), the limited geographical Evolution of Ca Considering the high con area covered (distances being of the order of centrations of E C02 measured in the water from 10km), and the chemical similarity of the Vichy, the quantity of dissolved Ca is certainly springs favour the hypothesis that the chemical determined by the solubility of calcium car composition of all fluids, as observed at the surface, is entirely constrained by subsuperficial reactions of water originating from a common deep seated source. log Al

The evolution of K While Na concentra tion is constant for all the samples, potassium concentration varies, and the temperature in -4 ferred from Na/K varies also from 135 to 145°C in Vichy and from 140 to 172°C in St Yorre. -5 Moreover, we found a relation between K and Mg concentrations, though it is known that -6 Mg is practically absent in hot waters (ELLIS and MAHON, 1967). The Mg solubility imposed by the mineral couple chlorite-quartz, decreases log Si with increasing temperature. In contrast, as suming (Nat) to be constant, K+ concentration Fig. 4. Si02-Al inverse correlation of the springs from would increase with increasing temperature. The Vichy. 160 G. MICHARD et al. bonate. Since the latter is less important at and K(CaHCO3+), the degree of saturation E2 higher temperatures, the mechanism providing of the system CaCO3 -water before and after the modification is probably due to a strengthen degassing can be calculated. Taking the CaCO3 ed dissolution of calcium carbonate during the measured in the issuing water, n turns out to cooling of water. This can be supported in be always lower than 1 before and generally considering the following three arguments: higher than 1 after degassing (Table 5). To (1) the concentrations of the trace elements decide whether or not precipitation may ac strontium and manganese in the different springs count for the observed Ca variations, we will are compatible with the dissolution of a common lay emphasis on the spring Dome. This well carbonate containing 1,500ppm of Sr and 750 discharges from a 145m deep drilling hole tapp ppm of Mn. Nevertheless, an equilibrium co ing a reservoir embedded in a lens of sand. precipitation of CaCO3 with partition coef Fifteen days after the installation of the dis ficients of 0.15 for Sr, and 5 for Mn cannot be charge tube, the outflow was 121/min., cor ruled out. responding to a daily deposition of about 6 kg (2) the isotopic composition of Sr now might of CaCO3 in the precipitation hypothesis. As be considered as a useful tool to distinguish a concequence, a period of 20 days should have between the two processes. In the case of pre been sufficient to block up the tube. In fact, cipitation only, all the water samples should the tube has to be exchanged every 4 years. give identical isotopic ratios, whereas variable Hence, we can assume that the composition of dissolution of calcite would yield a gradual water studied at the surface is representative modification for the isotopic composition of for the water in the subsurface aquifer. This the water originating at depths. Our results water is therefore undersaturated with CaCO3 (Table 4) show that the sample mostly depleted and the precipitation hypothesis can be refuted. in Sr has a S7Sr/86Srratio significantly higher than all others. Instead of using the 87Sr/86Sr Table 5. Degree of saturation of CaCO3calculated for versus 1/Sr diagram to establish an eventual some Vichysprings mixing line (cf. e.g. LANCELOTand ALLEGRE, S21before degassing, &22after degassing 1974), we plot the isotopic ratio as a function The ratio gas/water were taken from Annales of Rb/Sr. Since Rb is a convenient dilution des Mines (1951) tracer (see above), this allows to account for 0 dilution effects. The ordinate intercept of the n F24 F25 F31 F34 reference line drawn in Fig. 5 is 0.712, which &21 0.58 0.50 0.41 0.25 might correspond to the Sr isotopic ratio of 02 2.83 3.25 2.09 3.18 the contaminating surface rock. (3) In view of the known parameters, Ca con The Genereuse water, most depleted in Ca, centration, pH, alkalinity and partial CO2 pres approaches closest the composition of the origi sure, and with the use of the constants, KA nal fluid. Moreover it has to be noted that there (H2 CO3/HCO3), KA (HCO31CO3), Ks (CaCO3) exists a relation between the modifications 87Sr observed for Ca, Mg and Si: for all three ele 86Sr ments, Genereuse exhibits the minimal altera tion, whereas Lucas and Lardy are most modi

0.7145 fied. Assuming CaCO3 saturation for Genereuse, a corresponding temperature of 130 ± 5°C is calculated, in excellent agreement with the temperatures inferred from the silica content and Na/K ratio. We conclude therefore this spring to , have preserved almost entirely the 0.7140 chemical properties of the deep seated fluid.

SUMMARY

Considering the results presented in Inter 0.4 0.5 R b Sr pretation, we propose the following four step Fig. 5. 87Sr/86Srversus Rb/Sr plot of the samplesfrom model for the genesis of the water of the Vichy Vichy compiled in Table 4. The referenceline basin: yields an ordinate intercept of 0. 712. Subsuperficial changes in chemical composition 161

(1) The initial water equilibrates at 135°C in du Plateau Central. Revue de 1'Industrie Minerale granitic rocks with quartz and potash feldspars 300, 270-278. and may provoke an albitization of plagioclase, BIRCK, J. L. and ALLEGRE, C. J. (1973) 87Sr87Rb leading to An 15 and calcite (MICHARDand systematics in Muntche Tundra mafic pluton, Kola FOUILLAC,in prep.). The Ca content is con peninsula, USSR. Earth Planet. Sci. Lett. 20, 266 trolled by calcite solubility, whereas the Mg 274. concentration is maintained at a low level, BIRCK, J. L. and ALLEGRE,C. J. (1976) Chronology presumably by the chlorite-quartz equilibrium. and chemical history of basaltic achondrites parent (2) The water rises in subsurface strata of body studied by 87Rb87Sr method (to be published). almost entirely sedimentary origin and is cooled CAILLEAUX, P., FOUILLAC, C., MICHARD, G. et down. The temperature drop does not signi OUZOUNIAN, G. (1976) Etude geochimique des ficantly affect the pH, which is maintained at a sources thermales de Chaudes Aigues. Consequences rather low value by the H2CO3/HC03 buffer. gdothermiques. C.R. Acad. Sci. Paris 282 D, 1237 (3) The water remains in a shallow aquifer 1240. situated in porous sedimentary rocks, where the DECROS, P. (1963) Etude des eaux minerales de la following reactions might take place. region de St Yorre. Wallon, Vichy. 104 pp. a) As a consequence of the temperature drop, ELLIS, A. J. (1970) Quantitative interpretation of the water originally saturated with CaCO3 begins chemical characteristics of hydrothermal systems. U. to dissolve CaCO3 of the host rock. Apparently N. Symp. Development and Utilization of Geothermal resaturation is not achieved. Resources. Pisa. Geothermics 2, 516-528. b) The cooling down provokes also a modifica ELLIS, A. J. and MAHON, W. A. J. (1967) Natural tion of Si concentration coupled with Al con hydrothermal systems and experimental hot water centration. rock interaction. Geochim Cosmochim Acta 31 c) Magnesium and more or less also potassium 519-532. are subjected to a remobilization. FOUILLAC, C., CAILLEAUX, P., MERLIVAT, L. and (4) Issuing at the surface, CO2 is discharged MICHARD,G. (1975) Preliminary studies on mineral in the atmosphere. The solution remains again water in French Massif Central. U. N. Symp. Develop oversaturated with calcite, which is deposited ment and Utilization of Geothermal Resources, San in minor proportions. Francisco. Considering the usefulness of the different FOURNIER,R. 0. and ROWE, J. J. (1966) Estimation geothermometers, one can make the following of underground temperatures from silica content of remarks: water from hot springs and wet steam wells. Am. J. The temperature of the deep water is 135°C. Sci. 264, 685-697. The temperatures inferred from Na/K can FOURNIER,R. 0. and TRUESDELL,A. H. (1973) An show important variations (135-172°) for a empirical Na K Ca geothermometer for natural waters. group of springs having the same origin, due to Geochim. Cosmochim. Acta 37, 1255-1276. subsuperficial chemical alterations. Magnesium HEM, J. D. (1968) Graphical methods for studies of turns out to be a very useful tool for the study aqueous aluminium hydroxide, fluoride and sulfate of those alterations. complexes. US Geol. Surv. Water Supply Paper The silica thermometer gives acceptable results 1827B, 33. only in those cases where practically no deposi LANCELOT,J. R. and ALLEGRE,C. J. (1974) Origin of tion of clay minerals has occurred. carbonitic magma in the light of the Pb-U-Th isotopic The Na.K.Ca thermometer fails completely, system. Earth Planet. Sci. Lett. 22, 233-238. yielding unreasonable high values. THUIZAT, R. (1973) Les sources thermominerales du In contrast, in this well-buffered system, cal Puy de Dome, de l'Allier, du Cantal et de la Creuse. cite saturation can be used as an auxiliary These 3° cycle Universite de Clermont Ferrand. 128 thermometer. pp WHITE, D. F. (1965) Saline waters of sedimentary REFERENCES rocks. Am. Ass. Petrol. Geol. 4, 342-366. Annales des mines (1951) Statistique des sources ARMAND, L. (1933) Sur quelques sources minerales d'eaux minerales francaises, Paris, 268 pp.