Isotopic and chemical composition of water and steam discharges from volcanic-magmatic-hydrothermal systems of the Guanacaste Geothermal Province, Costa Rica

Applied Geochemistry, Vol. 7, pp. 309-332, 1992

0883-2927/92 $5.00+ .00 (~) 1992 Pergamon Press Ltd

Printed in Great Britain

Isotopic and chemical composition of water and steam discharges from volcanic-magmatic-hydrothermal systems of the Guanacaste Geothermal Province, Costa Rica

WERNER F. GIGGENBACH DSIR Chemistry, Private Bag, Petone, New Zealand and RODRIGO CORRALES SOTO Instituto Costarricense de Electricidad, San Jos6, Costa Rica

(Received 26 February 1991; accepted in revised form 23 January 1992) Abstract--The Guanacaste Geothermal Province encompasses three major geothermal systems, each

centered on its respective volcanic structure: Rinc6n de la Vieja to the NW, Miravalles in the center and Tenor/o to the SE. Each shows corresponding sets of surface manifestations: vapor discharges from fumaroles and steam-heated pools at altitudes >500 m; lower temperature SOa-CI springs on the lower slopes of the respective volcano; and cooler neutral CI springs to the S of the volcanic chain, at altitudes <500 m. The production of HCO3-rich waters is limited to a narrow belt stretching to the S of Miravalles volcano. Chemical and isotopic evidence suggests that the neutral CI waters, also discharged from deep wells, are derived from a more primitive CI-SO4 water formed by transfer of readily mobilised, originally magmatic constituents to deeply circulating groundwater. Isotopic evidence suggests that this groundwater is derived largely from areas to the north of the volcanic chain. The presence of immature waters within drillable depth is indicated by the incursion of an acid SO4 water into a well at Miravalles after deepening to 2000 m. As the CI waters rise, they start to boil. The separated vapors feed the fumaroles and steam-heated features at the surface, the residual water travels underground to be discharged some 5-25 km to the S. The uniformity in chemical and isotopic compositions of the neutral C1 waters is compatible with the assumption that the individual thermal systems are linked at depth to form large, contiguous geothermal reservoirs beneath the Guanacaste Geothermal Province.

INTRODUCTION

INVESTIGATIONSto assess the potential of the Guanacaste G e o t h e r m a l Province for power production began in 1975. A pre-feasibility study in 1976 revealed that a thermal reservoir with temperatures up to 240°C is likely to underlie the Miravalles area (GARDNER and CORRALES, 1977). Following this promising finding, three wells were drilled during 1979 and 1980. They all proved commercial producers. A further five wells were completed during the second stage of drilling starting in 1984. The state of investigations up to July 1985 and the geology of the area are described by MANIERI et al. (1985); a further update is given by MANIERI and VACA (1990). Construction of a 55 M W geothermal power plant is expected to be completed during the early 1990s. The investigation reported here was carried out within the I A E A Coordinated Research Program on the Application of Isotopic and Chemical Techniques to G e o t h e r m a l Exploration in Latin America. Isotopic and chemical analyses of most of the waters were carried out at the I A E A laboratories in Vienna, Austria, the isotopic analyses of S species and chemical analyses of some of the waters, of the gases,

hydrocarbons and minor solute components at D S I R , New Zealand. The major purpose of this study is the delineation of an internally consistent hydrological model of the geothermal systems of the Guanacaste Geothermal Province by use of comparatively simple techniques for the interpretation of the isotopic and chemical compositions of geothermal fluid discharges.

ISOTOPIC COMPOSITION OF WATERS The isotopic compositions, together with tritium contents, of waters discharged from springs and wells over the Guanacaste G e o t h e r m a l Province are listed in Table 1; sample locations are shown in Fig. 1. The data for deep wells are weighted means of separated water and steam compositions and are therefore those of total discharge. A standard plot of 62H vs 61So is given in Fig. 2. The waters are classified in terms of their major anions into predominantly CI waters (circles), H C O 3 (hexagons) and SO4 (squares) waters. Data points for most of the H C O 3 and SO4 waters lie close to the meteoric water line, those for the C1 waters plot to 309

310

W.F. Giggenbach and R. Corrales S. Table 1. Isotopic composition of waters from the Guanacaste Geothermal Province from NW to SE. For wells, enthalpies are given in kJ/kg, estimates in brackets. Water types are characterized by their predominant anion: s--SO4, c---C1, b - - H C O 3 62H Temperature (°C)

3H TU

Type

-6.90 - 1.70 -5.63 -6.57 -4.50 +0.86 +6.21 -5.00 -4.36 -4.50 -4.52 -6.44 -5.20 -5.04 -6.52 -5.00 -5.20 -6.40

---------0.0 ---4.1 6.2 2.7 3.5 0.7

s s c b s s s s s s s s s b b b b b

-26.5 -31.4 -26.3 -29.6 -31.4 -30.0 -39.6 -23.8 -34.0 -29.3 -33.0 -29.6

-3.60 -3.35 -2.67 -3.14 -2.77 -3.65 -4.12 -2.31 -3.67 -3.00 -3.49 -2.97

-0.0 -0.6 -0.6 -------

c c c c c c c c c c c c

-38.1 -24.1 -48.9 -54.1 -34.9 -50.0 -48.8 -48.3 -50.8 -44.0 -25.0 -17.8 -39.7 -40.9

-6.50 -4.90 -7.46 -7.68 -3.92 -7.26 -7.12 -7.43 -7.49 -6.68 -4.50 -2.70 -4.95 -3.87

6.2 6.1 2.3 1.1 0.2 1.7 1.1 0.4 0.0 4.0 -----

s s b c c b b b b s s s c c

Location

Date

AH BQ SN SW L1 L2 L3 QA GB GB 36 GS LT QC LQ 34 34 EG

Los Ahogados Borinquen Salitral Norte Salitral Norte, well Las Pailas, pool Las Pailas, lake Las Pailas, mud pool Quebrada Azufral Guayabal, pool Guayabal, pool PH-36, 140 m well Guayabal, stream La Torte Quebrada de la Chepa Los Quesos PH-34,280 m well PH-34 El Gomes

18 August 1987 18 August 1987 17 March 1982 17 March 1982 17 March 1982 17 March 1982 17 March 1982 14 August 1982 16 March 1982 30 June 1984 16 March 1982 16 March 1982 14 August 1987 6 September 1984 6 September 1984 27 June 1984 12 September 1984 6 September 1984

37 98 74 26 95 90 96 40 64 70 67 25 32 20 18 115 111 37

-43.7 - 16.7 -42.5 -42.1 -26.6 -11.0 +6.6 -27.0 -24.1 -24.8 -24.0 -38.1 -29.9 -27.9 -40.6 -36.1 -33.6 -38.0

P1 P1 P2 P2 P2 P5 P5 P10 P10 P10 Pll PI1

PGM-1 PGM-1 PGM-2 PGM-2 PGM-2 PGM-5 PGM-5 PGM-10 PGM-10 PGM-10 PGM-11 PGM-11

16 March 1982 28 June 1984 16 March 1982 28 June 1984 10 January 1985 28 June 1984 16 February 1985 21 September 1985 25 July 1985 18 September 1985 13 March 1985 18 September 1985

1065 1011 957 963 994 1020 (1000) 1000 (1000) (1000) I000 (1000)

QG HA BN RB SB PL BB BJ RO PP MZ TM RS SC

Quebrada Gaveteros Hornillas Arriba Benaqon Rio Bagaces Salitral Bagaces Pozo Loras San Bernardo Arriba San Bernardo Abajo Rio Blanco Punta de Palo Montezuma Tierra Morenas Rio Salitral Salitral Cafias

27 June 1984 6 September 1984 31 August 1984 31 August 1984 31 August 1984 31 August 1984 20 June 1984 14 September 1984 6 September 1984 6 September 1984 19 August 1987 20 August 1987 15 March 1982 15 March 1982

the right of this line c o r r e s p o n d i n g to a significant oxygen shift in the waters discharged from d e e p wells and neutral CI springs (Salitrals). Taking into account the geographical distribution of the H C O 3 and SO 4 springs, t h r e e groups can be distinguished: isotopically lightest waters are discharged from springs to the South of the Miravalles area ( R O , PL, BN, BB, B J, Fig. 1). Isotopically s o m e w h a t heavier waters are e n c o u n t e r e d in springs over the N W part of the study area ( A H , SW, L Q , GS) and the shallow well PH-34 (34), most at elevations <500 m. A group of isotopically considerably heavier waters is r e p r e s e n t e d by the p r e d o m i n a n t l y

20 20 50 49 72 22 46 50 45 25 62 91 28 39

6180

(%0 SMOW)

S O 4 waters discharged from springs (GB, H A , LT,

QC) and a shallow well PH-36 (36) on the flanks of the Miravalles and Rinc6n de la Vieja (L1, Q A ) volcanic structures at elevations a p p r o a c h i n g 1000 m. T h e s e isotopic trends run c o u n t e r to the general observation that g r o u n d w a t e r s b e c o m e isotopically lighter with altitude and distance inland. A n explanation is p r o v i d e d by the occurrence of p r e d o m i nantly N E winds over the study area (VIALE et al., 1986). The a t m o s p h e r i c moisture carried by t h e m loses its heavier isotopes on a p p r o a c h i n g the chain of volcanoes, leaving isotopically d e p l e t e d precipitation to fall in their shadows to the SW. A n essentially

Geochemistry of water and steam, Guanacaste Geothermal Province, Costa Rica

steam-heated pool showing maximum enrichment (L3), e' the effective kinetic enrichment factor, in the 6-notation, of 50%0 for 2H and 16%o for 180 and e230o the equilibrium fractionation factor at 230 ° of +2%o for 180, and 0%0 for 2H. The value 230 ° is the temperature at which most of the steam is likely to have separated from the water at depth as discussed by GIGGENBACHand STEWART(1982). Possible isotopic compositions of the deep parent water, from which the steam heating the steam-heated pools may have separated, are represented by a line extending from (~dw,min,parallel to the line connecting data points for the steam-heated pools. In case the deep water is derived essentially from local groundwater, their 62H values are equal. For Miravalles, 618Odw was found to be -2.0%0; its deuterium content is that of the local groundwater of -25%0. The composition of the deep parent water may, of course, also be derived directly from the isotopic composition of the waters discharged from the deep wells drilled over the Miravalles area. The total discharge data points, however, show considerable variability (Fig. 2). The approximately linear trend suggests mixing between two endmember components: an isotopically depleted groundwater, possibly represented by the little mineralised HCO 3 waters encountered to the south of the Miravalles area, and an isotopically enriched, especially in 180,

recent and local origin for the SO 4 and H C O 3 waters discharged over the MiravaUes area is supported by their high tritium contents (Table 1). Another process leading to isotopic enrichment is surface evaporation from predominantly SO4, steamheated pools. The isotopic trends caused by this process are a function of the degree of evaporation, the composition of the original groundwater and that of the steam heating the pools (GIGGENBACHand STEWART,1982). Some of the higher SO4, low C1 waters (L2, L3, TM, BQ) fall indeed along a line expected for such surface evaporation from steamheated pools. By extrapolating the line connecting these data points onto the meteoric water line, the composition of the local groundwater is obtained. It corresponds, as expected, to that for the least enriched steam-heated pool at Las Pailas (L1) and therefore also to that for the northern, isotopically heavier groundwater over the Miravalles area. The isotopic composition of the deep parent water, from which the steam may have boiled off, may be evaluated by use of a procedure outlined by GIGGENBACH and STEWART (1982) as shown in Fig. 3. The minimum isotopic composition of the deep parent water (6dw,min) is obtained according to t~dw,min = t~ . . . . . .

-

(1)

~'' + e230 °

-

where Owo,max is the isotopic composition of the ,

"EL

I

'\'\

RINC(}N

I

\

X

\ \.

X

X

\

"

1 \~

-~

IEG7"~ I,,

"\

,J,

~c,~.~.

)

w-

/ ~_

\¢m "~.

~

~

d~~"~F

GUANACASTE

.\

X'\ 0

10

20

'

'

'

311

30km

'

" - - >

\~: 4

FIG 1. Sketch map of Guanacaste Geothermal Province. Dashed lines link surface discharges of the three major types of thermal waters: CI waters (circles), acid SO4-CI waters (squares) and HCO3 waters (hexagons). For symbols see Table 1. Elevation contours in m.

312

W.F. Giggenbach and R. Corrales S. I

i

I

I

I

i

. ~

I

I

-10S2H (%0)

I

.

-20-

N

-

.

-30.

-40-

-50-

]

sulfate waters

FIG. 2. ISOtOpiccomposition of waters from the Guanacaste Geothermal Province. For symbols see Table 1. water with a composition close to that of -2.0%0 for 180 and -25%0 for 2H as derived above for the "parent water". The linear mixing relation thus indicated is also reflected in the behavior of the major solute species, C1, as shown in Fig. 4. By extrapolating the trend lines linking southern groundwaters and deep well discharges to 6dw as derived above, a C1 content for the original deep water of 3500 mg/kg is obtained. The deviations in the position of some of the data points for the well discharges from the simple mixing lines are likely to be due to secondary steam separation processes or possibly to sampling and analytical errors. On the basis of their isotopic compositions alone (Fig. 2), the waters discharged from the three natural C1 springs at Salitrals Bagaces (SB), Norte (SN) and Cafias (SC) might be assumed to be derived from the deep CI water, as discharged from wells at Miravalles,

through simple mixing with isotopically depleted, southern groundwater. By taking into account C1 contents, however, only the C1 waters discharged at Salitral Bagaces (SB), to the south of the Miravalles area, fit this simple pattern. Chloride contents for the other two saline waters (SN, SC) are much too high or 2H and 180 contents too low to be compatible with a common origin for all three Salitral discharges. The differences in isotopic compositions suggest that the C1 waters to the NW and SE of Miravalles contain larger proportions of isotopically depleted groundwater derived either from higher altitude precipitation or from areas farther to the south. The good agreement between spring and well discharges strongly supports the assumption that Salitral Bagaces represents indeed the natural discharge of CI waters from the Miravalles area. Accepting this possibility, the waters have travelled underground for some 15 km from their origin in the vicinity of

~2H

i

0 steam-heated pools 2 0 - ( ~ ) chLoride welters

•

44~ "

source water,

-20"

/

_6o<~ Y

N

~

/

43

~

0.2 •

.c

-8

.

0.'.

-6

(

0.6 .

-4

.

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-2

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0.8 .

/"

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. . . . . . . . . . .

~ , , , ~ ~ ~ ' Z . f ~ f/-

-40.

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0

f

L0 .

2

.(

~:

X0 .

4

.

.

.

6

.

.

.

8

.

8leO ( ~

FIG. 3. Evaluation of origin and isotopic composition of deep parent water supplying wells and steam-heated pools of the Guanacaste Geothermal Province.

Geochemistry of water and steam, Guanacaste Geothermal Province, Costa Rica i

i

i

313

direct degassing from sub-terranean, but freely "boiling" magma surfaces possibly followed by absorption of the highly acid and oxidising vapors into shallow 0w -30 I groundwater, and indirect degassing at sub-solidus ® temperatures from the crystallising shell enclosing the magma body, sufficiently cool to allow thermal cracks to remain open, and occluded vapors to escape E or groundwater to invade. -5o~ The former process is likely to be active at comN .... ® paratively shallow levels and leads to the formation -2of highly immature, acid, volcanic brines capable of [ complete rock destruction and quantitative leaching -3. of rock components. The second process is likely to take place at greater depth at temperatures between -4, 350 and 450°C and is greatly assisted by the incursion of deeply circulating groundwaters. In case such liquid water is involved, the process resembles more @U7 .: ~"> -5 ~"r' leaching of originally magmatic volatiles concentrated in vesicles or along grain boundaries. The fluids formed are already partly neutralised and less /// ~'f acidic, but are likely to still reflect the comparatively high acidity and redox state of the still quite high io / / ® temperature environment (GIGGENBACH,1991). ~oe ~o0 ~cc c~c~'9/kg/ In order to be able to quantify either the direct or FI6. 4. Plots of 2H (A) and 180 (B) vs CI contents for indirect magma degassing process, it is essential to thermal water discharges from the Guanacaste Geothermal obtain an estimate of the isotopic and chemical comProvince. Dash--dotted lines correspond to possible absorp- position of the volcanic fluid. Because of its high tion of volcanic-magmatic vapors. temperature origin, the 180-content of the aqueous part is likely to be close to that of the original magma Miravalles volcano, becoming somewhat diluted, but of - + 10%O. Its 2H content can be expected to vary otherwise preserving much of their chemical and with the type of parent magma. On the basis of the isotopic identity. isotopic composition of condensates from a wide In agreement with findings on other high tempera- range of high temperature fumaroles on andesitic ture geothermal systems around the world, the deep volcanoes, however, the deuterium content of water in Fig. 2 shows a considerable oxygen shift by "andesitic" water was found to be quite uniform and ~2.5%O. Such a shift is generally ascribed to close to - 2 0 + 10%o (GIGGENBACH,1992); its Ci 1SO-exchange of the rising waters with rock and content is likely to be close to that of typical vapor therefore should be a function of the initial isotopic discharges from andesitic volcanoes of 1.5-2.0% composition of water and rock contacted, the tem- b.w. or 15,000-20,000 mg/kg (GIGGENBACH et al., perature of interaction and a somewhat mysterious 1990). The corresponding range of compositions of parameter called "water-rock ratio". The latter is strictly defined only in stagnant systems; for dynamic "andesitic" waters, dia, is shown in Fig. 3 together systems any value obtained is largely a function of the with lines representing the fraction xa of this water possibly contributing to the formation of a thermal assumptions made in driving it (GIGGENBACH,1992). Based on the pattern described by the isotopic water. This fraction may also be calculated from compositions and C1 contents of a wide range of xo = (dig - di,,)/(dio - dim) (2) waters discharged from high temperature geothermal systems, another process giving rise to "180-shifts" where dig is the isotopic composition (180 or 2H) of has been proposed. It is assumed to consist of the any geothermal water and dim that of the local addition of highly acid, 1SO-enriched "volcanic- meteoric recharge. According to this evaluation, the deep Miravalles water contains about 18% of magmatic" fluids to deeply circulating groundwaters (GIGGENBACH,1992). The magnitude of any "oxygen- andesitic fluid. A t a C1 content of 15,000-20,000 rag/ shift" then is largely a function of the mixing ratio of kg of this fluid, the resulting thermal water should have a C1 content of between 2700 and 3600 mg/kg, volcanic fluid with groundwater. As discussed in more detail for the volcanic- values bracketing the CI content of 3500 mg/kg magmatic-hydrothermal systems associated with the derived by use of Fig. 2. For the waters discharged at White Island (GIGGENBACH,1987) and Nevado del the other two Salitrals (SN, SC) addition of volcanic Ruiz (GIGGENBACHet al., 1990) volcanoes, two major fluids to isotopically lighter, probably more southern modes of degassing of magma bodies residing within groundwaters is indicated; the fractions x~ are ~ 10% or beneath volcanic structures can be distinguished: for SN and again 18% for SC. .~0 I

314

W.F. Giggenbach and R. Corrales S.

The above evaluation does not take into account secondary processes, such as boiling, possibly leading to additional changes in CI and isotope contents. Also the composition of the volcanic fluid added may vary considerably. In spite of these potential sources of error, the generally good agreement of the compositions of the C1 waters encountered over the Guanacaste Geothermal Province with those expected, suggests that direct or indirect addition of magmatic vapors has to be considered a viable process for the formation of the thermal waters. Conversion of initially acid waters to neutral C1 waters requires extensive water-rock interaction accompanied by virtually complete removal of magmatic S species in the form of sulfates or sulfides. For waters affected by direct absorption of magmatic vapors at shallow levels, having undergone only limited water-rock interaction, part of the original magmatic S and acidity may still be present at the surface. This may explain the formation of the acid SO4-C1 waters discharged, for example, at Guyabal and from well PH-36. In Fig. 4B, the line describing the formation of the deep Miravalles well water passes indeed through the data points for Guayabal (GB), PH-36 (36) and also for Montezuma (MZ) on the slopes of the volcano Tenorio. Summing up isotopic findings: there is a marked decrease in the heavier isotope content of groundwaters over the Guanacaste Geothermal Province from north to south. The deuterium content of the deep water at Miravalles points to a meteoric recharge from the north. The "1SO-shift" of the deep waters of 2.5%0 may be explained in terms of the addition of - 1 8 % of "andesitic" vapor to such a northern groundwater. The well discharges are diluted to varying degrees by groundwater derived to the south of Miravalles Volcano. The acid SO4-C1 waters encountered at Guayabal are immature solutions formed by the direct absorption of - 2 . 5 % of volcanic vapors. Chloride waters discharged naturally at Salitrals Norte (SN) and Cafias (SC) are likely to have formed through acquisition of originally magmatic fluids by isotopically more depleted groundwaters.

RELATIVE CHLORIDE, SULFATE AND BICARBONATE CONTENTS

The contents of major chemical species in thermal waters discharged over the Guanacaste Geothermal Province are given in Table 2. An initial classification of the waters and discussion of possible genetic relations is initially carried on the basis of their major anions, CI, SO 4 and HCO3, as shown in Fig. 5. The dominant anion in the deep well discharges is C1; data points for wells (P), therefore, plot close to the C1 corner. The position of data points for the Salitral waters reflects increased formation of HCO 3 due to more prolonged interaction of the CO2 con-

taining waters with rock contacted during the long passage from their source regions, probably close to the volcanic chain, to the south. In agreement with their relative positions to their potential sources (Fig. 1) and their increased HCO 3 contents, the waters produced at Salitrals Bagaces (SB) and Cafias (SC) are likely to have travelled for much longer distances than that discharged from Salitral Norte (SN), right at the foot of the volcano Rinc6n de la Vieja. The HCO 3 waters proper fall into three groups: 1. The HCO3-CI waters discharged to the NW of Salitral Bagaces at San Bernardo (BB, B J) and Rio Blanco (RO) are likely to have formed from CO 2 charged C1 waters moving toward Salitral Bagaces after considerable dilution and even more prolonged interaction with rock. Their long residence time underground is reflected in their low tritium contents (0-1.1 TU). 2. The very dilute HCO 3 waters at Quebrada de le Chepa (QC), Benaqon (BN) and Pozo Loras (PL) are essentially local groundwaters only little affected by the admixture of deeper thermal waters. Their elevated tritium contents (1.7--4.1 TU) suggest addition of recent precipitation or short residence times underground. 3. The mixed HCO3-SO 4 waters encountered at El Gomes (EG) and Los Quesos (LQ) and produced from the shallow well PH-34 (34) are likely to have formed through mixing of varying amounts of highly immature SO4-C1 waters and local groundwater followed by partial equilibration with rock. The main representatives of the original immature SO4-Cl waters are the thermal discharges at Guayabal (GB) and from the nearby shallow well PH-36 (36). As discussed above, they may have formed through direct absorption of magmatic vapors into local groundwater. Indeed, their position in Fig. 5 is quite close to that expected for waters produced through the absorption of "andesitic" magmatic gases, the "andesitic waters" of GIGGENBACH et al. (1990). The somewhat lower relative SO 4 contents of GB and 36 point to removal of some S in the form of anhydrite or iron sulfides following minor water-rock interaction. At Ahogados (AH) and Montezuma (MZ), relative SO 4 contents of the SO4-C1 waters are even lower suggesting a higher degree of "maturation". It is interesting to note that each of the volcanic structures, Rinc6n de la Vieja, Miravalles and Tenorio appears to produce similar sets of waters: SO4-C1 waters close to the volcanoes and neutral C1 waters at the Salitrals. As indicated in Fig. 1, the former are arranged along a straight line stretching from Ahogados (AH) to Montezuma (MZ), the neutral C1 waters along another straight line to the south linking the Salitrals. Bicarbonate waters are restricted to a north--south trend line from Quebrada de la Chepa (QC) across the Miravalles area to Rio Blanco (RO). A fourth set of waters, present at all three systems, are high-SO4, low-C1 waters produced in steam-

Geochemistry of water and steam, Guanacaste Geothermal Province, Costa Rica h e a t e d p o o l s . A t R i n c 6 n d e la V i e j a t h e y a r e r e p resented by the water collected from a boiling crater at B o r i n q u e n ( B Q ) , at T e n o r f o b y t h o s e p r o d u c e d in b o i l i n g p o o l s at T i e r r a s M o r e n a s ( T M ) . T h e s t e a m heated pools associated with the Miravalles areas are t h o s e at L a s H o r n i l l a s . T h e s e a r e a s o f v a p o r disc h a r g e , m a r k e d by t h e p r e s e n c e o f f u m a r o l e s , s t e a m h e a t e d p o o l s o r s t e a m i n g g r o u n d , a r e likely to b e f e d by steam having separated from the deeper, high t e m p e r a t u r e C1 w a t e r s . T h i s v a p o r g e n e r a l l y t r a v e l s c l o s e to v e r t i c a l l y a n d q u i t e r e l i a b l y m a r k s t h e " e p i centers" of deeper upflow zones where the rising w a t e r s first s t a r t to boil. T h e r e m a i n i n g , l a r g e l y degassed waters may travel considerable horizontal d i s t a n c e s u n d e r g r o u n d b e f o r e t h e y a r e a b l e to r e a c h the surface.

315

T h i s l e a v e s t h e b y far m o s t i m p o r t a n t a r e a o f s t e a m discharge from the Guanacaste Geothermal Prov i n c e , t h a t at L a s P a i l a s o n t h e s o u t h e r n s l o p e s o f R i n c 6 n d e la V i e j a , " u n a t t a c h e d " to a n y o b v i o u s Cl w a t e r d i s c h a r g e . T o p o g r a p h i c a l l y b o t h Salitral N o r t e a n d B a g a c e s c o u l d q u a l i f y as t h e d i s c h a r g e p o i n t s f o r t h e a s s o c i a t e d C1 w a t e r s . I n t h e a b s e n c e o f a n y c o n c l u s i v e i s o t o p i c o r c h e m i c a l e v i d e n c e , h o w e v e r , it a p p e a r s b e s t to a s s u m e t h a t t h e l a r g e q u a n t i t i e s o f t h e r m a l w a t e r s p r o b a b l y rising b e n e a t h L a s Pailas c o n t r i b u t e to a c o m m o n e x t e n s i v e r e s e r v o i r o f n e u tral C1 w a t e r s s t r e t c h i n g a l o n g a n e n t i r e belt to t h e S W o f t h e c h a i n o f v o l c a n o e s to b e e v e n t u a l l y disc h a r g e d at a n y o f t h e Salitrals. T h e c l o s e s p a t i a l a s s o c i a t i o n o f n e u t r a l C1 w a t e r s a n d acid 5 0 4 - C 1 w a t e r s at M i r a v a l l e s is i l l u s t r a t e d by

Table 2. Chemical composition of waters discharged over the Guanacaste Geothermal Province. Major components, in mg/kg. For wells, pH values are those measured in the separated water at room temperature, solute contents are on total discharge basis Temperaturc (°C)

pH

Li

Na

K

Mg

Ca

B

AH BQ SN SW LI L2 L3 QA GB GB 36 GS LT QC LQ 34 EG

37 98 74 26 95 90 96 40 64 73 67 25 32 20 18 115 37

6.6 2.3 8.0 9.2 7.11 3.5 1.6 6.8 1.9 2.1 3.4 9.7 3.5 8.7 9.4 7.8 8.2

0.126 0.008 5.0 <0.1 <0.1 <0.1 d0.1 0.002 0.10 0.11 0.1 <0.1 0.1t02 <0.l <0.1 <0.1 <0.1

58 21 1720 8 6 17 7 20 56 57 59 23 22 5 7 22 85

12 8 167 4 2 9 2 5 2 3 1 6 5 2 2 16 23

17.0 18.1 17.9 5.4 4.2 4.2 11.1 10.6 48.3 48.2 48.3 14.7 15.4 1.6 2.3 2.1 81.0

63 35 181 15 8 23 23 24 99 104 99 93 64 6 11 9 68

P1 P1 P2 P2 P2 P5 P5 P10 P10 PI0 P11 P11

-------------

7.8 7.8 7.6 7.4 2.8 7.6 7.8 7.8 7.9 7.4 7.6 7.5

4.3 5.0 5.3 3.9 6.2 4.4 3.9 4.9 4.8 5.1 4.5 2.7

1425 1732 1842 1520 2050 1540 1310 1790 1580 1640 1435 1670

181 214 222 160 266 190 152 216 200 216 175 212

0.06 0.04 0.16 0.01 12.15 0.08 0.04 0.11 0.02 0.04 0.16 0.14

32 41 50 52 29 50 42 59 51 58 35 44

7.5 2.5 7.7 7.5 8.5 7.5 8.5 8.5 8.6 7.5 2.3 5.4 8.5 8.1

<0.1 <0.1 <0.1 0.2 3.6 <0.1 0.3 0.2 <0.1 <0.1 0.018 <0.002 0.5 1.7

6 5 59 83 2060 7 144 121 78 25 57 4 900 2757

2 2 17 13 70 12 25 22 17 6 6 1 25 72

1.6 1.8 33.6 1.7 5.7 2.0 29.1 26.7 16.8 16.0 45.7 1.0 20.4 28.3

7 5 54 15 98 6 56 52 31 41 168 1.1 49 93

QG HA BN RB SB PL BB BJ RO PP MZ TM RS SC

20 20 50 49 72 22 46 50 45 25 62 91 28 39

HCO 3

SiO 2

SO 4

C]

6 3 35 <5 <5 <5 <5 <5 5 2 <5 <5 <5 <5 <5 <5 <5

3 -201 411 9 --6 ---1 -28 17 36 428

115 305 110 83 57 176 380 61 297 276 311 70 118 31 55 369 121

229 861 59 17 37 135 3360 147 2830 2850 2860 273 305 3 16 43 261

101 10 30711 9 7 7 4 7 684 651 7111 7 12 3 4 9 52

4l 48 54 41 59 42 39 50 42 -38 --

20 35 9 16 <1 27 33 27 37 -35 --

467 465 511 393 530 520 360 570 460 457 430 465

47 38 87 71 62(I 36 22 40 30 29 27 32

2320 2820 3120 2940 3210 2550 2190 2910 2780 2790 2440 2820

<5 <5 <5 <5 49 <5 <5 <5 <5 <5 <1 1.2 24 74

4 -429 42 912 58 423 389 243 42 -1 340 699

58 54 161 132 120 107 177 153 151 55 205 15 80 144

25 182 54 71 118 3 21 23 16 133 593 61 106 355

4 4 8 73 2560 3 155 112 81 37 416 4 1220 3530

316

W.F. Giggenbach and R. Corrales S.

®

""T"

FIG. 5. Classificationof thermal waters in terms of relative CI, S O 4 and HCO 3contents.

the sudden increase in SO4 and acidity after deepening of well PGM-2 from 1200 to 2000 m in 1984. The composition of the resulting mixed water is given in Fig. 5 by the square (P2). A more detailed discussion of the ingression of acid 804-C1 waters into PGM-2 is carried out in conjunction with the isotopic composition of dissolved SO4. Beside providing valuable information on possible origins, the above classification of the waters, on the basis of their major anions, is a prerequisite to any further thermodynamic evaluation, such as the application of solute geothermometers.

APPLICATION OF Na-K-Mg-Ca AND DISSOLVED SiO2 GEOINDICATORS

At the advanced state of development of the Miravalles Geothermal Field, application of geothermometers, to obtain by now already known, deeper temperatures, is only of secondary importance. Application of these techniques, however, may provide a check as to their reliability and possibly additional information useful in the construction of a conceptual model of the field. In Fig. 6 analytical results, in mg/kg, are plotted in

. . . . .

tkm

v,n o FI6.6. Evaluation of water-rock equilibration temperatures by use of Na, K and Mg contents.

Geochemistry of water and steam, Guanacaste Geothermal Province, Costa Rica 60*

100 °

200 °

317

300*

co2

0

:)or)

L~

).01

2

).10

IO

4

0

1

2

3

4

5

6

7

Lkm

FIG. 7. Evaluation of equilibration temperatures and CO2 partial pressures by use of K, Mg and Ca contents. Dash-dotted line shows likely path of CI waters during their migration to the surface.

terms of a modified diagram proposed by GIGGENBACH (1988). In the original diagram the Na contents were divided by 1000, and K contents by 100. The present version is more suitable in the evaluation of conditions in lower temperature (<200 °) systems. As expected, all the SO 4 and HCO 3 waters plot in the area marked "immature waters". For these, the evaluation of K-Na temperatures is unreliable. For close to neutral HCO 3 or even SO4 waters, K-Mg temperatures, tkm in °C, may still be evaluated by use of the relation tkm = [4410/(14 -- log ( C 2 / C M g ) ) ]

-- 273

(3)

where ci are the concentrations of K and Mg as measured, in mg/kg. Values of tkm are somewhat higher than those measured, possibly reflecting the higher temperatures prevailing during interaction of the waters with clay at depth. Evaluation of K-Na temperatures is advisable only in the case of neutral Cl waters. Their occurrence is limited to the deep well discharges and the Salitral waters. All the well discharges plot, with the exception of PGM-2 in 1985 (P2, square), on the full equilibrium line. Their K-Na equilibration temperatures,/kn in °C, may be obtained from tkn = [1390/(1.75 -- log ( C K / C N a ) ) ] -- 273.

(4)

They are with 248 _+ 5°C well within the range observed for the wells. Concentrations ci are again in mg/kg. In the case of the Salitrals, K-Na temperatures are highest (230 °) for S. Norte (SN), followed with 160° by S. Bagaces (SB) and S. Caries (SC) with 144°. The high temperature indicated for S. Norte (SN) is likely

to be due to the short distance travelled by the water from its likely equilibration zone, possibly below Borinquen, to the surface. The much longer paths travelled by the two other waters, and correspondingly longer residence times, allowed their chemistry to readjust to lower temperatures. Both SB and SC lie along the same "slow cooling" trajectory, subparallel to the full equilibrium line with both &m and /kn readjusting. Much faster rise of the Salitral Norte (SN) water allows only the K-Mg temperature to adjust; the K-Na ratio remains close to that at depth. It is obvious that K-Na temperatures indicated for slowly rising or far-travelled waters will generally be well below those actually prevailing within the major upflow zones. They should, therefore, only be considered minimum temperatures. The K-Mg temperatures reflect even cooler conditions at some later stage during the migration of the waters. The chemical composition of neutral Cl waters may also be used to obtain an estimate of the CO 2 content of the deeper equilibration fluids. Again, following a procedure proposed earlier (GIGGENBACH, 1988), analytical data are plotted in Fig. 7 in terms of Lkc = log (C2K/CCa)VS Lkm = log (C2K/CMg). The technique is applicable only to systems where calcite can be expected to form. The formation range of calcite in Fig. 7 is delineated by two boundaries: one marked "rock dominated", the other "fluid dominated". Data points lying above this range represent systems where CO 2 partial pressures are too low to induce alteration of minerals of the primary rock phase, such as feldspars. Data points in the "fluid dominated" field have CO 2 pressures too high for calcite to be stable. Contours represent the CO2

318

W.F. Giggenbach and R. Corrales S.

contents, in terms of the mole ratio rco 2 = Xco 2/XH20, of coexisting liquid and vapor phases. Data points for the wells plot above the full equilibrium line indicating that the CO2 content of the waters is lower than that expected for attainment of full equilibrium with average crustal rock. This in turn implies that minerals such as feldspars are stable in contact with these waters and that calcite is not formed actively through alteration of Ca-A1silicates. The presence of secondary prehnite in drillcores (ROCHELLE et al., 1989) also points to CO 2 partial pressures well below those of full equilibrium (GIGGENBACH, 1988). The main cause of CO 2 contents being below full equilibrium values is likely to be earlier loss of CO2-rich vapor to feed the fumaroles or steamheated areas. Loss of CO2 at depth, however, moves or keeps the waters also close to saturation with respect to calcite. This may explain the tendency of well discharges to deposit this mineral as reported by VACA et al. (1989). An exception is the acid water produced from PGM-2, after deepening in 1984. Because of the position of its data point (P2, square), well below the stability range of calcite, the CO 2 partial pressure or temperature indicated are probably not realistic. Data points for the natural C1 water discharges from the Salitrals plot close to the lower boundary of calcite stability. The trend continues to the HCO 3 waters of San Bernardino (BB) as indicated by the dash-dotted line in Fig. 7. The considerable reductions in equilibrium temperatures and liquid phase CO 2 contents again reflect prolonged interaction of the waters with rock or vapor loss during

i

I

i

their passage underground. Both loss of CO2 and conversion to HCO 3 are in agreement with the lack of any free CO2 in these springs and their high pH

(>8.5). Another technique for the evaluation of deeper water-rock equilibration temperatures is based on the SiO 2 content of thermal waters. General application of the technique, however, is limited by the uncertainty regarding the nature of the SiO2 polymorph, quartz, chalcedony or opal, actually controlling dissolved SiO2 contents and the sensitivity of this geothermometer to secondary processes, such as dilution or steam loss. A n initial evaluation, however, may be carried out by assuming SiO2 contents to he controlled by chalcedony at lower temperatures and quartz at t > 150°C. The relation for the evaluation of SiO2 temperatures, in °C, incorporating this compromise, then becomes t~ = [1000/(4.55 - log (Cs~o_0)] - 273.

A technique allowing the effects of dilution and steam loss or addition on the SiO2 geothermometer to be evaluated consists of plotting total discharge SiO2 contents against corresponding CI contents as shown in Fig. 8. Data points for most wells fall on a line reflecting the effects of both dilution by low C1, low SiO 2 waters, and addition or loss of vapor. The individual effects of these processes may be disentangled by also taking into account variations in discharge enthalpies, gas contents and isotopic compositions. In this study dealing with the overall hydrology of the Guanacaste Geothermal Province no attempt will be made to evaluate secondary processes i

,

I

600 "

0

Si021

0

i

dw

200

/ira

f C)

--

10C0

(5)

2000

3000 CI (mg/kg)

FIG. 8. Dissolved S i O 2 c o n t e n t s as a function of CI contents.

Geochemistry of water and steam, Guanacaste Geothermal Province, Costa Rica affecting well discharges. T h e bulk of these discharges occupy positions c o r r e s p o n d i n g to SiO2 temp e r a t u r e s close to those actually e n c o u n t e r e d of 2 3 0 250°C. E x t r a p o l a t i n g to a C1 c o n t e n t of 3500 mg/kg, as derived a b o v e for the p a r e n t water, a SiO 2 temp e r a t u r e of 285°C is o b t a i n e d . T h e very low SiO 2 c o n t e n t s of the Salitral discharges (SB, SC, SN), at C1 c o n t e n t s similar to those of the well discharges, are likely to reflect the effects of conductive cooling during the long passages of these waters u n d e r g r o u n d . C o r r e s p o n d i n g t e m p e r a tures are similar to those indicated in Fig. 6, except that t h e i r o r d e r i n g a p p e a r s to b e inversed. Lowest N a - K t e m p e r a t u r e s are indicated for SC, highest for SN, while SiO2 t e m p e r a t u r e s are lowest for SN a n d highest for SC. T h e K - M g t e m p e r a t u r e s are uniformly - 6 0 ° a b o v e those m e a s u r e d a n d t h e r e f o r e m i r r o r most closely the o b s e r v e d trend. T h e inversion in silica t e m p e r a t u r e s is likely to reflect the effects of s e c o n d a r y processes, such as dilution, control of SiO2 c o n t e n t s by a m o r e soluble p o l y m o r p h or differences in the cooling paths for the waters. C o n t r o l of SiO 2 c o n t e n t s by a m o r e soluble polym o r p h may b e the r e a s o n for the high SiO 2 c o n t e n t s of the acid SO4-CI waters of G u a y a b a l ( G B , 36). T h e r e rapid rock dissolution a n d stabilisation by low p H values is likely to lead to s u p e r s a t u r a t i o n of the waters with respect to the less soluble phases. O n the o t h e r h a n d , the m u c h s t e e p e r slope of SiO2/CI may be due to heating of local g r o u n d w a t e r s by Cl-depleted, m o r e shallow vapors. T e m p e r a t u r e s indicated by the N a - K - M g and

319

SiO 2 g e o t h e r m o m e t e r s for well discharges agree well with those m e a s u r e d . T h e t e m p e r a t u r e s suggested for the n a t u r a l CI w a t e r discharges reflect considerable conductive cooling during their extensive travel u n d e r g r o u n d . Most of the o t h e r waters discharged over the G u a n a c a s t e G e o t h e r m a l P r o v i n c e are too i m m a t u r e for any solution g e o t h e r m o m e t e r to be applied reliably.

INFORMATION PROVIDED BY MINOR CHEMICAL COMPONENTS

F o r a selected set of samples, analytical data are given in T a b l e 3 for a series of m i n o r solution comp o n e n t s . S o m e , such as A1, are of interest in m o r e detailed t h e r m o d y n a m i c investigations into the stability of minerals in equilibrium with the waters; others, such as F a n d B, in the assessment of the e n v i r o n m e n t a l impact of various options for the disposal of the effluent from any future d e v e l o p m e n t for power production. T h e chemistry of the t h r e e rare alkalies is c o m p a r a tively simple. T h e y can be a s s u m e d to h a v e b e e n acquired by the waters during the early stages of their f o r m a t i o n , the p e n e t r a t i o n of deeply circulating g r o u n d w a t e r into high t e m p e r a t u r e rock. This process is likely to be a c c o m p a n i e d by intense rock alteration a n d alkalies are a d d e d to the resulting solutions in p r o p o r t i o n s close to those in the initial rock (GocuEL, 1983; GIGGENBACH, 1988). T h e Li/Cs weight ratios d e p e n d to some degree o n the type of

Table 3. Minor chemical components in selected waters from the Guanacaste Geothermal Province, as analysed at DSIR Chemistry, New Zealand, in mg/kg. For wells, compositions are those of the liquid phase separated at the temperatures given, uncorrected for vapor loss. For PGM-2 (P2), the first analysis represents a separated water collected on 28 June 1984 (pH = 7.5), the second on 10 January 1985 (pH = 2.8). The average compositions of basic and acid crustal rocks are those reported by ROSLER (1983) Temperature (°C)

Li

Rb

Cs

Sr

Ba

B

AI

F

CI

0.008 <0.002 0.600 <0.002 <0.002 0.004

0.33 0.18 -0,12 0.07 0.05

0.025 0.066 -0.033 0.015 0.007

6.20 2.50 34.1 0.05 0.08 4.50

0.05 22.7 -<0.05 0.06 480

--1.4 --26.0

101 10 3070 7 12 689

0.710 0.470 0.640 0.600 0.750 0.720

--0.51 1.59 2.07 1.57

--0.060 0.220 0.380 0.390

60.0 48.0 73.0 54.0 57.0 44.0

--0.25 0.11 0.10 <0.10

1.4 1.3 -1.2 ---

4100 2980 3567 3330 3710 3780

0.140 <0.010 <0.010 <0.002 <0.002 0.200

---0.17 0.02 --

0.400 --0.072 0.019 0.045

48.0 2.2 1.8 <0.05 1.2 65.0

<0.10 <0.10 -24.9 0.40 <0.10

0.7 0.3 0.3 --0.3

2600 170 112 416 4 3530

1.0 5.0

440 300

300 830

5 15

87,600 77,000

370 800

50 240

AH BQ SN QA LT GB

37 98 74 40 32 73

0.126 0.008 5.00 0.002 0.002 0.110

0.043 0.017 0.950 0.009 0.013 0.060

P1 P2 P2 P5 P10 P11

98 159 139 136 98 98

7.30 4.40 7.20 5.70 6.40 3.40

1.25 0.83 1.14 1.05 1.02 0.95

SB BB BJ MZ TM SC

73 46 50 62 91 39

3.40 0.30 0.23 0.018 <0.002 1.40

0.21 0.04 0.04 0.04 0.002 0.20

Contents in average crustal rocks (rng/kg) Basic 15 45 Acid 40 200

320

W.F. Giggenbach and R. Corrales S.

rock dissolved. For basic to intermediate (andesitic) rocks it is - 2 0 , and 5 for acid rocks (ROsLER, 1983). Following partial neutralisation of the acid solutions, secondary minerals, largely layer silicates, such as illite, are able to form. These are generally K-rich due to the preferential removal of this element from the rising waters. Rubidium behaves similarly to K and is, therefore, also taken up by early formed illites at quite high temperatures. Dissolved Li and Cs survive these early stages of water-rock interaction. The tendency of Li to be incorporated into the lattice of secondary quartz depositing from the cooling waters may lead to its removal from solution with decreasing temperature. The ability of quartz to accommodate Li, however, also decreases with temperature and efficient Li uptake is likely to be limited to an intermediate temperature range. Cesium shows a tendency to partition into secondary zeolites, a process apparently favored by decreasing temperatures (GocoEL, 1983). Figure 9 represents a graphical presentation of these variations in relative Li, Rb and Cs contents of thermal waters. Dissolved crustal rocks occupy a position close to the Rb corner. For the acid SO4, probably steamheated waters BQ, LT, and Q A , Cs contents are below the detection limit. Their Li/Rb ratios and limiting positions, quite close to the Rb corner, however, are fully compatible with derivation of all three elements through straightforward dissolution of rock. The position of data points for the SO4-C1 waters GB, A H and possibly MZ, reflect considerable removal of Rb even from these quite acid waters into clays. The position of the well discharges corresponds to an even higher degree of Rb uptake in secondary clays, as expected for these fully equilibrated waters. Their Li/Cs ratios are in agreement with leaching of

an average crustal rock. A possible explanation for the low relative Li content of the water from PGM-11 ( P l l ) is uptake of Li in secondary quartz. The positions of the remaining waters may be explained in terms of Cs uptake in secondary zeolites. In the case of the HCO 3 waters (BB, B J), the Li/Rb ratio is that of the deep well waters. For the water from Salitral Bagaces, the low Cs content may indicate uptake in zeolites together with further uptake of Rb in secondary clays during the long passage of the waters underground, a process apparently affecting waters from the other two Salitrals (SN and SC) only little. In the case of Salitral Norte, this may be due to the short residence of the waters underground. The behavior of another set of "conservative" components useful in the delineation of formation processes of waters, that involving C1, Li and B, is investigated in Fig. 10. As pointed out above, Li is the alkali element least affected by secondary absorption processes. Together with CI, it provides a useful parameter to tag thermal waters. Boron may have several origins. It may be leached from sedimentary rocks; due to its volatility in high temperature steam, it may also be introduced with any high temperature vapor phase absorbed into a water. Absorption of B-rich vapors is probably responsible for the very high relative B contents of the two obviously steam-heated waters BQ and TM, and to a minor degree for that of the Ahogados water (AH). The other group of acid, high-SO4 waters comprising GB, LT and Q A then must have formed through absorption of lower B, or higher C1 vapors. Another possibility is loss of B with vapor having boiled off from these waters. Rock dissolution leads to waters with low relative B and C1 contents. In order to produce the well discharge waters, C1 and B are probably introduced

ti

10 Os Fro. 9. Evaluation of possible processes affecting relative Li, Rb and Cs contents.

Geochemistry of water and steam, Guanacaste Geothermal Province, Costa Rica

321

CI/100

0001 0007

LqCI

~°SB CI

0005

01

001.~.

02

O0 2

05

20Li

B FIG. 10. Relative C1, Li and B contents.

through "leaching" of readily mobilised magmatic components in the zones of early water-rock interaction. The resulting solutions have a B/C1 weight ratio of - 0 . 0 2 . For comparison the positions of geothermal and volcanic waters produced over the Taupo Volcanic Zone in New Zealand are shown. The general pattern for these agrees well with that observed at Guanacaste discharges. The differences in the position of data points for waters of probably similar origin, MZ, GB and A H , may reflect a marked increase in the B/CI ratio of the absorbed magmatic vapors from SE to NW. Because of its volatility, B can be expected to be expelled from a thermal system during its early stages of emplacement. The trend in B/CI therefore may suggest that the NW systems are considerably "younger" than those in the SE, with Maravalles occupying an intermediate position. The crustal concentrations of the remaining three "minor" elements, Sr, Ba and F, are quite high (Table 3). Their comparatively low concentrations, and that of A1, in even acid waters then are likely to be due to the precipitation of insoluble secondary minerals such as sulfates and fluorides. Because of their pH and temperature dependent, rather complicated chemistry, these elements have found only very limited applications in geothermal exploration. They are, however, together with a series of transition and heavy elements of interest in assessing the environmental impact of any future geothermal development. Summing up the information provided by the minor components, the well discharges at Miravalles are likely to have formed through leaching of newly crystallised volcanic rock containing B and C1 in the ratio of -0.02. Interaction of the initially quite acid solutions with rock at depth led to the acquisition of

the rare alkalies. Uptake of Rb in secondary illite gave rise to the production of Rb-depleted waters discharged from the wells and at Salitral Norte (SN) and CaIlas (SC). Incorporation into secondary zeolites may explain the low relative Cs contents of the water discharged at Salitral Bagaces (SB) and San Bernardo (BB). Relative Li, Rb and Cs contents for most of the acid SO 4 water are compatible with their formation by simple rock dissolution. The marked increase in the ratio B/C1 in the SO4-C1 waters from SE to NW may reflect the decreasing age of the thermal systems.

ISOTOPIC COMPOSITION OF DISSOLVED SULFATE The impetus for carrying out an investigation into the isotopic composition of dissolved SOn and H2S in discharges over the Guanacaste Geothermal Province, as given in Table 4, was provided by the sudden change in the chemistry of waters discharged from well PGM-2 after deepening in 1984. Previously the well produced a neutral, low-SO 4 water, similar to that discharged from the other wells. After deepening from 1200 to 2000 m, the pH of the weirbox water changed from - 7 . 5 to 2.8 and SO 4 contents from 70 to 550 mg/kg. Another species showing a drastic change was Mg, from very low values typical of high-temperature neutral waters of 0.02 to >12 mg/kg (Table 2). On the basis of simple mixing relationships involving SO4 alone, the change in chemistry of PGM-2 may be explained in terms of the admixture of some 20% of a water similar to that encountered at Guayabal (GB) and PH-36. The increased total discharge C1 contents of PGM-2 after 1984, however, can only be

322

W.F. Giggenbach and R. Corrales S.

e x p l a i n e d if the a d d e d acidic w a t e r h a d a m u c h h i g h e r C1 c o n t e n t t h a n the G u a y a b a l w a t e r or if the G u a y a bal w a t e r h a d b e e n a d d e d to a p r e c o n c e n t r a t e d well water. O t h e r e v i d e n c e for possible a d m i x t u r e of G u a y a bal w a t e r was e x p e c t e d to b e p r o v i d e d by the 34S c o n t e n t s of the respective dissolved SO 4. U n f o r t u nately no data are available for P G M - 2 prior to d e e p e n i n g . Because of the similarities in its earlier discharge compositions to those of the o t h e r wells, it a p p e a r s quite safe to assume t h a t the isotopic c o m p o sition of its SO4, b e f o r e d e e p e n i n g , was close to the average of that for wells P G M - 5 , 10 a n d 11 of + 14.3%o. A f t e r addition of 20% of a G u a y a b a l type w a t e r with a 34S c o n t e n t of a b o u t +22.5%o, the 34S c o n t e n t of the mixture should have b e e n +21.9%o, a value well a b o v e that of +17.5%o actually f o u n d for P G M - 2 in 1985. E v e n taking into account possible sampling and analytical errors, the d i s a g r e e m e n t virtually rules out simple a d m i x t u r e of G u a y a b a l water. In view of these discrepancies, a d m i x t u r e of an entirely different w a t e r has to be considered. A c c o r d i n g to T a b l e s 1 and 2, the e n t h a l p y a n d total discharge C1 c o n t e n t s of the well r e m a i n e d largely u n c h a n g e d before and after 1984. T h e w a t e r responsible for the increase in acidity a n d SO 4 a n d Mg c o n t e n t s must, t h e r e f o r e , have a d e e p t e m p e r a t u r e a n d C1 c o n t e n t very similar to t h a t of the neutral well discharges and t h e r e f o r e may simply r e p r e s e n t a highly i m m a t u r e p r e d e c e s s o r of the n e u t r a l waters. I n d e p e n d e n t evidence is p r o v i d e d by the species listed in T a b l e 3 for P G M - 2 waters before and after d e e p e n i n g . In Figs 9 and 10, the position of data points of this well for b o t h the neutral (circle) a n d acid (square) w a t e r are virtually indistinguishable. In

the case of significant addition of G u a y a b a l water, t h e data point for the acid discharge in 1985, should b e shifted t o w a r d t h a t of the G u a y a b a l water. A c c e p t i n g the w a t e r a d d e d to P G M - 2 to be of different a n d possibly m u c h d e e p e r origin, such waters may b e quite c o m m o n b e n e a t h , at least, part of the Miravalles area. In addition to t h e i r use as isotopic tracers, the isotopic compositions of dissolved SO4 may also provide the basis for a n u m b e r of g e o t h e r m o m e t e r s . T h e t e m p e r a t u r e d e p e n d e n c e (in K) of the fractionation of aSO b e t w e e n S O 4 and H 2 0 (in %0) is given by the m e a n of fractionation factors ASO~_H20 = 1000 In Ctso~_mo = (3,065,500/T 2) - 4.9

(6)

as d e t e r m i n e d by LLOYD (1968) and MIZVTANI a n d RAFTER (1969). I s o t h e r m s r e p r e s e n t i n g this relation are s h o w n in Fig. 11, t o g e t h e r with data points for G u a n a c a s t e waters. T h e acid SO4-C1 springs G1, G2, LT, Q A , M Z a n d p r o b a b l y also A H form quite a tight group with a p p a r e n t S O 4 - H 2 0 equilibration t e m p e r a t u r e s of 90-120°C. T h e s e t e m p e r a t u r e s are only a little a b o v e those m e a s u r e d a n d reflect the rapid rate of 180 equilibration in acid waters. T h e small fractionation for the B o r i n q u e n w a t e r is p r o b a b l y due to nonequilibrium f o r m a t i o n of SO 4 by air oxidation of H2S in this boiling pool. T h e low t e m p e r a t u r e s indicated for the well discharges may be due to c o n t a m i n a t i o n with later formed S O , or to some r e - e q u i l i b r a t i o n during prolonged storage of the samples. T h e t e m p e r a t u r e indicated for P G M - 2 (P2), after d e e p e n i n g , agrees well with that likely to prevail at depth. T h e large

Table 4. Isotopic composition of dissolved SO4 and H2S in water and steam discharges from the Guanacaste Geothermal Province, as analysed by INS, DSIR, New Zealand (in %0) SO 4

Temperature (°C) AH BQ LM LV QA LT G1 G2

Los Ahogados Borinquen Las Pailas, mud Las Pailas, vent Quebrada Azufral La Torte Guayabal, June 1984 Guayabal, September 1985

PI P2 P5 P10 Pll

PGM-I, June 1984 PGM-2, January 1985 PGM-5, February 1985 PGM-10, July 1985 PGM-11, March 1985

GV HM SB MZ TM

Gaveteros Hornillas Peq. Salitral Bagaces Montezuma Tierras Morenas

C]

(mg/kg)

d348

6180

U2S

SO 4

SO 4

H20

37 98 98 98 40 32 73 69

229 861 --147 305 2700 2365

101 10 --7 12 651 630

-5.0 -3.1 -6.1 -3.9 -----

+ I 1.8 -3.0 --+ 11.7 + 15.5 +23.0 +22.1

+8.5 +7.4 --+ 12.0 + 12.4 +12.6 + 12.9

-6.9 - 1.7 +6.2 -4.5 -5.0 -5.2 -4.7 -4.5

136 139 138 134 136

38 620 22 30 27

2820 3210 2190 2780 2440

-1.5 -----

-+ 17.5 + 12.5 +15.5 +14.9

-+4.8 + 10.3 +9.1 +8.0

-3.4 -2.8 -4.1 -3.7 -3.5

36 98 73 62 91

--118 593 61

--2600 416 4

- 14.2 -3.3 --7.2 -0.3

--+13.3 +25.6 --

---+13.5 --

--8.2 -3.9 -4.5 -2.7

Geochemistry of water and steam, Guanacaste Geothermal Province, Costa Rica

323

6~Osc ~'~)

-5

61s0

H20

O

-10

E3~SHzs

O

FIG. 11. Evaluation of SO4-H2O(A) and SO4-H2S(B) equilibration temperatures. Horizontal arrows indicate data points for which 34S contents are available for only SO4, vertical arrows only for H2S. differences in the position of this data point from that of Guayabal waters again suggests that the SO4 in these systems has quite different origins. The other system allowing the evaluation of equilibration temperatures is based on the exchange 0f34S between SO 4 and co-existing H2S. This exchange reaction is much slower than that involving lSO and therefore can be expected to provide information on conditions at deeper levels; its temperature dependence is given by ASO~_H2s = 10(t0 In aso ~ H2S = ( 6 , 0 4 0 , 0 0 0 / T 2) + 2.6

(7)

as reported by ROBINSON(1973). Corresponding isotherms are shown in Fig. l l B . Only for a few discharges are 34S contents available for both SO4 and H2S. They are all acid springs. The temperature indicated for Montezuma (MZ) of 180°, and even that of 380° for Ahogados (AH), appear quite reasonable; the extremely high apparent temperature for BQ is again likely to reflect the effects of non-equilibrium oxidation of H2S at shallow levels, probably within the pool. Estimates of equilibrium temperatures for samples, for which only the 34S contents of SO 4 are available, may be obtained by assuming equilibration at depth with a common H2S. Its isotopic composition is most likely to be represented by the average of discharges probably least affected by secondary reactions, the vigorously boiling spring BQ, well PGM-1 at Miravalles (P1) and the steam vents at Miravalles (HM) and Las Pailas (LV), of - 3.0%o.

In Fig. l l B , the 34S content of dissolved SO4 for discharges, where no values for H2S are available, are marked by horizontal arrows and are plotted at the average value of -3.0%0. The temperatures indicated for the well discharges, P5, P10 and P l l , and Salitral Bagaces (SB) of between 320 and 420°C may reflect conditions in the primary zones of water-rock interaction where the originally magmatic fluids are converted to geothermal fluids and where most of the magmatic S is deposited as secondary sulfates or sulfides. The compositions of the dilute sulfate waters LT and QA bracket the well compositions. Their low CI/SO 4 ratios, however, suggest that they are not derived by simple dilution of the waters supplying the wells. The 348 content of the SO 4 discharged from well PGM-2 in 1985, after deepening, is higher than that of the neutral well discharges and in a position compatible with admixture of Guayabal SO4 with an even higher 34S content. The large increase in SO4 content in PGM-2 from 71 to 620 mg/kg (Table 2), however, implies that - 9 0 % of the sulfate in the mixed water would have to come from the Guayabal water resulting in a 34S content of the mixture of +21.9%o. As pointed out above, the much lower value of +17.5%o suggests that the acid water added to PGM-2 is different from that discharged at Guayabal. The 34S equilibration temperature for the SO4 from PGM-2 of 310° is also well above that of 230 ° indicated for Guayabal. The 34S contents of LV, HM, P1 and BQ are assumed to represent the isotopic composition of H2S in geothermal steam of the area. The lower

324

W.F. Giggenbach and R. Corrales S.

values observed for the vapor collected from a boiling mud pool at Las Pailas (LM), and a low temperature gas discharge on the flanks of Miravalles volcano (GV), are likely to be due to removal of isotopically heavier SO 4 within the pool or at depth. The heavier value of the boiling pools at Tierras Morenas may reflect removal of S in the form of secondary sulfides or elemental S. Isotopic evidence suggests that the increase in acidity and SO4 contents in well PGM-2, after deepening in 1984, is due to admixture of a deep, highly immature water still reflecting formation from magmatic fluids. It is similar, but not identical, to that encountered at Guayabal. Sulfate-water 18Oequilibration temperatures for the 504-C1 waters are 80-100°C, close to those measured. Those for the wells are possibly affected by re-equilibration after sampling, but are still considerably higher, 140-220 ° . The 345 ratio of H2S is likely to be -3.0%o, close to that expected for volcanic-magmatic systems. The SO4-H2S equilibration temperatures for the wells range from 310 to 420 ° and are likely to reflect conditions at considerable depth.

CHEMICAL COMPOSITION OF VAPOR DISCHARGES

The chemical compositions of 17 gas samples collected over the Guanacaste G e o t h e r m a l Province are given in Table 5. The data for individual components are reported in terms of mole fractions xi (mmol/mol) in the "dry" gas; they may be converted to concentrations in the vapor actually sampled, Xvi, in/~mol/ mol, by use of Xvi = XgXi, where Xg is the fraction of gas, again in mmol/mol, also given in Table 5. For

well samples the fraction of steam separated under the sampling conditions is reported. It is obtained from Ys = ( H a - n , ) / ( n v - n , ) (8) where H d is the measured discharge enthalpy of the well, and Hv and Hj the enthalpies of steam and water at the separation temperature. Total discharge gas c o n t e n t s Xd,i, in ~mol/mol, then correspond to Xd, i =

ysXgXi.

(9)

The data of Table 5 are reported as measured, uncorrected for the entrainment of air, as indicated for some samples by the presence of 02. In any further evaluation, the effects of this obviously shallow contamination are, at least to some degree, eliminated by subtracting 0.000087Xo2 from the Ne contents, 0.0446Xo2 from the A r contents and 3.73Xo~ from the N2 contents, and assuming the 02 content of the original fluids to be zero. E v e n after correction for air contamination, relative N2, Ne and A r contents of most discharges are still very close to that of air as shown by Fig. 12 suggesting an essentially atmospheric origin of these gases. In this case, however, the gases would have been introduced most likely dissolved in groundwater and data points should plot in the vicinity of that representing air saturated waters (asw). There are two major e n d m e m b e r processes controlling the exsolution of gases from geothermal solutions: total stripping of dissolved gases, for example in the case of vigorously boiling or effervescing pools, and equilibrium degassing from only very gently bubbling pools (GIGGENBACH, 1990). In the first case, the composition of the liberated gas approaches that of the gas dissolved in the water and N 2, Ne and A r are released in proportions to those

Table 5. Chemical composition of gas discharges over the Guanacaste Geothermal Province, in mmol/mol. S T is sample collection temperature in °C, y~ the steam fraction in the case of well discharges, xg is the fraction of gas in the steam sampled, in mmol/mol, R / R A the 3He/aHe ratio with respect to air. The composition of air saturated water, asw, at 20°C, is given in ~mol/mol Date

ST

y~

Xg

-10 3.7 9.7 1000 1000 720 3.6 3.7 2.8 20 2.8 4.6

AH BQ LM LV GB 36 34

August 1987 August 1987 June 1984 June 1984 November 1990 March 1982 March 1982

37 96 98 98 70 67 98

--------

P1 P2 P5 PR P10 Pll

June 1984 June 1982 June 1984 September 1985 September 1985 September 1985

136 159 136 dry 146 145

0.20 0.14 0.21 1.00 0.20 0.20

HM GV MZ TM

August 1987 June 1984 August 1987 August 1987

96 36 62 91

-----

asw

.

.

.

.

52 1000 950 380

CO2

H2S

NH3

He

Ne

963 932 946 975 982 969 991

3.8 35.2 24.3 14.9 <0.1 0.5 2.9

<0.01 0.02 0.51 0.03 <0.01 0.04 0.06

0.0018 0.0030 0.0130 0.0033 0.0027 0.0040 0.0040

0.00032 0.00056 0.00035 0.00006 0.00036 ---

967 870 870 989 890 977

7.6 3.2 2.5 0.8 7.9 5.8

3.40 5.70 3.10 0.30 3.20 2.90

0.0011 0.0012 0.0021 0.0014 0.0015 0.0016

0.00040 0.00240 0.00250 0.00021 0.00190 0.00030

983 963 987 986

8.8 31.6 1.5 1.6

0.01 <0.01 <0.01 <0.01

0.0025 0.0110 0.0004 0.0026

0.00004 0.00008 0.00020 0.00015

--

--

0.00004

0.00016

0.25

Geochemistry of water and steam, Guanacaste Geothermal Province, Costa Rica in air saturated groundwater (asw). In the second case, the degassing process is a reversal of the equilibrium dissolution process and the gases are, at least initially, released in proportion to those of the gas absorbed, air. The only two samples approaching the composition of air-saturated groundwater are those collected at Guayabal and from a submerged spring, at the bank of a flooded river, at Montezuma (MZ). In both cases, pool surfaces were well exposed to the atmosphere allowing air to dissolve and some of the atmospheric gases present in the samples may be such recycled air. Gases from andesitic centers usually show considerably increased relative N2 contents. A quite minor increase is indicated for samples, LV, HM and AH, the remainder of the surface samples have relative N2, Ne and Ar contents close to those of air. In the case of well samples, it is possible to evaluate total discharge gas contents by use of Eqn (9). They are about twice that of air-saturated water, also given in Table 5. A possible, at least partial, explanation is entrainment of "excess air" during formation of the groundwaters, as suggested by HEATON and VOGEL (1981). Another diagnostically useful diagram, involving N2, Ar and He, is represented by Fig. 13. All data points fall within the triangle defined by three endmember components: a He-poor meteoric component represented by air-saturated groundwater (asw), a N2-rich "andesitic" component and a Herich "crustal" component. As stated above, contributions of N2 from the "andesitic" component appear to be comparatively small. They are apparently highest for the two fumarolic samples HM and LV. All surface samples from the Miravalles-Las Pailas area show highly increased relative He contents. The 3He/4He ratios of 6.4-7.3 times that of air, as reported by POREDA and CRAm (1989) for four Table 5. Continued Ar

H2

02

N2

CH4

R/RA

0.26 0.36 0.25 0.03 0.30 0.08 0.05

0.0001 4.1 5.3 3.9 <0.0001 25.3 0.6

<0.02 <0.04 2.30 0.02 1.42 <0.01 0.05

33 27 18 5 15 4 4

0.12 0.61 0.36 0.42 1.13 0.47 0.08

----7.2 ---

<0.01 14.0 18,0 <0.02 13.0 <0.01

17 101 102 9 84 13

0.11 0.39 0.49 0.35 0.25 0.20

--6.4 7.3 -7.0

<0.02 0.77 0.99 <0.02

3 3 9 9

3.12 0.31 1.50 2.94

-----

5.62

11

--

1.0

0.20 1.30 1.30 0.10 0.90 0.20 0.02 0,03 0.21 0.11 0.28

3.3 3.5 1.4 0.3 0.3 0.3 1.2 0.004 <0.0001 0.7 --

325

samples collected from wells and the pool at Guayabal (Table 5), are those expected for systems along convergent plate boundaries. There appears to be a significant difference in the ratios between the two well discharges from PGM-5 (7.0) and PGM-11 (6.4) and the dry well PGM-5R (7.3) and the acid SO4-CI spring at Guayabal (7.2). Possible causes of variations in 3He ratios within a geothermal system have recently been discussed by HEARN et al. (1990). Increasing 3He contents with increasing HCO 3 and decreasing CI content at a thermal area in Yellowstone Park were ascribed to deep vapor and associated He loss from the high-Cl waters and deep dilution of the high HCO3-waters preventing vapor loss, thus preserving their high 3He ratios. In the case of Miravalles, high 3He ratios appear more likely to reflect closer association with magmatic fluids, lower values increasing contributions of radiogenic He at the periphery of the major upflow zones. The high chemical He contents observed for "shallow" vapors at Miravalles and Las Pailas also suggest that their parent waters are less degassed or are more effective in accumulating magmatic He than the waters supplying the deep wells and areas to the NW and SE (AH, BQ, TM and MZ). There relative He contents are much lower (Fig. 13) indicating smaller contributions of He from deep magmatic sources or earlier loss of He containing vapors. The fact that He contents are high for features generally higher up on the volcanic structures suggests that they are associated more closely with major upflow zones of magmatic vapors. The lower He contents and 3He ratios of the wells then would indicate that they tap waters on the periphery of the respective systems or from their outflow zones. Chemical data on geothermal gas discharges form also the basis for a number of geothermometers allowing the evaluat;an of deeper equilibration temperatures. Because of large differences in the origins and in the relative rates of equilibration of the reactive species, techniques involving as few species as possible at a time are preferable in the application of gas geothermometers to dynamic geothermal systems. Of the species listed in Table 5, H 2 was found to readjust most rapidly to changes in the temperature and redox state of the rising fluids. The gases CO2, CH 4 and N 2 react much more slowly and, therefore, are likely to reflect conditions at deeper levels (GIGGENBACH, 1987), The two most important redox buffers controlling the chemistry of gases in hydrothermal systems with close volcanic-magmatic associations involve Fe in its two oxidation states, present in minerals of the rock phase, and S in the lower ( - 2 ) and higher (+4, +6) oxidation states, present in the fluid phase (GIGGENBACH, 1987). AS stated above, H 2 contents adjust most rapidly to changes in temperatures; absolute H 2 content by themselves, therefore, provide a good indication of shallow equilibration tempera-

W. F. Giggenbach and R. Corrales S.

326

N2/100

1000Ne

Ar

FI6. 12. Relative N2, Ne and Ar contents of vapor discharges from the Guanacaste Geothermal Province. Solid circles represent compositions of air and air-saturated water (asw).

tures and redox conditions. A more quantitative technique is obtained by assuming H 2 to have equilibrated dissolved in an aqueous liquid phase of essentially meteoric origin. In this case, Ar contents correspond to those of air-saturated groundwater as shown above. By combining theoretical liquid phase H 2 contents with known saturation Ar contents, a H2/Ar geothermometer may be derived (GIOcENBACHand GocuEL, 1989) according to /HA = 70(2.5 + log

(XH,JXAO)

(10)

where tHA is the Hz/Ar equilibration temperature, in

°C, and XH~and XAr the analytical mole fractions of H~ and Ar as given in Table 5. In Fig. 14 lines expected for redox control by the two most important crustal buffers, the Fe(II)Fe(III) "rock buffer" and the H2S-SO 2 "fluid buffer'" are shown. Data points for the wells are plotted at their measured deep temperatures (230-250°C). Their position close to, but somewhat below the rock buffer line suggests attainment of equilibrium with the rock at measured well temperatures, but under slightly more oxidising conditions, possibly reflecting the residual effects of the presence of nearby, much

N21100

fumoroLe ///

oir"

10He Ar FIG. 13. Relative N2, lie and Ar contents. Solid circles mark composition of like]y endmember components.

Geochemistry of water and steam, Guanacaste Geothermal Province, Costa Rica I

i

i

i

I

i

I

327

oxidation states present in the rock phase. The temperature dependence of the reaction is given by tMc = [4625/(10.4 + log ( x c n J x c o , ) ) ] - 273 (12)

L~,:

Og( H2, At:

"OOo 2OC~ 3CC~ ;~Ci Fro. 14. Evaluation of H2-Ar temperatures.

more oxidising magmatic fluids as represented by the "'fluid buffer" line. The H2/Ar ratios for many of the surface gas discharges are even higher than those of the well discharges. Fluid-rock equilibration temperatures calculated by use of Eqn (10) range from 250 to 350°C, The extremely high value indicated for PH36, however, may be due to addition of H 2 through corrosion of the well casing by the acid fluids. This well was sampled after only a short period of discharge, after it had been shut in for a considerable time. The much lower H2/Ar-ratio observed for TM may reflect lower equilibration temperatures or more oxidising formation conditions for this gas associated with Tenorfo volcano. Definitely more oxidising conditions, and probably quite low temperatures, are likely to be responsible for the very low values of H2/Ar for each of the gases associated with the acid SO4-C1 springs AH, GB and MZ. The highly oxidising conditions indicated are in agreement with the highly immature nature of these discharges (GIGGENBACH, 1987). Other potential gas geothermometers are based on reactions possibly controlling CO 2 and CH 4 contents in geothermal systems. As pointed out above, reactions involving CH 4 are very slow and any temperatures obtained are likely to be those at greater depth. Again a technique based on the ratio between two components and on the assumption of full fluid-rock equilibrium is used (GIGGENBACH and GOGUEL, 1989). In Fig. 15 a line is given representing variations in the ratio XcnJXco,_ expected for equilibration of the two gases dissolved in an aqueous liquid phase according to the reaction CO 2 + 8(FeO) + 2H20(1 ) = CH 4 + 8(FEO1.5) (11) where (FeO) and (FeO1.5) represent Fe in its two

where tmc is the CH4/CO 2 equilibration temperature in °C. Apparent temperatures of equilibration for both well and surface samples over the Miravalles and Rinc6n de la Vieja areas are >350°C suggesting that CH4 equilibration takes place in much higher temperature zones, well below drilled depth, probably within the zones of initial interaction of the invading groundwaters with rock of a magmatic intrusion. Somewhat lower temperatures of 320°C (TM) and 340°C (MZ) are indicated for the system centered on the volcano Tenorio. Another temperature sensitive process is the interaction of CO2 with framework silicates of the primary rock phase to form secondary layer silicates and calcite (Gm6ENBACH, 1988) according to the reaction CO2 + Ca-aluminium silicate + feldspar = calcite + clay.

(13)

Again, assuming attainment of equilibrium of the gases completely dissolved in an essentially meteoric water phase, another geothermometer may be devised. It is also based on the normalisation of equilibrium CO 2 contents to Ar contents. The temperature dependence of the resulting indicator ratio Xco]XAr is given by

i

I

I

[]

I

.,,

3" 2-

I

}/

&

/~'"

\

0

-1 i LMC-2 MC= °g(XcHjXco2 )_ ~. -- ~

quencNng

1(~0.

FIG. 15. Evaluation of

~0"

300" (°C)

CO2-Ar and CH4-CO2 temperatures.

328

W.F. Giggenbach and R. Corrales S. XH2s = Cso4(50 -- Acr)/5444Aa.

LCA = log (XcoJXAr) = 0.0277t-- 7.53 + 2048(t + 273).

(14)

The corresponding line is shown in Fig. 15. The technique is inherently only applicable to systems where formation of calcite at some stage is at least likely. In the case of the acid SO4-C1 waters, associated with gas samples from PH-36 (36) and Montezuma (MZ), this assumption is probably not valid. In this case, CO2/Ar ratios are likely to reflect relative proportions of originally magmatic vapors and meteoric waters making up the discharge. In the case of the fumaroles (HM, LV) and steam-heated pools (BQ, LM, TM), the vapors may well have separated from neutral waters in equilibrium with calcite and apparent temperatures of equilibration may be quite realistic. They range from 260 to 300°C, lower than those indicated by CH4/CO2. Some of the high values of LCA may in part be due again to earlier loss of little soluble Ar. Data points for the wells plot quite close to the "full equilibrium" line; apparent temperatures of equilibration are 250-280 °, suggesting attainment of full equilibrium, and by implication, formation of calcite close to the zones reached by the drillholes. A check on the internal consistency of the thermodynamic techniques used here is provided by comparing total discharge CO 2 contents obtained by use of the analytical data of Table 5 and Eqn (9) with those suggested by Fig. 7. There data points plot close to the line corresponding to a ratio CO2/H2 O of 0.0005 + 0.0002. Values for this ratio, obtained from the well discharge analyses, are 0.0006 + 0.00025. Both are internally consistent and suggest attainment of overall vapor-liquid-rock equilibrium for reactions involving CO 2. The CO2/1-12O ratio of the steam discharge from the dry well PGM-5R (PR) of 0.02 is also very close to that of vapor in equilibrium with the deep water (Fig. 7). The somewhat higher ratio of 0.05 measured for sample HM from a natural vent in the Miravalles area may be due to condensation of some of the water vapor on its way to the surface. The lower CO2/H20 ratios of the two Las Pailas samples, 0.0035 for LM and 0.0095 for LV, may indicate equilibration at lower temperatures or steam separation from the already somewhat degassed, upper strata of the rising waters. Another check on the internal consistency of the approaches used here consists of comparing analytical H2S contents of the steam samples listed in Table 5 with those inferred from the SO4 contents and isotopic enrichments in waters from two steam heated pools at Las Pailas. Based on relations derived by GIGGENBACHand STEWART(1982), the mole fraction of H2S present in the steam heating steamheated pools (in mmol/mol) is related to the SO4 content of the pool waters, Cso4 (in mg/kg), the slope cr of the line connecting data points for the steamheated pools in Fig. 3 (3.2 at Las Pailas), and the 180 enrichment for a given pool A in %o, according to

(15)

Inserting measured values for L2 and L3 from Tables 1 and 2, H2S contents of the steam heating the pools of 0.05 and 0.29 mmol/mol respectively are obtained. They may be compared to those actually observed for the two samples LV and LM of 0.09 and 0.14 mmol/ mol, respectively. Both follow the same trend and are of similar magnitudes suggesting that the overall approach is valid. The H2S contents at Las Pailas may also be compared to those of the steam produced from wells at Miravalles. They range from 0.007 mmol/mol at PGM-5 to 0.027 at PGM-1 and PGM11. The values are below the range delineated by the steam-heated pools and may reflect the higher dilution by steam in the well samples or loss of some H2S as a sufide (pyrite) or with the vapor feeding the fumaroles. A t Borinquen quite high H2S contents are indicated, 0.74 mmol/mol by the SO 4 content and the isotopic composition of the pool waters, 0.35 mmol/ mol by the steam analysis. For the corresponding steam discharge at Tierras Morenas, the composition of the waters suggests a H2S content of the vapor of 0.09 mmol/mol; the high analytical gas content, Xg = 380 mmol/mol, is probably due to condensation of steam within the comparatively low temperature (91°C) pool sampled. The overall trend corresponds to a decrease in H2S contents of steam discharges from about 0.5 mmol/mol in the NW (Borinquen), over 0.1 mmol/mol at Las Pallas, to 0.02 mmol/mol at Miravalles. Other minor, but ubiquitous, constituents of geothermal vapor discharges are the higher hydrocarbons as listed in Table 6. Analytical data are reported in terms of mmol/moi of total hydrocarbon present. These concentrations may be converted to absolute concentrations, in/~mol/mol, by multiplication with the CH4 contents of the vapors sampled, also given in Table 6. Based on the analyses of a wide range of geothermal and volcanic gas discharges, the overall pattern exhibited by the Guanacaste samples corresponds closely to that expected for discharges from geothermal systems with close volcanic-magmatic associations. The presence of unsaturated (ethene, propene) and aromatic (benzene, toluene, xylenes) hydrocarbons in easily determined amounts is a clear indication for the interaction with sedimentary material at close to magmatic temperatures. G E N E R A L DISCUSSION AND SUMMARY

A n intriguing feature of the distribution of natural thermal discharges over the Guanacaste Geothermal Province is the occurrence of corresponding sets of water types centered on each of the three major volcanic structures Rinc6n de la Vieja, Miravalles and Tenorio: a set of acid SO4-C1 water discharges as

Geochemistry of water and steam, Guanacaste Geothermal Province, Costa Rica represented by Los Ahogados (AH), Guayabal (GB) and Montezuma (MZ), a set of high temperature steam discharges accompanied by steam-heated mud pools at Borinquen (BQ), Las Hornillas (HM) and Tierras Morenas (TM); and a set of intermediate temperature neutral C1 water discharges at Salitrals Norte (SN), Bagaces (SB) and Cafias (SC). The acid, SO 4 rich discharges occur in a straight line, following the regional tectonic trend (HEALY, 1969). They are generally located on the lower slopes of the volcanic structures, but at altitudes >500 m. The neutral C1 waters are discharged from springs well to the south and at altitudes <500 m (Fig. 1). The production of HCO 3 waters appears to be limited to a narrow belt stretching from springs to the north of Miravalles volcano (QC, LQ), to the south (BN, BB, B J, RO). The position of this belt coincides with that of a north-south trending graben structure (MANIERI et al., 1985). The most common mode of formation of HCO 3 waters consists of the absorption of CO 2 liberated from deeper thermal waters into local groundwater followed by water-rock interaction. The occurrence of HCO 3 waters therefore implies the presence of a tongue of deeper gas-rich waters moving underneath. The narrow distribution of HCO 3 waters suggests that the north-south trending graben may provide the major outflow channel of thermal water from the Guanacaste Geothermal Province. Another group of thermal areas over the southern slopes of Rinc6n de la Vieja comprises only the first two types of thermal features: fumaroles and steamheated pools at Las Hornillas and Las Pailas and a number of cooler SO4 springs as represented by Quebrada Azufral (QA). Judging from the extent and intensity of surface activity, Las Pailas is probably the most important area of heat discharge from the Guanacaste Geothermal Province. As pointed out above, fumaroles and steam-heated features are likely to mark the epicenters of major zones of upwelling, where rising, probably neutral Cl waters first start to boil. Accepting this scenario, the large vapor discharge at Las Pailas can be expected to be associated with the upwelling of a correspondingly large volume of deep water. In the absence of any obviously associated discharge of neutral C1 waters farther downhill, it has to be assumed that the waters rising beneath Las Pailas spread out over permeable strata underlying the Guanacaste Geothermal Province to join the flows of Cl waters emerging from the other three major thermal systems and to be discharged together with these at the Salitrals. The great similarity in the chemical composition of the Salitral discharges is fully compatible with the assumption of large, possibly contiguous bodies of neutral C1 water underlying the GGP. The presence of an extensive body of thermal waters was invoked by FURGERSON and ALFONSO (1977) to explain the occurrence of a thick zone (880-1100 m) of low resistivity underlying most of the area to the south of the volcanic chain.

329

Isotopic evidence shows that the waters discharged at Miravalles are derived largely from groundwater to the north, the Atlantic side, of the volcanic chain. The most likely reason for this is the much higher rate of precipitation there and the topography of the Guanacaste Geothermal Province favoring drainage to the south, toward the Pacific. Secondary waters, such as the diluted HCO 3 and SO 4 waters and even the well discharges, contain varying proportions of southern groundwater. The general flow of thermal waters, therefore, is likely to be from north to south with some of the deeper, neutral C1 waters travelling for considerable distances ( > 15 km) from their major zones of upflow, as marked by areas with fumaroles and steam-heated pools, before emerging at the surface. Depending on the distance travelled, the chemistry of the water adjusts to lower temperature conditions, their overall character, however, remains close to that of the deep waters as represented by the well discharges. The incursion of an acid SO4 water in well PGM-2, after deepening to 2000 m in 1984, may well point to the presence of a body of such immature waters close to the C1 waters, at least over parts of the Miravalles area. In this case the SO4-C1 water may be considered to represent the parent water of the neutral C1 waters. The differences in 34S ratios of dissolved SO4 and in relative Li, Rb, Cs, B and CI contents (Figs 9 and 10), suggest only a very tenuous link between the neutral C1 waters and the other group of acid SO4 waters, those discharged at the surface and therefore quite different origins and histories for these two groups of SO4 waters. The latter most likely represent waters formed through absorption of volcanic-magmatic vapors into groundwater at quite shallow levels, the former waters resulting from leaching of magmatic constituents at high temperatures by deeply circulating groundwater. In most aspects, the geothermal hydrology of the Guanacaste Geothermal Province resembles closely that indicated for other geothermal systems in Latin America such as those at Los Azufres (Mexico) and Zunil, Moyuta and Amatitl~in in Guatemala (Giggenbach, unpublished results), and that at Nevado del Ruiz, Colombia (GIGGENBACItet al., 1990). A general model depicting the distribution of the various water types within such geothermal systems, compatible with chemical and isotopic evidence and applicable in somewhat modified forms to all three sub-systems over the Guanacaste Province, is shown in Fig. 16. The overall flow of waters within the Guanacaste Geothermal Province is dominated by the northsouth hydrological gradient. The model reflects the generally observed trend in the evolution of fluids with close volcanic-magmatic associations along the sequence magmatic fluids > acid SO4-C1 water > neutral CI water > HCO 3 water (GIGGENBAcn, 1988). Over the Guanacaste area and elsewhere,

330

W.F. Giggenbach and R. Corrales S.

Table 6. Hydrocarbon contents in vapor discharges from the Guanacaste Geothermal Province, in mmol/mol of total hydrocarbons. The molefractions of CH 4 of the first column are those in the vapor sampled, also in mmol/mol. Absolute hydrocarbon contents, in/~mol/mol (ppm V), correspond to Xi = XcHa, xcH 4

met

ete

eta

pre

AH BQ LM LV GB 36 34

0.1200 0.0061 0.0013 0.0041 1.1300 0.4700 0.0580

996 890 904 976 998 997 972

<0.1 0.2 2.1 0.1 <0.2 0.3 <0.1

1.4 14.6 18.3 15.4 1.2 1.7 15.3

<0.1 0.1 0.6 <0.2 <0.2 <0.2 <0.2

P1 P2 P5 PR P10 Pll

0.0004 0.0026 0.0014 0.0069 0.0007 0.0009

892 858 896 941 888 903

6.7 4.6 2.4 0.8 3.2 1.9

38.0 30.9 46.8 19.2 50.0 47.4

HM

0.1600

997

<0.1

GV MZ TM

0.3100 1.4300 1.1200

998 998 997

0.1 <0.1 <0.1

pra

ibu

nbu

ipa

npa

0.2 2.8 4.1 3.1 0.3 0.2 4.0

<0.1 0.2 0.2 0.2 <0.2 <0.2 <0.2

<0.1 0.8 1.0 0.8 <0.2 <0.2 0.5

<0.1 0.1 1.0 0.1 <0.2 <0.2 <0.2

<0.1 0.2 2.1 0.2 <0.2 <0.2 <0.2

0.2 3.0 <0.3 0.2 0.4 0.2

11.9 16.0 14.7 5.5 15.7 15.4

2.8 3.3 1.0 0.3 2.3 1.3

3.9 9.1 4.5 1.8 4.9 5.0

0.4 2.1 1.1 0.2 0.8 1.0

1.8 7.9 3.9 0.9 2.5 2.8

1.I

<0.1

0.2

0.001

0.05

0.003

0.015

0.7 0.6 4.8

0.3 <0.1 <0.1

<0.1 0.1 0.5

<0.1 0.018 0.022

<0.2 0.06 0.10

<0.2 0.013 0.008

<0.2 0.026 0.025

met: methane; ete: ethene; eta: ethane; pre: propene; pra: propane; ibu: i-butane; nbu: n-butane; ipa: i-pentane; npa: n-pentane; ben: benzene; tol: toluene; ebe: ethyl-benzene; pmx: pm-xylene; ox: o-xylene; - - : not determined.

the a b s o r p t i o n of v o l c a n i c - m a g m a t i c v a p o r s into g r o u n d w a t e r is likely to occur at two distinct levels. A t c o n s i d e r a b l e d e p t h a n d t e m p e r a t u r e , the process leads first to the f o r m a t i o n of highly i m m a t u r e fluids, similar to those e n c o u n t e r e d in P G M - 2 after d e e p e n i n g . I n t e r a c t i o n with rock consists initially of close to isochemical rock dissolution (GIGGENBACH, 1988); f u r t h e r i n t e r a c t i o n converts these partially n e u t r a l i s e d waters to the n e u t r a l CI waters suitable for g e o t h e r m a l p o w e r p r o d u c t i o n . T h e C1 waters p r o d u c e d from wells c o n t a i n high relative c o n t e n t s of m e t e o r i c gases and, t h e r e f o r e , are likely to be de-

~

bicarbonatew. chloride waters liquid.vapor ~..~,~,..... ,^~

rived f r o m an e n v i r o n m e n t d o m i n a t e d by groundw a t e r a n d not from the v o l c a n i c - m a g m a t i c system itself (GIGGENBACH, 1987; GIGGENBACH et al., 1990). Isotopic e v i d e n c e (Figs 2 a n d 4) suggests t h a t part of the waters m a k i n g up the h y d r o t h e r m a l e n v e l o p e is derived from s o u t h e r n g r o u n d w a t e r . As the CI waters ascend, they start to boil. T h e s e p a r a t e d vapors t e n d to rise close to vertically, are discharged at the surface in the form of fumaroles or interact with shallow g r o u n d w a t e r to lead to the establishm e n t of s t e a m - h e a t e d boiling pools or m u d pools, characterised by high SO4, but low CI contents. T h e positions of these s t e a m - h e a t e d features are likely to

[at erat fumaroles

summit fumaroles craterLake

Fro. 16. Potential distribution of water types in a typical volcanic-magmatic-hydrothermal system. The regions marked liquid + vapor and sulfate waters overlap with the zones of groundwater penetration into cooling rock.

Geochemistry of water and steam, Guanacaste Geothermal Province, Costa Rica

Table 6. Continued ben

tol

ebe

pmx

ox

0.1 17.7 17.0 3.2 <0.5 0.6 8.5

0.2 6.0 . . <0.5 . .

0.5 23.4 . . <0.5 . .

0.6 31.0

0.2 9.3

. . <0.5 . .

<0.5

17.5 18.3 10.3 12.9 8.5 6.5

. . . 6.8 7.7 4.5

(1.8 0.6 0.3 5.4

0.05 . 0.10 0.09

. . . . . . .

.

. . . 8.0 8.3 5.2 0.16 . 0.13 0.09

. . . 4.8 4.8 2.9

2.2 2.6 1.7

0.21

0.08

0.16 0.11

0.06 0.06

.

mark the epicenters of major zones of upflow of geothermal waters. A fraction of the magmatic vapors may be able to rise within each of the volcaoic structures to shallow levels to be absorbed directly into higher altitude groundwater, to drain back to lower altitudes and to emerge as the dilute, low temperature SO4-C1 waters encountered at Los Ahogados ( A H ) to the north, Guayabal (GB) on the slopes of the Miravalles and M o n t e z u m a (MZ) to the south. Isotopic compositions and increased He contents show that the higher altitude thermal waters over the Miravalles area are more directly related to the major upflow zones of magmatic gases than the well discharges. The formation of these "volcanic" fluids is only indirectly related to that of the " g e o t h e r m a l " fluids. The assumption that many of the lower temperature SO4-C1 water discharges are not directly associated with any major upflows of high temperature vapors is confirmed by the lack of Hg anomalies in the soils around many of these acid SO4 spring areas (LESCINSKYet al., 1987). A large Hg anomaly was found at Las Pallas, lending support to the assertion that this area overlies an important zone of upwelling of deep thermal fluids. The H C O 3, very low CI waters, QC, BN and PL, are thought to form through absorption of CO2-rich vapors having separated from deeper flows of neutral C1 waters. On the other hand, the HCO3-CI waters discharged along Rio Blanco, BB, BJ and R O , have probably formed by direct conversion of residual C O 2 dissolved in the neutral CI waters through interaction with aquifer rock at shallow levels, followed by dilution. The distribution of the H C O 3 waters outlines the major zones of outflow from the geothermal system. The intermediate HCO3-SO4-C1 waters,

331

LQ, 34 and E G , discharged in the Miravalles area are probably dilute mixtures of waters affected to minor degrees by the absorption of magmatic vapors. The model of Fig, 16 is compatible with chemical and isotopic evidence, but was constructed by assuming uniform and isotropic permeability conditions and therefore is likely to represent only the potential distribution of fluids within the system. The actual distribution is largely governed by generally littleknown geological factors such as the nature and distribution of igneous and sedimentary rocks and the presence, direction and rate of formation of fissures and faults facilitating fluid movement. As more geochemical, geophysical and geological information becomes available, it should be possible to locate the nature and position of these fluid conduits and to construct increasingly detailed models of the system. Acknowledgements--The present investigation was carried

out within the framework of the 1AEA Coordinated Research Program on the "Application of Isotopic and Chemical Techniques to Geothermal Exploration in Latin America" (COSl8/002) with financial support from the Government of Italy. The authors wish to thank R. L. Goguel, DSIR Chemistry, for the atomic absorption analyses of some of the waters and B. W. Robinson, Institute of Nuclear Sciences, DSIR, for the isotopic analysis of the sulfur species. Editorial handling: Brian Hitchon.

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