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Geothermal Resources Council Transactions, Vol22, September 20-23,1998

About The Origin of C02 in Some HCOdNdCOz-Rich Portuguese Mineral Waters

J.M. Marques ', P. M. Carreira ', 1. Aires-Barrod, R. C. Craga'

' lnstituto Superior TCcnico. Laboratdrio de Mineralogia e Petrologia (LAMPIST). Av. Rovisco Pais, 1096 Lisboa Codex. Portugal. ' lnstituto Tecnoldgico e Nuclear (ITN). Laborathi0 de Is6topos Ambientais. Estrada Nacional No 10,2686 SacavCm Codex. Portugal.

ABSTRACT

The present paper aims at increasing lmowledge on the identification of the origin of carbon dioxide (6°C- origin of C02 in hot and cold HCOJ/Na/C02-rich mineral values) present in those hot and cold C02-rich mineral waters issuing in the northern part of the Portuguese waters. The role of carbonaceous slates (well displayed in mainland. The main chemical and isotopic signatures of these Chaves, Vidago and Pedras Salgadas areas) in the waters are presented. 6''C,, values observed in hot and hydrochemical signature of the underground waters has been cold mineral groundwaters range between -6.OO%o and - 1.OO considered. %o vs PDB indicating a deep-seated (mantle) origin for most of the C02. In the case of the heavier 6"Crn~~)values, the I tf contribution of metamorphic C02 or the dissolution of carbonate rock levels at depth cannot be excluded. Concerning the hot waters, the lack of a positive '80-shif€ should be attributed to water--rock interaction with short contact times, rather than to the isotopic influence of C02on the S'*o-values of the waters.

Introduction On the Portuguese mainland, the greatest number of hot (Chaves) and cold (Vilarelho da Raia, Vidago and Pedras Salgadas) HCOJ/Na/C02-rich mineral waters flow fiom Figure 1. Sketch map of the region, showing the position of main natural springs and drilled wells located either in granitic water samples. (1) Vilarelho da Raia area ; (2) Chaves thermal area ; (3) outcrops or in the peribatholitic boundaries concordant to the and (4) Vidago area ;(5) and (6) Pedras Salgadas area. (B)stands for hot main "E-SSW fault trend (Figure 1) which attains 30 km saline waters ;(0) stands for cold saline waters depth in the study area (Baptista et al., 1993). Chaves thermal waters have been well-known since Roman times and today Field and Laboratory Methods they are mainly used for balneotherapy and heating a swimming pool. These thermal waters are currently being Thermal and cold C02-rich mineral waters and cold utilized to heat an experimental greenhouse. superficial low mineralized waters were sampled and Geochemical and isotopic studies enabled us to develop a chemically and isotopically analyzed. Some literature data conceptual hydrogeologic model of the low temperature were used for Chaves thermal waters (Almeida, 1982). The geothermal system of Chaves. Isotope techniques (l80, 2H Environmental Isotopes Laboratory of the Nuclear and and were used to identify groundwater origin, altitude of Technological Institute (1TN)Portugal carried out the 'H) 18 recharge and underground flow paths (Aires-Bmos et al., determinations of 6 0, S2H and6l'C values in waters by 1991, 1994, 1995, Marques, 1993, Marques et al., 1995, mass-spectrometry. 6% analyses have been performed on 1996). Recently, special attention has been given to the total dissolved inorganic carbon (TIDC) precipitated "in situ"

113 Marques, et al. as BaC03 at pH higher than 9.0. The gas used in l3C/I2C along a belt extending 150km in Portuguese mainland, and by measurements was C02. Carbonates have been reacted with the N70°-800E faulted system, crossing the area in the 100% phosphoric acid to liberate C02. 3H analyses were also neighbourhood of Chaves (Portugal Ferreira et al., 1992). 18 Vidago and Pedras Salgadas areas are also mainly composed performed at the ITN. 8 0 and S2H were measured of Hercynian granites with some outcrops of metamorphic following the analytical methods of Epstein and Mayeda rocks of Silurian age covered by Cenozoic deposits. The high (1 953) and Friedman ( 1953), respectively. Tritium C02 content of Vidago and Pedras Salgadas mineral waters determinations were performed using electrolytic enrichment could be associated with the fact that Chaves thermal waters and subsquent measurement of counting rates by liquid emerge within a wide graben, whereas the Vidago and Pedras scintillation. 813C values in rocks (carbonaceous slates) have Salgadas cold mineral waters are found in areas where the been measured by mass-spectrometry at Delta Isotopes "E-SSW megalineament does not exhibit such Laboratory/The Netherlands. The standard 6 notation in per morphological structure (Portugal Ferreira et al., 1992). mil, used throughout the paper, is relative to the references V- 13 3 Geochemistry of the waters SMOW for 8l80 and S2H, and PDB for 6 C. H is in T.U. The hot and cold mineral waters, of HC0fla/C02-rich (tritium Temperature ("C), pH and electrical units). type, emerge fiom boreholes and springs, exhibiting conductivity (pS/cm) were determined in the field at the time outgassing phenomena. Chaves hot (= 76 "C) mineral waters of collection. Total alkalinity was measured a few hours afier collection. The following methods were applied for chemical and Vilarelho da Raia cold (~16°C)mineral waters are analyses performed at the Laboratory of Mineralogy and. characterized by high mineralization (TDS = 1800 mg/l) and Petrology of Instituto Superior T6cnico free C02 of about 350 and 1100 mg/l, respectively. The (LAMP1ST)Dortugal: atomic absorption spectrometry for Ca Vidago and Pedras Salgadas cold (= 17°C) mineral waters are and Mg; emission spectrometry for Na, K and Li; also characterized by high mineralization (TDS from 1135 to colorimetric methods for Si02 and Al; ion chromatography 4395 mgh), higher concentrations of Mg2+ and Ca" and higher flee C02 contents (up to 2500 mg/l). Almeida (1982) for SO4 and C1; potentiometry for alkalinity, here referred to presented the analykal results of the gas phase associated as HC03. The data on free C02 content relative to the with Chaves thermal waters. The following gases are presenl Vilarelho da Raia, Chaves and VidagoDedras Salgadas (% vol): C02 = 99.5 %, 02 = 0.05%, Ar = 0.02%, N2 = waters were kindly supplied by the Aguas de Carvalhelhos 0.28%, = 0.009%, CZH6 = 0.005%, H2 = 0.005% and Company, the Municipality of Chaves and Vidago - Melgago He = 0.01%. The low H2 concentrations suggest low & Pedras Salgadas Company, respectively. equilibration temperatures for the gas phase at shallow depths, as referred by Duck et al., (1995). Study Area The 8l80 and ij2H values of the hot and cold C02-rich Geological background mineral waters indicate a meteoric origin for these waters (Figure 2b). The low 8l80and S2H values of Chaves thermal The geology of Chaves geothermal area has been waters require that these waters were derived fiom meteoric described in Portugal Ferreira et al., (1992), Baptista et al., waters at more than 900m a.s.1.. These elevations are realised (1993) and Sousa Oliveira and Portugal Ferreira, (1995). The in the Bolideira granitic outcrop (NE-Chaves), which geomorphology of the study area is dominated by the presumably feeds the local infiltration (Aires-Barros et al., "Chaves Depression", a graben whose axis is oriented "E- 1994). Vilarelho da Raia cold mineral waters have an isotopic SSW. It is bounded at the east side by the edge of Padrela (6'*0 and S2H) composition similar to that of Chaves thermal Mountain escarpment with a 400m throw. The western block waters, suggesting a common origin for these waters (Figure is formed by several grabens coming from the Heights of 2b). The heavier S2H and 6l80values found in Vidago and Barroso towards the "Chaves Depression". The region under Pedras Salgadas cold mineral waters can be attributed to research is situated in the Ante-Mesozoic Hesperic Massif different recharge altitudes (Marques et al., 1996). that consists mainly of Hercynian granites and Paleozoic metasediments. The oldest formations correspond to a pre- Ordovician schisto-graywacke complex. In Ordovician and Results and Discussion Silurian times quartzites and schists were formed. Stable carbon isotope geochemistry Intercallated in the schistoid complex there are bands of carbonaceous slates. At the end of Palaeozoic these In mineral waters, carbon dioxide (or total carbon) can be formations were affected by the Hercynian granites intrusion. present in various forms: C02 (g), C02 (as), HCO; and CO: The most recent formations are Miocene-Pleistocene . The proportion of these species is related to the pH and the sediments with variable thickness, showing their maximum temperature of the fluid. The carbon dioxide present in development along the central axis of Chaves graben. Alpine themo-mineral waters can be atributed to two main origins: Orogeny has caused extensive tectonic features responsible organic and inorganic (Panichi and Tongiorgi, 1975). for the formation of several hydrothermal circuits. At Chaves, Concerning the organic source, C02 can be produced fiom the ascending hydrotermal circuits are structurally controled decay of organic matter with mean 613C values around - by the "E-SSW megafault, which is hydrothermally active 26%0 and -22%". Among the inorganic sources, carbon in

114 Marques, et al. hydromineral systems originate from: deep-seated (upper mixing of mantle and limestone C02 (Chaves and Vidago mantle) carbon showing 613C values ranging between -SO/OO AC18). An alternative int~re~tionis that the heavier 6I3C and -l0/oo, dissolution of limestones which have mean 613C values could be explained by metamorphic decarbonation reactions of country rocks that rnixed with magmatic CO2. values close to f 2O/00, or metamorphism of carbonates 13 C values (-26.9 and -27.Zo/oo) measured two producing COz with slightly positive 6°C values (Hoefs, 6 on carbonaceous slates of the study area indicate that the role of 1997). these rocks for the hydrochemical signature of mineral waters should be considered very limited. So, one has to look for the existence of carbonate rocks levels at depth, in the -40 a stratigraphic section crossed by the circulating waters. Some calcareous lenses have been reported by Brink (1960) in Vila Real area. They have a width of about 30 m and a length of about 1.5 km, and occur in a zone of low- grade rne~o~~smthat mainly comprises the chlorite- sericite schists and phyllites. Much larger limestone intercalations cross the Douro river, some kilometers to the 8 -0.5 8 -7.5 -7 4.5 -6 south of the studied area. A limestone zone traversing the 6 Osygen-18 Douro river could be traced for more than 13 km (Medeiros and Faria, 1935). Schermerhorn (1955) correlates the limestones intercalated in the Beira schists (the so-called Ante-Ordovician schisto-graywacke complex) of northern P Portugal (TrCis-os-Montes, Dom region) with those / occurring in NW Spain (Galicia). This author states that "in view of the fact that in the adjacent area of NW Spain thick limestones occur only at the Georgian-Acadian boundary, it seems not unlikely that the limestones of northern Portugal I I 1 represent this level, while thinning and disappeking towards Q -8.5 8 -7.5 -7 -6.5 -6 the southwest." 6 Oxygen-18 In the logarithmic Ca2+-HCO{ diagram (Figure 3), the hot and cold C02-rich mineral waters do not plot on the line of Figure 2. 6'H vs S'"0 (''/a0 vs V-SMOW) relationship in (a) local co superficial low mindised waters and (b) hot and cold Cqrrich mineral electroneutrality [HCO;] =2 [Ca2"]. This trend indicates waters of Vilarelho da Raidchaves and VidagoPedras Salgadas areas. Data that most HCO; should be related to the solution of deep- from Marques (1993). (X) stands for cold superficial low mineralised waters seated COz in water. However, the presence of carbonate (recharge waters), (A) for Vilarelho da Raia, (N) fbr Chaves, (0)for rocks in levels percolated by some of the xnineral waters Vidago and (0)for Salgadas €202-rich mineral waters. Pedras (those showing high Ca2" concentrations) should not be excluded. Almeida (1982) concluded fkom his 6I3C values measured t i t on C02 gas samples (613C(co2)= -5.72'/00 vs PDB) that most i of the 13C in the fiee C02 related &I the Chaves hot mineral loo00 i t waters was of upper mantle origin. It should be notd that th6 lop0 use of measurements on free C02 could be misleading Q because it may lead to wrong conclusions if the carbon c remaining in water is not con side^^ (R. Kreulen, personnel communication). In order to cl* the origin of C02 in the B COZ-rich cold mineral waters of Vilarelho da Raia, Vidago I and Pedras Salgadas, 6I3C dete~tio~were carried out on I 10 tal 1 I total dissolved inorganic carbon (TIDC) at four drilled well I Ca (lasn) ! waters associated with the deeper hidromineral circuit, and two cold superficial low mineralized waters (recharge wa~) Figure 3. Lo~rithm~~ca2'-Hco3- diagram for Vilarelho da Raia, located in Vilarelho da Raia and Pedras Salgadas areas. The Chaves, Vidago and Pedras Salgadas waters, including data from - Marques (1993) and unpublished data. Symbols as in Fig. 2. Sl3Cvalues (-23.4%0 and -22.4%0) associated with the cold superficial low mineralized spring waters indicate organic 13 an 6 C data from the mineral waters analysis plotkd against source for the C02. The 6I3C values associated with the C02- the logaritm of the saturation indexes (log S.I.) with respect rich mineral waters, lying in the range from -6"/00 to -lo/oo, to calcite (a) and aragonite (b) show remarkable regularities could be ascnied to a deep-seated (mantle) origin (Vilarelho for all the water points (Figure 4). The saturation indexes da Raia, Pedras Salgadas AC17 and Vidago AC16), or to have been calculated using the computer program HIDSPEC

115 Marques, et ai. (Carvalho and Almeida, 1989), which is an Chaves thermal waters), and the corresponding value in hy~ogeoche~calmodel that calculates the speciation of C02(&was +25.62 "/oo vs SMOW. Using this data, we natural waters. The saturation index (S.I.), for a given mineral calculated the fiactionation factor GCO~(& - H~o(I). The value is defmed (at any temperature) as the ratio between the obtained (+ 33.66 %o) indicates that equilibrium temperature activity product of the species involved in the hydrolysis (Friedman and 0' Neil, 1977) is close to the measured reaction and the equilibrium constant. The linear correlations temperature of 75°C at sampling. Concerning oxygen observed indicate that the presence of carbonate rock levels at isotopes, it seems that H20(,) and C02(, are in equilibrium. In depth as a possible carbon contributer should not be this case we have no evidence of waterlrock interaction at excluded. very high temperatures (Aires-Bmos e? al., 1995). The results of the several chemical geothermometers applied to Chaves them1 waters produce low temperature estimates (= 12OOC). So, we have to admit that the lack of an 180-shift,in Chaves thermal waters, should be mainly related to (i) the 0 lack of a high temperature reservoir at depth and (ii) relative small waterhock contact time. Isotopic influence of CO, on the 6l8Ovalue of the waters should be excluded.

Concluding Remarks -7 / 13 -1.5 -1 -0.5 0 0.5 1 -1.0 PDB) seems log S.I. Recent 613C data (-6.0 6 C 'loo vs 13 (calcite) to indicate that the light 6 C values could be assumed to be derived exclusively fiom a deep-seated (mantle) source, whereas a more complex origin could be proposed for the 13 R $(b) heavy 6 C values: mixing of mantle C02 with carbonate -1 0 CO, (derived hm thermal dec~bonation or rock dissolution). The most probable explanation for carbon dioxide transport fiom deep sources (mantle) to the surface involves tion on as a separate gas phase incorporated in the -6 infiltrated meteoric waters (at considerable depth in the case 1 I I I -7 ! I I I I' i of the hot C02-richwaters and at shallow levels in the case of -15 -1 -0.5 0 0.5 1 cold C02-rich waters). This hypothesis is suported by the fact log s.i. (aragonite) that the 6l80 and a2H data related to the thermal and cold COZ-rich mineral waters studied does not show evidences of mixing with magmatic waters. Most of the CO2-rich hot and Figure 4. Log S.I.-S'JC diagram for the COZ-rich mineral waters. cold springs studied are located near faults. These tectonic Chaves S'% value is the G'3C~s~31value from Almeida, 1982. Symbols as in Fig. 2 features must have access to deep levels in the study area, explaining the formation of the carbon dioxide waters. We Isotopic waterCO2 equilibrium note that, despite its peculiar lithochemistry, the In many cases, deep groundwaters related to geothe~l carbonaceous slates that are displayed itl Chaves, Vidago and Pedras Salgadas areas does not influence the hydrochemical systems display a positive shift in their oxygen- 18 content as a consequence of isotopic waterhock interaction with the signature of the underground mineral waters. reservoir rocks. Deuterium content is usually identical to that of local meteoric recharging waters. This is the most Acknowledgments chararacteristic for high temperature geothermal systems (Ellis and Mahon, 1977). Chaves them1 waters showing no This work has been supported by the oxygen-18 shift could represent either an old geothermal P~S~~~/~/7/96Project. We would like to thank system in which the isotopic composition of the rock had Dr, R. Kreulen for comments and suggestions on some topics adjusted to equilibn~with the recharge waters, or a deep presented in this manuscript. An early draft of this reservoir recharged by local meteoric waters with short manuscript was critically read by A. Tmesdell and we circulation time and small isotope exchange with rocks. The gratefully acknowledge his contribution. lack of an '80-shiA could also be explained by a compensating oxygen isotope exchange between H20(1) and COZ(~)(D'Amore and Panichi, 1987). References Almeida (1982) presented 6l80values of water and C02 Aires-Barns, L., Graqa, R. C. and Marques, J. M. (1991) Nota of Chaves thermal waters. 6l8O value of H20(1)was -8.04 '/oo preliminar sobre a geoquimica das bguas tertnais de Chaves. Revistn dn vs SNOW (similar to the isotopic composition of present-day Universidade de Aveh 6 (1.2), 59-69.

116 Marques, et al.

Aires-Barns, L., Graqa, R. C. and Marques, J. M. (1994) The low Friedman, I. and O'Neil, J. R (1977) Compilation of stable isotope temperature geothermal system of Chaves (Northem Portugal): a fractionation of geochemical interest. Data of geochemistry (6th edn). US. geochemical approach. In Proceedings. of the Symposium Geothermics 94 in Geological Survey Professional Paper 440-KK. Europe (Edited by BRGM), 230,67-73, OrlCans. Hoefs, J. (1 997) Stable Isotope Geochemistry. Completely Revised, Aires-Barns, L., Marques, J. M. and Graqa, R C. (1995) Elemental and Updated and Enlarged Edition. Springer-Verlag,201 pp. isotopic geochemistry in the hydrothermal area of Chaves / Vila Pouca de Aguiar (Northern Portugal). Environmental Geology ,25 (4), 232-238. Marques, J. M. (1993) As hpns termais e minernis de Vilnrelho dn Rain, Chnves, Vidago e Pedras Salgadas. Uma perspectiva global; Almeida, F. M. (1982) Novos dados geotermomitricos sobre iguas de aplicaqio geot6rmica. Trabalho elaborado no ambit0 das Provas de Aptidiio Chaves e de S. Pedro do Sul. Comunicnq'es dos Serviqos Geolbgicos de Pedag6gica e Capacidade Cientifica. Instituto Superior Tkcnico, 148 pp. Portugal, 68 (2), 179-1 90. Marques, J. M., Aires-Barros, L. and Graqa, R. C. (1995) A geoquimica Baptista, J., Coke, C., Dias, R. and Ribeiro, A. (1993) Tect6nica e isotbpica ("0,D e 'H) das hguas minerais de Vilarelho da Raia, Vidago e geomorfologia da regiiio de Pedras Salgadas / Vidago e as nascentes minerais Pedras Salgadas. Universidade do Porto, Faculdade de Cihcias Museu e associadas. Comunicaq6esda XII Reunitio de Geologia do Oeste Peninsular, Laboratbrio Mineral6gico e Geolbgico. Memdria. 4,473 - 477. I, 125-139. Marques, J. M. , Aires-Barns, L. and Gnqa, R. C. (1996) The origin Brink, A. H. (1 960) Petrology and ore geology of the Vila Real - Sabrosa and relation among hot and cold COZ-rich mineral waters in Vilarelho da Raia Pedras Salgadas region, northern Portugal: a geochemical approach. - Vila Pouca de Aguiar region, northem Portugal. Comunicaqtjes dos - Serviqos Geolbgicos de Portugal, XLIII, 143 pp. Geothennal Resources Council Transactions, 20, 363 - 369. Carvalho, M.R. and Almeida, C. (1989) HIDSPEC, um programa de Medeiros, A. C. and Faria, F. L. (1953) Estudo de algumas formaqaes especiaqHo e cilculo de equilibrios iguahocha. Revista da Universidade de calcirias dos distritos de Vila Real, Viseu e Guarda. Estudos, Notas e Aveiro, 4 (2), 1-22. Trabalhos do Serviqo de Foment0 Mineiro, 8,297 - 3 13. D'Amore, F. and Panichi, C. (1987) Applied Geothermics: Geochemistry Panichi, C. and Tongiorgi, E. (1975) Carbon isotopic composition of in geothermal exploration. M. J. Economides and P.O. Ungemach Eds, 69- COZErom springs, fumaroles, moffetes and travertines of Central - Southern 88. Italy: a preliminary prospection method of geothermal area. Proceedings of the Second United Nations Symposium on the Development and Use of Duchi, V., Minnissale, A., Vaselli, 0. and Ancillotti, M. (1995) Geothermal Resources, S. Francisco, C.A, 8 15 - 825. Hydrogeochemistry of the Campania region in southem Italy. Journal of Volcanology and Geothermal Research, 67,3 13 - 328. Portugal Ferreira, M., Sousa Oliveira, A. and Trota, A. N. (1992) Chaves geothermal pole. Geological Survey. I and 11. Joule I Program, DGXII, CEE. Ellis, A. J. and Mahon, W. A. J. (1977) Chemistry and geothermal UTAD (Universidade de T&-os-Montes e Alto Douro) internal report. systems. Energy Science and Engineering: resources, technology, management. An International Series. Academic Press, 392 pp. Schermerhom, L. J. G. (1956) The age of the Beira Schists (Portugal). Boletim da Sociedade Geolbgica de Portugal, XII, (1 /2), 77 - 100. Epstein, S. and Mayeda, T. (1953) Variation of "0 content of waters from natural sources. Geochimica et Cosmochimica Acta, 4,213-224. Sousa Oliveira, A. and Portugal Ferreira, M. P. (1995) Controlo estrutural das emerghcias hidrominerais da reg30 de Pedras Salgadas (Vila Friedman, I. (1953) Deuterium content of natural waters and other Pouca de Aguiar - Norte de Portugal). Universidade do Porto, Faculdade de substances. Geochimicn et Cosmochimicn Acto, 4 ,89-103. Cihcias Museu e Laboratcjrio Mineralcjgico e Geologico. Merndria. 4,485 - 489.

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