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This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. 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).