Assessment of Chaves Low-Temperature C[O.sub.2]-Rich Geothermal System Using an Interdisciplinary Geosciences Approach

• Published in: Geofluids Vol. 2019
• Main Authors: Marques, J.M, Carreira, P.M, Aires-Barros, L.A, Santos, F.A. Monteiro, da Silva, M. Antunes, Represas, P
• Format: Journal Article
• Language: English
• Published: John Wiley & Sons, Inc 31.03.2019
• Subjects: Analysis; Case studies; Earth; Geomorphology; Mantle; Tectonics (Geology); New Zealand; New York; France; Portugal; Spain
• ISSN: 1468-8115
• DOI: 10.1155/2019/1379093

This paper reviews the results of a multi- and interdisciplinary approach, including geological, geomorphological, tectonic, geochemical, isotopic, and geophysical studies, on the assessment of a Chaves low-temperature (77[degrees]C) C[O.sub.2]-rich geothermal system, occurring in the northern part of the Portuguese mainland. This low-temperature geothermal system is ascribed to an important NNE-trending fault, and the geomorphology is dominated by the "Chaves Depression," a graben whose axis is oriented NNE-SSW. The study region is situated in the tectonic unit of the Middle Galicia/Tras-os-Montes subzone of the Central Iberian Zone of the Hesperic Massif comprising mainly Variscan granites and Paleozoic metasediments. Chaves low-temperature C[O.sub.2]-rich geothermal waters belong to the Na-HC[O.sub.3]-C[O.sub.2]-rich-type waters, with pH [approximately equal to] 7. Total dissolved solids range between 1600 and 1850 mg/L. Free C[O.sub.2] is of about 500mg/L. The results of Si[O.sub.2] and [K.sup.2]/Mg geothermometers give estimations of reservoir temperature around 120[degrees]C. [[delta].sup.18]O and [[delta].sup.2]H values of Chaves low-temperature C[O.sub.2]-rich geothermal waters indicate a meteoric origin for these waters. No significant [sup.18]O- shift was observed, consistent with the results from the chemical geothermometry. [delta][sup.13][C.sub.CO2] values vary between -7.2 and -5.1%o vs. V-PDB, and C[O.sub.2]/[sup.3]He ratios range from 1*[10.sup.8] to 1*[10.sup.9], indicating a deep (upper mantle) source for the C[O.sub.2]. [sup.3]He/[sup.4]He ratios are of about 0.9 (R/Ra). The Chaves low- temperature C[O.sub.2]-rich geothermal waters present similar [sup.87]Sr/[sup.86]Sr ratios (between 0.728035 and 0.716713) to those of the plagioclases from granitic rocks (between 0.72087 and 0.71261) suggesting that water mineralization is strongly ascribed to Na-plagioclase hydrolysis. Geophysical methods (e.g., resistivity and AMT soundings) detected conductive zones concentrated in the central part of the Chaves graben as a result of temperature combined with the salinity of the Chaves low-temperature C[O.sub.2]-rich geothermal waters in fractured and permeable rock formations. This paper demonstrates the added value of an integrated and multi- and interdisciplinary approach for a given geothermal site characterization, which could be useful for other case studies linking the assessment of low-temperature C[O.sub.2]- rich geothermal waters and cold C[O.sub.2]-rich mineral waters emerging in a same region.