Geosciences Journal Vol. 14, No. 4, p. 423 − 442, December 2010 DOI 10.1007/s12303-010-0034-0 ⓒ The Association of Korean Geoscience Societies and Springer 2010 The role of geosciences in the assessment of low-temperature geothermal resources (N-Portugal): a review ABSTRACT: The aim of this paper is to review the results of the assessment of low-temperature geothermal resources (issue tem- peratures between 41 and 77 °C) that occur in the Portuguese mainland. For this purpose, a multidisciplinary approach, includ- ing geologic, tectonic, geochemical, geophysical and isotopic (δ 2 H, δ 18 O, δ 13 C, 3 H and 14 C) techniques, was applied in order to update local and/or regional conceptual circulation models. Three case studies of N-Portugal are presented and discussed. This paper describes different low-temperature geothermal waters presenting similar hydrogeological conceptual models but rather different geochemical signatures (e.g., HCO 3 –Na with pH ≈ 8, HCO 3 /Na/ CO 2 -rich with pH ≈ 7 and HCO 3 –Na with pH ≈ 9, type waters). In fact, in the studied low-temperature geothermal systems, local/ regional high altitude sites associated with highly fractured rocks play an important role in conducting the infiltrated meteoric waters towards the discharge zones near the Spas. The discharge zones are mainly related to the intersection of major regional fault lineaments (and conjugate structures), responsible for creating the mineral waters ascent. In some cases, geochemical and isotopic data point out to the existence of anthropogenic contamination of some geothermal spring waters related to the intense use of fer- tilizers in areas of widespread agricultural practices. Key words: geothermal waters, geochemistry, isotopes, geophysics, N-Portugal 1. INTRODUCTION Surface manifestations of geothermal fluid circulation are a subject of great scientific and economic interest. Explora- tion data (e.g., geotectonical, hydrogeological, geochemical, isotopic, and geophysical) should be used to build up a “pic- ture” of a given low-temperature geothermal system, ena- bling important decisions to be addressed in order to develop future deep drilling operations (Dickson and Fanelli, 2003). In Figure 1, one can observe the distribution of the geother- mal heat flow density (mW m −2 ) in Europe (Haenel and Hurter, 2002). The thermal models for the region indicate a mean heat flow value of 95 mW m −2 , derived from borehole measurements (Duque et al., 1998). Concerning fracture and structural main features several authors (e.g., Arthaud and Matte, 1975; Ribeiro et al., 2007) outline three main Late-Variscan strike-slip fault systems in the northern sector of Iberia: the dominant NE-NNE (always sinistral), and the subordinate and conjugate NW-NNW (Late-Variscan dextral) and the E-ENE (mainly sinistral). The geometry and kinematics related to the N-S maximum compressive stress field was responsible for the develop- ment of the process of active deep-crustal shortening and related main geomorphic patterns in Iberia (Vicente and Vegas, 2009). The multidisciplinary approach has provided important information to answer the most usual addressed questions associated with low-temperature geothermal systems, namely: i) what is the origin of the waters? ii) where are the recharge areas located? iii) which are the main water-rock interaction processes occurring at depth? iv) are the low-temperature geothermal waters the result of mixing processes in the system?. Several studies carried out in the northern part of the Por- tuguese mainland (Fig. 2) have provided a comprehensive José M. Marques* P.M. Carreira J.E. Marques H.I. Chaminé P.E. Fonseca F.A. Monteiro Santos H.G.M. Eggenkamp J. Teixeira Technical University of Lisbon (IST), Centre of Petrology and Geochemistry (CEPGIST). Av. Rovisco Pais, Lisbon 1049-001, Portugal Nuclear and Technological Institute (ITN), Estrada Nacional nº 10, Sacavém 2686-953, Portugal Centre and Department of Geology, Faculty of Sciences, University of Porto, Rua do Campo Alegre, 687, Porto 4169-007, Portugal Laboratory of Cartography and Applied Geology, Institute of Engineering of Porto, ISEP, Rua Dr. A. Bernardino de Almeida, 431, Porto 4200-072, Portugal Centre GeoBioTec, University of Aveiro, Aveiro 3810-193, Portugal Centre and Department of Geology, Faculty of Sciences, University of Lisbon, Ed. C6, Campo Grande, Lisbon 1749-016, Portugal Centre of Geophysics (IDL), University of Lisbon, Campo Grande, Ed. C8, Lisbon 1749-016, Portugal Technical University of Lisbon (IST), Centre of Petrology and Geochemistry (CEPGIST). Av. Rovisco Pais, Lisbon 1049-001, Portugal Laboratory of Cartography and Applied Geology, Institute of Engineering of Porto, ISEP, Rua Dr. A. Bernardino de Almeida, 431, Porto 4200-072, Portugal Centre GeoBioTec, University of Aveiro, Aveiro 3810-193, Portugal *Corresponding author: jose.marques@ist.utl.pt