Fluid Geochemistry of the Acqui Terme-Visone Geothermal Area (Piemonte, Italy)
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Applied Geochemistry 15 (2000) 917±935 www.elsevier.com/locate/apgeochem Fluid geochemistry of the Acqui Terme-Visone geothermal area (Piemonte, Italy) Luigi Marini a,*, Vittorio Bonaria a, Massimo Guidi b, Johannes C. Hunziker c, Giulio Ottonello a, Marino Vetuschi Zuccolini a aDipartimento per lo Studio del Territorio e delle sue Risorse, University of Genova, Corso Europa 26, I-16132 Genova, Italy bIstituto di Geocronologia e Geochimica Isotopica, CNR, Via Cardinal Ma 36, I-56127 Pisa, Italy cUniversite de Lausanne, Institut de MineÂralogie et PeÂtrographie, BFSH-2, CH-1015 Lausanne, Switzerland Received 17 November 1998; accepted 4 August 1999 Editorial handling by H. Armannsson Abstract The main geothermal reservoir of Acqui Terme-Visone hosts Na±Cl waters, which are in chemical equilibrium at 120±1308C with typical hydrothermal minerals including quartz, albite, K-feldspar, illite, chlorite (or smectite), anhydrite, calcite and an unspeci®ed Ca-Al-silicate. In the Acqui Terme-Visone area, these geothermal waters ascend along zones of high vertical permeability and discharge at the surface almost undiluted or mixed with cold, shallow waters. To the SW of Acqui Terme, other ascending geothermal waters, either undiluted or mixed with low-salinity waters, enter relatively shallow secondary reservoirs, where they reequilibrate at 65±708C. Both chemical and isotopic data indicate that bacterial SO4 reduction aects all these waters, especially those discharged by the secondary reservoirs. Therefore, geothermal waters must get in contact with oil, acquiring the relatively oxidized organic substances needed by SO4-reducing bacteria. This oil±water interaction process deserves further investigations, for potential economic implications. 7 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction and geophysical (geoelectric and seismic methods) sur- veys to assess its geothermal potential. In particular, The thermal waters of Acqui Terme and Visone results of geochemical investigations have been have been known and used therapeutically since reported by Dominco et al. (1980) and Bortolami et al. Roman times. This is testi®ed by the remnants of the (1983, 1984). Following these surface exploration monumental aqueduct, which was built by consul Sta- eorts, a deep geothermal well was drilled at the end tilio Tauro during the empire of Augustus (27 B.C.±14 of the 1980s. It was a ®asco and brought about the A.D.) to bring cold water to the spas. end of geothermal activities in the Acqui Terme-Visone In more recent times, the Acqui Terme-Visone area area. However, some shallow wells, which were drilled was investigated by means of geological, geochemical afterwards for domestic uses, encountered thermal waters, sometimes mixed with cold water. After almost 10 a, this paper revisits the isotopic * Corresponding author. Tel.: +39-10-353-8136; fax +39- and chemical characteristics of the waters discharged 10-352-169. at Acqui Terme, through the application of recent E-mail address: [email protected] (L. Marini). geochemical techniques, and formulates an updated 0883-2927/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(99)00094-3 918 L. Marini et al. / Applied Geochemistry 15 (2000) 917±935 conceptual geochemical model. To achieve these A section of the paper discusses the chemical and objectives over 50 samples of thermal and cold water isotopic evidence supporting the occurrence of bac- discharges were collected and analyzed both chemi- terial reduction of SO4 to sul®de, accompanied by oxi- cally and isotopically. The authors hope that these dation of organic substances to carbonate species. The new ®ndings may stimulate a new interest in the thermal waters of Acqui Terme and Visone provide a evaluation and exploitation of the natural resources clear example of this important process, which deserves of the study area. further attention for potential economic implications. Zurich Bern JF PF EL Geneve SCL LRS TL Lyon GL ARL IL Milano Ivrea PF Torino Acqui N Genova Nice Mediterranean sea 050 100 km Quaternary deposits Pennidic Domain Tertiary magmatism a (a) Ossola-Ticino Dome (Bergel intrusion) Neogene series Austroalpine Domain Apennines Helvetic Domain (a) cover rocks Thermal springs a Southern Alps b (b) crystalline basement Fig. 1. Locations of main thermal springs (triangles) in the western Alps (from Perello, 1997). PF PennidicFront; JF GiuraFront; SCL Simplon-CentovalliLine; RSL Rhone-SimplonLine; EL EngadinaLine; TL TonaleLine; ARL Aosta-RanzolaLine; GL GiudicarieLine: L. Marini et al. / Applied Geochemistry 15 (2000) 917±935 919 2. Geological framework waters is locally permitted by conditions of high verti- cal permeability, which are governed by the NW- to W-trending normal and strike±slip faults belonging to As recognized by Perello (1997, and references the transtensive Bagni-Visone fault system (Piana et therein) most thermal springs of the western Alps are al., 1997). located external to the mountain chain and close to two regional lithospheric discontinuities, which have been tectonically active until present: (1) the Pennidic Front, a ductile deformation zone reactivated by brittle shearing during late Neogene, and (2) the Rhone-Sim- 3. Field work, laboratory analyses and data presentation plon shear zone (Fig. 1). The thermal area of Acqui Terme-Visone is the only one located in the inner sec- Sample locations are shown in Fig. 2. Field charac- tor of the western Alps, but in a peculiar situation that teristics are given in Appendix A for the most import- is comparatively close to the Alps±Apennines bound- ant thermal and mineral springs and wells only. ary. In February 1997, 45 water samples were collected In a close-up view, the thermal area of Acqui from dierent sites, comprising springs and shallow Terme-Visone is located in an E±W trending sector of wells. The main thermal manifestations and 5 new sites the Bormida valley, where the mainly marine sedimen- (labelled 46 to 50) were sampled again in June 1997. tary rocks of the Tertiary Piemonte Basin (TPB for Repeated samples are identi®ed by the same codes of the ®rst survey followed by the letter b. short, Lower Oligocene±Lower Pliocene) outcrop. This Outlet temperature, pH, Eh, alkalinity (acidimetric sequence includes the gypsum-bearing evaporites of titration) and sul®de (methylene blue colorimetric Messinian age that are exposed north of Acqui Terme. method) were determined in the ®eld. Raw, ®ltered To the south of Acqui Terme, the transgressing marine (0.45 mm) and ®ltered-acidi®ed (with HCl 1:1) samples sediments of the TPB unconformably overlie the inner were collected and stored in polyethylene bottles, from sector of the Ligurian Alpine edi®ce. The latter largely each sample-site, for the analysis of major dissolved consists of: (1) ophiolites (mainly serpentinites, gabbros species, some minor constituents and the 2H/1H and and ultrama®c rocks) and related marine metasedimen- 18O/16O isotope ratios. Water samples were chemically tary rocks (chie¯y calc-schists and quartz-schists) of analyzed in the laboratory of the Institute of Geochro- the Voltri Group (Chiesa et al., 1975; Capponi et al., nology and Isotope Geochemistry, CNR, Pisa, Italy as 1994) and (2) mainly carbonate rocks of Triassic±Jur- follows: assic age, including Upper Triassic evaporites, as observed in the Sestri-Voltaggio area (Cortesogno and . Li, Na, K, Mg, Ca by atomic absorption spectro- Haccard, 1984). Locally, metamorphic and crystalline photometry and/or atomic emission spectropho- rocks, such as those of the Valosio Massif (paragneiss, tometry, orthogneiss, garnet-mica-schists, amphibolites, silicate- . Cl, SO4,NO3 by ion chromatography, bearing marbles; Cabella et al., 1991), also make up . B, SiO2 by visible spectrophotometry, the Ligurian Alpine edi®ce. F by ionselective electrode. In the absence of direct observations, hints of the The 2H/1H and 18O/16O isotope ratios of 24 selected rocks actually present below the marine sediments of samples were determined at the Institut de Mine ralogie the TPB at Acqui Terme are provided by geophysical et Pe trographie of Lausanne University, Switzerland data and regional geological models. According to by means of a Finnigan MAT 251 mass spectrometer, Cassano et al. (1986), an anomaly of high magnetic which is calibrated with an internal standard. This, in susceptibility, possibly related to buried ophiolites, is turn, is calibrated against SMOW and SLAP inter- present in a wide sector of southern Piemonte includ- national reference materials and GISP intercalibration ing Acqui Terme. Regional geological models indicate material following the recommendation of Coplen that the ultrama®c rocks, serpentinites and metasedi- (1988). Deviation of the intralaboratory INHOUSE ments of the Voltri Group as well as the carbonate standard is 21- for dD and 20.05- for d18O. rocks of Triassic±Jurassic age (including Upper Trias- All the analytical results are given in Table 1, sic evaporites) underlie Acqui Terme (Cassano et al., together with total carbonate and total ionic salinity. 1986; Biella et al., 1988; Piana et al., 1997). The marine Total carbonate (TC) represents the sum of the molal 2 sediments of the TPB, as a whole, represent an concentrations of CO2,aq, HCO3 ,CO3 , and related impermeable sequence, whose thickness is approxi- aqueous complexes and was computed through specia- mately 2±3 km in the study area. Nevertheless this seal tion calculations carried out by means of SOLVEQ is locally inecient and comparatively high ¯uxes of (Reed and Spycher, 1984). These calculations are lar- ascending thermal waters go through it, such as in the gely based on pH and titration (total) alkalinity and Acqui Terme-Visone area. The up¯ow of these thermal take into account the contributions of inorganic acid 920 L. Marini et al. / Applied Geochemistry 15 (2000) 917±935 34 32 anions (such as H3SiO4 ,H2BO3 , etc.), but neglect the The S/ S isotope ratio of dissolved SO4 was deter- contributions of organic acid anions (such as formate, mined in 4 selected samples and of dissolved sul®de in acetate, propanoate, oxalate, etc.).