surface waters, through an analysis of the chemistry of the .... Acqua Passante sp. 8. 1.9. 0.15 ..... Table 1 shows analyses of it, sampled at 5 different points with.
Journal of Volcanology and Geothermal Research, 31 (1987) 321-332 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
321
CHEMICAL COMPOSITION OF THERMAL SPRINGS, COLD SPRINGS, STREAMS, AND GAS VENTS IN THE MT. AMIATA GEOTHERMAL REGION (TUSCANY, ITALY) V. D U C H I 1, A.A. M I N I S S A L E 2 a n d F. P R A T I 1 IDipartimento di Scienze della Terra, Universit& di Firenze, Via La Pira 4, 50121 Firenze, Italy 2C.N.R. Centro di Studio per la Mineralogia e la Geochimica dei Sedimenti, c/o Dipartimento di Scienze della Terra, Via La Pira 4, 50121 Firenze, Italy
(Received December 2, 1985; revised and accepted August 15, 1986)
Abstract Duchi, V., Minissale, A.A. and Prati, F., 1987. Chemical composition of thermal springs, streams, and gas vents in the Mt. Amiata geothermal region (Tuscany, Italy). J. Volcanol. Geotherm. Res., 31: 321-332. A geochemical study of thermal and cold springs, stream waters and gas emissions has been carried out in the Mt. Amiata geothermal region. The cold springs and stream waters do not seem to have received significant contribution from hot deep fluids. On the contrary, the thermal springs present complex and not clearly quantifiable interactions with the hot fluids of the main geothermal reservoir. The liquid-dominated systems in the Mt. Amiata area, like most of the high-enthalpy geothermal fields in the world, are characterized by saline, NaC1 fluids. The nature of the reservoir rock (carbonatic and anhydritic), and its widespread occurrence in central Italy, favor a regional circulation of "Ca-sulfate" thermal waters, which discharge from its outcrop areas. Waters of this kind, which have been considered recharge waters of the known geothermal fields, dilute, disperse and react with the deep geothermal fluids in the Mt. Amiata area, preventing the use of the main chemical geothermometers for prospecting purposes. The temperatures obtained from the chemical geothermometers vary widely and are generally cooler than temperatures measured in producing wells. Other thermal anomalies in central Italy, apart from those already known, might be masked by the above-mentioned circulation. A better knowledge of deep-fluid chemistry could contribute to the calibration of specific geothermometers for waters from reservoirs in carbonatic rocks.
Introduction The Bagnore and Piancastagnaio geotherm a l fields (Fig. 1 ) w e r e d i s c o v e r e d in t h e late 1950s in t h e M t . A m i a t a region ( C a l a m a i et al., 1970). P r i o r to e x p l o r a t i o n , t h e s e fields were c h a r a c t e r i z e d b y a gas a n d s t e a m c a p o v e r l y i n g a c o n t i n u o u s liquid p h a s e ( C e l a t i et al., 1975; A t k i n s o n s et al., 1978). T h e m a i n r e s e r v o i r is l o c a t e d in c a l c a r e o u s - a n h y d r i t i c f o r m a t i o n s of
0377-0273/87/$03.50
T r i a s s i c age. T h e s m a l l a m o u n t of c h e m i c a l d a t a a v a i l a b l e s h o w s t h a t t h e fluid d i s c h a r g e d b y p r o d u c i n g wells is saline, w i t h N a + = 1950 p p m , K ÷ = 370 ppm, C1-=3200 ppm, H3BO3=14,000 ppm, SiO2 = 1200 p p m a n d highpco2 ( d a t a a f t e r B e r t i n i et al., 1985). W h i l e t h e fluid c h e m i s t r y in t h e r e s e r v o i r is s a l i n e - a l k a l i n e a n d CO2c h a r g e d , t h e t h e r m a l s p r i n g s of t h e Mt. A m i a t a a r e a d i s c h a r g e w a t e r s of a l k a l i n e - e a r t h sulfate
© 1987 Elsevier Science Publishers B.V.
322 character (Bencini et al., 1977). These contrasting chemical characteristics make classical geothermometers ( Na/K, Na-K-Ca,..etc.) unreliable for prospecting purposes in this area. This study is an attempt to evaluate the interaction between the deep saline fluids and surface waters, through an analysis of the chemistry of the thermal and cold springs, stream waters, and natural gas manifestations, taking into account the structural setting of the reservoir rock, and its hydraulic behavior.
Regional setting and hydrothermal activity The geotectonic setting of central Italy is the result of compressional forces active during the Miocene, which led to the piling of nappes transported northeast and east. At the end of the Miocene, and during the Pliocene, tensional tectonics, with strong vertical movements, led to the development of N W - S E horstgraben structures. The emplacement of magmas, ore-forming mineralization, and a high heat-flow anomaly characterizes this period. The volcanic complex of Mt. Amiata (being 0.18-0.29 m.y. of age, Bigazzi et al., 1981 ), and the two Bagnore and Piancastagnaio geothermal systems, are the "younger" products of this recent intense geodynamic activity. Hydrogeologically, the regional stratigraphic sequence comprises, from top to bottom: (a) a practically impermeable cover of Neogene clastic formations (upper Miocene-Pliocene) and allochthonous flysch "Ligurian" nappes (Cretaceous-Eocene), 3 and 4 respectively in Fig. 1; (b) a reservoir of Mesozoic carbonate (upper part) and anhydritic (lower part) formations, overlaid locally by Cretaceous pelitic and Oligocene arenaceous series (Tuscan nappe: 7, 6 and 5 in Fig. 1 ). Cuttings recovered from below the anhydritic formation present a thick sequence of metapelites, of probable Carboniferous-Devonian ( ? ) age ( Bagnoli et al., 1980). The Mt. Amiata rhyodacitic lavas (2 in Fig. 1 ) are considered to have a very high permea-
bility, and to be connected, via volcanic chimneys and volcano-tectonic faults, to the underlying reservoir rock (Calamai et al., 1970). Many mercury and some antimony mines have been worked in the Mt. Amiata region (Dessau, 1952; Dehm et al., 1983), for more than a hundred years. The mineralization has always assumed to be connected with the regional thermal anomaly. The mineralized horizon which worked as a trap was, for the most part, the carbonate-anhydritic horizon of the Tuscan nappe. A long history of hydrothermal activity in this area is evidenced by the presence of poliphase metamorphic sedimentary xenolites in the Mt. Amiata volcanites (Van Bergen, 1983). The distribution of hydrothermal minerals (albite, calcite, chlorite, quartz, K-feldpar and K-mica) in core drill samples has been described by Bertini et at. (1985). Fluid inclusions in these hydrothermal minerals were discussed by Betkin et al. (1985).
Chemistry Figure 1 shows where samples were taken, from thermal springs, cold springs, stream waters, and natural gas vents, Geothermal fields of Bagnore and Piancastagnaio, SW and SE of Mt. Amiata volcano, are marked by circles. Table 1 shows the analytical results for the main components and some trace elements, Na +, K +, and Li + were determined by A.A. spectrophotometry; Ca 2+ by complexometric titration with calceine as indicator; Mg 2+ by complexometric titration with eriochrome black as indicator; HC03- by titration with HC1 and methyl orange as indicator; C1 by argentometric back titration according to Volhard; SO42- by complexometric back titration of Ba 2 ~ in excess, after passing the samples through cation-removing ion-columns; SiO2 by colorimetry with the molybdate method; H3BO3 by colorimetry with the azomethine-H method; NH4 + by colorimetry with Nessler method; Fby colorimetry with the alizarin-complexone
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Marroneto Orcia Formone Bianco Rondinaia Cassia S. Lucia Bagno Vignoni Bagni S. Filippo Bagni S. Filippo ~ - m i S. Filippo Bagni S. Filippo Bagni S. Filippo Bagni S. Filippo Rigo S. Caaciano B. S. Casciano B. Acqua Passante Acqua Passante Acqua p a ~ a n t e Capannacce Capannacce Saragiolo Bagnore Bagnore Bagnoli Bagnoli Casteldelpiano Paglia Acqua Forte Bagnore 2 Poggio La Bella Poggio La Bella Flora Selvena Miniera Se|vena Puzzole Zancona Bagni S. Filippo Campiglia d'Orcia Fiora 2 Bagni di Saturnia Sarteano
1 2 3 4 5 6 7 8 9a 9b 9c 9d 9e 9f 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
sp rv rv rv rv rv sp tap tap tap tap tap tap tsp rv rv tap sp rv gpl rv sp sp gpl rv tap sp sp rv gpl rv rv sp rv rv tap gpl rv sp rv tsp tap
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T(°C) 6.33 6.97 7.64 6.65 7.65 7.65 7.30 6.91 6.5 6.7 6.66 6.29 6.12 6.14 7.5 6.54 6.7 6.16 5.86 5.99 7.5 7.8 6.4 6.17 7.19 6.35 6.54 5.85 6.46 6.02 6.45 6.91 5.36 6.86 7.34 6,99 5.75 5.6 6.64 6.46 6.3 6.7
pH 0.11 0.78 0.81 1.01 0.50 0.70 0.56 3.10 3.01 2.6 2.3 2.25 2.15 1.76 1.1 0.43 2.02 0.75 0.22 1.6 0.55 1.2 0.11 1.3 0.35 0.12 0.08 0.1 0.69 2.4 0.21 0.39 0.55 0.5 0.55 2.5 1.0 0.72 0.07 0.6 2.7 1.49
cond 137 761 804 982 471 667 546 3900 4063 3088 2747 2708 2648 2001 1102 457 2200 830 193 2108 531 1385 79 1421 380 93 72 96 805 1854 201 383 630 485 546 2775 1178 775 53 600 2910 1867
TDS 0.52 5.1 6.1 11.1 4.5 4.4 5.4 35.0 39.9 28.0 25.1 26.0 23.2 19.0 7.2 4.2 22.0 10.0 2.2 24.0 3.7 1.0 0.5 16.0 3.1 0.54 0.38 0.59 5.0 16.0 1.6 3.4 5.8 3.5 3.5 28.4 13.7 7.3 0.32 5.3 27.8 18.0
Ca 2 + 0.16 3.5 3.3 1.2 1.4 2.5 1.0 17.2 15.0 15.0 13.0 10.1 11.0 8.1 4.3 1.5 7.8 0.62 0.23 0.7 1.7 0.5 0.14 3.5 1.4 0.11 0.18 0.16 1.2 4.1 0.7 0.9 1.4 1.6 2.2 8.8 1.0 1.6 0.08 1.5 9.9 7.1
Mg ~÷ 0.85 2.3 1.1 1.5 0.48 2.0 0.35 3.8 1.2 1.1 1.0 1.0 1.0 0.75 3.9 0.45 2.7 0.2 0.14 0.41 1.5 15.1 0.37 0.22 0.2 0.43 0.3 0.38 1.0 2.7 0.2 0.32 0.23 0.38 0.75 0.67 0.19 1.1 0.23 0.5 3.1 0.31
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K÷ 0.24 6.0 6.3 3.5 1.7 3.7 0.22 36.0 25.0 25.0 21.1 18.0 18.9 12.3 8.6 0.64 24.0 1.9 0.24 0.3 0.06 1.7 0.1 6.6 0.2 0.24 0.04 0.1 5.6 0.03 0.09 0.2 0.93 0.88 1.3 21.8 6.6 2.61 0.01 0.52 29.6 18.0
S042 0.14 1.3 0.47 0.68 0.21 0.67 0.27 1.8 0.41 0.40 0.38 0.47 0.46 0.34 2.0 0.24 2.6 0.15 0.07 0.12 0.32 0.41 0.27 0.06 0.14 0.06 0.13 0.2 0.5 0.35 0.23 0.2 0.26 0.38 0.3 0.32 0.09 0.6 0.2 0.24 1.9 0.36
Cl 1100 120 170 150 120 200 170 500 770 470 600 450 470 380 83 100 330 430 350 850 120 220 920 270 67 1100 830 1050 430 440 52 170 23 110 50 170 270 100 670 230 330 200
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Chemical composition of waters from Mt. Amiata geothermal region, Italy
TABLE 1
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