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2 Geothermal Resources Council TRANSACTIONS, VOL 9 - PART I, August 1985 DEEP STRUCTURE,AGE AND EVOLUTION OF THE LARDERELLO-TPAVALE GE0THEIL"IAL FIELD Fausto Batini"), Giovanni Bertini"), Giovanni Gianelli"! Enrico Pandeli''), Mariano Puxeddu (2) and Igor M.Villa(3) (l)enel, Unit2 Nazionale Geotermica,P.zza Bartolo da Sassoferrato 14, Pisa, Italy (2)Istituto Internazionale per le Ricerche Geotermiche, CNR, Via del Buongusto 1, Pisa, Italy (3)Istituto di Geocronologia e Geochimica Isotopica, CNR, Via C.Maffi no.36, Pisa, Italy ABSTRACT A main seismic reflecting horizon (K), occurring in the depth range 3-6 km of the Larderello- Travale geothermal field, probably consists of fractured levels, sometimes filled with hot fluids and/or authigenic minerals present at the contact between granites and the overlying basement, which is seen in drill cores to have undergone a Hercynian metamorphism followed by a late Hercynian thermal event. After a polyphased Alpine metamorphism, a new thermal event then gave origin to the geothermal field and produced corundum and chiastolite in micaschist, leucogranitic dikelets and the resetting of metamorphic biotite ages to values in the range Ma. After the climax of the thermal event (3.5 Ma ago), the geothermal field underwent a monotonic cooling,with cooling rates in the range 16"-40"C/fIa, typical of a batholith. The different thermal regime in different wells easily explains the variations in the apparent ages of biotite, which is still partly open to 40Ar and 87Sr volume diffusion. INTRODUCTION The productive horizons of the Larderello geothermal field are mainly located in shallow permeable levels represented by Triassic dolostone and anhydrite of the Tuscan Nappe and locally by the uppermost part of r2 metamorphic basement lying below allochthonous flysch sequences (Fig.1). A gradual decrease in productivity, observed during the last tens of years, emphasized the compelling need to find new productive horizons in the peripheral areas of the field and in deeper levels of the metamorphic basement. For this purpose the Italian National Electricity Agency (ENEL) carried out a deep exploration program of deep wells and geophysical and geological studies. The results obtained so far are discussed briefly in this paper. Figure 1. Geological sketch map of the Larderello area with trace of seismic reflection profiles LAR 5 and LAR 12. 1) Neogene sediments: Upper Miocene-Middle Pliocene. 2) Flysch nappe: Lower Cretaceous. 3) Flysch nappe: Upper Cretaceous. 4) Flysch nappe: Paleocene-Upper Eocene. 5)Tuscan Nappe: Upper Trias-Lower Miocene. 6) Main overthrust. 7) Fault. 8) Seismic profile. Wells: SS = Sperimentale Serrazzano; VC= VC 11; SA = Sasso 22; SP = San Pompeo 2. GEOLOGICAL OUTLINE The geological studies based on deep drilling data revealed the presence of a complex of tectonic slices below Neogene sediments (Mio-Pliocene), flysch nappes (Lower Cretaceous-Upper Eocene) and the Tuscan Nappe (Upper Trias-Lower Miocene). This complex was formed by the Alpine orogeny that affected the uppermost part of the basement and its 253

3 Batini, et al. Piesozoic cover (Gianelli et al., 1978; Batini et al., 1983). This tectonic setting can be recognized throughout the Larderello-Travale geothermal field. The tectonic slices include basal levels of the Triassic evaporitic series (anhydrite with alternating chaotic breccia of dolomicrite and cnlcareous dolostone), terrigenous (Verrucano) and evaporitic formations of the Upper-Middle Trias and the Upper-Middle Paleozoic units of the metamorphic basement. This complex is overlain by klippes of the Tuscan Nappe (Upper Trias to Lower Miocene), sometimes thinned or replaced during the overthrusting of flysch nappes. A tectonic surface divides the complex of slices from an underlying metamorphic basement that includes the Boccheggiano Formation, the Buti, Filladi Inferiori and the almandine-bearing Micaschist Groups and other amphibolite and gneiss; the age of these units is Middle Paleozoic to Proterozoic (?) (Puxeddu et al., 1984). GEOPHYSICAL DATA Seismic reflection profiles (Batini et al., 1985, this volume) and relative geological sections are shown in Figs.2,3. Clear reflecting horizons were found inside the basement at different levels. The most evident are those termed 'K', associated with strong amplitude and frequency anomalies due to a high reflection coefficient. Figure 4 reports the isodepth contour lines of the K horizon, whose structures are characterized by "E-SSW trending structural highs. The K depth shows a minimum (2800 m b.s.1.) in correspondence to well San Pompeo 2, increasing northwards and especially eastwards ( m b.s.1.) in the Travale area. It is cut by subvertical discontinuities that sometimes also affect the overlying formations as far as the surface. The metamorphic units of the Tuscan basement are made up of rocks showing only slight differences of acoustic impedance, inadequate to explain the occurrence of a K horizon with a high reflection coefficient. Only mafic rocks could produce reflections with comparable intensity, but their presence can be excluded, according to a recent review of gravity data (OGS, 1983) and to seismic refraction data (Giese et al., 1980). The only reliable explanation for the observed reflections is a change in physical properties in metamorphic and/or intrusive rocks of quite similar seismic behaviour. This change can be ascribed to a different degree of fracturation and to the possible presence of fluids and hydrothermal minerals inside the fractures. This would generate a medium with a very low acoustic impedance compared to that of normal surrounding rocks, and would produce reflections of the 'bright spot' type. In the southern part of the Larderello field the K marker separates into different branches (see Figs.2,3). These seismic markers have been cored in San Pompeo 2 well (H and KI). The former corresponds to a highly fractured level rich in hydrothermal minerals up to a complete replacement of the original rock(epidosite after phyllite) at 2200 m depth. The latter (KI) is a strongly fractured micaschist horizon full of high-temperature, high-pressure fluids and rich in hydrothermal and contact metamorphic mineral assemblages. Analysis of a gas sample collected inmediately after one of the blow-outs revealed anomalous concentrations. The pressure of the fluid CH~ encountered in the fractured horizon at 2930 m was estimated at more than 240 bar (Cappetti et al., this volume). PETROLOGIC AND RADIOMETRIC DATA The presence of mineral assemblages such as 1) andalusite, cordierite, biotite, quartz f muscovite f chlorite and 2) cordierite, biotite, K-feldspar, sillimanite f andalusite indicates that a thermal event metamorphosed the Larderello basement rocks up to high grade. This event was late Hercynian ( Ma: Rb/Sr age on muscovite (Del Mor0 et al., 1982)). During the Alpine orogeny, a retrogressive metamorphism produced chlorite from biotite, albite from labradorite and tremolite-actinolite from horneblende. A new thermal event then gave origin to the geothermal field and to a contact aureole in the basement: post-tectonic biotite crystals with triple joints were formed after Hercynian kinked and chloritized biotite (Del Mor0 et a1.,1982); the appearance of andalusite, blue corundum and K-feldspar (Puxeddu, 1984) in the micaschist SP m (well San Pompeo 2) testifies to the attainment of about 610"-620 C for P = 1 Kb (Chatterjee and Johannes, 1974). The temperature in San Pompeo is now 394'C at 2560 m depth, which demonstrates that the Larderello field is cooling. The very high temperature reached in the Larderello basement suggests the presence of an underlying hot and still cooling intrusive body, acting as the heat source of the geothermal field. Recent confirmation came from the discovery of granitic-aplitic dikelets, some cm thick, cutting gneiss in well VC 11 at 2946 m (Batini et al., 1983). Radiometric ages were obtained on several well samples, part of which was published in 1982 (Del Mor0 et al., 1982). New data allow a better de-

4 0 I Urn m sa=lo 22 - N Batini, et al. SW LAR 5 NE 500 Sl Figure 2. Seismic profile (LAR 5) and geological interpretation. For symbols see Fig

5 Batini, et al. '1 \ as -c I LO - 2, -ej S S.POMPE0 2 I LAG0 8 LAR. 12 vc 11 COLLlNE 3 I No148 l N S.l krn Figure 3. Seismic profile (LAR 12) and geological interpretation. 1) Neogene sediments. 2) Flysch nappes. 3) Tuscan Nappe. 4) Tectonic slice complex. 5) Filladi Inferiori Group. 6) Micaschist Group. 7) Gneiss and amphibolite. 6) K-seismic reflecting horizon. 256

6 Batini, et 31. TRAVALE 3OO0- Temperature "C Depth contour in m Fault 0 5 km Figure 4. Isodepth contour lines of the K horizon and temperature distribution at 3000 m. Datum plane: 200 m a.s.1. finition of a consistent pattern of apparent age variation with well temperature. Since isotherms are difficult to constrain, a helpful observation is that there is a qualitative correlation between the temperature distribution and the isodepth contour lines of the K horizon (Fig.4). The best correlation is observed near the top of the K horizon, which is also the thermal top (e.g., in SP 3000, T is about C higher than in SA 3000). The well furthest from the thermal top, Sasso 22 (SA) is expected to yield the highest apparent K/Ar and Rb/Sr mineral ages, because the thermally-induced diffusive loss of radiogenic daughter isotopes is smallest (see discussion in Del Moro et al., 1982). Indeed, seven SA biotite Rb/Sr and K/Ar ages are concordant and range between 3.0 and 3.5 Ma. One biotite from well SS (Sperimentale Serrazzano) has an apparent K/Ar age of 2.5 Ma. An unreported K/Ar measurement (see Table 1) on a biotite from the bottom (2962 m) of well SP, at the thermal top, yields 1.6 Ka. This young age is obviously due to the high temperatures undergone by this sample during the last 3.5 Ma: the present-day T is > 397OC, whereas Tma, exceeded 61OoC. The coherent age pattern is consistent with the emplacement of an intrusive body at some depth (see below) approximately 3.5 Ma ago and subsequent monotonic cooling; the wells cooled differently and therefore record different cooling ages. Further evidence of the 3.5 Ma age is provided by an unreported K/Ar measurement in the VC 11 well. An aplitic dikelet cuts the gneiss: biotite was separated from the gneiss within 3 cm of the dikelet. There is little doubt that biotite was totally re- 257

7 ~~ ~ ~~ Batini, et al. Table 1. K/Ar ages in some minerals from the Larderello field Well Depth llineral T K 40Ar 40Ar Age f 1 m b.g.1. ("C) (2) (10-7mi/g) (XI (Ma) vc 2946 biotite f 0.03 SP 2962 biotite f SP 2962 tourmaline f 0.20 set by the dikelet, Its slightly younger age, 2.9 Ma, is more than reliable for emplacement of an aplitic differentiate. The main body is definitely older, i.e. '3.5 Ma. Finally, a hydrothermal tourmaline from SP yielded an age of 1.3 Ma, which may be the age of the hydrothermal event; however, as little is known of the behaviour of Ar in tourmalines, this preliminary datum requires further confirmation. DISCUSSION The intrusion of a magmatic body in shallow levels of the crust produced intense fracturation of the wall rocks through different processes. First, the upheaval of the region, due to the ascent of granitic melts (see Marinelli, 1969) and the swelling of the underlying mantle, generated tensile stresses and, hence, fracturing of the overlying rocks. Recent movements along active faults, evidenced by a continuous seismic activity in the Larderello-Travale region (Batini et al., 1984), then repeatedly opened new fissures and compensated for the decrease in permeability caused by self-sealing. The circulation of fluids characterized by high temperatures and pressures caused hydraulic fracturing in the wall rocks (Kissingler, 1976; Batini et al., 1984). Retrograde boiling also produced an expansion of the intrusive body in an external rind partially solidified and water saturated. This volume increase resulted in stockworks and strongly shattered rocks in the roof of the magma chamber and in the adjacent wall rocks (Burnham and Ohmoto, 1980). The result of all these processes is the presence of intensely fractured rocks to a thickness of some hundreds of metres, just above and within the top of the Alpine batholiths. This band is rich in still open fractures, which are either empty or filled with hot fluids (see San Pompeo 2 well), rich in dikes of granite and related differentiates, in contact metamorphic assemblages and in anastomosed veins of hydrothermal minerals, sometimes capable of completely replacing the original rock. As a consequence, the physical parameters (density and seismic wave velocity) must greatly differ from those of granite and metamorphic rocks with few or no fractures and secondary minerals. The presence of an acidic intrusion, at least below the San Pompeo area, is suggested by the domelike structure of the K horizon, by a negative gravity anomaly and by the maximum concentration of microseismic events whose hypocentres never fall below 8-10 km (Batini et al., 1984). This last observation indicates a ductile behaviour of the rocks below 8-10 km, owing to high temperatures (600'-700 C). The presence of an intergranular melt is very probable. Such a hypothesis is in agreement with the increase in Poisson's coefficient and the lack of S-waves observed by Nicolich and Pellis (1979) in a seismic refraction profile cutting the geothermal field. REFERENCES Batini, F., Bertini, G., Gianelli, G., Pandeli, E., and Puxeddu, M., Deep structure of the Larderello field: contribution from recent geophysical and geological data. Mem.Soc. Geol.It., 25, pp Batini, F., Console, R. and Luongo, G., Seismologicalstudy of Larderello-Travale geothermal area. Seminar on Utilization of Geothermal Energy for Electric Power Production and Space Heating. BHRA, Florence, May 1984, in press. Batini, F., Duprat, A. and Nicolich, R., Contribution of seismic reflection for the study of geothermal reservoirs in Tuscany (Italy). This volume. Burnham, C.W. and Ohmoto, H., Late-stage processes of felsic magmatism. Mining Geology, Spec.issue 8, pp Chatterjee, N.D. and Johannes,W., Thermal stability and standard thermodynamic properties of synthetic 2M1-MuscoviteY KA12 AlSi3010 (0H)z. Contrib.Miner.Petro1.,48,pp

8 Batini, et al. Del Moro, A., Puxeddu, M., Radicati di Brozolo,F. and Villa, I.M., Rb-Sr and K-Ar ages on minerals at temperatures of 300"-400 C from deep wells in the Larderello geothermal field (Italy). Contrib.Mineral.Petrol., 81, pp Gianelli, G., Puxeddu, M. and Squarci, P.,1978. Structural setting of the Larderello-Travale geothermal region. Mem.Soc.Geol.It., 19, pp Giese, P., Wigger, P., Fforelli, C. and Nicolich,R., Seismic studies for the determination of the crustal structure in the area of the geothermal anomaly in Tuscany. Advances in European Geothermal Research. Proc. 2nd International Seminar on Results of EC Geothermal Energy Research, Strasbourg, 4-6 March 1980, Reidel, Dordreeht, pp Kissingler,.,1976. A review of the theories of mechanisms of induced seismicity. Engineering Geology, 10, Nicolich, R. and Pellis, G., I1 contributo dei dati geofisici per lo studio delle strutture crostali della provincia geotermica tosco-laziale. 1st Geol. Appl. Mineral., University of Trieste, Contrib. no. 41, pp Osservatorio Geofisico Sperimentale,l983. Unificazione de2 rilievi gravimetrici nell'area geotermica toscana per ENEL EL. Final report no Puxeddu, M., Structure and Late Cenozoic evolution of the upper lithosphere in southwest Tuscany (Italy). Tectonophysics, 101, pp Puxeddu, M., Saup6, F., Dechomets, R., Gianelli, G. and Moine, B., Geochemistry and stratigraphic correlations: application to the investigation of geothermal and mineral resources of Tuscany, Italy. Chem. Geol., 43, in press. Marinelli, G., Some geological data on the geothermal areas of Tuscany. Bull.Volcan., 33, pp