CONTACT METAMORPHISM IN THE LARDERELLO GEOTHERMAL SYSTEM

Transcription

1 CONTACT METAMORPHISM IN THE LARDERELLO GEOTHERMAL SYSTEM Giovanni Gianelli 1 and Giovanni Ruggieri 1 1 Istituto Internazionale Ricerche Geotermiche, Via Alfieri, 1, 56010, Ghezzano (Pisa), Italy Key Words: geothermal, mineralogy, deep drilling, Larderello field ABSTRACT The potential reservoir of the steam-heated Larderello geothermal field consists of various metamorphic rocks and granite. These rocks have been found in the deepest part of the field in wells reaching depths close to 4.5 km. The intrusion of granite into the wall rocks, the circulation of magmatic fluids and their evolution to the present-day conditions of superheated steam involve complex processes which are reconstructed by fluid-inclusion and mineral studies. The results obtained through a study of core samples of granite and contact metamorphic rock indicate that: 1) the granitic rocks are highly differentiated, and their solidus temperature may be as low as 600 C; 2) the thermal metamorphism developed at temperatures of C and pressures of approximately MPa and the fluids permeating the contact aureole evolved from magmatic to meteoric; 3) the chemistry of the early magmatic stage is characterised by the presence of elements such as F, Cl, Li and B, derived from the anatectic magma and therefore of crustal origin; 4) at least part of the He and CO 2, and probably other gases as well, have a deep origin, possibly from the Earth s mantle. These findings depict a complex scenario for the deepest part of the Larderello geothermal field. When deep exploration is successful, the well produces superheated steam. However, projects for deep exploration, in particular those in the vicinity of the major seismic reflector present throughout the geothermal region, should take into consideration the possibility of encountering very hot fluids, with pressures greater than hydrostatic and unusual and undesired chemistry. Moreover, the prospect of finding melted rock (possibly as dykes of differentiated granite) should also be considered. 1. INTRODUCTION In the geographic region hosting the geothermal field of Larderello there is a remarkably good correspondence between the positive heat-flow anomaly, the Bouguer gravity minima, the marked negative magnetic anomaly, the high conductivity of the crust and the maxima of the isodepth contour lines of a bright-spot seismic reflector, known as a K horizon. Moreover, seismic tomography has defined a major lowvelocity body, km in diameter, rooted in the mantle and rising to 3-10 km depth. These findings led to the conclusion that acidic magma and/or fluids derived from magma are present at relatively shallow depth (Gianelli et al., 1997a, Manzella et al., 1998 and references therein). Granite and contact metamorphic rocks with K/Ar and 40 Ar/ 39 Ar ages ranging from 3.8 to 1.6 Ma have been found in fact in the deepest part of the field (Villa and Puxeddu, 1994). This suggests long-standing igneous activity lasting up to recent times and explains the exceptional temperatures measured in some deep wells (above 420 C). Projects for deep drilling (in order to explore the K seismic reflector) and enhanced heat recovery from deep hot-wet rocks at Larderello are currently under assessment (Gianelli et al., 1997b). Therefore, studies of the potential deep reservoirs are crucial in order to understand the characteristics and evolution of the deep fluids, assess the risk of finding magma or dangerous fluids during field operations and properly appraise the technical and financial resources needed for such operations. 2. METHOD OF STUDY Petrographic observations have been carried out in order to distinguish the different metamorphic stages and identify the equilibrium textures among the minerals present. Microprobe analyses have been made in order to better define the pressure and temperature conditions of the contact metamorphism. Available fluid-inclusion data have been reviewed and used, together with the mineral data, to reconstruct the composition of the metamorphic fluid. On the basis of the results, a conceptual model of the fluid s evolution is proposed. 3. IGNEOUS ROCKS So far, the igneous rocks found in the deep wells drilled in the Larderello field are high-al, S-type granite with significant F and B content (Gianelli et al., 1997a; Manzella et al., 1998). On-going research on further samples confirms these chemical features and indicates that differentiates of major acidic intrusions may be present at relatively shallow depths. (Cavarretta and Puxeddu, 1999). It has been estimated that at present melting conditions for some leucogranitic rocks of Larderello may occur at approximately 6 km depth in many parts of the geothermal field, where the geothermal gradient is about 100 C/km (Gianelli et al., 1997a; Manzella et al., 1998). This conclusion is based on the fact that the original load pressure of the shallow granite dyke of Monteverdi 7 (3483 m depth) can be estimated at MPa, considering an uplift rate of approximately 800 m in the last 4 Ma (Del Moro et al., 1982). The occurrence of muscovite in the granite at such low pressures (muscovite is not stable at P<350MPa) in water-saturated granites) can be explained by the presence of B, F and Li in the magma. Since these elements shift the granite solidus towards low T values (see London, 1995, and references therein), the granite solidus could intersect the curve of the muscovite at approximately 600 C. This temperature can be attained at relatively shallow depth, and, in some places, in correspondence to the K reflector. 4. CONTACT METAMORPHIC ROCKS 4.1 Petrography The high-temperature (HT), low-pressure (LP) metamorphism in the Larderello geothermal area affects units of Triassic, Palaeozoic or unknown age (Pandeli et al., 1994, and related bibliography), mostly of pelitic composition (phyllite, micaschist and gneisses). Metagraywake, quartzite, amphibolite and siliceous limestones are also present. The rocks underwent low-grade Alpine regional metamorphism. Relics of a Hercynian metamorphic phase are also present in micaschists and gneisses. The thermo-metamorphism of phyllite and micaschist under HT and LP conditions gave rise to crystallisation of biotite, andalusite and cordierite, which, apart from quartz and muscovite, are the most common mineral assemblages in the core samples of hornfelses. Corundum and K-feldspar (K-Fsp) are present in places. Metagraywakes and albite-schists become plagioclase-bearing hornfelses. Chlorite 1163

2 (Chl) is usually present as a relict mineral, transformed into biotite. However, textural evidence indicates that some chlorite is newly-formed after the hornfels biotite due to retrograde metamorphism. Garnet, chloritoid and kyanite are relict phases from the regional metamorphism. The texture of the hornfelses derived from phyillite and micaschist shows post-tectonic crystallisation of biotite that is mimetic on previous folds and crenulations. The oriented, inequigranular texture of the rocks passes to an equigranular texture, sometimes with triple-point grain boundaries, though not wide-spread throughout. Metasomatism on phyllite, micaschist and gneiss is revealed by the occurrence of veins with biotite, tourmaline, quartz and plagioclase. The thickness of the contact metamorphic aureole has been estimated to be approximately 600 m. 4.2 Metamorphic reactions Under the P-T conditions for the HT-LP metamorphism at Larderello (pressures ranging approximately from MPa, and temperatures from C, but usually C), andalusite (And) plus cordierite (Crd) and cordierite, plus biotite (Phl) (plus quartz, Q, and muscovite, Ms) are the stable metamorphic minerals. This is in agreement with the equilibrium textures observed among these minerals through petrographic analysis. The crystallisation in pelitic rocks of andalusite, cordierite and, at temperatures over 600 C, corundum (Cor) is characteristic of the thermal metamorphism at Larderello. According to Gianelli (1999), the reactions forming these minerals are of the type: 3Chl+5Ms = 5Phl +8 And +Q +12H 2 O (1) Ms+2Q+Chl = Crd+Phl+4 H 2 O (2) 2Chl +8And +11Q = 5Crd + 8 H 2 O (3) Ms = K-Fsp + Cor + H 2 O (4) Rare marbles and calc-silicate hornfelses have been found in a few geothermal wells. Their mineralogy is important to define the P-T-XCO 2 conditions of the contact metamorphism. In the well Selva 4 at 3370 m depth the mineral assemblages are: Calcite+Dolomite+Quartz+Diopside+Forsterite Calcite+Dolomite+Anhydrite+Phlogopite Calcite+Dolomite+Quartz+Plagioclase Carbonate layers within pelitic hornfelses show different mineral assemblages, (e.g. the Sasso 22 well at 1600 m depth), and wollastonite is a widespread contact metamorphic mineral whose presence suggests high temperatures (>500 C) and very low XCO 2 (<0.05). Actinolite is present in some wells. This mineral can crystallise over a large XCO 2 range in silicic dolostone and limestone. On the contrary, the mineral assemblages found in the well Selva 4 at 3177 m depth, with calcite and dolomite coexisting with diopside and forsterite (in the absence of tremolite), indicate intermediate to high XCO 2 (approximately 0.48 to 1.0, at 100 MPa in a temperature range of C). The occurrence of phlogopite in the quartzsaturated part of the core sample also indicates CO 2 -dominated fluids. The retrograde metamorphic re-crystallisation of the limestones and dolostones found in the Larderello contact aureole are characterised by minerals that are stable only under very low XCO 2 conditions, such as prehnite and serpentine minerals. 4.3 Mineral chemistry Biotite from pelitic hornfelses shows Mg/(Mg+Fe) values in the range and variable amounts of F/(F+OH) (Fig. 1). Low F content is found in gneissic rocks probably unaffected by significant contact metamorphism. Higher values are related to F-metasomatism. This is also revealed by the very high F content of the sample Carboli , where biotite occurs associated with tourmaline in veins. Fluorine is of magmatic origin, as shown by the F content of the granite and the relatively high values of F in the magmatic biotites (Fig. 1). The hydrothermal biotites also show a significant F content. Our interpretation is that major F-metasomatism occurred in the Carboli 11 wall rock. The other metamorphic rocks probably maintained their original F content or underwent only minor F-enrichment. It should also be noted that the F values are typical of highly differentiated crustal anatectic melts and associated hydrothermal suites (Brimhall and Crerar, 1987), in conformity with the peraluminous nature of the granite found at Larderello. Cordierite often shows retrograde replacement by a very finegrained white K-mica. When unaltered, the Mg/(Mg+Fe) ratio is approximately Rocks such as the metapelites of Larderello (comparatively rich in Fe) can reach the cordierite isograde at the relatively low temperature of about 500 C through reaction (2) above. Metamorphic feldspars in pelitic hornfelses at Larderello have been studied in detail in the well San Pompeo 2 (Cavarretta et al., 1983). The feldspar geothermometry of two samples indicates a minimum temperature of approximately C for a core sample at 2580 m depth, and C for a sample at 2900 m depth. Analysis of the coexisting K-feldspar and albite in a core sample at 2389 m depth indicates an intermediate structural state. In addition, albite and feldspar coexist with oligoclase, indicating temperatures near 480 C. Amphibolites are characterised by granoblastic textures in which the original oriented fabric has more or less disappeared. Amphibole, plagioclase (An10-85), biotite, quartz and Fe-Tioxides make up these rocks. In the sample Lumiera1b-3110, a green edenite coexists with a colourless grunerite. The presence of the latter and the labradoritic composition of the plagioclase indicate attainment of the amphibolite zone and temperatures possibly in excess of 600 C. Similar conclusions can be drawn for the Capannoli 2b-3170 amphibolite, in particular, on the basis of the high Al(IV) content of the bluegreen pargasite. 4.4 Reconstruction of the metamorphic fluid composition On the basis of the abundance of graphite-bearing phyllite in the metamorphic complexes of the Larderello field, Cathelineau et al. (1994) have suggested redox conditions corresponding to the graphite-oxygen equilibrium for metapelitic rocks. Magro and Ruggieri (1999), in a recent study of the isotopic composition of gases trapped in the fluid inclusions of hydrothermal minerals cored in the geothermal wells, found CO 2 δ 13 C values (from 4.10 to 0.47 ), suggesting a variable contribution of different CO 2 sources: graphite-water reactions in the C-rich metamorphic units and/or mantle degassing for the more negative values; thermometamorphic decarbonation and/or carbonate rocks hydrolysis for the less negative and positive values. The measured 3 He/ 4 He (R) isotopic ratios, normalised to the atmospheric ratio ( Ra = 1.4 x 10-6 ) of the He trapped in the fluid inclusions, is particularly significant. R/Ra ratios are usually >1 and indicate that part of the He has been supplied by the mantle. It is therefore possible that also some of the CO 2 at 1164

3 Larderello originating in the mantle rises to shallow crustal levels, where it mixes with CO 2 of crustal origin (decarbonation, graphite-water reaction, carbonate rocks hydrolysis). Redox conditions are better defined by analysing the CO 2 /CH 4 ratio of the fluid inclusions which trapped fluids present during the contact metamorphism (Tab.I). Let us consider the following equilibrium reactions: CH 4 +2H 2 O=4H 2 + CO 2 2H 2 O=2H 2 +O2. and The first reaction defines the equilibrium between the main gas components in the COH system. We can compute the H 2 fugacity using the mole fraction in Table I and the activity coefficients χ of the gas phase, considering that x i P tot =f i /χ i for the i-th gas component (P tot = total pressure, f i = fugacity and x i = mole fraction). We account for both ideal mixing and non-ideal mixing in a saline solution. The activity coefficients of H 2 O, CO 2, CH 4 and H 2 have been computed with a MRK equation of state (Holloway, 1981; Labotka, 1991) for ideal H 2 O-CO 2 mixing. For non-ideal mixing and saline solutions, the fugacity coefficients of CO 2 and H 2 O computed by Bowers and Helgeson (1983) have been used. The values of H 2 O fugacity is controlled by the buffering reactions (1) to (4) above. The redox conditions (computed via water s dissociation into hydrogen and oxygen) are characterised by the log(fo 2 ) values corresponding to the QFM buffers for the VC11 and MV7 samples and result to be about 1.5 orders of magnitude lower than the QFM buffers for Sasso 22 and SP2. For the latter two samples, the oxygen fugacity value is consistent with an equilibrium with graphite, controlled by the reaction: 2C+2H 2 O,g = CH 4,g + CO 2,g (5) Pyrite (py) and pyrrhotite (po) are sometimes found in association in the hornfelses, the second being common in the graphite-bearing schist of the SP2 well. At the py-po equilibrium, the fh 2 /fh 2 S ratio should range from approximately 0.17 to 0.02, respectively at 450 and 650 C, and at a pressure of 100 MPa. The high XH 2 /XH 2 S ratios for the samples MV7, Sasso 22 and SP2 indicate that the po, rather than the py phase should be the stable one at the temperature assumed for contact metamorphism. 5. CONCLUSIONS The P-T conditions for contact and hydrothermal metamorphism at Larderello are reported in figure 2. It is possible to distinguish two fields. 1) The high-pressure field is that of the early phase, characterised by saline fluids with magmatic connotations. Part of this field is contained between the two solidus curves, namely that for a water saturated granite and that for a saturated granitoid rich in volatiles, such as F, B and Li. The shift in the granite solidus due to the presence of these volatiles has been estimated by Manzella et al. (1998) on the basis of existing experimental data (London, 1995) and the fluid composition assumed for the Larderello granite. Carbon dioxide is another important component of the fluid phase. Part of this gas (as well as the He) may derive from an external source (Magro and Ruggieri, 1999). We have no evidence of a deep origin for the other gases, such as methane, which may originate in the contact metamorphic aureole through the gas equilibrium reaction with H 2 O, H 2 and CO 2. Temperature values are quite high and indicate that the wall rock is near the brittle-ductile rheologic transition. Fournier (1991) assumes a value of 450 C as the temperature at which the potential reservoir rocks of active geothermal systems exhibit a quasi-plastic response to deformation. Brittle deformation, enhanced by fluidoverpressure processes and low strain rates, is supported by evidence in the Larderello geothermal field of vein minerals formed at temperatures close to 500 C (Gianelli, 1994). A temperature of C at a depth of km probably defines a transition zone at Larderello, where the rocks pass from plastic to brittle mechanical behaviour when sporadic violent explosions associated with the expansion of magmatic gases occur at this depth. The overpresssure may derive from the tremendous volume increases of a fluid phase when magma crystallises (the so-called retrograde boiling, see Burnham, 1979). Recurrent processes of: a) fracturing of an impervious, plastic wall rock; b) fluid circulation of fluids with magmatic connotations and mixing with meteoric fluids, and c) closure of the pore spaces and return to plastic, impervious conditions, have been active since 4 Ma ago, and continue today, most likely at low to moderate rates. 2) The low pressure field in figure 2 corresponds to the recent hydrothermal fluid circulation. The P-T and salinity conditions of the fluids are generally characteristic of liquid or two-phase fluids. Mixing and boiling processes have also been demonstrated by fluid inclusion studies (Ruggieri and Gianelli, 1999; Ruggieri et al., 1999). The fluids assume a meteoric imprint because of the repeated circulation of meteoric waters at depth. The fluid composition generally ranged from moderate to low salinity and was probably richer in gas than the present-day geothermal fluid. The formation of a gas cap (largely CO 2, with minor CH 4 ) was enhanced by the impervious geologic units present on top of the reservoir rocks. The proposed conceptual model depicts the evolution of a contact metamorphic aureole within wall rocks around relatively shallow granite intrusions. The system is still active today, and the occurrence of deep fluids is indicated by geophysical data, in particular, the presence of the major, bright-spot type reflecting horizon at 4-6 km depth and the spontaneous blow out of a deep well which encountered fluid at more than 420 C at 2900 m depth. Larderello provides a unique possibility to directly explore a plutonic-metamorphic complex. The possibility of encountering magma should therefore not be ruled out when drilling is to be performed at depths approaching the K seismic reflector. ACKNOWLEDGEMENTS The author is indebted to Lorenzo Gori for the figure preparation. REFERENCES Belkin, H., De Vivo, B., Gianelli, G. and Lattanzi, P. (1985) Fluid inclusions in minerals from the geothermal fields of Tuscany, Italy. Geothermics, Vol. 14, Bowers, T.S. and Helgeson, H.C. (1983). Calculation of the thermodynamic and geochemical consequences of nonideal mixing in the system H 2 O-CO 2 -NaCl on phase relations in geological systems: Equation of state for H 2 O-CO 2 -NaCL 1165

4 fluids at high pressures and temperatures. Geochimica et Cosmochimica Acta, Vol. 47, pp Brimhall, G.H. and Crerar, D.A. (1987). Ore fluids: magmatic to supergene. In: I.S.E. Carmichel and H.P. Eugester (Eds): Thermodynamic modeling of geological materials: minerals, fluids and melts, Reviews in Mineralogy Vol. 17, ed. Mineralogical Society of America, pp Burnham, C. W. (1979) Magmas and hydrothermal fluids. In: Geochemistry of hydrothermal ore deposits, H.L. Barnes (Ed), ed. Jhon Wilet & Sons, New York, pp Cathelineau, M., Marignac, C., Boiron, M.C., Gianelli, G. and Puxeddu, M. (1994). Evidence for Li-rich brines and early magmatic fluid-rock interaction in the Larderello geothermal system. Geochim. Cosmochim. Acta, Vol. 58, pp Cavarretta, G. and Puxeddu, M. (1999). Two-mica F-Li-Brich monzonite apophysis of the Larderello batholith cored from 3.5 km depth. In preparation. Cavarretta, G., Gianelli, G. and Puxeddu, M. (1993). Hydrothermal and contact metamorphism in the Larderello geothermal field (Italy): a new contribution from San Pompeo 2 deep well. In: 4 th International Symposium on Water-Rock Interaction, August 29-September 3, 1883, Misasa, Japan, Del Moro, A., Puxeddu, M., Radicati di Brozolo, F. and Villa, I. (1982). Rb-Sr and K-Ar ages of minerals at temperatures of C from deep wells in the Larderello geothermal field (Italy). Contrib. Mineral. Petrol., Vol. 81, pp Fournier, R.O. (1991). The transition from hydrostatic to greater than hydrostatic fluid pressure in presently active continental hydrothermal systems in crystalline rocks. Geophysical. Res. Letters., Vol. 18, pp Gianelli, G. (1994). Brittle-ductile transitionin geothermal systems: a contribution from the Tuscan geothermal fields. Memorie Società Geologica Italiana, Vol. 48, pp Gianelli, G., Manzella, A. and Puxeddu, M. (1997a). Crustal models of the geothermal areas of southern Tuscany (Italy). Tectonophysiscs, Vol. 281, pp Gianelli, G. (1999). Condizioni di pressione e temperatura del termometamorfismo nel sistema geotermico di Larderello. Atti Società toscana di scienze naturali, in press. Gianelli, G., Squarci, P., Capocecera, P., Baldi, P., Cappetti, G. and Console, R. (1997b). Secondary heat recovery from low-permeability high-temperature reservoir: a possible project in the Larderello field, Italy. Geothermal Resource Council Transactions.., Vol.21, pp Holloway, J.R (1981). Composition and volumes of supercritical fluids in the Earth s crust. In: L.S. Hollister and M.L. Crawford (Eds.): Fluid inclusions: applications to petrology. Mineral. Association of Canada, Calgary, pp Labotka, T.C. (1991) Chemical and physical properties of fluids. In: D.M. Kerrick (Ed.): Contact metamorphism, Reviews in Mineralogy, Vol. 26, pp London, D. (1995). Chemical features of peraluminous garnites, pegmatites, and rhyolites as sources of lithophile metal deposits. In: Magmas, fluids and ore deposits, J. F. K. Thompson (Ed)., Vol.23rd ed. Mineralogical Association of Canada, Victoria, British Columbia, pp Manzella, A., Ruggieri, G., Gianelli, G. and Puxeddu, M., Plutonic-geothermal systems of Southern Tuscany: a review of the crustal models. Memorie Società geologica italiana, Vol. 52, pp Magro, G. and Ruggieri, G. (1999). Noble gases and carbon isotopic composition in fluid inclusions as indicators of the sources of hydrothermal fluids circulation in the crust In: Congresso Federazione Italiana Scienze della Terra, Bellaria, 1999, in press. Pandeli, E., Gianelli, G., Puxeddu, M. and Elter, F.M. (1994). The Paleozoic Basement of the Northern Apennines: stratigraphy, tectono-metamorphic evolution and alpine hydrothermal processes. Memorie Società geologica italiana, Vol. 48, pp Ruggieri, G., Cathelineau, M., Boiron, M.Ch. and Marignac, Ch. (1999). Boiling and fluid mixing in the chlorite zone of the Larderello geothermal system. Chem. Geol., Vol. 154, pp Ruggieri, G. and Gianelli G. (1999) Multi-stage fluid circulation in a hydraulic fracture breccia of the Larderello geothermal field (Italy). J. Volcanol. Geotherm. Res., Vol. 90, Ruggieri, G. and Gianelli, G. (1995) Fluid inclusion data from the Carboli 11 well, Larderello geothermal field, Italy. In: Proceedings of the World Geothermal Congress, Florence, May 1995, Vol. 2, Valori, A., Cathelineau, M. and Marignac, Ch. (1992) Early fluid migration in a deep part of the Larderello geothermal field:a fluid inclusion study of the granite sill from well Monteverdi. J. Volcanol. Geotherm. Res., Vol. 51, Villa, I. and Puxeddu, M. (1994). Geochronology of the Larderello geothermal field: new data and the closure temperature issue. Contrib. Mineral. Petrol., Vol. 315, pp Table I. Computed global COHNS fluid composition at 100 MPa. On the basis of Raman fluid inclusion analysis, Cathelineau et al. (1994). Sample T( C) XH 2 O XCO 2 XCH4 XH 2 S XN 2 XNaCl XH 2 Log fo 2 VC nd MV S < SP <

5 0.70 Metamorphic rocks Mg/Mg+Fe Granites Hydrothermal vein F/F+OH Carboli C bis 4304 Carboli C bis Castel di Pietra Radicondoli 26b Radicondoli 30b Sughere Monteverdi Carboli VC Sasso Sasso Sasso San Pompeo San Pompeo Padule Serrazzano Colline 5 Radicondoli 30b Bruciano Badia 1b Canneto 4a Fig. 1. Mg/Mg+Fe vs. F/F+OH diagram for biotites present in different rocks. Geothermal wells of the Larderello field. 1167

6 Fig. 2. Pressure and temperature conditions present during different stages in the geothermal system of Larderello. Dashed fields are established on the basis of fluid inclusion data (Belkin et al, 1985; Valori et al., 1992; Cathelineau et al., 1994; Ruggieri and Gianelli, 1995; 1999; Ruggieri et al., 1999). The high temperature, high pressure conditions developed during an early stage, characterised by magmatic and metamorphic fluids. Within rocks of pelitic compositions, cordierite, biotite and corundum are mineral markers of this water-rock interaction stage. The P-T conditions assumed for the contact metamorphism in the marbles of the Selva 4 well are also reported. Lower temperatures and pressures characterise the hydrothermal circulation of metamorphic fluids of prevalently meteoric origin. Present day conditions are steam-dominated. 1168