Geological Field Trips

Società Geologica Italiana

2011 Vol. 3 (2.2) ISPRA Istituto Superiore per la Protezione e la Ricerca Ambientale

SERVIZIO GEOLOGICO D’ITALIA Organo Cartografico dello Stato (legge N°68 del 2-2-1960) Dipartimento Difesa del Suolo

ISSN: 2038-4947

Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene - and the Larderello areas 85° Congresso Nazionale della Società Geologica Italiana - Pisa, 2010

DOI: 10.3301/GFT.2011.03 Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri and the Larderello areas A. Brogi - D. Liotta GFT - Geological Field Trips geological fieldtrips2011-3(2.2) Periodico semestrale del Servizio Geologico d’Italia - ISPRA e della Società Geologica Italiana Geol.F.Trips, Vol.3 No.2.2 (2011), 39 pp., 25 figs. (DOI 10.3301/GFT.2011.03)

Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri and the Larderello areas

Andrea Brogi(1), Domenico Liotta(2)

(1) Dipartimento di Scienze della Terra, University of Siena, Via Laterina 8, 53100 Siena. (2) Dipartimento di Geologia e Geofisica, University of Bari, Via Orabona 4, 70125 Bari. Corresponding author e-mail address: [email protected] - [email protected]

Responsible Director Editorial Board Claudio Campobasso (ISPRA-Roma) M. Balini, G. Barrocu, C. Bartolini, Editor in Chief D. Bernoulli, F. Calamita, B. Capaccioni, 2 Gloria Ciarapica (SGI-Perugia) W. Cavazza, F.L. Chiocci, R. Compagnoni, D. Cosentino, Editorial Responsible S. Critelli, G.V. Dal Piaz, C. D’Ambrogi, Maria Letizia Pampaloni (ISPRA-Roma) P. Di Stefano, C. Doglioni, E. Erba, publishing group Editorial Manager R. Fantoni, P. Gianolla, L. Guerrieri, Mauro Roma (ISPRA-Roma) M. Mellini, S. Milli, M. Pantaloni, V. Pascucci, L. Passeri, A. Peccerillo, Convention Responsible L. Pomar, P. Ronchi (Eni), Anna Rosa Scalise (ISPRA-Roma) B.C. Schreiber, L. Simone, I. Spalla, Alessandro Zuccari (SGI-Roma) L.H. Tanner, C. Venturini, G. Zuffa.

ISSN: 2038-4947 [online] http://www.isprambiente.it/site/it-IT/Pubblicazioni/Periodici_tecnici/Geological_Field_Trips

The Geological Survey of , the Società Geologica Italiana and the Editorial group are not responsible for the ideas, opinions and contents of the guides published; the Authors of each paper are responsible for the ideas, opinions and contents published. Il Servizio Geologico d’Italia, la Società Geologica Italiana e il Gruppo editoriale non sono responsabili delle opinioni espresse e delle affermazioni pubblicate nella guida; l’Autore/i è/sono il/i solo/i responsabile/i. DOI: 10.3301/GFT.2011.03 Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri and the Larderello areas A. Brogi - D. Liotta geological fieldtrips2011-3(2.2)

INDEX

Information Itinerary Riassunto ...... 4 5. Field Stops Abstract ...... 5 Stop 1.1: The Boccheggiano Fault ...... 22 Microstructural features ...... 28 3 Excursion notes 1. Introduction ...... 6 Stop 1.2: Montieri hydrothermal veins ...... 28 2. Gelogical Setting ...... 7 Stop 1.3: Rocks hosting the Present first reservoir ....30 The paleogeographic domains ...... 8 Compressional features ...... 11 Stop 1.4: Natural gas vents at Extensional features ...... 13 Monte Rotondo Marittimo ...... 31 3. Fossil geothermal system: the Bocchegiano-Montieri area ...... 16 Stop 1.5: Visit to the Larderello Museum and 4. Present geothermal system: “touristic” well ...... 31 the Larderello geothermal field ...... 19 index The reservoirs ...... 21 6. Conclusions ...... 35 The recharge area ...... 21 The geothermal fluids ...... 21 References...... 38

DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2)

4 information A. Brogi - D. Liotta Montieri Boccheggiano Larderello geotermia, fluidi Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas DOI: 10.3301/GFT.2011.03 Riassunto i sistemi geotermici fossili ed attuali, le similitudini fra illustra Questa escursione, di un solo giorno, di Boccheggiano-Montieri e del campo geotermico dell’area mineraria l’assetto strutturale paragonando da circolazioni di fluidi nelle cataclasiti delle faglie dirette del Ambedue le aree sono caratterizzate Larderello. e che hanno costituito i percorsi preferenziali per la circolazione. A partire da questi canali Pliocene-Attuale principalmente ubicate in strutturali le trappole verso migrati i fluidi sono successivamente strutturali, superiore). Sia l’area di Boccheggiano - (Triassico delle Anidriti di Burano corrispondenza della Formazione un diffuso magmatismo medio-crostale si meridionale, dove Montieri che di Larderello sono ubicate in Toscana strutture La presenza quindi di magmatismo, alla tettonica distensiva. è sviluppato contemporaneamente la formazione di circolazione fluidi e circolazione di acqua essenzialmente meteorica ha favorito distensive ha quindi anche lo scopo di L’escursione di Boccheggiano che in quella Larderello. idrotermali sia nella zona di Boccheggiano-Montieri, per meglio cioè l’area mineralizzata sottolineare l’importanza di studiare il “passato”, cioè il campo capire il “presente”, geotermico di Larderello. La prima parte dell’escursione sarà quindi dedicata alle relazioni fra e strutture nella mineralizzazioni di Boccheggiano-Montieri; la zona sarà invece seconda parte, invece, dedicata alla visita del campo geotermico di Larderello e alle sue Alcuni dati manifestazioni naturali. infine del sottosuolo saranno e discussi. illustrati chiave: Parole idrotermali, mineralizzazioni, Toscana meridionale geological field trips 2011 - 3(2.2)

5 information A. Brogi - D. Liotta geothermics, hydrothermal fluids, ore deposits, Southern structures and dominated meteoric water fluid flow circulation enhanced hydrothermal both in the Boccheggiano area and Larderello field. During this field trip we like to stress the importance study Past, i.e. the fossil geothermal systems, in order to better understand the Present, i.e. Larderello field. The first part of this field trip is dedicated to the relationships and structures in between mineralization the Boccheggiano-Montieri mining area, is whereas the second part of day addressed to the visit of Larderello geothermal area and to its natural manifestations. Furthermore, data from the and discussed. subsurface will be illustrated words: Key Stop 1.2 Stop 1.1 Stop 1.3 Stop 1.5 Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Stop 1.4 ARDERELLO L DOI: 10.3301/GFT.2011.03 Abstract fluid paths, the similarieties between fossil and Present hydrothermal aims to illustrate excursion This one-day area and the setting of the Plio-Pleistocene Boccheggiano-Montieri mineralized comparing the structural by fluid flow throw cataclasites hosted within Present Larderello geothermal field. Both areas are characterized traps structural toward channels, fluids migrated the Pliocene-Present normal faults. From these structural for the one of the main detachment levels Fm., Anidriti di Burano predominantly located the Upper Triassic where a diffuse of the Northern Apennines. Both areas are located in Southern Tuscany, tectonic evolution to extensional tectonics. Magmatism, contemporaneously magmatism developed intra-crustal geological field trips 2011 - 3(2.2)

6 excursion notes A. Brogi - D. Liotta Fig. 1.1 - Geological sketch map of the area with Fig. 1.1 - Geological sketch mineralized area. mineralized highlighted the Larderello geothermal field and Boccheggiano Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas DOI: 10.3301/GFT.2011.03 1. Introduction The understanding of the fluid flow path in geothermal areas is a continuous task to better address the exploitation of terrestrial heat, by geothermal fluids through a transported between Relationships network of fractures. geological structures and fluid flow in present through geothermal fields are mostly derived interpretation of borehole logs and geophysical similar information can be data. However by studying exhumed geothermal achieved areas, systems, now resulting in mineralized in epithermal and mesothermal developed conditions. is an ideal region for Southern Tuscany comparing information from present and fossil geothermal systems. This field trip is organized we will visit the in two parts: the first half, main outcrops of the Boccheggiano mineralized 2010), 2008; Liotta et al., et al., area (Rossetti during the cooling of Montieri developed during emplaced at mid-crustal level granitoid, et al., Late-Pleistocene at about 3-2 My (Boyce 2005); in the second part of 2003; Dini et al., we will look at the Larderello our excursion geothermal field, visiting the so-called “first cropping out close to the geothermal reservoir”, locality (Fig. 1.1). geological field trips 2011 - 3(2.2)

7 excursion notes , 2 ).The 2 A. Brogi - D. Liotta and 600 mW/m 2 Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 2.1 - Structural sketch map of Southern Tuscany with regional heat flow contour lines (equidistance: 50 mW/m map of Southern Tuscany sketch Fig. 2.1 - Structural respectively (from Bellani et al., 2004). (from Bellani et al., respectively plus signs show the Larderello and Monte Amiata geothermal fields where heat low reaches 1000 mW/m DOI: 10.3301/GFT.2011.03 The main goal of this field trip is to discuss the relationships between geological structures and fluid flow, comparing fossil and Present geothermal fields, that are supposed to be different just only for the age of cooling mid- crustal magma. 2. Geological Setting The Larderello and Boccheggiano areas are (Figs. 2.1, located in Southern Tuscany located in the 2.2, 2.3), that is structurally inner part of the northern Apennines. Its setting is consequence of two structural processes: the first related to contrasting and collision of the Adria the convergence microplate and the European plate, represented by the Sardinia-Corsica Massif (Molli, 2008, for a review); the second related to the extensional tectonics, which been acting since the Early-Middle have 1990; Carmignani et al., Miocene (Jolivet 2000). This 1994; Brunet et al., et al., affecting eastwards, extension migrated the entire northern Apennines. geological field trips 2011 - 3(2.2)

8 excursion notes A. Brogi - D. Liotta on average, with local peaks up on average, 2 in the Larderello geothermal area (Fig.2.1). 2 Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas The Ligurian Units, composed of remnants of Jurassic-Cretaceous oceanic crust and of its Jurassic- The Ligurian Units, composed of remnants Jurassic-Cretaceous The metamorphic Tuscan Unit, belonging to the external Tuscan Domain, consists of greenschist-facies Unit, belonging to the external Tuscan The metamorphic Tuscan The Umbria-Marche Domain consists of continental-margin deposits from Triassic to Late Miocene; this The Umbria-Marche Domain consists of continental-margin deposits from Triassic The Subligurian Units made up of arenaceous and calcareous turbidites Late Cretaceous-Oligocene age. The Tuscan Nappe including sedimentary rocks, ranging from Upper Triassic evaporites (Anidriti di Burano Fm) to (Anidriti di Burano evaporites from Upper Triassic Nappe including sedimentary rocks, ranging The Tuscan DOI: 10.3301/GFT.2011.03 The paleogeographic domains domains The collisional process determined the stacking of tectonic units deriving from palaeogeographic of the inner northern Apennines. These are, from top (Fig. 2.2): (a) Cretaceous sedimentary cover; (b) Domain during Late Oligocene-Early the Tuscan over Ligurian and Subligurian Units were thrust eastwards Miocene. (c) carbonates and Cretaceous-Lower Miocene marine clastic deposits (scaglia toscana group Jurassic-Cretaceous in duplex structures, detached along the Nappe, already involved Macigno Fm.). During the Early Miocene, Tuscan Domain and the inner part of Umbria-Marche the external Tuscan to thrust over level evaporite Upper Triassic Domain. (d) sedimentary succession, similar to that one characterizing metamorphic rocks deriving from a Triassic-Oligocene Domain. the inner Tuscan in the collisional stage determining isoclinal folds and involved Domain was of the Tuscan The substratum group) and (verrucano quartzite and phyllite of Triassic duplex structures. It is composed, from the top, phyllite. Palaeozoic (e) Domain represents the external Zone of the Northern Apennines, where a fold-and-thrust belt developed from belt developed of the Northern Apennines, where a fold-and-thrust Domain represents the external Zone 2002). et al., the Middle Miocene (Brozzetti The present crustal and lithospheric thicknesses, ranging between 22-24 km and 30-40 respectively The present crustal and lithospheric thicknesses, ranging (Locardi and Nicolich, 1988), are the clearest evidence of importance this extensional process. accompanied extension. Magmas with both eastwards, Since Late Miocene magmatism, becoming younger 1993). crustal and mantle geochemical signatures, intruded mostly at mid-crustal depths (Serri et al., 120 mW/m by high heat flow, is characterized Southern Tuscany Presently, to 1000 mW/m geological field trips 2011 - 3(2.2)

9 excursion notes A. Brogi - D. Liotta Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 2.2 - Left: relations among the different tectonic units of Northern Apennines and related palaeogeographical domains. Fig. 2.2 - Left: relations among the different tectonic units of Northern Apennines and related palaeogeographical Apennines (after Carmignani et al., 1994). Apennines (after Carmignani et al., Right: schematic crustal geological cross-sections showing the collisional and post-collisional evolution through the Northern evolution Right: schematic crustal geological cross-sections showing the collisional and post-collisional DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) excursion notes 10 : Late 2 A. Brogi - D. Liotta : quartz metaconglomerate, quartzite : quartz metaconglomerate, 3 Larderello areas. Symbols: (1) Q: Quaternary Larderello areas. Symbols: (B) : Late Triassic evaporites; (8) MRU evaporites; : Late Triassic 1 : Palaeozoic phyllite; (10) normal fault; (11) trace of the geological section (10) normal fault; (11) trace phyllite; : Palaeozoic 2 Southern Tuscany and Southern Tuscany (A) Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 2.3 - Geological sketch maps of Fig. 2.3 - Geological sketch given in Fig. 4.1. given continental deposits; (2) MR: Pliocene-Quaternary magmatic rocks; (3) P: Pliocene marine (4) M: Middle-Upper Nappe: (6) TN - Oligocene); Tuscan and marine sediments; (5) L: Ligurian Units (Jurassic Miocene continental, brackish and phyllite (Triassic Verrucano Group); (9) MRU Verrucano (Triassic and phyllite Triassic-Early Miocene sedimentary sequence; (7) TN Triassic-Early DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) excursion notes 11 A. Brogi - D. Liotta Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas DOI: 10.3301/GFT.2011.03 Compressional features metamorphic rocks cropping out both in the southern Middle These are recognizable in the greenschist-facies (i.e., Ridge) and in the sedimentary cover Montagnola Senese and - (i.e., Range Tuscan Larderello and Monte Amiata areas). In the metamorphic rocks of Montagnola senese area (Fig. 2.4) Late Oligocene-Early Miocene orientation of mainly defined by sub-parallel schistosity, pervasive compressional phase determined a planar, (Liotta, 2002). micas and carbonate minerals to the NE- These are roughly parallel This schistosity is an axial plane foliation associated to isoclinal folds axes. direction show a transport trending stretching lineation present on the S1 foliation. Kinematic indicators invariably the external Group overthrust rocks belonging to the Verrucano event, the NE. During this contractional toward the succession, thus determining the metamorphic conditions in which deformation occurred (i.e., Tuscan Montagnola Senese area, Liotta, 2002; Fig. 2.4). Group in the greenschist within the Verrucano duplex structures (Fig. 2.5) developed deeper levels At 1990; Brogi, 2008). 1988; Elter & Pandeli, metamorphic facies (Costantini et al., succession in the Larderello and thrusts were detected within the Tuscan Concerning the sedimentary cover, Monte Amiata area, now cropping out in isolated geological bodies (Nuclei toscani, Auctt.), surrounded by the conditions (Hillite-smectite low-metamorphic 2005). Thrusting occurred in very Ligurian Units (Brogi et al., marls (scaglia toscana and the Cretaceous-Oligocene clayey evaporites The Upper Triassic paragenesis). as played Anhydrites Burano the Upper Triassic Particularly, group) represented the main detachement levels. (Fig. 2.6). cataclastic level of a sub-horizontal the development favoring a regional decoupling horizon, geological field trips 2011 - 3(2.2) excursion notes 12 d A. Brogi - D. Liotta - Verrucano Group (Ladinian-Carnian); Tuscan Nappe: el - Anidriti di Group (Ladinian-Carnian); Tuscan - Verrucano 2 ) deformational events, respectively. Symbols: vr - Verrucano Group vr - Verrucano Symbols: respectively. ) deformational events, 2 ) and second (D 1 foliations. B: sections showing the pattern of bedding. Stereonets (lower hemisphere, equiangular net) 2 Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas and S 1 Fig. 2.4 - Geological cross section through the central part of Montagnola Senese area. A: sections showing the pattern Fig. 2.4 - Geological cross section through the central Burano Fm. (late Triassic); nq - Neogene and Quaternary sediments. Fm. (late Triassic); Burano structures related to the first (D the S (Ladinian-Carnian); d - Grezzoni Formation (Rhaetian-Norian); m - massive marble (Lower Liassic); mc - layered marble with marble (Lower Liassic); mc - layered (Rhaetian-Norian); m - massive Formation (Ladinian-Carnian); d - Grezzoni (sp) interlayere cherts (Middle-Upper Liassic); p - metamorphic scaglia toscana fm. with marble breccias and siliceous phyllite (Cenomanian). -Monte Quoio sub-unit: vr DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) excursion notes 13 A. Brogi - D. Liotta - Civitella M.ma 1 - anageniti minute fm.; VR 3 - tocchi fm.; VR 4 - upper member; Pal - Farma fm. (From Brogi, 2008). - Farma - upper member; Pal 1c - intermediate member; VR 1b - Tuscan Nappe, Anidriti di Burano Fm.; VR Nappe, Anidriti di Burano - Tuscan 1 Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas - basal member; VR 1a Fig. 2.5 - Geological sections across the Monte Leoni area showing the thrust affecting the Verrucano Group itself. Key: L Key: Group itself. Fig. 2.5 - Geological sections across the Monte Leoni area showing thrust affecting Verrucano fm.: VR - Ligurian Units; TN DOI: 10.3301/GFT.2011.03 Extensional features segmentation of the Tuscan After the emplacement of tectonic units, extension determined lateral normal faults (Figs. 2.6 and 2.7). The main Group through low-angle succession and of the basal Verrucano and within underneath the sedimentary cover, evaporites, are located: in the Upper Triassic detachment levels in duplex structures. already stacked Group, underneath the Verrucano phyllite, the Palaeozoic of tectonic This process, occurred in the Burdigalian-Messinian time span, determined development depressions (Fig. 2.8), where continental to marine Middle-Late Miocene sediments deposited (Brogi & Liotta, 2008). 1994). Therefore, relics of the 98 km, at least (Carmignani et al., Stretching is estimated in about 120% over Nappe and in the isolated geological bodies, made up of Tuscan compressional structures remained preserved the deeper the Ligurian Units directly overlain Group (Figs. 2.5 and 2.6). In this framework, Verrucano phyllite. Group and the Palaeozoic the Verrucano evaporites, such as the Late Triassic levels, structural The previously formed compressional and extensional structures, were dissected by a new event, normal faults. In the resulting tectonic depressions, marine sediments deposited which determined NW-SE geological field trips 2011 - 3(2.2) excursion notes 14 A. Brogi - D. Liotta Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 2.6 - Geological sections through the Montieri and Poggio Prugnoli areas, illustrating the Tuscan Nappe as isolated the Tuscan Prugnoli areas, illustrating Fig. 2.6 - Geological sections through the Montieri and Poggio - Upper Triassic Anhydrites (from Brogi et al., 2005). (from Brogi et al., Anhydrites - Upper Triassic geological bodies, where the Late Oligocene-Early Miocene compressional and Middle-Late extensional structures Nappe: T - Upper Oligocene Lower Miocene L - Ligurian Units; Tuscan (indicated by thick arrows) are recorded; Symbols: NL - Lower-Middle Radiolarites; terrigeneous turbidites; P Creataceous-Oligocene pelagic succession; R - Upper Jurassic limestone; EV massive bearing marls; M - Lower Jurassic nodular reddish limestone, cherty Posydonomia Jurassic DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) excursion notes 15 ie A. Brogi - D. Liotta Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 2.7 - Tuscan Nappe segmentation and Middle-Late Miocene sediments in Southern Tuscany, as reconstructed from Nappe segmentation and Middle-Late Miocene sediments in Southern Tuscany, Fig. 2.7 - Tuscan two Tuscan Nappe segments. two Tuscan the Late Triassic evaporites and/or the metamorphic rocks. The Middle-Upper Miocene deposits are located in gap between evaporites the Late Triassic fieldwork, borehole data, and interpretation of seismic reflection lines. Gray pattern indicates where the Ligurian Units overl fieldwork, borehole data, and interpretation of seismic reflection lines. Gray DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) excursion notes 16 A. Brogi - D. Liotta Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 2.8 - Geological cross section through the Larderello area and surroundings based on borehole stratigraphy, seismic Fig. 2.8 - Geological cross section through the Larderello area and surroundings based on borehole stratigraphy, Liotta, 2008). reflection lines and field mapping, showing the lateral segmentation of the Tuscan Nappe and Verrucano Group (after Brogi and Nappe and Verrucano segmentation of the Tuscan reflection lines and field mapping, showing the lateral DOI: 10.3301/GFT.2011.03 during Early-Middle Pliocene (Fig. 2.8). This latter extensional event, mainly characterized by high-angle mainly characterized Pliocene (Fig. 2.8). This latter extensional event, during Early-Middle 1994). The main tectonic 98 km (Carmignani et al., normal faults, produced a stretching in at least 7% over zones. transfer depressions are dissected by SW-NE is joined with widespread magmatism. Therefore, the the Pliocene extensional event In Southern Tuscany, processes resulting from the interplay by the hydrothermal of the area is characterized geologic evolution at depth. between ongoing extensional structures and cooling of granitoids 3. Fossil geothermal system: the Boccheggiano-Montieri area In the following a summary of geological setting Boccheggiano-Montieri is reported, on basis data and interpretation proposed by Liotta et al. (2010). In the Boccheggiano-Montieri area cataclasites deriving from Late Oligocene-Early Miocene stacking of normal faulting and from the Pliocene-Pleistocene high-angle tectonic units, from the Miocene low-angle normal faults are widely exposed (Fig. 3.1 and 3.2). geological field trips 2011 - 3(2.2) excursion notes 17 A. Brogi - D. Liotta Fig. 3.1 - Geological sketch map Fig. 3.1 - Geological sketch Liotta et al., 2010). Liotta et al., of the Boccheggiano-Montieri area. to other figures and References geological sections are given. Miocene and Pliocene Mineralized normal faults are highlighted (from All cataclastic levels are locally All cataclastic levels The structure is mineralized. by two isolated characterized Nappe geological bodies, Tuscan internally deformed during the collisional stage of the northern Apennines, as indicated by the occurrence of a Late Oligocene Nappe thrust doubling the Tuscan succession (Fig. 3.2). Middle-Late Miocene extensional low-angle tectonics developed the normal faults, separating presently named as Poggio Montieri Prugnoli and Poggio geological bodies. These faults localized show cataclastic levels at the base of Ligurian Units and within the Late Triassic Fm). (Anidriti di Burano evaporite Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) excursion notes 18 A. Brogi - D. Liotta NE dipping Boccheggiano Pliocene normal fault dissected the previously formed structural Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas DOI: 10.3301/GFT.2011.03 Fig. 3.2 - Composite geological section through the study area. Location in Fig. 3.1. The Pliocene Boccheggiano normal fault affects the 3-2.3 Ma-old monzogranite, encountered at depth. The location of the samples for fluid inclusions analyses are also shown 2010). (from Liotta et al., In between the Poggio Prugnoli and Poggio Montieri areas, the Ligurian Units directly overlie the Upper Triassic Montieri areas, the Ligurian Units directly overlie Prugnoli and Poggio In between the Poggio The evaporites. setting, thus juxtaposing the Ligurian Units (i.e., “Argille a Palombini”, Early Cretaceous) to the Palaeozoic a Palombini”, “Argille setting, thus juxtaposing the Ligurian Units (i.e., 2003), drilled, for about 2,000 m, a et al., (Fig. 3.2). The 3,721-m deep Montieri 4 borehole (Boyce phyllite 2006). Interpretation of seismic reflection 2005; Villa et al., with an age of c.a. 3-2 My (Dini et al., monzogranite 2008) suggests that the Boccheggiano normal fault crossed, at depth, Montieri et al., lines (Rossetti (Fig. 3.2). monzogranite the Boccheggiano fault zone characterizing features, therefore, indicate that the mineralization These structural at depth. recognized and the entire Boccheggiano-Montieri area are related to cooling of monzogranite geological field trips 2011 - 3(2.2) excursion notes 19 A. Brogi - D. Liotta - 2 : (c) : (b) and , reflections with high Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas (b) (a) - Mesozoic-Paleozoic Group; - Mesozoic-Paleozoic group; - phyllite-quartzitic micaschist group; - Paleozoic 3 2 1 - Upper Triassic evaporites; - Upper Triassic 1 Fig. 4.1 - 2003). Monticiano Roccastrada Unit (UMR): Monticiano Roccastrada UMR unmigrated seismic line and its unmigrated In drawing. UMR UMR BA - Gneiss Complex. Black triangles: intersections with other reflection seismic lines. of this geological section is The trace shown in Fig. 2.3 (From Brogi et al., Lower Miocene-Rhaetian succession; FT contrast of acoustic impedance are contrast highlighted by thick lines; dotted lines denote geological contacts; light grey lines indicate normal faults related to the third extensional event. Q geological interpretation. Symbols: - Quaternary sediments; P Pliocene sediments; M - Miocene deposits; P Pliocene deposits: M - Miocene Nappe (FT): FT deposits; Tuscan DOI: 10.3301/GFT.2011.03 4. Present geothermal system: the Larderello field by normal faults related to the Pliocene-Present time The structure of the geothermal area is characterized period. These structures cross-cut the already pre-existing compressional and extensional tend The upper boundary of this to die out at depth, in correspondence of the present brittle-ductile transition. (Fig. 4.1), interpreted referred to as the K-Horizon by a reflection seismic horizon, is marked zone transitional 1999). 1993; Liotta & Ranalli, (Cameli et al., level as a fluid-filled fractured geological field trips 2011 - 3(2.2) excursion notes 20 A. Brogi - D. Liotta geothermal fields. are exploited in the Larderello- extensional structures (Bellani et al., 2004). Two different reservoirs 2004). Two extensional structures (Bellani et al., peaks and depressions (Fig. 4.2), thus implying that fluid flow is controlled by the Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 4.2 - Geological cross sections, isotherms, and surface heat flow along a geological through the Larderello area control are denoted by a broken dashed line (From Bellani et al., 2004). dashed line (From Bellani et al., control are denoted by a broken (location is given in Fig. 2.3). Black circles represent temperature measurements in °C. Isotherms subject to the weakest in Fig. 2.3). Black circles represent temperature (location is given DOI: 10.3301/GFT.2011.03 The lateral continuity of the K-horizon is interrupted by intersection with brittle shear zones where Pliocene- is interrupted by intersection with brittle shear zones continuity of the K-horizon The lateral Present normal faults coalesce (Fig. 4.1). with data, measured at bottom holes, indicate local variations, temperature Along these brittle shear zones, geological field trips 2011 - 3(2.2) excursion notes 21 is 2 m -14 A. Brogi - D. Liotta or 10 -15 . 2 m -15 Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas DOI: 10.3301/GFT.2011.03 estimated. The bulk of porosity derives from tectonic fractures, although the dissolution of Late Triassic from tectonic fractures, estimated. The bulk of porosity derives in presence of hot saline solutions, with the consequence collapse and rupture overlying anhydride exploited through The second and deeper reservoir, carbonate rocks, can contribute to increase permeability. to the Pliocene-Present extensional faults, boreholes down to 4000 m, is located in cataclastic rocks, linked within the metamorphic rocks. Here, permeability is estimated in about 10 The reservoirs evaporites. corresponding to the Late Triassic is mostly located within the cataclastic level The first reservoir during the main detachment level was This level between 500 and 1000 m, approximately. Its depth ranges Miocene extensional stages. A permeability of about 10 the collisional and Early-middle The recharge area limestones and cherty The main recharge area is located in the Cornate area, where fractured Nappe, largely crop out. belonging to the Tuscan The geothermal fluids is the main The meteoric water in both reservoirs. The Larderello geothermal system is vapor-dominated with magmatic and metamorphic fluids, in a minor extent. A mantle signature is mixed source for the vapor, has became an injection of wastewater 2000). Since 1980’s also present in the gas phase (Minissale et al., important feature of the exploitation strategy. geological field trips 2011 - 3(2.2) itinerary 22 A. Brogi - D. Liotta schistosity (Chl+Ms+Qtz+FeOx±Ab), dipping 15° to SW. This folding event and the coeval This folding event dipping 15° to SW. schistosity (Chl+Ms+Qtz+FeOx±Ab), 2 Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas DOI: 10.3301/GFT.2011.03 5. Field Stops Stop 1.1: The Boccheggiano Fault village (Fig. 5.1). Going ahead for about 4 km the road from Massa Marittima to Siena, through Prata Take to the the Merse creek) will bring you (with an old bridge passing over left side, a little track more, on your This stop (43°05'47.7"-11°01'54.4") is located in the Boccheggiano pyrite mining district and we are outcrop. Boccheggiano normal fault. This is a regional structure, almost looking at the Pliocene-Pleistocene mineralized striking segment. The attitude of by an E-W segments, linked by two NNW-SSE 20 km long and characterized at depth trough borehole data. In the abandoned Campiano mine, it trends 170° the fault plane is constrained here estimated. throw of about 900 m was N and dips 45°NE (fig. 5.2). A vertical is constituted by while the hangingwall phyllite, is made up of Paleozoic In this outcrop (Fig. 5.3), the footwall Early Cretaceous shale and limestone belonging to the argille a palombini fm. metasiltite, of quartzose with levels is interlayered phyllite quartz-rich the Palaeozoic In the footwall, by isoclinal folds and The rock fabrics are characterized metasandstone and minor metaconglomerate. S pervasive synkinematic metamorphism is related to the Late Oligocene-Early Miocene collisional stage. shale and limestone of the is made up of Early Cretaceous interlayered The Boccheggiano fault hangingwall up to 20 m thick. Occurrence argille a palombini fm. Limestone predominates on the shale beds, defining levels at low angle with respect to the bedding, indicates a pre- pressure solution axial planar cleavage, of pervasive faulting deformation, deriving from the collisional stage. and a hangingwall a 1-m-thick core zone, 25-m-thick damage zone, is constituted of a footwall The fault zone extensively in the hangingwall, about 40 m thick (Fig 5.3b). Pyrite is mainly concentrated damage zone distributed in the 13-m-thick silicified rock mass, where original sedimentary fabrics are totally lost. On where pyrite damage zone the sedimentary features are maintained in rest of hangingwall the contrary, fault incohesive consists of a black clay-rich The core zone quartz veins. is allocated within millimeter-thick and quartz veins. phyllite (<30%) of Palaeozoic gauge with 1-10 mm fragments as suggested by nearest to the fault core, intensity of deformation is higher, damage zone, In the hangingwall spaced about 5 cm. here indicated by about 1 cm open fractures, frequency, the fracture geological field trips 2011 - 3(2.2) itinerary 23 A. Brogi - D. Liotta Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 5.1 - Location of the stops described in text. The geological map is from Giannini et al., 1971 (modified). Giannini et al., DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) itinerary 24 A. Brogi - D. Liotta Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 5.2 - Schematic geological map and sections of the Boccheggiano mine district. The mineralization (quartz - pyrite, Fig. 5.2 - Schematic geological map and sections of the Boccheggiano mine district. The mineralization the Late Triassic evaporite (from Liotta et al., 2010). (from Liotta et al., evaporite the Late Triassic mainly) occurs within the damage zone of the Boccheggiano fault and along the boundary between the Palaeozoic phyllite and phyllite of the Boccheggiano fault and along boundary between Palaeozoic mainly) occurs within the damage zone DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) itinerary 25 A. Brogi - D. Liotta : lithologic and fracture c and b : fracture frequency in the damage zone. frequency in the damage zone. : fracture d : The Pliocene Boccheggiano fault and its damage zone close to the Campiano mine. Location of samples : The Pliocene Boccheggiano fault and its damage zone a Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 5.3a-d - scan line along the indicated segment; BOC1 and BOC7 for fluid inclusions image analyses (from P1 to P12) are also shown. DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) itinerary 26 A. Brogi - D. Liotta : diagrams showing the high permeability of fault following methodology proposed by Caine : diagrams f and : the minimum percentage of mineralized veins in the hangingwall varies from about 60% to 4% in 40 m, varies in the hangingwall veins : the minimum percentage of mineralized e g Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 5.3e-g - the core zone to the hangingwall protolith (from Liotta et al., 2010). protolith (from Liotta et al., to the hangingwall the core zone et al. (1996); DOI: 10.3301/GFT.2011.03 Fracture frequency tends to decrease toward the undeformed rock (protolith), resulting in fractures spaced up the undeformed rock (protolith), resulting in fractures frequency tends to decrease toward Fracture frequency distribution (Fig. 5.3c and 5.3d) indicates that: (1) the footwall to 260 cm (Fig. 5.3c). The fracture close to the core zone; (2) the frequency peak is in hangingwall, is less deformed than the hangingwall; is asymmetric, suggesting a lithological control during brittle deformation. (3) the damage zone of this tendency to the undeformed rock. An evaluation tend to decrease from the core zone veins Mineralized obtained by means of image analyses procedure. The outcomes (Fig. surface ratio, by the vein/rock is given decreases from about 60 to in the rock-mass veins 5.3g) indicate that the percentage of mineralized density decrease, as it was protolith, following the fracture to the hangingwall 4%, from the fault zone expected. can be related and hangingwall in the footwall Considering their attitude and mutual relationships, fractures R and P damage zone, during the fault activity (Fig. 5.4). In footwall developed to Riedel shear fractures, during indicating that these acted as pathways by pyrite and iron oxides, surfaces are characterized fracture attitudes Their average are related to R and R' fractures. fractures damage zone, In the hangingwall fluid flow. geological field trips 2011 - 3(2.2) itinerary 27 A. Brogi - D. Liotta : : j d - h and c : Palaeozoic b : Three photos from g - e : Sketch of the relationships among : Sketch Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas a Fig. 5.4 - mantled with pyrite (from Liotta et al., 2010). mantled with pyrite (from Liotta et al., the Riedel shear fractures and the surface of the Riedel shear fractures Boccheggiano fault. The fault kinematics is given by the relationships between shear foliations in fault gouge and the plane. displaying damage zone, from the footwall phyllite R and P shear fractures. usually oxidized the no-silicified hangingwall rock mass: pyrite and the no-silicified hangingwall in millimetric veins. quartz are concentrated Two photos from the hangingwall silicified rock photos from the hangingwall Two along the concentrated mass; pyrite is generally margin of the quartz veins. Three photos, at different magnification, of the the cataclastic elements are often fault core zone: DOI: 10.3301/GFT.2011.03 (Fig. 5.4a) are 150/30 and 240/70, became loci for These fractures respectively. thus resulting in intense fluid flow, with pyrite and quartz, both in mineralization the silicified (Fig. 5.4c and 5.4d) not- in the clay- silicified rock mass. Fractures gauge are SW gently dipping shear foliations. relationships Their attitude and structural with the fault plane indicate that kinematics of the Boccheggiano normal fault component by a left lateral characterized was the quartz elements (Fig. 5.4a). Noteworthy, dispersed within the fault gouge are often mantled with pyrite (Fig. 5.4j), indicating during the fluid flow and mineralization cataclasite development. geological field trips 2011 - 3(2.2) itinerary 28 A. Brogi - D. Liotta ; the second generation of fluid ; the second generation equiv. . equiv Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas DOI: 10.3301/GFT.2011.03 inclusions documents a later stage, with homogenization temperature from 124 to 288°C and salinity 0.2 inclusions documents a later stage, with homogenization temperature to 1.9 wt.% NaCl Microstructural features with elongate (about 0.4 mm long) blocky show syntaxial veins of the footwall Thin sections from the pyrite pockets along the median line mainly concentrated pyrite crystals (about 0.3 mm, on average), quartz texture and euhedral network, among isolated sharp and smooth form a pervasive margins are generally (Fig. 5.5). Vein of the vein crystals (about or at their margins, small euhedral (Fig. 5.5a). Within these latter, phyllite elements of Palaeozoic shows two different textures, one in the hangingwall (Fig. 5.5c). Mineralization 0.1 mm) of pyrite can be recognized (Fig. 5.5b). Regarding for the silicified rock mass and other rest of rocks belonging to damage zone the original quartz texture highlights the areas where silicifing fluid infiltrated the silicified rocks, a fine grain limestone, produced metasomatism and deposited widespread small crystals of pyrite (Fig. 5.5d-f). with a blocky elongate quartz The boundaries of the silicified elements constitute margins syntaxial veins located along the margins with a 0.4-mm-long side, and it is generally is euhedral, texture. Pyrite in the veins the rest of characterize antitaxial veins median line (Fig. 5.5e-f).and/or along the vein Differently, quartz a massive display further from the fault plane (Fig. 5.5g-i). These veins damage zone, hangingwall pyrite with euhedral from polygonal to irregular, with margins ranging equant grains, texture, with relatively quartz. surrounded by unstrained indicate a main type of fluids that are veins Fluid inclusions studies carried out on quartz from the hydrothermal by two phase liquid rich-fluid inclusions (L+V): the first one, with homogenization temperatures characterized between 172 and 331°C salinity 0.0 8.8 wt% NaCl ranging Stop 1.2: Montieri hydrothermal veins Siena for about 1 km, reaching the cross-road to Montieri. Follow toward Back to the main road and move Montieri foot-hill. this windy street and cross all the village park close to chapel located at Poggio The Late for about 300 m, reaching the outcrop. this way the hill. Follow climbs over From here a track Nappe, and the cataclasites during doubling of the Tuscan Oligocene-Early Miocene cataclasites, developed Nappe boundary and located at the Ligurian Units/Tuscan deriving from the Miocene extensional events, became channels for the flow of geothermal fluids. evaporite, within the Late Triassic geological field trips 2011 - 3(2.2) itinerary 29 : g A. Brogi - D. Liotta : clasts of Palaezoic phyllite surrounded phyllite : clasts of Palaezoic a : silicified rock mass; the syntaxial veins are characterized by large quartz are characterized : silicified rock mass; the syntaxial veins d : massive fabrics of the antitaxial quartz veins with pyrite (from Liotta et al., 2010). with pyrite (from Liotta et al., fabrics of the antitaxial quartz veins : massive i and h : syntaxial quartz vein with pyrite. Pyrite can be concentrated along the vein median line or, along the vein with pyrite. Pyrite can be concentrated : syntaxial quartz vein c and b : minute quartz grains in those parts where the metasomatism of Early Cretaceous limestone occurred; : minute quartz grains f Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas and e Fig. 5.5 - Microphotographs of the footwall and hangingwall damage zone. Footwall. Footwall. damage zone. and hangingwall of the footwall Fig. 5.5 - Microphotographs network of quartz veins with pyrite; network of quartz veins along the vein margins, as it is shown in c. Hangingwall: margins, as it is shown in c. Hangingwall: along the vein grains; grains; by quartz veins with pyirite; by quartz veins DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) itinerary 30 A. Brogi - D. Liotta Fig. 5.6 - (A) Vacuolar carbonatic Fig. 5.6 - (A) Vacuolar evaporite levels. evaporite breccia (Calcare Cavernoso Fm.). breccia (Calcare Cavernoso were produced by The vacuoles dissolution of (B) the dolostone clasts, scattered in a carbonate of the from alteration matrix derived Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas DOI: 10.3301/GFT.2011.03 We are looking at hydrothermal quartz veins hosted in the Macigno Fm, i.e. close to boundary with quartz veins are looking at hydrothermal We Ligurides Units, located at the top (43° 07' 49''- 11° 00' 54''). Fluid inclusions studies carried out on samples from extensional and compressional cataclasites, indicate two defined by a two phase liquid rich fluid inclusions (L+V), with temperatures types of fluid inclusions, the older, typified by V+L and V with homogenization temperatures from 172°C and 280°C; the younger ranging between 197° and 248°C. ranging Stop 1.3: Rocks hosting the Present first reservoir From Montieri, continuing along the Massa main road, toward Marittima, we reach the cross-road where the directions to Monte and M.mo, Rotondo Larderello localities are indicated. Here (43° 07' 52.4" - 10°58' 50"), Fm. crops the Calcare Cavernoso from the out. These rocks derive (Anidriti di anhydrides Late Triassic Fm.), sited at the bottom of Burano Nappe. The Late Triassic the Tuscan represented the main evaporites during the detachment level collisional and post-collisional stages of the northern Apennines activity and Tectonic evolution. chemical processes, also induced of and dehydration by hydration in a vacuolar the evaporite transformed the anhydrides, Fm.) (Fig. 5.6). carbonatic breccia, (Calcare Cavernoso geological field trips 2011 - 3(2.2) itinerary 31 A. Brogi - D. Liotta ) constitutes 2 m -14 Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas needles. DOI: 10.3301/GFT.2011.03 The Calcare Cavernoso Fm., because of its brecciated texture (estimated permeability 10 Fm., The Calcare Cavernoso the main level hosting the Present first reservoir (both in the Larderello and Monte Amiata areas). The hosting the Present first reservoir the main level in the Colline Metallifere region. sulphides) horizon (mixed Fm. is also the main mineralised Calcare Cavernoso Stop 1.4: Natural gas vents at Monte Rotondo Marittimo the power plant and go ahead, up to hill, reaching Arriving to the Village, turn right side, toward the Biancane locality (43°09'08.61" - 10°51'13.07"), a large whitish area with fumaroles (Fig. 5.7), located of a tectonic discontinuity. along the trace rocks of the Larderello geothermal area crops out. This consists fractured the first reservoir In this stop, Nappe (Fig. 5.8 and 5.9). and Cretaceous succession of the Tuscan Jurassic related to the Upper Triassic, the top of hill, carbonatic succession is exposed in the southern side of “il Monte” hill. At The Tuscan is The gas vent of this reservoir. the Macigno and “Scaglia toscana” fms. represent impervious cover interrupted at the boundary with scaglia toscana group (Fig. 5.10). These are deeply altered by hydrothermal radiolarites. The fumaroles are located in the Middle Jurassic network, mostly with an almost vertical circulation of hot fluids channelled to surface through a fractures rise to loci for gas emission and growth of tiny gave and layering attitude. Intersection between fractures crystals Stop 1.5: Visit to the Larderello Museum and “touristic” well The Museum, located in one of the first Larderello. follow indication towards Monterotondo Marittimo, Leaving edifices built up during the initial period of industrial exploitation (XVIII century), hosts finds and tools from periods, when the geothermal resources were mainly used for thermal baths, passing Etruscan and Roman to production of the XVII and XVIII centuries, to Present, with exploitation only devoted through the borax from the Larderello area are also proudly exhibited. Samples with minerals electricity. provide This well, when it is opened, The so-called “touristic well” is a geothermal well drilling down the first reservoir. column of steam, at about 200°C. rise to an impressive gives geological field trips 2011 - 3(2.2) itinerary 32 A. Brogi - D. Liotta Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 5.7 - The fumaroles at Monterotondo Marittimo in the “Biancane” locality. DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) itinerary 33 - - 2 g 2m g - Calcari 2 t A. Brogi - D. Liotta - Diaspri Fm., (Malm); - Diaspri Fm., 3 g - Macigno Fm. (Upper Oligocene O - Anidriti di Burano e Calcare Cavernoso Fm. e Calcare Cavernoso - Anidriti di Burano 1 t - “Calcare Massiccio” Fm., (Lower Liassic); - “Calcare Massiccio” Fm., - Calcare Selcifero Fm. (Middle-Upper Liassic); 1 and the below formations. g 2s 2 g g , (“Scaglia toscana” fm.) represent the impervious cover of the (“Scaglia toscana” fm.) represent the impervious cover 3 g ce fm. (Rhaetian); (Macigno Fm.) and O - “Scaglia toscana” fm. (Cretaceous Oligocene); ce Rhaetavicula Contorta Fig. 5.9 - Geological cross section through the Monterotondo M.mo area (From Lazzarotto, carbonatic reservoir, indicated by carbonatic reservoir, 1967). Key: 1967). Key: Fig. 5.8 - Tuscan Nappe stratigraphic succession. Key: succession. Key: Nappe stratigraphic Fig. 5.8 - Tuscan (Norian - Rhaetian). e marne a Aquitanian); Calcari e Marne a Posidonia Fm. (Dogger); Calcari e Marne a Posidonia Liassic); Ammonitico Fm. (Lower-Middle Rosso Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) itinerary 34 A. Brogi - D. Liotta Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 5.10 - In the Monterotondo M.mo area the impervious cover of the uppermost geothermal reservoir corresponds to the of the uppermost geothermal reservoir Fig. 5.10 - In the Monterotondo M.mo area impervious cover “scaglia toscana” and Macigno fms. DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) itinerary 35 2010). originated surface and A. Brogi - D. Liotta hydrothermal pervading the pervading monzogranite, and trough the with deep fluids porous rocks. At permeable Tuscan and locally mixing depth through the mineralizing fluids, mineralizing (From Liotta et al., Boccheggiano fault depth, down to the heating and mixing existing cataclasites with meteoric water damage zone of the damage zone Nappe fractured and Nappe fractured upflowing toward the upflowing toward Meteoric water infiltrated to infiltrated Meteoric water based on the results of our study. illustrating the path of fluid flow, illustrating Fig. 6.1 - Conceptual cartoon, not to scale, Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas DOI: 10.3301/GFT.2011.03 6. Conclusions is to underlain the similarity between a fossil, Pliocene-Pleistocene field excursion The aim of this one-day geothermal system, and the Present Larderello field. data suggest that fluids flew through the In the Boccheggiano-Montieri area, fluid inclusions and structural circuit (Fig. 6.1). channels of a single hydrothermal cataclasites, representing structural geological field trips 2011 - 3(2.2) itinerary 36 limited A. Brogi - D. Liotta Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas Fig. 6.2 - Geological interpretation of the Larderello geothermal area. The extensional shear zones and the top of brittle- Fig. 6.2 - Geological interpretation of the Larderello geothermal area. The extensional shear zones at the top by extensional structures. ductile transition are shown (thick arrows indicate the sense of shear). The shear zones act as hydraulic channels where act as hydraulic are shown (thick arrows indicate the sense of shear). The shear zones ductile transition Light are exploited and named as deeper reservoir. geothermal fluid circulation is enhanced (dashed arrows). These shear zones including the Upper Triassic Fm. is located at the base of sedimentary cover the Calcare Cavernoso grey: sedimentary cover; is de hosting geothermal fluids are located. An already cooled granitoid traps structural at its base. In this level evaporites DOI: 10.3301/GFT.2011.03 A similar scenario (Fig. 6.2) is envisaged for the present day Larderello geothermal system where cooling for the present day A similar scenario (Fig. 6.2) is envisaged is representing the Present the deep reservoir at depth of about 6-8 km. In this view, are hypotized granitoids is defined by the as well the shallower reservoir analogue of the Boccheggiano fault damage zone, evaporites. cataclasite within the late Triassic Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri and the Larderello areas A. Brogi - D. Liotta g e o l o g i c a l f f i e l d t t r i p s 2 2 0 1 1 -3 3 ( 2 . 2 )

37 m e m o r a n d u m

DOI: 10.3301/GFT.2011.03 geological field trips 2011 - 3(2.2) references 38 , : e lo om mal the sional A. Brogi - D. Liotta Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas 1007-1010. extensional tectonics (Southern Tuscany, Italy). Tectonophysics 224, 413-423. Italy). Tectonophysics extensional tectonics (Southern Tuscany, 238, 295-315. (Northern Apennines, Italy). Tectonophysics, Tuscany 31, 382-420. Tic. Sci. Terra, meridionale). Atti il Monte Leoni (Toscana Italy). Lithos, 81, 1-31. Magmatic Province, geothermal field (Tuscan 83 (2), 241-264. Italy). Eclogae Geol. Helv., (Larderello geothermal field, Southern Tuscany, insights from the CROP 18 deep seismic reflection lines. J. Volcan. Geotherm. Res., 148, 60-80. Geotherm. Res., Volcan. insights from the CROP 18 deep seismic reflection lines. J. doi:10.1029/2007/TC002188. 27, TC5002. Basin (inner Northern Apennines, Italy). Tectonics middle-Late Miocene Radicondoli 1, 319-331. analysis. Boll. Soc. Geol. It., and structural insights from stratigraphy 321, 127-155. Tectonophysics, from Corsica to Tuscany. 40Ar/39Ar ages on micas along a transect Larderello Geothermal Field (Italy): constraints on geothermal fluid flow. J. Volcan. Geotherm. Res., 132, 15-29. Geotherm. Res., Volcan. J. on geothermal fluid flow. Larderello Geothermal Field (Italy): constraints 126, 243-262. Geotherm. Res., Volcan. fluid inclusions and stable isotopes. J. from mineralogy, area (Italy): constraints extension. International Journal of Earth Sciences 97, 677-703. compression versus 363, 127-139. Italy). Tectonophysics Larderello geothermal field (central DOI: 10.3301/GFT.2011.03 Carmignani L., Decandia F.A., Fantozzi P.L., Lazzarotto A., Liotta D. & Meccheri M. (1994) - Tertiary extensional tectonics in & Meccheri M. (1994) - Tertiary Liotta D. Lazzarotto A., P.L., Fantozzi Decandia F.A., Carmignani L., la Montagnola Senese tra di Monticiano-Roccastrada (1988) - L’unità Lazzarotto A. & Sandrelli F. Decandia F.A., Costantini A., from the Larderel of Pliocene-Pleistocene granites G. (2005) - Origin and evolution M. & Ruggieri Puxeddu Gianelli G., Dini A., E. (1990) - Alpine and Hercynian orogenic phases in the basement rocks of Northern Apennines & Pandeli Elter F.M. meridionale in scala 1:200.000. LAC, Firenze. Lazzarotto A. & Signorini R. (1971) - Carta Geologica della Toscana Giannini E., 18 Michard A. & Jourdan C. (1990) - Ductile extension in alpine Corsica. Geology, Goffè B., R., Fournier Dubois R., L., Jolivet Brogi A. & Liotta D. (2008) - Highly extended terrains, lateral segmentation of the substratum, and basin development: the and basin development: segmentation of the substratum, lateral (2008) - Highly extended terrains, Brogi A. & Liotta D. Umbria foredeep system: Nappe-western of the Tuscan Miocene evolution & Pialli G. (2002) - Early-middle Boncio P. F., Brozzetti Sea, insights fr of compression and extension in the Tyrrhenian (2000) - Migration L. & Cadet J.P. Jolivet Monié P., Brunet C., (1993) - Upper crustal structure of the Larderello geothermal field as a feature post-colli Dini I. & Liotta D. Cameli G.M., References and extensional structures in deep temperatures G. (2004) - Heat flow, & Ranalli Liotta D. Lazzarotto A., Brogi A., Bellani S., Boyce A.J., Fulignati P. & Sbrana A. (2003) - Deep hydrothermal circulation in the granite intrusion beneath Larderello geother circulation in the granite A. (2003) - Deep hydrothermal & Sbrana Fulignati P. A.J., Boyce area (Northern Apennines, Italy): Neogene-Quaternary Brogi A. (2008) - The structure of the Monte Amiata volcano-geothermal as imaged by reflection seismic lines: the G. (2003) - Extensional shear zones & Ranalli Liotta D. Lazzarotto A., Brogi A., (Italy) G. (2005) - Crustal structures in the geothermal areas of Southern Tuscany & Ranalli Liotta D. Lazzarotto A., Brogi A., geological field trips 2011 - 3(2.2) references 39 , te rains: A. Brogi - D. Liotta Fluid flow paths in fossil and active geothermal fields: the Plio-Pleistocene Boccheggiano-Montieri Larderello areas inner Northern Apennines, Italy. Tectonophysics, 315, 109-122. Tectonophysics, inner Northern Apennines, Italy. 99, 623-644. Earth Sci. (Geol. Rundsch.), Italy). Int. J. the ore deposits of Boccheggiano-Montieri area (Southern Tuscany, 41, 121-140. Soc. Geol. Ital., 319, 199-222. and tectonic implications. Tectonophysics, Italy): Fluid genesis and migration (central-northern of the Larderello geothermal field (Southern Tuscany, intrusions in the southern branch metamorphism during multiple granitic Geol. Soc. Lond. 165, 247-262. Italy). J. 223, 117-147. Tectonophysics, Italy. continental lithosphere in the genesis of Neogene-Quaternary magmatism central 152, 20-50. Geotherm. Res., Volcan. geothermal system (Italy). J. from the Larderello-Travale doi:10.1016/j.jvolgeores.2005.09.011 ). Mem. Soc. Geol. It. 6, 151-197. 24 (9),1479-1490. Geology, Italy). Journal of Structural Apennines, central DOI: 10.3301/GFT.2011.03 Liotta D., Ruggieri G., Brogi A., Fulignati P., Dini A. & Nardini I. (2010) - Migration of geothermal fluids in extensional ter Dini A. & Nardini I. (2010) - Migration Fulignati P., Brogi A., G., Ruggieri Liotta D., Mem. carta della Moho. centromeridionale: la nuova Locardi E. & Nicolich R. (1988) - Geodinamica del Tirreno e dell’Appennino in the Northern Apennines (2000) - Fluid geochemical transect G.F. & Tassi O. Vaselli Martinelli G., Magro G., Minissale A., 298, 413-442. Geol. Soc. London Spec. Publ., Molli G. (2008) - Northern Apennine-Corsica orogenic system: an updated review. C. & Funiciello R. (2008) - Pliocene-Pleistocene HT/LP Faccenna Boujbauenne M., Villa I.M., Balsamo F., F., Rossetti (1993) - Geochemical and petrological evidence of the subduction delaminated Adriatic & Manetti P. Innocenti F. Serri G., systematics in a subsurface grani M. & Bertini G. (2006) - Geochronology and isotope transport Puxeddu G., Ruggieri Villa I.M., Lazzarotto A. (1967) - Geologia della zona compresa fra l’alta Valle del Fiume Cornia ed il Torrente Pavone (Prov. di Pisa e (Prov. Pavone del Fiume Cornia ed il Torrente l’alta Valle compresa fra Lazzarotto A. (1967) - Geologia della zona meaning in the Montagnola Senese metamorphic rocks (inner Northern (2002) - D2 asymmetric folds and their vergence Liotta D. G. (1999) - Correlation between seismic reflectivity and rheology in extended lithosphere: Southern Tuscany & Ranalli Liotta D.