6.a. Faults, palaeo-fluids and mineralization

Two main mineralized fault systems have been recognized in both study areas (Figs 3, 5). The first fault system is defined by NNE-trending, E-dipping low- (15–30°) to middle- (30–50°) angle faults. These are well exposed in the Rio Albano area, where fault planes control the morphology, and in the northeastern part of the Valle Giove stope (Fig. 3). The fault architecture is defined by slip surfaces and fault cores up to 50 cm thick, characterized by cataclastic rocks cemented by haematite–quartz–adularia and/or Fe-hydroxides (Fig. 9a–g). Damage zones consist of fractured rock volumes up to 10 m thick mainly cross-cut by haematite veins (Fig. 9h). In some cases, the hydrothermal mineral assemblage was involved in progressive deformation, hence, in turn, comminuted and cemented by newly formed haematite/magnetite (Fig. 9i).

Fig. 9.

Examples of mineralized faults. (a) Middle-angle normal fault deforming the Triassic metaconglomerate and metasiltstone of the Verrucano Group exposed in the Topinetti area. (b) Middle-angle normal fault controlling the slope morphology along the sea coast of the Topinetti area. (c) Normal fault dissecting haematite and relative hydroxides exposed at the Calendozio stope. (d) Conjugate high-angle normal faults exposed at the Calendozio stope. (e) High-angle normal fault cutting the quartz-metaconglomerate of the Verrucano Group at the Valle Giove stope. (f) Mineralized damage zone associated with high-angle normal fault at the Calendozio stope. (g) Cataclasite cemented by haematite exposed in the Topinetti area. (h) Haematite shear veins developed in the damage zone of the middle-angle normal fault in the Topinetti area. (i) Brecciated haematite and pyrite within the massive Fe-ore body characterizing the Valle Giove stope. (j) Example of breccia cemented by haematite related to the hydrofracturing process.

Shearing was often accompanied by hydrofracturing during faulting events (Fig. 9j). Kinematic indicators consist of shear veins made up of haematite + quartz + adularia and/or epidote, calcite and quartz (Fig. 10) and slickenlines, ENE-trending and plunging dominantly (Fig. 11).

Fig. 10.

Kinematic indicators on the fault planes, consisting of (a, b) mechanical striation covered by Fe-hydroxides; (c) mechanical striation and chatter marks; (d) haematite mirror fault plane; (e) syn-kinematic adularia developed within an extensional jog; (f) syn-kinematic epidote on a fault plane. Mineral abbreviations: Adl – adularia; Ep – epidote; Fe –  iron.

Fig. 11.

(a) Conceptual sketch showing the relationships among low-, middle- and high-angle normal faults. (b) Stereographic diagrams (lower hemisphere, equal-area projection) of the whole structural and kinematic dataset collected in the study areas.

Kinematic data have been collected at structural stations (Figs 3, 5); results are reported in Figure 11 where the structural relationships among the analysed faults are summarized.

Geometrical relationships between mineralized low- and middle-angle normal faults are well exposed in the Topinetti area (Fig. 12a) where middle-angle faults (Fig. 12b) lie on low-angle ones (Fig. 12c, d), the latter representing the main detachment. Both low- and middle-angle normal faults host a common mineralization, mainly made up of cataclastic haematite and quartz, indicating a contemporaneity among fluid circulation, mineralization and fault activity (Fig. 12e–g). A tectonic transport towards the east characterizes these faults (Fig. 11).

Fig. 12.

(a) Structural sketch-map of the Topinetti area where the main structures and the stations of structural analysis are indicated. The relationships between low- and middle-angle normal faults are also highlighted. (b) Panoramic view of the Topinetti sea coast where major middle-angle normal faults are indicated. (c) Panoramic view of the Topinetti area highlighting the relationships between low- and middle-angle normal faults. (d) Detail of fault relationships enlarged from (c). (e) Enlarged area of the detachment showing a rock fault deriving from a combined effects of cataclasis and hydrofracturing processes. (f) Detail of zoned mineralized fault rock enlarged from (d). (g) Detail of kinematic indicators recognized on the detachment slip surface.

Mineralized NE- and WNW-trending high-angle normal to oblique-slip faults (Fig. 11) dissect the low-angle faults in both study areas. The NE- and WNW-trending high-angle faults host haematite + quartz mineralization in their fault zone and associated splay structures (Fig. 13). The geometric relationship between splay structures and the main fault accounts for a relevant strike-slip component. Accordingly, kinematic indicators consist of mineralized shear veins and mechanical striations, indicating a left-lateral and right-lateral oblique-slip shear sense for the NE- and WNW-trending faults, respectively (Fig. 11).

Fig. 13.

Geological sketch-map showing NE-trending high-angle normal fault with associated splay structures both mineralized mainly by haematite and magnetite.

In the Valle Giove area, the principal Fe-ore bodies consist of W-dipping strata-bound bodies (Figs 3, 14a). Their areal distribution is however concentrated in a limited rock volume, WNW-oriented, where the exploitation was carried out (Fig. 14a).

Fig. 14.

(a) Panoramic view of the Valle Giove stope where the distribution of the Fe-ore bodies is indicated. (b–e) Brecciated haematite and pyrite along the WNW-striking faults. (f) Brecciated volumes of haematite + pyrite and metre-thick strata-bound body strictly confined within metasomatized metasiliciclastic levels. (g) Detail of the brecciated body shown in (f).

In this framework, the central part is formed by a massive Fe-ore body, up to 20 m thick, often composed by brecciated haematite and pyrite (Fig. 14b, c). This suggests contemporaneity among fluid circulation, mineralization and tectonic activity, distributed along the WNW-trend (Fig. 14d, e).

The Fe-ore body laterally extends forming metre-thick strata-bound bodies strictly confined within metasomatized metasiliciclastic levels (Fig. 14f, g). In these strata-bound bodies, the ore body is not cataclastic, although local minor brittle deformation can occur.

Fe-ore bodies are also surrounded by decimetre to metre thick greenish zones, where quartz, phlogopite and chlorite are the main mineral phases. These formed during a diffuse circulation of geothermal fluids that, from the fault zones, permeated into selected volumes, determining intense metasomatism in metasiliciclastic rocks with a medium to coarse grain size. In contrast, metasiltstone and phyllite are only partially altered (Fig. 15).

Fig. 15.

(a) Example of metasiltstone affected by metasomatism. The greenish zones are mainly composed by phlogopite, chlorite, haematite and quartz and formed during the diffuse circulation of geothermal fluid in selective volumes. (b) Detail of haematite pods developed within the metasomatic volumes.

6.b. Secondary permeability

The above reported data highlight the role of fractures in determining the geothermal fluid pathways. To evaluate the permeability (k) values which favoured the fluid flow along the fault zones, and to compare the results with similar ones from active geothermal contexts, we selected the most accessible outcrops where mineralized faults are exposed, and where the exposures are useful for collecting data along scan lines and scan boxes. The study outcrops are located in the Topinetti area, close to the sea coast (Fig. 5). The methodology is based on various steps, as described in Zucchi et al. (Reference Zucchi, Brogi, Liotta, Rimondi, Ruggieri, Montegrossi, Caggianelli and Dini2017), and on the assumption that permeability can be evaluated assuming fractures as described by two spaced parallel planes (parallel plate model in Zimmerman & Bodvarsson, Reference Zimmerman and Bodvarsson1996; Nicholl et al. Reference Nicholl, Rajaram, Glass and Detwiler1999; Leung & Zimmerman, Reference Leung and Zimmermann2012). For the goals of our work, the methodological approach can be described as follows: (a) a detailed geological map at 1:100 scale of the selected area (scan area) is realized, reporting the structures to be analysed; (b) within the scan area, three scan lines are disposed (Fig. 16a, b) orthogonally to the trend of the main structures. Scan line length was up to 9 m, sufficient to quantify the minimum and maximum fracture spacing; (c) scan boxes of 0.4 m per side, regularly spaced (1 m), are then settled along each scan line. Within each scan box, fracture parameters (length, maximum and minimum aperture, frequency) were collected. Veins formed by a clear contribution of hydrofracturing (i.e. the occurrence of hydraulic breccia embedded in the mineralization) are not compatible with the parallel plate model, resulting in unreasonable values of permeability (≥10⁻¹² m², see scan line C in Fig. 16). To avoid this issue, we statistically considered only those results where k < 10⁻¹² m², thus representing the maximum possible permeability estimates, in the frame of the model we assumed.

Fig. 16.

(a) Structural map of the scan area in which scan line locations have been reported. (b) Scan line results. The blue dashes located on the scan line bars indicate the scan boxes in which geometrical data were collected. (c) Diagrams showing the results obtained by geometrical parameters collected within each scan box. In detail diagrams show: (i) frequency; (ii) total vein length; (iii) minimum and maximum aperture; (iv) secondary permeability; (v) efficient porosity versus distance. (d) Statistical distribution of the permeability values.

The measurements were therefore processed in order to fit the following equation (Zucchi et al. Reference Zucchi, Brogi, Liotta, Rimondi, Ruggieri, Montegrossi, Caggianelli and Dini2017), assuming the plate boundary model (Gale, Reference Gale and Narasimhan1982; Cox et al. Reference Cox, Knackstedt, Braun, Richards and Tosdal2001) and laminar flow (Zimmerman & Bodvarsson, Reference Zimmerman and Bodvarsson1996; Leung & Zimmerman, Reference Leung and Zimmermann2012):

$$k = \left( {{2 \over 3} \times {{{b^3}} \over L}} \right) \times F \times {10^{ - 6}}$$

where b and L represent the mean of fracture aperture and length, respectively, and F is the factor of fracture connectivity, here considered to be 0.4 in a possible range of variation between 0 and 1, from parallel to orthogonal angular relationships among fractures (Nicholl et al. Reference Nicholl, Rajaram, Glass and Detwiler1999).

The results are reported in Figure 16c together with all collected data. In this way, permeability was computed for each scan box, considering the harmonic mean that provides the best approximation for a system dominated by long fractures. Diagrams (Fig. 16c) and statistical evaluations (Fig. 16d) indicate permeability values ranging between 5 × 10⁻¹³ and 5 × 10⁻¹⁶ m². Efficient porosity, deriving from the ratio between the area occupied by the mineralized fractures and the area of the scan box, shows variable values encompassed between 0.02 % and 3.69 %, except for a spike of ∼35 % in correspondence with the higher permeability values along the scan line C in points (Fig. 16c), located at the intersection with a fault zone.

6.c. Inherited structures and fluids

The faults and associated fluid flow (i.e. present mineralization) cross-cut all the previously formed structures. In the study areas, the inherited structures are related to two folding events, referred to as F₁ and F₂, respectively. The best evidence of F₁/F₂ interference (dominantly Type 2 in Ramsay, Reference Ramsay1967) is visible on the benches of the Valle Giove stope (Fig. 3). Here, the contact separating the Rio Marina Fm from the Verrucano Group is deformed by an F₁ sub-isoclinal anticline, with a sub-vertical axial plane, NW-striking. The F₁ line shows a dominant sub-vertical attitude. At outcrop scale, minor F₁ folds (i.e. second-order folds) are rare and consist of decimetre to metre isoclinal folds characterized by steeply dipping axial planes and sub-vertical hinge lines (Fig. 17). A main tectonic foliation (main schistosity, S₁) is associated with these folds (Fig. 17a, b). The angular relationship with the bedding (S₀) is well recognizable in the fold hinge zone, whereas S₁ becomes mostly parallel, or sub-parallel, to S₀ in the fold limbs (Fig. 17c). S₁ consists of a pervasive and penetrative slaty-type foliation (sensu Durney & Kisch, Reference Durney and Kisch1994), or penetrative slaty cleavage (sensu Williams, Reference Williams1972; Hobbs et al. Reference Hobbs, Means and Williams1976; Williams et al. Reference Williams, Means and Hobbs1977), well developed in the fine-grained lithotypes such as phyllite and metasiltstone. Analyses of thin-sections highlight that S₁ is a continuous foliation (sensu Passchier & Trouw, Reference Passchier and Trouw1996) with a mineral assemblage mainly consisting of quartz + phengite + muscovite + kaolinite ± calcite ± Fe–Ti oxides (Fig. 17d). Phengite, associated with S₁, displays a Si content of ∼3.5 atoms per formula unit (apfu) indicating that S₁ developed during high-pressure (HP) conditions (Bianco et al. Reference Bianco, Brogi, Caggianelli, Giorgetti, Liotta and Meccheri2015).

Fig. 17.

F₁ and F₂ folds and related tectonic foliations (S₁ and S₂). Valle Giove stope: (a) F₂ fold affecting metasandstone and inter-layered phyllite and metasiltstone (Verrucano Group); (b) detail of S₁ and S₂ relationships in the F₂ hinge zone; (c) isoclinal F₁ fold deformed by F₂. Topinetti area: (d) thin-section (plane polarized light) showing the S₂ foliation as derived by pressure-solution mechanism; (e) isoclinal F₁ fold deformed by F₂ folds and details shown in (f) and (g).

F₁ folds are deformed by metre to decimetre open to tight asymmetric F₂ folds, with both sub-horizontal NNE-trending hinge lines and axial planes (Figs 3, 17). F₂ folds are characterized by a spaced and non-penetrative (sensu Durney & Kisch, Reference Durney and Kisch1994) tectonic foliation (S₂), also defined as crenulation cleavage (Rickard, Reference Rickard1961; Gray, Reference Gray1977), mainly affecting metapelite and phyllite (Fig. 17). On the contrary, this foliation developed discontinuously in the metasandstone and is not visible in the metaconglomerate levels. Thin-section analyses highlight that S₂ consists of a rough and smooth crenulation cleavage (Fig. 17d), as derived by pressure-solution mechanisms (sensu Passchier & Trouw, Reference Passchier and Trouw1996). F₁ and F₂ folds exposed in the Calendozio and Topinetti areas show similar features to those already described.

Ore lenses (pyrite, quartz and haematite, adularia, quartz) together with the greenish envelopes, made up of neoformational quartz, phlogopite and chlorite, are mimetic on the metasiliciclastic rock-texture, therefore following the geometry of both the F₁ and F₂ folds (Fig. 18).

Fig. 18.

Examples of ore and metasomatic lenses developed mimetically to folded S₀ parallel to S₁ foliation from the Valle Giove stope: (a) folded and mineralized metasandstone and (b) detail of the hinge zone; (c) folded and mineralized metasandstone and (d) detail of the hinge zone.

7.a. Deformational events in the framework of the Elba and Northern Apennines tectonic evolution

Concerning the folding events described in the study areas, similar structures have been recognized in the metamorphic rocks exposed in the inner Northern Apennines, where two main folding events have been described by several authors (e.g. Costantini et al. Reference Costantini, Decandia, Lazzarotto and Sandrelli1988; Carmignani & Kligfield, Reference Carmignani and Kligfield1990; Conti et al. Reference Conti, Costantini, Decandia, Elter, Gattiglio, Lazzarotto, Meccheri, Pandeli, Rau, Sandrelli, Tongiorgi and Di Pisa1991; Liotta, Reference Liotta2002; Molli et al. Reference Molli, Giorgetti and Meccheri2002; Carosi et al. Reference Carosi, Iacopini and Montomoli2004; Aldinucci et al. Reference Aldinucci, Brogi and Sandrelli2005; Brogi, Reference Brogi2006; Brogi & Giorgetti, Reference Brogi and Giorgetti2012; Capezzuoli et al. Reference Capezzuoli, Spina, Brogi, Liotta, Bagnoli, Zucchi, Molli and Regoli2021). The F₁ event, producing the isoclinal folds mainly recognized in the Valle Giove stope (Fig. 17e–g), is related to the stacking of the tectonic units forming the Northern Apennines during the collisional stage. S₁ schistosity in fact displays the HP mineral assemblage as inferred for the Verrucano Group exposed in the Monte Giove, at the north of the Valle Giove mine (Bianco et al. Reference Bianco, Brogi, Caggianelli, Giorgetti, Liotta and Meccheri2015). Similar evidence has been described in southern Tuscany, along the Middle Tuscan Ridge (Costantini et al. Reference Costantini, Decandia, Lazzarotto and Sandrelli1988), where S₁ consists of a composite tectonic foliation associated with HP assemblages (1.5–1.2 GPa and 400–350 °C; Theye et al. Reference Theye, Reinhardt, Goffé, Jolivet and Brunet1997; Brunet et al. Reference Brunet, Monié, Jolivet and Cadet2000; Rossetti et al. Reference Rossetti, Faccenna, Jolivet, Goffé and Funiciello2002) and carpholite-bearing quartz shear veins parallel to the S₁ (Theye et al. Reference Theye, Reinhardt, Goffé, Jolivet and Brunet1997; Rossetti et al. Reference Rossetti, Faccenna, Jolivet, Funiciello, Tecce and Brunet1999; Brogi & Giorgetti, Reference Brogi and Giorgetti2012; Giuntoli & Viola, Reference Giuntoli and Viola2021). On Elba Island, the S₁ assemblage has been dated at 19.68 ± 0.15 Ma and 20.99 ± 0.73 Ma by ⁴⁰Ar/³⁹Ar on phengite (Deino et al. Reference Deino, Keller, Minelli and Pialli1992; Ryan et al. Reference Ryan, Papeschi, Viola, Musumeci, Mazzarini, Torgersen, Sørensen and Ganerød2021) and at 19.8 ± 1.4 Ma by ⁴⁰Ar/³⁹Ar on glaucophane (Bianco et al. Reference Bianco, Godard, Halton, Brogi, Liotta and Caggianelli2019), thus ascribing an Aquitanian–early Burdigalian (at least) age to the later stage of the collisional event (Pertusati et al. Reference Pertusati, Raggi, Ricci, Duranti and Palmeri1993). Additional radiometric dates on similar assemblages obtained for the surrounding areas indicate ages between ∼27 and 25 Ma (Brunet et al. Reference Brunet, Monié, Jolivet and Cadet2000 with references therein), therefore suggesting that the F₁ folds are structures deriving from a polyphase deformational event (Molli et al. Reference Molli, Conti, Giorgetti, Meccheri and Oesterling2000; Brogi & Giorgetti, Reference Brogi and Giorgetti2012) at deep structural levels. Conversely, F₂ folds are asymmetric and developed at shallower structural levels, as indicated by the texture and structure of the S₂ foliation. This folding event is younger than 19.68 ± 0.15 Ma and older than the age of mineralization (5.39 ± 0.46 Ma and 5.53 ± 0.14 Ma), developed during the activity of the normal faults, now dissecting the F₁/F₂ folds. The F₂ event is interpreted as the effect of the tectonic activity ongoing during exhumation and uplift of the previously stacked tectonic pile (Carmignani & Kligflied, Reference Carmignani and Kligfield1990; Carmignani et al. Reference Carmignani, Decandia, Disperati, Fantozzi, Lazzarotto, Liotta and Meccheri1994; Montomoli et al. Reference Montomoli, Ruggieri, Boiron and Cathelineau2001; Liotta, Reference Liotta2002; Carosi et al. Reference Carosi, Iacopini and Montomoli2004).

7.b. Permeability evaluation

Suitable secondary permeability is essential for geothermal fluid flow (Rowland & Sibson, Reference Rowland and Sibson2004; Zucchi et al. Reference Zucchi, Brogi, Liotta, Rimondi, Ruggieri, Montegrossi, Caggianelli and Dini2017; Liotta et al. Reference Liotta, Brogi, Ruggieri and Zucchi2021). The method we adopted to estimate permeability (k) is based on measurement of vein length and thickness, assuming that veins with the same mineral association were contemporaneously formed, simplifying the polyphase development of the mineralized veins. This is a necessary assumption, since we deal with shear veins, where cataclasis is contemporaneous with mineral deposition, preventing the typical banded vein texture indicative of different events of crystallization (e.g. Oliver & Bons, Reference Oliver and Bons2001; Bons et al. Reference Bons, Elburg and Gomez-Rivas2012).

Nevertheless, considering the limit of the parallel plate model and the above-mentioned simplifications, the results are comparable with the permeability measured in active geothermal systems (Rowland & Sibson, Reference Rowland and Sibson2004). Moreover, taking into account that the study area can be considered as having developed in the same tectonic framework of the Larderello geothermal field (Puxeddu, Reference Puxeddu1984; Liotta et al. Reference Liotta, Brogi, Meccheri, Dini, Bianco and Ruggieri2015; Zucchi et al. Reference Zucchi, Brogi, Liotta, Rimondi, Ruggieri, Montegrossi, Caggianelli and Dini2017), it is significant that the permeability values we have obtained are in the range of those measured within the so-called first reservoir exploited in the Larderello geothermal area, at 1–2 km depth (Cappetti et al. Reference Cappetti, Parisi, Ridolfi and Stefani1995; Batini et al. Reference Batini, Brogi, Lazzarotto, Liotta and Pandeli2003; Romagnoli et al. Reference Romagnoli, Arias, Barelli, Cei and Casini2010), located at the same structural level we studied in the mining area. Another positive confirmation derives from the experimental studies conducted on fluid-assisted brittle shear zones, which provide typical values of permeability of between 10⁻¹⁴ and 10⁻¹⁶ m² (Townend & Zoback, Reference Townend and Zoback2000; Wibberley & Shimamoto, Reference Wibberley and Shimamoto2003).

7.c. The tectonic control on the palaeo-fluid circulation

Oblique-slip to normal faults dissected the F₁ and F₂ folds, indicating that a brittle deformation occurred in an extensional setting and affected the previously formed contractional structures. This setting, described by several authors for the whole inner Northern Apennines (e.g. Carmignani et al. Reference Carmignani, Decandia, Disperati, Fantozzi, Lazzarotto, Liotta and Meccheri1994; Liotta et al. Reference Liotta, Cernobori and Nicolich1998; Brogi, Reference Brogi2008, Reference Brogi2020; Brogi & Liotta, Reference Brogi and Liotta2008; Molli, Reference Molli, Siegesmund, Fügenschuh and Froitzheim2008; Barchi, Reference Barchi, Beltrando, Peccerillo, Mattei, Conticelli and Doglioni2010), has also been reconstructed for Elba Island (e.g. Keller & Pialli, Reference Keller and Pialli1990; Pertusati et al. Reference Pertusati, Raggi, Ricci, Duranti and Palmeri1993; Collettini & Holdsworth, Reference Collettini and Holdsworth2004; Smith et al. Reference Smith, Holdsworth and Collettini2011; Liotta et al. Reference Liotta, Brogi, Meccheri, Dini, Bianco and Ruggieri2015; Zucchi et al. Reference Zucchi, Brogi, Liotta, Rimondi, Ruggieri, Montegrossi, Caggianelli and Dini2017; Mazzarini et al. Reference Mazzarini, Musumeci, Viola, Garofalo and Mattila2019; Zucchi, Reference Zucchi2020) where extensional tectonics interplayed with magmatism and related hydrothermalism (Dini et al. Reference Dini, Innocenti, Rocchi, Tonarini and Westerman2002; Westerman et al. Reference Westerman, Dini, Innocenti, Rocchi, Breitkreuz and Petford2004; Viti et al. Reference Viti, Brogi, Liotta, Mugnaioli, Spiess, Dini, Zucchi and Vannuccini2016). Nevertheless, this setting has been challenged by some authors who support a compressional setting active until Pliocene time, at least for Elba Island (e.g. Ryan et al. Reference Ryan, Papeschi, Viola, Musumeci, Mazzarini, Torgersen, Sørensen and Ganerød2021). We disagree with this interpretation simply because the normal faults controlling the mineralization exposed in the study areas, and particularly in the Calendozio–Topinetti localities (Fig. 2), were active during late Miocene – Zanclean time as indicated by the ages (5.39 ± 0.46 Ma: Lippolt et al. Reference Lippolt, Wernicke and Bahr1995; 5.53 ± 0.14 Ma: Wu et al. Reference Wu, Stuart, Heizler, Benvenuti and Hu2019) obtained for the syn-faulting haematite–adularia hydrothermal assemblages. This unequivocal consideration allows us to ascribe the faults controlling the geothermal fluid circulation to an extensional setting. A similar conclusion was reached, for the same areas and faults, by Mazzarini et al. (Reference Mazzarini, Musumeci, Viola, Garofalo and Mattila2019) who described an E–W extensional stress field at the time of faulting (see also Spiess et al. Reference Spiess, Langone, Caggianelli, Stuart, Zucchi, Bianco, Brogi and Liotta2022 for a discussion).

The low- and middle-angle faults were dissected by NE- and WNW-trending high-angle faults mineralized by the same hydrothermal assemblage. The concomitant circulation of the common geothermal fluid in both the low- and middle-angle normal and high-angle faults indicates that the fault systems were contemporaneously active at the time of the mineralization, reasonably increasing the secondary permeability (Debenedetti, Reference Debenedetti1952; Gilliéron, Reference Gilliéron1959; Deschamps et al. Reference Deschamps, Dagallier, Macaudier, Marignac, Maine and Saupé1983; Mazzarini et al. Reference Mazzarini, Musumeci, Viola, Garofalo and Mattila2019). In this view, the coexistence of all the faults should be explained in the same kinematic framework, as illustrated in Figure 11, from which a W–E extensional direction is derived.

However, as already described, the same mineralization, in addition to fault damage zones, also took place in strata-bound bodies. The temperature of the mineralizing fluids is >310 °C (Deschamps et al. Reference Deschamps, Dagallier, Macaudier, Marignac, Maine and Saupé1983) and, coupled with Pb isotopic data (Lattanzi et al. Reference Lattanzi, Benvenuti, Gale, Hansmann, Köppel and Stos-Gale1997), converges towards a deep interaction with fluids of a magmatic origin (Tanelli et al. Reference Tanelli, Benvenuti, Costagliola, Dini, Lattanzi, Manieri, Mascaro and Ruggieri2001; Orlando et al. Reference Orlando, Ruggieri, Chiarantini, Montegrossi and Rimondi2017). It implies that the secondary permeability related to the fault zones played a role in channelling deep fluids. This is favoured when the fault kinematics range from strike- to oblique-slip, since fluids are preferentially channelled along the intermediate stress axis (Sibson, Reference Sibson2000).

Our kinematic database indicates that the strike- to oblique-slip faults are WNW- and NE-trending (Fig. 11). The WNW-trend is also the orientation of the elliptic wrapping of the Fe-ore body exposed in the Valle Giove stope (Fig. 3). We note that in the central part of the mining area, along the WNW-trend, cataclastic mineralization occurs in discontinuous sectors. This framework accounts for extensional jogs developed in the linkage zone of WNW-trending fault segments. Deep fluids flowed up through these extensional jogs, laterally permeating the suitable lithotypes and triggering selective metasomatic processes within quartz-metasandstone and metaconglomerate (Fig. 19). The lateral migration was reasonably supported by repeated episodes of veining and circulation of fluids occasionally overpressured, as testified to by zones affected by hydrofracturing (cf. also Mazzarini et al. Reference Mazzarini, Musumeci, Viola, Garofalo and Mattila2019). Thus, the fluids permeating within the quartz-rich rocks, being confined at the top and bottom by impervious metapelite levels, reacted with the silica (Deschamps et al. Reference Deschamps, Dagallier, Macaudier, Marignac, Maine and Saupé1983), promoting metasomatic processes followed by the deposition of Fe-oxides (magnetite and haematite) and Fe-sulphide (pyrite), strongly depending on the fluid chemical–physical features, i.e. temperature, redox capacity and pH value (Anderson & Burnham, Reference Anderson and Burnham1965; Sommerfeld, Reference Sommerfeld1967; Fournier et al. Reference Fournier, Rosenbauer and Bischoff1982; Manning, Reference Manning1994; Shmulovich et al. Reference Shmulovich, Yardley and Graham2006).

Fig. 19.

Conceptual model showing the geothermal fluid migration and related ore body development reconstructed for the Valle Giove stope: the deep fluids flowed up through extensional jogs formed in the realizing step-over zone of WNW-trending faults; these fluids laterally permeated in suitable lithotypes and produced selective metasomatic processes especially within quartz-metasandstone and metaconglomerate.

This process leads to metres-thick strata-bound bodies (Zuffardi, Reference Zuffardi1990) having a geometry shaped by the F₁ and F₂ folding events. Consequently, the interpretation should be abandoned that the apparently folded strata-bound bodies are related to a syn-diagenetic/hydrothermal or metamorphic origin (Bodechtel, Reference Bodechtel1965; Cortecci et al. Reference Cortecci, Lattanzi and Tanelli1985; Zuffardi, Reference Zuffardi1990; Lattanzi et al. Reference Lattanzi, Benvenuti, Costagliola and Tanelli1994) responsible for a concentration of metals formerly deposited in a sedimentary/hydrothermal setting during Palaeozoic and Triassic times (see also Tanelli et al. Reference Tanelli, Benvenuti, Costagliola, Dini, Lattanzi, Manieri, Mascaro and Ruggieri2001 for a discussion).