New Insights into Graphite Deposits in Chisone and Germanasca Valleys (Dora-Maira Massif, Western Italian Alps): Scientific Advances and Applied Perspectives

Abstract

Graphite is a critical raw material due to its pivotal role in the green transition; hence, there is a renewed interest in its exploration across Europe. The Chisone and Germanasca Valleys (Piemonte, IT) were home to significant graphite exploitation until the 20th century, owing to the widespread presence of graphite ore bodies hosted in the metasedimentary succession of the Pinerolo Unit in the Dora-Maira Massif (Western Alps). This contribution presents a renewed study on the geology, mineralogy, petrography, and geochemistry of graphite ores and their host rocks, employing OM, SEM-EDS, and BSE, μRaman, and ICP-OES/MS and INAA analyses. Mineralization occurs in two metasedimentary successions: (i) the Bourcet-type succession (meta-conglomerates and meta-sandstones intercalated with meta-siltstones/metapelites) and (ii) the Pons-type succession (meta-siltstones/metapelites intercalated with minor meta-arenites). Graphite occurs as (i) high-purity, fine-grained crystals dispersed within or concentrated in layers along the regional schistosity, or (ii) low-purity, coarse-grained crystals within shear zones. Based on crystallinity, three types of graphite were distinguished: high (Type I), intermediate (Type II), and poor (Type III) crystalline graphite, likely formed under different genetic conditions. The comparison of these findings has implications for future exploration and provides new insights into the metallogeny and geological evolution of the area.

1. Introduction

Graphite is widely used in modern industry, owing to its specific crystal structure, consisting of carbon atoms densely arranged in parallel-stacked, hexagonal honeycomb-lattice sheets. This configuration provides exceptional physical and chemical properties, making the graphite suitable for many applications. In detail, graphite has high electrical and thermal conductivity, high resistance to thermal chemical inertness, compressibility, elasticity, and lubricity [1,2,3,4,5]. For the former reasons, natural graphite is globally used for the manufacture of refractories, electrodes, lubricants, foundries, and batteries [6]. The final use of graphite in a specific industrial application, however, is strictly dependent on several key factors, i.e., carbon content, purity, shape, and crystal size; for instance, highly crystalline graphite is mainly used as refractory material in several industrial processes, whereas low-crystallinity graphite is preferentially used for lubricant [3,7]. Finally, the production of graphene requires crystalline graphite with an exceptionally high purity level (99.9% C) [8,9,10].

In recent years, natural graphite has been added to the list of critical and strategic raw materials [11,12,13,14] (CRMs/SRMs), which includes a range of metals, rocks, and minerals of high economic importance, essential for technological development but subjected to likely severe disruption along the supply chain [14]. Natural graphite is pivotal in the manufacture of electric components, aerospace applications, and batteries for the e-motive industry [13,15]. Global market projections predict a 500% increase in graphite demand by 2050, driven by the intensification of transportation electrification. This surge is the result of the growing emphasis on global climate neutrality, in alignment with the 17 Sustainable Development Goals [16]. Currently, anode material used in lithium-ion batteries for the e-motive industry and storage devices is derived from synthetic graphite, produced through the heat treatment of petroleum coke or coal tar pitch [2,17]. This process requires high energy consumption and production costs [18]. In contrast, natural graphite is a more energy and cost-efficient alternative. Short-term forecasts predict that batteries are expected to account for the largest growth in demand for natural graphite, with 700,000 metric tons of natural graphite demanded in 2025 compared to 133,000 metric tons in 2018 [19].

The world’s leading graphite mine producer is China, with 78% of total world production in 2024, followed by Madagascar, Mozambique, Brazil, Russia, Norway, Tanzania, India, North and South Korea, Canada, Turkey, Mexico, Ukraine, USA, and Austria, accounting for a total of 1.6 million metric tonnes for 2024 [20]. Remarkably, China, Brazil, Madagascar, and Mozambique also hold the largest reserves of natural graphite [20]. As mentioned above, apart from Norway, Ukraine, and Austria (whose mine production for 2024 accounts respectively to 7.000, 1200, and 500 metric tonnes), none of the other EU countries concur to graphite production, exposing the EU to high supply risks. In this context, EU countries are called to respond proactively to the future increase in demand and likely shortages, with new exploration programs starting from ancient mining districts. Driven by these concerns, in 2024, the European Commission drew up the “Critical Raw Material Act” [21], which emphasizes the importance of enhancing a sustainable and domestic supply of raw materials for the development of strategic industrial sectors of the European Community. In alignment with this directive, several EU countries (France, Spain, Portugal, etc.) have renewed legislation to facilitate the exploration of likely domestic resources and, in June 2024, Italy also issued the law DL-28 June 24 n.84 [22] to prioritize and promote the exploration and assessment of domestic mineral resources.

Italy has long been home to the exploitation of graphite. Important mining sites were located in the Chisone and Germanasca Valleys of the Piemonte Region (Northern Italy). Here, graphite was mined from the early 19th century to the mid-20th century, when the operations ceased due to the drop in economic feasibility, despite conspicuous reserves still being unexploited. Since the closure of the mines, no modern geological, mineralogical, petrographic, or geochemical studies have been conducted on graphite mineralization, and all the existing information on the ores dates back to the 20th century.

In line with the new European directives on CRMs/SRMs and Italian actions towards resource exploration, this work presents the results of a new study on graphite mineralization outcropping in several areas of the Chisone and Germanasca Valleys, belonging, from a geological point of view, to the Dora-Maria Massif, western Alps. The study employs modern analytical techniques to examine the geochemistry, mineralogy, and petrography of graphite ores and their host rocks. The primary aims are to better define the graphite occurrences, describe the petrographic and geochemical features of the graphite mineralization related to the host rocks, and possibly explore the presence of strategic co-products often associated with graphite mineralization. The results of this work are of significant interest from both a practical and scientific perspective. Indeed, this study can serve as a starting point for planning a more extensive exploration activity aimed at defining the economic potential of graphite ore bodies. Furthermore, some of the findings from this work may open new research perspectives concerning the onset and evolution of geological events that have affected the area.

2. Overview of Graphite Types, Genesis, and Mineral Association

Graphite deposits are due to two distinct processes [23]: (i) metamorphic, syngenetic graphite forms via the in situ metamorphic recrystallization of organic matter, whereas (ii) fluid-deposited, epigenetic graphite, precipitates from C-O-H fluids by saturation. The crystallinity of metamorphic graphite is influenced by the metamorphic grade at which the rock re-equilibrated, increasing from greenschist- to granulite-facies conditions. On the contrary, the crystallinity of the fluid-deposited graphite depends mainly, though not exclusively, on the temperature of the fluid from which it is precipitated. Epigenetic high-crystallinity graphite deposits are most common and form through precipitation from high-temperature fluids at relatively high pressure (e.g., granulite-facies terranes). Conversely, epigenetic low-crystallinity graphite deposits are rare due to the very low solubility of carbon in moderate-temperature fluids at low pressure (e.g., upper crust). More recently, a classification that combines graphite morphology and genetic processes has been proposed, with subdivision into three graphite types: (i) microcrystalline graphite, (ii) vein-type graphite, and (iii) flaky graphite [24]. The quality of the graphite ores, and hence the economic importance of the three types of deposits, is different.

The deposits of microcrystalline graphite, commercially known as amorphous graphite, and scientifically referred to as “cryptocrystalline graphite” [25], “semi-graphite”, or “graphitic carbon” [26,27], are characterized by fine-grained graphite particles associated with numerous impurities; consequently, the carbon content is generally low, ranging from 75 to 97% [28]. The aspect can be either earthy and friable, unctuous to the touch, or in massive aggregates, hard and a little oily with semi-metallic luster [25]. This microcrystalline graphite is mainly used as a lubricant. The mineralization often occurs in several repeated seams, from centimeters to a few meters in thickness, hosted in metasedimentary rocks and parallel to the main foliation. The ore bodies are typically lens-shaped or stratiform with ore grades ranging from 30 to 95% [24]. This graphite is syngenetic, being derived from low-grade metamorphism (sub-greenschist to greenschist-facies) [2] of organic matter originally deposited in sedimentary basins.

The vein-type graphite deposits, also known as “lump and chip graphite deposits”, consist of coarse-grained flakes, rosettes, fibers, or needle-shaped crystals arranged perpendicularly within veins cutting through the host rocks [29]. The graphite flakes are generally several centimeters in size; fragments of 0.5–0.8 cm in diameter are commercially known as “lump” and “chip” graphite [24]. This graphite type is highly pure, with carbon content reaching ~99% [28], making it ideal as a refractory material. The mineralization occurs in veins with variable thicknesses (from a few mm up to ~1 m, with an average of 30 cm) cutting high-grade metamorphic rocks (mainly granulites), or acid to ultrabasic igneous rocks [30]. The carbon source may either derive from organic matter assimilated by magma or be mantle-derived, or it can derive from the decarbonation of carbonate-bearing granulite-facies rocks [28]. Graphite precipitation from high-temperature fluids rich in CO₂ or CH₄ can be triggered by variations of temperature (T) and/or pressure (P) conditions, fluid buffering, or mixing with fluids of different compositions [24,26,28,31,32].

The flaky graphite deposits consist of coarse-grained, lamellar graphite crystals with a scaly appearance and high metallic luster [25]. Commercially, it is subdivided into two subtypes, according to the crystal size: coarse flake (150–850 μm in diameter) and fine flake (45–150 μm in diameter), with further commercial categories arranged for fine-flake graphite [33]. Due to its high crystallinity and purity (generally >90% carbon), flaky graphite is highly valued in industrial applications. Currently, it accounts for ca. 49% of the global natural graphite consumption [33,34,35,36,37]. These deposits are commonly found in paragneiss or marble that experienced amphibolite- or granulite-facies metamorphism. Similar to microcrystalline graphite, the carbon source derives from the organic matter originally present in the protoliths. The ore grades vary with the host rock type: in paragneisses, the graphite is finely dispersed, with ore grade ≤3%, while in granulite-facies marbles, graphite is commonly distributed in the whole rock with grades generally <0.5%, despite grades of 1–3% having been observed in some deposits [24]. Crystalline flaky graphite was also observed in marble with porphyroblastic textures; the ore grades in these latter range from a few % up to 25%. Graphite deposits are currently a target for vanadium (V) exploration as a graphite co-product. Several examples of V-enriched graphite mineralization occur worldwide [38,39,40,41,42,43]. Vanadium enrichment derives from biochemical processes occurring before the transformation into organic matter or during the degradation of organic matter [44] in sedimentary basins, and it is linked to the V mobility in the water column at different oxidation states (i.e., +3, +4, and +5), [38,40,41,44,45]. Later processes, such as hydrothermal alteration and/or metamorphism, may further fix V into minerals through secondary transformations [38,46].

3. Geological Background

The graphite ore bodies investigated in this study are located in the Chisone and Germanasca Valleys, approximately 65 km West of Torino, northwest Italy (Figure 1). Geologically, the ores lie within the rocks of the Pinerolo Unit, which is the lowermost tectono-metamorphic unit of the Dora-Maira Massif, within the Pennidic realm of the Western Alps [47,48]. The Dora-Maira Massif (Figure 1a), along with Monte Rosa and Gran Paradiso, is one of the Internal Crystalline Massifs, representing the Briançoinnais microcontinent sited at the distal margin of the European Plate [47,48,49]. It covers an area of ~1000 km², extending for ~70 km in length (N-S) and ~25 km in width (E-W). The Dora-Maira Massif is overlaid by the oceanic units of the Piemonte Zone toward the northwest and by the Mesozoic carbonatic metasedimentary successions of the pre-Piemonte Units toward the south [49,50], whereas, to the east, it is bounded by discordant Quaternary sediments of the Po Valley (Figure 1b) [48,51].

The Dora-Maira Massif comprises two distinct complexes which differ in tectono-metamorphic evolution and lithological features [52,53,54,55,56]. (i) At the upper structural position, a polymetamorphic Paleozoic basement, also known as Dora-Maira Basement Complex [49] (Figure 1b), mostly consists of micaschists and paragneisses with minor lenses of marbles and metabasites. This basement experienced both the Variscan tectono-metamorphic cycle at amphibolite-facies conditions [57,58,59] and the Alpine subduction-exhumation orogenic cycle, reaching a metamorphic peak at high- to ultra-high pressure (HP-UHP) eclogite-facies conditions [60,61,62,63]. (ii) At a lower structural position, a Carboniferous monometamorphic metasedimentary succession, referred to as Pinerolo Graphite Complex [64] or Pinerolo Unit (Figure 1b), only records the tectono-metamorphic evolution related to the Alpine subduction-exhumation stages, with a metamorphic peak at blueschist to eclogite-facies conditions [62,65,66,67]. Lithologically, the Pinerolo Unit mostly consists of meta-conglomerates, meta-sandstones (paragneiss), meta-siltstones and metapelites (micaschists), representing the host rocks of the graphite mineralizations. Both complexes were intruded with Permian dioritic and granitic stocks (Figure 1c), which were transformed into metadiorites and orthogneisses during the Alpine metamorphic cycle. In the study area, these (meta)-igneous rocks are mostly represented by the “Monte Freidour Ortogneiss” [68,69] and the “Malanaggio Metadiorite” [69,70,71].

Figure 1. (a) Simplified tectonic sketch map of the Western Alps with the location of the Dora-Maira Massif. (b) Simplified tectonic and lithological map of the Dora-Maira Massif (modified from [48,63,72]. (c) Simplified geo-lithological map of the northern Dora-Maira Massif in the Chisone and Germanasca Valleys, modified after [73] and integrating the new data from [63,74]. The map shows the location of the sampling areas (black boxes), the main ancient mines (numbered), the graphitic levels, and the location of the three succession types identified by [74]. The R1 ratio, calculated using micro-Raman spectra, is reported in the map and refers to the degree of crystallinity of the graphite (see text for further explanations).

Figure 1. (a) Simplified tectonic sketch map of the Western Alps with the location of the Dora-Maira Massif. (b) Simplified tectonic and lithological map of the Dora-Maira Massif (modified from [48,63,72]. (c) Simplified geo-lithological map of the northern Dora-Maira Massif in the Chisone and Germanasca Valleys, modified after [73] and integrating the new data from [63,74]. The map shows the location of the sampling areas (black boxes), the main ancient mines (numbered), the graphitic levels, and the location of the three succession types identified by [74]. The R1 ratio, calculated using micro-Raman spectra, is reported in the map and refers to the degree of crystallinity of the graphite (see text for further explanations).

3.1. Lithostratigraphy of the Pinerolo Unit

The lithostratigraphic features of the Pinerolo Unit have been described in detail by Manzotti et al. (2016) [64] and Nosenzo et al. (2024) [74], who also provided an interpretation of the paleo-environmental depositional settings. Three types of sedimentary successions have been identified in three type-localities: Ponte Raut, Bourcet, and Pons [74] (Figure 1c). At Ponte Raut, a graphite-rich conglomerate–sandstone succession is exposed, which consists of alternating layers of meta-conglomerates and meta-sandstones. Both lithologies are gray due to the presence of abundant, finely dispersed graphite. A conglomerate–sandstone succession is also exposed in the type-locality of Bourcet, which differs from that of Ponte Raut because graphite is no more dispersed but, rather, concentrated in thin layers of fine-grained black graphitic schists (up to 40 cm in thickness) interbedded with layers of graphite-free meta-conglomerates and meta-sandstones up to several meters thick. At Pons, the third type-locality, a graphite-rich siltstone succession is exposed, which consists of fine-grained, dark gray, meta-siltstones rich in finely dispersed graphite, alternating with coarser-grained, graphite-poor meta-sandstones. Meta-conglomerates are typically absent from this third type of succession. The three lithological sequences as a whole have been interpreted as a (meta-)sedimentary succession formed by the deposition of continental terrigenous sediments within endorheic basins in an “intramontane” setting during the Carboniferous [64]. More specifically, (i) deposition in a high-energy fluvial environment has been suggested for the Ponte Raut-type succession, characterized by laterally discontinuous, alternating layers of coarse (conglomerates) to medium (sandstones) -grained sediments, (ii) sedimentation in flat marshy areas cyclically affected by fluvial floods has been proposed for the Bourcet-type succession, due to the typical association of thick layers of coarse-grained terrigenous sediments alternating with thin layers of fine-grained materials enriched in organic matter, and (iii) the Pons-type succession would have been deposited in lower energy environments such as endorheic basins and ephemeral lakes, in which the sedimentation of fine-grained materials (clays, silts, and organic matter) was episodically interrupted due to coarser-grained contributions (sandstones) from the nearby rivers.

3.2. Tectono-Metamorphic Evolution of the Pinerolo Unit

The metamorphic evolution of the Pinerolo Unit is relatively poorly known, especially in the northern portion of the Dora-Maira Massif. The observed mineral assemblages, which include chloritoid, phengite, and garnet in metasediments, and glaucophane, epidote, and garnet in mafic rocks, are broadly consistent with an HP metamorphic peak, at either blueschist [53,65,75,76] or eclogite-facies conditions [62,67]. In the study area, Westin et al. (2022) [77] applied the Raman Spectroscopy on Carbonaceous Material (RSCM) geothermometer on graphite from the graphitic schists to constrain the peak-T registered by these rocks. Their results indicate temperatures ranging from 531 to 568 °C, comparable with data from literature [62,65,67]; however, pressure estimates are not available so far for the Pinerolo Unit in the northern sector of the Dora-Maira Massif.

Different deformation events are recorded in the study area, related to the subduction (D1 event), and exhumation (D2, D3, and D4) stages of the Alpine orogeny. The D1 event is generally poorly visible, being partially overprinted by the later events. The D2 deformation event was responsible for the formation of a pervasive axial-plane foliation (S2) that transposed the earlier S1 foliation and developed isoclinal folds with E-W axes. The regional foliation corresponds to the S2 foliation [78]. The following deformation events (D3 and D4) are only locally recorded in some lithologies; D3 developed an axial-plane cleavage with E-W direction due to open cylindric folds (D3), while D4 was responsible for the formation of N-S fractures linked to a gentle W-verging folding [79].

3.3. Graphite Mineralization of the Pinerolo Unit

The graphite ore bodies occur in seams and lenses within the graphitic paragneisses and micaschists of the Pinerolo Unit (Figure 1c) [73]. The mineralized horizons extend from South to North, from the Po Valley (Saluzzo and Paesana Municipalities), through the Chisone and Germanasca Valleys (to the north-west), being the focal points of the ancient mining activities, up to the Sangone Valley (Giaveno and Cumiana Municipalities). The graphitic horizons are parallel to the regional foliation S2, hence confirming the sedimentary origin of the graphitic source. All the galleries of the old mines follow an E-W orientation, aligned with the regional lineation (L2) and with the orientation of the fold axes derived from the deformation stage D2.

The most economic ore bodies are located in the Pramollo Valley (i.e., a later valley of the main Chisone Valley). Here, the graphitic mineralization occurs in three main seams on both the northern (Siassiera, Timonsella, and Dormigliosi Mines) [80] and the southern (Icla and Brutta-Comba Mines) slopes of the valley (Figure 1c). The majority of graphite extraction originated from the southern slope mines, being the last to chase the works in 1983. In these mines, the graphite seams have an E-W direction with 30°deep S. The thickness of the seams is highly variable, from a few cm up to several m [81]. Furthermore, the graphitic bodies are laterally continuous for more than 15 km to the east, towards San the Germano Chisone Municipality, and to the south towards the Campasso locality (Figure 1c), although lateral heterogeneities, local thinning, and pinch-outs with metamorphic foliation were also observed.

These three main graphitic seams continue northward along the western side of the Chisone Valley; evidence of artisanal and familial mining activities has been documented in the proximity of the Inverso-Pinasca and Dubbione villages. The seams have the same orientation and dip as those of the Icla Brutta Comba mines; however, the lateral continuity is minor. Smaller graphite lenses also occur in other localities: at the confluence between the Germanasca and Chisone Valleys, at several localities near the municipalities of Perosa Argentina, and Pomaretto, and up to the north near Meano Village (Figure 1c).

Previous studies classified the graphite of the study area as an “amorphous-type” with a sedimentary origin; these studies suggested that the graphitization occurred during Alpine orogeny, but they also raised the possibility that graphitization was partially linked to the intrusion of dioritic bodies [25]. However, no systematic study was conducted with a modern approach. In the literature, the graphite was characterized by a relatively low grade of graphitic carbon—typically ranging from 60% to 65%—due to high levels of impurities. Although higher-quality graphite was found locally, its graphitic carbon content averaged only 70%–75%, which was still insufficient for use in electrode manufacturing [25]. Due to the low quality and the challenges associated with upgrading the ore through purification processes, this graphite has primarily been used as a refractory material in foundries and as a lubricant in paint production [25]. None of the previous studies investigated the presence of economic minerals or valuable metals (i.e., CRMs) impurity.

4. Analytical Methods

This study was carried out by sampling graphite mineralization and host rock lithologies in five areas in the Chisone and Germanasca Valleys (see below). A total of 35 samples were selected, from which thin sections for petrographic observations in transmitted and reflected light were prepared. Appendix A presents a list of the samples with the provenance and a short description.

Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX or EDS) analyses were carried out on graphite specimens using the facilities of the Earth Science Department at the University of Torino (Italy). Morphological analyses were carried out on raw graphite particles coated with Au. The images were acquired using an SEM TESCAN VEGA, (Brno, CR) equipped with an Explora 30 mm² EDX (Oxford Instrument, Cambridge, UK) operating at 5 KeV. Detailed chemical analyses on graphite from mineralization (Appendix A) were then carried out using an SEM-EDS system JEOL JSM IT300LV (High Vacuum–Low vacuum 10/650 P- 0.3–30 kV) (Tokyo, JP), equipped with Aztec Live software 6.1 (Oxford Instrument, Cambridge, UK) and an OXFORD ULTIMAX65 silicon drift detector (40 mm²) (Oxford Instrument, Cambridge, UK), SATW Light Element Window high-resolution: MnKa (127 eV @130,000 cps) FKa (64 eV@ 130,000 cps) CKa (56 eV @ 130,000 cps). The analyses were acquired at the energy of 20 KeV, with a 1nA beam current and 10 mm WD; EDS spectra were acquired using a time of 30 s for spectrum, a process time of 1, and a dead time of 30% corresponding to ~1,000,000 cs for spectrum. The calibration EDS acquisitions were performed every 30 min via a Co standard.

The crystallinity degree of graphite was studied through the use of micro-Raman Spectroscopy (μRaman) of the Interdepartmental Center “G. Scansetti”, at Earth Science Department of the University of Torino (Italy). The analyses were run on a total of eleven hand specimens using an Optical Microscope (Olympus BX40) connected to a Horiba-Jobin Yvon LABRAM HR800 spectrometer (FR), equipped with a Nd, and He/Ne lasers and a CCD di 1040 × 256 pixels (26 μm pixel size) multichannel detector. The system is equipped with a Peltier air-cooling system reaching −70 °C. The operating conditions were as follows: a solid-state Nd laser (λ = 532, nm), grating of 600 grooves/mm, a hole at 200 μm, a slit at 100 μm, and 50×/20× objective magnification. The highest spectral resolution ranges between 1 and 3 cm⁻¹. To prevent heating and graphite recrystallization, laser filters D1–D2 were applied to reduce the incident power on the sample to 0.008–0.01 mW, depending on the objective magnification. The Raman spectra were processed using the Fytik1.3.1. software and the Pearson7 function. The processing of data allowed for the obtaining of clean spectra with well-defined D1, G, and D2 bands, which intensities and areas are used, respectively, for R1 [R1 = D1/G] and R2 [R2 = D1/(G + D1 + D2)] parameter calculation [82,83]. These ratios are indicative of the degree of crystallinity (e.g., [82,83]). For all the graphite specimens, the spectra were systematically acquired on the basal plane of the graphite crystals (001), easily recognizable in hand specimens through the highest luster. The systematic data acquisition on the basal planes reduced the likely incongruences derived from the crystallographic orientation effect, hence allowing to obtain quantitative, detailed, repeatable, and comparable results among the samples of this study. On the contrary, our data cannot be compared to those obtained on thin sections by other authors because, in these cases, the samples are cut perpendicular to the foliation, i.e., perpendicular to the graphite basal planes [84]. For the same reason, and also because we have evidence that fluids could precipitate part of the studied graphite, we did not use the graphite Raman spectra as geothermometers [83,84].

Geochemical analyses were carried out on 2 different graphite ore samples with different crystallinity degrees and macroscopic features. The two samples derive from two different areas (i.e., Pramollo and Pons; see below). The analyses were carried out at Actlabs (Ontario). Graphitic C was evaluated using a 0.5 g sample, subjected to a multistage furnace treatment before infrared (IR) spectroscopy analyses. The C was measured as CO₂ gas flow in the IR cell using ELTRA Instruments. Major and trace elements were analyzed through multi-method analyses, making use of Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and Instrumental Neutron Activation Analysis (INAA). Major elements (Al₂O₃, CaO, Fe₂O₃, K₂O, MgO, MnO, Na₂O, P₂O₅, SiO₂, and TiO₂ + LOI) were analyzed using whole-rock ICP-MS after lithium metaborate/tetraborate fusion. Trace element geochemistry (Ag, Al, As, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Ga, Gd, Ge, Hf, Ho, In, K, La, Li, Lu, Mg, Mn, Mo, Na, Nb, Nd, Ni, P, Pb, Pr, Rb, Re, S, Sb, Sc, Se, Sm, Sn, Sr, Ta, Tb, Te, Th, Ti, Tl, Tm, U, V, W, Y, Yb, Zn, and Zr) was analyzed through ICP-OES + ICP-MS after 4-acid “near total” digestion (hydrochloric, nitric, perchloric, and hydrofluoric). INAA analysis, making use of a nuclear reactor to measure gamma-ray fingerprints emitted after suitable decay, was used to better constrain Au, Co, As, Sb, W, Ta, U, Th, Cs, In, Re, and Cl, and lower levels of most Light Rare Earth Elements (LREEs).

5. Results

5.1. Field Observations

Field observation and sampling were generally conducted in proximity to the access galleries of the ancient graphite mines and, in some cases, within the galleries that are still accessible (Figure 2 and Figure 3). The sampling areas are mostly located on the right side of the main Chisone Valley. From South to North, these areas are as follows: (1) Pramollo (i.e., Timosella-Dormigliosi and Siassiera Mines), (2) Inverso-Pinasca, (3) Pomaretto (Clot di Boulard and Pons Mines), and (4) Garnier. An additional sampling area is located on the left side of the Chisone Valley, i.e., (5) Grandubbione (Figure 1c) (Appendix A).

In these areas, two types of metasedimentary successions have been recognized, which can be correlated with the Bourcet-type and Pons-type successions described by Nosenzo et al. (2024) [74]. The Bourcet-type succession is exposed in the Pramollo and Garnier areas and at the upper structural levels of the Inverso-Pinasca and Grandubbione areas, while the Pons-type succession occurs at lower structural levels in the Grandubbione and Inverso-Pinasca areas, and it is the dominant succession in the Pomaretto area.

5.1.1. Bourcet-Type Succession

The Bourcet-type succession consists of alternating layers of meta-conglomerates and meta-sandstones (Figure 3a) locally intercalated with minor meta-siltstones/meta-pelites. At the mesoscale, the meta-conglomerates are characterized by pluri-centimetric quartz clasts, often elongated and aligned parallel to the main foliation (Figure 3a), set in a fine-grained matrix consisting of paragneisses and/or gneissic micaschists. Locally, pebbles of graphitic schists, up to a few cm in length, are observed in addition to the quartz clasts (Figure 3b) (e.g., in the Inverso-Pinasca and Pomaretto areas). The meta-sandstones are mostly fine-grained paragneisses and quartzitic micaschists (Figure 3c), while the minor meta-siltstones and metapelites are fine-grained micaschists, either black to dark grey due to the abundant disseminated graphite (Figure 3d) or light gray/green in color due to the absence of graphite and abundance of white mica. The graphite ores occur as lenses or discrete horizons ranging in thickness from some cm to m (Figure 3e,f,h), and locally, they are remobilized in quartz-bearing veins within highly deformed meta-conglomerates, meta-arenites, and meta-siltites (Figure 3g). The exploration galleries and mine entrances are generally E–W-oriented, parallel to the regional lineation (L2) and the orientation of the fold axes (Figure 2a,b). The distribution of graphitic mineralization seems to be unrelated to the lithology; in the Timonsella Mine (Pramollo area; Figure 2a), for example, the graphite ore is hosted in the meta-conglomerates, whereas, at the nearby West Siassiera Mine (Pramollo area; Figure 3h), the ores occur within intercalated meta-sandstones and meta-siltites/pelites.

5.1.2. Pons-Type Succession

The Pons-type succession consists of graphitic meta-siltites and metapelites (i.e., white mica and chlorite-rich micaschists) interbedded with minor meta-sandstones (i.e., paragneisses; Figure 3i). Graphite is generally finely disseminated in these fine-grained lithologies, which are typically dark gray in color (Figure 3j,l,m); less frequently, it is concentrated in discrete mm- to cm- thick graphite-rich, dark layers alternated to graphite-poor layers (Figure 3k), oriented parallel to the regional foliation. In some exploration galleries (e.g., Pomaretto area; Figure 2c), shear zones filled with graphite and cementing fault breccia are also visible, likely related to a later graphitic remobilization (Figure 3n–p).

5.2. Petrography of the Host Rock

5.2.1. Bourcet-Type Succession

The Bourcet-type succession is dominated by lithologies derived from coarse- to medium-grained protoliths, i.e., meta-conglomerates and meta-sandstones, while meta-siltites and metapelites are volumetrically minor. Graphite is generally absent in the meta-conglomerates and meta-arenites (Figure 4a–f), while it is sporadically present in the meta-siltstones (Figure 4g,h) and metapelites (Figure 4i).

The meta-conglomerates are medium- to fine-grained paragneisses characterized by a strongly heterogeneous grain size (Figure 4a) and mostly consisting of quartz, white mica, chlorite, and biotite with minor albite and garnet. The main foliation (S_m) is defined by white mica, chlorite, ± biotite, concentrated in sub-millimetric thick, discontinuous layers (Figure 4b) alternating with pluri-millimetric fine-grained quartzitic layers. The S_m envelops pluri-millimetric to cm elongated medium-grained quartzitic domains, which derive from the recrystallization of the former quartzitic pebbles of the protolith (Figure 4a). Albite and garnet are only locally present; albite appears in equilibrium with S_m, while the fine-grained garnet is almost completely replaced with chlorite and Fe-oxides, and its relations with S_m remain ambiguous.

The meta-sandstones are fine-grained paragneisses showing the same mineral assemblage as the meta-conglomerates, but with a finer and more homogeneous grain size (Figure 4c,e). The main schistosity is defined by sub-millimetric thick layers of white mica, chlorite, ± biotite alternating with pluri-millimetric layers of quartz and albite (Figure 4d,f). Albite generally forms millimetric-sized ocelli syn-kinematic with respect to S_m, as suggested by the occurrence of an internal foliation continuous with the S_m (Figure 4d,f); it is locally rimmed by a thin rim of oligoclase. Garnet is more abundant than in the meta-conglomerates; its relations with the S_m are ambiguous due to the very fine-grained size. It is idioblastic when included in albite (Figure 4f), while in the matrix, it is systematically replaced with chlorite ± biotite.

The meta-siltstones are mostly quartzitic micaschists characterized by a more pervasive foliation compared to the other lithologies (Figure 4g); their mineral assemblage consists of quartz, white mica, chlorite, ± chloritoid and contains higher proportions of Al-rich minerals, which reflect the increasing pelitic component of the protoliths. The main foliation, S_m, is mostly defined by white mica and chlorite and transposes an earlier foliation S_m−1, defined by white mica, chlorite, and chloritoid and often preserved in microlithons and intrafoliar folds (Figure 4g,h). In this lithology, biotite is scarce and, if present, is a late phase, developed at the expense of chlorite.

The metapelites are rare and limited to thin and discontinuous horizons in this succession. These are strongly foliated micaschists with a high modal amount number of phyllosilicates, which are concentrated in thick, pluri-millimetric continuous layers (Figure 4i) alternated with mm discontinuous quartzitic layers. They are characterized by a low-variant mineral assemblage consisting of quartz, white mica, chlorite, garnet, ± chloritoid, ± lawsonite, with late albite and epidote and accessory rutile. The S_m, defined by white mica and chlorite, envelops pluri-millimetric garnet porphyroblasts (Figure 4i) and transposes an earlier S_m−1 foliation defined by white mica, chlorite, chloritoid, and lawsonite. The S_m−1 is preserved either in microlithons or as an internal foliation within garnet porphyroblasts (Figure 4i,j), discordant with respect to the external S_m. Lawsonite (now completely replaced with lozenge-shaped epidote + white mica pseudomorphic aggregates) and chloritoid occur as inclusions within garnet (Figure 4j), but they are absent in the matrix.

5.2.2. Pons-Type Succession

The Pons-type succession consists of lithologies derived from fine-grained terrigenous sediments, i.e., meta-siltstones and metapelites, with minor intercalations of meta-sandstones. It differs from the Bourcet-type succession for the absence of meta-conglomerates and for the more widespread occurrence of graphite, which is finely disseminated in most of the lithologies (Figure 4m–t).

The meta-sandstones are medium- to fine-grained paragneisses consisting of quartz, albite, white mica, chlorite, and garnet. The main foliation S_m, defined by mm-thick white mica + chlorite layers, is not particularly pervasive due to the abundance of albite ocelli partially overgrowing it. In the Pomaretto area, this lithology is locally crenulated, with the development of a discontinuous axial-plane schistosity S_m+1, defined by white mica and chlorite (Figure 4k,l). If present, garnet is fine-grained and idioblastic, locally replaced by chlorite at its rim.

Meta-siltstones and metapelites are intimately associated and often intercalated at a cm, or even mm, scale (Figure 4o). They mostly differ from each other in the relative proportion of phyllosilicates, which are less abundant in the meta-siltstones compared to the metapelites. This translates into a less pervasive schistosity in the meta-siltstones, which can be mostly defined as fine-grained paragneisses or quartzitic micaschists (Figure 4m–o), compared to the metapelites which are mostly micaschists (Figure 4q,s). The mineral assemblages are similar in the two lithologies and consist of quartz, white mica, chlorite, albite, ± garnet, ± chloritoid, ± biotite. The S_m is defined by white mica, chlorite, ± biotite; this last is generally associated with the quartz-rich domains. The S_m transposes an earlier S_m−1 foliation defined by white mica, chlorite, ± chloritoid, which is locally preserved in the metapelites within microlithons or intrafolial folds (Figure 4r). The S_m is locally crenulated (e.g., in the Pomaretto area) with the development of a S_m+1 axial-plane schistosity, more pervasive in the metapelites compared to the meta-siltstones (Figure 4o). Albite appears in equilibrium with the S_m, and it often includes an internal foliation continuous with the external one (Figure 4t). Garnet shows the same microstructural relations as albite, i.e., it is syn-kinematic with respect to the S_m and locally includes an internally rotated foliation continuous with the external one (Figure 4p,t). Garnet occurs in idioblasts with variable sizes, ranging from sub-millimetric in the meta-siltstones (Figure 4n) to millimetric in the metapelites (Figure 4p,t). It is generally absent from the chloritoid–bearing lithologies (Figure 4q,r). White mica, chlorite and, locally, biotite also form late flakes statically overgrowing the S_m (Figure 4t).

5.3. Petrography of Graphite Ores

The observation via optical microscopy (OM both transmitted and reflected lights; on graphitic mineralization reveals a typical mineral association defined by graphite, ranging from 30 to 55 vol%, depending on the sampling locality, with a variable concentration of white mica, chlorite, and quartz. In some specimens, minor albite, biotite replacing chlorite, garnet, ilmenite, pyrite, and Fe-(oxy)hydroxides crusts were also observed. The samples generally exhibit a pervasive schistosity (S_m), again depending on the sampling localities. Later quartz and chlorite veins cutting the graphitic mineralization and the S_m were often observed in several samples, both from Bourcet-type and Pons-type successions.

Under OM observation two graphite types were identified based on their grain size. (i) “Fine-grained graphite” is the most common and occurs as crystals dispersed in host rocks or concentrated in sub-millimetric- pluri-millimetric layers alternating to quartz, white mica, chlorite with minor albite layers, defining the S_m. (ii) “Coarse-grained graphite” does not exhibit a clear schistosity. It occurs in samples directly collected in mine galleries within or in the proximity of shear zones, such as, respectively, in Pons, and Clot di Boulard.

High-resolution morphological observations (SEM-BSE) highlight that both graphite types may occur either with a more massive aspect (Figure 5a), or as lamellae aggregates, often highly deformed and interlayered with silicates (Figure 5b–d). The morphology is unrelated to the sample provenance area, the succession types (i.e., Bourcet-type vs. Pons-type), and the grain size.

5.3.1. Graphite Mineralization in Bourcet-Type Succession

The graphite mineralization in Bourcet-type successions varies in terms of graphite content according to the sampling locality. The graphite is always “fine-grained” and displays low-to-pervasive S_m schistosity, consisting of graphitic layers alternated to white mica and quartz levels and local Fe-Mg chlorite (Figure 6a,b) at West Siassiera Mine (Pramollo area). In some samples of the Pramollo area (e.g., Dormigliosi and Timonsella Mines), the S_m overlies a previous schistosity (S_m−1). This latter is highlighted by graphite crystals and preserved in microlithons (Figure 6c). At the Garnier area, the alternation between graphite-rich and graphite-poor layers is more evident, and the relation with grain size is clear: graphite-poor levels have a coarser grain size and alternate with graphite-rich levels with finer grain size. The graphite-rich levels often exhibit two foliations with a highly crenulated S_m and an axial-plane foliation (S_m+1) defined by thicker quartz, white mica, chlorite, and graphite layers (Figure 6d).

5.3.2. Graphite Mineralization in Pons-Type Succession

Similar to the Bourcet-type succession, the graphite mineralization in Pons-type succession may be ± concentrated within host rocks, with millimetric-to-centimetric layers of “fine-grained graphite” crystals alternated with graphite-poor levels. In the Pons-type succession, characterized by finer-grained host rocks compared to the Bourcet-type succession, graphite is also commonly finely dispersed in the host rock, forming graphitic micaschists. In these rocks, the S_m is defined by sub-millimetric levels of fine-grained graphite alternated to white mica, quartz, Fe-Mg-chlorite, and minor biotite levels, locally surrounding albite ocelli with a discordant internal foliation (Figure 6e).

The same schistose texture is also found in samples collected within graphite mineralization, where the graphitic layers are thicker (pluri-millimetric scale) (Figure 6f). In some samples, such as those from Inverso-Pinasca and Pons areas, the S_m is highly crenulated, resulting in an axial-plane crenulation cleavage (S_m+1) defined by white mica and highlighted by graphite (Figure 6g).

The “coarse-grained graphite” was observed in only one mineralized sample from the Pons Mine (Pomaretto area). The graphite crystals, locally including ilmenite, are tabular, coarser-grained, and fractured (Figure 6h). They do not define a foliation but constitute the cement of a tectonic breccia within a shear zone.

Weathered ilmenite is a common accessory mineral in high-grade graphite mineralization (Figure 6i,j). Finally, most mineralizations are cut by late veins consisting of quartz and chlorite (Figure 6k,l).

5.4. Degree of Crystallinity

Representative graphite spectra, comprehensive of the second-order region, are reported in Figure 7,

Table 1. Results from Raman spectra processing. The table reports the D1, D2, and G peak positions, intensities, FWHM (full width at half maximum) and areas. The R1 parameter is the ratio between the intensities of the D1 and G bands, whereas the R2 parameter is the ratio between the area of the D1 band and the sum of the areas of G, D1 and D2 bands [83].

Provenance Area	Analysis ID	D1 Position	D1 Intensity	FWHM D1	Area D1	G Position	G Intensity	FWHM G	G Area	D2 Position	FWHM D2	D2 Area	R1	R2
Garnier	GN06	1351.89	734.10	41.12	39,652.50	1580.84	1205.61	21.41	35,124.10	1621.50	12.78	2935.70	0.61	0.51
Garnier	GN07	1352.60	226.17	34.81	8,818.23	1581.66	556.42	17.07	11,907.60	1622.96	11.54	1039.08	0.41	0.41
Pomaretto (Pons)	PS01a (DM1887)	1348.21	467.07	51.19	27,510.60	1592.03	347.09	33.91	14,441.70	1614.61	22.93	11,621.30	1.35	0.51
Pomaretto (Pons)	PS01b (DM1887)	1350.37	656.08	42.90	32,113.70	1587.83	611.14	33.45	24,491.60	1616.17	23.47	7823.48	1.07	0.50
Pomaretto (Clot di Boulard)	PS12	1352.64	433.14	41.76	21,008.20	1582.70	851.66	19.71	22,522.40	1622.77	12.61	2211.91	0.51	0.46
Grandubbione	GR01	1352.48	351.97	46.12	18,883.70	1584.38	647.15	26.07	19,307.40	1619.16	26.22	4972.21	0.54	0.44
Inverso-Pinasca	IP05	1354.22	79.93	44.90	3994.37	1582.40	214.80	20.28	5390.77	1622.71	12.97	560.35	0.37	0.40
Inverso-Pinasca	IP01	1352.05	480.34	43.76	25,890.30	1582.62	832.82	22.94	23,443.20	1621.36	15.16	3304.74	0.58	0.49
Inverso-Pinasca	DM1886	1356.37	253.52	43.56	13,586.10	1583.60	520.94	20.72	14,470.60	1623.14	12.84	3476.96	0.49	0.43
Pramollo (West Siassiera)	PR02	1351.94	319.55	44.75	20,728.40	1581.59	2867.85	17.75	63,147.70	1622.86	17.73	5025.54	0.11	0.23
Pramollo (Timonsella)	DM1867a	1349.95	140.77	32.65	1405.76	1581.64	1258.05	15.60	68,148.20	1610.77	28.84	5141.40	0.11	0.02
Pramollo (Timonsella)	DM1867b	1354.16	54.41	42.23	6495.24	1581.46	3488.75	17.08	29,868.40	1621.82	11.10	966.78	0.02	0.17
Pramollo (Icla Brutta Comba)	PR01	1351.01	199.39	43.80	10,895.70	1582.07	433.67	23.36	13,122.60	1616.78	28.80	2100.45	0.46	0.42

, and Figure 8, displaying the results of μRaman spectroscopy for 13 graphite samples; in particular, the calculation of R1 and R2 ratios [83] were used as an indication of the degree of crystallinity for each analyzed graphite. The increase in the R2 ratio is indicative of a decrease in crystallinity at T > 330 °C, whereas the increase in the R1 ratio is indicative of an increase in crystallinity at lower temperatures (e.g., [83,84,86,87,88]).

The Raman data allowed us to distinguish between three types of graphite based on the shape of their spectra (Figure 7) and, in more detail, on the relationships between the R2 parameter and the Full Width at Half Maximum (FWHM) of the graphite (G) band and the R1 parameter, both of whose shapes are very sensitive to the crystallinity of the carbonaceous material (Figure 8). The first graphite type (Type I) corresponds to graphite characterized by the presence of a sharp, high-intensity G peak with a low D1 peak in the first-order region and of a single sharp S1 peak in the second-order region (Figure 7), and by low values of R2 (from 0.02 to 0.23), R1 (from 0.02 to 0.11), and FWHM of the G band (from 15.60 to 17.75; Figure 8). This graphite derives only from the samples collected in the Pramollo area (Dormigliosi-Timonsella and West Siassiera Mines).

The second type (Type II) corresponds to graphite that shows a rising of the D1 and D2 bands in the first-order region and of the S2 band in the second-order region, and as revealed by R2 values from 0.40 to 0.51, by R1 values from 0.37 to 0.61, and by FWHM values of the G band from 17.07 to 26.07 (Figure 8). These samples are collected in the Pramollo (Icla-Brutta Comba Mine), Inverso-Pinasca, Grandubbione, Pomaretto (Clot di Boulard locality), and Garnier areas. The third graphite type (Type III) is characterized by large and intense D1, D2, and S2 bands compared to the G and S1 ones. In detail, the G band is less intense than the D1 band, whose shape is due to the presence of the D4 band underneath, and the G band is asymmetric toward the high wave-numbers (Figure 7). The R2 values (0.50–0.51) are similar to the highest ones of Type II graphite, whereas the FWHM values of the G band and the R1 ratio are the highest measured (33.45–33.91, 1.07–1.35, and 0.50–0.51, respectively). This Type III graphite constitutes only the Pons Mine samples (Pomaretto area; Figure 8).

5.5. Mineral Chemistry and Bulk Rock Composition of Graphite Ores

SEM-EDS analyses of graphite mineralization specimens (Appendix A) confirmed the overall fine-grained nature of the mineralization already observed at OM, and several accessory minerals were identified other than ilmenite and pyrite, such as rutile, zircon, xenotime, monazite, and apatite. The analyses also revealed the local occurrences of clays finely intergrown to graphite (Figure 9a). The carbon content of graphite ranges from a minimum of 97.39 wt. % to a maximum of 99.59 wt. % in Pramollo area (both West Siassiera and Timonsella-Dormigliosi localities) with low impurities of Si (up to a maximum of 2.11 wt. %), Al, K, and Fe, minor than 0.5 wt. % (

Table 2. Representative EDS analyses of graphite.

	C	Mg	Al	Si	K	Fe	Ba	Ti	Ca	Mn	Na	S	F
	Wt. %
Pramollo (West Siassiera)	98.59			1.41
	99.51		0.14	0.17
	99.29		0.21	0.26	0.10	0.14
Pramollo (Dormigliosi-Timonsella)	98.32		0.60	0.90		0.17
	97.39			2.11
	99.56		0.12	0.21
Inverso-Pinasca	54.05	1.39	12.28	21.44	8.18	1.97	0.08	0.39		0.03	0.19
	53.82	1.34	12.68	21.49	8.44	0.10		0.27	0.05	0.03	0.16
Garnier	89.01	0.72	2.31	3.67	0.82	2.81			2.26
	90.71	0.53	2.22	3.41	1.07	1.70			0.12
	92.94	0.13	0.33	6.07	0.14	0.28			0.10
	91.67	0.28	1.29	1.95	0.29	4.09			0.11
Pomaretto (Pons)	88.94		0.16	10.38		0.20						0.30
	86.63		2.65	6.89	1.43	0.89					0.23		0.67

). Inverso-Pinasca graphite is very fine-grained and finely intergrown with muscovite; hence, the analyses of graphites are influenced by the surrounding silicates: the C content is around 54 wt. %, versus high Al, Si, and K (up to 12. 68 wt. % and 21.49 wt. %, and 8.44 wt. %, respectively). Minor Mg (up to 1. 39 wt. %) occurs. Other minor impurities are Fe, Ti, Ca, Mn, Na, and Ba (always less than 0.5 wt. %). Garnier graphite analyses display lower C contents ranging from 89.01 wt. % to 92.94 wt. %; the amount of impurity is higher with Si, Al, and Fe up to a maximum of 6.07 wt. %, 2.22 wt. %, and 2.81 wt. %, respectively. Other impurities are Mg and Ca. Notably, the samples collected in the correspondence of the shear zones with coarse-grained graphite from the Pomaretto area (Pons Mine) are highly impure with a minimum of 86.63 wt. % to 88.94 wt. % C and high Al (up to 8.09 wt. %), Si (up to 10.40 wt. %), Fe (up to 10. 81 wt. %), and K (up to 3.4 wt. %), with minor Ba and S (Table 2). Since no intercalation with muscovite was observed, the impurities belong to graphite.

Ilmenite (FeTiO₃) is the most common accessory mineral: it is generally discordant to the foliation and strongly weathered (Figure 9b,c). In some samples, relics of rutile can be observed within ilmenite (Figure 9c). In ilmenite, the main impurities are Mn (up to 4 wt. % MnO) and V (<0.5 V₂O₃); the rutile inclusions may present higher V contents (with ranges between 0.73 and 1.21 wt. % V₂O₃) (

Table 3. Representative EDS analyses of the accessory minerals.

	Al2O3	MgO	CaO	SiO2	SO2	K2O	TiO2	MnO	NiO	FeO	V2O3	ZrO2
	Ilmenite
	Wt. %
Pramollo (Timonsella)	0.14			0.21			52.16	4.49		42.57	0.44
Grandubbione	0.12			0.2			48.31	2.87		48.13	0.38
Inverso-Pinasca	0.1			0.21			47.99	2.94		48.34	0.42
Pomaretto (Pons)	0.17			0.16			47.16	1.59		50.44	0.48
	Rutile
	Wt. %
Pramollo (Timonsella)				0.42			98.43			0.19	0.97
Pramollo (Timonsella)				0.31			97.83			0.65	1.21
Pomaretto (Pons)	0.14			0.18			98.48			0.47	0.73
	Zircon
	Wt. %
Pramollo (Timonsella)	0.31			31.32						0.75		67.62
Grandubbione	0.13			31.41		0.21						68.21
Pomaretto (Pons)	0.73			32.59						0.41		66.07
	Fe.(oxy)hydroxides
	Wt. %
Pomaretto (Pons)	0.23			1.56	0.32				0.22	68.44
	0.94			1.34	0.72				0.26	68.59
	O	F	P	K	Ca	Si	Fe	S	Ni	Ti
	Pyrite
	Wt. %
Grandubbione						0.14	58.86	39.2		1.56
Pomaretto (Pons)						0.24	55.83	43.36	0.33
	Apatite
	Wt. %
Pomaretto (Pons)	37.41	4.62	19.92	0.23	41.83

). Detrital zircons (ZrSiO₄) are stoichiometric with minor Fe as the main impurity (Table 3; Figure 9d). Among the REEs phosphates, monazite ((Ce,La,Nd,Th)PO₄)(Figure 9e) shows variable ranges of Ce, La, and Nd with minor Pr, Sm and Gd, whereas Dy, Tb, and Y can be neglected (

Table 4. Representative EDS analyses of REEs phosphates.

	Monazite		Xenotime
	Grandubbione	Pomaretto (Pons)	Grandubbione
	Wt. %
Al2O3	0.26	0.39	0.21
MgO	0.45
CaO		0.54
SiO2	0.29	0.63	1.44
MnO	0.26	0.13	0.14
FeO		1.26
ZrO2			0.28
PO2	30.95	29.14	33.44
CeO2	29.02	30.51
La2O3	14.33	14.86
Pr2O3	3.37	3.36
Sm2O3	3.37	2.24	1.35
Gd2O3	2.77	1.41	6.23
Dy2O3	0.82	0.51	9.22
Y2O3	0.63		40.36
ThO2		1.83
UO2		0.21
Yb2O3			1.28

); impurities of U and Th (up to 0.21 wt. % UO₂ and 3.8 wt. % ThO₂ respectively) were observed. In xenotime (YPO₄), the major REEs are Dy (9.22 wt. % Dy₂O₃), Gd (6.23 wt. % Gd₂O₃), and Er (2.54 wt. % Er₂O₃). Pyrite (FeS₂) is rare and occurs dispersed in some samples; it shows minor impurities of Ni (up to 0.33 wt. %, Table 3) and it is commonly weathered and armored with Fe-(oxy)hydroxides (jarosite-goethite) deriving from pyrite oxidation (Figure 9f). Major impurities are Si (from ~1 to ~4 wt. % SiO₂) and Al (from <1 to ~6 wt. % Al₂O₃), while minor impurities are S, Ni, Ca, and Mg (Table 3).

Wet chemical bulk ore analyses (Appendix B; Figure 10) were performed on two samples, one from the Timonsella Mine (DM1867; Pramollo area) and the other from the Pons Mine (DM1887; Pomaretto area), containing graphite with the highest and lowest degrees of crystallinity, respectively. Regarding the major elements (Figure 10a), both samples are depleted in MnO, CaO, and Na₂O with respect to the average upper crust [88], whereas the Pons sample is also depleted in Fe₂O₃ and MgO. Except for SiO₂, all the other oxides are higher in the Timonsella sample compared to the Pons one.

Regarding the trace elements (Figure 10b), the Pons sample is slightly enriched in all elements with respect to the average upper crust, except for a slight depletion in P and a relevant depletion in Sr. Compared to Pons sample, the Timonsella one shows higher P contents but depletions in typical fluid-mobile elements, i.e., REE (with the fractionation of LREE and MREE with respect to HREE) and Sr, Cs, Rb, Ba, and U. Interestingly, the fluid-immobile elements, i.e., high-field-strength elements (HFSE) and Th, have the same values in both samples or show very slight variations.

Regarding ore elements, the amount of graphitic carbon in Timonsella samples is more than double that in the Pons sample (14 wt. % vs. 6 wt. %, respectively; Appendix B) and V, which could be a co-product of graphite mineralization, is very low (<200 ppm). Figure 10c shows that, in the Pons sample, most ore elements are present in quantities corresponding to those of the average upper crust [89] or slightly higher or lower, with the notable exceptions of enrichments in Ag, Au, and Sb. Also in this case, the Timonsella sample shows enrichments (Pb, Mo, Ag, W, Bi, and Au) and depletions (S, Co, Ni, Sn, and Sb) of the typical fluid-mobile elements without significant variations of the fluid-immobile ones, with respect to the Pons sample. For Zn, the values fall in the range of the average upper crust.

6. Discussion

6.1. Metamorphic Evolution of the Host Lithology

The detailed petrographic investigation of the different lithologies hosting the graphite mineralization allowed for recognizing the evidence of a high-pressure metamorphic peak, broadly compatible with blueschist-facies conditions. The observed equilibrium mineral assemblages are mostly controlled by the nature of the protoliths (i.e., coarser-grained metasediments have different assemblages compared to the finer-grained ones), whereas we did not find significant differences among lithologies derived from the same protolith type in the two different successions (e.g., meta-siltstones from the Bourcet-type succession have the same assemblages as meta-siltstones from the Pons-type succession). Overall, the lithologies derived from the finer-grained protoliths (i.e., meta-siltstones and metapelites) better preserve the evidence of the Alpine peak metamorphic assemblage, while the lithologies derived from the coarser-grained protoliths (i.e., meta-conglomerates and meta-sandstones) were more pervasively re-equilibrated during exhumation. The blueschist-facies peak assemblages developed earlier than the main foliation S_m and generally define the S_m−1 foliation. Chloritoid is the diagnostic mineral for the peak metamorphic stage in most of the meta-siltstones and metapelites; it defines the S_m−1 and is often re-oriented along the S_m due to the transpositive nature of the S_m. The low-variant garnet + chloritoid + lawsonite peak assemblage has been recognized in a few metapelites only. In all the lithologies, the most pervasive deformation event occurred during exhumation under greenschist-facies conditions and was responsible for the development of the S_m. The mineral assemblages developed during this stage mostly consist of white mica, chlorite, ± biotite, albite, and garnet. A later deformation event is locally registered in some areas (i.e., Pomaretto and Garnier), which was responsible for a pervasive crenulation of the S_m and the local development of an S_m+1 axial-plane foliation defined by greenschist-facies phases. A late heating event is possibly testified to in some samples by the occurrence of an oligoclase rim around albite and by the development of biotite at the expense of chlorite.

6.2. Genesis of the Graphite Deposits

The multidisciplinary and multiscale approach (i.e., fieldwork, optical and electronic microscopy, μRaman analysis, and mineral and bulk chemistry) applied in this study allows for constraining the possible genesis of the graphite deposits in the Chisone and Germanasca Valleys.

At the mesoscale, the graphite mineralizations display similar features and settings that seem unrelated to the succession types within which they are hosted. The graphite bodies mainly occur as stratiform bodies, seams, or lenses parallel to the regional schistosity. Microstructural observations confirm this setting: the graphitic layers are parallel to the S_m. The distribution of the graphite layers is consistent with a genesis linked to the concentration and sedimentation of conspicuous organic matter in fluvial–lacustrine sedimentary environments during the Carboniferous period. The graphitization process, which requires relatively high temperatures, therefore occurred after sedimentation.

Textural observations on graphite ores, coupled with geochemical analyses, reveal the occurrence of two graphite groups. (1) The first and most common group occurs as “fine-grained graphite” crystals dispersed in meta-siltites/metapelites of both Bourcet-type and Pons-type successions, or variably concentrated in layers alternated to quartz, white mica, chlorite, and minor albite layers defining the S_m; it is commonly pure, with a C content ranging from 92 to 97 wt. % for almost all graphite samples, with the exception of the samples from Timonsella and West Siassiera Mines (Figure 1, Pramollo area) having the highest purity with C contents reaching ~99.5 wt. %. (2) The second graphite group, classified as “coarse-grained graphite”, was observed in samples collected from shear zones (e.g., Pons Mine). The samples do not show any clear schistosity, and geochemical analyses reveal high impurities (Al, Si, Fe, and K), whereas the C content reaches the maximum value of ~86 wt. %.

On the basis of μRaman analyses on the degree of crystallinity, three distinct types of graphite are recognized (Figure 8): a graphite with low R1 and R2 ratios (Type I), a graphite with intermediate R1 and R2 ratios (Type II), and a graphite with high R1 and intermediate R2 ratios (Type III).

Merging the results from petrographic observations and geochemical analyses with μRaman data, we can reconsider the graphite types based on likely genetic conditions as well. Type I graphites are “fine-grained graphites” characterized by high purity; the low R1 and R2 ratios (Figure 8) are indicative of high crystallinity likely acquired at high-grade metamorphic conditions (e.g., [83]). This graphite type only occurs in the Pramollo area (Timonsella and West Siassiera Mines). As evident from Figure 1, these are the only two sites located near the Permian dioritic plutons (i.e., a few hundred meters from the metadiorites). Interestingly, the bulk-chemical composition of the sample from this area (DM1867) reveals a higher % of graphitic carbon (14%) and depletion in fluid-mobile elements (Figure 10). These data suggest that the contact metamorphic event and the hydrothermal circulation associated with the intrusion of diorite bodies (Malanaggio Diorite) in the Permian age likely have triggered the process of high-crystallinity graphitization within the Carboniferous sedimentary sequence closest to the intrusive bodies.

The “fine-grained graphite” from the other localities (e.g., Garnier, Inverso-Pinasca, Grandubbione, Figure 1 and Figure 8) are characterized by higher chemical impurities and belong to the Type II group defined by μRaman analyses because they show intermediate R1 and R2 ratios (Figure 8) that are indicative of intermediate crystallinity (e.g., [83]). These features are relatively uniform among all the localities and might have been acquired during the Alpine metamorphic peak. Being the peak temperature conditions of Alpine metamorphism lower than those related to the Permian contact metamorphism, the crystallinity of the Type I graphite from the Pramollo area was not modified by this event.

The “coarse-grained graphites”, found only in the Pons Mine and Clot di Boulard (Pomaretto area), are characterized by high chemical impurities. Whereas the samples from Clot di Boulard show an intermediated degree of crystallinity (Type II; Figure 8), those from Pons Mine are the only ones displaying Type III μRaman spectra (Figure 8), i.e., spectra with high R1 and intermediate R2 ratios, with the presence of a D4 band and with an asymmetric G band toward the high wave-numbers. Although a direct comparison with the spectra of other studies cannot be applied and our data cannot be used for geothemometry, due to the different orientation of the analyzed crystals, the parameters of the Type III graphites from Pons Mine are typical of relatively high-crystallinity carbonaceous material formed at low temperatures (ca. 300 °C) [86,87]. Interestingly, the Pons graphitic ore body constitutes the cement of tectonic breccias within the shear zone (Figure 3n–p). A recent experimental study [90] indicates that μRaman spectra similar to those of Type III graphite could indicate carbonaceous material for which high crystallinity was acquired via deformation and not via a temperature increase, in a semi-brittle deformation regime, at temperatures lower than 300–350 °C. Thus, for Pons Mine, we assume that at least part of the graphite could have been formed along semi-brittle shear zones that were active during the late Alpine stage, i.e., at a lower thermal gradient with respect to the Alpine metamorphic peak. Note that, for the Clot di Boulard samples, collected in proximity to a shear zone, the graphite crystallinity was not affected by the strain, leading to an intermediate degree of crystallinity (in agreement with Figure 1 [90]). However, although the bulk trace elements of the sample from this area (DM1887) indicate that most of the fluid-mobile elements are in concentrations similar to or higher than the average upper crust (Figure 10), we cannot exclude moderate remobilization and precipitation of the Pons graphite along the faults from saline, low-temperature fluids (likely involving meteoric water) circulating during the later stages of the deformation.

7. Conclusions

This study, conducted on the graphitic mineralization and hosting lithologies of the Pinerolo Unit, has revealed new insights into their texture, mineralogy, and geochemistry, which allowed for the reconstruction of the geological paleo-environment and the tectono-metamorphic evolution of this metasedimentary sequence. The findings can be summarized as follows:

(a)

The graphitic mineralization was derived from the deposition of organic matter during the Carboniferous, in a continental sedimentary environment, likely ascribable to a marshland area in an intramontane basin replenished by rivers transporting and depositing its detrital charge during low-energy/low-oxidation periods in agreement with Nosenzo et al., 2024 [74].

(b)

The hosting lithologies are characterized by blueschist-facies peak mineral assemblages formed during Alpine subduction, only preserved in the finer-grained lithologies (i.e., meta-argillites and meta-siltstones). The greenschist-facies mineral assemblage, associated with the pervasive schistosity (S_m), is related to Alpine exhumation, and it widely overprints the metamorphic peak paragenesis.

(c)

The studies on the morphology, chemistry, and crystallinity of the graphite reveal a difference between the Pramollo (Timonsella Mine) and Pomaretto (Pons Mine) areas. Graphite characterized by high purity and a high crystallinity typical of high-grade metamorphism (Type I) occurs in the Pramollo area. On the contrary, graphite with low purity and a high crystallinity typical of low-grade metamorphism (Type III) is observed in the Pons Mine. This difference likely reveals a difference in their genesis. Interestingly, the samples from the Pramollo area were collected near a Permian metadiorite stock, whose thermo-metamorphic aureole could have induced the graphitization of the organic matter. On the contrary, the graphite collected from several shear zones from the Pons Mine may have been generated via late growth and/or the remobilization of graphite along shear zones active during the exhumation stages of the Alpine orogenesis. Additional detailed analytical works would confirm this hypothesis.

(d)

The geological setting, the genetic process, and the main features of the mineralization indicate that the graphite of the Pinerolo Unit belongs to graphite I of Simandl et al., 2015 [24]

(e)

Although graphite mineralization is a common exploration target, especially for CRMs (such as V), given the capability of the organic matter to scavenge metallic cations, which can later crystallize as ore minerals during metamorphism, bulk ore geochemical analyses did not reveal economic amounts of CRMs, apart from graphite itself. However, a more extensive geochemical campaign is advisable before excluding the area as an exploration target.