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Article

Ore Genesis Based on Microtextural and Geochemical Evidence from the Hydrothermal As–Sb Mineralization of the Matra Deposit (Alpine Corsica, France)

by
Danis Ionut Filimon
1,
John A. Groff
2,
Emilio Saccani
3,* and
Maria Di Rosa
1
1
Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, 56126 Pisa, Italy
2
Physical Sciences Department, Tuscaloosa Academy, Tuscaloosa, AL 35406, USA
3
Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, Via Ludovico Ariosto 35, 44121 Ferrara, Italy
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(8), 814; https://doi.org/10.3390/min15080814 (registering DOI)
Submission received: 25 June 2025 / Revised: 18 July 2025 / Accepted: 29 July 2025 / Published: 31 July 2025
(This article belongs to the Special Issue Using Mineral Chemistry to Characterize Ore-Forming Processes)
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Abstract

The Matra As–Sb deposit (Alpine Corsica, France) is hosted in the normal N–S trending Matra Fault. Sulfide minerals in ore consist of realgar, stibnite, and pyrite with minor orpiment and hörnesite. The gangue includes quartz, dolomite, and calcite. In this study, the microstructural analysis of selected ore samples has been combined with the geochemical characterization of the sulfides. The results depict a succession of events that record the evolution of the ore deposit related to fault movement. In the pre–ore stage, plumose, crustiform, jigsaw, and feathery textures of quartz testify to a short–lived boiling event. The mineral assemblage of the main–ore stage includes an Fe(–Zn) substage dominated by the formation of different textures of pyrite. In general, pyrite samples contain significant concentrations of As (≤32,231 ppm) and Sb (≤10,684 ppm), with lesser amounts of by Tl (≤1257 ppm) and Ni (≤174 ppm). This is followed by an Sb–As–Fe substage of pyrite–stibnite–realgar ±orpiment. The precipitation of the sulfides was mainly driven by changes in ƒS2. The increasing level of oxidation is attributed to a progressive influx of meteoric water resulting from reactivation of the Matra Fault.

1. Introduction

The Matra deposit (MD), located in eastern Corsica, France, produced a total of ~30,000 tons of ore containing 30% As until 1945 [1]. Realgar is the main ore mineral with stibnite and minor orpiment. The deposit was discovered between 1880 and 1890 as a zone of green siliceous clay–like material containing realgar and orpiment exposed in a stream bank [2,3,4]. The mineralization occurs along the post–Alpine normal N–S–trending Matra Fault, which juxtaposes the Jurassic–Cretaceous(?) Upper Castagniccia and Santo Pietro di Tenda Units, both belonging to the Schistes Lustrés Complex [5]. The Matra Fault is part of a larger network of normal faults that accommodated the extension of the Tyrrhenian Sea [6].
The Early Oligocene in the Corsica–Northern Apennine–Tuscany sector marked an important transition from compression to extensional tectonics [7]. This shift was associated with igneous activity and the formation of mineralization at a large scale in the Tuscan Archipelago and the Tuscany Magmatic Province. Ore deposits in the Tuscany Magmatic Province include carbonate–hosted Sb (±Au) mineralization that is characterized by the presence of invisible gold in pyrite, a trace element signature of As–Hg–Tl–Ba, jasperoid alteration, and dilute low–temperature fluids [8,9,10]. Sillitoe and Brogi [10] proposed that carbonate–hosted Sb (±Au) mineralization, related to the Tuscany Magmatic Province, was Carlin–style based on ore characteristics and the evolution of plutons emplaced at depths of 5–7 km. Ore produced from the MD has a similar mineral assemblage and geological setting, formed during the transition from compressional to extensions tectonics along the Corsica–Tuscany transect.
The objectives of this study are therefore to establish the ore paragenesis of the MD and to document the geochemistry of the different sulfides. Data obtained using various analytical techniques such as scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM–EDS), electron probe microanalyses (EPMA), and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) are used to produce a genetic model, including the tectonic setting, for the MD As–Sb mineralization. Finally, as both As and Sb are included in the list of European Critical Raw Materials [11], this contribution may provide new insights that will benefit future exploration for this type of ore deposit.

2. Geological Setting

According to Durand–Delga [12], Corsica can be divided into two distinct geological domains (Figure 1): the Autochthonous Hercynian Domain (to the west) and Alpine Corsica (to the east). The Hercynian Domain is primarily composed of Carboniferous to Permian granitoids intruding a Paleozoic basement, which is unconformably overlain by Jurassic to Middle Eocene sedimentary units (e.g., [13]). Alpine Corsica comprises a complex stack of tectonic units derived from both oceanic and continental domains. The oceanic units represent remnants of the Ligure–Piemontese oceanic basin, which existed between the European and Adria continental margins during the Middle to Late Jurassic (e.g., [12,13,14,15,16,17,18,19,20]). The oceanic–derived units include an ophiolitic sequence covered by associated sedimentary covers, and based on their structural position and the degree of deformation and metamorphism, they compose the Upper Units and the Schistes Lustrés Complex [12]. The tectonic units of Alpine Corsica are unconformably overlain by Miocene (Burdigalian–Langhian) sedimentary rocks hosted in basins that are exposed in the Saint–Florent and Francardo regions (e.g., [21]).
The Upper Units (e.g., Balagne Nappe) occupy the highest structural level of Alpine Corsica [22]. They are primarily composed of basalts with associated sedimentary cover rocks, marked by a significant input of continental–derived sediments [23], ranging in age from the Dogger to the Cretaceous [12]. Basalts within the Balagne Nappe exhibit ocean–floor metamorphism [23,24,25].
The Schistes Lustrés Complex is widely exposed in eastern Corsica and Cap Corse and lies below the Upper Units [26]. It is composed of several tectonic units having oceanic affinity and includes serpentinized ultramafic rocks, troctolites, gabbros, ferrogabbros, plagiogranites, dolerites, and basalts—most of which display N–MORB–type magmatic affinities [24,27,28]. The associated ophiolitic sedimentary cover rocks range in age from the Late Jurassic to the Mid–Cretaceous and are composed of pelagic deposits (radiolarian cherts, shales, and micritic limestones) that are typical of oceanic sequences formed in the deeper parts of the Ligure–Piemontese oceanic basin [29]. Metamorphism of the Schistes Lustrés Complex varies from low blueschist to the eclogite facies [30,31,32].
Between the Late Cretaceous and Late Eocene, convergence between the European and Adria plates led to the progressive closure of the Ligure–Piemontese oceanic basin [33,34,35]. This tectonic evolution began with intra–oceanic subduction, which was followed by the subduction of the continental margins and subsequent collision–related processes (e.g., [36]). Plate convergence ceased in the Early Oligocene and was replaced by widespread extensional tectonics (e.g., [37]). Evidence of these subduction–related events is well–preserved in the Schistes Lustrés Complex, which reached metamorphic peak conditions represented by the eclogite facies between ~83 Ma and 35 Ma [16,38].
During the Early Oligocene, a shift in plate kinematics transferred the convergence zone east of Alpine Corsica, initiating a new westward–dipping subduction system [7,39,40]. This change generated a broad extensional regime that caused the collapse of the previously formed orogenic wedge in Corsica and led to the opening of the Liguro–Provençal back–arc basin [37,41,42].
This was followed by Late Miocene extension that triggered the opening of the Tyrrhenian back–arc basin, effectively separating Corsica from the Apennine orogenic system [43,44,45]. In Alpine Corsica, this episode of extension was accompanied by magmatic activity (~14 Ma), based on the age of lamproitic rocks at Sisco, which are attributed to the Tuscany Magmatic Province ([46]; Figure 1a). The Tuscany Magmatic Province comprises a suite of mafic to silicic intrusive and extrusive rocks that become progressively younger toward the east, culminating in ages of 0.2–0.3 Ma at Mt. Amiata [46,47].
The Alpine Corsican tectonic units in the Matra area include the Santo Pietro di Tenda and the Upper Castagniccia Units, which are part of the Schistes Lustrés Complex [5,48]. The Santo Pietro di Tenda Unit comprises an ophiolitic sequence consisting of metaserpentinites, metagabbros, metaophicalcites, metabasalts, and metaophiolitic breccias overlain by metacarbonates and schists (e.g., [28,29]), whereas the Upper Castagniccia Unit is mainly composed of calc–schists, schists, and quartzites [49]. Both of these units were affected by Alpine ductile deformation and metamorphism that reached the eclogite facies [5]. In the post–Alpine deformation, these units were juxtaposed along the Matra normal fault, a brittle structure trending N170° and dipping 75° eastward (Figure 1b; [5]). The Matra Fault developed during Miocene extension in association with the opening of the Tyrrhenian Sea and hosts the MD composed of Sb–As–Fe sulfides and the related dolomite and minor quartz gangue. Given the kinematic indicators (e.g., slickenfibers) formed within both the sulfides and the dolomite gangue, Filimon et al. [5] demonstrated that the faulting activity and the formation of MD and related gangue were coeval.

3. Sampling and Analytical Methods

Forty–one samples of gossan and metamorphic rocks hosting mineralization were collected from mine waste and outcrops surrounding the MD in the Presa Valley (Figure 1c and Figure 2, Table 1). The mine was inaccessible to sampling due to rock collapse at its entrance. Twenty–three samples representing different ore types, host rocks, and gossan were studied using optical microscopy (e.g., transmitted and reflected light) and X-ray fluorescence spectroscopy (XRF) at the Ferrara University (Supplementary Table S1).
These samples were then studied using a combination of SEM, EPMA, and LA–ICP–MS. The evaluation of these data allowed us to define mineral assemblages, microtextures, and the geochemistry of the sulfide and gangue minerals (Supplementary Tables S2–S4). Scanning electron microscope analyses were undertaken using a Hitachi TM3030Plus Tabletop Electron Scanning Microscopy laboratory coupled with a TM3030 Swift ED3000 microanalysis system (EDS) in the Dipartimento di Scienze della Terra di Pisa (Pisa, Italy). The WDS analyses (spot and map modes) of ore minerals (e.g., realgar, stibnite, and pyrite) were performed using a JEOL JXA–8800 microprobe in the Dipartimento di Scienze della Terra “A. Desio” at the Università di Milano Statale.
Spot analyses were performed at 15 kV and 2 nA with a beam size diameter of 1 µm. The standards used for sulfides were Ni2Si (Ni), pyrite (Fe, S), cuprite (Cu), stibnite (Sb), and GaAs (As). Back–scattered electron mapping conditions were 15 kV and 1 nA, with a counting time of 20 ms per grid point. Mineral phases documented in the maps were identified using the spot analysis mode of XMapTools [51]. The EPMA of carbonate gangue was performed using a beam diameter of 3 µm, with a 15 kV voltage and a 2 nA sample current at the ISTerre of Grenoble (Grenoble, France). The standards used included calcite (Ca), dolomite (Mg), siderite (Fe, Mn), and strontianite (Sr). Additional analytical information is given in the Supplementary Materials, Table S3.
The trace element contents of pyrite and stibnite were determined using a PerkinElmer NexION 2000 ICP–MS coupled with a New Wave Research–193 Ar–F 193 nm excimer laser in the Centre for Instrument Sharing at the University of Pisa (CISUP). The laser operated at a repetition rate of 10 Hz with a spot size between 35 and 40 µm, depending on the mineral size, and 5 J/cm2 of energy density. The trace elements analyzed included 49Ti, 53Cr, 55Mn, 57Fe, 59Co, 60Ni, 65Cu, 66Zn, 71Ga, 72Ge, 75As, 77Se, 95Mo, 107Ag, 111Cd, 115In, 118Sn, 121Sb, 125Te, 182W, 195Pt, 197Au, 205Tl, 208Pb, and 209Bi. Data reduction was performed with the IOLITE v4.10.7 software [52]. Iron and sulfur from EPMA were used as internal standards in pyrite and stibnite, respectively. The sulfide analyses were calibrated using the standard STDGL3 [53], and the data quality was verified using UQAC5.
X-ray diffraction (XRD) was used to identify minor mineral phases (e.g., hörnesite and orpiment). Owing to the small amount of material, X-ray powder diffraction data were collected using a Bruker D8 Venture single–crystal diffractometer in the Dipartimento di Scienze della Terra di Pisa (Pisa, Italy), equipped with a Photon III CCD area detector (microfocus CuKα radiation) simulating a Gandolfi–like geometry (Supplementary Table S5).

4. Mineralogy and Geochemistry of Ore Minerals

4.1. Pyrite

In mineralized samples from the MD, pyrite was categorized as either Py–I, Py–II, or Py–III based on textural features and mineralogical associations (Figure 3). Py–I occurs as large euhedral crystals ≤1 cm in size and is associated with sphalerite (Figure 3a,b or Figure 4a). The cubic shape is a characteristic of both single crystals and aggregates disseminated in the quartz and dolomite gangue. No zoning was identified in Py–I, as compositional variations are limited (Fe0.780–0.989As0.020–0.380Sb0–0.001Ni0–0.035Cu0–0.001S2). The most abundant trace elements were As, Sb, and Tl. Note that Ni and Zn were also detected (Table 2; Figure 5).
Py–II crystals are anhedral and range in size from 100 µm to 2 mm. These occur in the dolomite gangue, often in association with Py–I. Visible banding was evident (Figure 3c,d), and EPMA identified rhythmic zoning in some crystals, while others displayed a mottled texture (Figure 4b). This is reflected in the compositional range of Fe (Fe0.934–1.005As0.005–0.191Sb0–0.042Ni0–0.052Cu0–0.002S2; Supplementary Table S4). Analyses by LA–ICP–MS revealed high concentrations of As and Sb (Table 2). Other elements such as Tl, Ni, and Zn were also detected (Figure 5). This type of pyrite occurs either as crystal aggregates in veins or disseminated within the dolomite gangue.
The most common type of pyrite identified in ore samples is Py–III. It is characterized by highly fractured crystals ranging in size from 50 µm to 1 mm (Figure 3e,f). The occurrence of these crystals is confined to veins that are distinguished by the presence of dolomite–calcite, stibnite, realgar, and, to a lesser extent, quartz. This pyrite formed through the fracturing of Py–I and subsequent displacement of these fragments. The EPMA micro–maps show that Py–III crystals have rims enriched in As and Sb. Although Ni shows a uniform distribution, the back–scattered electron (BSE) images for As show inner zones that are lighter in color and have high As concentrations relative to the darker zones. The LA–ICP–MS analyses also recorded significant variations in several elements, including As, Sb, Ni, and Tl (Table 2). The highest recorded concentrations of Cu and Co were 441 ppm and 2868 ppm, respectively (Figure 5). Although Py–III has a wider range of element concentrations, there is significant overlap between the compositions of Py–I and Py–II (Table 2; Supplementary Materials Table S2). What readily distinguishes Py–II is that the intense fracturing of pre–existing pyrite and displacement of the small fragments (≤15 µm) formed parallel layers within the dolomite gangue (Figure 3g,h or Figure 4d).

4.2. Stibnite

Stibnite (Sb2S3) is a common mineral that is spatially associated with other sulfides in Matra ore samples. As stibnite formed in association with distinct mineral assemblages, this was used as a basis to distinguish three types of stibnite (e.g., Sbn–I, Sbn–II, and Sbn–III). Sbn–I occurs either as large (≤1 cm) anhedral crystals or small (≤100 µm) acicular crystals (Figure 6a,b). It is associated with Py–I, realgar, sphalerite, and quartz. The results of the LA–ICP–MS analyses show significant ranges in concentrations of As, Tl, and Zn (Figure 7).
Sbn–II occurs as interstitial fillings (≤100 µm; Figure 6c,d) and veinlets (≤3 cm thick; Figure 6c,d). It is associated with dolomitic gangue and locally encapsulated by realgar. Sbn–II consists of 70.8 wt.% Sb and 28.1 wt.% S (compositional range of Sb1.897–2.068As0.032–0.281Ni0–0.004Fe0–0.004Cu0–0.004S3). The trace element composition of Sbn–II is dominated by As, Tl, and Ni (Figure 7).
Crystals of Sbn–III range in size from 50 µm to 1 cm and occur in veins also containing Py–III and quartz that cut the dolomite gangue (Figure 6e,f). Positive anomalies of As and Fe were recorded by EPMA analyses, but no compositional zoning was identified. Median concentrations for Sb and S were 71.32 wt.% and 28.47 wt.%, respectively (compositional range of Sb1.748–2.030As0.041–0.282Ni0–0.003Fe0.001–0.012Cu0–0.004S3). There was significant variation in the concentrations of As, Tl, Fe, and Ni (Figure 7).

4.3. Arsenic Minerals

The most abundant arsenic sulfide phase is realgar (As4S4). It occurs in veinlets ≤1 cm wide (Figure 2d–f,h–l) as nodule–like masses ≤5 cm across (Figure 2f) or as interstitial fillings (Figure 2i). Petrographic studies indicate that realgar grew on different types of pyrite and is intergrown with dolomite and quartz (Figure 3a). EPMA data for realgar indicate 68.1 wt.% for As and 30.6 wt.% for S, with an Fe content of ≤0.18 wt.% (compositional range of As3.682–3.813Sb0–0.012Fe0–0.005Ni0–0.004S4). These data record a range of 0.09–0.17 wt.% for Sb and Ni values ≤0.05 wt% (Supplementary Materials, Table S3).
Another primary arsenic–bearing minerals, orpiment (As2S3), was identified in a limited number of samples. The breccia samples contain yellow prismatic orpiment crystals ≤80 µm in length, which have a vitreous luster and partially fill vugs. The occurrence of orpiment was verified by XRD (Figure 2f). Pristine realgar crystals occur in some vugs in contact with orpiment (Supplementary Materials, Table S5). A similar relationship was identified in one sample between pristine realgar crystals and hörnesite filling a vug.

4.4. Gangue

Ore samples contain gangue consisting of dolomite and quartz. Dolomite is more abundant and occurs in crystal aggregates or veins (Figure 2h,i). The SEM–EDS analyses recorded peaks of Ca, Mg, and Fe. EPMA analyses confirmed elevated Fe concentrations of 3.35–5.73 wt.% to support its classification as ferroan dolomite ([54] Supplementary Materials, Table S3).
Macroscopic and petrographic studies of hydrothermal quartz veinlets identified distinct textures. Plumose and crustiform textures (Figure 8a,b) were evident at a macroscopic scale compared with mosaic and jigsaw textures at a microscopic scale (Figure 8c,d). Veins containing Sbn–I, Py–II, and Py–III, quartz grains tend to be small (<100 µm).
Late calcite occurs in veins (<1 cm) that cut gangue containing ore minerals or as crystals filling cavities. The cavity fillings consist of large, euhedral crystals with lesser quartz and dolomite.

4.5. Supergene Minerals

Secondary minerals formed by surface weathering include pararealgar, hörnesite [Mg3(AsO4)2·8(H2O)], gypsum, and unidentified Fe–Mn–Ni oxides that contain minor amounts of As and Sb. Hörnesite in several ore samples was identified by XRD (Figure 2l) and is characterized by globular, white crystals ≤100 μm. It is locally associated and intergrown with pristine realgar crystals and likely formed by the weathering of primary As–bearing sulfide minerals occurring as large masses of radiating crystals on mine waste. The Ni oxide is recognized by an aqua–green color and globular masses that are ≤1 cm across. In contrast, gypsum was identified as small, transparent, tabular crystals. These minerals dominantly occur as thin films on outcrops of gossan in metaophicalcites and metaserpentinites.

5. Discussion

The mineralization studied in this work is located along the Matra Fault, which displaced different tectonic units of the Schistes Lustrés Complex (Figure 1; [5]). These units include ophiolitic rocks and their carbonate sedimentary cover, which were both faulted and underwent hydrothermal mineralization processes [2,4].

5.1. Ore Textures and Geochemistry

From the meso– to the microscale, the mineralogy and tectonic setting of the Matra deposit indicates that this ore deposit formed in a dynamic system where the multi–stage activation of the Matra Fault drove fluid circulation and sulfide crystallization. Ore minerals occur in breccia clasts, veinlets, gangue, and along the fault plane as slickensides. Specifically, analyses of pyrite and stibnite were conducted to characterize their microtextures and geochemistry.
Pyrite is ubiquitous in ore deposits and can host various trace elements that reflect the physico–chemical conditions of formation (e.g., redox state, pressure, and temperature) and the nature of their sources. Therefore, pyrite is often used to study ore–forming processes and to develop genetic models for different types of ore deposits [56,57]. As stibnite is also an important ore constituent of the MD, it could provide additional information to develop a genetic model.
A proposed sequence of mineralization at the MD was developed relative to reactivation of the Matra Fault. Py–I has the largest size and often exhibits euhedral forms, suggesting it probably formed during an early stage of faulting (Figure 3). This is supported by the results of George et al. [58] that demonstrated how continuous, or renewed, fluid circulation and crystallization can produce euhedral pyrite crystals. Conditions in the Matra hydrothermal system likely changed with the crystallization of anhedral Py–II, which is widespread and occurs either as aggregates with Py–I or within gangue (Figure 3).
The evolution of the textures from Py–I to Py–II might be driven by the changing thermal condition that existed during formation of the MD, with temperature decreasing over time. As shown by relationships in other sulfide deposits (e.g., [59,60]), colloform pyrites occur in low–temperature (<300 °C) hydrothermal settings, forming the characteristic texture we observe in MD for Py–II (Figure 3). Yet the different and highly disjointed texture of Py–III suggests mechanical alteration of Py–II due to microfracturing during faulting. Continued or repeated movement along the Matra Fault produced strain partitioning, which resulted in the fracturing of Py–III to form the characteristic highly fractured and fine-grained crystals.
Although these differences in pyrite textures are proposed to record changes in the mineralizing system, there is little chemical variation from Py–I to Py–III (Figure 9 and Figure 10). Due to this and documentation that mineralization was syndeformational, there is not compelling evidence to indicate that these represent different generations of pyrite.
Trace elements hosted by pyrite can exist in the mineral structure or occur as micro– to nanoscale particles (NPs; [61,62,63]). At MD, the elements that likely underwent isovalent substitutions with Fe2+ include Mn, Co, Ni, Cu, and Zn. The Ni enrichment observed in the pyrite of Matra ranges between 50 and 5000 ppm, with higher values for Py–III (Figure 5). A similar Ni concentration was identified by Picot and Johan [64] in Alpine Corsica close to Matra. These authors described Ni–bearing minerals in the ophiolitic rocks of the Santo Pietro di Tenda Unit. We therefore suggest that the Ni in Matra pyrite may derive from fluid interaction with mafic and ultramafic host rocks widely exposed in the area (Figure 1).
The Co/Ni ratios can provide information regarding the different settings in which pyrite forms [65]. Those determinated for Matra are very low (Co/Ni < 0.1) (Figure 10a) and are incompatible with sedimentary, hydrothermal, or magmatic/volcanological origins [66,67]. Although there is a linear relationship between these two elements, the ratio is influenced by a positive Ni anomaly due to the mafic nature of country rocks hosting mineralization.
The occurrence of a second group of elements that are abundant in Matra samples (e.g., As3+, Sb3+, and Tl+) can be explained by As enrichment in pyrite. In fact, As can cause structural defects, such as heterovalent substitutions, and promote metal enrichment in the pyrite structure [68,69]. The substitution process involving 2Fe2+ ↔ Tl+ + Sb3+ or (Cu+ and Ag+) was proposed by D’Orazio et al. [70] and George et al. [68]. This suggests that structural substitutions of Tl, Sb, and As may occur via isovalent and heterovalent processes or NPs. In addition, our data show that the LA–ICP–MS spectra follow a linear trend and lack any signs of peaks (Figure 11). This supports the absence of micro–inclusions of other minerals and is confirmed by the petrographic observations (Figure 3).
Arsenic in pyrite can have different oxidation states. This is represented by As1− [71], As2+ [72], As3+ [73], or amorphous As–Fe–S nanoparticles (As0; [74]). The negative correlation between As and S (R = −0.91; Figure 10b) indicates that As enters the structure as an anion [74]. This is confirmed by plotting data generated by EPMA analyses of Py–I, Py–II, and Py–III in a ternary diagram (Figure 12). Deditius et al. [73,74,75] stated that, as is evidenced in the MD, the majority of pyrite exhibits an oxidation state of 1+, which is the case for Matra pyrite (Figure 12). However, a secondary distribution typical of Matra pyrite, in which the As has a 3+ oxidation state, appears to occur (Figure 12). This suggests an incipient evolution of the system’s oxidation state that led to the crystallization of realgar, orpiment, and hörnesite during the hydrothermal event.
The different types of pyrite in samples from the MD are also enriched in Sb and Tl. This is illustrated by the broad linear relationship (R = 0.91) between As and Tl concentrations (Figure 10c,d), as reported by D’Orazio et al. [70]. Nevertheless, the relationship between the As, Sb, and Tl (R = 0.82) is less clear, yet it suggests a close relation. What is evident is that data for Py–I and Py–II plot as two distinct clusters, whereas Py–III is easily distinguished due to the large range in compositions (Figure 10d).
George et al. [58] proposed that it is possible to verify whether Sb controls how elements (e.g., Cu and Zn) are concentrated in the pyrite structure. This was deduced by hypothesizing solubility lines and considering the following relationships: CZn = CSb + 10−4 and CCu = 2 × CSb + 10−4 (where CZn, CCu, and CSb are the mol. % concentrations of Zn, Cu, and Sb in pyrite). If data plot below the line (e.g., a solubility curve), it may be expected that a solid solution occurs. Conversely, where data plot above the line, this likely indicates the presence of nanoparticles or larger inclusions. Data for pyrite from the MD are located below the line in a plot of Cu vs. Sb, suggesting that Cu occurs in solid solution with all the pyrite types (Figure 10e). Similarly, data in a Zn vs. Sb plot, excluding one point, are located below the line and suggest that Zn occurs in solid solution with pyrite (Figure 10f). The anomalous point likely indicates the presence of sphalerite nanoparticles.
Stibnite mainly occurs as small euhedral crystals that are either disseminated in gangue or combine to form veins. Although the three types of stibnite are associated with different mineral assemblages, there is little variation in their geochemical composition (Figure 7). As with pyrite, there is no compelling evidence to indicate there are different generations of stibnite. Rather, they are probably the same generation, with slightly different textures.
Element substitutions in stibnite are not as well understood compared with pyrite [76,77,78,79,80,81,82,83,84,85,86]. The Sb3+ ↔ As3+ replacement mechanism in stibnite is favored because of the similar ionic radii and chemical properties of these elements [87,88]. In fact, a linear relationship between As and Sb (R = –0.97; Figure 13a) is observed in MD stibnite, supporting this replacement mechanism. Linear relationships are also evident for Pb and Cu (R = 0.48; Figure 13b), as previously demonstrated [76]. Despite these relationships, it is unclear how the elements enter stibnite, and it is also unknown how high concentrations of Fe, Zn, and Tl can occur. Substitution mechanisms such as Cu+ + Pb2+ + As3+ ↔ 2Sb3+ + ↔ and 3Sb3+ ↔ As3+ + 2Cu+ + Hg2+ + Pb2+ were proposed by Fu et al. [76] and Song et al. [78], respectively. However, these are not applicable to stibnite from the MD. When comparing trace element compositions for stibnite documented in the literature, we observe that for MD ore, As and Pb + Hg are at least one order of magnitude higher and lower, respectively (Figure 13c). The abundance in As is not surprising for the MD, and the small contents of Pb + Hg in stibnite are supported by a lack of Pb–Hg–bearing minerals [49]. Additionally, Hg was not measured during LA–ICP–MS acquisition, as Hg contents in bulk ore samples were below 31 ppb [89]. In contrast, Tl has average concentrations of 100–1000 ppm (Figure 13d). Unlike mineralizations in other hydrothermal systems where Cu + Ag + Tl contents in stibnite define different trends when compared with Fe, the influence of Cu and Ag is negligible and no linear relationship between Fe and Tl can be observed in data for the MD (Figure 13e).
Although the mechanisms by which metals are incorporated into the pyrite structure are not entirely clear, Reich et al. [71] suggested that As–rich pyrites formed under nonequilibrium conditions may have precipitated rapidly at low temperatures (T < 250 °C), incorporating trace metals into the mineral structure. This incorporation is facilitated by the presence of extra vacancies and surface defects resulting from rapid growth and disequilibrium. Due to the limited reference data available, it is possible that during formation under nonequilibrium conditions, stibnite precipitates rapidly in low–temperature systems (T < 250 °C) to become enriched in trace metals.

5.2. Paragenetic Sequence

The mineral paragenesis developed for the MD records pre–ore and main–ore stages, which are subdivided into a first and second event, followed by a supergene stage (Figure 14).
The pre–ore stage is represented by the formation of quartz devoid of sulfide minerals. Plumose, crustiform, jigsaw, and feathery textures in quartz (Figure 8) record a late, short–lived boiling event [55,90]. This was most likely due to tectonic decompression caused by the reactivation of the Matra Fault. Although the boiling event was likely brief, there were two important consequences. The first is that Hg partitioned into the vapor phase, which explains the low concentrations of Hg (≤31 ppb) in ore samples [89], while Sb and As remained in the aqueous phase at low temperatures [91]. The second consequence is that a loss of CO2 prompted the precipitation of ferroan dolomite.
The main–ore stage began with the Fe (–Zn) substage with the deposition of dolomite containing Py–I, Py–II, and minor sphalerite (Figure 14). Although the textures of pyrite changed over time (from euhedral to anhedral), the only potentially important geochemical change was a slight decrease in As content from Py–I to Py–II. There is no evidence that boiling occurred during the formation of gangue quartz and dolomite or associated sulfides.
Subsequently, the main–ore stage evolved into the Sb–As–Fe substage, which is represented by the deposition of As–Sb sulfides and Py–III (Figure 14). At this point, the hydrothermal system was open due to reactivation of the Matra Fault, which brecciated Py–III and permitted continuous fluid flow to allow mineralization, including stibnite veins (e.g., Sbn–I, Sbn–II, and Sbn–III). An important point is that the deposition of realgar indicates a change in the oxidation state of As relative to As in earlier pyrite. The formation of As–As bonds in realgar suggests that As was not completely oxidized by S, and the average oxidation state is considered to be As2+. In realgar, three different scenarios may be envisaged for the bonding of As atoms: (1) one bond to three S atoms; (2) two bonds to two S atoms; and (3) one bond to two As atoms and one S atom (e.g., [92]). These scenarios result in oxidation states of 3+, 2+, and 1+, with the average oxidation state for realgar being considered 2+ [93]. The main–ore stage ended with crystallization of orpiment and hörnesite. These minerals contain As atoms in different oxidation states. Macroscopic observations of orpiment and realgar crystals deposited in the same vugs indicate that both minerals were stable based on the pristine nature of the crystals, even though As atoms have oxidation states of 3+ in orpiment and 2+ in realgar. A similar relationship is documented by the coexistence of hörnesite and realgar in unoxidized ore samples. Even though the oxidation states of As atoms are 5+ for hörnesite and 2+ in realgar, the realgar crystals exhibit a vitreous luster and appear pristine.
The main mechanism for the precipitation of sulfides is therefore considered to be changes in sulfur fugacity. These changes occurred because the reactivation of the Matra Fault allowed an influx of oxygenated meteoric water into the hydrothermal system that resulted in significant fluid mixing. After the deposition of pyrite at the MD, mineralogical changes in the main–ore stage document a transition from reducing to oxidizing conditions that caused changes in the oxidation state of As atoms (Figure 14). The distribution of data in a Fe–As–S ternary diagram (Figure 12), interpreted by following Deditius et al. [75], shows that As enters the structure of pyrite in a reduced state (As1−) compared to an oxidized state (As2+) for realgar. Due to the continued influx of meteoric water, the oxidation state of As changed from 3+ to 5+ during the deposition of orpiment and hörnesite, respectively. An important point is that the meteoric water did not introduce significant quantities of other elements to alter the composition of the hydrothermal fluids. Instead, cooling caused by fluid mixing decreased the solubility of As and Sb [91], and a concurrent decrease in ƒS2 can explain the formation of stibnite and realgar in late–stage hydrothermal systems. A salient characteristic of the hydrothermal fluids that produced mineralization was that they were neutral or slightly reduced, as indicated by the widespread occurrence of dolomite gangue during the main–ore stage. Fluid mixing and interaction with the carbonate gangue likely caused changes in the fugacity of oxygen (ƒO2) and carbon dioxide (ƒCO2). This could explain how arsenic sulfides (As2+ and As3+) and arsenates (As5+) can coexist.
Another possible factor that could contribute to the precipitation of sulfides is a decrease in temperature. Stibnite could be most sensitive to temperature, as demonstrated by other authors [94,95,96]. To verify this hypothesis, particular attention was paid to the twinning of dolomite gangue, which has been shown to provide a simple constraint to discriminate <350 °C [97]. The results seem to be consistent with temperatures <200 °C, but it is not clear whether calcite forms at the same time as stibnite or only later. Therefore, the role of temperature in the precipitation of sulfides along the Matra Fault requires future investigation.
As noted above, the reactivation and movement of the Matra Fault allowed increasing amounts of oxidized meteoric water to enter the hydrothermal system. Cooling associated with fluid mixing is an efficient mechanism for the deposition of stibnite [97] and As–sulfide minerals [91]. When hydrothermal fluids ascended along the Matra Fault, changes in physico–chemical conditions (e.g., temperature and pH) caused a decrease in the solubility of metals and precipitation of As and Sb minerals. Even if Sb was transported as a hydroxide complex [e.g., Sb(OH)3], significant precipitation would occur once temperatures were below 250–300 °C [98,99,100]. The sensitivity of Sb to temperature is further demonstrated by how the solubility of stibnite decreases with temperature [98]. The absence of base metals at the MD, except for minor sphalerite (Figure 14), supports the hypothesis that chloride ions were not complexing agents and that fluid temperatures were <300 °C [101,102]. Therefore, it is possible that several parameters could have changed the valence state of a metalloid, allowing the same element to crystallize with different valence states.
The supergene minerals are not related to the hydrothermal system and formed due to surface weathering. This is evident in the widespread occurrence of hörnesite in mine waste and outcrops of gossan containing As–bearing minerals. Gypsum likely formed due to the oxidation of sulfides. Other metals that were mobilized by weathering formed thin films consisting of Ni and Fe–Mn oxides.

5.3. Geodynamic Evolution of the Matra Deposit

A comparison of the characteristics of mineralization at the MD and Tuscany Carlin–style deposits is made in this section. Similarities and differences in their geochemistry are examined in the context of tectonic and magmatic evolution. Regarding ore mineralogy, many similarities between the MD and Carlin–style gold deposits in Tuscany can be observed, which could suggest a possible genetic link. A common sulfide assemblage at both localities consists of stibnite and pyrite ± realgar ± sphalerite. However, there are important differences in geochemical signatures, with Tuscany represented by the assemblage of Au–Hg–Tl–Ba [8,9] compared with the MD, which is characterized by Tl and the absence of the other elements. Significant differences also exist in the composition of hydrothermal fluids that formed quartz prior to the realgar–orpiment–stibnite–carbonate mineralization. In Tuscany, acidic ore fluids decalcified ferroan limestones that released the Fe necessary for sulfidation and Au deposition in arsenian pyrite hosted by jasperoids [10,103]. At the MD, there is no evidence of pre–existing ferroan carbonates, acidic ore fluids, or decalcification of the host rock. As a consequence, jasperoids did not form at the MD. Another important feature of the MD is the pre–ore quartz, which displays textures that are indicative of boiling, which has never been reported in Tuscany mineralization. The two–phase liquid–rich fluid inclusions in the Tuscany deposits have Th values of 123°–245 °C and salinities <7 wt.% NaCl equiv. [8]. All these factors support a significant meteoric water component in the hydrothermal system, as we have proposed for the MD. Currently, there are no fluid inclusion or stable isotope data for the MD to allow direct comparison.
Structures also had a prominent role in mineralization in both the MD and Tuscany deposits. Filimon et al. [5] established that mineralization at the MD was structurally controlled by the Matra Fault, which developed during a post–Alpine event linked to the opening of the Tyrrhenian Sea [104]. At a regional scale, the larger N–S trending fault network is well–preserved in Alpine Corsica [6] and the northern Tyrrhenian Sea [105]. It formed due to slab rollback and the consequent northeastward migration of the Apennine frontal thrust [45]. This extension also triggered the post–orogenic magmatism of the Tuscany Magmatic Province, from eastern Corsica to southern Tuscany.
Magmatic evolution is the last point to be examined. The only magmatic activity in Corsica is represented by the small lamproite outcrops of Sisco (~14 Ma; [46]). Due to the absence of large–scale and prolonged magmatic activity in eastern Corsica, there was no differentiation in a deeper magma to supply a metal–rich (e.g., Au) vapor phase. Instead, extensional tectonism provided pathways for fluid circulation and the leaching of metals from mafic–ultramafic country rocks by hydrothermal solutions of meteoric origin.
After sporadic manifestations of igneous activity in the Tyrrhenian Sea (e.g., Capraia: ~7 Ma, Elba: 8.4–5.9 Ma, Montecristo: 7 Ma, and Giglio: 5 Ma), magmatism migrated eastward. This is represented by the emplacement of intrusions; the formation of volcanic complexes (e.g., San Vincenzo and Campiglia Marittima: 5.7–4.3 Ma, Gavorrano: 4.4 Ma, Roccastrada: 2.5–2.2 Ma, Amiata: 0.3–0.2 Ma, and Latera: 0.6–0.2 Ma); and the active geothermal fields of Larderello, Monte Amiata, and Latera ([10,106]; Figure 1a). This shows how the intensity of igneous activity increased with time in the Tuscany Magmatic Province and elevated its role in the formation of the Tuscany Carlin–style gold deposits [10]. The greater areal extent of hydrothermal alteration and the existence of large volumes of more–evolved igneous rocks are factors that support the enrichment of ore metals that generated Carlin–style gold deposits proximal to the geothermal fields at Larderello, Monte Amiata, and Latera [10].
The above comparison highlights how mineralization in the MD and Tuscany ore deposits formed in different ways and at different times. However, given their common origin related to Tyrrhenian Sea extension from the Miocene onward, further constraints related to the metallogenesis in the peri–Tyrrhenian area are needed.

6. Conclusions

The As–Sb mineralization at the MD formed in a hydrothermal system partly controlled by reactivation of the Matra Fault, which was linked to the opening of the Tyrrhenian back–arc basin. This normal fault juxtaposes mafic–ultramafic rocks, with associated carbonate sedimentary covers. The structurally controlled mineralization consists of pre–ore quartz that transitioned to main–ore–stage dolomite after a short–lived boiling event. Sulfide mineralization in the Fe (–Zn) substage of the main–ore stage is characterized by three types of pyrite that include idiomorphic (Py–I) and anhedral (Py–II) forms. The disjointed/fractured texture of (Py–III) pyrite records activation of the Matra Fault prior to the Sb–As–Fe substage of the main–ore event. This event coincided with the influx of meteoric water into the hydrothermal system. Supporting evidence is a change from reducing (high–ƒS2) to oxidizing (low–ƒS2) conditions indicated by oxidation numbers of As1−,3+ (pyrite), As2+ (realgar), As3+ (orpiment), and As5+ (hörnesite). Three types of stibnite were also identified by distinct textures, but there is little variation in geochemistry. The trace elements with the highest concentrations in pyrite and stibnite are As–Sb–Tl–Ni and As–Fe–Tl, respectively. These elements entered the lattice of sulfide minerals under a nonequilibrium state at a low temperature (<250 °C). The combination of these conditions with rapid mineral precipitation can promote the scavenging of metals and allow metals to be incorporated into sulfides. Analogous pyrite + stibnite ± realgar ± sphalerite mineralization is found in the Tuscany Magmatic Province, developed east of the Tyrrhenian Sea and during the migration of the Apennine orogenic system. Although there is an indisputable tectonic link between the MD and Carlin–style gold mineralization in Tuscany, the MD has unique features that distinguish it from all the other peri–Tyrrhenian age deposits.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/min15080814/s1: Table S1: X-ray fluorescence (XRF) analyses of major (Wt%) and trace (ppm) element composition of samples from the Matra mine; Table S2: Trace elements concentration (ppm) in pyrites from the Matra mine; Table S3: Trace elements concentration (ppm) in stibnite from Matra mine; Table S4: Major elements (wt. %) data of sulfides and dolomite from Matra mine; Table S5: XRD spectra and data of orpiment and hörnesite from Matra mine (* https://rruff.info).

Author Contributions

Conceptualization, D.I.F. and M.D.R.; methodology, J.A.G. and E.S.; software, D.I.F.; validation, J.A.G. and E.S.; formal analysis, D.I.F. and E.S.; investigation, J.A.G.; resources, M.D.R.; data curation, D.I.F. and E.S.; writing—original draft preparation, D.I.F.; writing—review and editing, E.S.; visualization, J.A.G.; supervision, J.A.G. and E.S.; project administration, M.D.R.; funding acquisition, E.S. and M.D.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by “PRA project, grant number PRA2022–Zarone” by the University of Pisa and “FAR–2024–Saccani” by the University of Ferrara.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank two anonymous reviewers for their constructive reviews and the editor for managing this paper. Technical support and discussions with A. Risplendente, B. Ledérsert, C. Biagioni, S. Pagnotta, and M. Masotta also improved the manuscript. Financial support was provided by V. Zarone (PRA2022 project, Università di Pisa) and the University of Ferrara (FAR–2024–Saccani).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Regional geological map including the studied mine area within a small section of northwestern Italy. The inset panel at the upper right shows the location of the study area relative to a larger section of the western Mediterranean area (modified after [10,17]). (b) Geological setting of the Matra area (modified after [5]). (c) Geological map of the area of the Matra deposit (modified after [5]).
Figure 1. (a) Regional geological map including the studied mine area within a small section of northwestern Italy. The inset panel at the upper right shows the location of the study area relative to a larger section of the western Mediterranean area (modified after [10,17]). (b) Geological setting of the Matra area (modified after [5]). (c) Geological map of the area of the Matra deposit (modified after [5]).
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Figure 2. (a) Landscape around the Matra deposit showing mine waste along the Presa valley. (b) Field occurrence of gossan proximal to the Matra Fault and hosted by metaserpentinites of the Santo Pietro di Tenda Unit. (c–l) Photographs illustrating the different rock types and ore located along the Matra Fault. (c) Example of gossan that contain a dolomite vein displaced by a microfault that developed within the larger normal fault network. Relics of unaltered ophicalcite are also shown. (d) Ore breccia associated with altered metaserpentinite of the Santo Pietro di Tenda Unit, sample MAT24. (e) Realgar (Rlg) and stibnite (Sbn–II) veins in the calc–schist of the Upper Castagniccia Unit, sample MAT4. (f) Ore breccia characterized by vugs filled with realgar (Rlg) and orpiment (Orp), sample MAT11. (g) Ore breccia with pyrite (Py–I), sample MAT6. (h) Dolomite (Dol) and realgar (Rlg) ore breccia, sample MAT24c. (i) Brittle deformation in the ore breccia, sample MAT9. (j) Ore sample cut by realgar (Rlg) and stibnite (Sbn–II) veins, sample MAT10. (k) Ore sample with stibnite (Sbn–I) and quartz (Qz) characterized by vugs filled with realgar (Rlg), sample MAT12. (l) Euhedral crystals of realgar (Rlg) and hörnesite (Hns) filling a vug in sample MAT6b. Mineral abbreviations are from Warr [50].
Figure 2. (a) Landscape around the Matra deposit showing mine waste along the Presa valley. (b) Field occurrence of gossan proximal to the Matra Fault and hosted by metaserpentinites of the Santo Pietro di Tenda Unit. (c–l) Photographs illustrating the different rock types and ore located along the Matra Fault. (c) Example of gossan that contain a dolomite vein displaced by a microfault that developed within the larger normal fault network. Relics of unaltered ophicalcite are also shown. (d) Ore breccia associated with altered metaserpentinite of the Santo Pietro di Tenda Unit, sample MAT24. (e) Realgar (Rlg) and stibnite (Sbn–II) veins in the calc–schist of the Upper Castagniccia Unit, sample MAT4. (f) Ore breccia characterized by vugs filled with realgar (Rlg) and orpiment (Orp), sample MAT11. (g) Ore breccia with pyrite (Py–I), sample MAT6. (h) Dolomite (Dol) and realgar (Rlg) ore breccia, sample MAT24c. (i) Brittle deformation in the ore breccia, sample MAT9. (j) Ore sample cut by realgar (Rlg) and stibnite (Sbn–II) veins, sample MAT10. (k) Ore sample with stibnite (Sbn–I) and quartz (Qz) characterized by vugs filled with realgar (Rlg), sample MAT12. (l) Euhedral crystals of realgar (Rlg) and hörnesite (Hns) filling a vug in sample MAT6b. Mineral abbreviations are from Warr [50].
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Figure 3. Photomicrographs (reflected light) showing the different pyrite types (e.g., Py–I, Py––II, Py–III, and late Py–III). (a) Pyrite (Py–I), stibnite (Sbn–I), and realgar in quartz gangue, sample MAT12. (b) Pyrite (Py–I) and stibnite (Sbn–I) in quartz gangue, sample MAT12. (c) Zoned Pyrite (Py–II) in dolomite gangue, sample MAT10. (d) Pyrite (Py–I and Py–II) with stibnite (Sbn–II) in dolomite gangue, sample MAT10. (e) Transition from pyrite (Py–II to Py–III) in dolomite gangue, sample MAT10. (f) Disjointed pyrite (Py–II with Py–III) and dolomite gangue, sample MAT10. (g) Pyrite (Py–III and late Py–III) in dolomite gangue, sample MAT10. (h) Pyrite (late Py–III) in dolomite gangue, sample MAT10.
Figure 3. Photomicrographs (reflected light) showing the different pyrite types (e.g., Py–I, Py––II, Py–III, and late Py–III). (a) Pyrite (Py–I), stibnite (Sbn–I), and realgar in quartz gangue, sample MAT12. (b) Pyrite (Py–I) and stibnite (Sbn–I) in quartz gangue, sample MAT12. (c) Zoned Pyrite (Py–II) in dolomite gangue, sample MAT10. (d) Pyrite (Py–I and Py–II) with stibnite (Sbn–II) in dolomite gangue, sample MAT10. (e) Transition from pyrite (Py–II to Py–III) in dolomite gangue, sample MAT10. (f) Disjointed pyrite (Py–II with Py–III) and dolomite gangue, sample MAT10. (g) Pyrite (Py–III and late Py–III) in dolomite gangue, sample MAT10. (h) Pyrite (late Py–III) in dolomite gangue, sample MAT10.
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Figure 4. EPMA micro–maps showing the contents of major elements in pyrite (Py–I, Py–II, Py–III, and late Py–III). The distributions expressed in counts per second (cps, see the bar in the figure) range between a minimum (MIN) and a maximum (MAX) value, which are as follows: (a) Py–I, S: 0–536 cps; Fe: 0–671 cps; As: 49–788 cps; and Sb: 266–1421 cps; (b) Py–II, S: 140–514 cps; Fe: 253–636 cps; As: 0–88 cps; and Sb: 0–116 cps; (c) Py–III, S: 244–546 cps; Fe: 0–687 cps; As: 0–169 cps; and Sb: 0–101 cps; (d) late Py–III, S: 0–530 cps; Fe: 0–678 cps; As: 0–89 cps; and Sb: 0–1437 cps.
Figure 4. EPMA micro–maps showing the contents of major elements in pyrite (Py–I, Py–II, Py–III, and late Py–III). The distributions expressed in counts per second (cps, see the bar in the figure) range between a minimum (MIN) and a maximum (MAX) value, which are as follows: (a) Py–I, S: 0–536 cps; Fe: 0–671 cps; As: 49–788 cps; and Sb: 266–1421 cps; (b) Py–II, S: 140–514 cps; Fe: 253–636 cps; As: 0–88 cps; and Sb: 0–116 cps; (c) Py–III, S: 244–546 cps; Fe: 0–687 cps; As: 0–169 cps; and Sb: 0–101 cps; (d) late Py–III, S: 0–530 cps; Fe: 0–678 cps; As: 0–89 cps; and Sb: 0–1437 cps.
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Figure 5. Whisker diagram showing composition ranges of selected trace elements in different pyrite types (Py–I, Py–II, and Py–III) using LA–ICP–MS data. The boxes represent data from the 25th–75th percentiles, the whiskers indicate the minimal and maximum values, the horizontal lines represent the medians, and the lines represent outlier values (Supplementary Materials, Table S2).
Figure 5. Whisker diagram showing composition ranges of selected trace elements in different pyrite types (Py–I, Py–II, and Py–III) using LA–ICP–MS data. The boxes represent data from the 25th–75th percentiles, the whiskers indicate the minimal and maximum values, the horizontal lines represent the medians, and the lines represent outlier values (Supplementary Materials, Table S2).
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Figure 6. Photomicrographs showing the different stibnite types (e.g., Sbn–I, Sbn–II, and Sbn–III). (a) Stibnite (Sbn–I) in dolomite–quartz gangue, sample MAT12 (reflected light). (b) Stibnite (Sbn–I) and pyrite (Py–I) in quartz gangue (Qz), sample MAT12 (BSE image). (c) Stibnite (Sbn–II) and realgar (Rlg) intergrown in dolomite gangue (Dol), sample MAT9 (reflected light). (d) Sbn–II and realgar (rlg) vein associated with Py–II in dolomite gangue (Dol), sample MAT10 (reflected light). (e) Stibnite (Sbn–III) and pyrite (Py–III) in dolomite gangue (Dol), sample MAT10 (reflected light). (f) Stibnite (Sbn–III) and pyrite (Py–III) in dolomite (Dol) and quartz gangue (Qz), sample MAT10 (reflected light).
Figure 6. Photomicrographs showing the different stibnite types (e.g., Sbn–I, Sbn–II, and Sbn–III). (a) Stibnite (Sbn–I) in dolomite–quartz gangue, sample MAT12 (reflected light). (b) Stibnite (Sbn–I) and pyrite (Py–I) in quartz gangue (Qz), sample MAT12 (BSE image). (c) Stibnite (Sbn–II) and realgar (Rlg) intergrown in dolomite gangue (Dol), sample MAT9 (reflected light). (d) Sbn–II and realgar (rlg) vein associated with Py–II in dolomite gangue (Dol), sample MAT10 (reflected light). (e) Stibnite (Sbn–III) and pyrite (Py–III) in dolomite gangue (Dol), sample MAT10 (reflected light). (f) Stibnite (Sbn–III) and pyrite (Py–III) in dolomite (Dol) and quartz gangue (Qz), sample MAT10 (reflected light).
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Figure 7. Whisker diagram showing composition ranges of selected trace elements in different stibnite types (Sbn–I, Sbn–II, and Sbn–III) using LA–ICP–MS data. The boxes represent data from the 25th to 75th percentiles, the whiskers represent the minimal and maximum values, the horizontal lines represent the medians, and the lines indicate outlier values (Supplementary Materials, Table S3).
Figure 7. Whisker diagram showing composition ranges of selected trace elements in different stibnite types (Sbn–I, Sbn–II, and Sbn–III) using LA–ICP–MS data. The boxes represent data from the 25th to 75th percentiles, the whiskers represent the minimal and maximum values, the horizontal lines represent the medians, and the lines indicate outlier values (Supplementary Materials, Table S3).
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Figure 8. Photomicrographs (cross–polarized light) of the pre–ore quartz showing different textures: (a) plumose, (b) crustiform, (c) jigsaw, and (d) feathery. Sketches adapted and modified by Moncada et al. [55].
Figure 8. Photomicrographs (cross–polarized light) of the pre–ore quartz showing different textures: (a) plumose, (b) crustiform, (c) jigsaw, and (d) feathery. Sketches adapted and modified by Moncada et al. [55].
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Figure 9. Schematic representation depicting the textural evolution of sulfides in the Matra Mine. (a) Pyrite (Py–I) and sphalerite in prevalent quartz gangue. (b) Pyrite (Py–II) in prevalent dolomite gangue. (c) Pyrite (Py–III) in prevalent dolomite gangue. (d) Pyrite (late Py–III) and stibnite (Sbn–III) in prevalent dolomite gangue. (e) Stibnite (Sbn–I and Sbn–II) and realgar in dolomite and quartz gangue.
Figure 9. Schematic representation depicting the textural evolution of sulfides in the Matra Mine. (a) Pyrite (Py–I) and sphalerite in prevalent quartz gangue. (b) Pyrite (Py–II) in prevalent dolomite gangue. (c) Pyrite (Py–III) in prevalent dolomite gangue. (d) Pyrite (late Py–III) and stibnite (Sbn–III) in prevalent dolomite gangue. (e) Stibnite (Sbn–I and Sbn–II) and realgar in dolomite and quartz gangue.
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Minerals 15 00814 g010
Figure 10. (a) Ni (ppm) versus Co (ppm) diagram showing data generated by LA–ICP–MS analysis of pyrite (R2 = 0.55); (b) linear relationship between S (mol. %) and As (mol. %) based on data from EPMA point analysis (R2 = −0.91); (c) relationship of As (mol. %) and Tl (mol. %) in pyrite samples shown by data from LA–ICP–MS analysis (R2 = 0.91); (d) relationship of As (mol. %) and Sb+Tl (mol. %) in pyrite samples shown by data from LA–ICP–MS analysis (R2 = 0.84); (e) Sb (mol. %) and Cu (mol. %) plot with a solubility line (CCu = 2 × CSb + 10−4, where CCu and CSb are the mol. % concentrations of Cu and Sb in pyrite), proposed by George et al. [58]; and (f) Sb (mol. %) and Zn (mol. %) plot with a solubility line (CZn = CSb + 10−4, where CZn and CSb are the mol. % concentrations of Zn and Sb in pyrite), proposed by George et al. [58].
Figure 10. (a) Ni (ppm) versus Co (ppm) diagram showing data generated by LA–ICP–MS analysis of pyrite (R2 = 0.55); (b) linear relationship between S (mol. %) and As (mol. %) based on data from EPMA point analysis (R2 = −0.91); (c) relationship of As (mol. %) and Tl (mol. %) in pyrite samples shown by data from LA–ICP–MS analysis (R2 = 0.91); (d) relationship of As (mol. %) and Sb+Tl (mol. %) in pyrite samples shown by data from LA–ICP–MS analysis (R2 = 0.84); (e) Sb (mol. %) and Cu (mol. %) plot with a solubility line (CCu = 2 × CSb + 10−4, where CCu and CSb are the mol. % concentrations of Cu and Sb in pyrite), proposed by George et al. [58]; and (f) Sb (mol. %) and Zn (mol. %) plot with a solubility line (CZn = CSb + 10−4, where CZn and CSb are the mol. % concentrations of Zn and Sb in pyrite), proposed by George et al. [58].
Minerals 15 00814 g010
Minerals 15 00814 g011
Figure 11. Representative LA–ICP–MS depth–concentration profiles of selected trace elements in pyrite (Py–I, Py–II, and Py–III) and stibnite (Sbn–I, Sbn–II, and Sbn–III). In most cases, the spectra appear flat and only rare peaks possibly related to the presence of NP are documented.
Figure 11. Representative LA–ICP–MS depth–concentration profiles of selected trace elements in pyrite (Py–I, Py–II, and Py–III) and stibnite (Sbn–I, Sbn–II, and Sbn–III). In most cases, the spectra appear flat and only rare peaks possibly related to the presence of NP are documented.
Minerals 15 00814 g011
Minerals 15 00814 g012
Figure 12. Ternary As–Fe–S diagram showing the data from EPMA points and micro–maps for pyrite samples from the Matra Mine. The main trends related to the As oxidation states are taken from Deditius et al. [75].
Figure 12. Ternary As–Fe–S diagram showing the data from EPMA points and micro–maps for pyrite samples from the Matra Mine. The main trends related to the As oxidation states are taken from Deditius et al. [75].
Minerals 15 00814 g012
Minerals 15 00814 g013
Figure 13. (a) Plot of Sb (mol. %) versus As (mol. %) in stibnite of the Matra deposit based on EPMA data (R2 = −0.97). (b–e) Comparison of the stibnite from the Matra deposit with other published data [76,77,78,79,80,81,82,83,84,85,86]. (b) Cu (ppm) versus Pb (ppm). (c) As (ppm) versus Pb + Hg (ppm). (d) As (ppm) versus Tl (ppm). (e) Fe (ppm) versus Cu + Ag + Tl (ppm).
Figure 13. (a) Plot of Sb (mol. %) versus As (mol. %) in stibnite of the Matra deposit based on EPMA data (R2 = −0.97). (b–e) Comparison of the stibnite from the Matra deposit with other published data [76,77,78,79,80,81,82,83,84,85,86]. (b) Cu (ppm) versus Pb (ppm). (c) As (ppm) versus Pb + Hg (ppm). (d) As (ppm) versus Tl (ppm). (e) Fe (ppm) versus Cu + Ag + Tl (ppm).
Minerals 15 00814 g013
Minerals 15 00814 g014
Figure 14. Paragenetic sequence for the Matra deposit showing pre–ore, main–ore, and supergene stages.
Figure 14. Paragenetic sequence for the Matra deposit showing pre–ore, main–ore, and supergene stages.
Minerals 15 00814 g014
Table 1. Summary and description of samples collected around the Matra mine that were used in this study.
Table 1. Summary and description of samples collected around the Matra mine that were used in this study.
SampleTypeDescription
MAT 4, 4bHost rockFoliated calc–schist of Upper Castagniccia Unit, characterized by different veins of realgar and stibnite.
MAT 6, 6bBrecciaBreccia with quartz and dolomite clasts with the matrix containing traces of pyrite, realgar, and orpiment.
MAT 7OreMineralized sample with dolomite gangue, showing veins of realgar and disseminated stibnite and pyrite.
MAT 8OreFramework of calcite veins with massive realgar.
MAT 9OreDolomite gangue with interstitial stibnite, realgar, and crosscutting calcite veins.
MAT 10, 10bOreDolomite gangue with stibnite, realgar, and pyrite in veinlets or disseminations.
MAT 11BrecciaBreccia with quartz and dolomite clasts in a matrix containing traces of mineralization, including pyrite, realgar, and hörnesite.
MAT 12, 12bOreQuartz gangue with disseminated stibnite and pyrite, along with nodules of realgar.
MAT 14, 14bGossanNon–cohesive altered and mineralized rock, showing iron–oxide coloration with traces of green zones enriched with nickel oxides.
MAT 15, 15bHost rockSerpentinite sample of the Santo Pietro di Tenda Unit, with dolomite and realgar veins cutting the main foliation.
MAT 24, 24b, 24cOre brecciaMineralized breccia composed of drusy dolomite gangue with clasts of pyrite, stibnite, and realgar, featuring slickensides and slickenfibers on quartz and stibnite.
MAT 25GossanAltered sample with iron–oxide coloration, containing drusy calcite veins.
MAT 26GossanAltered sample with iron–oxide coloration and quartz veins.
MAT 28OreMineralized sample with dolomite and calcite, containing massive realgar and disseminated stibnite and pyrite.
MAT 31GossanWeathered, altered, with a reddish–brown iron–oxide coating. Contains residual quartz and calcite veinlets.
Table 2. Summary of trace element data (%adl: percentage of data above the detection limit) for pyrite (Py–I, Py–II, and Py–III) and stibnite (Sbn–I, Sbn–II, and Sbn–III) using LA–ICP–MS.
Table 2. Summary of trace element data (%adl: percentage of data above the detection limit) for pyrite (Py–I, Py–II, and Py–III) and stibnite (Sbn–I, Sbn–II, and Sbn–III) using LA–ICP–MS.
MineralsN TiCrMnFeCoNiCuZnGaGeAsSeMoAgCdInSnSbTeWPtAuTlPbBi
Py–I20max 3.83n.d.1.63n.d.3.172191.46386n.d.n.d.49,417n.d.5.59n.d.n.d.0.100.3926,876n.d.0.12n.d.n.d.17950.10n.d.
min2.00n.d.0.85n.d.0.1811.50.341.74n.d.n.d.11,601n.d.0.63n.d.n.d.0.030.268999n.d.0.12n.d.n.d.9050.06n.d.
median3.00n.c.1.25n.c.0.5335.50.512.91n.c.n.c.19,793n.c.2.91n.c.n.c.0.050.3312,699n.c.0.12n.c.n.c.10750.10n.c.
mean2.86n.c.1.23n.c.0.8858.20.6222.5n.c.n.c.23,286n.c.2.92n.c.n.c.0.050.3313,261n.c.0.12n.c.n.c.11650.09n.c.
stdev0.55n.c.0.21n.c.0.9252.40.3485.6n.c.n.c.10,443n.c.1.32n.c.n.c.0.020.094458n.c.n.c.n.c.n.c.2370.02n.c.
%adl100n.c.100n.c.85.010045.0100n.c.n.c.100n.c.100n.c.n.c.10010.0100n.c.5.00n.c.n.c.10015.0n.c.
Py–II20max 12.7n.d.627n.d.55.024136.79257n.d.n.d.154,785n.d.1160n.d.0.601.1611.976,967n.d.7.28n.d.n.d.13,6040.93n.d.
min1.93n.d.15.1n.d.1.240.580.344.73n.d.n.d.26,996n.d.3.99n.d.0.600.140.412285n.d.0.11n.d.n.d.7800.07n.d.
median5.84n.c.154n.c.2.5534.41.1146.2n.c.n.c.46,566n.c.33.8n.c.0.600.341.2815,055n.c.1.32n.c.n.c.36620.46n.c.
mean5.64n.c.224n.c.13.03401.9072.8n.c.n.c.63,066n.c.115n.c.0.600.402.1123,539n.c.2.90n.c.n.c.45890.49n.c.
stdev2.50n.c.199n.c.20.07102.0770.7n.c.n.c.39,703n.c.263n.c.n.c.0.232.6420,007n.c.3.44n.c.n.c.37510.43n.c.
%adl100n.c.100n.c.40.075.050.0100n.c.n.c.100n.c.100n.c.5.0095.090.0100n.c.30.0n.c.n.c.10015.0n.c.
Py–III37max 768n.d.780n.d.286911,405442907n.d.n.d.320,1873.7025800.231.851.562.06251,5131.07145n.d.n.d.47,4435.50n.d.
min1.98n.d.0.68n.d.0.4710.30.431.87n.d.n.d.44543.700.570.230.670.030.493891.070.07n.d.n.d.9.670.11n.d.
median7.82n.c.40.9n.c.68.0238143.012.2n.c.n.c.32,2323.706.590.231.010.161.2423191.071.69n.c.n.c.9280.59n.c.
mean40.0n.c.87.2n.c.153306879.452.8n.c.n.c.76,1383.703030.231.070.281.2228,7071.078.17n.c.n.c.66201.32n.c.
stdev130n.c.144n.c.4663217114151n.c.n.c.90,896n.c.619n.c.0.420.300.5261,702n.c.26.1n.c.n.c.11,8421.56n.c.
%adl91.9n.c.100n.c.10010086.5100n.c.n.c.1000.0081.12.7018.997.316.21002.7081.1n.c.n.c.10073.0n.c.
Sbn–I19max n.d.3.330.19n.d.n.d.n.d.3.048.870.261.3044,07828.60.76n.d.n.d.0.059.30n.d.n.d.0.04n.d.n.d.16060.06n.d.
minn.d.1.040.19n.d.n.d.n.d.0.261.550.030.6912,2475.300.24n.d.n.d.0.011.15n.d.n.d.0.04n.d.n.d.1260.03n.d.
mediann.c.1.750.19n.c.n.c.n.c.1.204.080.051.1615,5488.730.42n.c.n.c.0.034.62n.c.n.c.0.04n.c.n.c.4320.04n.c.
meann.c.2.130.19n.c.n.c.n.c.1.364.070.081.0619,68112.80.46n.c.n.c.0.034.92n.c.n.c.0.04n.c.n.c.5750.04n.c.
stdevn.c.0.96n.c.n.c.n.c.n.c.0.731.800.060.18975510.60.24n.c.n.c.0.013.65n.c.n.c.n.c.n.c.n.c.4210.02n.c.
%adln.c.36.85.26n.c.n.c.n.c.10010084.268.410021.121.1n.c.n.c.52.631.6n.c.n.c.5.26n.c.n.c.10010.5n.c.
Sbn–II18max n.d.0.701.011480.038.717.194.690.021.2034,8857.096.800.020.230.0410.5n.d.0.290.03n.d.n.d.8980.130.005
minn.d.0.420.0919.00.031.940.100.580.010.6545080.780.690.020.140.010.19n.d.0.290.02n.d.n.d.75.60.020.005
mediann.c.0.560.3163.00.034.061.563.350.010.9814,1801.531.400.020.140.022.25n.c.0.290.03n.c.n.c.3020.050.005
meann.c.0.560.4069.30.035.011.852.980.010.9314,0052.362.340.020.160.022.98n.c.0.290.03n.c.n.c.3780.060.005
stdevn.c.0.200.32n.c.n.c.2.912.141.430.010.1687742.671.98n.c.0.050.013.20n.c.n.c.0.01n.c.n.c.2330.05n.c.
%adln.c.11.150.038.95.5627.888.910033.383.310027.81005.5622.250.044.4n.c.5.5611.1n.c.n.c.10022.25.56
Sbn–III19max n.d.21.327325840.5920.51.784.530.051.5421,2371.525.72n.d.n.d.0.048.02n.d.0.254.32n.d.n.d.6813.820.01
minn.d.1.100.3064.50.093.630.200.950.030.7743831.520.38n.d.n.d.0.010.57n.d.0.110.02n.d.n.d.0.080.030.01
mediann.c.2.780.632760.3512.80.282.060.051.1213,7751.521.90n.c.n.c.0.024.13n.c.0.180.06n.c.n.c.19.180.180.01
meann.c.5.8930.66550.3412.30.432.100.051.1613,4031.522.40n.c.n.c.0.024.21n.c.0.180.82n.c.n.c.1130.660.01
stdevn.c.6.6681.88410.218.440.510.860.010.244122n.c.1.70n.c.n.c.0.013.34n.c.0.101.72n.c.n.c.1941.29n.c.
%adln.c.52.657.963.221.0515.80.0094.726.380.01005.2673.7n.c.n.c.31.621.1n.c.10.531.6n.c.n.c.10042.15.26
N = number of analyses; n.d. = not detected; n.c. = not calculated.
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Filimon, D.I.; Groff, J.A.; Saccani, E.; Di Rosa, M. Ore Genesis Based on Microtextural and Geochemical Evidence from the Hydrothermal As–Sb Mineralization of the Matra Deposit (Alpine Corsica, France). Minerals 2025, 15, 814. https://doi.org/10.3390/min15080814

AMA Style

Filimon DI, Groff JA, Saccani E, Di Rosa M. Ore Genesis Based on Microtextural and Geochemical Evidence from the Hydrothermal As–Sb Mineralization of the Matra Deposit (Alpine Corsica, France). Minerals. 2025; 15(8):814. https://doi.org/10.3390/min15080814

Chicago/Turabian Style

Filimon, Danis Ionut, John A. Groff, Emilio Saccani, and Maria Di Rosa. 2025. "Ore Genesis Based on Microtextural and Geochemical Evidence from the Hydrothermal As–Sb Mineralization of the Matra Deposit (Alpine Corsica, France)" Minerals 15, no. 8: 814. https://doi.org/10.3390/min15080814

APA Style

Filimon, D. I., Groff, J. A., Saccani, E., & Di Rosa, M. (2025). Ore Genesis Based on Microtextural and Geochemical Evidence from the Hydrothermal As–Sb Mineralization of the Matra Deposit (Alpine Corsica, France). Minerals, 15(8), 814. https://doi.org/10.3390/min15080814

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