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Article

Geological, Mineralogical, and Alteration Insights of the Intermediate-Sulfidation Epithermal Mineralization in the Sidi Aissa District, Northern Tunisia

by
Jamel Ayari
1,
Maurizio Barbieri
2,*,
Tiziano Boschetti
3,
Ahmed Sellami
1,
Paolo Ballirano
4 and
Abdelkarim Charef
5
1
National Office of Mines, La Charguia I, Tunis 3023, Tunisia
2
Department of Chemical Engineering Materials Environment, University of Rome “La Sapienza”, Via Eudossiana, 1, 00185 Rome, Italy
3
Department of Chemistry Life Sciences and Environmental Sustainability, University of Parma, Parco Area delle Scienze 157/A, 43124 Parma, Italy
4
Department of Earth Sciences, University of Rome “La Sapienza”, P. le Aldo Moro, 5, 00184 Rome, Italy
5
Laboratoire Géoressources, Centre des Recherches et Technologies des Eaux, B.P. 273, Soliman 8020, Tunisia
*
Author to whom correspondence should be addressed.
Geosciences 2025, 15(7), 269; https://doi.org/10.3390/geosciences15070269 (registering DOI)
Submission received: 1 June 2025 / Revised: 29 June 2025 / Accepted: 10 July 2025 / Published: 12 July 2025
(This article belongs to the Section Geochemistry)
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Abstract

The Sidi Aissa Pb-Zn-(Ag) District, located within the Nappe Zone of northern Tunisia, has been reinterpreted as a typical intermediate-sulfidation (IS) epithermal mineralization system based on field observations and lithogeochemical analyses. Previously described as vein-style Pb-Zn deposits, the local geological framework is dominated by extensional normal faults forming half-grabens. These faults facilitated the exhumation of deep Triassic autochthonous rocks and the extrusion of 8-Ma rhyodacites and Messinian basalts. These structures, functioning as pathways for magmatic-hydrothermal fluids, facilitated the upward migration of acidic fluids, which interacted with the surrounding wall rocks, forming a subsurface alteration zone. The mineralization, shaped by Miocene extensional tectonics and magmatic activity, occurred in three stages: early quartz-dominated veins, an intermediate barite-rich phase, and late-stage supergene oxidation. Hydrothermal alteration, characterized by silicification, argillic, and propylitic zones, is closely associated with the deposition of base metals (Pb, Zn) and silver. The mineral assemblage, including barite, galena, sphalerite, and quartz, reflects dynamic processes such as fluid boiling, mixing, and pressure changes.

1. Introduction

Epithermal deposits are essential sources of precious metals including gold (Au) and silver (Ag), in addition to base metals like lead (Pb), zinc (Zn), and copper (Cu) [1,2]. These deposits are genetically linked to subaerial calc-alkaline to alkaline magmatism, which typically occurs in volcanic arc and rift settings but may also be associated with adakitic magmatism in collisional environments and in Circum-Pacific accretionary belts [1,3,4]. Additionally, they yield a variety of minerals, particularly alunite, silica solid phases (e.g., quartz, chalcedony, opal), and kaolinite [1,2]. Based on the characteristics of gangue and ore minerals, alteration assemblages, and the properties of hydrothermal fluids, epithermal veins are classified into three major subtypes: high sulfidation (HS), low sulfidation (LS), and intermediate sulfidation (IS) [1,2]. HS epithermal deposits are typically found within lithocaps and are distinguished by quartz, alunite, and kaolinite. Conversely, LS deposits are often located at the margins or peripheries of these systems, characterized by hydrothermal alterations with adularia, sericite, quartz, and calcite. An intermediate mineralogical composition distinguishes the IS deposits between the HS and LS types. Generally, HS deposits exhibit a spatial relationship with porphyry copper systems [5,6,7]. Nonetheless, HS systems do not always correlate with deeper porphyry deposits and represent unique mineralized systems [1]. The oxidation of existing epithermal deposits can lead to supergene enrichment, contingent upon favourable tectonic, climatic, and lithological conditions, particularly in HS deposits where vuggy residual quartz facilitates the dispersion of acidic solutions [1].
The comprehensive exploration project of the polymetallic resource base in the Nappe Zone of northern Tunisia, organized by the National Office of Mines (ONM) in collaboration with Rome University, involved a regional geological and mineralogical survey in the Sidi Aissa Pb-Zn District. Previous investigations within this district have predominantly relied on unpublished reports from mining companies [8,9,10], which indicated vein-style mineralization.
Previous studies on geochemistry, geochronology, fluid inclusions, and stable isotopes suggest that shallow boiling conditions facilitated epithermal deposit formation in the Nappe Zone. In particular, [11,12,13] identified small-scale shallow convective hydrothermal cells in northern Tunisia linked to mafic intrusions. Using stream sediment geochemical data, [14,15] showed that arsenic and antimony anomalies align with major east–west lineaments, controlling magma migration and hydrothermal fluid flow. The authors of [16] further attributed arsenic in geothermal waters to the oxidation of arsenic-rich sulfides, a hallmark of high-temperature environments [17]. Other authors [18] reported reported that alunite is the dominant hydrothermal mineral at the abandoned Jalta Mine, suggesting steam-heated conditions.
Despite these insights, the recognition of epithermal textures and paleosurfaces remains limited. Textural features such as colloform-crustiform banding, bladed calcite, and hydrothermal breccias provide key evidence of episodic processes like fluid boiling, mixing, and pressure fluctuations, which are crucial in both LS and IS systems [19,20,21]. Paleosurfaces often exhibit silicified ledges, sinters, and hydrothermal eruption breccias, marking past fluid discharge zones [22,23]. These are particularly evident in LS systems, where hot springs and geysers produce silica sinters and precious metal anomalies [24]. Hydrothermal alteration near paleosurfaces, including silicification and advanced argillic alteration, is a key indicator of fluid pathways. In IS systems, deeper barite-rich assemblages are juxtaposed with near-surface silicification, reflecting structurally controlled fluid ascent [2]. Identifying these features is critical for exploration but is complicated by rugged terrain and erosion, significantly altering the geological record.
This study describes the structural influences on veins and the distribution patterns of textures, alterations, and paragenesis in the Sidi Aissa District. It includes geological mapping of the deposit and an assessment of the area’s structural attributes at a 1:20,000 scale. Additionally, the research involves a detailed mineralogical and textural examination of the veins and alteration zones, integrating field observations with subsurface data from unpublished reports and maps archived at the National Office of Mines [8,9,10], as well as new data derived from XRD analysis of the vein minerals.

2. Regional-Scale Geologic, Tectonic, and Metallogenic Settings

Extending from the Moroccan Rif in the west to southern Italy in the east, the Nappe Zone of northern Tunisia (Figure 1) represents a wide structural domain within the easternmost portion of the Maghrebide-Alpine belt, originating from the collision between the African and Eurasian plates during the Langhian [25,26,27]. The Nappe Zone consists of Miocene folds and Tellian and Numidian thrust sheets emplaced over the Atlasic foreland to the south. The Tellian thrust sheets are primarily composed of Upper Cretaceous to Eocene limestones and shales, while the Numidian thrust sheet is mainly composed of Oligocene to Burdigalian claystones and sandstones. The Atlasic foreland contains limestone, dolomite, and clay, with significant exposures of Triassic evaporites (gypsum, halite, and anhydrite) through the NE–SW and E–W trending faults [28,29,30].
During the Late Miocene, northern Tunisia experienced extensional collapse caused by two ENE- and SE-directed systems [30]. During the late Tortonian to Messinian, the earlier extensional system featured low-angle normal faults and high-angle faults with ENE-directed movement, resulting in half-grabens and hanging-wall syncline basins [30,31]. These extensional structures largely contributed to the exhumation of the deep autochthonous Triassic rocks in the footwalls of extensional faults and also the extrusion of 8-Ma rhyodacites and Messinian basalts [13,25,28,32]. During the Plio-Quaternary, the boundary extensional fault zones experienced a phase of tectonic inversion [30,31].
In these settings, magmatic-related environments having structural weaknesses have acted as zones of enhanced permeability, facilitating focused fluid flow and ore deposition [33,34]. This structural control is particularly relevant to the formation of Mississippi Valley-type (MVT), sedimentary exhalative (SEDEX), and skarn deposits in proximity to magmatic intrusions.
Geosciences 15 00269 g001
Figure 1. Generalized geologic, tectonic, and metallogenic map of northern Tunisia adapted from [25,35,36].
Figure 1. Generalized geologic, tectonic, and metallogenic map of northern Tunisia adapted from [25,35,36].
Geosciences 15 00269 g001

3. Sidi Aissa Pb-Zn-(Ag) District

The Sidi Aissa Pb-Zn-(Ag) District is approximately 15 km northwest of Mateur City in the western upstream region of the Ichkeul Plain, northern Tunisia (Figure 1 and Figure 2). Historical studies in the district have primarily relied on unpublished mining company reports [8,9,10]. This district includes the Sidi Aissa and Chakaria deposits and two notable prospects at Cheneg Roha and Lahmeri (Figure 2).
Pb-Zn mineralization in the area was first identified in 1900 and mined through open pits and subsurface workings until 1963. Between 1971 and 1973, La Société Équipement Hydraulique, in collaboration with a Bulgarian mission, carried out an exploration programme in the region. This initiative included drilling four exploratory boreholes up to 150 m and extensive trenching activities [37]. This work estimated the proven and probable lead-zinc reserves at approximately 200,000 tons [38].
The mineralization at Sidi Aissa predominantly consists of sulfides, specifically galena (PbS) and sphalerite (ZnS) enriched with silver (30 g/t; [8]) (Supplementary S1), within a barite-dominant gangue. These sulfide minerals occur as fine grains, typically not exceeding 0.2 mm in size. Despite the extensive exploitation history, the genesis of these deposits remains controversial, with debates surrounding their age, origin, mineralization style, and classification. Earlier geological interpretations categorized the mineralization style as vein-veinlet, strongly associated with fault zones within Triassic dolostones and Campanian–Maastrichtian limestones. These deposits were considered typical of Mississippi Valley-type (MVT) mineralization in northern Tunisia, reflecting the prevailing understanding of the time. However, [8], based on subsurface and trenching data, proposed an alternative hypothesis suggesting that the mineralization is largely strata-bound and primarily controlled by sedimentary enrichment within Paleocene marls. This interpretation is further supported by observations that the contact zones between the Triassic dolostones and Campanian–Maastrichtian limestones are notably barren. The divergence in these interpretations highlights the complexity of the Sidi Aissa District. It underscores the need for further studies to clarify its geological history and refine exploration and mining strategies.

4. Materials and Methods

4.1. Fieldwork

The ore deposits in the study region remain poorly defined in terms of their geological framework, geomorphological features, host rock characteristics, hydrothermal alteration minerals suite, and the role of primary volcanic rocks. Consequently, fieldwork was carried out between 2022 and 2024 as part of a 3-year exploration programme. This programme involved compiling regional data, geological mapping, and regional lithogeochemistry. A 1:20,000-scale geological map of the study area was produced based on the aforementioned investigations.

4.2. Materials and Methods

Mineralogical studies of 14 selected bulk rock and clay fraction samples from the mineralized veins and alteration zones were performed using X-ray Diffraction (XRD) techniques (Figure 2, Table 1) to identify the mineralogical composition. The clay mineral assemblage was quantified under three different conditions: as an untreated oriented clay sample, after glycolation, and after heating at 550 °C for 4 h, following the methodology established by [39]. Data were collected using a focusing-beam AXS D8 Advance (Bruker Corporation, Billerica, MA, USA) operating in q/q geometry, transmission mode, using capillaries as sample holders. The instrument is fitted with a PSD VÅntec-1 and radial soller slits along the diffracted beam. Patterns were measured in the 5–145 °2q angular range, 0.022 °2q step size, and 3 s counting time per step. Identification of mineral species was performed with the help of PDF2-2021 and the DIFFRAC suite of programmes (Bruker AXS, 2019; Bruker Corporation, MA, USA).

5. Results

Figure 2 and Figure 3 present updated geological maps and cross sections of the Sidi Aissa-Chakaria and Lahmari-Cheneg Roha ore zones (key mineralized structures in the Sidi Aissa District), integrating data from previously unpublished mining reports (field surveys, drill-hole data, and seismic surveys). Additionally, the characteristics of these ore deposits and their corresponding hydrothermal alterations are thoroughly examined through whole-rock geochemical data (Table 1).

5.1. Deposit Characteristics

5.1.1. Sidi Aissa-Chakaria Ore Zone

Based on field observations (Figure 4), drill hole data, and seismic surveys (Figure 3), the sulfide ore deposits in the Sidi Aissa-Chakaria ore zone are predominantly found as veins and fracture fillings with significant alteration (Figure 4a–c). The semi-massive to massive orebodies, composed of a fine-grained mineral matrix (Figure 4d), are hosted within Paleocene marls interbedded with thin silicified black shale layers (Figure 3b and Figure 4e). These deposits, exposed in an abandoned quarry, are aligned along E–W trending normal faults that define the boundaries of a half-graben structure (Figure 4a). They are buried at an approximate depth of 20 m below the surface, concentrated over an area of approximately 600 m2. Vein thickness generally ranges from 1 to 5 m, occasionally reaching up to 10 m, with vein lengths varying between 200 m and 1 kilometre (Figure 3b).
XRD analysis of four rock samples from an exposed profile shows a distinct mineralogical change with depth. Surface layers primarily comprise quartz, illite, chlorite, natrojarosite (Figure 4c), and barite, whereas deeper layers consist of barite, cerussite, and mimetite (Table 1). The silicified black shale primarily comprises quartz, illite, and chlorite. Drill core samples from the main ore-bearing veins reveal a mineral assemblage dominated by galena and sphalerite, enriched with silver. The alteration patterns surrounding the ore deposits exhibit a distinct zonation, transitioning from intense silicification (quartz-dominated) near the core to argillic alteration (illite, chlorite, and quartz) outward. The E–W trending fault responsible for the half-graben formation also hosts basaltic volcanism and is infilled with finely laminated maar-fall and/or lacustrine sediments (Figure 4f) that have undergone extensive silicification (Figure 4g). This fault represents a localized expression of regional extensional tectonics, characterized by NE–SW trending faults and fractures. The footwall comprises Eocene marls, while the hanging wall consists of Paleocene marls interbedded with thin black shale layers, underscoring a stark lithological contrast across the fault zone. Basaltic flows and tectonised Triassic evaporites mark the contact zone between the footwall and the hanging wall, indicating significant structural disruption. Evidence of significant uplift and denudation in the upper plate of the footwall block has been extensively documented in northern Tunisia [29,30,31].
The landscape is characterized by isolated silicified ridges extending in an E–W direction along the mineralized vein system (Figure 4h). These resistant features consist of sheeted quartz and stockwork veins and hydrothermal breccias that have undergone intense silicification (Figure 4i,j,k). The breccias exhibit diverse textures, including colloform-crustiform (Figure 4l,m), cockade (Figure 4p), and quartz pseudomorphing bladed carbonate textures (Figure 4n,o). At the periphery of the silicified zone, the exposed wall rock contains a complex network of stockwork and sheeted veins, primarily composed of banded quartz. Surrounding intrabasins are filled with fluvial and lacustrine sediments, comprising predominantly subrounded sandstone and conglomerate blocks (Figure 4q).
A trench in the northwest part of the Sidi Aissa ore zone exposes bimodal basalt-rhyolite sequences formed during multicycle volcanic activity (Figure 5a,b). Basalt occurs as lava flows and dikes, while rhyolite forms shallow subaqueous domes. As observed in other regions, these rhyolitic domes are commonly associated with mafic volcanic sequences. The flanks of these domes are heavily silicified and brecciated, forming carapaces (Figure 5b). Mafic lava flows exhibit glassy selvedges and hyaloclastite particles (Figure 5c), indicative of rapid quenching [41]. These features are embedded in illite rinds formed through the alteration of mafic glass. The bimodal basalt-rhyolite sequences exhibit extensive silicification, reflecting intense hydrothermal alteration.
A well-preserved profile illustrates two clearly defined alteration horizons. The uppermost horizon, associated with argillic alteration, comprises calcite, dolomite, quartz, illite, and chlorite (Table 1). The wall rock presents a variety of textures, ranging from chalky to crumbly, with occasional areas of well-consolidated material (Figure 5d). The lowermost horizon, exhibiting propylitic alteration, is notably rich in chlorite and can be readily identified by the greenish colouring of the host rock (Figure 5d), which is a consequence of the considerable presence of chlorite.

5.1.2. Cheneg Roha-Lahmari Prospect Zone

The Cheneg Roha-Lahmari prospect zone (Figure 6a) is structurally influenced by the NE–SW Ras El Korane-Thibar master fault (Figure 3), which has played a key role in the formation of a prominent silicified ridge, constituting a resistant high topography or ledge (Figure 6b). Extensive silicification (Figure 6c) and brecciation (Figure 6d), including pipe breccia (Figure 6e) and collapse breccia (Figure 6f), are observed along the fault, reflecting intense fracturing and episodic hydrothermal fluid activity associated with extensional tectonism. A smaller pebble-dike breccia body having a 2-m thickness contrasts abruptly with nearby sedimentary and basaltic rocks and is a distinctive feature of the Cheneg Roha prospect zone (Figure 6e). The breccia dike consists primarily of rounded to subordinately angular clasts from a variety of rock types, embedded in a sand-to-silt matrix. It is minimally altered and is thought to have been emplaced in close connection with poorly mineralized and fractured intrusive bodies. Pebble-dike breccias are also considered indicative of decompressive occurrences and so are associated with phreatic/phreatomagmatic diatremes [4,5,6].
Silicification has significantly enhanced the resistance of the ridge to erosion, contrasting with the surrounding rock, which is characterized by widespread argillic alteration. Additionally, hydrothermal processes have facilitated the precipitation of barite, commonly infilling fractures and voids within the brecciated zones, providing further evidence of substantial fluid–rock interaction (Figure 6g). This process commonly results in the infill of veins, fractures, and brecciated zones, effectively cementing and reinforcing the host rock [7,42]. The resulting mineralization strengthens the rock mass, reducing permeability and increasing resistance to erosion, thereby supporting the development and persistence of resistant ledges [2]. Furthermore, late-stage silicification frequently accompanies barite deposition, further enhancing the durability of ledge structures by increasing rock hardness and limiting the effects of weathering. The spatial and genetic association of barite with fault-controlled breccias and veins highlights its critical role in the geomechanical evolution of epithermal deposits, where episodic fluid flow, structural deformation, and mineral precipitation collectively influence ore deposition and long-term structural integrity [34,43].
The hydrothermal breccias that have erupted are exposed along the lower eastern slopes of the ledge and the adjacent plain, where they serve as a substrate for altered tuffs and are interspersed with basaltic flows.
In the vicinity of the eastern edge of the fault-controlled ledge, remnants of gossanous materials, including hematite and goethite, are exposed in outcrops. The fault strikes within the Cheneg Roha ledge demonstrate a significant eastward curvature, particularly in the northern region near the Lahmari prospect zone. This area features gossan (Figure 6h) that is heavily altered and leached, rich in goethite, quartz, and kaolinite (Table 1), and displays boxwork structures covering an area of approximately 50 m by 100 m.
In the northern periphery of the Lahmari-Cheneg Roha prospect zone, travertine bodies have been identified overlying extensional fault systems (Figure 6i), reflecting structurally controlled deposition associated with subsurface fluid migration [44]. These travertine deposits exhibit distinct banding having alternating layers of white crystalline calcite and reddish-brown material, likely composed of iron oxides and clays. The well-defined lamination indicates variations in fluid chemistry and flow dynamics during deposition, a hallmark feature of hydrothermal travertines [45]. The reddish-brown layers suggest periods of increased iron content or oxidizing conditions, while the white calcite bands point to purer calcium carbonate deposition. Such banding is often associated with episodic precipitation, potentially driven by seasonal changes, fluctuations in fluid flow, or intermittent tectonic activity [46]. This type of travertine is typically found in tectonically active regions where hydrothermal fluids, enriched with dissolved minerals, ascend to the surface through fractures and faults [44].
These banded travertine deposits are significant indicators of active or past geothermal systems and serve as potential surface expressions of deeper hydrothermal mineralization processes. Their presence can provide valuable insights into the dynamics of subsurface fluid flow and the geothermal history of tectonically influenced regions.

6. Discussion

6.1. Tectonomagmatic Setting

Extensional deformation in the Sidi Aissa District is characterized by two dominant, perpendicular fault systems trending E–W to NW–SE (Figure 3a), consistent with extensional regimes identified in adjacent basins [30]. These systems formed a series of half-grabens (Figure 3a,b), enabling the deposition of lacustrine and volcanic sediments (Figure 4f,g). Magmatic activity, dominated by 8-Ma rhyodacites and Messinian basalts, is closely linked to fault-controlled conduits. Cryptodome formation (Figure 5b) and hydrothermal breccias (Figure 6b,e,f) suggest significant magma uplift, influencing local stress fields and facilitating mineralization.
Extensional movements resulted in the uplifting and exhumation of the footwall and the subsidence of the hanging wall. On the surface, the footwalls of the normal faults form NE–SW and E–W trending ridges (Figure 6b), disconnected by argillic zones, while the hanging walls are characterized by the development of synclinal depocentres (Figure 4f). The structural setting of this region is coherent with regional-scale features, as defined by both geomorphological observations and seismic data [29,30]. Vertical normal faults along Cheneg Roha are easily identified by the prominent ridge that develops due to the footwall uplifting and hanging wall subsidence. In contrast, the ridges along Sidi Aissa are less pronounced as the lithology of hanging walls is dominated by Eocene marls, which are more prone to erosion.
On the other hand, the striking nearby E–W and NE–SW vertical faults, active during the Miocene, have played a crucial role in the emplacement of magmatism in the district. The main magmatic phases in northern Tunisia—8-Ma rhyodacites and Messinian basalts [13,25,28,32]—are reflected in the study region’s volcanism, which is dominated by basalt eruptions, shallow rhyolite domes, and hydrothermal breccias. Half-graben depocenters are infilled with air-fall deposits. The geometry of the magmatic bodies and their interaction with the basement suggests extrusion as crypto domes, as evidenced by significant regional uplift.

6.2. Mineralization Characteristics

At the surface, secondary mineralization after oxidation of primary hypogene sulfides includes natrojarosite, barite, cerussite, mimetite, quartz, illite, and chlorite (Table 1). Natrojarosite forms under acidic conditions and is a hallmark of oxidation zones above sulfide-rich ore bodies [23]. Barite, often associated with late-stage mineralization, indicates a neutral to slightly alkaline environment and suggests fluid mixing between hydrothermal and meteoric waters [47]. Cerussite and mimetite are secondary weathering products that contribute to the lead enrichment of the surface mineral assemblage. These minerals are consistent with the epithermal environment and weathering, where secondary enrichment processes concentrate lead and arsenic [24]. The presence of illite and chlorite in the alteration zone reflects the interaction of hydrothermal fluids with host rocks under moderate temperature and near-neutral pH conditions, typical of IS systems [48].
At depth, mineralization is confined by silicified black shales acting as aquitards (Figure 3b), which inhibit fluid movement and create pressure differentials conducive to mineral precipitation [49]. The primary minerals in this zone include barite, galena, sphalerite, and quartz. The association of barite with galena and sphalerite in the deeper zones reflects the cooling and neutralization of metal-bearing hydrothermal fluids [1]. Barite precipitation in such settings typically occurs due to the mixing of sulfate-rich fluids with Ba-bearing hydrothermal fluids, a process often linked to the late stages of mineralization [1,50]. Sphalerite and galena deposition is facilitated by boiling and pressure fluctuations within the hydrothermal system, leading to metals precipitation from magmatic-derived fluids [51]. The dike breccia body commonly accompanies the end stage of an epithermal system [52]. Phreatomagmatic breccias, often surrounded by intermediate- to high-sulfidation epithermal stockwork-disseminated mineralization, may contain higher-grade mineralization than their surroundings [52].

6.3. Hydrothermal Alteration

Prominent silicification in the region forms resistant knobs, ledges, and carapaces (Figure 6b and Figure 7), surrounded by argillic alteration halos (Figure 7). Original rock textures are rarely preserved. Silicification occurs primarily along E–W to NE–SW fractures and extensional faults, marked by steeply dipping quartz veins. It is closely associated with base-metal mineralization in the Sidi Aissa ore zone, evidenced by significant quartz influx (fine- to coarse-grained) forming large siliceous bodies and breccia cement. A final silicification phase displays crustiform, comb, cockade, and mosaic textures in veins and veinlets (Figure 4l–p), quartz networks, filled voids, and matrix-supported hydrothermal breccias.
Argillic alteration is widely distributed and can be readily observed in the field, marked by a yellow-to-white colouration that is typically found in proximity to silicified zones (Figure 7). This alteration is most pronounced in rocks having higher permeability, especially in breccia formations. The extent of argillization varies, resulting in wall rock textures that range from chalky to crumbly and occasionally well consolidated. The mineralogical composition of this alteration is primarily characterized by illite and quartz (Table 1). Propylitic alteration is frequently observed at the periphery of argillic alteration (Figure 7) and is identified by the greenish tint of the host rock, attributed to the significant presence of chlorite (Table 1).

6.4. Deposit Classification

The sulfide mineralization in the Sidi Aissa District consists of a vein and fracture filling system with silicification and argillic and propylitic alteration types. The system is mainly controlled by fractures and extensional faults trending east–west to northeast–southwest. The ascent of hydrothermal fluids was facilitated by the networks of extensional normal faults, leading to significant transformations of the original rock formations into various facies. Many of the mineral occurrences display typical epithermal textures, specifically crustiform-colloform banded quartz. The presence of hydrothermal breccia, pseudomorphs of bladed carbonates, and crustiform-colloform textures is evidence of boiling within this hydrothermal system. During the boiling process, there is a notable decrease in temperature, the liberation of H2O vapour, and an increase in pH, which collectively serve as a significant mechanism for the precipitation of base metals [1,51,53]. In the up-flow region of epithermal systems, the loss of CO2 to vapour during the boiling process leads to the immediate precipitation of calcite, which promotes the development of bladed crystal structures [19,54]. The colloform/crustiform banded quartz texture indicates episodic and rapid deposition of chalcedonic quartz within voids found in shallow epithermal settings [50,54]. This texture arises from the rapid opening of fractures that induce a pressure drop and rapid cooling, which are associated with boiling or flashing phenomena [53,55].
The mineralized sulfide veins typically exhibit well-defined, sharp contacts with their volcaniclastic host rocks. This feature implies that the ore veins developed under hydrostatic pressure conditions [54], which is associated with the formation of hydrothermal breccias [54,56]. Such characteristics generally indicate episodes of significant pressure drops that may have triggered intermittent boiling within the hydrothermal system [53,57]. The mineralization in the Sidi Aissa District, characterized by base metals and silver, represents a late-stage, silver- and base metal-rich example of intermediate-sulfidation epithermal deposits (IS). This contrasts with high-sulfidation systems, which feature advanced argillic alteration and vuggy quartz, and low-sulfidation systems, which are dominated by adularia and sericite [2,48]. The widespread occurrence of illite and calcite alteration minerals suggests that the base metal mineralization at Sidi Aissa originated from near-neutral-pH magmatic-derived fluids, a hallmark of IS. Furthermore, the sulfidation state, as indicated by the sulfide assemblage (5–20 vol%), aligns with IS, according to [48].
In IS, the absence of pyrite in late-stage mineralization associated with silver and base metals reflects specific geochemical processes of fluid evolution. Oxidized, sulfate-rich hydrothermal fluids inhibit pyrite precipitation while promoting galena, sphalerite, and barite formation [1,58].
As a common gangue mineral, the presence of barite suggests that sulfur predominantly occurs as sulfate, S(VI), with insufficient reduced sulfur as S(-I) for pyrite formation. Furthermore, Fe depletion due to earlier mineralization stages and S-bonding with Pb and Zn further suppresses pyrite crystallization [48]. Additionally, fluid inclusion studies indicate that late-stage mixing of hydrothermal fluids with meteoric water lowers temperature and pH, destabilizing pyrite while favouring silver sulfosalts, galena, and sphalerite precipitation [59]. This phenomenon is evident in deposits like Pachuca-Real del Monte (Mexico) and San Cristóbal (Bolivia), where late-stage fluids are oxidized and barite-rich, excluding pyrite from the paragenetic sequence [48,60]. Therefore, the absence of pyrite provides insight into IS’s evolving redox state and fluid chemistry.
In summary, the geological context, sulfide assemblage, ore textures, and hydrothermal alteration assemblages support the classification of the base metal epithermal mineralization at Sidi Aissa as an IS-type deposit.

6.5. Mineralization Stages

Mineralization in the Sidi Aissa District occurs in four distinct stages, each exhibiting a range of characteristics, including variations in ore–gangue mineral assemblages and textures, metal ratios, types of alteration, and the nature of host rocks.
The abundant development of quartz represents the first stage of mineralization, as stockwork and sheeted veins occur in more massive structures, such as bladed, crustiform-colloform, and cockade textures. This stage is well represented in the Sidi Aissa-Chakaria ore zone. These veins are largely quartz-dominated, but they later evolve to feature quartz–clay assemblages. Additionally, late-stage clay matrix breccias are present. According to [4], a vein of this type (massive to semi-massive sulfide) is commonly fault-controlled and characterizes the lower parts of the epithermal system. This later finding implies that at Sidi Aissa, the uppermost part of the epithermal systems is exposed at the surface by erosion and tectonically induced uplift, which is largely supported by the tectonomagmatic evolution of the study region. This is consistent with the large-scale As and Sb anomalies outlining the E–W trending faults in northern Tunisia, as reported by Ayari et al. in 2022 [15]. As and Sb belonging to the epithermal suite of indicator elements are not present within the surficial steam-heated zone because they are not carried by low-pressure vapour but rather fractionate into the liquid at deeper levels where the hydrothermal fluid boils, releasing H2S and other volatiles [24].
The second-stage paragenetic sequence is marked by predominant barite, distinguishing it from the first stage. While sphalerite and galena occur throughout, they are mainly associated with late-stage hydrothermal assemblages alongside barite and other sulfides [47,50]. Barite formation suggests fluid mixing, typically occurring in magmatic–hydrothermal systems as cooling hydrothermal fluids interact with meteoric water [1,47].
The third stage of mineralization is characterized by the presence of travertines in the Lahmari-Cheneg Roha prospect zone, marking the last phase of hydrothermal activity with lower temperatures and altered fluid compositions. Travertines form when hydrothermal fluids rich in calcium carbonate emerge along faults, degas CO2, and precipitate calcite [61]. Their formation signals the depletion of metal-rich fluids, with carbonate precipitation replacing ore deposition [22]. This shift reflects the transition from active mineralization to residual geothermal activity, with fault-controlled pathways maintaining fluid flow [44]. As surface markers of waning hydrothermal systems, travertines help to identify potential underlying ore bodies.
Supergene mineralization represents the fourth paragenetic phase, driven by weathering and oxidation. High permeability around quartz veins facilitates the dispersion of oxidized mineral complexes over several metres in the Cheneg Roha prospect zone. At Sidi Aissa, supergene mineralization appears as localized natrojarosite-rich spots. Oxidation above the water table promotes the formation of goethite and lepidocrocite. In epithermal systems, mineralized vein sets and stockworks typically develop along secondary splays adjacent to major faults [50,62], as observed in the Cheneg Roha zone. These environments are prone to rapid erosion, which can lead to epithermal system telescoping onto underlying porphyry-related mineralization, driven by late-stage alteration or the interplay of uplift and hydrothermal processes.

6.6. Implications for Exploration

The tectonomagmatic and mineralization characteristics of the Sidi Aissa District provide critical insights for exploration strategies in analogous geological settings. The extensional tectonics, dominated by the E–W and NW–SE fault systems, facilitated the formation of half-grabens that supported sedimentary deposition and magma emplacement. These fault systems acted as primary conduits for hydrothermal fluid migration and magmatic activity, as evidenced by the occurrence of rhyodacites, Messinian basalts, and hydrothermal breccias. Structural corridors where these fault trends intersect represent priority exploration targets due to their enhanced permeability and potential for mineral deposition.
The mineralization’s epithermal intermediate-sulfidation (IS) characteristics suggest a system governed by magmatic-derived fluids operating under moderate temperatures and near-neutral pH conditions. These fluids interacted with host rocks along extensional faults, creating distinct zones of silicification, argillic alteration, and propylitic alteration. The silicification, characterized by resistant quartz veins and breccias, along with intermediate-sulfidation textures such as crustiform and colloform banding, highlights the role of dynamic boiling processes in ore formation. Exploration should focus on the quartz-veined and silicified zones, especially near the fault-controlled breccias, as these areas are highly prospective.
At depth, silicified black shales function as aquitards, exerting stratigraphic control on mineralization by facilitating pressure gradients conducive to barite, galena, and sphalerite precipitation. This underscores the importance of integrating stratigraphic and structural mapping in exploration programmes to identify analogous aquitard configurations. Geophysical techniques, such as resistivity and induced polarization, can aid in delineating silicified zones and sulfide-rich ore bodies obscured by surface alteration.
Surface mineral assemblages, including natrojarosite, barite, cerussite, and mimetite, indicate oxidized zones with secondary enrichment potential. These assemblages, combined with hydrothermal breccias, provide valuable indicators of underlying sulfide-rich mineralization. Geochemical surveys targeting these minerals, coupled with advanced spectral analysis to identify alteration halos, represent effective tools for delineating prospective zones.
The presence of intermediate-sulfidation textures and associated mineral assemblages emphasizes the critical role of boiling zones in metal deposition. Exploration should prioritize the areas displaying evidence of episodic pressure drops, such as breccias, crustiform quartz, and bladed calcite pseudomorphs, as these indicate fluid mixing and metal precipitation. Fluid inclusion studies and sulfur stable isotope ratio analyses will be valuable for enhancing the understanding of the temporal and spatial evolution of the hydrothermal system, thereby improving exploration models and facilitating target identification in this mineral-rich district.

7. Conclusions

The Sidi Aissa District exemplifies a complex interplay between Miocene extensional tectonics, magmatic activity, and intermediate-sulfidation (IS) epithermal mineralization (IS). Extensional deformation, characterized by E–W and N–S trending normal faults, played a critical role in magma emplacement, hydrothermal fluid flow, and mineral deposition. The region’s magmatism, dominated by 8-Ma rhyodacites and Messinian basalts, was associated with crypto dome formation, hydrothermal breccias, and significant uplift.
Mineralization in the district is defined by a multi-stage evolution, including base metals (Pb, Zn) and silver deposition. Surface mineralization reflects secondary enrichment, while subsurface mineralization involves barite, galena, sphalerite, and quartz, controlled by hydrothermal processes such as boiling, fluid mixing, and pressure fluctuations. Distinctive mineralogical and textural features, such as crustiform quartz and bladed calcite, highlight episodic fluid dynamics.
The hydrothermal alteration in the district is extensive, with silicification, argillic, and propylitic zones closely linked to the fault systems. The silicified zones form resistant ledges, while the argillic and propylitic alterations outline structural controls on fluid pathways. Due to oxidized, sulfate-rich hydrothermal fluids, the absence of pyrite in the late-stage mineralization underscores unique fluid evolution processes. It highlights the dominance of barite, galena, and sphalerite as primary minerals.
These findings enhance our understanding of the tectonomagmatic and hydrothermal processes in the Sidi Aissa District and underscore its potential for further exploration for silver and base metals. Targeting analogous extensional systems with bimodal volcanism and structural controls in the Nappe Zone is strongly recommended to identify additional mineralized zones.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/geosciences15070269/s1, Supplementary S1: Chemical composition of Sidi Aissa ore deposit; Supplementary S2: XRD-representative of the different sampling points (Table 1).

Author Contributions

Conceptualization, J.A., A.S., and A.C.; methodology, J.A., A.S., M.B., T.B., and P.B.; software, T.B.; validation, J.A., M.B., T.B., P.B., and A.C.; formal analysis, J.A., T.B., and P.B.; investigation, J.A., A.S., and A.C.; data curation, J.A., M.B., T.B., A.S., P.B., and A.C.; writing—J.A., A.S., and A.C.; writing—review and editing, J.A., M.B., T.B., P.B., A.S., and A.C.; visualization, J.A., M.B., T.B., P.B., A.S., and A.C.; supervision, J.A., M.B., and T.B.; project administration, J.A.; funding acquisition, J.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data from our research are available upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 2. Detailed geological map showing location of the main deposits and prospects, drill holes, cross section, seismic line and sampling sites at Sidi Aissa mining district.
Figure 2. Detailed geological map showing location of the main deposits and prospects, drill holes, cross section, seismic line and sampling sites at Sidi Aissa mining district.
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Figure 3. Surface and subsurface geological data: (a) ENE–WSW–trending interpreted seismic line (L1 in Figure 2) [40]; (b) summary geology logs for three drill holes; (c) NW–SE–trending geological and structural interpretative cross section (C1 in Figure 2).
Figure 3. Surface and subsurface geological data: (a) ENE–WSW–trending interpreted seismic line (L1 in Figure 2) [40]; (b) summary geology logs for three drill holes; (c) NW–SE–trending geological and structural interpretative cross section (C1 in Figure 2).
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Figure 4. Field photographs showing view from the Sidi Aissa ore zone and representative epithermal textures. (a): general E–W view from the main abandoned quarry; (b): vein and fracture fillings mineralization; (c): exposed natrojarosite; (d): semi-massive to massive mineralization; (e): exposed black shales; (f): finely laminated maar-fall and/or lacustrine sediments; (g): silicified and laminated air-fall sediments; (h): silicified E–W trending zones (ridges); (i): sheeted veins; (j): stockwork veins; (k): breccia; (l,m): crustiform-colloform textures; (n,o): quartz pseudomorphing bladed carbonate textures; (p): cockade textures; (q): conglomerate blocks.
Figure 4. Field photographs showing view from the Sidi Aissa ore zone and representative epithermal textures. (a): general E–W view from the main abandoned quarry; (b): vein and fracture fillings mineralization; (c): exposed natrojarosite; (d): semi-massive to massive mineralization; (e): exposed black shales; (f): finely laminated maar-fall and/or lacustrine sediments; (g): silicified and laminated air-fall sediments; (h): silicified E–W trending zones (ridges); (i): sheeted veins; (j): stockwork veins; (k): breccia; (l,m): crustiform-colloform textures; (n,o): quartz pseudomorphing bladed carbonate textures; (p): cockade textures; (q): conglomerate blocks.
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Figure 5. Field photographs of the northwest part of the Sidi Aissa ore zone. (a): main trench; (b): rhyolitic dome; (c): glassy selvages and hyaloclastite particles; (d): argillic alteration overlying propylitic alteration.
Figure 5. Field photographs of the northwest part of the Sidi Aissa ore zone. (a): main trench; (b): rhyolitic dome; (c): glassy selvages and hyaloclastite particles; (d): argillic alteration overlying propylitic alteration.
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Figure 6. Field photographs showing view from the Cheneg Roha-Lahmari prospect zone and representative structures and lithologies. (a): general view of the prospect; (b): ridge structure in Cheneg Roha; (c): silicification along fault zone; (d): breccia along fault zone; (e): pebble-dike breccia; (f): breccia collapse; (g): barite deposition; (h): exposed gossan at Lahmari zone; (i): travertine geobodies.
Figure 6. Field photographs showing view from the Cheneg Roha-Lahmari prospect zone and representative structures and lithologies. (a): general view of the prospect; (b): ridge structure in Cheneg Roha; (c): silicification along fault zone; (d): breccia along fault zone; (e): pebble-dike breccia; (f): breccia collapse; (g): barite deposition; (h): exposed gossan at Lahmari zone; (i): travertine geobodies.
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Figure 7. Schematic cross section of Sidi Aissa hydrothermal system and associated alteration.
Figure 7. Schematic cross section of Sidi Aissa hydrothermal system and associated alteration.
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Table 1. Sites, features, and mineralogical composition of samples in the Sidi Aissa District.
Table 1. Sites, features, and mineralogical composition of samples in the Sidi Aissa District.
Sampling PointSample IDDescriptionMineralogical Composition
CDQChIlKMGLNBCeMi
A: Oxidized profile exposed in the main quarry at Sidi Aissa ore zone3833Uppermost zone of the profile × × ×
3834Upper zone of the profile × × × ×
3835 *Intermediate zone of the profile × × × × ×
3839Lower zone of the profile × × ×
3837Black shale × × ×
B: Trench in western Sidi Aissa ore zone3832 *Silicified zone with sheeted textures × × × × ×
C: Intrabasins in southern Sidi Aissa ore zone3822Quartzites × × × × ×
3823 *Conglomerates × × × × ×
D: Trench in northwestern Sidi Aissa ore zone3827Quartzites × × ×
3828 *Altered volcanic glass × × × × ×
E: Alteration profile exposed in trench in northern Sidi Aissa ore zone3824Lower zone of the profile × × × × ×
3825 *Upper zone of the profile × × × × ×
3826Oxidized vein × × × ×
F: Gossan exposed in Lamari prospect zone3829 *Gossanous crust × × × ×
(C: calcite; D: dolomite; Q: quartz; Ch: chlorite; Il: illite; K: kaolinite; M: magnetite; G: goethite; L: lepidocrocite; N: natrojarosite; B: barite; Ce: cerussite; Mi: mimetite, XRD-representative of the different sampling points, * Supplementary S2).
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MDPI and ACS Style

Ayari, J.; Barbieri, M.; Boschetti, T.; Sellami, A.; Ballirano, P.; Charef, A. Geological, Mineralogical, and Alteration Insights of the Intermediate-Sulfidation Epithermal Mineralization in the Sidi Aissa District, Northern Tunisia. Geosciences 2025, 15, 269. https://doi.org/10.3390/geosciences15070269

AMA Style

Ayari J, Barbieri M, Boschetti T, Sellami A, Ballirano P, Charef A. Geological, Mineralogical, and Alteration Insights of the Intermediate-Sulfidation Epithermal Mineralization in the Sidi Aissa District, Northern Tunisia. Geosciences. 2025; 15(7):269. https://doi.org/10.3390/geosciences15070269

Chicago/Turabian Style

Ayari, Jamel, Maurizio Barbieri, Tiziano Boschetti, Ahmed Sellami, Paolo Ballirano, and Abdelkarim Charef. 2025. "Geological, Mineralogical, and Alteration Insights of the Intermediate-Sulfidation Epithermal Mineralization in the Sidi Aissa District, Northern Tunisia" Geosciences 15, no. 7: 269. https://doi.org/10.3390/geosciences15070269

APA Style

Ayari, J., Barbieri, M., Boschetti, T., Sellami, A., Ballirano, P., & Charef, A. (2025). Geological, Mineralogical, and Alteration Insights of the Intermediate-Sulfidation Epithermal Mineralization in the Sidi Aissa District, Northern Tunisia. Geosciences, 15(7), 269. https://doi.org/10.3390/geosciences15070269

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