Genesis of the Aït Abdellah Copper Deposit, Bou Azzer-El Graara Inlier, Anti-Atlas, Morocco

Abstract

The Aït Abdellah copper deposit in the Bou Azzer-El Graara inlier of the Moroccan Anti-Atlas provides key insights into structurally and lithologically controlled mineralization in Precambrian terranes. The deposit is hosted in feldspathic sandstones of the Tiddiline Group, which unconformably overlie the Bou Azzer ophiolite, and is spatially associated with a NE–SW-trending shear zone. This zone is characterized by mylonitic fabrics, calcite veining, and an extensive network of fractures, reflecting a two-stage deformation history involving early ductile shearing followed by brittle faulting and brecciation. These structural features enhanced rock permeability, enabling fluid flow and metal precipitation. Copper mineralization includes primary sulfides such as chalcopyrite, bornite, pyrite, chalcocite, digenite, and covellite, as well as supergene minerals like malachite, azurite, and chrysocolla. Sulfur isotope values (δ³⁴S = +5.9% to +22.8%) indicate a mixed sulfur source, likely derived from both ophiolitic rocks and volcano-sedimentary sequences. Carbon and oxygen isotope data suggest fluid interaction with marine carbonates and meteoric waters, potentially linked to post-Snowball Earth deglaciation processes. Fluid inclusion studies reveal homogenization temperatures ranging from 195 °C to 310 °C and salinities between 5.7 and 23.2 wt.% NaCl equivalent, supporting a model of fluid mixing between magmatic-hydrothermal and volcano-sedimentary sources. The paragenetic evolution of the deposit comprises three stages: (1) early hydrothermal precipitation of quartz, dolomite, sericite, pyrite, and early chalcopyrite and bornite; (2) a main mineralizing stage characterized by fracturing and deposition of bornite, chalcopyrite, and Ag-bearing sulfosalts; and (3) a late supergene phase with oxidation and secondary enrichment. The Aït Abdellah deposit is best classified as a shear zone-hosted copper system with a complex, multistage mineralization history. The integrated analysis of structural features, mineral assemblages, isotopic signatures, and fluid inclusion data reveals a dynamic interplay between deformation processes, hydrothermal alteration, and evolving fluid sources.

1. Introduction

Fault and shear zones are critical in the spatial distribution of hydrothermal ore deposits, serving as primary conduits for mineralizing fluids. These structures enhance permeability, facilitating fluid migration, metal transport, and ore deposition in a wide range of geological settings (e.g., [1,2]). In addition to fault-controlled mineralization, metal precipitation commonly occurs at physicochemical gradients, such as oxidation-reduction interfaces, where moderate-temperature, oxidized, and saline fluids trigger the deposition of sulfides. This process has remained active since the Paleoproterozoic [1].

Fluid-rock interactions play a crucial role in both primary mineralization and secondary enrichment, where hydrothermal alteration modifies host rocks, concentrating metals over time. In copper systems, chalcopyrite and bornite typically form during the initial mineralization stages. Conversely, supergene processes, driven by weathering and oxidation, result in the remobilization and enrichment of copper as chalcocite, digenite, covellite, and malachite (e.g., [3,4]).

The Bou Azzer-El Graara inlier has been extensively studied for its Neoproterozoic ophiolites and associated Co-bearing arsenides (e.g., [5,6,7,8,9,10,11,12,13]). However, beyond Co, the region hosts Cu, U, Mn, Fe, REEs, Ag, and Au-Pd mineralization [6,14,15,16,17,18]. Copper mineralization occurs at Bleïda and Takroumt in Neoproterozoic basement rocks [19,20,21] and in Cambrian carbonates at Assif N’Zaid and Jbel Laasal [22]. The central Bou Azzer-El Graara inlier hosts the Tichibanine and Aït Abdellah deposits, key targets for mining and exploration.

The Aït Abdellah copper deposit, 70 km east of Bou Azzer, is hosted within the eastern serpentinized peridotites of the Bou Azzer ophiolite, an oceanic crust remnant emplaced onto the West African Craton during the Pan-African orogeny [10,11,23,24,25]. Mineralization occurs within the Tiddiline Group siltstones and feldspathic sandstones, overlying ophiolitic gabbros, volcanic-sedimentary units, and quartz diorites. The genetic link between ophiolitic sequences and copper deposits is key to understanding regional metallogeny, and previous studies suggest mineralization resulted from hydrothermal circulation along shear zones, where fluids interacted with ophiolitic and carbonate sequences to precipitate the deposit [19,26,27,28,29]. However, a comprehensive geochemical, isotopic, and fluid evolution analysis remains absent.

This study provides the first comprehensive investigation of the Aït Abdellah deposit, integrating structural analysis, mineralogy, stable isotope geochemistry, and fluid inclusion data to constrain deposit-forming processes. By comparing Aït Abdellah with other copper deposits in the Bou Azzer-El Graara inlier and the broader Anti-Atlas, particularly those hosted in the Neoproterozoic basement and the Early Cambrian Adoudou Formation, this study offers new insights into the controls, distribution, and evolution of copper mineralization in the region.

2. Geological Setting

2.1. The Anti-Atlas Belt

The Anti-Atlas belt is divided into two primary domains, separated by the Anti-Atlas Major Fault (AAMA) (Figure 1; [19]). The southwestern (Eburnean) domain is characterized by Paleoproterozoic siliciclastic sedimentary sequences and intrusive complexes [30]. The oldest rocks in the Anti-Atlas, particularly in its western inliers, consist of metasedimentary sequences intruded by ~2 Ga aged Eburnean granitoids [31,32,33,34,35,36,37,38,39]. In contrast, Paleoproterozoic rocks are absent in the Pan-African domain across the central and eastern Anti-Atlas and the Moroccan Meseta. However, the presence of inherited Paleoproterozoic zircons in Neoproterozoic magmatic rocks [40,41,42,43,44,45] suggests that these older crustal components may exist at depth.

Several studies have revealed evidence of Mesoproterozoic magmatism in the Anti-Atlas. In the western part, mafic dykes within the Zenaga inlier have been dated at ~1656 ± 9 Ma and 1655 Ma [46], while doleritic dykes in the Bas Drâa inlier have been dated at 1381 ± 8 Ma and 1384 ± 6 Ma [39,47]. Additionally, a new age of ~1710 Ma from quartzites in the Igherm inlier indicates that the Taghdout Group formed during the late Paleoproterozoic to early Mesoproterozoic [48,49].

The Neoproterozoic evolution of the Anti-Atlas was shaped by a complex history of rifting, subduction, and collision, culminating in post-orogenic magmatism. The Bou Azzer-Siroua axis, marked by ophiolite complexes, represents a supra-subduction zone (SSZ) setting and a classic Wilson cycle suture [19,50,51]. This cycle involved four key stages [10,52,53,54]. The first stage was rifting and oceanic basin formation, initiated during the Tonian-Cryogenian period, which led to the development of oceanic crust. This was followed by subduction and arc accretion, where the consumption of oceanic lithosphere resulted in the formation of offshore island arcs and back-arc basins, as recorded in the Bou Azzer and Siroua ophiolites. During the obduction stage, these ophiolites were thrust onto the West African Craton (WAC), accompanied by the emplacement of quartz-diorite plutons around 680–660 Ma [19,43,55]. The final stage of Neoproterozoic crustal evolution was marked by post-collisional magmatism, during which the Ouarzazate Group developed between 580 and 540 Ma, forming an extensive silicic igneous province (Figure 1).

2.2. The Bou Azzer-El Graara Inlier

The Bou Azzer-El Graara inlier exposes a well-preserved Proterozoic sequence with several key stratigraphic groups (Figure 2). The oldest units, part of the Tachdamt-Bleïda formations, include stromatolitic limestone, quartzite, mafic sills, felsic tuffs, and volcanic rocks, dating to the Tonian-Cryogenian period, with the Tachdamt Formation dated at ~883 Ma [56,57]. Overlying this is the Tazegzaout-Assif n’Bougmmane Group, revised to the Early Cryogenian (~750 Ma; [52,58]. This group consists of orthogneiss, metagabbro, schists, and pegmatite, with zircon U-Pb ages ranging from 755 ± 9 Ma to 695 ± 7 Ma [52,59].

Figure 2. Geological map of the Bou Azzer-El Graara inlier, modified after [52,60,61,62,63,64].

Figure 2. Geological map of the Bou Azzer-El Graara inlier, modified after [52,60,61,62,63,64].

The Tichibanine-Ben Lgrad Group in the northern part of the inlier includes volcanic sequences with basaltic to rhyolitic compositions, dated at 761 ± 2 Ma and 762 ± 2 Ma [55], interpreted as a rift-affected intra-oceanic arc [60]. The central Bou Azzer-El Graara Group, marked by an ophiolite complex, is considered either a fragment of a back-arc basin or fore-arc complex [65,66,67]. Isotopic data suggest a juvenile mantle source for the basalts [68].

The late Neoproterozoic Tiddiline Group unconformably overlies the ophiolitic rocks and exceeds 1000 m in thickness [19,69]. It includes volcanic rocks, conglomerates, feldspathic sandstones, and varved glaciomarine deposits, indicating episodic glaciation [60,70,71]. Massive diamictites and sandstones confirm glacial erosion and ice-rafted debris [72]. The Tiddiline Group is exposed along two WNW-trending corridors separated by Cryogenian terrains. The northern corridor extends for 40 km, while the southern corridor includes isolated units at Bou Azzer, Tachdamt, and Trifya. This fragmentation is due to left-lateral strike-slip tectonics. Originally thought to be Late Neoproterozoic, the Tiddiline Group now forms the basal section of the Ouarzazate Supergroup [73]. In the Aït Ahmane region, the Tiddiline Group forms a 35 km long NW-SE belt.

Post-collisional deposition of the Ouarzazate Group (~560–543 Ma) represents metacratonic evolution, comprising volcanic, volcanoclastic, and clastic sediments [73,74,75]. New studies highlight the complex tectonic evolution of the inlier, with Mesoproterozoic mafic dykes and revised geochronology for volcanic and sedimentary sequences [39,47,48]. This supports a polyphase evolution of the Anti-Atlas, emphasizing rifting, subduction, and post-collisional magmatism [10,74].

The Carapace d’Ambed Formation, previously defined by [19], corresponds to quartz-carbonate lenses developed at the top of serpentinite massifs. These lenses are the result of meteoric alteration processes and are characterized by interbedded siliciclastic and carbonate units. The Ambed Formation is heavily deformed due to Pan-African and Hercynian orogenies, with major fault systems facilitating mineralizing fluid flow [76]. These formations guide exploration in the Bou Azzer district, particularly for Co and Ni deposits [8,11,20,25,76,77,78,79,80,81]. Siliceous-carbonate rocks in the Ambed Formation host significant concentrations of Co, Ni, Fe, and Cu due to serpentinite leaching [19].

2.3. Deposit Geology and Field Observations

The Aït Abdellah area is characterized by Ediacaran volcanic and sedimentary sequences, including basalts, andesites, rhyolites, tuffs, shales, and siltstones, reflecting a complex volcano-sedimentary environment. These rocks indicate significant mineral potential in the region [56]. Structurally, the Tiddiline Formation [69] unconformably overlies the Bou Azzer-El Graara ophiolitic complex, which underwent folding, schistosity, and metamorphism during the Pan-African tectonic phase B₁ [82]. Phase B₂ caused WNW-ESE folds with subvertical axial planes and fracture schistosity, suggesting a low compression rate and alignment parallel to the craton edge [83].

The western side of the Aït Abdellah zone features early Ediacaran carbonates, known as the “carapace d’Ambed,” formed from the serpentinization of peridotites into serpentinites, carbonates, and siliceous shells [81]. The area is dominated by thrust faults, shear zones, and folds, which were integral to mineral deposit emplacement and hydrothermal system development [10,84]. The structural elements likely provided fluid pathways essential for mineralization. Metamorphic conditions in the Aït Abdellah inlier range from greenschist to amphibolite facies, especially near ophiolitic complexes.

Copper mineralization in Aït Abdellah is closely linked to fault zones and fracturing, with copper, cobalt, and gold deposits concentrated in these zones. Geochronological data place mineralization events between the Neoproterozoic and early Cambrian periods (750–540 Ma), coinciding with tectonic phases that shaped the Anti-Atlas [85,86,87,88,89]. U-Pb zircon dating and isotopic analyses support this timeline, further linking mineralization to significant tectonic events (Figure 2; [61,74]).

The Aït Abdellah area is composed of sedimentary, volcano-sedimentary, and magmatic rocks (Figure 2). The sedimentary succession of the Tiddiline Formation, dated to the Ediacaran, consists primarily of bedded pelites, siltstones, and feldspathic sandstones. These deposits unconformably overlie the Cryogenian ophiolitic complex of the Bou Azzer Group, which includes gabbro, quartz diorite, and volcaniclastic rocks. The Tiddiline Formation is deformed, showing sub-vertical axial-plane slaty cleavage resulting from the late Pan-African orogenic phase. Structurally, it follows a NW–SE orientation and dips gently eastward.

The Aït Abdellah copper mine is situated within a structurally complex zone where mineralized sandstones of the Tiddiline Formation (Figure 3a) are dissected by faults and shear zones trending ENE–WSW, WNW–ESE, and NW–SE (Figure 4a). The emplacement of doleritic dikes with NE–SW to ENE–WSW orientations likely influenced the thermal and chemical evolution of the system. Field mapping and underground data indicate that the copper mineralization is hosted in lens-shaped ore bodies reaching approximately 100 m in extension, with grades ranging from 0.8 to 1.2% Cu, and an estimated resource potential of 5 million tonnes (5 MT). These ore bodies are primarily developed within sandstone and siltstone units (Figure 3b). This is illustrated in Figure 3a,b, which show representative cross-sections and field exposures of the mineralized structures, highlighting their geometry and structural control.

Two distinct mineralization styles are recognized. The first is vein-type copper mineralization (Figure 4b,c), characterized by quartz-carbonate veins that crosscut sandstones near shear zones. These veins, typically a few centimeters thick, contain centimeter-sized sulfides within stockwork system fracture-veins and veinlets (Figure 5e,f) and larger veins. The ore mineralogy consists of pyrite, chalcopyrite, and bornite (Figure 5a–c), hosted within a quartz-rich gangue (Figure 5f). The second style is disseminated sandstone-hosted mineralization (Figure 4e,h), occurring in zones adjacent to the veins and forming horse-tail structures associated with shear zones (Figure 4f,g). Sulfides are found in intergranular spaces, with phyllitic films enveloping quartz grains. This style, restricted to feldspathic sandstones, shares a mineral assemblage similar to the vein-type mineralization (Figure 4 and Figure 5).

3. Methods

3.1. Microscopic Studies

Twenty mineralized rock samples were collected throughout the Aït Abdellah mining district, specifically from the mineralized facies of the Tiddiline Formation, ensuring a comprehensive analysis of mineralization at different levels. For the mineralogical study, the samples were first examined macroscopically, and then polished thin sections were prepared at the Reminex Research Center in Marrakech, Morocco. Petrographic observations were conducted at the Department of Geology, Faculty of Sciences in Agadir, Morocco, using transmitted and reflected light microscopy with an Olympus BX41 metallographic microscope (Olympus Corporation, Tokyo, Japan) equipped with an integrated Sony TOUPCAM camera (Sony, Tokyo, Japan).

3.2. Mineralogical Studies

The mineralogical analysis involved petrographic analysis of the collected samples to identify mineral phases and textural relationships. To further complement this study, scanning electron microscopy (SEM) was employed. A Philips XL 30 scanning electron microscope (Philips Group, Amsterdam, The Netherlands), coupled with a BRUKER X-Ray Microanalysis System (Bruker, Billerica, MA, USA), was used for qualitative and semi-quantitative identification of silicate and metallic minerals at the Research Center of the Faculty of Sciences, Agadir, Morocco. Seven thin sections from Aït Abdellah were analyzed using a Cameca SX100 electron microprobe (CAMECA, Gennevilliers, France) equipped with five wavelength-dispersive spectrometers at the Centre for Scientific Instrumentation (CIC) at the University of Granada, Spain. These samples were carbon-coated to improve conductivity and reduce charging effects during SEM analysis, which was enhanced with energy-dispersive X-ray spectroscopy (EDS) for detailed mineralogical and geochemical data.

3.3. Stable Isotopes Analysis

Stable isotope analyses were conducted on 10 sulfide samples (chalcopyrite and bornite) from the Aït Abdellah deposit at the Isotope Science Lab (ISL) of the University of Calgary, Canada. Sulfur isotope ratios (³⁴S/³²S) were determined for sulfide minerals using Continuous Flow-EA-IRMS (Elemental Analyzer-Isotope Ratio Mass Spectrometry) (Thermo Fisher Scientific, Waltham, MA, USA). In this process, samples were first converted to sulfur dioxide (SO₂) gas via combustion in an elemental analyzer. The resulting SO₂ was then introduced into the mass spectrometer for isotopic ratio determination, ensuring precise δ³⁴S measurements and providing valuable insights into the sulfur sources and processes within the deposit.

For the carbonate minerals, 7 carbonate samples (calcite and quartz) were analyzed for their δ¹³C and δ¹⁸O compositions using the Continuous Flow-GasBench-IRMS (Thermo Fisher Scientific, Waltham, MA, USA). The samples were reacted with phosphoric acid to produce CO₂ gas, which was then analyzed for its isotopic composition. This method provides accurate measurements of C and O isotopic ratios, essential for interpreting fluid sources and diagenetic histories. All isotopic results are reported in per mil (%) relative to internationally recognized standards: Vienna Pee Dee Belemnite (VPDB) for carbon and Vienna Standard Mean Ocean Water (VSMOW) for oxygen. The ISL maintains rigorous calibration protocols using certified reference materials to ensure data accuracy and comparability. The precision of sulfur isotope analyses was 0.3% (1σ), while carbon and oxygen isotope analyses had precisions of 0.2% (1σ). Data normalization was performed using internationally accepted standards, including IAEA-S1, IAEA-S2, and IAEA-S3 for sulfur and NBS 18, NBS 19, and IAEA-CO-8 for carbon and oxygen.

3.4. Fluid Inclusion

Fluid inclusion studies were conducted at the Analytical Center for Multi-Elemental and Isotope Research of the V.S. Sobolev Institute of Geology and Mineralogy, SB RAS, in Novosibirsk, Russia. The primary objective was to determine the pressure-temperature-composition (PTX) conditions governing mineral formation in sulfide ore mineralization from three Aït Abdellah rock samples.

Twenty doubly polished thick sections were analyzed, focusing on fluid inclusions hosted within vein quartz and quartz-calcite minerals associated with copper deposits. The study employed a Linkam THMSG-600 thermal-cryo-chamber (Linkam Scientific Instruments Ltd., Surrey, UK) to assess phase transition temperatures and a LabRam HR800 Horiba Jobin Yvon Raman spectrometer (HORIBA, Kyoto, Japan) for phase composition analysis.

4. Results

4.1. Structural Control and Hydrothermal Alteration

The Aït Abdellah copper deposit is controlled by a NE-SW-oriented shear zone that extends for several kilometers, dipping 70° to 75° SE. Field observations indicate that the central part of the shear zone exhibits a well-developed mylonitic fabric with calcite-filled veins. The surrounding host rocks display penetrative deformation that intensifies near the shear zone. An S-C fabric is evident, where shear planes (C) trend N73°, parallel to the main shear plane, while foliation (S) is oblique, with an average N120° trend (Figure 5). The host rocks also contain fault zones, dilatational fractures, and brecciated quartz veins with quartz-carbonate gangue and sulfides. Outcrops reveal reddish quartz lenses associated with hematite mineralization. The structural evolution of the deposit is characterized by two deformation stages: ductile and brittle. The ductile phase is characterized by penetrative flow schistosity that transposes primary bedding (S0) and quartz-feldspar veins, indicating that vein emplacement occurred syn-tectonically with strong mylonitization of the host rocks in the shear zone core. This phase is also associated with chloritization and disseminated pyrite mineralization in the surrounding rocks (Figure 6d). The brittle phase is defined by faulting, fracturing, and brecciation, with brecciated quartz veins reflecting intense shearing. Hydrothermal alteration increases toward the center of the shear zone, showing distinct phases of chloritization, carbonatation, silicification, and hematitization. Chloritization occurs as elongated chlorite crystals linked to pyritization in mylonitized zones. Silicification alters sandstones by filling pore spaces with silica, while carbonatation manifests as calcite and dolomite veins and veinlets (Figure 7h). Hematitization is observed as reddish quartz lenses, suggesting hematite and magnetite deposition (Figure 7g).

4.2. Mineralogy, Texture, and Paragenesis

The Aït Abdellah deposit displays a complex paragenetic sequence characterized by two distinct stages of sulfide mineralization. The early stage is dominated by pyrite (Figure 6f), chalcopyrite I, and bornite I (Figure 6e). In contrast, the later stage introduces a suite of secondary sulfides, including late-stage chalcopyrite II (Figure 6b–d), bornite II, chalcocite, covellite (Figure 6a), and digenite, in addition to supergene phases such as malachite (Figure 6g), azurite (Figure 6h), chrysocolla, and iron oxides. Mineralization is manifested in multiple forms, encompassing vein-hosted, disseminated, and stockwork styles. Disseminated sulfides are spatially associated with quartz-carbonate veins that form a dense network of mineralized fractures and veinlets hosted within feldspathic sandstones and carbonate rocks.

Chalcopyrite is the predominant sulfide phase and occurs in two distinct textural generations (Figure 6b,d). The earlier generation is characterized by exsolution blebs enclosed within primary bornite, while the later generation comprises coarse aggregates and disseminations aligned along shear zones and fracture planes (Figure 6d). This late-stage chalcopyrite is commonly intergrown with bornite, chalcocite, covellite, and malachite, and is hosted within a gangue composed of quartz and carbonate minerals. It frequently forms clusters on the millimeter to centimeter scale and often exhibits replacement textures, where it is partially to completely altered to chalcocite and covellite. Inclusions of bornite, sphalerite, and tennantite–tetrahedrite are also commonly observed within chalcopyrite.

Bornite occurs as veins, bands, and disseminated grains within the quartz-carbonate matrix, primarily in feldspathic sandstone and carbonate-rich lithologies. It exhibits a range of textures, including massive, disseminated, and exsolution features (Figure 6a,c–e and Figure 7a,e). The early bornite generation contains chalcopyrite blebs, whereas the later stage is texturally intergrown with secondary chalcopyrite. Pyrite is comparatively scarce and occurs as fine-grained euhedral crystals, typically in close association with early-stage chalcopyrite.

Chalcocite is abundant in the secondary mineral assemblage, preferentially replacing chalcopyrite and bornite within quartz-carbonate veins under supergene conditions (Figure 6a,c and Figure 7b,c,e). Covellite and digenite typically form as alteration rims and along micro-fractures within primary chalcopyrite and bornite.

Sphalerite is recognized as a minor inclusion within early-stage chalcopyrite and displays evidence of the “chalcopyrite disease” [90], characterized by submicron chalcopyrite inclusions (Figure 6b).

The supergene assemblage is marked by the presence of copper carbonates and hydrous copper silicates, including malachite, azurite, and chrysocolla. Among these, malachite is the most widespread, occurring along fractures and in veinlets that range in width from millimeters to centimeters. It is commonly associated with residual chalcopyrite and hosted in a gangue of quartz and dolomite. Malachite-rich veinlets may occur in isolation or as part of more extensive stockwork systems, often accompanied by iron oxides such as hematite and goethite, marking the extent of oxidation zones.

The paragenetic sequence of the Aït Abdellah copper deposit, based on mineralogical investigations and textural relationships, is composed of three distinct ore stages (Figure 8). The first stage is characterized by the precipitation of hydrothermal minerals such as quartz, dolomite, and sericite, indicating the circulation of fluids rich in silica and carbonates. During this phase, pyrite, trace amounts of sphalerite, and the first generations of bornite (I) and chalcopyrite (I) are formed, suggesting a sulfur- and base metal-rich environment under medium- to high-temperature conditions.

The second stage represents the main economic mineralization phase, which was initiated by intense fracturing and shearing, leading to hydrothermal circulation and the redistribution of minerals. This stage is marked by the precipitation of chlorite and calcite, while quartz, dolomite, and sericite continued to form in smaller quantities. Bornite II and chalcopyrite II are the dominant economic sulfides present during this stage, along with minor amounts of Ag included in the lattice of tennantite-tetrahedrite. These phases reflect changes in physicochemical conditions, likely associated with decreasing temperatures and interactions with circulating ore fluids.

The final stage of mineralization corresponds to supergene alteration, where primary sulfides undergo oxidation and transformation due to the interaction with meteoric fluids. Key secondary minerals formed during this stage include chalcocite (±Ag), digenite, covellite, malachite, azurite, and chrysocolla, which reflect the oxidizing conditions and supergene copper enrichment. Hematite, formed under these conditions, further indicates an oxygen-rich environment that promotes the oxidation of earlier minerals.

4.3. Mineral Chemistry

EPMA analysis provided detailed insights into the mineral compositions and zoning characteristics of samples from the Aït Abdellah mineralization zone (Figure 6 and Figure 7). Microprobe analysis was conducted on seven thin sections (AB1, AB3, AB3-2, AB4, AB5, and AB6). The findings confirm that the primary mineral assemblage consists of bornite, chalcocite, chalcopyrite, and pyrite, with Cu and S as the dominant elements, minor Fe content, and no detectable Au or Ag. Bornite contains approximately 26 wt.% S, 11 wt.% Fe, and 61 wt.% Cu, while chalcopyrite consists of 34 wt.% S, 29 wt.% Fe, and 33 wt.% Cu. Pyrite is mainly composed of S and Fe, with 53 wt.% S and 45 wt.% Fe. Chalcocite exhibits the highest Cu content in the assemblage, with 21 wt.% S and 77 wt.% Cu. These compositional values allow for the calculation of structural formulas, offering insights into crystal chemistry and the physicochemical conditions of mineral formation.

SEM analyses further characterize the sulfide mineralization in the Aït Abdellah area, confirming the dominance of Cu, S, and Fe (Figure 7a–d), with notable traces of Ti and P in certain samples (Figure 7e,f). The data reveal variable Cu enrichment, with Cu concentrations reaching up to 78 wt.% and S around 22 wt.%. Bulk composition analysis of selected samples yields average values of 63.98 wt.% Cu, 21.70 wt.% S, and 10.51 wt.% Fe, consistent with the presence of Cu-rich sulfides such as chalcocite and bornite.

4.4. Stable Isotopes

The δ¹⁴S, δ¹³C, and δ¹⁸O isotopic data for sulfide and carbonate samples from the Aït Abdellah copper deposit are summarized in

Table 1. Sulfur isotopic data of bornite and chalcopyrite from the Aït Abdellah copper deposit.

Samples ID	Mineral	δ34S
AB-01	Bornite	14.3
AB-02	Bornite	15.7
AB-04	Bornite	19.2
AB-05	Bornite	14.2
AB-07	Bornite	6
AB-G3	Bornite	5.9
AB-G4	Chalocopyrite	22.8
AB-G5	Chalocopyrite	20.6
AB-G6	Chalocopyrite	15.6
AB-G7	Chalocopyrite	11.1

and

Table 2. Carbon-oxygen isotopic compositions of calcite from the Aït Abdellah copper deposit.

Samples ID	δ13C-carbs	δ18O-carbs	δ18O-carbs (VSMOW))
AB-G1	−5.2	18.4	−12.2
AB-G2	−5.1	16.3	−14.2
AB-G3	−4.6	19.5	−11.1
AB-G4	−4.9	19.7	−10.9
AB-G5	−4.9	21.4	−9.3
AB-G6	−4.4	20.5	−10.1
AB-G7	−5.1	19.9	−10.7

and are presented in Figure 9 and Figure 10. The sulfur isotopic composition of ten bornite and chalcopyrite samples ranges from 5.9% to 22.8%. For the carbonate samples, the δ¹⁸O and δ¹³C values of seven calcite samples associated with copper mineralization range from 16.3% to 21.4% and −5.2% to −4.4%, respectively.

Figure 9. δ³⁴S sulfur isotope composition of bornite-chalcopyrite from the Aït Abdellah copper deposit. The diagram compares δ³⁴S values with those of igneous sulfides, sedimentary, metamorphic, and evaporitic sulfur sources [91], as well as with δ³⁴S values from various copper ore deposits in the Anti-Atlas. Data for the Anti-Atlas deposits are sourced from [12,28,33,49,92,93,94].

Figure 9. δ³⁴S sulfur isotope composition of bornite-chalcopyrite from the Aït Abdellah copper deposit. The diagram compares δ³⁴S values with those of igneous sulfides, sedimentary, metamorphic, and evaporitic sulfur sources [91], as well as with δ³⁴S values from various copper ore deposits in the Anti-Atlas. Data for the Anti-Atlas deposits are sourced from [12,28,33,49,92,93,94].

Figure 10. Carbon and oxygen isotopic compositions of calcite from the Aït Abdellah ore deposit. (a) Plot of δ¹³C _PDB vs. δ¹⁸O _SMOW illustrating isotopic variations and potential fluid sources [95], (b) δ¹⁸O values of the Aït Abdellah calcite [91], (c) C isotopic characteristics of the ore-forming fluids in the Aït Abdellah copper deposit, compared to carbon sources from magma or mantle [91], marine carbonates, and sedimentary organic matter [96].

Figure 10. Carbon and oxygen isotopic compositions of calcite from the Aït Abdellah ore deposit. (a) Plot of δ¹³C _PDB vs. δ¹⁸O _SMOW illustrating isotopic variations and potential fluid sources [95], (b) δ¹⁸O values of the Aït Abdellah calcite [91], (c) C isotopic characteristics of the ore-forming fluids in the Aït Abdellah copper deposit, compared to carbon sources from magma or mantle [91], marine carbonates, and sedimentary organic matter [96].

4.5. Fluid Inclusion Studies

Fluid inclusion analyses were conducted on quartz samples from host rocks (AB2101 and AB2102) and quartz-calcite veins (AB2103). The inclusions are primarily two-phase, consisting of liquid and gas bubbles (Figure 11), with some single-phase inclusions containing only liquid or gas (Figure 12). The primary inclusions in the quartz from the host rocks are typically 1–5 μm in size and lack zonation, though some inclusions in a quartz grain from AB2101 reach up to 7 μm. These inclusions are mostly isometric in shape, with a few exhibiting negative crystal shapes (Figure 12). Variations in gas-liquid ratios are present, and all primary inclusions homogenize into the liquid phase, with the highest homogenization temperatures (T_h) found in inclusions with a higher gas-to-liquid volume ratio (V ≥ L) (Figure 12).

Secondary inclusions within the quartz are associated with healed fractures and exhibit isometric or elongate shapes. These inclusions are primarily liquid-rich, though some contain significant gas phases. The homogenization temperatures of secondary inclusions show variability, with higher Th values in inclusions where the gas volume significantly exceeds the liquid, leading to homogenization into the gas phase (Figure 11). Cryometric analyses of inclusions from the host rocks reveal eutectic temperatures ranging from −38 °C to −41 °C, while the last ice crystal melting temperature (T_{m ice}) ranges from −3.5 °C to −7.6 °C, corresponding to salinities of 5.5–11.22 wt.% NaCl _equiv. (Table 3).

Raman spectroscopy of the inclusions identified CO₂ and N₂ in the gas bubbles, with CO₂ exhibiting a shift difference (∆) that corresponds to a density of 0.135 g/cm³ (Figure 12) [97,98]. Secondary inclusions in the quartz-calcite vein (AB2103) exhibited irregular or elongated shapes and relatively low T_h values ranging from 174 °C to 208 °C, alongside high salinity levels. The eutectic temperature of these inclusions ranged from −60 °C to −56 °C, with T_{m ice} at −21.1 °C, indicating a salinity of 23.16 wt.% NaCl _equiv. (cf. [99]) (Table 3). Raman spectroscopy did not detect any volatiles, such as CO₂ or N₂, in the gas bubbles of these inclusions.

Table 3. Summary of microthermometric data of fluid inclusions in the Aït Abdellah deposit.

Sample	Host Mineral	Type	Phase	Th. C°	Tm(ice). C°	Salinity (wt.% NaCl equiv.)	N
AB2101	Quartz	primary	L + V	219 ± 0.5. 254 ± 0.5. 311 ± 1.2	−4.5–−7.5	7.2–11.1	7, 10, 4
	Quartz	secondary	L + V	205 ± 1.5. 263 ± 0.5	−4.1–−7.1	6.7–10.6	10, 6
	Quartz	secondary	V >> L	309 ± 1.2 *			9
AB2102	Quartz	secondary	L + V	132 ± 2.5. 244 ± 0.5. 275 ± 0.5	−3.5–−7.6	5.7–11.22	20, 3, 6
AB2103	Quartz	secondary	L + V	174 ± 1.5. 195 ± 0.5. 208 ± 2.5	−21.1	23.2	22, 13, 8

L–liquid, V—gas bubble, T_h—homogenization temperature, T_{m (ice)}—temperature of last ice crystal melting, N—number of inclusions. *—homogenization into gas. Salinity was calculated according to [99].

Table 3. Summary of microthermometric data of fluid inclusions in the Aït Abdellah deposit.

Sample	Host Mineral	Type	Phase	Th. C°	Tm(ice). C°	Salinity (wt.% NaCl equiv.)	N
AB2101	Quartz	primary	L + V	219 ± 0.5. 254 ± 0.5. 311 ± 1.2	−4.5–−7.5	7.2–11.1	7, 10, 4
	Quartz	secondary	L + V	205 ± 1.5. 263 ± 0.5	−4.1–−7.1	6.7–10.6	10, 6
	Quartz	secondary	V >> L	309 ± 1.2 *			9
AB2102	Quartz	secondary	L + V	132 ± 2.5. 244 ± 0.5. 275 ± 0.5	−3.5–−7.6	5.7–11.22	20, 3, 6
AB2103	Quartz	secondary	L + V	174 ± 1.5. 195 ± 0.5. 208 ± 2.5	−21.1	23.2	22, 13, 8

L–liquid, V—gas bubble, T_h—homogenization temperature, T_{m (ice)}—temperature of last ice crystal melting, N—number of inclusions. *—homogenization into gas. Salinity was calculated according to [99].

5. Discussion

5.1. Genetic and Structural Framework of Mineralization

The copper mineralization at Aït Abdellah is structurally and genetically linked to a complex geodynamic history involving both Pan-African and Variscan tectonic events [11,82]. The mineralized system is controlled by a major NE–SW-oriented shear zone that extends over several kilometers and is characterized by intense mylonitization, S–C fabrics (shear planes trending N73° and foliation planes at N120°), and increasing deformation intensity toward its core. This shear zone not only focused tectonic strain but also facilitated the ascent and circulation of metal-bearing fluids, making it the principal conduit and structural control for copper mineralization.

The mineralization process is associated with both ductile and brittle deformation phases. The ductile phase involved pervasive shearing and mylonitization that created permeable zones enhancing fluid migration, leading to the formation of disseminated and stockwork-style copper mineralization [2,82,100,101,102]. Subsequent brittle deformation introduced faulting and fracturing that enhanced permeability, allowing for further hydrothermal fluid influx and sulfide precipitation, especially along NE–SW-trending fractures and breccia zones [11]. These structural features preserved the geometry and coherence of the mineralized bodies, despite multiple tectonic reactivations.

Mineralization appears to have initiated during the pre-Pan-African stage (B1) in a post-tectonic, possibly submarine volcanic setting [11,86,89]. During the Pan-African orogeny, early mineralization was structurally remobilized while retaining its original orientation. Copper deposition occurred in both fissural veins and as disseminations within microfractures and cavities in the host rocks. The mineralization is closely associated with hydrothermal veins composed of quartz, dolomite, and chlorite (Figure 13). It is interpreted to result from a single hydrothermal event involving fluid-assisted fracturing and sulfide deposition in a structurally controlled setting.

The paragenetic evolution can be subdivided into three main stages. The first stage includes early precipitation of quartz, dolomite, sericite, pyrite, and initial generations of bornite and chalcopyrite, under medium to high temperatures. The second stage, which represents the main economic mineralization phase, is marked by intense hydrothermal activity and fracturing, with the deposition of chlorite, calcite, bornite II, chalcopyrite II, and Ag-bearing sulfosalts. The third, supergene stage corresponds to the oxidation and enrichment of primary sulfides into chalcocite, covellite, malachite, chrysocolla, and iron oxides. Microprobe and SEM analyses confirm the dominance of Cu–Fe sulfides including chalcopyrite (34 wt.% S, 29 wt.% Fe, and 33 wt.% Cu), bornite (26 wt.% S, 11 wt.% Fe, and 61 wt.% Cu), pyrite, and chalcocite (21 wt.% S, 77 wt.% Cu), indicating a low- to moderate-temperature hydrothermal origin. Bornite appears to reflect a secondary phase of mineralization linked to renewed fluid circulation or remobilization. Minor inclusions of Ag-bearing tennantite–tetrahedrite within chalcopyrite and trace amounts of silver in chalcocite are also noted. Additionally, Ti-oxides (rutile, ilmenite) and iron oxides (hematite, goethite) reflect late-stage alteration processes.

Hydrothermal alteration is an integral component of the mineralization system and is most intense within the core of the shear zone. The alteration assemblage includes pervasive chloritization, carbonatization, silicification, and hematitization [103]. Chloritization, often associated with pyritization, is particularly strong within mylonitic zones, suggesting close interaction between deformation and fluid activity. Hematitization, often visible as reddish quartz lenses in outcrop, marks the influence of oxidizing conditions and likely reflects supergene processes.

The upward migration of saline, metal-rich fluids through fault zones triggered metasomatic reactions with feldspathic sandstones, resulting in cooling, chemical evolution, and precipitation of copper sulfides alongside gangue minerals such as quartz and dolomite. Supergene enrichment processes later overprinted the primary mineralization along fault-controlled zones where oxidizing meteoric fluids leached primary sulfides and redeposited copper as malachite, chalcocite, and covellite. These secondary minerals are concentrated in structurally favorable domains that promote fluid focusing and localized enrichment.

5.2. Isotopic Constraints on Ore Fluid Evolution

Sulfides formed by magmatic-hydrothermal processes typically display sulfur isotopic compositions close to zero (e.g., [91,104,105]). Sulfur from igneous rocks generally has an average δ³⁴S value of 1.0 ± 6.1%, reflecting a broad range from −11 to +14.5% [105]. These values result from complex processes like assimilation and partial melting, where pyritic sedimentary rocks (low δ³⁴S) and evaporites (higher δ³⁴S) contribute to the sulfur isotopic signature [106,107,108]. This sulfur signature is consistent with that observed in Ni-Cu-PGE sulfide-bearing mafic intrusions [109].

In the Aït Abdellah deposit, sulfur isotopic analyses of chalcopyrite and bornite show a range of δ³⁴S values from +5.9% to +22.8%, with an average of 14.5%. These results suggest a heterogeneous sulfur composition with contributions from both magmatic and hydrothermal sources. Lower δ³⁴S values near +5.9% likely reflect magmatic sulfur, while higher values (up to +22.8%) indicate hydrothermal processes, such as sulfur degassing or interaction with volcanic and crustal reservoirs, leading to ³⁴S enrichment.

The carbon and oxygen isotopic signatures of carbonates in the Aït Abdellah copper deposit indicate a carbon source derived from mantle rocks, likely related to serpentinization during the Pan-African orogeny. The δ¹³C values of calcite associated with copper mineralization range from −5.2% to −4.4% (VPDB), with an average of −4.9%, which are similar to values reported for nearby Bou Azzer ore deposits [23]. These values suggest a carbon isotope signature characteristic of mantle-derived fluids, potentially influenced by serpentinization processes in the surrounding ultramafic rocks. Carbon and oxygen data also suggest fluid interaction with marine carbonates and meteoric waters, possibly associated with post-Snowball Earth deglaciation.

The δ¹⁸O values of carbonates in the Aït Abdellah deposit range from 16.3% to 21.4% (VSMOW), consistent with values observed in other ophiolitic-hosted copper deposits, including Bou Azzer [23]). These values suggest a magmatic or metamorphic fluid source, with the spread in δ¹⁸O values likely reflecting fluid mixing or boiling [110].

The structural architecture of the Aït Abdellah copper deposit is fundamentally controlled by tectonic events associated with the Pan-African and Variscan orogenies [43,62]. The NE–SW-trending shear zone, developed in response to these orogenic episodes, served as a principal conduit for hydrothermal fluids responsible for the transport and deposition of copper sulfides and associated gangue minerals. Mineralization at Aït Abdellah is typified by a multi-stage evolution, beginning with early hydrothermal precipitation of copper sulfides and followed by a supergene enrichment phase, during which secondary copper minerals such as malachite and covellite formed under oxidative near-surface conditions.

The ascent and emplacement of mineralizing fluids were governed by a combination of magmatic and crustal fluid sources, with structural permeability exerting primary control on fluid migration and ore localization. This interplay between fluid dynamics and tectonic structures facilitated the focused deposition of copper within favorable lithological and structural traps, offering critical insights into the ore-forming processes operative in the region.

Isotopic data, particularly sulfur and carbon isotopic signatures, further constrain the origin and evolution of the mineralizing fluids. The S–C isotopic compositions indicate a dual fluid source, with contributions from both mantle-derived and crust-modified components [23]). The role of the shear zone as a long-lived, high-permeability pathway, coupled with the complex polyphase tectonic history, was instrumental in the formation, enrichment, and preservation of copper mineralization at Aït Abdellah.

6. Conclusions

Integrated structural, mineralogical, geochemical, and fluid inclusion analyses yield the following key insights into the genesis of the Aït Abdellah copper deposit:

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The mineralization is structurally controlled by a major NE–SW-trending shear zone, which served as a primary conduit for hydrothermal fluid flow and facilitated ore emplacement.

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Three distinct mineralizing stages have been identified: (1) an early hydrothermal phase marked by the deposition of quartz, dolomite, pyrite, bornite, and chalcopyrite; (2) a subsequent stage characterized by intense fracturing and fluid flow, leading to the precipitation of bornite, chalcopyrite, and Ag-bearing sulfosalts; and (3) a supergene enrichment phase during which primary sulfides were oxidized to form secondary copper minerals such as chalcocite and malachite.

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Sulfur isotope compositions (δ³⁴S values of chalcopyrite and bornite ranging from +5.9% to +22.8%) suggest a mixed magmatic-hydrothermal sulfur source.

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Carbon isotope signatures in calcite (δ¹³C from –5.2% to –4.4%, VPDB) are consistent with a mantle-derived carbon source. In contrast, oxygen isotopes (δ¹⁸O between 16.3% and 21.4%, VSMOW) reflect fluid mixing between magmatic/metamorphic and marine sources.

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Fluid inclusion data reveal homogenization temperatures ranging from 195 °C to 310 °C and salinities between 5.7 wt.% and 23.2 wt.% NaCl equivalent, supporting a model involving mixing of magmatic-hydrothermal fluids with basin or volcano-sedimentary fluids.

Collectively, these findings support a genetic model of multi-stage, structurally focused copper mineralization within Neoproterozoic ophiolitic and volcano-sedimentary assemblages. The results have important implications for guiding exploration strategies targeting similar metallogenic systems in the central Anti-Atlas region.