Copper Mineralization in the Lower Cambrian Cover of Jbel N’Zourk (Central Anti Atlas, Morocco): Tectonic-Mineralizations Relationships ()
1. Introduction
Many copper occurrences are present in the Neoproterozoic-Cambrian cover, also known as the Adoudounian cover [1] (Figure 1). These indices are poorly documented and have generally been considered to be the result of syngenetic processes [2] [3] [4] . The great interest of mining groups in copper resources guides the need for the best constraint of the geological history of such mineralizations. The origin of these ores, which are hosted at different stratigraphic levels of the Adoudounian cover, remains in most cases poorly known even if few of them are currently being mined. Few of these copper orebodies are currently mined. Active mines include the Jbel Laassel mine whose reserves are estimated to be 7,500,000 t with 1% Cu, [5] the Agoujgal mine whose reserves are estimated to be 5,200,000 t with 1.11% Cu and 21 g/t Ag, [6] and the Ouanssimi mine whose reserves are estimated to 1,300,000 t with 2.65% Cu [5] . Other ore bodies are in exploration phase (Jbel N’Zourk, Tarmmant, Tizi N’Mekraz, Assif N’Zaid, Oued Rthem, Tizert…) [2] [5] [6] . The Jbel N’Zourk copper body is the largest copper deposit in the Adoudounian cover in Central Anti-Atlas. It is located at the southern flank of the Central Anti-Atlas between the Bou Azzer El Graara and Zenaga inliers (Figure 1). Exploration studies performed by Managem group since 2016, are presently estimating the resources at 1,250,000 t with 1.12% Cu [7] . The aims of the present work are to characterize the copper mineralizations in Jbel N’Zourk, the nature and structural analysis of their host rocks, and the relationships between the mineralized rocks and tectonics structures, Which will contribute to highlight the role of the deformation of the Adoudounian cover in the emplacement of copper mineralizations, based on structural, microstructural, and petro-graphic observations.
2. Regional Geological Setting
2.1. The Anti-Atlas Belt
Located along the northern border of the WAC (West African Craton) [10] , it is limited by the alpine chain of the High Atlas to the North, by the Tindouf basin to the South, by the Atlantic Ocean to the West and by the Tafilalet and Bechar basins in the East. It is a large Atlasic bulge oriented ENE-WSW, superposed on a Hercynian fold whose anticlinal core reveals the Proterozoic crystalline basement through “inliers” [Bas Dràa, Ifni, Kerdous, Tagragra of Akka, Igherm, Tata, Izazen, Iguerda, Ouzellarh, Sirwa, Zenaga, Bou Azzer el Grara, Saghro and Ougnat] (Figure 1), distributed along two major fault zones (South Atlas Fault, and Central Anti-Atlas Fault). This belt has recorded Eburnean, Pan-African and Hercynian orogenesis, which are related to the Paleoproterozoic, Neoproterozoic and Paleozoic respectively. The Precambrian basement is formed by diversified sedimentary and metamorphic complexes, often intruded by granitic complexes dated at 2050 ± 10 MA [11] , and more rarely, by basic and ultrabasic intrusive rocks while the Pan-African belt shows ophiolitic complexes are preserved along the Anti-Atlas Major Fault (AAMF) in the Bou Azzer and Siroua inliers [12] - [21] . All these formations are also associated with many acid and basic volcanic rock bodies of variable nature and size. The rest of the Anti-Atlas region is generally covered by Paleozoic terrain rarely deformed [15] [16] [22] [23]
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Figure 1. Generalized geological map of the Anti-Atlas, simplified from the Geological Map of Morocco, scale1:1,000,000 [8] [9] ; (a): Location of the Anti-Atlas within the structural domains of NW Africa, the studied region is indicated by red square.
[24] [25] [26] . The latter is largely developed on the southern flank of the Anti-Atlasic anticlinal [23] [27] .
2.2. The Adoudou Formation
Its name is related to the Adoudou Oued located to the west of the Anti-Atlas [24] . This for-mation crops out largely in the Anti-Atlas belt and southwest of the High Atlas with a 1000 m thick series and represents the oldest volcano-clastic complex unconformably covering the Neoproterozoic Ouarzazate group [28] [29] . The Adoudou Formation is associated with a major marine transgression from the West to the Est. It has been subdivided into two groups: the Taroudant Group, with Lower Dolostones and Lower Sandstones units, and the Tata Group, with Upper Dolostones and Upper Sandstones units [23] [30] [31] [32] [33] . The Adoudounian series shows significant variations in facies and thickness. Moving eastward (Saghro, Ougnat, etc.), these series become thinner and dominated by increasingly coarser terrigenous facies, as opposed to the carbonate facies deposited to the west (Tiznit region) [27] [23] . A calico-alkaline volcanism is initiated during the same period as the sedimentation of the Adoudou formation. Two volcanic complexes, Jbel Boho and El Gloa, are located in the Central Anti-Atlas [22] [34] [35] . Old lava flows belonging to the Jbel Boho volcanic system, interspersed in the Adoudou formation, have been dated at 534 ± 10 MA (U/Pb Shrimp on Zircon) [35] and at 531 ± 5 MA [35] .
3. Local Geological Setting
The Jbel N’Zourk deposit is hosted in the Lower Cambrian part of the late Ediacaran to Cambrian volcano-sedimentary formations at the southern flank of the Central Anti-Atlas between the Zenaga and Bou Azzer El Graara inliers, 50 km NW of the Bleida mine. Geological mapping, fieldwork and lithological description of drill cores have allowed us to construct the lithostratigraphic column of the Jbel N’Zourk region (Figure 2 & Figure 3), which consists of a thick succession (approximately 800 m) of the Adoudounian cover. We distinguish from bottom to top:
• The basal series: represented in the studied region by a breccia and conglomeratic unit with a thickness of 5 to 100 meters, composed of subangular elements which origin is the underlying volcano-sedimentary series. Horizontal rough bedding appears locally in these facies, indicating a high energy deposition process in a fan-delta system.
• The lower limestone: with a 150 - 250 m thickness series composed of a succession of dolomitic limestones levels separated by red and white clays siltstones showing some local breccia recurrences. The dolomitic levels, when they are not massive, are frequently characterized by the presence of horizontal laminates of microbial origin and more rarely by stromatolitic domed structures. Intense secondary silicifications affect the dolomites in Jbel N’Zourk.
• The Jbel Boho volcanic complex: with levels of pyroclastic tuffs, lava and volcanic breccias interspersing at the base of the lower limestones in Jbel N’Zourk. These volcanic manifestations are mainly found in the river area (Mechkerj River). This volcano-sedimentary series with a thickness varying from 100 to 200 m indicates a synsedimentary volcanic system, which was to constitute an emerging relief, with dismantling products transported in the form of debris flows interbedded within the platform deposits of the Adoudounian formation.
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Figure 2. Lithostratigraphic columns of the Jbel N’Zourk area.
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Figure 3. Geological and structural map of the Jbel N’Zourk area.
• The “Lie de Vin” Series: are lower sandstones that are part of the Tikirt Formation (150 to 200 m thick) which is composed essentially by a red claystones of sand-stones frequently interlayered by centimetric clayed siltstones and purple argillites representing a significant sea level regression.
The vertical change in facies from the lower limestone to the “Lie de Vin” series is rapid and progressive; on the other hand, this series is brutally surmounted, without any visible discordance, by the upper limestones, which initiates a new negative sedimentary sequence.
4. Sampling and Analytical Methods
In order to establish a detailed survey of the Jbel N’Zourk copper mineralized veins and to understand their distribution and tectonic control, a structural and microstructural study has been performed in the prospect area. Samples were collected from outcrops and drill cores were used for petrographical, mineralogical, and geochemical studies. Polished thin sections were prepared from these samples and have been first studied using a petrographic microscope with transmitted and reflected light modes. Complementary observations and mineral’s analyses were carried out using JSM IT100 (JEOL) Scanning Electron Microscope (SEM) with 0.5 to 30 kV accelerating voltage. The SEM system is coupled to Energy-Dispersive X-ray fluorescence (XRF) spectrometer (EDS) to make qualitative compositions of sulphide minerals from ten polished sections. Whole-rock geochemical data of samples collected at different prospect area were determined at the Reminex Research Center (MANAGEM Group). For the major elements analysis (SiO2, Al2O3, Fe2O3, CaO, MgO, MnO, K2O, TiO2, P2O5), 0.5 g of the sample is crushed at less than 100 μm and it is dissolved by fusion at 500˚C during 40 min with 2.5 g of sodium peroxide in a zirconium crucible. The melt mixing is then dissolved with 100 mL of hydrochloric acid (28% HCl) and the solution is analyzed using an ICP-AES ULTIMA 2C using the Jobin Yvon-HORIBA device. For the other elements (Cu, CuOx, As, B, Ba, Be, Bi, Cd, Co, Cr, Ge, Li, Mo, Nb, Ni, Pb, Sb, Se, Sn, Sr, W, Y, Zn, Ag), 0.25 g of the sample is dissolved by microwave-assisted acid attack (50% HCl and 50% HNO3) during 40 min at 220˚C. The solution is then analyzed by ICP-MS Thermo X’Serie 2.
5. Results
5.1. Structural Study
The Jbel N’Zourk area is located in a large anticlinal structure. This structure only affects the higher terms compared to the basic silty sandstone and ferruginous micro-conglomerates. The whole sequence in Jbel N’Zourk undergoes at least two deformation stages. Two types of folds are present. The first family corresponds to very tight folds characterized by axial planes poured eastward or kneeling folds, with an N-S orientation. Fracture schistosity, absorbed essentially by the levels of siltstones between dolomitic limestones, is associated with these folds. The second fold family characterized by very open is opaque folds oriented NE-SW; of pluri-metric to plurikilometric amplitude spatially associated with overlaps of the same direction, and are distributed mainly in the western flank of Jbel N’Zourk. Sometimes we can observe in the same folding axis the passage from tight folds to kneeling folds. This disharmony is due to the effect of discontinuity and variability in the power of the siltite levels between the limestones. These silty horizons played a soapy role when the limestone bars were folded. Some of these folds are therefore interpreted as fault propagation folds that have formed at the tip of blind thrust and back thrust that terminate upward from a flat decollement localized along the upper boundary of the Jbel Boho formation.
One decametric extension thrust fault can be observed at the top of the Jbel N’Zourk relief, where the unit of the lower dolomitic limestones in underlying the unit of siltstones and sandstones of the series of “Lie de Vin”, along a NNW-SSE trending thrust dipping 80˚ to the east (Figure 4 and Figure 5).
The western flank of Jbel N’Zourk is the most deformed part; the survey of the fracturing made it possible to distinguish two families of faults, a NNW-SSE orientation family that generally ranges from N150˚ and N180˚, parallel to the major Jbel N’Zourk fault, and a second E-W family characterized by an orientation between 45˚ and N 70˚ (Figure 6).
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Figure 4. Geological cross section showing the structure of the Jbel N’Zourk deposit.
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Figure 5. Panoramic view showing the passage of the Jbel N’zourk fault in the study area.
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Figure 6. Fracturing in the study area: (A) reverse fault N45; (B) reverse fault N40; (C) normal fault at the crest of Jbel N’Zourk; (D) reverse fault on the western flank of Jbel N’Zourk.
In addition to the large structures of Jbel N’Zourk, microstructures have been identified within the mineralization, which essentially corresponds to veins, veinlets, microfails, tension slots and veins (Figure 7).
These microstructures sometimes show indications that provide information on the movement and cinematic properties of the tectonic deformations that have affected the region. Jbel N’Zourk is a region characterized by intense fracturing that sometimes makes the entire structure difficult to identify due to large scree overlaps (Figure 7).
5.2. Copper Mineralization
5.2.1. Geometries and Orientation
In the Jbel N’Zourk copper deposit, the economic mineralized zone is present in the western part of the Jbel N’Zourk fault in a thick series of carbonates of the Adoudou formation. The sulphides are mainly hosted in the base of the lower carbonate formation in veins and sometimes in the base siltstones of the series. The lateral and vertical extent of ore bodies was delimited by cartography and drilling studies. The thickness of the mineralized zone reaches 40 m. Locally,
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Figure 7. Photographic view of micro-structures at Jbel N’zourk. (A) sigmoidal Z tension fractures; (B) micro-fault in the Jbel Boho volcanic formation; (C) quartz veins and fractures; (D) malachite vein; (E) Stockwork secant on stratification; (F) hydraulic breccias with angular fragments.
later copper sulphide minerals are distributed in both families of the faults NNW-SSE and E-W in the Adoudou formation. In this formation, sulphides occurrence are essentially observed in fractures, in quartz veins and in quartz and calcite geodes. The eastern boundary of ore bodies present the Jbel N’Zourk fault zone, the southern and western boundaries of the deposit are not explored and are not yet fully defined, but ongoing drilling exploration activities aim to define the boundaries of the economic mineralized zones in this deposit.
5.2.2. Associated Mineralogy and Paragenetic Succession
Without distinction between primary and secondary origin, the copper mineralization in Jbel N’Zourk deposit is composed of bornite, calcocite, chalcopyrite, covellite, pyrite, digenite, malachite, azurite, cuprite, tenorite and chrysocolla, associated with quartz, dolomite and calcite as gangue mineral. This mineral gangue in the veinlets or voids systematically has a banding texture. When these three phases are present, it forms ribbons within the veins with quartz on the epontes, then dolomite and finally calcite in the center. This gangue chronology is a great reference point for determining the succession of copper minerals in this deposit. Bornite is the most abundant and developed copper sulphide in the deposit as evidenced by petrographic observations (Figures 8(A)-(C) ), being partially or totally replaced by chalcocite. Chalcopyrite often appears as a corona around the bornite and rarely included in the bornite (Figure 8(A), Figure 8(C)). This association can be explained by exsolution between the two phases or by the replacement of the bornite by chalcopyrite. Chalcopyrite and bornite are transformed into dark blue colored covellite, digenite and chalcocite (Figure 8(A), Figure 8(B)). These secondary alteration minerals are observed mainly in the rims of the bornite and chalcopyrite minerals. However some Cristals can be totally replaced by late chalcocite, digenite and covellite. The mineral phases of copper carbonates are generally associated with the supergenic alteration of primary sulphides and are composed of malachite, azurite and chrysocolla (Figures 8(D)-(F)). These copper carbonates are present as secant veinlets on all structures replacing sulphides and dolomites. All these relationships between minerals allow the distinction of a primary paragesesis represented by: quartz, bornite, chalcopyrite, dolomite and calcite, and a secondary one represented by: covellite, chalcocite, digenite, malachite, azurite, chrysocolla, tenorite and cuprite. Based on the intersection relationships between the different mineral phases, several episodes of fracturing have been observed (Figure 8; Table 1).
5.2.3. Multiscale Observations of the Ore Texture
In the field and at macroscopic scale, two types of textural mineralization have been identified; the most frequent texture of mineralization in ore deposit is a stockwork with veins, fractures and veinlets are oblique to the stratification. The veins of this stockwork have locally combed quartz textures. The second type of texture corresponds to disseminations within the rock groundmass in the basic siltstones. Locally it is spatially related with the stockwork texture. At the microscopic scale by observation of polished thin sections, both types of mineralization appear composed by the same ore and gangue minerals. Textural, chronological relationships and paragenetic sequence has being also similar (Figure 8).
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Figure 8. Paragenetic associations and replacement textures. (A) Disseminated chalcopyrite (Cpy), associated with Gitite (Gi) and calcite (Cl); (B) Vein of bornite (Bn), covellite (Cv), diginite (Dg) and calcite (Cl); (C) bornite remplaced by chalcopyrite; (D) “Peigne texture”; (E) Malachite veinlets; (F) Azurite veinlets.
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Table 1. Paragenetic succession of the Jbel N’Zourk deposit.
5.2.4. Principal Component Analysis (PCA) of Chemical Data
The database of the JBEL N’ZOURK prospect is composed of 97 samples that correspond to the sampling by lithology of the copper mineralized zone (Table 2 and Table 3). This database contains analyses of Total copper (Cu) (Figure 9). The results of the PCA show that two factors F1 and F2 explain 60.07% of the total variability (Figure 10(A)). Three groups of elements discriminated in the F1 - F2 plane (Figure 10(C)) corresponding in the observation plan to siltstones, dolostones and potentially to tectonic breccias for the group represented by the rare elements (Figure 10(D)). The copper variable, not well represented in the F1 - F2 plane, would be considered to have an affinity for the siltstone group (Figure 10(D)). This representation of the variables suggests that copper is not related to any of the three types of lithology. Indeed, copper is independent of sedimentary siltstones and dolostones, and it is anticorrelated with the lavas formation. The factors F2 and F5 are the most correlated with the copper variable (Figures 10(D)-(F) ). Moreover, an examination of the F2 - F5 plane, suggests that copper could be associated with titanium, with correlation coefficient between total copper and titanium is 0.62 (Figure 10(E), Figure 10(F)). In a binary representation, the coefficient of determination, between these two elements, is 0.4 which shows that the correlation between these two elements is limited (Figure 9). Therefore, the PCA highlights the independence of copper (total and oxidized) relative to the others variables present in the available database, especially that corresponding to lithology (Figure 10).
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Figure 9. Binary graph showing TiO2, SiO2, K2O and Al2O3 content as a function of total copper (Cu) content. The straight lines correspond to the regression lines, their colors to the different variables compared to the copper contents. R2 is the coefficient of determination. The ordinate scale is logarithmic.
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Figure 10. Results of PCA analyses of the database of Jbel N’Zourk deposit. (A) Eigenvalues and cumulative variance function of factors. (B) Table of correlation between the copper variable and the factors. (C) Variables projection in the F1 - F2 plane. (D) Samples lithology project in F1 - F2 plane. (E) Variables projection in the F2 - F5 plane. (F) Samples lithology project in F2 - F5 plane.
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Table 2. Whole rock major element chemical data of the studied samples from the Jbel N’Zourk deposit.
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Table 3. Whole rock trace element chemical data of the studied samples from the Jbel N’Zourk deposit.
6. Discussion
In the Jbel N’Zourk copper deposit, the mineralization is present at a macroscopic scale as a stockwork, disseminations and in karst pockets. The stockwork is composed by veins and veinlets filled with two successive mineralization phases:
1) A primary paragenesis composed by: bornite, chalcopyrite, associated with dolomite, quartz and calcite;
2) A secondary paragenesis composed by: covellite, chalcocite, digenite, malachite, azurite, chrysocolla, tenorite and cuprite associated with a gangue composed mainly of quartz-dolomite.
The veins and veinlets of this stockwork are oblique to the stratification plane. The majority of fractures families always present the same paragenetic evolution, mineralogy and textural. In consequence these different observations suggest that, with respect to the primary paragenesis of sulphides, all stockwork veins and veinlets are co-genetic (formed during in the same mineralization event). Microscopic studies reveal that the disseminations mineralizations correspond to microcracks and micro-fractures filled with the same mineralogical content, and texture characteristics as stockwork veins. In addition, disseminated copper containing mineral grains are directly connected to the stockwork veins. Consequently, all primary mineralization, from macroscopic to microscopic scale, is connected to the stockwork and deposited during a same hydrothermal fluid-assisted fracturing event of the host sedimentary rocks. Currently, according to the All these observations favor an epigenetic origin of the Jbel N’Zourk copper mineralization. In addition, the principal component analysis (PCA) of chemical data shows that copper does not have affinities with any of the lithological groups (siltstone, dolostone and breccias) present in the area. This result is also consistent with an epigenetic origin of the mineralization.
In comparison with most of the copper occurrences hosted in the Adoudounian cover of the Central Anti-Atlas, the Jbel N’Zourk deposit shows a particularity that is reflected in the association of mineralization with tectonic structures, this is confirmed by structural, microstructural and metallographic study. Both families of fractures NNW-SSE and E-W, also the folds associated with the main fault of Jbel N’Zourk, served as a main drain that controlled the placement of the mineralization. Many authors evoke a Variscan remobilization of a primary syngenetic mineralization within the Adoudou formation to explain the stockwork textures intersecting sedimentary structures [1] [4] [9] .
[1] describes kink bands and a rough cleavage developed in the vicinity of the previous fractures along the borders of the Bou Azzer El Graara inlier in the late Palaeozoic compressional event, generating in meter to decameter detachment folds scale which are common in the Adoudou formation, having a dominant NW-SE trending axes with subordinate NE-SW structures [1] [17] . These authors interpret these folds in terms of reactivation of Panafrican structures of the basement during the variscan orogenesis. In the Jbel N’Zourk the folds are associated locally with kilometre-scale sinistral strike-slip faults and SW-verging transpressional reverse faults, where the Adoudou formation coming, via an inverse play, on the unit of siltstones and sandstones of the series of “Lie de Vin”. This fault probably served as principal drain for the hydrothermal fluids that formed the mineralization of the Jbel N’Zourk deposit. Under such conditions, the age of the copper mineralization event at the Jbel N’Zourk deposit could probably be contemporary to the deformation of the cover during late Paleozoic compression. Currently, we propose that the copper mineralization presented in the Adoudou formation of the late Ediacaran to lower Cambrian cover is related to the variscan reactivation of the basement structures and addition to the variscan faults that drained the hydrothermal fluids which precipitated its copper content as a stockwork in the deformed terrains of the Jbel N’Zourk.
7. Conclusions
The copper mineralization in the Jbel N’Zourk deposit is epigenetic and is controlled by variscan tectonic structures that are associated with folding in the Adoudou formation. As a consequence, the copper mineralizing event occurred during this period. Most of the copper mineralization is present as stockwork, in fractures, faults and karst fill pockets. The location of the better-mineralized zone near faults and fractures indicates that it was epigenetic mineralization; it is possible that it occurred during the late variscan shortening. The position of the Jbel Boho volcanic series just below the mineralization can explain the origin of hydrothermal fluids.
The benefits of such an approach have a dual interest; 1) Scientific, contributes to a better understanding of the mechanism of vein formation process and understanding of the relationship between copper deformation-mineralization in the Adoudounian cover of the Central Anti Atlas, and 2) economics, contributes to a better knowledge of specific ore body geometry and distribution, thus the deformation affected the terrain and hence highly recommended in exploration program, but can also provide solutions to the exploitation problems.
Acknowledgements
The authors would like to thank all those who have contributed, directly or indirectly, to its elaboration, to who have been teaching, guiding and encouraging. The thanks are also addressed to the MANAGEM group (SOMIFER) for their collaboration and help during the field survey, as well as the reviewers for their helpful comments and suggestions.