Analysis of tectonic fracturing in the Mibladen ore deposit (Upper Moulouya, Morocco) and its impact on the Pb–Ba mineralization emplacement

The MVT Pb–Ba mineralizations of the Mibladen ore deposit are hosted by Jurassic carbonates as well as Infracenomanian conglomerates and sandstones. The mineral paragenesis is mainly composed of galena and barite with lesser chalcopyrite and pyrite, accompanied by supergene oxidation minerals. This ore deposit is the result of a major epigenetic mineral stage with economic orebodies occuring as replacement of pre-existent carbonate rocks, fillings of karst cavities, interstratal joints, collapse-breccias, fractures, and faults. Structural and microtectonic analyses we carried out in this ore deposit, allowed us to highlight two main fracture networks controlling ore deposition within karst cavities and interstratal joints: (i) NNW-SSE to NNE-SSW trending tension gashes and normal faults; (ii) ENE-WSW to E-W trending reverse faults with strike-slip components and transtensive relay zones. All of these structures are developed under a regional compressional tectonic regime divided into extensional and transtensional episodes (σ1–σ2 and σ2–σ3 permutations) with sub-meridian σ1 axis and sub-equatorial σ3 axis. This compressive tectonic event caused the uplift of Mibladen area and favored the circulation of mineralizing fluids along the NE-SW and ENE-WSW major faults such as Aouli and Amourou Faults, during the Infracenomanian period (Upper Jurassic–Early Cretaceous).


Introduction
The western Mediterranean domain contains several Pb-Zn-Ba-F-Mn deposits (e.g., Barbieri et al. 1990;Perseil 1998;Tlil et al. 2017;Issaad et al. 2019) which have made it one of the most important Tethyan-Atlantic metallogenic provinces. The Moroccan Atlas system, part of this domain, includes several Mississippi Valley-type (MVT) ore deposits with economic Pb-Zn-Ba orebodies. These MVT occurrences are hosted in large metallogenic provinces in north-western Africa from Tunisia to Morocco (Bouabdellah and Sangster 2016).
The MVT Pb-Ba ore deposit of Mibladen, subject of this study, is located south of the Aouli inlier, 15 km north-east of Midelt (Fig. 1). It has become world famous for its beautiful vanadinite crystals (Jahn et al. 2003;Praszkier 2013). Currently, the Mibladen mine is closed, its total production (from 1938 to 1983) is estimated about 6.253.000 t of ore at an average grade of ≈ 5 wt% Pb (Annich and Rahhali 2002;Rahhali 2002), accounting for approximately 30% of Moroccan Pb production (Bouabdellah and Sangster 2016). The Pb-Ba mineralized zone covers an ENE-WSW trending band. It extends over a strike length of 15 km and is 1-4 km wide. This mineralized band is framed by two major faults: the ENE-WSW Amourou Fault in the south and the NE-SW Aouli Fault in the north (Fig. 2).
Pb-Ba mineralizations of Mibladen ore deposit are structurally controlled (Dagallier and Macaudiere 1987;El Jaouani 2001;Naji 2004). Prior to the present work, no detailed study concerning fracturing analyses, paleostress reconstructions, and the determination of the regional tectonic regime associated to the emplacement of Mibladen mineralizations has been established, except for a few very limited and not in-depth studies. For this reason, we focused our work on the analysis of tectonic fracturing at different scales and paleostress reconstruction in this ore deposit. This paper aims to: (i) determine the relationships between tectonic episodes and ore depositional stage in the Mibladen deposit; (ii) determine the relative age of the Pb-Ba mineralization emplacement; (iii) propose a model for the Pb-Ba mineralization emplacement in the Mibladen ore deposit based on the results of this work and available data from previous studies. The distribution of Pb-Ba mineralization in the Mibladen sedimentary series is represented in a set of lithostratigraphic columns. The results of structural analyses and paleostress reconstruction are presented on a set of rose diagrams and stereoplots.

Methods
For each mining site, we carried out multi-scale structural and microtectonic analyses for the Pb-Ba mineralization and its host rocks. We also carried out lithostratigraphic sections showing the arrangement of Pb-Ba mineralization in its host sedimentary units.
To determine the set of the structural systems controlling ore deposition and to reconstruct the associated paleostress fields, we used the Win-Tensor program (Delvaux 1993(Delvaux , 2012Delvaux et al. 1997;Delvaux and Sperner 2003) to process field data measurements collected from  (Raddi et al. 2013, modified) 1 3 faults and tension gashes. The calculated stress tensors are based on four parameters: the main stress axes σ1, σ2, σ3 with σ1 ≥ σ2 ≥ σ3 ≥ 0 and the stress ratio R = ( 2 − 3)∕( 1 − 3) with 0 ≤ R ≤ 1 (e.g., Angelier 1989Angelier , 1991Angelier , 1994Gephart and Forsyth 1984;Vandycke and Bergerat 1992;Lund and Townend 2007;McFarland et al. 2012). The automatic processing of microtectonic measurements is first carried out using the "Right Dihedron method" developed by Angelier and Mechler (1977) and improved by Delvaux and Sperner (2003). The obtained stress tensor using this method is then used in the "Rotational Optimization method" (Delvaux and Sperner 2003). This method is based on a composite function F5 which must be minimized. For slickensided fault planes, this function minimizes the slip deviation between the observed and resolved slip vectors, and it maximizes the resolved shear stress magnitude to favor slip on the fault plane (Angelier 1991;Delvaux and Sperner 2003). For tension gashes, it minimizes both the resolved normal and shear stress magnitudes to favor fracture opening and prevent slip on the plane (Angelier 1991(Angelier , 1992Delvaux and Sperner 2003). Results showing a value of F5 > 20 for tension gashes and F5 > 22 for striated fault planes are excluded from the calculations (Delvaux and Sperner 2003).

Gitology and distribution of Pb-Ba mineralizations in the Mesozoic sedimentary series of Mibladen
In the Mibladen ore deposit, economic orebodies are arranged in an ENE-WSW elongated band (Fig. 4). Spatial distribution of mineralized occurrences shows the existence of two Horizons or "Faisceaux" corresponding to two different stratigraphic levels (Fig. 4). These mineralized Horizons are organized into several mining sites ("Faisceau Inférieur" or "Lower Horizon": Bou el Maden, Anne Marie, the S and the R mining sites; "Faisceau Supérieur" or "Upper Horizon": Adeghoual, the Dalles, the T Ouest, the O, the AB2, AB3, AB4 and AB5 mining sites; Emberger 1965b; Felenc and Lenoble 1965) (Fig. 4). The 2ème Horizon and Marguerite mining sites ( Fig. 4) occupy probably an intermediate position between the two Horizons (Emberger 1965b). Lithostratigraphic sections we carried out in each of the Mibladen mining sites (Fig. 5), allowed us to highlight the distribution of Pb-Ba mineralizations in the different terms of the Mesozoic series in this ore deposit. The majority of the mineralized bodies are hosted by argillaceous limestones, dolomitic limestones, dolostones, and limestones of Middle Liassic. Early Liassic dolostones are also mineralized, they contain stratiform barite orebodies with thickness which can reach 100 cm (Fig. 6a). Pb-Ba mineralizations are also hosted by Bajocian bioclastic limestones in the AB3 mining site (Fig. 5). The last mineralized term of the Mesozoic series of Mibladen corresponds to Infracenomanian conglomerates and sandstones in the AB2 and AB5 mining sites. Pb-Ba mineralizations hosted in Early Liassic and Bajocian formations are highlighted for the first time in this work. In the S, Bou el Maden, Anne Marie and AB2 mining sites, barite ± galena are finely interbedded or disseminated within Middle Liassic marls and claystones (Fig. 5).
The morphology of the economic orebodies in the Mibladen deposit is very diversified; it is represented by stratiform bodies corresponding to interstratal joint and karst fillings (Fig. 6c, e and f), dissolution cavities characterized by the substitution and replacement of the carbonate host rock by metalliferous fluids (metasomatic processes) ( Fig. 6b and g). Pb-Ba mineralizations hosted in the Infracenomanian conglomerates and sandstones are characterized by the impregnation of intergranular spaces and the substitution of conglomerate carbonate cement (Fig. 6d). In addition to the strata-bound orebodies, Pb-Ba mineralizations occur as fracture and vein fillings. These fractures are sub-vertical to vertical (Fig. 7) and affect all series of the mineralized zone.
The mineralogy of the Mibladen ore deposit is not complicated, it is one of the simplest mineral assemblages ever found in a MVT district (Bouabdellah and Sangster 2016). The primary mineral paragenesis is mainly made up of barite and galena ( Fig. 6) with lesser chalcopyrite and pyrite, sphalerite is virtually absent (Margoum 2015;Bouabdellah and Sangster 2016). Barite has two aspects: (i) poorly crystalized masses occupying interstratal joints, fractures and karst cavities; (ii) well-crystallized masses with crested barite developed within geodes and open-spaces. Its color is white, pink or honey-pink, it is sometimes upholstered with brown shades corresponding to iron oxides. Galena is generally well crystallized with cubic or octahedral shapes; the crystal size can sometimes exceed one centimeter. Its silver content can reach 500 ppm in some mining sites (Emberger 1965b). The supergene mineralization stage results from sulphide oxidation (Bouabdellah and Sangster 2016). It is represented by mineral assemblages with cerussite, anglesite, wulfenite, vanadinite, quartz, calcite, aragonite, gypsum, manganese oxides (cryptomelane, hollandite, and coronadite; Jahn et al. 2003), phosgenite (Pb 2 CO 3 Cl 2 ), mottramite [PbCu (VO 4 ) (OH)], and paralaurionite (PbCl(OH)) (Praszkier 2013). This is the reason for the worldwide reputation of the Mibladen ore deposit as one of the best destinations for mineral collectors.

Structural control of Pb-Ba mineralizations in the Mibladen ore deposit
The Jurassic and Infracenomanian series hosting Pb-Ba mineralizations in the Mibladen ore deposit are intensely fractured. This tectonic fracturing is favorable to metalliferous fluid circulations (Fig. 7). In all of the mining sites, we observed vein orebodies related to sub-vertical and vertical fractures. These affect all strata bearing Pb-Ba mineralizations. They act as pathways for the circulation of Pb-Ba mineralizations, and they are responsible for their deposition into open-spaces such as ravinement surfaces (Fig. 7a), karst cavities (Fig. 7b), and interstratal joints (Fig. 7c-e). The mineralized structures thickness ranges from centimeters to decimeters, their intersection with the strata-bound orebodies shows significant mineral enrichment zones. Structural analyses of economic orebodies we carried out in the Mibladen mining sites show a similitude between all these sites. Pb-Ba mineralizations are controlled by two main fracture networks. The distribution of these fractures is characterized by two preferred direction systems (Fig. 8): -ENE-WSW to E-W (N60-N100) system characterized by vein structures developed along reverse faults with strike-slip component. These faults show slickensided Fig. 4 Spatial distribution of mineralized occurrences in the Mibladen ore deposit (Felenc and Lenoble 1965, modified) planes filled by Pb-Ba mineralizations ( Fig. 9a and b). These fault planes are generally non rectilinear, the strikeslip components allow the appearance of transtensive relays and pull-apart openings (Fig. 9c-g) favorable to the Pb-Ba mineralization concentrations.
-NNW-SSE to NNE-SSW (N160-N15) system represented by vein structures developed along very frequent tension gashes and less frequent normal faults. Tension gashes ( Fig. 9h and i) have centimetric thickness, they are characterized by the development of mineral-rich geodes of the supergene mineral stage.
Field observations in the Mibladen mining sites, allowed us to conclude that these different mineralized structures have the same relative age and are developed simultaneously. Their intersection zones do not show any offset where these fractures are interconnected.

Tectonic setting of the emplacement of Pb-Ba mineralizations in the Mibladen ore deposit
The determination of the tectonic setting responsible for Pb-Ba mineralizations emplacement in the Mibladen ore deposit is based on stress tensor calculations, paleostress reconstruction, and the definition of the regional tectonic regime of the emplacement of these mineralizations. This procedure is based on field data collected from microtectonic measurements along slickensided faults (Fig. 10a) and tension gashes (Fig. 10b-l) in the different mining sites of Mibladen. Automatic stereographic projection of more than 250 microtectonic measurements, allowed us to determine a tectonic regime characterized by a NNW-SSE to N-S trending σ1 axis and an ENE-WSW to E-W trending σ3 axis. The calculated stereoplot from reverse faults with dextral strikeslip component in the S mining site (Fig. 10a) demonstrates a pure compressive tectonic regime with a horizontal NNW-SSE trending σ1 axis (N162, 06° SS W) and a sub-vertical ENE-WSW trending σ3 axis (N55, 71° WSW). The stress ratio R = 0.5; indicating that 2 = ( 1 + 3)∕2.
In the different mining sites of Mibladen, kinematic context of the development of mineralized tension gashes is consistent with a paleostress tensor showing transtensive (Fig. 10b-k) or transpressive (Fig. 10l) tectonic regime. This tectonic regime is characterized by NNW-SSE to N-S trending σ1 axis (N168-N001) and ENE-WSW to E-W trending σ3 axis (N76-N88). The value of the stress ratio R ranges from 0 to 1, indicates that the tectonic phase is divided into several episodes with σ1-σ2 and σ2-σ3 axis permutations. These permutations, due to relative variations in principal stress magnitudes, allow the appearance of extensive and transtensive episodes during the single compressive tectonic phase (e.g., Angelier 1984;Angelier et al. 1985;Hu and Angelier 2004). These are responsible for tension gashes opening and the appearance of normal faults synchronous with reverse faults developed under the NNW-SSE compressive regime. Statistical analyses of the principal stress axes σ1, σ2, and σ3 (Fig. 11) demonstrate also that this single compressive tectonic phase is divided into several episodes with σ1-σ2 and σ2-σ3 permutations.

Age of Pb-Ba mineralization emplacement in the Mibladen ore deposit
Pb-Ba mineralizations of Mibladen ore deposit are clearly epigenetic. They occur as replacement of pre-existent carbonate rocks post-dating the lithification of the host rocks (Bouabdellah and Sangster 2016). The lack of radiometric dating for these Pb-Ba mineralizations is the cause of permanent debates between researchers. In a metallogenic synthesis for the Upper Moulouya lead district, Emberger (1965b) proposed a post-Cretaceous or Atlasic age for the epigenetic stage of Mibladen mineralizations, because the Infracenomanian conglomerate is impregnated with these Pb-Ba mineralizations. Naji (2004) assigned this epigenetic mineralization stage to two distinct events: (i) a NW-SE to N-S extensive tectonic event responsible for ore deposition within faults and karsts affecting the Mibladen Jurassic series; (ii) a post-Cretaceous event responsible for the remobilization of the strata-bound mineralizations and their impregnation in Infracenomanian conglomerates. Combining petrographic and geochemical data, field relationships and previous tectonic studies together with the comparison to the Touissit-Bou Beker district, Bouabdellah and Sangster (2016) proposed an Atlasic (Late Miocene) age of the Mibladen mineralizations.
The results of the present study show that Pb-Ba mineralizations in the Mibladen ore deposit were deposited under a NNW-SSE compressive tectonic regime responsible for the appearance of ENE-WSW to E-W reverse faults with strikeslip component. These faults are mineralized with galena and barite which crystallize within transtensive relays and pull-apart openings developed along fault planes. The tectonic compressive regime is divided into several transpressive and extensive tectonic episodes responsible for tension gashes and normal fault openings which are also mineralized with galena and barite. These episodes are due to stress axes permutations. During the Infracenomanian period (Upper Jurassic-Early Cretaceous), the Mibladen region and its peripheries underwent a major compressive and paroxysmal NNW-SSE tectonic phase with transtensive and transpressive episodes (Hinaje 2004;Yaagoub et al. 2021). It led to the folding, tilting and uplift of the Jurassic series. These series underwent intense erosion with the deposition of Infracenomanian conglomerates unconformably overlying the underlying formations (Amade 1965;Emberger 1965a, b;Felenc and Lenoble 1965;Dagallier and Macaudière 1987;Hinaje 2004;Yaagoub et al. 2021). In the Middle Atlas and the Central High Atlas, this compressive NNW-SSE phase is post-Bathonian and ante-Barremian (Hinaje 2004). Combining these available results with the results of the present work, we assign the emplacement of the epigenetic Pb-Ba mineralizations of the Mibladen deposit to this Infracenomanian NNW-SSE compressive phase. The impregnation of Infracenomanian conglomerates and sandstones by Pb-Ba mineralizations is not necessarily a condition for these mineralizations to be assigned to a post-Cretaceous period. We propose a model where these permeable detrital formations are traversed and impregnated by metalliferous fluids which replace conglomerate and sandstone cement and fill out open spaces (Fig. 12). The hypothesis of an epigenetic mineralization stage divided into two different events separated in time and space (Naji 2004) is unlikely in our opinion, because structures controlling ore deposition in the Jurassic and Infracenomanian formations have the same age and are developed under the same compressive tectonic regime with transtensive episodes but not under a pure extensive one. The Upper Miocene age of Pb-Ba mineralizations under a N-S compressive tectonic regime proposed by Bouabdellah and Sangster (2016) is also questionable; during this period (Upper Miocene), the tectonic regime is extensive and oriented NE-SW in the northwestern part of the Middle Atlas belt (Hinaje et al. 2001), it is related to the Sefrou-Tahla-Skoura-Tazarine corridor opening (Hinaje 2004). In the Atlas belt, this regime is consistent with a NE-SW trending compression during the Middle Upper-Tortonian period and with a NW-SE trending compression during the Tortonian-Messinian time (Aït Brahim et al. 2002). In the Upper Moulouya, it is consistent with a NW-SE trending compression (Morel et al. 1993;Zouine 1993). The post-Cretaceous sub-meridian compressive regime in the Atlas and the Upper Moulouya domains is rather assigned to: the Plio-Quaternary (Aït Brahim et al. 2002), the Early Quaternary (Morel et al. 1993;Zouine 1993) or the Middle-Late Quaternary (Hinaje et al. 2001;Hinaje 2004). Moreover, the hypothesis of a post-Cretaceous age of the ore deposition is contradicted by the presence of non-mineralized Cenomano-Turonian limestones directly resting on the mineralized Liassic carbonates of Jbel Argoud (Fig. 2), or on the Hercynian granitoids hosting Pb-Ba veins in the Assaka Ijdi syncline west of Mibladen (Emberger 1965a). In these regions, Cenomano-Turonian limestones are karstified and recorded post-Cretaceous tectonic events. Cenozoic faults and fractures affecting these limestones are barren. Moreover, Dagallier and Macaudière (1987) assumed a post-Dogger, but ante-Cretaceous, age for the sub-meridian compression responsible for karstic recovery and Pb-Ba mineralization redistributions in the Mibladen deposit. Jébrak et al. (1998) have not excluded the possibility that Mibladen mineralizations are related to the tectonic phase associated with the closure of the Tethyan basin starting from the Middle Jurassic time.
The MVT Pb-Ba mineralizations of the Mibladen ore deposit are thus involved a major epigenetic stage assigned to the Infracenomanian period (Upper Jurassic-Early Cretaceous) and synchronous with the long period of uplift and erosion of the Upper Moulouya region during the NNW-SSE trending compression.

Proposed model for the Pb-Ba mineralizations emplacement in the Mibladen ore deposit
The Mibladen Pb-Ba mineralizations are independent in time and space from Zeida and Aouli mineralizations. They are related to a distinct metallogenic event post-dating the Triassic rifting stage, and represent a remobilization of the Zeida-Aouli concentrations, or a more recent leaching of the same sources (Jébrak et al. 1998). The study of available data from fluid inclusions together with S and Pb isotopes of the Upper Moulouya district (Duthou et al. 1976;Jébrak et al. 1998;Bouabdellah and Sangster 2016) allowed Bouabdellah and Sangster (2016) to highlight the following conclusions about the genetic model of the epigenetic mineralization stage in the Mibladen ore deposit: (i) ore fluids were moderately hot (> 100 °C) and derived from basins, that scavenged lead and associated metals from the underlying Hercynian granitoids and clastic rocks; (ii) Domerian carbonates were the main regional aquifer for the ore brines; (iii) the mechanism of ore deposition is inconsistent with a single model: there is a mixing of two incompatible fluid reservoirs; a deep-seated, basement-equilibrated hydrothermal fluid, and a surficial formation and/or meteoric water; (iv) an uplift episode is necessary to create topographic slopes and favor metalliferous fluid circulations along deep ENE-WSW and NE-SW faults.
In the light of these conclusions and the results of the present work, we propose a model for the Pb-Ba mineralizations emplacement (Fig. 12), based on the following points: (i) during the Upper Jurassic-Early Cretaceous period, the NNW-SSE compressive regime divided into several episodes led to the Mibladen region uplift. This is due to reverse and strike-slip faults directed ENE-WSW and NE-SW, respectively, such as Amourou and Aouli Faults which bound the Pb-Ba mineralized zone of Mibladen; (ii) this uplift allowed  the erosion of the underlying series and the deposition of Infracenomanian continental succession represented by conglomerates and sandstones; (iii) the creation of topographic slopes by this tectonic uplift favored a gravity-driven flow of surface fluids towards deep zones along major tectonic faults; (iv) in their paths, these fluids cross the Liassic aquifer, leach lead accumulated in Triassic clastic rocks and Aouli veins, then they mix together with deep fluids leaching Hercynian granitoids; (v) after their mixing, these two fluids produce a metalliferous fluid loaded with Pb and Ba which migrates along major faults; (vi) finally, this fluid is deposited within karst cavities, interstratal joints, pull-apart openings developed along ENE-WSW faults, tension gashes and Infracenomanian syntectonic conglomerates in process of deposition.
These hydrothermal and mineralizing events are probably related to large-scale crustal processes triggered during the Upper Jurassic-Early Cretaceous period in the Moroccan Atlas system such as the emplacement of magmatic intrusions in the Central High Atlas and the Middle Atlas belts (e.g., Fedan 1988;Charroud 1990;Samir 1991;Beraâouz and Bonin 1993;Lhachmi et al. 2001;Zayane et al. 2002;Ibouh 2004;Frizon de Lamotte et al. 2008;Bensalah et al. 2013;Michard et al. 2013;Guezal et al. 2014). In addition to syn-metamorphic deformations, and their associated hydrothermal manifestations in the High Atlas belt during this period (Laville et al. 1991(Laville et al. , 1994Laville and Piqué 1992;Hinaje 2004).

Conclusion
Gitological, structural, and microtectonic analyses associated to paleostress reconstruction in the MVT ore deposit of Mibladen allowed us to highlight the following results: -This ore deposit involved a major mineral epigenetic stage economically more important with orebodies hosted by Jurassic carbonates and Infracenomanian continental series and occupying karst cavities, interstratal joints, fractures, and faults.
-Structural control of ore deposition is favored by two main fracture networks: (i) NNW-SSE to NNE-SSW tension gashes and normal faults; (ii) ENE-WSW to E-W trending reverse faults with strike-slip components and transtensive relay zones. -The tectonic setting of the mineralization emplacement is consistent with a NNW-SSE trending compression with extensional and transtensional episodes related to σ1-σ2 and σ2-σ3 axis permutations during the Infracenomanian period (Upper Jurassic-Early Cretaceous). -This compressive regime favored metalliferous fluid circulations along major ENE-WSW and NE-SW trending faults such as Amourou and Aouli Faults.