Comment on “Geological mapping and mining prospecting in the Aouli inlier (Eastern Meseta, Morocco) based on remote sensing and geographic information systems (GIS)”by Benaissi et al. (China Geology, 5 (2022))

Driss Yaagoub, Saïd Hinaje

Intelligent Systems, Georesources & Renewable Energies Laboratory, Sidi Mohammed Ben Abdellah University, Faculty of Sciences and Techniques, B.P. 2202, Route d’Imouzzer, Fez,Morocco

1. Introduction

The Aouli inlier, located at the Western extremity of the Eastern Moroccan Meseta is one of the most important metallogenic provinces of Morocco. It is known for the striking diversity of its geological formations and structures,as well as its numerous ore deposits, and its various industrial rocks and minerals (Emberger AN, 1965a; Margoum DA et al., 2015; Yaagoub DR et al., 2021a, with references therein).For this reason, this inlier has been the subject of numerous studies in the various disciplines of Earth Sciences, since the beginning of the 20th century until today.

In a newly published paper, entitled “Geological mapping and mining prospecting in the Aouli inlier (Eastern Meseta,Morocco) based on remote sensing and geographic information systems (GIS)”, Benaissi LA et al. (2022) present the results of some remotely sensed and GIS methods used to map geological formations, lineaments and hydrothermal alteration minerals in the Aouli inlier. Consequently, the authors propose a new updated geological map of the Aouli inlier, lineament map, mineral alteration maps and the location of potential zones for an eventual tactical mining exploration. Moreover, the authors attempt, in some sections of the manuscript, to discuss the structural control of hydrothermal mineralization and the role of Late Hercynian brittle deformation in the emplacement of the major ore bodies of the region.

In the present contribution, we would like to discuss some points that have not been considered by authors to help the reader better constrain the complexity of the geology and the mineralization vein swarms of the Aouli inlier. For this, we focus our discussion-comments on the following sections as presented in the concerned paper: (i) the geological and metallogenic setting of the Aouli inlier; (ii) the updated geological map of the Aouli inlier; (iii) the mapping of lineaments and hydrothermal alteration minerals in the Aouli inlier and (iv) the structural control of hydrothermal mineralization in the Aouli inlier.

2. Geological and metallogenic setting of the Aouli inlier

The bibliographic synthesis of Benaissi LA et al. (2022)about the geological, tectonic and metallogenic setting of the Aouli inlier appears poorly constructed. This is reflected in the missing description of facies, especially of the Meso-Cenozoic cover, and their precise ages (series and stages)which are widely established in previous works (Fig. 1). In the tectonic setting, the authors have only discussed the Variscan and Late Variscan deformation phases, totally ignoring the later phases related to the Triassic rifting stage and the Atlas Orogeny, which have direct impacts on the structuring of the Aouli inlier and the emplacement of its ore deposits. For the metallogenic setting, the authors did not mention that the Aouli vein-type mineralizations are also hosted by the Triassic formations, and that the Mibladen MVT-type mineralizations are also hosted by the Dogger and Infracenomanian series (Yaagoub DR et al., 2021a, 2021b,with references therein). All of these missing points are due to the lack of several bibliographic references that were not consulted by the authors and are not mentioned in the list of references (all the references about the Upper Moulouya massif cited in Yaagoub DR et al., 2021a, 2021b, 2021c).

Fig. 1. Updated synthetic lithostratigraphic column of the Aouli inlier and its Meso-Cenozoic cover.

3. The updated geological map of the Aouli inlier

The authors attempted to produce an updated geological map for the Aouli inlier (Fig. 5 in their published paper).However, several points need discussion in this map, both for outcrops of the Paleozoic basement and those of the Meso-Cenozoic cover.

3.1. Paleozoic basement

In their updated geological map, the authors assigned to El Hassir granitic apophyse (the northeastern part of the geological map) a single granodiorite facies. Indeed, this massif includes a large variety of magmatic facies known in previous works and which were completely ignored by the authors. These facies are: gabbro-diorites, quartz diorites,granodiorites, quartz monzodiorites and quartz monzonites(Dahire MO, 2004). In the Hassan II Lake area, West-Northwest of Sidi Saïd, the authors reported the existence of a migmatite massif (point: X/4.79750°W, Y/32.7800°N) based,most likely, on the article by Elabouyi MU et al. (2019). In this article (Fig. 4a), the migmatites outcrop only as dispersed masses occupying very small areas within this massif. The latter is mainly dominated by cordierite-bearing granites,mingling granite zones, in addition to small microgranite dykes. The authors also note the lack of contact metamorphic zones in the updated geological map, which are established in previous works. These zones are : biotite zone, cordierite zone, andalusite zone and sillimanite zone (Elabouyi MU et al., 2019). Moreover, the authors did not succeed in removing or discussing contradictions between the geological map at 1∶125000 (Emberger AN, 1965a) and that at 1∶50000(Raddi YO et al., 2013) for the Paleozoic basement of the Aouli inlier (the shape of El Hassir apophysis and its outcropping area, the presence of hornfels around the twomica granites, the existence of a granodiorite stock South of Souk El Hajar area (point: X/4.68259°W, Y/32.84401°N), the amphibolite outcrops, ···). The authors attributed “Aouli Granites” to only one part of the granitic massif corresponding, in the literature, to the grey biotite granites (or calc-alkaline granites). Indeed, the “Aouli Granites”corresponds to all granitic facies outcropping in the Aouli inlier (Elabouyi MU et al., 2019, with references therein).Finally, the authors did not explain the elimination of two biotite schist massifs outcropping near the Aouli area from the geological map at 1∶50000 (Raddi YO et al., 2013) (points:X/4.58000°W, Y/32.8000°N and X/32.58000°W,Y/32.82000°N).

3.2. Meso-Cenozoic cover

The Meso-Cenozoic cover represented in the updated geological map of Aouli appears to be very simplified with many details missing as compared to the available geological maps and recently published works (Yaagoub DR et al.,2021a, 2021b, 2021c). Moreover, the geological contours of the Meso-Cenozoic formations on the available geological maps are not respected by the authors.

The upper part of the Triassic series is not represented by micro-conglomerate, red clay with sandstone and dolomite levels as mentioned by the authors. The last Triassic term in the Aouli inlier is represented by tholeiitic basalts with ages ranging up to Senumerian (ca. 210-196 Ma;40Ar/39Ar on plagioclase; Fiechtner L et al., 1992). Indeed, the sedimentary series assigned by the authors to the end of the Triassic period corresponds to an Infraliassic suprabasaltic continental formation outcropping in the eastern bank of Moulouya River between Aouli and the Akebab plateau areas. The Jurassic formations are not well subdivided and the series and stages are not represented in the updated geological map although this is well established in previous works not included in the reference list of the authors. In fact, the Early and Middle Liassic formations are characterized by shallow platform dolostone, limestone and dolomitic limestone interbedded with thin marl levels (Yaagoub DR et al., 2021b, 2021c). The Upper Liassic series are represented by Early Toarcian red marls overlain by Middle-Upper Toarcian thin limestones benches (Yaagoub DR et al., 2021c). The Dogger succession is mainly consisting of Aalenian bioclastic limestones with belemnites and Bajocian greenish marls with interbedded carbonate beds (Yaagoub et al., 2021c). Moreover, the authors assigned the Infracenomanian continental series and the Cenomanian-Turonian carbonates to the “Cretaceous”, which contradicts the subdivisions established in previous works as well as the position of these outcrops (geological contours) in the available geological maps (Yaagoub DR et al., 2021b,2021c). Thus, the Infracenomanian period in the Aouli inlier is characterized by continental sedimentation with three members: (1) an undated lower fluvio-lacustrine member;(2) an intermediate azoic member corresponding to dolomitic and evaporitic Sebkha; and (3) an upper member represented by marls with gypsum and marl-limestones. The upper part of these marls is assigned to the Upper Cenomanian, then, the Mesozoic series ends with lumachellic limestones deposited during a generalized Cenomanian-Turonian marine transgression over the entire Aouli inlier (Yaagoub DR et al.,2021c, with references therein). The Cenozoic series are also poorly represented in the updated geologic map of the Aouli inlier by Benaissi LA et al. (2022). These series are well subdivided and their ages are established in the available geological maps (Raddi et al., 2013; Yaagoub DR et al., 2022,with references therein). They start with Oligo-Miocene molasses, puddingstones, sandstones and marls surmounted by Mio-Pliocene marls and lacustrine limestones. The series ends with Quaternary fluvial terrace conglomerates, marly limestones, siltstones, claystones and alluvium interbedded by volcanic cinders, pyroclastic breccia and lava.

4. The mapping of lineaments and hydrothermal alteration minerals in the Aouli inlier

To produce the lineament map (geological fracturing) of the Aouli inlier, Benaissi LA et al. (2022) first automatically extracted the lineaments. Then, the authors compared these“automatic” lineaments with the topographic map of Midelt at 1∶200000 (Emberger A, 1965b in their paper) to make adjustments and to rule out lineaments that coincide with roads, streams, cliffs, and those that also coincide with the boundaries of lithological formations. The result is a lineament map containing only the geological lineaments and in particular those which correspond to geological fractures or veins. However, when we refer to the article by Emberger AN(1965b) (cited by the authors), we note the lack of such a topographic map of Midelt at 1∶200000 in this article, which questions the validity of the lineaments map obtained by the authors. Moreover, and to our knowledge, there is no topographic map of Midelt at 1∶200000. Actually, the Midelt area and Aouli inlier are part of two geological maps at 1∶200000 carried out on topographic sheets at the same scale: (1) the geological map at 1∶200000 of the Northern Middle Atlas (Termier HE and Dubar GO, 1940) and (2) the geological map at 1/200000 of the High Atlas of Midelt(Dubar GO, 1939).

The obtaining of such a lineament map by Benaissi LA et al. (2022) led to speculative interpretations, especially when superimposing this map with the mineral alteration map. This is expressed in the location of new favorable areas for possible mining exploration such as the NNE-SSW shear zone of Ansegmir and the NW-SE corridor of Assaka Ijdi-Tijouit. Indeed, the Ansegmir Pb-Zn ± F ± Ba vein system in the Aouli inlier is characterized by ENE-WSW to E-W mineralized veins not NNE-SSW veins (Emberger AN,1965a; Yaagoub DR et al., 2021a). The newly NW-SE corridor of Assaka Ijdi-Tijouit has never been mentioned in previous works and the authors did not discuss this point in any further detail. In our opinion, these two areas correspond only to the hydrographic lines of Ansegmir River (or the barren NNE-SSW Ansegmir Fault Zone) and Moulouya River (between Assaka Ijdi and Tijouit areas), which is very consistent with the topographic maps and satellite images.

5. Structural control of hydrothermal mineralization in the Aouli inlier

According to Benaissi LA et al. (2022), the mineralized structures of the Aouli ore deposit show an orientation ranging from N-S to ESE-WNW, and the Late Hercynian dextral and sinistral faults have contributed to the opening of the extensional structures that represent the major ore bodies of the region. These two points are not accurate and are not in agreement with what was proposed in previous studies for Aouli ore deposit. Indeed, structural analysis of the Aouli deposit show that mineralizations are mainly controlled by three fracture sets: (1) NNE-SSW to NE-SW system; (2)ENE-WSW to E-W system and (3) WNW-ESE system(Yaagoub DR et al., 2021a). The N-S mineralized vein swarms are not mentioned in the Aouli inlier. Moreover, the Aouli ore bodies are developed under an extensional tectonic regime consistent with a NW-SE to NNW-SSE sub-horizontal σ3 axis. This extensive tectonic regime, related to the Atlas rifting stage during the Upper Triassic-Early Liassic period,allowed the development of vein structures along NNE-SSW to E-W trending normal faults and tension gashes, as well as WNW-ESE trending normal faults with strike-slip component(Yaagoub DR et al., 2021a). The Late Hercynian phase is characterized by a pure strike-slip tectonic regime with submeridian shortening responsible for NE-SW sinistral strikeslip faults and NW-SW dextral strike-slip faults, which cannot give rise to extensional structures (normal faults, tension gashes, …) as mentioned by the authors. Moreover, some segments of the Aouli vein swarms are hosted by Triassic-Infraliassic series (Emberger AN, 1965b; Yaagoub DR et al.,2021a), which post-dates the Late Hercynian tectonic events.

6. Conclusions

In this paper we discussed and commented some sections of the newly published article by Benaissi et al. (China Geology, 5 (2022)). We focused our discussion-comments on the following sections as presented in the concerned article:

(i) The geological and metallogenic setting of the Aouli inlier where we clarified some points poorly constructed by Benaissi LA et al. (2022) because of the lack of several bibliographic references that were not consulted by the authors.

(ii) The updated geological map of the Aouli inlier proposed by the authors where we clarified several points concerning the outcroppings of the Paleozoic and Meso-Cenozoic formations, their precise ages and their positions on the geological map.

(iii) The mapping of lineaments and hydrothermal alteration minerals in the Aouli inlier where we discussed the validity of the obtained lineament map, as well as the location of new favorable areas for possible mining exploration as proposed by the authors.

(iv) The structural control of hydrothermal mineralization in the Aouli inlier where we have re-emphasized the main fracture systems responsible for the mineralization emplacement, the associated tectonic regime and the relative age of these mineralization, according to previous works totally ignored by the authors.

?

?