New Targets of Potential Mining Interest Using Gravimetric and Satellite Data: Case Study of Hercynian Rehamna Massif, Morocco

Abstract


Introduction
The Rehamna Massif is made of Paleozoic terrains resting on a Precambrian base and covered by discordant Meso-Cenozoic sediments.Structured by a polyphase Hercynian deformation, this Massif is intruded by different acidic and basic magmatic bodies.These conditions make this area favourable for the development of mineralization, therefore, mining research.
Geophysics methods are among forceful indirect investigation techniques.They are of improvement contribution to different earth sciences disciplines, among this mining prospecting.Physical characteristics of researched target require adequate exploration process.Many studies demonstrate that discovered mineralization in the Rehamna Massif is depending on late orogenic leucogranitic bodies (Jenny 1974;Marconnet et al., 1987;El Mahi et al., 2000).Ore deposit exploited is commonly vein -type; hence the usefulness of detecting fractured zones.The purpose of our investigation is to study the relationship between mineral occurrences and geophysical signature of known and presumed granitic bodies using gravimetric data, and determining geological lineaments of the study region via satellite image filtering, to highlight potentially mineralized areas.

Geology of the Study Area
The Western Moroccan Meseta is a segment of Moroccan Variscan belt, on the north of West African Craton, containing massifs of Paleozoic age.The Rehamna Massif, belonging to this area, is located between the Jebilet Massif in the south and the Moroccan Central Massif to the north (Fig. 1).It is made up of Paleozoic age terrains, eroded and peneplanated, structured by the Variscan orogeny and based on Paleoproterozoic (Cocherie, 2001;Beaudin et al., 2003, Perira et al., 2015) and Neoproterozoic basement (Corsini, 1988;).These lands are covered at the borders of this Massif by tabular formations of the Meso-Cenozoic.(Michard, 1982, El Mahi et al., 2000) modestly deformed (Guezou and Michard, 1976).This zone is bounded to the east by the NNE-SSW Median Fault (Piqué et al., 1982).The deformation gradient increases from west to east, and it's accompanied by anchi to epizonal metamorphism (El Mahi et al., 2000).• The Eastern Rehamna, bounded to the west by the Oulad Zednes Fault.It consiste essentially of Lalla Tittaf and Ouled Hassine lower metamorphic Units.The age of the first one is controversial; it is attributed to the Viséen-Namurian by facies analogy with the Sarhlef Shale Formation of Central Jebilet (Huvelin, 1977;Michard, 1982).However, Baudin et al. 2003, suggest a Paleoproterozoic age after dating a metagabbro at 2136 ± 17 Ma by U / Pb on zircons (Cocherie, 2001).According to Michard et al. (2010), this dating corresponds to zircons from the underlying Precambrian crust.Whereas, Devonian age is proposed for Oulad Hassine metapelit unit (Michard, 1982, Wernert et al., 2016).This part of Rehamna is the site of most leucogranitic apexes and old mines.
• The outcropping of leucogranitic apexes is responsible for fracturing the surrounding terrains.
Residual hydrothermal fluids of magmatic differentiation process, occasionally carrying mineralization and fill fractures.Leucogranites prospecting is a research metallotect of tin and tungsten (Marconnet et al., 1987).The following table gives an inventory of the Rehamna massif mineralization:

WGM Data
To implement our study, Bouger gravity anomaly of World Gravity Map (WGM), release 1.0 (2012) has been used, deliverable by the BGI (Bureau Gravimétrique International) (Bonvalot et al., 2012).It's derived from Earth Geopotential Model (EGM2008) (Pavlis et al., 2008) and ETOPO1 Global Relief Model (Amante et al., 2009).This grid is provided with 1'x1' resolution.Bouguer anomaly map has been developed based on a crustal density of 2.67g/cm3.Mining prospecting and structural geological studies are among of several applications using WGM data.Following WGM map preparation, Bouger Anomaly (BA) was gridded with minimum curvature algorithm (Fig. 3).We used BA support to achieve various treatments, in order to get structures anomalies of studied area .The BA is the sum of the Regional Anomaly (RgA) and the Residual one (RsA).The RsA is obtained by removing RgA assimilated to a polynomial surface of first order.The regional field expresses deeper anomalies associated to long wavelengths, while the residual one expresses high frequencies related to shallow anomalies.Data filtering is used extensively to isolate and enhance anomaly features of interest.The main purpose of the filters used is to highlight shallow structures.The first filter applied is the Horizontal Gradient (HG) (Cordell, 1979;Blakely & Simpson, 1986).It represents a forceful tool for mapping contacts of density contrast and shallow features (Dentith and Mudge 2014).This method is appropriate even with noised gravity data (Phillips 1998).HG is obtained by deriving residual gravity from x , and from y .The total horizontal gradient (THG) is calculated by using pythagora's theorem as seen in Fig. 4 and equations 1 and 2. , The second process applied to emphasise gravimetric data is the Vertical Derivative (VDR).This filter wich is defined by , represents the first derivative of potential gravity according to Z direction.This method enhances the shallowest geologic sources and suppresses deeper sources.Contrasting to horizontal gradient, the vertical one delineates anomalies irrespective of source orientation (Dentith and Mudge, 2014).
The third filter used is Tilt Derivative (TDR).It's a powerful notion proposed by Miller and Singh (1994) for location of potential field sources and mining targets research (Verduzco at al., 2004).This operator is defined as the Arctan value of the ratio of the vertical potential field derivative (VDR) to its horizontal derivative (THG), as indicated in equations 3 and 4 (Miller and Singh, 1994).

Remote Sensing Data
Satellite data is actually a powerful tool for geological mapping and structural studies.It provide a excellent support for mineral exploration through hydrothermal alteration mapping and faults detection.To enhance geological lineaments, several manual, semi-automatic, and automatic techniques were developed (Gannouni and Gabtni, 2015;Bonetto et al., 2017, Enoh et al., 2021;Ahmadi and Pekkan, 2021).
In order to get structural lineaments of study area, band 6 of spacecraft Landsat_8 has used (ID: LC82020382016150LGN01, sensor OLI_TIRS, Path/row 202/38) acquired on 29 May 2016.Data preparation consists of extracting the interest region (Fig. 5), then, a directional filter has been applied.This convolution filter is a first derivative edge enhancement (Haralick et al., 1987)

Regional and residual anomaly
From the RgA map and the RsA one (Fig. 6a, 6b), we notice that all anomalies are generally oriented NE-SW.This orientation is in accordance with the Hercynian structuring.The intensities of BA are ranging from +70 mGal to 140 mGal, and gradually decrease from NW to SE.

Horizontal gradient
We proceeded to a horizontal derivative filters to emphasize changes of density in the horizontal gradient according to both directions X and Y.This process gives more details and discriminate several shapes of anomalies as seen in Fig. 7.The analysis of THG map show four main low anomalies: the first one at NW related to Doukkala basin, the second in the center includes the most of Permian leucogranites apexes, indicate the mass deficiency of the granite.The third anomaly in the Southwest of Bengurir could indicate Moulay Karkour granite, the fourth at the SE designate Cretaceous and Eocene formations of phosphates.High anomalies express perhaps the uprising of basement.We notice also a tendency of forms to be E-W, mainly in Eastern Rehamna.

Vertical derivative (VDR)
VDR is used to better discriminate the anomalies of high frequencies.From the analysis of the VDR (Fig. 8), we distinguish several shapes of anomalies with differences amplitude and direction.Two types of negative anomalies stand out: • A global negative anomaly that begins with the SB Granite, and continues eastward to subdivided of into two branches towards the Oulad Zednes shear zone: the first branch continues eastward; beyond RA granite, and the second extends northward (Kef el Mouneb and Dar Khalifa Brik).This anomaly brings together the Granite of SB and the leucogranites of RA, SBH and BG which indicates that these granitic bodies could have a genetic relationship.The Koudiat Rmel granite escapes this distinction.• The negative anomaly related to the RA Granite, continues eastward beyond the El Bandira Mine in the Sidi Ben Azzouz region.El Bandira and Rhaichet mines are located in the borders of this anomaly.• Restricted anomalies: case of the negative anomaly linked to the Hassine Mine which extends to the east of El Menaat near of Guelb Boualla.This anomaly is most likely related to presence of a leucogranitic apex.This result confirms the hypothesis of Marconnet et al., 1987.Two other negative anomalies, the first one located in SE of Skhour Rehamna, extending between Jorf el Beîda and Cherfa.A Pb index is bordering west of this anomaly.The second anomaly at Kef el Mounib area.Michard, (1968a) predict the presence of a thermal front in the South of Skhour and at Kef el Mounib (Piqué, 1972), which confirms the occurrence of leucogranitic apexes in these sites of low density.• On the other hand, positive anomalies are generally related to uprising of basement and metamorphic units.

Tilt derivative
The analysis of tilt derivative map (TDR) shows that all anomalies of first vertical derivative are generally conserved; with a better delineation of sources edges (Fig. 9).Values of TDR angle are ranging from to radians as a result of Arctan trigonometric role.It discriminate areas of known and presumed granites and basement as described above.TDR show furthermore the relationship between SB granite and leucogranitic apexes.

Landsat Data Processing
The directional filters results of Landsat data are summarized in Fig. 10.From the analysis of the synthetic image (e), we can distinguish four main family directions, which explain the polyphase tectonic style of Rehamna massif: N-S to NNE-SSW at Western Rehamna, NNE-SSW to NE-SW at Central zone, NE-SW to E-W (rarely NNW-SSE) at Eastern Rehamna.Similar directions was obtained using aeromagnetic and gravimetric data (Benyas et al., 2021;Benyas et al., 2022).

Fig. 1 .
Fig.1.Situation of Rehamna Massif in Moroccan mesetian domain (Northern Provinces of Morocco, extracted from the geological map 1000000 e )

Fig. 5 .
Fig. 5. Band 6 of Landsat image covering area of study

Fig. 11 Fig. 11 .
Fig.11includes lineaments, mineral occurrences and old mines surimposed to VDR map, illustrate the relationship between these elements and anomalies of existent and presumed leucogranite apexes in the study area.As described above (negative anomalies), and the linaments results, we can select four main targets of potential mining interest related to likely leucogranite apexes (Fig.11).

Table 1 .
Geological setting of the mineralized area in the Rehamna Massif