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Contribution of multispectral remote sensing to mining exploration in the Rehamna Massif, Moroccan Meseta

. The Western Moroccan Meseta contains mining sites in operation for several decades and others in development. The Rehamna Massif belonging to this, is the subject of this study. The present study reveals new results on the mineralization in this massif. It is based on the synergy of field investigation data and ASTER image analysis (L1T) covering this massif. Through spectral processing, namely the calculation of the band ratios, ACP and MNF, applied to the nine VNIR and SWIR bands of this image, it was possible to reveal the distribution of hydrothermal alteration minerals in the study area. The conjuncture of these data allowed us to select geological targets likely to be of potential mining interest in the massif .


Introduction
Except the phosphate deposits on its borders (Cenozoic cover), the Hercynian Rehamna Massif does not contain lot of current mining activity, unlike the other massifs of the Western Meseta, although they have a common geological history. Admittedly, the outcrops in this massif do not lend themselves well to direct observation; fact that prompted us to use other methods and techniques of investigation to get geo-mining information in this site.
Our study is based on the zones of hydrothermal alteration mapping by the identification of the minerals resulting from this phenomenon, namely kaolinite, chlorite, allunite and epidote using the opportunities offered by the multispectral satellite imagery. Several studies have shown the importance of mapping hydrothermal alteration minerals in mining exploration and identification of mining deposits [1][2][3].
The images generated by the multi and hyperspectral sensors have shown great utility in geological and mineral mapping, and therefore offer a decision support tool upstream of any mining exploration project. The use of these images in the detection of mining sites has become fruitful after the launch of the ETM + (Enhanced Themaic Mapper plus) and ASTER (Advanced Spectral Thermal Emission and Reflection Radiometer) satellites in 1999 [1,4,5].

Geological setting
The Rehamna Massif is located in the center of the Western Moroccan Meseta, in the middle position between the Moroccan Central Massif to the north and the Jebilet domain to the south. It is a segment of the Hercynian belt, containing Paleozoic formations, intruded by late hercynian granitic bodies, and covered, in part, by discordant, Meso-Cenozoic and Quaternary formations [6 -9]. In the Rehamna, the authors distinguish two parts: the North Rehamna or the Mechraa Ben Abbou basin and the Southern Rehamna or Rehamna s.str [6,7]. The Rehamna s.str, object of this study, is subdivided into three structural zones: i) The Western Rehamna, part belonging to the coastal bloc, the least deformed zone of the Moroccan Meseta. This zone consists of Combro-Ordovician terrains limited to the east by the NNE-SSW median fault [10]. ii) The Central Rehamna, a narrow NE-SW band, between the Oulad Zednes shear zone in the east and the median fault in the west. This part of the Rehamna contains orthogneisses [8,11,12] dated from the Upper Precambrian [13] covered by Cambrian arkoses, limestones and cipolin, surmounted by the metaconglomerate of kef Elmounib [14] attributed to the lower Devonian, followed by Devonian terrains of Skhour containing phyllites and quartzites [12,15]. iii) Eastern Rehamna, zone limited to the west by the Oulad Zednes fault, and represented by two metamorphic units: lower unit of Lalla Tittaf and upper unit of Ouled Hassine. The first consists of micaschists, marbles and quartzites is attributed to probable Devonian [6,12,16], however, the age of Lalla Tittaf formation is controversial. It is constituted of micaschists with amphibolites, metagabbros, rare acid meta-tuffs and some levels of marbles [17] is attributed to the Lower Carboniferous (Viséen-Namurian) [6,17]. However, zircon dating attributes this formation to Paleoproterozoic [12,13].

Ore deposit in the Rehamna massif
The Rehamna Massif, zone of the province with perbatholitic mineralization in the Hercynian domain, has been involved in mineral exploration for several decades [18,19]. It can be traced back to the Roman era [12]. This works led to mining Sn, W, Be, Cu and Mo [20]. Many abandoned mines since the 1970s are known, mainly belonging to the Devonian unit of the Oulad Hassine of the East Rehamna: -the Oulad Hassine Plomb-Zinc mine; -the Raichet Wolfram mine ; and -Ouled Salah Barytine and Plomb mine. We also know several mining indices, namely Uranium, Beryl, Copper and Barytine. These Hercynian late-Hercynian mineralizations, generally vein-type, of pneumatolytic and hydrothermal origin, are essentially related to magmatic intrusions [21]. [22] Used, for the first time in the Rehamna, the remote sensing technique by means of Landsat MSS image for the prospection of tungsten mineralized leucogranitic apexes, by using of phenomenon of "transparency" of the cover metamorphic.
Until now, all data indicate that the mineralization discovered in the Rehamna Massif is essentially related to granitic intrusions, and is concentrated east of the Oulad Zednes Accident.

Data and method
In order to conduct the geological investigations, we relied on the ASTER satellite image data. The reputation of these data in the mining targeting and exploration as well as the sensitivity of certain bands to alteration minerals justify our use of it. In the present study, the ASTER level L1T image (AST_L1T_00303012007112752_6821), (Path 202 -Row 38), acquired on March 01 2007, is used. This data orthorectified and projected in the WGM-1984 system, obtained on the site of the United States Geological Survey (USGS). It composed of three bands in the near-infrared visible (VNIR), six bands in the short-wave infrared (SWIR) and five bands in the thermal-infrared (TIR) ( Table 1).
The analysis of the image included three spectral methods applied to the nine VNIR and SWIR bands: first, band-ratio calculations to identify hydrothermal alteration minerals from possible zones with mining potential, then principal component analysis (PCA) and a minimum noise fraction transformation (MNF) were applied on the same bands to highlight selected areas.

Bands ratios
Band ratios are used in geology to discriminate types of rocks or minerals. The principle is to execute arithmetic operations on the digital number (DN) of each pixel of the bands considered in ratio. Several band ratios have been developed to highlight certain minerals or groups of minerals. Fig. 4 shows that the iron oxides (hematite, geothite) are detectable by bands 1 and 3, the Al-OH group of clay minerals by bands 5 and 6, band 7 detects Fe-OH (jarosite, muscovite) and band 8 detects Mg-OH (chlorite, epidote, carbonates) [23]. In our case study, we used the ratios of [24]; (Band 4 + Band 6) / Band 5 for the argillic zone (Allunite, kaolinite, Pyrophylite), (Band 7 + Band 9) / Band 8 for the prophylactic zone (Carbonate, Chlorite, Epidote) and (Band 5 / Band 3 + Band 1 / Band 2) for the oxidation zone (Fe2 +), as well as the ratio (Band 5 + Band 7) / Band 6 for the phyllic zone (Sericite, Muscovite, Illite, Smectite) [25] ). The results of this methode are shown in Figure 5.  We also tested the band ratios proposed by [26]: OHI, KLI, ALI and CLI index; with: Where OHI is the mineral alteration index, KLI is the kaolinite index, ALI is the allunite index, and CLI is the index of calcite. Figure 6 shows the results of these ratios.

PCA technique
Principal Component Analysis (PCA) is a multivariate statistical decomposition that consists in transforming correlated variables into new variables uncorrelated [27], with concentration of most of the spectral information in the first components. This technique is frequently used in alterations mapping field related to metallogenic provinces [28][29][30].
We applied this technique to our data using covariance matrix on all nine bands. Figure 8 shown the resulting components. The image eigenvalues obtained from PCA are indicated in figure 9. The first three principal components (PC1, PC2, PC3) contain more than 90% of the spectral information, the last 3 components contain useless information.    From this MNF transformation (figure 13), we notice a good discrimination of the three structural zones of the Rehamna massif and fault zones. The areas of interest appear in a blue green color (tiffany blue). Nevertheless, this method did not lead to the desired result.

Field work and results
Our field investigations allowed us to discover new mining occurrences at the scale of the study area ( Figure 14 and Figure 15). Several indices sampled in the field are in agreement with the areas selected by image processing.

Conclusion
The synergy of both remote sensing and field data in the Rehamna Massif allowed us to draw the following conclusions. Field investigations revealed the existence of sulphide mineralization. The minerals we were able to collect (pyrite, chalcopyrite, goethite) are related to the magmatic bodies. The position of the field indices is confirmed by the existence of the mining indices polygons obtained by image data processing. The method of processing supported by VNIR and SWIR band ratios, in the Rehamna Massif reveals a potential mining capital in relation to the indices thus demonstrated. The ACP, and MNF treatments report results converge to almost the same areas. The band ratio method is very efficiency rather than other methods uses in this study.
The localization of the zones with mining occurences shows an association with a structural component that are the faults. It also shows an association with the lithologic component mainly in granite margins and the basic vein magmatic bodies.
This study can be considered as a support and orientation for exploration and mining research in this Rehamna area. It can be fortified by other geophysical methods.