## 2 Material and Methods
### Geophysical and Remote Sensing Data  
#### 2.4.1 Radiometric Data  
Airborne geophysics based on gamma-ray spectrometry estimates the concentration of Potassium (K), Uranium (eU) and Thorium (eTh) at the earth's surface down to 30-45 cm [52; 53; 54; 55]. These radioelements occur in widely varying concentrations in the rocks that form the earth's crust. The Table 1 shows the concentrations of the three radioelements in the main rocks categories. The airborne gamma-ray spectrometry data used in this study were acquired in the Moroccan Anti Atlas in 1998 by the company Geoterrx-Dighem for the Ministry of Energy and Mines. The measurements were made with an average ground clearance of 60 m by the Exploration GR-830/3 spectrometer. The flight lines are oriented N315\({}^{\circ}\) and spaced at 500 m. The flight line crossings are N45\({}^{\circ}\) and 4000 m apart. Gamma-ray emissions were recorded from the ground and air over an energy range of 0 to 3 MeV with 8 downward and 2 upward crystals, respectively. Count rates were determined within three windows corresponding to natural radiogenic concentrations of Potassium (K, 1.46 MeV), Uranium (U, 1.76 MeV and Thorium (Th, 2.62 MeV). The radiometric data were recorded at a lower frequency (1 Hz) and with an average spacing of 63 m. The recorded data then underwent a series of corrections which are: (i) activity time correction; (ii) calculation of effective ground clearance at standard temperature and pressure conditions; (iii) subtraction of cosmic and helicopter noise; (iv) subtraction of radon background (assessed by upward facing detector measurements); (v) Compton effect correction and (vi) attenuation corrections. These radiometric data were provided to us in the form of digital maps. Using ArcGIS 10.3 software, these maps were first georeferenced and digitized to create a digital database (by digitizing the intersection of the iso-value curves and the flight lines). They were then interpolated using the inverse distance weighting (IDW) interpolation method to obtain maps representing the horizontal variation of radioactive concentrations of threeelements: (i) Potassium (K in %), (ii) equivalent Thorium (eTh in ppm), and (iii) equivalent Uranium (eU in ppm). Furthermore, ratios of K/eTh and K/eU were calculated to delineate Potassium enrichment zones as indicators of potential mineral resource-related alteration zones [25; 28; 56; 57; 58] (Figure 3). The image was pre-processed using the Fast Line-of-sight Atmospheric Analysis of Hypercubes (FLAASH) method [59; 60; 61]. This correction process requires the luminance image and generates a reflectance corrected image. Subsequently, we calculated the band \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Rock Type**} & \multicolumn{2}{c}{**Potassium (\%)**} & \multicolumn{2}{c}{**Uranium (ppm)**} & \multicolumn{2}{c}{**Thorium (ppm)**} \\ \cline{2-7}  & **Range** & **Mean** & **Range** & **Mean** & **Range** & **Mean** \\ \hline Acid Extrusives & 1.0-6.2 & 3.1 & 0.8–16.4 & 4.1 & 1.1–41.0 & 11.9 \\ \hline Acid Intrusives & 1.0-7.6 & 3.4 & 0.1–30.0 & 4.5 & 0.1–253.1 & 25.7 \\ \hline Intermediate Extrusives & 0.01–2.5 & 1.1 & 0.2–2.6 & 1.1 & 0.4–6.4 & 2.4 \\ \hline Intermediate Intrusives & 0.1–6.2 & 2.1 & 0.1–23.4 & 3.2 & 0.4–106.0 & 12.2 \\ \hline Basic Extrusives & 0.06–2.4 & 0.7 & 0.03-3.3 & 0.8 & 0.05–8.8 & 2.2 \\ \hline Basic Intrusives & 0.01–2.6 & 0.8 & 0.01–5.7 & 0.8 & 0.03–15.0 & 2.3 \\ \hline Ultrabasic & 0–0.8 & 0.3 & 0–1.6 & 0.3 & 0–7.5 & 1.4 \\ \hline Chemical Sedimentary Rocks & 0.02–8.4 & 0.6 & 0.03–26.7 & 3.6 & 0.03–132.0 & 14.9 \\ \hline Carbonates & 0.01–3.5 & 0.3 & 0.03–18.0 & 2 & 0.03–10.8 & 1.3 \\ \hline Detrital Sedimentary Rocks & 0.01–9.7 & 1.5 & 0.1–80.0 & 4.8 & 0.2–362.0 & 12.4 \\ \hline Metamorphosed Igneous Rocks & 0.1–6.1 & 2.5 & 0.1–148.5 & 4 & 0.1–104.2 & 14.8 \\ \hline Metamorphosed Sedimentary Rocks & 0.01–5.3 & 2.1 & 0.1–53.4 & 3 & 0.1–91.4 & 12 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Radioelement concentrations in main categories of rocks [53; 56].