## 6 Discussion
For each endmember, we calculated the percentage of its participation (in terms of number of pixels) within each one of the general lithologic types present in the study area. In order to facilitate the following discussion, we grouped the 11 LSUs present within the study area (described in Section 2) into five general lithologic types, namely, (1) acidic to intermediary lava flows and breccias (LSUs 2, 3, 4, and 5), (2) basic to intermediary lava flows (LSUs 6, 7, 8, and 9), (3) lacustrine and debris flows (LSU1), (4) alluvial/beach deposits (LSU10), and (5) scree deposits (LSU11). Then, for each endmember, we counted the number of pixels with positive abundances within each one of the five aforementioned lithologic types and we computed the corresponding percentage over the total surface. These percentages are presented in Table 4.
3. For each one of the 11 LSUs, we calculated the percentage of surface containing pixels with \"high\" to \"very high\" hydroxyl-bearing alteration and sulfur abundance. To this end, we first quantized each of the hydroxyl-bearing alteration and the sulfur abundance maps into four groups. Specifically, we divided the range of values of the pixels of each map into four equally sized intervals and we counted the number of pixels lying in each one of them (according to their abundance value). Those that were in the lower-valued interval (they exhibited low abundance values) were labeled as \"low abundance\" pixels and those that were in one of the next three intervals were labeled as \"intermediate abundance\", \"high abundance\", and \"very high abundance\" pixels. Then, we counted the number of pixels showing high and very high abundance of hydroxyl-bearing alteration and sulfur inside each LSU and we calculated the corresponding percentages over the total number of pixels of each LSU. The results are presented in Table 5.  
The discussion is organized in the following four sections that concern: (1) the comparison of the results obtained by SU with those obtained by the use of the alteration SI (NHASI) in terms of accuracy using the reference map, (2) the area of each primary lithology that was affected by the hydrothermal alteration (LSUs as depicted in the geological map and grouped in the aforementioned five lithologic types) as well as the area affected by strong alteration within each specific LSU, (3) the comparison of the results obtained by the three sensors, and (4) various aspects of the proposed processing methodology. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **NHASI Low** & \begin{tabular}{c} **NHASI** \\ **Intermediary** \\ \end{tabular} & \begin{tabular}{c} **NHASI** \\ **High** \\ \end{tabular} & \begin{tabular}{c} **SU** \\ **Low** \\ \end{tabular} & \
\begin{tabular}{c} **SU** \\ **Intermediary** \\ \end{tabular} &
\begin{tabular}{c} **SU High** \\ \end{tabular} \\ \hline ASTER & & & & & & \\ \hline weak alteration & 165 & 212 & 69 & 233 & 159 & 44 \\ \hline middle alteration & 131 & 291 & 195 & 56 & 356 & 188 \\ \hline strong alteration & 36 & 114 & 182 & 19 & 85 & 204 \\ \hline OA & & 45.8\% & & & 59\% & \\ \hline Landsat-8/OLI & & & & & & \\ \hline weak alteration & 204 & 181 & 60 & 206 & 188 & 49 \\ \hline middle alteration & 91 & 317 & 209 & 47 & 337 & 216 \\ \hline strong alteration & 18 & 119 & 176 & 17 & 75 & 178 \\ \hline OA & & 50.7\% & & & 54.9\% & \\ \hline Sentinel-2 MSI & & & & & & \\ \hline weak alteration & 195 & 181 & 68 & 239 & 162 & 32 \\ \hline middle alteration & 92 & 317 & 203 & 39 & 368 & 203 \\ \hline strong alteration & 25 & 114 & 173 & 9 & 80 & 198 \\ \hline OA & & 50\% & & & 60.5\% & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Confusion matrices between the three alteration degree classes of the reference map (source [68]) (in lines) and the three categories of the quantized NHASI values and hydroxyl-bearing alteration abundances (issued from SU) (in columns). The overall accuracy is also shown in percentage. OA: overall accuracy.