Located at 4500m a.s.l in Andean cordillera (Northern Chile), Cerro Pabellón (CP) geothermal powerplant is the first commercial power plant in South America that exploits the Na-K-Cl fluid circulating in the fractured volcanic formations. This study focuses on hydrothermal alteration of the geothermal reservoir based on four production wells drilled on both sides of a graben structure and presenting unlike hydraulic properties. Cuttings were sampled and analyzed using X-ray diffraction (XRD) to observe the mineral distribution and variations of crystal-chemistry of clays (fraction <5 μm). Then, scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDX) were used to perform chemical microanalysis of hydrothermal minerals. These results suggest a high heterogeneity of the reservoir in term of alteration: a pervasive alteration dominated by trioctahedral clays (saponite to chlorite sequence through corrensite and C-Crr mixed layers) was observed in all samples whereas a multi-event fracture-controlled alteration was only observed locally. A first event was observed at shallow depth in the active part of the reservoir composed of adularia + Ba-rich feldspar + feathery quartz + chalcedony + calcium arsenates + illite suggesting boiling processes. Another event dominated by dioctahedral clays (illite-smectite mixed layers with more than 90% of illite and illite) occurs in all permeable fracture zones intersected by wells. This multi-event alteration seems strongly controlled by the eastern graben fault and the wide fracture network connected to this structure.