The epithermal alteration, affecting the Tertiary pyroclastic rocks of NW Sardinia and forming large kaolin deposits, can be attributed to the action of acid fluids circulating through faults and fractures and crossing a paleo-surface at about 200 m depth, where fluids flowed laterally in the upper part of the groundwater. The NW part of Sardinia, has been cited recently in several works (Gold mines of Sardinia, 1998; Lattanzi, 1999; Simeone & Simmons, 1999) as a suitable environment for the formation of precious metal deposits. The Romana and Tresnuraghes areas, where the largest kaolin deposits of the island outcrop, were investigated using mineralogical and stable isotope studies, for defining the environment of formation and the characteristics of the mineralizing fluids, and aimed at applications in mining exploration. Five different alteration facies were recognized. Silicization (silica veins, silicified rocks, sinter), advanced argillic (kaolinite 1T-1Md + silica minerals + or - alunite) and intermediate argillic (montmorillonite + cristobalite + kaolinite 1 Md) alterations, were recognized in both investigated areas. The argillic (illite + montmorillonite + quartz) and sericitic alterations (illite + quartz + or - potassium feldspar), only occur at Tresnuraghes. The presence of these two alterations, in association with the silicization (sinter), suggests a low-sulfidation type epithermal system for Tresnuraghes. At Romana, the hydrothermal mineral alteration, reflecting the circulation of lower pH fluids than those of Tresnuraghes, probably originated in a high-sulfidation type epithermal system. Studies on hydrothermal mineralogical associations, performed for each alteration facies, suggest: i) a steam-heated environment of formation, ii) temperatures ranging from 100 degrees to 200 degrees C, iii) mineralizing fluids circulating near to the Miocene paleo-surface, and iv) a shallow extension in depth of the alterations, which can be related to the neutralization of fluids by water-rock interaction phenomena. Estimated temperatures and stable isotope data (delta <sup>18</sup> O and delta <sup>2</sup> H) indicate that low temperature meteoric waters, mixed with steam, circulated at shallow depths and formed the kaolin mineralization. The downward decrease in the intensity and rank of alteration suggest that the movement and neutralization of the acid fluids were mainly per descensum. As acid leaching zones capping high-sulfidation and low-sulfidation systems are common in epithermal precious metal deposits around the world (Sillitoe, 1993; Simeone & Simmons, 1999), the presence of the advanced argillic alteration, in both investigated areas, suggests that further research, for recognizing possible precious metal deposits, should be undertaken. The mineralogical and geochemical features of Locchera kaolin quarry (Romana), formed by hotter and more acid fluids than the other deposits, suggest a suitable area for detailed prospecting aimed at precious metal deposits.