Widespread and intense zones of hydrothermal alteration are present in the Cretaceous Saplica volcanics of NW Turkey. ropylitic, phyllitic (quartz-sericite, pyrite-sericite) and argillic types of alteration, along with hematite, silica polymorphs and two different types of tourmaline mineralization, have developed as products of these intense alteration processes. In addition, many springs have been detected in the vicinity of the alteration zones. Moderately warm, generally acidic, sulfate-dominated and meteoric in origin, the highly conductive and oxidizing spring waters of Saplica (Giresun,Turkey) were sampled at different times over the past 2 years, especially for chemical and isotopic analyses. Molalities, activities, activity coefficients and log activity ratios of ionic species and mineral saturation indices have been calculated for the sampled spring waters. Minerals actually observed in rocks of the alteration zones yield positive saturation indices in corresponding water samples, while mineral species absent in the alteration zones accordingly display negative saturation indices in corresponding water samples. In other words, the presence/absence of minerals in the field corresponds well to the mineral saturation indices determined from our water samples.