Relation between spring-water chemistry and hydrothermal alteration in the Şapli{dotless}ca volcanic rocks, Şebinkarahisar (Giresun, Turkey)

Widespread and intense zones of hydrothermal alteration are present in the Cretaceous Şapli{dotless}ca volcanics of NW Turkey. Propylitic, 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 Şapli{dotless}ca (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. Based on δ¹⁸O and ²H data, the spring waters observed in the argillic alteration zones are generally meteoric in origin. The TU value of the spring waters is between 3.40 and 11.2, indicating that young waters have mixed with older waters from different sources. Some of the analyzed samples show high Fe³⁺, Al³⁺, Na, Ca, K and Mg concentrations, but their cationic contents vary over time. Amongst the trace elements, the highest dissolved concentrations were those of Sr, Ba, B, Mn, Zn and U. The lowest concentrations were measured for Mo, V, Cd, Be and Ce. Increases in Al and Fe concentrations parallel increases in total and mineral acidity values. Significantly, precipitation of recent gypsum, alunite, native sulfur, and goethite has been observed around the spring-water sources.