Mineralization of the Kozan-8 thermal well (Kruja Province, central Albania): a mineralogical and water geochemical studya)

In Albania, the presence of high mountain chains and active fault systems favors the ascent of deep waters producing several thermo-mineral springs. The most important springs, rising from deep karst aquifers, are located in the Kruja tectonic zone (central Albania), that consists of elongated anticline structures of Cretaceous–Eocene limestones and dolomites covered by Oligocene flysch deposits, and is affected by longitudinal NW-SE faults intersected by transversal faults. The carbonate rocks have been buried up to ca. 10 km depth, and are underlain by Triassic-Permian evaporites. The geothermal gradient ranges from 7 to 11 mK/m. Five deep wells are present in the area (depth range 600-3000 m) tapping thermal waters of carbonate aquifer. Thermal waters in the central area of the Kruja province are mainly of the Cl–SO4 –Na–Ca type, with temperatures up to 56 °C for springs and 65 °C for wells, and high H2S concentrations. Recent scientific activities carried out at the Kruja Province focused on Kozan-8 well (1837 m depth) at Elbasan locality, including water and deposited minerals sampling. Thermal waters discharge exhibits EC = 6340 µS/cm, temperature of 55.4 °C, pH = 6.73, and reducing Eh. Water stable isotopes results (d18O = -8.4 ‰ and dD = -53.9 ‰) indicate a main meteoric origin of the water. Water composition shows similar high Na+ and Ca2+ contents among cations, and high Cl- and (SO4)2- among anions. Together with H2S, enrichment in CO2, CH4 and He has also been detected. This water composition may be related to the presence of deep fluids with long/deep circulation in the local anticline structures, involving Mesozoic carbonates and the underlying Triassic evaporites. At the emergence, the water results oversaturated with respect to Ca-Mg carbonates, and slightly undersaturated with respect to sulphates. Around the water discharge abundant mineral deposits are present, consisting of banded encrustations of different colours, formed from the outgoing hot fluids. Combined optical microscopy and X-ray powder diffraction analyses evidenced that the studied encrustations mainly consist of micrometric-sized crystals of Ca- and Mg-sulphates (gypsum, CaSO4·2H2O; bassanite, CaSO4·0.5H2O; epsomite, MgSO4·7H2O), calcium carbonates (aragonite and calcite, CaCO3), and sulphur (S8).