Fluid geochemistry of the Mondragone hydrothermal systems (southern Italy): water and gas compositions vs. geostructural setting

Abstract

The geochemistry of natural thermal fluids discharging in the Mondragone Plain has been investigated. Thermal spring emergences are located along the Tyrrhenian coast in two different areas: near Padule-S. Rocco (41°7.5′N 13°53.4′E) at the foot of Mt. Petrino, and near Levagnole (41°8.5′N 13°51.3′E) at the foot of Mt. Pizzuto. The water isotopic composition of both thermal discharges is lighter than the one of local shallow groundwater (δ18O ≅ −6.3‰ SMOW vs. ≅ −5.9‰; δD ≅ −40‰ SMOW vs. ≅ −36‰, respectively) as a consequence of inland higher altitude of recharge by rainfall, suggesting that thermal water undergoes a deep and long flow-path before emerging along the coast. The chemical composition of the highest temperature samples of two areas points that fluids in the hydrothermal reservoir(s) interact with similar lithologies, since they are both hosted in the lower sedimentary carbonate formations of the Campanian–Latial Apennine succession. However, the two spring systems are different in terms of temperature and salinity (Levagnole: ≅50 °C and 8.9 g/L vs. Padule: ≅32 °C and 7.4 g/L, respectively). The higher salinity of Levagnole springs is due to a longer interaction with evaporite material embedded in Miocene sedimentary formations and to the eventual mixing, during rising, with fresh seawater close to the seashore. The chemical and isotopic composition of the free gases associated with the springs, again suggests a different source of the two hydrothermal systems. Comparing the 3He/4He measured ratios with other gas emissions located NE and SE of Mt. Massico-Roccamonfina alignment, it is evident that the Levagnole thermal springs are related to the northern Latial mantle wedge where the 3He/4He is about 0.5 R/Ra, whereas the Padule-S. Rocco springs, although being only 3.5 km south of Levagnole, are related to the Campanian mantle wedge where R/Ra is always ≥2.0. Such a difference in 3He/4He ratio in a very short distance, clearly, suggests a different source of the Padule-S.Rocco gas phase rising to the surface through the deep transpressive regional fault(s) system related to the NE–SW Ortona–Roccamonfina tectonic alignment.