Fluid Geochemistry of the Mondragone hydrothermal systems (southern Italy): water and gas composition versus geostructural setting

The geochemistry of natural thermal fluids discharging in the Mondragone Plain has been investigated. Thermal waters discharge as springs along the Tyrrhenian coast in two different areas: near Padule-S. Rocco at the foot of Mt. Petrino (41°7.5'N 13°53.4'E), and in Levagnole at the foot of Mt. Pizzuto (41°8.5'N 13°51.3'E). The water isotopic composition of both thermal discharges is lighter than the one of local shallow groundwater (?18O is -6.3? vs. -5.9? of SMOW and ?D is -40? vs. -36? of SMOW) as a consequence of inland (higher altitude above sea level) recharge by rainfall, suggesting that thermal water undergoes a deep and long flow-path before emerging along the coast. The chemical composition of highest temperature samples evidences that the hydrothermal reservoirs interact with similar lithologies, since they are both hosted in the sedimentary basement of the Campanian - Latial Apennine geologic succession. However, the two spring systems are different in terms of temperature and salinity (Levagnole: 50°C and 6 mS/cm vs. Padule: 32°C and 5 mS/cm). The higher salinity of the Levagnole springs is due to a longer interaction with evaporitic material embedded in the Miocenic formations and to the eventual mixing with 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 composition with other gas emissions 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 in a very short distance, clearly suggests a different source of the gas phases rising to the surface through deep transpressive regional faults related to the Ortona-Roccamonfina (NE and SW) tectonic alignment.