Fault-related fluid circulation in the seismically active Irpinia region (southern Italy): insights from fluid inclusions and calcite veins

The analysis of fault-related mineralization, with particular emphasis on fluid inclusions (FIs) trapped in syn-kinematic minerals, provides crucial insights into fluid circulation modality and fluid–rock interactions, so furnishing new tools to investigate the relationship between fluids and active tectonic. This study investigates the genesis, microstructural characteristics, and geochemical signatures of calcite veins associated with dip-slip faults in the Irpinia region (southern Apennines, Italy), a seismically active area located very close to the epicentral zone of the 1980 Mw 6.9 Irpinia earthquake. A comprehensive approach combining field observations, petrographic and microstructural analyses, fluid inclusion microthermometry, and geochemical profiling based on isotopic (δ¹³C and δ¹⁸O) and rare earth element (REE+Y) data reveals that the calcite veins precipitated from low-salinity H₂O–NaCl fluids, derived from the mixing of shallow and deep groundwater. These fluids, rich in CO₂ and coming from deep crustal reservoirs (8–12 km), migrated episodically through fault zones and were modified by mixing with post-depositional fluids produced during carbonate diagenesis, under varying thermal conditions (100–320 °C). Our study also proposes a computational model that reconstructs the isotopic evolution of the mineralizing fluids, capturing the sequential processes of fluid equilibration with dolostones, interaction with aquifer waters, and CO₂ degassing prior to calcite precipitation forming the mineralization. The good agreement between model predictions and measured isotopic data demonstrates the robustness of the model and highlights the dynamic fluid mixing processes within the fault zone. Furthermore, these findings highlight the role of episodic fluid migration, driven by fault-valve processes, in promoting calcite oversaturation and precipitation during seismic events. The integration of structural, geochemical, and modelling data refines our understanding of CO₂-rich fluid ascent, fault-related mineralization, and their link to fluid–rock interaction processes. This multidisciplinary approach offers new insights into fault mechanics and seismo-genesis, with implications for seismic hazard assessment and geochemical monitoring in active fault systems