Structurally controlled genesis of caprock in volcanic hydrothermal systems

The mechanical and hydraulic behavior of faults in geothermal systems is strongly impacted by fluid-induced alteration. For instance, hydrolytic alteration of felsic volcanic rocks deeply affects the frictional and permeability properties of fault rocks, controlling the hydraulic behavior of faults. We investigated fault rocks from the caprock of a fossil hydrothermal system in the Northern Apennines, by combining field structural observations with mineralogical and microstructural analyses, friction experiments and permeability tests on fault rocks. Hydrolytic alteration promoted general weakening of fault rocks by enrichment of kaolinite-alunite-group minerals in the fault core, favoring strain localization. Enrichment of kaolinite along major faults induces a local decrease in permeability of three orders of magnitude (1.62x10^-19 m²) with respect to the unaltered protolith rocks (1.96x10^-16 m²) transforming faults from fluid conduits into barriers.

Alunite-kaolinite-rich rocks shows a velocity-strengthening frictional behavior, suggesting that hydrolytic alteration favors stable slip of faults at low temperatures (160-270°C).