Effects of hydrothermal alterations on physical and mechanical properties of rocks in the Kuril–Kamchatka island arc

• Thermal waters alter volcanic rocks transforming them into hydrothermally altered rocks.
• High-temperature fluids cause consolidation and strengthening of rock.
• Low-temperature subsurface fluids lead to rock property “deterioration”.
• The subsurface horizon of hydrothermal clays is the most adverse zone.

The hydrothermal systems of the Kuril–Kamchatka island arc are hosted in volcanic formations of Neogene–Quaternary age. Thermal water alters volcanic rocks and transforms them into hydrothermal rocks such as secondary quartzites, various propylites, zeolitic rocks, argillic rocks, clays, opalites, and quartz–feldspar metasomatites. Mineralogical alteration of rocks contributes to changes of their physical and mechanical properties. The relationship between type of hydrothermal alteration and rock properties is quite variable and depends on a number of factors including parent rocks, РТ-conditions, chemical composition and type of fluid, and duration of fluid–rock interaction. The study showed that high-temperature fluids cause consolidation and strengthening of rock, decrease in their porosity and permeability, and remove hygroscopic moisture. This tendency is observed independently of fluid chemical composition. Property variations caused by low-temperature subsurface fluids are more complicated and diverse but their “deterioration” prevails i.e. a decrease in density, strength and elastic modulus, formation of secondary porosity, high hygroscopy, softening and swelling in water saturated environment.Special attention is given to the near-surface alteration zones which affects the selection of sites for power plant construction and can impact the integrity of their foundation. The subsurface horizon of hydrothermal clays is the most problematic zone due to high porosity, plasticity, hygroscopy, compressibility and occasionally swelling.