Argillization and Its Influence on Physical and Mechanical Properties of Andesites on the Eastern-Pauzhetsky Thermal Field, Kamchatka Peninsula

Abstract

The research was done on the Pauzhetsky hydrothermal system, which is located inside a volcano-tectonic depression in the south of Kamchatka Peninsula, and belongs to the Eastern Volcanic Belt. The superheated steam of the system is used to generate electricity at the Pauzhetskaya GeoPP. Due to the discharge of thermal water and steam, several thermal fields were formed on the surface. The object of present research is the Eastern-Pauzhetsky field. This field is characterized by the natural regime of steam and thermal waters discharge, since it is located at a distance from production wells. The field was studied with boreholes and open pits down to the depth 7-8 m. It is formed in andesites, which are dense, low porous rocks with high strength. Due to the action of thermal water, the andesites are argillized with developing of smectites and high-silica zeolites. Hydrothermal argillization of andesites causes their decompression and leaching, an increase in porosity, while their elastic and strength properties are significantly reduced. This trend in changing properties contributes to further progressive argillization. Upwards along the section, andesites transform to metasomatic breccia consisting of andesite fragments cemented with secondary minerals (smectites, zeolites, opal, quartz, calcite, and pyrite). The maximum alteration of initial andesites is achieved in the near-surface horizon where they totally transform to hydrothermal clays forming insulating cover above the field. Clay horizon has a zonal structure, which is expressed in a change in color, texture, grain size, mineral composition, consistency, and properties. In particular, smectites prevail in the lower part of the clayey horizon, and kaolinite in the upper part. Thus, three horizons are distinguished along the section: argillized andesites, metasomatic breccia, and hydrothermal clays. The degree of hydrothermal alteration of rocks strongly depends on their spatial position in a certain part of the field and the depth of their occurrence. The main factor controlling hydrothermal alteration is the permeability of rocks and fractured zones.