The influence of hydrothermal argillization on the physical and mechanical properties of tuffaceous rocks: a case study from the Upper Pauzhetsky thermal field, Kamchatka

Abstract

The development of geothermal sites for electricity production, direct utilization, or tourist destinations has increased in recent decades worldwide. Such development requires investigation of the engineering geological setting, including an assessment of the physical-mechanical properties of rocks. Geothermal regions in the Kuril-Kamchatka arc are composed of Neogene-Quaternary volcanic formations with relatively high strength and deformation properties. Thermal fluids circulating within rocks change their compositions, porosities, and physical-mechanical properties. The most intense alterations occurring in the thermal fields are due to argillization. This study investigates the effects of hydrothermal alteration on the physical-mechanical properties of tuffites in the Upper Pauzhetsky thermal field located in South Kamchatka. The aims are to consider the entire range of hydrothermal argillization and to analyze the relationships between physical-mechanical properties and degrees of alteration. The variations in mineralogy, microstructure, particle size distribution, consistency, and rock properties along the section of the thermal field are studied. The results show that argillic alteration gradually decomposes tuffites to weak clay-rich soils, forming a cover several meters thick. The alteration is accompanied by decreases in strength and rock weakening; minor variations are observed between fresh and altered tuffites, whereas a dramatic reduction in strength occurs when tuffites disintegrate and are converted into clays. The heterogeneity of the clayey layer and the presence of secondary siliceous minerals and residual components of parent rocks are the factors that distinguish hydrothermal clays from sedimentary deposits.