Hydrothermal alteration and physical and mechanical properties of rocks in a volcanic environment: A review

Abstract

Volcanic rocks are the prominent host rocks in geothermal and volcanic systems in general, displaying heterogeneity. Although various external factors such as temperature, pressure, time, fluid chemistry, and subsurface geology have been thoroughly researched regarding the source of hydrothermal minerals in geothermal fields, the effect of hydrothermal alteration on volcanic hosts is still controversial in the literature. This review compiles data on the physical and mechanical properties of the host rocks composing volcanic environments exhibiting hydrothermal alteration or remaining unaltered. The considered data is originated from hydrothermal areas from Kuril-Kamchatka (Russia), Los Humeros (Mexico), Ngatamaraki, Rotokawa, Kawerau and Ohakuri geothermal fields and Mt. Ruapehu, Mt. Taranaki, and Whakaari volcanoes (New Zealand), Solfatara (Italy), Reykjanes, Nesjavellir, and Theistarereykir geothermal fields (Iceland), La Soufrière de Guadeloupe (Caribbean) volcano, and Merapi volcano (Indonesia). Analysis of average values displayed in several graphical representations and correlations finds that dense rocks (such as lavas and intrusive rocks) exhibit greater competence and lower porosity than fragmental rocks. However, altered dense rocks display greater variability in mechanical properties compared to pyroclastic rocks, primarily influenced by mineral dissolution leading to rock weakening. Exceptions occur for high-temperature hydrothermal alteration, such as advanced silicification and propylitic alteration, with the latter influenced by minor types of alteration. Fragmental rocks have diverse behaviour with the extent of hydrothermal alteration and welding/compaction. According to the compiled data, an overall strengthening of pyroclastic rocks develops as hydrothermal alteration increases, regardless of the type of hydrothermal alteration. The complexity of hydrothermal systems, the variability shown by different hydrothermal settings and histories in terms of temperature, fluid chemistry and secondary mineral assemblage, and the variety of rock materials with different microstructures contribute to moderate correlations between properties compared to those established in an unaltered state. However, the same trends (linear, nonlinear, positive, negative) are preserved along hydrothermal alteration. This review emphasizes the significance of the type and degree of hydrothermal alteration, along with the rock type and pre-existence of fractures, in shaping the development of alteration in volcanic environments and modifying the properties of host rocks. The relevance of the review relies on the fact that these properties are considered to enhance the productivity of geothermal fields and improve the assessment of volcanic hazards. Future research is expected to expand on this groundwork.