- © The Mineralogical Society Of America
The main goal of geothermal geochemistry research is to identify the origin of geothermal fluids and to quantify the processes that govern their compositions and the associated chemical and mineralogical transformations of the rocks with which the fluids interact. The subject has a strong applied component: Geothermal chemistry constitutes an important tool for the exploration of geothermal resources and in assessing the production characteristics of drilled geothermal reservoirs and their response to production. Geothermal fluids are also of interest as analogues to ore-forming fluids. Understanding of chemical processes within active geothermal systems has been advanced by thermodynamic and kinetic experiments and numerical modeling of fluid flow. Deep drillings for geothermal energy have provided important information on the sources and composition of geothermal fluids, their reaction with rock-forming minerals, migration of the fluids, and fluid phase separation and fluid mixing processes.
Based on findings to date, geothermal fluids may be classified as primary or secondary. Primary fluids are those found in the roots of geothermal systems. They may constitute a mixture of two or more fluids, such as water of meteoric origin, seawater and magmatic volatiles. Several processes can lead to the formation of secondary fluids, such as the boiling of a primary fluid that separates it into liquid and vapor and the un-mixing of a very hot brine by its depressurization and cooling. Further, secondary geothermal fluids form by the mixing of deep fluids with shallow ground water or surface water. In this chapter we summarize the geo-hydrological and geochemical features of geothermal systems and delineate the processes that produce the observed chemical composition of the various types of geothermal fluids found in these systems. The main emphasis is, however, on gas chemistry and the assessment of fluid phase separation below hot springs and around discharging wells drilled into liquid-dominated …