chemistry of geothermal waters and hydrothermal alteration Rainwater and snow always contain small amounts of dissolved solids, which are largely derived from sea-water spray and aerosols. Aerosols are small suspended particles in the air that are washed down with the precipitation. Sea-water spray forms when the wind rips small droplets off wave crests. When transported into the air the droplets may evaporate, leaving the dissolved solids floating in the air to form aerosols.
Rainwater and melted snow are undersaturated with all common rock-forming minerals. Thus, upon contact with the soil and bedrock this water starts to dissolve the minerals, and in this way its content of dissolved solids increases. Additionally, carbon dioxide and organic acids may be added to the water from the soil, where they form by decay of organic matter. Dissolution of the rock and soil by rainwater seeping into the ground is generally sufficient to saturate it with some minerals, that will consequently tend to be deposited from solution.
The process of rock dissolution and secondary mineral deposition that occurs in the near-surface environment is collectively termed
chemical weathering. When these chemical processes occur deep in the ground at elevated temperatures and pressures, they are termed
hydrothermal alteration. Weathering and hydrothermal alteration invariably lead to changes in the mineralogy of the soil and rock, and sometimes also to changes in their chemical composition.
The process of weathering and hydrothermal alteration is often termed ‘hydrogen-ion metasomatism’ because it may be viewed as a chemical reaction between acids and bases. The aqueous solution acts as an acid and the rock as a base. Many silicates, which are outstandingly important rock-forming minerals, behave as quite strong bases when dissolving in water. Their dissolution thus entails consumption of hydrogen ions. Various cations are simultaneously released into solution. The reactivity of natural water, that is, its tendency to dissolve the rock minerals, depends on the supply of acids to this water. In most natural waters the most important acid is carbonic acid (i.e. dissolved carbon dioxide). In geothermal waters, hydrogen sulphide, silicic acid, and boric acid may also be important. Waters containing acids in rather high concentrations are relatively reactive and have a relatively high hydrogen ion content (low pH). By contrast, waters with a low content of dissolved acids, and in contact with very reactive rocks, attain low hydrogen ion concentrations (high pH) through the dissolution of rocks.
The chemical composition of geothermal waters is extremely variable. The content of dissolved solids ranges from a few hundreds of milligrams per litre to as much as 30 per cent. The main factors affecting the composition are temperature and the chemical composition of the rocks through which the water flows. However, infiltration of sea-water into the bedrock and degassing of a magma heat source to many geothermal systems may also contribute to the chemical composition of geothermal waters. Although all common rocks are mostly composed of various oxide and silicate minerals, they contain a small but variable content of soluble salts. The amount of these soluble salts in the rock has a profound effect on the concentration of dissolved solids in the water. Certain rocks, such as evaporite sediments, are solely composed of soluble salts. Water associated with such rocks is hypersaline and referred to as brine.
Studies of the chemical composition of the water in many drilled geothermal fields around the world, together with the alteration mineralogy, indicate that chemical equilibrium is closely approached between these minerals and the aqueous solution, at least if temperatures exceed 50–100°C.
Hydrothermal alteration has been studied extensively in active geothermal systems, which have been drilled for the purpose of exploiting geothermal resources, and also in fossil systems of this kind that have been exposed by erosion. Temperatures increase with depth in the drilled areas and may reach as much as 350°C; even more in a few instances. The hydrothermal minerals in active geothermal systems typically display a depth-zonal distribution, suggesting that each of these minerals, or an assemblage of minerals, is stable over a certain range of temperature. Other minerals, such as quartz, may be stable over almost the whole temperature range. In fossil geothermal systems the zonal distribution of the hydrothermal minerals defines an aureole around intrusive formations that are considered to represent the heat source for the fossil geothermal system. In some geothermal systems total reconstitution of the primary rock minerals by hydrothermal minerals may have occurred; alteration in others is limited. The age of the system, the contact area between water and rock, and the degree of instability of the primary rock minerals determine the extent of the alteration.
The solubility of many of hydrothermal minerals, such as quartz, is temperature-dependent. Geothermal waters reside for a considerable time, probably years, at depth in geothermal reservoirs, or long enough to come close to chemical equilibrium with the hydrothermal minerals. The water may cool extensively in upflow zones between hot springs and the reservoir, either through conductive heat loss to the wall rock or by boiling. The residence time in the upflow is relatively short and is often not sufficient to allow the water to re-equilibrate much as it cools. In other words, the chemical composition of the water emerging in hot springs is very much the same as that in the reservoir. This has been made use of for geothermal exploration. By collecting samples of hot-spring waters and analysing them, (e.g. for silica) one can predict reservoir temperature (Fig. 1). For example, if the analysis of a water sample yields a silica concentration of 400 mg per litre (mg l
−1), the reservoir temperature can be predicted to be 235°C; a significant conclusion considering that one of the most important exploitation characteristics of a geothermal reservoir is its temperature.
The quartz geothermometer was the first to be developed for geothermal exploration. Many other geothermometers have since been developed, particularly those that are based on the aqueous concentration ratios of various cations, such sodium/ potassium, sodium/lithium, and magnesium/potassium.
In many geothermal fields, particularly, those located on high ground, the groundwater table is so low that no hot-water springs exist; only fumaroles (steam vents) are present. The steam contains various gases, including carbon dioxide, hydrogen sulphide (easily recognized by its notorious smell), hydrogen, and nitrogen. Gas geothermometers have been developed for geothermal exploration that relate the gas content of steam in fumaroles to temperature in the underlying geothermal reservoir where the steam originated.
Stefán Arnórsson
Bibliography
Ellis, A. J. and and Mahon, W. A. J. (1977) Chemistry and geothermal systems. Academic Press, New York.
Nicholson, K. (1993) Geothermal fluids. Springer-Verlag, Berlin.
