Page 117 - Geothermal Energy Systems Exploration, Development, and Utilization

2.5 Geochemistry 93
the surface without any interaction with shallow or surface waters, in the form of
fumaroles and steam jets. Alternatively, separated vapor may condensate, at least
partly, in shallow groundwaters or surface waters to form steam-heated waters.
In this environment, atmospheric oxygen oxidizes H 2 S to sulfuric acid producing
acid–sulfate waters. These are characterized by low chloride contents and low pH
values (0–3) and react quickly with host rocks to give advanced argillic alteration
paragenesis, which are dominated by kaolinite and alunite. Dissolved cations and
silica are mainly leached from the surrounding rocks, whose compositions may be
approached by these acid waters. Shallow steam-heated waters may themselves boil,
separating secondary steam, which reaches the surface in the form of low-pressure
steaming grounds.

2.5.5.3 Sodium–Bicarbonate Waters
Bicarbonate-rich waters originate through either dissolution of CO 2 -bearing gases
or condensation of geothermal steam in relatively deep, oxygen-free groundwaters.
Because the absence of oxygen prevents oxidation of H 2 S, the acidity of these
aqueous solutions is given by dissociation of H 2 CO 3 . Although it is a weak acid, it
converts feldspars to clays, generating neutral aqueous solutions, which are typically
rich in sodium and bicarbonate, particularly at medium–high temperature. In fact,
the low solubility of calcite prevents the aqueous solution to increase in calcium
content; potassium and magnesium are fixed in clays and chlorites, respectively;
and sulfate concentration is limited by the low solubility of anhydrite.
Sodium–bicarbonate waters are generally found in the condensation zone of
vapor-dominated systems and in the marginal parts of liquid-dominated sys-
tems. However, sodium–bicarbonate waters are also present in deep geothermal
reservoirs hosted in metamorphic and/or sedimentary rocks.

2.5.5.4 Acid Chloride–Sulfate Waters
These acid waters do not come from separate reservoirs, but are produced through
inflow of acid magmatic gases into the deepest portions of convecting neutral pH,
NaCl systems. The acidity and the chemistry of the aqueous solution depends
on the extent of water–rock titration, which is also a function of the amount of
magmatic gases added to the water and of the availability of minerals that are able
to neutralize acids.
This type of waters is commonly found in crater lakes. The chemistry of crater
lake waters, especially during periods of intense volcanic activity, is obviously
dominated by inflow and absorption of magmatic gases rich in HCl and S species,
mainly SO 2 and H 2 S, leading to the production of strongly reactive aqueous
solutions with respect to cation leaching or rock dissolution, leading to deposition
of alunite, anhydrite, pyrite, and kaolinite.
At greater depths, magmatic gases interact with water and masses of rocks much
larger than in crater lakes, and at higher temperatures and for longer periods of
time, with respect to crater lakes, thus leading to higher extents of neutralization
and ultimately to the formation of neutral NaCl waters (Giggenbach, 1997a; Reed,
1997).