Page 108 - Geothermal Energy Systems Exploration, Development, and Utilization

84 2 Exploration Methods
developed from fluid inclusions for a limited number of geothermal systems
have greatly aided in developing exploration models for additional geothermal
resources.
Although the sequence of alteration minerals varies from system to system,
there is a general relationship between hydrothermal alteration minerals and
temperature ranges, as summarized by Henley and Ellis (1983); (Figure 2.14).
Some hydrothermal minerals (such as pyrite, calcite, and quartz) are of little
use for evaluating deep temperatures and permeabilities, because these minerals
are stable over large temperature intervals. The most informative minerals are
the authigenic feldspars that are sensitive to both temperature and permeability.
The occurrence of hydrothermal minerals typical of active geothermal systems
depends on several factors such as temperature, pressure, fluid composition, and
permeability (Browne, 1970).

Amorphous SiO 2
Quartz
K - Feldspar
Albite
Calcite
Mont morillonite
Mont−Illite
Illite
Chlorite
"
Biotite
Actinolite
Tremolite
Diapside
Garnet
Epidote
Prehnite
Heulandite
Stilbite
Ptilolite
Laumontile
Wairakite

100 200 300 °C
Figure 2.14 Temperature ranges for typical hydrothermal
alteration minerals observed in active geothermal systems.
(Henley and Ellis, 1983). Solid and dashed lines indicate the
most and less frequent temperature ranges of occurrence,
respectively.