Page 108 - Geothermal Energy Renewable Energy and The Environment

94 Geothermal Energy: Renewable Energy and the Environment

FIGUre 6.3 (See color insert following page 17.0..) Travertine deposits at Yellowstone National Park.
(Photo by David Monniaux.)

FIGUre 6.4 Exposed tufa towers. (Photo by Doug Dolde.)

that geothermal activity may have been persistent in a region. Borates are especially sig-
nificant because geothermal fluids tend to have elevated boron contents, relative to meteoric
groundwaters (Coolbaugh et al. 2006). This compositional characteristic reflects the fact that
boron is preferentially partitioned into the fluid phase, relative to most minerals because of its
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small size and high charge (3 ). The most common boron-bearing minerals associated with
geothermal fluids that evaporate at the surface are borax, Na B O ·10H O, and tincalconite,
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4
2
2
Na B O ·5H O. These minerals are often associated with halite (NaCl) and other salts common
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4
2
2
in evaporite settings.
Using surface mineralogy as a means to undertake reconnaissance exploration efforts is a cost
effective way to identify geothermal resource targets. However, this approach is necessarily limited
by the local climate patterns. Areas of high rainfall can prevent deposition of noticeable volumes
of minerals that require high evaporation rates. Conditions that favor formation of such deposits
are readily achieved in the arid western United States, the East African Rift Valley, and central
Australia, for example.