geothermal resource potential of the safford-san simon basin, arizona

may be erroneous. However, silica concentration is frequently used in geothermal
exploration to infer deep geothermal reservoir temperatures with the
assumption that silica concentration in shallow thermal wells and spring waters
is
derived from leakage of water out of a geothermal reservoir and that the
silica concentration has not been diluted by mixing, or that if mixing has
occurred, the mixing ratio can be determined in order to calculate silica
concentration before mixing (Fournier and others, 1974; Fournier and Truesdell,
1974). The main requirements for silica geothermometry are that silica in
solution is determined by temperature-dependent equilibrium with a solid form
of silica in the reservoir and that no
equilibration occurs after the thermal
waters leak from the geothermal reservoir and cool.
This latter requirement
is reasonable because water flow or leakage rates probably exceed the kinetics
of quartz equilibria in water at lower temperatures.
Because Cactus Flat-Artesia thermal waters originate from confined aquifers
which are apparently heated in a conductive heat regime, silica concentrations
should be in temperature-dependent equilibrium with either quartz,
chalcedony, or alpha-cristobalite.
Silica concentrations for ground waters in the Cactus Flat-Artesia area
are all supersaturated with respect to quartz at measured temperatures.
In
addition, inspection of the chemical analyses of Cactus Flat-Artesia ground
waters listed in Table 2 shows relatively high concentrations of dissolved
silica (up to 50 mg/l) in cold sodium bicarbonate ground waters, which have
high bicarbonate concentrations and neutral pH.
Several studies of low-temo
perature «50 C)
ground waters in contact with felsic rocks or sediment derived
from these rocks have shown that silica concentration is a function of
dissolved carbon dioxide concentration following the reaction in equation 1
(Garrels, 1967; Garrels and MacKenzie, 1967; Paces, 1972).
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