One of the major applications of water geochemistry in the exploration of the 
potential geothermal reservoirs involves estimation of their temperature using 
chemical and isotopic geothermometers. These tools are based on empirical or semi-
empirical laws and commonly use data obtained from chemical and isotopic analyses of 
surface thermal waters. Unfortunately, the estimations of reservoir temperatures 
using these tools are not always concordant. The mixing of the deep geothermal 
fluids with surface waters or their cooling and the associated 
precipitation/dissolution processes during their rising to the surface are often 
responsible of these discordances. Other factors such as the presence of sea water, 
the water salinity or the nature of the rocks surrounding the reservoirs can also 
influence the temperature values given by geothermometry. For instance, the silica 
geothermometer underestimates the reservoir temperature when applied to deep 
geothermal fluids diluted by surface waters or after silica precipitation due to a 
fluid cooling. Conversely, for dilute thermal waters collected from volcanic or 
granite areas, the Na/K geothermometer often yields overestimated reservoir 
temperatures. The Na/K and Na/K/Ca geothermometers cannot be used with sea water. 
Through some examples, the aim of this study is to better understand the use of the 
Na/Li geothermometer and more especially, the behavior of the lithium and its 
isotopes, which can result relevant and decisive tools in numerous cases to estimate 
or validate the temperature of the fluids in the geothermal reservoirs. The behavior 
of this geothermometer is relatively complex because it doesn’t only depend on 
temperature but also on other factors such as the dissolved chloride concentration 
or the nature of the rocks interacting with the reservoir fluid.

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