Geochemistry of Thermal Water in the Mont-Dore Area

Summary

Geochemical study of thermal and mineral springs of the Mont-Dore region gives useful informations on subsuperficial and deep aquifers of the area.

By the light of water geochemistry, the area presents all qualitative signs of a geothermal field :

• an important and widely dispersed C0₂ flux characterised by several cold mineral springs.

• a sulfate rich aqueous system in the Chaudefour valley

• hot and mineralised waters : the Mont Dore springs and the obviously Dordogne valley springs.

These four systems obviously not interelated can be distinguished by their chemical and isotopic properties.

The thermal area of Dordogne valley springs constitute the most interesting zone for geothermal purposes.

On these waters, confident temperature determination can be made (~ 150°). Occurence of deep a water level at higher temperature (180°) is also possible.

Water geochemistry studies have been used in the Mont-Dore geothermal project to get all possible informations about geothermal fluids of this area. This includes deep temperatures, homogeneity or heterogeneity of supposed deep reservoirs, fluids and solutes origin and circulation paths.

Twenty four springs were sampled in October 1978. Locations of springs are given in figure 1. Each spring was analysed for major and minor ions, and isotopic compositions. Chemical and isotopic analyses of the gaseous phase were also carried out. Tables listing the complete analytical results can be found in the paper of GERARD and al (1980), presented at the same symposium. In this present work are only given the useful data for interpretation ; a detailed study of gas analyses has been already published (LELEU, DEGRANGES, 1979).

At first look, waters of the area exhibit great differencies in their chemical composition. As for their isotopic behaviour, none shows a δ¹⁸O geothermal shift (figure 2) and the domain of variation is too small to distinguish different aquifers. On the contrary, the isotopic composition of sulfur in dissolved sulfate displays different fields. Using a δ³⁴S-f[SO₄=] plot to avoid possible dilutions effects (figure 3), shows that the Mont-Dore city waters differ from the Dordogne valley waters.

Considering these chemical and isotopic variations we have split the samples into four groups :

• I Cold, CO₂ rich, springs

• II Dordogne valley waters

• III Mont Dore city springs

• IV Chaudefour valley springs and sulfurated springs of Puy de Sancy.