Hydrogen Metasomatism in Geothermal Fields : A Geothermometric Approach Involving Fluids and Mineral Compositions

Abstract

Temperature in an active hydrothermal system may be estimated by a good number of techniques. Today distribution of temperature is given by direct measurements in the wells by Kuster equipment. Downhole temperature may be estimated using different dissolved species geothermometers and chemical composition of the pumped up fluids. Temperature data on crystallization of authigenic minerals produced by water-rock interactions are obtained from microthermome- tric study of trapped fluids in quartz, carbonates or epidote. These different approach were used on a same geothermal field, Los Azufres (Mexico). There is a good agreement among the values calculated from different geothermometers. In general, there is a little underestimation (-10 to -15°C) of direct measurements compared to geothermometric esimations, which may be due to the delay in reaching complete thermal equilibrium.

As chlorites represent a major alteration product of andesites in the field, composition — temperature relations were investiga- ted. Firstly, a statistical approach of chlorite solid solution was performed in order to determine the major correlations among chemical constituents. The site occupancy, especially AI(IV) and the octahedral occupancy : 6 — AI(VI) — (Mg+Fe(2⁺)) shows respectively positive and negative correlations with temperature. It is considered that these variations depend mainly of temperature ; consequently the linear expression of composition variations as a function of T can be used as a geothermometer in the field (Cathelineau and Nieva, 1985). Satisfactory results in the application of the geothermometer to other chlori- tes from alteration zones of metallic (U) deposits were obtained.

An attempt to correlate epidote composition with temperature was also made ; main result is the decrease of X pistachite with increasing temperature. But the wide range of chemical variations in a single crystal induce considerable uncertainties of temperature estimation.

Cathelineau M. and Nieva D. (1985) — A chlorite solid solu tion geothermometer. The Los Azufres (Mexico) geothermal system. Contrib. Mineral. Petrol. 91, 235–244.