| Abstract |
The chemical composition of hydrothermal silicates at Los Azufres geothermal field (Mexico) was studied using microprobe techniques. The main objective of this study was to study the compositional variations of secondary silicates in relation to changes of the physico-chemical parameters in the field, in order to obtain a better understanding of the water/rock interaction processes occurring in this active hydrothermal system. The most abundant minerals are chlorite, epidote, quartz, calcite und sericite. Zeolite, several types of clay minerals, potassium feldspar, albite, prehnite, amphibole, hematite, pyrite and hydrothermal garnet are also present. Chlorites show a relatively homogenous Feclinochlore composition. Epidotes are solid solutions with Feepidote as the dominant end member. Hydrothermal garnets are grossular-rich solid solutions with higher andradite content in core areas. Amphiboles show an Fe-rich tremolite composition. On the basis of the investigated hydrothermal minerals and their changes with depth it is possible to delimit three zones in the altered rocks from Los Azufres. Using the stability conditions of significant mineral assemblages it is also possible to stimate the temperatures of these zones. Zone I. Main paragenesis: stilbite + chlorite + calcite; max. depth approx. 500 m; temperature lower than 170?C and pressure lower than 50 bar. Zone II. Main paragenesis: heulandite + chlorite + calcite or wairakite + chlorite + calcite; max. depth 1500 m approx; temperature between about 170?C and 250?C and pressure up to 150 bar. Zone III. Main paragenesis: epidote + tremolite + chlorite; max. depth 3000 m approx; temperature higher than 250?C and pressure between about 150 and 200 bar. |