Element Mobility Due to Hydrothermal Alteration in Los Azufres Geothermal Field, Mexico

Abstract

We report the chemical composition (major and trace elements, including REE) of a drill well core sample from the Los Azufres geothermal field, Mexico, which presented a hydrothermally altered section and an almost fresh part. This special sample allowed us to study the mineralogical and chemical effects imposed by hydrothermal alteration on the mm scale in this important geothermal system. The altered part of the sample shows intense silicification, hematization, and dissolution boundaries. Most major and trace elements were mobilized from the original rock. Major element composition reflects the silicification, chloritization, and epidotization processes taking place in the geothermal system. High-field strength elements such as Zr, Ti, and P show a significant concentration decrease in the highly-altered rock. The rare-earth elements La and Ce, as well as Yb and Lu were probably partially re-deposited during alteration. The positive anomaly of Eu in the altered section may suggest that Eu is being concentrated in hydrothermal epidote after its release from plagioclase to the geothermal fluid. The geothermal fluid responsible for this hydrothermal alteration was probably oxidizing, of high temperature (> 250ºC), and enriched in REE and other trace elements.