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To investigate the effects of hydrothermal alteration on the chemistry of volcanic rocks, we analyzed the whole rock chemical composition (major and trace elements, including rare–earth elements–REE) of two distinct portions of a drill well core sample from the Los Azufres geothermal field, Mexico. This highly hydrothermally–altered sample allowed us to study, for the first time, the mineralogical and chemical effects imposed by hydrothermal alteration on the mm scale in this important geothermal system. Mineralogically, hydrothermal alteration in the sample is mainly represented by chloritization of primary olivine and pyroxene, argillitization of primary plagioclase, as well as banded chlorite and quartz, with significant amounts ofepidote and hematite. The" altered" part of the sample contains intensely altered remnants of the fresh rock, which show 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. 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 may suggest that Eu is being concentrated in hydrothermal epidote after its release from plagioclase to the geothermal fluid. The high–field strength elements such as Zr, Ti, and P, show as well significant hydrothermal alteration–related decrease in the highly–altered rock. The geothermal fluid responsible for this hydrothermal alteration was probably oxidizing, of high temperature (> 250 C), and …