The Los Humeros geothermal system is composed of more than 2200 m of Quaternary altered volcanic rocks and an underlying Cretaceous sedimentary sequence. The low salinity of the fluids discharged at present (Na+ and Cl- concentrations <500 ppm), and the excess steam, indicate that the reservoir contains a mixture of steam and dilute groundwater. Water-rock equilibrium is not attained. Hydrothermal minerals are present in veinlets, vugs, and replacing primary minerals, Three mineral zones are recognized: 1) a shallow argillic zone (<400 m depth), 2) a propylitic zone (ranging between 500 and 1800 m) and 3) a skarn zone (>1800 m). Petrographic examination of cuttings from five wells and temperature data indicate at least two stages of hydrothermal activity. Temperature is the main factor that affects the chemical composition of chlorite, epidote and biotite. Fe2+ and Al-IV increase in chlorite with temperature [from 1.4 formula position unit (fpu) to 2.8, and from 03 to 2.4 fpu, respectively]. The pistacite content of epidote varies from 18 to 33 mol% in high-temperature regions (>270 degreesC) and from 13 to 26 mol% in low-temperature regions (<250 &DEG;C). Biotite displays a slight increase in Al-IV contents (1.55-2.8) and octahedral occupancy (5.93-6.0 fpu) with temperature. Whole rock composition and variations in oxygen fugacity conditions are factors that also affect the concentrations of Fe, Al and Mg in the octahedral sites of chlorite, epidote, biotite and amphiboles. Chemical variations observed in alteration minerals at different depths in the Colapso Central-Xalapazco region could be used as indicators of relict physico-chemical conditions in the reservoir, before the present economic exploitation. (C) 2002 CNR. Published by Elsevier Science Ltd. All rights reserved.