X-Ray Diffraction Analysis of Hydrothermal Minerals from the Los Azufres Geothermal System, Mexico

Abstract

Los Azufres is an active geothermal field located in the middle of the Mexican Volcanic Belt (MVB), a vast Miocene-Recent, E-W-oriented volcanic province spanning central Mexico. Three wells—Az-5, Az-28, and Az-31—from this geothermal field were selected to investigate the distribution, alteration sequence, and thermal stability of hydrothermal minerals. Maximum in situ measured temperatures are 280°C at 1493 m, 265°C at 1700 m, and 288°C at 1300 m depths in Az-5, Az-28, and Az-31, respectively. The host rocks in these wells are dominantly andesite followed by dacite, rhyolite, and basalt. Rock cuttings from different depths were analyzed for clay and non-clay minerals by X-ray Diffraction (XRD) methods. Hydrothermal quartz, calcite, and pyrite, as well as other alteration mineral phases (e.g., chabazite and chlorite) that are difficult to identify by traditional petrography were identified and their abundances semi-quantitatively estimated by XRD. We show that these mineral data present a better perception of distribution trends of hydrothermal minerals in geothermal wells than the qualitative mineral identifications generally used for this purpose. Homogenization temperatures measured in fluid inclusions of hydrothermal minerals, in situ measured temperatures in the wells, and K+/H+ vs. Mg2+/(H+)2 activity diagrams for the chemical characteristics of the present geothermal fluids were used to define the thermal regime and the resultant stability conditions of the clay minerals. Smectite, illite, and chlorite are present in the <2 mu;m size fraction. Gradual variations in relative abundances of clay minerals range from smectite dominant at shallow well depths to a combination of smectite, illite, and chlorite at intermediate depths, and to illite and chlorite in the deepest levels. Excellent crystallinity and lack of mixed-layered clay minerals support a model involving a discontinuous change from smectite to chlorite and/or illite, rather than that involving continuous mixed-layering of smectite-illite and/or smectite-chlorite. Mineralogical and fluid inclusion data suggest that mineral distribution trends documented for the Los Azufres geothermal system reflect the prevailing thermal regime. The mineral parageneses of the Los Azufres geothermal field are broadly comparable with those reported in other geothermal systems of the world.