San Bartolomé de los Baños is a geothermal zone located in the southeast of the Guanajuato state in Mexico, and hosts near 150 thermal surface manifestations (hot springs), with discharge temperatures up to 92 °C and pH of 6.0-9.2. These manifestations, as well as surface hydrothermal alteration patches, are distributed along the N-S San Bartolomé fault, which in the study area has a displacement of ~100 m. This fault crosscuts a calc-alkaline volcanic sequence of Upper Miocene age that consists of a basal pyroclastic unit overlaid by basaltic lavas. Three suites of hydrothermal alteration occur at the surface: (a) silicification; (b) argillic; and (c) advanced argillic. Other hydrothermally-derived mineral assemblages are: (d) silica sinter; (e) silica- and sulfate-rich geyserite; and (f) quartz-calcite-fluorite-barite veinlets. Fluid inclusion microthermometry analysis of the veinlets show homogenization temperatures (Th) between 62 and 139 °C (with modal value at ~110 °C), and salinity between 2.4 and 10.4 % (weight NaCl equivalent). A rough correspondence between Th and salinity suggests that hydrothermal fluid mixed with meteoric surface water close to the discharge (at depths lesser than 50 m). Hydrothermal alteration assemblages indicate fluid temperature near the surface of ~100 °C and slightly alkaline pH, whereas silica sinter suggests a reservoir temperature >200 °C. Both hydrothermal alteration and hot springs are spatially restricted to the San Bartolome fault, which, along with the apparent absence of modern volcanic activity, indicates that the geothermal system is non-magmatic and fault controlled (extensional domain type).