The Apache Mines sulfur-sulfate deposits lie near the Gulf coast of Baja California, about 35 km south of San Felipe. The deposits were formed by now extinct fumaroles. This is a study of near surface hydrothermal wall rock alteration. Granitic rocks (biotite, tonalite, and granodiorite), metamorphic rocks (schist, quartzite, and marble), andesitic volcanic rocks and fluvial sandstones make up the country rock. Block faults in the study area provided passage for hydrothermal fluids. All but the volcanic rocks were altered by these fluids. The rising sulfurous hydrothermal fluids became oxidized near the surface, depositing native sulfur and forming sulfuric acid and solfataric conditions. Downward percolating sulfuric acid leached large cavities near the surface and hydrogen metasomatism altered the adjacent country rock. Primary alteration produced expandable clays by altering plagioclase. Further hydrothermal attack produced kaolinite and natroalunite and finally anhydrite and gypsum. When biotite was present in the country rock, jarosite was produced instead of natroalunite. The silica released from the sulfate replacement formed hydrothermal opal. The most extreme alteration left only a porous opaline residue. The most probable origin of these deposits is a small pluton at depth. Volatiles left in the crystalizing magma chamber would have mixed with meteoric water and the hot, low-density fluids would have risen towards the surface along the fractures in the country rock. The possibility of a major economic sulfide deposit at depth is unlikely. Pyrite was found at depth, but trace element analyses indicate there are no base-metals such as Ag, Pb, or Zn. There were no Hg or As anomalies, although they are commonly associated with hot spring deposits. A copper anomaly suggests there might be some porphyry copper, but it would probably be at a depth of at least 1,000 m. Native sulfur crystals, which are sold for museum specimens, are the best economic possibility in these deposits. Other possibilities include silica for use in the manufacturing of glass and sulfates for use as soil conditioners.