Hydrothermal alteration and associated sulfur-sulfate-sulfide mineralization were determined for five sulfurous hot springs located along the northeast coast of Baja California. Rock samples showed no alteration due to interactions with the hydrothermal waters. Kaolinite and mixed layer illite-smectite clays were present in sediment samples collected adjacent to Puertecitos and Punta Estrella sulfurous (PES) and nonsulfurous (PENS) hot springs. This hydrothermal alteration of rock and sediment samples is indicative of low temperature processes and is limited to the immediate vicinity of each spring locale. Many commonly occurring precipitates found associated with hydrothermal activity elsewhere are found at these springs. These include pyrite, hydrotroilite, barite, anhydrite, and opal. All occur in the sediments at the surface and may be more abundant at depth. H2S concentrations in the springs are comparable to those found at the Salton Sea geothermal area, which suggests that minor sulfide mineralization may be occurring in the subsurface. Sulfur isotope ratios were determined for sulfur-bearing precipitates at four springs. δs34 values for sulfates of approximately +20 permil indicate a seawater origin. Sulfide δs34 values varied considerably from spring to spring: δs34 = -0.91 permil to -20.88 permil. This large variation is attributed to leaching of preexisting sulfide deposits at depth. Elemental sulfur precipitating at El Coloradito hot spring has a δs34 value equal to that of the dissolved H2S in the hydrothermal solutions suggesting near-surface oxidation of H2S as the source.
