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Description
Five sulfurous warm springs discharge slightly below mean low tide along the northeast coast of Baja California. These warm spring waters were analyzed to determine: (1) the origin of the waters feeding the hydrothermal systems, (2) rock-water interactions at depth, (3) if near-surface mixing with a cold groundwater occurs, (4) if ore deposition at depth occurs, and (5) reservoir temperatures. Sulfur isotopic ratios were measured for H2S and SO4 dissolved in the warm spring waters to determine their origin. The thermal waters at El Coloradito and Punta Estrella Nonsulfurous most likely ascend to the surface without mixing with a nonthermal groundwater. The spring waters at Puertecitos, Punta Estrella Sulfurous and San Felipe are the product of near-surface mixing between the ascending hot waters and seawater. The San Felipe springs may also form by the boiling of a completely seawater-composed reservoir fluid. Surface temperatures of the five spring waters range from 31°C to 63°C. Estimated reservoir temperatures based on the Na-K-Ca geothermometer are 115°C at El Coloradito and 215°C to 240°C at the other four warm springs. The five thermal waters have a slightly acidic Na-Ca-Cl-dominated composition, and are enriched in K and especially Ca, and depleted in Na, Mg, and SO4 relative to seawater. This is the result of rock-seawater interactions at depth under hydrothermal temperatures. The high salinities of the spring waters require the reservoir fluids to be partially or totally composed of seawater. Total heavy metal content (Ag, Cr, Cu, Fe, Mn, Ni, and Zn) of 1-65 ppm was measured in the warm spring waters, which is about 1,000 times greater than the metal content of seawater. These metals are leached from wallrock by the Cl-rich thermal fluids. Metal sulfides are believed to be precipitating at depth. dS34 values for SO4 dissolved in the warm spring waters are uniform and indicate a marine origin. dS34 values for dissolved H2S are quite variable, and may be due to: (1) a kinetic reduction of marine SO4 in the reservoir fluids that produces a sulfur isotope fractionation between the reaction product, H2S, and original SO4, and (2) leaching of sulfide minerals in wallrock at depth. The dS34 value for S° in the warm spring water at El Coloradito indicates formation by near-surface oxidation of H2S.
