“Sour gas” hydrothermal jarosite: ancient to modern acid-sulfate mineralization in the southern Rio Grande Rift

Abstract As many as 29 mining districts along the Rio Grande Rift in southern New Mexico contain Rio Grande Rift-type (RGR) deposits consisting of fluorite–barite±sulfide–jarosite, and additional RGR deposits occur to the south in the Basin and Range province near Chihuahua, Mexico. Jarosite occurs in many of these deposits as a late-stage hydrothermal mineral coprecipitated with fluorite, or in veinlets that crosscut barite. In these deposits, many of which are limestone-hosted, jarosite is followed by natrojarosite and is nested within silicified or argillized wallrock and a sequence of fluorite–barite±sulfide and late hematite–gypsum. These deposits range in age from ∼10 to 0.4 Ma on the basis of 40Ar/39Ar dating of jarosite. There is a crude north–south distribution of ages, with older deposits concentrated toward the south. Recent deposits also occur in the south, but are confined to the central axis of the rift and are associated with modern geothermal systems. The duration of hydrothermal jarosite mineralization in one of the deposits was approximately 1.0 my. Most Δ18OSO4–OH values indicate that jarosite precipitated between 80 and 240 °C, which is consistent with the range of filling temperatures of fluid inclusions in late fluorite throughout the rift, and in jarosite (180 °C) from Pena Blanca, Chihuahua, Mexico. These temperatures, along with mineral occurrence, require that the jarosite have had a hydrothermal origin in a shallow steam-heated environment wherein the low pH necessary for the precipitation of jarosite was achieved by the oxidation of H2S derived from deeper hydrothermal fluids. The jarosite also has high trace-element contents (notably As and F), and the jarosite parental fluids have calculated isotopic signatures similar to those of modern geothermal waters along the southern rift; isotopic values range from those typical of meteoric water to those of deep brine that has been shown to form from the dissolution of Permian evaporite by deeply circulating meteoric water. Jarosite δ34S values range from −24‰ to 5‰, overlapping the values for barite and gypsum at the high end of the range and for sulfides at the low end. Most δ34S values for barite are 10.6‰ to 13.1‰, and many δ34S values for gypsum range from 13.1‰ to 13.9‰ indicating that a component of aqueous sulfate was derived from Permian evaporites (δ34S=12±2‰). The requisite H2SO4 for jarosite formation was derived from oxidation of H2S which was likely largely sour gas derived from the thermochemical reduction of Permian sulfate. The low δ34S values for the precursor H2S probably resulted from exchange deeper in the basin with the more abundant Permian SO42− at ∼150 to 200 °C. Jarosite formed at shallow levels after the pH buffering capacity of the host rock (typically limestone) was neutralized by precipitation of earlier minerals. Some limestone-hosted deposits contain caves that may have been caused by the low pH of the deep basin fluids due to the addition of deep-seated HF and other magmatic gases during periods of renewed rifting. Caves in other deposits may be due to sulfuric acid speleogenesis as a result of H2S incursion into oxygenated groundwaters. The isotopic data in these “sour gas” jarosite occurrences encode a record of episodic tectonic or hydrologic processes that have operated in the rift over the last 10 my.