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-bariteFsulfide-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-bariteFsulfide and late hematitegypsum. These deposits range in age from~10 to 0.4 Ma on the basis of 40 Ar/ 39Ar dating of jarosite. There is a crude northsouth 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 D 18 O SO 4 -OH values indicate that jarosite precipitated between 80 and 240 8C, which is consistent with the range of filling temperatures of fluid inclusions in late fluorite throughout the rift, and in jarosite (180 8C) from Peña 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 H 2 S 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 d 34 S values range from À24x to 5x, overlapping the values for barite and gypsum at the high end of the range and for sulfides at the low end. Most d 34 S values for barite are 10.6x to 13.1x, and many d 34 S values for gypsum range from 13.1x to 13.9x indicating that a component of aqueous sulfate was derived from Permian evaporites (d 34 S=12F2x). The requisite H 2 SO 4 for jarosite formation was derived from oxidation of H 2 S which was likely largely sour gas derived from the thermochemical reduction of Permian sulfate. The low d This article is a U.S. government work, and is not subject to copyright in the United States.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 H 2 S incursion into oxygenated groundwaters. The isotopic data in these bsour gasQ jarosite occurrences encode a record of episodic tect...

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The pollen 14 C age and oxygen isotopic composition of siliceous sinter deposits from the former Beowawe geyser field reveal evidence of two hydrothermal discharge events that followed relatively low-magnitude (10 À11 m 2 ) following each earthquake. However, the timescale for onset of thermal convection implied by an overturned temperature profile in a geothermal well 300 m from the Malpais fault is much shorter: 200-1000 years. We speculate that individual segments of the Malpais fault become clogged on shorter timescales and that upward flow of groundwater subsequently follows new routes to the surface.

The mobilization of arsenic (As) from riverbank sediments affected by the gold mining legacy in north-central South Dakota was examined using aqueous speciation chemistry, spectroscopy, and diffraction analyses. Gold mining resulted in the discharge of approximately 109 metric tons of mine waste into Whitewood Creek (WW) near the Homestake Mine and Cheyenne River at Deal Ranch (DR), 241 km downstream. The highest concentrations of acid-extractable As measured from solid samples was 2020 mg kg−1 at WW and 385 mg kg−1 at DR. Similar sediment mineralogy between WW and DR was identified using XRD, with the predominance of alumino-silicate and iron-bearing minerals. Alkalinity measured in surface water at both sites ranged from 1000 to 2450 mg L−1 as CaCO3 (10–20 mM HCO3− at pH 7). Batch laboratory experiments were conducted under oxidizing conditions to evaluate the effects of NaHCO3 (0.2 mM and 20 mM) and NaH2PO3(0.1 and 10 mM) on the mobilization of As. These ions are relevant for the site due to the alkaline nature of the river and nutrient mobilization from the ranch. The range of As(V) release with the NaHCO3 treatment was 17–240 μg L−1. However, the highest release (6234 μg L−1) occurred with 10 mM NaH2PO3, suggesting that As release is favored by competitive ion displacement with PO43− compared to HCO3−. Although higher total As was detected in WW solids, the As(V) present in DR solids was labile when reacted with NaHCO3 and NaH2PO3, which is a relevant finding for communities living close to the river bank. The results from this study aid in a better understanding of As mobility in surface water sites affected by the mining legacy.