GROUNDWATER-STREAMWATER RARE EARTH ELEMENT DYNAMICS IN THE SULPHUR CREEK WATERSHED, VALLES CALDERA, NEW MEXICO

The Jemez Mountains volcanic field, lying at the intersection of the Jemez Lineament and the Rio Grande Rift, contains the world-class, high-temperature (≤ 300 ^oC) Valles Caldera geothermal system (VC). The anatomy of the VC is a textbook example of volcanogenic geothermal systems, given its defined deep reservoir, vapor core, and distributed outflow features. The only surface expressions present in the caldera are acid-sulfate springs and fumaroles, which emerge from Quaternary rhyolite through intersections of densely faulted regions in the Sulphur Creek Graben. This study characterizes rare earth elements (REE) in these thermal features and the surface waters of the Sulphur Creek watershed (SC).

Acid-sulfate springs (pH: 1.63-3.33, [Al]: 33-110ppm, & [Fe]: 9-80ppm] and ubiquitous condensates above the geothermal boiling zone acidify (from pH 6.2 to 1.61) and salinize (e.g., [Al]: 0.03-21ppm & [Fe]: 0-10ppm) upstream waters of the SC. The greatest single ΣREE concentrations are at 350 ppb, confirming a notion from early work (late ‘80s) that the SC waters contain the highest REE contents ever found in geothermal waters. All spring waters appear to have inherited REE profiles from some felsic end-member, as typified by enriched HREEs, depleted LREEs, and a negative Eu anomaly to define a ‘gull-wing’ pattern. Influenced stream waters retain and reflect identical signatures. Water-to-rock patterns appear relatively flat and enriched in Eu; this normalization may indicate these waters readily leach REEs, especially Eu, from assumed felsic parent rock (Bandelier Tuff). Results of lanthanide (Ln or any REE) speciation modeling reveal that free ions (Ln³⁺) and sulfate ([x̄_stream]: 210ppm & [x̄_springs]: 1280ppm) complexes (LnSO₄⁺ and Ln(SO₄)^2-) account for ~98% of dissolved Ln species.