New Mexico Geological Society Annual Spring Meeting — Abstracts

The Jemez Watershed (JW) is in north-central New Mexico within the Jemez Mountains, which overlies the intersection between the Rio Grande Rift and the Jemez Lineament. A prominent feature of the JW is the Valles Caldera, which houses a world-class, high-temperature, liquid-dominated geothermal system (≤ 300 ^oC). Such highly mineralized geothermal systems usually have a significant environmental footprint. Geologic evidence suggests that the JW is influenced by the Valles Caldera geothermal system (VC). Multiple outflow thermal expressions, namely Jemez Springs Hot Springs and Soda Dam Springs, of the VC surround the main tributary (Jemez River or JR) of the JW. Furthermore, just southeast of the VC Ring Fracture Zone, headwaters of Sulphur Creek mixing with intra-caldera, acid-sulfate springs contribute to the JW further downstream. We aim to characterize hydrochemical mass movement between JW and VC waters with refined major-ion chemistry and new trace element analyses.

Recent Fall 2017 sampling during low-flow (~23-26 cfs) conditions reflects impairments of VC geofluids on JW water quality. There is bulk salinization (net ~500 ppm increase in total dissolved solids) of JR waters from just above Soda Dam Springs to San Ysidro Bridge. Upstream JW acidification (from pH 6.2 to 1.61) and trace metal salinization ([Al]: 0.03-21ppm & [Fe]: 0-10ppm) occur in the Sulphur Creek subbasin, and similar downstream influences are observed in Redondo Creek (from pH 7 to 4.7; [Al]: 0.01-1.5ppm), a JW tributary. We display these geochemical influences spatially and through multivariate statistics (principal component analysis). A tentative binary Cl/Br mixing model reveals bulk-salinized waters contain 19-30% geothermal spring/seepage contributions; this mixing model is not applicable to acidified waters, as Cl and Br lose conservative behavior under pH of 2.

From the same sample suite, concentrations of twenty-four trace metals and rare earth element (REEs) are detected through inductively coupled plasma mass spectrometry. Tentatively, these results show that ~71% of analyzed metals (Sb, Cr, Pb, Se, Cd, V, etc.) appear the most concentrated in the acidified and SO₄-rich waters. Comparatively, the outflow thermal springs and bulk-salinized JR waters contain the greatest concentrations of As, U, Li, Rb, and Cs. Interestingly, among these two classes, Ni and Ti appear in equal amounts. Analyzed REEs data reveal patterns congruent with those observed by previous investigators. The acidic waters show the highest REEs content (at least one order of magnitude more concentrated) and exhibit positive Eu anomalies. The outflow thermal and bulk-salinized fluids are characterized by the documented negative Eu anomalies, but REEs patterns for these waters appear the most incomplete (Nd is nonexistent in some) and fluctuating, which may indicate below-detection values.

These observations set up the upcoming stages of this study, which will employ radiogenic isotopic systems for more sensitive mixing models and reaction path modeling for determining sinks and sources. Further development of trace element and coupled major-trace element interpretations and necessary repeat analyses are also in view. This research aims to ultimately further understanding of continental geothermics, in hopes of serving as an analog to environmental assessments of other hydrothermal systems.