Sources of chloride in hydrothermal fluids from the Valles caldera, New Mexico: a Cl study

The Valles caldera in New Mexico hosts a high-temperature geothermal system, which is manifested in a number of hot springs discharging in and around the caldera. In order to determine the fluid pathways and the origin of chloride in this system, we measured ³⁶Cl/Cl ratios in waters from high-temperature drill holes and from surface springs in this region. The waters fall into two general categories: recent meteoric water samples with low Cl⁻ concentrations (< 10 mg/L) and relatively high ³⁶Cl/Cl ratios (300–1000) × 10⁻¹⁵]; and geothermal brines with high Cl⁻ concentrations (800–9400 mg/L) but low ³⁶Cl/Cl ratios (11–26) × 10⁻¹⁵]. The ³⁶Cl/Cl ratios for meteoric waters are slightly higher than expected for this region, suggesting a small addition of anthropogenic ³⁶Cl. Because of low ³⁶Cl/Cl ratios and high Cl⁻ concentrations in the brines, chloride in these waters must be derived from subsurface sources. A comparison between the observed ³⁶Cl/Cl ratios in the brines and those calculated for potential source formations in this region indicates that the present host formations, mainly volcanic tuffs, cannot be major sources of chloride, and that formations at greater depth, such as the Paleozoic and Precambrian formations are more likely to be sources of chloride in the brines. The results suggest that brines are meteoric waters which penetrated into the basement where they derive chloride from leaching of basement rocks and/or from saline pore fluids trapped there, along with likely addition of chloride from Paleozoic strata. Although these fluids have since come to reside in the intracaldera volcanic sequence after convective upwelling, they do not derive much Cl⁻ from the volcanic strata; and residence times of fluids in the volcanics are < 100,000 years.