New geochemical evidence constraining the water-rock-gas interaction on geothermal fluids of the Querétaro Graben, northern Trans-Mexican Volcanic Belt

This study examines the water-rock-gas interaction of the Los Geysers geothermal system in the northern Trans-Mexican Volcanic Belt (TMVB). Physico-chemical parameters, δ13C of total dissolved inorganic carbon (TDIC), and Rare Earth Elements (REE) composition on 14 thermal water discharges are reported and discussed. Waters had temperatures ranging from 40 °C to 98 °C, pH from 7.4 to 9.2, and total dissolved solids from 598 mg L−1 to 992 mg L−1. Signals of thermophilic biota, mainly diatom frustules, were found in the suspended particulate matter of spring waters. The range of δ13CTDIC was between −7.55‰ and −4.13‰ (avg. −7.10‰). Although potentially affected by secondary processes, the dissolved C species of the studied waters likely have a predominant deep, mantle-related component. The lanthanoids concentrations (∑REE) were variable over practically one order of magnitude, with average values between 1463 pmol L−1 for Ce and 9 pmol L−1 for Lu. Post-Archean Australian Shale (PAAS)-normalized patterns showed a slight enrichment in middle lanthanides, numerically expressed as [Sm/La]N and [Sm/Yb]N, ranging from 1.43 to 2.06 and 0.84 to 2.03, respectively. We propose that the overall signature of the REE is inherited during the regional migration of the liquid and from the primary aquifer. Moreover, PHREEQC computer code was run to calculate the dominant REE complexing ligands in the geothermal system at different temperatures. Our calculations indicate that F− complexes dominate over CO3 species at outlet temperatures, but oxyhydroxide complexes (REE-O2H, REE-O2–, and REE-O⁺) act as sinks after leaching from the source rock. The 143Nd/144Nd ratio of one of the studied samples was measured (0.512320 ± 0.000019; εNd = −6.20 ± 0.37, 2s.e.). The conceptual model in this paper highlights a mixing waters process between geothermal waters and cold shallow waters in the Queretaro Graben.