Deep Groundwater Circulation within Crystalline Basement Rocks and the Role of Hydrologic Windows in the Formation of the Truth or Consequences, New Mexico Low-Temperature Geothermal System

Jeff D Pepin, New Mexico Institute of Mining & Technology, Socorro, NM, United States, Mark Austin Person, New Mexico Tech, Earth & Environmental Science, Socorro, NM, United States, Fred M Phillips, New Mexico Institute of Mining and Technology, Socorro, NM, United States, Shari Kelley, New Mexico Bureau of Geology and Mineral Resources, Socorro, United States, Stacy Timmons, Aquifer Mapping Program, New Mexico Bureau of Geology and Mineral Resources, Socorro, NM, United States, Lara Owens, RMI, Boulder, CO, United States, James C Witcher, Witcher and Associates, Las Cruces, NM, United States and Carl W Gable, Los Alamos National Laboratory, Earth and Environmental Sciences Division, Los Alamos, NM, United States
Abstract:
Hot Springs are common in amagmatic settings, but the mechanisms of heating are often obscure. We have investigated the origin of the Truth or Consequences, New Mexico low-temperature (~ 41 °C) hot springs in the southern Rio Grande rift. We tested two mechanisms that could account for the geothermal anomaly. The first scenario is that the anomaly is the result of lateral forced convection associated with a gently-dipping carbonate aquifer. The second scenario is that high permeability of crystalline basement rocks permits circulation of groundwater down to depths of 8 km prior to discharging in Truth or Consequences. To test these hypotheses, we constructed a two-dimensional hydrothermal model of the region using FEMOC. Model parameters were constrained by calibrating to measured temperatures, specific discharge rates and groundwater residence times. We collected 16 temperature profiles, 11 geochemistry samples and 6 carbon-14 samples within the study area. The geothermal waters are Na+/Cl- dominated and have apparent groundwater ages ranging from 5,500 to 11,500 years. Hot Springs geochemistry is consistent with water/rock interaction in a silicate geothermal reservoir, rather than a carbonate system. Peclet-number analysis of temperature profiles suggests specific discharge rates beneath Truth or Consequences range from 2 to 4 m/year. Geothermometry indicates maximum reservoir temperatures are around 167 °C. We were able to reasonably reproduce observed measurements using the permeable-basement scenario (10-12 m2). The carbonate-aquifer scenario failed to match observations. Our findings imply that the Truth or Consequences geothermal system formed as a result of deep groundwater circulation within permeable crystalline basement rocks. Focused geothermal discharge is the result of localized faulting, which has created a hydrologic window through a regional confining unit. In tectonically active areas, deep groundwater circulation within fractured crystalline basement may play a more prominent role in the formation of geothermal systems than has generally been acknowledged.
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