Interpreting the Plumbing of the Rincon Geothermal System using Electromagentic Surveys and Hydrothermal Models

The goal of this study is to assess the plumbing of the upflow zone of a forced convection geothermal system. The Rincon geothermal system is located in the Rincon Hills about 56 km north of Las Cruces, NM, at a topographic saddle dividing the Rio Grande and Jornada del Muerto basins. Rincon is a blind geothermal system that has no surface expression. Its exact size and extent are unknown. Temperatures measured in exploration wells are as high as 99 °C at 371 m depth on the hanging wall side of the east-dipping East Rincon Hills Fault (ERHF). The geothermal fluids are brackish (~ 1900 mg/l). Five thin (<1 m thick) opal beds have been mapped adjacent to the ERHF. An overturned temperature profile in one well (SLH-1) and the distribution of opal deposits is likely associated with geothermal discharge within a three-dimensional groundwater flow system. We recently completed 3 transient electromagnetic (TEM) transects and 4 magnetotelluric (MT) soundings within the discharge area of Rincon geothermal system. Preliminary electrical resistivity maps based on 1D TEM inversions show a laterally extensive electrically conductive (1-10 Ω*m) feature near the water table that may be associated with a geothermal outflow plume of hot, brackish water. We developed preliminary hydrothermal models of fluid, heat, and solute transport in the vicinity of the fault zone. These idealized cross-sectional models include a thermal sink term to account for out-of-plane groundwater flow and are constructed perpendicular to the fault zone. Model results suggest that the overturned temperature profile is due to discharge of nearly boiling fluids at the water table and flow of relatively cool (10 °C) fluids moving parallel to the fault zone within a deeper confined aquifer. Preliminary modeled electrical resistivity fields estimated using Archie's Law indicate that the distribution of these brackish geothermal fluids alone may not be responsible for the formation resistivity patterns, which have values as low as 1 Ω*m. This suggests hydrothermal processes such as pyrite mineralization or clay alteration may take place within the plume. Development of 3D hydrologic models and inversion of additional MT soundings are ongoing and should help to elucidate the plumbing of the Rincon geothermal system.