PRELIMINARY RESULTS OF JOINT ANALYSIS IN PROTEROZOIC TERRAINS IN NEW MEXICO

Recent model-based studies of two geothermal systems in New Mexico (Socorro and Truth or Consequences (TorC)) suggest that the measured temperatures and chemical compositions of the fluids in these systems are best explained by deep (4–8 km) circulation of groundwater within permeable (10^-14 to 10^-12 m²) Proterozoic basement. A detailed investigation of fracture density and connectivity in Proterozoic basement outcrops in areas with known geothermal potential and areas with no known potential was initiated to determine if circulation in Proterozoic crystalline rocks in New Mexico is an important geothermal process. Two areas with little potential are in the northern Sierra Nacimiento west of the Hot Dry Rock site and along Tolby Creek east of Eagle Nest. Three areas with high potential are in the Mud Springs Mountains, TorC, and the Caballo Mountains. A relict geothermal system in the Tajo Granite east of Socorro was also examined. Fracture densities, which are determined by counting the number of joints intersecting a meter stick, range from 6.6 ± 4.4 to 26.6 ± 14.9/m (range 0 to 58/m) in geothermal areas and 3.2±1.2 to 7.4± 3.9/m (range 1 to 16/m) in non-geothermal areas. The highest values are in TorC and the lowest values are in the Sierra Nacimiento. Connectivity, which is based on the number of systematic joint intersections (nodes) in a square meter, is highest in TorC (21 to 40/m²) and is lowest in the Sierra Nacimiento (2 to 9/m²). Fracture densities vary from 27/m adjacent to the western boundary fault of the Tajo Granite exposure to 3/m in the center of the exposure. Connectivity ranges from 42 to 23/m² in the relict geothermal system east of Socorro. Mineralized (fluorite and barite) and iron-rich fluids clearly flowed through fractures in the Tajo Granite. Iron-rich fluids also flowed through granite in the Caballo Mountains, and both calcite- and quartz-filled fractures cut the Proterozoic granite and the overlying Cambro-Ordovician Bliss Sandstone.