New Mexico Geological Society Annual Spring Meeting — Abstracts

Subsurface fluid flow and heat transport in the Animas graben and Animas horst between Hillsboro and Lake Valley in south-central New Mexico are evaluated using older industry and new temperature gradient data, and published and new water chemistry and isotopic data. Thermal manifestations include warm springs (31-38°C) and a borehole with a high geothermal gradient (80°C at 74 m) near Hillsboro. In addition, three shallow water wells drilled during the past decade near Lake Valley have encountered warm water (31-38°C) at shallow depth (<24 m). These warm waters and a nearby warm spring indicate the presence of a previously undocumented geothermal system north of Lake Valley along Berrenda Creek where the creek crosses the Animas horst.  A hydrogeologic model, using temperature as a groundwater tracer, was constructed to test the hypothesis that groundwater originating from the Black Range flows eastward, circulates to depths up to 4 km to be heated, and flows upward to discharge along the western boundary faults of the Animas horst, particularly the north-striking Berrenda normal fault. Although the steady-state hydrogeologic model does produce elevated temperatures along the Berrenda fault, the temperature-depth profiles predicted by the model do not match the observed data. Instead, the shapes of the measured temperature-depth profiles from the Hillsboro area are more consistent with a shorter lived, dynamic hydrothermal system, with hot water leaking up along the Berrenda fault and associated buried faults to the west. The warm water along Berrenda Creek just north of Lake Valley is leaking up along a fault zone located about 300 m to the east of the main strand of the Berrenda fault. The estimated reservoir temperatures at Hillsboro using the chalcedony and quartz geothermometers from springs and wells is 73 to 137°C and 68 to 161°C, respectively, compared to a temperature of 300°C at 4 km predicted by the model. The estimated reservoir temperature near Lake Valley using the chalcedony geothermometer is 64 to 83°C. Most of the sampled groundwater has similar chemistry and a meteoric water isotopic composition. Exceptions are waters from wells near Hillsboro, which have elevated sulphate and low magnesium and calcium concentrations compared other groundwater in the area.