STRUCTURAL CONTROL OF WARM SPRINGS AND WELLS IN THE HILLSBORO-LAKE VALLEY-PALOMAS BASIN AREA

Subsurface fluid flow and heat transport in the Animas graben and horst between Hillsboro and Lake Valley and in the Palomas Basin 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 geothermal system north of Lake Valley along Berrenda Creek where the creek crosses the Animas horst. Three wells in the Palomas Basin tap into a 28-30°C north-striking confined aquifer at a depth of 280 m that appears to be associated with a small intra-basin graben and localized gravel deposits. A numerical hydrogeologic model of the Animas horst and graben system, 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 6 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 consistent with more complicated shallow and local hydrothermal processes. The warm water along Berrenda Creek just north of Lake Valley is leaking up along a fault zone located about 1.5 km to the east of the main strand of the Berrenda fault within the Animas horst. The maximum estimated reservoir temperatures at Hillsboro using the chalcedony and quartz geothermometers are 137°C and 161°C, respectively. The maximum estimated reservoir temperatures near Lake Valley using the chalcedony and quartz geothermometers are 83°C and 112°C, respectively. Most of the sampled groundwater has similar chemistry with a meteoric water isotopic composition.