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Analysis of Borehole Temperature Data from the Mt. Princeton Hot Springs Area, Chaffee County, Colorado

Paul Morgan
Colorado Geological Survey/Colorado School of Mines

Mt. Princeton Hot Springs are a group of thermal springs in an accommodation zone in the Sawatch Fault, the western bounding fault of the Upper Arkansas (half) graben in the northern Rio Grande rift. The springs include Hortense Hot Spring, the hottest spring in Colorado with a temperature of about 82°C. A cluster analysis of the chemistry of the waters of the springs indicated that two thermal reservoirs may be feeding the springs. AMAX Exploration drilled 31 thermal gradient holes in the Upper Arkansas Valley in the mid-1970s to investigate the geothermal potential of the area for power production, but abandoned the prospect. Contours of the geothermal gradient based on these data leave the anomaly open to the west, including the highest contours of the anomaly. Five additional temperature gradient holes were drilled in 2009 by Mt. Princeton Geothermal LLC. Drilling problems prevented penetration to the planned depth in the two western holes but the western margin of the anomaly was probably defined. More interesting was the penetration of an isothermal zone in one of the holes at 65°C, slightly hotter than the outflow temperature of the main Mt. Princeton Hot Springs. The topography in the Mt. Princeton area is rugged. On a small-scale, geothermal gradients from individual boreholes form an irregular contour pattern. Much of this irregularity correlates with topographic irregularities. A crude approximation has been used to remove the effects of topography: at each borehole site the elevation of the surface and the geothermal gradient was used to calculate the elevation of the 65°C isotherm (assuming 1-D vertical heat flow). These elevations were then contoured as the top of the warm (65°C) Mt. Princeton Hot Spring aquifer. These contours were much smoother than the geothermal gradient contours and were found to dip gently from the west to Mt. Princeton Hot Springs, where they intersected the surface, and then plunged steeply below the valley to the east. A hotter (82°C) aquifer from Hortense Hot Springs could not be contoured as this aquifer was not penetrated by any of the drill holes. Geothermometry of the Hortense and Mt Princeton Hot Spring waters indicates reservoir temperatures of ~150°C, mixing with a cold meteoric component in the ratio of 1 part hot with 2 to 3 parts cold.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013