Chemical and structural dynamics of a geothermal system

In an ongoing project to relate surface hydrothermal alteration to structurally controlled geothermal aquifers, we mapped a 16 km swath of the eastern front of the Stillwater Range using Hyperspectral fault and mineral mapping techniques. The Dixie Valley Fault system produces a large fractured aquifer heating Pleistocene aged groundwater to a temperature of 285/spl deg/ C at 5-6 km. Periodically over the last several thousand years, seismic events have facilitated flow of heated fluids to the surface, leaving a rich history of hydrothermal alteration in the Stillwater Mountains. At Dixie Hot Springs, the potentiometric surface of the aquifer intersects the surface, and 75/spl deg/ C waters flow into the valley. We find a high concentration of alunite, kaolinite, and dickite on the exposed fault surface directly adjacent to a series of active fumaroles on the range front fault. This assemblage of minerals implies interaction with water temperatures in excess of 200/spl deg/ C. Field spectra support the location of the high temperature mineralization. Fault mapping using a digital elevation model in combination with mineral lineation and Held studies show that complex fault interactions in this region are improving permeability in the region leading to unconfined fluid flow to the surface. Seismic studies conducted 10 km to the south, at Dixie Hot Springs, show that the range front fault dips 25-30/spl deg/ to the southeast [R. E. Abbott et al. (2001)]. At Dixie Meadows the fault dips 35/spl deg/ southeast showing that this region is part of the low angle normal fault system that produced the Dixie Valley Earthquake in 1954 (M=6.8). We conclude that this unusually low angle faulting may have been accommodated by the presence of heated fluids increasing pore pressure within the fault zone. We also find that younger synthetic faulting is occurring at more typical high angles. In an effort to present these findings visually, we created a cross-section, illustrating our interpretation of the subsurface structure and the hypothesized locations of increased permeability. The success of these methods at Dixie Meadows will greatly improve our understanding of other Basin and Range geothermal systems.