EVALUATING POTENTIAL INFLUENCES OF FAULT-CONTROL ON THE CHARACTER OF HYDROTHERMAL SPRINGS IN THE ATLANTA LOBE OF THE IDAHO BATHOLITH, USA

We examined hydrothermal springs in two general areas located within the Atlanta Lobe of the Idaho Batholith: the Vulcan/Molly Springs area, located along the Middle Fork Payette/South Fork Salmon fault zones, and the Bonneville/Kirkham Springs area within the Idaho Trans-Challis Fault Zone. Individual springs showed some variation in pH and temperature, which we attribute in part to variations in the granitic rocks hosting the hydrothermal aquifers, and in part to differences in the permeability structure underlying the different springs, which promotes greater or lesser amounts of water-rock interaction and controls the mean residence time and length of flow paths between the springs and their recharge areas. In spite of the variability observed between individual springs, however, statistical comparisons of pH and temperature do not indicate significant differences between the two areas.

To further investigate possible differences between hydrothermal reservoirs in the two areas, we developed estimates of the depth of circulation for selected springs in the study areas. To accomplish this, we calculated the spacing of major spring-controlling faults and along-strike spacing of spring groups, using data from the Idaho Department of Water Resources geothermal GIS database and digital geologic maps from the Idaho Geological Survey. On the basis of the separation distances and spring location data measured in the field, we arrived at rough estimates of the area contributing recharge to individual spring groups, and calculated the depth of circulation required to produce the observed fluxes of enthalpy using a calculated volumetric heat generation rate for Atlanta Lobe granite and an assumed basal heat flux of ~90mW/m². Although structural controls on spring formation are different between springs controlled by the Middle Fork Payette/South Fork Salmon fault zones and springs controlled by the Trans-Challis fault zone, our preliminary analyses do not indicate substantial differences in the characters of the hydrothermal reservoirs. Additional studies will be required to definitively ascertain the source of heat for hydrothermal systems in the Atlanta Lobe of the Idaho Batholith, and to better understand how faults control the character of hydrothermal springs in central Idaho.