Non-magmatic fracture-controlled hydrothermal systems in the Idaho Batholith: South Fork Payette geothermal system

Abstract

Non-magmatic, fracture-controlled hydrothermal systems located within and near the Idaho batholith have been examined and evaluated for potential application as natural analogues to high-level nuclear waste repositories. Detailed geochemical, petrologic, and structural studies of hot springs along the South Fork of the Payette River (SFPR) in central Idaho, USA, have been combined to assess the nature of a hydrothermal aquifer system along a major brittle shear zone. Geochemical modeling of thermal spring water chemistry with SOLVEQ [Reed, M.H., 1992. SOLVEQ: Users Manual. Unpublished Report, Dept. Of Geological Sciences, University Of Oregon, Eugene, OR.] indicates reservoir temperatures of 85°C to 160°C, and an alteration assemblage consisting of several clays, chlorite, serpentinites, quartz, albite, microcline, and micas. Geothermometry calculations, coupled with independent estimates of geothermal gradients based on heat flow and regional geology, allow estimation of reservoir depths ranging from 2.4 to 6.7 km. Comparison of trace element, rare earth element, and SOLVEQ alteration assemblages supports the hypothesis that waters equilibrated with granitic rocks at depth and ascended to the surface in a short time span. Stable isotope data (δD=−138.4‰ to −148.4‰ and δ 18O=−17.5‰ to −19.4‰) indicate that thermal waters of meteoric origin have interacted with country rocks at a very high water/rock (W/R) ratio. Systematic variations in δD values and the amount of δ 18O shift for thermal waters suggest that recharge probably occurs at much higher elevations than discharge. Fracture orientations, petrologic relations, and geochemical differences between adjacent springs delineate separate hydrothermal systems along the major structure defining the course of the SFPR. Hydrothermal systems along the SFPR exist primarily as small circulation cells which are probably influenced by minor changes in geothermal gradients associated with different rock types. These subsystems are dependent on periodic seismic or microseismic activity to maintain flowpaths. A working model of deep, fracture-controlled hydrothermal convection systems based on the SFPR Geothermal System may be used to evaluate the long-term behavior of artificial hydrothermal systems associated with high-level nuclear waste repositories.