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

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, Ž.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. wGeochemical modeling of thermal spring water chemistry with SOLVEQ Reed, M.H., 1992. SOLVEQ: Users Manual. xUnpublished Report, Dept. Of Geological Sciences, University Of Oregon, Eugene, OR. indicates reservoir temperatures of 858C to 1608C, 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 Ž 18and ascended to the surface in a short time span. Stable isotope data dDsy138.4 ‰ to y148.4 ‰ and d Osy17.5 ‰ to.y19.4 ‰ indicate that thermal waters of meteoric origin have interacted with country rocks at a very high waterrrock Ž. 18WrR ratio. Systematic variations in dD values and the amount of d O 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 cours