SELF-ORGANIZING THERMAL FLUID FLOW IN FRACTURED CRYSTALLINE ROCK:AN EXAMPLE IN THE IDAHO BATHOLITH

Thermal springs in the Idaho batholith discharge at discrete locations along a 50+ km reach of the Middle Fork of the Boise River (MFBR). Recharge water flows through Basin and Range fractures where it is heated by the geothermal gradient and ultimately discharges from the damage zone of Trans-Challis faults located near the bottom of the MFBR. Stable isotopes of water, ¹⁴C groundwater ages, fracture and fault orientations, fracture volume changes due to chemical evolution, and recharge area calculations suggest that the thermal springs issue from individual hydrothermal systems and that are self-organizing. Water evolves chemically along flow paths dissolving feldspars and precipitating secondary minerals. Secondary minerals accumulate in less efficient fractures and are flushed from the more efficient ones. Flow area calculations using heat flow, exponential decay of porosity, and curve intersection methods show that the many of the thermal systems extend beyond their immediate topographic watershed, and that some capture water from adjacent watersheds. Geochemical/flow feedback loops that provide a mechanism for self-organization are modeled using PHREEQC and positive and negative fracture volume changes are calculated. Suggested criteria for identifying self-organizing granitoid thermal groundwater systems include: 1) multiple discharge locations that are spatially distributed along the damage zone of a major fault, 2) chemical disequilibrium between higher elevation cool groundwater and thermal discharge water, 3) thermal discharges that are not associated with the intersection of the damage zones of two or more major structural features, 4), thermal groundwater that has appreciable age, and 5) thermal discharge fluxes that are greater than the volume of recharge that can be supplied by the drainage basins recharge area. It is likely that these criteria can also be applied to non-granitic thermal groundwater systems.