Analytical and numerical models of hydrothermal fluid flow at fault intersections

Fault intersections are the locus of hot spring activity and Carlin-type gold mineralization within the Basin and Range, USA. Analytical and numerical solutions to Stokes equation suggest that peak fluid velocities at fault intersections increase between 20% and 47% when fracture apertures have identical widths but increase by only about 1% and 8% when aperture widths vary by a factor of 2. This suggests that fault zone intersections must have enlarged apertures. Three-dimensional finite element models that consider intersecting 10- to 20-m wide fault planes resulted in hot spring activity being preferentially located at fault zone intersections when fault zones were assigned identical permeabilities. We found that the onset of convection at the intersections of the fault zones occurred in our hydrothermal model over a narrow permeability range between 5 x 10-13 and 7 x 10-13 m2. Relatively high vertical fluid velocities (0.33 m year-1) extended away from the fault intersections for about 0.51.5 km. For the boundary conditions and fault plane dimensions used, peak discharge temperatures of 112 degrees C at the water table occurred with an intermediate fault zone permeability of 5 x 10-13 m2. When fault plane permeability differed by a factor of 2 or more, the locus of hot spring activity shifted away from the intersections. However, increasing the permeability at the core of the fault plane intersection by 40% shifted the discharge back to the intersections. When aquifer units were assigned a permeability value equal to those of the fault planes, convective rolls developed that extend about 3 km laterally along the fault plane and into the adjacent aquifer.