Silica precipitation in fractures and the evolution of permeability in hydrothermal upflow zones.

Analytical models are used to compare the rates at which an isolated fracture and vertical, parallel fracture sets in hydrothermal upflow zones can be closed by silica precipitation and thermoelastic stress. Thermoelastic sealing is an order of magnitude faster than sealing by silica precipitation. In vertical fracture sets, both the amount of silica precipitation resulting from cooling and the total thermal expansion of the country rock may be insufficient to seal cracks at depth. These crack systems may ultimately close because the pressure dependence of silica solubility maintains precipitation during upflow even after the temperature gradient vanishes.