| Abstract |
In high-temperature geothermal systems, the lower limit of hydrothermal circulation is controlled by a decrease in permeability. This limit is a fundamental aspect of the circulation volume and the heat budget, and, as a consequence, it will influence the ultimate heat recovery from geothermal systems and the pro-cedures that are applicable for heat mining below the permeability interface. In this paper, we consider the apparent termination of permeability with depth as documented in corehole Matalibong-25 in the Tiwi geothermal field. The bottom of convective circula-tion at Tiwi cannot be related to either the transition from brittle to ductile behavior which occurs at a tem-perature of about 400?C, or to changes in rock lithol-ogy. We suggest that the reduction in permeability is due to a stress reorientation with depth. The termina-tion of permeability with depth, whatever the cause, requires that heat be transferred to the convective por-tion of the system by conduction. Temperatures with-in the overlying convective zone are buffered by the boiling point to depth curve. We use time-tempera-ture constraints imposed by 40 Ar/39 Ar age spectra on adularia, hydrothermal alteration mineralogy, and fluid-inclusion geothermometry to constrain the intru-sive history of the system and the transfer of heat across an impermeable barrier. In the Tiwi system, three intrusive episodes are required to match the observed time-temperature relationships |