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

Press the Back button in your browser, or search again.

Copyright 1999, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.