EFFECT OF THERMALLY-INDUCED DILATATION ON THE PERMEABILITY OF GRANITE IN CASES IN WHICH THE LITHOSTATIC PRESSURE SLIGHTLY EXCEEDS THE FLUID PRESSURE

Abstract

Geological data suggest that in the evolution of many post-magmatic ore deposits the pressure of hydrothermal fluids approached the lithostatic pressure, and at some times may actually have exceeded it. The authors therefore undertook to investigate the pressure and temperature dependence of the permeability of granites in the range of greatest interest, i.e., at relatively low effective pressures. The unit used to measure in-situ permeability is described elsewhere; its fundamental advantages from our point of view was the ability to study percolation of liquids rather than gases, and the wide range of fluid pressures that can be achieved in it, so that we could produce low pore-pressure gradients along the specimen at high absolute fluid pressures (up to 1500 bar). The experimental technique involved direct observation of the temperature dependence of the permeability. Experiments with a single specimen were performed at a constant effective pressure and successively increasing temperature.