Geological control of fracture permeability in the Carnmenellis granite, Cornwall: implications for radionuclide migration

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Abstract
Multi-packer hydraulic tests and radioactive tracer experiments carried out in boreholes in the Carnmenellis granite have shown that the flow of water through the rock is largely confined to narrow zones separated by zones of very low permeability. Correlation of the hydraulic data with geological data from oriented cores has shown that most of the flow is associated with discrete geological features, including pegmatitic and other veins, and elvan (quartz-feldspar porphyry) dykes, characteristic features of the granite of the area. Joints have also been found to conduct water, particularly in the upper 250 m of the granite. The permeability of the granite has been found not to be simply a function of fracture frequency, long sections of highly fractured rock having no associated flow under test conditions. While flow paths have been found to become less frequent with depth, flow rates do not vary over the depths studied, the highest flow rate recorded at the site being associated with a vein at 637 m. Water-conducting fractures have been found to have certain preferred orientations which vary with depth. A set striking 155°C is particularly well developed in the upper 250 m of the granite. This orientation is significant in that it is parallel to the cross-courses in the nearby mineralized belt with which the younger (Tertiary) mineralization tends to be associated and from which thermal brines issue into several local mines. These fractures are also approximately parallel to the maximum horizontal stress which affects the granite and which appears to be responsible for the selective opening of joints of certain orientations.

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