Discussion on ‘Hyper-permeable granite: lessons from test pumping in the Eastgate Geothermal Borehole, Weardale, UK’ by P.L. Younger & D.A.C. Manning Quarterly Journal of Engineering Geology and Hydrogeology, 43, 5–10

摘要

The authors have described some unexpected properties from a mineralized fracture system associated with Slitt Vein in the Weardale Granite. They ask whether the presence of open fissures might be more commonplace in granites than is currently supposed. Their suggestion that the main cavity, intersected at 410–413 m in the granite, might result from dissolution by deep weathering at the Carboniferous–Devonian unconformity is interesting, but other possibilities exist. The borehole was sited on a complex quartz–fluorspar mineral vein, and had passed through several mineral-filled veins. Examples of dissolution of one phase of ‘Pennine Style’ mineralization and replacement by later phases are known, and so it is conceivable that in this instance this was a mineral vein from which the infill has been dissolved and not replaced. Given the absence of any sign of hydrothermal minerals, however, it is far more likely that the cavity is structural, and formed either as a pre-mineralization void space that was somehow cut off from the hydrothermal plumbing system or as a result of post-mineralization strike-slip movement.