The first deep geothermal exploration borehole (995 m) to be drilled in the UK for over 20 years was completed at Eastgate (Weardale, Co. Durham) in December 2004. It penetrated 4 m of sandy till (Quaternary), 267.5 m of Lower Carboniferous strata (including the Whin Sill), and 723.5 m of the Weardale Granite (Devonian), with vein mineralization occurring to 913 m. Unlike previous geothermal investigations of UK radiothermal granites that focused on the hot dry rock concept, the Eastgate Borehole was designed to intercept deep fracture-hosted brines associated with the major, geologically ancient, hydrothermal vein systems. Abundant brine (≤46 °C) was encountered within natural fracture networks of very high permeability (transmissivity c . 2000 darcy m) within granite. Evidence for the thermal history of the Carboniferous rocks from phytoclast reflectance measurements shows very high values (≥3.3%) indicating maximum temperatures of 130 °C prior to intrusion of the Whin Sill. Geochemical analysis of cuttings samples from the Eastgate Borehole suggests radiothermal heat production rates for unaltered Weardale Granite averaging 4.1 μW m −3 , with a mean geothermal gradient of 38 °C km −1 . The Eastgate Borehole has significant exploitation potential for direct heat uses; it demonstrates the potential for seeking hydrothermal vein systems within radiothermal granites as targets for geothermal resources.
After initial mapping of the fluorite/barite zones (1926) then discovery of the underlying unexposed Weardale Granite (1953), and proving its early Devonian age by the Rookhope borehole, extensive geochemical/mineralogical investigations indicate a likely alkali magmatic contribution to the post-Whin Sill orefield, contrasting with most other Hercynian Mississippi-type orefields in UK. Creaney's mapping of rank of early Namurian coals above the two central cupolas of the granite implied high temperatures of over 200°C just prior to the Whin Sill emplacement (297.4 Ma) in concord with an underlying magmatic source lasting 1 – 5 Ma. Their sharp boundaries unequivocally indicate that heat rose through the granite by hydrothermal convection, not conduction. A Stephanian source of quartz-tholeiitic magma is consistent with the Scottish Midland Valley succession (302 – 295 Ma) and contemporaneous Whin magmatism. The magma underplated the granite because of its higher density. It prepared conditions for the subsequent post-Whin Sill emplacement of the mineral deposits, producing the hot convection cell which drew in saline fluids from adjacent deep Carboniferous troughs. Following Whin Sill emplacement, highly pressurized early Permian alkali basaltic magma also underplated the granite. This was contemporaneous with the succession in the Scottish Midland Valley (age 298 – 292 Ma). Hot mineralizing fluids were forced upwards into the established convection cell in the granite and then into the Carboniferous of the Fluorite Zone, forming the mineral deposits, and then outwards beyond the granite cupola regions to deposit barite at c . 50°C.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.