The role of the Devonian Weardale Granite in the emplacement of the North Pennine mineralization

摘要

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 degrees 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 degrees C.