The Palaeogene Armathwaite-Cleveland Dyke in upper Teesdale, northern England; magnetic characteristics and relationship to mineralization

A magnetometer survey, conducted with the initial aim of delineating the extent of a magnetite-rich orebody within the Teesdale Fault in the Harwood Valley of Upper Teesdale, unexpectedly revealed a Palaeogene remanent magnetization. This is interpreted as reflecting the presence at depth of a hitherto unrecorded section of the Armathwaite-Cleveland Dyke, a member of the Palaeogene Mull Swarm of intrusions. The dyke has now been continuously mapped magnetically from the easternmost outcrop of its Armathwaite section in the South Tyne Valley, near Garrigill, to the westernmost outcrop of the Cleveland section in Ettersgill, Teesdale, where it follows the Teesdale Fault and undergoes a small dextral en-echelon offset. We retain the originally proposed mechanism for the formation of the orebody, by the interaction between Northern Pennine mineralizing fluids and the still hot, recently intruded Permo-Carboniferous Whin Sill and its contact rocks. We find no evidence of a wider distribution of ore, or of Permian magnetization. We conclude that the orebody was emplaced along the Teesdale fault and that its magnetic signature was thermally re-set during the Palaeogene intrusion.