Magnetic investigation of igneous intrusions in Teesdale, Northern England

Abstract

A total field magnetic survey was conducted in Harwood valley in Northern Teesdale, and the neighbouring area, to investigate the igneous intrusions which could be the cause of a magnetic anomaly and metasomatism in the mineralization of the area. Niccolite and other nickel-bearing minerals had been found in magnetite-rich ore at three localities in Harwood valley by a team of the British Geological Survey. This mineral assemblage has only been sighted in a few other zones and the cause of mineralization was assumed to be metamorphism due to the emplacement of the Whin Sill and associated dykes during the late Carboniferous-early Permian age. Magnetic profiles recently acquired in the area confirm the existence of a dyke intruding the Teesdale fault along the Harwood valley. The average amplitude of the anomaly is 350nT, reversed with respect to the present field. Further investigations to the south-east and north-west of the valley relate the intrusion with the known outcrops of the Cleveland dyke (Tertiary). An outcrop of an igneous intrusion that bears similar properties with samples of the Cleveland dyke rock was recently sighted by the author at lower Langdon Beck near the confluence of the Tees and Harwood rivers. Thin sections of this rock displayed an identical match in grain size and pigment composition with the Cleveland dyke samples from other known outcrops, confirming the dyke's presence along the Teesdale Fault.2.5D modelling of the magnetic profiles was carried out and the results reflect the depth to the top of the dyke at Harwood to be about 30m. Thickness of the body varies from 5 to 25m but is 12m at Harwood and about 14m in Etters Gill. Further south of the valley at Langdon Beck is another geological formation; the Burtreeford Disturbance. This is a series of faulting and folding that was a result of compressional stress (Permian age) from a WSW direction. It also produced the E- facing monoclinal folds of the Dent Fault zone and the easterly directed Pennine thrusts of the Cross Fell Inlier before, during and after the emplacement of the Whin. The study here reflects the Disturbance as a magnetic high and its confluence with the Teesdale fault, the inferred Cleveland dyke and the Whin Sill offers a complex interpretation challenge. A GIS data base for the project area was created using Arc-GIS, GRAVMAG, SURFER,DIDGER and other Microsoft Office applications. General interpretation of the anomalies is conducted but conclusions raise several questions. The cause of the mineralization in Harwood Valley is still uncertain, though it could be due to the cooling Cleveland dyke (58Ma) or the dyke could be forming a passage for the fluids of unknown age from the lower crust or mantle that cool and crystallize at shallower depths.