Palaeomagnetic dating of the west Cumbrian hematite deposits and implications for their mode of formation

Palaeomagnetic analysis of six orientated hematite samples from the Florence Mine, Egremont, west Cumbria is described. A total of 21 specimens were subjected to incremental thermal demagnetization and the components of magnetization isolated by principal components analysis. All the specimens displayed a single component of magnetization with no evidence of either multicomponent magnetizations or significant low-temperature secondary magnetic components. The mean direction of the characteristic remanence based on the isolated components has a declination of 203 degrees, and an inclination of -13 degrees (alpha(95) = 10 degrees, k = 12) with the corresponding pole position at 38.6 degrees S, 333.5 degrees E (D-p = 5.2, D-m = 10.3). A single primary component is responsible for the characteristic remanent magnetism (ChRM) directions. The initial susceptibilities and magnetic intensities are compatible with petrographic observations that the samples comprise virtually pure hematite. The ChRM originated as chemical remanent magnetism when the hematite precipitated at temperatures over 100 degrees C. Comparison of the calculated pole position for the west Cumbrian hematite deposits with the apparent polar wander path for Britain south of the Iapetus Suture reveals that the ore deposits formed during the Late Carboniferous and Early Permian. The west Cumbrian hematite deposits therefore are not precipitates of a brine expelled from the Triassic sediments of the East Irish Sea Basin. The hematite may have precipitated from a brine expelled from older sediments (Devonian?), a fluid associated with igneous activity resulting from Permian rifting in the Irish Sea area, or they may be associated with ingress of surface-derived fluids during the Permian.