PALEOMAGNETIC DATING OF THE WEST CUMBRIAN HEMATITE DEPOSITS AND IMPLICATIONS FOR THEIR MODE OF FORMATION

Palaeomagnetic analysis of six orientated hematite samples from the Fl orence Mine, Egremont, west Cumbria is described. A total of 21 specim ens were subjected to incremental thermal demagnetization and the comp onents of magnetization isolated by principal components analysis. All the specimens displayed a single component of magnetization with no e vidence of either multicomponent magnetizations or significant low-tem perature secondary magnetic components. The mean direction of the char acteristic remanence based on the isolated components has a declinatio n of 203 degrees, and an inclination of -13 degrees (alpha(95) = 10 de grees, 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 i s responsible for the characteristic remanent magnetism (ChRM) directi ons. The initial susceptibilities and magnetic intensities are compati ble with petrographic observations that the samples comprise virtually pure hematite. The ChRM originated as chemical remanent magnetism whe n the hematite precipitated at temperatures over 100 degrees C. Compar ison of the calculated pole position for the west Cumbrian hematite de posits with the apparent polar wander path for Britain south of the Ia petus Suture reveals that the ore deposits formed during the Late Carb oniferous and Early Permian. The west Cumbrian hematite deposits there fore are not precipitates of a brine expelled from the Triassic sedime nts of the East Irish Sea Basin. The hematite may have precipitated fr om a brine expelled from older sediments (Devonian?), a fluid associat ed with igneous activity resulting from Permian rifting in the Irish S ea area, or they may be associated with ingress of surface-derived flu ids during the Permian.