Magnetic anisotropy and remanent magnetism in hemo-ilmenite from ore deposits at Allard Lake, Quebec

Abstract

The anisotropism of magnetic susceptibility of nearly pure hemo-ilmenite ore from deposits in the Allard Lake area consists of a distinct plane of maximum susceptibility (defined by maximum and intermediate axes along which the susceptibility is of similar magnitude) with a minmum susceptbility axis at right angles. This plane coincides with a preferred crystallographic grain-orientation indicated by the parallelism of titanhematite lamellae which have exsolved on the basal plane of the ferrianilmenite host grains. Measurement of the remanent magnetism (RM) shows a striking tendency for the RM vectors to lie in or near this preferred plane. On the assumption that hematite (if not ilmenite as well) is potentially ferromagnetic in the basal plane only (paramagnetic parallel to the c-axis), it was inferred that the RM vectors could represent the resolved component of the magnetizing field which tended to lie in the plane of maximum susceptibility. Under this hypothesis, it is possible to resolve the broad spread shown by the RM vectors of samples from one deposit in terms of a single magnetizing field direction. The variation of the intensity of RM of otherwise similar samples can also be explained on this basis. With one exception, the RM of 47 measured samples of hemo-ilmenite ore was reversely polarized (N-seeking pole up). Additional ‘natural history’ evidence bearing on the problem of self-reversal of thermo-remanent magnetism (TRM) is provided by this study.