Palaeomagnetic dating of mineral deposits: a brief review and a Western Australian perspective

Abstract

Published examples show that the palaeomagnetic method may furnish constraints upon the timing of a mineralisation for a variety of ore-deposits. Success of the palaeomagnetic method relies upon: 1. Knowledge of the apparent polar wander path (APWP) for the continent or terrane in which the deposit occurs. 2. Crystallisation of new magnetic minerals in a deposit or its wall-rocks during a principal phase of mineralisation, constituting a magnetisation event. 3. Duration of the mineralisation event having been sufficiently long to average palaeosecular variation of the Earth's magnetic field. 4. The ore deposit having a sufficiently isotropic magnetic fabric to record accurately the Earth's magnetic field direction. 5. Knowledge of the history of tectonic disturbance of the mineral deposit. 6. A lack of complete alteration of the magnetic phases, leading to total palaeomagnetic overprinting in a later, and different, magnetic field-direction. Even when a number of these requirements for dating are not met, the palaeomagnetic method still has the potential to discriminate syngenetic from epigenetic mineralisation or provide a minimum estimate of mineralisation-age. By sampling both the ore deposit and the country rocks surrounding the zone of mineral alteration, it can be established whether the two have comparable or different magnetisation histories. The former relationship characterises a syngenetic deposit; the latter an epigenetic deposit. In the case of total remagnetisation of an ore, only a minimum mineralisation age can be estimated from the remagnetisation direction. The usefulness and precision of the palaeomagnetic method varies from continent to continent and with mineralisation age. In favourable circumstances, a precision of within half a geological period is estimated during Phanerozoic times. Precision generally decreases with increasing mineralisation age. Potential applications of the method to Western Australian base-metal and iron-ore deposits serve to illustrate the dependence of the method on (i) the density of existing palaeomagnetic data and (ii) the rate and course of APW at the approximate time of mineralisation.