The Paradox of Scale: Reconciling magnetic anomalies with rock magnetic properties for cost-effective mineral exploration

Abstract

Targeting magnetic anomalies is a common practice in the mineral industry. However, it is uncommon for anomalies to be reconciled with their causative lithologies after a hole has been drilled. Furthermore, the effects of remanent magnetization are seldom considered, even though they are likely to be significant. This study explores how timely rock magnetic property measurements coupled with magnetic field modelling can be used to explain the anomaly whilst drilling is underway, thus saving critical exploration expense. The Rover 3 anomaly, near Tennant Creek in the Northern territory, Australia, was inverted using three different source geometries; an ellipsoid, a plunging elliptic pipe, and a plunging polygonal pipe, all with assumed homogeneous magnetization. All the modelled bodies have resultant magnetization vectors with moderate inclinations down to the southeast, and the modelled tops of the bodies are in the range of 190–240 m below surface. Analyses of remanent magnetization in the intersected lithologies found the primary causative lithology to be a rhyodacitic unit with only moderate magnetic susceptibility (K < 0.01 SI) and strong remanent magnetization (Koenigsberger ratio (Q) > 10), also directed down to the southeast, with moderate inclination. Some samples of the mafic units near the top of the volcanic pile also displayed a large component of remanent magnetization. However, much of it was found to be “soft” low coercivity remanence, carried by multidomain magnetite. This detailed knowledge of the rock properties was used to calculate the relative contribution of each unit by separating both the remanent magnetization and induced magnetization into X, Y, and Z vector components, attributing the resultant components to specified thicknesses of rock, weighting the contribution according to its distance from the observation point and adding the resultants for each layer. This analysis determined a bulk magnetization vector oriented at moderate inclination down to the southeast. In order to reconcile the measured properties with the observed anomaly, we constructed a model in ModelVision Pro TM using stratigraphic units defined by drilling, with measured magnetization vectors attributed to each layer. The shape and lateral extent of the layers are unknown, but using only simple elliptical prisms the modelled anomaly matched the actual anomaly to within 10% rms, illustrating that the anomaly could be reconciled with the rock property measurements. In this case, if the remanent magnetization had been measured on site during drilling, it may have been possible to recognise that the anomaly was due to remanence by the time drilling had reached a depth of approximately 400 m. This may have resulted in a saving of approximately two weeks and the significant cost associated with drilling a further 350 m.