Constrained 3D modelling and geochemical analyses of the Horseshoe Range BIF: tools for evaluating magnetic signatures under cover

Abstract

The Horseshoe Range is a banded iron formation (BIF) in the Southern Capricorn Orogen, WA and is associated with a large horseshoe-shaped positive magnetic anomaly. Electron microscope mineralogy identified ubiquitous goethite and magnetite/hematite. This study focussed on measuring the magnetic properties of the rocks at Horseshoe Range in order to accurately predict their geophysical responses when buried beneath cover. Remanent magnetisation intensities of the rocks were high (up to 1300 A/m) and vectors measured in the rocks were oriented predominately downward which typically result in negative anomalies which is inconsistent with the observed anomaly. Due to the positive nature of the magnetic anomaly and the ability to accurately model the response without remanent magnetisation it appears that the high intensity remanent magnetisations may be volumetrically insignificant and likely limited to the near surface. The remanence may be caused by near surface formation of maghemite during bushfires and/or induced by lightning strikes. The BIF can be modelled using a single homogenous layer with a susceptibility of 0.8 SI. However, this is not geologically consistent with BIFs which typically display variable iron-oxide mineralogy and associated petrophysical properties. One way to more accurately model BIFs is to use the first vertical derivative as the model input. Using this approach, a 4 layer model was generated which matched the anomaly to an RMS of ~1%. Modelled susceptibilities ranged from 0.01 – 0.55 SI which are consistent with the measured properties. However, this model did not take into account the measured high intensity downward magnetisation vectors.