Integrated petrophysics linked with structural geology and mineralogy are powerful tools for mineral system characterisation. In this study, open-pit samples and drill core from the Osborne IOCG deposit were measured for anisotropy of magnetic susceptibility (AMS) and remanence. This study focused on the relationships between petrophysical properties and the high amplitude magnetic anomaly of the deposit. The magnetite-rich 'iron formation' typically has high density and magnetic susceptibility, which show a near linear relationship related to magnetite content (R2=0.77). Samples are magnetically anisotropic, with P-factors ranging from 1.1 to 1.4 and define a NNW-SSE-oriented magnetic foliation. However, the K1 (i.e. the maximum susceptibility) principal component of the AMS ellipsoid is close to horizontal, and therefore does not add to the vertical component of the magnetic field or contribute significantly to the TMI anomaly over the deposit. Koenigsberger ratios (Q) are commonly <0.5. Some Q values above 2 are observed in mineralisation. However, there is no monoclinic pyrrhotite, and the elevated Q-ratio is instead a result of the very low magnetic susceptibilities, and more likely associated with pseudo-single-domain magnetite. Remanence is subordinate to magnetic susceptibility at Osborne because multidomain magnetite grains are the main carrier. Therefore, remanence does not contribute significantly to the total magnetisation of the deposit. Statistical linear relationships between density and magnetic susceptibility break down at high K-values of 1.5 SI. This suggests that neither the AMS fabric nor remanence is the cause of the mismatch between the location of the orebody and the TMI magnetic anomaly, which subsequently made the initial drill siting difficult. Instead, it is related to the self-demagnetisation effect and local modification of the magnetic field, which are both due to the extreme magnetic susceptibilities observed.

Open-Access Online Publication: March 03, 2023