An analysis of the Broken Hill exploration initiative petrophysical database

Abstract

An analysis of the Broken Hill Exploration Initiative petrophysical database reveals that most rock types display a bimodal magnetic susceptibility distribution. Iron in the weakly magnetic subpopulation is incorporated into the paramagnetic silicate minerals e.g. Fe2+, whereas similar lithologies, which are strongly magnetic, contain more Fe3+ in magnetite. Analysis of magnetic susceptibility values shows a regional increase in magnetite formation in amphibolites with metamorphic grade as the Broken Hill orebody is approached. This is coincident with an overall increase in density of the amphibolites as metamorphic grade increases. Graphs of magnetic susceptibility versus Koenigsberger Ratio (Q) reveal that most lithologies show three distinct sub-populations: low susceptibility with low remanence (46% of population); low susceptibility with high remanence (42% of population); and high susceptibility with high remanence (11% of population). Very few samples represent a fourth population with high magnetic susceptibility and low remanence. It is concluded that most highly magnetically susceptible rocks sampled have a significant component of natural remanent magnetisation (NRM) to affect magnetic field modelling. There is little difference between outcrop and drillcore values. From outcrop data there is a general remanent declination trending towards the North with a preferred inclination of between ?45 to ?90 degrees (near vertically upward). As these directions are generalIy near the earth's present magnetic field the effect of remanence approximately enforces the present day field. Eighty percent of drillcore samples reveal a very slight anisotropic susceptibility.