Magnetic susceptibility mapping at Broken Hill, Australia

Abstract

A magnetic susceptibility meter has been used as a mapping tool on outcrops of magnetic metasediments at Broken Hill. On each outcrop, magnetic susceptibility readings were taken on a 0.1 m grid, contoured, and compared with geological features drawn at the same scale. The area chosen, known as the Monuments, coincides with substantial aeromagnetic anomalies generated by magnetite-bearing metasediments of the Sundown Group, part of the Early Proterozoic Willyama Supergroup. The magnetic susceptibility mapping shows that concentrations of magnetite follow bedding, and do not follow high grade or retrograde schistosity. The magnetite concentrations follow around folds with high grade axial plane schistosity (S2), and are truncated by high grade shears that are parallel to the same undeformed high grade schistosity. The magnetite owes its existence to sedimentary or diagenetic processes, which formed concentrations of magnetite or another ferric mineral precursor. The characteristics of many other aeromagnetic anomalies in metasediments at Broken Hill indicate that they have the same origin.