IRON: Magnetic properties and magnetic signatures of BlFs of the Hamersley Basin and Yilgarn Block, Western Australia

Abstract

Magnetite-rich banded iron-formations (BIFs) exhibit characteristic magnetic properties, including strong anisotropy. Interpretation of the geological structure of BIF units from the associated magnetic anomalies is complicated by anisotropy of susceptibility and, frequently, by remanent magnetisation. The timing of remanence acquisition relative to folding exerts a crucial influence on the form of the anomalies. Neglect of susceptibility anisotropy and remanence can lead to large errors in interpreted dips and thicknesses of BIF units. The effective susceptibility of BlFs parallel to bedding exceeds the susceptibility normal to bedding, typically by a factor of 2 to 4. Bedding-parallel susceptibilities of magnetite-rich BlFs are typically 0.5 to 2.0 SI (0.05-0.16 G/Oe). Remanence directions in BlFs usually lie close to the bedding plane. Koenigsberger ratios (Q) of BlFs vary widely, but characteristic values can often be determined for individual units. Q values in the range 1 to 2 are common. The magnetisations of the haematite-rich supergene-enrichment iron ores are much lower than those of their BIF precursors. Magnetic properties of outcropping BlFs are usually greatly modified by weathering, which substantially decreases the bulk susceptibility, the degree of anisotropy and the remanence intensity. Deeper and more intense weathering of BlFs is encouraged by faulting and can be associated with reduced magnetic response over intensely faulted zones. The remanence of BlFs from the Hamersley Basin is carried by late diagenetic to low-grade metamorphic magnetite after primary haematite. At Wittenoom and Paraburdoo, the remanent magnetisation of Brockman Formation BlFs is pre-folding, whereas in an area of higher metamorphic grade, the Turner Syncline, the remanence of the BlFs is probably post-folding. Aeromagnetic signatures over the Turner Syncline clearly reflect anisotropy and remanence. Anisotropy and remanence effects are also evident in observed magnetic signatures over Archaean BlFs in the Yilgarn Block. Magnetic property measurements on samples from the Mount Magnet area and elsewhere confirm the high anisotropy and strong remanent magnetisations of these rocks.