Abstract
Closely spaced drillcore susceptibility measurements for the complete Dales Gorge Member of the Brockman Iron Formation were made as part of a study of oxide mineralogy and rock magnetic properties of unmineralized banded iron formations (BIFs) in the Hamersley Basin, Western Australia. Magnetic properties of BIFs are dominated by magnetite because of its much higher intrinsic magnetization even when the opaque oxides are volumetrically dominated by hematite. Bulk magnetic susceptibility varies considerably throughout the BIF units due to variations in thickness and composition of oxide/silicate mesobands, variations in the proportion of microbands, and changes in the hematite/magnetite ratio. This local-scale variability and the presence of layering effects at several scales means that the drill-hole susceptibility data are non-stationary, and conventional tools for analysis such as autocorrelation, spectral analysis, and semivariogram techniques have limited value. Both continuous wavelet (CWT) and discrete wavelet (DWT) techniques proved to be very powerful for analysis of these non-stationary data. CWT analysis using a Morlet wavelet helped to quantify the periodicity of the layering and DWT analysis using a Haar wavelet provided an effective means of 'blocking' the log so that susceptibilities of individual BIF macrobands can be compared. The results suggest that wavelet analysis can provide definitive information on the scaling properties of magnetic susceptibility needed to determine 'formation response' or facies variation within the major BIF units. Rapid changes in magnetic susceptibility in BIFs are mirrored by the oxide mineralogy. Opaque-oxide petrography and rock magnetism results show that replacement of depositional or diagenetic hematite by magnetite within and between sedimentary band structures by reduction during prograde metamorphism was incomplete, and equilibrium in the magnetite stability field was not reached even at the mesoband scale.
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