Identification of marker shale horizons in banded iron formation: linking measurements of downhole natural gamma-ray with measurements from reflectance spectrometry of rock cores
K.L. Silversides and R.J. Murphy
Journal name: Near Surface Geophysics
Issue: Vol 15, No 2, April 2017 pp. 141 - 153
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Marker shale horizons are used in stratigraphic interpretation and for modelling and mining of the banded-iron-formation-hosted iron ore deposits in the Hamersley Province in the Pilbara region of Western Australia. These deposits contain marker shale horizons that are highly consistent throughout the Hamersley Province and are used to define lithology and provide context to enable the separation of visually similar rock units. The locations of these shales are normally determined from natural gamma-ray logs in exploration holes, which provide a coarse resolution boundary due to their wide spatial separation. Hyperspectral imagery can provide detailed information at high spatial resolution on the location and strike of shale horizons as they are presented on benches and walls in open-pit mines. The relationship between measurements of downhole gamma and shale horizons as detected by hyperspectral imagery must be understood if these very different kinds of measurements are to be used in a complementary way. The ability to identify marker shale horizons using their spectral features would allow them to be mapped using hyperspectral imagery. Multivariate analysis of shales from a typical Marra Mamba deposit showed absorption features relating to ferric iron, OH, H2O, and Al-OH, which allowed different marker shales to be spectrally separated. The kaolinite and Al2O3 abundance was estimated from the hyperspectral data using the intensity of the feature at 2202 nm to within 5.8% and 2.6%, respectively. Comparison of measurements of downhole gamma-ray with a proportion of kaolinite estimated from hyperspectral data showed both similarities and differences that required examination. Peaks in the gamma were located at the same depth in the core as peaks in the proportion of kaolinite. However, the relative magnitudes of these peaks were not consistent. Additionally, the gamma-ray baseline measurements between peaks varied at different depths in the core, with some sections of the core having a much higher baseline than others. This was not true for estimated kaolinite. These differences would lead to different interpretations of the distribution of material types down the hole. Similarities in the location of peaks in both types of data provide a basis to link these very different types of data to improve the resolution and accuracy of ore body boundary models.