Austin, James1, Schmid, Susanne 2 and Foss, Clive1
1Potential Fields Geophysics, CSIRO Mineral Resources, Lindfield, NSW 2070, 2Multidimensional Geoscience, CSIRO Mineral Resources, Kensington, WA 6151
The Amadeus Basin in central Australia is prospective for stratiform base metal deposits and hydrocarbons. The Basin displays subtle magnetic anomalies that trace strata for considerable distance, highlighting complex folding patterns. Magnetic modelling techniques can be utilised on these stratiform anomalies to extrapolate the near-surface structure of the basin. Generally magnetic anomalies are assumed to have predominantly induced magnetisation, and with this assumption dip can be reasonably estimated using magnetic data alone. However, where the magnetisation is not purely induced (i.e., includes remanent magnetisation) the mathematical trade-off between the dip and magnetisation of bodies means that the dip of a body cannot be known unless the magnetisation is also known. Normally it would be optimal to measure the magnetisation, but this is not always possible or feasible, e.g., due to land access issues. In this study, we investigate the relationships between dip and magnetisation using an approach that would generally be considered a little backward. Rather than constrain structure using petrophysics, we use structural geology to constrain petrophysics. Three study areas were chosen to investigate numerous stratigraphic horizons in three major study areas, the Waterhouse Range, Glen Helen Station and Ross River areas. Modelling results suggest that a paucity of layers retain predominantly induced magnetisation, remanence is dominant in some, but both induced and remanent magnetisation are typically present. Remanence is mainly associated with relatively oxidised units that contain only hematite (e.g. Arumbera Sandstone), and comparisons with known apparent polar wander paths suggest that these magnetisations pre-date major folding in the basin. In some cases, magnetic anomalism reflects redox zonation within units, e.g. the Pertatataka Formation near Glen Helen, where discrete magnetic layers coincide with thin grey (reduced, magnetite-rich) horizons interbedded with more prevalent red (oxidised, hematite-rich) horizons, which are only very weakly magnetised. We also found that where magnetised units are relatively thin and occur near the surface, their magnetic response is sharp. However, in coincident aeromagnetic data, adjacent anomalies commonly overlap to form a single anomaly, thus misrepresenting the magnetic field, and mis-mapping the dip of the magnetic horizons. This study highlights some major pitfalls in attempting to map structure using magnetics. Near surface sedimentary units tend to be variably oxidised, and their petrophysical properties are inconsistent along strike. Their total magnetisation is commonly comprised of a significant component of remanent magnetisation, and therefore due to the mathematical trade-off between dip and magnetisation direction, industry standard inversions will commonly mis-map surface structure. Remanent magnetisation pre-dates major folding in many cases therefore, opposite limbs of the same fold can have completely different magnetic signatures. Our ability to target mineral systems in sedimentary systems is contingent on our ability to map the structure of such systems. This study demonstrates that petrophysical knowledge is a pre-requisite constraint for successfully informing structural and tectonic studies using geophysics.
James Austin is specialised in structural geology and potential fields geophysics, but he dabbles in many aspects of geology. His research is focused on understanding relationships between crustal processes and geophysical fields. His recent work is focused on the development of integrated technologies for mapping mineral systems.