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A 3D model for the Koonenberry Belt from geologically constrained inversion of potential field data

A 3D model of the poorly outcropping Koonenberry Belt in northwestern New South Wales was produced by linking a series of 2.5D geological cross-sections in a computer-aided 3D geological modelling package. The 2.5D sections resulted from inversion of magnetic and gravity data by iterative modifications of forward models, starting with initial cross-sections that represented a best first estimate of the geology in a setting of limited geological data constraints. The process involved repeated two-way exchange between the geophysicist and geologists. Profiles longer than 50 km were simultaneously inverted for magnetic and gravity data with rigorous attention paid to the closeness of fit of the model anomaly to data, particularly on the flanks of anomalies, and to the geological justifications of the model. A reflection seismic profile across the centre of the belt helped to constrain the key Wonnaminta profile. Confidence in the outcome of the inversion process was boosted by confirmation of a prediction of the model for remanence in rocks in the basement of the Bancannia Trough. Wavelet edge detection traces ('worms') played a pivotal role in defining the starting models, testing the validity of final inversions, and linking the 2.5D profiles to construct the 3D model. The resulting 3D model played a crucial role in the tectonic interpretation of the Koonenberry Belt.

Origin of negative magnetic anomalies on the southwestern margin of the Thomson Orogen

Negative aeromagnetic anomalies characterise the belt of Yancannia Formation rocks, which occur in drillcore and scattered outcrop along the southwestern margin of the Thomson Orogen. These anomalies are distributed along the axes of folds, and have been targets for diamond drilling. Petrophysical measurements of 11 pyrrhotite-bearing specimens, taken from a high magnetic susceptibility zone in drillhole YCRCD-02, include high-precision magnetic susceptibility and magnitude and inclination of natural remanent magnetisation (NRM), allowing calculation of the Köenigsberger ratio and determination of NRM polarity. Polarities are mixed, but the uniformly high Köenigsberger ratio suggests that normal polarity specimens may be dominated by a characteristic remanence antipodal to the reversed polarity remanence, rather than by viscous remanence in the direction of Earth's magnetic field. NRM inclinations match both a pyrrhotite-hosted remanence responsible for similar negative anomalies in the Stawell gold field in Victoria, and local inclinations corresponding to Australia and Gondwana palaeomagnetic poles for the Benambran through Kanimblan-Alice Springs orogenies, which have deformed the Yancannia Formation. Combining the palaeomagnetic study and aeromagnetic imagery analysis suggests that pyrrhotite mineralisation accompanied Benambran deformation, although remanence of the pyrrhotite may have been acquired or reset during the later deformation events. A dyke near the base of the mineralised zone has been inferred to be correlative with Permo-Triassic alkaline intrusives in the southeastern Thomson Orogen, and diatremes in the adjoining Koonenberry Belt that have a mid to late Permian age, but inclinations corresponding to Permian poles lie outside the range of specimen inclinations, and the likely mixed polarity of the characteristic remanence argues against acquisition of remanence during the early to middle Permian range of the younger part of the Kiaman Reversed Superchron. The deduced association between negative magnetic anomalies and structurally controlled pyrrhotite mineralisation, associated with gold enrichment, will be useful as a guide to future exploration drilling.

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