Geostatistically and Drilling Constrained Magnetic Inversion for Predicting Mineralisation at the Basil Cu-Co Deposit

Abstract

The Basil Cu/Co deposit comprises a 26.5Mt JORC-compliant inferred resource of copper and cobalt, grading 0.57% Cu and 0.05% Co. It lies in the Harts Range, central Australia, within the Riddock Amphibolite of the Irindina Province. The deposit coincides with a prominent anomaly in aeromagnetic data. Intersections of mineralisation at depth follow the magnetic anomaly trend. Analysis of drilling within the mineralised zone determined a spatial association between pyrrhotite with high magnetic susceptibility and chalcopyrite, with no other significant magnetic mineralisation present. A study was commissioned to examine if geophysical inversion could predict the distribution of mineralisation, using pyrrhotite as a proxy for chalcopyrite, from the surface to the drillhole intersections, as well as predicting further mineralisation at depth or in the vicinity of the deposit. Commercial software was chosen for performing geostatistical analysis, 3D geological modelling, forward modelling and stochastic inversion. Petrophysical data from core and information on mineralisation from drilling were used to constrain 3D geological modelling of mineralisation based on domain kriging of susceptibility data and sulphur assays. Magnetic data was conditioned for inversion in Intrepid software. Sensitivity testing of results to source depth and distribution was performed using 3D forward modelling. Alternative 3D geological models were tested during inversion for their behaviour and adherence to observed drilling data and the limitations imposed by the sulphide distribution in the geostatistics. The resulting initial geological model had known property voxels from drilling fixed and surrounding property voxels locally interpolated from kriging. Using these as a seed, geophysical inversion was performed alternating property and lithology inversion, until a desired minimum misfit with the observed magnetics signal was reached. The new predicted mineralisation distribution was compared with estimated mineralisation shells from conventional geostatistical modelling and found to be in good agreement, with reliability increasing closer to the surface. Predictions of mineralisation at greater depth and beneath weaker anomalies were more diffuse and showed less tendency to change during inversion, and were limited by the flight acquisition specifications. Deep targets have not yet been tested by follow-up drilling.