Efficient regional scale 3D potential field geophysical modelling to redefine the geometry of granite bodies beneath prospective, geologically complex, northwest Tasmania

Abstract

In this study, a regional model that defines the three-dimensional geometry of the subsurface geology beneath the complex, prospective northwestern Tasmania has been developed. This has been achieved using a series of potential field inversions constrained by surface geology, geological sections, seismic interpretations and a newly extended petrophysical dataset. Three major episodes of granitic magmatism are preserved in Tasmania: in the Neoproterozoic, Cambrian and Devonian. Granite bodies are hence considered important indicators of mineralization for explorers in an area of challenging vegetation, topography and cover sequences. Forward modelling and property-based inversions of the pre-existing geological model show that the previously interpreted subsurface geometry is not compatible with potential field data. Four sub-regions displayed a large discrepancy between calculated and observed data. This study redefines the subsurface geometries of these sub-regions through individual geometry inversions. The density and magnetic susceptibility ranges of units are further refined through property inversions. The modified geometry of the Devonian granites in the four sub-regions may be summarized as follows: 1) the Housetop Granite is relatively thin (≤5 km thickness), whereas 2) the Heemskirk and Meredith Granites are very thick and granite extends to a shallower depth between these bodies than previously interpreted. This region between plutons is thus a more prospective region than previously thought. 3) For the first time, an intrusive body underlying the eastern part of the Rocky Cape Group has been identified. Its petrophysical properties are similar to that of a granite, and its top is interpreted at a depth of >3 km. This interpreted low density (granitic) unit may be either Neoproterozoic or Devonian. 4) A new non-magnetic, low density Cambrian granite, with a minimum burial depth of 1 km, is also modelled in 3D, within the Mount Read Volcanics, in the south of the study area. Our approach, whereby sub-regions are identified for more detailed modelling, enables new constraints to be introduced in a computationally efficient way, and has general application to refining the geometry of key structures in prospective regions.