Imaging a buried terrane with district-scale ambient noise tomography and gravity modelling: insights for scale-reduction in mineral exploration

Mineral exploration proceeds by a process of scale-reduction. Decisions need to be made at all stages as the physical search space becomes progressively reduced from regional through to district, camp and tenement/deposit scale. While there are many regional scale datasets available to explorers through public geoscience agencies, one of the most difficult steps to make is from the regional-scale down to the district- and camp-scale, particularly in areas with post-mineral cover. Three dimensional models of subsurface geology at the district- to camp-scale can provide a means to filter for areas of enhanced prospectivity, for example by looking for broad structural or lithological features that are favourable for mineralisation. For the most part, 3D models of subsurface geology are developed by relying on inversions of potential field data coupled with other constraints such as surface mapping, drilling, electrical geophysics and, if available, reflection seismic data. Passive seismic methods are less commonly used by the exploration industry as a means to image subsurface geology, despite the method being able to provide depth-constrained, three dimensional seismic velocity information that is complementary to potential field data. As an example of the use of a district-scale passive seismic array to image the upper crust and architecture of possible mineral systems, we present a 3D velocity model derived from an ambient noise tomography (ANT) survey across an area of 1,800km2 in the northern Macquarie Arc under >100m of Mesozoic to Cenozoic cover. The ANT velocity model extends to depths of ~5km and reveals velocity features that are ~1-1.5 km thick and are spatially correlated with granitic plutons evident in potential field data. These larger, apparently magmatic bodies may represent multi-stage granitic plutons, and could represent source zones for possible magmatic-associated hydrothermal mineralisation. The ANT model also clearly delineates the steep, westerly dip on a major shear zone that runs through the centre of the survey area. When this type of ANT velocity model is available and combined with potential field data, exploration decisions can proceed with more confidence since they are based on consideration of multiple petrophysical properties - density, magnetism and velocity.