21 Basalt Street: Undercover mineral exploration of the Mount Read Volcanics in north-western Tasmania

The difficulty of searching for economically viable mineral deposits that occur beneath geological cover sequences has long driven innovation in exploration geophysics. Here, we present a case study of a novel passive seismic exploration technique that can be used to accurately image geological structure within the top 5 km of the Earth's crust. We deployed a dense array of 426 portable seismic sensors in the vicinity of Waratah in north-western Tasmania. This part of Tasmania has long been inferred to be underlain by Paleozoic basement rocks that contain the famed Mount Read Volcanics, a geological terrane that is highly prospective for volcanogenic massive sulphide deposits. To date, exploration near Waratah has been largely discouraged by the presence of a cover sequence of Paleogene basalt that is estimated to be up to 400 m thick in some locations. The presence of strong remanent magnetism in the basalt poses further difficulties to exploration efforts, and innovative geophysical methods are clearly required. To overcome the challenge posed by the thick cover sequence, we apply ambient noise surface wave tomography with the aim of creating a 3D S-wave velocity model of the top 4 km of the crust beneath the seismic array. Our seismic observations allow us to image the depth extent of the cover sequence, and identify a region of low seismic velocity (< 2.2 km/s) that likely corresponds to a pre-Paleogene topographic depression that contains a thick section of Paleogene basalt. Furthermore, we interpret a region of increased seismic velocity (> 2.5 km/s) as an area of thin basalt cover in the vicinity of the existing Hellyer mine site, as well as possible fault structures in the Paleozoic basement. We demonstrate that passive seismic methods can be effectively used to target follow-up geophysical studies of prospective areas even in the presence of thick geological cover.