Inverse modeling constrained by potential field data, petrophysics, and improved geologic mapping: A case study from prospective northwest Tasmania

Abstract

The Heazlewood-Luina-Waratah area is a prospective region for minerals in northwest Tasmania, Australia, associated with historically important ore deposits related to the emplacement of granite intrusions and/or ultramafic complexes. The geology of the area is poorly understood due to the difficult terrain and dense vegetation. We construct an initial high-resolution 3D geological model of this area using constraints from geological maps, and geological and geophysical cross sections. This initial model is improved upon by integrating results from 3D geometry and physical property inversion of potential field (gravity and magnetic) data, petrophysical measurements, and updated field mapping. Geometry inversion reveals that the Devonian granites in the south are thicker than previously thought, possibly connecting to deep sources of mineralization. In addition, we identified gravity anomalies to the northeast that could be caused by near-surface granite cupolas. A newly discovered ultramafic complex linking the Heazlewood and Mount Stewart Ultramafic Complexes in the southwest has also been modeled. This implies a greater volume of ultramafic material in the Cambrian successions and points to a larger obducted component than previously thought. The newly inferred granite cupolas and ultramafic complexes are targets for future mineral exploration. Petrophysical property inversion reveals a high degree of variation in these properties within the ultramafic complexes indicating a variable degree of serpentinization. Sensitivity tests suggest maximum depths of 2-3 km for the contact aureole that surrounds major granitic intrusions in the southeast, while the Heazlewood River complex is likely to have a deeper source up to 4 km. Our case study illustrates the value of adding geological and petrophysical constraints to 3D modeling for the purpose of guiding mineral exploration. This is particularly important for the refinement of geological structure in tectonically complex areas that have lithology units with contrasting magnetic and density characteristics.

Insights and Lessons from 3D Geological and Geophysical Modeling of Mineralized Terranes in Tasmania

“…The potential field modeling [15] resulted in a number of improvements to the 3D geological understanding of the region. Combined gravity and magnetic signatures indicate the Housetop Granite as relatively thin (≤5 km thickness), with a mafic-ultramafic complex near its base at a depth of ~3-6 km, similar to the model of [62]. This granite form contrasts with other plutons in the region, such as the Heemskirk and Meredith Granites, whose gravity signature shows them to be more deeply rooted, in excess of 10 km, as well as being connected at depth, extending the area of granite-related mineralization potential.…”

“…The Heazlewood-Luina-Waratah region (Figure 1C) hosts several significant mineral systems. The 20 × 20 km model developed by [62] encompasses Proterozoic metasedimentary rocks, Cambrian allochthonous mafic-ultramafic complexes (including the Luina Group), Ordovician-Devonian sedimentary rocks, and Devonian granitic intrusives [62] (p. K16). The area also comprises the eastern zone of the Arthur Metamorphic Complex, which consists of various Proterozoic protoliths metamorphosed during the Cambrian Tyennan Orogeny [63].…”

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

“…Model construction and refinement can be achieved through the reconciliation of field observations, geophysical interpretation, incorporation of geological data uncertainty and the inclusion of prevailing tectonic evolution (Lindsay et al, 2013). A brief discussion of commercially available 3D potential field inversion packages is available in Eshaghi et al (2020).…”

“…Therefore, new petrophysical measurements were needed to improve the database for detailed geophysical modelling. A geophysical case study was presented by Eshaghi et al (2020) that shows the benefit to geophysical modelling of incorporating improved petrophysical information for a highresolution case.…”