Reid, Anthony1,2, Pawley, Mark1
1Geological Survey of South Australia, Department for Energy and Mining, 11 Waymouth St, Adelaide, South Australia, 5001, Australia 2Department of Earth Sciences, University of Adelaide, South Australia, 5005, Australia
Nickel is back in high demand, with the result that the risk equation for greenfield exploration is significantly improved. Of Australian Proterozoic terranes, the Gawler Craton in South Australia stands out as a terrane with a perceived lack of nickel potential. But is this really the case, or is the current prospectivity telling us more about a lack of data, than a lack of metals?
Recent exploration drilling in the western Gawler Craton intersected over 200m (down-hole length) of nickel and copper bearing sulphides hosted by a pyroxenite intrusive. This greenfields discovery by Western Areas in JV with Iluka Resources is highly encouraging. While previous drilling has intersected mafic and ultramafic intrusions with minor sulphide, this has been the first drilling to demonstrate that significant sulphide mineralisation is present in the region.
A review of the geological and geophysical features of the western Gawler Craton demonstrates why this intersection is unlikely to be the last. Key features of the western Gawler Craton include:
- Curvature of the lithosphere-asthenosphere boundary upwards towards the western edge of the craton.
- Topography on the Moho, with up to 5-10 km offsets across major structures imaged in crustal reflection seismic.
- Trans-lithospheric conductivity zones images in magnetotelluric data that correspond to major structures in regional aeromagnetic data.
- A complex network of anastomosing shear zones, with evidence for poly-metamorphism and syn-deformation intrusion.
- The localisation of different magmatic events into specific regions, with the boundaries between these magmatic domains corresponding to major structural boundaries in the region.
- Variation in (admittedly sparse) radiogenic isotope composition of these magmatic rocks suggesting variation in composition of the deep crust across the region.
- Geochemistry of mafic and ultramafic rocks suggests an enriched mantle source coupled with crustal interaction and assimilation.
- A metasedimentary component to the region, including an Archean to earliest Paleoproterozoic succession (Mulgathing Complex) and sedimentary rocks deposited at c. 1700 Ma and c. 1660 Ma.
- Variable depth of exhumation across the region, with upper crustal lithologies juxtaposed against lower crustal granulites across major structures.
Joining the dots along these lines of evidence suggests a broad geological setting that is comparable to other terranes with proven nickel-copper-PGE resources. In the Gawler Craton melting of previously fertilised lithospheric mantle produced mafic and ultramafic magmas that were able to ascend towards the crust. Assisted by trans-lithospheric structures and complex deformation these magmas were likely focused into the crust during one or more tectonothermal events, where they underwent crustal assimilation. Variation in exhumation depth across the region as a result of the complex anastomosing shear zone network means magma chambers, magma transfer zones and the root zones are likely present in different parts of the region providing opportunities for the formation and preservation of sulphide accumulation zones. Future work in this region including mapping of sparse outcrop, geophysical interpretation, systematic geochronology and ultimately further exploration drilling will assist to better understand this mineral system.
Anthony has an interest in tectonics, magmatism, orogenic processes and the relationship of these to the formation of mineral deposits. Anthony works at the Geological Survey of South Australia.