Selway, Kate1, Dentith, Michael2, Gessner, Klaus3
1Department of Earth and Environmental Sciences, Macquarie University, Australia; 2Centre for Exploration Targeting, School of Earth Sciences, The University of Western Australia, Crawley, WA 6009, Australia; 3Geological Survey of Western Australia, East Perth, WA 6004, Australia
The Eastern Goldfields Superterrane, in the Yilgarn Craton, Western Australia, is one of the most highly mineralised regions on Earth, hosting world-class orogenic gold and nickel-sulfide deposits. Mineral systems models for both of these deposit types suggest that lithospheric-scale processes are involved in their formation. Therefore, lithospheric-scale geophysical imaging is a promising tool to improve understanding of the formation of the deposits and to aid future exploration.
Long-period magnetotelluric (MT) data were collected over an approximately 250 km x 200 km area covering the western part of the Eastern Goldfields Superterrane and the eastern Youanmi Terrane. The survey region covers the Kalgoorlie and St Ives gold camps and the Kambalda nickel camp, as well as the Ida Fault, a prominent isotopic boundary between the older Nd model ages of the Youanmi Terrane and the younger Nd model ages of the Eastern Goldfields Superterrane. A 3D conductivity model was produced from the data, with good resolution to depths of 150 to 200 km.
Results show that the lithospheric mantle from depths of approximately 100 to 150 km is more conductive (~10 to 100 ohm m) beneath the Youanmi Terrane than the Eastern Goldfields Superterrane (>100 ohm m). Crustal conductivity is more heterogeneous but most of the strongly conductive regions (<100 ohm m) are located in the Eastern Goldfields Superterrane. The resolution of the model in the near-surface is insufficient to make a detailed comparison with the locations of known deposits, but most upper crustal conductors are spatially correlated with regional-scale faults, which are inferred to be important in the formation of orogenic gold deposits.
Anomalously conductive zones in tectonically stable regions often indicate past metasomatism, either through the hydration of nominally anhydrous minerals or the growth of conductive mineral phases such as amphibole or phlogopite.
Quantitative interpretation of the MT model shows that the mantle conductors in the Youanmi Terrane are too conductive to be explained purely by hydrated peridotite and imply the presence of hydrous metasomatic minerals. The observed patterns of lithospheric conductivity suggest a more complex relationship between mantle metasomatism and gold and nickel mineral systems than expected from previous studies.
Kate, Mike and Klaus are all interested in the multi-disciplinary application of geophysical, geological and geochemical data to understanding tectonic evolution and mineral systems.