Shedding light on sedimentary hosted copper, lead and zinc potential.
The Geological Survey of South Australia (GSSA) is placing considerable emphasis on a holistic and comprehensive approach to the mineral exploration challenge. One of the main pillars is the whole-of-lithosphere and bottom-up approach to mineral exploration, which guides precompetitive data acquisition programs to map mineral systems in South Australia from the deep mantle to the surface of the earth.
One such example of deep-probing methods has been the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) to map the lower crust to upper mantle beneath the state. The project has improved our understanding of the control of the iron oxide – copper–gold (IOCG) systems across the state and spectacular correlations between upper crustal conductors and IOCG deposits near the surface have provided a powerful exploration tool.
Recently, the Exploring for the Future program under the auspice of Geoscience Australia in the Northern Territory has uncovered a deep mantle control on other types of deposits – sedimentary hosted base metals. In a remarkable outcome published June 2020 in the venerable Nature Geoscience journal, Mark Hoggard, Karol Czarnota and co-authors show that the transition between thick and thin mantle lithosphere (marked by the ~170 km depth contour of the lithosphere–asthenosphere boundary) controls the location of ~85% of all global sediment-hosted base metal deposits, including all giant deposits (>10 megatons of metal). These deposit types include sedimentary copper, lead–zinc sedimentary exhalatives, and Mississippi Valley type lead–zinc. Cooler geothermal gradients along craton margins and longer time of rifting during the genesis of deep sedimentary basins result in favourable conditions to scavenge and precipitate metals in sedimentary packages.
For the South Australian exploration challenge – extensive cover across large swaths of greenfield terranes – these new insights provide a novel tool to shed light on the sedimentary hosted deposits in our state through mapping the boundary between thin and thick cratonic lithosphere. While the Northern Territory has been mapped with the acquisition of the AusArray seismic tomography experiment in detail, South Australia so far relies on low-resolution continental-scale models to constrain the location of this elusive boundary.
The GSSA will embark on a seismic AusArray experiment across the central-eastern Gawler Craton. One of the many outcomes will be a map of the lithosphere–asthenosphere boundary thickness in unprecedented detail across this prospective region. In addition to the IOCG potential of the region, it will comprise a precompetitive dataset that informs the potential for sedimentary copper deposits and lead–zinc occurrences. A detailed description of this work and its multiple outcomes for South Australia is presented as a feature article in this issue of the MESA Journal by our new seismologist, Dr John Paul O’Donnell, and co-authors.
These outcomes will additionally complement the GSSA’s new sedimentary copper project in collaboration with CSIRO, which will undertake a basin-wide analysis of the sedimentary cover and its architecture across the Stuart Shelf.
Access the EarthArXiv Preprint of Hoggard et al.’s Nature Geoscience article (provided under Creative Commons Attribution 4.0 International Public License).
– Stephan Thiel, October 2020
