Understanding the cover in the Gawler Craton, South Australia: combining landscape variability, linear structures, and regolith mapping to assist mineral exploration

Ignacio González-Álvarez1,2, Carmen Krapf3, Uli Kelka1, Cericia Martínez1, Thomas, Albretch4, Tania Ibrahimi1, Mark Pawley3, Jonathan Irvine3, Anna Petts3, Jens Klump1

1CSIRO, Mineral Resources, Discovery Program, Perth, Western Australia, Australia; 2University of Western Australia, Centre for Exploration Targeting, Perth, Western Australia, Australia; 3Geological Survey of South Australia, Department for Energy and Mining, Adelaide, Australia; 4Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, Australia

Successful mineral exploration in Australia has sharply decreased over the past few decades. With >75% of South Australia’s land surface covered by transported regolith material, exploration is a significant challenge. Regolith integrates the expression of geology, climate, groundwater, topography, and geomorphological processes. Landscape domains and their stratigraphy record the 4D architecture of the overburden, and capture the relationship of the surface and cover to the underlying basement geology. To understand regolith composition and distribution, as well as the landscape variability, surface mapping can potentially be a cost-effective and powerful exploration tool. To date, this has involved traditional field-mapping techniques accompanied by remote sensing and geophysical data.

The Archean basement in South Australia has numerous major structures that are expressed at surface as lineaments, landforms and possibly as boundaries between different landscape domains. Several studies on neotectonic features highlighted the potential significance of these structures as geochemical pipelines that connect basement rocks with the surface.

In this study, we integrate regolith mapping, linear surface features, and landscape variability in the central Gawler Craton, South Australia. We aim to enhance the understanding of how basement structures such as faults and shear zones may affect present-day landscapes and surface lineaments. We also explore if the integration of landscape variability, surface lineaments, basement features and regolith domains, can provide sample areas that may efficiently record geochemical footprints from underlying basement rocks.

This project tests the conceptual variability of landscape domains across the study area, and offers insights into possible geochemical dispersion pathways from depth through cover to surface.


Biography

Carmen is working for the Geological Survey of South Australia as a Senior Geologist focusing on characterising South Australia’s cover and emerging mineral systems in cooperation with CSIRO utilising machine learning and automated mapping. Carmen is also involved in the National Exploration School Under Cover School (NeXus).

(U-Th)/He-dating of Iron oxides: Towards establishing a temporal framework for landscape evolution and regolith development in Southwest Western Australia

Wells, MA1, González-Álvarez, I2,3 and Danišík, M4

1John de Laeter Centre (JdLC), Curtin University, Perth, WA 6102, Australia; 2CSIRO, Mineral Resources, Discovery Program, Perth, Australia; 3University of Western Australia, Centre for Exploration Targeting, Perth, Australia; 4John de Laeter Centre for Isotope Research, Applied Geology, Curtin University, Perth, Australia

Understanding the temporal formation of iron oxides can provide important clues for understanding landscape evolution, weathering processes and past climatic conditions. Recent application of (UTh)/He-dating to iron oxide assemblages in iron ore deposits1 and in weathering systems in Western Australia, analysed in the John de Laeter Center (JdLC) GeoHistory Facility, have demonstrated the efficacy of the technique for the geochronological analysis of iron oxide-rich systems. Hence, the iron oxide-rich duricrust, capping the deeply weathered regolith of the Darling Range in the south-west of Western Australia, provides a suitable geochronological proxy that provides a time-integrated record of the weathering processes that have shaped regolith formation.

Despite numerous studies focused on laterite formation in the Darling Range , there is a paucity of data pertaining to the age or timing of laterite formation. The exceptions are for exposures of lateritic duricrust at two sites along the Darling Scarp. Early work on lateritic duricrust near Toodyay (50 km northeast of Perth), yielded (U-Th)/He ages of 10.0–7.5 Ma (Late Miocene) and more recently, findings arising from a pathfinder JdL-GSWA project , measured comparable Late-Miocene/Early Pliocene, to possible Early-Pleistocene (U-Th)/He ages of 5.7–1.3 Ma for pisolitic and fragmental duricrust in lateritic profiles at the Boddington Gold Mine (100 km southeast of Perth).

Further east, in the central Yilgarn, a recent JdL-CSIRO pilot study also measured Late Miocene/Pliocene ages (7.9–3.5 Ma) for nodular duricrust exposed near the Blue Haze Au mine, Forrestania (120 km southeast of Southern Cross). An exception in this study was an Early Oligocene age (34.8 +/- 3.8 Ma) for one example of nodular duricrust. Given the small number of
samples examined in this study, an explanation for the much older age is currently open to interpretation. However, the generally comparable (U-Th)/He ages for lateritic duricrust across these
widely spread locations in southern WA, suggest that the processes of regolith formation and/or modification were, broadly, regionally synchronous. These initial findings help to underscore and
provide the impetus in establishing a temporal framework for duricrust formation, which may also provide a record of past processes that have helped in shaping landscape evolution in the broader, Southwest Western Australia.

Acknowledgements
The authors would like to thank Classic Minerals for their support and permission in publishing the Forrestania dating work. The Boddington dating results derive from a project funded by the Geological Survey of Western Australia (GSWA) from the Exploration Incentive Scheme. Martin Danišík acknowledges the support of a Curtin Senior Research Fellowship.


Biography

In 1997, Martin joined CSIRO to study WA’s Ni laterite and Pilbara iron ore deposits. In 2017, Martin joined the John de Laeter Centre to investigate (U-Th)/He-dating of WA regolith and characterize e-metal (Li,V) ores. Throughout his career, Martin has investigated the element and mineral distribution of these varied deposits.

A fresh Look at the Stratigraphy of the Lefroy Palaeodrainage System, Eastern Goldfields, WA.

Lynham, Leah1

1James Cook University, Townsville, Australia

Buried palaeovalley sediments within the Lefroy and Cowan palaeodrainage systems and other palaeodrainage systems in the wider Eastern Yilgarn craton contain economically viable deposits of alluvial gold (Oxenburgh et al., 2017). The Lefroy Palaeodrainage System and other, parallel systems are said to have formed pre-Jurassic (Clarke, 1994), potentially from glacial scouring during the Late Palaeozoic (Eyles and de Broekert, 2001), before a depositional regime led to ~100m of buried fluvial, marginal marine and marine sediments being deposited in the buried palaeovalleys.  Little is known about the geological controls on the activation of the system, the origin of the basal placer deposits or the sedimentary controls on the channel infill. Unravelling the geological controls on the incision of the palaeovalley network and infill of the palaeochannel sediments within the Lefroy palaeodrainage System could have positive implications for future alluvial gold exploration within the area.  

Previous studies have suggested that the Lefroy and Cowan systems became separated due to uplift along the Jarrahwood axis prior to Eocene deposition (eg (Clarke, 1994, de Broekert and Sandiford, 2005), although studies disagree on how far North the Jarrahwood Axis transects the Cowan System. Building on the foundations of already completed work, new palaeocurrent data from field observations, and new stratigraphic data from the Neptune Open pit, and diamond drill cores from the area help to refine the stratigraphy of the Lefroy Palaeodrainage System, and the palaeoenvironment leading to the deposition of the Lefroy Palaeodrainage System sediments.

CLARKE, J. D. A. 1994. Evolution of the Lefroy and Cowan palaeodrainage channels, Western Australia. Australian Journal of Earth Sciences, 41, 55-68.

DE BROEKERT, P. & SANDIFORD, M. 2005. Buried Inset‐Valleys in the Eastern Yilgarn Craton, Western Australia: Geomorphology, Age, and Allogenic Control. The Journal of Geology, 113, 471-493.

EYLES, N. & DE BROEKERT, P. 2001. Glacial tunnel valleys in the Eastern Goldfields of Western Australia cut below the Late Paleozoic Pilbara ice sheet. Palaeogeography, Palaeoclimatology, Palaeoecology, 171, 29-40.

OXENBURGH, S., FALCONER, M., DOUTCH, D., EDMONDS, P., FOLEY, A. & JANE, M. 2017. Kambalda – St Ives goldfield. In: PHILIPS, N. (ed.) Monograph 32 – Australian Ore Deposits. The Australasian Institute of Mining and Metallurgy.


Biography

Leah is Researcher with the Graduate Research School at James Cook University. Her current work concentrates on reconstructing the middle-Late Eocene palaeoenvironment of the Eastern Goldfields Region, Prior to her position at JCU she worked as an exploration geologist on various projects throughout WA.

Southwestern Australian paleovalleys evolution: A case of landscape evolution on Archean basement

Heilbronn, Kelly1, González-Álvarez, Ignacio2 and Klump, Jens2

1Geosciences, College of Science and Engineering, James Cook University, Townsville, Queensland, Australia. 2CSIRO, Mineral Resources, Discovery Program, Kensington, Western Australia, Australia

 Landscape features (e.g. paleovalleys) result from the interaction of geology, climate, sea-level changes, sedimentary dynamics and tectonism though time. The Digital Elevation Model (DEM) in SW Australia displays a dendritic to sub-dendritic pattern, interpreted as the surface expression of large buried paleovalleys that cut across geological provinces within the Yilgarn Craton. These paleovalley patterns terminate at the southern boundary of the Yilgarn Craton and are not traceable into the Albany-Fraser Orogen. Paleovally traces which existed in the Albany-Fraser region were later removed. The separation of Australia from Antarctica (Cretaceous) resulted in the deposition of large volumes of eroded sediment from the Albany-Fraser region into the Bremer Sub-basin. Any final traces of paleovalleys from within the Albany-Fraser region were later eliminated due to erosional action of transgression-regression cycles. The Yilgarn Craton, however, has preserved traces of paleovalley due to lack of sea-level influence, and a landscape evolution dominated by river systems.

The dendritic pattern of the large paleovalleys in SW Australia is similar to modern glacial dominated valleys. This observation supports that paleovalleys in SW Australia were carved by glaciers before the breakup of Gondwana, during the Carboniferous-Permian glaciation when ice sheets progressed NNW from Antarctica towards Australia, over the Albany-Fraser Orogen and into the Yilgarn Craton. Additionally, some paleovalleys exhibit unusual morphologies displaying tributaries that narrow downstream and divert abruptly from the overall NNW drainage trend. These peculiar paleovalleys are up to 6 km wide upstream. This study links these unusual morphologies, such as the sudden directional changes from overall NNW drainage trend, with underlying Archean basement architecture.  We related magnetic data with DEM sub-products (Multi-resolution Valley Bottom Flatness map (MrVBF), Gallant and Dowling, 2003), revealing that linear features in the magnetics correlate with these unusual paleovalley features in the MrVBF map. This supports that basement structures influenced the narrower width and abrupt change in water flow direction of these paleovalleys when they were active, and were fundamental in the drainage and landscape development of this part of Australia.


Biography

Kelly Heilbronn is a PhD Candidate at James Cook University working on the Mesozoic tectonic evolution offshore eastern Australia. Kelly completed an internship with CSIRO in 2018 under supervision of Dr Ignacio González-Álvarez and Dr Jens Klump focusing on the evolution of southwestern Australian paleovalleys from the Palaeozoic until today.

Australia and Brazil: Contrasting Weathering and Erosion Histories but Similar Cratonal Landscapes

Paulo Vasconcelos1

1School of Earth and Environmental Science, the University of Queensland, Brisbane, Qld, Australia.

Australia is sparsely vegetated and the flattest, hottest, most tectonically stable but fastest latitudinally moving continent on Earth; it has migrated from high to low latitudes in ~50 Ma. In contrast, Brazil is a densely vegetated, wet, high relief cratonal terrane that has moved slowly longitudinally along the Equator for the past ~70 Ma. Despite these contrasting underlying geological and geographical characteristics, landscapes in these two regions are remarkably similar, share analogous ancient weathering histories, and are marked by plateaus surrounded by dissected plains that erode similarly and at equivalent rates.  In situ cosmogenic isotopes show that plateaus eroded at less than 2 m.Ma-1, while dissected plains erode at 5-20 m.Ma-1. Cosmogenic isotope concentrations in sediments show greater erosion rates, suggesting that erosion focusses preferentially along escarpments. 40Ar/39Ar geochronology on Mn-oxides and (U-Th)/He-4He/3He geochronology on goethite show that ancient weathering profiles blanketing plateaus in both continental areas are as old as ~90-70 Ma. These results are substantiated by in situ cosmogenic 3He concentrations in hematite. Geochronological results for the surrounding plains, on the other hand, indicate that the dissected areas are typically younger than ~30 Ma. Weathering profiles in both continental areas, on opposite sides of the planet, host supergene mineral populations that record analogous and often contemporaneous events of water-rock interactions through time. Importantly, these plateaus have been continuously emergent throughout their entire histories, hosting in the weathering profiles underneath minerals precipitated under contrasting climatic conditions through time. Interestingly, the analogous weathering histories of the two continental areas are mostly recorded in oxides, hydroxides, and clay mineral assemblages. The distribution and abundance of sulphates, carbonates, and silica-minerals, on the other hand, mark significant contrasts between the two landmasses. The most striking contrast is weathering under water deficient conditions in Australia while Brazil weathered under pronounced oversupply of rainwater. It is remarkable that such differences in climatic conditions produced such similar resulting landscapes. This is only possible because in Brazil, iron oxyhydroxides and ferricretes provided the landscape scaffolding that is provided in Australia by silica minerals and silcretes.


Biography

Paulo Vasconcelos is a geologist specialised in the development and application of novel geochronological tools for investigating earth and planetary processes.

About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

As a broadly based professional society that aims to represent all Earth Science disciplines, the GSA attracts a wide diversity of members working in a similarly broad range of industries.