Mineral exploration in Australia faces the challenge of declining discovery rates despite continued exploration investment. The UNCOVER roadmap, developed by stakeholders from industry, government and academia, has highlighted the need for discovering mineral resources in areas of cover. In these areas, potentially prospective basement is covered by regolith, including transported sediment, challenging many traditional exploration methods designed to probe outcrop or shallow subcrop. Groundwater-mineral interaction in the subsurface has the potential to give the water geochemical and isotopic characteristics that may persist over time and space. Geoscience Australia’s hydrogeochemistry for mineral exploration project, part of the Exploring for the Future Programme, aims to use groundwater chemistry to better understand the bedrock-regolith system and develop new methods for recognising mineral system footprints within and below cover. During the 2017 dry season (May to September), ~150 groundwater samples (including QC samples) were collected from pastoral and water supply bores in the regions of Tennant Creek and McArthur River, Northern Territory. The Tennant Creek region has a demonstrated iron oxide-hosted copper-gold-iron(-bismuth) mineral potential in the Paleoproterozoic and Mesoproterozoic basement and vast areas of regolith cover. Among the critical elements of this mineral system, the presence/absence of redox contrasts, iron enrichment, presence of sulfide minerals, and carbonaceous intervals can potentially be diagnosed by the elemental and isotopic composition of groundwater. The McArthur River region, in contrast, has demonstrated sediment-hosted stratiform lead-zinc-silver mineral potential in the Paleoproterozoic to Neoproterozoic basement and also vast areas of regolith cover. Here, critical mineral system elements that have the potential to be identified using groundwater geochemistry include the presence of felsic rocks (lead source), carbonate rocks (zinc source), basinal brines, dolomitic black shales (traps), and evaporite-rich sequences. Preliminary results will be presented and interpreted in the context of these mineral systems.

Modern geochemical exploration arguably started in the early XXth century when Victor Moritz Goldschmidt developed the geochemical laws of the distribution of chemical elements in Earth systems. These laws allowed an unprecedented understanding of element mobility, trapping, and associations that revolutionized the appreciation of processes including ore deposit formation, hydrothermal alteration, supergene enrichment, and geochemical dispersion. Direct or indirect application of these laws underpinned countless mineral deposit discoveries by geochemical techniques for decades. Many of those deposits, however, were located at or close to the Earth’s surface. The relentless growth in world population, combined with societal pressure to deliver ongoing economic growth and aspirations to better living standards by poorer nations, drives rising demand for resources worldwide. Discoveries of major mineral resources that are easily accessible at or near the Earth’s surface are becoming rarer and those that are known are being produced relentlessly. This inexorably drives current and future mineral exploration toward deeper resources that are more difficult to discover. There is a continuum of scenarios from deep deposits in bedrock-dominated terrain, through those covered by but a few meters of in-situ weathering profiles or soils, to those concealed by 10s to a few 100s of meters of transported (allochthonous) sedimentary cover. The current arsenal of geochemical techniques includes (1) targeting sampling media that are or have been in contact with deep environments (groundwater, soil gas, biota including deep-rooted plants and termitaria), and (2) using chemical extractions on surface materials that have the potential to isolate ions or molecules that may have moved up through a regolith profile post-mineralisation (e.g., weak extractions, nanoparticles, soil hydrocarbons). The future challenges of exploration geochemistry will be dominantly in these covered terrains. I believe that a combination of (1) more powerful data analysis techniques, including advanced multivariate statistics accounting for the compositional and geospatial nature of geochemical data and machine learning, and (2) the integration of geochemical data with other geoscientific data, such as geophysics, geology and spectroscopy, hold exciting promise for the future. This Abstract was presented at the 2017 Goldschmidt Conference (https://goldschmidt.info/2017/)

The mineral resources sector plays a vital role in Australia’s ongoing economic prosperity. The sector dominates the nation’s export earnings, provides substantial direct and indirect employment and investment in regional and indigenous communities, supports downstream and service industries, and delivers essential revenue to governments.

Real advances in understanding geology for mineral, energy and groundwater resource potential of the Australian continent will come from unveiling what lies at depth, especially in the extensive under-explored regions that are obscured by cover. In this context, airborne electromagnetics (AEM) is a geophysical method at the forefront in addressing the challenge of exploration undercover. In collaboration with the state and territory geological surveys, Geoscience Australia has led a national initiative whose goal is to acquire AEM data at 20 km line spacing across Australia. This initiative, AusAEM, represents the world’s largest AEM survey flown to date; it has covered ~2.5 million km2, a substantial part of northern Australia, and is providing new insights in areas that have not been extensively explored. Regional models of subsurface electrical conductivity derived from AusAEM data support a range of applications that include geological mapping, mineral and petroleum exploration, watershed management and environmental studies. <b>Citation:</b> Ley Cooper, A.Y.. and Brodie, R.C.., 2020. AusAEM: imaging the near-surface from the world’s largest airborne electromagnetic survey. In: Czarnota, K., Roach, I., Abbott, S., Haynes, M., Kositcin, N., Ray, A. and Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.