<div>A groundwater chemistry, regolith chemistry and metadata record for legacy geochemical studies over the southern Curnamona Province done by GA and partners as part of CRC LEME from 1999 to 2005, that was never fully released. This includes comprehensive groundwater chemistry from more than 250 bores in the Broken Hill region, containing physicochemical parameters, major and trace elements, and a suite of isotopes (34S, Pb, Sr, 18O, D). Recent work on this dataset (in 2021) has added hydrostratigraphic information for these groundwater samples. Also included is a regolith geochemistry dataset collected adjacent to some of the groundwater bores which tests the geochemical response of a range of different size fractions, depths and digests.</div>

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.

Exploring for the Future (EFTF) is a four-year (2016-20) geoscience data and information acquisition program that aims to better understand on a regional scale the potential mineral, energy and groundwater resources concealed under cover in northern Australia and parts of South Australia. Hydrogeochemical surveys utilise groundwater as a passive sampling medium to reveal the chemistry of the underlying geology including hidden mineralisation. These surveys also potentially provide input into regional baseline groundwater datasets that can inform environmental monitoring and decision making. Geoscience Australia, as part of the Australian Government’s EFTF program, undertook an extensive groundwater sampling survey in collaboration with the Northern Territory Geological Survey and the Geological Survey of Queensland. During the 2017, 2018 and 2019 dry season, 224 groundwater samples (including field duplicate samples) were collected from 203 pastoral and water supply bores in the Tennant Creek-Mt Isa EFTF focus area of the Northern Territory and Queensland. An additional 38 groundwater samples collected during the 2013 dry season in the Lake Woods region from 35 bores are included in this release as they originate from within the focus area. The area was targeted to evaluate its mineral potential with respect to iron oxide copper-gold, sediment-hosted lead-zinc-silver and Cu-Co, and/or lithium-boron-potash mineral systems, among others. The 2017-2019 surveys were conducted across 21 weeks of fieldwork and sampled groundwater for a comprehensive suite of hydrogeochemical parameters, including isotopes, analysed over subsequent months. The present data release includes information and atlas maps of: 1) sampling sites; 2) physicochemical parameters (EC, pH, Eh, DO and T) of groundwater measured in the field; 3) field measurements of total alkalinity (HCO3-), dissolved sulfide (S2-), and ferrous iron (Fe2+); 4) major cation and anion results; 5) trace element concentrations; 6) isotopic results of water (δ18O and δ2H), DIC (δ13C), dissolved sulfate (δ34S and δ18O), dissolved strontium (87Sr/86Sr), and dissolved lead (204Pb, 206Pb, 207Pb, and 208Pb) isotopes; 7) dissolved hydrocarbon VFAs, BTEX, and methane concentrations, as well as methane isotopes (δ13C and δ2H); and 8) atlas of hydrogeochemical maps representing the spatial distribution of these parameters. Pending analyses include: CFCs and SF6; tritium; Cu isotopes; and noble gas concentrations (Ar, Kr, Xe, Ne, and 4He) and 3He/4He ratio. This data release (current as of July 2021) is the second in a series of staged releases and interpretations from the Northern Australia Hydrogeochemical Survey. It augments and revises the first data release, which it therefore supersedes. Relevant data, information and images are available through the GA website (https://pid.geoscience.gov.au/dataset/ga/133388) and GA’s EFTF portal (https://portal.ga.gov.au/).

<div>Throughout geological history, marine organic-rich shales show variable but appreciable enrichment in uranium (U), < 5 to > 500 ppm. Here we report the results of high-energy resolution fluorescence detection (HERFD) x-ray absorption spectroscopy at U L3 and M4 edges to characterize U speciation in marine sediments.</div><div><br></div><div>We characterised U oxidation state in samples from the Cretaceous Toolebuc Formation of the Eromanga Basin, Australia. Nine samples were carbonaceous shales with high total organic carbon (TOC) content of 5.9 to 13.4 wt&nbsp;% and with low maturity organic matter. Two samples of coquinite were selected for comparison (TOC 0.3 and 2.4 wt %).</div><div><br></div><div>Our results suggest that a significant proportion of U in marine black shales (~20 to 30%) exists as U(VI) (Figures 1-2), despite the extremely reducing (anoxic to euxinic) conditions during sediment precipitation and diagenesis. Within individual samples, spot analyses indicate variation in the estimated oxidation state within a range of ~20% of U(VI). Uranium is unevenly distributed at mm to nanoscale. Nanoscale secondary ion mass spectrometry (NanoSIMS) reveals different associations that often coexist in single samples; nano-particulate uranium is associated with organic matter matrix or sulphide minerals, whereas phosphate minerals display diffuse uranium enrichment. The coquinite has a higher proportion of U(VI), consistent with the dysoxic depositional environment (Boreham and Powell, 1987).</div><div><br></div><div>The unexpectedly enhanced proportion of U(VI) relative to U(IV) within marine organic-rich shales implies that U might not be immediately fixed by reduction processes during sedimentation, but adsorbed by accumulating organic matter, at least in part as U(VI). This is consistent with the behaviour of uranium reported within the water column of the anoxic Black Sea (Anderson, 1989), experiments on U(VI) sorption by organic matter (e.g., Bhat et al., 2008), and previously documented redox state of U from continental organic-rich Eocene (56-34 Ma) sediments of paleochannel and lacustrine origin (Cumberland et al., 2018).</div><div><br></div><div>The results are significant for improving hydrocarbon exploration in known fields (covering the gap to a carbon-free economy without development of new greenfield oil provinces); economic geology (uranium, base-metal, and critical-metal deposits); and environmental management (evaluating potential mobilization of U by groundwaters).</div><div><br></div>This Abstract was submitted and presented to the 2023 Goldschmidt Conference Lyon, France (https://conf.goldschmidt.info/goldschmidt/2023/meetingapp.cgi)

With the increasing need to extend mineral exploration undercover, new approaches are required to better constrain concealed geology, thereby reducing exploration risk and search space. Hydrogeochemistry is an under-utilised tool that can identify subsurface geology and buried mineral system components, while also providing valuable insights into environmental baselines, energy systems and groundwater resources. With this aim, 238 water bores spanning seven geological provinces in the Northern Territory and Queensland were sampled and analysed for major cations and anions, trace elements, stable and radiogenic isotopes, organic species, and dissolved gases. Here, we demonstrate the utility of this dataset for identifying carbonate-rich aquifers and mineral system components therein. First, we use trends in major element ratios (Ca+Mg)/Cl– and SiO2/HCO–3, then strontium isotope ratios (87Sr/86Sr), to define subpopulations that reflect both spatial and compositional differences. We then apply mafic-to-felsic trace element ratios (V/Cs and Cu/Rb) to reveal elevated base metal concentrations near Lake Woods caused by water–rock interaction with dolerite intrusions. Correlated Sr concentrations between groundwater and surface sediments suggest that the geochemical evolution of these mediums in carbonate-dominated terrains is coupled. Our work develops an approach to guide mineral exploration undercover via the characterisation and differentiation of groundwaters from different aquifers, resulting in improved identification of geochemical anomalies. <b>Citation:</b> Schroder, I., de Caritat, P. and Wallace, L., 2020. The Northern Australia Hydrogeochemical Survey: aquifer lithologies, local backgrounds and undercover processes. 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.