Desem, Candan1, Maas,Dr Roland1, Woodhead, Professor Jon1, Carr,Dr Graham2, de Caritat, Dr Patrice3
1The University Of Melbourne, Parkville, Australia, 2CSIRO, North Ryde, Australia, 3Geoscience Australia, Canberra, Australia
The lead (Pb) isotopic composition of the regolith reflects contributions from bedrock geology, mineralisation, wind-blown dust and anthropogenic contamination (industry, transport, agriculture,
residential, waste handling). Evaluating the relative roles of each contribution is critical to many studies ranging from attempts to capture the history and extent of Pb pollution through to mineral
resource exploration programs. To this end we have produced the first Pb isotope map of Australia’s regolith at a continental scale. Catchment outlet (~ floodplain sediment) samples collected for
Geoscience Australia’s National Geochemical Survey of Australia (NGSA) program, with an average sampling density of 1 site/5200 km2 and covering ca. 81% of the Australian continent, were utilised here. The coarse grain-size fractions (<2 mm) of the top outlet sediment (0-10 depth) samples were selected.
A number of acid leaching protocols have been devised to separate loosely bound Pb (e.g. aerosol from anthropogenic contamination) from Pb structurally bound in minerals (from the underlying
geology, mineralisation, or their weathering products). We utilised a sequential leach protocol with ammonium acetate followed by aqua regia (HNO3-HCl), originally developed by Mike Korsch and Graham Carr at CSIRO. Ammonium acetate is expected to extract loosely bound Pb sourced from windblown dust (including anthropogenic contributions) or from shallow groundwater interaction. Pb extracted using the more aggressive aqua regia step represents the underlying geological signature. Pb isotope compositions were acquired using a sector-field ICP-MS, which provided ‘fit for purpose’ levels of precision/accuracy and the high throughput required in order to process large sample sets – more than 1500 samples in this case.
Our research program aims to (i) compare the Pb isotope signatures released by the two leach protocols, (ii) examine if soil Pb isotope mapping can identify underlying geology and metallogenic
provinces, and (iii) investigate anthropogenic signals across the continent. Preliminary analysis of the data obtained for the aqua regia leachates shows clear and distinct trends reflecting the underlying bedrock geology. For example, the Archaean rocks of the Yilgarn and Pilbara Cratons are marked by regolith with more radiogenic Pb isotope compositions, while regolith samples from younger geological provinces (e.g. Tasmanides of eastern Australia) have less radiogenic Pb isotope signatures. Other geological elements (e.g. Gawler Craton, Curnamona Province) are also clearly distinguished. Preliminary Pb isotope maps of the continent will be presented.
Candan is a PhD candidate at the University of Melbourne, School of Earth Sciences. In a collaboration with Geoscience Australia, Candan’s PhD investigates the use of Pb isotopes in the regolith as tracers for environmental contamination and mineral exploration. Candan has developed the first Pb isotope regolith map of Australia.