Australia’s national geoscience agency, Geoscience Australia, and the corresponding agencies from the States and the Northern Territory commenced a collaborative project in 2007 aimed at delivering a layer of pre-competitive geochemical data at the national scale. The resulting dataset will provide the energy and mineral exploration industry with a picture of the background geochemical concentration levels and patterns in Australia.
• The sampling strategy was developed during three pilot projects, and takes advantage of natural weathering and transport processes that form well-mixed, fine-grained sediments, which represent, as much as possible, the average composition of large catchments. To this end, outlet sediments, which in most cases are similar to floodplain or overbank sediments, are being collected at ~1400 target sampling sites determined by terrain and hydrologic modelling. The resulting average sampling density is ~1 site/5500 km2.
To ensure internal consistency, a detailed field manual was produced, field training was provided to sampling teams in all the States and the Northern Territory, and equipment and consumables were centrally purchased and distributed to the field teams. At each site, outlet sediment samples are collected from the surface (0-10 cm) and from a deeper level (between ~60 and 80 cm). Field data, e.g. GPS coordinates, Munsell® soil colour and field pH, are measured and recorded electronically; digital photographs are taken at each site and collated.
In the laboratory, the collected samples are dried, split and sieved prior to analysis for more than 60 elements on two grain size fractions (<2 mm and <75 µm). The analytes include all major elements and a comprehensive suite of minor and trace elements determined by XRF and ICP-MS (after total digestion) in Geoscience Australia’s laboratories. Particular elements (F, Se, PGE’s) will be analysed separately using dedicated methods in external laboratories. The concentrations of Au and other elements will be determined on aqua regia leaches, also in external laboratories.
In addition, bulk analyses, such as pH and EC of 1:5 (soil:water) solutions, and laser particle size analysis will be carried out at GA. Quality control measures include field duplicates, laboratory duplicates, blanks, internal standards and certified reference materials interspersed at regular intervals in the sample suites presented to the laboratories. A split of each bulk sample is archived for future investigations.
As at January 2008, almost a quarter of the sites in the national coverage have been sampled and sample preparation is under way. Analyses are planned for late 2008 and 2009, after which interpretation and reporting will commence. The project is scheduled for completion in mid-2011. Products will include a database, a geochemical atlas and reports, which will be made available via Geoscience Australia’s website. For more information please see www.ga.gov/ngsa.