ABSTRACT: Building on method developments achieved during a series of precursor pilot projects, the National Geochemical Survey of Australia (NGSA) project targets catchment outlet (overbank) sediments as a uniform sampling medium. These transported, fine-grained materials are collected (from a shallow and a deeper level) near the lowest point of 1390 catchments, which cover 91% of the country. Dry and moist Munsell® colour, soil pH and electrical conductivity and pH of 1:5 (soil:water) slurries are recorded and laser particle size analysis and infrared spectroscopy are performed. The dried samples are sieved into two grain-size fractions (<2 mm and <75 mm) that are analysed by x-ray fluorescence (XRF) and inductively-coupled mass spectrometry (ICP-MS) (multi-element, total analyses), by ICP-MS after aqua regia digestion (multi-element, including low level gold), and specialised methods for platinum group elements, fluorine and selenium. At the time of writing, 78% of the samples have been collected and most analyses are completed for the first 25% of samples. The project is due for completion in June 2011.

Outlet sediment (or overbank) samples from 99 catchments in the Thomson region have been examined by conventional geochemical analytical methods and by partial extraction using Mobile Metal Ion (MMI) leach. Elements such as Pb have good correlation with known mineral deposits using conventional (near-total digestion) methods, whilst elements such as Cu, Au and Ag show a better correlation with known mineral deposits when MMI concentrations are used. This study shows that very low density sampling of catchment outlet sediments (1 site/1540 km2) provides useful and possibly predictive geochemical information for mineral exploration in areas dominated by transported regolith.

Legacy product - no abstract available

The National Geochemical Survey of Australia (NGSA) project aims to provide pre-competitive data and knowledge to support exploration for energy resources in Australia (www.ga.gov.au/ngsa) and is part of Geoscience Australia's Onshore Energy Security Program. The NGSA project, which is carried out in collaboration with the geological surveys of all States and the Northern Territory, will deliver the nation's first geochemical atlas, a comprehensive geochemical database and preliminary reports when it concludes in June 2011. Catchment outlet sediments (similar to floodplain sediments in most cases) were sampled during 2007-2009 near the outlet of 1186 catchments covering ~80% of the country (average sample density 1 sample per 5500 km2). Samples were collected at two depths: 0-10 cm below the surface (top outlet sediment, TOS) and between on average 60 and 80 cm depth (bottom outlet sediment, BOS). Samples were dried and sieved to either <2 mm (coarse) or <75 mm (fine) fractions, yielding four samples per site. The geochemical data presented here is from total element content analysis by x-ray diffraction after lithium borate fusion (for potassium) and collision cell inductively coupled plasma-mass spectrometry after lithium borate fusion followed by hydrofluoric and nitric acid digestion (for thorium and uranium). (.../... truncated)

Map(s) of Al (aluminium) concentration (Total content, Aqua Regia soluble content, and/or Mobile Metal Ion soluble content) in Top Outlet Sediment (TOS) and/or Bottom Outlet Sediment (BOS) samples, dry-sieved to <2 mm and/or <75 um grain size fractions. Source: The Geochemical Atlas of Australia (Caritat and Cooper, 2011)

Map(s) of Au (gold) concentration (Total content, Aqua Regia soluble content, and/or Mobile Metal Ion soluble content) in Top Outlet Sediment (TOS) and/or Bottom Outlet Sediment (BOS) samples, dry-sieved to <2 mm and/or <75 um grain size fractions. Source: The Geochemical Atlas of Australia (Caritat and Cooper, 2011)

Map(s) of Bi (bismuth) concentration (Total content, Aqua Regia soluble content, and/or Mobile Metal Ion soluble content) in Top Outlet Sediment (TOS) and/or Bottom Outlet Sediment (BOS) samples, dry-sieved to <2 mm and/or <75 um grain size fractions. Source: The Geochemical Atlas of Australia (Caritat and Cooper, 2011)

Map(s) of Ca (calcium) concentration (Total content, Aqua Regia soluble content, and/or Mobile Metal Ion soluble content) in Top Outlet Sediment (TOS) and/or Bottom Outlet Sediment (BOS) samples, dry-sieved to <2 mm and/or <75 um grain size fractions. Source: The Geochemical Atlas of Australia (Caritat and Cooper, 2011)

Map(s) of Cd (cadmium) concentration (Total content, Aqua Regia soluble content, and/or Mobile Metal Ion soluble content) in Top Outlet Sediment (TOS) and/or Bottom Outlet Sediment (BOS) samples, dry-sieved to <2 mm and/or <75 um grain size fractions. Source: The Geochemical Atlas of Australia (Caritat and Cooper, 2011)

Map(s) of Cu (copper) concentration (Total content, Aqua Regia soluble content, and/or Mobile Metal Ion soluble content) in Top Outlet Sediment (TOS) and/or Bottom Outlet Sediment (BOS) samples, dry-sieved to <2 mm and/or <75 um grain size fractions. Source: The Geochemical Atlas of Australia (Caritat and Cooper, 2011)