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National Geochemical Survey of Australia

Results of the National Geochemical Survey of Australia Project

Geochemical and bulk properties maps produced by the survey can be viewed and downloaded online from the Geoscience Australia website.

A comprehensive geochemical stream sediment survey has been carried out over most of Australia during the past 4 years as part of the Onshore Energy Security Program, a collaborative project between Geoscience Australia (GA), DMITRE (now part of the South Australian Department of State Development) and other state Geological Surveys.

The objective has been to provide a reference geochemical data set for as wide a range of elements and readily-measured sample parameters as practical. Based on orientation surveys, overbank samples collected from major drainage were chosen as the sampling medium.

The survey is the first, Australia-wide, stream sediment reconnaissance project of its type. Although funding was obtained on the basis of its value to energy minerals exploration, the results have a much wider application being of interest not only to geologists but potentially also environmentalists, pastoralists and those involved in agriculture and community health.

Due to its analytical consistency, the survey provides a geochemical baseline against which comparisons can be made and a source of archival material against future needs.

Relative to most stream sediment surveys, this project had a number of unusual aspects:

  • As already noted, the sampling density was low and this was based on orientation surveys done in SA and NSW by GA. It was found that meaningful data could be obtained from sampling densities as low as one sample per 5000 square kilometres and this sampling density was adopted.
  • The sampling sites were largely pre-selected on the basis of catchment area using a digital elevation model of Australia (from the NASA Shuttle Radar Topography Mission). This allowed exit points from catchment basins to be identified electronically.
  • Overbank samples, rather than material from the active channel, were chosen. This generally had the effect of increasing the proportion of fine sediment in the sample, an important consideration given the large size of the catchment areas.

The notes given here are intended as an index to this work. A more comprehensive coverage can be found in the Geoscience Australia reports noted within each section.

Sampling

At each sample site two samples were taken: one sample from about 15cm depth (Top Outlet Sample) and another from a nominal depth of 60cm (Bottom Outlet Sample) or as conditions dictated.

These were later subdivided into -2mm and -75µm fractions and so, sample permitting, there were four samples from each site available for analysis.

The basic physical and chemical characteristics of each sample were recorded at the time of collection, along with a brief geomorphological description and photograph(s) of the site - see National Geochemical Survey of Australia: Field Manual (File GA10307.pdf) and National Geochemical Survey of Australia: Sample Preparation Manual (File GA14098.pdf).

Analyses

In all, 67 elements were determined (some by more than one method) although the nature of the analysis and the subsample to which it was applied varied, being largely a function of the availability of suitable material and the capacity of the method to deliver a suitable result. There were essentially 5 sample attacks.

  1. Metaborate/tetraborate fusion:  followed by XRF or ICPMS using the HF/HNO3-dissolved bead, where XRF was not suitable. Those done by XRF (12) include Al, Ca, Cl, Fe, K, Mg, Mn, Na, P, S, Si and Ti.  ICPMS elements (43) include Ag, As, Ba, Be, Bi, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Ga, Gd, Ge, Hf, Ho, La, Lu, Mo, Nb, Nd, Ni, Pb, Pr, Rb, Sb, Sc, Sm, Sn, Sr, Ta, Tb, Th, U, V, W, Y, Yb, Zn and Zr
  2. Peroxide fusion: followed by specific ion electrode: used for F
  3. Fire assay (lead fusion):  used for Au, Pt and Pd
  4. Aqua regia: followed by ICPMS (57 elements) Ag, Al, As, Au,  B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Hf, Hg, Ho, In, K, La, Li, Lu, Mg, Mn, Mo, Na, Nb, Nd, Ni, Pb, Pr, Rb, Sb, Sc, Se, Sm, Sn, Sr, Ta, Tb, Te, Tl, Tm, U, V, W, Y, Zn and Zr
  5. MMI™ A proprietary partial leach:  used for 54 elements  (determined by the MMI-ME method) Ag, Al, As, Au, Ba, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Hg, K, La, Li, Mg, Mn, Mo, Nb, Nd, Ni, P, Pb, Pd, Pr, Pt, Rb, Sb, Sc, Se, Sm, Sn, Sr, Ta, Tb, Te, Th, Ti, Tl, U, V, W, Y, Yb, Zn and Zr

Details of the procedures are given in National Geochemical Survey of Australia: Analytical Methods Manual (File GA17349.pdf) and the quality assurance work in National Geochemical Survey of Australia: Data Quality Assessment (Files GA19631.pdf and GA19632.pdf).

Use of the data

In using the data, the nature of the sampling and mode of analysis should be borne in mind. The object has been to provide a representation of the makeup of regional scale areas of approximately 5000 square kilometres and as a consequence of the averaging process, the presence of small and perhaps noteworthy sources of particular elements may become much less obvious. Simplistically, a source which is approximately one square kilometre and averaging 1% of an element should increase the result over background in an outlet sample by something of the order of 2ppm. While there are a large number of other factors which might also reasonably be considered, clearly the results will tend to best reflect those features with a large surface expression, high levels of the element of interest or some combination of the two.

The analytical methods used in this survey represent the best balance that could be achieved for such a broad range of elements but no single analytical method can be ideal for all elements. The existence of a result for a particular method therefore does not guarantee that the result will necessarily be optimal for any particular purpose. Quite apart from the inherent differences in sensitivity of a method with respect to each element, where a method produces results for a large number of elements, the choice of analytical arrangements must necessarily be a compromise since a laboratory procedure which is ideal for say, aluminium, which may be present in tens of per cents, could well ensure that those results for elements present in much smaller amounts, such as silver, are uniformly below the detection limit. In addition, where the method depends on extracting the element from the sample, the nature of the solvating agent/s also needs to be taken into account. This is of particular note when considering the MMI results which by design are a measure of that part of the total concentration that is easily removed and are intended to highlight the transported component. It is worthwhile, therefore, to be selective and choose those results which best suit whatever task is in hand.

Geochemical survey: Zirconium
Results

Given the broad-scale reconnaissance nature of the survey, variations in level might reasonably be expected to be relatively subtle and this was generally the case. However, there were a number of somewhat unexpected and rather eye-catching results among the SA data.

Perhaps the most interesting were those for zirconium and titanium, both of which had maxima well in excess of 10,000ppm in the -75µm fraction in samples from a nominal depth of 60cm (borate fusion). The corresponding results for the surface samples and/or the -2mm fraction were much more subdued, indicating that the zirconium and titanium-rich phase(s) are largely confined to the fine fraction (silt) of the deeper samples and that this fraction constitutes only a very small part of the whole sample. Many of the rare earth elements are to a greater or lesser extent, anomalous in the same general area as is silver. If the magnitude of these results was unexpected, the size of the catchment areas represented by these results, is equally so. 

While it would be misleading to attempt to infer a resource on the basis of a reconnaissance stream sediment programme and a particularly broad-scale survey at that, the results do compare favourably with those from the Ooldea area where there is an established zirconium-titanium sands industry.