Theory
In spite of recent hype and rhetoric, partial dissolution of elements from samples as a tool to aid exploration is not new. In a paper in "Journal of Geochemical Exploration, April 1984", by TT Chao, of the US Geological Survey, there are references to 195 papers dealing with the extraction of elements from soil and rock samples and many of these papers were published in the 1940/50's. As an exploration tool, these techniques have a primary aim of enhancing soil element anomalies as an aid to exploration.
Partial dissolution is always less vigorous than those digests which will extract the bulk of an element and is often targeted at specific mineral phases associated with the sample. It has long been established that the oxides of Iron and Manganese exercise significant influence on the distribution of element dispersion haloes surrounding mineralisation in the weathered profile. Iron and Manganese ions, which are more soluble at depth, migrate to the surface and precipitate in the more oxygenated environment forming an amorphous coating over mineral grains. These coatings have many active reaction sites that facilitate the adsorption of migrating ions onto these phases and over a period of time, these coatings are enriched with other transported metal ions. If the whole sample was digested and analysed, the concentration of transported ion would be only a small fraction of the total concentration of the element in question and potential anomalous indicators would be lost in the total concentration of the element. However if a technique could be devised where only the amorphous coatings on the grains could be dissolved, then an indication of only the mobile ions could be seen. Partial digests of the sample attempt to address this issue.
Chao's paper examines a range of partial dissolution techniques, ranging from water soluble components through to acid mixtures that would approach total extraction of many elements. He also reports on the approach of using increasingly stronger reagents to extract specific phases in sequence.
One of the difficulties experienced by the early researchers of these techniques was the absence of highly sensitive instruments that could be used for large numbers of samples and for a wide range of elements. The directors of Ultra Trace Pty Ltd purchased the first ICP-MS instrument in Australia in 1985 and this technique has filled this analytical void. Using this instrumentation, Ultra Trace now offers a range of partial digestion techniques for sample evaluation. Detection limits are very low and in general, most elements will be present in detectable quantities. Such low detection limits can be achieved for two reasons. Firstly we do not have to dilute the digest a great deal because not all the sample is dissolved in the digest, and secondly, our later generation ICP-MS coupled with Flow Injection technology further enables the analysis of stronger solutions.
Because these partial digests target the coatings on mineral grains, the techniques by their nature are usually applied to soil samples. To ensure the maximum surface area is available during digestion we recommend that soil samples be sieved to -80 mesh. In general terms, the finer the sample the better since as the grain size is reduced, the surface area per unit mass increases and then the amount of coating extracted compared to the bulk of the grain is improved. Sample submissions to date have ranged from -200 mesh (75 micron), up to 2 mm (2000 micron). The analysis only requires a small sample weight (though at least 50 grams should be submitted) and at some sites it may be well nigh impossible to gather enough -75 micron material. Most submissions to date have been of -80 mesh size fraction and the technique seems to work extremely well on this material. The technique comes into its own where overburden is thick and with poor outcrop, which makes it ideal for many parts of Australia.
One aspect of the technique we have noticed is that indicator elements (mobile elements) will be different from one geological region to another. For this reason,we recommend that the full appraisal suite of elements is analysed on a number of samples from a suspected mineralised area and anticipated barren areas. Full scale regional exploration can then be carried out by analysing only for those elements, which are relevant to the target being sought in this environment.
In general, the detectable dispersion halo for Gold is greater in our partial leach than in the more conventional techniques. It is however recommended that anomalous samples be analysed for Gold by the Low Level Aqua-Regia method (detection limit 0.2ppb) to determine total Gold.
Ultra Trace are very well placed to advise on the application of these techniques since the person involved in the development of the methodology, Mr Alex Christ, is a geologist. Alex gained his degree at the University of Adelaide but for the last few years has become a leader in the application of ICP-Mass Spectrometry as applied to analysis of samples for mineral exploration. Alex would be pleased to discuss any of the above issues and can assist in interpretation of these and other analytical techniques.