UltraFine+® The Method

The next-generation soil sampling method for transported cover

Greenfields mineral exploration in transported cover is often hindered by a failure to detect, understand, and evaluate near surface geochemical anomalies. The UltraFine+® method was designed specifically to address this challenge.

Developed by CSIRO, LabWest and 8 industry sponsors as part of the MRIWA M462 Project 2016-2018.

Soil chemistry is often the first pass physical prospectivity sampling conducted in areas of shallow transported cover. However, for too long we have applied techniques that are successful in residual and outcropping areas (e.g., 4-acid digestion lithogeochemistry). While great for these settings, if your sampled material is 95 % barren quartz in sand dunes, then 95 % of your assayed materials are not adding value to your analysis. If an ore deposit weathers and its components disperse into the transported cover above, those target and pathfinder elements of interest are charged, mobile, small, and keen to adsorb to surfaces.

These adsorptive surfaces are provided by clays, Fe-oxides and other scavenging phases. These also happen to be the smallest particle size fractions. The UltraFine+® method was developed to separate the <2 µm “ultrafine” soil fraction for multielement analysis, effectively concentrating the phases we are interested in. Now that it is physically separated (not chemically desorbed like other partial extractions focus on), the ultrafine material is aggressively digested to recover nearly all the elemental signature possible. In addition, there is much more information that can be gleaned from our soil samples and the method is amenable to measuring these parameters. To add context and understanding to our elemental signatures, we collect physico-chemical parameters like pH, electrical conductivity, bulk soil particle size and various spectral reflectance measurements to give an indication of the mineralogy. Armed with all this information, we can potentially explain more variance in the data, look for more meaningful trends (e.g., by using machine learning approaches) and more.

Pioneering soil scientists and mineralogists have long established the link between surface area and absorptive capacity and particle sizes.  Early test work by CSIRO and collaborators separated these size fractions in sand dunes and other materials and show where Au, Cu, Zn and other metals are preferentially located.

Pie chart showing in which size fraction of sand dune samples (average of 14 samples) most of the Au is concentrated (modified from Noble et al., 2013). Even though ~95 % of these samples were sand (>53 µm), most of the Au is contained in the <2 µm, ultrafine size fraction. The UltraFine+® soil analytical method was developed to capitalise on this stark difference and address this challenge.

The new method shows more reproducible and reliable results, with 100 – 250 % increased concentrations of Au, Cu and Zn than from the traditional <250 µm fraction, and removal of the nugget effect for Au (Noble et al. 2018). Of most relevance to gold explorers, the study showed a marked decrease in censored results for Au (~67 % to 10 % below the detection limit). Successful detection of mineralisation through cover has been shown for Au, Cu and base metal deposits, and we continue to expand into other commodity targets with new research.

Comparison of traditional soil sample analyses by GSWA in 2000 (Pye et al. 2000) via fire assay on the <2 mm to > 0.45 mm size fraction to analyses via UltraFine+® in 2020 (Noble et al. 2020). (A) Only 18 samples out of 300 returned Au above the detection limit in with the traditional soil analysis method. (B) The same samples returned detectable Au in most samples with the UltraFine+® method.
Boxplot comparisons of UltraFine+® with four-acid digest (Cu, Pb, Zn) analyses on samples collected by the NTGS over the MacDonnell Ranges in the Northern Territory (Dunster and Mügge 2001). The 2001 survey results are displayed in red and the 2020 results from re-analysis via UltraFine+® are displayed in green.

Further comparison studies were conducted on 4-acid digestion, various aqua regia extractions, hydroxylamine hydrochloride, MMI, water leach and other commercially available clay separation techniques. Head to the Publications tab to learn more.

This method was developed by CSIRO in collaboration with LabWest during the CSIRO/MRIWA Project M462 completed in September 2018.

The UltraFine+® method has since been further refined and improved during CSIRO/MRIWA Project M462a. Head to UltraFine+® Next Gen Analytics to learn more.