Refining fine fraction soil extraction methods and analysis for mineral exploration

Abstract

Soils are widely used as geochemical sample media. In transported cover, the soils that host the mobile element signature are the smallest size fractions, so we should consider concentrating the clay size fraction (<2 µm) as an improved sample medium. Twenty-seven bulk reference soils were collected in the vicinity of known mineral deposits (including background areas) that reflect common soil types of Western Australia. A selection of these soils was used in replicate testing to assess differences in particle size, sample weight, dispersants, and how this relates to the Au and Cu geochemistry. Using multiple analysis of variance tests, the submicron fractions showed that, although the <0.2 µm fraction was slightly different to the <2 µm and <0.75 µm fractions, there was no additional value in the extra effort required to extract these submicron fractions, and the <2 µm fraction was favoured. Ultrasonics and rinsing were not required for Au analysis, and during wet separation no Au is lost in solution and only a small fraction of the overall Cu (0.5%) is lost. Using a dispersant was critical for solid recovery and Na-hexametaphosphate (technical or laboratory grade) was found to be the most effective reagent. The <2 µm size fraction showed no significant differences for assays of Au using 0.1, 0.2, 0.5, 1 and 4 g ( P < 0.05) with a microwave-assisted aqua regia digestion being the best analytical method for Au recovery. In addition to the geochemistry, the spectral reflectance variation was investigated with respect to particle size and weight, and showed more variation in the smaller weight fraction of material and a loss of quartz in the fine materials as expected. The integration of results from this study generated a new workflow called UltraFine+™. This workflow provides 40 elements, spectral mineralogy, particle-size distribution, pH, electrical conductivity (EC) and specific surface area, offering more quality data to improve mineral exploration using soil and regolith samples. Supplementary material: Spreadsheets of soils and regolith data from orientation sites and regional map sites analysed with the UltraFine+ method is available at https://doi.org/10.25919/5d1058f8b05fc