Application of ultrafine fraction soil extraction and analysis for mineral exploration

Abstract

The need to innovate to advance exploration success is ongoing and, while regional geochemical surveys are common, increasing the data we collect and improving survey results has stagnated. A new adaption of existing methods known as UltraFine+™ extracts the <2 µm fraction of soils and sediments, which is then analysed and combined with spectral mineralogy proxies and physicochemical properties to improve targeting for Au and base-metal exploration. We applied the UltraFine+™ workflow to three small orientation site studies in Western Australia, and reprocessed archived regional soil samples from the Geological Survey of Western Australia to test the method. The orientation programme ( c . 200 samples) involved samples from the Leonora and Sir Samuel 1:250 000 map sheets, an area that hosts known major Au and base-metal deposits. We then applied this approach to the Kingston 1:250 000 map sheet area, analysing a further 300 samples in a region on the Yilgarn Craton margins that is essentially greenfields, as there has been little exploration and the original geochemical survey data were heavily censored due to the abundance of transported regolith dominated by quartz-rich sand. The archived samples were specifically selected soils (sheetwash or sandplain) with a component of Quaternary–Tertiary transported cover. Results at the orientation test sites (Telfer West, Tooting Bec and DeGrussa) were promising. Importantly, the study revealed a marked decrease in censored results for Au (from 63% to 10% below detection limit). Geochemistry and some example indices for mineral exploration and lithology mapping, as well as example maps using the additional spectral mineralogy proxies or particle-size variation, are presented. The application of the <2 µm particle-size separation and the UltraFine+™ workflow importantly demonstrate the additional value from (re-)assaying regional soil and sediment samples to generate new targets and improve regional geochemical maps. This is an exercise that can be applied to new greenfields surveys and, when exploration budgets are lean, to archived samples.