Strong versus weak digestions: ligand-based soil extraction geochemistry

ABSTRACT: Ligand-based soil extraction geochemistry is a high resolution, multi-element soil analysis technique for mineral exploration. The use of strong complexing ligands in the extraction procedure limits matrix element interference, reduces background analyte values (noise) and overcomes potential pH effects. Improvements in the spatial and amplitude (signal to noise) resolution of this technique compared to total digestion or more aggressive partial digestions is useful not only in areas with exotic cover, but in residual and weathered terrain where duricrust development and mechanical transport can cause dispersion of anomalies. At the Green Tree Frog Prospect in Queensland, Australia, superior amplitude contrast and more accurate delineation of sub-surface mineralization is observed for ligand-based geochemistry compared to fire assay in terrain with exotic tertiary cover. In Guinea, West Africa, ligand-based soil extraction of gold has outlined five new economic zones since its introduction. Comparison of a range of weak and strong digestions and extractions from glacial till at Cross Lake, a VMS (volcano-genic massive sulphide) deposit in the Abitibi belt of Canada, provides very good evidence as to the value of strong ligands in the extraction. More recent develop-ments of this technique utilize compatible and incompatible elements (principally Ni and Ce) to obtain inferred geology images, which are of direct assistance to exploration where outcrop and sub-crop are limited or of low density. A prospect scale image of inferred geology from The Mount SE, Widgiemooltha in the Eastern Goldfields of the Yilgarn Craton, Western Australia obtained from a ligand based soil geochemistry program provides superior detail to 1:40 000 standard geological mapping. A semi-regional scale roadside sampling program at Wongan Hills on the western side of the Yilgarn Craton illustrates that this type of geochemistry has a role to play in early stage, regional-scale exploration where geological outcrop and sub-crop are limited