An approach to the optimisation of partial extractions for use in geochemical exploration

Geochemical exploration for mineral deposits has generally been restricted to regions dominated by residual regolith or where transported regolith cover is thin. A variety of partial geochemical extraction techniques, linked to new predictive models for element migration through transported regolith cover, have demonstrated a potential to detect deeply buried mineralisation under certain circumstances. Problems with the reliability and further development of such partial extractions are linked to the lack of information on either the form of metals or the factors that control the extraction of metals from regolith materials.This study quantitatively examines the mechanisms of a range of partial extraction methods, based on a suite of surface regolith samples from the Mandamah Cu-Au deposit that is covered by 50m of transported regolith. Samples at were subjected to acetate, hydroxylamine.HCI and Na-pyrophosphate extractions at various reagent concentrations, pH values, temperatures and durations, following various sample preparation and chemical pretreatment permutations. The data were modelled, and reaction conditions subsequently optimized, on the basis of central composite designs.Conventional partial extraction (acetate, hydroxylamine and aqua regia) data, displayed high variability for some major and trace metals surrounding the periphery of mineralisation at Mandamah but little indication of direct vertical migration of ore-related metals. The buffered acetate extraction is primarily controlled by the capacity of the solution to generate acid rather than exchange induced by the cation of the acetate salt. Trace metals were highly susceptible to re-adsorption effects. Acidic hydroxylamine extraction is driven by kinetically limited acid hydrolysis and the hydroxylamine concentration has little effect on metal extractability. Alkaline Na-pyrophosphate extraction proved difficult to model. Two new partial extractions developed in this study - alkaline hydroxylamine.HCI and pH-static calcium nitrate - offer a different functionality to conventional extractions and provide more coherent geochemical patterns at Mandamah related to the location of buried mineralisation. These patterns are also related to the capacity of samples to resist pH neutralization.Systematic optimisation of geochemical extraction procedures is demonstrated to be an effective approach to improving detection of geochemical patterns in surface regolith that can be spatially related to the effects of mineralisation on the chemistry and mineralogy of overlying transported regolith cover