The reaction conditions chosen for most partial geochemical extractions remain largely unjustified in terms of optimizing their ability to isolate element signatures related to the effects of mineralization. This study demonstrates the use of central composite designs to study the effects of variation in temperature, pH and reagent concentrations on metal extraction for a buffered acetate leach and a hydroxylamine leach. These designs have been used on nine soil samples from a traverse over porphyry Cu–Au mineralization in central NSW, Australia. For both leaches, metal extractability varies significantly with reaction conditions and the strongest responses to mineralization are generally provided under a very restricted range of digestion conditions. The optimum extraction conditions at Mandamah for the acetate leach are the low pH of the acetate buffer (3.76) and a high acetate concentration at any temperature. The optimum conditions at Mandamah for the hydroxylamine leach are pH = 0, T = 100°C and a moderate to high hydroxylamine concentration (although not for its ability to reduce metal oxides).