Studies were conducted by Minera Escondida, Ltda. (MEL) between 1995 and 2003 to support an investment decision to open Escondida Norte, an enriched porphyry copper deposit adjacent to the Escondida mine and processing facilities. Due to the importance of copper-iron sulphide minerals in predicting metallurgical performance in both process alternatives, a quantitative spatial model of copper-iron sulphide mineral abundances was determined to be a critical study component.
A suite of total and partial copper analyses already in common use was chosen to maximize relative selectivity among the major copper sulphide minerals in the mineral deposit. These analyses were merged with partial and total sulphur analysis and combined with a novel approach to interpret the analytical results as normative mineral estimates of chalcocite, covellite, chalcopyrite and pyrite content. Development and validation of the normative mineral estimates included verification on the analytical stability and reproducibility of the chemical analyses, determination of extraction rates on synthetic mineral standards, and comparison with modal mineral abundance on natural ore samples. Uncertainty of individual normative copper sulphide minerals is estimated at ±5% (95% confidence interval), while uncertainty on pyrite content is ±12%. The uncertainty of the normative mineral estimates are comparable to uncertainty of corresponding modal mineral estimates, and differences between modal and normative estimates are largely within the estimation uncertainty.
While the methods were originally developed for a simple association based on supergene chalcocite ± covellite replacing and overprinting hypogene chalcopyrite + pyrite, subsequent work developed normative mineral models for bornite + chalcopyrite hypogene and mixed copper oxide-sulphide supergene mineralization.