Building a Tonnage-Surface Function for Metal Grades and Geological Dilution: An Application to the Massive and Stockwork Zambujal Ore Deposit, Neves–Corvo Mine, Portugal

Classical metal tonnage cut-off curves are an important output of mineral deposit evaluation. However, such curves omit local morphological variations such as those found in sulphide stockwork-type deposits, in which sulphides and host rock exhibit variable proportions. This paper presents a methodology for building a stochastic model of the massive and stockwork Zambujal ore deposit with respect to both morphology and metal content. The model allows a metal tonnage cut-off surface to be constructed conditional both to metal content and to local proportions of sulphides relative to host rock. Two random variables are modelled using stochastic simulation: (i) a variable (P(x)) that represents the local proportion of sulphides within each mining block and (ii) an auxiliary variable (Y(x)) representing the relative contents of metal, that is, the metal contents recalculated for the matrix sulphides. The results are validated by comparing the global quantities obtained using ordinary kriging of the effective grades with those obtained by modelling the variables P(x) and Y(x).