Cut-off Grade Based Stope Optimisation to Maximise Value at the Garpenberg Mine

Abstract

This research was aimed at determining the influence parameters have on the cut-off grade of sublevel stopes in the Garpenberg underground mine, and combing these to maximise value at the mine. Garpenberg is a multi-deposit poly-metallic mine owned by Boliden AB. Cost calculations were performed on data from 2018 to derive costs for each deposit and activity. Ore transport and rock bolting were two parameters which were suitable to be used for variable cost implementation. These costs were assigned to the different orebodies and cut-off grades were adjusted to reach a break-even level. Stopes were created using the Stope Optimizer software in Deswik, using the variable cut-offs and the technical constraints previously used for Garpenberg's Life of Mine Plan. Cut-offs were elevated or depressed to optimise NPV for each deposit separately and the optimal combination yielded an NPV increase of 530 Mkr, a 4\% increase, compared to using static cut-off grades. More value can be obtained by combining cut-off strategies in Garpenberg as whole, in contrast to separate optimisation. Scheduling of newly created stoping areas within the Lappberget deposit proved to be beneficial for value. Finally, a financial analysis into double stopes of 50 m high was performed. The elimination of production levels provides the main cost reduction. However, lower selectivity and expected dilution will limit the financial gain from this method.