Application of Surpac and Whittle Software in Open Pit Optimisation and Design

Abstract

This paper studies the Surpac and Whittle software and their application in designing an optimised pit. Exploration data from Mpeasem Gold Mining Project (MGMP) is used as the primary input for the work. The work entails: block modelling of MGMP deposit; pit optimisation; analysis of the Pit NPV’s sensitivity to changes in gold price and mining cost; and detailed pit design. The deposit has undergone intense weathering, forming an oxidised gold deposit up to about 50 m in depth. Sections through the deposit were used to create a solid model of the orebody, which was divided into blocks to form a block model comprising unit blocks measuring 20 m x 20 m x 10 m. The block model grade was estimated using the Inverse Distance Weighting (IDW) method, giving an average grade of 1.533 g/t with 22.79 Mt of ore. During the optimisation, a total of 82 optimal pit outlines were generated using the 3D Lerchs-Grossmann algorithm. Pit 36 was chosen having the highest the Net Present Value (NPV) @ 10% of $338.60 million. The optimal pit had 21.19 Mt of ore at an average grade of 1.557 g/t. The NPV was very sensitive to gold price changes but marginally sensitive to mining cost changes. From the optimal pit, a detailed pit designed with circular ramp was selected over one with all-cut ramp since it had a higher expected revenue and lower stripping ratio. It is concluded that Surpac and Whittle software combine as a powerful tool for designing an optimal pit.