Geological and Geotechnical Considerations in Open Pit Planning and Design; a Case of Okote Gold Deposit, Southern Ethiopia

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Addis Ababa University


This manuscript particularly presents the geological geotechnical factors and parameters to be considered for the designing and planning of Okote gold project which is located in Borena, southern Ethiopia. The deposit is currently under feasibility study by National Mining Corporation (NMiC). Geologically, the area lies within the southern extension of the Megado meta-volcano-sedimentary terrain. Carbonate amphibole schist, chlorite schist/ carbonate chlorite schist, metagabbro, metagranodiorite, metadiorite, are identified as the main lithological units. They are all aligned to the NNE-SSW direction and dipping to the west at about ~65-780. There are two distinctly identified mineralization types: chlorite schist hosted gold mineralization and metagranodiorite hosted mineralization. The resource estimation using ordinary kriging reveals the Okote gold deposit has 77 million tons of ore at 1.18g/ton average grade and 0.49 cut-off grade. Thus, the contained value is calculated to be 23,062Kg of gold. Geotechnical study shows that the rock mass in the area is strong enough for hard rock mining. According to the RMR89 classification system, waste and ore body are rated 65 and 65.5 respectively. All the factor of safety calculated for bench, inter-ramp, and overall slope angles for the proposed open pit range between 1.12 to 1.30 for dynamic condition and 1.27 to 1.43 for static condition. The bench face angle should not exceed 700. Bench height is 14m and has width of 7m. The selected ramp width is 18m (without ditch on both side of road, 2m each) depending on the preferred size of dump track. Depth of the pit is ~190m, that means starting from 1100m to 1290m. The calculated stripping ratio for the proposed open pit is 7.43 with 13 years of mine life. The pit has total enclosed volume of 89x106 m3 with longest diameter of 1.19Km.



Okote, RMR89, Open Pit, Inter-Ramp Slopes, Overall Slopes, Factor of Safety