Identification of suitable areas for multireef operation at Thorncliffe Chrome Mine

Abstract

Thorncliffe Chrome Mine, one of Glencore’s Eastern Chrome operations, is currently mining the Middle Group No.1 (MG1) chromitite layer and is conducting a pre-feasibility study for an expansion project to mine the Middle Group chromitite No.2 (MG2) layer. This paper focuses on identifying viable areas within the MG2 layer that adhere to the mining, geological, and rock engineering parameters for a safe multiple-reef operation; and establishing efficient ways of accessing and extracting the MG2 chromitite layer. The findings from the multi-reef project at Samancor’s Tweefontein Mine (Gouvea, 2013) and Maritz (2015) emphasized that the best layout for a safe and stable multi-reef operation is one where pillars are superimposed. The guidelines by Salamon and Oravecz (1976) were key in the selection criteria for pillar stability. A stage gate analysis was used to identify mineable areas using the success selection criteria for mineability obtained from geotechnical and geostatistical analysis of the assay data and isopach plan. During this study, it was found that the most cost-efficient way of accessing the MG2 chromitite layer is from the MG1 mining horizon through a connecting ramp instead of sinking a shaft. Moreover, using the MG1 infrastructure during the development of the MG2 mining horizon will reduce the project development cost. A bord-and-pillar layout with superimposed pillars will assist in reducing the stress concentrations and interactions between the two mining horizons. With all the considerations being satisfied, it can be concluded that the MG2 layer is viable to mine in a safe manner along with the MG1 layer in a multi-reef environment.