Rock structure characterization of a magnetite gneiss with foliation-parallel discontinuities for footwall slope design

The Warramboo magnetite iron ore deposit in South Australia is predominantly hosted by gneiss. Design of the footwall slope requires an assessment of the role of foliation-parallel discontinuities as planes of weakness parallel to the slope. A review of published studies of joint spacing and joint persistence in gneiss shows spacing and persistence of foliation-parallel discontinuities relative to non-foliation-parallel discontinuities varies greatly. Foliation-parallel discontinuities form a spectrum between simply being one joint set in a blocky rock mass through to being a significant anisotropic weakness. Based on detailed analysis of the characteristics and frequency of foliation in drill core in the Warramboo magnetite deposit, it is inferred, based on analysis of spacing, that foliation-parallel discontinuities are no more frequent or persistent than other joints. Therefore the rock mass is blocky rather than anisotropic and footwall slopes steeper than the foliation dip are viable.