Geotechnical slope design in hard rock lithium deposits

Abstract

The increasing demand for commodities such as lithium is driven by the accelerating transition to a green economy. Currently, there are two primary types of lithium mines: brine and hard rock or, more specifically, pegmatite (spodumene) deposits. This paper synthesises the geotechnical controls on slope design and the associated risks for hard rock lithium deposits. Typical controls are illustrated, using specific examples from recent experience at three lithium deposits and operations in Australia and Africa. Geological models of hard rock lithium deposits comprise the pegmatite orebody and the surrounding country rocks. The orebody is typically an inclined pegmatitic intrusion, creating a footwall and hanging wall open pit setting. The country rocks vary in the degree of metamorphism of sedimentary and igneous protoliths.Weathered or transported material up to a 5-60 m depth may pose localised geotechnical risks.Fresh rock mass conditions are typically favourable with high intact strengths. This means that the structural geology is critical to geotechnical slope design. For hard rock lithium deposits, key elements of the structure model are:• Orientation of pegmatite intrusions and potential shearing at the contacts with country rocks.• Presence of foliation or bedding.• Regional scale faults and shears.Geotechnical conditions on the hanging wall are typically favourable for slope design, while the footwall interacts with potentially unfavourable structural conditions where careful geotechnical design is required. Slope stability risks are typically bench to multi-bench scale issues. Examples of common risks, stability issues and techniques for managing them are presented in this paper.