2021 Recipients

Many of the world’s open pit mines plan to transition to underground block cave mining to extend mine life and continue to exploit resources at depth. The optimum time and depth to transition between open pit mining and cave mining is highly dependent on economic factors due to the differing cost of operations and capital investment. Any lapse in production due to the transition can be detrimental to the operation.
The failure to understand the geomechanical relationship between the open pit and the caving operation can result in further costs due to surface disturbance of critical infrastructure, the loss of viable resources, and possible dilution of the resource. Deformation and failure of pit slopes from cave-induced subsidence can also impact concurrent surface and underground mining operations and bring a premature end to the open pit. This research aims to investigate the simultaneous advancement of an open pit mine with an underlying caving operation, as well as identify the geomechanical risks associated with different, but discrete, transition depths. Through a series of conceptual hybrid continuum-discontinuum models, different transitioning scenarios (including concurrent development) will be studied to identify the effects of both geological and mining factors. Examples include rock strength, fault structures, joint networks, draw rate, design sequence and layout, and preconditioning effects. The conceptual nature of the models allows for increased flexibility to vary inputs. Results from these models will be analysed to identify relationships and draw conclusions regarding the controlling factors. Certain factors remain flexible between different operations and can be optimized, while varying geological factors can increase understanding of the geomechanical uncertainty and risk. Statistical analyses and machine learning algorithms will be exploited to understand the implication of poor data quality and the confidence in quantifying risk, as well as the post-processing of models to further explore the importance and interdependence of input parameters.

The Howard’s Pass Pb-Zn district is located within the Canadian Selwyn Basin and is home to the world’s largest undeveloped Pb-Zn deposits. Sulfide deposition occurred in the Silurian and the deposit was deformed during the Cretaceous Cordilleran orogeny. The Pb-Zn deposits have been incorporated into a series of upright WNW striking folds on a 40km trend along the border of the Yukon and Northwest Territories. Owing to the structural complexity of the deposits, multiple structural models for their evolution have been proposed. These range from Silurian soft-sediment deformation to the proposal of a regional shear zone hosting the deposits. In the shear zone model, bedding is completely transposed and previously interpreted stratigraphic boundaries are tectonic in origin. This study aims to test
this model and is focused on the group of deposits at the eastern end of Howard’s Pass. New lithostratigraphic and structural mapping will be combined with logging of drill core, microstructural analysis of oriented samples, and the analysis of sulfide textures, to establish the structural controls on mineralization and how the structural evolution of this region relates into the larger-scale effect of Cordilleran deformation on the Selwyn Basin. Additionally, a comparison of the geometry and sulfide deformation will be compared to the deposits at Macmillan Pass to help understand how diachronous Pb-Zn deposits responded differently to the Cretaceous orogeny. This will help aid in a mining environment by helping define how the deformation has affected the ore bodies in terms of texture, grain size, remobilization, and reprecipitation. From a greenfield’s perspective, understanding the geometry of the deposits will help predict where to focus efforts to help expand the current resource and find new deposits within Howard’s Pass and the Selwyn Basin as a whole. From the first field season, lithostratigraphic mapping and structural observations indicate one main phase of folding, F1, and the XY group of deposits is located on the southern limb of a macroscopic syncline. F1 folds are upright and gently plunging to the WNW-NNW. A regionally developed, steep NE dipping, cleavage, S1, is axial planar to the
F1 folds across Howard’s Pass. S1 typically manifests as a slaty cleavage comprising pervasively developed dissolution seams. Within the mineralized sections at XY, S1 forms spaced sulfide-rich dissolution seams that were previously interpreted as dewatering structures. A later kink fabric, Sk, is seen locally and may be genetically linked to a movement on WNW and NNE striking faults that overprint the F1 folds. Mapping shows that the geometry of the deposits is primarily controlled by folding and that contacts between units are stratigraphic in origin. S1 and Sk are the only foliations mapped. No shear zone fabrics were identified and there is no evidence that bedding has been transposed. Ongoing work will focus on identifying fabrics in oriented thin sections to compare to the macroscopic field evidence. Additionally, analytical methods, such as Electron Backscatter Diffraction (EBSD), Micro-XRF, and SEM, are being considered to look at how the different sulphides differ from each other in mechanics of remobilization.