Case Study 4: Eastern Fold Belt

GMEX EFB1

The Mount Isa Inlier is host to many economic deposits, some world class, such as Mount Isa Cu, Mount Isa Pb-Zn-Ag, Cannington Ag-Pb-Zn, Century Zn-Pb and Ernest Henry Cu-Au. The Eastern Succession is well known for its endowment of Fe-oxide Cu-Au and Cu-related deposits and prospects, many of which remain controversial in both ore genesis mechanisms and timing of mineralisation. Epigenetic processes are predominantly responsible for the majority of the Cu and Cu-Au deposits in the eastern Mount Isa Inlier, with most models favouring a role for igneous-related hypersaline fluids. Copper-Au mineralisation and the majority of the deposits in the Eastern Succession have a close spatial relationship with faults and strike-slip deformation. The structural architecture and deformational history of the area is reasonably well known, providing a good basis for application of discrete element techniques incorporating the interaction of fault movement (slip) and stress partitioning. The ability to numerically simulate and ‘map’ the effects of deformation can provide important data in understanding the spatial and temporal consequences of such processes. This technique has been used in determining the magnitude and distribution of stress in several mineralised and faulted terranes, and has been successfully applied to other well known areas such as the Laverton Tectonic Zone and the Sukumaland Greenstone Belt.

GMEX EFB2GMEX investigated the role of both fault architecture and competency contrasts in the localisation and distribution of stress and strain. The geomechanical analysis used a discrete element approach to look at the regional response to predominantly strike-slip deformation and its associated stress and strain partitioning. Stress and strain anomalies were identified and the majority of these had a close correlation with known deposits and prospects. Areas of high differential stress (Ds) were closely associated with known areas of mineralisation and shear zones such as the Selwyn High Strain Zone. The geomechanical analysis also correlated well with areas of increased prospectivity that had been identified in a 12-layer prospectivity analysis undertaken during the pmd*CRC by Mustard et al., (2004) and also by the review and combination of the Weights of Evidence (WOFE) and Geomechanical modelling results by McLellan et al. (2010). Due to complex geometrical relationships in the region there is minimum confidence that any generalised fault orientation within the whole region could be used or identified as being more prone to failure or more prospective due to fault block rotation and heterogeneous patterns of deformation and stress partitioning.

GMEX EFB3Several new prospective areas for Cu and Cu-Au, based on structural and geomechanical outputs, were identified, which had not been previously identified by other means. These areas were highlighted to have the most favourable structural and geomechanical combinations suitable as exploration targets for structurally controlled Cu and Cu-Au mineralisation in the region. The combination of low values of s3, PfF (low fluid pressure required for failure), and increased Δσ (promoting shear failure) or low Δσ (promoting tensile failure) provide the best targeting criteria for mineralisation. This modelling technique is an excellent method to better understand fault movement and block rotation and its effect on stress and strain partitioning. It is a very effective exploration tool.