Numerical modelling of an evolving gold-lode system: structural and lithological controls on ore-shoot formation in the Magdala goldmine, western Victoria

Variable distribution of elevated gold grades in the Magdala Central Lode system is controlled by preferential and localised reactivation of pre-existing faults in a progressively rotating stress field. Populations of slickenline lineations on fault surfaces in combination with extension vein arrays associated with the faults indicate Central Lode formed initially during southwest – northeast compression, but was subsequently locally reactivated during east – west compression on lower angle fault segments to produce a series of ore-shoots within the overall lode system. A series of 2D cross-section numerical models have been used to examine how deformation and fluid flow are portioned at different stages of the lode evolution. Models have been constructed in the plane estimated to contain maximum and minimum principal stresses during the events leading up to, and during, mineralisation as indicated by the lineation and extension vein data. Results of the models show that during southwest – northeast compression, a major shear zone forms along the boundary between the Stawell Facies and the overlying Albion Formation consistent with the extent and location of the Central Lode Shear Zone. In a second series of models it is shown that during east – west compression, shear failure is considerably more localised to regions overlying lower angle lithological contacts. This localisation of shear failure in these east – west compression models shows strong correlation with the distribution of gold in grade shell models, confirming the structural and lithological factors identified as controlling the mineralisation.