The Sunrise Dam Gold Mine (SDGM), owned and operated by AngloGold Ashanti Australia Pty Ltd. (AGA), is located on the edge of Lake Carey, 45 km southeast of Laverton, Western Australia. The mine is located within the Laverton Tectonic Zone and sits within a basin sequence of intermediate volcaniclastics, banded iron formations and turbiditic sediments, which overlie a greenstone and granitic basement. Host rocks for gold mineralisation range from volcaniclastics to banded iron formations and felsic porphyry rocks. Mineralisation throughout the mine has a strong structural control. There are two main types of mineralisation styles, shear hosted and extensional brittle veining, which have relationships to two distinct local deformational events D3 and D4. GMEX was requested by AGA to assist them to better understand the geomechanical response of the mine sequence during deformation events, by employing Finite Element Analysis (FEA) in an attempt to help them delineate targets for their within mine exploration programme.
At the local mine scale the overall structural architecture of the deposit consists of shallow to moderately dipping shear zones which preserve a penetrative shear zone fabric variably dipping to the west-to-northwest. This fabric consists of a sericite+-chlorite schistosity and plunges to the north. Quartz and carbonate slickenfibres within faults and breccia veins indicate shear sense similar to the penetrative fabrics (variably dipping to the west). Flat laminated quartz-carbonate veins are also evident and are moderately dipping towards the NE, many of which are associated with en-echelon arrays and sigmoidal geometries. On the larger scale deep porphyry bodies may have provided a competency contrast and a possible supra-hydrostatic fluid source, and may have played an important role in the reactivation of moderately dipping shear zones and formation of steep brittle structures. The partitioning of stress and strain during the D3 and D4 deformation events, particularly around the porphyry and mafic bodies, may have been very important in the localisation and variability in gold grades.
1st stage Results – A solid state 3D model was constructed and then converted to a hexahedral grid for geomechanical modelling. The modelling results indicated a distinct partitioning of stress and strain as a result of the deformation events, with localisation of high values of shear strain and volumetric strain (dilation) correlating very well with known mineralisation and higher grades.
The modelling highlighted that high fluid pressures were a critical component in the system, having a strong influence in not only the distribution of dilation but also the longevity of dilation through the deformation cycle. Increased fluid pressure gradients result in increased dilation and failure in the SDGM, and most importantly sealing the system (by reducing the permeability of the SSZ) allowed a greater build up of pressure resulting in increased failure (tensile) higher in the system. During D4 a strong SW plunge/trend in favourable geomechanical outputs was apparent around and below the Dolly Porphyry. These areas were identified as targets based on the geomechanical predictions and followed up by deep drilling. The targeting was successful with the discovery of the 12 Mt (~2 Moz) Vogue orebody.
2nd stage Results – Following the successful modelling at SDGM a couple of years earlier, AGA later requested GMEX to follow up on target definition using newly acquired structural data. A new and more complicated 3D solid state 3D model was constructed to help AGA define lode-scale targets in and around the Dolly Porphyry. The geomechanical modelling predicted several target shoots that are currently being drilled. The project outcomes provided a clearer understanding of how the deformation events resulted in rock failure and fluid flow to deposit gold in current locations, and have provided several high value targets for underground drilling exploration.