Numerical simulation of deformation and controls on mineralisation at the Sunrise Dam Gold Mine, Western Australia. John G McLellan, Thomas Blenkinsop Economic Geology Research Unit and Predictive Mineral Discovery Cooperative Research Centre, School of Earth and Environmental Sciences, James Cook University, Townsville 4811, Queensland, Australia Michael Nugus, Michael Erickson Anglogold Ashanti Australia Ltd., Laverton 6440, Western Australia, Australia ABSTRACT: Economic mineralisation at the Sunrise Dam Au mine has occurred as a result of two main deformation events. The first event, D 3 (NW-SE compression) involved predominantly reverse shearing and reactivation on shallow dipping structures and strike slip deformation on steep dipping structures, and formed important NE-SW shear hosted structures in the form of tension and breccia vein styles that host gold mineralisation. The second mineralising deformation was D 4 (NE-SW compression) which again reactivated existing structures, resulting in both steep and flat dipping sets of veins and brecciation, and which hosts the majority of the known high grade gold mineralisa- tion. One of the main shear zones, the Sunrise shear zone, may have acted as a seal, allowing fluid pressure to increase towards lithostatic levels, which in combination with deformation has resulted in tensile failure and precipitation of ore below the Sunrise shear zone. Three dimensional numeri- cal models predict areas of high shear strain, dilation and areas most likely to fail as a result of in- creased fluid pressure. Some of these areas coincide with existing mineralisation, and these results have great potential to be used in a predictive capacity for future exploration strategy. KEYWORDS: numerical modelling, deformation, Au mineralisation, Sunrise Dam 1 INTRODUCTION The world-class Sunrise Dam Gold Mine (SDGM) is located within the Laverton Tec- tonic Zone, of the Western Australian Yilgarn Craton. The mine is situated 850 km ENE of Perth and 45 km SE of Laverton, WA. The SDGM deposit has produced 4.8 Moz of gold at an average grade of 4.2 g/t Au. As of the 1st January, 2007, remaining resources stood at approximately 36.7 Mt @ 3.1 g/t (3.6 Moz), and reserves of 16.7 Mt @ 3.5 g/t Au (1.9 Moz: Erickson et al, 2006; unpublished data). The structural evolution and deformation responsi- ble for the main mineralisation periods of the SDGM can be categorised into two major events, which are locally known as D 3 and D 4 . During D 3 (NW-SE compression) reactivation of pre-existing structures such as the Sunrise shear zone (SSZ) resulted in NE-SW shear- hosted structures in the form of tension veins and brecciation which are host to moderate Au grades with a notable lack of coarse Au associ- ated with shear failure. During D 4 (NE-SW compression) reactivation of pre-existing struc- tures was also important with precipitation of coarse Au (with sulphides and tellurides) in both steep and flat dipping sets of veins and brecciation (e.g. the Dolly lodes). Numerical modelling has been proven as a use- ful tool in simulating coupled deformation and fluid flow processes (e.g. McLellan et al, 2004), and in targeting mineralisation (e.g. S- chaubs et al, 2006). The main aims of this study were to firstly ascertain the distinct structural events and primary stress vectors that led to mineralisation by conducting a 3D-stress analy- sis for inputs into numerical models, and secondly to simulate numerically both major deformation events with a view to identifying areas requiring minimum fluid pressures