Gold Potentials Estimation using 3D Restoration Modeling – Mount Pleasant Area, Western Australia

International audienceGold deposits are used to be related to fault systems that control metals’ transport and accumulation through relatively high permeable discontinuous structures. However, some coeval gold deposits occur at locations far from the main faults. In those cases, fractures appear due to the rock mass heterogeneity. This work may explain the presence of such gold accumulations in Mount Pleasant region, Western Australia, by applying restoration tools to estimate strains during the stretching tectonic phase. Firstly, a 3D Gocad/Skua solid-model was built by using available geological maps and cross-sections. The backward model was obtained by flattening a stretching-representative reference surface selected from the litho-time scale sequence. The present 3D model was restored regionally assuming a flexural slip deformation style and using a full geomechanical finite-element-based approach. Rock heterogeneities drive damages in locations far from the fault systems. The off-fault damage areas which follow the regional deformation trend are located preferentially in lithological contacts. Based on a logistic regression method, it is shown that high gold occurrences off-fault zones coincide spatially with locally-high-gradients of the first deformational parameter calculated on the restored strain tensor. This methodology could be used for inferring favorable areas in green exploration fields

Estimation of gold potentials using 3D restoration modeling, Mount Pleasant Area, Western Australia

International audienceA broad variety of gold-deposits are related to fault systems developed during a deformation event. Such discontinuities control the metals transport and allow the relatively high permeability necessary for the metals accumulation during the ore-deposits formation. However, some gold deposits formed during the same deformation event occur at locations far from the main faults. In those cases, the fracture systems are related with the rock heterogeneity that partially controls the damage development on the rock mass. A geo-mechanical 3D restoration modeling approach was used to simulate the strain developed during a stretching episode occurred in the Mount Pleasant region, Western Australia. Firstly a 3D solid-model was created from geological maps and interpreted structural cross-sections available on the studied region. The backward model was obtained flattening a stretching-representative reference surface selected from the lithology sequence. The deformation modeling was carried out on a 3D model built on Gocad/Skua and restored using a full geo-mechanical modeling based on a finite element method used to compute the volume restoration in a 600 m tetrahedral-mesh-resolution solid. The 3D structural restoration of the region was performed flattening surfaces using a flexural slip deformation style. Results show how the rock heterogeneity allows damages in locations far from the fault systems. The distant off-fault damage areas are located preferentially in lithological contacts and also follow the deformation trend of the region. Using a logistic regression method, it is shown that off-fault zones with high gold occurrences correlate spatially on locations with locally-high-gradient first deformational parameter, obtained from the restoration strain field. This contribution may provide some explanation for the presence of gold accumulations away from main fault systems, and the method could be used for inferring favorable areas in exploration surveys

Gold Potential Maps using 3D Restoration, Mount Pleasant Area, Western Australia

International audienceA broad variety of gold-deposits are related to fault systems developed during a deformation event. Such discontinuities control the metals transport and allow the relatively high permeability necessary for the metals accumulation during the ore-deposits formation. However, some gold deposits formed during the same deformation event occur at locations far from the main faults. In those cases, the fracture systems are related with the rock heterogeneity that partially controls the damage development on the rock mass. A geo-mechanical 3D restoration modeling approach was used to simulate the strain developed during a stretching episode occurred in the Mount Pleasant region, Western Australia. Firstly a 3D solid-model was created from geological maps and interpreted structural cross-sections available on the studied region. The backward model was obtained flattening a stretching-representative reference surface selected from the lithology sequence. The deformation modeling was carried out on a 3D model built on Gocad/Skua and restored using a full geo-mechanical modeling based on a finite element method used to compute the volume restoration in a 600 m tetrahedral-mesh-resolution solid. The 3D structural restoration of the region was performed flattening surfaces using a flexural slip deformation style. Results show how the rock heterogeneity allows damages in locations far from the fault systems. The distant off-fault damage areas are located preferentially in lithological contacts and also follow the deformation trend of the region. Using a logistic regression method, it is shown that off-fault zones with high gold occurrences correlate spatially on locations with locally-high-gradient first deformational parameter, obtained from the restoration strain field. This contribution may provide some explanation for the presence of gold accumulations away from main fault systems, and the method could be used for inferring favorable areas in exploration surveys

Aseismic deformations perturb the stress state and trigger induced seismicity during injection experiments

International audienceFluid injections can trigger seismicity even on faults that are not optimally oriented for reactivation, suggesting either sufficiently large fluid pressure or local stress perturbations. Understanding how stress field may be perturbed during fluid injections is crucial in assessing the risk of induced seismicity and the efficiency of deep fluid stimulation projects. Here, we focus on a series of in-situ decametric experiments of fluid-induced seismicity, performed at 280 m depth in an underground gallery, while synchronously monitoring the fluid pressure and the activated fractures movements. During the injections, seismicity occurred on existing natural fractures and bedding planes that are misoriented to slip relative to the background stress state, which was determined from the joint inversion of downhole fluid pressure and mechanical displacements measured at the injection. We then compare this background stress with the one estimated from the inversion of earthquake focal mechanisms. We find significant differences in the orientation of the stress tensor components, thus highlighting local perturbations. After discussing the influence of the gallery, the pore pressure variation and the geology, we show that the significant stress perturbations induced by the aseismic deformation (which represents more than 96 per cent of the total deformation) trigger the seismic reactivation of fractures with different orientations