Laboratory measurements of ultrasonic wave velocities and anisotropy across a gold-hosting structure: A case study of the Thunderbox Gold Mine, Western Australia

Abstract

Most lode gold deposits worldwide are associated with structures such as shear zones. Thanks to their capacity to couple resolution and depth of investigation, seismic methods can identify these indirect indicators of mineralization and help extend gold exploration targets to greater depths. Rocks from shear zones are usually seismically anisotropic. Seismic anisotropy is generally related to the intrinsic texture of the rock and the presence of cracks at depth, although the impact of the latter relative to the former fades out with increasing depth. Understanding the seismic impedance contrast between the rocks and determining seismic anisotropy in relation to the texture of the rock, and its evolution with depth (pressure) is therefore necessary to help interpret exploratory seismic surveys. To do so, we characterized in the laboratory 126 core samples representing different stages of alteration and different lithologies from the Thunderbox Gold Mine in Western Australia. Laboratory measurements consist of ultrasonic wave velocity at ambient and in situ pressure, density, mineralogy, texture analysis, and estimation of P-wave anisotropy by inverting the ultrasonic data. These experimental data were used as input in different rock physics models to calculate texture-derived velocities and to model the effects of seismic anisotropy on seismic reflectivity. Except for massive basalt samples that present a remarkable elastic impedance contrast with all lithologies and for the talc schist samples (shear zone samples) that present high seismic anisotropy values, the seismic reflectivity between the lithological units encountered at the Thunderbox Gold Mine is low. According to seismic reflectivity modelling, seismic anisotropy along the shear zone significantly affects the seismic reflectivity, with values of reflectivity reducing more with the increasing angle of seismic reflection than if the shear zone is considered as an isotropic interface. The good agreement between calculated texture-derived velocities with experimental measurements shows that the crystallographic preferred orientation of minerals of the shear zone samples is the main source of seismic anisotropy. This study seeks to improve the understanding of the seismic response across mineral deposits that are structurally controlled by shear zones.