Understanding seismic reflectivity across hydrothermal alteration zones associated with gold: Example of the Karari gold deposit, Western Australia

Abstract

In this study, we examined whole core samples from a gold deposit in Western Australia, the Karari mine, located south of the Carosue Dam processing plant. By analyzing the mineralogy and performing petrophysical and acoustic measurements on 21 core samples representing different stages of hydrothermal alteration, our goal was to document the values of acoustic impedance as a function of the degree of alteration and identify which parameters (e.g., mineralogy, porosity, foliation) affect the seismic reflectivity across the alteration zones. To do so, we measured the ultrasonic velocities and bulk density at ambient conditions for all samples as well as at confining pressures for two samples. The acoustic measurements were conducted in the directions perpendicular and parallel to the foliation of the samples. A subset of samples was selected to identify the mineral phases, perform automated mineral mapping, measure grain density, and compute porosity using the bulk density–grain density relation and backscattered electron (BSE) images. Our results show that, regardless of the direction of acquisition, the acoustic impedance in the background samples is consistently smaller than that in the altered samples. This difference in acoustic impedance is attributed to a porosity difference of around 2% between the background and altered samples and is not owing to the presence of alteration minerals. Seismic reflectivity at the background/hydrothermal alteration interface decreases with increasing pressure owing to the closure of microcracks. For the Karari site, with a targeted exploration depth between 200 m and 800 m, the altered zones can be differentiated from the host rock by their acoustic impedance, as the reflection coefficients are expected to vary between 0.05 and 0.13, depending on the direction of seismic propagation and depth.