Alexander Hansson details how a CT scanner reveals gold and scheelite in drill core samples
With a novel drill core scanner, continuous acquisition of X-ray CT data from drill cores can be performed on-site, at a speed compatible to core logging. This type of data was often retrieved by scanning selected core pieces cut in suitable lengths, to study sections of a drill hole in detail – often a time-consuming procedure. Using the full data from the whole volume and length of the hole makes continuous data simultaneously available for x-ray CT, density and elemental concentrations, over the length of the hole. The scanning can also be done by sending core boxes to one of Orexplore’s labs.
High Attenuation Particles
High attenuation particles can be detected and quantified in 3D x-ray CT data. Minerals with elements of high atomic number, Z, e.g. gold (Z = 79) and scheelite (CaWO4, W: Z = 74) show up as high attenuation particles in GeoCore X10 data in case the particles are resolved with the 3D pixel grid (with 0.2 x 0.2 x 0.2mm pixels in the standard scanning scenario of up to 6m/h).
Studying Gold In Drill Cores
In Fig.1, 3D x-ray CT data of 3cm from a drill core is shown, highlighting quartz (blue), pyrite (yellow) and gold (red) in a host rock (grey) based on the measured x-ray attenuation values. In this example, one of the gold particle candidates appears on the surface of the core and its composition has been verified by a reference method. In total, seven high attenuation particles with average volumes of about 0.018mm3 are present in this section.
So, gold particles show up as high attenuation particles, but what can the data be used for? An interesting path is to use the data to increase the understanding of the mineralisation e.g. in terms of whether the gold is associated with quartz or sulphides, in turn guiding the process steps needed after excavation. Another use case is to identify exactly which veins contain coarse gold and to measure their orientations using the Orexplore Insight software for further use in geological modelling software. Moreover, depending on the gold particle size distribution, and the presence of other kinds of high attenuation particles, it may even be possible to estimate gold concentration values, while, being a non-destructive method, still preserving all drill core. Maintaining spacial information may be critical for a structurally controlled deposit.
Quantification Of High Attenuation Particles
Scheelite is a good example of a mineral that will show up as high attenuation particles if resolved (Fig.2.) with scheelite (orange) and quartz (blue) in a host rock (grey). The mineral can occur at rather high concentrations and can therefore cover a fair fraction of the CT volume. Detailed studies of particle shapes and size distributions is possible, which gives valuable input to the mineral processing chain. Optimisation of processes such as crushing, grinding and sieving by means of GeoCore X10 data may yield considerable economic benefit.
Alexander Hansson is principle researcher at Orexplore