Stellenbosch University’s (SU’s) Central Analytical Facilities department launched the first and only nano computer tomography (CT) scanner in Africa in September last year, with the technology having the potential to benefit the mining industry in a cost-effective and time-efficient way.

Small samples – such as a rock chip or a small stone – can be analysed at a very high resolution of up to 500 nm using the NanoCT scanner, enabling scientists and engineers to visualise the inner structure of an intact object, seeing shapes and connectivity in three dimension (3D).

As a result of its precision and increasing awareness of the power of the technology, SU CT scanner facility manager Dr Anton du Plessis says that, in Europe and the US, many mining companies are using the technology to investigate orebodies to better analyse their deposits in 3D and two-dimensional (2D) slice images or virtual thin sections. In the oil and gas industry, the porosity and pore connectivity of rocks can be investigated in 3D.

“This has proven to work well for many mining companies internationally, owing to the technology using a nondestructive technique. As a result, it provides useful and fast structural information about the inside of the rock, which, in many cases, allows for more selective analysis to take place,” says Du Plessis.

Being aware of its ability to affect the mining industry positively, consulting firm SRK Consulting corporate consultant and partner Hennie Theart notes that the nano CT scanner provides a new method to determine the 3D size distribution of minerals.

This will be possible in cases where the desired mineral has an atomic density that contrasts with that of the surrounding minerals in the rock and there are no complex assemblies of ore minerals with a similar atomic density.

He adds that, while this method is more suitable for initial mineralogical studies for mineral extraction purposes than for direct exploration work, “there might be some applications where such information can be useful in exploration, such as to determine the grain size fraction that needs to be sampled in stream-sediment regional geochemical surveys”.

SU CT facility analyst Stephen le Roux indicates that, in South Africa, most analyses in exploration are currently conducted using field mapping and drilling programmes. As these methods entail only visual analysis of the surface, only 2D analysis of a project can be obtained.

However, exploration and mineralogical investigations can be done in 3D using the NanoCT scanner, allowing for not only the minerals to be viewed in accurate dimensions but also the findings to be revisited at anytime after leaving site, as the data can be stored in a laboratory.

“The scanner is more of a preprocessing type of tool that enables you to see if all the mineral samples that you bring into the plant are in actual fact what you need,” says Le Roux.

Theart states that using the scanner becomes pivotal during the initial phases of mineralogical investigations of some types of orebody samples when they are collected and submitted for mineralogical studies.

“Also, when the economic mineral is disseminated in trace amounts, such as in the case of gold deposits, the mineralogist has to study a large number of polished sections using either conventional micro- scopic studies or advanced equipment, such as the quantitative evaluation of minerals by the scanning electron microscopy method,” Le Roux explains.

Theart says the NanoCT scanner could, therefore, provide an alternative and cost- effective method of scanning such minerals. However, he warns that it should be noted that the observations on the grain size of a specific mineral has to be considered in combination with observations on whether the mineral is situated on the boundary between other minerals or occurs as inclusions in other minerals.

Illustrating the effectiveness of the NanoCT scanner in the diamond industry, exploration and mining consulting services company Mineral Services Group chairperson John Gurney says using the scanner has demonstrated that some diamonds are formed by metasomatic – the chemical alteration of a rock by hydrothermal and other fluids – processes in veins that have crystallised in the cratonic upper mantle roots of peridotite and eclogite.

Peridotite is a dense, coarse-grained igneous rock comprising olivine and pyroxene minerals and eclogite is a metamorphic rock comprising pyroxene omphacite and pyrope-rich garnet.

Moreover, this has indicated that the concentration of diamonds in a metasomatic vein can be extremely high and a greater bulk of these mantle rocks are barren of diamonds and have either not been metasomatised or have not been metasomatised by the correct diamond-producing fluids.

“Once the understanding of this notion is accepted, it conceptually enables one to develop an understanding of what the problems are in the establishment of diamond distribution throughout a kimberlite pipe,” Gurney explains.

Gurney states that, for the mining industry holistically, there are a lot of other applications for which the scanner would be useful. These include assessing beach sand, silicate deposits and platinum or gold distribution in drill core spring, as well as many other applications where the distribution of a primary ore or the detection of minor phases is of interest and commercially important.

Theart adds that the scanner will be useful for those elements and associated minerals that have a relatively high atomic mass, compared with the enclosing silicate minerals and relatively monomineralic ore mineralogy. Silicate minerals include gold ores, tin, tungsten, niobium, certain types of molybdenum, uranium ores, platinum-group element ores and rare-earth minerals.

Skills Development
Although the scanner enables the mining industry to use an effective method that could affect the industry positively, Du Plessis highlights that, in some cases, where the investigation is not routine, there is a need for skills development for all personnel using the scanner, especially in 3D image analysis techniques, in order to make the most of the technology.

“In as much as the scanner is useful for the sector, companies still need someone who can set up the scanner correctly to view the desired samples. Secondly, the person viewing the sample has to know which software procedures are needed for the samples so that this yields positive results for the investigations or analysis,” he explains.

The SU CT facility offers analytical and consulting services to mining companies, where it conducts samples to mitigate the current lack of skills to operate the NanoCT scanner. Everything is effectively provided to make best use of this technology.

For more information on the scanner, log on to http://www.sun.ac.za/ctscanner.