Broekmans, MATM (editor) Proceedings, 10th International Congress for Applied Mineralogy (ICAM) 1-5 August 2011, TRONDHEIM, Norway ISBN-13: 978-82-7385-139-0

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THE USE OF 3D X-RAY COMPUTED TOMOGRAPHY FOR GOLD LOCATION IN EXPLORATION DRILL CORES

Deshenthree Chetty1*, Wilma Clark, Charles Bushell1, Tlou Piet Sebola1, Jakobus Hoffman2,

Robert Nshimirimana2, Frikkie De Beer2

1Mintek, Mineralogy Division, Private Bag X3015 Randburg, 2125 South Africa

2Necsa, Radiation Sciences Department, P. O. Box 582, Pretoria, South Africa Abstract Three-dimensional computed tomography (3DCT) is a non-destructive characterisation technique that was applied to the study of gold-bearing ore from the Witwatersrand Deposit, South Africa. The ability to pinpoint gold occurrence prior to downstream comminution and leaching would potentially reduce processing costs. The aim of the study was therefore to determine to what extent gold, typically fine-grained in occurrence, could be identified in situ. Two gold-bearing drill core pieces were investigated using 1-mm focal spot X-ray tomography and micro-focus X-ray tomography (µXCT). Using the derived data, the cores were physically cut and polished for examination by conventional automated scanning electron microscopy (SEM) to detect gold grains. The SEM results were then compared against the µXCT data. Gold was, to an extent, located by µXCT and validated against SEM data. These first findings suggest that areas rich in gold can be pinpointed by 3DCT prior to conventional assessment, hence potentially reducing processing costs. Keywords: 3D computed tomography, micro-focus X-ray tomography, gold 1 INTRODUCTION The use of 3D computed tomography (3DCT) is well known in the medical industry, where CAT-Scan (computed axial tomography) technology is routinely used. Some other applications for 3DCT may be found in the fields of anatomy, palaeontology, archaeology and automated manufacturing. The technique has only recently gained more attention in the field of minerals processing, however. With the exception of a steady stream of publications from the University of Utah [1], the literature is otherwise sparse in contributions on 3DCT applied to minerals processing. Since 3DCT is a non-destructive method, it shows potential benefits in reduced sample preparation costs, analytical time and data quality compared with conventional 2D microscopy (e.g., optical and SEM-based techniques). The method uses, usually, X-rays that penetrate a sample. In the interaction with the components of the sample the radiation changes its nature – a shift in the energy spectrum due to absorption and scattering interactions with the components in the sample. This change is referred to as attenuation. Less commonly, neutrons and γ-rays may be also used to pass through the sample, and are thus also attenuated by the components through which they pass. The detected attenuated signals (commonly referred to as radiographs) are then processed and related to the components in the sample, thus allowing identification. Cone beam

*Correspondence to: deshc@mintek.co.za

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technology involves the collection of X-rays from a point source, passing through a sample that is rotated as the beam passes through it, such that a number of 2D projections are obtained, along with their corresponding images (Figure 1; [2]). These images result from the attenuation of the beam by the components. Differences in attenuation may be viewed as grey levels. Together, the images obtained are reconstructed to render a 3D volume of the sample (tomogram), such that the various components within the sample can be viewed in 3D, and thus described. In the gold industry, assays currently provide the best means of determining gold grade, but not how and where the gold occurred prior to crushing and milling in obtaining the assay sample. Conventional methods of assessing this would involve preparation of polished/thin sections for analysis under an optical microscope or SEM. The use of 3DCT provides an opportunity to assess potential for pinpointing gold, in situ, with minimal to no sample preparation. Locating gold in these ores, prior to further comminution for extraction by leaching, would be of benefit for exploration, mining and beneficiation purposes. Costs would be reduced in the downstream processing if only gold-bearing masses of ore were treated, as opposed to large amounts of barren material that are mixed in. As such, the potential would exist for 3DCT as the ultimate ‘sorting’ mechanism to upgrade ore prior to leaching. To this end, this contribution details and assesses preliminary methodology using 3DCT for the location of gold grains in drill core that originates from the Carbon Leader Reef in the Witwatersrand Basin. 2 METHODS OF APPROACH Two pieces of quartered drill core (~3 cm thickness, ~10 cm long) were selected from the gold-bearing Carbon Leader Reef of the Witwatersrand Deposit, and conveniently labelled A and B. The pieces were selected on the basis of high gold grades in their opposite quarters, and their presence in the lowest portions of the Reef, which is known to be bottom loaded with gold. Each sample was subjected to tomography in the SANRAD facility located at Necsa, using X-rays generated on a 1 mm focal spot [3] as the source, to determine to what extent minerals could be distinguished. Furthermore, the derived images were analysed to guide which areas should be focussed on as best opportunities to encounter gold. For the tomography, each sample was mounted on a stand and rotated while exposed to an X-ray beam generated at 100kV. A full rotation yielded 375 projections. Reconstruction of the images to generate a tomogram (3D virtual digital image of the sample) was done using the Octopus reconstruction software [4]. On the basis of the 3D tomography data, the cores were physically cut at appropriate positions, so as to intersect the perceived gold. The appropriate surfaces were polished and subjected to conventional automated scanning electron microscopy (SEM) using a trace mineral search on a QEMSCAN instrument at Mintek. The trace mineral search involved a bright phase search used to locate and map gold occurrence. On completion of the SEM examination, the smaller cut pieces were further investigated using microfocus X-ray tomography for higher spatial resolution results. An X-Tek microfocus system was used at X-Sight X-ray services in South Africa, to obtain tomograms for analysis. An X-ray voltage of 200 kV was applied at 125 μA, with a 1 mm Cu filter material to reduce the detrimental effect of X-rays in the tomograms by eliminating X-rays in the lower energy spectrum. A full rotation yielded 4000 projections, each with a 1 s exposure time. For the reconstruction, a beam hardening and noise reduction algorithm was applied, with no binning. Reconstructed images were imported into the VGStudioMAX graphics visualisation package [5], so as to assess the 3D mineral grains in the drill cores.

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3 RESULTS 3.1 Volume rendering and analysis of X-ray data Frontal, sagittal and axial projections, each perpendicular to the other, of each 3D volume, are presented in the software (Figure 2), allowing visualisation of the minerals at various locations within the sample. Virtual ‘slicing’ through the sample is therefore possible, showing different projections through the sample. Grey levels are distinguished on the basis of the mineral specific X-ray attenuation coefficients (μ; Table 1). Gold displays a particularly high attenuation coefficient relative to the other minerals in the assemblage, with uraninite displaying the next highest coefficient. On this basis, gold and uraninite ought to stand out (brighter) against the silicate matrix minerals (darker) when exposed to X-ray energy at 100 kV. The grey level histogram, however, did not display a distinct peak that could be related to gold, or pyrite, for that matter. Instead, a large broad peak was present, representing the silicate components of the cores (Figure 3). Since gold shows the highest attenuation, however, it was possible to create an interval at the highest end of the grey level spectrum (to the right) that should delineate areas containing gold. The areas with the highest attenuation were determined from the histogram and separated from the other regions by disabling the unwanted regions with the software. The data corresponding with these high attenuating areas were then further analysed. These are large areas, however, unable to resolve individual gold grains, owing to an achieved spatial resolution of 80 μm. Such images were lined up against a co-ordinate axis (Figure 4) to facilitate the location of areas of interest in the actual sample. This was necessary to understand which mineral/s contributed to the highest attenuation areas, on which conventional mineralogical techniques were applied. 3.2 Conventional mineralogy for gold location For the complementary evaluation of the tomogram results, physical sectioning was undertaken. An axial slicing orientation of the sample was chosen over the frontal or sagittal, for locating levels at which to physically section the samples, because it represents vertical distribution in the core, which is meaningful in a stratigraphic sense, but also because physical cutting and grinding were easier in this orientation. Figure 5 shows an example of a cut portion from core A, labelled A1; similarly, the cut B portion was labelled B1. The QEMSCAN bright phase search yielded seven gold grains (in the form of electrum) in A1, and 99 grains in B1. For A1, the largest gold grain was 22 μm in equivalent circle diameter (ECD; i.e., the diameter of a circle of equivalent area to the grain exposed in section). Gold grains were mostly associated with pyrite. In B1, the largest of the grains was 104 μm ECD, occurring in a stringer of pyrite grains amongst which a number of other gold grains were located. 3.3 Microfocus X-ray computed tomography Having narrowed the portions down to single surfaces, microfocus X-ray computed tomography (μXCT) was attempted to provide better spatial resolution to help in locating gold. The topmost axial slices for A1 and B1 would coincide with their respective polished surfaces, so that high attenuation portions of the topmost axial slices could then be compared against the QEMSCAN-located gold. Immediately obvious was a small peak on the grey level histogram, representing pyrite. The highest spatial resolution achieved was 20 μm. No gold was reliably located that would coincide with QEMSCAN observations for the topmost axial slice (i.e., the polished surface) of sample A1. Considering that the largest encountered grain was 22 μm ECD, it was not considered large enough to be spatially resolved from the μXCT data. Possible larger grains were encountered in other portions of the sample, however.

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Sample B1 yielded a number of possible gold grains on the topmost axial surface that could be correlated with the QEMSCAN-located gold (Figure 6). This is because such grains were larger than the 20 μm spatial resolution obtained through the micro-focus X-ray tomogram. A total of five gold grains was therefore correlated (Figure 6). 4 DISCUSSION Two aspects of importance for these ores in terms of the objective are firstly the attenuation of gold relative to the other minerals, and secondly the grain size of the gold. Under the conditions employed for both ordinary and μXCT, gold is easily distinguished from other minerals in the assemblage on the basis of its higher attenuation coefficient. For grain size, the methodology outlined showed that gold can be located to a certain extent, dependent on the resolution of the system that is used. Ordinary XCT spatially resolves grains to 80 µm, whereas μXCT resolves grains down to 20 µm in this specific instance. Since most of the gold is finer in size than 20 µm, such resolutions would be inadequate at distinguishing all gold grains, resulting in only five grains larger than 20 µm being correlated with QEMSCAN data. As pointed out by Miller [1], increased resolution can only be achieved by analysis of smaller sample size. The width of the samples is of particular importance, as such width must always lie in the beam path as the sample rotates. The wider the sample scanned, the more the geometry is compromised, so that resolution can only be optimised for the width of sample scanned. The drill core pieces considered would therefore need to be smaller if smaller gold grains are to be identified using the given system and scanning parameters. The method, nevertheless, has proved to be useful in locating gold in these ores. The use of ordinary XCT appears feasible, however, only in the instances where gold occurs with pyrite, since pyrite, having a higher attenuation than the silicates, stands out against the matrix, and together with gold, would create an intermediate grey level intensity, higher than pyrite, but lower than gold, allowing one to focus on such areas. The disadvantage expected is where small gold grains occur in a silicate matrix, such that a grey level higher than silicate, but lower than gold, is obtained. Depending on the gold grain size relative to the volume of the silicate matrix, it should be noted that this grey level might well be mistaken for that of pyrite or another mineral and will consequently not be investigated further. Microfocus XCT certainly reduces these problems with better spatial resolution, so would consequently need to be used as the primary technique to identify all gold grains in the rock matrix. As the scanning parameters are not yet fully optimised, results could improve when all variables are optimised through further investigations. 5 CONCLUSIONS The methodology employed in this study showed that gold can be located by μXCT, but that smaller grains require a higher spatial resolution to be located, which in turn, requires smaller sample size. The potential for XCT to be a ‘sorting’ mechanism is therefore a challenge at present owing to the resolution vs. sample size problem. Crushing to smaller particle size, and scanning of such particles would be required, which could be time consuming if not automated. These represent points of departure for future investigation with this evolving technique. 6 ACKNOWLEDGMENTS

Mr Paul Keanly of X-Sight X-ray Services, and Mr Louis Mudalahothe of the Mineralogy Division, Mintek, are acknowledged for providing the microfocus data, and precision preparation of the cores, respectively.

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7 REFERENCES [1] Miller, J.D. (2010): Characterisation, analysis and simulation of multiphase particulate systems using

high resolution X-ray micro tomography (HRXMT). XXV International Mineral Processing Congress (IMPC) 2010 Proceedings, Brisbane, Queensland Australia, 6–10 September, 2010.

[2] Kak, A.C., and Slaney, M. (1999): Principles of computerised tomographic imaging. The institute of Electrical and Electronics Engineers, Inc., New York, pp329.

[3] De Beer, F.C. (2005): Characteristics of the neutron/X-ray tomography system at the SANRAD facility in South Africa, Nuclear Instruments and Methods in Physics Research A, (542): 1–8.

[4] Masschaele, B., Dierick, M., Vlassenbroeck, J., and De Witte, Y. (2007): Octopus V8. Ghent: Institute for Nuclear Science (INW), XRayLab. Available online http://www.xraylab.com.

[5] Volume Graphics GmbH VGStudioMax 1.2.1. Germany (2006) Available online http://www.volumegraphics.com/products/vgstudio/index.html

TABLE 1: X-ray attenuation coefficients (theoretical derived for a tungsten target) for minerals present in the cores assessed at 100 kV

Mineral Chemical formula μ (X-rays) cm-1 Density g.cm-3

Quartz SiO2 1.84 2.65

Chlorite Mg5Al2Si3O10(OH)8 1.53 2.5

Chloritoid FeAl2OSiO4(OH)2 6.61 3.61

Pyrite FeS2 18.4 5.01

Uraninite UO2 283 10.97

Brannerite (Ca,U)(Ti,Fe)2O6 89.6 6.37

Gold (electrum) (Au,Ag) 358 17

Carbon C 0.47 2.26

Zircon ZrSiO4 45.9 4.71

Source: http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html. Accessed on 24 March 2010.

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Figure 1: Cone beam configuration for CT with X-rays (a) and image information as a function of projection angle (b, after [2]).

Figure 2: 3D rendered volume of core piece A (right), with axial (top left), sagittal (middle left) and frontal

(bottom left) projections displayed. The red cursor in each refers to the same point viewed from all projections in a perpendicular axis system, shown on each.

X-ray point source

Detector

Axis of rotation

Cone beam Sample

a b

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Figure 3: Histogram of grey level values (corresponding with darker portions to the left and brighter portions to the right) vs. frequency, showing a large, broad, asymmetric peak related to silicates, with a tail

incorporating grey levels of pyrite, sample A. Intervals are distinguished based on visualisation of mineral grains in the rendered volume.

Figure 4: Volume rendered image showing areas of highest X-ray attenuation (in this case the darker patches)

in sample A (left), with cut-away portion indicating levels chosen for physical cutting of the core piece to expose an area of interest (right).

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Figure 5: Example of a cut and polished section (right) of sample A1, with its corresponding axial tomogram

image that was used to guide the cutting (left). The highest attenuation (brightest) areas in the axial] image correspond with pyrite-rich areas that contain fine-grained gold in the polished section.

Figure 6: False colour maps from tomography (left) and QEMSCAN (right) imaging, showing the pyrite stringer (yellow grains) in which five gold grains (ringed) could be correlated between the two images.

Note the different orientations of the maps.

10 mm 10 mm

3m