The Application of High Resolution 3D X-ray Computed Tomography scanning in the study of base and precious metal ore deposits

Transcription

1 The Application of High Resolution 3D X-ray Computed Tomography scanning in the study of base and precious metal ore deposits Dr David Holwell University of Leicester, UK

2 The Application of High Resolution 3D X-ray Computed Tomography (HRXCT) scanning in the study of base and precious metal ore deposits Dr David Holwell Daryl Blanks, Emily Firth, Tara Stephens (University of Leicester) Steve Barnes, Margaux Le Vaillant (CSIRO, Perth, Australia)

3 HRXC T Non-destructive imaging of the internal structures of rocks Detects density differences Ideal for imaging ores Precious metals Sulphides Oxides Resolution down to 1µm

4 HRXC T Advantage over traditional 2D techniques Non destructive Quick 3D textural data Eliminates/identifies nugget effect Identifies where to section

5 How it works Nikon XT H 225 high resolution X-ray CT scanner

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7 Instant radiography

8 Thousands of 2D image files captured as sample rotates Typical scan 2-8 hours depending on resolution required

9 Processing Software processes the 2D images and renders into 3D Processing takes around minutes Intel Xeon CPU GBP GHz, 16 Gb RAM 6 Graphics cards

10 Attenuation of x-rays X-rays attenuated when they pass through a material. The denser the material is, the higher the attenuation. X-rays must be able to see through a sample. So energy (kv) can be controlled to ensure some x-rays penetrate the full sample.

11 Attenuation of x-rays However, increasing the kv, reduces the difference in attenuation between different phases Thus, lower kv is better for resolving similar density phases Godel 2013 (Econ Geol) But for lower kv to penetrate the sample, the sample needs to be small.

12 Attenuation of x-rays Thus, for the best resolution of different phases, the sample size should be fairly small Also, the resolution of the images are better if the sample is closer to the x-ray emitter, so again, a smaller sample is optimum Trade off between sample size (density) and resolution

13 VGStudio MAX

14 3D viewing

15 THE USE OF HRXCT IN GEOLOGICAL RESEARCH

16 Use in geological research Applied sparsely to a range of geological materials including metamorphic rocks, fossils, sedimentary rocks and meteorites Recent advances in process time and resolution have generated much more application Carlson et al 2003 (Geol Soc Sp Pub)

17 Porosity studies

18 Use in archaeological research King Richard III Discovered by University of Leicester

19 Use in geological research: ores Due to the clear densities in the normal phases (sulfides and/or oxides in silicate/carbonate gangue The study of base and precious metal ores is where HRXCT can add significant knowledge and value Pyrite in quartz vein (2 cm across)

20 Used as a package The technique will not identify exact compositions, but relative densities HRXCT in concert with SEM/microprobe is a powerful tool for mineral characterisation 3D texture Chemical composition Full mineralogical characterisation

21 APPLICATION TO BASE AND PRECIOUS METAL ORES: IMAGING

22 Epithermal precious metal deposits

23 Introduction Kiziltepe low sulphidation epithermal Au-Ag deposit in the Sindirgi district, Balikesir, Turkey. Mining to commence shortly The veins show colloform banding of chalcedony and quartz, quartz pseudomorphing platey calcite and bonanza grade sulphide rich mineralisation. Hosted by Early Miocene dacitic-andesitic volcanic and pyroclastic formations TURKEY

24 Textural characteristics 1 cm

25 Style 1 1 cm Sulphide cluster Lattice blading 2 cm Associated with bladed carbonate in which mineralisation is absent. Quartz Quartz Silver rich Present as acanthite, native silver, stephanite (Ag 5 SbS 4 ), billingsleyite (Ag 7 AsS 6 ) and minor silver halides. Au-Ag-S Stephanite Acanthite Electrum Gold forms as Uytenbogaardtite (Ag 3 AuS 2 ), Au-acanthite and electrum. 20 µm 20 µm

26 Style 2 1 cm

27 Style 2 Mineralisation associated with altered dacite Pyrite rich with inclusions of galena and minor sphalerite Gold and silver present in quartz vein but absent in wallrock occurring as electrum, Au-stephanite and uytenbogaardtite, acanthite and Agsulphosalts 12 cm py ac py gl el qtz 200 µm 50 µm

28 Style 3 1 cm

29 Style 3 Mineralisation present in chalcedony as disseminated sulphides Gold present as native gold (84 89 Au wt. %), uytenbogaardtite, electrum, Au-acanthite and Austephanite. Ag-sulphosalts, Ag halides, and minor (Fe/Cu)Ag-sulphides. Au qtz Quartz chlorargyrite Chalcedony Sulphides Bladed quartz 50 µm 200 µm Au

30 Style 4 Depth m

31 Style 4 Mineralisation hosted by narrow chalcedony veins (<1 cm) which cross cut stage 1. Minor gold Richer in base metals, with sphalerite, chalcopyrite and pyrite with Ag-As- Cu-Sb sulphosalts mainly billingsleyite and polybasite. Acanthite Chalcopyrite Acanthite Quartz Quartz Sphalerite Ag-Au-Sb-As-S 25 µm Cu-billingsleyite 20 µm

32 Example of HRXCT used in concert with compositional analysis to produce a fully characterised ore paragenesis

33 Porphyry Cu-Au-Mo deposits

34 Porphyry Cu-Au-Mo ores Disseminated sulfides Gold Silicate/ carbonate gangue Perfect combination! El Teniente, Chile

35 Porphyry Au sample - China

36 pyrite pyrite Altered volcanic host gold chalcopyrite chalcopyrite

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38 Example of HRXCT used in to identify textural distribution of ore minerals

39 Magmatic Ni-Cu-PGE sulfides

40 Magmatic Cu-Ni-PGE ores Disseminated sulfides PGM Silicate gangue Perfect combination! Mogalakwena mine, Platreef, South Africa

41 Sulphide PGM Plagioclase 1cm Orthopyroxene Quarter core of Ni-Cu-PGE sulfide deposit, Bushveld Complex Progressive stripping of lower density material.

42 100µm Chromite and sulfide-bearing pyroxenite, Bushveld Complex Interstitial sulfide AND inclusions in chromite

43 Way-up structures in dykes Sulphide droplets in gabbroic dyke Invasion of coarse silicates in the upper margins of blebs Trapped sulfide droplets and hydrous phase

44 Sulphide bleb Clinopyroxene Plagioclase Ilmenite/ magnetite 2 cm

45 Sectioning 1 mm sulphide droplet in basalt with silicates stripped away Lower density chalcopyrite Higher density pyrrhotite

46 Equivalent 2D sections. A single scan, done over minutes to hours can be equivalent to thousands of 2D sections

47 Representivity Much higher degree of confidence Vastly increased statistical significance Especially for nuggety samples or small sample numbers

48 Skaergaard intrusion host to Platinova Reef Pd-Au stratiform deposit

49 Mineralisation Hosted by gabbros of the Triple Group Tiny droplets of Cu sulfide Pd-Cu, Au-Cu alloys Invisible in core cpx 100µm bn dg

50 Micro CT Unequivocal 3D evidence of most inclusions along grain boundaries Some present as inclusions in oxide with some silicates 2 mm

51 Eyeballs Single PMM on the margins of rounded sulfide droplets Variable sulfide:pmm ratio Cu sulfide, PMM Godel et al. (2014) Lithos

52 High PMM:sulfide grains Single PMM with little to no associated sulfide Variable sulfide:pmm ratio Cu sulfide, PMM

53 Variability in PMM/sulfide ratio Cabri et al. (2005) Min Eng 2D studies show this BUT, could be due to angle of sectioning 3D data allows for unequivocal confidence in variability being real

54 Ultra-high tenor sulfides Classic R factor models do not work. Tiny droplets trapped in situ cannot equilibrate with enough magma. Dissolution of sulfides? PGE-bearing sulfide droplets re-dissolve back into magma D values mean that PGE (high D sul/sil) will remain in the sulfide Loss of Fe-Cu-S will enrich the droplet Or? Pre-formed PM nanoclusters, or saturation in PM alloys which are then collected by sulfide droplets?

55 Cu sulfide PMM

56 Example of HRXCT used to provide evidence of the physical processes involved in ore formation using data equivalent to thousands of 2D sections

57 Orogenic gold deposits

58 Orogenic gold deposits Disseminated sulfides Gold Nuggety Silicate (carbonate) gangue Perfect combination! Talca, Chile

59 Gold-sulphide vein sample Vein sample containing pyrite, galena and gold as electrum Remove silicates Pyrite Galena Electrum 2 cm box

60 Gold-sulphide vein sample Vein sample containing pyrite, galena and gold as electrum Remove silicates Pyrite Galena Electrum 1mm

61 Multi stage gold paragenesis Gold with pyasp Gold with gnsp-cpy Nuggety gold in qtz Mineral de Talca, Chile Firth et al. (2015) Min Dep

62 High grade dendritic gold

63 APPLICATION TO BASE AND PRECIOUS METAL ORES: QUANTITATIVE ANALYSIS

64 Quantitative grain analysis Ability to analyse the size and shape of individual grains Allows for detailed 3D grain size analysis Quantification of grade without destructive assay

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66 Quantitative grain/pore analysis Ability to perform volumetric proportion analysis Alternative to point counting, done in seconds Quicker and more data than quantitative SEM

67 SUMMARY Non destructive 3D textural analysis Increased confidence over 2D analysis Ideal for nuggety ores Require complimentary compositional analysis Paragenetic studies Metallurgical studies Quantitative data analysis

68 GRACIAS Dave Holwell Jueves, Chachani 2, Gawen Jenkin Novel environmentally friendly ore processing techniques for the 21st century