AEGC 2019: From Data to Discovery – Perth, Australia 1

Application of micro-XRF to characterise diamond drill-core from lithium-caesium-tantalum pegmatites Naomi Potter Nigel Brand Portable XRF Services Pty Ltd Geochemical Services Pty Ltd Level 2, 9 Colin St, West Perth, 6005 1/5 Colin St West Perth 6005 njpotter@pxrfs.com.au nwbrand@bigpond.net.au

INTRODUCTION

X-ray fluorescence (XRF) is a well-established analytical technique that is commonly used to generate semi-quantitative rock compositions (Jenkins 1999; De Vries and Vrebos, 2002). The advances in XRF technology has led to the increasing popularity of the technique and the conception of µXRF instrumentation. Until recently, one of the primary difficulties for micro-XRF has been focusing the X-ray beams, which had been limited to specialist synchrotron facilities where the high flux of X-rays and collimator optics enabled a small X-ray spot size (<25 µm). However, the addition of capillary optics in the micro-XRF has enabled the spot size to be reduced to tens of micrometres and can consequently measure small areas without

sacrificing intensity or sensitivity, facilitating the commercial application of micro-XRF instruments. Micro-XRF technology can be utilised in many geological applications, on various sample types (e.g. drill core, pulps, thin sections) with minimal sample preparation. By scanning the surface with a focused X-ray beam, the elemental composition and spatial distribution of major, minor and some abundant trace element elements can be determined (Fritz et al., 2016; Hoehnel, 2018). The large sample chamber (600mm x 350mm x 260mm) and internal stage (330mm x 170mm) enable an assortment of sample shapes and sizes to be accommodated, as samples up to ~190 mm can be measured without the need for re-orientation. Precise measurements can be made at a rate of millimetres per second. The instrument can perform these measurements “on-the-fly” with data collection simultaneous to the movement of the sample stage, reducing the total time needed for analysis completion. These measurements can provide a qualitative overview of the elemental distribution of large samples, and are acquired as either single-point or multi-point line scanning. Point scanning, with a defined dwell time, is required for accurate chemical quantification. These features of the µXRF enable a multitude of information to be collected covering the chemical, textural, and mineralogical characteristics of the sample. The technology has been successful utilised in the study of geological materials, principally with regards to temporal and spatial variations of the chemical compositions of minerals (Fritz et al., 2016; Genna et al., 2009; Hoehnel et al., 2018). The measurement of millimetre scale chemo-mineralogical variability of diamond drill core and interpretation of resultant elemental compositional maps is shown to be important for the correct evaluation of hydrothermal alteration, deformation and solution-precipitation processes (Genna et al., 2009; Hoehnel et al., 2018). For example, Genna et al., (2009) highlights the µXRF as a tool to decipher alteration zonation in a fragmental rhyolite complex for volcanic massive sulphide (VMS) exploration. Most µXRF instruments use a theoretical quantification method called fundamental parameterisation. Fundamental parameters are derived from the Sherman equation (Sherman, 1955) and use the atomic parameters of individual elements (e.g. absorption, scattering and emission parameters) to calculate the X-ray intensities that would be expected for any given concentration. Using this database, the fundamental parameter algorithms can calculate the concentration of each element in weight percent, which is then normalised to 100%. The use of a fundamental parameter (FP) model enables the collection of quantitative data for heterogeneous samples. In comparison, conventional XRF instruments use empirically determined models to calculate influence coefficients to correct for matrix effects and essentially calibrate XRF data. A large selection of

SUMMARY Micro X-ray fluorescence (µXRF) is a rapid and non-destructive technique used to acquire qualitative and quantitative data at high spatial resolution (i.e. µm scale). To date this technology has yet to be utilised to its full potential in the exploration, mining and metallurgical industries. Micro-XRF instrumentation can acquire a wealth of geochemical, mineralogical and petrological information for a versatile range of sample types and sizes, due to the large sample chamber and small X-ray beam size. The non-destructive nature of the instrument combined with the minimal sample preparation required is a major advantage over other geochemical techniques and makes µXRF ideally suited for rapid and inexpensive studies aimed at investigating micromorphology, chemical variability and mineral alteration in rock samples. In this paper, we present the findings of an ongoing case study on Lithium-Caesium-Tantalum (LCT) pegmatites, primarily focused on the Sinclair Caesium Deposit. The analyses were conducted on split diamond drill-core and highlight the quality of data that can be acquired by µXRF and the ability to accurately identify the mineralogy and quantify the chemical compositions of geological samples. The resulting element distribution and mineral maps enabled mineralogical discrimination, the identification of previously unknown mineralogical features and the detection of chemical differences at a micro-scale. Therefore, µXRF is a versatile tool that can be utilised in the exploration and mining industry for the rapid acquisition of high-quality qualitative and quantitative geochemical data on a variety of geological and environmental samples. Key words: µXRF, mineralogy, petrology, geochemistry, pegmatite.

Application of Micro-XRF to LCT pegmatites Potter and Brand

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well-characterised reference materials that are comparable to project lithology and matrix-densities are required (Kanngießer, 2003). For these instruments, the larger spot size (3 to 8 mm) mean that heterogeneity and grain size issues are pervasive, especially in diamond drill core, and as such there is a general acceptance that pulverising, sieving, homogenisation is best practice. However, for µXRF the accuracy of fundamental parameter models is shown to be fit for purpose (Elam et al., 2004; Flude et al., 2017), and can overcome heterogeneity and matrix variations commonly observed in traditional XRF instrumentation Investigating the spatial chemical variability of bulk samples is important, and can be used to aid the interpretation of crucial textural and mineralogical features that may otherwise not be visible or readily distinguishable using other methods. Although there are benefits to the use of an empirical standard-based model compared to fundamental parameters in µXRF instrumentation, the additional analytical effort that is required has not yet been justified (Flude et al., 2017). In this study, we investigate the application of µXRF technology to lithium-caesium-tantalum (LCT) pegmatites to evaluate its capability to accurately identify the mineralogy and quantify the chemical compositions of the samples. Additionally, we demonstrate various ways that µXRF can contribute to geological and mineralogical research and projects, as well as the requirements and limitations of the instrument. The preliminary results clearly demonstrate the use of the µXRF for mineralogical discrimination, identification of previously unknown and visually indistinguishable mineralogical features and the detection of chemical differences at a micro-scale.

METHOD Split diamond drill core samples from two separate LCT pegmatites are examined. The mass and dimensions of the samples were ideal for this investigation, and furthermore sample preparation was not required. Sample 1 Amblygonite, albite, quartz and lepidolite

6.35cm diameter (HQ), ~17cm length μXRF analysis area of ~108cm2, Approximate analysis time: 3 hours Sample 2 Spodumene, quartz, jadite, illite, apatite 5.06cm diameter (NQ2), ~11cm length μXRF analysis area of ~31.2cm2 Approximate analysis time: 6 hours Chemical data were acquired using a bench-top Bruker Nano Analytics 2D-micro-XRF spectrometer (Bruker M4 Tornado). The instrument has a 50kV 30-Watt Rh anode target, two simultaneously operating 30mm2 XFlash® silicon drift detectors via beryllium windows and poly-capillary optics that can focus a beam spot size down to 25 µm. Sample location is recorded on two cameras (10x and 100x) enabling the precise location of the X-ray beam on the sample to be identified. Measurements can be achieved under normal atmospheric conditions (air) or under vacuum. Air readily attenuates X-ray energies, notably low energy X-rays so the evacuation of air and operation of the sample chamber under a vacuum significantly improves light element analysis (i.e. Z>11). Elements ranging from sodium (Na) to uranium (U) can be measured with quantification limits ranging from percentages to parts per million.

Areas selected for analysis were determined with the use of an internal camera in the chamber. The stage can be moved in three dimensions, providing full access to the core and thereby greatly increasing the mapping capacity. The cores were measured under vacuum with a spot size of 25 µm. The instrument was turned on 1.5 hours before quantitative analyses were carried out to reduce errors from beam instability whilst the X-ray tube is warming up (Flude et al., 2017). The data acquisition for each sample was performed in separate runs. Two-dimensional element distribution maps are gridded by pixel intensity. The instrument collects an entire X-ray spectrum for each pixel in the grid, with pixel intensity proportional to the intensity of the X-ray spectrum for that element. The largest X-ray intensities are generally yielded from Kα absorption energies (usually Kα) but alternative peaks can be manually selected. This is important for the construction of meaningful element maps when elements with overlapping characteristic X-ray energies are present in the sample. Elements that are associated with LCT pegmatite exploration are evaluated. These elements include the major elements: Al, Si, K, Fe, Ca, P, Mn, S, Na, and Mg and the minor and trace elements: Cs, Rb, Sr, Ba and Ta. Evaluation of the data and preparation of mineral distribution maps were undertaken with the Advanced Mineral Identification and Characterisation (AMICS) software provided by Bruker Nano Analytics. The software database is comprised of more than 2,000 minerals and is designed to produce quick and easy automated identification and quantification of minerals. Library spectra are computer generated; however individual spectra for specific minerals of interest can be added to the library. X-ray penetration depth varies with the atomic number of the fluorescing element and the sample matrix. This can be an advantage for heavier elements as good quality maps can be produced from unpolished or rough surfaces with minimal effect on sample precision. However, caution must be exercised when interpreting areas pertaining to lighter-matrices as X-ray critical penetration depths will be smaller and surface conditions become more important. The AMICS software employs a simulated mineral mixture gradient algorithm that provides robust mineral boundary identification.

RESULTS

Elemental maps provide real insight into the visually indistinguishable chemical and mineral variations and the locations of elements of interest e.g. caesium. As shown in Figure 1, caesium is concentrated in the lepidolite and can therefore be associated with the primary mineralogy of the rock and not a later hydrothermal process. Another visually unrecognisable feature includes Ca-Sr-concentrated veins that cut amblygonite which are likely to indicate the commencement of the weathering of amblygonite by penetrating ground waters. Additionally, nuggets of tantalum are observed within the lepidolite and are not consistently scattered throughout the mineral phase. The ability to rapidly identify the deportment of elements within mineral phases without sample preparation highlights the benefits of μXRF applications.

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Figure 1. (a) photograph of Sample 1 from a LCT pegmatite at the Sinclair Caesium Deposit, (b-f) False colour images showing the highest intensity of different elements per pixel normalised to the analysed area (b) Al and Si, (c) Cs and P, (d) Rb, Sr, and Ca, note the Ca-Sr veins cutting amblygonite, (e) Cs, (f) Ta. The application of AMICS automated mineral classification software was also examined. The resultant mineral map (Figure 2) provides information on mineral distribution, grain size, and visually indistinguishable gangue minerals. These data are crucially used in the development of appropriate metallurgical circuits. Spodumene is the primary mineral of interest due to its high Li contents. Five minerals were identified, with spodumene as the dominant phase (~57%). Yet, by visual identification it is difficult to differentiate spodumene and quartz and could lead to inaccurate estimates of the spodumene content. Furthermore, illite, apatite and jadite may have been unidentified or misidentified by visual inspection alone.

Figure 2. (a) photograph of Sample 2, red box denotes the analysed area, (b) mineral classification map using the AMICS software, note the high abundance of spodumene, (c) frequency histogram of classified mineral abundances of analysed area. Micro-XRF measurements are currently being made on additional samples from zones in the Sinclair Caesium Deposit, Pioneer Dome LCT Project, Western Australia. Samples from a high-grade pollucite zone (with lepidolite veinlet texture), a lepidolite-rich zone (with cleavendite and quartz), and an albite-muscovite wall zone (with accessory garnet) are being investigated. The analysis of these samples will enable the characterisation of the major zones and improve the understanding of the deposit, the findings of which will be discussed subsequently. The ability to identify these mineralogical, textural and chemical features after a few hours of scanning with no sample preparation required highlights the benefits of this new technology. The data presented in this abstract does not exhaust the chemical database generated by the M4 Tornado, and instead provides a preview of the quality and quantity of chemical data that can be collected by an µXRF instrument.

CONCLUSIONS

Micro-XRF scanning is a versatile tool used to acquire qualitative and quantitative geochemical and mineralogical data at an unprecedented speed. Qualitative element maps provide useful information on spatial variation and concentration of major, minor and trace elements, which may not be visibly distinct, but are important for understanding the paragenesis and mineralisation. The technique may also foster the ability to provide more detailed geochemical investigations of other geological and environmental applications that have not yet been explored.

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ACKNOWLEDGEMENTS The authors wish to thank the Pioneer Dome Lithium-Caesium-Tantalum Project team for providing LCT pegmatite samples for this study. We would like to acknowledge Bruker for their assistance with the collection of data. Additionally, we thank both Bruker and Pioneer Resources for allowing and encouraging these data to be presented.

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