Louise Fisher

Maia X-ray fluorescence imaging: Capturing detail in complex natural samples

Motivated by the challenge of capturing complex hierarchical chemical detail in natural material from a wide range of applications, the Maia detector array and integrated realtime processor have been developed to acquire X-ray fluorescence images using X-ray Fluorescence Microscopy (XFM). Maia has been deployed initially at the XFM beamline at the Australian Synchrotron and more recently, demonstrating improvements in energy resolution, at the P06 beamline at Petra III in Germany. Maia captures fine detail in element images beyond 100 M pixels. It combines a large solid-angle annular energy-dispersive 384 detector array, stage encoder and flux counter inputs and dedicated FPGA-based real-time event processor with embedded spectral deconvolution. This enables high definition imaging and enhanced trace element sensitivity to capture complex trace element textures and place them in a detailed spatial context. Maia hardware and software methods provide per pixel correction for dwell, beam flux variation, dead-time and pileup, as well as off-line parallel processing for enhanced throughput. Methods have been developed for real-time display of deconvoluted SXRF element images, depth mapping of rare particles and the acquisition of 3D datasets for fluorescence tomography and XANES imaging using a spectral deconvolution method that tracks beam energy variation.

Resolution of geochemical and lithostratigraphic complexity: a workflow for application of portable X-ray fluorescence to mineral exploration

Portable X-ray fluorescence (pXRF) technology can be used to collect large amounts of multi-element data rapidly at relatively low cost and has been widely embraced within the minerals industry. However, to date, it has been difficult to compare data-sets collected by different users or at different times because there is no standardized approach to the collection of these data. The absence of information on standardization and calibration procedures raises concerns about a lack of internal consistency within these data-sets and precludes comparison of different data-sets. This paper seeks to address this issue by developing a workflow for the collection of pXRF data in an exploration or mining setting. Two case studies highlight the robustness and possible applications of pXRF data collected following QA/QC protocols. A good correlation between conventional laboratory analyses and pXRF data is demonstrated through comparison of analysis methods for a drill-hole at the Plutonic Gold Mine, Western Australia, and fine-scale lithostratigraphic variation is recognized in pXRF data collected on grade control pulps from a drill fan at the Agnew Gold Mine, Western Australia. The Agnew data precision is sufficient to distinguish alteration signals from background lithology, and to discern which alteration signals are associated with gold mineralization.

Use and calibration of portable X-Ray fluorescence analysers: application to lithogeochemical exploration for komatiite-hosted nickel sulphide deposits

Portable X-Ray Fluorescence (pXRF) analysers allow on-site geochemical analysis of rock powders and drill core. The main advantages of pXRF analysis over conventional laboratory analysis are the speed of data collection and the low cost of the analyses, permitting the collection of extensive, spatially representative datasets. However, these factors only become useful if the quality of the data meets the requirements needed for the purposes of the study. Here, we evaluate the possible use of portable XRF to determine element concentrations and ratios used in exploration for komatiite-hosted nickel sulphides. A portable XRF analyser was used to measure a series of chalcophile and lithophile element concentrations (Si, S, K, Ca, Ti, Cr, Fe, Ni, Cu, Zn, As, Sr, and Zr) of 75 samples from three komatiite units associated with nickel sulphide ores in the Yilgarn Craton, Western Australia. Crucial steps in the study were the development of a strict calibration process as well as numerous data quality checks. The 670 analyses collected in this study were compared with conventional laboratory XRF data on discriminant diagrams commonly utilized in exploration for komatiite-hosted nickel sulphides (Cr vs Ni and Ni/Ti vs Ni/Cr). After comparing the results obtained with pXRF during this study with the laboratory values, we can conclude that portable XRF analyses can be used for rapid assessment of the nickel sulphide prospectivity of komatiites provided that strict control protocols are followed. Supplementary Material: is available at http://www.geolsoc.org.uk/SUP18706

3D representation of geochemical data, the corresponding alteration and associated REE mobility at the Ranger uranium deposit, Northern Territory, Australia

Interrogation and 3D visualisation of multiple multi-element data sets collected at the Ranger 1 No. 3 uranium mine, in the Northern Territory of Australia, show a distinct and large-scale chemical zonation around the ore body. A central zone of Mg alteration, dominated by extensive clinochlore alteration, overprints a biotite–muscovite–K-feldspar assemblage which shows increasing loss of Na, Ba and Ca moving towards the ore body. Manipulation of pre-existing geochemical data and integration of new data collected from targeted ‘niche’ samples make it possible to recognise chemical architecture within the system and identify potential fluid conduits. New trace element and rare earth element (REE) data show strong fractionation associated with the zoned alteration around the deposit and with fault planes that intersect and bound the deposit. Within the most altered portion of the system, isocon analysis indicates addition of elements including Mg, S, Cu, Au and Ni and removal of elements including Ca, K, Ba and Na within a zone of damage associated with ore precipitation. In the more distal parts of the system, processes of alteration and replacement associated with the mineralising system can be recognised. REE element data show enrichment in HREE centred about a characteristic peak in Dy in the high-grade ore zone while LREEs are enriched in the outermost portions of the system. The patterns recognised in 3D in zoning of geochemical groups and contoured S, K and Mg abundance and the observed REE patterns suggest a fluid flow regime in which fluids were predominately migrating upwards during ore deposition within the core of the ore system.

Application of portable X-ray fluorescence analysis to characterize dolerite dykes at the Plutonic Gold Mine, Western Australia

The amphibolite-facies, Au-mineralized mafic rocks at the Plutonic Gold Mine are intruded by a suite of dolerite dykes of unknown age. The zones between these intrusive units often host significant Au mineralization. It is unclear whether this enrichment in Au mineralization is a function of the intrusion of the dolerites themselves or the influence of pre-existing structures (e.g. faults or shears). Geochemical characterization of the different microcrystalline dolerite units is important to the understanding of the structural architecture of the deposit and to the possible relationship of the dolerites to Au mineralization. The collection of a large geochemical dataset (n = 497) from the dolerite dykes from across the deposit using portable X-ray fluorescence technology allows us to break them into four distinct geochemical groupings. Thus we can define their geometries with greater confidence than was possible using lithology alone. Traverses across individual dolerite dykes indicate that the chill margins are the most geochemically homogenous and most likely to represent the chemistry of the source magma. Plots of Ti v. Zr combined with principal component analysis (PCA) define four geochemically distinct suites of dolerites. By applying this understanding to dolerites in a small area of the deposit, a new interpretation was generated whereby significant amounts of rock that were previously modelled as being dolerite were reclassified as potential host-rock, thus increasing the potential for Au in this area.

The Maia detector array and x-ray fluorescence imaging system: locating rare precious metal phases in complex samples

X-ray fluorescence images acquired using the Maia large solid-angle detector array and integrated real-time processor on the X-ray Fluorescence Microscopy (XFM) beamline at the Australian Synchrotron capture fine detail in complex natural samples with images beyond 100M pixels. Quantitative methods permit real-time display of deconvoluted element images and for the acquisition of large area XFM images and 3D datasets for fluorescence tomography and chemical state (XANES) imaging. This paper outlines the Maia system and analytical methods and describes the use of the large detector array, with a wide range of X-ray take-off angles, to provide sensitivity to the depth of features, which is used to provide an imaging depth contrast and to determine the depth of rare precious metal particles in complex geological samples.

Primary cumulus platinum minerals in the Monts de Cristal Complex, Gabon: magmatic microenvironments inferred from high-definition X-ray fluorescence microscopy

An unusual occurrence of Pt-enriched pyroxenites in the Monts de Cristal igneous complex is characterized by unusually high ratios of Pt to other platinum-group elements (PGEs) and very low Cu and sulfide contents. Synchrotron X-ray fluorescence microscopy was used to identify over a hundred discrete grains of platinum minerals and relate their occurrence to textural associations in the host heteradcumulate orthopyroxenites. Element associations, backed up by FIB-SEM and PIXE probe observations, indicate that most of the Pt is associated with either As- or trace Cu–Ni-rich sulfides, or both. Some of the Pt–As grains can be identified as sperrylite, and most are likely to be Pt–Fe alloy. The relative abundances and volumes of Pt minerals to sulfide minerals are very large compared with typical magmatic sulfides. Almost all of the grains observed lie at or within a few tens of μm of cumulus orthopyroxene grain boundaries, and there is no significant difference between the populations of grains located inside or outside plagioclase oikocrysts. These oikocrysts are inferred to have crystallized either at the cumulus stage or very shortly thereafter, on the basis of their relationship to Ti enrichment in the margins of pyroxene grains not enclosed in oikocrysts. This relationship precludes a significant role of trapped intercumulus liquid in Pt deposition or mobilization and also allows a confident inference that Pt-rich and Pt–As-enriched phases precipitated directly from the magma at the cumulus stage. These observations lead to the conclusion that fractionation of Pt from other PGEs in this magmatic system is a consequence of a solubility limit for solid Pt metal and/or Pt arsenide.

3D visualisation of portable X-ray fluorescence data to improve geological understanding and predict metallurgical performance at Plutonic Gold Mine, Western Australia

Abstract The amphibolite-facies metabasaltic rocks of the Mine Mafic Package at Plutonic Gold Mine, Western Australia, contain an estimated endowment of 10·5 Moz of Au. A preliminary study based on portable X-ray fluorescence (pXRF) analyses identified a geochemical stratigraphy which strongly controlled the location of Au mineralisation. The present study incorporates a significantly larger pXRF dataset and presents the data in a three-dimensional framework. This dataset allows an investigation of the mineralogy of Au mineralisation with varying geochemical associations across the deposit. Historically, high As content in the mill feed resulted in poor metallurgical performance. Seamless data integration of the pXRF dataset allows for recognition of the different styles of Au mineralisation based on Au/As ratios, and visualisation of the distribution of these different mineralogical associations in three dimensions. This work enables us to better predict the As concentration of underground ore blocks, and to be proactive in optimising the mill configuration to improve metallurgical performance.

Mapping bedrock lithologies through in situ regolith using retained element ratios: a case study from the Agnew-Lawlers area, Western Australia

Large, high-quality multi-element geochemical datasets are becoming widely available in the exploration industry, and afford excellent opportunities to investigate geochemical processes. A dataset of over 2500 analyses of unweathered and variably weathered mafic and ultramafic rocks for over 50 elements has been collected by Gold Fields Ltd. in the auriferous Agnew-Lawlers area of the eastern Yilgarn Craton of Western Australia. This dataset is used to investigate changes in element abundances and inter-element ratios through varying degrees and styles of weathering in an area of thick regolith characterised by deep in situ weathering. Systematic interrogation of the data, using lithostratigraphic controls derived from regional mapping and geophysics, reveals that a suite of elements, including Ti, Al, Zr, Th, La, Sc and Nb, and to a lesser extent Cr and Ni, behave as essentially immobile components during saprolite formation. In some cases diagnostic element ratios persist into siliceous duricrust. Ratios of these elements are used as reliable discriminants of bedrock type, and delineate features such as cryptic layering within fractionated sills and subtle geochemical variants in a sequence of tholeiitic and komatiitic basalts. Mapping on the basis of discriminant element ratios greatly extends previous trace-element ratio-based schemes for rock type discrimination. The potential to determine several of these elements with adequate precision and accuracy using portable XRF technology opens a potentially useful technique for rapid geochemical bedrock mapping in residual terrains.

A comprehensive approach to understanding ore deposits using portable X-ray fluorescence (pXRF) data at the Plutonic Gold Mine, Western Australia

Since 2009 all underground face samples and diamond-drill core samples at the Plutonic Gold Mine (Plutonic), Marymia Inlier, Western Australia have been analysed by portable X-ray fluorescence (pXRF) following a systematic approach. This method is rapid and cost-effective and provides analyses of a large suite of chemical elements which can be used to characterize lithology and alteration. The delivery of a comprehensive workflow for sample preparation, analysis, results correction, and rapid processing enabled a quantum leap in the way mine geologists use geochemistry in modelling the ore body. Interpretation of the mine-site dataset of over 200 000 multi-element analyses has resulted in significant improvements in the understanding of the Plutonic deposit. In this contribution, we review how our understanding of Plutonic has been significantly improved through the use of pXRF geochemical analyses. Incorporation of pXRF data into routine geological modelling at Plutonic has resulted in improved confidence in the models of the ore bodies themselves and late-stage dolerite intrusives. It has allowed better management of milling processes through the development of metallurgical proxies and for significant insights into the role of stratigraphy in controlling the location of gold mineralization. We highlight the potential that a systematic approach to collecting pXRF data can have in a mining environment. These same techniques could be adapted and used in other mine and/or exploration settings.