Bélinda Godel

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.

Platinum ore in three dimensions : insights from high-resolution X-ray computed tomography

Platinum-group elements (referred to as PGE, comprising Pt, Pd, Rh, Ru, Ir, and Os) are strategic metals with a wide variety of industrial applications. Most of the worlds PGE production is mined from large mafic-ultramafic intrusions such as the Bushveld Complex in South Africa, which currently provides 75% of the worlds Pt production. The PGE mineralization is found within distinctive layers, tens to hundred of centimeters thick but extending laterally for many tens of kilometers, where the PGE occur at low parts per million levels as platinum-group minerals (PGM) and in solid solution within disseminated base-metal sulfides. There is still heated debate at the most fundamental level about the mode of formation of this class of deposit; genetic models range from primary magmatic sulfide collection to concentration by migrating halogen-rich fluids. A crucial line of evidence is the spatial relationship between the PGM, which are the most important PGE-bearing phases, and the base-metal sulfide aggregates or blebs. So far, all observations have been carried out using two-dimensional mineralogical studies where textural relationships with other minerals are ambiguous, and with statistical limitations owing to sampling of trace phases intersecting random surfaces. We present the first detailed three-dimensional in situ analysis of the PGM at the sample scale using high-resolution X-ray computed tomography coupled with conventional microscopic and mineralogical study. We find a striking and highly consistent relationship of PGM grains with the edges of complex-shaped magmatic sulfide blebs, and the intersection of these blebs with chromite-silicate grain boundaries. These new three-dimensional observations strongly support an orthomagmatic model coupled with nucleation and growth of PGM at the margins of sulfide liquid droplets.

Extremely Ni-rich Fe–Ni sulfide assemblages in komatiitic dunite at Betheno, Western Australia: results from synchrotron X-ray fluorescence mapping

Fresh unserpentinised komatiitic dunite at Betheno (Western Australia) contains a distinctive sulfide assemblage of pentlandite, pyrite and millerite. The Ni tenor (i.e. Ni concentration in the original sulfide liquid) of this assemblage is in excess of 30 wt%, and the bulk sulfide composition falls within the compositional range of monosulfide solution (MSS) above 800°C. Such assemblages have conventionally been interpreted as the result of hydrothermal upgrading of normal lower Ni-rich magmatic assemblages, but this explanation is not applicable at Betheno. Subtle zonation of Ni concentration in the host olivine, revealed by high-resolution X-ray fluorescence mapping using the Maia detector on the Australian Synchrotron, suggests that coupled subsolidus re-equilibration of Ni and Fe between olivine and sulfide is not a plausible explanation, and the olivine appears to have gained Ni from sulfides rather than the other way around. This leaves a primary magmatic origin as the favoured interpretation, and supports the existence of a stable pyrite–millerite tie-line in the Fe–Ni–S system at low temperatures. Further evidence for this comes from the existence of similar assemblages in fresh dunites from the nearby Perseverance nickel deposit. Hydrothermal alteration is evidently not necessary to form unusually Ni-rich sulfide assemblages. The exceptionally high Ni tenors are attributed to open-system equilibration of sulfide liquid with typical Ni-undepleted olivine, under conditions where sulfide compositions are essentially buffered by the olivine composition, and to the known positive correlation between the Fe/Ni distribution coefficient between olivine and sulfide and the Ni tenor. Other Ni-rich, millerite-bearing assemblages, such as those from the Black Swan nickel deposit, may also have primary origins.

Sulfide-silicate textures in magmatic Ni-Cu-PGE sulfide ore deposits: Disseminated and net-textured ores

Abstract A large proportion of ores in magmatic sulfide deposits consist of mixtures of cumulus silicate minerals, sulfide liquid, and silicate melt, with characteristic textural relationships that provide essential clues to their origin. Within silicate-sulfide cumulates, there is a range of sulfide abundance in magmatic-textured silicate-sulfide ores between ores with up to about five modal percent sulfides, called “disseminated ores,” and “net-textured” (or “matrix”) ores containing about 30 to 70 modal percent sulfide forming continuous networks enclosing cumulus silicates. Disseminated ores in cumulates have various textural types relating to the presence or absence of trapped interstitial silicate melt and (rarely) vapor bubbles. Spherical or oblate spherical globules with smooth menisci, as in the Black Swan disseminated ores, are associated with silicate-filled cavities interpreted as amygdales or segregation vesicles. More irregular globules lacking internal differentiation and having partially facetted margins are interpreted as entrainment of previously segregated, partially solidified sulfide. There is a textural continuum between various types of disseminated and net-textured ores, intermediate types commonly taking the form of “patchy net-textured ores” containing sulfide-rich and sulfide-poor domains at centimeter to decimeter scale. These textures are ascribed primarily to the process of sulfide percolation, itself triggered by the process of competitive wetting whereby the silicate melt preferentially wets silicate crystal surfaces. The process is self-reinforcing as sulfide migration causes sulfide networks to grow by coalescence, with a larger rise height and hence a greater gravitational driving force for percolation and silicate melt displacement. Many of the textural variants catalogued here, including poikilitic or leopard-textured ores, can be explained in these terms. Additional complexity is added by factors such as the presence of oikocrysts and segregation of sulfide liquid during strain-rate dependent thixotropic behavior of partially consolidated cumulates. Integrated textural and geochemical studies are critical to full understanding of ore-forming systems.

Morphology and microstructure of chromite crystals in chromitites from the Merensky Reef (Bushveld Complex, South Africa)

The Merensky Reef of the Bushveld Complex consists of two chromitite layers separated by coarse-grained melanorite. Microstructural analysis of the chromitite layers using electron backscatter diffraction analysis (EBSD), high-resolution X-ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals: fine-grained idiomorphic and large silicate inclusion-bearing crystals. The lower chromitite layer contains both populations, whereas the upper contains only fine idiomorphic grains. Most of the inclusion-bearing chromites have characteristic amoeboidal shapes that have been previously explained as products of sintering of pre-existing smaller idiomorphic crystals. Two possible mechanisms have been proposed for sintering of chromite crystals: (1) amalgamation of a cluster of grains with the same original crystallographic orientation; and (2) sintering of randomly orientated crystals followed by annealing into a single grain. The EBSD data show no evidence for clusters of similarly oriented grains among the idiomorphic population, nor for earlier presence of idiomorphic subgrains spatially related to inclusions, and therefore are evidence against both of the proposed sintering mechanisms. Electron backscatter diffraction analysis maps show deformation-related misorientations and curved subgrain boundaries within the large, amoeboidal crystals, and absence of such features in the fine-grained population. Microstructures observed in the lower chromitite layer are interpreted as the result of deformation during compaction of the orthocumulate layers, and constitute evidence for the formation of the amoeboid morphologies at an early stage of consolidation. An alternative model is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals, formed as a result of supercooling during emplacement of the lower chromite layer against cooler anorthosite during the magma influx that formed the Merensky Reef. The upper chromite layer formed from a subsequent magma influx, and hence lacked a mechanism to form dendritic chromite. This accounts for the difference between the two layers.

Chromite in komatiites: 3D morphologies with implications for crystallization mechanisms

High-resolution X-ray computed tomography has been carried out on a suite of komatiite samples representing a range of volcanic facies, chromite contents and degrees of alteration and metamorphism, to reveal the wide range of sizes, shapes and degrees of clustering that chromite grains display as a function of cooling history. Dendrites are spectacularly skeletal chromite grains formed during very rapid crystallization of supercooled melt in spinifex zones close to flow tops. At slower cooling rates in the interiors of thick flows, chromite forms predominantly euhedral grains. Large clusters (up to a dozen of grains) are characteristic of liquidus chromite, whereas fine dustings of mostly individual ~20-μm grains form by in situ crystallization from trapped intercumulus liquid. Chromite in coarse-grained olivine cumulates from komatiitic dunite bodies occurs in two forms: as clusters or chains of euhedral crystals, developing into “chicken-wire” texture where chromite is present in supra-cotectic proportions; and as strongly dendritic, semi-poikilitic grains. These dendritic grains are likely to have formed by rapid crescumulate growth from magma that was close to its liquidus temperature but supersaturated with chromite. In some cases, this process seems to have been favoured by nucleation of chromite on the margins of sulphide liquid blebs. This texture is a good evidence for the predominantly cumulus origin of oikocrysts and in situ origin of heteradcumulate textures. Our 3D textural analysis confirms that the morphology of chromite crystals is a distinctive indicator of crystallization environment even in highly altered rocks.

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.

Platinum-Group Element Deposits in Layered Intrusions: Recent Advances in the Understanding of the Ore Forming Processes

The major deposits of platinum-group elements (PGE) of the Earth are associated with ultramafic and mafic igneous rocks. The bulk of current PGE production is extracted from narrow stratiform horizons referred to as reefs located in the lower to central portions of large layered intrusions and is dominated by the Bushveld Complex in South Africa. The PGE-mineralized horizons occur in most case as laterally continuous and uniform layers that can extend over hundreds of kilometres along strike. The origins of these extensive PGE-rich layers remain controversial and subject to debate. Over the past ten years, technological developments have allowed the acquisition of multidisciplinary dataset at spatial resolution, detection limits and precisions that were impossible to achieve in the past. The results obtained provide additional insights into the complexity of the ores and the variability both within and between different ore deposits and highlight the necessity of adapting (for each deposit) models of formation based on all of the information available. Models of formation of PGE-reefs need to consider and integrate the superimposition of both physical and geochemical processes, over a range of scales and over a wide range of temperatures.

Microscale data to macroscale processes: a review of microcharacterization applied to mineral systems

Abstract Microanalysis can provide rapid, quantitative characterization of mineral systems that complements the field- and core-scale observations traditionally made in ore deposits. We review recent innovations in microanalytical procedures and their application to studies of ore deposits. Case studies are presented examining how microanalysis can provide constraints on macroscopic processes within mineral systems. Synchrotron X-ray fluorescence shows centimetre-scale chemical variations associated with proximity to mineralization in samples from Sunrise Dam Gold Mine, Western Australia. Pseudomorphs of igneous plagioclase and chemically driven recrystallization interpreted from electron backscatter diffraction suggest that the system was dominated by fluid-driven brecciation with very little shearing. Both the fluid chemistry and fluid pressure evolved during a protracted sequence of vein formation and alteration accompanying gold mineralization. A second case study of sulphide mineralogy at the Mt Keith nickel sulphide deposit, Western Australia demonstrates how X-ray computed tomography combined with trace element mapping can constrain the chemistry and dynamics of magmatic systems. Large-scale interaction between silicate and sulphide melts, shown by homogenous palladium enrichment in pentlandite, leads to a large proportion of globular ores with a high nickel content. Increasing use of microanalysis in ore deposit geology is resulting in the constant reassessment of established models for ore genesis though a combination of micro- and macroscale datasets.