Search Results (29)

An enrichment in nickel (Ni) or platinum group elements (PGE) is seldom observed in ores of the iron oxide–copper–gold (IOCG) type. This phenomenon is, however, known from a few deposits and prospects in the Carajás Mineral Province, Brazil. The Ni-PGE enrichment is explained, in part, by the spatial association of the IOCG-type ores with altered mafic-ultramafic lithologies, as well as by reworking and remobilization of pre-existing Ni and PGE during multiple mineralization and tectonothermal events across the Archean-Proterozoic. One such example of this mineralization is the Jatobá deposit in the southern copper belt of the Carajás Domain. This is the first detailed study of the Ni and PGE mineralization at Jatobá, with implications for understanding ore genesis. Petrographic and compositional study of sulfides shows that pyrite is the main Ni carrier, followed by pyrrhotite and exsolved pentlandite. Measurable concentrations of palladium (Pd) and platinum (Pt), albeit never more than a few ppm, are noted in pyrite. More importantly, however, the trace mineral signature of the Jatobá deposit features several platinum group minerals (PGM), including merenskyite, naldrettite, sudburyite, kotulskite, sperrylite, and borovskite. These PGM occur as sub-10 µm-sized grains that are largely restricted to fractures and grain boundaries in pyrite. All Pd minerals reported contain mobile elements such as Te, Bi, and Sb and are associated with rare earth- and U-minerals. This conspicuous mineralogy, differences in sulfide chemistry between the magnetite-hosted ore and stringer mineralization without magnetite, and microstructural control point to a genetic model for the sulfide mineralization at Jatobá and its relative enrichment in Ni and PGE. Observations support two alternative scenarios for ore genesis. In the first, an initial precipitation of disseminated or semi-massive Ni-PGE-bearing sulfides took place within the mafic rock pile, possibly in a VHMS-like setting. Later partial dissolution and remobilization of this pre-existing mineralization by mineralizing fluids of IOCG-type, possibly during the retrograde stage of a syn-deformational metamorphic event, led to their re-concentration within magnetite along structural conduits. The superposition of IOCG-style mineralization onto a pre-existing assemblage resulted in the observed replacement and overprinting in which PGE combined with components of the IOCG fluids like Sb, Bi, and Te. An alternative model involves leaching, by the IOCG-type fluids, of Ni and PGE from komatiites within the sequence or from ultramafic rocks in the basement. The discovery of PGM in Jatobá emphasizes the potential for additional discoveries of Ni-PGE-enriched ores elsewhere in the Carajás Domain and in analogous settings elsewhere. Full article

The cooling paths and kinetics in the system Cu-Fe-O are investigated by the empirical micro- and nanoscale analysis of slags from the flash furnace smelter at Olympic Dam, South Australia. We aim to constrain the exsolution mechanism of delafossite (Cu¹⁺Fe³⁺O₂) from a spinel solid solution (magnetite, Fe₃O₄) and understand why cuprospinel (CuFe₂O₄) is never observed, even though, as a species isostructural with magnetite, it might be expected to form. Flash furnace slags produced in the direct-to-blister copper smelter at Olympic Dam contain four Cu-bearing phases: Cu-bearing magnetite, delafossite, metallic copper, and cuprite. Delafossite coexists with magnetite as rims and lamellar exsolutions, as well as bladed aggregates, associated with cuprite within Si-rich glass. The empirical compositions of magnetite and rim delafossite are (Fe²⁺_6.89Cu²⁺_0.86Co_0.13Mg_0.15Si_0.02)_8.05 (Fe³⁺_15.52Al_0.41Ti_0.01Cr_0.01)_15.95O₃₂, and (Cu¹⁺_0.993Co_0.002Mg_0.002)_0.997(Fe³⁺_0.957Al_0.027Ti_0.005Si_0.004)_0.993O₂, respectively. The measured Cu content of magnetite represents a combination of a solid solution (~6 mol.% cuprospinel endmember) and exsolved delafossite lamellae. Atomic-resolution high-angle annular dark field scanning transmission electron microscope (HAADF STEM) imaging shows epitaxial relationships between delafossite lamellae and host magnetite. Defects promoting the formation of copper nanoparticles towards the lamellae margins suggest rapid kinetics. Dynamic crystallization under locally induced stress in a supercooled system (glass) is recognized from misorientation lamellae in delafossite formed outside magnetite grains. The observations are concordant with crystallization during the cooling of molten slag from 1300 °C to <1080 °C. Melt separation through an immiscibility gap below the solvus in the system Cu-Fe-O is invoked to form the two distinct delafossite associations: (i) melt-1 from which magnetite + delafossite form; and (ii) melt-2 from which delafossite + cuprite form. Such a path also corroborates the published data explaining the lack of cuprospinel as a discrete phase in the slag. Delafossite rims form on magnetite at a peritectic temperature of ~1150 °C via a reaction between the magnetite and copper incorporated in the oxide/Si-rich melt. The confirmation of such a reaction is supported by the observed misfit orientation (~10°) between the rim delafossite and magnetite. HAADF STEM imaging represents a hitherto underutilized tool for understanding pyrometallurgical processes, and offers a direct visualization of phase relationships at the smallest scale that can complement both experimental approaches and theoretical studies based on thermodynamic modelling. Full article

The Mount Weld rare earth element (REE) deposit, Western Australia, is one of the largest of its type on Earth. Current mining exploits the high-grade weathered goethite-bearing resource that lies above, and which represents the weathering product of a subjacent carbonatite. The mineralogy, petrography, deportment of lanthanides among the different components, and variation in mineral speciation, textures, and chemistry are examined. Microanalysis, involving scanning electron microscope (SEM) imaging, electron probe microanalysis (EPMA) and laser ablation inductively coupled-plasma mass spectrometry (LA-ICP-MS), was conducted on sized fractions of three crushed and ground laterite ore samples from current and planned production, and a representative sample from the underlying carbonatite. High-magnification imaging of particles in laterite samples show that individual REE-bearing phases are fine-grained and extend in size well below the micron-scale. Nanoscale inclusions of REE-phosphates are observed in apatite, Fe-(Mn)-(hydr)oxides, and quartz, among others. These have the appearance, particularly in fluorapatite, of pervasive, ultrafine dusty domains. Apart from the discrete REE minerals and abundant nano- to micron-scale inclusions in gangue, all ore components analysed by LA-ICP-MS contain trace to minor levels of REEs within their structures. This includes apatite, where low levels of REE are confirmed in preserved igneous apatite, but also Fe- and Mn-(hydr)oxides in which concentrations of hundreds, even thousands of ppm are measured. This is significant given that Fe-(Mn)-(hydr)oxides are the most abundant component of the laterite and points to extensive mobility and redistribution of REEs, and especially HREE, during progressive lateritisation. Late-formed minerals, notably tiny grains of cerianite, reflect a shift to oxidising conditions. REE-fluorocarbonates are the main host for REEs in carbonatite and are systematically replaced by hydrated, Ca-bearing REE-phosphates (largely rhabdophane). The latter displays varied compositions but is characteristically enriched in HREE relative to monazite in the same sample. Fine-grained, compositionally heterogeneous rhabdophane is accompanied by minor amounts of other paragenetically late, hydrated phosphates with enhanced MREE/HREE relative to LREE (although still LREE-dominant). Minor, relict xenotime and zircon are significant HREE carriers. Ilmenite and pyrochlore group members contain REE but contribute only negligibly to the overall REE budget. Although the proportions of individual mineral species differ, the chemistry of key ore components are similar in different laterite samples from the current resource. Mineral signatures are, however, subtly different in the lower grade southeastern part of the deposit, including higher concentrations of HREE relative to LREE in monazite, rhabdophane, florencite and Fe-(Mn)-(hydr)oxides. Full article

Bi₈Te₃ is a member of the tetradymite homologous series, previously shown to be compositionally and structurally distinct from hedleyite, Bi₇Te₃, yet inadequately characterized structurally. The phase is identified in a sample from the Hedley district, British Columbia, Canada. Compositions are documented by electron probe microanalysis and structures are directly imaged using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). Results confirm that Bi₈Te₃ has an 11-atom layer structure, in which three Bi-Bi pairs are placed adjacent to the five-atom sequence (Te-Bi-Te-Bi-Te). Bi₈Te₃ has trigonal symmetry (space group R$\overline{3}$m) with unit cell dimensions of a = ~4.4 Å and c = ~63 Å calculated from measurements on representative electron diffraction patterns. The model is assessed by STEM simulations and EDS mapping, all displaying good agreement with the HAADF STEM imaging. Lattice-scale intergrowths are documented in phases replacing Bi₈Te₃, accounting for the rarity of this phase in nature. These results support prior predictions of crystal structures in the tetradymite homologous series from theoretical modeling and indicate that other phases are likely to exist for future discovery. Tetradymite homologues are mixed-layer compounds derived as one-dimensional superstructures of a basic rhombohedral sub-cell. Each member of the series has a discrete stoichiometric composition and unique crystal structure. Full article

We used high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) to image the crystal structures of four minerals in the Bi₄X₃ isoseries (X = Te, Se, S), a subgroup of the tetradymite homologous series: ikunolite (Bi₄S₃), laitakarite (Bi₄Se₂S), joséite-B (Bi₄Te₂S), and joséite-A (Bi₄TeS₂). The four minerals are isostructural and interpretable in terms of regular stacking of seven-atom packages: [Bi–S–Bi–S–Bi–S–Bi], [Bi–Se–Bi–S–Bi–Se–Bi], [Bi–Te–Bi–S–Bi–Te–Bi], and [Bi–S–Bi–Te–Bi–S–Bi], respectively. The four phases are mixed-layer structures representing the Bi_2kTe₃ (k = 2) module within the tetradymite series. Diffraction patterns confirm they are seven-fold superstructures of a rhombohedral subcell with c/3 = d~1.89–1.93 Å. Modulation along the d* interval matches calculations of reflection intensity using the fractional shift method for Bi₄X₃. Internal structures can be discerned by high-resolution HAADF STEM imaging and mapping. Paired bismuth atoms are positioned at the outside of each seven-atom layer, giving the minerals a modular structure that can also be considered as being composed of five-atom (X–Bi–X–Bi–X) and two-atom (Bi–Bi) sub-modules. The presence of mixed sites for substituting cations is shown, particularly for Pb. Moreover, Pb may be important in understanding the incorporation of Ag and Au in Bi–chalcogenides. Visualisation of crystal structures by HAADF STEM contributes to understanding relationships between phases in the tetradymite homologous series and will play an invaluable role in the characterization of potential additional members of the series. Full article

The Blackbush uranium prospect (~12,580 tonnes U at 85 ppm cut-off) is located on the Eyre Peninsula of South Australia. Blackbush was discovered in 2007 and is currently the single example of sediment-hosted uranium mineralisation investigated in any detail in the Gawler Craton. Uranium is hosted within Eocene sandstones of the Kanaka Beds and, subordinately, within a massive saprolite derived from the subjacent Hiltaba-aged (~1585 Ma) granites, affiliated with the Samphire Pluton. Uranium is mainly present as coffinite in different lithologies, mineralisation styles and mineral associations. In the sandstone and saprolite, coffinite occurs intergrown with framboidal Fe-sulphides and lignite, as well as coatings around, and filling fractures within, grains of quartz. Microprobe U–Pb dating of coffinite hosted in sedimentary units yielded a narrow age range, with a weighted average of 16.98 ± 0.16 Ma (343 individual analyses), strongly indicating a single coffinite-forming event at that time. Coffinite in subjacent saprolite generated a broader age range from 28 Ma to 20 Ma. Vein-hosted coffinite yielded similar ages (from 12 to 25 Ma), albeit with a greater range. Uraninite in the vein is distinctly older (42 to 38 Ma). The 17 ± 0.16 Ma age for sandstone-hosted mineralisation roughly coincides with tectonic movement as indicated by the presence of horst and graben structures in the Eocene sedimentary rocks hosting uranium mineralisation but not in stratigraphically younger sedimentary rocks. The new ages for hydrothermal minerals support a conceptual genetic model in which uranium was initially sourced from granite bedrock, then pre-concentrated into veins within that granite, and is subsequently dissolved and reprecipitated as coffinite in younger sediments as a result of low-temperature hydrothermal activity associated with tectonic events during the Tertiary. The ages obtained here for uranium minerals within the different lithologies in the Blackbush prospect support a conceptual genetic model in which tectonic movement along the reactivated Roopena Fault, which triggered the flow of U-rich fluids into the cover sequence. The timing of mineralisation provides information that can help optimise exploration programs for analogous uranium resources within shallow buried sediments across the region. The model presented here can be predicted to apply to sediment-hosted U-mineralisation in cratons elsewhere. Full article

Pyrite is the most common sulphide in a wide range of ore deposits and well known to host numerous trace elements, with implications for recovery of valuable metals and for generation of clean concentrates. Trace element signatures of pyrite are also widely used to understand ore-forming processes. Pyrite is an important component of the Olympic Dam Cu–U–Au–Ag orebody, South Australia. Using a multivariate statistical approach applied to a large trace element dataset derived from analysis of random pyrite grains, trace element signatures in Olympic Dam pyrite are assessed. Pyrite is characterised by: (i) a Ag–Bi–Pb signature predicting inclusions of tellurides (as PC1); and (ii) highly variable Co–Ni ratios likely representing an oscillatory zonation pattern in pyrite (as PC2). Pyrite is a major host for As, Co and probably also Ni. These three elements do not correlate well at the grain-scale, indicating high variability in zonation patterns. Arsenic is not, however, a good predictor for invisible Au at Olympic Dam. Most pyrites contain only negligible Au, suggesting that invisible gold in pyrite is not commonplace within the deposit. A minority of pyrite grains analysed do, however, contain Au which correlates with Ag, Bi and Te. The results are interpreted to reflect not only primary patterns but also the effects of multi-stage overprinting, including cycles of partial replacement and recrystallisation. The latter may have caused element release from the pyrite lattice and entrapment as mineral inclusions, as widely observed for other ore and gangue minerals within the deposit. Results also show the critical impact on predictive interpretations made from statistical analysis of large datasets containing a large percentage of left-censored values (i.e., those falling below the minimum limits of detection). The treatment of such values in large datasets is critical as the number of these values impacts on the cluster results. Trimming of datasets to eliminate artefacts introduced by left-censored data should be performed with caution lest bias be unintentionally introduced. The practice may, however, reveal meaningful correlations that might be diluted using the complete dataset. Full article

Bi-Pb-chalcogenides of the aleksite series represent homologous mixed-layer compounds derived from tetradymite (Bi₂Te₂S). Considering tetradymite as composed of five-atom (Bi₂Te₂S) layers, the named minerals of the aleksite homologous series, aleksite (PbBi₂Te₂S₂) and saddlebackite, (Pb₂Bi₂Te₂S₃) have been considered as phases formed by regular stacking of longer seven- and nine-atom layers. High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) imaging of thinned foils prepared in-situ on domains deemed homogeneous from electron probe microanalysis, STEM energy-dispersive X-ray spectrometry (EDS) element mapping and fast Fourier transforms (FFTs) from the images offer new insights into these structures. The hitherto-unnamed phase, PbBi₄Te₄S₃, previously interpreted as regular intergrowths of five- and seven-atom layers, is characterized in terms of regular repeats of five- and seven-atom layers over distances of at least 350 nm, defining the (57), or 24H polytype. Imaging of other domains in the same lamella with identical composition at the electron microprobe scale reveals the presence of two additional polytypes: (5559), or 48H; and (557.559) or 72H. Unit cell dimensions for all three polytypes of PbBi₄Te₄S₃ can be both measured and predicted from the HAADF STEM imaging and FFTs. STEM EDS mapping of each polytype confirm the internal structure of each layer. Lead and S occur within the centre of the layers, i.e., Te–Bi–S–Pb–S–Bi–Te in the seven-atom layer, Te–Bi–S–Pb–S–Pb–S–Bi–Te in the nine-atom layer, and so on. Polytypism is an intrinsic feature of the aleksite series, with each named mineral or unnamed phase, except the simple five-atom layer, defined by several alternative stacking sequences of different length, readily explaining the differing c values given in previous work. Homology is defined in terms of layer width and the stacking arrangement of those layers. Coherent structures of the same composition need not only be built of layers of adjacent size (i.e., five- and seven-atom layers) but, as exemplified by the (5559) polytype, may also contain non-adjacent layers, a finding not anticipated in previous work. New polysomes remain to be discovered in nature and each potentially exists as multiple polytypes. The present study further emphasizes the utility of HAADF STEM imaging and atomic-scale STEM EDS element mapping as an optimal tool for tracking stacking sequences and characterising structures in mixed-layer compounds. Full article

Titanomagnetite from Fe-Ti-V ores of the Lanjiahuoshan deposit, Panzhihua layered intrusion, Southwest China, was investigated at the nanoscale. The objectives were to establish the composition of exsolution phases and their mutual relationships in order to evaluate the sequence of exsolution among oxide phases, and assess mechanisms of ore formation during magma emplacement. At the micron-scale, titanomagnetite shows crosscutting sets of exsolutions with ilmenite and Al-Mg-Fe-spinel (pleonaste), as well as overprint, both in terms of phase re-equilibration and remobilization of trace elements. Most complex textures were found in titanomagnetite surrounded by ilmenite and this was selected for high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) imaging and STEM energy-dispersive X-ray spectrometry (EDS) spot analysis and mapping on a thin foil prepared in situ on a focused ion beam scanning electron microscope platform. Titanomagnetite revealed two sequential sets of exsolutions, {111} crosscutting {100}, which are associated with changes in phase speciation and trace element distribution patterns. Qandilite is the dominant spinel phase inside titanomagnetite; magnesioferrite is also identified. In contrast, Fe-poor, Al-rich, Mg-bearing spinel is present within ilmenite outside the grain. Vanadium enrichment in newly-formed magnetite lamellae is clear evidence for trace element remobilization. This V-rich magnetite shows epitaxial relationships with ilmenite at the contact with titanomagnetite. Two-fold super-structuring in ilmenite is evidence for non-redox re-equilibration between titanomagnetite and ilmenite, supporting published experimental data. In contrast, the transformation of cubic Ti-rich spinel into rhombohedral ilmenite imaged at the nanoscale represents the “oxy-exsolution” model of titanomagnetite–ilmenite re-equilibration via formation of a transient ulvöspinel species. Nanoscale disorder is encountered as vacancy layers in Ti-rich spinel, and lower symmetry in the Fe-poor, Al-Mg phase, suggesting that slow cooling rates can preserve small-scale phase equilibration. The cooling history of titanomagnetite ore can be reconstructed as three distinct stages, concordant with published models for the magma plumbing system: equilibrium crystallization of Al-rich, Mg-bearing titanomagnetite from cumulus melts at ~55 km, with initial exsolutions occurring above 800 °C at moderate fO₂ conditions (Stage 1); crosscutting {111} exsolutions resulting in formation of qandilite, attributable to temperature increase due to emplacement of another batch of melt affecting the interstitial cumulus during uplift. Formation of 2-fold superstructure ilmenite + V-rich magnetite exsolution pairs representing non-redox equilibration indicates resetting of the cooling path at this stage (Stage 2); and ilmenite formation from pre-existing Ti-rich spinel and ulvöspinel, illustrative of redox-driven cooling paths at <10 km (Stage 3). HAADF STEM provides direct imaging of atomic arrangements, allowing recognition of processes not recognizable at the micron-scale, and can thus be used to constrain exsolution models during ore formation. Full article

Metal nanoparticles (NP) in minerals are an emerging field of research. Development of advanced analytical techniques such as Z-contrast imaging and mapping using high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) allows unparalleled insights at the nanoscale. Moreover, the technique provides a link between micron-scale textures and chemical patterns if the sample is extracted in situ from a location of petrogenetic interest. Here we use HAADF STEM imaging and energy-dispersive X-ray spectrometry (EDX) mapping/spot analysis on focused ion beam prepared foils to characterise atypical Cu-As-zoned and weave-twinned hematite from the Olympic Dam deposit, South Australia. We aim to determine the role of solid-solution versus the presence of discrete included NPs in the observed zoning and to understand Cu-As-enrichment processes. Relative to the grain surface, the Cu-As bands extend in depth as (sub)vertical trails of opposite orientation, with Si-bearing hematite NP inclusions on one side and coarser cavities (up to hundreds of nm) on the other. The latter host Cu and Cu-As NPs, contain mappable K, Cl, and C, and display internal voids with rounded morphologies. Aside from STEM-EDX mapping, the agglomeration of native copper NPs was also assessed by high-resolution imaging. Collectively, such characteristics, corroborated with the geometrical outlines and negative crystal shapes of the cavities, infer that these are opened fluid inclusions with NPs attached to inclusion walls. Hematite along the trails features distinct nanoscale domains with lattice defects (twins, 2-fold superstructuring) relative to hematite outside the trails, indicating this is a nanoprecipitate formed during replacement processes, i.e., coupled dissolution and reprecipitation reactions (CDRR). Transient porosity intrinsically developed during CDRR can trap fluids and metals. Needle-shaped and platelet Cu-As NPs are also observed along (sub)horizontal bands along which Si, Al and K is traceable along the margins. The same signature is depicted along nm-wide planes crosscutting at 60° and offsetting (012)-twins in weave-twinned hematite. High-resolution imaging shows linear and planar defects, kink deformation along the twin planes, misorientation and lattice dilation around duplexes of Si-Al-K-planes. Such defects are evidence of strain, induced during fluid percolation along channels that become wider and host sericite platelets, as well as Cl-K-bearing inclusions, comparable with those from the Cu-As-zoned hematite, although without metal NPs. The Cu-As-bands mapped in hematite correspond to discrete NPs formed during interaction with fluids that changed in composition from alkali-silicic to Cl- and metal-bearing brines, and to fluid rates that evolved from slow infiltration to erratic inflow controlled by fault-valve mechanism pumping. This explains the presence of Cu-As NPs hosted either along Si-Al-K-planes (fluid supersaturation), or in fluid inclusions (phase separation during depressurisation) as well as the common signatures observed in hematite with variable degrees of fluid-mineral interaction. The invoked fluids are typical of hydrolytic alteration and the fluid pumping mechanism is feasible via fault (re)activation. Using a nanoscale approach, we show that fluid-mineral interaction can be fingerprinted at the (atomic) scale at which element exchange occurs. Full article

The petrography and geochemistry of zircon offers an exciting opportunity to better understand the genesis of, as well as identify pathfinders for, large magmatic–hydrothermal ore systems. Electron probe microanalysis, laser ablation inductively coupled plasma mass spectrometry, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging, and energy-dispersive X-ray spectrometry STEM mapping/spot analysis were combined to characterize Proterozoic granitic zircon in the eastern Gawler Craton, South Australia. Granites from the ~1.85 Ga Donington Suite and ~1.6 Ga Hiltaba Suite were selected from locations that are either mineralized or not, with the same style of iron-oxide copper gold (IOCG) mineralization. Although Donington Suite granites are host to mineralization in several prospects, only Hiltaba Suite granites are considered “fertile” in that their emplacement at ~1.6 Ga is associated with generation of one of the best metal-endowed IOCG provinces on Earth. Crystal oscillatory zoning with respect to non-formula elements, notably Fe and Cl, are textural and chemical features preserved in zircon, with no evidence for U or Pb accumulation relating to amorphization effects. Bands with Fe and Ca show mottling with respect to chloro–hydroxy–zircon nanoprecipitates. Lattice defects occur along fractures crosscutting such nanoprecipitates indicating fluid infiltration post-mottling. Lattice stretching and screw dislocations leading to expansion of the zircon structure are the only nanoscale structures attributable to self-induced irradiation damage. These features increase in abundance in zircons from granites hosting IOCG mineralization, including from the world-class Olympic Dam Cu–U–Au–Ag deposit. The nano- to micron-scale features documented reflect interaction between magmatic zircon and corrosive Fe–Cl-bearing fluids in an initial metasomatic event that follows magmatic crystallization and immediately precedes deposition of IOCG mineralization. Quantification of α-decay damage that could relate zircon alteration to the first percolation point in zircon gives ~100 Ma, a time interval that cannot be reconciled with the 2–4 Ma period between magmatic crystallization and onset of hydrothermal fluid flow. Crystal oscillatory zoning and nanoprecipitate mottling in zircon intensify with proximity to mineralization and represent a potential pathfinder to locate fertile granites associated with Cu–Au mineralization. Full article

Scheelite (CaWO₄) and powellite (CaMoO₄) are isostructural minerals considered as a non-ideal solid solution series. Micron- to nanoscale investigation of a specimen of skarnoid from Zhibula, Gangdese Belt, Tibet, China, was carried out to assess the identity of the phases within a broad scheelite-powellite (Sch-Pow) compositional range, and to place additional constraints on redox changes during ore formation. An electron probe microanalysis shows that Mo-rich domains within complex oscillatory-zoned single crystals, and as thin sliver-like domains, have a compositional range from 20 mol.% to 80 mol.% Pow. These occur within a matrix of unzoned, close-to-end-member scheelite aggregates (87 mol.%–95 mol.% Sch). Laser-ablation inductively coupled plasma mass spectrometry spot analysis and element mapping reveal systematic partitioning behaviour of trace elements in skarn minerals (grossular₅₀, diopside₈₀, anorthite, and retrograde clinozoisite) and scheelite-powellite aggregates. The Mo-rich domains feature higher concentrations of As, Nb, and light rare earth elements LREE, whereas W-rich domains are comparatively enriched in Y and Sr. Transmission electron microscopy (TEM) was carried out on focused-ion-beam-prepared foils extracted in situ from domains with oscillatory zoning occurring as slivers of 20 mol.%–40 mol.% Pow and 48 mol.%–80 mol.% Pow composition within an unzoned low-Mo matrix (20 mol.% Pow). Electron diffractions, high-angular annular dark field (HAADF) scanning-TEM (STEM) imaging, and energy-dispersive spectroscopy STEM mapping show chemical oscillatory zoning with interfaces that have continuity in crystal orientation throughout each defined structure, zoned grain or sliver. Non-linear thermodynamics likely govern the patterning and presence of compositionally and texturally distinct domains, in agreement with a non-ideal solid solution. We show that the sharpest compositional contrasts are also recognisable by variation in growth direction. Atomic-scale resolution imaging and STEM simulation confirm the presence of scheelite-powellite within the analysed range (20 mol.%–80 mol.% Pow). Xenotime-(Y) inclusions occur as nm-wide needles with epitaxial orientation to the host scheelite-powellite matrix throughout both types of patterns, but no discrete Mo- or W-bearing inclusions are observed. The observed geochemical and petrographic features can be reconciled with a redox model involving prograde deposition of a scheelite+molybdenite assemblage (reduced), followed by interaction with low-T fluids, leading to molybdenite dissolution and reprecipitation of Mo as powellite-rich domains (retrograde stage, oxidised). The observation of nanoscale inclusions of xenotime-(Y) within scheelite carries implications for the meaningful interpretation of petrogenesis based on rare earth element (REE) concentrations and fractionation patterns. This research demonstrates that HAADF-STEM is a versatile technique to address issues of solid solution and compositional heterogeneity. Full article

Many analytical techniques for trace element analysis are available to the geochemist and geometallurgist to understand and, ideally, quantify the distribution of trace and minor components in a mineral deposit. Bulk trace element data are useful, but do not provide information regarding specific host minerals—or lack thereof, in cases of surface adherence or fracture fill—for each element. The CAMECA nanoscale secondary ion mass spectrometer (nanoSIMS) 50 and 50L instruments feature ultra-low minimum detection limits (to parts-per-billion) and sub-micron spatial resolution, a combination not found in any other analytical platform. Using ore and copper concentrate samples from the Olympic Dam mining-processing operation, South Australia, we demonstrate the application of nanoSIMS to understand the mineralogical distribution of potential by-product and detrimental elements. Results show previously undetected mineral host assemblages and elemental associations, providing geochemists with insight into mineral formation and elemental remobilization—and metallurgists with critical information necessary for optimizing ore processing techniques. Gold and Te may be seen associated with brannerite, and Ag prefers chalcocite over bornite. Rare earth elements may be found in trace quantities in fluorapatite and fluorite, which may report to final concentrates as entrained liberated or gangue-sulfide composite particles. Selenium, As, and Te reside in sulfides, commonly in association with Pb, Bi, Ag, and Au. Radionuclide daughters of the ²³⁸U decay chain may be located using nanoSIMS, providing critical information on these trace components that is unavailable using other microanalytical techniques. These radionuclides are observed in many minerals but seem particularly enriched in uranium minerals, some phosphates and sulfates, and within high surface area minerals. The nanoSIMS has proven a valuable tool in determining the spatial distribution of trace elements and isotopes in fine-grained copper ore, providing researchers with crucial evidence needed to answer questions of ore formation, ore alteration, and ore processing. Full article

A comprehensive nanoscale study on magnetite from samples from the outer, weakly mineralized shell at Olympic Dam, South Australia, has been undertaken using atom-scale resolution High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF STEM) imaging and STEM energy-dispersive X-ray spectrometry mapping and spot analysis, supported by STEM simulations. Silician magnetite within these samples is characterized and the significance of nanoscale inclusions in hydrothermal and magmatic magnetite addressed. Silician magnetite, here containing Si–Fe-nanoprecipitates and a diverse range of nanomineral inclusions [(ferro)actinolite, diopside and epidote but also U-, W-(Mo), Y-As- and As-S-nanoparticles] appears typical for these samples. We observe both silician magnetite nanoprecipitates with spinel-type structures and a γ-Fe_1.5SiO₄ phase with maghemite structure. These are distinct from one another and occur as bleb-like and nm-wide strips along d₁₁₁ in magnetite, respectively. Overprinting of silician magnetite during transition from K-feldspar to sericite is also expressed as abundant lattice-scale defects (twinning, faults) associated with the transformation of nanoprecipitates with spinel structure into maghemite via Fe-vacancy ordering. Such mineral associations are characteristic of early, alkali-calcic alteration in the iron-oxide copper gold (IOCG) system at Olympic Dam. Magmatic magnetite from granite hosting the deposit is quite distinct from silician magnetite and features nanomineral associations of hercynite-ulvöspinel-ilmenite. Silician magnetite has petrogenetic value in defining stages of ore deposit evolution at Olympic Dam and for IOCG systems elsewhere. The new data also add new perspectives into the definition of silician magnetite and its occurrence in ore deposits. Full article