Carbonatitic versus hydrothermal origin for fluorapatite REE-Th deposits: Experimental study of REE transport and crustal “antiskarn” metasomatism

Nolans-type ore deposits contain REE and Th mineralization hosted in fluorapatite veins. These veins intrude granulite facies rocks and are surrounded by a diopside selvage. Nolans-type deposits are thought to form by REE, F and P-rich hydrothermal fluids derived from alkali or carbonatitic intrusions. However, REE are not effectively transported in F and P-rich systems. REE ore deposits are commonly hydrothermally overprinted, possibly obscuring the igneous nature of the primary mineralization. We conducted a series of piston cylinder “sandwich” experiments, testing the hydrothermal fluid hypothesis, and a newly suggested process of carbonatite metasomatism. Our results confirm theoretical predictions that REE are hydrothermally immobile in these systems and the experimental phase assemblage is not compatible with the natural rocks. Our results show that fluorapatite can only host several weight percent levels of REE at temperatures higher than ∼600 °C. Below that temperature, a miscibility gap exists between REE-poor fluorapatite and REE-rich silicates such as britholite or cerite. In contrast, experiments reacting P and REE-rich carbonatite with silicate rock above 700 °C closely resemble natural rocks from Nolans-type deposits. Selvage mineralogy is sensitive to the MgO content of the carbonatite. A diopside selvage formed at carbonatite MgO/(CaO+MgO) ≈ 0.2 while wollastonite and forsterite formed at lower and higher ratios, respectively. Phosphate solubility in carbonatites decreases with decreasing MgO contents. As diopside formed, REE-rich fluorapatite preferentially crystallized from the selvage inwards. Thus, carbonatites are effective at simultaneously mobilizing REE, F and P to the site of deposition. Nolans-type deposits are the cumulate residue of this reaction, with the carbonatite liquid migrating elsewhere. At temperatures below 700 °C the carbonatite–silicate reaction additionally formed monticellite, cuspidine and magnesioferrite, resembling a skarn assemblage. Whereas skarns form by infiltration of silicate magmas or related fluids to carbonate rocks, our experiments are the opposite: intrusion of carbonatite into silicate rock. These mid-crustal skarn-like rocks may host elevated ore elements of carbonatitic affinity, such as F, P, Y, REE, Th, Ba, Sr, and Nb. We propose the term “antiskarn” to describe such systems, and suggest they trace the migration of carbonatite liquids through the crust. Hydrothermal reworking, retrogression, or metamorphism of antiskarns may obscure the carbonatitic genesis of the rocks. These metasomatic zones are the crustal equivalent of wehrlites that form by peridotite–carbonatite reaction at mantle depths.This research is supported by an Australian Government Research Training Program (RTP) Scholarship. Michael Anenburg acknowledges a Ringwood Scholarship from the Research School of Earth Sciences, Australian National University

Quantifying the Tetrad Effect, Shape Components, and Ce–Eu–Gd Anomalies in Rare Earth Element Patterns

Plots of chondrite-normalised rare earth element (REE) patterns often appear as smooth curves. These curves can be decomposed into orthogonal polynomial functions (shape components), each of which captures a feature of the total pattern. The coefficients of these components (known as the lambda coefficients—λ) can be derived using least-squares fitting, allowing quantitative description of REE patterns and dimension reduction of parameters required for this. The tetrad effect is similarly quantified using least-squares fitting of shape components to data, resulting in the tetrad coefficients (τ ). Our method allows fitting of all four tetrad coefficients together with tetrad-independent λ curvature. We describe the mathematical derivation of the method and two tools to apply the method: the online interactive application BLambdaR, and the Python package pyrolite.We show several case studies that explore aspects of the method, its treatment of redox-anomalous REE, and possible pitfalls and considerations in its use.This study was supported by Australian Research Council Linkage grant LP190100635

Iron cation vacancies in Pt(iv)-doped hematite

Platinum-doping of hematite (α-Fe2O3) is a popular method to increase the performance of hematite in photoelectrochemical applications. The precise mode of Pt incorporation is however unclear, as it can occur as Pt0, Pt2+ or Pt4+, either on the surface, as dispersed inclusions, or as part of the hematite crystal lattice. These different Pt-doping varieties can have major effects on the hematite performance. Here, we employ a high-pressure synthesis method assisted by silicate liquid flux to grow Pt-doped hematite crystals large enough for elemental analysis by wavelength dispersive spectroscopy (WDS). We find that the total cations are lower than the expected 2 atoms per formula unit, and together with Fe, they are inversely correlated with Pt contents. Linear regressions in compositional space reveal that the slopes are consistent with 4Fe3+ = 3Pt4+ + VFe as the charge-balanced substitution mechanism. Therefore, Pt4+-doping of hematite at high oxygen fugacities, which does not allow Fe2+ to form, will lead to removal of Fe and formation of cation vacancies. Our hematite also contains significant Al3+, Ti4+ and Mg2+, raising the possibility of fine tuning the hematite properties by co-doping with other elements. Photoelectrochemical performance of cation vacancy bearing hematite is experimentally understudied and is a potentially promising future field of study

Controls on critical metals in magmatic and hydrothermal systems

Metals with high demand and a supply risk are called critical metals. One solution to the criticality problem is source diversification. We do not fully understand critical metal enrichment in ore deposits. I focus on two groups: the rare earth elements (REE) and the platinum group elements (PGE). At Nolans, NT, REE are hosted in fluorapatite veins up to several metres thick. I show experiments testing two formation hypotheses. First, I tested whether REE are hydrothermally mobile. I ran layers of rock-like compositions, REE-free apatite, REE, and a saline solution in piston cylinder experiments at 2-5 kbar and 550-700 C. The results show that the REE layer bonded with the more soluble elements (Si, Al, Mg, Fe) to produce insoluble REE minerals such as allanite or britholite. Formation of REE-bearing apatite was limited to a thin zone between the REE and apatite. In a P-F dominated fluid, the solubility of REE is negligible. Second, I tested the reaction between a REE-P-F-bearing carbonatite layer and a silicate layer, at 6 kbar and 650-900 C. The experiments resulted in a reaction zone consisting of diopside and REE-rich apatite. This indicates that carbonatites can carry P and REE, and form REE-rich apatite in reaction with silicate rocks. These textures closely reproduce the Nolans ore. The Nolans REE are now hosted in alteration products consisting of carbonates, phosphates and silicates. This study documents the alteration mineralogy. I show a decoupling between Ce and La, caused by oxidation of Ce(III) to Ce(IV), leading to its incorporation in Th-bearing minerals. La, Nd and Pr are concentrated in Ce-free phases. This has implications for mineral processing. Thorium is an unwanted by-product of REE production, and Ce is a lower-value product. The concentration of Ce and Th in a single mineral may allow its separation before processing, increasing the monetary value and reducing the environmental hazard. Analysis methods for fluorapatite compositions typical for carbonatite (LREE-enriched, carbonate bearing) are discussed. Rhenium is another critical metal that commonly occurs in molybdenite. Carbonatite-hosted molybdenites commonly host 100s ppm of Re. Carbonatites are usually considered as deposits for REE or Nb, and the occurrence of Re-rich molybdenite is perplexing. I performed experiments which tested whether (1) carbonatites can recrystallise molybdenite powder, and (2) whether rheniite can crystallise in carbonatites. The powder was recrystallised to coarse crystals, and similarly sized rheniite crystals formed by reaction of perrhenate and sulfate. Thus, carbonatites flux molybdenite and rheniite growth. It is a first step in understanding the reason for the occurrence of molybdenite in carbonatites. These experiments also tested the behaviour of other PGE in peralkaline melts. It is uncertain whether PGE must be concentrated from silicate melts by sulfide saturation, or can they be transported as nanonuggets in silicate melts independently of any presence of a sulfide phase. Nanonuggets are well known from PGE solubility experiments in which they are treated as experimental artefacts. I exploited the tendency of these metals to form nanonuggets to further explore their behaviour. These experiments were conducted in Ag-Pd, Au, or Pt capsules at 5 kbar and 1050-1100 C. Textural evidence indicates that the nanonuggets formed by reduction of an initially oxidised melt. They preferentially stick to magnetite and coarsen by consumption of existing nanonuggets. PGE can be transported as nanonuggets in silicate melts regardless of the presence of sulfide, and their concentration can be greater than that allowed by equilibrium solubility. Re-bearing sodalite that formed in those experiments was in equilibrium with Re metal, suggesting the role of the peralkaline melt for stabilising the higher oxidation state

Molybdenum and rhenium disulfide synthesis via high-pressure carbonate melt

Various applications of transition metal dichalcogenides (TMDs) require preparation by exfoliation of precursor bulk materials. However, bulk TMDs are not always available in suitable forms and current synthesis methods may not result in appropriate crystals. This study reports synthesis of large crystals (50–100 μm) of MoS2 and ReS2, by recrystallisation of MoS2 powder or reaction of sulfate with perrhenic acid, respectively. The reactions have been performed at high pressure and temperature (≥1 GPa; ≥800 °C) in a liquid Ca-carbonate flux. The resulting crystals were characterised by electron microscopy imaging, EDS and WDS chemical analyses, and Raman spectroscopy. The carbonate matrix can be easily dissolved to recover the product TMDs. This method allows synthesis of large well-crystalline TMD compositions that are otherwise challenging to obtain.This research is supported by an Australian Government Research Training Program Scholarship and a Ringwood Scholarship. This work was supported by Australian Research Council grant FL130100066 to Hugh O'Neill who provided constructive comments on the manuscript

Rare earth mineral diversity controlled by REE pattern shapes

The line connecting rare earth elements (REE) in chondrite-normalised plots can be represented by a smooth polynomial function using lambda shape coefficients as described by O'Neill (2016). In this study, computationally generated lambda combinations are used to construct artificial chondrite-normalised REE patterns that encompass most REE patterns likely to occur in natural materials. The dominant REE per pattern is identified, which would lead to its inclusion in a hypothetical mineral suffix, had this mineral contained essential REE. Furthermore, negative Ce and Y anomalies, common in natural minerals, are considered in the modelled REE patterns to investigate the effect of their exclusion on the relative abundance of the remainder REE. The dominant REE in a mineral results from distinct pattern shapes requiring specific fractionation processes, thus providing information on its genesis. Minerals dominated by heavy lanthanides are rare or non-existent, even though the present analysis shows that REE patterns dominated by Gd, Dy, Er and Yb are geologically plausible. This discrepancy is caused by the inclusion of Y, which dominates heavy REE budgets, in mineral name suffixes. The focus on Y obscures heavy lanthanide mineral diversity and can lead to various fractionation processes to be overlooked. Samarium dominant minerals are known, even though deemed unlikely by the computational model, suggesting additional fractionation processes that are not well described by lambda shape coefficients. Positive Eu anomalies only need to be moderate in minerals depleted in the light REE for Eu to be the dominant REE, thus identifying candidate rocks in which the first Eu dominant mineral might be found. Here, I present an online tool, called ALambdaR that allows interactive control of lambda shape coefficients and visualisation of resulting REE patterns

Perrhenate sodalite growth from alkali silicate melts by noble metal catalysis

Perrhenate sodalite (Na8Al6Si6Re2O32) which contains the perrhenate anion (ReO4−) has been synthesised in equilibrium with Na–Fe-rich aluminosilicate melt and Re metal at 1100 °C and 500 MPa. Quenched glasses contain a homogeneous distribution of nepheline and magnetite, whereas sodalite only forms a crust on noble metal surfaces (Pt, Pt–Rh, Ag–Pd, and Re) in contact with the glass. The sodalite have been characterised by wavelength-dispersive X-ray spectroscopy and Raman spectroscopy. The experimental products contain both Re(0) and Re(VII), but other oxidation states of Re were not detected, suggesting the role of Na as a higher oxidation state stabiliser. Sodalite has potential uses for immobilising volatile Re and Tc during nuclear waste vitrification. Our study demonstrates the thermodynamic stability of perrhenate sodalite at Na-rich aluminosilicate melts at high temperature and pressure conditions.This work was supported by Australian Research Council grant FL130100066 to Hugh O’Neill

Noble metal nanonugget insolubility in geological sulfide liquids

Noble metals (NMs) in Earth’s magmatic systems are thought to be controlled entirely by their strong partitioning to sulfde liquids. This chemical equilibrium is at the root of various models, ranging from NM deposit formation to planetary differentiation. Noble metals commonly occur as sub-micrometer phases known as nanonuggets. However, the assumptions that nanometer-scale thermodynamic equilibrium partitioning is attained and that NM nanonuggets are soluble in sulfde liquids have never been validated. Using novel experimental methods and analytical techniques we show nanometer-scale NM ± Bi phases attached to exterior surfaces of sulfde liquids. Larger phases (≤1 µm) show clear liquid immiscibility textures, in which Fe, Cu, and Ni partition into sulfde liquids whereas NMs partition into bismuthide liquids. Noble metal compositions of sulfdes and their associated NM phases vary between adjacent droplets, indicating NM disequilibrium in the system as a whole. We interpret most nanometer-scale NMs contained within sulfdes to be insoluble as well, suggesting that previously reported sulfde–silicate partition coeffcients are overestimated. Consequently, sulfde liquids likely play a secondary role in the formation of some NM ore deposits

Experimental observations on noble metal nanonuggets and Fe-Ti oxides, and the transport of platinum group elements in silicate melts

Platinum group element (PGE) nanonuggets are a nuisance in experimental studies designed to measure solubility or partitioning of noble metals in silicate melts. Instead of treating nanonuggets as experimental artifacts, we studied their behaviour motivated by recent discoveries of PGE nanonuggets in a variety of natural settings. We used an experimental setup consisting of AgPd, Pt or AuPd capsules and Fe(-Ti) oxide-saturated hydrous peralkaline silicate melts to maximise nanonugget production. TABS (Te, As, Bi, Sb, Sn) commonly occur in PGM (platinum group minerals), prompting addition of Bi to our experiments to investigate its properties as well. Three-dimensional optical examination by 100× objective and immersion oil reveals variable colour which correlates with nanonugget size and shape due to plasmon resonance effects. We observe two textural types: (1) intermediate-sized nanonuggets dispersed in the glass and adhering to oxides, and (2) abundant fine nanonuggets dispersed in the glass with coarse euhedral crystals in contact with oxides. Slow cooling removes dispersed nanonuggets and greatly coarsens existing oxide-associated metal crystals. Nanonugget-free halos are commonly observed around oxide grains. All metal phases are composed of major (Ag, Pd) and trace (Pt, Ir, Au) capsule material. Our results show reduction processes, imposed by growing oxides, causing local metal saturation in the oxide rich zones with preferential nucleation on smaller oxide grains. The redox gradient then blocks additional metals from diffusing into oxide rich zones, forming halos. As the entire experimental charge is reduced throughout the run, nanonuggets form in the distal glass. Bismuth contents of metal phases do not depend on Bi2O3 amounts dissolved in the melt. Further PGM crystallisation consumes nanonuggets as feedstock. We conclude that the appearance of metallic PGE phases happens in two stages: first as nanonuggets and then as larger PGM. Once formed, nanonuggets cannot be removed by oxidation or dissolution, but only by PGM coarsening. Sulfur-poor PGE ore deposits commonly contain more PGE than permitted by existing estimates of equilibrium solubility in silicate melts. This is commonly explained by initial scavenging of PGE by sulfide liquids followed by S-loss, but evidence for S-loss is not conclusive. We suggest that nanonuggets may be a means to transport PGE from source regions to form ore deposits, followed by direct PGM crystallisation from silicate melt without intermediate concentration by sulfide liquids

Sub-seafloor self-metasomatism and REE mobility in and around plagiogranites: Allanite and epidote of the Troodos ophiolite epidosites

Epidosides in the oceanic crust are rocks composed almost entirely of quartz and epidote that form by alteration of primary igneous minerals. They are common in the diabase of the sheeted dykes and form in regions that experienced high volumes of fluid flow. This hydrothermal fluid begins as seawater and is subsequently modified by reaction with heated rocks during circulation in the crust. The amount and nature of magmatic fluid input to this hydrothermal circulation systems is poorly constrained.<br>Plagiogranites also contain epidosites on occasion. Here we show examples from the Troodos Ophiolite in Cyprus of plagiogranite-hosted epidosides. Zoning in epidote and allanite (A REE-rich member of the epidote group) indicates that the epidotising fluids were initially derived from the magma and not from modified seawater and only later the fluid evolved to seawater-like characteristics. Similar magmatic signature is also observed in epidote–allanite veins observed in diabase-hosted epidosites, suggesting that magmatic fluids can migrate from the plagiogranites to other regions of the oceanic crust.<br><br>Further reading:<br><br>Anenburg, M., Katzir, Y., Rhede, D., Jöns, N., and Bach, W., 2015, <i>Rare earth element evolution and migration in plagiogranites: a record preserved in epidote and allanite of the Troodos ophiolite</i>: Contributions to Mineralogy and Petrology, v. 169, p. 25