Yuanming Pan’s research while affiliated with University of Saskatchewan and other places

Niobium and tantalum are the so-called “geochemical twins” with similar properties and have long been used to constrain magmatic sources and trace geological and planetary processes. Significant fractionations of Nb and Ta have recently been documented in diverse magmatic-hydrothermal and metamorphic systems, but their origin remains unclear. Niobium and Ta are preferentially incorporated in Ti-rich minerals such as ilmenite, rutile, and titanite. In this study, representative samples of Nb-rich titanite and ilmenite exhibiting large Nb/Ta variations (5 to 65) from two anorogenic alkaline suites in the Fangcheng and Ziyang regions of China and associated hydrothermal alteration have been investigated by Raman spectroscopy, synchrotron X-ray adsorption spectroscopy (SXAS) and X-ray photoelectron spectroscopy (XPS). The Raman spectra of Nb-rich titanite and ilmenite show only minor degrees of metamictization. The Nb K-edge SXAS spectra of Nb-rich ilmenite with Nb/Ta > 30 are similar to that of Nb2O5, indicative of a dominant presence of Nb5+ with minor Nb4+, and an Nb5+/ΣNb ratio range from 0.59 to 1. In contrast, the Nb K-edge SXAS spectra of titanite and ilmenite with Nb/Ta < 30 indicate mixed valences of Nb5+ and Nb4+, with Nb5+/ΣNb in the range of 0.22 to 0.55. Similarly, XPS measurements of titanite and ilmenite further support the presence of mixed valences of Nb, with Nb5+/ΣNb in the range of 0.49 to 0.85. In addition, Ta L3-edge SXAS spectra of titanite suggest the occupancy of Ta5+ at the octahedral site. Fitting results from Nb K-edge and Ta L3-edge extended X-ray adsorption fine structure (EXAFS) data further suggest that both Nb and Ta replace Ti via coupled substitutions of the type 2Ti4+ ↔ (Nb5+, Ta5+) + (Al, Fe)3+) and an isovalent substitution (Ti4+ ↔ Nb4+). A positive correlation between the oxidation states of Nb and the Nb/Ta values has been observed in titanite and ilmenite, but not for Ta. These results suggest that the extreme enrichment and large variations in Nb contents as well as significant fractionation of Nb from Ta in titanite and ilmenite are controlled by the prevailing oxygen fugacity in magmatic-hydrothermal systems. Given these complexities involving variable valence states, circumspection is required to use Nb and Ta anomalies to interpret geotectonic settings and apply Nb/Ta systematics to constrain magmatic-hydrothermal sources.

Except for the recently discovered stable U4+ chloride complex under reduced conditions at high temperatures, genetic models for the formation of uranium deposits had almost invariably invoked the pivotal roles of soluble U6+ species for the transport of uranium in fluids and their reduction to sparingly soluble U4+ as the deposition mechanism. However, the questions of when and how this reduction occurred in most uranium deposits, such as those in the Athabasca basin, Saskatchewan, Canada, are often not clear. The unconformity-related uranium deposits in the Athabasca basin are commonly accompanied by extensive and intensive alteration halos, including hematite-rich alteration or hematitization. Previous U L3-edge X-ray absorption near-edge structure (XANES) studies of uranium-bearing fluid inclusions and thermodynamic modeling demonstrated uranium transport as uranyl (UO22+) species in hypersaline fluids in the Athabasca basin. Electron microprobe analyses reveal that hematite inclusions in quartz overgrowths, as well as some disseminated hematite in clay mineral (illite-chlorite) matrices, in both orebodies and associated alteration halos from five uranium deposits (Arrow, Cigar Lake, Key Lake, McArthur River, and Phoenix) in the Athabasca basin contain elevated contents of uranium (up to 2.16 wt.% UO3). Synchrotron U L3-edge X-ray absorption spectroscopy (XAS) and U 4f X-ray photoelectron spectroscopy (XPS) analyses show that uranium in hematite occurs dominantly as the uranyl species, providing unambiguous evidence for direct uranyl deposition in the Athabasca basin. However, direct uranyl deposition with hematite during a single episode of hydrothermal alteration can account for only low-grade uranium mineralization. High-grade uranium deposits in the Athabasca basin required multiple episodes of hydrothermal alteration and/or other deposition mechanisms, such as those related to reduction.

Heavy rare earth elements (HREE), as critical materials for the global transition to a low-carbon economy, are mostly supplied by regolith-hosted ion adsorption deposits formed from the weathering of granites. Magmatic-hydrothermal titanite of pronounced HREE enrichment in granites has been suggested to play an important role in forming HREE-dominated regolith-hosted deposits. However, the crystal-chemical mechanism and geological process responsible for HREE enrichment in titanite remain unknown. This study investigated two texturally distinct types of HREE-enriched titanite (titanite I and II) in granites from the Gucheng regolith-hosted HREE deposit in South China. Secondary ion mass spectrometry (SIMS) U-Pb dating of rutile associated with titanite I and II yielded similar ages of 101.0 ± 1.7 Ma and 97.9 ± 6.2 Ma, respectively, supporting HREE remobilization during auto-metasomatism. Microbeam synchrotron X-ray absorption spectroscopic analyses show that titanite I and II have distinct Y K-edge X-ray absorption near-edge structure spectra. The best-fit results of Y K-edge extended X-ray absorption fine structure data suggest that titanite I and II have a Y-O first shell with coordination numbers and distances of 7.8 and 7.2 and 2.31–2.51 Å and 2.30–2.47 Å, respectively. Measured Y 3d and Dy 3d X-ray photoelectron spectroscopic data also support different Y,HREE-O first shells between titanite I and II. These results suggest that Y3+ and REE3+ occupy the 7-coordinated Ca site via three substitutions (Y,REE)3+ + (Al,Fe)3+ ↔ Ca2+ + Ti4+, 2(Y,REE)3+ + □Ca ↔ 3Ca2+, and 2(Y,REE)3+ + O2− ↔ 2Ca2+. First-principle calculations and the lattice strain model predict preferential uptakes of Y3+ and HREE3+ as the (Y,HREE)O8 polyhedra arising from the latter substitution mechanism at the Ca site in titanite. Also, we link the 2Y3+ + O2− ↔ 2Ca2+ substitution in titanite to highly oxidized and HREE-enriched melts/fluids originating from the subducted slab. These findings further support our previous suggestion that the crystallization of HREE-enriched titanite in granites plays an important role in forming HREE-dominated regolith-hosted deposits.

Regolith-hosted rare-earth-element (REE) deposits are the world’s primary source of heavy REEs (HREEs) critical to the global clean-energy transition. Previous studies suggested that REEs in regolith-hosted deposits are largely inherited from their parent granites. However, several HREE-dominated deposits occur in the weathering crusts of LREE-enriched granites, where the mechanisms of REE fractionation remain poorly understood. Also, the conventional mining method of regolith-hosted REE deposits has limited efficiencies in REE recovery while causing enormous environmental contaminations. Herein we have investigated the distribution and speciation of Y and REEs in three representative regolith-hosted REE deposits (i.e., Gucheng and Shangyou, HREE-dominated; and Renju, LREE-dominated) as well as Y-sorbed birnessite from batch experiments. Our results show that birnessite in all three deposits is a minor constituent but contains anomalously high REE concentrations, and contributes to 25.3%, 23.4%, and 26.5% of the HREE contents of mineralized saprolites. Measured Y K-edge X-ray absorption spectroscopic data suggest that Y3+ (representing HREE3+) is adsorbed on birnessite as YO8 complexes in all three deposits but via different linkages: i.e., the bidentate corner-sharing mode in the HREE-dominated deposits but a mixture of both bidentate corner-sharing and edge-sharing modes in the LREE-dominated deposit. These binding mechanisms are also observed in Y-sorbed birnessite prepared at different ionic strengths. Therefore, different binding mechanisms of Y and HREE sorption on birnessite together with its preferential adsorption of HREE not only are responsible for the formation of HREE-dominated deposits from LREE-enriched granites but have important implications for the sustainable development of regolith-hosted REE deposits.

This publication presents a study on the mineral chemistry and luminescence properties of quartz samples from pegmatites of the Tørdal region in Norway. A total of 12 samples were analyzed using Secondary Ion Mass Spectrometry (SIMS), Electron Paramagnetic Resonance Spectroscopy (EPR), and Cathodoluminescence (CL) to gain insights into their trace element concentration and distribution as well as their luminescence behavior. The samples are characterized by different Cl emissions at 450 nm, 500 nm 650 nm and an additional shoulder at 390 nm, which is only partially visible due to the absorption of the glass optics. Of these luminescence bands, the 500 nm band is the most dominant in most samples and it is characterized by an initial blue-green luminescence, which is not stable under electron irradiation. Moreover, it is characterized by a heterogeneous distribution within the samples. This luminescence can be mostly assigned to [AlO 4 /M + ] 0 defects, with charge compensation mostly achieved by Li +. Analyses by EPR spectroscopy prove the dominance of structurally bound Al, Li, and Ti ions in the investigated samples. Further analyses using SIMS mapping demonstrate that Na and K are mainly bound to micro fractures or inclusions, suggesting a limited role in the compensation of the luminescence centers. Additionally, the SIMS mappings show that some samples contain Al-rich clusters of 10 to 20 μm in diameter, whereas other trace elements are characterized by a homogeneous distribution. These clusters correspond to bright luminescence areas in size and shape and could potentially indicate H + compensated [AlO 4 /M + ] 0 defects.

Prolonged and episodic magmatic-hydrothermal processes have been proposed to be important in forming large-tonnage Cu volcanogenic massive sulfide (VMS) deposits but are difficult to document and remain poorly understood in ancient VMS systems. In this study, we combine textural evidence, in situ analysis of sulfur isotopic composition with trace elements of pyrite from the well-preserved, large-tonnage Ashele VMS deposit (Central Asian Orogenic Belt, NW China) to elucidate the origin of the ore-forming materials and controlling factors of VMS deposits. The distribution of hydrothermal alteration and mineralization allows the Ashele deposit to be divided into five zones: massive sulfide, quartz-pyrite, chlorite-chalcopyrite-quartz-pyrite, quartz-chlorite-sericite-pyrite, and quartz-sericite-pyrite zones. Cu mineralization is mainly hosed in the massive sulfide and chlorite-chalcopyrite-quartz-pyrite zones. Three texturally and compositionally distinct types of pyrite from the Cu mineralization zones have been recognized. The first type of growth zones with high Cu, As, and volatile elements (e.g., Hg and Tl) concentrations and negative δ34S values (-7.83‰–-0.35‰) observed in pyrite grains from the massive sulfide zone most likely formed from the input of magmatic volatiles from degassing of the underlying magmatic systems. The second type of Cu-As-rich growth zones in pyrite grains from the chlorite-chalcopyrite-pyrite zone is not enriched in volatile elements and has positive δ34S values (2.59‰–6.56‰), which is interpreted to have precipitated during fluid boiling. The third type of growth zones, without Cu-As enrichment but having positive δ34S values (0.30‰–9.76‰) in pyrite grains from both mineralization zones, was probably formed without boiling or magmatic volatile input. Our results suggest that multi-stage magmatic degassing or fluid boiling accompanied by Cu precipitation recorded in individual pyrite grains can reveal episodic magmatic-hydrothermal activities, which are essential factors in forming large-tonnage VMS deposits.