Hydrothermal Alteration of Eudialyte-Hosted Critical Metal Deposits: Fluid Source and Implications for Deposit Grade

Abstract

Eudialyte-hosted critical metal deposits potentially represent major sources of rare earth elements (REE), zirconium and niobium. Here, we study the chemical and isotopic composition of fresh and altered eudialyte in nepheline syenite from the Ilímaussaq Complex, Greenland, one of the world’s largest known eudialyte-hosted deposits. Late-magmatic hydrothermal alteration caused partial replacement of primary magmatic eudialyte by complex pseudomorph assemblages of secondary Zr-, Nb-, and REE-minerals. Three secondary assemblage types are characterised by the zirconosilicates catapleiite, gittinsite and zircon, respectively, of which the catapleiite type is most common. To investigate elemental exchange associated with alteration and to constrain the nature of the metasomatic fluids, we compare trace elements and Sm/Nd isotope compositions of unaltered eudialyte crystals and their replaced counterparts from five syenite samples (three catapleiite-type, one gittinsite-type, and one zircon-type assemblage). Trace element budgets for the catapleiite-type pseudomorphs indicate a 15–30% loss of REE, Ta, Nb, Zr, Sr and Y relative to fresh eudialyte. Moreover, the gittinsite- and zircon-type assemblages record preferential heavy REE (HREE) depletion (≤50%), suggesting that the metasomatic fluids mobilised high field strength elements. Initial Nd isotope ratios of unaltered eudialyte and catapleiite- and gittinsite-type pseudomorphs are indistinguishable, confirming a magmatic fluid origin. However, a higher initial ratio and stronger HREE depletion in the zircon-type pseudomorphs suggests a different source for the zircon-forming fluid. Although alteration reduces the metal budget of the original eudialyte volume, we infer that these elements re-precipitate nearby in the same rock. Alteration, therefore, might have little effect on overall grade but preferentially separates heavy and light REE into different phases. Targeted processing of the alteration products may access individual rare earth families (heavy vs. light) and other metals (Zr, Nb, Ta) more effectively than processing the fresh rock.

Structural state of rare earth elements in eudialyte-group minerals

“…This particular spectrum shows a characteristic shoulder at 17,065 eV and additional features at 17,099 and 17,115 eV, corresponding closely to the XANES features of zircon (Fig 2 a ). This may suggest the presence of 8-coordinated Y in microscopic zircon as an alteration product of eudialyte (van de Ven et al ., 2019).…”

“…Furthermore, the open literature is inconsistent as to whether REE substitute on the Na, Ca or Zr sites, or a combination of these (Johnsen and Grice, 1999; Johnsen et al , 2001; Rastsvetaeva, 2007; Pfaff et al , 2008; Möller and Williams-Jones, 2016). Such considerations are more than academic – EGM deposits are commonly altered (Mitchell and Liferovich, 2006; Borst et al , 2016; Möller and Williams-Jones, 2017; Estrade et al , 2018; van de Ven et al , 2019) and some replacement processes are topotactic (i.e. retaining structural templates from the primary structure to the secondary).…”

Carbonate complexation enhances hydrothermal transport of rare earth elements in alkaline fluids

“…Based on the absence of carbonate minerals in the early fluid inclusions, Vasyukova et al 20,28 speculate that early LREE mobilization in the high T Cl-and F-rich fluids are probably due to hydroxy-fluoride complexation similar to that suggested for Zr and Nb 77,78 rather than carbonate complexation such as described here. Alkaline alteration at the Illimaussaq nepheline syenite Complex in Greenland instead leads to preferential loss of the HREE and HFSE from their primary source, as evidenced by their strong depletion in eudialyte pseudomorphs 15,16 . The new hydroxyl-carbonate species identified here may be the first of many unsuspected complexes and advocate for the reevaluation of the role of alkaline fluids in the genesis of REE deposits, via a more systematic characterization of fluid inclusions volatile and trace elements composition, as well as new high P-T solubility studies.…”

“…These unusual HREE enrichments have been suggested to result from (1) the preferential leaching of LREE out of the carbonatite magmas and/or magmatic ores 7 ; (2) sequential precipitation of HREE then LREE-bearing minerals upon cooling and mixing with meteoritic water 13 ; or (3) a combination of primary source and magmatic enrichment processes (e.g., HREE concentration in magmatic calcite by fractional crystallization) followed by non-selective secondary hydrothermal remobilization of the REE 12,14 . In peralkaline syenites and granites, hydrothermal processes have also been reported to promote either the dissemination of HREE through the formation of less concentrated pseudomorph assemblages within the magmatic intrusion (e.g., Illimaussaq and Greenland) 15,16 or their enrichment at the boundaries of said pseudomorphs or in hydrothermal veins (e.g., Thor Lake and Strange Lake and Canada) [17][18][19] while LREE may be further mobilized and deposited in distal fractures.…”

Editorial for Special Issue “Rare Earth Deposits and Challenges of World REE Demand for High-Tech and Green-Tech at the Beginning of the 3rd Millennium”

“…No and Park [16] investigate the origin of zircon-allanite-magnetite bands in alkaline rocks intruding the Chungju Unit, South Korea, and develop hypotheses for the role of crystal settling during magmatic crystallisation. Van de Ven and co-workers [17] examine the processes of hydrothermal alteration of eudialyte from the Illimaussaq complex, Greenland, resulting in the formation of HREE and LREE-enriched phases, and discuss the potential for the targeted processing of alteration minerals. Guastoni et al [18] present the results of studies of Nb-Y-F bearing pegmatites in the Italian Alps, including both their genesis and the detailed crystallography of Nb and REE-bearing phases.…”