Geology and mineralogy of the Blue Beryl Dyke

The Julianna pegmatite system crops out in an active amphibolite–migmatite quarry (50.70327, 16.73677) at Piława Górna in the Góry Sowie Block (Szuszkiewicz et al., Reference Szuszkiewicz, Szełęg, Pieczka, Ilnicki, Nejbert, Turniak, Banach, Łodziński, Różniak and Michałowski2013). The unit constitutes a central part of the Sudetes at the NE margin of the Bohemian Massif in SW Poland. The pegmatites crystallised from anatectic melts produced by partial melting of metasedimentary–metavolcanic rocks during retrograde amphibolite-facies metamorphism ca. 370–380 Ma (Van Breemen et al., Reference Van Breemen, Bowes, Aftalion and Żelaźniewicz1988; Timmermann et al., Reference Timmermann, Parrish, Noble and Kryza2000). Most of the anatectic melts were trapped in neosomes in migmatites, whereas the rest occur in pegmatite bodies located along tectonic zones in the Góry Sowie metamorphic rocks (Szuszkiewicz et al., Reference Szuszkiewicz, Szełęg, Pieczka, Ilnicki, Nejbert, Turniak, Banach, Łodziński, Różniak and Michałowski2013; Pieczka et al., Reference Pieczka, Szuszkiewicz, Szełęg, Janeczek and Nejbert2015). The pegmatite system consists of a network of co-genetic dykes and vein- and nest-type apophyses that vary in size and degree of geochemical evolution and represent barren and rare-element pegmatites. They individually represent various subclasses that correspond to the niobium–yttrium–fluorine (NYF) and lithium–caesium–tantalum (LCT) petrogenetic families in the pegmatite classification of Černỳ and Ercit (Reference Černý and Ercit2005), however overall, the system exhibits hybrid NYF + LCT characteristics. Wise et al. (Reference Wise, Müller and Simmons2022) classified the Góry Sowie pegmatites as an example of type 1 DPA (direct product of anatexis) pegmatites, showing a typical Be–Nb–Ta–P–Li–B geochemical signature.

The Blue Beryl Dyke, which was exposed in 2010, extended over a few tens of metres horizontally with a maximum thickness of ~3–4 m. The zoning was typical up to the blocky unit with interstices among feldspars filled with quartz, but without a massive core. The characteristic features of the dyke were the presence of short prismatic crystals of pale-bluish low-Na,Cs beryl, abundant cassiterite and columbite-group minerals, and subordinate, few millimetre-sized, greenish crystals of gahnite associated with ferro- and zinconigerites-2N1S, (Al,Fe,Zn)₂(Al,Sn)₆O₁₁(OH) and (Zn,Al,Fe)₃(Al,Fe,Ti)₈O₁₅(OH), and -6N6S, (Al,Fe,Zn)₃(Al,Sn,Fe)₈O₁₅(OH) and (Zn,Al)₇(Al,Fe³⁺,Ti,Mg)₁₆O₃₁(OH), as well as genthelvite, Be₃Zn₄(SiO₄)₃S (Pieczka et al., Reference Pieczka, Szełęg and Szuszkiewicz2019). Tourmaline-supergroup minerals were represented by Zn-enriched foitite, □(Fe²⁺₂Al)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH), evolving into schorl, NaFe²⁺₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH); Li-bearing tourmalines were unique, occurring only as small dark greenish crystals of Fe-bearing elbaite, Na(Li_1.5Al_1.5)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃OH, and fluor-elbaite, Na(Li_1.5Al_1.5)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃F, in a few-centimetre-sized muscovite flakes, and as tiny inclusions in quartz and albite (Pieczka et al., Reference Pieczka, Gołębiowska, Jeleń, Włodek, Szełęg and Szuszkiewicz2018). Lithium-bearing mineralisation was also represented by minute aggregates of spodumene, LiAlSi₂O₆, veinlets of a Cs-dominant dark mica, and single aggregates of partly altered lithiophilite, LiMn²⁺(PO₄), associated with ferrowodginite, Fe²⁺ Sn⁴⁺Ta₂O₈, and wodginite, Mn²⁺Sn⁴⁺Ta₂O₈. The REE-bearing phases were represented by rare to accessory monazite-(Ce), monazite-(Nd), monazite-(Sm), xenotime-(Y), allanite-(Ce) and traces of rhabdophane-group phosphates.

Genetic implications

To the best of our knowledge, the monazite-(Nd) and monazite-(Sm) from Piława Górna are the two most Nd- and Sm-enriched crystals worldwide. This could imply that these elements, with abundances in the Earth's continental crust of ~20 and 3.9 ppm, respectively (Rudnick, Reference Rudnick and White2018), have been concentrated by ~1.5×10⁴ times in monazite-(Nd) and from up to 8×10³ times (Nd) to above 70×10³ times (Sm) in monazite-(Sm). Thus, it is clear that geochemical fractionation of REE itself within a bulk volume of the pegmatite-forming melt was not the only crucial factor for the chemical composition of these crystals. When explaining the crystal-chemistry of monazite-(Gd), Ondrejka et al. (Reference Ondrejka, Uher, Ferenc, Milovská, Mikuš, Molnárová, Škoda, Kopáčik and Bačík2023) suggested the role of a localised process in a chemically near-closed system leading to the progressive concentration of MREE in fluids as a result of the promoted crystallisation of early monazite-(Ce) and xenotime-(Y), which selectively decreased the LREE and HREE contents in the crystallisation environment. The abnormally high Sm- and Nb-enrichment of the Blue Beryl Dyke monazite-(Sm) and monazite-(Nd), and exceptional rarity of these monazite species in the dyke also suggest local REE geochemical anomalies in the pegmatite-forming melt–fluid system related to variations of its chemical and physical parameters, even at the micro scale. These led to the rapid growth of accessory phases under supercooling conditions which scavenged selected elements leading to their local depletion that was not balanced by diffusion processes. We suggest that the outstripping crystallisation of xenotime-(Y) progressively increased the local abundances of LREE (Gd to La), first enabling the occasional crystallisation of monazite-(Sm) with a large negative Eu anomaly; this then led to the crystallisation of (Sm,Nd)-bearing monazite-(Ce) with greater Eu³⁺ concentrations than in the monazite-(Sm), and finally to the formation of monazite-(Nd) with a smooth REE_CN pattern. As is known, the main reason for the Eu anomaly is in the presence of Eu²⁺ or Eu³⁺ species in the crystallisation environment (Weill and Drake, Reference Weill and Drake1973; Bau, Reference Bau1991; Deer et al., Reference Deer, Howie and Zussman2001). At higher temperatures and in more reducing magmas, Eu occurs as Eu²⁺ which is compatible with Ca-bearing melt components, mainly becoming incorporated into plagioclase crystals. At lower temperatures and under more oxidising conditions, the element will occur as Eu³⁺, becoming incompatible with the melt components but compatible with other trivalent REE. Thus, the decreasing negative Eu anomaly in monazite-(Sm) through monazite-(Ce) to monazite-(Nd) [from 0.06(2) to 0.33(12) and 0.97, respectively] indicates increasing amounts of Eu³⁺ in probably more oxidising fluids coexisting with the pegmatite-forming melt, which were subsequently remobilised by the coeval local hydrothermal alteration of plagioclase. The fluid also affected the REE fractionation, decreasing the tetrad effect on the third, second, and first tetrads. These observations provide additional insights into the evolution of the crystallisation environment of the monazites.

Monazite-(Sm) exhibiting a negative Eu anomaly has been found as an intergrowth with partly digested columbite-(Mn) undergoing alteration to fluor- and oxy/hydroxycalciomicrolite and finally to F-free plumbomicrolite associated with ferrowodginite, wodginite, F-bearing tourmaline, fluorapatite and Cs-bearing mica. This indicates relatively early crystallisation of the mineral from a pegmatite-forming melt evolving into a residual melt coexisting with aqueous fluids. High-temperature hydrothermal fluids are commonly associated with the presence of strong REE-complexing ligands such as F^–, Cl^–, OH^–, and PO₄^3–, which are reported to be the most effective agents in the mobilisation, transport, fractionation, and deposition of REE species (Gieré, Reference Gieré1990; Jiang et al., Reference Jiang, Wang, Xu and Zhao2005; Migdisov and Williams-Jones, Reference Migdisov and Williams-Jones2014; Williams-Jones, Reference Williams-Jones, Simandl and Neetz2015; Migdisov et al., Reference Migdisov, Williams-Jones, Brugger and Caporuscio2016). Among these anions, Cl^– forms the weakest complexes with REE and F^– the strongest ones (Migdisov et al., Reference Migdisov, Williams-Jones, Brugger and Caporuscio2016). However, as chloride is a dominant ligand in natural hydrothermal systems, it is mainly responsible for hydrothermal transport of the REE. Thus, Nd and Sm may have undergone additional fractionation within the Blue Beryl Dyke during the hydrothermal stage due to the different stabilities of LREE-chloride complexes, and differences in their mobility and solubility. This fractionation should decrease with increasing atomic number of the REE, which was corroborated in the Blue Beryl Dyke monazites by the decreasing tetrad effect on the third, second, and first tetrads from monazite-(Sm) through monazite-(Ce) to monazite-(Nd). Due to significant differences between the stability/mobility of LREE-chloride complexes and those involving HREE, this process may have led to Sm- and Nd-enrichment resulting in the local saturation in SmPO₄ and NdPO₄ at different stages of the system's evolution. The destabilisation of Nd–Cl and Sm–Cl complexes could also have been related to the neutralisation of the solution's acidity, e.g. through F consumption during the formation of F-bearing tourmaline, fluorapatite, or fluor-calciomicrolite.

In melt–fluid systems, Th and U, the two elements that are commonly associated with REE in monazite-group minerals, are highly compatible. Among the studied monazite crystals, monazite-(Ce) was the most enriched in Th and U, containing up to 14.91 wt.% ThO₂ and 1.76 wt.% UO₂. The amounts of ThO₂ in monazite-(Sm) were <2 wt.% and UO₂ was absent altogether; in turn, small amounts of UO₂ were measured in monazite-(Nd) (0.15 wt.%), whereas ThO₂ was absent. According to Keppler and Wyllie (Reference Keppler and Wyllie1990, Reference Keppler and Wyllie1991), the Th and U partitioning coefficients between fluid and melt are very low if water is the only volatile component present. However, this partitioning increases significantly as the F^– concentration increases, indicating the extraction of certain amounts of Th and U from the melt through the formation of fluoride complexes in the fluid. Chloride anions may form complexes with U, but not with Th; the latter will not be extracted under such conditions. Consequently, fluids with higher Cl^– concentrations coexisting with more peraluminous silicate melts will reduce the initial U contents of the melts as the latter will partition into the fluid phase. These data on the behaviours of Th and U in melt–fluid systems corroborate the crystallisation of monazite-(Sm) from a melt which became progressively enriched in volatiles, including F and Cl. The system evolved progressively into a hydrated F-bearing residual melt from which the (Sm,Nd)-bearing monazite-(Ce) crystallised and which lost U through the formation of (U–Cl)-complexes in the fluid. Monazite-(Nd), occurring as ~10 μm inclusions in columbite-(Fe), had a different REE_CN pattern shaped by λ coefficients that differed from those characteristic of monazite-(Sm) and monazite-(Ce); this monazite also had the weakest tetrad effect only on the third tetrad, and a negligible Eu anomaly. The small UO₂ contents and the absence of ThO₂ as well as its coexistence with inclusions of other phases crystallised from fluids (cassiterite, scheelite, uraninite and a BiVO₄ phase) indicate late crystallisation from fluids devoid of fluorine, which potentially could be coeval with the formation of F-free oxy/hydroxy-plumbomicrolite, as well as possible formation through secondary remobilisation and in situ crystallisation. These data agree with the observations of Gysi et al. (Reference Gysi, Harlov and Miron2018) who showed that the REE_CN profiles of monazites could be classed into two groups based on their LREE contents: one group showing the limited fractionation of La, Ce, Pr and Nd, and the other group showing a strong geochemical fractionation and including enhanced concentrations of Sm, Eu and Gd. These authors explained the differentiation as resulting from the incorporation of REE with varying ionic radii to match the geometric parameters of the monazite structure.