Pegmatite Minerals Research Articles

Comparative Study of Gemological and Spectroscopic Features and Coloration Mechanism of Three Types of Spodumene

Spodumene is a characteristic mineral in lithium-rich granitic pegmatites, serving both as a valuable mineral resource and an important gem material. This study incorporates three different color varieties of spodumene—pink to violet, yellow-green, and colorless—into a unified research framework. X-ray powder diffraction (XRD), electron probe microanalyzer (EPMA), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, ultraviolet–visible spectroscopy (UV-Vis), and photoluminescence spectroscopy (PL) were employed to systematically analyze the chemical composition, crystal structure, and spectroscopic properties of spodumene. Furthermore, the coloration mechanism and fluorescence emission of the different color samples were investigated and analyzed. The results indicate that the presence and mixed valence states of the transition metals Fe and Mn primarily influence the color and photoluminescence of the three types of spodumene. Mn3+ is the primary color-causing element in pink to violet spodumene, while Fe3+ is the primary color-causing element in yellow-green spodumene. Photoluminescence in all three color varieties is dominated by Mn2+. These findings contribute to a deeper understanding of the color and luminescence mechanisms of spodumene, expanding its potential applications as both a gem material and a luminescent material.

Lithium pegmatite formation in Kelumute-Jideke pegmatite field, Chinese Altai: Insight from geochronology, petrology and lithium isotope geochemistry

The Kelumute-Jideke pegmatite field is an important lithium (Li)-rich pegmatite mineralization area within the Chinese Altai orogenic belt. There have been four large (Kalu’an No.803, No. 805, No.806, No.807 veins), one medium (Kelumute No.112 vein), nine small deposits (such as Jiamukai, Qunkuer, Azubai) with totally 130,000 tons Li2O found in this area. However, it remains enigmatic whether Li-rich pegmatite formed through fractional crystallization of coeval granite or direct anatexis of metasedimentary rocks. Here we selected pegmatites and their surrounding rocks of metasedimentary rocks and granitic batholith in this pegmatite field to conduct geochronology, whole major and trace element, and Li isotope analyses. The granite intrusions in this field exhibits multiple-stage emplacement ages, including 447.7 ± 4.2 Ma, 405.8 ± 0.0 – 397.4 ± 4.1 Ma, 358.4 ± 3.1–308.4 ± 4.6 Ma, 239.5 ± 1.6 – 213.7 ± 1.5 Ma. The youngest magmatism is coeval with the formation age of rare metal pegmatites. Geochemistry analyses of granites show a peraluminous S-type granite signature with high SiO2 (71.21–75.01 wt%) contents and A/CNK (> 1) ratios. The metasedimentary rocks of Kulumuti Group in this field have experienced weak weathering and exhibit extremely high Li content (maximum of 1193 ppm), which is much higher than that of the Traissic granite (96.1 ppm). The average composition of metasedimentary rocks is used to calculate the partition coefficient of different mineral proportions during the modelled equilibrium melting of the metamorphic phase. This approach determines the mineral proportions and partition coefficients under various conditions. Subsequently, Rayleigh dehydration melting simulations were performed on the Li-rich and Li-poor metasedimentary rocks. The Rayleigh dehydration melting simulations show that melts produced from partial melting of Kulumuti Group display Li content of 80–3435 ppm and δ7Li values of 1.0–8.3 ‰, consistent with natural pegmatites. The low-degree partial melting of claystone-rich metasedimentary rocks (Li-rich) in the Kulumuti Group can produce a preliminary Li-rich melt. However, through Rayleigh fractional crystallization simulation of coeval granite, it is found that the calculated δ7Li value (+6‰) of pegmatites is much higher than that of natural Li-rich pegmatite (average + 3.0 ‰). Therefore, the formation of Li-rich pegmatites in the Kelumute-Jideke pegmatite field is predominantly attributed to the partial melting of Li-rich protoliths with low δ7Li values, rather than high differentiation of Halong granite. The study highlights the importance of anatexis of metasedimentary rocks in formation of Li-rich pegmatite, which is key to mineral exploration for Li pegmatite in Altai orogenic belt.

Mineral composition and trace element characteristics of pegmatites from Smilovene, Sredna Gora Mountain, Bulgaria

The study is focused on the Smilovene pegmatites, targeting estimation of their rare element potential. These bodies bring rare-metal mineralization with LCT geochemical signature and mineral association of albite, oligoclase, microcline, quartz, muscovite, beryl, garnet, fluorapatite, zircon, columbite, samarskite-(Y), monazite-(Ce), magnetite, and gahnite. Late chlorite, prehnite, titanite, and bertrandite were established. Beryl is characteristic with high amounts of FeO, Na2O, MgO, ZnO, F, and traces of Cs, and Li. Spessartine-almandiness is enriched in Y2O3, TiO2, ZnO, P2O5, REE, Li, Ge, Ga with typical inclusions of apatite, monazite-(Ce), zircon, magnetite, gahnite, ilmenite, uraninite, and Nb-bearing phases. Significant contents of HfO2, UO2, REE2O3, ThO2, and traces of Y, Sc, Ta, Zn, Nb, Be, Li and inclusions of uraninite and cheralite are common in zircon. Minor amounts of Cl, MnO, Y2O3, and REE2O3 attribute to fluorapatite. Except the dominant Ce2O3 in monazite-(Ce), La2O3, ThO2, and Gd2O3 are established. The presence of rare-metal mineralization in the Smilovene pegmatites confirms their potential as a critical/strategic elements concentrator, feasible for further exploration and possible exploitation.

Tourmaline geochemical and B isotopic constraints on pegmatite Li mineralization and exploration

Pegmatite–related deposits represent one of the most significant types of mineral deposits housing rare–metal elements such as Li, Be, Nb, Ta, Rb, Cs, and Sn. Although extensively studied for almost two centuries, the mechanism controlling the rare–metal mineralization in pegmatites remains controversial. In addition to the enrichment of rare–metal elements in the source region, differentiation processes (e.g., fractional crystallization and liquid immiscibility) after emplacement may have also contributed to the concentration and mineralization of rare–metal elements. However, compared to fractional crystallization, the role of liquid immiscibility in pegmatite mineralization has received limited attention. In this study, the element and boron (B) isotopic compositions of tourmalines from different textural zones (Zones I–VI) of the rare–metal–mineralized Koktokay No. 3 pegmatite, Altai, NW China, as well as from the altered country rock and the border zone, were analyzed to evaluate the role of liquid immiscibility in the generation of Li–mineralized pegmatites. Tourmalines display a variety of compositions, ranging from schorl and elbaite in the outer zones to elbaite in the inner zones of the Koktokay No. 3 pegmatite. Tourmalines from Zones I–III exhibit no obvious internal textures, whereas some tourmalines from Zones IV–VI have replacement textures or abrupt zonations. The distinction is attributed to the absence and presence of exsolving fluids during their formation, respectively. Tourmalines in Zones I–III and VII–VIII display less variable δ11B values (–15.07 ‰ to –12.21 ‰ and –14.16 ‰ to –13.10 ‰, respectively), reflecting a negligible B isotope fractionation produced by fractional crystallization during the pegmatite evolution. By contrast, tourmalines in Zones IV–VII exhibit more significant variations in δ11B values (–14.83 ‰ to –8.09 ‰) compared to those in Zones I–III and VII–VIII. The high δ11B tourmalines in Zones IV–VII were most likely crystallized from the fluids exsolving from the highly evolved pegmatite–forming magma. Their occurrence indicates the fluid exsolution occurring between zones IV and V, where Li mineralization began in the Koktokay No. 3 pegmatite. The mineralization of rare–metal elements is closely linked to the evolution of magma into a coexisting magma–fluid system. In addition, Li–mineralized pegmatites are characterized by tourmalines with Fe3+Al-1 substitution and higher Zn, Li, Li/Sr, and V/Sc than barren pegmatites. These differences are believed to be due to the higher fO2 and greater extent of magmatic differentiation in Li–mineralized pegmatites compared to the barren ones. These findings provide new insights into using the geochemical compositions of tourmalines as a guide for exploring Li–mineralized pegmatites.

Principal components analysis and K-means clustering of till geochemical data: Mapping and targeting of prospective areas for lithium exploration in Västernorrland Region, Sweden

To achieve the demand for elements used for the green transition energy, such as lithium, it is necessary to recognize the spatial distribution of the concentrations of these elements in different earth materials such as bedrock and soil and to identify areas with anomalous concentrations of such elements (i.e., mineralization) for further exploration and hopefully exploitation. This study carried out multivariate statistical analyses on compositional (i.e., element concentration) data from till samples to recognize areas that likely contain lithium pegmatite mineralization in the Västernorrland region, central Sweden. We applied principal components analysis (PCA) and K-means clustering techniques to reveal regional-scale patterns in the till geochemical data. We demonstrate that these two methods have potential for recognition of geochemical anomalies related to the underlying bedrock geology as well as to mineralization. The results of PCA- and K-means clustering were validated using known occurrences of lithium mineralization. Two different datasets were compared; one containing all available geochemical data and the second containing only available trace elements in the dataset and it was found that anomalous clusters of samples defined by K-means clustering have anomalous multi-element signatures defined by robust PCA. This demonstrated that principal components are the continuous solutions to the discrete cluster members for the K-means clustering. The results show that both PCA and K-means clustering of till geochemical datasets at the early stages of exploration and target generation may reveal useful information that can be used to identify potential areas for more detailed mapping or exploration activities.

Evolution of beryllium minerals in granitic pegmatite Maršíkov D6e, Czech Republic: Complex breakdown of primary beryl by internal and external hydrothermal-metamorphic fluids

Beryllium mineralization was studied by EPMA and XRD techniques in the beryl-columbite pegmatite D6e from the Maršíkov District, Bohemian Massif, Czech Republic. A detailed study of microtextures in BSE images revealed a complex formation of fine-grained secondary Be-silicates at the expense of primary beryl and earlier secondary Be-minerals in the following proximal and distal assemblages: (A) primary magmatic beryl; (B) proximal secondary beryl; (C) proximal bertrandite + K-feldspar and minor muscovite, chamosite, gismondine-Ca and quartz; (D1) proximal assemblages of milarite + gismondine-Ca and bavenite-bohseite + epidote; (D2) distal assemblages on brittle tectonic cracks including milarite, bavenite-bohseite, albite, K-feldspar, quartz and rare phenakite, and (D3) epidote, bavenite-bohseite, quartz, albite, K-feldspar and minor milarite. A formation of secondary Mg,Fe,V,Na-enriched beryl (B) is connected with a mixing of residual (pegmatite) and external Ca,Mg,Fe,V-enriched fluids from the host amphibole gneiss at T ~ 300–400 °C and P ~ 200–400 MPa. The assemblage (C) formed due to an income of K,Mg,Ca-enriched fluids (residual + external) at T ~ 150–300 °C. The subsequent proximal (D1) and distal (D2, D3) assemblages formed during an moderate to strong income of Ca-rich external fluids from the host rocks related to retrograde hydrothermal-metamorphic overprint manifested by the Alpine-type hydrothermal veins. A common presence of epidote in the assemblages with bavenite-bohseite suggests crystallization at T < ~200–300 °C. Detailed textural and paragenetic study of primary and secondary Be-minerals is a useful tool to recognize and study various processes proceeded during subsolidus evolution of granitic pegmatites.

Reevaluation of the K/Rb-Li Systematics in Muscovite as a Potential Exploration Tool for Identifying Li Mineralization in Granitic Pegmatites

A dataset of &gt;1190 published compositional analyses of muscovite from granitic pegmatites of varying mineralogical types was compiled to reevaluate the usefulness of K-Rb-Li systematics of muscovite as a tool for distinguishing mineralogically simple pegmatites from pegmatites with potential Li mineralization. Muscovite from (i) common, (ii) (Be-Nb-Ta-P)-enriched, (iii) Li-enriched, and (iv) REE- to F-enriched pegmatites contain Li contents that vary between 10 and 20,000 ppm depending on the degree of pegmatite fractionation. Common pegmatites are characterized by low degrees of fractionation as exhibited by K/Rb ratios ranging from 618 and 25 and Li contents generally being &lt;200 ppm but infrequently as high as 743 ppm in muscovite. Moderately fractionated pegmatites with Be, Nb, Ta, and P enrichment contain muscovite having K/Rb ratios mostly between 45 and 7 plus Li contents between 5 to &gt;1700 ppm. Muscovite from moderately to highly fractionated Li-rich pegmatites exhibit a wide range of K/Rb ratios and Li values: (i) K/Rb = 84 to 1.4 and Li = 35 to &gt;18,100 ppm for spodumene pegmatites, (ii) K/Rb = 139 to 2 and Li = 139 to &gt;18,500 ppm for petalite pegmatites, and (iii) K/Rb = 55 to 1.5 and Li = 743 to &gt;17,800 ppm for lepidolite pegmatites. Pegmatites that host substantial REE- and F-rich minerals may carry muscovite with K/Rb ratios between 691 to 4 that has Li contents between 19 to 15,690 ppm. The K/Rb-Li behavior of muscovite can be useful in assessing the potential for Li mineralization in certain granitic pegmatite types. The proposed limits of K/Rb values and Li concentrations for identifying spodumene- or petalite-bearing pegmatites as part of an exploration program is reliable for Group 1 (LCT) pegmatite populations derived from S-type parental granites or anatectic melting of peraluminous metasedimentary rocks. However, it is not recommended for application to Group 2 (NYF) pegmatites affiliated with anorogenic to post-orogenic granitoids with A-type geochemical signatures or that derived by the anatexis of mafic rocks that generated REE- and F-rich melts.

Iron isotope systematics of the Jiajika granite-pegmatite lithium deposit, Sichuan, China

The Jiajika deposit in western Sichuan is the largest pegmatite-type lithium deposit in China, but the mechanism of rare metal element enrichment in this giant deposit remains a topic of debate. This study presents a comprehensive Fe isotope study of granites, aplites, pegmatites, metamorphic rocks, and associated mineral separates from the 3211 m long deep Jiajika scientific drilling core (JSD-1) to constrain the magmatic-hydrothermal processes in the Jiajika deposit. Aplites and pegmatites have variable δ56Fe values that range from –0.12 ± 0.02 ‰ to 0.38 ± 0.02 ‰, by contrast, two-mica granite samples show limited variation in δ56Fe compositions (0.11 ± 0.04 ‰ to 0.21 ± 0.04 ‰). The Fe isotope data of the coexisting Fe-bearing minerals can impose critical constraints on crystal fractionation. Garnet shows the lightest δ56Fe compositions (–0.19 ± 0.06 ‰ to –0.06 ± 0.02 ‰) and tourmaline has the highest δ56Fe values (0.15 ± 0.01 ‰ to 0.22 ± 0.02 ‰) among all the investigated minerals, whereas the δ56Fe values of biotite range from 0.06 ± 0.02 ‰ to 0.18 ± 0.02 ‰. According to the compositional homogeneity of minerals and the relatively consistent offsets in δ56Fe between the three Fe-bearing mineral pairs, the co-existing Fe-bearing minerals in the Jiajika granitic pegmatites are interpreted to be in Fe isotope equilibrium. Additionally, based on the differences in mineral composition and geochemistry of bulk-rock samples at different observation depths, as well as the modeling results, the significant Fe isotope variation in Jiajika deposit reflect the combined results of biotite fractionation, hydrothermal alteration (tourmalinization), and garnet accumulation during multistage magmatic-hydrothermal processes. Spodumene-bearing pegmatites located in the upper portion of the drill core exhibit enrichment in heavy Fe isotopes (0.18 ± 0.04 ‰ and 0.30 ± 0.03 ‰) which can be attributed to their higher degree of magma evolution and the most abundant magmatic fluids related to enrichment and migration of rare metals. Furthermore, we conducted a comprehensive statistical analysis on the available Fe isotope data of granitic pegmatites, providing new insights into granite-pegmatite systems and related rare-metal mineralization processes from a Fe isotope perspective.

Whole-Rock Geochemistry and Mica Compositions in Lijiagou Pegmatite Spodumene Deposit, Western Sichuan, China

The Lijiagou pegmatite spodumene deposit, located in the middle of the Songpan–Garze Fold Belt and southeast of the Ke’eryin ore field, is a newly discovered super-large deposit. In order to reveal the metallogenic tectonic environment and evolution process of pegmatite, based on the study of the geological characteristics of pegmatite, we carried out a whole-rock geochemical analysis of Ke’eryin two-mica granite and Lijiagou pegmatite and carried out a detailed electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis of mica minerals in each zonal pegmatite. The results show that the Ke’eryin two-mica granite is mainly formed in the transition period from syn-collision to post-collision. After the end of the continental collision, the crust is squeezed and thickened in the post-collision extensional transition tectonic environment. Mica from the microcline pegmatite zone (MP) to the albite spodumene pegmatite zone (ASP) in pegmatite show different compositions and structural characteristics, with the evolution trend in the direction from muscovite to Li-bearing mica. The type of mica from MP to AP is mainly muscovite, and Li-bearing mica appears in ASP, which is secondary and metasomatic at the edge of primary muscovite. From MP to ASP, there was a negative correlation between Nb/Ta, K/Rb and the Li, Rb, and Cs contents of mica, while the contents of Li, Rb, Cs, and F in the Li-bearing mica of ASP increased sharply. This evidence illustrates that the favorable tectonic environment contributed to the formation of the Lijiagou pegmatitic spodumene deposit. Lijiagou pegmatite experienced the magmatic–hydrothermal evolution process and has a high degree of differentiation and evolution from MP to ASP, which gradually increased. Combined with the change in mica type, it is considered that ASP formed from the stage of magmatic transition to hydrothermal and was a hydrothermal environment, and Li, Rb, and Cs mainly began to enrich at the stage of magmatic–hydrothermal transition.

Muscovite as a potential tool for magmatic differentiation and identifying Li mineralization in the Jiajika regional pegmatites, West Sichuan

Muscovite as a potential tool for magmatic differentiation and identifying Li mineralization in the Jiajika regional pegmatites, West Sichuan

The influence of crystal chemistry on lithium isotopic fractionation in pegmatite minerals

The understanding of lithium (Li) isotopic fractionation controlled by mineral species, especially those containing trace or minor amounts of Li (e.g., a few to tens of ppm), is hindered by lack of knowledge about the crystal chemistry of Li-bearing minerals. In this study, we performed solid-state Nuclear Magnetic Resonance (NMR) experiments, employing the Li7 Magic Angle Spinning (MAS) to investigate the local coordination environments of Li in minerals and their impact on Li isotopic fractionation. Our investigation focused on four coexisting minerals sourced from the Koktokay No. 3 pegmatite dike in the Xinjiang, China, namely quartz, K-feldspar, muscovite, and tourmaline, which has Li content and δLi7 value of 14.6 ppm and 3.90‰, 19.3 ppm and 7.09‰, 430 ppm and − 14.2‰, and 108 ppm and 13.42‰, respectively. Muscovite displayed the lowest δLi7 value, whereas tourmaline exhibited the highest value, and a considerable variation of 27.62‰ in δLi7 value was observed between muscovite and tourmaline. The analysis of Li7 MAS-NMR spectra further revealed distinct Li coordination environments among these minerals. A notable counterintuitive negative correlation exists between the δLi7 value and the chemical shift for the four Li-bearing minerals. This negative correlation contrasts to the expected Li isotopic fractionation under equilibrium conditions, in which enrichment of heavy Li isotopes is accompanied with large chemical shifts in Li-bearing minerals. This unexpected observation can be potentially attributed to nonequilibrium conditions of crystallization of the investigated minerals, coupled with fast diffusion of Li6 in minerals, boasting abnormally high coordination numbers due to weak LiO chemical bonds. Essentially, our study suggests that Li isotopic fractionation among Li-bearing minerals in pegmatite can be significant and is intricately influenced by both crystal chemistry and chemical diffusion.

Geophysical Constraints to the Geological Evolution and Genesis of Rare Earth Element–Thorium–Uranium Mineralization in Pegmatites at Alces Lake, SK, Canada

This investigation establishes an integrated method for rare earth elements (REE) exploration through a very promising and advanced exploration prospect in the Alces Lake area (SK, Canada) by assessing the integrated analysis of several multisource geophysical datasets. The resulting outcome provides important lithostructural information to the well-exposed, mineralized middle-to-lower crust at Alces Lake, comprising deep-seated poly-phase folds, ductile shear zones, and brittle faults. Geophysical–geological models of the Alces Lake property were constructed at different scales. The area of interest is located within the Beaverlodge Domain, about 28 km north of the Athabasca Basin’s northern margin. It contains some of the highest-grade rare earth elements (REE) in the world with the REE hosted predominantly in monazites within quartzo-feldspathic granitic to biotite–garnet–monazite–zircon-rich restite-bearing/cumulate mush melt pegmatites of anatectic origin (abyssal). Geophysical magnetic, gravity, and radiometric data were used together with Shuttle Radar Topography Mission (SRTM) images to facilitate the processing, modeling, and interpretation. Consequently, major structures were identified at different scales; however, the emphasis was given to studying those at the district/camp scale. The REE zones discovered to date occur within a large district-scale refolded synformal anticline. The eastern limb of this folded structure comprises a 30–40 km long, NW-trending shear zone/fault corridor with deep-seated structural crustal roots that may have served as the major pathway for ascending fluids/melts and facilitated the emplacement of mineralization. Thus, shear zones, faults, and folds in combination with lithological contacts/rheological contrasts appear to control residual/cumulate pegmatite emplacement and monazite deposition. Anomalies obtained from the airborne equivalent thorium survey data prove to be the most useful for REE pegmatite exploration. The results herein provide new interpretation and modeling perspectives leading to a better understanding of the distribution and lithostructural controls of REE on the property, and to new guidelines for future exploration programs at Alces Lake and elsewhere in northern Saskatchewan.

Redistribution of incompatible elements during hydrothermal alteration of pegmatite from the Djurkovo Pb-Zn deposit, Central Rhodopes

Mineralogical and geochemical study was done on weakly deformed, relatively thin pegmatites that intruded the marbles of the Rhodope metamorphic complex in the Djurkovo Pb-Zn deposit, Central Rhodopes. The pegmatites consist mainly of K-feldspar and quartz and contain minor garnet; the pegmatites lack clear zonation except for the preferable crystallization of the garnet in the central parts. The main accessory minerals are ishikawaite, zircon, monazite-(Ce), apatite, and titanite. The textural relationships indicate later hydrothermal alteration of pegmatites which led to the formation of albite, epidote, sericite, chlorite, carbonate, quartz and leucoxene. The (REE+Y, ACT, Nb, Ta, Ti)-bearing pegmatite minerals (oxides, silicates and phosphates) are partly leached and/or replaced by secondary ones. The newly-formed phases precipitated as anhedral grains along fractures and dissolved zones onto/or close to the primary minerals due to the limited mobility of the incompatible elements in fluids conditioned by pH, ligand activity and temperature.

Magmatic–hydrothermal evolution of the Koktokay No. 3 pegmatite, Altai, northwestern China: Constraints from in situ boron isotope and chemical compositions of tourmaline

Lithium–Cs–Ta-enriched (LCT-type) pegmatites are characterized by zoning texture and extreme enrichment of rare-metals. The role of fluids in zonation development and rare-metal mineralization remains a topic of debate. The Koktokay No. 3 pegmatite in northwestern China represents a typical LCT-type pegmatite characterized by economically significant Li–Be–Nb–Ta–Cs mineralization. This pegmatite exhibits a complete evolutionary sequence from outer to inner zones, with diverse rare-metal mineralization. Tourmaline is ubiquitously present in all textural zones of LCT-type pegmatites. In the Koktokay No. 3 pegmatite, the tourmalines exhibit a range of compositions from dravite and schorl to elbaite and rossmanite. The tourmalines in this pegmatite are characterized by significantly enriched light B isotope compositions (δ11B = −14.8‰ to −8.1‰), similar to those typically observed in most pegmatites derived from continental crust sources. The tourmalines in the outer zones have less variable δ11B values (−13.8‰ to −11.5‰) and are interpreted to have crystallized in a viscous magma. The tourmalines in the inner zones exhibit larger variations in δ11B values (−14.8‰ to −8.1‰) than those from the outer zones. The decreasing δ11B values of tourmalines from the outer to inner zones are attributed to the separation of an immiscible fluid phase that preferentially incorporated heavy B isotopes. The occurrence of hydrothermal tourmaline as veinlets indicates the exsolution of fluids in inner zones, with high δ11B values ranging from −10.2‰ to −9.3‰. Therefore, the hydrothermal fluid can be saturated when the magma has been strongly evolved. In addition to fractional crystallization, exsolved fluids play a significant role in the formation of internal zones and rare-metal mineralization in LCT-type pegmatites. The rare-metals are gradually enriched through fractional crystallization of magma, and the fluid contains mobile elements such as Cs that facilitate precipitation of rare-metal minerals.

Do Pegmatites Crystallise Fast? A Perspective from Petrologically-Constrained Isotopic Dating

Most recent studies consider the formation of individual pegmatite bodies to be a fast process with estimated crystal growth rates reaching a walloping 10 m/day. This opinion is presumably underpinned by the traditional way of thinking of them as the end products of magmatic fractionation. Indeed, modelling has shown that if a pegmatite-forming substance with a temperature near granitic solidus intrudes into a much colder host rock, as recorded in some outcrops, it must cool rapidly. From here, a conclusion is made that the crystallisation must likewise be rapid. However, this view is challenged by several studies that published isotopic dates supported by petrological characterisation of the analysed materials, which suggested or can be used to suggest that some minerals in pegmatites grew over millions of years. Surprisingly, such in-depth work on the geochronology of individual pegmatite bodies is relatively uncommon, so it is early to make generalisations. Here, I highlight some of the existing evidence with the aim to stimulate further research into the timescales of pegmatite crystallisation, including the use of petrologically constrained isotopic dating.

In-situ elemental and boron isotopic variations of tourmaline from the Koktokay pegmatitic rare-metal deposit, China: Insights into external contamination and the source of the granitic pegmatite

This study presents petrochemical and boron isotope compositions of tourmalines from the fertile (No. 3) and relatively barren (No. 1, 2b) pegmatite in the Koktokay pegmatitic rare-metal deposit, to better understand the influence of external contamination on pegmatite mineralization and the source of these pegmatites. There are four types of tourmalines that have been studied: (1) tourmaline in the tourmaline-mica nest of pegmatite No. 1 (Tur-N); (2) tourmalines in the comb-like structure of pegmatite No. 1 (Tur-C1), 2b (Tur-C2), and 3 (Tur-C3), respectively. Tur-N tourmaline is heterogeneous in backscattered electron (BSE) images, Tur-C1 and Tur-C2 appear to be homogenous crystals, while Tur-C3 exhibits to have a complicated zonal structure. These tourmalines are members of the alkali group tourmalines representing mostly dravite-schorl solid solutions. Tur-N contains the lowest Ca content. There is a general increase in the contents of Mg and Ca from Tur-C1 to Tur-C3, implying that the pegmatite melt was contaminated by the host rock (metagabbro) during the emplacement. The contamination's diffusive influx of Mg, Ca, and Fe components into the pegmatite melt impelled the crystallization of tourmaline during the emplacement, which subsequently triggered the deposition of rare element minerals. The external contamination implies that the closed system of melt is extremely favorable for the enrichment and mineralization of rare elements in the pegmatitic rare-metal deposit. The boron isotopes vary slightly among the four types of tourmalines. The B-isotope composition of Tur-C3 is the heaviest (–11.63 ± 0.05 ‰ to –13.16 ± 0.05 ‰), followed by Tur-C1 (–12.29 ± 0.06 ‰ to –12.93 ± 0.05 ‰) and Tur-C2 (–12.51 ± 0.05 ‰ to –13.21 ± 0.05 ‰), Tur-N contains the lowest δ11B value (–13.20 ± 0.05 ‰ to –13.76 ± 0.05 ‰). Synthesized with the previous research results, we hold that the boron isotope of tourmaline (Tur-C3) in the comb-like structure of No. 3 pegmatite might represent the isotope composition of its initial magma, indicating that the tourmaline and associated pegmatite may derive from the melting of clastic sedimentary rock from the Habahe Group forming in Mid Ordovician to Early Devonian in the Chinese Altay orogenic belt.

Petrogenesis of the Weiling beryl-bearing granitic pegmatite – A giant LCT-type pegmatite in the Northern Wuyi area, South China

The Weiling granitic pegmatite is a super-large (ca. 9.20 million tons) LCT-type pegmatite body in the Northern Wuyi area of South China, with the typical mineral assemblage quartz, plagioclase, K-feldspar and muscovite as well as tourmaline (schorl-dravite), garnet (almandine-spessartine) and beryl. This overall unzoned pegmatite with locally layered texture has economic amounts of the rare metal mineral beryl and ultrapure quartz, and porcelain clay in its uppermost weathering parts. The 40Ar/39Ar dating of pegmatitic muscovite yielded a precise age of 405.33 ± 3.38 Ma, indicating this beryl-bearing pegmatite body was formed in the Early Devonian rather than in the Jurassic-Cretaceous as previously thought. Boron and Nd isotope data demonstrate that the pegmatitic melts have solely crustal sources and are most likely derived from partial melting of the ambient metasedimentary rocks from the Zhoutan Formation. Geochemical characteristics of pegmatitic minerals muscovite, beryl, tourmaline, garnet and K-feldspar and the absence of coeval fertile granites around this pegmatite suggest that the Weiling pegmatite most likely evolved from anatectic melts that did not experience significant fractionation. The pegmatitic melts were generated by low degrees of muscovite dehydration partial melting of local metasedimentary rocks rich in fluxing elements and rare metals. These melts experienced limited differentiation during ascent in the crust. The beryl mineralization formed in the medium-temperature (222–357 ℃), low-salinity (3.3–10.9 wt% NaCl equiv.) and high-density (0.62–0.86 g/cm3) H2O-NaCl-KCl-CO2-N2 system. These temperatures are much lower than typical granitic pegmatites, possibly reflecting late fluids derived from the pegmatitic magma and experienced post-crystallization alteration. The Weiling pegmatite represents a Be-bearing pegmatite that formed by partial melting of local metasedimentary rocks.

Mineral chemistry and in-situ U-Pb geochronology of altered pegmatite from the vicinity of the Strashimir Pb-Zn vein deposit

The current study presents new mineralogical, geochemical and geochronological data for a pegmatite body hosted in gneisses and marbles from the vicinity of the Strashimir Pb-Zn vein deposit, Madan ore district, South Bulgaria. The mineral composition of the studied pegmatite is represented by oligoclase–andesine (An10.1–33.2) and albite (An0–7.6), which prevail over K-feldspar (Or87.9–92.4), and quartz. The established accessory minerals are allanite-(Ce), titanite, apatite and zircon. The pegmatite/marble contact is affected by later hydrothermal silicate-carbonate alteration without detected ore mineralization, despite the spatial proximity with the Strashimir Pb-Zn vein deposit. Epidote-group minerals in pegmatite are defined as members of the clinozoisite–epidote series. As a major constituent of the hydrothermal alteration zone, they are manifested in two well-distinguished generations along with chlorite, quartz and carbonates. The calculated temperature of chlorite mineralization yields T° of crystallization in the range of 223–266 °C. As a result of the hydrothermal fluid circulation, the accessory allanite-(Ce) is transformed to REE-rich epidote-clinozoisite, marked by depletion of REE and Fe and enrichment of Si, Al, and Ca. Due to the limited mobility of REE in fluids, after leaching these elements are incorporated in nearby crystallized epidotes. According to the occurrence, mineral association and chemical properties, three titanite populations are distinguished in the pegmatite. Two of them were in-situ dated by the LA-ICP-MS U-Pb method and reveal overlapping ages of 39.9±2.1 Ma and 39.5±2.2 Ma (39.3±1.2 Ma combining all titanite analyses). The ages are interpreted as titanite growth in a pegmatite body related to granitic melts in the Late Alpine high-metamorphic units (Madan Unit) of the Central Rhodopes. Hydrothermal fluids either did not affect the U-Pb isotope system of the titanites or were derived from the same fluid-rich melt.

Intra-granitic pegmatites of the Las Chacras-Potrerillos batholith (Sierra de San Luis, Argentina): Geochemistry, dating, and a new model of magma evolution

The chemical composition of primary minerals and fine-grained border zones from intra-granitic pegmatites have been analyzed to constrain the last stages of magmatic differentiation in the Devonian Las Chacras-Potrerillos batholith, central Argentina. Based on accessory phase assemblages and mineral chemistry, the pegmatites are assigned to the NYF (Nb – Y – F) family. They were emplaced in all six units of the batholith but there is an increase in pegmatite abundance, size and complexity from north to south in the batholith. A similar N–S trend in pegmatite melt evolution is defined by fractionation indicators in pegmatitic K-feldspar (e.g., K/Rb: 68–301, Rb/Sr: 1–47, Cs: 8–71 ppm, Ba: 66–800 ppm), in white mica (e.g., K/Rb: 36–99, Rb/Sr: 40–826, Cs: 30–206 ppm, Ba 18–1889 ppm) and in tourmaline (Fetot + Mgtot: 2.6–4.0 apfu, Altot: 5.0–6.4 apfu). The regional fractionation patterns of pegmatite mineral chemistry are consistent with a general southward increase in the fractionation degree of the host granite units, confirming that compositional characteristics of the parental magmas are passed on to the pegmatitic phases. A new zircon U–Pb age of 388.3 ± 1.9 Ma from the southern granites constitutes the oldest crystallization age of the batholith. This resolves the previously-debated emplacement sequence and has important implications for the magmatic evolution of the largest and most evolved pegmatites, which cluster around the contact zone between the southern granites and the younger, central unit (biotite porphyritic granite). The age constraints combined with chemical data from the pegmatitic minerals show that pegmatites in the southern region of the batholith share a common magma source, which we suggest is the central unit of biotite porphyritic granite. The increased degree of fractionation in the southern pegmatites is attributed to elevated concentrations of volatiles in the parental magmas, which derive from dehydration partial melting and assimilation of fluids from metaclastic rocks in the crust.

Sm-Nd and U-Pb isotope behavior of REE-rich accessory minerals in pegmatite during overprinted metamorphic and hydrothermal events: Evidence from the Paleoproterozoic rare-earth pegmatite in the lesser Qinling district of China

Sm-Nd and U-Pb isotope systems are widely used to dating various magmatism and tracing geological processes, but their behaviors during post-magmatic metamorphic and/or hydrothermal disturbances remain unclear. Here, we investigated the trace elements, U-Pb and Sm-Nd isotope systems of accessory minerals (allanite, titanite, and apatite) from the Huangjiagou Paleoproterozoic rare-earth pegmatite, to determine the timing of magmatism and also the overprinted metamorphic and hydrothermal events, with a particular emphasis on the isotopic behavior of these accessory minerals under metamorphic event and fluid-rock reaction. Two-period accessory minerals are identified: Group I allanite, crystallized in the magmatic period but was modified by high-temperature metamorphism and hydrothermal metasomatism; and Group II allanite, titanite and apatite, which newly precipitated in the late hydrothermal event. In-situU-Pb dating of the Group I allanite (type Ia) yielded a magmatic age of ca. 1.82 Ga, whereas the type Ib allanite record the high-temperature anhydrous thermal metamorphism at ca. 1.75 Ga. Group II allanite and titanite yielded U-Pb ages of ca. 131 Ma, indicating a Mesozoic hydrothermal metasomatism overprinting, due to the emplacement of neighboring granites. Group I allanite exhibits different zones and inconsistent U-Pb ages, but they exhibit an identical Sm-Nd isochron age (∼1.83 Ga) broadly consistent with the pegmatite age, indicating immobility nature of the Sm-Nd system. Group II titanite and allanite exhibit a Sm-Nd isochron age of ∼130 Ma with initial εNd(t) values of −27.5 to −29.8, consistent with the whole-rock εNd(t = 130 Ma) values of granitic rocks, implying a total redistribution of the REEs. While, Group II apatite and coexisting allanite did not produce geologic meaningful Sm-Nd ages with a wider initial εNd(t) range of −15.8 to –23.7, indicating the fluid has incompletely equilibrated Sm-Nd isotopic system due to various fluid-rock reaction. Our study provides a typical example to illustrate the robustness of the Sm-Nd isotopic system of magmatic allanite under the disturbance of later metamorphism and metasomatism, and shows the different degree equilibration of Sm-Nd isotopes in hydrothermal systems under various fluid-rock reactions.