Rare-element Pegmatites Research Articles

Petrogenetic relationships between Paleoproterozoic granitoids and rare-element pegmatites from Dibilo (Liptako, West Niger, West African Craton)

The rare-element pegmatites of Dibilo form intrusions in a tonalitic gneiss of the Paleoproterozoic Téra-Ayorou pluton about 4 km from the contact zone with the Diagorou-Darbani greenstone belt, in the Nigerien Liptako (NE portion of the Man Ridge of the West African Craton). The general objective of the present study is to determine the petrogenetic relationships between the granitoids and the Dibilo pegmatites through a multidisciplinary approach combining petrology, geochemistry, structural geology and metallogeny. Two generations of granitoids have been distinguished at Dibilo. A first generation of metaluminous to weakly peraluminous granitoids (tonalitic gneiss, metadiorite and two-mica granitic gneiss) constitute a calc-alkaline series. Trace element geochemical signatures suggest that these granitoids were emplaced in an island arc and subduction zone context. A second generation of granitoids (leucogranites), encompassing two-mica, muscovite and two-mica-garnet granites, are peraluminous and calc-alkaline. These leucogranites are fertile, i.e., rich in incompatible elements, with Li concentrations attaining 250 ppm in the richest. A model of partial melting shows that they could derive from the partial melting of Li-rich metadiorite, which is itself highly enriched in lithium (150 ppm). One sample of migmatite has Li contents of 300 ppm, which evidences the fertile nature of the tonalitic-dioritic-granitic gneiss complex.Three types of pegmatite have been identified at Dibilo: sterile pegmatites (type I), peraluminous and poorly evolved; Nb-Ta pegmatites (type II-a) and Nb-Ta, Mo pegmatites (type II-b), moderately evolved, peraluminous, respectively mineralized with columbite-group minerals (type II-a) and columbite-group minerals and Mo-bearing sulfide (type II-b); and highly evolved, peraluminous Li and Nb-Ta pegmatites (type III) mineralized with spodumene and columbite-group minerals. The type I barren pegmatites share similarities with the leucogranites and could also originate from the partial melting of the dioritic gneiss. The highly evolved type II-a, type II-b and type III pegmatites correspond to the residual liquids resulting from the fractional crystallization of leucogranites. Therefore, we propose that the Dibilo rare-element pegmatites derive from both fractional crystallization and partial melting processes within the tonalitic-dioritic-granitic complex.

Genesis studies of Li–Rb deposits in pegmatites from Bailongshan, western China: Evidence from chronology, fluid inclusions, and H–O isotope analysis

The Bailongshan Pegmatite deposit, located in the West Kunlun Orogenic Belt, Northwest China, is a newly discovered, super-large Li–Rb (Be–Ta–Nb) rare-metal deposit. Since complex magmatic-hydrothermal processes are responsible for the mineralization of such rare-element pegmatites, it is desirable to study the evolution and sources of ore-forming fluids to analyze the genesis of ore deposits. In this study, the 40Ar/39Ar plateau ages of muscovite and biotite were determined to be 171.36 ± 1.87 and 172.39 ± 1.66 Ma, respectively, indicating that the duration of hydrothermal mineralization was approximately 170 Ma. Based on the zonal nature of the mineral assemblage, the Bailongshan area was divided into four zones and stages (I–IV), namely the albite–quartz–lithium tourmaline (AQT, stage I), albite–quartz-bearing mica (AQM, stage II), albite–quartz–spodumene (AQS, stage Ⅲ), and spodumene–quartz (SQ, stage IV) zones. Among these, AQS and SQ were the main ore-bearing areas. In terms of the fluid inclusions found in quartz and spodumene, the different types include a gas-rich phase (V-type), a liquid-rich phase (L-type), a daughter mineral-bearing three-phase (S-type), and a carbon dioxide-bearing phase (C-type). In stage I, the homogeneous temperatures of the V- and S-type fluid inclusions varied from 365 to 415 °C, while their corresponding salinities were 8.5–12.9 and 44.8–47.2 wt% NaCl equiv., respectively. In stage II, the homogeneous temperature and salinity of the L-type inclusions were 315–365 °C and 9.9–13.3 wt% NaCl equiv., respectively, while in stages Ⅲ and IV, the homogeneous temperatures of the L- and S-type fluid inclusions were between 235 and 335 °C, while their salinities were 7.2–12.3 and 32.1–37.0 wt% NaCl equiv., respectively. Furthermore, for the C-type inclusions, the homogeneous temperature and salinity were 235–320 °C and 4.9–10.6 wt% NaCl equiv., respectively. The laser Raman results showed that the fluid in the metallogenic stage was an H2O–NaCl–CO2–CH4 system. Based on the homogeneous temperature and salinity results, the fluid capture pressure from stage III to stage IV was calculated to be 280–150 MPa, and the depth of the capture was >6 km. Moreover, the H–O isotope results suggested that the early ore-forming fluids are mainly magmatic hydrothermal fluids, whereas the later (stage IV) mineralizing fluids may be mixed with a small amount of meteoric water. The subsequent immiscibility of the fluid may be one of the factors responsible for the discharge and precipitation of minerals.

Boron sources of tourmaline-rich Nb-Y-F-pegmatites in south Norway: Implications for pegmatite melt origin

Tourmaline is common in rare element pegmatites of the Nb-Y-F (NYF) type in the south-central part of the Proterozoic Sveconorwegian orogen in southern Norway. In the global context, however, tourmaline appears rare in this type of pegmatite. This study aims to explain the unusual tourmaline abundance in these pegmatites and the origin of boron (B) in the respective melts, and to raise awareness of tourmaline in NYF pegmatites generally. Tourmalines from six pegmatites in three Sveconorwegian lithotectonic units: Bamble, Kongsberg and Idefjorden, were investigated in terms of their mineral chemistry and δ11B values, in addition to bulk rock analyses of pegmatites and host rocks. Tourmalines in pegmatites from Bamble and Kongsberg record B isotopic compositions (δ11B = -1.0 to + 9.9 ‰) that are heavy relative to continental crust and mantle sources. In contrast, tourmaline in pegmatites and host rocks from Idefjorden have light B isotopic ratios (δ11B = -14.8 to −12.5 ‰) that are typical crustal values. We suggest that the latter melts were sourced from orthogneisses at depth. We relate the heavy B isotopic composition of Bamble and Kongsberg pegmatites to regional Na-metasomatism by fluids sourced from Mesoproterozoic shallow marine sediments. This is supported by previously published δ11B ratios from metasomatized Bamble host rocks. The spatial association of pegmatites with Na-metasomatism in the basement rocks suggests that metasomatism enhanced the fertility and B-concentration in the affected lithologies, favouring partial melting and the formation of tourmaline-bearing pegmatites.These findings contribute to understanding the petrogenesis of Sveconorwegian pegmatites but they also imply that B can play a greater role in the formation of NYF pegmatites than previously thought and that tourmaline has value as a petrogenetic tool in this type of pegmatites as well as in the Li-Cs-Ta (LCT) type to which is it is more commonly applied.

First in situ Lu–Hf garnet date for a lithium–caesium–tantalum (LCT) pegmatite from the Kietyönmäki Li deposit, Somero–Tammela pegmatite region, SW Finland

Abstract. The in situ Lu–Hf geochronology of garnet, apatite, fluorite, and carbonate minerals is a fast-developing novel analytical method. It provides an alternative technique for age dating of accessory minerals in lithium–caesium–tantalum (LCT) rare-element (RE) pegmatites where zircon is often metamict due to alteration or radiation damage. Currently most dates from Finnish LCT pegmatites are based on columbite-group minerals (CGMs), but their occurrence is restricted to mineralised zones within the pegmatites. Accessory minerals such as garnet and apatite are widespread in both mineralised and unmineralised LCT pegmatites. Lu–Hf dating of garnet and apatite provides an exceptional opportunity to better understand the geological history of these highly sought-after sources for battery and rare elements (Li, Nb, Ta, Be) that are critical for the green transition and its technology. In this paper we present the first successful in situ Lu–Hf garnet date of 1801 ± 53 Ma for an LCT pegmatite from the Kietyönmäki deposit in the Somero–Tammela pegmatite region, SW Finland. This age is consistent with previous zircon dates obtained for the region, ranging from 1815 to 1740 Ma with a weighted mean 207Pb / 206Pb age of 1786 ± 7 Ma.

Metallogenic mechanism of the rare-element pegmatite dykes in Guanpo area from the East Qinling Orogen

Metallogenic mechanism of the rare-element pegmatite dykes in Guanpo area from the East Qinling Orogen

The magmatic-hydrothermal transition in P-rich pegmatitic melts: Crystal-melt-fluid interactions recorded by phosphate minerals

Rare-element pegmatites are the crystallization product of melts strongly enriched in incompatible elements. Some of these elements, e.g., Li, B, P, F, and H2O, function as melt structure modifiers, allowing such melts to reach unusually low crystallization temperatures and regulating the saturation of aqueous-carbonic fluids from the melt. However, the timing of fluid saturation is highly debated, and the processes that mark the magmatic-hydrothermal transition in rare-element pegmatites are still elusive. The crystallization sequence of phosphate minerals can be used to assess the timing and nature of fluid-related processes in pegmatitic systems. In the Buranga pegmatite, western Rwanda, magmatic phosphates are replaced by (hydrous) alkali-rich phosphates and later by strongly hydrated phosphates as the dike cools. This reaction series indicates a disequilibrium in the crystal-melt-fluid system and records complex fluid-rock interactions during dike consolidation. This contribution compares these interactions with the fluid record preserved in fluid inclusions in the main phosphates of the crystallization sequence. Three major fluid inclusion types are observed, aqueous-carbonic (mostly-two-phase, L-V), carbonic-aqueous (two-phase, V-L, or three-phase, L-L-V), and crystal-rich inclusions (multiphase, L-V-Solids). Crystal-rich fluid inclusions in trolleite [Al4(PO4)3(OH)3], the last primary phosphate to precipitate from the melt, show a paragenesis of solid phases similar to the secondary phosphates observed in the dike. LA-ICPMS analyses of individual fluid inclusions show high concentrations of Na and K, with some content of Li, Rb, Cs, B, Fe, and Mn in the fluid. A similar composition is present in all inclusion types within the same mineral. Crystal-rich fluid inclusions demonstrate that during the magmatic-hydrothermal transition, primary phosphates react with the fluid in the presence of melt to generate secondary phosphates. No evidence of external fluid sources has been observed during the magmatic-hydrothermal transition. The fluid inclusion record shows that the fluid salinity is kept constant due to fluid-mineral interactions and corroborates the hypothesis that fluid-soluble elements are internally remobilized in the melt-crystal-fluid system during the magmatic-hydrothermal stage. These interpretations have implications for the general evolution of P-rich pegmatitic melts and the mineralization of Nb and Ta in these systems.

Unraveling 800 Ma of pegmatite formation in the western Grenville Front tectonic Zone near Sudbury, Canada

The origin of pegmatites has been at the core of petrological research for decades, but despite much effort their origin remains controversial – protracted fractionation of a causative felsic magma versus anatexis of an appropriate protolith. To address this controversy, we present the results of U-Pb analyses (zircon (n = 774), monazite (n = 230), xenotime (n = 295)) for 32 well characterized pegmatites from an extensive swarm of felsic dike rocks outcropping, along a 12 km transect ca. 12 km south of Sudbury (Ontario, Canada) in the westernmost part of the Grenville Front Tectonic Zone (GFTZ). Collectively these rocks exhibit a remarkably large morphological, textural and mineralogical variation related to the crystallization of three distinct pegmatite-forming events that span 800 Ma. The derived concordant U-Pb ages of the dated phases coincide with the late Paleoproterozoic (ca. 1740 Ma) Yavapai orogeny, the Mesoproterozoic (ca. 1450–1430 Ma) Chieflakian event, and the early Neoproterozoic (ca. 1000–970 Ma) Grenville orogeny. Field observations and related geochemical data suggest the two older events are genetically related to regional magmatism (i.e., fractionation of a progenitor), whereas the Neoproterozoic pegmatites were generated via partial melting of an appropriate protolith at depth. The data therefore support a duality of models for pegmatite genesis. We emphasize the important role of imaging (CL and BSE) as an integral part of our protocol, as this revealed widespread coupled dissolution-precipitation (CDP) replacement textures and metamictization in both zircon and xenotime, the latter due to high U contents (i.e., >1000 pm). Interestingly, the Neoproterozoic pegmatites show similar U(±Th), Nb(±Ta) and REE enrichments like their contemporaneous Sveconorwegian counterparts. Many zircons record U-Pb system resetting at ca. 220–200 Ma which is possibly related to the far-field effects of the onset of the Central Atlantic Magmatic Province (CAMP) related to the opening of the Atlantic Ocean. Finally, this study shows that anatexis is a viable process of rare-element pegmatite formation, even in areas rich in potential parental granites, which suggests that anatectic rare-element pegmatites might be globally under-recognized.

Regional Zoning of a Li-Cs-Ta Pegmatite Field: Insights from Monazite-Cheralite Chemistry, U-Th-Pb and Sm-Nd Isotopes

Abstract Li-Cs-Ta (LCT ) rare-element pegmatites occur as late-stage and highly fractionated bodies at the margins of regionally zoned granite pegmatite fields. The evolution of the granitic pegmatite system, including its rare-metal metallogeny, is often difficult to determine due to complex textures involving variable crystal size and a heterogeneous chemical composition. The Renli-Chuanziyuan pegmatite field (South China) displays a well-developed regional zonation sequence, involving a core of biotite-, two-mica- and muscovite monzogranites (MMs) that grades outward into microcline (K-zone), microcline-albite (K-Na-zone), albite (Na-zone) and albite-spodumene (Na-Li-zone) pegmatites. Monazite and the Th, Ca–end-member (i.e. cheralite) provide valuable age, rare earth element (REE) geochemical and Sm-Nd isotopic data for understanding the regional zoning process within the Renli-Chuanziyuan pegmatite. Monazite (from the MM and the K-, K-Na- and Na-zone pegmatites) and cheralite (from the Na-Li-zone pegmatite) have variable compositions and complex internal microtextures. The monazite and cheralite grains contain irregular areas with subtle heterogeneous BSE response along cracks and grain margins, suggesting that they have experienced alkali-bearing fluid-aided modification. However, these features are rarely seen in monazite from the K-zone pegmatite. Common Pb contamination and/or Pb loss during fluid-aided modification may have disturbed the monazite and cheralite U-Th-Pb isotopic system, due to the differential mobility of U, Th and Pb. The unaltered Na-zone monazite and Na-Li-zone cheralite yielded Th-Pb ages of 140.42 ± 2.30 Ma (2 σ, mean standard weighted deviation (MSWD) = 2.4, n = 14) and 139.58 ± 2.15 Ma (2 σ, MSWD = 2.9, n = 21), respectively. The unaltered MM, K-zone and K-Na-zone monazite yielded 206Pb-238U ages of 138.03 ± 2.18 (2 σ, MSWD = 2.5, n = 18), 140.39 ± 2.18 (2 σ, MSWD = 3.0, n = 20) and 140.58 ± 2.14 Ma (2 σ, MSWD = 2.0, n = 52), respectively. These ages for the four pegmatite zones are temporally consistent with a syngenetic origin for the magmatic sequence of biotite-, two-mica- and MM and the pegmatite system and rare-metal (Li-Nb-Ta-Rb-(Cs)-(Be)) mineralization. The Sm-Nd isotopic analyses of the unaltered monazite and cheralite from the MM and four pegmatite zones yield similar initial Nd isotopic composition with εNd(t) = −9.9 to −7.9, indicating an identical single-source region (i.e. the Neoproterozoic South China lower crust). The Sm/Nd ratios display a gradual decrease across the four pegmatite zones from the unaltered K-zone monazite to Na-Li-zone cheralite, i.e. 0.39–0.63 (avg. = 0.43) for K-zone, 0.29–0.35 (avg. = 0.31) for K-Na-zone, 0.26–0.30 (avg. = 0.28) for Na-zone and 0.21–0.27 (avg. = 0.24) for Na-Li-zone. Such progressive variations suggest their derivation from the same parental magma, which experienced varying degrees of fractionation before the extraction of pegmatitic melts. Comprehensive monazite and cheralite geochemistry, as well as in situ U-Th-Pb and Sm-Nd isotopic results indicate that Rayleigh-type fractional crystallization controls the mineralogical and geochemical evolution from a chemically zoned granite source.

MINERAL EXPLORATION IN THE VICINITY OF THE LATE EDIACARAN BOUTEGLIMTAPLITE AND PEGMATITE DIKES INIMITER INLIER USING ASTER DATA: PRELIMINARY RESULTS

In the southern part of the Imiter inlier, the early Ediacaran basement of the Saghro Group is crosscut by an important swarm of aplitic and pegmatitic dikes that display an extension of several meters and which can exceed 10 in thickness. They are distributed according to two main directions: NE-SW and NW-SE. Petrographic analysis reveals that the pegmatites and aplite primarily consist of quartz, plagioclase, K-feldspar, muscovite, biotite, allanite, and sphene. The BouTeglimt pegmatites, related to the large BouTeglimt granitic and granodioritic plutons, are exterior pegmatites type, hosted in the metamorphic country rocks. Pegmatite zoning is recognized at two scales: internal zoning, representing variations in mineralogy and texture within individual pegmatite bodies, and regional zoning, characterized by increased mineralogical complexity with distance from the granitic source. Using the ASTER sensor, the study investigated significant clayey, phyllitic, and propylitic alterations of the pegmatites, aplite, and their host rocks in the Imiter mine area. Field investigations and mineralogical characterization highlight four new mineralized zones in base and precious metals. These zones are commonly situated at the peripheries of granites or at the contact between the SG series and the late Ediacaran rocks of the Ouarzazate Group. The mineralization is observed in steeply-dipping veins along major E-W faults parallel to the Imiter fault, as well as in stockwork ore within tuffs, lapilli tuff, rhyolite, and disseminated mineralization within various Ouarzazate Group pyroclastic rocks. The ore mineralogy comprises sulfides and oxide minerals. Furthermore, the muscovite-rich BouTeglimt pegmatite and aplite can be considered an excellent exploration indicators due to their occurrence in fertile granites and rare-element pegmatites. They may hold potential as indicators of tantalum mineralization.

Mineral chemistry and genesis of monazite-(Sm) and monazite-(Nd) from the Blue Beryl Dyke of the Julianna pegmatite system at Piława Górna, Lower Silesia, Poland

AbstractMonazites are one of the most interesting groups of accessory mineral components of crystalline rocks due to the information on geochemical evolution of the crystallisation environment coded in their chemical compositions, in addition to comprising one of the most valuable objects for geochronology studies. This paper presents monazite-(Sm) and monazite-(Nd) from the Blue Beryl Dyke of the Julianna system of rare-element pegmatites at Piława Górna, Lower Silesia, Poland. These monazites are unique due to their unusually high Sm and Nd contents, reaching 33.22 wt.% Sm2O3 and 34.12 wt.% Nd2O3, respectively. We consider the most significant factors of the enrichment in Sm and Nd to be the occurrence of highly fractionated pegmatite-forming melts during the final stages of solidification and associated hydrothermal fluids that were strongly enriched in rare earth element REE–Cl and REE–F complexes. Local disequilibria allowed for the rapid growth of accessory phases under supercooling conditions associated with the scavenging of selected elements, leading to their local depletion, which was not balanced by diffusion processes. As a consequence, the depletion of light rare earth elements (LREE) led to the incorporation of available middle rare earth elements (MREE, Sm–Dy) in the case of Sm and Nd, which could occupy an acceptable structural position in minerals of the monazite group.

Quantitative evaluation of metamictisation of columbite-(Mn) from rare-element pegmatites using Raman spectroscopy

AbstractRaman spectroscopic analysis was performed on columbite-(Mn) samples from a variety of previously studied rare-element pegmatites in Xinjiang, China, including the Jing'erquan No. 1 spodumene-subtype, Dakalasu No. 1 beryl–columbite-subtype and Kalu'an spodumene-subtype pegmatites, to quantify the relationship between the degree of metamictisation of columbite and Raman spectra. For all of the analysed columbites-(Mn), the position (p) and the full width at half maximum (FWHM) of the strongest band, A1g vibration mode related to the Nb/Ta–O bond, in the Raman spectra have a negative correlation. Combined with previously determined U–Pb isotopic data and major–minor-element data for the columbites-(Mn), the degree of metamictisation was quantified using the alpha-decay dose (D) and displacement per atom (dpa), both of which were corrected for effects caused by annealing. The results demonstrate that the columbite-(Mn) from Jing'erquan and Kalu'an are very crystalline, whereas those from Dakalasu are transitional between crystalline and amorphous stages. The main factor influencing the key parameters, i.e. band position and FWHM, of the strongest Raman band of columbite-(Mn) is metamictisation caused by radiation damage, whereas composition and crystal orientation have limited influence. A set of equations are established to quantify the degree of metamictisation of columbite using the band position and the full width at half maximum: FWHM = 8.309 × ln(aD) + 30.11 (R2 = 0.9861); p = –5.187 × ln(aD) + 867.09 (R2 = 0.966); FWHM = 8.1453 × ln(adpa) + 48.425 (R2 = 0.9822); and p = –5.078 × ln(adpa) + 855.67 (R2 = 0.9594).

Lithium pegmatite of anatectic origin – A case study from the Austroalpine Unit Pegmatite Province (Eastern European Alps): Geological data and geochemical modeling

There is an ongoing debate as to whether rare element pegmatite is always related to fractional crystallization of huge fertile granite bodies or whether it can also form directly from limited portions of enriched anatectic melt. In this contribution, we present a case study from the Eastern European Alps, showing the continuous evolution from melt generation in staurolite bearing micaschist to spodumene (LiAlSi2O6) bearing pegmatite. The investigated Austroalpine Unit Pegmatite Province formed during Permian lithospheric extension and all levels of the Permian crust are now accessible in a Cretaceous nappe stack. This nappe stack is mapped in detail, subdivided lithostratigraphically and the internal tectonic structure is well understood. It is clear that the described pegmatites are neither spatially nor genetically related to large fertile granite bodies. A review of geochronological data proofs that emplacement of pegmatite and leucogranite is broadly contemporaneous with high temperature-low pressure metamorphism of the country rocks. Field observations give clear evidence of a genetic link between simple pegmatite, leucogranite, evolved pegmatite and albite-spodumene pegmatite, on the one hand, and the subsolidus or suprasolidus metasediment hosting them on the other hand. These relations are underpinned by geochemical major and trace element investigations on all types of rock and the minerals therein. As source rock an Al-rich metapelite enriched in Li (70–270 ppm) with respect to the average upper continental crust is identified. The main Li-carrier is staurolite with up to 3000 ppm Li. The pegmatite and leucogranite originate in anatectic melts that formed between 0.6 and 0.8 GPa and 650–750 °C, corresponding 18–26 km depth. During melt formation staurolite was consumed by sillimanite forming reactions. Subsequently, the melts were enriched in Li by fractional crystallization of quartz and feldspar during their ascent to higher crustal levels. While simple pegmatite and inhomogeneous leucogranite formed in lower and intermediate levels, evolved and albite-spodumene pegmatite crystallised at high levels at conditions between 0.3 and 0.4 GPa and 500–570 °C, corresponding to about 12 km depth. Based on these data we develop a geochemical model for showing that Li can be transferred from a metasediment into an anatectic melt if staurolite is stable or metastable at the onset of melting. Such a melt can contain between 200 and 1000 ppm Li and the melt can be further enriched with up to 5000 to 10000 ppm Li through fractional crystallization of quartz and feldspar, allowing crystallization of spodumene. Estimated fractionation degrees vary between 81 and 99 %, depending on the protolith composition, the melting scenario and the partitioning coefficients. Li-Al-rich metasediments are therefore a rich source of certain rare elements when they first melt. However, the spectrum of elements contained in the melt depends on a wide variety of conditions. This anatectic model provides an alternative explanation for the formation of Li-rich pegmatite if no fertile parent granite can be identified.

Geochronological constraints for a two-stage history of the Sveconorwegian rare-element pegmatite province formation

Most pegmatites of southern Norway seem to be derived from anatectic melting of metamorphic rocks during the Sveconorwegian orogeny rather than to be highly evolved residual melts derived from granites. We test this hypothesis by providing new age data for thirteen pegmatites and one granite. Based on these new age data, we distinguish two age groups of Sveconorwegian pegmatites (>1,000 m3 in size); Group 1: 1100–1030 Ma and Group 2: 930–890 Ma. All pegmatites except those from the Østfold area crystallized significantly earlier or later than adjacent granites. The Tørdal granite, yielding an age of 946 ± 4 Ma, is about 40 Ma older than the adjacent pegmatites. Field evidence and the age difference between pegmatites and granites supports an anatectic origin for these pegmatites. Sources of these pegmatite melts are biotite- and biotite-amphibole gneisses and amphibolites. Group 1 pegmatites formed in transpressional regimes after peak metamorphism, whereas Group 2 pegmatites formed in an extensional regime and the required heat for partial melting was provided by mafic magma underplating.Differences in the rheological behavior of amphibolite and granitic gneiss during extensional tectonics are the major reason why Group 2 pegmatites occur preferentially in large amphibolite bodies. Under mid-crustal conditions, amphibolite reacts brittle to semi-brittle forming open structures in an extensional tectonic regime where partial melts drained into. Granitic gneisses react in a ductile manner and do not have the ability to drain partial melt.Pegmatite formation in the Grenville Province, i.e., the Laurentian part of the Grenville–Sveconorwegian orogenic belt, formed between ca. 1090 and 980 Ma peaking at 1010 to 980 Ma. Thus, the Grenville peak postdates the Sveconorwegian Group 1 peak by about 30 Ma. These pegmatites formed in similar orogenic settings, implying that similar tectono-metamorphic developments along the Grenville–Sveconorwegian orogenic belt were diachronous.We conclude that local anatexis is the major pegmatite-melt forming process in the Sveconorwegian as well as Grenville orogen. Local anatexis also may be important in other pegmatite provinces.

An evolutionary system of mineralogy, Part VII: The evolution of the igneous minerals (>2500 Ma)

AbstractPart VII of the evolutionary system of mineralogy catalogs, analyzes, and visualizes relationships among 919 natural kinds of primary igneous minerals, corresponding to 1665 mineral species approved by the International Mineralogical Association—minerals that are associated with the wide range of igneous rock types through 4.566 billion years of Earth history. A systematic survey of the mineral modes of 1850 varied igneous rocks from around the world reveals that 115 of these mineral kinds are frequent major and/or accessory phases. Of these most common primary igneous minerals, 69 are silicates, 19 are oxides, 13 are carbonates, and 6 are sulfides. Collectively, these 115 minerals incorporate at least 33 different essential chemical elements.Patterns of coexistence among these minerals, revealed by network, Louvain community detection, and agglomerative hierarchical clustering analyses, point to four major communities of igneous primary phases, corresponding in large part to different compositional regimes: (1) silica-saturated, quartz- and/or alkali feldspar-dominant rocks, including rare-element granite pegmatites; (2) mafic/ultramafic rock series with major calcic plagioclase and/or mafic minerals; (3) silica-undersaturated rocks with major feldspathoids and/or analcime, including agpaitic rocks and their distinctive rare-element pegmatites; and (4) carbonatites and related carbonate-bearing rocks.Igneous rocks display characteristics of an evolving chemical system, with significant increases in their minerals’ diversity and chemical complexity over the first two billion years of Earth history. Earth’s earliest igneous rocks (>4.56 Ga) were ultramafic in composition with 122 different minerals, followed closely by mafic rocks that were generated in large measure by decompression melting of those ultramafic lithologies (4.56 Ga). Quartz-normative granitic rocks and their extrusive equivalents (>4.4 Ga), formed primarily by partial melting of wet basalt, were added to the mineral inventory, which reached 246 different mineral kinds. Subsequently, four groups of igneous rocks with diagnostic concentrations of rare element minerals—layered igneous intrusions, complex granite pegmatites, alkaline igneous complexes, and carbonatites—all first appeared ~3 billion years ago. These more recent varied kinds of igneous rocks hold more than 700 different minerals, 500 of which are unique to these lithologies.Network representations and heatmaps of primary igneous minerals illustrate Bowen’s reaction series of igneous mineral evolution, as well as his concepts of mineral associations and antipathies. Furthermore, phase relationships and reaction series associated with the minerals of a dozen major elements (H, Na, K, Mg, Ca, Fe, Al, Si, Ti, C, O, and S), as well as minor elements (notably Li, Be, Sr, Ba, Mn, B, Cr, Y, REE, Ti, Zr, Nb, Ta, P, and F), are embedded in these multi-dimensional visualizations.

The Li-Bearing Pegmatites from the Pampean Pegmatite Province, Argentina: Metallogenesis and Resources

The Li-bearing pegmatites of the Pampean Pegmatite Province (PPP) occur in a rare-element pegmatite belt developed mainly in the Lower Paleozoic age on the southwestern margin of Gondwana. The pegmatites show Li, Rb, Nb ≤ Ta, Be, P, B, Bi enrichment, and belong to the Li-Cs-Ta (LCT) petrogenetic family, Rare-Element-Li (REL-Li) subclass; most of them are of complex type and spodumene subtype, some are of albite-spodumene type, and a few of petalite subtype. The origin of the pegmatites is attributed predominantly to fractionation of fertile S-type granitic melts produced by either fluid-absent or fluid-assisted anatexis of a thick pile of Gondwana-derived turbiditic sediments. Most of the pegmatites are orogenic (530–440 Ma) and developed during two overlapped collisional orogenies (Pampean and Famatinian); a few are postorogenic (~370 Ma), related to crustal contaminated A-type granites. The pegmatites were likely intruded in the hinterland, preferably in medium-grade metamorphic rocks with PT conditions ~200–500 MPa and 400–650 °C, where they are concentrated in districts and groups. Known combined resources add up 200,000 t of spodumene, with variable grades between 5 and 8 wt.% Li2O. The potential for future findings and enlargement of the resources is high, since no systematic exploration program has yet been developed.

Geochronology of the Chakabeishan Li–(Be) rare-element pegmatite, Zongwulong orogenic belt, northwest China: Constraints from columbite–tantalite U–Pb and muscovite–lepidolite 40Ar/39Ar dating

Lithium–(beryllium) are important strategic metals in modern industries. The Markam–Yajiang–Karakoram (MYK) giant Li ore belt (>14 Mt Li2O) is located in the central Qinghai–Tibetan Plateau, and is a world-class Li deposit belt. The reported pegmatite deposits in the belt are mainly found in the Tianshuihai–Songpan–Garzê Terrane (TSGT) and are generally considered to be related to Late Triassic S-type granites and flysch sedimentary rocks. Recently, abundant pegmatite veins have also been discovered in an adjacent terrane (i.e., the North Qaidam Terrane) to the north of the MYK belt, but it is unclear whether these newly discovered pegmatites are genetic related to the MYK belt. Here, we report high-precision columbite–tantalite (i.e., coltan) U–Pb ages and muscovite–lepidolite 40Ar/39Ar ages for the Li–(Be) rare-element pegmatites from the North Qaidam Terrane (NQT) in the Chakabeishan area of the Zongwulong orogenic belt. Coltan grains from three spodumene-bearing pegmatite samples yielded weighted-mean 207Pb-corrected U–Pb ages of 214.9 ± 1.7, 217.0 ± 2.3, and 215.0 ± 1.5 Ma. Muscovite 40Ar/39Ar dating of two spodumene-bearing pegmatite samples and one barren pegmatite sample yielded plateau ages of 213.00 ± 0.97, 211.67 ± 0.35, and 211.78 ± 0.29 Ma, respectively. A lepidolite 40Ar/39Ar age for a lepidolite-bearing pegmatite is 216.62 ± 0.88 Ma. The subtle age differences obtained by the different methods are within reasonable error estimates or may be due to different closure temperatures. The ore-forming ages of the Chakabeishan Li–(Be) rare-element pegmatites are similar to those of Li–(Be) pegmatite deposits in the MYK belt. This and data from previous studies suggest that the Chakabeishan Li–(Be) rare-element pegmatites were also formed in a post-collisional setting after the closure of the Paleo-Tethys (Anyemaqen) Ocean. This study has important implications for understanding the origins of the MYK Li ore belt, indicating that the formation of these Li deposits was not solely associated with the Late Triassic S-type granites and flysch sedimentary rocks. Our results may also have implications for exploration, which extends the potential area of Li deposits from the Tianshuihai–Songpan–Garzê Terrane to the North Qaidam Terrane.

Study on the Controlling Factors of Li-Bearing Pegmatite Intrusions for Mineral Exploration, Uljin, South Korea

Recently, the demand for lithium (Li) as an energy storage element has increased, owing to the rapid increase in the number of electric vehicles. To meet this demand, Li exploration has become increasingly important. The Boam deposit is located in the Uljin area of eastern South Korea, where several rare-element pegmatites (0.24% Li) intrude the Precambrian Janggun Limestone Formation. In this study, we performed petrographical and geometrical analyses of the rare-element pegmatites recognized in the vicinity of the Boam deposit, through which the Li-mineralization process was identified and the factors controlling intrusion studied. Our results are summarized as follows: (1) the pegmatites exhibit regional and internal zoning based on their mineral assemblages; (2) Li mineralization is restricted to pegmatites; (3) the geometry and distribution of the pegmatites are strongly controlled by fracture, bed contact, and post-intrusive deformation; and (4) exploration should be concentrated in the ENE–WSW-trending zone of the upper part of the Janggun Limestone Formation. These results provide valuable understanding to guide the development of strategies for early-stage mineral exploration in the Uljin area.

Geological, Mineralogical and Geochemical Study of the Aquamarine-Bearing Yamrang Pegmatite, Eastern Nepal with Implications for Exploration Targeting

The Yamrang Pegmatite in the Ikhabu Pegmatite Field, Eastern Nepal is Nepal’s primary source of aquamarine. This paper reports detailed mineralogy and whole rock granite and pegmatite geochemistry, and major and trace element data for alkali feldspar and muscovite in order to classify the aquamarine-bearing Yamrang Pegmatite, elucidate beryl-saturation processes and evaluate potential geochemical exploration tools for beryl-pegmatites. Five internal mineralogical/textural zones were identified in the Yamrang Pegmatite; zone 1 (saccharoidal albite); zone 2 (blocky perthitic microcline); zone 3 (muscovite–microcline–quartz); zone 4 (beryl-quartz), and zone 5 (miarolitic cavities). Zones 1–4 represent the magmatic stage, while zone 5 formed during the hydrothermal stage of pegmatite genesis. Spectacular aquamarines are recovered from miarolitic zone 5, while beryl saturation is found in zones 3, 4, and 5. Based on beryllium (Be) content, Be partition among co-existing minerals at the magmatic stage is beryl > muscovite > tourmaline > alkali feldspar > quartz. In contrast, the sequence at the hydrothermal stage is beryl > muscovite > albite > tourmaline > quartz. The Be content in rock-forming minerals decreases from pegmatite margin to core, and tourmaline could have played a significant role in Be enrichment processes in the marginal pegmatite zone. High temperature, a low degree of fractionation, and the dominance of Be-compatible mineral phases such as muscovite, calcium-rich alkali feldspar and tourmaline resulted in beryl undersaturation in marginal zones. However, low temperature, high fractional crystallization, and low abundance of Be-compatible mineral phases resulted in beryl saturation in inner zones. The strongly peraluminous nature, low total REE content (<500 ppm), mineral assemblage of beryl, tourmaline, spessartine, columbite-tantalite, depletion of Ba, Nb, and enrichment of Pb, Rb, Cs in the primitive mantle normalized multi-element plots suggest that the beryl-bearing Yamrang Pegmatite corresponds to the LCT pegmatite family. Alkali feldspar with K/Rb values of 30–150, Rb ~3000 ppm, Cs >100 ppm, and muscovite, with K/Rb ranging 18–50, Rb ~6000 ppm, Cs > 500 ppm, and Ta > 65 ppm in inner zones (3–5), indicate that the Yamrang Pegmatite is an intermediate-fractionated, beryl-type rare-element (REL) pegmatite. It is probable that whole rock Be content of >10 ppm could be considered an exploration guide to beryl mineralization in the region.

A Combined EMPA and LA-ICP-MS Study of Muscovite from Pegmatites in the Chinese Altai, NW China: Implications for Tracing Rare-Element Mineralization Type and Ore-Forming Process

The mineralogical studies of rare-element (REL) pegmatites are important for unraveling the ore-forming process and evaluating REL mineralization potential. The Chinese Altai orogenic belt hosting more than 100,000 pegmatite dykes is famous for rare-metal resources worldwide and diverse REL mineralization types. In this paper, we present the results of EMPA and LA-ICP-MS for muscovite from the typical REL pegmatite dykes of the Chinese Altai. The studied pegmatites are Li-Be-Nb-Ta, Li-Nb-Ta, Nb-Ta, Be-Nb-Ta, Be and barren pegmatites. The Li+ accompanied with Fe, Mg and Mn substitute for Al3+ at the octahedral site in muscovite from the REL pegmatites, and the substitution of Rb by Cs at the interlayer space is identified in muscovite from the Be pegmatites. The P and B contents increase with evolution degree and the lenses from the Nb-Ta pegmatite are produced at late fluid-rich stage with high fluxes (P and B). The enrichment of HFSE in muscovite indicates a Nb-Ta-Sn-W rich pegmatite magma for the Be-Nb-Ta pegmatite. From barren pegmatite, beryl-bearing zone, to spodumene-bearing zone, the evolution degrees of pegmatite-forming magmas progressively increase. In the Chinese Altai, the possible indicators of muscovite for REL mineralization types include Rb (ca. 400–600 ppm, barren pegmatite; ca. 1200–4000 ppm, Be pegmatite; >4500 ppm, Li pegmatite), Cs (ca. 5–50 ppm, barren pegmatite; ca. 100–500 ppm, Be pegmatite; >300 ppm, Li pegmatite) and Ge (<3 ppm, barren pegmatite; ca. 4–6 ppm, Be pegmatite; ca. 6–12 ppm, Li pegmatite) coupled with Ta, Be (both <10 ppm, barren pegmatite) and FeO (ca. 3–4 wt%, Be pegmatite; ca. 1–2.5 wt%, Li pegmatite). The plots of Nb/Ta vs. Cs and K/Rb vs. Ge are proposed to discriminate barren, Be- and Nb-Ta-(Li-Be-Rb-Cs) pegmatites. The Li, Be, Rb, Cs and F concentrations of forming liquid are evaluated based on the trace element compositions of muscovite. The high Rb and Cs contents of liquid and lower Be contents than beryl saturation value indicate that both highly evolved pegmatite magma and low temperature at emplacement contribute to beryl formation. The liquids saturated with spodumene have large variations of Li, possibly related to metastable state at Li unsaturation–supersaturation or heterogeneous distribution of lithium in the system.

Two-stage regional rare-element pegmatite formation at Tysfjord, Norway: implications for the timing of late Svecofennian and late Caledonian high-temperature events

Pegmatite fields within granite plutons are commonly considered to have formed from residual melts of their host. This is not always true as demonstrated by the Tysfjord granite gneiss and its two groups of pegmatites. The Tysfjord granite gneiss, exposed in a tectonic window of the Caledonides of northern Norway, is part of the transscandinavian igneous belt (TIB) that includes several phases of granitic magmatism. In the northern Hamarøy area (Drag-Finnøy), where most rare-element pegmatites occur, Paleoproterozoic and metamorphosed Group 1 allanite–(Ce)–fluorite metapegmatites have similar bulk rock chemical composition as the TIB granite gneiss rocks, indicating that these pegmatites are residual melts. Group 1 metapegmatites, which are up to 400 m in size, are among the largest known intra-plutonic pegmatites with Nb–Y–F (NYF) signature. The formation of these unusually large granite-hosted NYF pegmatites may have been facilitated by the overall high F content of TIB granite gneisses. Undeformed Group 2 amazonite–tourmaline pegmatites yield columbite and zircon U–Pb ages in the range 400–379 Ma. These pegmatites are interpreted to be anatectic melts that formed from the partial melting of Tysfjord granite gneiss. Group 2 pegmatites, including those from Træna Island and the Sjona tectonic window (400 and 414 Ma), formed during late Caledonian ductile shearing and incipient unroofing of the central Scandinavian Caledonides and record progressively younger ages of this event from SW to NE.