Pegmatite Field Research Articles

Genesis of LCT Pegmatites during Early Paleozoic Orogeny of the North Qinling Orogenic Belt, China: Emplacement Conditions and Structural Control

AbstractThe Guanpo pegmatite field in the North Qinling orogenic belt (NQB), China, hosts the most abundant LCT pegmatites. However, their emplacement conditions and structural control remain unexplored. In this contribution, we investigated it combining pegmatite orientation measurement with oxygen isotope geothermometry and fluid inclusion study. The orientations of type A1 pegmatites (Pf < σ2) are predominantly influenced by P‐ and T‐fractures due to simple shearing in Shiziping dextral thrust shear zone during D2 deformation, whereas type A2 pegmatites (contemporaneous with D4) are governed by hydraulic fractures aligned with S0 and S0+1 stemming from fluid pressure (Pf < σ2). Additionally, type B pegmatites (Pf ≤ σ2) exhibit orientations shaped by en echelon extensional fractures in local ductile shear zones (contemporaneous with D3). The albite‐quartz oxygen isotope geothermometry and microthermometric analysis of fluid inclusions in elbaites from the latest pegmatites (including types B and A2) suggest that the crystallization P‐T for late magmatic and hydrothermal stages are 527.5–559.2°C, 320°C, 3.1–3.6 kbar and 2.0 kbar, respectively. Our observations along with previous studies suggest that the genesis of the LCT pegmatites was a long‐term, multi‐stage event during early Paleozoic orogeny (including the collision stage) of the NQB, and was facilitated by various local fractures.

Crystallization Sequence of the Spodumene-Rich Alijó Pegmatite (Northern Portugal) and Related Metasomatism on Its Host Rock

The Barroso–Alvão region is an excellent setting for studying Li mineralization associated with granitic pegmatites and developing Li exploration techniques. Among the distinguished pegmatite types in this pegmatite field, the spodumene-bearing dyke from Alijó is a representative example of an Iberian Li–Cs–Ta (LCT) pegmatite currently under exploitation. In this work, we examine the internal evolution of the Alijó dyke and its external metasomatic effect on the surrounding metasediments, contributing to lithium exploration techniques. Electron microprobe analyses provided clues about the crystallization conditions and the degree of differentiation of the pegmatitic melt, whereas the external metasomatism induced by the spodumene-bearing pegmatite was studied through whole-rock geochemistry. The obtained results indicate that the primary crystallization of the studied dyke likely occurred at temperatures between 450–500 °C, with emplacement at shallow crustal levels of about 2–3 kbar. The high concentrations of trace elements such as Li, Cs, Rb, Be, Sn, Nb, Ta, Ge, U, and Tl in the pegmatitic melt suggests high availability of these elements, allowing their partitioning into an early exsolved fluid phase. The exsolution of this fluid phase, subtracting components such as F and B, from the pegmatitic melt would cause a significant undercooling of the melt. Moreover, the interaction of this expelled fluid with the country rock generated a metasomatic overprint in the surrounding metasedimentary host rocks. The metasomatic effect in Alijó is strongly influenced by the nature of the host metasediments, with a significantly higher grade of metasomatism observed in pelitic (mica-rich) samples compared to psammitic (mica-poor) samples collected at same distances from the dyke. The greisen developed close to the pegmatite contact reflects this metasomatic signature, characterized by the mobilization of at least B, F, Li, Rb, Cs, Sn, Be, Nb, Ta, and Tl. We cautiously suggest that whole rock Li concentrations greater than 300 ppm, combined with a minimum value of 1000 ppm for the sum of B, F, Li, Rb, Cs, and Sn in pelitic metasediments of Barroso–Alvão, may be indicative of a mineralized pegmatite in this region.

Neoarchaean and Palaeoproterozoic tectono-metamorphic events along the southern margin of the Zimbabwe craton: Insights from muscovite 40Ar/39Ar geochronology from rare-metal pegmatites, Zimbabwe

Muscovite 40Ar/39Ar geochronology on ten samples from complex-type, rare-metal (e.g., Li, Cs, Ta, Rb, Be, Nb, Sn and W) pegmatites from the Bikita and Mweza pegmatite fields in Zimbabwe yield ages that range from ca. 2622–1940 Ma. One pegmatite from the Bikita field yield a relict age of ca. 2622 Ma which is similar to the ca. 2.62 Ga emplacement age of the Main Bikita Pegmatite previously determined by laser ablation analyses of Ta–Nb–Sn oxides. Younger muscovite 40Ar/39Ar ages of ca. 2580–2539 Ma in the Bikita field may reflect post-pegmatite emplacement tectono-thermal events under fluid-mediated conditions. Data from the Mweza pegmatite district yield dates of ca. 2027–1940 Ma which reflect a Palaeoproterozoic overprint. Palaeoproterozoic 40Ar/39Ar ages are spatially restricted to near the Northern Marginal Zone–Zimbabwe Craton boundary and are absent towards the interior of the craton in the Bikita pegmatite field.

Analiza strukturalna i zróżnicowanie pegmatytów w starożytnym twierdzeniu, Giraúl IV, dystrykt pegmatytów Namibe, Angola

Since the early 60´s, in the past XX century, the Giraúl pegmatites have been known for their resources of beryl, mica and feldspars, which were exploited in a regular basis from Giraúl claims I to IV till 1974, during the Portuguese administration of Angolan territory. A broader exploration of this pegmatite field was performed by the ancient Lobito Mining Company (LMC) engaged in detailed geological mapping of the granitic pegmatites and the structural constraints of their location. A structural map of the region was than elaborated, combining the interpretation of aerial photographs with field work performed by the LMC geologists. Recently, a growing economic interest is attributed to these claims, in the region of Bulamucolocai, Pitau and Muvero desert-dry rivers (locally known as “Mulolas”), considering the Li, Cs, and Ta (LCT) metallic specialization of some pegmatite bodies and the occurrence of beryl and tourmaline gemstones, mainly, morganite (Cs-beryl), aquamarine and also elbaite-liddicoatite. Giant crystals of spodumene, up to 6 m in length, define individualized quartz + spodumene units inside some of the more typical LCT pegmatite bodies. Pollucite was identified in the main pegmatites of Giraúl IV claim and not in the adjacent igneous leucocratic breccias. These, in turn, correspond to a complex pegmatite assemblage, very peculiar in what concerns its selective metasomatic effect over some surrounding rocks, with the formation of rims of holmquistite amphibole in contact with gabbro and norite and schorl-dravite tourmaline in contact with gneissic to metapelitic hosts. The breccia-like granitic rock combines clasts of spodumene an K-feldspar with a matrix mainly composed of some quartz, albite and mica including tourmaline, garnet and F-apatite, as accessory minerals. In the same area, huge potassic pegmatites hold giant crystals of microcline and orthoclase and very little quartz, being unusual due to its high content of triplite – zwieselite and triphylite – lithiophilite primary phosphates. The overall composition of these pegmatites is more likely syenitic (low quartz content) than truly granitic. A high-resolution structural analysis of the LCT ensemble (pegmatite plus related lithotypes) is now proposed enhancing the unusual relations between granite breccia plugs, sill-like more typical pegmatites, irregular shaped isodiametric bodies and products of metasomatism. This approach will lead to the understanding of the true dimension, anatomy and inner fraccionation of the different LCT facies and rare-metal deposits with obvious consequences regarding mineral detection and resource – reserve estimation, through the proposal of a more suitable conceptual model to rule its exploration.

Cassiterite trace element discrimination diagrams to facilitate critical mineral exploration

Cassiterite is a weathering-resistant mineral, which can incorporate a variety of trace elements. Trace elements in cassiterite samples collected from twelve deposits in the Herberton Mineral Field, Australia, were measured with the use of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The results were combined with published data from other tin fields, including the Andean Sn belt in South America; the Karagwe Ankole belt in Rwanda; and, from China, the Kangxiwa-Dahongliutan pegmatite field, the Youjiang basin, the Nanling belt and the Da Hinggan Range belt. Tin deposits in the dataset can be subdivided into four deposit types: 1) greisen and veins; 2) skarns; 3) Li-Cs-Ta pegmatites; and 4) polymetallic veins. The cassiterite dataset was analyzed using basic descriptive statistics, principal component analysis (PCA), and cluster analysis. Cassiterite grains from greisen and vein deposits are characterized by high concentrations of Ti (avg. 1751 ppm) and moderate concentrations of Al (avg. 97 ppm), whereas cassiterite grains from skarn deposits generally contain lower concentrations of Ti and Al. Chemical compositional boundaries in cassiterite from different deposits were recognized with cluster analysis. The relative enrichment of Al and Ti in cassiterite grains from greisen and vein deposits is likely due to greisenization reactions. The Ti vs. Al diagram can be used to differentiate between cassiterite grains derived from greisen and vein deposits, as compared to cassiterite grains derived from skarn deposits, whereas Sb vs. V diagram can be used to differentiate between cassiterite grains from polymetallic vein deposits. Zirconium and Nb concentrations are useful in identifying cassiterite grains sourced from LCT pegmatite deposits. The discrimination diagrams developed in this study through cluster analysis indicate that cassiterite grains sourced from different deposit types can be differentiated based on their trace element geochemistry and this can be a useful tool in critical mineral exploration. Therefore, these diagrams can be used effectively to understand metal association and deposit types in a region with detrital cassiterite from stream sediments, till and heavy mineral placer deposits.

Comment on “The Volyn biota (Ukraine) – indications of 1.5 Gyr old eukaryotes in 3D preservation, a spotlight on the `boring billion' ” by Franz et al. (2023)

Abstract. Franz et al. (2023) report a diverse and three-dimensionally preserved suite of mid-Proterozoic microfossils from miarolitic cavities within the granitic Volyn pegmatite field, a major granitic plutonic complex in NW Ukraine. The biota is dated at between ∼ 1.76 and ∼ 1.5 Ga and includes fungus-like objects. This biota is reported as evidence of organisms living within the continental lithosphere, illuminating part of a ∼ 1.8–0.8-billion-year interval of the Proterozoic Eon characterised by relatively low climatic variability and slow biological evolution. We show that at least some of this putative diversity represents modern contamination including plant hairs, a distinctive pollen grain assignable to the extant conifer genus Pinus, and likely later fungal growth. Comparable diversity is shown to exist in modern museum dust, presented as an example of potential airborne contamination and calling into question whether any part of the Volyn “biota” is biological in origin. We emphasise the need for scrupulous care in collecting, analysing, and identifying Precambrian microfossils.

Characteristics and formation of rare-metal pegmatites and granites in the Duanfengshan-Guanyuan district of the northern Mufushan granite complex in South China

The Mufushan area, which has abundant rare-metal pegmatites within and around the Mufushan Granite Complex, has become a major target for Ta-Nb-(Li-Be) exploration in South China. The age and origin of the pegmatites and associated rare-metal mineralization are still under debate. Here, we report the in situ U-Pb ages and geochemical characteristics of granites and pegmatites in the Guanyuan and Duanfengshan districts, which are located in the central and northern parts of the Mufushan Complex. Combined zircon, apatite and monazite U-(Th)-Pb dating revealed that biotite, two-mica, and muscovite granites from the Guanyuan and Duanfengshan districts were emplaced at 143–139 Ma, which overlaps with the U-Pb ages of columbite-group minerals (CGM) from different internal zones of the Duanfengshan pegmatites (142–140 Ma). Whole-rock major and trace element compositions and Sr-Nd-Hf isotope data reveal that the granites and pegmatites experienced continuous evolution from biotite, two-mica, and muscovite granites to pegmatite and that the magma originated from the partial melting of mica schists that are abundant in the Mufushan area. Temporal, chemical and mineralogical evidence indicates a genetic link between muscovite granite and Ta-Nb pegmatites. The textures and chemical compositions of CGM from different pegmatites exhibit features typical of magmatic CGM, indicating that fractional crystallization was the driving force that promoted Ta-Nb enrichment. The increasing alumina saturation index [ASI: molar Al/(Ca–1.67P + Na + K)] of pegmatitic melt due to albite crystallization may have been the main factor controlling CGM deposition, explaining why major Ta-Nb mineralization is bound to albite pegmatites. The Duanfengshan and other rare-metal pegmatites in the Mufushan area are derivatives of the most evolved granitic facies (i.e., muscovite granite) of the Mufushan Complex. The Duanfengshan and Renli pegmatite fields indicate that the Early Cretaceous (∼140 Ma) may have been an important, underappreciated epoch for the formation of pegmatite-related rare-metal resources in the Mufushan area and beyond in South China.

Testing the reproducibility of in situ Lu[sbnd]Hf dating using Lu-rich garnet from the Tørdal pegmatites, southern Norway

In situ LuHf geochronology offers the potential for direct dating of garnet within petrographic context. However, the method requires matrix-matched standards to calibrate measured Lu/Hf ratios. In order to assess the accuracy of this calibration, as well as the reproducibility of the resulting LuHf dates, two Lu-rich (up to 1 wt% Lu) garnet samples from the Tørdal pegmatite field in southern Norway were analysed over six analytical sessions. Multi-session isochron LuHf dates of 930.3 ± 1.4 Ma (MSWD = 1.0, 164 analyses) and 930.3 ± 1.7 Ma (MSWD = 1.5; 125 analyses) were obtained, conform with previously published age constraints. The relative standard deviation of 0.1% between the analytical sessions indicates that the dates are highly reproducible. For each individual analytical session, age uncertainties of 0.3–0.6% were achieved by measuring high Lu count rates in analog detection mode. We further describe a calibration strategy that deals with LuHf datasets where Lu is measured in both pulse and analog detector mode. Given the high analytical precision and reproducibility, we suggest that the Tørdal garnets are suitable reference materials for future LA-ICP-MS/MS LuHf studies.

The Borborema pegmatite Province, NE-Brazil revisited

The Borborema pegmatite Province, NE-Brazil revisited

Zimbabwe's coloured gemstone endowments - A regional geological overview

Zimbabwe hosts a varied array of coloured gemstones. With the exception of emerald deposits and several world-class pegmatites, few of the gemstone occurrences have received detailed attention from the scientific or mineral exploration communities. In the present contribution we summarize the status of knowledge of the gemstone deposits and occurrences in Zimbabwe, paying particular attention to the geological settings in which they were formed. Synthesis of this regional geological approach reveals that there may be significant exploration potential for further gemstone occurrences, particularly in the extensive pegmatite fields and in Al-enriched orogenic belts that have undergone greenschist to granulite facies metamorphism. Further socio-economic and developmental initiatives will aid in optimizing the value generation from this important sub-sector.

Petrogenesis and tectonic-magmatic context of emplacement of lepidolite and petalite pegmatites from the Fregeneda-Almendra field (Variscan Central Iberian Zone): clues from Nb-Ta-Sn oxide U-Pb geochronology and mineral geochemistry

The Fregeneda-Almendra pegmatite field of the Iberian Massif represents a typical expression of peraluminous rare-metal magmatism that occurred over western Europe at the end of the Variscan orogeny. It is the host for two main types of Li-mineralized intrusions, identified at the scale of the Variscan belt, including petalite- or spodumene-rich pegmatites, as well as Li-mica-rich pegmatites, for which the origin of mineralogical-chemical differences is not yet understood. Here, we provide cassiterite and columbite-group mineral (CGM) U-Pb ages along with oxide, mica and phosphate mineral compositions for Li-pegmatites from the Fregeneda-Almendra field in order to assess their petrogenesis and tectonic-magmatic context of emplacement. U-Pb geochronology indicates that petalite-rich and Li-mica-rich pegmatites were mostly emplaced sub-synchronously from 315 ± 6 to 308 ± 6 Ma, during strike-slip deformation and granitic magmatism within an anatectic dome bounding the pegmatite field. U-Pb data and pegmatite geographic zonation suggest that Li-pegmatites were sourced from buried equivalents of leucogranites and migmatites from the dome. Li-pegmatites experienced a complex crystallization including K-feldspar, petalite, topaz, Nb-Ta-Fe-Mn-rich cassiterite, amblygonite-group minerals (AGM) and CGM as early magmatic phases, followed by lepidolite for Li-mica-rich pegmatites. At the magmatic-hydrothermal transition, notably leading to the formation of Nb-Ta-Mn-Fe-poor cassiterite hosting CGM inclusions, earlier minerals were resorbed by muscovite and albite. A later F-rich hydrothermalism is locally reflected by zinnwaldite overgrowths on muscovite. Cassiterite, CGM and micas from petalite-rich pegmatites show lower Mn/Fe ratios and higher Ti contents, along with lower Zr-Ga contents for cassiterite, than that from Li-mica-rich pegmatites. Such behavior is consistent with a magmatic differentiation process whereby Ti content decreased and the degree of Mn-Fe geochemical fractionation and solubilities of Ga and Zr increased in the melts, possibly in relation with high fluorine activity. In Li-mica-rich pegmatites, AGM equilibrated with a melt with up to 2 wt% F, similar to that in equilibrium with lepidolite (1–3 wt%). In petalite-rich pegmatites, the relatively high F concentration of the melts equilibrated with AGM (≤ 1.5 wt% F) contrasts with the liquid equilibrated with muscovite (< 0.5 wt% F). This can be accounted for by muscovite crystallization after the exsolution of a F-rich aqueous phase at the magmatic-hydrothermal transition. Relatively similar F contents in the initial melts of petalite- and Li-mica-rich pegmatites support the hypothesis that the stability of lepidolite does not only involve high F but also a low H2O/F activity ratio. For the Fregeneda-Almendra Li-mica-rich pegmatites, this could be explained by a decrease of melt H2O solubility due to a relatively low pressure of emplacement.

U–Pb Dating, Gemology, and Chemical Composition of Apatite from Dara-e-Pech, Afghanistan

Minerals of the apatite group commonly occur in granite pegmatites, and their ability to incorporate a wide range of trace elements makes them a good indicator of magma composition and magmatic–hydrothermal processes. Gem-quality purple apatite crystals from the Dara-e-Pech pegmatite field in Afghanistan have rarely been reported. Here, we investigated apatite crystals originated from this locality, using gemological testing, chemical analysis, and in situ U–Pb dating, with the purpose of identifying their origin, the constraints on the magma source in which the apatite crystals were formed, and the timing of the magmatic–hydrothermal activity. Our findings demonstrate that the purple apatite crystals were impure fluorapatite, characterized by heavy rare-earth element (HREE) enrichment, intermediate Eu anomalies, and non-CHARAC Y/Ho ratios. The results showed that these apatite crystals yielded a lower intercept age of 135.8 ± 6.9 Ma. We proposed that the pegmatitic apatite samples formed in a transitional magmatic–hydrothermal pegmatitic system with moderate fO2 in the Early Cretaceous (~135 Ma). Our study helps to constrain the magmatic–hydrothermal activities of the little-known Dara-e-Pech pegmatite field.

Structural Controls on the Origin and Emplacement of Lithium-Bearing Pegmatites

Abstract Exploration and mining of lithium pegmatites has increased in recent years to meet the growing demand for critical metals, which are required for electric-powered transportation as well as sustainable energy production and storage. Pegmatite deposits produced approximately 60% of global lithium production in 2018, the vast majority of which was from spodumene pegmatites. However, there is lack of comprehensive models linking mineral system evolution and structural controls of lithium pegmatites. The majority of studies on lithium pegmatites have focused on chemical fractionation and assume that pegmatites have evolved from a parental granitic source, yet problems emerge when the connection between the granitic source and the pegmatite bodies is not obvious. This lack of connection has given rise to an alternative model of pegmatite formation, the anatectic model. In the anatectic model, granitic pegmatites form from granitic melts produced by partial melting of a suitable lithology, typically pelitic metasediments, without forming a large granitic body followed by fractional crystallization. Here, we first examine the formation of granitic melts related to anatexis, as well as how regional crustal structures influence chemical composition, crustal migration, and melt accumulation within the crust. We next examine the Wekusko Lake pegmatite field located in Manitoba, Canada, in terms of the probable melt source migration, the relationship between crustal-scale strain-zones and spatial pegmatite emplacement, and the chemical fractionation trend that records the formation of lithium-enriched pegmatites.

Pegmatite fields of São Pedro de Ferros, Antônio Dias and Marilac, Minas Gerais State, Brazil: petrographic characterization

Pegmatites are rocks of granite or metastatic origin with gigantic crystals that are formed in fluid phase of the magmatic liquid enriched with incompatible elements, thats why they can be considered as gemstones and industrial minerals deposits. Both studied areas in the present article were genererated by anatexia process or partial fusion. The Jatobá Mine in the Pegmatitic Field of Ferros-Antônio Dias, in Santa Maria de Itabira, explores the aquamarine gem, a beryl variety, in a pegmatite that is located closer to the granite system of the quartz core and is docked in a gnaisse granitic rock from the Guanhães Complex. The collected sample was denominated like Quartzo alkali feldspar sienite in the Diagram QAPF. The blue color of the beryl is given by the intrusion of Fe. The Duas Cores Mining represents the Pegmatitic Field of Marilac, near Governador Valadares, and the exploration of bi-color tourmaline will begin soon. This mineral is in a very weathered pegmatite, positioned more at the end of the granite system and docked in the mica-schist rock of the São Tomé Formation. The sample was named Tonalite by the QAPF Diagram. The color variation of the explored gem is related to the enrichment of the Li element at the site.

Magmatic-hydrothermal evolution of the aquamarine-bearing Yamrang Pegmatite, Eastern Nepal: Insights from beryl, garnet, and tourmaline mineral chemistry

The Ikhabu Pegmatite Field (IPF) in the Nepal Himalaya is a newly discovered rare-element (REL) pegmatite field that hosts the Yamrang pegmatite, the primary source of the famous Nepalese aquamarines. In this contribution, we provide a comprehensive description of the geology of both the IPF and the Yamrang pegmatite, offering insights into the evolutionary stages of the Yamrang pegmatite based on its internal texture and the mineral chemistry of beryl, garnet, and tourmaline. The Yamrang pegmatite is classified as a beryl-columbite subtype of REL pegmatite, exhibiting axial symmetric zonation and is differentiated into five mineralogical-textural zones during magmatic and hydrothermal stages. The magmatic stage involves undercooling, disequilibrium crystallization, and fractional crystallization whereas the hydrothermal stage comprises alkali-enriched fluid exsolution, the formation of miarolitic cavities, and the crystallization of aquamarine. Yamrang aquamarines are enriched in FeO, up to 1.1 wt%, with Fe2+ as the primary chromophore, resulting in their blue color. Two types of beryl have been identified: magmatic beryl in zones 3 and 4 and hydrothermal beryl in miarolitic cavities (zone 5). Magmatic beryls are characterized by their alkali-poor to sodic composition, prismatic to columnar habit, homogeneous texture, and the presence of Fe-Al octahedral substitution. In contrast, hydrothermal beryls belong to the sodic to Li-Cs beryl group, with tabular to acicular habit, weak compositional zoning, and channel-tetrahedral substitution. Yamrang garnet belongs to the almandine-spessartine series, while the tourmalines are Mg-Ca rich dravite-schorl to Fe-Mn rich schorl variety. Alkali-poor to Li-Cs beryl with low REE contents, spessartine garnet (spessartine component, Xsps ∼ 52) with high HREE, and schorl with high Ti but low Li, Pb, and REE contents suggests that the Yamrang Pegmatite is an intermediate-fractionated REL pegmatite. Potential pegmatite evolution indicators for IPF include high alkali, Fe, Mn, and Rb contents in beryl, high Xsps in garnet, and an increase in Li, Fe, Mn, Zn, Ga, and Nb, alongside depletion of Mg, Ca, Ti, and Ta in schorl. The identification of a rare metal pegmatite field in the Nepalese Himalaya reinforces the general rare metal mineralization potential of the entire Himalaya region.

Pegmatite zonation and the use of muscovite as a geochemical indicator for tin-tantalum-tungsten mineralization: Case studies from the Kalehe and Idjwi areas, Democratic Republic of Congo

The central part of the Karagwe-Ankole Belt (KAB) in the Great Lakes area of Central Africa is rich in various rare and strategic elements. The area north of Kalehe and Idjwi island within the Sud-Kivu Province of eastern Democratic Republic of the Congo (DRC) contains Nb, Ta, Sn and W mineralization hosted in pegmatites and quartz veins that are related to leucogranite intrusions. In this work, two granite-pegmatite fields were studied: the Bugarula field in the northern part of Idjwi island and the Kalehe field, along the west coast of Lake Kivu, with a view to determine the co-genetic link between different pegmatites and leucogranites in these regions. Based on mineralogical assemblages, five pegmatite zones were identified, occurring with some spatial overlap, but with an overall increasing distance from the inferred parental granite: 1) albite pegmatites, 2) two-mica pegmatites, 3) muscovite pegmatites, 4) beryl-bearing Nb–Ta–Sn mineralized pegmatites and 5) tourmaline-bearing Nb–Ta–Sn mineralized pegmatites. Muscovite compositions were analyzed from each zone and used to calculate pegmatite fractionation trends by using Rayleigh fractionation modelling. The model uses published partition coefficients (Kd) and measured contents of K, Rb and Cs in muscovite. Muscovites from the consecutive pegmatite zones in both the Bugarula and Kalehe pegmatite fields record a continuous fractionation trend, indicating a co-genetic magmatic evolution. A parental leucogranitic composition of 3.6 wt% K2O, 8 ppm Cs and 300 ppm Rb was iteratively determined to yield a best fit fractionation trend with the data. This composition is within the range of exposed leucogranites sampled in this study, which average at 4.2 wt% K2O, 7 ppm Cs and 400 ppm Rb (n = 12). It should be noted that individually, none of the sampled leucogranites gave satisfactory fractionation trajectories that would suggest a direct co-genetic link between them and the pegmatites. In the Idjwi granite-pegmatite field, the degree of fractionation with respect to the inferred parental leucogranite ranges from 20% to 99% for the least fractionated and most evolved Nb–Ta–Sn mineralized pegmatites, respectively. In the Kalehe area, fractionation values also range from 22% up to 99% for the least fractionated and most evolved (mineralized) pegmatites, respectively. Muscovites from mineralized pegmatites of zones 4 and 5 show high concentrations of Nb, Ta and Sn. As such, we show that muscovite compositions in these pegmatites provide a powerful tool to track Nb–Ta, Sn (and W) enrichment and potential mineralization. These high degrees of fractionation associated with the enrichment in economically interesting elements are similar to reported values for pegmatite fields elsewhere in the Karagwe-Ankole Belt.

Characterisation and rare-metal potential of the Winneba-Mankoadze pegmatites, Southern Ghana: Evidence of two pegmatite fields

In southern Ghana, the region along the coast between Accra and Cape Coast hosts a large number of pegmatites mineralized in lithium, niobium-tantalum and tin. The pegmatites occur in many distinct groups, each extending over several kilometers. They intrude metasedimentary units of the Birimian Supergroup, and are associated with early to late orogenic granite intrusions which are metaluminous, sterile, and too old to be potential parental granites for the pegmatites. In this study, we characterized the Winneba-Mankoadze group of geographically coeval pegmatites, using field description, petrography, rare-metal mineralogy and accessory mineral geochemistry on micas, garnet and Nb–Ta–Sn minerals, in order to determine its rare-metal potential and to investigate its origin. The results indicate that the pegmatites are part of the albite-spodumene type of the Lithium–Cesium–Tantalum (LCT) family. The rare metal mineral assemblages are particularly complex and display relevant oxide species such as columbite- and wodginite-group minerals, tapiolite, microlite, cassiterite and rutile, which are evidences of an extremely evolved magmatic system. Based on mineral assemblages, whole-rock geochemistry, and mineral geochemistry on garnet, micas and the CGM, two pegmatite fields are distinguished in the Winneba-Mankoadze group, and an anatectic origin is proposed. For the first time in West Africa, we fully describe a highly fractionated LCT-family pegmatite field comparable to the most evolved pegmatite bodies in the world.

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.

Lithium, Phosphorus, and rare earth elements in magmatic garnets from granitic Pegmatites: Coupled substitution and petrogenetic implications

The general chemical formula of garnet can be expressed as X3Y2(ZO4)3 where X, Y, and Z represent dodecahedral, octahedral, and tetrahedral sites, respectively. Lithium, phosphorus, and rare earth elements (including yttrium) in garnets from the Luanzhuang-Guanpo and Ziyugou (ZYG) pegmatite fields in the East Qinling orogen and the Kalu’an (KLA) Li pegmatites in the Chinese Altay orogen have been investigated both at a mineral scale and in the context of pegmatite types to identify the substitution mechanisms for these elements. Our results show that Y + REE or P can enter the garnet lattice through coupled substitution with Li (+Na) that can compensate the charge differences due to Y + REE in the X-site or P in the Z-site of the garnet. Two substitutions, namely [X(Li,Na)+1X(Y,REE)3+1(R2+)–2] and [X(Li,Na)1ZP1XR2+-1ZSi-1], are likely the most important and can account for incorporation of the majority of Y + REE, P, and Li + Na in pegmatite garnets. Whichever substitution becomes predominant in garnets is strongly dependent on the bulk chemistry of pegmatite-forming melts. Consequently, the first substitution dominates in garnets from NYF (niobium-yttrium-fluorine) pegmatites and mixed-type pegmatites with NYF affinity whereas the other one dominates in garnets from relatively P-rich LCT (lithium-cesium-tantalum) and barren pegmatites. Incorporation of Sc and Ti in garnet may also need Li + Na, although Sc and Ti are not as much as Y and REE in pegmatite garnet. The zonings of Y, REE, and Sc in garnet are mainly controlled by the composition of pegmatite-forming melt and consumption of these elements due to the growth of garnet itself and other minerals enriched in Y + REE (e.g., xenotime, fergusonite, monazite, etc.) or Sc (e.g., biotite). Additionally, our results, combined with the data in the literature, show that garnets from NYF pegmatites and mixed-type pegmatites with NYF affinity commonly contain elevated Y (up to ∼ 20000 ppm), REE (up to ∼ 20000 ppm), and Sc (up to ∼ 2400 ppm) but low P (<100 ppm) compared to those from LCT and barren pegmatites. Nevertheless, individual garnets with low concentrations of Y, REE, Sc, and P can occur in any types of pegmatites. Therefore, the coupled substitution between Li + Na and Y + REE or P along with zonings and concentrations of Y, REE, and Sc in garnet should be investigated on a regular basis to interpret pegmatite petrogenesis.

Geochronology of plagioclasite veins from the Lipovskoye pegmatite field (Middle Urals)

The relevance of the research is due to the need to improve the method of chemical dating as applied to high-thorium and high-uranium accessory minerals, which are difficult to date by isotopic research methods. The purpose of the research is to study the chemical composition of monazite and uraninite from plagioclasite veins of the Lipovskoye pegmatite field and to determine their age. Research Methodology: quantitative analysis of the chemical composition of monazite and uraninite was performed on an X-ray spectral electron probe microanalyzer CAMECA SX 100. Measurement conditions: accelerating voltage 15 kV, current strength 250 nА, electron beam diameter 2 µm. Sample chamber pressure 2 ∙ 10–4 Pa. The spectra were obtained on tilted wave spectrometers, the intensity was measured using analytical lines: Th Ma, U Mb, Pb Ma, Y La, Si Ka, Ca Ka, P Ka, Ce La, La La, Pr Lb, Nd La, Sm Lb, Dy La, Gd Lb. Age calculation was carried out according to well-known methods of foreign authors in addition to their own developments. Results. According to the chemical composition, monazite belongs to the cerium variety and is characterized by high contents of ThO2 up to 13,4 wt. %, UO2 up tо 0,8 wt. % and PbO up to 0,18 wt. %. According to chemical dating, monazite-(Ce) gives an age of 266 ± 17 Ma. In undeformed plagioclasites, uraninite contains impurities of ThO2 up to 11,6 wt. % and PbO up to 3,0 wt. %, its calculated age is 258 ± 3 Ma. In tectonized plagioclasites, uraninite is characterized by impurities of ThO2 from 26 up to 45 wt. % and PbO up to 2,4 wt. %. According to chemical dating data, two ages are calculated in these uraninites: 255 ± 10 Ma in the central part of the grains, and 244 ± 4 Ma in the marginal parts of the grains. Conclusions. Microprobe data on the composition of these minerals have been obtained, and the complex geochronology of these rocks has been established by chemical dating. These plagioclasite bodies were formed after the Middle Permian quartz-potassium feldspar (topaz-beryl) granitic pegmatites, widely developed within the Lipovskoye field, and later underwent superimposed changes twice in the Upper Permian and Triassic times.