Geochemistry of K‐feldspar and Muscovite in Rare‐element Pegmatites and Granites from the Totoral Pegmatite Field,San Luis,Argentina

The geochemistry of K‐feldspar for K, P, Sr, Ba, Rb, Cs, Ga, and of muscovite for the same elements plus Nb and Ta, was used for proving the parental relationships of S‐type granites and LCT (Li, Cs, Ta) rare‐element pegmatites in the southernmost pegmatitic field of the Pampean pegmatite province in Argentina. The variation of K/Rb‐Cs, K/Cs‐Rb, K/Rb‐Rb/Sr, K/Rb‐Ba in K‐feldspar from the granites and pegmatites show that they form an association with the evolutional sequence: granites → barren‐ to transitional pegmatites → beryl type, beryl‐columbite‐phosphate pegmatites → complex type of spodumene subtype pegmatites → albite‐spodumene type → albite type pegmatites. This sequence reflects the regional distribution of the different magmatic units. The Ta‐Cs diagram for muscovite reveals that none of the studied pegmatites exceed the threshold established in previous studies for being considered with important tantalum oxide mineralization. The granites and pegmatites constitute a rare‐element pegmatitic field in which different magmatic units form a continuous fractionation trend, extended from the less evolved granitic facies to the most geochemically specialized pegmatites     

Mineralization of pegmatites in parts of the Oban Massif, Southeastern Nigeria： A preliminary analysis

The Oban Basement Massif of southeastern Nigeria is composed of metamorphosed rocks including phyllites, schists, gneisses and amphibolites cut by pegmatitic dykes of varying length and thickness, which intruded the metamorphic rocks. Preliminary geochemical study and analysis of these pegmatites from western Oban Massif at Uyanga, Akwa Ibami, Iwuru I, Iwuru Ⅱ and Igbofia showed that the pegmatites are highly albitized. This is incon-sistent with earlier postulations that the pegmatites in this part of Nige...     

Rare-metal pegmatites of Wamba,central Nigeria - their formation in relationship to late Pan-African granites

he Sn-(Nb, Ta) mineralization of the Wamba field (central Nigeria) occurs in muscovite-quartz-microcline pegmatites, which are related to the late-orogenic Pan-African (f 550 Ma) "Older Granites". The emplacement of granites and pegmatites was controlled by late Pan-African shear tectonics. The granitoid magmatism was multiphase and has produced peraluminous biotite granite, biotite-muscovite granite, and muscovite granite plutons. Sodic metasomatism has altered highly evolved granite cupolas and many of the pegmatite dikes. The pegmatitic mineralization of predominantly cassiterite is closely associated with albitization. Chemical data of granites and granitic and pegmatitic muscovites show that Rb, Cs, Sn, Nb, and Ta are enriched during both magmatic and postmagmatic evolution, with highest contents of these elements in early muscovites of the albitized and mineralized pegmatites. Trace-element chemistry of the pegmatitic muscovites reveals a chemical zonation of the pegmatite field related to the late-orogenic shear system.     

珠峰地区锂成矿作用:喜马拉雅淡色花岗岩带首个锂电气石-锂云母型伟晶岩

喜马拉雅淡色花岗岩具有较好的稀有金属成矿前景。珠穆朗玛峰位于该淡色花岗岩带的中部,其中大量的淡色花岗岩和伟晶岩出露,并成为珠穆朗玛重要的岩石组成部分。近期,我们在珠峰前进沟地区发现并采集了锂成矿伟晶岩,在手标本上可以清晰看到浅褐红色的铁锂云母。进一步的全岩地球化学以及矿物学研究表明,前进沟锂成矿伟晶岩为锂电气石-锂云母型伟晶岩,具有稀有金属元素（Be-Nb-Li）含量高、Rb/Sr比值高、Zr/Hf和Nb/Ta比值低等特征。所有的矿物学和地球化学特征都表明该伟晶岩经历了高度的岩浆分异作用。矿物成分上看,云母由铁锂云母演变为锂云母,电气石由黑电气石演变为锂电气石,Fe、Mg含量降低,Li含量升高,这一特征直接指示着演化过程中岩浆成分的变化。这次发现,是首次在该地区发现锂成矿作用,也是我国喜马拉雅首次报道锂电气石-锂云母型伟晶岩的存在。结合珠穆朗玛峰周围（普士拉、热曲）近期发现的锂辉石-透锂长石型伟晶岩,珠穆朗玛地区很可能成为我国重要的一个锂（Li）成矿远景区。     

Neoproterozoic U-Th-REE-bearing Pegmatites in Wadi Ibib,South Eastern Desert,Egypt:Structural and Geochemical Measures for a Syn-Tectonic Anatectic Model of Formation

The Wadi Ibib area is situated in the northern part of the Neoproterozoic Hamisana Shear Zone (HSZ), which is a high strain zone evolved during the late stages of the Pan-African orogeny, likely as a tectonic escape structure. Amphibolite facies pelitic metasedimentary windows crop out in the axial parts of the HSZ and are noticeably associated with numerous N-trending pegmatite dikes. Whole-rock geochemistry of the pegmatites reveals a peraluminous (S-type) affinity, with low K/Rb ratios and elevated concentrations of U, Th, REE, Rb, Li, Cs, Y, Nb and Ta. Structurally, the pegmatite sets intrude along the shear plane of the HSZ, corresponding to the regional N-trending tectonic fabrics, such as axial planar foliation and dextral-shearing in the metasedimentary host rock. Field relationships, including structural context, coupled with geochemical characteristics of the Wadi Ibib pegmatites, do not support their formation as a complementary part of evolved granitic magmas. Space-localized decompression-induced partial melting of peraluminous garnet-bearing metapelites was alternatively the underlying process for formation of these pegmatites. Such decompression was associated with regional escape tectonics and stress axes permutations during the late deformation stage (D3) in the evolution of the south Eastern Desert terrane, due to end-orogeny system pressure-release.     

The Kenticha rare-element pegmatite,Ethiopia: internal differentiation,U–Pb age and Ta mineralization

The Kenticha rare-element pegmatite, a globally important tantalite source in the Neoproterozoic Adola Belt of southern Ethiopia, is a highly fractionated, huge (2,000 m long and up to 100 m thick), subhorizontal, sheet-like body, discordantly emplaced in ultramafic host rock. It corresponds to the spodumene subtype of the rare-element pegmatite class and belongs to the lithium–cesium–tantalum petrogenetic family. The Kenticha pegmatite is asymmetrically zoned from bottom to top into granitic lower zone, spodumene-free intermediate zone, and spodumene-bearing upper zone. A monomineralic quartz unit is discontinuously developed within the upper zone. Whole-rock data indicate an internal geochemical differentiation of the pegmatite sheet proceeding from the lower zone (K/Rb ~36, K/Cs ~440, Al/Ga ~2,060, Nb/Ta ~2.6) to the upper zone (K/Rb ~19, K/Cs ~96, Al/Ga ~1,600, Nb/Ta ~0.7). The latter one is strongly enriched in Li₂O (up to 3.21%), Rb (up to 4,570 ppm), Cs (up to 730 ppm), Ga (up to 71 ppm), and Ta (up to 554 ppm). Similar trends of increasing fractionation from lower zone to upper zone were obtained in muscovite (K/Rb 23–14, K/Cs 580–290, K/Tl 6,790–3,730, Fe/Mn 19–10, Nb/Ta 6.5–3.8) and columbite–tantalite (Mn/Mn + Fe 0.4–1, Ta/Ta + Nb 0.1–0.9). The bottom-to-top differentiation of the Kenticha pegmatite and the Ta mineralization in its upper part are principally attributed to upward in situ fractionation of a residual leucogranitic to pegmatitic melt, largely under closed system conditions. High MgO contents (up to 5.05%) in parts of the upper zone are the result of postmagmatic hydrothermal alteration and contamination by hanging wall serpentinite. U–Pb dating of Mn-tantalite from two zones of the Kenticha pegmatite gave ages of 530.2 ± 1.3 and 530.0 ± 2.3 Ma. Mn-tantalite from the Bupo pegmatite, situated 9 km north of Kenticha, gave an age of 529.2 ± 4.1 Ma, indicating coeval emplacement of the two pegmatites. The emplacement of the pegmatites is temporally related to postorogenic granite magmatism, producing slightly peraluminous, I-type plutons in the area surrounding the Kenticha pegmatite field. Fractionated members of this suite might be envisaged as potential parental magmas.     

The Northeast Kingdom batholith,Vermont: magmatic evolution and geochemical constraints on the origin of Acadian granitic rocks

Five Devonian plutons (West Charleston, Echo Pond, Nulhegan, Derby, and Willoughby) that constitute the Northeast Kingdom batholith in Vermont show wide ranges in elemental abundances and ratios consistent with major crustal contributions during their evolution. The batholith consists of metaluminous quartz gabbro, diorite and quartz monzodiorite, peraluminous granodiorite and granite, and strongly peraluminous leucogranite. Contents of major elements vary systematically with increasingSiO<₂ (48 to 77 wt.%). The batholith has calc-alkaline features, for example a Peacock index of 57, and values for K<₂O/Na₂O (<1), K/Rb (60–350), Zr/Hf (30–50), Nb/Ta (2–22), Hf/Ta (up to 10), and Rb/Zr (<2) in the range of plutonic rocks found in continental magmatic ares. Wide diversity and high values of minor- and trace-element ratios, including Th/Ta (0.5–22), Th/Yb (0–27), Ba/La (0–80), etc., are attributed to intracrustal contributions. Chondrite-normalized REE patterns of metaluminous and relatively mafic intrusives have slightly negative slopes (La/Yb_cn<10) and negative Eu anomalies are small orabsent. The metaluminous to peraluminous inter-mediate plutons are relatively enriched in the light REE (La/Yb_cn>40) and have small negative Eu anomalies. The strongly peraluminous Willoughby leucogranite has unique trace-element abundances and ratios relative to the rest of the batholith, including low contents of Hf, Zr, Sr, and Ba, low values of K/Rb (80–164), Th/Ta (<9), Rb/Cs (7–40), K/Cs (0.1–0.5), Ce/Pb (0.5–4), high values of Rb/Sr (1–18) low to moderate REE contents and light-REE enriched patterns (with small negative Eu anomalies). Flat REE patterns (with large negative Eu anomalies) are found in a small, hydrothermally-altered area characterized by high abundances of Sn (up to 26 ppm), Rb (up to 670 ppm), Li (up to 310 ppm), Ta (up to 13.1 ppm), and U (up to 10 ppm). There is no single mixing trend, fractional crystallization assemblage, or assimilationscheme that accounts for all trace elementvariations from quartz gabbro to granite in the Northeast Kingdom batholith. The plutons originated by mixing mantle-derived components and crustal melts generated at different levels in the heterogeneous lithosphere in a continental collisional environment. Hybrid rocks in the batholith evolved by fractional crystallization and assimilation of country rocks (<50% by mass), and some of the leucogranitic rocks were subsequently disturbed by a mild hydrothermal event that resulted in the deposition of small amounts of sulfide minerals.     

Trace element chemistry of lithium-rich micas from rare-element granitic pegmatites

Summary Granitic pegmatites characterized by advanced accumulation and fractionation of incompatible rare lithophile elements (Li, Rb, Cs, Be, Ta Nb, B, P and F), often contain mineral assemblages which host lithium-rich micas. Lepidolite and lithian muscovite occur in high-pressure spodumene, low-pressure petalite, phosphorus-enriched amblygonite and fluorine-rich lepidolite subtypes of orogenic affiliated complex type granitic pegmatites and rarely in anorogenic affiliated amazonite-bearingTrace element data determined by X-ray fluorescence for lepidolite of various pegmatite subtypes, morphology (book, scaly, fine-grained), position within the pegmatite (primary zones, replacement units, pockets), mineral assemblages and tectonic affinity (orogenic vs anorogenic) show extreme fractionation of Rb and Cs; modest levels of T1, Ga, Nb, Ta, Sn and Zn; and typically low abundances of Ba, Sr, Ni, Pb, Y, V, W and Zr. Extreme fractionation is indicated by low values of K/Rb, K/Cs and Nb/Ta which are lowest in lepidolite from petalite subtype pegmatites.No systematic differences in trace element content is evident among the different lepidolite morphologies or paragenetic position. Lepidolite from spodumene subtype pegmatites are generally slightly less fractionated than those from petalite or lepidolite subtype pegmatites.

---

Spurenelement-Chemie von Lithium-reichen Glimmern aus granitischen Pegmatiten

Zusammenfassung Granitische Pegmatite, die durch fortgeschrittene Anreicherung und Fraktionierung von inkompatiblen, seltenen, lithophilen Elementen (Li, Rb, Cs, Be, Ta Nb, B, P und F) charakterisiert sind, enthalten häufig Mineralparagenesen mit Lithium-reichen Glimmern. Lepidolith und Li-Muskowit treten in Hochdruck-Spodumen, in Niedrigdruck-Petalit, in mit Phosphor angereichertem Amblygonit und in Fluor-reichen Lepidolith-Unterarten aus komplexen orogenen granitischen Pegmatiten und selten auch aus anorogenen, Amazonit-führenden Pegmatiten, auf.Spurenelement-Daten aus der Röntgenfluoreszenzanalyse von Lepidolith aus verschiedenen Pegmatit-Untertypen, die Morphologie (tafelig, schuppig, feinkörnig), die Position innerhalb des Pegmatits (primäre Zonen, verdrängte Einheiten, Taschen), Mineralbestände und tektonische Affinität (orogen gegen anorogen) zeigen eine extreme Fraktionierung von Rb und Cs, bescheidene Gehalte an TI, Ga, Nb, Ta, Sn und Zn; und typischerweise geringe Häufigkeiten von Ba, Sr, Ni, Pb, Y, V, W und Zr. Die extreme Fraktionierung wird durch niedrige Werte von K/Rb, K/Cs und Nb/Ta angezeigt, die in Lepidolith von Pegmatiten des Petalit-Subtyps am niedrigsten sind.Aus den verschiedenen Morphologien oder paragenetischen Positionen von Lepidolith sind keine systematischen Unterschiede im Spurenelementgehalt ersichtlich. Lepidolith aus Pegmatiten des Spodumen-Subtyps sind generell etwas weniger fraktioniert als jene von Pegmatiten des Petalit- oder Lepidolith-Subtyps.

---

---

With 4 Figures     

阿尔泰可可托海3号脉伟晶岩型稀有金属矿床云母和长石的矿物学研究及意义

可可托海3号脉伟晶岩型稀有金属矿床是阿尔泰造山带产出的规模最大的伟晶岩脉,其完美的同心环状结构分带举世闻名。云母和长石作为3号脉9个结构带的贯通性矿物,由外向内表现不同的结构和成分特征。其中,云母由白云母系列向锂云母系列演化,白云母呈黄-绿色中细粒→白色或绿色中粗粒-巨晶→白色或绿色书状集合体→白色或绿色中粗粒-巨晶,锂云母呈玫瑰紫中细粒鳞片状或楔状集合体,BSE图像下云母表现出成分分带及不平衡和交代结构;长石主要为钾长石和钠长石,及少量斜长石,钾长石主要呈块体产出,钠长石呈细粒→叶片状→薄片状产出。本次研究通过电子探针(EMPA)和激光剥蚀等离子质谱(LA-ICP-MS)获得3号脉各结构带云母和长石的主微量成分。3号脉云母具有高Li(249×10^-6~35932×10^-6)、Rb(1240×10^-6~22825×10^-6)、Cs(35.9×10^-6~13980×10^-6)、Ta(13.3×10^-6~447×10^-6)含量、低K/Rb值(4.23~59.4)和K/Cs值(6.53~2368),钾长石具有低K/Rb值(35.4~1865),且由外向内,随K/Rb值降低,云母的Li、Rb、Cs、F、Ta含量升高,表明3号脉是一个由外向内结晶的分异演化程度较高的伟晶岩脉。另外,连续相邻结构带中云母和长石的主微量成分呈振荡变化,该现象主要受熔体不混溶过程的控制,也受矿物结晶不平衡影响,而熔体不混溶过程也是控制3号脉结构分带的机制之一。外部带(I-IV带)和内部带(V-VIII带)的云母和碱性长石在成分(FeO、Li、Rb、Cs、F、Ta含量和K/Rb值及K/Cs值)和结构(不平衡和交代结构)上具有明显差异,内部带演化程度明显加大,流体组分比例升高,表明体系由以熔体为主的阶段(外部带)进入以熔流体为主相对不稳定的阶段(外部带)。结合野外观察的证据,促使体系在IV带和V带间发生突然转变而进入熔流体阶段的是一个泄压事件。     

Mineralogy,geochemistry, and genesis of co-genetic granite-pegmatite-hosted rare metal and rare earth deposits of the Kawadgaon area,Bastar craton,Central India

Co-genetic pegmatites associated with the granite of the Kawadgaon area in the Bastar craton, Central India, contain a wide range of ore minerals of Nb, Ta, Be, Sn, Zr, Ti, and REE, including columbite-tantalite, ixiolite, pseudo-ixiolite, wodginite, tapiolite, microlite, fersmite, euxenite, aeschynite, beryl, cassiterite, monazite, xenotime, zircon, ilmenite, triplite, and magnetite. There is a distinct vertical zonation between the rare metal and tin pegmatites in apical parts of the host granite. Geochemically, these are LCT-S type, beryl-columbite-phosphate pegmatites that have notably high contents of SiO₂ (av. 73.80%), Rb (av. 381 ppm), and Nb (av. 132 ppm). The investigated granites probably were derived from the melting of older crustal rocks, as indicated by a high initial ⁸⁷Sr/⁸⁶Sr isotopic ratio, and the major-element geochemistry of the granites and pegmatites. Plots of mol. CaO/(MgO+FeO^t) vs. mol. Al₂O₃/(MgO+FeO^t) suggest that the source rock was pelitic metasediments. Based on the available data, it is postulated that the derivation of pegmatites from the parent granite occurred shortly after granite emplacement in the late Archaean-early Proterozoic (~2500 Ma). The K/Rb, Ba/Rb, and Rb/Sr ratios of the felsic bodies reveal that a substantial part of the granite formed from evolved melts, and further fractionation produced the co-genetic pegmatites and associated rare metal and rare earth deposits.     

Nb–Ta–(Ti–Sn) oxide mineral chemistry as tracer of rare-element granitic pegmatite fractionation in the Borborema Province,Northeastern Brazil

The Borborema Pegmatitic Province (BPP), northeastern Brazil, is historically important for tantalum mining and also famous for top-quality specimens of exotic Nb–Ta oxides and, more recently, for the production of gem quality, turquoise blue, ‘Paraíba Elbaite.’ With more than 750 registered mineralized rare-element granitic pegmatites, the BPP extends over an area of about 75 by 150 km in the eastern part of the Neoproterozoic Seridó Belt. The Late Cambrian pegmatites are mostly hosted by a sequence of Neoproterozoic cordierite–sillimanite biotite schists of the Seridó Formation and quartzites and metaconglomerates of the Equador Formation. The trace-element ratios in feldspar and micas allow to classify most pegmatites as belonging to the beryl–columbite phosphate subtype. Electron microprobe analyses (EMPA) of columbite, tapiolite, niobian–tantalian rutile, ixiolite and wodginite group minerals from 28 pegmatites in the BPP are used to evaluate the effectiveness of Nb–Ta oxide chemistry as a possible exploration tool, to trace the degree of pegmatite fractionation and to classify the pegmatites. The columbite group mineral composition allows to establish a compositional trend from manganoan ferrocolumbite to manganocolumbite and on to manganotantalite. This trend is typical of complex spodumene- and/or lepidolite-subtype pegmatites. It clearly contrasts with another trend, from ferrocolumbite through ferrotantalite to ferrowodginite and ferrotapiolite compositions, typical of pegmatites of the beryl–columbite phosphate subtype. Large scatter and anomalous trends in zoned crystals partially overlap and conceal the two main evolution patterns. This indicates that a large representative data set of heavy mineral concentrate samples, collected systematically along cross-sections, would be necessary to predict the metallogenetic potential of individual pegmatites. Other mineral species, e.g. garnets and/or tourmaline, with a more regular distribution than Nb–Ta oxides, would be more appropriate and less expensive for routine exploration purposes. The currently available Nb–Ta oxide chemistry data suggest the potential for highly fractionated Ta–Li–Cs pegmatites in the BPP, so far undiscovered, and encourages further, more detailed research.     

Origin and evolution of topaz-bearing granites from the Nanling Range, South China: a geochemical and Sr–Nd–Hf isotopic study

Summary The F-rich Hongshan pluton in the eastern Nanling Range, southern China, is a topaz-bearing albite leucogranite. It is distinctive from other topaz-bearing felsic rocks in South China with respect to age, size, geochemical evolution and topaz mode and morphology. The Hongshan granites are highly peraluminous and characterized by high K₂O/Na₂O, Si, Rb, Cs, Nb, Ta and F, and low Ca, Ba, Sr, Zr, Hf, P, K/Rb, Zr/Hf and Eu/Eu^*. The granites show significant trace-element variations with magma evolution, with increasing Rb, Cs, Nb, Ta, Sn, W and decreasing Sr, Ba, Zr, Hf, Y, REE, Pb, Th, K/Rb, Zr/Hf, Th/U and Eu/Eu^*. These changes dominantly reflect fractional crystallization of plagioclase, biotite and accessory minerals such as zircon and monazite. The granites also exhibit a decrease in ɛNd(t = 225 Ma) from −7.9 to −11.7 with magma evolution. Modeling shows that the Nd isotopic variation could result from assimilation of the Taoxi Group wall rocks during fractional crystallization. The Hongshan pluton also shows spatial geochemical variations; the most evolved parts are located in the southeastern part of the pluton, which would be the most likely target area for rare-metal mineralization commonly associated with other topaz-bearing granites. Zircon grains from two rock types in the Hongshan body were analyzed in situ for U–Pb ages and Hf isotopic values. The concordant zircon grains mostly range from 218 to 230 Ma with an average of 224.6 ± 2.3 Ma (Indosinian). Some zircons with different internal structures and Hf isotope compositions, as well as monazite fragments, yield U–Pb ages of ca. 280 to 240 Ma, suggesting older thermal events in the studied area. The ɛHf(t) of these older zircons is strongly negative (−12.3), implying a crustal source with a Paleoproterozoic model age, similar to that for the Proterozoic Zhoutan Group. The main (∼225 Ma) zircon population exhibits less negative ɛHf(t) (−3.0 to −7.6) and Mesoproterozoic model ages, suggesting that the original magma of the Hongshan granite was generated from deeper Mesoproterozoic crust.     

华北克拉通北缘显生宙含石榴石淡色花岗岩:特征、时代及成因初探

淡色花岗岩是一种特殊的花岗岩类型,其暗色矿物含量低,且多含有白云母、电气石或石榴石等富铝矿物。通常认为,淡色花岗岩是大陆碰撞造山带最具标志性的岩石类型,主要来自于地壳内沉积物的部分熔融,虽然一些最新研究强调它只是岩浆高度分异与演化的产物。显生宙时期,华北克拉通北缘花岗质岩浆作用强烈,然而对于区域存在的少量具有淡色花岗岩特征的岩体却关注较少。本文在对冀东麻地含石榴石白云母二长花岗岩详细研究的基础上,对华北北缘显生宙含石榴石淡色花岗岩的特征、时代进行了总结与对比,并对其成因进行了初步探讨。结果表明,这些岩石具有淡色花岗岩的典型矿物组成与地球化学特征,但已有数据还不足以充分论证其是否由变沉积岩部分熔融形成。与此同时,稀土元素四分组效应与Nb/Ta、Zr/Hf、Y/Ho、K/Rb、Rb/Sr等比值以及CaO、Ba、Sr等元素含量的差异性指示,这些花岗岩经历了不同程度的岩浆分异作用,部分岩石受到了岩浆演化晚期熔-流体相互作用的影响。岩浆分异程度的不同,是导致这些淡色花岗岩具有不同稀有金属成矿潜力的重要因素,高度分异演化有利于成矿元素的富集。由于仅部分淡色花岗岩经历了较高程度的分异,因此岩浆分异可能并非淡色花岗岩形成的必要机制。华北北缘含石榴石淡色花岗岩集中出现在中-晚二叠世、中-晚侏罗世。前者的形成,与古亚洲洋闭合时的碰撞造山有关;后者与东北、华南等中国东部相似岩石同时代产出,形成于古太平洋俯冲的大地构造背景下,其地球动力学内涵值得进一步探讨。     

The genesis of felsic-mafic plutonic associations: a Sr and Nd isotopic study of the Hercynian Braga Granitoid Massif (Northern Portugal)

The late tectonic Braga plutonic complex in the “Centro Iberian Zone”, North Portugal, was emplaced during the Hercynian orogeny within a Silurian sedimentary sequence and displays an acid-basic association which consists of three well-defined intrusions: the biotite-dominant Braga granite, minor bodies of gabbro to granodiorite composition and the mildly peraluminous Gonça leucogranite. These three plutonic suites present field relationships indicating a synchronous magmatic emplacement for which a Rb---Sr age of 310±10 Ma is obtained. The distribution of the three plutonic units along regular curves in major and trace element diagrams suggests that the different units can be genetically related. However, the Rb---Sr and Sm---Nd isotopic results do not indicate a unique homogeneous source and a simple fractional crystallization process. The gabbros have chemical characters (high K, Ba, Sr and light rare-earth elements and low Nb, Ti and Zr contents) and isotopic compositions (Sr_i=0.70497 and ε_Nd=−2.5), which suggest an alkaline magmatic affinity of shoshonitic type. They are probably derived from an enriched mantle source. In contrast, the peraluminous Gonça leucogranite (Sr_i=0.70933 and ε_Nd=−6.8) is more likely the result of crustal anatexis. The Braga granite and the evolved members of the basic series have an intermediate isotopic composition (Sr_i=0.70532 to 0.70733 and ε_Nd=−3.0 to −6.2) which can be interpreted in terms of an hybridization process between the two previous end-members. The chemical and Sr---Nd isotopic compositions of the Braga plutonic series can be explained by an assimilation-fractional crystallization (AFC) model between a mantle-derived magma (equivalent to the gabbros) and a crust-derived magma (the Gonça granite).     

Classification and mineralization potential of the pegmatites of the Eastern Brazilian Pegmatite Province

The potential for Nb, Ta, Li, Sn-mineralization as well as for precious stones for the Eastern Brazilian Pegmatite Province (EBPP) has been evaluated on the basis of 530 K-feldspar and 550 muscovite major and trace element analyses. The EBPP is situated mainly in the State of Minas Gerais, but encompasses also parts of the States of Bahia, Espírito Santo and Rio de Janeiro. The EBPP is the largest pegmatite province of South America. It was divided into the pegmatite districts of Itambé, Araçuaí, Safira, Nova Era, Aimorés and Espera Feliz. This was done to test whether the pegmatites of these districts differ in their mineralization potential and how geotectonic setting influences mineralization potential. The fractionation diagrams such as Cs, Zn, Li, Be, Ba versus K/Rb, Cs versus Ta/(Ta + Nb), and U, Na₂O versus K/Cs for the pegmatite districts of Araçuaí and Safira show the widest range in fractionation. These pegmatite districts are leaders in the production of gem-quality tourmaline, aquamarine, morganite, and contain abundant spodumene, tantalite and columbite. In contrast, the Espera Feliz and Aimorés pegmatite districts are the most primitive districts examined and have a corresponding lack of rare-element mineralization. Literature data indicate that all studied pegmatites are of Brasiliano age, i.e., formed between 600 and 480?Ma. The pegmatites of Transamazonic age (1.9?Ga), found rarely in the study area, are of economic importance in the context of emerald mineralization, but seem to be of less importance for rare metal and other gemstone mineralization.     

喜马拉雅琼嘉岗超大型伟晶岩锂矿的形成时代、源区特征及分异特征

喜马拉雅新生代淡色花岗岩带是近年来提出的与高度结晶分异、异地深成淡色花岗岩有关的稀有金属战略远景区,目前其金属组合以铍-铌-钽（-锡-钨）为主。秦克章等（2021a）报道了在高喜马拉雅带珠峰地区发现的琼嘉岗锂矿,是喜马拉雅首例具有工业价值的伟晶岩型锂矿。本次研究重点揭示喜马拉雅琼嘉岗伟晶岩型锂矿的成矿特征、形成时代和源区特征。琼嘉岗矿区矿石矿物主要为锂辉石、铌铁矿-铌锰矿、少量锡石和绿柱石,特征性长柱状锂辉石主要产于块体微斜长石+锂辉石带和分层细晶岩带内。琼嘉岗锂辉石伟晶岩各结构分带的K/Rb含量较为相似,锂含量从边部细粒钠长石带（~100×10^-6）到分层细晶岩带（~1000×10^-6）,再到块体微斜长石+锂辉石带（>3000×10^-6）逐渐升高,而Cs含量逐渐降低。独居石和铌钽铁矿族矿物LA-ICPMS定年结果显示,琼嘉岗锂辉石伟晶岩形成于新喜马拉雅阶段早期（25~24Ma）,与高喜马拉雅地区淡色花岗岩时代相近。矿物化学和独居石Nd同位素结果显示该稀有金属伟晶岩结晶于高度演化的花岗伟晶岩熔体,源区特征与高喜马拉雅结晶岩系一致。本研究所揭示的琼嘉岗成矿特征、形成时代和源区特征将为高喜马拉雅其它地区找寻大型花岗伟晶岩型锂矿提供重要借鉴意义。     

青藏高原东北缘茶卡北山印支期(含绿柱石)锂辉石伟晶岩脉群的发现及Li-Be成矿意义

青藏高原东北缘茶卡北山地区首次发现锂辉石伟晶岩脉群。这些伟晶岩脉沿宗务隆山南缘断裂北侧密集出露,并呈狭窄带状北西向展布。到目前为止,已发现9条含绿柱石锂辉石伟晶岩脉(Li2O平均品位为1.11%~3.13%,BeO平均品位为0.06%)和13条含绿柱石伟晶岩(BeO平均品位为0.044%~0.056%)。伟晶岩锆石U-Pb测年确定其成岩成矿年龄为217 Ma,含绿柱石伟晶岩具有高SiO2(71.62%~77.34%)、Al2O3(15.57%~17.55%)和富K2O(1.99%~2.02%)、Na2O(6.09%~6.24%),稀土元素总量非常低(ΣREE=5.2~9.1μg/g),轻稀土元素略微富集((La/Yb)N=6.8~10.1),Eu具负异常(δEu=0.25~0.92),具有Cs、Rb、Ta、P和Pb富集,以及Ba、Th、La、Ce、Sr、Nd和Ti的强烈亏损特征。含绿柱石锂辉石伟晶岩具有高SiO2(75.73%~77.34%)、Al2O3(15.58%~17.52%)和富Na2O(3.0%~3.16%)、贫K2O(0.36%~0.79%),稀土元素总量也很低(ΣREE=5.3~6.0μg/g),轻稀土元素略微富集((La/Yb)N=3.1~4.6),Eu具强烈负异常(δEu=0.17~0.23)。相对于含绿柱石伟晶岩,含绿柱石锂辉石伟晶岩更加富集Cs、U、Nb、Ta、Th、Sn和B,更亏损K和P。含绿柱石伟晶岩和含绿柱石锂辉石伟晶岩锆石具有相似的Hf同位素组成,εHf(t)值分布范围在–15.1~–12.9之间,对应的Hf同位素地壳模式年龄tDM2为1.99~2.22 Ga,表明伟晶岩源于全吉地块古元古代地壳物质的重熔再造。茶卡北山(含绿柱石)含绿柱石锂辉石伟晶岩的发现可推断宗务隆山构造带东段是青藏高原北部一条新的、重要的锂铍成矿带,除Li和Be外,Nb、Ta、Cs和Sn可能也是有潜力的成矿元素。     

Chemical and isotopic (Pb,Sr) zonation in a peraluminous granite pluton: role of fluid fractionation

The Davis Lake pluton (DLP, ~800 km²) of southwestern Nova Scotia, Canada, part of the large peraluminous South Mountain batholith of ca. 380 Ma (U/Pb zircon, Ar/Ar mica), consists of granite and subordinate topaz–muscovite leucogranite that hosts greisen tin-base metal mineralization. A new Pb–Pb isochron age for leucogranite from the most evolved part of the DLP indicates a crystallization age of 378±3.6 Ma, coincident with other radiometric ages of the DLP (Rb–Sr, Re–Os, Pb–Pb). The intrusion displays a compositional zonation defined by lead and strontium isotopic ratios, as well as some major elements (e.g., Si, F), incompatible trace elements (e.g., Li, Rb, Ta, U, Sn), and elemental ratios (e.g., K/Rb and Nb/Ta). The greisens and the leucogranites that host them are characterized by extreme radiogenic compositions for Pb and Sr, and their chemical-isotopic trends are extensions of the trends displayed by the less evolved granites. The covariations of the isotopic ratios with several major and trace elements and elemental ratios as well as the Pb–Pb and Rb–Sr isochrones indicate that all phases of the intrusion originated from a homogeneous parental magma. The granitoid magma underwent extensive fractional crystallization of feldspars, minor biotite and accessory minerals (monazite, apatite and zircon) in a compositionally zoned magma chamber that was subsequently accompanied by fluid fractionation, during which time the internally derived fluorine-rich fluids modified the Rb/Sr, U/Pb and Th/Pb ratios, leading to distinct variations of ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb isotopic ratios. These data therefore document the evolution of a granitic magma through magmatic (i.e., crystal fractionation), orthomagmatic (i.e., crystal-fluid fractionation) and hydrothermal (i.e., fluid fractionation) stages that culminated in the formation of a tin-base metal deposit. The Pb isotope data also constrain the source region for the DLP as being Avalonian basement that, by inference, must underlie much of the Meguma Terrane.Editorial responsibility: T.L. Grove     

Pristine vs. contaminated trends in Nb,Ta-oxide minerals of the Jihlava Pegmatite District,Czech Republic

Summary Columbite-tantalite is widespread in the lepidolite-subtype rare-element pegmatites of the Jihlava pegmatite district, western Moravia, Czechoslovakia. In most pegmatites, fractionation of columbite-tantalite shows initial enrichment in Mn followed by increasing Ta, in accordance with the usual trend from ferrocolumbite through mangano-columbite to manganotantalite, typical of the lepidolite pegmatites. Many dikes, however, show local deviations toward Fe- and Ti-rich compositions. In extreme cases, all columbite-tantalite is strongly enriched in Fe and Ti, and is associated with ixiolite and tantalian rutile. The degree of enrichment of the Nb,Ta oxide-mineral assemblage in Fe and Ti is proportional to tectonic introduction of wallrock xenoliths of mafic pyroxene-biotite syenite into the pegmatites during late stages of their consolidation. Extensive reaction of the residual pegmatite melt with the xenoliths contaminated the near-by melt, and generated Nb,Ta oxide minerals and tourmaline of non-typical chemistries. Late fersmite probably formed after thermal equilibration of the pegmatites with their syenitic country rocks, from Ca-bearing regional interstitial fluids pervading through the solidified pegmatites.

---

Zusammenfassung In Selten-Element Pegmatiten (Lepidolith-Subtyp) des Jihlava Pegmatit Distriktes, West-Mähren, Tschechien, ist Columbit-Tantalit weitverbreitet. In den meisten Pegmatiten führte die Fraktionierung von Columbit-Tantalit zur einer anflinglichen Anreicherung von Mn gefolgt von einer Zunahme in Ta, vergleichbar mit dem für Lepidolith-Pegmatit bekannten Trend Ferrocolumbit-Manganocolumbit-Manganotantalit. Viele Gänge zeigen jedoch lokale Abweichungen zu Fe- und Ti-reichen Zusammensetzungen. In extremen Fällen ist Columbit-Tantalit stark an Fe und Ti angereichert und mit Ixiolith und Tantalo-Rutil vergesellschaftet. Das Ausmaß der Fe und Ti Anreicherung in (Nb,Ta)-Oxid-Assoziationen ist proportional dem tektonisch bedingten Eindringen von maischen Pyroxen-Biotit-reichen Syenit-Nebengesteinsxenolithen in die Pegmatite gegen Ende ihrer Verfestigung. Tiefgreifende Reaktion der Pegmatit-Restschmelze mit den Xenolithen kontaminierte die Schmelze und führte zur Bildung von (Nb,Ta)-Oxid-Mineralen und Turmalin ungewöhnlicher Zusammensetzung. Möglicherweise nach der thermischen Gleichgewichtseinstellung der Pegmatite mit ihren syenitischen Nebengesteinen bildete sich Fersmit unter Beteiligung von Ca-führenden regionalen Porenlösungen, welche die verfestigten Pegmatite durchdrangen.

---

---

With 6 Figures     

Multistage Growth of a Rare-Element, Volatile-Rich Microgranite at Argemela (Portugal)

The small Argemela microgranite body in central Portugal displaysmany of the mineralogical and chemical features characteristicof peraluminous, Li, P-rich, rare-element pegmatites. Its mineralogyconsists predominantly of quartz, albite, white mica (partlyreplaced by lepidolite) and a phosphate of the amblygonite series.K-feldspar is noticeably absent or scarce. Cassiterite, beryland columbite are the main accessories. The microgranite showsextreme enrichment in incompatible elements such as F, P, Rb,Cs, Li, Sn and Be, and extreme depletion in Sr, Ba, Zr and REE.It is highly sodic and strongly peraluminous. The micrograniteoverall is interpreted as a mixture of two components: a crystalmush injected from below (seen in narrow dykes intersected duringdrilling, composed of quartz, albite and phengite) and interpretedas ‘feeders’, overprinted by a second highly evolvedcomponent dominated by Li, F, P (Rb, Cs, Be, Sn, Nb, Ta, etc.)considered as a ‘lubricant’ medium for the ascendingmush and occasionally quenched (quartz, albite, skeletal lepidoliteand amblygonite). This second component has the mineralogicaland chemical characteristics of rare-element pegmatites. Allthese petrological characteristics are magmatic. Only a fewnarrow cross-cutting veinlets with quartz, K-feld-spar and F-pooramblygonite are considered as fluid derived. A model of crystallizationin successive steps is proposed where concentration in fluxingagents (F, Li, P, etc.) is progressively enhanced up to saturationwith the crystallization of magmatic lepidolite and amblygonite. KEY WORDS: petrogenesis; microgranite; pegmatite; volatiles; Portugal ^*Corresponding author.