东秦岭稀有金属伟晶岩的类型、内部结构、矿化及远景——兼与阿尔泰地区对比

东秦岭地区和阿尔泰造山带均产出大量稀有金属伟晶岩,是中国重要的稀有金属产地。前者工作程度低,远景尚不明朗;后者规模巨大。开展成矿条件对比研究十分必要。东秦岭地区产出铍矿、锂矿和复杂稀有金属矿,以锂矿化为主,伟晶岩类型复杂,包括绿柱石-铌铁矿型、复杂型锂辉石亚型、复杂型锂云母亚型和钠长石-锂辉石型。阿尔泰稀有金属伟晶岩发育多种稀有金属矿化组合,伟晶岩类型为绿柱石-铌铁矿型、复杂型锂辉石亚型和钠长石-锂辉石型。东秦岭稀有金属伟晶岩的内部结构分带型式包括对称分带结构、均一结构和分层结构,阿尔泰稀有金属伟晶岩以对称分带结构为主,也见均一结构。东秦岭与阿尔泰稀有金属矿石矿物相近,东秦岭产出更多含锂磷酸盐矿物。东秦岭稀有金属伟晶岩分异演化程度相对集中且高,阿尔泰稀有金属伟晶岩分异演化程度跨度大。东秦岭和阿尔泰锂矿的锂矿化主要发生于岩浆就位前,复杂稀有金属矿稀有金属富集作用发生在岩浆就位前和就位后,但阿尔泰复杂稀有金属矿经历了更为复杂和极度的分异演化过程。东秦岭稀有金属伟晶岩可能与同期花岗岩为同一熔融事件的产物,与早期花岗岩来自同一物质来源。阿尔泰稀有金属伟晶岩与花岗岩关系复杂,但大量早期花岗岩的形成提高了地壳成熟度,有利于形成晚期稀有金属伟晶岩。东秦岭稀有金属伟晶岩产出于北秦岭单元中,形成于晚造山和造山后阶段,集中于造山后阶段,稀有金属矿化呈多期断续叠加特征。阿尔泰稀有金属伟晶岩主要产出于琼库尔-阿巴宫地体和中阿尔泰山地体内,集中于造山后和非造山阶段。伟晶岩岩浆活动受控于物质来源和造山作用。储存稀有金属的岩石在造山作用中熔融,发生多期的大规模花岗质岩浆活动,稀有金属通过长期复杂的分异演化过程在残余熔体中不断富集。这种富挥发分和稀有金属的过铝质硅酸盐岩浆随后上升就位,可经后续冷却结晶和不混溶作用进一步富集稀有金属,从而形成稀有金属伟晶岩。东秦岭具有形成含稀有金属高度分异演化岩浆的有利条件,该区具有寻找铍矿和复杂稀有金属矿的潜力。     

Geochemical characteristics and spectrometric prospecting in the muscovite-bearing pegmatites and granites,southeastern Aswan,Egypt

Rare metal mineralization of Sn, Nb-Ta and W is encountered in the Gebel Dihmit area （GDA）, southeastern Aswan, Egypt. The mineralization is related to muscovite granites and their pegmatite derivatives. The pegmatites are divided into three types according to their main mineral assemblages： K-feldspar-muscovite-tourmaline, K-feldspar-albite-muscovite and albite-K-feldspar-lepidolite veins. Petrogenetic studies indicate that Sn and Nb-Ta mineralization extends from the late-magmatic stage to the pegmatite and hydrothermal stages of the （GDA） suite. The albite-K-feldspar-lepidolite granite is composed dominantly of albite, lepidolote, and quartz, with topaz, K-feldspar and amblygonite. The accessory minerals are zircon, monazite, pollucite, columbite-tantalite, microlite and Ta-rich cassiterite. Phenocrysts of quartz, topaz and K-feldspar contain abundant inclusions of albite laths and occasional lepidolite crystals along growth zones （snowball texture）, indicating simultaneous crystallization from a subsolvus, residual magma. The origin of the pegmatites is attributed to extreme differentiation by fractional crystallization of a granitic magma. The economic potential for rare metals was evaluated in the geochemical discrimination diagrams. Accordingly, some of the pegmatites are not only highly differentiated in terms of alkalis, but also the promising targets for small-scale Ta and, to a less extent, Sn. The pegmatites also provide the first example of Fe-Mn and Nb-Ta fractionation in successive generations of granites to cassiterite-bearing pegmatites, which perfectly ex- hibit similar fractionation trends established for primary columbite-tantalite in the corresponding categories of pegmatites. Uranium and Th of magmatic origin are indicated by the presence of thorite and allanite, whereas evidence of hydrothermal mineralization is the alteration of rock- foring minerals such as feldspar and the formation of secondary minerals such as uranophane..     

Geochemistry of the uranium-thurium-bearing granitic rocks and pegmatites of wadi haleifiya area, southeastern Sinai, Egypt

There are two main granitic rocks cropping out in the study area:1) the syn-orogenic granites are moderately weathered,jointed,exfoliated and characterized by low relief.These rocks are subdivided into tonalite and granodiorite.They are essentially composed of plagioclase,quartz,biotite,hornblende and potash feldspar;and 2) the post-orogenic granites,characterized by high relief terrain and represented by monzogranite,syenogranite and alkali granite.The monzogranites suffered hydrothermal alteration in particular along joints,faults,shear zones and fractures,which recorded the highest values of radioactivity,reflecting the role of post-magmatic alteration processes in the enhancement of radioactivity.The hydrothermal alteration(desilicification and hematitization) resulted in the formation of mineralized(altered) granites.The altered granites are enriched in TiO 2,Al 2 O 3,FeO T,MnO,MgO,Na 2 O,Rb,Sr,Y,Zr,Zn,Ga and Co and depleted in SiO 2,CaO,P 2 O 5,Nb,Pb,Cu,Ni and Cr relative to the fresh monzogranite.The investigated granites contain basic xenoliths as well as pockets of pegmatites.Perthites,quartz,plagioclase and sometimes biotite,represent the essential constituents.Some accessory minerals like zircon are metamicted reflecting their radiogenic nature.The alkali granites are characterized by the presence of aegirine,rebeckite and arfvedsonite.Both syn-and post-orogenic granites show some variations in their bulk chemical compositions.The older granitoids are metaluminous and exhibit characteristics of I-type granites and possess an arc tectonic environment.On the other hand,the younger granites are peraluminous and exhibit the characteristics of post-collisional granites.It is interpreted that radioactivity of the studied rocks is mainly controlled by both magmatic and post-magmatic activities.Frequently,the post-orogenic granites host zoned and unzoned pegmatite pockets.Some of these pockets anomalously attain high radioactivity.The syenogranites and the pegmatites are characterized by high contents of SiO 2 and K 2 O and low CaO and MgO.They have transitional characters from highly fractionated calc-alkaline to alkaline.The alkali granites related to A2-subtype of A-type granites.The post-orogenic granites were originated from magma of dominant crustal source materials and related to post-collisional setting under extensional environment.     

SWIR ASTER band ratios for lithological mapping and mineral exploration: a case study from El Hudi area, southeastern desert, Egypt

This study aims to discriminate and to map the basement rocks as well as the barite mineralization exposed at El Hudi area, Southeastern Desert, Egypt using the processed short-wave infrared bands of advanced space-borne thermal emission and reflection radiometer (ASTER) in collaboration with the field verification and petrographic analysis. El Hudi area is covered dominantly by the Late Precambrian high-grade metamorphic complex of metasedimentary rocks (gneisses, schists, migmatites, and minor amphibolites) which are intruded by the younger granitoids. Nubian sandstones unconformably overlie the basement outcrops and occur as a remnant caps. The metasedimentary rocks cover the area of interest forming a belt of biotite gneisses and migmatites intercalated with hornblende biotite schists and minor amphibolites. Their exposures exhibit well-foliated and banded structures. The metasedimentary rocks have gray and dark gray image signatures on the ASTER band ratio image 8/5, which correspond to biotite gneiss, migmatites, and hornblende biotite schists, respectively. Presence of absorption feature near band 8 (2.295 – 2.365 μm) for the chlorite alteration product is probably responsible for the lowering of the 8/5 band ratio value and the dark gray image signature exhibited by hornblende biotite schists. The granitoid rocks in El Hudi area are late to postorogenic younger granitoids including three main rock types, Abu Aggag granites, El Hudi garnetiferous muscovite granites, and coarse-grained biotite granites. The acidic dykes are cutting across the granitoids and the gneisses and they form a highly elevated ridges and peaks showing sharp contact with the invaded rocks. Abu Aggag granites are highly dissected by great number of both strike- and dip-slip faults as well as joints trending in NNW–SSE, NNE–SSW, N–S, ENE–WSW, and WNW–ESE directions. On 7/8 band ratio image, Abu Aggag granites have dark gray image signature whereas postgranitic dykes have white image signature. Under the microscope, Abu Aggag granites are homogenous medium to coarse-grained rocks composed mainly of quartz, plagioclase, microcline, and biotite. Zircon, apatite, and opaques are accessories, while chlorite, kaolinite, and epidote are secondary minerals. Presence of absorption feature around band 7 (2.235–2.285 μm) for the kaolinite mineral may be responsible for the dark gray image signature exhibited by Abu Aggag granites. El Hudi garnetiferous muscovite granites are hosting El Hudi barite veins which extend mainly in NNW–SSE and NW–SE. Garnetiferous muscovite granites have gray image signature on 5/4 band ratio image whereas pegmatites and postgranitic dykes have black image signature. Barite veins can be distinguished within garnetiferous muscovite granites by their dark gray image signature on 5/4 band ratio image. The spectral reflectance curve of barite exhibits absorption feature around 2.1 μm (band 5), which leads to lower the ratio value and yields the dark image signature to barite veins. The above-described ASTER band ratio images were integrated into one false-color composite image (8/5:R; 5/4G; and 7/8B) which was used to produce 1:100,000 geological map for El Hudi area and to locate the barite mineralization.     

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     

The Durulgui rare-metal granite‒pegmatite system in the eastern Transbaikal region: Petrological and geochronological aspects

The Durulgui granite?pegmatite system unites the Dedova Gora granite massif and pegmatite field with the Chalotskoe beryl deposit. New geochronological data on micas from porphyric biotite granites, fine-grained biotite granites, two-mica granites, and Be-bearing pegmatites are discussed. The plateau age of 128.5(±1.5)–131.2(±1.5) should be considered as indicating the formation time of the granite?pegmatite system as a whole. The age of the system implies the possibility of its formation owing to several magmatic pulses. This assumption concerns porphyric and fine-grained biotite granites and two-mica and muscovite granites, the contact between which is locally sharp. At the same time, the succession “two-mica granites → muscovite granites → granite?pegmatites → microcline pegmatites → microcline?albite pegmatites → albite pegmatites” demonstrates gradual facies transitions between rocks, which indicates their emplacement during a single magmatic pulse.     

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.     

Interaction of crustal and mantle materials,sources of trace elements during the formation and evolution of Early Paleozoic Li-rich granite-pegmatite systems in southeastern Tuva

We present new data on the age, composition, and environments of formation of granites of the Kystarys complex and the associated Li-rich rare-element pegmatites of the South Sangilen pegmatite belt including the large Tastyg lithium deposit. It has been established that they formed during the Early Paleozoic collisional orogeny in the Tuva-Mongolian massif at the Cambrian-Ordovician boundary. The granites of the Kystarys complex are moderately alkaline high-K rocks and are enriched in Zr, Nb, Y, and REE; therefore, they are classified as postcollisional, transitional to within-plate (A-type). The spodumene pegmatites of the South Sangilen pegmatite belt are similar to the above granites in age and isotopic and geochemical parameters, which suggests a paragenetic relationship between these rocks. Pegmatites form several pegmatite fields within the belt, which differ in trace-element signatures. In addition to predominant Li, Cs, and Ta, specific to all spodumene pegmatites (LCT family), pegmatites of two fields have high contents of Nb, Y, REE, and Zr, which are indicator elements of NYF family pegmatites. It has been established that the formation of spodumene pegmatites with combined LCT-NYF geochemical signatures was preceded by the intrusion of dikes of monzogabbro with the geochemical characteristics of OIB and of alkali aegirine granites and by the formation of associated metasomatites enriched in Zr, Nb, Y, and REE. Based on the geological, mineralogical, and geochemical data, we substantiate the hypothesis of the formation of Li-bearing granite-pegmatite melts from a mixed source resulted from the influence of fluids of an alkaline igneous complex of mantle genesis on the crustal protolith.     

Origin of the younger granites of northern Nigeria

The similarity of the lead isotopic composition in older granites, pegmatites and the younger granites, together with the distribution of tin and the absence of tourmaline in the Mesozoic ring-complexes, are utilised as evidence to establish that the Nigerian younger granites were derived from an initial peralkaline melt by fusion of local basement rocks. It is also suggested that the associated metaluminous trend is caused simply by more complete melting, and that peraluminous granites developed during subsequent cooling. When feldspar normative compositions of younger granites are plotted in the quartz-saturated ternary feldspar system (James and Hamilton, 1969), they support a cyclic event of fusion and cooling to account for the variation in rock-type. The actual source material has not been established but it could be similar to bauchite, a quartz-fayalite monzonite, which was melted by a linear zone of high heat flow from the mantle during the disruption of Gondwanaland.     

福建云霄晶洞花岗岩及含石榴子石伟晶岩的硅氧同位素研究

在运用常规方法和激光烧蚀同位素分析方法分别对云霄晶洞花岗岩和伟晶岩(长石、石英和石榴子石)的硅、氧同位素组成进行研究的基础上,探讨了晶涧花岗岩及其含石榴子石伟晶岩的物质来源及形成条件.伟晶岩中的石英和长石的氧、硅同位素组成分别较之花岗岩中的石英和长石的氧、硅同位素组成,均未发生明显变化,表明二者岩浆来源一致.云霄县的乌...     

Mineralogical and chemical evolution of tantalum–(niobium–tin) mineralisation in pegmatites and granites. Part 2: Worldwide examples (excluding Africa) and an overview of global metallogenetic patterns

Columbite-group minerals (CGM) account for the majority of the production of tantalum, an important metal for high-technology applications. Along with other Ta–Nb oxides such as tapiolite, wodginite, ixiolite and pyrochlore supergroup minerals, CGM are recovered from rare-metal granites and granitic rare-element pegmatites. In this paper mineralogical and geochemical data with a focus on CGM, tapiolite, wodginite and ixiolite are presented for rare-element granites and pegmatites from worldwide occurrences except Africa that has been covered in a previous contribution (Melcher et al., 2015). Major and trace element data of the Ta–Nb oxides are presented and compared for a total of 25 granite/pegmatite provinces, and one carbonatite for comparison. Based on CGM compositions, the data allow to distinguish between various subgroups of Li–Cs–Ta (LCT)-family pegmatites, Nb–Y–F (NYF)-family pegmatites, mixed LCT–NYF pegmatites, and rare-element granites.Each period of Ta-ore formation in Earth history is characterised by peculiar mineralogical and geochemical features. Some of the largest and economically most important rare-element pegmatite bodies are located within Archean terrains and intruded ultramafic and mafic host rocks (e.g., Tanco/Canada, Wodgina and Greenbushes/Western Australia, Kolmozero/Kola). They are highly fractionated, of LCT affinity throughout and yield complex mineralogical compositions. The variety of minor and trace elements incorporated attests to a rather insignificant role of the immediate host rocks to their geochemical signature and rather points to the significance of the composition of the underlying crustal protoliths, internal fractionation and the processes of melt generation. Many of the Archean pegmatites carry significant Li mineralization as spodumene, petalite, and amblygonite, and all of them are also characterised by elevated Li in CGM. In addition, Sb and Bi are important trace elements, also reflected by the occasional presence of stibiotantalite and bismutotantalite. REE_N patterns of CGM are dominated by the MREE or HREE, and range from very low to high total REE concentrations. Negative Eu anomalies are omnipresent. Scandium contents are also highly variable, from very high (Tanco) to very low concentrations (Wodgina, Kolmozero).A second period of worldwide pegmatite formation was in the Paleoproterozoic. All CGM analysed derive from LCT-family pegmatites except samples from the Amazonas region where Ta is mined from rare-metal granites at Pitinga. Pegmatites intruded highly variable lithologies including metasediments, metabasites, gneiss, granite and quartzite within a variety of structural and paleogeographic settings; however, most of them are syn- to post-orogenic with respect to major Paleoproterozoic orogenic events. Minor and trace element signatures are similar to CGM from Archean pegmatites. Some are characterised by considerable REE enrichment (São João del Rei/Brazil; Amapá/Brazil; Finnish Lapland/Finland), whereas others have normal to low total REE concentrations (Black Hills/USA, Bastar/India). Examples with high REE commonly are enriched in Sc and Y as well, and are often transitional to NYF-family pegmatites.The Mesoproterozoic period is comparatively poor in rare-element pegmatites and rare-metal granites. Mineralogical and chemical attributes of ixiolite–wodginite, tapiolite, CGM and rutile from placer material in Colombia point to an unusual pegmatite source of NYF affinity, yielding high total REE, Sc and Th at low Li and Bi. REE patterns have typical negative Eu and Y anomalies.A third major period of pegmatite formation was the Early Neoproterozoic at around 1 Ga, documented in the Grenvillian (North America), the Sveconorwegian (northern Europe) and the Kibaran in central Africa. CGM are present in numerous, mostly small pegmatites, although larger examples also occur (e.g., Manono in the D.R. Congo; Melcher et al., 2015). Pegmatite fields often display a zonal arrangement of mineralised pegmatites with respect to assumed “fertile” parent granites. They intrude metasediments, metabasites, gneiss and granite of middle to upper crustal levels and display a variety of mineralogical and chemical characteristics. Pegmatites of the Sveconorwegian and Grenville domains are usually of the NYF type and CGM are characterised by elevated Y, REE, Th and Sc. In contrast, the pegmatites of central (Kibara Belt) and southwestern Africa (Orange River Belt) are commonly of LCT affinity carrying spodumene, beryl and cassiterite (Melcher et al., 2015). These CGM have elevated conce ntrations of Li, Mg, Sn and Hf. Total REE concentrations are low except for the Sveconorwegian, and exhibit a variety of shapes in normalised diagrams.The fourth major pegmatite-forming event coincides with amalgamation of Gondwana at the Neoproterozoic/Paleozoic boundary around 550 Ma ago. This event is omnipresent in Africa (“Panafrican”) and South America (“Brasiliano event” documented in the Eastern Brazilian pegmatite and Borborema provinces). Pegmatites often intruded high-grade metamorphic terrains composed of metasediments including schist, marble, quartzite, as well as gneiss, amphibolite, ultramafic rocks, and granite. Within the Neoproterozoic, rare-metal granites of NYF affinity are locally abundant. Pegmatites show both LCT and NYF affinities, and mixed types occur in Mozambique. The Alto Ligonha and Madagascar provinces are characterised by abundant REE and Sc both within Ta–Nb-oxides and as separate mineral phases. Notably, some pegmatite provinces are almost devoid of cassiterite, whereas others carry cassiterite in economic amounts.In the Phanerozoic (younger than 542 Ma), pegmatites formed at all times in response to orogenetic processes involving various continents and terranes during the long-time amalgamation of Pangea and the Alpine orogenies. Whereas some activity is related to the Pampean, Acadian and Caledonian orogenies, the Variscan/Hercynian and Alleghanian orogenies are of utmost importance as manifested in pegmatite formation associated with Sn–W mineralised granites in central and western Europe as well as in the Appalachians. Most of the Variscan and Alleghanian pegmatites are of LCT affinity, although NYF and some mixed types have been described as well. Variscan pegmatite formation culminated at ca. 330 to 300 Ma, whereas Alleghanian pegmatites range in age from about 390 Ma to about 240 Ma. Most are syn- to post-orogenic and were emplaced at different crustal levels and into a variety of host rocks. Degree of fractionation as well as minor and trace element geochemistry of Ta–Nb oxides are rather variable and cover the complete field of CGM compositions. REE patterns are characterised by prominent negative Eu anomalies.Some Mesozoic and Cenozoic pegmatites and rare-metal granites from Southeast Asia and the Russian Far East are included in the compilation. Rare-metal granites of the Jos Plateau (Nigeria) were previously investigated (Melcher et al., 2015). The proportion of NYF pegmatites and rare-metal granites in the Mesozoic is striking, i.e. illustrated by Jos, Orlovka, Ulug Tanzek as well as the southeast Asian deposits related to tin granites. CGM from these areas are invariably rich in REE, Sc, Y and Th. In all rare-metal granites, Ta–Nb oxides are characterised by high total REE concentrations and both, negative Eu and Y anomalies in chondrite-normalised REE diagrams.Although constituting a vastly different magmatic system compared to rare metal pegmatites and granites, we included the Upper Fir carbonatite from the Canadian Cordillera, for comparison, because it is characterised by unusal high Ta contents. As expected, the CGM differ from the pegmatitic CGM by having high Mg and Th, and low U concentrations in columbite-(Fe) and lack an Eu anomaly. However, they also show similarities to primitive CGM from rare metal pegmatites of the NYF family in terms of the REE pattern and the increase in #Ta and #Mn towards the margins of the CGM. Our findings support recent results presented in Chudy (2014) indicating that the Ta enrichment in some carbonatites might be attributed to magmatic processes and conditions that are similar to the pegmatitic systems.     

New evidence of cassiterite-bearing Precambrian basement rocks of the Jos Plateau,Nigeria — the Gurum case study

The major sources of cassiterite in Nigeria are the alluvial and eluvial deposits from the biotite granites within the Jurassic alkaline ring complex (the Younger Granites) of the Jos Plateau. Less than 5% of the total production has been recovered from pegmatites within the largely Precambrian basement complex consisting of migmatites, gneisses, the Pan-African (Older) Granites and pegmatites, but with the rapidly depleting reserves of the former source new reserves have to be found.This study presents the first report on an important new source of cassiterite in basement-complex gneisses, migmatites, pegmatites and aplites in the Gurum area near Jos. The cassiterite, which has been economically concentrated by the leaching and weathering of these rocks, is generally finer grained and darker than the alluvial cassiterite derived from the Jurassic biotite granites. This new find has important petrogenetic and economic implications, suggesting that the basement complex could be a suitable parent rock for the Younger Granites and a significant potential source of cassiterite in Nigeria.     

Central and west african rare-Metal granitic pegmatites,related aplites,quartz veins and mineral deposits

The author summarises the results of his studies of Central and West African rare-metal pegmatites and supplements them by the newest datas obtained by H. Adam in Ivory Coast. He compares then the classification of African rare-metal pegmatites with the classifications obtained in U.S.S.R., in U.S.A. and in Canada for the same kind of pegmatites. The author's conclusions may be summarised as follows: (a) The rare-metal pegmatite types succeed each other in a more or less defined order which is independent from the ages of the orogenies as well as from the nature and from the degree of metamorphism of enclosing rocks. (b) The spatial distribution of the rare-metal pegmatite types in or around the granitic intrusions depends from the depth at which the parent granites are crystallizing. — When the granites are crystallizing at relatively shallow depths, the rare-metal pegmatites and partially the quartz veins and the associated mineralizations are located in the granitic bodies them selves. When the granites are crystallizing at more and more greather depths, the rare-metal pegmatites, the quartz veins and the associated mineralizations as well as the secondary phenomena such as albitization and greisenization are no more restricted to the granitic intrusions, but with the increasing depth of their crystallization are more and more penetrating in the roofs of the granitic cupolas. In addition it must be underlined that in granites crystallizing at shallow depths, the dimensions of the individual pegmatites are small and have no economic value; the rare-metal pegmatites associated with granitic intrusions cyrstallizing at greather depths may reach gigantic dimensions and certain types may be mined for tin, niobium, tantalum, beryl and lithium minerals; (c) In the same metallogenic province may co-exist granitic intrusions having crystallized at diffrent depths or at diffrent geological ages. To each depth of crystallization of the granitic intrusions corresponds a specific spatial distribution pattern of rare-metal pegmatites characterized by the distance of different pegmatite types to the granitic contacts as well as by the dimensions of the pegmatite types and by the size of their minerals. This may outline a special zonation within a metallogenic province. — At the end of the paper, the author gives a comparison of Central and West African rare-metal pegmatite type classification with similar classifications obtained in U.S.S.R., in U.S.A. and in Canada. From this comparison it results that the evolution of the pegmatitic process remains the same throughout geological epochs and is independent of the nature of the enclosing rocks. The magnitude and the fullness of this process are depending from the depth of crystallization of the granitic intrusions.

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Résumé L'auteur résume ses observations sur les pegmatites à métaux rares du centre et de l'ouest africain, il les complète par des nouvelles données et notamment par celles de H. Adam, relatives à la Côte d'Ivoire et les compare aux données obtenues sur d'autres continents et notamment en URSS, au Canada et aux Etats Unis. — Les conclusions relatives aux pegmatites à métaux rares du centre et de l'ouest africain sont résumées ci-dessous: (a) les types de pegmatites à métaux rares se succèdent dans un ordre déterminé indépendant de l'âge, de la nature et du degré de métamorphisme des roches dans lesquelles elles cristallisent; (b) la répartition spatiale ou la zonéographie des types de pegmatites à métaux rares dépend de la profondeur de la mise en place des intrusions granitiques auxquelles les pegmatites sont associées. Lorsque les intrusions granitiques cristallisent à faible profondeur, les pegmatites et partiellement les filons de quartz avec leurs minéralisations sont contenus dans les granites. Lorsque les granites cristallisent à des profondeurs de plus en plus grandes, les pegmatites à métaux rares, les filons de quartz et les minéralisations de columbo-tantalite, de béryl, de minerais lithiques, de cassitérite et de wolfram, quittent les granites et, à mesure que la profondeur de mise en place augmente, se localisent de plus en plus loin des contacts dans les roches encaissantes; de plus, dans les granites de faible profondeur, les dimensions des pegmatites à métaux rares sont très faibles; les pegmatites associées avec des intrusions plus profondes peuvent atteindre des dimensions géantes de même leurs minéraux. Seules ces dernières offrent un intérêt économique; (c) dans une même province métallogénique, on peut rencontrer des intrusions granitiques mises en place à des profondeurs différentes et, parfois, à des époques différentes. A chaque profondeur de mise en place correspond une distribution spatiale particulière des types de pegmatites à métaux rares, caractérisés par leurs dimensions et par la grandeur des minéraux individuels. — Ces distributions spatiales différentes des types de pegmatites, jointes aux différences des dimensions des pegmatites et de leurs minéraux peuvent, à leur tour, dessiner une zonation à l'intérieur d'une province métallogénique. — A la fin de la note, l'auteur fait une comparaison rapide de la classification des types de pegmatites à métaux rares du centre et de l'ouest africain avec les classifications de la même catégorie des pegmatites de l'URSS, du Canada et des Etats Unis. De cette comparaison il en résulte que le processus pegmatitique évolue de manière très semblable à travers les époques géologiques indépendamment de la nature des roches encaissantes; mais l'ampleur du processus dépend de la profondeur de la mise en place des intrusions granitiques auxquelles les pegmatites sont associées.

     

The Albera zoned pegmatite field,Eastern Pyrenees,France

Summary Four types of pegmatites comprise the zoned pegmatite field in the eastern sector of the Albera Massif. Type I is represented by barren pegmatites with graphic textures; type II comprises transitional varieties with Li-Fe-Mn phosphates, Be (chrysoberyl) and scarce Nb-Ta and U oxide minerals; type III consists of pegmatites with significant zones of replacement containing Li-Fe-Mn phosphates, beryl and more abundant Nb-Ta oxide minerals; and type IV, muscovite-quartz-albite pegmatites are highly mineralized with Be, Nb-Ta and HREE. REE mineralization is strongly related to abundance of graphite in the late pegmatite units and in the host-rock. The individual pegmatite types are distributed within four subparallel zones concentric around anatectic muscovite-biotite leucogranites, with type I within the granites or close to the contact, and type IV pegmatites in the outermost areas. The zoning from type I to type IV could relate to fractionation processes which generated the pegmatites and is characterized by an enrichment of Mn, Ta, Na, Li, P, Be and REE. According to the pegmatite distribution and their fractionation trends, we propose an origin by differentiation of a granitic melt.

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Résumé On a établi quatre types de pegmatites dans le champ pegmatitique zoné du secteur est du Massif des Albères (Pyrénées Orientales, France). Celles de type I sont des pegmatites non minéralisées avec des textures graphiques, celles de type II sont des variétés intermediaires avec des phosphates à Li-Fe-Mn, Be (chrysobéryl) et des rares oxides à Nb-Ta et U; celles de type Ill sont des pegmatites avec des zones de réplacement bien dévéloppées et qui contiennent des phosphates à Li-Fe-Mn, du béryl et des oxides à Nb-Ta plus abondants; celles de type IV sont des pegmatites bien minéralisées à Be, Nb-Ta et des T.R. La minéralisation à T.R. est liée à des phénomènes de graphitisation répandus dans les unités tardives de la pegmatite et dans l'encaissant. La distribution de chaque type de pegmatite correspond à quatre zones à peu près parallèles et concentriques autour des granites anatectiques à muscovite-biotite, avec le type I dans les granites ou prochain au contact, et les pegmatites à type IV dans la bande plus externe. La zonation serait due à des processus de fractionnement qui auraient généré les pegmatites et qui sont caracterisés par un enrichissement en Mn, Ta, Na, Li, P, Be et T.R. dès les pegmatites de type I vers celles de type IV. On propose un origine par différentiation des granites en vue de la distribution des pegmatites.

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With 5 Figures     

美国Spruce Pine与新疆阿尔泰地区高纯石英伟晶岩的对比研究

     对美国北卡罗来纳州 Spruce Pine 地区白岗岩 / 伟晶岩和新疆阿尔泰白云母花岗岩 / 伟晶岩进行了对比研究。这两个地区的伟晶岩可能是过铝质花岗岩浆通过熔体 - 蒸汽分异作用的产物。美国产高纯石英的伟晶岩发育在片岩、片麻岩的背景之上,主要造岩矿物中富含斜长石,Na 大于 K,大离子亲石元素 Sr 和 Ba 含量高,高场强元素和稀土元素含量低,Eu 正异常,石英中杂质元素含量低。这些特征可作为高纯石英伟晶岩的判别标志。我国阿尔泰地区的伟晶岩脉十分发育,所研究的样品中,有些样品已经具有这些特点,阿尔泰伟晶岩区具有产高纯石英的成矿前景。     

Mineral chemistry and geochemical aspects of Gebel Filat granites, South Eastern Desert, Egypt

Gebel Filat granites form one of Egyptian younger granite intrusions in Wadi Allaqi region, South Eastern Desert of Egypt. They are perthitic monzogranites composed mainly of K-feldspars, plagioclase, and quartz with minor biotite. Plagioclase feldspars are Na-rich and have low anorthite content (An_2–3). Potash feldspars are mainly perthitic microcline and have chemical formula as (Or_96–96.6 Ab_3.4–4 An₀). Biotite is Mg-rich and seems to be derived from calc-alkaline magma. Chlorite is pycnochlorite with high Mg content, revealing its secondary derivation from biotite. The estimated formation temperatures of biotite and chlorite are (689–711°C) and (602–622°C), respectively. Gebel Filat monzogranites are metaluminous, high-K calc-alkaline, I-type granites. They are late orogenic granites related to subduction-related volcanic arc magmatism. They are enriched in LILE and depleted in HSFE indicating highly differentiation character. The REE patterns display an enrichment in LREE due to presence of zircon and allanite as accessories and depletion in HREE with slight negative Eu anomaly $ \left( {{\text{Eu}}/{\text{Eu}} * = 0.51 - 0.97} \right) $ . The parent magma of Gebel Filat monzogranites were emplaced at moderate depths (20–30 km) under moderate conditions of water-vapor pressure (1–5 kbar) and crystallization temperature [700–750°C]. The source magma of these granites seems to be derived from partial melting of lower crust material rather than upper mantle. The geochemical characteristics of pegmatites revealed that they are related to post orogenic within plate magmatism and not genetically related to the parent magma of Gebel Filat monzogranites. Distribution of radioactive elements (U and Th) in the studied rocks indicates normal U–Th contents for Filat monzogranites and U–Th bearing pegmatites. The positive correlations of each of Zr and Y versus U and Th are attributed to presence of zircon and allanite as accessories which incorporate U and Th in their crystal lattice.     

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.     

Age,Composition, and Geodynamic Environments for the Formation of Granites and Lithium-rich Rare-Element Pegmatite of Khusuingol Field (Sangilen Highlands)

The composition, age, and genetic relationships of spodumene pegmatites of the Khusuingol field (South Sangilen pegmatite belt, SW Sayan), encompassing the large Tastyg lithium deposit, with granitoids of the neighboring polygenic Dzos-Khusuingol batholith have been studied. SHRIMP-II U–Pb zircon analyses yielded an Early Paleozoic age of the granites and pegmatites. The tectonic settings of the formation of these rocks are discussed.     

The significance of europium anomalies in the REE spectra of granites and pegmatites,Mont Laurier,Quebec

Negative Eu anomalies in the REE spectra of granitic rocks are usually interpreted as evidence of earlier separation of a mineral phase such as plagioclase. Our study of the behaviour of the REE, and of U and Th, during late stage alteration of granites and the formation of pegmatites, suggests an alternative means for producing Eu anomalies. Albitization of earlier-formed plagioclase has depleted the granites in Eu and enriched the pegmatites in this element. This process is linked to the mobilization of U in the granites which is dependent on the oxidation state of the fluids. A systematic correlation between the ratios $\text{Eu}\text{Ce}$ and $\text{U}\text{Th}$ leads us to suggest a similar explanation for irregularities in the abundances of Ce and other rare-earths in the REE spectra of these rocks.We suggest that anomalous behavior of Eu in other environments or processes such as the alteration of basalt and the formation of certain ore deposit types may be caused partly by this mechanism.     

新疆阿尔泰造山带中伟晶岩型稀有金属矿床成矿 规律、找矿模型及其找矿方向

文章对阿尔泰造山带中的主要伟晶岩类型、时空分布特征、形成物源以及稀有金属矿化类型、形成条件(包括温度、压力、侵位深度)、可能控制因素等进行了归纳和总结,进而提出了阿尔泰伟晶岩成因模式、稀有金属矿化机制、伟晶岩型稀有金属矿床找矿模型及其找矿方向。阿尔泰稀有金属伟晶岩显示2个期次(同造山和后造山)和4个阶段(泥盆纪—早石炭世、二叠纪、三叠纪、早侏罗世)的成岩成矿特征。其中,以后造山阶段的三叠纪伟晶岩成岩及其Be、Li成矿作用最为显著。不同期次和阶段的伟晶岩显示规律的时空分布特征,稀有金属伟晶岩的成岩成矿明显受"构造-变质-物源-岩浆"的控制,而伟晶岩与周边花岗岩存在时代或物源上的解耦,表明阿尔泰伟晶岩不是由花岗质岩浆分异演化晚期的残余岩浆固结形成,由此提出阿尔泰不同时代伟晶岩的成因模式,即造山过程中加厚的不成熟地壳物质在伸展减压背景下发生小比例部分熔融(深熔)形成独立伟晶岩。通过对形成伟晶岩初始岩浆中磷含量、伟晶岩分异演化程度的评价以及基于围岩蚀变过程中全岩及蚀变矿物电气石中稀有金属Li、Rb、Cs含量特征,建立了阿尔泰伟晶岩型稀有金属矿床找矿模型、地质-地球化学找矿指标体系,并提出不同尺度的找矿方向。