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

The 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(P_(f)<σ_(2))are predominantly influenced by P-and T-fractures due to simple shearing in Shiziping dextral thrust shear zone during D_(2)deformation,whereas type A2 pegmatites(contemporaneous with D_(4))are governed by hydraulic fractures aligned with S_(0)and S_(0+1)stemming from fluid pressure(P_(f)<σ_(2)).Additionally,type B pegmatites(P_(f)≤σ_(2))exhibit orientations shaped by en echelon extensional fractures in local ductile shear zones(contemporaneous with D_(3)).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℃,320℃,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.

Rare-Element Pegmatites Rocks Rich and Li of the Aïr Massif of the Tchirozerine: Mineralogy and Chemical Composition Agadez Region-Northem Niger

The lithium potential in the Aïr massif is represented by mineral index of spodumene pegmatites and, lepidolite pegmatites. The mineral deposits of lithium occur in cluster or veins that cut the host rock or are located near the contact between the greenstone belt and granitic massif. The evidence of lithium is in the form of clusters or disseminated and stockwerk. Mineralogical characteristics show similarities between the Air Massif pegmatites and indicate the same homogenous source during the magma-generation process. The pegmatite rocks attracted the attention due to their wide exposure and composition, well appearance, and economically hosting of significant rare earth metals such as Sn and W. The mineralogical and petrographical investigations on the eight pegmatites rocks samples observed have a relative similarity, while a little difference in the shapes attributed to the ratio in the pegmatite rocks of the minerals. The occurrence of the kink band indicates the influence of the tectonic processes which affected the Aïr massif after the emplacement of late magmatic or post-magmatic pegmatites by injection into fractured rocks in the upper part of the crust. The Air Massif pegmatite has higher concentrations Li and of all trace elements except Hf and occasionally Zr, Ti, Sn and Mg of for the economic exploration.

Age and geochemical evolution of granite magmatism in Olkhon region from Caledonian syncollisional ore-free granite to the rare metal granite and pegmatite of Middle Paleozoic intraplate setting

The detailed description of two granite complexes in the Olkhon subterrane is given.The Early Paleozoic Sharanur complex was formed by granitization of gneisses of the Olkhon series.It includes migmatites,granite-gneisses,granites and pegmatites of normal alkalinity;they belong to the type of syncollisional granites.The Middle Paleozoic Aya granite complex includes mother Aya massif of amazonite-bearing granites and several types of rare-metal pegmatites.They have elevated alkalinity,low of Ba,Sr,and high LILE and HFSE elements contents.The Aya pegmatites lie in northwest cracks of stretching and associated with the rise of the territory under the influence of the North Asian plume.These cracks and pegmatites mark the beginning of a new intraplate geodynamic setting.Two geochemical types are distinguished among the pegmatites of this complex.These are amazonite pegmatites of Li-F type with Ta mineralization and complex type pegmatite with Be-Rb-Nb-Ta and Li-F mineralization(the Ilixin vein).The Tashkiney pegmatite vein is similar to Ilixin,but lies in the gneisses of the Olkhon series.It shows high concentrations of Be,Nb,Ta,as well as W,Sn,but lacks Li and F,due to a greater depth and higher temperature of the melt crystallization of this pegmatite.

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.

Mineralogy and radioactivity of pegmatites from South Wadi Khuda area,Eastern Desert,Egypt

Radioactive minerals in pegmatites associated with granitic rocks are commonly encountered in the south of the Wadi Khuda area and found as dyke-like and small bodies.They are observed within garnet-muscovite granites near the contact with older granitoids.Field surveys indicated that the studied pegmatites vary in dimensions ranging from 2 to 10 m in width and from 10 to 500 m in length.They are composed mainly of intergrowth of milky quartz,reddish-pink K-feldspar and plagioclase together with small pockets of muscovite.Field radiometric measurements indicated that radioactivity in pegmatites is more than twice that of their enclosing country rocks.Radionuclide measurements revealed that the average contents of U and Th increase gradually from rocks of dioritic to granodioritic composition(1.5×10-6 U and 4.3×10-6 Th) and increase significantly in biotite granites(5.8×10-6 U and 15.2×10-6 Th) but drastically decrease in muscovite granites(2.2×10-6 U and 5.6×10-6 Th).The average contents of U and Th of anomalous pegmatites are 95.3×10-6 and 116.9×10-6,respectively,indicating their uraniferous nature.In the south of the Wadi Khuda area,pegmatites are low in average Th/U(1.4) and high in average U/K(35.6),which suggests that uranium concentrating processes did not affect the pegmatites,indicating poor source-rocks.Mineralogical investigations of the studied pegmatites revealed the presence of secondary uranium minerals(kasolite and autunite),in addition to zircon,thorite,apatite,garnet and biotite.Primary and secondary radioactive mineralizations indicated that the mineralization is not only magmatic,but also post-magmatic.Electron microprobe analyses showed distinct cryptic chemical zoning within thorite where UO2 decreases from core to rim.This feature in thorite is sporadic,suggesting non-uniform redistributions of UO2 within thorite during magmatic processes.

Rare metal-bearing pegmatites from the Southeastern Desert of Egypt:Geology,geochemical characteristics,and petrogenesis

The pegmatite province of the Southeastern Desert (SED) is part of a pegmatite district that extends from Egypt (extends to 1200 km2). Rare metal pegmatites are divided into (1) unzoned, Sn-mineralized; (2) zoned Li, Nb, Ta and Be-bearing; and (3) pegmatites and pegmatites containing colored, gem-quality tourmaline. The Rb/Sr data reflect a crustal origin for the rare metal pegmatites and indicate that the original SED magma was generated during the peak of regional metamorphism and predates the intrusion of post-tectonic leucogranites. These bodies developed an early border zone consisting of coarse to very coarse muscovite+quartz+alkali feldspar, followed by an intermediate zone of dominant quartz+feldspar+muscovite rock. Garnet, tourmaline, beryl, galena, pyrite, amblygonite, apatite and monazite are rare accessories in both zones. Cassiterite tends to concentrate in replacement zones and along fractures in albite+quartz+muscovite-rich portions. The highest concentrations of cassiterite occur in irregular greisenized zones which consist dominantly of micaceous aggregates of green Li-rich muscovite, quartz, albite and coarse-grained cassiterite. The different metasomatic post-solidification alterations include sodic and potassic metasomatism, greisenization and tourmalinization. Geochemically, the pegmatite-generating granites have a metaluminous composition, showing a differentiation trend from coarse-grained, unfractionated plagioclase-rich granite towards highly fractionated fine- to medium-grained, local albite-rich rock. Economically important ore minerals introduced by volatile-rich, rare metal-bearing fluids, either primarily or during the breakdown of the primary mineral assemblages, are niobium-tantalum oxides, Sn-oxides (cassiterite), Li-silicates (petalite, spodumene, euctyptite, and pollucite), Li-phosphates (amblygonite, montebrasite and lithopilite) and minor REE-minerals (Hf-zircon, monazite, xenotime, thorian, loparite and yttrio-fluorite). The pollucite is typically associated with spodumene, petalite, amblygonite, quartz and feldspar. The primary pollucite has Si/Al (at) ratios of 2.53-2.65 and CRK of 79.5- 82.2. Thorian loparite is essentially a member of the loparite (NaLREETi2O6)-lueshite (NaNbO3)-ThTi2O6-ThNb4O12 quaternary system with low or negligible contents of other end-member compositions. The mineral compositionally evolved from niobian loparite to niobian thorian and thorian loparite gave rise to ceriobetafite and belyankinite with high ThO2 contents. Thorian loparite is metamict or partly metamict and upon heating regains a structure close to that of synthetic loparite NaLaTi2O6.

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 Nigeria are barren. Indices of fractionation such as Ba/Rb, K/Rb, Na/K, K/Cs, K/Ba reveal that the pegmatites are mineralized and different discrimination plots show that they are enriched in Li, Be, Sn, Ba, Ta, Ni, Cs, Sn and Zn. Rare metal indicative elements such as Ta, Nb, Rb, Cs, and Sn are enriched in the pegmatites, confirming that the pegmatites are mineralized.

Petrological and mineralogical characteristics of younger granites and pegmatites in the Wadi Haleifiya area, southeastern Sinai, Egypt

This study is concerned with the radioactivity and mineralogy of the younger granites and pegmatites in the Wadi Haleifiya area, southeastern Sinai Peninsula, Egypt. The area is occupied by metasediments, migmatites, older and younger granites. Most of these rocks, especially granites, are dissected by mafic and felsic dykes as well as pegmatites. The younger granites are represented by three main varieties: monzogranites, syenogranites and alkali feldspar granites. The monzogranite consists essentially of quartz, plagioclase, potash feldspar and biotite with minor musco-vite. Iron oxide, titanite, zircon and allanite are the main accessory minerals. Syenogranite is massive, medium- to coarse-grained and commonly exhibits equigranular and hypidiomorphic textures. It is made up essentially of potash feldspar, quartz, plagioclase and biotite. Iron oxides, allanite, epidote, titanite, and zircon are accessory minerals. The alkali feldspar granite consists mainly of perthite, quartz, alkali amphibole (arfvedsonite and riebekite), biotite, sub-ordinate plagioclase and aegirine. Iron oxide, zircon and apatite are accessory minerals, whereas chlorite and sas-surite are secondary minerals. The altered monzogranite and pegmatite recorded high radioelement contents. The eU reaches up to 120 (av.=82×10-6) in the altered monzogranite and up to 55 (av.=27×10-6) in the pegmatites. The high radioactivity in the altered monzogranite is due to the presence of thorite, uranothorite and metamict zircon. In the pegmatites, it is re-lated to the presence of uranophane, uranothorite, thorite, zircon, samarskite, monazite, xenotime, magnetite, ilmen-ite, hematite and rutile.

Study on the origin and rock-forming simulation experiments of aquamarine-bearing pegmatites of Ailaoshan,Yunnan,China

The Ailaoshan aquamarine-bearing pegmatites are associated with Proterozoic metamorphic rocks in the southern portion of the Ailaoshan fault-folded complex.The gem-bearing pegmatite mineralization zones of the region occur in areas generally consistent with the regional tectonic trend.The pegmatites are found in metamorphic rocks,migmatites and in the inner/outer contact zones of gneissoid granites. The Rb-Sr isochron drawn for the pegmatites is 26～31 Ma,(i.e.in Himalayan).The homogenization temperatures of melt and liquid inclusions in minerals vary from 185 to 920℃,which are comparable to the inclusions observed in banded migmatites and ptygmatic quartz veins in the surrounding metamorphic rocks. The mineralization fluids of the pegmatite were rich in HCO_3 and CO_2,and their compositional assemblages are comparable to metamorphic fluids.Results of H,O,C,Si etc.isotopic analyses and REE,and Be analyses indicates that the sources of mineralization components that formed the pegmatites are closely associated with metamorphic fluids and the enclosing metamorphic rocks. A pegmatite structure simulation experiment was conducted at high temperature and pressure(840℃and 1,500×105Pa.),with various metamorphic rock samples in a water-rich and volatile-rich environment.When the liquidus was reached,the temperature was gradually decreased at the rate of 5～10℃/day over a time period of three months.SEM energy-dispersive spectrum analyses were performed on the experimental products.A series of pegmatoid textures were observed including zonal texture,megacryst texture,drusy cavities,crystal druses,and vesicular texture along with more than ten types of minerals including plagioclase,microcline,quartz and biotite.Different metamorphic rock melts generated different mineral assemblages.Experiment results revealed that the partial melting of metamorphic rocks could form melts similar to pegmatite magmas. Based upon the geological characteristics,geochemistry,and pegmatite texture simulation experimental results,it is concluded that the mineralization components of Ailaoshan aquamarine-bearing pegmatites came from metamorphic rocks.The petrogenetic model for the origin of pegmatites is related to ultrametamorphism and metamorphic anatexis.

Lithium-bearing Pegmatite Exploration in Western Altun,Xinjiang,using Remote-Sensing Technology

Western Altun in Xinjiang is an important area,where lithium(Li)-bearing pegmatites have been found in recent years.However,the complex terrain and harsh environment of western Altun exacerbates in prospecting for Li-bearing pegmatites.Therefore,remote-sensing techniques can be an effective means for prospecting Li-bearing pegmatites.In this study,the fault information and lithologyical information in the region were obtained using the median-resolution remotesensing image Landsat-8,the radar image Sentinel-1 and hyperspectral data GF-5.Using Landsat-8 data,the hydroxyl alteration information closely related to pegmatite in the region was extracted by principal component analysis,pseudoanomaly processing and other methods.The high spatial resolution remote-sensing data WorldView-2 and WorldView-3 short-wave infrared images were used and analyzed by principal component analysis(PCA),the band ratio method and multi-class machine learning(ML),combined with conventional thresholds specified the algorithms used to automatically extract Li-bearing pegmatite information.Finally,the Li-bearing pegmatite exploration area was determined,based on a comprehensive analysis of the faults,hydroxyl alteration lithology and Li-bearing pegmatite information.Field investigations have verified that the distribution of pegmatites in the central part of the study area is consistent with that of Li-bearing pegmatites extracted in this study.This study provides a new technique for prospecting Li-bearing pegmatites,which shows that remote-sensing technology possesses great potential for identifying lithium-bearing pegmatites,especially in areas that are not readily accessible.

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), south- eastern Aswan, Egypt. The mineralization is related to muscovite granites and their pegmatite derivatives. The peg- matites 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 crystal- lization 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 peg- matites. 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 sec- ondary minerals such as uranophane..

The long-lived partial melting of the Greater Himalayas in southern Tibet, constraints from the Miocene Gyirong anatectic pegmatite and its prospecting potential for rare element minerals

The Cenozoic Himalayan leucogranite-pegmatite belt has been a hotspot for rare metal exploration in recent years.To determine the genesis of the pegmatite in the Himalayan region and its relationship with the Greater Himalayan Crystalline Complex(GHC),the Gyirong pegmatite in southern Tibet was chosen for geochronological and geochemical studies.The dating analyses indicate that the U-Th-Pb ages of zircon,monazite,and xenotime exhibit large variations(38.6‒16.1 Ma),with the weighted average value of the four youngest points is 16.5±0.3 Ma,which indicates that the final stage of crystallization of the melt occurred in the Miocene.The age of the muscovite Ar-Ar inverse isochron is 15.2±0.4 Ma,which is slightly later than the intrusion age,showing that a cooling process associated with rapid denudation occurred at 16‒15 Ma.TheεHf(t)values of the Cenozoic anatectic zircons cluster between−12 and−9 with an average of−11.4.The Gyirong pegmatite shows high contents of Si,Al,and K,a high Al saturation index,and low contents of Na,Ca,Fe,Mn,P,Mg,and Ti.Overall,the Gyirong pegmatite is enriched in Rb,Cs,U,K,Th and Pb and depleted in Nb,Ta,Zr,Ti,Eu,Sr,and Ba.The samples show a high 87Sr/86Sr(16 Ma)ratio of ca.0.762 and a lowεNd(16 Ma)value of−16.0.The calculated average initial values of 208Pb/204Pb(16 Ma),207Pb/204Pb(16 Ma)and 206Pb/204Pb(16 Ma)of the whole rock are 39.72,15.79 and 19.56,respectively.The Sr-Nd-Pb-Hf isotopic characteristics of the Gyirong pegmatite are consistent with those of the GHC.This study concludes that the Gyirong pegmatite represents a typical crustal‒derived anatectic pegmatite with low metallogenic potential for rare metals.The Gyirong pegmatite records the long‒term metamorphism and partial melting process of the GHC,and reflects the crustal thickening caused by thrust compression at 39‒29 Ma and the crustal thinning induced by extensional decompression during 28‒15 Ma.

Neoproterozoic Rare Element Pegmatites from Gitarama and Gatumba Areas, Rwanda: Understanding Their Nb-Ta and Sn Mineralisation

The aim of this work was to study the petrography, geochemistry of the pegmatites, their relationship to the mineralisation in Gitarama and Gatumba areas, and current processes that occurred after the primary emplaced neoproterozoic rare element pegmatites. Previous works on pegmatites were geochemistry and geological maps which are not enough for focused exploration and mine planning. Therefore, geological, petrographic, geochemical studies of neoproterozoic rare element pegmatites of Gatumba and Gitarama areas in relation to their mineralisation were carried out. The samples were analysed for mineral assemblages by petrographic light microscope;major elements by ICP AES;trace and rare earth elements by ICP MS. Petrographic studies revealed the mineral assemblages included quartz, microcline, biotite and major muscovites, which implied that there was the process of muscovitisation occurred after the primary emplacement of pegmatites. The results of geochemical analysis revealed that the silica content (in wt%) ranges from 59.5 - 80.5 with an average of 67.13 (in wt%) for the weathered pegmatite in Gatumba area ,and high percentages of SiO2 (in wt%) range 73.9 - 75.0 with an average of 73.15 (in wt%) for fresh pegmatite in Gitarama area. The pegmatites from Gatumba area were altered and much enriched in Rb (227 - 3460 ppm), Cs (2.59 - 24.7 ppm), Ta (2.6 - 268 ppm), Li (40 - 9224 ppm), W (240 - 10,000 ppm), Nb (13 - 517 ppm), Sn (24 - 8870 ppm). Their enrichment is commonly used as a marker of a magmatic-hydrothermal alteration. Conversely, the pegmatites from Gitarama area showed the low to moderate concentrations in Rb (321 - 337 ppm), Cs (5.47 - 5.62 ppm), Ta (1.3 - 1.6 ppm), Li (~20 ppm), W (5540 - 6410 ppm), Nb (3.9 - 4.3 ppm), Sn (28 - 44 ppm). The variation plot of ratios: Al2O3/(Na2O + K2O) versus Al2O3/(CaO + Na2O + K2O) for the pegmatites from study areas are higher than one (A/NK vs. A/CNK > 1) indicating peraluminous, the other samples of pegmatites indicated metaluminous (A/NK > 1 and A/CNK &Sigma;REE varying between 12.1 - 72.78 ppm and 45 - 54.37 ppm respectively, signifying low to medium form of enrichment. The pegmatite from Gatumba and Gitarama areas showed the K/Rb ratios ranging from 15.74 to 80.26 and from 190.41 to 199.39 respectively. As the pegmatite samples show K/Rb ratios less than 100 are commonly accepted for mineralization, therefore the pegmatites from Gatumba area were found mineralised, conversely to the pegmatite samples from Gitarama area, which were found barren.

Rare Metals (Ta-Nb-Sn) Mineralization Potential of Pegmatites of Igangan Area, Southwestern Nigeria

The study was carried out to determine the rare metal mineralization potential of some pegmatites associated with metasediments in the Igangan 240 NW sheet. Geological mapping on a scale of 1:50,000 revealed the pegmatites intrude metasediments and geochemical analysis for major, trace and rare earth elements were carried out using ICP MS/AES. Petrographic studies reveal a mineral assemblage of quartz, microcline and tourmaline;SEM studies revealed garnet and tourmaline to be close to the spessartine end-member and schorl respectively with albite occurring as the dominant plagioclase feldspar in the pegmatites. Result of geochemical analysis revealed SiO2 with an average of 73.91% in the whole rock pegmatite Al2O3 with an average of 13.93%, and average concentration of 0.57%, 4.3% and 4.77% for CaO, Na2O and K2O respectively. It also revealed average concentration of 29 ppm, 153 ppm, 30 ppm, 118 ppm and 129 ppm for W, Li, Ta, Nb and Sn in the mica respectively which is above the average values in the whole rock, felspars and tourmaline extracts. REE abundance in the whole rock pegmatites is low to moderate with ∑REE varying between 8 - 220 ppm, 2 - 23 ppm in feldspars and 3 - 32 ppm in mica signifying no form of REE enrichment. Geochemical analysis results and trace elemental plots such as K/Rb vs. Rb, Ta vs. Ga, Ta vs. Cs were used to assess rare metal mineralization and it revealed the pegmatites have low level of rare metal and rare earth element mineralization with average k/Rb values of 177 indicative of low fractionation levels in the pegmatites.

Emplacement and Evolution History of Pegmatites and Hydrothermal Deposits, Matale District, Sri Lanka

Excellent outcrops in Matale Sri Lanka provide unique insight into the emplacement and evolution history of hydrothermal and pegmatitic rocks in the central highlands of Sri Lanka. Field, structural, petrological, thermo-barometric studies in the metamorphic basement rocks in the central highlands and related hydrothermal deposits are presented in this study. Detailed petrographic and mineralogical data reveal peak metamorphic conditions for the crustal unit in the study area as 854 ± 44oC at 10.83 ± 0.86 kbar. Hydrothermal veins consisting of quartz and mica are closely related to cross-cutting pegmatites, which significantly post-date the peak metamorphic conditions of the crustal unit. Field relations indicate that the veins originated as ductile-brittle fractures have subsequently sealed by pegmatites and hydrothermal crystallization. Geological, textural and mineralogical data suggest that most enriched hydrothermal veins have evolved from a fractionated granitic melt progressively enriched in H2O, F, etc. Quartz, K-feldspar, mica, tourmaline, fluorite and topaz bear evidence of multistage crystallization that alternated with episodes of resorption. It was suggested that the level of emplacement of pegmatites of the Matale District was middle crust, near the crustal scale brittle-ductile transition zone at a temperature of about 600oC. For this crustal level and temperature range, it is considered very unlikely that intruding pegmatitic melts followed pre-existing cracks. As such the emplacement temperatures of the pegmatites could be well below the peak metamorphic estimates in the mafic granulites. The metamorphic P-T strategy and position of formation of hydrothermal deposits and pegmatites is summarized in the modified P-T-t-D diagrams.

P-Li-Be Bearing Pegmatites of the South East Brazil

The P-Li-Nb pegmatites are located in the south of Brazil, in the states of Minas Gerais and Espirito Santos. They represent the largest pegmatite fields of Brazil and the richest in precious stones. Two types of pegmatites are characterized by their mineralogical characteristics and tectonic and magmatic relations. The first group occurred during a compressive deformation phase D1 about 582 Ma and 550℃ - 700℃ and 4 - 5 kb. The second pegmatites group was formed during the decompression phase D2 (520 - 500 Ma) of the Brasiliano metamorphic rock fusion. The geochemical parameters of the P-Li-Be bearing pegmatites of the first group show the same trend fractionation, as suggested by the mineralogical composition. The variation of tourmaline and columbite-tantalite composition of the first group game again applies a change of melt composition during the regional development of the pegmatites. A systematic compositional trend seems to suggest a petrogenetic link between the pegmatites of the region. The Fe/Mn ratio of tourmaline in samples of the first group shows the same behavior as in columbite-tantalite and garnets. The simple pegmatites are transiting north in the gem-rich pegmatites. The Fe/Mn ratio not only shows qualitatively the fractionation index, the degree of regional development, but also the internal development of the body. The ratio shows a negative correlation with lithium. The Co, Zn and Nb contents are rising at first group, but falling when starting crystallization of garnet, columbite-tantalite, and Behierit.

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.

Geology and Mineral Chemistry of Uranium-and Thorium-bearing Minerals in Rare-Metal(NYF)Pegmatites of Um Solimate,South Eastern Desert,Egypt

The garnet muscovite granitic pegmatite of Um Solimate,in southern Egypt,represents a promising asset for strategic and economic metals,especially Bi-Ni-Ag-Nb-Ta as well as U and Th.The ore bodies occur as large masses,pockets and/or veins of very coarse-grained pegmatites,which consist mainly of K-feldspar,quartz and albite with subordinate muscovite,garnet,and biotite.Radiometric data revealed that e U-and e Th-contents of the pegmatites reach up to 39 ppm and 82 ppm,respectively.The studied pegmatites are enriched in primary U and Th minerals(uraninite,coffinite,thorianite and uranothorite)as well as Hf-rich zircon and monazite,which give rise to anomalous radioactive zones.Niobium-tantalium-bearing minerals(i.e.ferrocolumbite,microlite and uranopyrochlore),xenotime,barite,galena,fluorite,and apatite are ubiquitous,and,consequently,the studied pegmatites belong tothe Niobium-Yttrium-Fluorine-type(NYF)family.The noble metal mineralization includes argentite(Ag_(2)S),native Ni and Bi as well as bismite and bismoclite.In addition,beryl and tourmaline are observed in pegmatites near the contact with metasediments and ultramafic bodies.The observed compositional variations of Ta/(Ta+Nb)and Mn/(Mn+Fe)ratios in columbite(0.08-0.45 and 0.11-0.57,respectively)and Hf contents in zircon(3.54-6.46 wt%)may reflectan extreme degree of magmatic fractionation leading to formation of the pegmatite orebody.

Petrogenetic and Metallogenetic Characteristics of the Xikeng Granitic Pegmatites, Fujian Province

The Xikeng pegmatite field lies on the eastern margin of the south China fold system in Fujian Province,and it is located at the junction of three major tectonic units. The distribution of pegmatites is obviously controlled by the fold system.There exists apparent in jectionrelationship between the pegmatites and the surrounding Sinian schist and granulitite.The granitoidsextensively distributed in the field belong either to the Variscan or to the Yenshanian cycle,and it is evidentthat the pegmatites are genetically related to Variscan migmatitic granites. The pegmatites can be grouped into four types:muscovite-orthoclase-albite pegmatite（I）.muscovitedlbite-orthoclase pegmatite（Ⅱ）,muscovite-orthoclase-albite pegmatite（Ⅲ）,and muscovite-albitespodumene pegmatite（IV）.Owing to strong metasomatism and multi-stage emplacement of pegmatitic meltsolution,the sequence of interior assemblage zones in the pegmatites does not always represent the sequenceof original crystallization. The mineral composition of the pegmatites is extremely complicated.81 kinds of minerals have so farbeen found、From type I to type IV,the mineral assemblage tend to get increasingly complex.together withthe synchronous intensification of rare-metal and Sn mineralizations.Most of the type-IV pegmatites are ofeconomic value The features of fluid inclusions in the minerals are significantly different not only in different typos ofpegmatite,but also in different parts of a single pegmatite vein.Theδ18O values of migmatitic granite andpegmatites are comparatively low（9.3-10.4‰）,and those of rock-forming fluids are higher than 9.5‰ Isotopic ages of the pegmatites brangs from 235 to 328 Ma with initial 87Sr/86Sr ratios being ”.715-0.746. According to the temporal and spatial relationships between the pegmatites and the migmatitic granitecombined with the features of the pegmatites themselves,it can be concluded that the Xikeng pegmatites arethe product of differentiation closely related to the migmatitic granite.

Aspect of the specialty metals potentials of pegmatites in western Oban Massif,SE Nigeria:A preliminary geochemical approach

Minerals in general and rare-metals in particular are of invaluable importance in the development and industrialization of any nation.This is true as they could affect a nation's national income,employment ratio,Gross National Product,and its entire economy.The study area(western Oban Massif-SE Nigeria) was investigated for its specialty metals bearing potentials because of(1) its geology,which is similar to other specialty metals bearing areas of the world;(2) past works,which points to the area as a likely source of economic deposit of such metals;and(3) the soaring cost of these rare metals in the light of recent upsurge in global information/telecommunication technology.Results from the study,show that only cesium(Cs) and beryllium(Be) show minute signs or indications of enrichment if properly harnessed.Thus these metals should be studied in details to ascertain their economic potentials.The rocks are too deficient in the other rare-metals(lithium,zirconium,strontium,tantalite,niobium,tungsten,gallium,and chromium) to warrant further analysis.