The Distribution of Rare Metals in the LCT Pegmatites from the Giraúl Field, Angola
António Olímpio GonçalvesJoan Carles Melgarejo i DraperMaría Pura Alfonso AbellaSandra AmoresA. PaniaguaAndrés Buta NetoEduardo Alves MoraisAntoni Camprubí
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Abstract:
The Giraúl granitic pegmatite field in Angola is composed of five pegmatite types, the most evolved belong to the beryl-columbite, beryl-columbite-phosphate and spodumene types. Pegmatites are concentrically zoned with increased grain size toward a quartz core; the most evolved pegmatites have well-developed replacement units. These pegmatites are rich in Nb-Ta oxide minerals and the field has a moderate interest for critical elements such as Ta and Hf. Tourmaline, garnet and micas occur as accessory minerals. The abundance of Zr and Nb-Ta minerals increases with the evolution of the pegmatites, as well as the proportions of beryl and Li-rich minerals. The Ta/(Ta + Nb) ratio in Nb-Ta oxide minerals and the Hf/(Hf + Zr) ratio in zircon also increase with the evolution of the pegmatites and within each pegmatite body from border to inner zones, and especially in the late veins and subsolidus replacements. Textural patterns and occurrence of late veins with Ta-rich minerals suggest that Nb and especially Ta can be enriched in late hydrothermal fluids exsolved from the magma, along with Hf and other incompatible elements as Sn, U, Pb, Sb and Bi.Keywords:
Pegmatite
Columbite
Tourmaline
Spodumene
Pegmatite
Tourmaline
Muscovite
Spodumene
Petrogenesis
Topaz
Cassiterite
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目前研究已经显示，喜马拉雅淡色花岗岩具有良好的铍-铌钽-锂等稀有金属成矿潜力。其中珠穆朗玛峰（后文简称珠峰）西侧的普士拉一带，是喜马拉雅地区锂辉石伟晶岩集中的区域。本文报道在普士拉东北的珠峰北侧热曲地区，发现有含锂辉石伟晶岩脉，这些伟晶岩呈透镜体状集中赋存于肉切村群黄带层大理岩与北坳组钙质硅酸岩的接触界线部位，同围岩一起经历了强烈的变形，且未出现明显内部分带结构，矿物组成中包含锂辉石、透锂长石、绿柱石、铌钽铁矿、锡石等锂-铍-铌钽-锡稀有金属矿物，其Li2O含量达1.30%~2.15%，显示经历过高程度分异演化的岩浆结晶特征。热曲含锂辉石伟晶岩的发现表明珠峰地区具有锂成矿的良好前景，是未来锂矿产勘查的重点靶区，而藏南拆离系韧性剪切带中的肉切村群黄带层下部与北坳组顶部位置，是锂辉石伟晶岩的重要富集层位，值得今后在锂资源寻找过程中予以充分关注。;Present studies have revealed the leucogranites from Himalayan orogen are vital potential for Be-Nb-Ta-Li rare-metal mineralization, and spodumene-bearing pegmatites dominantly concentrated in the Pusi La of the Mount Qomolangma region. In this study, the first spodumene-bearing pegmatite is discovered in the Ra Chu transect, north of the Pusi La. The foliated pegmatites present as lenses concordant with the prominent mylonitic fabric and mainly concentrated in the contact boundary between the calc-silicate of the North Col Formation and the marble in the Yellow Band of the Rouqiecun Group. Internal zonation is not observed from the pegmatites. The mineral assemblages of the pegmatite contain Li-Be-Nb(Ta)-Sn minerals including spodumene, petalite, beryl, columbite and cassiterite. Li2O concentrations of the pegmatite samples from the transect are 1.30%~2.15%. Mineralogy and geochemistry results suggest the pegmatites have experienced a high degree of crystal fractionation, which indicates that there is a sufficient prospect for lithium mineralization in the Mount Qomolangma region. The bottom of the Yellow Band and top of the North Col Formation corresponding to the high-strain shear zone of the South Tibetan Detachment System is probably the enrichment layer of spodumene-bearing pegmatites and it is an important target zone for lithium mineral exploration in the future.
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Spodumene
Cassiterite
Columbite
Mylonite
Greenstone belt
Tourmaline
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Rare-element pegmatites (with elevated contents of Li, Sn, Ta, and W) were studied at four localities: Osova Bitýska, Vlkov, Křižinkov and Křovi. Pegmatite at Osova Bitýska has Li mineralization (lepidolite, tourmaline) and high contents of F, Rb, Cs and elevated Sn (lepidolite, topaz, tourmaline, phosphates). Pegmatite of Vlkov contains elevated Nb (oxides) and F (tourmaline). The pegmatite Křižinkov 2 contains elevated REE and Zr (monazite, zircon, xenotime, cheralite) and increased contents of Nb and F (columbite, tourmaline). The Li-pegmatite at Křižinkov is well fractionated, contains elevated Li, F, Sn, and W (tourmaline – elbaite, cassiterite, wodginite, ixiolite); similar degree of fractionation is in the adjacent pegmatite at Křovi. All studied pegmatites have tourmalines with high contents of Fe and F and very low contents of Mg, indicating their increased fractionation.
Tourmaline
Pegmatite
Cassiterite
Columbite
Topaz
Spodumene
Wolframite
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Abstract 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
Pegmatite
Muscovite
Columbite
Spodumene
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Pegmatite
Spodumene
Tourmaline
Mineral resource classification
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Tourmaline is a common mineral in rocks. It can record the geological information during its formation and is a geochemical tracer. Its in-situ element and boron isotope tests provide a broader application perspective. Therefore, the chemical and boron isotope characteristics of tourmaline can be used to trace the mineralization of pegmatite-type rare metals. West Kunlun is an important Li-Be metallogenic belt, and many important Li-Be deposits have been developed. Longmenshan is one of the recently discovered pegmatite-type Li-Be deposits in the Dahongliutan area. There are barren and spodumene-bearing pegmatites that all contain tourmaline in Longmenshan. According to the occurrence, crystal form, and chemical composition, the tourmalines in Longmenshan are divided into two types: Tur-I tourmaline in the barren pegmatite close to the granite and Tur-II tourmaline from the spodumene-bearing pegmatite. Compositionally belongs to the alkali group and schorl-dravite solid solution series, with Tur-I being mainly schorl and Tur-II being schorl-dravite. Tur-I tourmalines are rich in Li, Sn, V, Cr, Zn and Mn while Tur-II tourmalines are rich in Be, Sr and Sc. The δ11B of the Tur-I tourmalines hosted in the barren pegmatite and Tur-II from the spodumene-bearing pegmatite ranges from −8.38‰ to −6.81‰ and from −10.00‰ to −6.41‰, respectively. These characteristics indicate that the formation fluid of tourmaline in Longmenshan is mainly derived from magma, and other fluids are rarely involved. The large range of B isotope variation indicates that the Tur-II tourmalines are more obviously affected by fluid activity.
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Cassiterite
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AssrRAcr Analysis of fluid inclusions in spodumene, beryl, tourmaline, and quartz from miarolitic pegmatites ofAfghanistan, coupled with lithium aluminosilicate stability relations and with previous studies from gem pegmatites of San Diego County, California, indicate that pocket development in tourmaline-rich, miarolitic rare-element pegmatites occurs between approximately 475 and, 425C and between 2800 and 2400 bars. This range of P and ?r is comparable to the conditions of late-stage crystallization in geochemically similar massive (nonmiarolitic) pegmatites. Whether gem pockets form may be dependent largely on the timing of tourmaline crystallization. Formation of tourmaline removes an alkali borate component from residual pegmatitic melt, with the consequent deposition of other alkali aluminosilicate and oxide-forming minerals and exsolution of copius amounts of HrO. If tourmaline crystallization is inhibited until the late stages of pegmatite consolidation, the large quantities of HrO that are liberated may form pegmatitic pockets. IxrnonucrroN
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Spodumene
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Tourmaline is a kind of accessory mineral widely distributed in acidic igneous rocks and metasomatites,and the most widely distributed Fe-Mg-Li tourmaline includes several end members like dravite,schorl and elbaite as well as a series of transitional minerals.In the same area,the same rock is often characterized by the development of the same type of tourmaline.Nevertheless,the end members and transitional minerals are well developed in Nanping pegmatite and altered wall rocks,except for elbaite.Such a phenomenon is very rarely observed in the same type of granitic pegmatites both in China and abroad.In Nanping,tourmalines with different components are distributed in different types of granitic pegmatites and different differentiation evolution stage of the pegmatite.The schorl (Fe tourmaline) is widely distributed in the four types of pegmatites as well as their altered wall rocks.In the rare metal mineralized pegmatite,the tourmaline can be divided into two types,i.e.,the middle member of the Fe-Li series (Li-Fe tourmaline) and the members of the Mg-Fe series comprising dravite and Mg-Fe tourmaline.The two types of tourmalines are apparently different in the formation age and the paragenetic association of minerals.Based on detailed descriptions of physical-optical characteristics,chemical components,X-ray powder diffractions,infrared absorption spectra and thermal spectra of tourmalines from Nanping pegmatites,this paper discusses the evolution regularity and formation environment of these tourmalines.The schorl in Type Ⅰ pegmatite was formed under the conditions of upper hydraulic pressure,low content of rare elements and absolute domination of crystallization.In contrast,the schorl in Type Ⅱ-Ⅲ pegmatites was formed in a relatively low depth suggesting the beginning of the transformation to the open system,with rare elements concentrated in the pegmatite melt-solution in such an environment.The Li-Fe tourmaline in Type Ⅳ pegmatite was formed in a relatively open system with wide metasomatism,with its formation depth apparently shallower than that of Type Ⅰ-Ⅲ pegmatites.In such a formation environment,elements such as Li,Rb,Cs,Nb,Ta and Sn are highly concentrated in the pegmatite melt-solution.The formation environment of dravite in Type Ⅳ pegmatite was no longer belonging to endogenic pegmatite mineralization,but this mineral inherited some characteristics of Li-Fe tourmalines in Type Ⅳ pegmatite in content of elements such as Li.Its modes of occurrence are very rarely seen in pegmatite areas both in China and abroad.The formation sequence of tourmalines in Nanping pegmatites and wall rocks is on the whole in order of black Fe-tourmaline→yellowish green Li-Fe tourmaline→yellowish brown Mg-Fe tourmaline→grayish blue dravite.This sequence will surely play an important indicating role in researches on the formation environment and evolution of the Nanping pegmatite
Tourmaline
Pegmatite
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The tourmaline group minerals (schorl to elbaite) typically occur in the Li-bearing pegmatites from the Sao Jose da Safira region (the Oriental Pegmatitic Province (OPP) in Minas Gerais State, Brazil). The Fe/Mn ratios of the tourmalines are compared to those measured in columbite-tantalite and garnet and decrease northward continuously from the beryl-bearing pegmatites to the spodumene-bearing ones. The Fe/Mn ratio has been used as a qualitative index of fractionation which seems to reflect the regional zoning of the pegmatites around a hidden granite body. The Fe/Mn ratio values correlate negatively with the Na and Li contents. The REE, Nb, Co and Zn contents are controlled by the mineral assemblages.
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Tourmaline
Columbite
Spodumene
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