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。     

Trace element geochemistry by laser ablation ICP-MS of micas associated with Ta mineralization in the Tanco pegmatite,Manitoba, Canada

In the Tanco pegmatite, one of the world’s major Ta deposits, tantalum mineralization shows a complexity that reflects the complex petrogenesis of its host pegmatite. Micas are common in most of the pegmatite units and are intimately associated with the successive stages of Ta mineralization, from the wall zone to the central zones where micaceous replacement is pervasive. Different generations of micas, both primary and secondary, associated with Ta oxides, were selected for electron microprobe and laser ablation ICP-MS investigation. Their chemical trends are used to constrain the magmatic versus hydrothermal processes that played a role in their crystallization and their associated Ta mineralization. Micas range from dioctahedral muscovite to trioctahedral lepidolite through Al↔Li substitution. Unexpectedly, the most evolved compositions (low K/Rb ratios and high Li contents) occur in the wall zone; they are interpreted to reflect nonequilibrium crystallization from an undercooled melt, with or without boundary layer effects. In the central zones, the fine-grained mica–quartz assemblage hosts some coarser-grained Li-muscovite, which has the highest Ta contents (up to 400 ppm). These Li–F–a-rich micas are interpreted to originate from a magmatic metasomatic event, which was also at the origin of the MQM-style Ta mineralization at Tanco. However, the Li–Ta-poor, muscovite end-member compositions of fine-grained alteration micas suggest crystallization from an aqueous fluid, during a metasomatic (hydrothermal) event involving late pegmatitic fluids. The low Ta concentrations (around 50 ppm) of this fine-grained muscovite suggest that this fluid transported at least small amounts of Ta.     

Volatile contents of phlogopite micas from South African kimberlite

Phlogopite micas from nodules in South African kimberlites were analyzed for major elements with the electron microprobe and for volatile contents by high temperature mass spectrometry. The micas are from primary- (deformed) and secondary- (undeformed) textured grains in perodotite xenoliths, glimmerites, MARID (mica-amphibole-rutile-ilmenite-diopside) suite nodules and a mica megacryst. The major element and volatile contents of micas exhibiting these modes of occurrence overlap to a greater extent than indicated in previous studies. Concentrations of volatile species occupying structurally defined crystallographic sites (H₂O, F, Cl) are greater for many of the micas than predicted on the basis of the mica formula, particularly for the glimmerite and MARID suite samples. A correlation exists between micas with tetrahedral and octahedral cation deficiencies and those with excess H₂O, F and Cl. Substitution of H⁺ for tetrahedral and possibly octahedral cations may be responsible for the excess H₂O in these micas. Except for one sample, the major element and volatile data for the peridotite, glimmerite and MARID suite micas indicate that they crystallized at oxygen fugacities below the quartz-fayalite-magnetite buffer. F and K₂O are in the correct proportion in the micas to provide the source for these elements in alkali basalts, but not in mid-ocean ridge basalts. Kaersutite amphibole is a more likely source of potassium and fluorine in mid-ocean ridge basalts.     

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。     

Fluorine and chlorine in apatites,micas, and amphiboles of layered trap intrusions of the Siberian Platform

Geochemistry of chlorine and fluorine in apatites, micas, and amphiboles in rocks from eight intrusive complexes of the Siberian Platform has been first studied on the basis of new factual and analytical data (more than 1000 analyses). The main attention is focused on minerals from layered intrusions. Most apatites show F > Cl; the maximum contents of halogens are specific to chlorapatite (6.97 wt.% Cl) and fluorapatite (6.04 wt.% F). The total f value (f = Fe/(Fe + Mg), at.%) of femic minerals varies from 2 to 98 at.% in micas and from 22 to 95 at.% in amphiboles. The Cl-f and F-f trends show an increase in the Cl content and a decrease in the F content in the minerals with increasing f. Chlorine clearly exhibits ferrophilic properties, and fluorine has magnesiophilic properties. The halogen-richest minerals are fluorophlogopite (F = 7.06 wt.%, f = 7 at.%), chlorannite (Cl = 6.30 wt.%, f = 89 at.%), and chloroferrihastingsite (Cl = 5.22 wt.%, f = 90 at.%). Coexisting micas and amphiboles in the rocks are close in f value, but the micas are richer in Cl than the amphiboles. We assume that the halogen-containing minerals crystallized at the high pressure of halogen-hydrocarbon fluids at the levels of the MW, IW, and QIF buffers. The reducing conditions of the magmatism process are also evidenced by the presence of graphite and native metals in the rocks. The similarity of the Cl-f and F-f trends of micas and amphiboles from different intrusive complexes indicates the same mechanisms of the melt differentiation and mineral crystallization.     

Genesis of yellow manganese-rich elbaite from the Canary mining area,Lundazi, Zambia

The most important source of yellow gem elbaite is the Canary mining area in the Lundazi District of eastern Zambia. The tourmaline has been mined since 1983 from both pegmatite and eluvial/alluvial deposits, in colors typically ranging from yellow-green to yellow to orange and brown; much of the orange-to-brown material is heated to attain a ‘golden’ or ‘canary’ yellow color. The elbaite is Mn-rich (up to 9.18 wt% MnO documented in the literature) and contains small amounts of Ti and little or no Fe. The distinctive composition of this tourmaline is probably the result of the early crystallization of abundant schorl from an unusual B-rich, Li-poor pegmatite melt, which depleted Fe while conserving Mn until the late-stage crystallization of gem pockets. The simple mineralogy of the pegmatite consists of feldspars, quartz, and tourmaline; the lack of micas, phosphates, or Li minerals, and the presence of very little garnet, allowed Mn to fractionate to high levels during pegmatite crystallization. The presence of abundant gem tourmaline in a Li-poor pegmatite is highly unusual.     

Structural formulae of oxidized and hydroxyl-deficient micas and decomposition of the hydroxyl group

Studies of the structural formulae of Canadian micas indicate that the anionic framework of most natural micas differs from that of ideal mica in the quantity and/or charge of the anions per unit cell. Seven chemically analysed micas having variable chemical properties were chosen for heating experiments under controlled laboratory conditions. Ferric and ferrous iron, water and fluorine were determined after heating. The structural formulae of the oxidized and altered micas were calculated on the basis of 44+z valiencies whre z is a charge difference between the original and altered mica, and depends mainly on the degree of oxidation of iron.By comparing the structural formulae and anionic compositions of dehydrated and altered micas with those of ideal mica, it is possible to study the process of decomposition of the hydroxyl group during oxidation and dehydration. The decomposition of the hydroxyl group takes place either as a result of the oxidation of iron or by loss of water (and/or fluorine). The process of decomposition thus depends on the quantity of ferrous iron in the mica, on the composition of the hydroxyl group, and on the available atmospheric oxygen, or environmental conditions. The iron-poor micas, phlogopite, muscovite and lepidolite, lose hydroxyl mainly as water, without changing the charge of the layers, whereas biotite may also oxidize, resulting in a change of charge balance. Where the hydroxyl group is composed predominantly of fluorine the biotite either remains in a semi-oxidized state, or oxidizes at the expense of atmospheric oxygen or water. Micas containing partly-defincient anionic frameworks are susceptile to adsorption of water and argon from the atmosphere. The stability of mica during physical-chemical changes of the environmental conditions depends not only on its cationic content but also on the composition of its anionic network and layer charges.     

Tourmaline from the rare-element Pinilla pegmatite, (Central Iberian Zone, Zamora, Spain): chemical variation and implications for pegmatitic evolution

Summary Tourmaline is an ubiquitous constituent in the Pinilla de Fermoselle rare-element pegmatite (Zamora, Spain), as well as in barren pegmatitic and quartz–tourmaline veins inside the associated leucogranite. The rare-element pegmatite shows internal zoning, evolving from a barren facies, in the lower border zone, in contact with the leucogranite, to a Li-rich facies in the upper border zone, close to the host-rocks.Tourmalines from the veins within the leucogranite have highest Mg contents, and belong to the schorl–dravite series. The tourmalines from the rare-element pegmatite mostly belong to the schorl–elbaite series, with chemical compositions within the range of the end-members, whereas the tourmalines associated with the most evolved zone in the pegmatite belong to the elbaite–rossmanite series. The broad compositional range shown by the tourmalines correlates quite well with the pegmatite zoning. The most plausible substitution mechanism for the chemical evolution of tourmalines during crystallization seems to be Mg_–1Fe²⁺₁, [X]_–1^YAl_–1^XNa_–1^YFe²⁺₁, for the foitite–schorl series; ^YFe²⁺_–3^YAl_1.5^YLi_1.5, for the schorl–elbaite vector; ^XNa_–1^YLi_–0.5[X]₁^YAl_0.5, for the elbaite–rossmanite series; and, (OH)₁F₁ for all the tourmalines except the pink elbaites. This chemical variation in tourmaline is consistent with a crystal fractionation model for the evolution of the Pinilla pegmatite.     

深熔成因伟晶岩初始岩浆中稀有金属富集特征

本文以新疆阿尔泰切木尔切克地区的混合岩和伟晶岩为例,开展区内混合岩、伟晶岩与周围的变沉积岩（二云母片岩）的野外地质、全岩地球化学以及云母类矿物化学组成研究,目的在于揭示深熔作用形成的熔体中稀有金属富集特征。研究结果显示,阿尔泰切木尔切克地区的伟晶岩与混合岩中浅色体有相似的矿物组合和主要化学组成,且伟晶岩与暗色体呈互补的微量元素组成,表明切木尔切克伟晶岩为变沉积岩深熔成因。从二云母片岩到混合岩中浅色体和伟晶岩,白云母中Li、Be、Nb、Ta、Rb、Cs显示增加趋势,表明深熔作用形成浅色体及其汇聚成伟晶岩脉过程中可促进稀有金属的富集,尤其是Be和Ta,富集程度达3倍以上。低温条件下白云母脱水熔融,导致黑云母作为残留相,明显制约Li、Rb、Cs等稀有金属在熔体中的富集,但对Be的影响非常有限。结合阿尔泰伟晶岩广泛的Be成矿作用,推断阿尔泰伟晶岩很可能是深熔成因的。     

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.     

从云母微量元素特征探讨华南花岗岩的成因和演化

云母中微量元素可分为浅源指示元素和深源指示元素。浅源指示元素Nb、Ta、Sn、Li、Rb在南岭系列云母中的含量高于长江系列,而深源指示元素Co、Ni的含量长江系列大于南岭系列。云母中Nb、Ta、Sn、Li、Rb变化特征是自身成矿演化的直接示踪剂,而Co、Ni变化特征是Fe、Cu、Au、Mo(W)、Pb、Zn成矿演化的重要示踪剂。云母中Cu、Zn、W、Mo、U、Th等元素呈波动性变化,不能灵敏指示花岗岩的成因与演化。微量元素在云母中的选择分布主要受岩浆物源化学背景的制约,同时也受岩浆侵位和分异演化程度高低以及结晶的物理化学条件变化等因素的影响。     

福建南平花岗伟晶岩型稀有金属矿田Nb-Ta-Sn地球化学研究

福建南平是我国重要的花岗伟晶岩型矿田之一,伟晶岩主要产出在中-新元古代变质岩系中.伟晶岩脉的形成和加里东期西芹花岗岩具有密切的成因联系.根据伟晶岩主要矿物成分和所含Nb、Ta、Sn等元素可分为白云母-钾长石-早期钠长石型（Ⅰ）、白云母-钠长石-钾长石型（Ⅱ）、白云母-钾长石-钠长石型（Ⅲ）和白云母-钠长石-锂辉石型（Ⅳ）.对矿田中的中-新元古代变质岩系、西芹花岗岩,花岗伟晶岩中的Nb、Ta、Sn地球化学做了详细研究.认为南平伟晶岩是在区域地层和相关花岗岩中Nb、Ta、Sn含量具有高背景值的环境中产生的.伟晶岩在形成过程中,Nb、Ta、Sn元素伴随伟晶熔体的分异、演化及交代作用,其含量向晚期趋于增高,在白云母-钠长石-锂辉石伟晶岩中,Nb、Ta、Sn构成了工业矿体,而且主要以独立矿物形式存在.     

Metasomatic overprinting by juvenile igneous fluids,Igdlerfigsalik, South Greenland

The late Igdlerfigsalik centre, part of the Igaliko nepheline syenite complex in the Gardar Province, S Greenland, is bounded to the N and E by compositionally similar, earlier syenites forming the early Igdlerfigsalik and South Qôroq centres. In a circa 1 km wide zone parallel to its contact with late Igdlerfigsalik, the South Qôroq centre shows recrystallisation and alteration of mafic phases. South Qôroq therefore comprises two zones, termed the “unaltered” and the “recrystallised” zones. A study of the biotites from the rocks of the area of the present study has demonstrated variations in biotite chemistry, notably variations in elements inferred to reside on the octahedral and hydroxyl sites. Samples were chosen to provide a representative selection from each centre, avoiding pegmatites and late-stage veins. Electron probe microanalyses demonstrate little Cl in these biotites. Fluorine contents of some biotites can be demonstrated to vary with Fe/(Fe+Mg) and ∑(octahedral Al and Ti). Statistical analysis of the data sets as a whole, however, does not demonstrate simple relationships between fluorine content and these parameters and more complex crystallographic controls over fluorine uptake are inferred. Despite these variations, the relative fluorine contents characteristic of each centre can be determined, which, it is believed, reflect the characteristic F activity of fluids associated with each period of magmatism. In South Qôroq, the unaltered zone has a distinctive F-content in biotite, reflecting the original F-content of fluids associated with South Qôroq magmatism. However in the recrystallised zone, the F-contents are distinct, more closely resembling F-contents characteristic of late Igdlerfigsalik rocks. Reaction between a fluorine-rich, younger fluid and older fluorine-poor rocks is inferred. Despite evidence in the micas for fluid-rock interaction, whole-rock chemical changes are irregular in nature and appear not to conform to simple trends. Mass-balance equations are therefore unable to predict the levels of fluid-rock interaction. Similar exchange between late-stage fluids from younger centres and pre-existing rocks may be commonplace in igneous plutonic environments, and fluorine in micas may provide a relatively sensitive guide to fluid-rock interactions.     

Radiogenic 87Sr,its mobility,and the interpretation of RbSr fractionation trends in rare-element granitic pegmatites

The extent of fractionation of Rb and Sr is routinely used in petrogenetic modelling of igneous processes, including internal fractionation of individual pegmatites as well as large-scale evolution of pegmatite groups and fields. However, highly evolved granitic pegmatites may contain as much as 14000 ppm Rb and less than 150 ppm Sr. The total Sr in K-feldspar and micas from geologically old and Rb-rich pegmatites may consist predominantly of radiogenic ⁸⁷Sr, which obscures the original relationship of Rb to common Sr at the time of crystallization. A subtraction of radiogenic ⁸⁷Sr calculated from the Rb content and age of emplacement is possible, but it commonly results in negative concentrations of Sr. The relative immobility of Rb, analytically determined isotopic composition of Sr, apparent ages of the Rb, Sr-bearing minerals, high concentration of ⁸⁷Sr in coexisting Rb-poor phases, and experimental evidence indicate that post-crystallization migration of radiogenic ⁸⁷Sr is significant. Where isotopic data are not available, RbSr trends in geologically old and highly fractionated pegmatites are misleading and cannot be used for geochemical interpretation of pegmatite derivation or evolution.     

Characterisation of Two New Reference Materials: STL-1, the Stewart Lepidolite and ZA-1, the Zapot Amazonite

The analysis of granitic pegmatites still remains a challenge because suitable natural reference materials are scarce or not available. Two new reference materials were prepared at the Smithsonian Institution, to provide an avenue to pursue the geochemical analysis of micas and feldspars in granitic pegmatites: STL-1, the Stewart lepidolite (NMNH 174041) and ZA-1, the Zapot amazonite (NMNH 174042). STL-1 was prepared from lepidolite collected from the lithium-rich Stewart pegmatite, San Diego County, California (33°22'52'N, 117°03'41'W). ZA-1 was prepared from an amazonite from the topaz-bearing Zapot pegmatite, Mineral County, Nevada, (38° 41'N, 118 °33'W). The results of this study indicated that STL-1 and ZA-1 are homogeneous and could be used as reference materials that would allow the expansion of calibration curves in XRF analysis up to 16000 μg g⁻¹ for Rb, 2000 μg g⁻¹ for Cs and 100 μg g⁻¹ for Tl. STL-1 and ZA-1 also contain unusually high concentrations of Ga and Tl, and STL-1 of Nb.     

川西扎乌龙-青海草陇花岗伟晶岩型稀有金属矿床磷灰石地球化学特征及地质意义

扎乌龙-草陇锂矿床位于松潘-甘孜造山带中西部,为典型的花岗伟晶岩型稀有金属矿床.前人基于矿区花岗岩和伟晶岩紧密的时空及成因关系,认为伟晶岩与白云母花岗岩同源且成矿与花岗质岩浆的分异相关.然而,岩浆分异演化过程中熔体的信息记录及其何种地质过程对成矿起主导作用,仍缺乏有效制约.本文对矿区花岗质岩浆来源及其演化过程开展了研究...     

辽西兴城地区首山南花岗伟晶岩地质特征及其地质意义

通过对辽西兴城地区首山南花岗伟晶岩进行岩相学、锆石LA-ICP-MS U-Pb年代学及岩石地球化学的研究,揭示其形成年代、与其围岩成因关系以及其形成环境。研究结果表明:花岗伟晶岩的形成时代为(2 486±8)Ma(NSWD=3.1)。锆石CL图像形态特征以及微量元素特征显示,伟晶岩锆石属于岩浆成因但受到热液改造。首山南伟晶岩同绥中花岗岩的黑云母二长花岗岩作对比,在主量元素、微量元素及稀土元素方面表现出与围岩极其相似的特征。伟晶岩样品表现出高硅(SiO_2=77.28%~77.9%)、富碱(K_2O+Na_2O=5.5%~5.69%)、过铝质(A/CNK=2.53~2.54)特征,在SiO_2-K_2O判别图解上属钾玄系列。稀土元素总量低(33.1×10~(-6)~41.01×10~(-6))。球粒陨石标准化配分模式显示出LREE相对富集,HREE相对亏损;(La/Yb)_N=16.33~32.41,平均值为24.37;δEu值为0.92~1.11,表现为无异常至轻微正异常。这些地化特征总和表明首山南伟晶岩是黑云母二长花岗岩高度演化的结果。结合前人Hf研究结果,认为伟晶岩及其母岩形成均与底板垫托作用有关。     

Chemical composition and physical properties of lithium-iron micas from the Krušné hory Mts. (Erzgebirge)

Compositions of natural lithium-iron micas are approximated best by the sidero-phyllite-polylithionite join. These micas contain little or no magnesium and manganese. Their octahedral sheets contain close to two trivalent cations (mainly aluminum) in small crystallographic sites and a variable quantity of lithium and R⁺² (mainly iron) in large sites. Octahedral vacancies are situated mostly in large sites. Lithium and R⁺² approach a 44 replacement relationship in micas with octahedral occupancy close to six. Lithium and fluorine show a good positive correlation (small excess of fluorine over lithium), which indicates a crystallochemical association between them. There is a less distinct positive correlation between lithium and R⁺⁴.Based on simplifications, a calculation shows that about two-thirds of octahedral vacancies are caused by substitutions within the octahedral sheet, one-third, by tetrahedral substitutions. Different methods of calculating the crystallochemical formula yield slightly different numbers of octahedral vacancies, but do not affect the mica's position in plots of physical parameters against composition. If a crystallochemical formula is calculated from analysis of a mica contaminated with quartz, topaz, or feldspar, the apparent number of octahedral vacancies increases; such a formula exhibits unusual behavior in composition plots.     

Compositional controls on <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar ages of zoned mica from a rare-element pegmatite

A high spatial resolution Ar–Ar dating study of compositionally zoned micas using UV laser extraction has been used to investigate the effect of composition and compositional boundaries upon argon diffusion in mica. The crystals are characterised by muscovite cores and Li–F–Rb–Cs-rich lepidolite rims produced by the interaction of a residual melt-vapour phase with the original muscovite during the late stages of pegmatite crystallisation. Single crystals exhibit dramatic intra-grain age variations, with a maximum range of 2,880–2,117 Ma. Backscattered electron images combined with UV-laser analyses reveal that apparent ages younger than 2,600 Ma are directly associated with the compositionally distinct rims, suggesting that Ar diffusion was faster in the lepidolite than in muscovite. Although it is not possible to be fully quantitative using the present data, it seems that argon diffusion rates in lepidolite are similar to those in biotite. Major rearrangements of the crystal structure needed to accommodate high concentrations of lithium, fluorine and large interlayer cations (Rb⁺, Cs⁺) in muscovite account for lower argon retentivity of the crystal rims. In most cases the age profiles show no disturbance at the compositional boundary, despite the transition from a di-octahedral to tri-octahedral mica, indicating that in general the boundaries do not act as fast diffusion pathways. However, in some cases there is a clear drop in ages at the compositional boundary, which acts as an effective sub-grain boundary. The difference between the two types may be related to the internal structure of the compositional boundaries or their degree of contact with the grain boundary network of the rock.     

Zur Spurenelementverteilung in pegmatitischen K,Na-Feldspäten als Prospektionshilfe am Beispiel Tsaobismund/SWA

Zusammenfassung Die Verteilung der Elemente Be, Rb und Cs in den K,Na-Feldspäten eines beryllführenden Pegmatits (SW-Afrika) wird auf ihre Eignung als Höffigkeitshinweis geprüft. Dabei wird ein logarithmisches Verteilungsgesetz angewandt, dessen Verteilungskoeffizient Aussagen über eine Eigenmineralisation dieser Elemente ermöglicht. Im Gehalts-Erstarrungsdiagramm weichen die analytisch ermittelten Verteilungskurven vom idealen Verlauf bei einem Solidifikationsgrad von ca. 0.7 der Pegmatithülle ab. Dieser entspricht zeitlich dem Beginn der Kristallisation von Beryll bzw. Hellglimmer (Rb-, Cs-Fallen) in tieferen Stockwerken des Pegmatits.

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The distribution of the elements Be, Rb, and Cs in the K,Na-feldspars of a beryllbearing pegmatite (SW-Africa) is proved in its possible role as proximity indicator. The distribution coefficient from a logarithmic distribution law applied in this paper can be used to make declarations about the mineralization of those elements. In a content-solidification plot the analytically ascertained distribution curves deviates from ideal ones at a degree of solidification of 0.7 of the pegmatite shell. This corresponds with the beginning crystallization of beryll and micas (Rb-, Cs-traps) in deeper levels of the pegmatite.