PEG 2013 Abstract Volume

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CONTRIBUTIONS TO THE

6th

INTERNATIONAL

SYMPOSIUMON GRANITIC PEGMATITES

EDITORSWILLIAM B.SIMMONS

KAREN L.WEBBER

ALEXANDER U.FALSTER

ENCARNACIN RODA-ROBLES

SARAH L.HANSONMARA FLORENCIA MRQUEZ-ZAVALA

MIGUEL NGELGalliski

GUESTEDITORSANDREW P.BOUDREAUX

KIMBERLY T.CLARK

MYLES M.FELCH

KAREN L.MARCHAL

LEAH R.GRASSI

JON GUIDRY

SUSANNA T.KREINIK

C.MARK JOHNSON

COVER DESIGN BY RAYMOND A.SPRAGUE

PRINTED BY RUBELLITE PRESS,NEW ORLEANS,LA


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PEG 2013: The 6th International Symposium on Granitic Pegmatites

PREFACE Page ii

PREFACE

Phosphate Theme Session

Dedicated to: Franois Fontan, Andr-Mathieu Fransolet and Paul Keller,

It is a pleasure for the organizing committee to

introduce the special session on phosphates, as animportant part of the program of the 6th

International Symposium on Granitic Pegmatites.

The complexity of phosphate associations,

commonly occurring as a mixture of several fine-grained phases, make their study difficult. However

over the last decades, the number of publications on

pegmatite phosphate minerals has increased

exponentially as a result of new techniques. The

early investigations focused on description,

composition and paragenesis. Now that most of the

phases have been extensively described and

replacement sequences of secondary phosphates arebetter understood, we are at the beginning of a new

frontier of investigations of the petrogenetic role of

phosphates during the evolution of the pegmatites

and on their relationship to other mineral phases,such as silicates. These kinds of studies are getting

more and more abundant, with some interesting

examples in this volume.

We take this opportunity to honor FranoisFontan, Paul Keller and Andr-Mathieu Fransolet.

These three exceptional researchers have

contributed enormously to the advancement in the

knowledge on phosphates during the last decades.

They worked individually and jointly, always in an

enthusiastic, effective and tireless way. They passed

their knowledge and interest in phosphates onto

many younger researchers. Now we would like tothank them sincerely for all their work.

Franois Fontan was born in Toulouse

(France) in 1942, and he passed away six

years ago, in July 2007, at the age of 64.

Franois spent his career in research with the

CNRS, at the Universit Paul-Sabatier in

Toulouse (France). He obtained his Doctorat

dEtat in 1971, under the guidance ofFranois Permingeat, a founder of the

International Mineralogical Association. He

was particularly involved in investigations of

the mineralogy and genesis of phosphate

minerals in granitic pegmatites. With other

specialists, he worked on the characterization

of complex assemblages of phosphates inpegmatites in several European and African

countries. Among his list of more than one

hundred articles are descriptions of eight new

mineral species. Fontanite,

(Ca[(UO2)3(CO3)2O2]6(H2O)) was named inrecognition of his accomplishments.


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PREFACE Page iii

Andr-Mathieu Fransoletwas born in

Heusy (Belgium) in May of 1947. He

finished his studies in Geology at the

Universit de Lige in October of 1969. In

1975 he defended his PhD Thesis on

Geological and Mineralogical Sciences, atthe same university, where he also

developed most of his fruitful research,

until he retired last year. During that time

he published more than one hundred

papers, most of them on phosphates

associated with pegmatites all over the

world and he contributed to the discovery

of ten new mineral species. There are two

mineral phases named after him:

Fransoletite (H2Ca3Be2(PO4)44(H2O)) and

parafransoletite

(Ca3Be2(PO4)2(PO3,OH)24(H2O))

Paul Kellerwas born in Sarata, Romania

in 1940. He finished his studies in Crystal

Chemistry of Phosphates and Arsenates at

The University of Stuttgart (Germany), in

1973. He worked for many years at the

Institut fr Mineralogie und Kristallchemie,

at the University of Stuttgart (Germany),where he retired in 2006. His research on

phosphates was very broad, with the

publication of more than one hundred

scientific papers, including the discovery of

close to a dozen new mineral species. The

phosphate paulkellerite

(Bi2Fe(PO4)O2(OH)2) was named after him,

in recognition of his productive career.

(photograph courtesy of Anthony Kampf).


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TABLE OF CONTENTS Page iv

TABLE OF CONTENTS

KEYNOTE SPEAKERS

Crystal Chemistry and Geothermometric Applications of Primary Pegmatite Phosphates

F.HATERT ........................................................................................................................................................................................... 1

Reading Pegmatites: What Minerals Say

D.LONDON ........................................................................................................................................................................................... 5The Late-Stage Mini-Flood of Ca in Granitic Pegmatites: A Working Hypothesis

R.MARTIN ............................................................................................................................................................................................ 7

Productive Zones in Gem-Bearing Pegmatites of Central Madagascar, Geochemical Evolution and Genetic InferencesF.PEZZOTTA ........................................................................................................................................................................................ 9

CONTRIBUTIONS

Pegmatitic Rocks in a Migmatite-Granite Complex (NW Portugal)M.AREIAS,M.RIBEIRO,A.DRIA ....................................................................................................................................................... 12

Unraveling the Fluid Evolution of Mineralized Pegmatites in NamibiaL.ASHWORTH,J.KINNAIRD,P.NEX .................................................................................................................................................... 14

The Phosphate Minerals Assemblages from Joco Pegmatite, Minas Gerais, Brazil

M.BAIJOT,F.HATERT,S.PHILIPPO ..................................................................................................................................................... 16Preliminary Crystallography and Spectroscopy Data of Euclase from Northeast of Brazil

S.BARRETO,DE B.,V.ZEBEC,AOBI,R.WEGNER,RBRANDO,P.GUZZO,L.SANTOS,V.BEGI,V.BERMANEC............................. 18

Two Generations of Microcline from Mount Malosa Pegmatite, Zomba District, MalawiV.BERMANEC,M.HORVAT,.IGOVEKI GOBAC,V.ZEBEC ............................................................................................................... 20

Chrysoberyl in Association with Sillimanite at Roncadeira in the Borborema Pegmatite Province, Northeastern Brazil:Petrogenetic and Gemological Implications

H.BEURLEN,D.RHEDE,R.THOMAS,D.SOARES,M.DA SILVA ............................................................................................................ 22

Cartography to Chemistry: Estimating the Bulk Composition of the Mt Mica Pegmatite via Map Analysis, Maine, USAA.BOUDREAUX,L.GRASSI,W.SIMMONS,A.FALSTER,K.WEBBER,G.FREEMAN ................................................................................ 24

Accessory Mineralogy of an Evolved Pegmatite, Dickinson County, MichiganT.BUCHHOLZ,A.FALSTER ,W.SIMMONS ............................................................................................................................................ 26

The Ponte Segade Deposit (Galicia, NW Spain): A Recently Discovered Occurrence of Rare-Element Pegmatites

F.CANOSA,M.FUERTES-FUENTE,A.MARTIN-IZARD ........................................................................................................................... 28

Contact Zone Mineralogy and Geochemistry of Mount Mica Pegmatite, Oxford Co., Maine, USAK.CLARK,W.SIMMONS,K.WEBBER,A.FALSTER,E.RODA-ROBLES,G.FREEMAN.............................................................................. 30

Preliminary57

Fe Mssbauer Spectroscopy Study of Metamict Allanite-(Ce) from Granitic Pegmatite, Fone, Aust-Agder,Norway

A.OBI,C.MCCAMMON,N.TOMAI,V.BERMANEC ........................................................................................................................... 32

Crystal Chemistry of M2+

Be2P2O8(M2+

= Ca, Sr, Pb, Ba) Beryllophosphates: A Comparison with Feldspar AnaloguesF.DAL BO,F.HATERT,C.RAO ........................................................................................................................................................... 34

Spatial Statistical Analysis Applied to Rare-Elements LCT-Type Pegmatite Fields: An Original Approach to ConstrainFaults-Pegmatites-Granites Relationships

S.DEVEAUD,C.GUMIAUX ,E.GLOAGUEN,Y.BRANQUET .................................................................................................................... 36

Be and Zn Behavior During Anatetic Formation of Early Pegmatoid Melts in Variscan TerrainsAn Example from theArga Pegmatite Field, Northern Portugal

P.DIAS,C.LEAL GOMES ..................................................................................................................................................................... 37Structural and Paragenetic Analysis of Swarms of Bubble Like Pegmatites in a Miarolitic Granite from Assuno South ViseuCentral Portugal

P.A.DIAS,P.ARAJO,M.PEREIRA,B.PEREIRA,J.AZEVEDO,J.OLIVEIRA,J.CARVALHO,C.LEAL GOMES......................................... 39

Petrogenesis of Peraluminous Anatectic PegmatoidsP.A.DIAS,C.LEAL GOMES ................................................................................................................................................................ 41

Beryl and Be-mineralization in Pegmatites of the Oxford Pegmatite Field, Maine, USAA.FALSTER, J.NIZAMOFF ,W.SIMMONS,R.SPRAGUE ........................................................................................................................ 43


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Compositional and Textural Evolution of Amphibole and Tourmaline in Anatectic Pegmatite Cutting Pyroxene GneissNear Miroov, Moldanubian Zone, Czech Republic

P.GADAS,M.NOVK,J.FILIP,M.GALIOV VAINOV ......................................................................................................................... 45

Regional Zoning in a LCT (Li, Cs, Ta) Granite-Pegmatite System in the Eastern Pampean Ranges of San Luis, ArgentinaM.GALLISKI,INVITEDSPEAKER ......................................................................................................................................................... 47

The Complex Nb-Ta-Ti-Sn Oxide Mineral Intergrowths in the La Calandria Pegmatite, Caada Del Puerto, Crdoba,Argentina

M.GALLISKI,M.F.MRQUEZ-ZAVALA,P.

ERN,R.LIRA,K.FERREIRA ............................................................................................. 49Granitic Pegmatites in the Yukon, Northwest Territories and British Columbia, Canada

L.GROAT,INVITEDSPEAKER,J.CEMPIREK,A.DIXON ......................................................................................................................... 51

LCT and NYF Pegmatites in the Central Alps. Exhumation History of the Alpine Nappe Stack in the Lepontine DomeA.GUASTONI,G.PENNACCHIONI ......................................................................................................................................................... 53

Mineral-Chemistry of Nb-Ta-Y-REE-U Oxides in the Pegmatites of Central AlpsA.GUASTONI ...................................................................................................................................................................................... 55

Mineralogy and Geochemistry of Pelitic Country Rock within the Sebago Migmatite Domain, Oxford Co., Maine

J.GUIDRY,A.FALSTER,W.SIMMONS,K.WEBBER.......................................................................................................................................... 57

Wodginite Group Species from the Emmons Pegmatite, Greenwood, Oxford County, Maine, USAS.HANSON,A.FALSTER,W.SIMMONS,R.SPRAGUE ........................................................................................................................... 59

Mineralogy, Petrology and Origin of the Kingman Pegmatite, Northwestern Arizona, USAS.HANSON,W.SIMMONS,A.FALSTER ................................................................................................................................................ 61

Rare Earth Minerals of the Mukinbudin Pegmatite Field, Mukinbudin, Western AustraliaM.JACOBSON ..................................................................................................................................................................................... 63

A Study of Heavy Minerals in a Unique Carbonate Assemblage from the Mt. Mica Pegmatite, Oxford County, MaineM.JOHNSON,W.SIMMONS,A.FALSTER,G.FREEMAN ......................................................................................................................... 65

Structural Insights Gleaned from Palermos Two Newest Minerals, Falsterite and NizamoffiteA.KAMPF............................................................................................................................................................................................ 67

Pan-African PegmatitesPossibly the Best Pegmatites in the World?J.KINNAIRD,INVITEDSPEAKER,P. NEX ............................................................................................................................................ 69

Sr-and Mn-enrichment in Fluorapatite from Granitic Pegmatites of Oxford County, MaineS.KREINIK,M.FELCH,W.SIMMONS,A.FALSTER,R.SPRAGUE ........................................................................................................... 71

Kystaryssky Granite Complex: Tectonic Setting, Geochemical Peculiarities and Relations with Rare-Element Pegmatitesof the South Sangilen Belt (Russia, Tyva Republic)

L.KUZNETSOVA .................................................................................................................................................................................. 73

The Influence of C-O-H-N Fluids on the Petrogenesis of Low-F Li-Rich Spodumene Pegmatites, Sangilen Highland, TyvaRepublic

L.KUZNETSOVA,V.PROKOFEV ........................................................................................................................................................... 75

Seixoso-Vieiros Rare Element Pegmatite Field: Dating the Mineralizing EventsA.LIMA,L.MENDES,J.MELLETON, E.GLOAGUEN,D.FREI ................................................................................................................. 77

Characterization and Origin of Common Pegmatites: the Case of Intragranitic Dikes from the Pavia Pluton (WesternOssa-Morena Zone, Portugal)

S.M.LIMA,A.NEIVA,J.RAMOS ........................................................................................................................................................... 79

Chamber Pegmatites of Volodarsk, Ukraine, the Karelia Beryl Mine, Finland and Shallow Depth Vein Pegmatites of theHindukush- Karakorum Mountain Ranges. Some Observations on Formation, Inner Structures, Rare and Gem Crystals inthese Oldest and Youngest Pocket Carrying Gem Pegmatites on Earth

P.LYCKBERG,V.CHOURNOUSENKO,A.HMYZ ..................................................................................................................................... 81

Compositional Evolution of Primary to Late Tourmalines from Contaminated Granitic Pegmatites; A Trend Towards Low-T

Fibrous TourmalinesI.MACEK,M.NOVK,R.KODA,J.SEJKORA ....................................................................................................................................... 84

Phosphates From Rare-Element Pegmatites of the East Sayan Belt, Eastern Siberia, RussiaV.MAKAGON....................................................................................................................................................................................... 86

Bismutotantalite from Pegmatites of the Western Baikal Region, East Siberia, RussiaV.MAKAGON,O.BELOZEROVA ............................................................................................................................................................ 88

Geochemistry, Mineralogy and Evolution of Mica and Feldspar from the Mount Mica Pegmatite, Maine, USAK.MARCHAL,W.SIMMONS,A.FALSTER,K.WEBBER,E.RODA-ROBLES,G.FREEMAN......................................................................... 90

The Secular Distribution of Granitic Pegmatites and Rare-Metal PegmatitesA.MCCAULEY,D.BRADLEY ................................................................................................................................................................ 92


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The Composition of Garnet as Indicator of Rare Metal (Li) Mineralization in Granitic PegmatitesL.MORETZ,A.HEIMANN,J.BITNER,M.WISE,D.RODRIGUES SOARES,A.MOUSINHO FERREIRA ......................................................... 94

Amazonite-Cleavelandite Replacement Units in NYF-Type PegmatitesResidual Fluids or Immiscible Melts?A.MLLER,B.SNOOK,J.SPRATT,B.WILLIAMSON,R.SELTMANN ....................................................................................................... 96

Feldspars, Micas and Columbite-Tantalite Minerals from the Zoned Granitic Lepidolite-Subtype Pegmatite at Namivo, AltoLigonha, Mozambique

A.NEIVA ............................................................................................................................................................................................. 98

The Paragenesis of Falsterite and Nizamoffite, Two New Zinc-Bearing Secondary Phosphates from the Palermo No. 1Pegmatite, North Groton, New Hampshire

J.NIZAMOFF,A.FALSTER,W.SIMMONS,A.KAMPF,R.WHITMORE ...................................................................................................... 99

Contamination Processes in Complex Granitic PegmatitesM.NOVK,INVITEDSPEAKER ........................................................................................................................................................... 100

Gormanite Distribution and Equilibrium Conditions in Pegmatite from Corrego Frio Fields, Galileia, Minas Gerais, BrazilF.PIRES,H.AMORIM ,M.FONSECA .................................................................................................................................................. 104

Chrysoberyl and Alexandrite Mineralization in the Oriental Pegmatite Province of Minas Gerais, BrazilDeposit ControlsF.PIRES,M.FONSECA,R.LIMA ........................................................................................................................................................ 106

Phosphate Minerals from the Galileia Pegmatite Field, Minas Gerais, Brazil: Equilibrium ConditionsF.PIRES,N.PALERMO,M.FONSECA,R.LIMA ................................................................................................................................... 108

Uranium in Pegmatites-Brazilian Case StudyF.PIRES,S.MIANO,R.LIMA ............................................................................................................................................................. 110

Zoning in the Urubu Pegmatite, Aracuai District, BrazilLi-Rich ParagenesesF.PIRES,J.SA,R.LIMA .................................................................................................................................................................... 112

Mineral Equilibria in the Volta Grande Ta-Nb-Sn-Li Pegmatite, Sao Joao Del Rei District, Minas Gerais, BrazilF.PIRES,D.ARANHA ,S.MIANO ,C.ASSUMPO ,L.SILVA .............................................................................................................. 114

Iron-Bearing Beryl from Granitic Pegmatites; EMPA, LA-ICP-MS, Mssbauer Spectroscopy and Powder XRD StudyJ.PIKRYL,M.NOVK,J.FILIP,P.GADAS,M.GALIOV VAINOV .................................................................................................... 116

Textural and Mineralogical Features of the Li-F-Sn-Bearing Pegmatitic Rocks from Castillejo de Dos Casas (Salamanca,Spain): Preliminary Results

E.RODA-ROBLES,A.PESQUERA,P.GIL-CRESPO,I.GARATE-OLABE,U.OSTAIKOETXEA-GARCA...................................................... 118

Pegmatites from the Iberian Massif and the Central Maine Belt: Differentiation of Granitic Melts versus Anatexis?E.RODA-ROBLES,INVITEDSPEAKER, W.SIMMONS,A.PESQUERA,K.WEBBER,A.FALSTER............................................................ 120

Fe-Mn-(Mg) Distribution in Primary Phosphates and Silicates from the Beryl-Phosphate Subtype Palermo No.1 Pegmatite(New Hampshire, USA)

E.RODA-ROBLES,J.NIZAMOFF,W.SIMMONS,A.FALSTER ............................................................................................................... 123Trace Element Content in Primary Fe-Mn Phosphates from the Triphylite-Lithiophilite, Graftonite-Beusite and Triplite-Zwieselite Series: Determination by LA-ICP-MS Methods and Preliminary Interpretation

E.RODA,A.PESQUERA,S.GARCA DE MADINABEITIA,J.I.GIL IBARGUCHI,J.NIZAMOFF,W.SIMMONS,A.FALSTER,M.A.GALLISKI... 125

New Insights into the Petrogenesis of the Berry-Havey Pegmatite from Tourmaline Petrography and ChemistryE.RODA-ROBLES,W.SIMMONS,A.PESQUERA,P.GIL-CRESPO,J.NIZAMOFF,J.TORRES-RUIZ ........................................................ 127

Lead-Rich Green Orthoclase from Broken Hill PegmatitesL.SNCHEZ-MUOZ,I.SOBRADOS,J.SANZ,G.VAN TENDELOO,A.CREMADES,XIAOXING KE,F.ZIGA,M.RODRIGUEZ ,A.DEL CAMPO,Z.GAN ..................................................................................................................................................................... 130

Twin and Perthitic Patterns of K-Rich Feldspars of Pegmatites from Different Geological EnvironmentsL.SNCHEZ-MUOZ,P.MODRESKI,V.ZAGORSKY,B.FROST,O.DE MOURA ..................................................................................... 131

Chemical Variation of Li Tourmaline from Nagatare Pegmatite, Fukuoka Prefecture, JapanY.SHIROSE,S.UEHARA .................................................................................................................................................................... 133

Mount Mica Pegmatite, Maine, USAW.SIMMONS,A.FALSTER,K.WEBBER,E.RODA-ROBLES............................................................................................................................ 135

Towards Exploration Tools for High Purity Quartz in the Bamble-Evje Pegmatite Belt, South NorwayB.SNOOK,A.MLLER,B.WILLIAMSON,F.WALL ............................................................................................................................... 137

Mineralogy Meets Mineral Economics: How Does Pegmatology Interface with the Mineral Industry, Society and MarketForces

M.SWEETAPPLE,INVITEDSPEAKER ................................................................................................................................................. 139

Internal Evolution of an Adirondack Pegmatite Dike, New YorkP.TOMASCAK,S.PRATT,C.SPATH III ............................................................................................................................................... 141


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Inclusions and Intergrowths in Monazite-(Ce) and Xenotime-(Y): Thermal Behavior and Relation to Crystal-ChemicalProperties

N.TOMAI,V.BERMANEC,R.KODA,M.OUFEK,A.OBI ........................................................................................................... 142

Namibite and Hechtsbergite from the Nagatare Mine, Fukuoka Prefecture, JapanS.UEHARA,Y.SHIROSE .................................................................................................................................................................... 144

A Single-Crystal Neutrons Diffraction Study of Brazilianite, NaAl3(PO4)2(OH)4

P.VIGNOLA,G.GATTA,M.MEVEN .................................................................................................................................................... 146Hydroxyl Groups and H2O Molecules in Phosphates: A Neutron Diffraction Study of Eosphorite, MnAlPO4(OH)2H2O

P.VIGNOLA,G.GATTA,G.NNERT ................................................................................................................................................... 148

Karenwebberite, Na(Fe2+

,Mn2+

)PO4, a New Phosphate Mineral Species from the Malpensata Pegmatite, Lecco Province,Italy

P.VIGNOLA,F.HATERT,A-MFRANSOLET,O.MEDENBACH,V.DIELLA,S.AND ................................................................................ 150

Crystal-Chemistry of A Near End-Member Triplite, Mn2+

2(PO4)F, from Codera Valley (Sondrio Province, Central Alps, Italy)P.VIGNOLA,G.GATTA,F.HATERT,A.GUASTONI,D.BERSANI .......................................................................................................... 152

History of Mount MicaK.WEBBER,W.SIMMONS,A.FALSTER,R.SPRAGUE,G.FREEMAN,F.PERHAM................................................................................. 154

The Discrimination of LCT and NYF Granitic Pegmatites Using Mineral Chemistry: A Pilot StudyM.WISE ........................................................................................................................................................................................... 156

The Composition of Gahnite as Indicator of Rare Metal (Li) Mineralization in Granitic PegmatitesJ.YONTS,A.HEIMANN,J.BITNER,M.WISE,D.SOARES,A.FERREIRA............................................................................................... 158

Miarolitic Facies of Rare-MetalMuscovite Pegmatites, Azad Kashmir, PakistanV.ZAGORSKY ................................................................................................................................................................................... 160

On the Problem Of Granite-Pegmatite Relationships: Types of Granite-Pegmatite SystemsV.ZAGORSKY,V.MAKAGON .............................................................................................................................................................. 162


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Abstracts Page 1

CRYSTAL CHEMISTRY AND GEOTHERMOMETRIC APPLICATIONS OF PRIMARY PEGMATITEPHOSPHATES

F. HatertUniversit de Lige, Laboratoire de Minralogie B18, B-4000 Lige, Belgium. [email protected]

Introduction

Iron-manganese phosphate minerals arewidespread in medium to highly evolved LCTgranitic pegmatites, ranging from the beryl-columbite-phosphate subtype to the spodumenesubtype. These phosphates play an essentialgeochemical role in the evolution processesaffecting pegmatites, and a good knowledge of theircrystal chemistry and of their stability fields isabsolutely necessary to better understand the genesisof pegmatites. Among iron-manganese phosphates,several groups are of peculiar interest, since theyform primary (magmatic) or high-temperature

hydrothermal minerals. These groups are phosphatesof the triphylite-lithiophilite [Li(Fe2+,Mn2+)PO4-Li(Mn2+,Fe2+)PO4], karenwebberite-natrophilite[Na(Fe2+,Mn2+)PO4-Na(Mn

2+,Fe2+)PO4], andsarcopside-zavalaite series [(Fe2+,Mn2+)3(PO4)2-(Mn2+,Fe2+)3(PO4)2], as well as alluaudite- andwyllieite-type phosphates. The crystal-chemicalfeatures of these minerals, as well as someexperimental data on their stability fields, will besuccessively presented in this lecture. Theydemonstrate how a global approach, combininglaboratory experiments with petrographic and

crystallographic measurements on natural samples,is necessary to decipher all aspects of these complexminerals.Primary Li- and Na-bearing phosphates of the

triphylite group

In pegmatites, primary phosphates of thetriphylite-lithiophilite series form masses that canreach several meters in diameter, enclosed insilicates. During the oxidation processes affectingthe pegmatites, these olivine-type phosphatesprogressively transform to ferrisicklerite-sicklerite[Li1-x(Fe

3+,Mn2+)(PO4)-Li1-x(Mn2+,Fe3+)(PO4)] and to

heterosite-purpurite [(Fe3+

,Mn3+

)(PO4)-(Mn3+,Fe3+)(PO4)], according to the substitutionmechanism Li++ Fe2+ []+ Fe3+.

The crystal structure of minerals of thetriphylite-lithiophilite series (triphylite: a= 4.690, b= 10.286, c= 5.987 ,Pbnm) has been investigatedfrom synthetic samples and natural minerals (Hatertet al. 2011a, 2012), and is characterized by twotypes of octahedral sites: the M(1) octahedra

occupied by Li, and the M(2) sites occupied by Fe

and Mn. A natural sample from the Altai Mountains,China, was recently investigated by Hatert et al.(2012), in order to understand the structuralvariations occurring during the oxidation oflithiophilite into sicklerite. Five single-crystals,corresponding to intermediate members of thelithiophilite-sicklerite series, were extracted from athin section and are orthorhombic, space groupPbnm, with unit-cell parameters ranging from a =4.736(1), b = 10.432(2), c = 6.088(1) (lithiophilite) to a = 4.765(1), b = 10.338(2), c =6.060(1) (sicklerite). The structures show a

topology identical to that of olivine-typephosphates, with Li occurring on the M(2) site andshowing occupancy factors from 0.99 Li atoms performula unit (p.f.u.) (lithiophilite) to 0.75 Li p.f.u.(sicklerite). These values are in good agreementwith the values measured by SIMS (Secondary IonMass Spectrometry), which indicate Li values from0.96 to 0.69 Lip.f.u.

Natrophilite, NaMnPO4, is another pegmatitephosphate with the olivine structure, in which theM(1) site is occupied by Na while the M(2) sitecontains the smaller divalent cations. Recently, the

Fe-analogue of natrophilite was found at theMalpensata granitic pegmatite, Colico commune,Lecco province, north Italy (Vignola et al. 2011).This phosphate, Na(Fe2+,Mn2+)PO4, has been namedkarenwebberite in honour of Dr. Karen LouiseWebber, Assistant Professor Research at theMineralogy, Petrology and Pegmatology ResearchGroup, Department of Earth and EnvironmentalSciences, University of New Orleans, Louisiana,U.S.A. (IMA 2011-015). It forms late stagemagmatic exsolution lamellae up to 100m thick,hosted in graftonite and associated with Na-bearing

ferrisicklerite and with an heterosite-like phase (Fig.1a). Karenwebberite is orthorhombic, space groupPbnm, a =4.882(1), b = 10.387(2), c= 6.091(1), V = 308.9(1)3, and Z = 4. The mineral showsthe olivine structure, withM(1) occupied by Na andM(2) occupied by Fe and Mn.

Karenwebberite is also a polymorph of mariite,NaFePO4 (a = 6.861(1), b= 8.987(1), c= 5.045(1), Pmnb), which shows a crystal structure distinct


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Abstracts Page 2

from that of olivine. This polymorphic relationshipis of particular interest, since the transformationbetween olivine-type and mariite-type phosphatesis temperaturedependant, as shown experimentallyby Corlett and Armbruster (1979). These authorsconfirmed that olivinetype Na(Fe,Mn)PO4

phosphates are low-temperature polymorphs ofmariitetype phosphates, and that the transitionbetween the two polymorphs of NaMnPO4 occurs

around 325C (P = 100 bars). Hydrothermalinvestigations performed on alluauditetypephosphates at 1 kbar (Hatert et al.2006 and 2011b)also produced several mariitetype phosphates withvarious Fe/(Fe+Mn) ratios; these results indicate atransition temperature of about 500550C between

karenwebberite and mariite. Consequently,karenwebberite certainly crystallized below 550Cin the Malpensata dyke.

Fig. 1:A) Exsolution lamellae of karenwebberite (light brown), oxidized into Na-bearing ferrisicklerite (dark brown) andincluded in graftonite. Malpensata pegmatite, Colico, Italy (plane polarized light; the length of the photograph is 1.5mm). B) Triphylite (dark grey) including lamellae of sarcopside (white), Caada pegmatite, Spain (sample SS-3, BSEimage). C) Lamellae of zavalaite included in lithiophilite, La Empleada pegmatite, San Luis, Argentina (crossed polars;the length of the photograph is 2.5 mm). D) Rim of qingheiite-(Fe2+) (brown) surrounding frondelite (red) in a quartz-albite matrix. Sebastio Cristino, Minas Gerais, Brazil (plane polarized light, sample SC-34).

Phosphates of the sarcopside group and their

exsolution texturesLamellar triphylite + sarcopside[(Fe,Mn)3(PO4)2] associations are well known innumerous rare-element granitic pegmatites (Fig. 1b).These intergrowths are traditionally interpreted asexsolution textures, and Moore (1972) suggests theexistence of a complete Li(Fe,Mn)(PO4)-(Fe,Mn)3(PO4)2 solid solution at high temperature.According to this hypothesis, exsolutions of

sarcopside into triphylite, or of triphylite into

sarcopside, would appear during cooling, dependingon the composition of the parent high-temperatureunique phase.

Hatert et al. (2007) performed hydrothermalexperiments between 400 and 700C (1 kbar), inorder to determine the stability field of the triphylite+ sarcopside assemblage. These experimentsindicate that the triphylite + sarcopside assemblageis a primary assemblage in granitic pegmatites, since


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it has been reproduced hydrothermally at 500 and700C (1 kbar). The electron-microprobe and SIMSanalyses of these synthetic phosphates show that theLi content of triphylites significantly increases withtemperature, from 0.72 a.p.f.u. at 400C, to 0.037a.p.f.u. at 700C, for the LiFe2.5(PO4)2 starting

composition. By comparison with the analytical datacollected on natural assemblages, these experimentalresults provide a relatively accurate determination ofthe temperature at which the exsolutionscrystallized.

Zavalaite, ideally (Mn2+,Fe2+,Mg)3(PO4)2, is anew phosphate mineral species from the La Empleadagranitic pegmatite, San Luis province, Argentina(Hatert et al.2013), which forms exsolution lamellaeoccurring within lithiophilite (Fig. 1c). Zavalaite isthe Mn2+-rich equivalent of sarcopside and ofchopinite [(Mg,Fe)3(PO4)2], and belongs to the

sarcopside group of minerals. Its single-crystal unit-cell parameters are a = 6.088(1), b = 4.814(1), c =10.484(2) , = 89.42(3), V= 307.2(1) 3, spacegroup P21/c. The mineral is named in honour ofMara Florencia de Ftima Mrquez Zavala (orMrquez-Zavala (1955-)), researcher and Head ofthe Department of Mineralogy Petrography andGeochemistry, IANIGLA, CCT Mendoza,CONICET, Argentina, for her contribution to theknowledge of Argentinean mineralogy.

The genesis of zavalaite lamellae is comparedto the genetical processes responsible for the

formation of sarcopside lamellae observed intriphylite. Exsolutions of zavalaite appeared duringthe cooling of primary, Li-poor lithiophilite;consequently, this mineral can be considered as aprimary phosphate, which crystallized underpegmatitic conditions similar to those of lithiophiliteformation. Starting from the proportions ofzavalaite in the exsolution textures (less than 10%zavalaite), and using the experimental triphylite-sarcopside geothermometer described by Hatert etal.(2007, 2009), a temperature of ca. 300C can beestimated for the crystallisation of zavalaite

exsolutions. This temperature is in good agreementwith those occurring in such lithiophilite-richevolved pegmatites.

Alluaudite- and wyllieite-type phosphates: the

transition between primary minerals and Na-

metasomatic phases

The alluaudite group of minerals consists of Na-Mn-Fe-bearing phosphates which are known tooccur in Li-rich granitic pegmatites. Due to their

flexible crystal structure, which is able toaccommodate Fe2+ and Fe3+ in variable amounts,alluaudites are very stable and crystallize from thefirst stages of pegmatite evolution to the latestoxidation processes. These minerals exhibitchemical compositions ranging from

Na2Mn(Fe2+

Fe3+

)(PO4)3 to NaMnFe3+

2(PO4)3, withMn2+ or some Ca2+ replacing Na+, Fe2+ replacingMn2+, and some Mg2+ or Mn2+ replacing iron. Thealluaudite structure was described on a naturalsample from the Buranga pegmatite, Rwanda. Themineral is monoclinic, space group C2/c, and thestructural formula corresponds toX(2)X(1)M(1)M(2)2(PO4)3, with four formula unitsper unit cell. The structure consists of kinked chainsof edge-sharing octahedra stacked parallel to {101}.These chains are formed by a succession of M(2)octahedral pairs linked by highly distorted M(1)

octahedra. Equivalent chains are connected in the bdirection by the P(1) andP(2) phosphate tetrahedrato form sheets oriented perpendicular to [010].These interconnected sheets produce channelsparallel to the c axis, channels which contain the Xsites.

Hatert et al. (2006) explored thegeothermometric potential of the Na2(Mn1-xFe

2+x)2Fe

3+(PO4)3 solid-solution series (x = 0 to 1),which represents the compositions of natural weaklyoxidized alluaudites; they performed hydrothermalexperiments between 400 and 800C, at 1 kbar.

Under an oxygen fugacity controlled by the Ni/NiObuffer, single-phase alluaudites crystallize at 400and 500C, whereas the association alluaudite +mariite appears between 500 and 700C. The limitbetween these two fields corresponds to themaximum temperature that can be reached byalluaudites in granitic pegmatites, because mariitehas never been observed in these geologicalenvironments. Because alluaudites are very sensitiveto variations of oxygen fugacity, the field ofhagendorfite, Na2MnFe

2+Fe3+(PO4)3, has beenpositioned in the f(O2)-T diagram, and provides a

tool that can be used to estimate the oxygen fugacityconditions which prevailed in granitic pegmatitesduring the crystallization of this phosphate.

Hydrothermal experiments were also performedby Hatert & Fransolet (2006) in the Na-Fe2+-Fe3+(+PO4) ternary system, between 400 and 700C, at 1kbar. Alluaudite-type phosphates were observedbetween 400 and 700C, and occupy the central partof the Na-Fe2+-Fe3+ diagram. Electron-microprobe


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analyses indicate that the compositional field ofalluaudites covers ca. 10 % of the diagram surface at400C, but only ca. 2-3 % at 500 and 600C. Theresults of these experiments are applicable to Fe-richalluaudites, as for example ferrohagendorfite fromAngarf-sud, Morocco, which exhibits an ideal

chemical composition Na2Fe2+

2Fe3+

(PO4)3.Alluaudite-type compounds with similarcompositions were obtained between 400 and 700Cin the hydrothermal experiments, thus confirmingagain the existence of primary alluaudites in graniticpegmatites.

In order to constrain the conditions oftemperature and oxygen fugacity which occurred inpegmatites, we also decided to reproduceexperimentally several associations of pegmatitephosphates. Primary alluaudite + triphyliteassemblages were reported in the Hagendorf-Sd

(Germany), Buranga, and Kibingo (Rwanda)pegmatites, and the hydrothermal experiments (P = 1kbar, T = 400-800C) lead to the crystallisation ofalluaudite + triphylite at 400 and 500C, and ofalluaudite + triphylite + mariite at 600 and 700C(Hatert et al. 2011b). In these experiments,significant amounts of Na were observed insynthetic triphylites; the Na content of triphylitesconsequently constitutes a geothermometric tool thatcan be used to constrain the temperature conditionsin which the alluaudite + triphylite assemblagescrystallized in pegmatites.

Qingheiite-(Fe2+

), ideally Na2Fe2+

MgAl(PO4)3,is a new mineral species recently described in theSebastio Cristino pegmatite, Minas Gerais, Brazil(Hatert et al. 2010). It occurs as rims aroundfrondelite grains, included in a matrix of quartz andalbite (Fig. 1d). The mineral exhibits a wyllieite-type structure, topologically identical to that ofalluaudite, with single-crystal unit-cell parameters a= 11.910(2), b = 12.383(3), c = 6.372(1) , =114.43(3), V = 855.6(3) 3, space group P21/n.Since frondelite from Sebastio Cristino is anoxidation product of primary triphylite, rims of

qingheiite-(Fe

2+

) can be interpreted as the result of areaction between this primary Mg-bearing triphylite(source of Fe, Mn, Mg, P) and albite from the matrix(source of Na, Al). This reaction certainly took placeat the albitization stage, during which high amountsof Na were available. The oxidation processesaffecting the pegmatite subsequently oxidizedtriphylite in ferrisicklerite and then in frondelite, and

provoked an oxidation of qingheiite-(Fe2+) followingthe substitution mechanism Na++ Fe2+= + Fe3+.This oxidation is responsible for the presence ofvacancies and Fe3+in qingheiite-(Fe2+).

ReferencesCorlett, M.I. and Armbruster, T. (1979): Structural relations

between mariite and natrophilite in the system NaFePO4-NaMnPO4. GAC-MAC Join annual meeting, 4(44), 1979.

Hatert, F. & Fransolet, A.-M. (2006): The stability of iron-richalluaudites in granitic pegmatites : an experimentalinvestigation of the Na-Fe(II)-Fe(III) (+PO4) system.Berichte der Deutschen Mineralogischen Gesellschaft,Beihefte zum European Journal of Mineralogy, 18, 53.

Hatert, F., Fransolet, A.-M., and Maresch, W.V. (2006): Thestability of primary alluaudites in granitic pegmatites: anexperimental investigation of the Na2(Mn1-xFe2+x)2Fe3+(PO4)3 solid solution. Contribution toMineralogy and Petrology, 152, 399-419.

Hatert, F., Roda-Robles, E., Keller, P., Fontan, F. & Fransolet,A.-M. (2007): Petrogenetic significance of the triphylite +sarcopside intergrowths in granitic pegmatites : an

experimental investigation of the Li(Fe,Mn)(PO4)-(Fe,Mn)3(PO4)2 system. Granitic pegmatites: the state ofthe art, Book of Abstracts, 44.

Hatert, F., Ottolini, L., Keller, P., Fransolet, A.-M. (2009):Crystal chemistry of lithium in pegmatite phosphates: ASIMS investigation of natural and synthetic samples.Estudos Geolgicos 19(2), 131-134.

Hatert, F., Baijot, M., Philippo, S. & Wouters, J. (2010):Qingheiite-(Fe2+), Na2Fe2+MgAl(PO4)3, a new phosphatemineral from the Sebastio Cristino pegmatite, MinasGerais, Brazil. European Journal of Mineralogy, 22, 459-467.

Hatert, F., Ottolini, L., Fontan, F., Keller, P., Roda-Robles, E. &Fransolet, A.-M. (2011a): The crystal chemistry of olivine-type phosphates. 5th International symposium on granitic

pegmatites,-PEG2011, Abstract book, 103-105.Hatert, F., Ottolini, L., and Schmid-Beurmann, P. (2011b):

Experimental investigation of the alluaudite + triphyliteassemblage, and development of the Na-in-triphylitegeothermometer: applications to natural pegmatite

phosphates. Contributions to Mineralogy and Petrology,161, 531-546.

Hatert, F., Ottolini, L., Wouters, J. & Fontan, F. (2012): Astructural study of the lithiophilite-sicklerite series.Canadian Mineralogist, 50, 843-854.

Hatert, F., Roda-Robles, E., de Parseval, P. & Wouters, J.(2013): Zavalaite, (Mn2+,Fe2+,Mg)3(PO4)2, a newmember of the sarcopside group from the La Empleada

pegmatite, San Luis Province, Argentina. Canadian

Mineralogist, 50, 1445-1452.Moore, P.B. (1972): Sarcopside: its atomic arrangement.American Mineralogist, 57, 24-35.

Vignola, P., Hatert, F., Fransolet, A.-M., Medenbach, O., Diella,V, and Ando, S. (2011): Karenwebberite, IMA 2011-015.CNMNC Newsletter No. 10, October 2011, page 2551;

Mineralogical Magazine, 75, 2549-2561.


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READING PEGMATITES:WHAT MINERALS SAY

D. LondonConocoPhillips School of Geology & Geophysics, University of Oklahoma, Norman, OK (USA) 73019:

[email protected]

Each of the families, groups, and species ofminerals found in granitic pegmatites contributes

some information to the whole understanding ofthese rocks. The petrologic significance of someabundant and accessory minerals from pegmatites isbriefly summarized below.

Feldspars: Sodic plagioclase (Pl) and alkalifeldspar (Kfs) constitute ~ 65% of pegmatites bymode or norm. The vast majority of pegmatitescontain solvus Pl-Kfs pairs from the start, whichconstrains pegmatite crystallization to temperaturesbelow the feldspar solvus crest (< ~ 700-685C).Accurate feldspar solvus thermometry, however,

puts the crystallization temperature of pegmatites at~ 450C, and feldspar crystallization is isothermal,following the ingress of the 450C isotherm intopegmatites (London et al.2012). In conjunction witha conductive cooling model, the primarycrystallization of feldspars and the consequentexsolution to perthite in an 80 cm-thick dike atRamona, California (USA) appears to have occurredover a span of a few of weeks.

The An content of Pl decreases smoothly andcontinuously from pegmatite margins to center, andthe K/Rb and K/Cs ratios of Kfs increase. Both

reflect sequential crystallization of pegmatites frommargin to center, and both are consistent withnormal igneous fractionation trends, uninterruptedby hydrothermal processes. Plagioclase is prevalentin marginal zones, whereas Kfs is more abundant incentral zones. This fractionation trend between Pland Kfs is NOT consistent with a thermally-drivendiffusion gradient (Jahns and Burnham 1969), inwhich Kfs precipitates in cooler regions and Pl inhotter zones (Orville 1963). It I S, however,consistent with the patterns of sequential feldsparfractionation that arise from the crystallization of

significantly undercooled granitic melts.Quartz: Quartz is basically unstudied, except

for an extensive but private data base in theindustrial realm. Pegmatitic quartz tends to containless Al, Ti, Fe, and alkalis than do quartzphenocrysts from rhyolites, and unlike phenocrysticquartz, pegmatitic quartz lacks appreciablecathodoluminescence (CL) or CL zoning. Thus,although the morphology of quartz in pegmatites is

igneous, its composition, fluid inclusion populations,and isotopic composition all suggest that pegmatitic

quartz does not preserve its original igneouscomposition well (i.e., it is extensively recrystallizedin the subsolidus).

Micas: Evolutionary trends in the compositionsof micas from LCT and NYF pegmatites have beenwell described for decades, and those trends areconsistent with normal igneous fractionation. Fromthe margins inward, the micas are first depleted inMg, then in Fe as Al replaces octahedral Fe,culminating in a final enrichment in Li, F, and Mn.Beyond that, the micas have provided little else in

the way of petrogenetic information.Lithium Aluminosilicates: The lithium

aluminosilicates eucryptite (-LiAlSiO4),spodumene (-LiAlSi2O6), and petalite (LiAlSi4O10)constitute an important petrogenetic grid for LCTpegmatites, because the stability relations amongthese minerals are functions only of P and T in thequartz-saturated environment of pegmatites (London1984). Where all three minerals occur (e.g., Tanco,Canada; Bikita, Zimbabwe), their sequentialcrystallization follows an isobar from ~ 525-350Cat ~ 280 MPa (London 1986); below ~ 350C, the

cooling path follows a geotherm into the stabilityfield of eucryptite + quartz. The inflection in thecooling curve from isobaric to the geothermalgradient at Tanco (London 1986) and Harding, NewMexico (USA)(Chakoumakos and Lumpkin 1991)denotes the temperature (~ 350C) of the host rocksupon emplacement of these pegmatites. Thattemperature at an emplacement depth of ~ 8.8 kmcorresponds to a geothermal gradient of ~ 35C/km,which unrealistically higher than the calculatedcratonic geotherm of the Canadian shield in theArchean (~ 24C/km: Chloe et al.2009). A tentativeconclusion is that the region of the Superiorprovince that contains the Tanco pegmatitepossessed local heat due to the emplacement ofnearby plutons. It is an important but heretoforeunstudied fact that some cratonic provinces aredominated by spodumene-bearing pegmatites,whereas others are petalite-bearing, with implieddifferences in temperature or depth of pegmatiteemplacement in the various continental settings.


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Garnet: Pegmatitic garnet consists mostly ofalmandine-spessartine solid solutions with minorgrossular and negligible pyrope components. Wherestudied, the spessartine content increases withpegmatite fractionation. Interestingly, the Nernstdistribution coefficient for Mn/Fe between

garnet/melt is > 1. Therefore, the crystallization ofgarnet does NOTcontrol the increasing Mn/Fe ratioof melt from granitic sources to the mostdifferentiated pegmatites (ern et al. 1985). Thecompositions of pegmatitic garnets, together withthe low modal abundance of biotite, render garnet-biotite thermometry meaningless and not applicableto pegmatites.

Tourmaline: Evolutionary trends in thecomposition of tourmaline in pegmatites mirrorthose of the micas, and have been known fordecades. The stability of tourmaline in pegmatites,

however, now has a firm basis for understanding therelationships between tourmaline crystallization,melt composition, and temperature and pressure.The Li-free tourmalines are stable to ~ 750C and atany pressure above ~ 50 MPa (London 2011). Thesolubility (saturation) of tourmaline in melt,however, is a function mostly of the activityproducts of B and Al in the melt, and of temperature.The solubility of Li-free tourmaline in granitic melt,as measured by the B content of melt at saturation,decreases from 2 wt% B2O3 in melt at 750C, 200MPa to only 0.1 wt% B2O3in melt at 450C.

Li-rich tourmaline is unstable below 200 MPa atany temperature. Based on existing work (London2011), the elbaite component increases withincreasing pressure; relationships with regard totemperature are more complex and are still beingworked out. However, the stability field for elbaiteappears to correspond to that of spodumene. Lithiumadopts octahedral coordination in both phases,which coordination is favored by increasing pressureand decreasing temperature.

Phosphates: Equilibria among feldspars, LiAl-

silicates, garnet, and their corresponding phosphatesbuffer melt compositions between ~ 0.5-2.5 wt%P2O5. This range of phosphorus concentration is alsorecorded by the alkali feldspars. Even the most P-rich pegmatites contain < 1 wt% P2O5 in their bulkcompositions, where known.

Beryl: The most beryl-rich pegmatites knowncontain an average of 205 ppm Be. At thatconcentration, a weakly peraluminous granitic meltbecomes beryl-saturated below 600C (Evensen etal. 1991). Beryl (and columbite) are commonaccessory minerals of the border zones of

pegmatites, consistent with a low temperature ofcrystallization along the margins.

Pollucite:Pollucite marks the highest degree ofchemical saturation in granitic melts. London (2008)calculated that the Tanco pegmatite achievedsaturation in pollucite at ~ 385C, after ~ 50% of thepegmatite-forming melt had crystallized. Based onits Cs content, London (2008) estimated that asmuch as 18,000 km3 of rock might have beeninvolved in the partial melting event that led to theformation of the Tanco pegmatite.

Referencesern, P., R. E. Meintzer, A. J. Anderson (1985) Extremefractionation in rare-element granitic pegmatites: selectedexamples of data and mechanisms. Canadian Mineralogist,vol. 23, 381-421.

Chakoumakos, B. C., G. R. Lumpkin (1990) Pressure-temperature constraints on the crystallization of the Harding

pegmatite, Taos County, New Mexico. CanadianMineralogist, vol. 28, 287-298.

Chloe, M. A., C. Jaupart, J.-C. Mareschal (2009) Thermalevolution of cratonic roots. Lithos, vol. 109, 4760.

Evensen J. M., D. London, R. F. Wendlandt (1999) Solubilityand stability of beryl in granitic melts. AmericanMineralogist, vol. 84, 733-745.

Jahns, R. H., C. W. Burnham (1969) Experimental studies of

pegmatite genesis: I. A model for the derivation andcrystallization of granitic pegmatites. Economic Geology,vol. 64, 843-864.

London, D. (1984) Experimental phase equilibria in the systemLiAlSiO4-SiO2-H2O: a petrogenetic grid for lithium-rich

pegmatites. American Mineralogist, vol. 69, 995-1004.London, D. (1986) The magmatic-hydrothermal transition in the

Tanco rare-element pegmatite: evidence from fluidinclusions and phase equilibrium experiments. AmericanMineralogist, vol. 71, 376-395.

London, D. (2008) Pegmatites. Canadian Mineralogist SpecialPublication 10, 368 p.

London, D. (2011) Experimental synthesis and stability oftourmaline: a historical overview. Canadian Mineralogist,vol. 49, 117-136.

London, D., G. B. Morgan VI, K. A. Paul, B. M. Guttery , G.B.(2012) Internal evolution of a miarolitic granitic pegmatite:the Little Three Mine, Ramona, California (USA). CanadianMineralogist, vol. 50, 1025-1054.

Orville, P.M. (1963) Alkali ion exchange between vapor andfeldspar phases. American Journal of Science, vol. 261,201-237


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THE LATE-STAGE MINI-FLOOD OF CA IN GRANITIC PEGMATITES:AWORKING HYPOTHESIS

R. MartinEarth & Planetary Sciences, McGill University, [email protected]

The textural and compositional data amassed inrecent years show convincingly that graniticpegmatites are indeed igneous rocks. What many failto realize, however, is that in general, themineralogy of these rocks is not igneous. What Iintend to show, in this essay, is that a fullunderstanding of the make-up of a graniticpegmatite requires knowledge of subsolidusmodifications that are largely orchestrated byassemblages beyond the outer contact of theintrusive body.

Phase-equilibrium investigations predict that thefractional crystallization of a high-temperaturefeldspar or feldspars from a batch of graniticmagma, be it of orogenic (LCT) or anorogenic(NYF) affiliation, will lead to a gradual increase inRb and Cs, and a gradual decrease in the amount ofCa. Thus one can expect the Ca content of theprimary feldspars to decrease progressively from thewalls inward, as the melt migrates inexorably towardthe pseudoternary minimum or eutectic in thecalcium-free system AbOrQtzH2O. Although thesodic plagioclase does attain An0 in most cases, thebulk composition of the pegmatite may still containa fraction of weight percent of CaO, which accountsfor the Ca complexed with part of the fluorine in themelt.

The Ca content of the primary feldspars is noteasily erased in going down-temperature, owing tothe coupled substitution Ca + Al for Na + Si. It tendsto be locked in. On the other hand, the (Al,Si)-disordered state of the early-formed feldspar ispartly or completely destroyed. The development ofmicrocline twinned according to the albite andpericline laws is a subsolidus inversion; it does notoccur in the presence of a silicate melt. Thisinversion initially involves the hydrogen ion as acatalyst, and may later involve molecules of H2O asa solvent participating in solution-and-redeposition

steps that coarsen the twin microstructure.In an LCT pegmatite, fluorapatite is confined to

the earliest stage of crystallization, near the outercontact, where it commonly forms acicular crystalsoriented perpendicular to the outer contact. It thendisappears, not because of a shortage of phosphorus,but rather of calcium. Nodules of a primaryphosphate mineral, typically manganoan triphylite,may well accumulate as the core zone is approached,

but these nodules are devoid of fluorapatite. At alater stage, coarse euhedral crystals of fluorapatiteare found in cavities that are attributed to dissolutionby the caustic orthomagmatic fluid. The lateappearance of striking crystals of fluorapatite isattributed to an influx of calcium from outside thesystem. The LCT pegmatites of the Oxford suite, inMaine, provide excellent examples of thisassociation.

In an NYF pegmatite, there may very well be alate development of calcium-bearing accessoryphases such as pyrochlore, microlite, fersmite, andfluorite. The elements contributed to the cavityenvironment from beyond the outer contact mayinclude Mg and B, which account for the strikingdevelopment of zoned crystals of liddicoatite, fluor-dravite and danburite in the well-studiedAnjanabonoina NYF pegmatite, in centralMadagascar. In general, these late minerals areatypical of an NYF pegmatite, and account for amixed NYFLCT signal that confusespetrogenetic interpretations.

A mini-flood of Ca, as Richard H. Jahnscalled it, is produced late in the development of thepegmatite. In the context of LCT pegmatitesemplaced into the calc-alkaline host-rocks of theSouthern California batholith, he considered that

these more calcic rocks were surely involved. Healso appealed to a leaching of the early-formedoligoclase or andesine produced by the pegmatite-forming magma in the wall zone and border zone.He was correct in identifying plagioclase as aprincipal reactant in the flood-producing reaction,but did not recognize the mechanism of massive andsudden liberation of Ca.

Everyone who learns about phase diagramsrelevant to igneous petrology gets to know thebinary system AbAn at one atmosphere. This wasan early chapter in the Ph.D. thesis of Norman Levi

Bowen, completed in 1912. The determination of theliquidus and solidus is clearly a solid contribution toan understanding of the differentiation of magmas.However, Bowen and his successors to this day,even those working with H2O as a component in thesystem, have been totally incapable of shedding lighton the phase diagram AbAn below the solidus. Allvaliant attempts fail because of kinetic reasons.There is agreement that there are three solvi, named


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Peristerite, Bggild and Huttenlocher, but no onehas been able to map these in TXspace. TEM-scalephotos show clearly that perthite-like exsolutionlamellae do form, and presumably coarsen to alimited extent with annealing on geological time-scales. To state the problem concisely, all

plagioclase compositions between An12and An95100are metastably stuck somewhere below the solidus.These bulk compositions have begun to order (AlSi) and to exsolve, but have not gotten very farbecause of the coupled substitution quoted earlier.

As Richard Jahns used to say, graniticpegmatites are prone to stew in their own juice. Thisjuice acts as the catalyst in transforming the early-formed disordered K-feldspar to microcline or amixture of microcline + orthoclase, both showing atwinned microtexture. As it becomes progressivelyenriched in fluorine, it becomes increasingly

aggressive, and can create cavities by dissolution.More importantly in this context, it can leak into thecountry rock laterally and from the roof of zonedpegmatites; there it meets the metastably stuckplagioclase in any lithology that it encounters. Theaggressive fluid, likely somewhat alkaline, promptlydissolves the plagioclase, and deposits in its placepure albite and K-bearing phases, possiblymuscovite and microcline. Metamorphic petrologists

agree that below 400C, the phase diagram for thesystem AbAn shows a stable coexistence of An01and An95100. The dissolution of a calcic plagioclasewill shift the pH of the fluid to acidic values owingto the local consumption of Na + K and the massiveliberation of Al + Ca. This acidic fluid then re-enters

the cooling pegmatite, and there starts a new cycleof solution-and-redeposition steps. It is aggressivetoward microcline and orthoclase, and efficientlyreplaces them by albite + muscovite, likely at atemperature in the range 250400C.

The reconstruction described above has majorramifications concerning the economic aspects ofgranitic pegmatites. The mini-flood of Ca (and Al) isclearly carried by an aqueous fluid that has lost K tothe exocontact but has picked up Na (from the netdissolution of plagioclase) to account for the Na-metasomatic reactions that are widespread and

indiscriminate within the zoned pegmatite. The factthat minerals of tantalum, lithium and tin areintimately associated with the albite-dominantreplacement units shows that mineralization is dueto a hydrothermal process, not a magmatic one. Theaqueous fluid evidently is able to leach and transportthese elements, and deposits tantalite-(Fe), tantalite-(Mn), micas of the lepidolite series and cassiterite aspart of the replacement assemblage.


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PRODUCTIVE ZONESIN GEM-BEARING PEGMATITES OF CENTRAL MADAGASCAR,GEOCHEMICALEVOLUTION AND GENETIC INFERENCES

F. PezzottaNatural History Museum, C.so Venezia 55, 20121 Milano, Italy. [email protected]

Most gem-bearing pegmatites are miarolitic, are

highly evolved, and belong to the LCT family. Gemcrystals of several mineral species (e.g. tourmaline-supergroup minerals, beryl varieties, spodumenevarieties, spessartine, topaz, etc.) may be present inpegmatites with two different occurrences: (1) inrare and small primary cavities dispersed in thecentral portions of pegmatitic bodies, providing verylimited amount of crystals, generally of highgemological quality; (2) in a series of more or lesslarge, partially interconnected primary cavities,occurring in core zones and characterizing limitedportions or sectors of pegmatitic bodies, providing

for large quantities of crystals, of low, to medium tohigh gem quality. Such pegmatite portions or sectorsare called productive zones or just zones by gemminers.

As described above, occurrence (1) is the mostcommon; small-scale gem mines can operate overlong periods tunneling in pegmatites and findingoccasionally only very small quantities (in the orderof tens of grams to a few hundred grams) of gem-quality crystals. In some rare cases, gem minesencounter a zone (occurrence 2), in which a seriesof large gem-pockets is discovered, with the

production of large quantities of gem-qualitycrystals (exceptionally in the order of hundreds ofkilos and even tons).

Zones physically correspond to the cores ofwell-structured and large pegmatitemacrostructures (PM), these last defined inPezzotta (2009) as the ensemble of rock structures ofa pegmatitic body formed under the sameevolutional process; which means, formed startingfrom the same pocket of homogeneous pegmatiticmelt or fluid, from which during crystallization thetypical pegmatitic heterogeneites formed. It is

important to notice that not all cores of gem-bearingpegmatites contain a zone, and that PM may ormay not (and in gem-pegmatites in general not)correspond to the entire volume of the pegmatiticbody. In many cases a single pegmatitic bodycontains several PM, of different sizes and,as aconsequence, with different structure (zoning), withthe largest PM generally being more complex. This

phenomenon has been related in Pezzotta (2009) to a

size factor.Gem bearing pegmatites in Madagascar aredistributed mostly in the central and south of theisland. Economically, the most important ones arethose characterized by the occurrence of red andmulticolored tourmaline (liddicoatite andelbaite, Pezzotta & Laurs, 2011). Tourmalinebearing pegmatites occur in pegmatitic fields hostedin many different rock units (paragneiss, quartzites,marbles, and migmatites), belonging to differentgeologic domains and sub-domains (geologicframework here used for reference from Tucker et

al., 2011): the Antananarivo domain, characterizedby the fields of Anjoma, Valozoro-Camp Robin,Alaka Misy Itenina; the Itremo sub-domain,characterized by the fields of Sahatany, Manapa,Anjanabonoina, Vohitrakanga, Ambalamahatsara;the Ikalamavony sub-domain, characterized by thefields of Bevoandrano, Ikalamavony, Tsitondroina,Mandrosonoro-Ambatovita. Only one importanttourmaline deposit (Anjahamiary, Pezzotta, 2003),and a few minor ones, are known in southMadagascar (Anosyen domain).

Gem bearing pegmatites of central and south

Madagascar are associated with a lateNeoproterozoic tectono-magmatic event (agebetween 570 and 540 Ma, Pezzotta, 2005, andreferences therein) characterized by the intrusion ofsyenite and granite plutons, emplaced in anextentional tectonic regime.

Rarity of these gem-bearing pegmatites, coupledwith scarce and/or incomplete exposure, alteration,and irrational mining, make the documentation andmapping of PM in Madagascar difficult.Nevertheless, several examples have been observedand sampled by the author in the Sahatany, Manapa,

Anjanabonoina, Valozoro, Ambatovita,Bevoandrano, and Tsitondroina fields. Gem-tourmaline bearing pegmatites in Madagascar are ingeneral of tabular shape and they are from sub-horizontal through gently to steeply dipping. Sizesrange from a few meters in thickness and some tensof meters in length up to some tens of meters inthickness and over 1 km in length. The thickestdikes have a more or less layered structure, with


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layers characterized by variations in the grain size(from fine to medium to coarse) and in the contentof minerals (sodic and potassic feldspars, quartz,tourmaline, garnet, Nb-Ta oxides, danburite,spodumene). Layers have thicknesses ranging fromcentimeters to about one meter. Very rare miarolitic

cavities of small size (a few decimeters across), withgem-quality crystals mostly of tourmaline, can occurdispersed inside the coarse-grained bands. Thinnerdikes (decimeters up to meters in thickness) aregenerally not layered and have lateral variations inthe rock grain size and mineral distribution.

PMs develop in more or less large sectors ofdikes, in many cases occupying the entire thicknessof the dike but in some thick dikes, only a portion ofit. In the large subhorizontal Tsarafara pegmatite inthe Sahatany valley, the author observed in onevertical section three well structured, complexly

zoned, and distinct PM corresponding to threeproductive zones. PM containing productivezones occurring in dikes belonging to differentfields, have similar features which include: a lowerborder zone, roughly layered with fine to mediumgrained aplitic-pegmatitic rock; an intermediate zoneof pegmatitic grain size enriched in tourmaline andaccessories; a lower portion of the core zone,characterized by abundant cleavelandite and large toenormous crystals of quartz, tourmaline, beryl,spodumene, and abundant blades of purple Li-richmica; an upper portion of the core zone composed

mostly by large to giant crystals of microcline, insome cases partially replaced by cleavelandite andan upper border zone of coarse to medium-finegrained aplitic-pegmatitic rock.

In most of the pegmatite fields, the largest PMare characterized by a complex structure and a corecontaining a zone. Nevertheless, in some fields,the most complex and large PM do not contain gem-bearing cavities. In these cases, gem-production isconfined to dispersed, small, and rare miaroliticcavities in coarse-grained lenses in the dikes, or insmall PM characterized by a more or less simple

structure. An example is the Bevoandrano pegmatitefield, south of Ikalamavony, in the Fianarantsoaprovince. In this field, as the thickness of the PMexceeds several meters, a series of rock unitsdevelop in the core zone, including masses ofgranular lepidolite, masses of milky quartz,extensive albitized volumes, more or less associatedwith amblygonite masses, and large taperingspodumene crystals. In this case, large quantities of

more or less opaque, polychrome, tourmalinecrystals form, frozen in the rock, and no cavitiesdevelop. In the authors experience, not a singlezone producing gem-quality crystals has beenfound until now in Bevoandrano.

The Tsitondroina field, located west of

Ikalamavony, Fianarantsoa region, represents adifferent case in which, miarolitic cavitiescontaining gem crystals occur only in the largestPM, which is the most important known deposit,composed of a gigantic PM, over 300 meters longand over 40 meters thick, hosted in a pegmatitic veinwhich can be traced in the field for over 1.5 km inlength.

Highly evolved to extremely evolvedgeochemical features of zones, are:

Mineralogy Exotic mineralogy typicallycharacterizes the most evolved portions of gem-

bearing pegmatitic dikes. Zones in gem-tourmaline pegmatites in Madagascar arecharacterized by the occurrence of the famous,multicolored crystals of fluor-liddicoatite (Lussier &Hawthorne, 2011). These are large tourmalinecrystals of first generation grown across all stages offormation of the core zones of PM, ranging incomposition from primitive dravite-schorl, , tohighly evolved liddicoatite-elbaite, both withvariable Ca, Na, in the X site. Zoning patternsevidenced by slicing the crystals across the C-axe,from the base to the termination (represented by the

antilogous pole), are the result of variableoccurrence and combination of crystal forms (moresteep pyramids and dipyramids produce morecomplex patterns across c-axe) and variations in theratio between prismatic and pyramidal-pediongrowth. Such compositional, structural andmorphological variations represent an extraordinaryrecord of the chemical-physical variations duringcrystal growth, and further studies are in progressabout this topic.

Boron isotopes- Pezzotta et al.(2010) sampleda series of gem-bearing pegmatitic dikes hosted in

paragneiss and marbles of the Itremo domain, Inthese pegmatites, minerals with B in 3-foldcoordination with O (3BC), dumortierite andtourmaline supergroup minerals (dravite-elbaite-liddicoatite), and danburite with B in 4-foldcoordination (4BC), formed during all stages ofcrystallization of the pegmatitic veins. Dravite anddumortierite are confined to the most primitive rockunits; elbaite-liddicoatite and danburite formed up


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until the latest stages of crystallization (miaroliticzone) of the veins. The central, most evolved zoneof numerous pegmatites is characterized by a suiteof borates that include the 4BC rhodizite-londonitesolid solution, with local strong Rb enrichments, the4BC behierite-schiavinatoite solid solution (Ta

borate and Nb borate, respectively), and,occasionally, the 3BC hambergite.B isotopic studies were conducted on: (1)

tourmaline, danburite, and hambergite sampledfrom the border zone to the mica-rich, highlyevolved, core zone of the famous and largeAnjanabonoina pegmatite (AJB); (2) tourmaline,danburite, rhodizite-londonite, and behierite-schiavinatoite sampled in the border zone and in thehighly evolved core zone (micafree) of tworepresentative pegmatites, in the Tetezantsio area(TTZ). For primitive rock units, the results indicate

for AJB: 11

Bdravite 4.70 and 11

Bdanburite -0.83;and for TTZ: 11Bdravite-elbaite-2.33 and 11Bdanburite-

8.45. For zoned crystals of tourmaline fromAJB formed in pegmatitic units of intermediategeochemical evolution, 11B is homogeneous withvalues between 6.21 and 7.38. Cogenetichambergite crystals have similar values (11B7.02). For the most evolved rocks from AJB:11Belbaite-schorl core6.94 and

11Bliddicoatite rim 14.72and 11Bdanburite from -0.26 to 8.03 (rim); and forTTZ: 11Belbaite-liddicoatite 0.60,

11Bdanburite -6.72,11Brhodizite-6.93,

11Bbehierite-10.70.

These data indicate that cogenetic 3BC(tourmaline, hambergite) and cogenetic 4BC(danburite, rhodizite, behierite) represent twodistinct B isotopic populations, characterized byrather similar values, with 11B of 4BCsystematically shifted to lower values with respect tothe cogenetic 3BC. Whatever was the nature of thecrystallizing medium, the data reported aboveindicate that an isotopic fractionation processoccurred in response of the type of boroncoordination in the two major minerals,tourmaline and danburite.

The difference in the original composition of thepegmatite-forming medium, and the differences inthe local geological conditions (nature of the hostrock, P-T conditions), are fundamental to themineralogy and zoning of PM and in the sizefactor characteristic of PM belonging to different

fields. As evidenced in Pezzotta (2005), in the samefield, different pegmatite populations develop indifferent rock units; moreover, the available dataobtained by geologic mapping do not show clearrelations between spatial distribution of pegmatiteand granitoid intrusions. The author proposes a

model for the formation of Malagasy gem-bearingpegmatites to be the result of the crystallization ofmagmatic liquids of residual compositionresulting from both magmatic fractionation ofmigmatitic granites or from migmatiticsegregations, contaminated at different proportionsby high-density metamorphic fluids originated at (orclose to) the level of intrusion, during the upliftaccompanying the late phases of the last upper-Neoproterozoic teconomagmatic event.

ReferencesLussier A.J., Hawthorne F.C. (2011) Oscillatory zoned

liddicoatite from Anjanabonoina, central Madagascar.II. Compositional variation and mechanism ofsubstitution. Canadian Mineralogist 49: 89-104

Pezzotta F. (2005) A first attempt to the petrogenesis andthe classification of granitic pegmatites of the ItremoRegion (central Madagascar). Abstracts of theCrystallization Processes in Granitic Pegmatites,International Meeting, May 23-29, 2005, Elba Island,Italy

Pezzotta F. (2009): Definition of pegmatiticmacrostructure and evidences of a size factorcontrolling evolutional processes in pegmatitic rocks.Etudos Geologicos, Contributions of the 4th

International Symposium on Granitic Pegmatites PEG2009BRAIL, 19 (2), 292-293.Pezzotta F., Dini A., and Tonarini S. (2010) Three- and

four-coordinated boron minerals in pegmatites hostedin dolomitic marble in Central Madagascar;

paragenesis and boron isotopes. IMA 2010 Bonds andBridges, Budapest, in press..

Pezzotta F. and Laurs B.M. (2011) Tourmaline: TheKaleidoscopic Gemstone. Elements, 7, 5, 333-338.

Pezzotta F., and Jobin M. (2003) The Anjahamiarypegmatite, Fort Dauphin area, Madagascar. Web siteof Mineralogical Society of America.www.minsocam.org/msa/Pegmatites.html.

Tucker R.D., Roig J.Y., Macey P.H., Delor C., AmelinY., Armstrong R.A., Rabarimanana M.H., and RalisonA.V. (2011) A new geological framework for south-central Madagascar, and its relevance to the out-of-Africa hypothesis. Precambrian Research, 185, 109-130.


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PEGMATITIC ROCKS IN A MIGMATITE-GRANITE COMPLEX (NWPORTUGAL)

M. Areias, M. Ribeiro, A. DriaCGUP/DGAOT-FCUP, Porto, Portugal ([email protected])

Geometry, structure and mineralogy

In NW Portugal a migmatite massif surroundinga synorogenic granite occurs within the axial zone ofthe Iberian Variscan Orogen along the westernborder of the Central Iberian Zone (CIZ). Thismassif includes metatexites and diatexites, both cutby a set of pegmatitic and granitic bodies and veins.The pegmatitic bodies exhibit variable geometry anddimension, both concordant or discordant with thestructure of the host migmatite rocks (N150 toN170; 90).

Three types of veins were investigated: (i)aplite-pegmatites (APG) veins; (ii); pegmatoid(PGTD) veins; and (iii) leucocratic patch (LCP)veins.

The APG veins (Fig. 1A) (thickness: cm to m;length: 10 to hundreds meters) have sharp contacts,striking N110 to N140 and internal zonation. Thezoning comprises: i) an aplitic central zone,composed by quartz, albite, biotite and aligned trailsof tourmaline (schorl) and garnet (Fe+Mn+Fbearing); ii) an intermediate coarser-grained zonewith perthitic feldspar, albite, quartz, tourmaline andmuscovite. The feldspar has irregular borders andabundant fine-grained matrix-mineral inclusions; iii)a medium-grained quartz-feldspathic zone with fine-grained mica, quartz and tourmaline agglomerates;

and iv) a quartz-albite fringe adjacent to host rock(comb structures). The albite contains abundantapatite inclusions.

The PGTD veins (Fig.1B) are coarse-grainedbiotite-rich without internal structure and withirregular geometry parallel to the migmatitestructure striking N120 to N150. These veins,associated with diatexites, have gradually coarseninggrain size towards the center of the body and arecomposed of plagioclase (An20), quartz, orthoclase,biotite, cordierite, sillimanite, garnet, rare andalusite,and pseudomorphic, fine-grained clusters of white

mica after biotite. The coarse-grained perthitic K-feldspar contains inclusions of plagioclase, biotite,cordierite and quartz with irregular or roundedmorphology. Some aligned bands of biotite-cordierite-sillimanite (schlieren) occur in the quartz-feldspathic zones.

The LCP veins (Fig. 1C) show diversegeometry, with tabular or irregular bodies, parallelor intersecting the migmatitic structure. They are

characterized by a leucocratic matrix with feldspar,plagioclase, quartz and medium to coarse-grainedmuscovite and biotite-chlorite-rutile-muscovitetourmalinefibrolite clusters. Theabundant coarse-grained and anhedral K-feldspar(microcline) develops intergrowths with plagioclaseand contains rounded inclusions of quartz,plagioclase and biotite.

Geochemistry

The pegmatoid bodies, leucosomes, diatexitesand associated granites show low concentration ofFe, Mg, Ca, Mn, Th and HSFE regarding the UpperContinental Crust UCC (Taylor & McLennan,1985). The contents of LILE are similar to the UCC

(Fig. 1D).The APG are most enriched in Mn, Mg, Ta, Nb,Cs and Be and depleted in K, Ba, Sr, Th and Ti. Thechondrite-normalized (Boynton, 1984) REE patternhas low REE, negative Eu anomaly and low HREEfractionation.

The chemical composition of PGTD veins issimilar to the associated diatexites. The REE patternis characterized by moderate REE, absence of a Euanomaly and variable fractionation trends of HREE.

The LCP veins and leucosomes show similarREE pattern, with positive Eu and Tb anomalies,

low REEand variable fractionation of HREE.The granitic rocks yield moderate REE, anegative Eu anomaly and high REE fractionation(16 La/Lu 45).

In all the lithologies, the REE is related tozircon and garnet content. The Eu anomaly shows apositive correlation with K/Na. In the garnet bearingrocks the fractionation of HREE is lower.

Discussion

The APG veins show mineralogical compositionand structural zoning similar to typical graniticpegmatites. The PGTD veins show chemical and

mineralogical affinity with diatexites and the LCPveins show the same affinity with metatexiticleucosomes. This suggests different time and originof the studied veins although they all have pegmatitictexture (coarse grain) and granitic composition.

The central aplitic zone of the APG seems to bethe earliest with nucleation of Na, Mn and Fe phaseswithout K. The growth of orthoclase and muscovite,indicating a period of K abundance, is confined to


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the intermediate zone. Lastly, the comb intergrowthof quartz-albite, with abundant apatite inclusionsand rare orthoclase+muscovite indicates high Na, Caand P content and low K content.

In diatexites the K-feldspar was the last phase,as indicated by the inclusions of all other

paragenetic minerals. In the leucosomes theintergrowths of plagioclase and K-feldspar indicatethat the first was replaced by the last one. Some ofthe leucosomes show replacement of biotite bytourmaline.

The sequence of minerals growing in the APGveins seems to be related to the sequential stages ofanatexis: i) a first melt without K-feldspar, resultingfrom the water-undersaturated breakdown of biotite,with anhydrous peritetic minerals (cordierite and

garnet) and rich in plagioclase + quartz; the apliticinner part of the APG is probably related to thisstage; ii) in a second stage, the progression ofmelting promoted the grow of large orthoclase andreplacement of plagioclase by K-feldspar inleucosomes and diatexites; these fluids have not

reached the entire migmatitic complex since thereare diatexites and metatexites without K-feldspar;iii) in a later stage, the APG growth was marked bya quartz-albite comb structure, with abundant apatiteinclusions indicating the effect of the K-poor andNa+P - rich fluid. This last phase could coincidewith the emplacement and crystallization of P-richgranites and release of B-rich fluids that gives rise tothe development of tourmaline-bearing veins.

Fig. 1:A1) Transgressiveaplitopegmatite vein in metatexite (MTX); A2) APG detail; B1) Pegmatoid veins (PGTD)associated with diatexite (DTX); B2) PGTD detail; C1) Transgressive leucocratic patch (LCP) vein in metatexite (MTX).C2) Leucosome (LCS). D) UCC-normalized multi-element diagram. E) Chondrite-normalized REE pattern diagram

including granite and metatexite.

AcknowledgmentsResearch integrated in the activities of CGUP, with

financial EU funds by FEDER/OPHP and FCT (fellowship nSFRH/BD/65509/2009) and PETROCHRON project(PTDC/CTE-GIX /112561/2009).

References

Taylor, S.R, McLennan S.M (1985): The continentalcrust. Its composition and evolution. Blackwell ScientificPublication, London, 312.

Boynton W.V. (1984): Geochemisytry of rare earth elements:metrorite studies. In Henderson P. (ed), Rare earth elementgeochemistry. Elsevier, pp 63-114.


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UNRAVELING THE FLUID EVOLUTION OF MINERALIZED PEGMATITES IN NAMIBIA

L. Ashworth1, J. Kinnaird1, P. Nex1,21Economic Geology Research Institute, School of Geosciences, University of the Witwatersrand

[email protected] Financial Services

Introduction

Namibia is renowned for its abundant mineralresources. A large proportion of these resources ishosted in the metasedimentary lithologies of theDamara Belt, the northeast-trending inland branch ofthe Neoproterozoic Pan-African Damara Orogen.Deposit types include late- to post-tectonic (~ 523 506 Ma) LCT (Li-Be, Sn-, and miarolitic gem-tourmaline-bearing) pegmatites, and uraniferouspegmatitic sheeted leucogranites (SLGs), whichhave an NYF affinity.

Fluid Inclusion Results

Fluid inclusion studies reveal that althoughmineralization differs between the different types ofpegmatites located at different geographic locations,and by extension, different stratigraphic levels, thefluid inclusion assemblages present in thesepegmatites are similar. Thorough fluid inclusionpetrography indicated that although fluid inclusionsare abundant in the pegmatites, no primary fluidinclusions could be identified, and rather thosestudied are pseudosecondary and secondary. Fluidinclusions are aqueous-carbonic, carbonic, andaqueous.

Three types of pseudosecondary fluid inclusionwere observed. Aqueous-carbonic pseudosecondaryinclusions contain halite and an unidentified,acicular opaque phase. In the Li-Be and gemtourmaline-bearing pegmatites, these inclusionscontain pure CO2, however in the Sn-bearing

pegmatites trace amounts of CH4are present. These

inclusions homogenize at temperatures (Th) rangingfrom 320330 C, and their density is intermediate(0.7 0.8 g/cc). The presence of halite crystals inthese inclusions indicates salinity in excess of 23.3equivalent wt % NaCl (Shepherd, et al., 1985).Qualitative PIXE micro-elemental maps show thepresence of Fe, Mn, Cu, and Zn, suggesting that thetrapped fluids are far more compositionallycomplicated than indicated by microthermometryalone. Saline aqueous inclusions containing haliteand two opaque phases homogenize at lowertemperatures (100 120 C), and their density

ranges from 0.9 1.0 g/cc. Pure CO2pseudosecondary inclusions are observed only in thepegmatitic SLGs. They homogenize at atemperatures ranging from 7 C to 15 C, and theirdensity is relatively high (0.80.9 g/cc).

Multiple secondary fluid inclusion populationsare present in the pegmatites. The three populationsobserved are of similar composition, Th, and densityto the pseudosecondary populations, however theyare less saline (1215 equivalent wt % NaCl).

Stable Isotopes

Oxygen isotope ratios in quartz show a bimodaldistribution (Fig. 1). This suggests an I-type affinityfor the samples showing lower 18O ratios (18O =1113 ) i.e. the Li-Be pegmatites and pegmatiticSLGs, and an S-type affinity for those with higherratios (18O = 1516 ) i.e. the miarolitic and

Fig. 1:18O ratios for quartz, feldspar, whole rock samples taken from mineralized pegmatites inNamibia, and their adjacent country rocks.


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Sn-bearing pegmatites. This is corroborated by thefact that the metapelitic and carbonate country rocksin the vicinity of the latter exhibit similar 18O ratiosto those of the pegmatites, while the same is not trueof the I-type pegmatites and their country rocks,which are carbonates, metapelites, and granodiorites.

The 18

O ratios of the S-type pegmatites are high,and this is consistent with the derivation of thesemagmas from a sedimentary source. However, theratios of I-type pegmatites are elevated above thoseof typical I-type granites, which typically range from18O = 79 , suggesting either a low-temperatureexchange with meteoric fluid, high-temperaturehydrothermal exchange with 18O country rocks, orthe derivation of these pegmatites from a non-peliticsource (Sharp, 2007). D values range from -40 to -90 indicating that the pegmatitic fluids areprimary magmatic.

GeochemistryTrace elements show that both LCT and NYF

pegmatites fall within the range of syn- to post-collisional granites (Fig. 2A). Rare earth elementconcentrations in all of the LCT pegmatites are low.

Although there is some scatter in REE abundance,all of the pegmatites show a relative enrichment inLREE in comparison to HREE, and a strong Euanomaly (Fig. 2B). The NYF pegmatitic SLGscontain higher abundances of REEs, and a depletionin LREE in comparison to HREE (

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PEG 2013 Abstract Volume