153
Thc CanadianMineralo gist
(1997)
Vol.35,pp. 153-165
THEVOLTAGRANDEPEGMATITES,
MINAS GERAIS,BRAZIL:
EXCEPTIONALLY
AN EXAMPLEOF RARE-ELEMENT
GRANITICPEGMATITES
ENRICHED
IN LITHIUMAND RUBIDIUM
rirAK-TINELAGACFIEI
05,France
ruellnmond 75231ParisCed.ex
Laboratoire
deGdotogie,
EcoleNormaleSup4rizure,
U\A 1316du CNRS,24,
JOFI QUEIvIEI.{EUR
Depanmentof Geology,IGC, UFMG, Av.Antonio Carl.os6627,31270BeIoHorizon e, Brazil
ABSTR,ACT
granites@arly hoterozoic) hostedby the Archean
The Volta Grandegnaitic pegmatitesareassociatedwith Trar:samazonian
g&enstonebelt of the Rio das Mortes Valley, which is situaledaJthe southemborder of the Sio FranciscoCraton,in Minas
Gerais,Brazil. The pegnatite bodies, which are usually large, show a dominant intermediatezone containing sp,odumene,
microcline, atbite and quartz,with an irregular borderof an aplitic faciessurroundedby an extensivemetasomaticaureolewith
zinnwaldite,phlogopiteandhobnquistite.The spodumene-rich
corezoneis continuousor segmented,andalsocontainslensesof
lepidolite. Thesepegmatitesare characterizedchemically by their high Rb and Li content Rubidium is concentratedin the
potassicminerals:microcline,muscovite,lepidolite andzinnwaldite.Thesemineralsshowexceptionallylow K/Rb weightratios,
from 1.5to 4.0. In micas,a positivelinear correlationexistsbetweenfluorine andrubidium.The evolutionof the concentrations
of Rb andCs in the minerals,in the coreof the pegmatiteandin the metasomaticaureole,canbe explainedusing experimentally
determinedpartition coefficients.
granitic pegmatite,lithium, rubidium, cesium,K/Rb ratios, zinnwaldite,holmquistite,Volta
Keywords:rare-element-enriched
Grande,Minas Gerais,Brazil.
Somaenr
(hot6rozo'iqueinf6rieur) intrusifs
Irs pegnatites granitiquesde Volta Grandesont associdesaux granitestransamazoniens
du Rio dasMortes et localis6si la bordurem6ridionaledu Cratondu SdoFrancisco,
dansla ceinturede rochesvertesarchdennes
i Minas Gerais,Brdsil. I,es cotps pegmafitiques,g6ndralementde grandesdimensions,pr6sententune zoneinterrn6diairetrbs
quarE et feldspaths,partiellemententourded'une zoneaplitiqueaux contoursfureguliers.Ils d6velopd6velopp6eI spodumBne,
pent une aureole€tenduede m6tasomatose
l zinnwaldite,phlogopiteet hotnquistite. Ic coeurriche en spodumdneest continu
ou segmentf,et contientdeslentilles de ldpidolite. Chimiquemen!cespegmatitessontcaractiris6espar leursteneurstrds6levdes
microcline,muscovite,l6pidolite et zinnwaldite
enrubidium et lithium. Le rubidium estconcentrdda:rsles min6rauxpotassiques:
qui pr6sententsouventdesteneursexcqfionnelles, conespondantI desrapportspond6rauxK/Rb trds bas"allant de 1,5 e 4,0.
Dansles micas,on note une corr6lationlin6aire entrele rubidium et le fluor. Les coefficientsde partaged6termin6sexltsrimentalementpermett€ntd'exptquer l'dvolution desconcentrationsen Rb et Cs dansle,smin6rauxdespegmatiteset de leur aur6ole
de m6tasomatose.
Mots-cl6s:pegmatitegranitique I 6l6mentsrares,lithium, rubidium, cdsium,rapport K/Rb, zinnwaldite, holmquistite,Volta
Grande,Minas Gerais,Brdsil.
INrRoDUc"noN
A field of large pegmatitebodiesis associatedwith
Early Proterozoic granites (Iransamazonian,1930 t
20 Ma; Qu6m6neur& Vidal 1989)of the 56o Jododel
Rei areain southemMinas Gerais,Brazil. One of the
pegmatite swarms in this field, the Volta Grande
pegnatites,canbe describedas spodumene-rich
bodies
displayng a very high degree of fractionation and
LE-mnil allress: mlagache@cristal.ens.ft
enrichmentof the rare lithophile elementsLi and Rb,
leading to the precipitation of spodumeneand lepidolite, and K-feldsparas well as muscoviterich in Rb.
These pegmatites were previously investigated by
Heinrich (L964), who was interested in the Sn-Ta
mineralization,for which they werebeing minedat that
time. Currently, someof the pegmatitesare mined for
spodumene.
t54
BambuiGroup
Fl
ffil
Afinasslrpororanp
FSI
granat€
Transamazonlan
f
Basicintrusbn
ffffll
Grsenstcnebtt
I---l
arcngangranit&€neFg
ffil
Pericatoncregion
BH - Bob ftodzonle
Ftc. 1. Location map of tle Transamazoniangranitesat fhe southernborder of the 56o FranciscoCraton. The boxed area
correspondsto Figure 2.
The exceptionalenrichmentof the mineralsin rubid- tact aureole.In addition,the experimentsperformedby
ium motivateda detailedstudy of the mineralogyand severalauthorson the determinationof partition coefnof the geochemisty of the Volta Grandepegmatitres,
to cients of alkali elementsbetweenminsmk or magrna
characterizethe fractionationas well as to evaluatethe and aqueousfluid allow us to verify if a chemical
extent of metasomaticfransfonnationsin the exocon- equilibrium amongthe solid phasesis attained.It will
/
Ix*
,*
BOM f,
.,1
t.
I It
7'r' ',' i t
{+ttL
fiki
=o
--'n
)'
r TABUoES
t
I
I.Rt'3)
.1 ,
P;**a*\il
-+++q
-+++
+
/-
7Zr.+**-ffi
O51o|(
E
sgoJgoDatRsiGBup
E
$
MinasSupercrcup
liEl
Fotiatedgmniteandt ondjhemie
IlFl
Pegmstts
EE
niigmariteano oiliogmisg
m
FinegninedgEnite
li-T'l cnnira
El
tronotriomire
El
Diortra-gabbD
Archsanisotfopisgnnits
ffi
uttmmatcpct
ffffll
Greenstonebott
@l
Undifierenriatedgreiss
Ftc. 2. Sketchmap of the geologicalenvironmentof the Volta Grandepegmatitefield.
155
THE VOLTA GRANDE GRANITIC PEGMATITFS. BRAZIL
extremely variable in composition: relatively basic
rocls suchas gabbro,diorite and trondhjemiteseemto
form zonedintrusions;yonngerganite, adamelliteand
granitic pegmatiteoccur to the north and west of the
basicintrusive complexes.
REGToNAL
GBotoctcal SErrD{G
Two large granitic massifs, the Tabu6esand the
The 56o Jododel Rei arealies on the southernborder Rit6polis massifs(Fig. 2), as well as two smaller and
of the Precambrian(Iransamazonian)56o Francisco apparentlyolder basic intrusions,the 56o Sebastianda
Craton (Fig. 1). This area consistsof a series of Victoria gabbroandthe lbitunga, RosarioandFazenda
Archean terranesmetamorphosedin the amphibolite Martin diorites and tonalites, are found in the study
facies, and locally in the granulite facies toward the area. The Ritdpolis massif consistspredominantlyof
(Qu6m6neur& Garcia
west. The terranesform part of the Rio das Mortes graniteand albite-richada:nellite
the
source of the Volta
have
been
may
1993),
and
greenstonebelt and are comprisedof gneisses,amphipegmatites.
Grande
gneisses
are
bolites and meta-ultrabasicrocks. The
approximatelygranodioriticto trondhjemiticin compoTIfi VoLTA GnANDShcMATruEs
sition.They coverlargeareasto the north andsouthand
formations.
representtle oldestpre-greenstone
Pegmatites of the Volta Grande swarm were
The Archean terrane is cross-cutby late Archean emplacedas flat subhorizontalsills that form large
eranite (2700 Ma" Qu6m6neur& Vidal 1989) and by lenticular bodies elongatedparallel to the foliation of
plutons ofthe Transanazoniansuite.They are aligned the metamorphichost rocks (Fig. 3).The largestintrudirectionalongthe enthesouthemborder sion (body A) measured1000 x 150 x 20 m before
in an east--west
of the craton,which is approrimately250 km long from exploitation.Neighboringlenticularpegmatites(bodies
the Quilombo ganite in the west to the Campotde B, C...) typically hadmaximumdimensionsof approxigranite in the east. The Transamazonianplutons are matelv700 x 120 X 10 m.
alsobe possibleto estimatethe compositionof the fluid
presentduring magmaticcrystallizationand to follow
the metasomaticprocessesin the exocontactaureole,
./-
./'
/\\
\ .z-V'Y-
\\( ,l
-\ry:A;
ro:mlsesmatite
'/
-
Flc. 3. The pegmatite field of Volta Grande.
o
F"rpn'iOorit"
5OO
IOOOm
t56
THE CANADIAN MINERALOGIST
Figure4 showsthe distributionof internalpegmatite
zones in ttre remaining parts of pegmafitebody A,
according to Qu6m6neur(1987). The subhorizontal
attitude of the pegmatites,subparallelto the erosional
surface,andthe extensivemining activitiesorenderthis
representationof intornal sfucture rather uninformative. Figure 5 shows a schematicvertical section
tbrough the A pegmatite,which can be taken as fairly
representativeof the intemal structure,types of zones
andzonalsuccessionin thepegmatitesexamined.Tbree
principal zonescanbe distinguished,from the contacts
c/ith the amphibolite wallrock inward: (1) wall zone,
lower andupper,with (a) apttic and (b) quartz-spotted
facies,(2) intermediatezone,lower andupper, and (3)
corezone,continuousor segmentedandlenticular.The
disfributionof the zonesis symmehicalinprinciple, but
asymmetical in termsof dimensionsandvolumes.The
intermediatezoneis volumetrically dqminanf,particularly its lower part.
Externalto the pegmatiteproper,an externalmetasomatic aureoleis developedin the amphibolitewallrock
adjacentto the pegmatitecontacts.
Thewall zone
This zoneshowsa variablethicknessfrom 0 to 4 m.
The main componentis an aplitic facies consistingof
fine-grained,mm-sizedalbite with subordinatelenses
of quartz (1-2 cm). This aplitic faciesis found mainly
alongthe footwall cortact, andlocally at the roof of the
pegmatite.However, the upper wall-zone commonly
containsa greatervariety ofmineral andtexturalfacies
thanthe lower segment.In the A body, a quartz-spotted
V71
faciesis developedin this location,with roundedblebs
of quartz (5-10 cm) in a matrix of K-feldspar, albite
and muscovite.In the Minas Brasil body, the common
aplitic facies adjoins the upper contactwith amphibolite, followed inwardby a layer of columnarspodumene
(0.8-1.2m) andquartz.
The inner contactofthe aplitic facies usually has a
thick border (1G-30 cm) of muscovite.In the C body,
this border also ca:ries rounded blebs (5-10 cm) of
K-feldsparandquartzin a matrix of spodumeneo
quartz,
albite, K-feldsparand muscovite.
Thc intermcdiatezone
This zone,which constiortes
75-80Voof theA pegmagranitic textwe (0.5-2 cm). It
tite, hasa coarse-grained
is composedof.2V30Vo qvafiz, 20-307o spodumene,
l5-2OVoalbite,lU-LSVoK-feldspar,and3-5Vomuscovite. This zone commonly showsa porphyritic aspec!
as it containsfairly abundantcrystals of spodumene
50 to 100 cm long. The lower segmentof this zoneis
distinctly thicker than the upper one. However, the
upperpart tendsto be texturally more diversifid as it
containsnumerouslenticular pods of K-feldspar and
accessoryquartz, L-5 m long and 0.3-5 m across.
The core zone
This zoneis centrally situatedin terms of the zonal
sequence,but is asymmetricallypositionedin the upper
central part of the pegmatite,mainly becauseof the
asymmetryin volumesof the lower andupperintermediate zones. It consists of large flat prisms of
angniwwa
Exocontaclaureole
V7
NnpnOotitewith hotmquistite
Wall zone
lll:lll
lTl
@l
Aptiffcfacies
lntermediatezone
corezone
++++++++
+++++++++
++++++++++
++++++++++
+
+
#e-dtiiiffi
+\
ii;;;i
+ ; i + i i i i i i i + + ++ +
n
+ + + + F + + ++ + ++ + ++ ! J ' + ++++
+++ + + + +
++++
+
+l +++
l ' [l +
+
+ + + t+ +
+
Ii +
ij +
-ii+++++++{@+++++++
;p65;195P.-:,j#V.
Wallzone
F-T
QuarE-spottedtacies
J?++++
Flc. 4. Skerchof the zoningof tle west part of tle pegmatitebody A, asit is currentlypeserved.
+
+++
f,++
ltt+
+++
{1I
THEVOLTA
157
GRANDE GRANMC PEGMATITES. BRAZL
spodumene,30 to 100 cm long, which constitute
6O-80vol.Voof the core.The interstitial spaceis filled
by quartzand K-feldspar.In the past,lenticularbodies
of lepidolite were exposedin the core zone of the B
pegmatite. Lepidolite was fine-flaked and compact.
Although not currently availablein the field, lepidolite
was preservedin laboratory samplesand examinedin
the presentstudy.
zinnwaldite and locally by zinnwaldite+ holmquistite.
The outer faciesis 2-3 m thick the extent of metasomatism of the amphibolite is 1afte1 limited, with
holmquistite,zinnwaldite and mainly U-bearing phlogopite dispersedtbroughout the fairly well preserved
country-rock.
Comparisonwith otherpegmatiteoccurrences
The exocontactaureole
The mineralogy and internal structureof the Volta
Grandepegmatitesresemblethoseof otheroccurrences
A metasomaticzone,a few metersacross,is devel- assembledby Cernf (1992)into the categoryofalbiteoped in the amphibolite wallrock. It consistsof two spodumenetype, suchasthe V/einebenepegmatitesof
distinct facies. The lrner facies is G-l m vride: it the Koralpe (GOd 1989) and the Kings Mountain
consists of a compact dark-colored ooglimmeriteo'pegmatitesin the Appalachians(Kunasz 1982). They
formed by a completereplacementof hornblendeby areall characterizedby predominanceof a spodumene-
amPhibolite
exocontact
aureole
nf-ril'.:El
1-'IX$\t - .':--'j'Nf'i *; i
\{all zone
upper
intermediate
zone
@arsezone
i.;.;'P1t't.;.El&
with
outerfacies(amPhibotite
andzinn$raldite)
holmquistite
innerfacies(zinnwaldite)
apliticfacies
quartzspottedfacies
blocksof microcline
r-
+
+ + + + + + + + + + + + . +. .++. '
+ + + + + + + + + + ++ ++ ++
.+.+
t- + + + + + + + + +
.+.'
+ + + + + + + + + + + ++ ++
+ + + + + + + + + + + + + . +. .++. .+'
+ + + + + + + + + +
+ + + + + + + + + + + + + + + + .++. .++. '
+ + + + + + + + +
+. + + + + + + + + + + + +.+.+
lovtFr + + + + + + + + + + + + + . +
intermediate + + + + + + + + + + + + + . + . +
+++++++++t--+
zone + + + + +
+ + + + + + + + + +.y'.I3ia.+
+ +--.L+++++++++\...\:
+ . ..:.,i,4''.: + + + +.,_+..+ t-t . +'\i.:+
t-
ff.il;oj*kiW
+rI
!)
wall zone
exo@ntact
aureole
muscovite-richaureole
aplitic facies
inner facies (zinnnaldite)
outer facies (amPhibolitewith
holmquistiteand zinnuraldite)
amphibolite
Frc. 5. Schematicvertical cross-sectionof the pep.atite body A showinga typical sequenceof intemal zones.
158
THE CANADIAN MINERALOGIST
rich zone and the presenceof minor aplitic units.
Ferromagnesianmicas andholmquistitein the exoHowever, the Volta Grandepegmatitesare distinctly contactconstitutethe metasomaticaureole.As shown
more stronglyzonedthanthe classicnear-homogeneousbelow, magnesianzinnwaldite is the main mineral in
albite-spodumenepegmatites,and they also lack the the inner faciesof the aureoleadjacentto the pegmatite
high degreeofprefened orientationof spodumeneand contacts,but it gradesinto a more or lesslithian phloK-feldspartypical of most pegmatitesin this category. gopite in the outer facies.Holmquistite(Qu6mdneur&
The albite-spodumenepegmatitestend to be steeply Lagache1994)is restrictedto the outerfaciesalone.
dipping to subvertical,but the Volta Grandebodiesare
subhorizontal;accordingto this feature,combinedwith
GEocHExv[srTrY
the aslmmetrical aspectsof internal zoning, the Volta
Grandeoccunenceshavethe characteristicsof lavered
The chemical composition of minerals was deterpegmatite(Cemf 1991).
minedby electronmicroprobe,using Camebaxand SX
50 insruments at the Universit6 Pierre et Marie Curie
(Paris),by llductively CoupledPlasryaflCP) specDEscRrP'rroN
oFTID MD[BRAr-s
tromety at Ecole des Mines at Saint-Etienne,an! by
Albite occutsin the aplite zone,mainly as mm-size wet chemistryat the Laboratoirede G6ologiedel'Ecoie
composicrystals.It is also presentin the intermediatezone as Normale Sup'6rieurede Paris.Representative
crystalsup to a few cm acrossor asmm-sizeinclusions tions of eachof the -ajor mineralsof the pegmatiteand
of its contactaureoleare given in Tables I to 5.
in K-feldspar.
Electron-microprobeanalysiswere performedwith
T\e K-feld.sparis a clear grey perthite-freemicrocline. It is found in all pegmatiteunits except in the the following standards:orthoclasefor I! Si and A1,
aplitic zone and in lepidolite lenses.In the C body, albite for N4 synthetic MnTiO3 for Mn and Ti, synroundedblocks of K-feldspar (3 to 10 cm in diameter, thetic Fe2O3for Fe, diopsidefor Ca and Mg, topaz for
in a matrix of quartz,albite.,spodumeneandmuscovite) F, syntheticRbCaF3for Rb, and Cs-rich beryl for Cs.
arepresentat the contactswith the aplitic zone.Micro- The acceleratingvoltage was 15 kV, the sample
cline also is found in lensesin associationwith quartz, current, 10 nA, and the countingtime was 10 seconds
wherecrystalscan reachseveralmetersin length.This for major elementsand 20 secondsfor minor elements.
microcline contains l0 to l1%o albite and approxi- Data reduction was done usine the PAP correction
mately 5Voquartzas discretemicro-inclusions.Al1 the procedure.
samplesstudiedcorespondto maximummicrocline,as
-ro
indicatedby the niclinic indicator 4(130) proposedby Whok ck compositions
MacKenzie(1954).
For reasonsevidentin Figure5, it is virtually imposSpodumcneis presentin all pegmatiteunits exceptin
sibleto collect a samplequantitativelyrepresentativeof
the aplitic facies of the wall zone and in the lensesof
the bulk compositionof any of the pegmatitesstudied.
microclineandlepidolite.It is a major constituentof the
Howeveroa few whole-rock analyseswere performed
intermediatezone,where it occursas acicularor prison the intermediatezone. It constitutesmost of the
matic crystalsseveralcentimetersin length. Superimvolume
of the pegmatites; it cannot be considered
posedon this texturearelarge rod-shapecrystalsup to
totally representativeof the bulk composition,but its
I m in length. Spodumenelensesconsistof 60 to 80Eo
of all the
modalcompositionshowsgoodrepresentation
spodumene,as prisms 0.2 to I m longowith the space
mineral components,andits chemistryshouldbe faidy
betweencrystalsfilled by quartzand microcline.
representativeof the degreeof fractionationand rareMuscovite typically is found in the intermediate elementaccumulationin the pegmatiteas a whole.
zone,particularly at the contactwith the aplitic facies
Five samples(3 kg each) of the intermediatszone
of the wall zone.It occursas mm- to cm-sizegreenish were crushed,split and analyzedby ICP spectometry.
yellow flakes. It is also locally present(in the westem The resultsare consistentwith the mineral modesand
part of bodies A and B) as plates a few centimeters relatively unifomr. They show a compositionrich in
acrossin a muscovite- quartz- cassiteriteassemblage SiO2 (75 to 78 wt.Vo),roughly equal proportions of
(crystalsof cassiteritein this zoneattain 10 cm across).
K2O and Na2O (2 to 3 wLToeach), and low Fe/I\4n
A secondgenerationofmuscovite is found in fractures weight ratio (between2 and 3). The most prominent
and microcavitiesin the intermediatezone,in the core characteristicof the whole-rock compositionsis the
of the pegmatite,and associatedwith lepidolite.
high Rb content,ranging from 0.7 to 1.2 wt.EoR'b2O.
Rose-mauvelepiblite is relatively rare. It is found Accordingly, the K/Rb weightratio averagesas low as
in bodies A and B, in lensesof a fine-grainedmica about 2.5. This is a value rarelv encounteredeven in
(0.1 to 1.0 mm) togetherwith muscoviteof the second highly fractionated rare-element pegmatites (demf
generation(5Vo)andscatteredquartz.
et al. I985a), and it comescloseto the valuescharacAccessoryminerals are cassiterite,tantalite, micro- teriz;rngthe most differentiatedpegmatitesanalyzedto
lite, monaziteand gamet.
date.The K/Rb ratio is slightly lower thanthe approxi-
THB VOLTA GRANDE GRANITIC PEGMATITES. BRAZIL
159
and in eachsample,rubidium contentsare more variRbzO,but the
able than in microcline, 2.1 to 4.4 v,rt.Vo
averagecontentis 3.1 wtvo,which is of the sameorder
of magnitude as in microcline. The resulting K/Rb
weight ratios rangefrom 5.3 to 2.0, with an averageof
2.7. T\e second type of muscovite associatedwith
lepidolite is iron-free and, in comparisonto the first
generation, has similar Rb but higher Cs contents
Mineral compositiow
(0.24 comparedto 0.09. vt.Vo'Cs2O).The Rb/Cs ratio
Representativeresults of electon-microprobe and is conespondinglylower, from 33 to I 1. The RbzOand
wet-chemicalanalysesof microcline fromtwo different K/Rb data match those for muscovite and lithian
pegmatitebodies are given in Table 1. Micl and muscovitefrom Tanco (Cem! et aI. 1996); however,
Mic2 samples come from lenses of microcline of the Cs contentof the Tancomicasis higher.
Compositionsof two samples of Li-rich micas
bodiesA andC. andMic3 comesfrom the intermediate
zone of body C. Thesecompositionshave beentaken LPl and LP2 (Table 3) have been deterrnined on
from a batch of about 15 samples.Wet-chemicalanal- variousflakesuken in two areasof the pegmatitefield.
ysesindicatethat the abundanceof albite inclusionsin Extensivevariations are evident from the microprobe
which would datafor eachindividuat flake, which presentsa random
microcline is on the order of l0 to 1.5Eo,
enhancethe K and Rb abundancesin pure microcline. mixture of compositions.The compositionvariesfrom
The chemical compositionof microcline within indi- that of the muscoviteof the secondgeneration,with
vidual samples and among the pegmatite bodies is which the lithian mica is associated,to that of a lepirather unifonn. For example,in sample Mic2, K2O
variesfrom 14.18to L4.58wt.Vo,
andRbzOvariesfrom
3.03 to 3.39 wt.%o.From one sampleto another,the
variation is of the sameorder as indicatedby the mean
valuesof the three sampleschosen.The averageK/Rb
TABI,E I. AVERAGE COMPOSMON OFALBITEAND OF MICROCUNE
INTIIRBBSAMPLES
weightrariois 4 t 0.5.Thisis thesa:neorderof magnitude
Mic3
Mlc3
Mlcz
Mic2
Micl
Mlcl
elbite
as cited for the Tanco pegmatite(Cen$ et aI. 1985a), ft1ds@
(c.B)
(6) Glm)
(4)
02) (cbm)
.(1?
but is greaterthan the valueof 1.9reportedfor feldspar 3d------rr-.65
u.29
61.81
62,w
63,14
o:.az
et-n
18.21 18.00 18,18 17,72
19.58 18.25 18.18
Alro3
in the lithian pegmatiteat Red CrossLake (Cem! et al.
0.07
0.05
0.U
0.02
0.06
o,o2
0.00
L985b, L994).The averagecompositionof microcline FEO
1,43
0.43
1.80
0.41
0.35
1.81
11.83 '14.41
NarO
13'11
14'14
12.83 14.31 l32a
0.07
K:O
correspondsto Or66Abe.9aRbfs.16.
0.00
0.00
0.00
0.00
0.00
0.00
O.V2
CsO
2.6s
Cesiumvaluesare more variable,between500 and Rho
326
2.97
3.14
2.87
3.n
o,0o
0.10
0'14
0.05
0.0J
0.09
0.00
0.09
Cs:O
1400ppm, which is an averageconcentrationfor rareo<L l34ppm
n.d. l4EFpo
rd. l54ppa
ad.
LizO
'total
98.01
9.37
w.29
98.23
elementgranitic pegmatite(Cer$ 1992),but relatively
99,26
98.07
100,33
low compared to t}te rubidium concentrations.The
(.e@}
@lys
@dsetd@iel
C@citi@@dtsiledt@@lbdd€rd@'ni6opFb€
averageRb/Cs value rangesfrom 22 to 61, and the lte@b{ofpoid @\zEdbyel$t@8ic.Fobeb gi@ inpsdhs* 4d: d d!ffiinod
Prcpod@of 6id9 q#Fne apl*d id wigir 79q.rpt I @bd
averagetrVCsvalue,from 89 to250 16-37and42-508,
respectively,for Tanco; Cem! et al. (L996)).The IVCs
range is, howeveq far from the minimum value of
22.4 g1venbyCern! et aI. (1985b,1996).Thesevalues
suggesta generallylow abundanceof Cs in the Volta
OF
TABLE2. AVERAGECOMPOSIIIONOFTI{REESAMPLES
FIRSTCENERA'NONANDONEOFSFTOND4ENEMTION MUSCOITTE
Grandepegmatites,and they may explain the absence
of polueite in thesebodies,despitetheir otherwisshigh
Mgr (9)
MSs(3)
mmvite
Msl (rcP) Ms2 (5) MS303)
degreeof fractionation.
lsf
Albite is found in all zonesof the pegmatitesexcept
30.72
39.97
33.89
35.12
34.30
AluOr
in the core zone and lepidolite lenses. The data
0.06
0.05
0.07
0.01
TiOz
0.04
1.87
2.59
2.75
0.00
2.35
presentedin Table I representthe averagecomposition FeO
0.40
0 08
0.52
n.d.
nd.
of albite micro-inclusionsin microcline.Albite associ- MnO
0.03
Mgo
0.43
0.02
0.00
0.o2
atedwith muscovitehasthe samecomposition.Albite
0.01
0.05
0.02
0.00
0.01
CaO
0.51
Na2O
0.33
0.18
0.34
0.15
is very pure and containsneither Ca, Rb nor Cs above
8.86
9.15
9,13
8,60
8.83
Kzo
the limits of detectionof the microprobe(100 ppm).
2.65
Rbro
3.22
3.18
2.70
0.09
o.o7
0.24
Cs2O
0.08
0.ll
Analysesof muscovitewere performedon samples
Lbo
0.29
nd.
nd.
n.d.
n.d.
taken from different bodies of the pegmatite field,
0.84
F
rd.
nd.
nd.
0.73
4.95
nd.
HrO
rd.
Id.
ld.
representativeof both types of muscovitedescribedin
Total
97.58
type is
the precedingsection.The fust coarsed-grained
distinguishedby elevatedcontentsof Fe (1 ta 3 wt.Vo Cqosid@ (€6I(Kdinwight 7o)re obtabcd by ol!.amnisr'probq Thetwtlts
alrc relised
FeO: Table 2). Among the various samplesanalyzed ofmICP-MS oabnir offtst€asdimMilg0CP)
mate value derived for the Tanco pegmatite,in Manitoba (3.6: Cernf et at. 1996),but the rangeof Fe/Mn
valuesis higher (0.9 at Tanco).The Volta Grandedata
are slightly surpassedonly by the geochemicalsignature of theRedCrossLakepegmatites,alsoin Manitoba
(Rb2O0.63-1.55wt.Vo,K/Rb2.6-l; Cem! et al. 1994).
160
TIIE CANADIAN MINERALOGIST
TABLE 3. AVEMGE
COMPOSITION OF TWO MUSCOVTTE.LEPIDOLM
Muscovite
LPI
LPI
LPI
LPt
Lpl
Lp2
(l t)
(ln
(2) (chenr")
:lplidolite (c!gqr") _G)
SiO2
49.90 43.32 44.91 a5.
Al:O:
26.14 34.89 28.92 26,02 22.52 26.09
FeO
0.07 0.06 o.O2 0.01 O.O2 O.t2
I,IIO
rld.
n.d. 0.38 0.50 O.O2 rt.d.
MgO
n d.
n.d. 0.01 0.A2 0.01
n.d.
Na2O
0.38 0.53 0.38 0.29 0.18 0.48
KzO
7.68 s.96 8.29 8.08 7.90 8.A
Rbo
4.65 3.24 4.?O 5.29 5.86 4.26
cs:o
0.49 0.23 0.52 0.68 1.18 0.38
LizO
3.30
rr"d, 3.34* 3.55* 4.08*
3,49
F
5.41 1.15 4.56 6.05 8.19 5.62
HrO
4.10
n.d.
n.d.
n.d.
n.d.
4,26
Total
102.12
LpZ
(sl
SAMPLES
Lpz
(a)
Lp2
(il)
31.03 29.gl 26.46
0.1I
0.09 0.tl
rr.d.
n.d.
n-d.
n d.
nd.
rLd.
0.34 0.27 o.2s
8.99 8.88 9.46
3.9s
4.1,9 4.92
0.24 0.28 o.2g
rr"d. 3.404 3.70*
2.68 4.tt
5.70
n.d.
n.d.
n.d.
lit!fum ooDlErscalculdedby thsEsrhodof rindl€ & v{e}b 0990). Norethd tnehigh€rsilioa
c) indioatos
co'tffiinthe resultsof tlewee{bmioal analyses
reslt fiom thepresence
of quirta henceall oft;el@S
ae relairely depleced"
TLooonrpositions
ce quot€din wsigfuo/o.
dolite. In the absenceof datafor lithium- fluorine is a
goodindicatorof this variation.The fluorine concenftation ranges,for example,from 0.7 vrt,Votn muscovite
of the secondgeneration,to 2 wt.Eoin Li-poor areasof
sample LPl, and can attain 8 wt.7o. Representative
electron-microprobedataon sampleLP1 are therefore
reported as averagesof four gtoups, based on their
averagefluorine concentation. The fust group has a
very low concenfrationof F (4, wt.7o), the second
correspondsto F between4 and 5.5 wt.7o, the third
between5.5 and 7 wt.Vo,andfle last one to concentrations of 8 wt.Vo.Membersof the first and last groups
are rare. The compositions obtained on LP2 are
presented in three groups, which average at ^2.7,
4ard^5.7 wt%oF.
Wet-chemical analyses,performed on the two
samplesof mica, show a higher concentrationof silica
owing to the presenceof small quantities of quartz.
Hence,all other elementsare relatively depleted.It is
not possibleto quantify a correctionbecausethe results
ofthe chemicalanalysisrepresentan averagecomposition of intergrown mica aggregates,with unknown
proportionsofindividual phases.The chemicalanalyses
estabhshan averagelithium contentat3.4wt.%oU2O.
Tindle & Webb (1990) proposedan equationto estimate the Li contentsof fiioctahedral micas basedon
silica content.This equationwas appliedto the F-rich
mica compositions,yielding Li2O contentson the order
of3to4wt.Eo.
Rubidium contents of the F-poor mica (3.2 wt.Vo
Rb2O) are slightly greater than those in the second
generation of muscovite (2.7 tx't.VoRb2O), but the
most F-rich domains contain up to 6.0 wt.7o Rb2O.
Figure 6 illustates the concentationof Rb2Oasafunction of fluorine. A positive linear relationship exists
betweenthe two elements,and thereforealso between
lithium and rubidium. The correlation is valid for
muscovite,lepidolite and zinnwaldite.The variation of
Cs is less consistent,but follows the sametrend. The
K/Rb weight ratio in lepidolite is between2.4 ar:d1..2,
whereasthe Rb/Cs value is about 10. The Rb content,
the K/Rb and Rb/Cs valuesof the Volta Grandelepidolite are closely comparableto those of the Tanco
'lepidolite" (heterogeneous
but so far analyzedin bulk:
Ce,m! et al. 7996).However,the Red Cros,slepidolite
acains K/Rb and I(Cs valuesbelow O.l.'(Cem! et al.
1994\.
It is clearthat lepidolite concenfrates
the largerrarealkali elementsin comparisonto muscovite.This is due
to the decreasein the teftahedralrotation anglecr from
11.6" in muscovite(Giiven & Burnham L967)to 6" n
trilithionite or zinnwaldite (Guggenheim & Bailey
1977) and 3' in polylithionite Clakeda & Bumham
1969);tlis inducesan increasein the K-O bondlength
in lithium micas and promotesan easierincorporation
of Rb and Cs in the lithian micas in general, and
polylithionite in particular. The proportion of muscovite, trilithionite andpolylithionite canbe calculatedfor
eachcomposition.For example,the averagecomposition of samFleLPI correspondsto a mixture of 42Vo
muscovite, 32Votrilifrriontte and 26Vopolylithionite,
wrth6 vtt.VoF.As the fluorine increases,the proportion
of muscovite decreasesand that of polylithionite increases,in accordwith the increasingF content.
A greyishwhite specimenof spodumsrcfrom the core
zonewas analyzedby wet chemistry.The composition
is closeto the oxide contentscalculafedfrom the ideal
formula:64.9VoSiO2,27.54Vo
N2O3,7.L5Vo
Li2O and
0.4Vowt.VoNa2O.However,LizO is lower by 0.9 wt.Vo,
and the deficit cannotbe fully compensated
by Na2O,
as determinedby the analysis.Among the ftace elements,spodumenecontains1080ppm K, 907 ppm Mn
T6I
T!{E VOLTA GRANDE GRANTTIC PEGMATITES. BRAZL
6
I
f.'
I
r r-lg
$s
o
N
!
f
I
raz
I rlrr
I
9A
tr'
3
/l
ar
I
zinrmnldite v
I
LPl
A
LP2
a
2
10
F wLolo
Ftc.6.TheRbrOwt.tloversusFwt Tocontents
pepatites.
of micasfromtheVoltaGrande
and 85 ppm Rb; Fe was not detected.This composition
The exocontactmicas of the Volta Grandepegmais similar to those cited in the papers of Cernf & tites aredistinctly moremagnesian@e/I\49weight ratio
Ferguson(lW2) nd Charoyet al. (1992), which also 2.2-1.4) rmtJnose
af Tanco(3.12.2;Morgan & Iondon
arc characteizedby a deficit in Li.
1987)or RedCrossLake(5.3-2.1,exceptionally1.5;P.
A few representativecompositionsof theexocontact Cernf, pers.comm. 1996).At Tanco,both the Rb and
micas were determinedas a function of distancefrom Cs abundancesdecrease(from 2.0 to 0.4VoRbzO,and
the pegmatite contact CIable 4). One sample at the from 1.5 to 0.3VoCs2O,respectively)with increasing
contactwas analyzedby both electon-microprobeand distancefrom the contactover 3 meters.The K/Rb and
wet-chemicalmethods;the results lead to an average K/Cs weight ratios correspondingly increase (from
stru ctural formula (N a0.02Kn
r&bo21Cq.eJx.6(Als.e 3.6 to L9.2 and from 4.7 to 25.3, respectively);the
Mg6.a5Fe?+eaTMnns2litl:"es(Sl3.rAbB)O1s(Otl0rF15),
cor- Rb/Cs weight ratio oscillates irregularly between
to a Mg-rich zinnwaldite. Its composition 1.2 and1.5(Morgan& London1987).The behaviorof
can be comparedto those of the yellow and brown Cs is distincly different at Tancofrom our observations
micasobservedby Semenov& Shmakin(1988)in the
outer contactof rare-metalpegmatitesfrom the Bastar
area Qndia), and to syntheticMg-rich zinnwaldite of
Monier & Robert (1986). Though we have no dataon
TABI,B4. AVERAGECOMPOSINONOF SD(MICA SAMPLES,
MNGINGFROMMgiRIGIaNNWAT.DITETOPHI'COPITE
the Li con0ents,we can assumethat micasarericher in
lithium at the contact,as their contentsof Mg and Fe
deol
Mg-nch Mg-rlch M€2
Ml€3
Nfr64
Ml@5 lv[@6
(8)
(2)
(5)
(O
zntu ze(22)
(8)
M
increaseoutward. The mica composition evolves to
al@le (cb@)
@m
@lffi 2O.50q 5GE06
lm
lG2m
sio,
44.39
4L6O
4t.54
40.78
4225
3927
38.56
phlogopiteeven at a distanceof 0.2-O.5m. Rubidium
Al:Q
20.92
20.90
16.0t
16.14
17.25
t6,U
16.34
concentralionsalsovary as a frrnctionof distance,being Trq
0.83
0.8s
t,49
0.56
0,72
1.01
l.r0
FeO
6.4
7.50
tt23
15.03
l2,ls
l4S3
16.41
highestat the contact.Rb abundancedecreasesfrom 4
FqO:
1.49
to 2 wt.VoRbO within 1 meter,whereasCs apparently MrO
0.37
0.30
0.2t
0.33
024
0.41
024
3.91
4.05
6.17
r2.ll
11,47
12,6t
11.15
increasesftom 0.2 to I wt.VoCs2O,but in a rather MgO
023
o2t
0.01
CaO
0.07
0.07
0.00
0,a2
0./O
o.t2
0.10
0.00
0.06
0.19
0.06
enatic pattem.Figure 6 showstlat Rb and F contents Na,o
'1.94
KrO
9.16
t20
7.54
8.41
6,97
8.57
of zirinwalditefollow the samecorrelationas do those Rb:O
3,96
4.4
4,82
3.21
1.97
1.91
226
0,21
C&O
0.35
0.47
0,51
0.54
0.36
t.U
of the micasfrom the core of the pegmatite.The K/Rb
LirO
3.33 L67(')
Ld.
rd.
nd.
a&
trd.
qd
F
4.M
496
&d.
[d.
rd.
2,79
weight ratio rangesfrom 1.5 to 4, and Rb/Cs values,
Hp
r.t8
od.
ed.
a<L
&d.
trd,
nd.
from 12 to 4. Zn waldite and phlogopite clearly
Totsl
100.86
replacethe primary hornblendeby reactionofthe rarefte8ql6@!@gCn@ll0tongh!i!ffi
dtbeirdili@Fft@tt!p.rdgffi
Th6
alkali-rich pegmatiticfluid.
syebol . iidirarF [&i@ @
dhdd
tu M9rirh ziEwsldib disg 6e e.rlod of ltndlo &
W6b (1990),irticn i! d valid fo t3oligbs cd..ddi@
wigbZd
olMg Tl' @'Frift
@ qs.@dh
162
THE CANADIAN MINERALOGIST
at Volta Grande.The chemisnyof the exocontactmica
at Red Cross Lake was not examinedin relation to
distancefrom thepegmatite,but tle biotite is invariably
stronglyzonedandattainsK/Rb andI?Cs weight ratios
as low as 0.18 and 0.05, respectively,in the ouler
zones(demf et at. 1994).The Rb/Csratio is extremely
variable.
Five different samples of haln4uistite Cfable 5)
were analyzedby electron microprobe,and one was
characterizedby wet chemistry.The five compositions
obtained are very similar (Qu6m6neur& Lagache
1994),so that only an averagecompositionis given in
Table 5. This compositionyields the structuralformula
(Lrr.O{ao.D@gr.+Fer.oXAlzSrs)Qz(OH,F)2,
which matches
the theoreticalformula givenby London& Burt (1982).
Comparisonwith compositionspublishedin the literature showsttrat it is richer in Fe andpoorerin Mg than
mostof thesamplessnrdid butvery closetoholmquistite
from Tanco,Manitoba (Morgan & London 1987).It is
very poor in Na, K Rb and Cs.
As generalobservations,we have documentedthat
amongthe pegmatitemineralssensustricto, Rb and Cs
are concenfratedin the potassicminerals,microcline,
muscovileand lepidolite. Li, which is one of the major
constituentsof spodumene,is also concentratedin the
micas. In the metasomaficexocontactassemblage,
fte
micas containhigh abundancesof Li, Rb and Cs, and
Li is alsopresentin holmquistite.
In general, the Rb-rich potassic minerals of the
pegmatitehave K/Rb weight ratios of approximately
1 to 4, dependingon the mineral. As we have stated
beforg the structuralfeaturesof zinnwalditearesimilar
to thoseof trilithionite, sothat the KlRb valuesfor these
two mineralsalso are similar and closeto 1.5-2. In the
exocontactaureole.this ratio is observedin the maene-
TABLE 5. AVEMGE COMPOSITIONOF
IIOLMQUI$NE
SiOz
Al:or
Tio?
FeO
FeaO:
lv1no
Mgo
CaO
Na?o
K:O
Rbo
Liro
F
Hzo
Total
'Ib
58.66
12,75
0.04
15.08
0.28
6.40
0.07
0.46
0.05
nd.
lI.d.
0,29
rd.
59.05
12.29
0.t7
10.13
3.66
0.22
6.60
0.56
0.29
0.06
94ppm
2.66
0.10
2.49
98.28
nwgo iemls'tetedAm the mp*itim
of
five sanplesobtainedby electm misoprobs.Ths
ccmpositionof mo of the sampl€sws 0st€(0ed
fo the pr€s€il$ ofquqtz
Tle nerage bulk
compositimis Aned in w€igh %.
sian zinnwaldite proper near the contact, but Rb
asa function of distancefrom the contact,and
decreases
the K/Rb value increasesaccordingly.ConcerningCs,
the Rb/Cs weight ratio is signifieal6y lower in late
mineralssuchaslepidolite,andit alsotendsto decrease
q/ith distancefrom the pegmatiteinto the metasomatic
aureole.
DIScUSSION
In order to explain the abovedata,we next examine
the experimentallydeterminedpartition coefficientsfor
Rb and Cs betweenhydrothermalfluids and minerals,
which have beenextensivelystudiedfor feldsparsand
micasby severalauthors.Someexperimentsperfonned
on equilibria between melt and aqueousfluid have
shownthat the partition coefficientsof Rb and Cs are
close to unity for silicate melts ranging from
QtzaoAbato QtzaaOra(Carron&Lagache 1980).The
partitioning of elementsbetwe€ntwo solidsdependson
T andP, but not on the environment(fluid or melt), that
led to the equilibration. Experimentaldata will then
provide information on the composition of pairs of
mineralsin equilibrium and also on the compositionof
the fluid coexistingwith melt during magmaticcrystallization or of the fluid that inducesre-equilibrationof
the minerals in the core of the pegmatite or in the
exocotrtact.
Rubi.dium
The partition coefficient for rubidium between
sanidine and hydrothermal solution has been determinedexperimentallyby severalauthors(.agache1968,
Volfinger 1969, Beswick 1973, Lagache & Sabatier
1973).Theseauthorshavedemonstratedthat the partition coefficient Csrsq expressedas atomic ratios
(K/Rb)*(K/Rb)* is constant and equal to 2.22 at
600'C and 1 kbar for concentations \p Io 4 wt.Eo
rubidium, i.e., Rb is weakly partitionedin favor of the
solution.The potassicfeldsparsfrom the Volta Grande
pegmatitescontain Rb concentrationssimilar to those
of the experimentalstudies.
In the caseof muscovite,the partition coefficient,
experimentally
expressed
in the6!svs manner,det€rmined
by Volfinger (1969),is 1.82for the sameexperimental
conditions.For a given solution,rubidfumis thus more
concentratedin muscovitethan in sanidine.The partition coefficientbetweenmuscoviteandsanidineCmusw
= (K/Rb)-J(K/Rb)* is 0.82. In the first-generation
muscovite of Volta Grande,the K/Rb atomic ratios
ranges from 11.6 to 4.4, with an averageof 5.9,
whereasthis ratio in the microcline is approximately
8.8 (Iable 6). The resultingpartition coefficientis thus
1.3to 0.5, valuesthat bracketthe experimentallydeterminedvalue of 0.82.In anotherrare-elementpegmatite
in the same area, the Urubu pegmatite,Qu6m6neur
et al. (1993) measureda partition coefficient of 1 be-
TTIEVOLTA GRANDEGRANMC PEGMATITES.BRAZL
(&RbANDRbrcs)ANDATOIdC(K1Rb)
TABIf 6. AVERAGEqEIGIIT
RAnOS
FORPEG{ATITBFORMINGMINERAUIRICIIINRARBAIXALIEIAMENTS
oi@Ine
@vitgl
stg@,
@vlte2!dg@.
l€pldoltte
&Rbrcigtr
4
2.7(5.3-2)
2.7
2.5-1.4
1.5
24
KlRbaton
8.8
5.9(11.6-4.4)
5,9
5.4-3.1
Rtr/Cswelgh
32
33
11
r5-7
4,5-9
4-2
t63
illustates the decreaseof rubidium contents in the
ex@ontact.
Cesium
The partitioncoefficientsfor cesiumbetweensanidine
or muscoviteandan aqueoussolutionhavebeenexperimentally determined(Lagache1969, Volfinger L969,
Ca:ron & Lagache1980)for the sameconditionsasfor
rubidium. In the caseof sanidine,the partition coeffrtween muscoviteand K-feldspar. Consideringall the cient is constantfor the concentationsobservedin the
simplifications inherent to the experimental results K-feldsparof the pegmatites,andCs is concentratedby
(ressure andtemperatureof equilibrationnot precisely a factor of40 in the fluid. In otherswords, the hydroknown in the natural pegmatites,the fact that experi- thermal fluid will be sfiongly enriched in Cs with
mentswereperformedwith sanidine,whereasthe peg- progressivecrystallization.
matite containsmicrocline,for example)as well as the
For muscovite,the partition coefflcient varies vrith
heterogeneous
disribution of Rb in muscovite,thereis cesium concenhation,from 11 at 10 ppm to 28 at
good agreementbetwecn the experimentally deter- 3 wt.Vo Cs (Volfinger 1969). The muscovite in the
minedpartition coefficientsandthosemeasured
between Volta Grande pegmatites generally contains 0.1 to
themineralsof thepegmafite.
concenfration
Cs;thuswe canestimaJethatCs
O.2wt.Vo
In conclusion,a largepreferentialpartitioning of Rb in the fluid was approximately25 times higher.
into one speciflcpotassicmineralis not observed.This
The increasein Cs concentrationin the fluid during
observationfits well with an equilibrium betweenthe tle course of crystallization is more pronounced,
compositionsof the two minerals.In contast, the fluid comparedto the modest increasesexpectedfor Rb,
coexisfingin equilibrium with thesemineralsshouldbe discussedabove.The Rb/Cs value CIable6) decreases
weakly enrichedin Rb. An estimationof the composi- from the first generationof pegmatite-formingminerals
tion of this fluid canbe calculatedby:
(microclineandmuscovite)to tle lepidolite andmuscovite of the secondgeneration,as well as with distance
(K/Rb),q= (K/Rb).h,,/C.io".vuc
from the contact in the minerals of the metasomatic
aureole.It is apparentthat the micasmost remotefrom
which resultsin a K/Rb value of 4.0 (atomic) for the the contactarethemostCs-rich.The Rb/Csweightratio
fluid in equilibrium with the minerals during crystal- attainsvery low values,down to 4, indicating a gradual
lization.
depletionof Rb in the fluid due to zinnwalditecrystalThere are no experimentalpartition data for lepi- lization, and an enrichmentof Cs, sincethis elementis
dolite. However,using the K/Rb ratio measuredin the invariably partitionedin favor ofthe fluid.
mostF-rich compositionsof lepidolite from the pegmatite, and the ratio estimatedfor the fluid above, the Lithium
partition coefflcient for rubidium between lepidolite
and solution canbe calculatedby:
Rubidium and cesium never form end-member
mineral phasesin the Volta Grande pegmatite,but
CW,. = (K/Rb)h/6/Rb)uq = 3.114= 0j8
occur in potassic minerals. In contrast lithium is a
major constituentef imFortantmineralsin the pegmaLepidolite thus weakly concenftatesrubidium, more tite (spodumeneand lepidolite) and contact aureole
than miqocline and muscovite.The value calculatedis (Mg-rich zinnwaldite and holmquistite). The original
closethe coefftcientof 0.65obtainedexperimentallyby magrnawas very rich in lithium, which resultedin the
Volfinger (1974) tor Rb partition betweenfluid and crystallization of the large mass of spodumene.The
anothertrioctahedralmica, phlogopite.Oncemore, we fluids that were generatedalso were Li-rich, which
must recall that the structue of lithian micas is more resultedin the crystallizationof lithian muscoviteand
favorable to the infroduction of large nlkali cations. lepidolite in the fractures,and in the transfomation of
Lepidolite andzinnwaldiie areamongthe few minerals minerals in the contact aureoleto lithian micas and
holmquistite (Wood & Williams-Jones1993, Qu6m6which concentrateRb from aqueousfluid.
The metasomaticaureoleis rich in micas. As the neur & LagacheL994).
partition coefficientfavorsthe micas,the Rb contentof
Concr-ustoNs
the metasomatic fluid must decreaseprogressively
during its flow acrossthe exocontactzone. Table 4
presents the compositions of exocontact micas
The Volta Grandepegmatitesare fractionatedto a
(zinnwalditeand phlogopite) as a function of distance degreerarely observedin rare-elementgranitic pegmafrom the pegmatitecontact, from 0.1 to 1.0 m, and tites.The K/Rb weight ratio in potassicmineralsranges
Mg-rtchzimmldtte (c!mact)
pblosopite(20@-1m)
Ig
TI{E CANADIAN MINERALOGIST
from 4.0 to 1.5,both in the pegmatiteandin the contact
aureole.The partition coefficientsfor Rb and Cs determined from experimentalstudieshave been successfully used to accountfor the observeddishibution of
these elements.In the core of the pegmatitesbodies,
thereis no preferentialpartitioningof Rb in the potassic
minerals.Crystal/meltor crystal./vaporpartitioning can
explain the emichmentof cesiumin the residualfluid
and,as a consequence,
in the mineralsofthe aureoleof
metasomatism.In any event the total concentrationof
cesi'm is too low to permit the crystallizationof pollucite, as describedin somerare-elementpegmatites.In the aureoleof metasomatism,rapid decreaseof rubidium and relative increaseof cesium with increasing
distancefrom the contactdependon the sameprocesses.
Finally, the high concentrationof Li resulted in the
crystallizationof spodumeneand lepidolite, as well as
in the widespread occurrence of magnesium-rich
zinnwaldite and holmquistite in the metasomatic
aureole.
ACIQ\TOWI-EDGHVH\NS
This work has been made possible tbrough the financial participation of a CAPES{OFECLIB
project and
of FAPEMG. We are indebted to the MIBRA Society
for giving us facilities in our field work. Technical
assistance from N. Catel and L. Maraou are gratefully
acknowledged. We also want to rhank R. Linnen for tre
first English version of the manuscript. We are very
gratefirl to P. Cernf, who carefully improved the final
version of the paper. We benefitted from his knowledge
of granitic pegmatites, as applied to a Brazilian pegmatite that deserved to be described. We also $ateflrily
aknov'71.6t" many helpful suggestions from R.F.
Martin and from an anon)mous reviewer.
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Received December 21, 1994, revised manuscript accepted
October 13, 1996.