chemistry, paragenesis and significance of tourmaline in pegmatites of the southern tin belt,...

Ž .Chemical Geology 158 1999 203–225

Chemistry, paragenesis and significance of tourmaline inpegmatites of the Southern Tin Belt, central Namibia

Paul Keller a, Encarnacion Roda Robles b,), Alfonso Pesquera Perez b,´ ´Francois Fontan c

a Institut fur Mineralogie und Kristallchemie, UniÕersitat Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany¨ ¨b Departamento de Mineralogıa y Petrologıa, UniÕersidad del Pais Vasco 644, E-48080 Bilbao, Spain´ ´

c Laboratoire de Mineralogie, UniÕersite Paul Sabatier, F-31000 Toulouse, France´ ´

Received 19 June 1998; accepted 25 February 1999

Abstract

Numerous pegmatite bodies occur in several distinct belts within the Damara orogen of central Namibia. Many of thepegmatites have been mined extensively for Sn, Li, Nb, Ta, Cs, mica, feldspar, and gem tourmaline and gem beryl.Tourmaline is widespread as an accessory and locally abundant mineral in different geological environments within the

Ž .Damara Orogen. In pegmatites from the Southern Tin Belt STB , it exhibits diverse habits and colours, from black to darkgreen, light green, and pink, as well as variations in composition depending on pegmatite type and location within thepegmatite. Electron microprobe analyses of the tourmalines show a significant chemical variation from Fe-rich compositions

Y Ž .with moderate Al in the Y sites to Li-rich compositions with high Al, and relatively high F. Fer FeqMg ratios for black,Ž .green, and pink tourmalines vary from 0.65 to 0.95, 0.66 to 0.99, and 0.50 to 0.87, respectively. Nar NaqCa ratios

Ž . Ž .increase from Li-bearing tourmaline 0.83–0.94 to black tourmaline 0.95–0.99 . Tourmaline compositions mostly plotalong trends involving proton-loss and alkali-defect substitutions from the schorl additive component. These substitutionsaccount for increases of Al in the Y site, and decreases in the X site and OH occupancies at the expense of the schorl

Ž .component. Amounts of Al Y range from 0.23–0.63 in black tourmalines to 0.82–1.44 in green and pink tourmalines.Within individual pegmatites, the abrupt transition from black tourmaline to green tourmaline, and later pink tourmaline,reflects decreasing Fe and increasing Li and Al via the substitution LiAlFe . Enrichment of Li in the tourmaline isy2

accompanied by increases of Be, Sn, Rb, Cs, and Pb. Insignificant amounts of Fe3q in the tourmalines suggest a lowoxidation state of the pegmatite-forming fluids. As the precipitation of cassiterite requires an increase in fO , such2

conditions may account for the distribution of tourmaline and cassiterite in different pegmatite zones. Field relations,Ž .distribution, petrographic features, and tourmaline chemistry provide evidence of two pegmatitic domains in the STB: 1 a

Ž .western one, characterized by pegmatites with Sn, Nb)Ta, Li, Be mineralization and containing Fe–Mn phosphates andŽ .black tourmaline, which may be explained in terms of simultaneous crystallization of two petrologenetic units; and 2 an

Ž .eastern one characterized by pegmatites with Li)Be, Sn, Nb)Ta mineralization and elbaite, which formed via

) Corresponding author. Tel.: q34-94-464 7700; fax: q34-94-464 8500; e-mail: [email protected]

0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2541 99 00045-5

( )P. Keller et al.rChemical Geology 158 1999 203–225204

fractionation processes and sequential crystallization from the margins inward to the cores. q 1999 Elsevier Science B.V. Allrights reserved.

Keywords: Tourmaline; Granitic pegmatites; Mineral chemistry; Central Namibia

1. Introduction

The importance of tourmaline for petrologic andmetallogenetic studies is well established and has

Žbeen the subject of a large number of studies seeHenry and Dutrow, 1996; London et al., 1996; Slack,

.1996; for reviews . This is due to the fact thattourmaline is stable over a wide regime of pressuresand temperatures, and has a variable compositionand is able to exchange components and volatilespecies with coexisting minerals and fluids as exter-nal conditions change. Tourmaline, therefore, is use-ful as a monitor of the physical and chemical envi-ronment in which it developed, particularly in gran-ites, pegmatites, and related hydrothermal settingsŽManning, 1982; Jolliff et al., 1986; Kassoli-Four-naraki, 1990; Gallagher and Kennan, 1992; Londonand Manning, 1995; Roda et al., 1995; Lynch and

.Ortega, 1997; Federico et al., 1998 . This studyfocuses on tourmalines associated with pegmatitesfrom the Southern Tin belt of central Namibia, exam-ining the petrographic characteristics and chemicalvariability of the tourmaline to decipher its petro-logic significance.

2. Regional geologic setting

The geology of central Namibia is dominated bymetasedimentary and igneous rocks that form the

Ž .Damaran orogen Fig. 1 . This region is of particulareconomic interest because of the occurrence of tinmineralization associated with a large number ofpegmatites. The Pan-African Damara orogen has beendivided into several zones on the basis of the stratig-raphy, structure, grade of metamorphism, types of

Ž .granitoids, and geochronology Miller, 1983 .Boundaries between the zones are faults or majorlineaments. Some zones are subdivided by otherlineaments; e.g., the Central zone comprises a North-ern portion and a Southern separated by the magneti-

Ž .cally-defined Omaruru lineament Corner, 1983Ž .Fig. 1 . This is a tectono-stratigraphic boundaryacross which are significant stratigraphic, tectonic,and magmatic differences. The tourmalines underdiscussion occur in pegmatites from the Southernportion.

The Central zone is a high-temperature and low-pressure terrain within the orogen that contains silli-manite–cordierite metamorphic assemblages in-truded by numerous granitoids. Deep stratigraphiclevels crop out in the Southern portion of the Centralzone, which are floored by a 1700 to 2000 Ma

Ž .granite gneiss basement Abbabis Formation . Thisbasement is overlain by quartzite of the Late Protero-zoic Nosib Group and metamorphic rocks of the

Ž .Swakop Group, mainly marble Karibib FormationŽand metapelite or metagreywacke Kuiseb Forma-

.tion . Stratiform and discordant tourmaline-rich rocksoccur at different stratigraphic levels within theKuiseb Formation as well as within other metasedi-

Žmentary units of the Damara orogen Behr et al.,.1983; Steven and Moore, 1995 . The origin of the

tourmalinites, nevertheless, does not appear relatedto the Sn-rich pegmatites or granitic intrusions of thearea because the tourmaline-rich rocks are older thanthe pegmatites and granites.

3. Geology of the pegmatites

The numerous occurrences of pegmatite in theDamara orogen can be grouped in various character-

Ž . Ž .istic belts Fig. 1 : 1 the Northern, Central, andSouthern tin belts, also known as Cape Cross–Uis,Nainais–Kohero, and Sandamap Noord–Erongo peg-

Ž .matite belts, respectively; 2 the Karibib pegmatiteŽ . Ž .belt; 3 the Okahandja pegmatite belt; and 4 the

Ž .Ugabmond–De Rust belt not shown in Fig. 1Ž .Frommurze et al., 1942; Keller, 1991 . Where thegeological and genetic relationships are sufficientlyknown, some of the belts can be subdivided into

Žswarms, e.g., within the Northern tin belt Diehl,

( )P. Keller et al.rChemical Geology 158 1999 203–225 205

Fig. 1. Location of different pegmatites in the Southern Tin Belt of the Damara orogen.


Tab

le1

Mai

nch

arac

teri

stic

sof

repr

esen

tati

vepe

gmat

ites

ofth

eS

outh

ern

Tin

Bel

tan

dth

eir

asso

ciat

edto

urm

alin

es

Far

mP

egm

.B

orde

rzo

neIn

tern

alzo

ning

and

min

eral

ogy

Nat

ure

ofT

ourm

alin

eT

ourm

alin

eE

cono

mic

min

eral

sb

aŽ

.Ž

.si

zesi

zeho

stro

ckte

xtur

egr

ain

size

Inte

rmed

iate

zone

Cor

ezo

neR

epla

cem

ent

unit

sb

Ž.

Ž.

Ž.

size

size

size

San

dam

apC

illi

erno

tex

pose

dK

sp,

Qtz

,m

sc,

Qtz

,tr

iph

msc

,Q

tz,

clv,

Ksp

,ca

ss,

beS

chis

tsof

the

an-

toeu

hedr

albl

ack

fine

toca

ss,

tan,

beŽ

.Ž

.?,

med

ium

?B

LA

CK

TO

UR

M4

=?

2=

?tw

oK

uise

bF

orm

atio

npr

ism

atic

crys

tals

wit

hm

ediu

mc

10=

30Q

tzan

dbo

ok-m

sc

San

dam

apP

etal

ite

poor

lyK

sp,

Qtz

,m

sc,

pet,

Qtz

,ca

sscl

v,Q

tzm

scS

chis

tsof

the

sub-

toeu

hedr

albl

ack

fine

tope

t,ca

ss,

col-

tan

Ž.

Ž.

Noo

rd?,

med

ium

expo

sed

Li-

Fe-

Mn

pho

trip

h2

=4

seve

ral

Kui

seb

For

mat

ion

pris

mat

iczo

ned

crys

tals

med

ium

Ž5

=?

BL

AC

KT

OU

RM

6=

?br

own

rim

san

dc

.20

=50

pale

gree

nco

re

Goa

beb

Cam

eroo

ngr

phgr

angr

phgr

anq

Qtz

,K

sp,

msc

Qtz

albi

tic

rock

san

dS

chis

tsof

the

sub-

toeu

hedr

albl

ack

very

fine

cass

cc

Ž.

?,la

rge

10=

40F

e-M

nph

o,B

LA

CK

10=

80‘‘

grei

sen’

’,ca

ssK

uise

bF

orm

atio

npr

ism

atic

zone

dcr

ysta

lsto

med

ium

Ž.

ŽT

OU

RM

3=

10se

vera

lbr

own

rim

san

dc

.10

=60

pale

gree

nco

rew

ith

Qtz

-Ksp

grap

hic

inte

rgro

wns

Dav

ib-O

stA

riak

asno

tob

serv

edgr

phgr

an,

Ksp

,Q

tz‘‘

grei

sen’

’,L

i-m

ica

Sch

ists

ofth

esu

b-to

euhe

dral

blac

kfi

neto

cass

,tan

cŽ

.?,

larg

e?

msc

,ab

,Q

tz,

amb

20=

200

cass

Kui

seb

For

mat

ion

pris

mat

iczo

ned

crys

tals

coar

seŽ

.Ž

BL

AC

KT

OU

RM

,2

=6

seve

ral

brow

nri

ms

and .

Fe-

Mn

pho

pale

gree

nco

rec

40=

200

DV

4no

tob

serv

edK

sp,

msc

,Q

tz,

amb,

Qtz

,cl

v,ca

ssle

pS

chis

tsof

the

sub-

toeu

hedr

algr

een

fine

toca

ss,

tan,

Ž.

Ž.

?,m

ediu

m?

Fe-

Mn

pho

CO

LO

UR

ED

3=

12tw

oK

uise

bF

orm

atio

npa

lepr

ism

atic

crys

tals

,m

ediu

mam

b,le

p10

=30

cT

OU

RM

AL

INE

and

subh

edra

lc

8=

30bl

ack

crys

tals

DV

1no

tob

serv

edK

sp,

ab,

msc

,Q

tz,

Ksp

,ca

ss,

yS

chis

tsof

the

subh

edra

lbl

ack

fine

toca

ssŽ

.Ž

.?,

larg

e?

Fe-

Mn

pho,

msc

,G

EM

2=

3se

vera

lK

uise

bF

orm

atio

ncr

ysta

lsm

ediu

mB

LA

CK

TO

UR

MT

OU

RM

,c

50=

200

?


Ž.

Bra

bant

20=

700

BL

AC

KT

OU

RM

,K

sp,

Qtz

,m

scQ

tz,

Ksp

,am

bcl

v,L

i-m

ica,

tpz,

fine

grai

ned

cone

-sha

ped

blac

km

ediu

mto

cass

,ta

n,am

bc

cŽ

Qtz

,ab

,K

sp,

12=

700

6=

700

cass

,am

b,co

l-ta

nD

amar

agr

anit

ezo

ned

crys

tals

brow

nco

arse

Ž.

msc

1=

8se

vera

lri

ms

and

pale

gree

n.

2=

?co

reth

eygr

owpe

rpen

dicu

lar

toth

eco

ntac

ts

Dah

eim

Dah

iem

Iy

Ksp

,Q

tz,

msc

lep,

Qtz

,cl

v,pe

t,P

INK

TO

UR

Mun

diff

eren

tiat

edsu

b-to

euhe

dral

pink

fine

tope

t,am

b,be

cc

Ž.

?,m

ediu

m?

15=

50am

b,be

,10

=40

mem

bers

ofth

epr

ism

atic

crys

tals

med

ium

?D

amar

ase

quen

ce

Dah

eim

IIap

liti

cro

ck,K

spQ

tz,

Ksp

,P

INK

TO

UR

Mno

tex

pose

d?L

i-m

icar

lep,

ab,

Mar

bles

and

calc

-su

b-to

euhe

dral

pink

fine

toge

mto

urm

alin

ec

Ž.

3=

120.

5=

?2

=12

?Q

tz,

pet

sili

cate

sof

the

pris

mat

iccr

ysta

lsm

ediu

m1

=?

Kar

ibib

For

mat

ion

Eti

roJo

oste

Qtz

,K

sp,

bi,

msc

,ab

,Q

tz,

Ksp

,m

sc,

be,

Qtz

,K

sp,

be,

Li-

mic

a,tp

z,fl

r,cl

v,S

alem

type

gran

ite

an-

tosu

bhed

ral

blac

kfi

neto

be,

Ksp

,am

b,ta

n,Ž

.Ž

.28

=85

0B

LA

CK

TO

UR

M,

apt,

BL

AC

KT

OU

RM

,am

b,co

l-ta

n,tr

ip,

2=

8se

vera

lcr

ysta

lsQ

tz-t

ourm

med

ium

Bi

ore,

Li-

mic

aab

,ap

ttp

z,co

l-ta

n,m

onB

im

in,

apt,

pseu

dogr

aphi

c5

=?

5=

?B

LA

CK

TO

UR

Min

terg

row

nsc

9=

195

are

pres

ent

Otj

imbo

joB

erge

ry

gran

itic

,K

spQ

tz,

lep,

cass

,L

i-m

ica,

Qtz

,am

b,ab

Mar

bles

ofth

esu

bhed

ral

zone

dcr

ysta

lsfi

neto

cass

,ge

mŽ

.Ž

.Ž

Ost

?,m

ediu

m?

BL

AC

KT

OU

RM

MU

LT

I-ap

t,gr

eise

nw

hite

mic

aK

arib

ibF

orm

atio

nda

rk-g

reen

core

sco

arse

tour

mal

ine

cŽ

..

30=

100

CO

LO

UR

ED

1=

4se

vera

lan

dpi

nkri

ms

TO

UR

MA

Lc

20=

50

Om

apyu

rN

estl

ery

Ksp

,Q

tz,

msc

,Q

tz,

lep,

clv,

yun

diff

eren

tiat

edeu

hedr

albl

ack

pris

mat

icfi

neto

gem

tour

mal

ine

cŽ

.O

tjim

bojo

?,m

ediu

mB

LA

CK

TO

UR

M?

PIN

K-T

OU

RM

,10

=20

mem

bers

ofth

ecr

ysta

lsan

dm

ediu

mw

hite

mic

aD

amar

ase

quen

ce‘w

ater

mel

on’

c20

=50

tour

mal

ine

In‘I

nter

nal

Zon

ing

and

Min

eral

ogy’

,th

efo

llow

ing

abbr

evia

tion

sha

vebe

enus

ed:

Qtz

—qu

artz

;K

sp—

K-f

elds

par;

msc

—m

usco

vite

;ab

—al

bite

;cl

v—

clea

vela

ndit

e;tr

iph

—tr

iphy

lite

;pe

t—pe

tali

te;

bi—

biot

ite;

pho

—ph

osph

ates

;am

b—

ambl

ygon

ite;

cass

—ca

ssit

erit

e;ap

t—ap

atit

e;be

—be

ryl;

tpz—

topa

ze;

lep

—le

pido

lite

;fl

r—fl

uori

te;

col—

colu

mbi

te;

tan

—ta

ntal

ite;

trip

—tr

ipli

te;

mon

—m

onaz

ite;

Bi

min

—B

im

iner

als;

grph

gran

—gr

aphi

cgr

anit

e.aG

rain

size

:ve

ryfi

nes

-6

mm

;fi

nes

6m

mto

2.5

cm;

med

ium

s2.

5cm

to10

cm;

coar

ses

)10

cm.

Siz

esof

pegm

atit

esar

egi

ven

inm

eter

sas

wid

ths=

leng

ths.

bT

hesi

zes

ofzo

nes

onbo

thsi

des

ofth

eco

reis

sum

mar

ized

;cex

pose

d,?

not

obse

rved

orun

know

n.


.1991 . Otherwise, they are subdivided into portionscontaining groups of pegmatites that are usuallynamed according to the geographic location in which

Ž .they occur Keller, 1991 .The pegmatites lacking economic importance con-

sist of K-feldspar, variable amounts of albite, quartz,and, locally, large quantities of muscovite, tourma-line, or almandine. These kinds of pegmatites wereemplaced within igneous and metamorphic rocks,and are related to both syntectonic and post-tectonic

Ž .granites Haack and Gohn, 1988; Diehl, 1991 .Most of the economic pegmatites were intruded

into tightly folded micaschist, dolomitic marble orquartzite, and only a few are situated within granite.The former pegmatites are usually dyke- or pod-likein morphology and commonly more or less concor-dant, but locally are clearly discordant to the regionalfoliation. They are unmetamorphosed and evidentlypost-tectonic, but generally it is impossible to relatethe pegmatites to a distinct post-tectonic parent gran-

Žite body Martin, 1983, Haack et al., 1983; Winkler,.1983 . Rb–Sr whole-rock ages are known only for

the leucogranite and pegmatites of Sandamap–NoordŽ .Steven, 1993 and no other pegmatites under discus-sion. The given data are 512"19 Ma for theleucogranite, 473"23 Ma and 468"14 Ma forpegmatites.

Internal zoning is well developed in most of thepegmatites and quartz cores are prominent in manydeposits. However, some important economic peg-

Ž .matites in the region e.g., the Uis swarm are poorlyzoned to unzoned. Although quartz cores and coremargins commonly are mineralized, economic min-erals also occur in other pegmatite zones at severallocalities.

Many of the pegmatites have been mined exten-sively for Sn, Li, Be, Nb, Ta, Cs, gem tourmalineand beryl, and mica and feldspar. The stanniferouspegmatites are grouped in the tin belts mentionedabove. The Southern tin belt is located in the South-ern portion of the Central zone, and follows with itsnorthern termination the Omaruru lineament up to

Žthe Karoo intrusives of the Erongo Mountains Fig..1 . The pegmatites usually occur in metasedimentary

rocks of the Kuiseb Formation that crop out on thenorthern slope of Karibib marble bands.

The Southern tin belt can be subdivided into threeportions from the west to east: the Ameib–Davib–

Sandamap portion, the Etiro–Daheim portion, andŽ .the Omapyu–Otjimbojo portion Fig. 1 . At the time

of our investigation, however, there is some doubt ifthe differentiation into ‘Etiro–Daheim’ and‘Omapyu–Otjimbojo’ portions is necessary or advan-tageous to build genetically-related models for thepegmatitic swarms. The main characteristics of thepegmatites in the Southern Tin belt are outlined inTable 1.

3.1. The Ameib–DaÕib–Sandamap portion

Pegmatites of this swarm form, together withbodies of pegmatitic granite, more or less prominentridges that strike generally SW–NE over a distanceof about 40 km. The host rocks are metagreywackeand metapelite of the Kuiseb Formation, mainlymicaschists with minor marble and calc-silicate rockŽ .Frommurze et al., 1942 . Post-tectonic, medium- tofine-grained leucogranite has intruded the metasedi-ments, but relations between the leucogranite and thepegmatites are unclear. The tourmalines studied arefrom pegmatites on the Sandamap, Sandamap Noord,Goabeb, Davib-Ost, and Brabant farms.

3.2. The Etiro–Daheim portion

In this area there is an almost continuous field oftin-bearing pegmatites that extend as a continuousband, typically only a few hundred meters in width.Host rocks are undifferentiated members of theDamara sequence, micaschist as well as marble, and

Ž .granite Frommurze et al., 1942 . The tourmalinesstudied occur in pegmatites from the Daheim andEtiro farms. Characteristic of these pegmatites is theoccurrence of elbaite associated with amblygonite,"petalite, lepidolite and beryl; Li–Mn–Fe phos-phates occur locally. Most of the pegmatites havebeen mined more for Li and Be than for Sn orNb–Ta.

3.3. The Omapyu–Otjimbojo portion

This is the easternmost part of the main SouthernTin Belt. The structural location nevertheless is not


as clear as for the other portions. Host rocks com-prise micaschist and marble of the Damara sequence.The pegmatites of this portion have been mined notonly for Sn but also for gem tourmaline, the latterbeing a common feature in the easternmost peg-matites of the Karibib pegmatite belt. Based on ownfield observations, the Sn andror tourmaline peg-matites of this portion lack the Li–Fe–Mn phos-phates. However, amblygonite–montebrasite gener-ally occurs in the gem tourmaline-bearing peg-matites.

4. Sampling and analytical methods

The tourmaline samples studied have been se-lected from the most representative bodies of peg-matite in the Southern Tin Belt. Samples were pre-pared by hand picking, and the separates were exam-ined with a binocular microscope to remove contam-inated grains, and then, were ground by hand in anagate mortar.

Major element analyses were performed on pol-ished thin sections by a Camebax SX-50 electron

Ž .Fig. 2. Schematic representation of some of the textural characteristics of the studied tourmalines: a comb-structured tourmaline from theŽ . Ž .Brabant pegmatite; b radial intergrowth of quartz and mica developed from a tourmaline crystals aggregate in the Sandamap pegmatite; c

Ž .irregular string and radial aggregates forming intergrowths of quartz–tourmaline in the Davib-Ost pegmatite; d tourmaline crystalsŽ .fractured and replaced by quartz in the Davib-Ost pegmatite; e ‘watermelon’ tourmaline rimmed by whitish mica in the Nestler pegmatite;

Ž .and, f vug scattered by cracks in the quartz core, filled with very fine-grained masses of albite and clay minerals, together with crashed andedged multicolored tourmaline in the Berger pegmatite.

()

P.K

elleret

al.rC

hemicalG

eology158

1999203

–225

210

Table 2Ž U .Average compositions of tourmaline from the different farms. N: number of analyses; trace element values in ppm; n.a.: not analyzed

Label Black-Range Sandamap Goabeb Davib-Ost Davib-Ost Brabant Daheim Daheim Etiro Berger Berger Nestler Nestler NestlerŽ . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .N 25 30 30 DV 20 Ariakas 26 24 green 14 pink 11 33 green 3 pink 22 black 17 green 20 pink 24

SiO 35.80 34.94 36.25 38.55 35.81 35.05 37.64 37.59 35.21 36.26 38.21 35.20 37.78 38.492

TiO 0.17 0.57 0.36 0.01 0.38 0.52 0.01 0.02 0.31 0.10 0.01 0.20 0.06 0.002

Al O 34.94 34.58 34.81 40.43 34.71 32.87 42.65 42.11 33.41 36.56 40.92 33.61 37.46 41.922 3

Cr O 0.01 0.01 0.01 0.02 0.02 0.02 0.01 0.01 0.02 0.05 0.01 0.01 0.01 0.012 3

FeO 11.52 13.35 10.03 0.59 11.70 13.90 0.03 0.03 13.23 9.23 0.11 14.98 3.80 0.03MnO 0.16 0.21 0.12 0.75 0.19 0.29 0.28 1.27 0.55 0.45 0.99 0.10 1.56 0.16MgO 2.05 1.12 3.02 0.01 2.27 1.26 0.01 0.02 0.96 0.03 0.01 0.40 0.03 0.00ZnO 0.14 0.13 0.07 0.07 0.07 0.21 0.05 0.04 0.29 0.36 0.06 0.21 0.11 0.03Li O 0.04 0.03 0.02 1.81 0.06 0.07 1.70 2.02 0.12 1.11 1.84 0.06 1.45 1.892

NiO 0.02 0.02 0.02 0.03 n.a. 0.02 0.03 0.04 0.03 0.02 0.01 0.03 0.01 0.03CaO 0.03 0.08 0.14 0.64 0.11 0.09 0.52 0.47 0.08 0.14 0.43 0.08 0.30 0.43Na O 1.65 1.78 1.57 1.83 1.69 1.99 1.65 1.80 2.08 2.47 1.93 1.61 2.40 1.742

K O 0.02 0.04 0.03 0.02 0.03 0.04 0.02 0.01 0.04 0.03 0.01 0.03 0.02 0.022

F 0.39 0.46 0.18 1.31 0.37 0.85 1.12 1.17 0.95 1.23 1.08 0.46 1.42 1.07Cl 0.01 0.01 0.01 0.01 n.a. 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01Total 86.95 87.33 86.64 86.08 87.41 87.19 85.73 86.62 87.29 88.05 85.63 86.99 86.42 85.83O5F 0.16 0.19 0.08 0.55 0.16 0.36 0.47 0.49 0.40 0.52 0.45 0.19 0.60 0.45Total 86.79 87.14 86.56 85.53 87.25 86.83 85.26 86.13 86.89 87.53 85.18 86.80 85.82 85.38Structural formula on the basis of 24.5 atoms of oxygenSi 5.863 5.769 5.921 5.899 5.846 5.807 5.756 5.723 5.807 5.749 5.878 5.875 5.909 5.867Al 6.747 6.731 6.704 7.293 6.681 6.421 7.689 7.559 6.496 6.834 7.419 6.613 6.907 7.533Al T 0.137 0.231 0.079 0.101 0.154 0.193 0.244 0.277 0.193 0.251 0.122 0.125 0.091 0.133Al Z 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000 6.000Al Y 0.610 0.500 0.625 1.192 0.527 0.228 1.444 1.281 0.303 0.584 1.297 0.487 0.816 1.400

()

P.K

elleret

al.rC

hemicalG

eology158

1999203

–225

211

Ti 0.021 0.071 0.044 0.001 0.046 0.065 0.002 0.002 0.039 0.012 0.002 0.025 0.008 0.000Cr 0.002 0.002 0.002 0.002 0.002 0.002 0.001 0.002 0.002 0.006 0.001 0.001 0.002 0.002

2qFe 1.579 1.844 1.371 0.076 1.598 1.927 0.004 0.004 1.825 1.224 0.014 2.091 0.497 0.004Mn 0.022 0.029 0.016 0.097 0.026 0.040 0.036 0.164 0.077 0.060 0.130 0.014 0.207 0.021Mg 0.501 0.275 0.734 0.003 0.553 0.312 0.002 0.004 0.234 0.006 0.002 0.100 0.008 0.001Zn 0.017 0.016 0.008 0.008 0.009 0.026 0.006 0.004 0.036 0.043 0.006 0.026 0.012 0.004Ni 0.003 0.002 0.003 0.004 n.a. 0.003 0.003 0.005 0.004 0.002 0.001 0.004 0.002 0.004Li 0.027 0.017 0.016 1.113 0.041 0.047 1.046 1.238 0.078 0.711 1.141 0.041 0.914 1.158Y total 2.783 2.757 2.820 2.496 2.801 2.651 2.545 2.706 2.599 2.649 2.595 2.791 2.447 2.595Ca 0.005 0.014 0.024 0.105 0.020 0.016 0.085 0.076 0.013 0.024 0.071 0.015 0.050 0.071Na 0.524 0.569 0.497 0.543 0.536 0.638 0.490 0.530 0.667 0.761 0.575 0.522 0.729 0.514K 0.005 0.007 0.006 0.004 0.007 0.009 0.003 0.003 0.008 0.006 0.003 0.006 0.003 0.004X total 0.524 0.590 0.507 0.652 0.562 0.663 0.578 0.609 0.688 0.781 0.649 0.543 0.782 0.589Cl 0.002 0.003 0.002 0.001 n.a. 0.003 0.002 0.005 0.003 0.003 0.003 0.004 0.003 0.002F 0.202 0.241 0.093 0.634 0.192 0.447 0.544 0.563 0.495 0.616 0.524 0.243 0.701 0.518Fer 0,76 0,87 0,65 0,96 0,74 0,86 0,66 0,5 0,88 0,99 0,87 0,95 0,98 0,80Ž .FeqMgNar 0,99 0,98 0,95 0,84 0,96 0,98 0,85 0,83 0,98 0,97 0,89 0,97 0,94 0,88Ž .NaqCaU

Be 5 6 5 5 n.a. 9 n.a. 52 10 5 13 5 n.a. 27Sc 5 22 5 5 n.a. 5 n.a. 5 5 24 5 29 n.a. 5Ga 65 71 n.a. n.a. n.a. 79 n.a. 330 135 n.a. n.a. n.a. n.a. 325Rb 81 33 n.a. n.a. n.a. 668 n.a. 1246 1 n.a. n.a. n.a. n.a. 106Sr 10 10 10 10 n.a. 35 n.a. 32 10 10 10 10 n.a. 20Nb 19 19 n.a. n.a. n.a. 6 n.a. 7 3 n.a. n.a. n.a. n.a. 19Sn 50 50 50 210 n.a. 50 n.a. 240 50 110 260 50 n.a. 260Cs 2 2 n.a. n.a. n.a. 2 n.a. 70 3 n.a. n.a. n.a. n.a. 59Pb 20 20 20 181 n.a. 20 n.a. 650 20 20 300 20 n.a. 168


microprobe, in the Laboratoire de Mineralogie et´Cristallographie of the Universite Paul Sabatier, in´Toulouse, France. In total, over 300 electron micro-probe analyses were made of the tourmalines. Oper-ating conditions were: voltage 15 kV and beam

Ž . Ž .current 10 nA. SiO Si , MnTiO Ti, Mn , wollas-2 3Ž . Ž . Ž .tonite Ca , corundum Al , hematite Fe , albite

Ž . Ž . Ž . Ž .Na , orthoclase K , fluorite F , periclase Mg ,Ž . Ž .synthetic chromite Cr , Ni metal Ni , and tugtupite

Ž .Cl were used as internal standards. Trace elementsincluding Be, Sc, Ga, Rb, Sr, Nb, Sn, Cs, and Pb,and rare earth elements were analyzed by ICP-AESand ICP-MS, respectively. The trace element analy-ses of tourmaline separates were performed by X-RayAssay Labs of Don Mills, Ontario. Samples werealso analyzed with an X-ray diffractometer using Sias the internal standard, by scanning over the interval5–708 2u using CuKa radiation. Cell parameterswere obtained with the program of Novak and

Ž .Colville 1989 .Infrared absorption spectra were collected with a

Nicolet-740 FTIR spectrometer using 0.1 mg of sam-Ž .ple as KBr pellets total weights50 mg in the

region 4000–400 cmy1. The scanning conditions

were: 1 cmy1 nominal resolution and 0.2 cmy1 scanspeed. Mossbauer spectroscopic studies were carried¨out at the Electricity and Electronic Department ofthe Universidad del Paıs VascorEHU. 57 Fe´Mossbauer spectra of Fe-rich tourmalines were per-¨formed at room temperature using a standard spec-trometer with a CorRh source. The isomer shift isreported relative to metallic iron. All of the spectrawere collected in a multi-channel analyzer with 512channels, and the experimental data were evaluatedby means of a least-squares fitting.

5. Tourmaline occurrence and petrography

Tourmaline commonly is a minor phase in peg-matites of the southern Tin Belt. Locally, however,relatively tourmaline-rich assemblages in differentzones of the pegmatite bodies can be observed. Onthe basis of textural characteristics and mineral asso-ciations, six types of tourmaline are distinguishedŽ .Table 1 .

Ž .1 Black tourmaline, fine to coarse grained, thatŽ .occurs in the border Brabant, Etiro pegmatites and

Ž . Ž . Ž .Fig. 3. Al–Fe tot –Mg diagram in molar proportions for tourmaline from the different farms, with fields after Henry and Guidotti 1985 .Ž . Ž . Ž .a Li-rich granitic pegmatites and aplites; b Li-poor granitic rocks and associated pegmatites and aplites; c Fe-rich quartz–tourmaline

Ž . Ž . Ž .rocks hydrothermally altered granites ; d metapelites and metapsammites coexisting with an Al-saturating phase; e metapelites andŽ . Ž .metapsammites not coexisting with an Al-saturating phase; f Fe-rich quartz–tourmaline rocks, calc-silicate rocks, and metapelites; gŽ . Ž .low-Ca meta-ultramafic and Cr, V-rich metasedimentary rocks; h : metacarbonates and metapiroxenites; i Ca-rich metapelites, metapsam-

Ž . Ž . Ž .mites, and calc–silicate rocks; j Ca-poor metapelites, metapsammites, and quartz–tourmaline rocks; k metacarbonates; l meta-ultra-mafic rocks.


Fig. 4. Unit-cell dimensions of representative tourmaline samples.Ž .Reference lines from Donnay and Barton 1972 .

Žintermediate zone Sandamap, Berger, Nestler peg-.matites . It is mainly associated with quartz, K-

feldspar, and muscovite. Border zones of the Brabantpegmatite show comb-textured tourmaline withcone-shaped crystals, up to 20 cm in length, that are

Žperpendicular to the contacts of the pegmatite Rose,

.1981 . In this case, the crystals exhibit an opticalŽzoning with brown rims and pale green cores Fig.

.2a . In the Sandamap pegmatites, tourmaline crystalsform aggregates from which radial intergrowths of

Ž .quartz and muscovite are developed Fig. 2b . Tour-maline crystals typically in Berger pegmatite radiateand flare outward from a common center; it alsoencrusts large crystals of feldspar. Black to deepgreen, coarse grained subhedral to euhedral tourma-line, up to 15 cm in sections perpendicular to thec-axis, forms a texturally distinct subtype. It occursin the outer zone of the Nestler pegmatite coexistingwith K-feldspar megacrysts, quartz, and mica.

Ž .2 Black tourmaline, very fine to coarse grained,associated with quartz, K-feldspar, muscovite, andnodules of Fe–Mn phosphates in the intermediate

Ž .zone Goabeb and Davib-Ost pegmatites . Tourma-lines display a subhedral to euhedral prismatic habitwith an optical zoning: brown rims to pale green

Žcores. Large crystals of tourmaline up to several cm.in length locally form irregular strings or radial

Ž .aggregates in intergrowths with quartz Fig. 2c . InŽ .some places e.g., Davib-Ost , tourmaline crystals

appear fractured and partially replaced by quartzŽ .Fig. 2d . Nests of quartz-tourmaline intergrowths or

Ž .Fig. 5. Plot of Mg vs. Fer FeqMg ratio in tourmaline from pegmatites of the different farms. Values for Mg are expressed in atoms performula unit.


dendrites may be found in masses of quartz, albite,Žand muscovite within the intermediate zone e.g.,

.Davib-Ost pegmatite .Ž .3 Black tourmaline, fine to medium grained,

which occurs as an accessory mineral within theintermediate zone of the Sandamap Noord peg-matites. This zone comprises an assemblage ofquartz–feldspar–muscovite with irregularly dis-

tributed nodules of petalite and huge crystals ofŽ .perthitic K-feldspar up to 2 m in size , as well as

large blocks of Li–Fe–Mn phosphates, some ofwhich show a dendritic morphology.

Ž .4 Black tourmaline, fine to medium grained,anhedral to subhedral, associated with quartz,feldspar, muscovite, beryl, columbite–tantalite, ap-atite, amblygonite–montebrasite, and Bi-minerals in

57 Ž . Ž .Fig. 6. Characteristics Fe Mosbauer spectra of tourmaline from the STB: a schorl from Brabant farm, with Fer FeqMg s0.86¨Ž Ž . . Ž .quadropole splitting D of the outer and inner doublets are respectively 2.32"0.002 and 1.42"0.02 ; b compositionally intermediate

Ž . Ž . Ž Ž .tourmaline from Davib-Ost DV , with Fer FeqMg s0.96 quadropole splitting D of the outer and inner doublets are respectively.2.384"0.005 and 1.71"0.05 .


the intermediate zones and cores of the Etiro peg-Ž .matites Miller, 1969 . The tourmaline occurs locally

as vermiform bands around masses of coarsequartz–feldspar–muscovite within a finer-grainedrock along the outer intermediate zone. Quartz-tourmaline intergrowths have been also observed.

Ž .5 Pink tourmaline, fine to medium grained,Žwhich occurs in quartz-rich pegmatite cores e.g.,

.Daheim Farm and Nestler . In the Daheim pegmatite,tourmaline is relatively abundant and appears inter-grown with lepidolite. In the Nestler pegmatite, tour-maline is associated with masses of lepidolite androrradial intergrowths of lepidolite-cleavelandite. Quartzcontains so-called ‘watermelon’ tourmaline. Some ofthese multicoloured tourmaline crystals are rimmed,

Ž .but not replaced, by a whitish mica Fig. 2e .Ž .6 Variably coloured tourmaline, fine to medium

grained, subhedral and showing pronounced zoning,Žrepresents the last texturally distinct type e.g., Berger

.pegmatite . Green to pink tourmaline occurs in lepi-dolite-bearing and greisen bands at the margins ofthe quartz cores. The greisen consists of pearly whitemica, quartz, amblygonite, and bluish albite. Severalvugs occur in the quartz cores, some of which aresurrounded by cracks. In such cases, the vugs arefilled with very fine-grained masses of albite andclay minerals, containing crushed multicoloured

Ž .tourmaline of high gem quality Fig. 2f .

6. Tourmaline chemistry

Results of electron microprobe analyses of thetourmalines associated with pegmatites of the South-ern Tin Belt are presented in Table 2. Cations performula unit were calculated on the basis of the

Ž .structural formula XY Z BO T O W , for which3 6 3 3 6 18 4

XsNa, Ca, K, vacancy; YsMg, Fe2q, Fe3q, Liq,Al, Mn, Ti4q, Cr 3q; ZsAl, Mg, Fe2q, Fe3q, Cr 3q;TsSi, Al; WsOHy, O2y, Fy, Cly.

Overall, proportionally large variations occur forŽ . Ž .Al O 32.87–42.65 wt.% , FeO 0.03–14.98 wt.% ,2 3Ž . Ž .MnO 0.10–1.56 wt.% and F 0.18–1.42 wt.% .

ŽSmaller variations are observed for SiO 34.94–2. Ž . Ž38.55 wt.% , MgO 0.01–3.02 wt.% , TiO 0.0–0.572

. Ž . Žwt.% , Na O 1.57–2.47 wt.% , CaO 0.03–0.642

. Ž .wt.% and ZnO 0.03–0.36 wt.% . The concentra-Ž . Žtions of K O 0.01–0.04 wt.% , Cr O 0.01–0.052 2 3

. Ž .wt.% and Cl -0.02 wt.% are negligible. Amountsof Li O shown in Table 2 were determined by ICP2

analysis of tourmaline separates, which range from0.02 to 0.40 wt.% for the Fe-rich tourmalines and1.11 to 2.02 wt.% for the Li-rich tourmalines. Repre-sentation of the microprobe data on an Al–Fe–Mg

Ž .ternary diagram Henry and Guidotti, 1985 shows aconsiderable compositional range, from schorl to

Ž .end-member elbaite Fig. 3 . Data for the Fe-richtourmalines plot in the field of Li-poor granitoids

Ž q 2q. 3q Ž . Ž . Ž .Fig. 7. R qR vs. R variation: a in tourmaline from the different farms average values and b Nestler tourmalines. The variationcan be related to alkali-defect, proton-loss, and octahedral-defect substitutions. Lines represent compositions between unsubstituted

Ž .schorl-dravite and elbaite, and the fully substituted end-members proton-loss and octahedral-defect end members .


and their associated pegmatites and aplites, whereaselbaites and intermediate compositions plot in thefield of Li-rich granitoid pegmatites and aplites. Lowcontents of Mg and Ca indicate minor amounts ofthe dravite and uvite components, respectively. Blacktourmalines, however, contain upwards of 30–50mol% of foitite and olenite components. This isreflected on the values of a and c cell parametersŽ .Fig. 4 that show a notable deviation from the

Žschorl-elbaite reference line Donnay and Barton,.1972 .

Black tourmaline is associated with the border orintermediate zone of the pegmatites, and in some

Ž .cases with the core e.g., Etiro ; intermediate compo-sitions and elbaite mainly occur in the intermediate

Ž .zone and core Table 1 . In general, the plot of MgŽ .vs. Fer FeqMg ratios shows a good correlation

for black tourmalines and some intermediate compo-sitions. The highest Mg contents are in tourmalinesfrom Goabeb farm, followed by those from Ariakas

Ž . Žand Sandamap farms Fig. 5 . The highest Fer Feq.Mg ratio is shown by tourmalines from Nestler

Ž . Ž . Ž . Ž . 3q q Ž . Ž 3q � 4. Ž q .Fig. 8. Plot of a AlqLi vs. FeqMgqMn , b R vs. R , and c R q2 X vs. 2R qLi in the Li-bearing tourmalines fromthe different farms. The vectors represent the possible exchange operator that could have been effective in these tourmalines.


pegmatite; the widest range in this ratio is in blacktourmaline from the Etiro farm pegmatite. Within

Ž .individual zoned pegmatites e.g., Nestler in whichŽdifferent types of tourmaline coexist, the Fer Feq

.Mg ratio increases from black tourmaline to thecompositionally intermediate varieties, whereas el-

Ž .baite shows typically highly variable Fer FeqMgŽ .ratios Table 2 . In general this ratio increases through

the sequence black tourmaline-bearing pegmatitesŽGoabeb, Ariakas, Sandamap, Etiro, Sandamap No-

.ord, Brabant to green tourmaline-bearing pegmatitesŽ . Ž .Davib-Ost, Daheim, Berger, Nestler . Nar NaqCa

Žratios increase from Li-bearing tourmaline 0.83–. Ž . Ž .0.94 to black tourmaline 0.95–0.99 Table 2 .

Calculations of site occupancies show that all ofthe tourmalines have sufficient Al to fill the octahe-

Ž . Ž .dral Z site Table 2 . However, Mossbauer spectro-¨scopic data acquired on some of the tourmalines

Ž . 3qreveal that: 1 the amount of Fe is insignificant,in agreement with the absence of tourmaline compo-

Ž .sitions plotting below the schorl-dravite line Fig. 3 ;Ž . 2q Ž .2 part of the Fe 10 to 30% of total iron is

Ž .believed to be partitioned into the Z site; and 3 thesmall number of doublets reflects a high degree of

2q Ž .Fe order, at least in the Y site Fig. 6 . Due todifferences in the ionic radii of Al3q and Fe2q, localoctahedral distortions will vary, thus yielding awidening of the Mossbauer absorption peaks. These¨results are consistent with the Mossbauer data re-¨

Ž .ported by Korovushkin et al. 1979 for tourmalinesfrom rare-metal pegmatites, indicating negligible ox-idation. The two Fe2q doublets in Fig. 6 have been

Ž .assigned according to Burns 1972 . The first doubletof the quadrupole splitting with a higher ^D valueis due to Fe2q in the Y site and the other, lessintense doublet with a lower ^D value is interpretedto represent Fe2q in the Z site. Relative amounts ofFe2q in the Y and Z sites were calculated usingareas of the respective Fe2q doublets, and the cationordering degree was evaluated by the number ofnon-equivalent Fe2q sites. Although some workersargue that the Fe2q occupies only the bigger Y siteŽ .Pieczka and Kraczka, 1994; Dyar et al., 1998 ,natural tourmalines with significant amounts of Fe inthe Z site have been described in different regionsŽ .Fuchs and Maury, 1995; Visona and Fuchs, 1997 .The introduction of Fe2q in the Z site can becharge-balanced by the substitution of Fe2q by Al in

Ž .the Y site Fuchs et al., 1998 , with a general com-positional variation toward the hypothetical end-

� 4Ž 2q .Ž 2q . Ž .member: X Fe Al Fe Al Si O BO -2 5 6 18 3 3Ž .OH,F .4

The total number of cations in the Y site rangesfrom 2.46 to 2.81 and suggests the presence of Y-sitevacancies. In similar cases, a loss of protons hasbeen postulated to explain apparent Y-site vacanciesŽFoit and Rosenberg, 1977; Rosenberg and Foit,

.1979 . Compositions of the pegmatite-hosted tourma-lines from the STB mostly plot along and betweenproton-loss and alkali-defect substitutions from the

Ž .schorl additive component Fig. 7a . The increase infoitite and olenite components is similar for theoverall variation found in tourmalines from south-

Ž .west England London and Manning, 1995 . Amountsof Al generally increase from 6.50 to 6.80 in blacktourmaline to greater than 7.35 in elbaite. The com-bination of proton-loss and alkali-defect substitutionsaccounts for the increase of Al in the Y site and thedecrease in X-site and OH occupancies at the ex-

Ž .pense of the schorl component Fig. 7b . The analyti-cal results show that all of the average compositionshave )0.5 mole Al pfu in the Y site, and X site

Ž .deficits of 30–50% with cation totals NaqCaqKŽ .of 0.53–0.79 Table 2 , compared with the ideal

Žvalue of 1.00 in schorl and elbaite e.g., Foit and. Ž .Rosenberg, 1977 . In general, the increase of Al Y

is accompanied by the increase of F, so that thefluorine contents are higher for Li-bearing tourma-

Ž .lines 0.50–0.70 apfu than for Fe-rich tourmalines

Fig. 9. Infrared spectra in the O–H stretching region of threeŽ . Ž . Ž .representative tourmaline. a Nestler and Goabeb; b Nestler; c

Nestler and Daheim. Horizontal scale is in cmy1.


Ž . Ž .-0.25 apfu Table 2 . The incorporation of Li ismostly controlled by the elbaite-type substitution

Ž .LiAlFe Fig. 8a . However, the Li contents ofy2

Li-bearing tourmalines are lower than the theoreticalŽ .Li content of elbaite, that is 1.5 per 31 OqOHqF

as derived from schorl by the substitution LiAlFe .y2Ž .According to Foit and Rosenberg 1977 , Li can be

reduced by means of three competing mechanisms:Ž .1 the dehydroxylation type substitution AlO Li -2 y1Ž . Ž .OH ; 2 formation of X-site vacancies via they2

� 4 Ž .alkali-defect substitution X AlNa Li ; and 32 y2 y1

formation of Y-site vacancies via the octahedral-de-� 4Ž .fect substitution Y OH Li O . The data for Li-y1 y1

bearing tourmalines from the STB fall on trends thatare roughly parallel to the exchange vectors:� 4 Ž . � 4 � 4X Al O Na Li OH Y , X Al O -2 5 13 y1 y1 y13 y4 4.5 12

Ž . � 4 Ž . � 4Na Li OH Y Fig. 8b , and X Al -y1 y0.5 y12 y4 2 5Ž . � 4 Ž .O Na Li OH Y Fig. 8c . These substi-12 y2 y1 y12 y4

tutions can be shown to be a linear combination ofŽ . � 4 � 4vectors such as AlO OH Y , X AlNa Li ,3 y3 y1 y2 y1

Ž . Ž .Fig. 10. Detailed compositional variation of zoned tourmaline crystals occurring in a the Berger pegmatite, and b the Goabeb pegmatite.


� 4 Ž .and X AlONa Li OH . Such trends imply ay1 y1 y1

significant proton loss, as well as decreasing ofY-site vacancies suggesting that the third mechanismindicated above was negligible. The amount of X-site

Ž .vacancies in pink tourmalines 0.35–0.41 apfu re-flects a moderate degree of substitution from elbaitetoward the rossmanite end-member, such as defined

Ž .by Selway et al. 1998 , via the exchange vector� 4X AlNa Li .y2 y1

Infrared spectra for elbaite in the O–H stretchingregion display a split in the single absorption maxi-mum shown by black tourmaline in this region. Thissplit is considered evidence of local ordering of Li

Ž .and Al in the Y site Grice and Ercit, 1993 . Fortourmaline with intermediate compositions, this peaksplit is not so evident, yielding a spectra intermediate

Ž .between those of schorl and elbaite Fig. 9 .Under the microscope, tourmaline crystals com-

monly exhibit an optical zoning involving two orthree distinct growth zones. Characteristic coloursare blue, grey, yellow, green, and brown. However,chemical variations from rims to cores of crystals arerelatively small. A common zoning trend in elbaiteand compositionally intermediate tourmalines shows

Ž .variations in which FeqMgqMn and Ti decreaseŽ .whereas AlqLi and Ca increase from cores to

Ž .rims Fig. 10 . A reciprocal zoning trend is observedin elbaite and intermediate tourmalines from peg-matite-hosted ‘watermelon’ tourmaline. Zoning inFe-rich tourmalines is characterized by increasingŽ .FeqMgqMn , Ti, and Ca, as well as a slight

Ž . Ž .decreasing of LiqAl , from cores to rims Fig. 10 .Concentrations of trace elements in tourmaline

depend on the pegmatite type and its internal evolu-Ž .tion Table 2 . However, we have not observed

systematic variations in the trace-element contents,except for Be, Ga, Rb, Sn, Cs, and Pb contents,lower in Fe-rich tourmalines than in elbaite. Thevalues for Sn are not related to the occurrence ofcassiterite in the pegmatites. Sn-poor tourmalinesmay be associated with cassiterite and vice-versaŽe.g., relatively Sn-rich tourmaline occurs in theNestler pegmatite but no cassiterite or other Sn-

.bearing mineral has been observed in the samples .In contrast, concentrations of V, Sc, and Zn as wellas La, Ce, Nd, and Dy in black tourmaline are higherthan in elbaite. REE contents are very low in all ofthe analyzed samples. Li, Rb, and Cs contents of

black tourmaline are higher than those tourmalinesfrom barren pegmatites elsewhere.

7. Discussion

Field, petrographic, and chemical data show aclear genetic relationship for the studied tourmalineswith pegmatitic environments in the STB. Tourma-line occurs as several texturally and compositionallydistinct types that are confined to different zones ofthe pegmatite bodies. The textural characteristics ofthe tourmaline, along with the occurrence of coarse-grained quartz–mica–feldspar assemblages and other

Žminerals typical of pegmatites e.g., beryl, petalite,.Fe–Mn phosphates, lepidolite , are evidence for

growth of tourmaline from Li, Be, B, F, and P richpegmatitic systems.

7.1. Compositonal Õariations

Analytical data reveal that the tourmaline compo-sitions fall mainly along and between the foitite andolenite substitutions reflecting the fractionation andinternal evolution of the pegmatites. Within individ-ual pegmatites, tourmaline becomes more and more

Ž .aluminous with fractionation Fig. 7b . The extent ofalkali-defect and proton-loss substitutions increases

Žwith decreasing temperature of crystallization Ro-.senberg and Foit, 1979 , decreasing X and OH occu-

pancies and increasing Al content in Y site at theexpense of the schorl component. The majority oftourmalines from the STB have significant X-sitevacancies that indicate either the partitioning of Nainto coexisting albite, or that alkali-deficient tourma-line crystallized from fluids that became depleted inalkalis as a consequence of albite precipitation. Theincrease of Al is observed in general in the sequence

Žblack tourmaline-bearing pegmatites Goabeb, Etiro,.Sandamap Noord, Brabant , to green tourmaline and

Želbaite-bearing pegmatites Davib-Ost-DV1, Da-.heim, Otjimbojo . Thus, the more evolved peg-

matites are those in the eastern part of the STB.Ž .Likewise, Fer FeqMg ratios for tourmalines from

Fe–Mn phosphate- and black tourmaline-bearingpegmatites are generally lower than those for tour-malines from pegmatites including Li-minerals, sug-

Žgesting major fractionation of the latter. The Fer Fe.qMg ratio, however, is not a simple indicator of


fractionation and formational processes of the peg-matites, because it also depends on partitioning of FeŽ .and Mn between coexisting tourmaline and phos-phates. This may be the reason for the overlapping of

Ž .Fer FeqMg values of the tourmalines from differ-Ž .ent pegmatites Fig. 5 . Possible explanation for the

lack of Fe–Mn phosphates in the eastern part fromSTB is that the crystallization of tourmaline removewhat little ferromagnesian components the meltsmight contain, so that the later phosphates to beformed are amblygonite–montebrasite.

The occurrence of black tourmaline, composition-ally intermediate tourmaline, and elbaite in the bor-der, intermediate zone, and core of the pegmatites,respectively, suggests that the chemical variation oftourmaline has been controlled, to some degree, bythe elbaite substitution LiAlFe . This trend is con-y2

sistent with previous studies of the compositionalŽevolution of tourmaline in pegmatites Staatz et al.,

.1955; Jolliff et al., 1986 , and is supported by thecrystal zoning patterns in tourmaline from individualpegmatites. However, tourmaline crystals from mi-arolitic cavities that exhibit contrasting composi-tional trends with pink centers and green rims are notconsistent with decreasing Fe and increasing Li–Al.

Ž .Such as Foord 1977 argued, the green rims formedafter rupture of the cavities as a result of the influxof Fe–Mg components from wallrocks to pegmatites.Fe-rich tourmaline in the border zones may haveprecipitated from the melt through undercooling atthe pegmatite margins, or by fluid-based diffusion ofFe from the host rocks into B-rich but Fe-poorpegmatite melt. Boron metasomatism and formationof tourmaline in the host rocks can be explained bythe opening of the pegmatite system at the last stagesof crystallization. Where crystallization proceeded Licontent may have increased to form Li-rich tourma-line in the intermediate zones; depletion of Fe andMg in the melt resulted in the formation of elbaite.Thus, black tourmaline was followed by composi-tionally intermediate tourmaline and later by elbaitein the crystallization sequence. Such variations mayhave been controlled by fractionation of silicate meltthrough sequential crystallization from the marginsinward to the cores of the pegmatites. However, theoccurrence in some pegmatites of inverse crystal

Ž .zoning patterns e.g., Nestler, Goabeb is inconsis-tent with fractional crystallization processes. The

formation of mineral zones, in these cases, may bedue to equilibrium crystallization in which the zonalassemblages developed simultaneously. Separationof the zones could have been promoted by incongru-ency in the solubilities of alkalis, Al, and Si between

Ž .melt and fluid London, 1992 . A simultaneous crys-Žtallization model for pegmatites in Namibia e.g.,

.Etiro, Brabant, Davib-Ost was also proposed byŽ .Uebel 1977 .

Tourmalines having a relatively high Mn contentare Li-rich, consistent with data reported in the

Ž .literature Slivko, 1961; Sahama et al., 1979 . SlivkoŽ .1961 suggested that the formation of Mn-rich tour-maline is restricted to pegmatites saturated in Li andNa. However, Mn-rich and Li-poor tourmalines also

Ž .exist as described by Ayuso and Brown 1984 andŽ . Ž .Jiang et al. 1997 , among others. Nemec 1973´

reported that the Sn content of tourmalines fromSn-mineralized areas is greater than that from un-

Žmineralized areas, and argued that high Sn s60.ppm is characteristic of schorl with Sn-rich associa-

tions. In the STB, however, black tourmalines fromŽstanniferous pegmatites have lower Sn contents 50

. Ž .ppm; Table 2 . Power 1968 noted that the Sncontent of tourmalines from SW England increaseswith the degree of differentiation. With this in mind,tourmalines from the Davib-Ost, Daheim, and Otjim-bojo pegmatites that show higher Sn, would reflect amore advanced degree of evolution in accordance

Y Žwith the trends defined by Al contents and Fer Fe.qMg ratios. The fact that Li, Be, Sn, Rb, Cs and

Pb contents in tourmaline are lower in Fe-rich tour-maline than in Li-rich tourmaline is apparently dueto fractionation processes during pegmatite forma-tion.

It is clear that in the STB there is a regionalassociation of boron with Sn and Nb–Ta mineraliza-tion. Nevertheless, in the various pegmatites, tourma-line and cassiterite are confined to different zonesŽ .e.g., Sandamap, Goabeb, Berger . This pattern sug-gests that boron does not directly affect the solubilityof Sn and Nb–Ta, but may play a significant role in

Žconcentrating metals through indirect effects London. Žet al., 1996 . Boron and other fluxing components F

.and P enhance the solubilities of high field strengthelements in the melt, depressing the solidus andpromoting more extended crystal fractionation andfurther concentration of metals in the residual hy-


Ždrothermal fluids Manning, 1981; Pichavant, 1987;.London, 1995 . Cassiterite and other ore minerals are

believed to precipitate due to increasing oxidationŽ .state Taylor and Wall, 1992 , and increasing alumi-

Ž .nosity of the melt Wolf and London, 1997 .

7.2. Textural features

Comb textures exhibited by tourmaline and alkaliŽ .feldspar e.g., Sandamap and Brabant pegmatites

may reflect crystallization via heterogeneous nucle-ation from supercooled melts. The growth directionof crystals in such an environment is controlled bytemperature gradients, which are perpendicular tocontacts. Conical thickening of crystals towards theinner parts of the pegmatites records a decreasingthermal gradient. Simple fractionation seems an un-likely history for pegmatite bodies that have tourma-line only at their margins, and dimensions larger than

Ž .that of the tourmaline fringe London et al., 1996 .An abrupt end of the tourmaline fringe means thatthe system became closed to the influx of ferromag-nesian components from the host rocks. For such

Ž .pegmatites the model of Uebel 1977 describing twopetrogenetic units is favoured.

Ž . Ž .1 the outer unit ‘pegmatite shell’ that shows ananisotropic fabric is characterized by large initialundercooling and heterogeneous nucleation. Combtextures commonly occur here together with graphicintergrowths, skeletal, dendritic, or arborescent crys-tal forms, and radial aggregates including tourma-line, quartz, Fe–Mn phosphates, and feldspars. Thesetextures are interpreted to be a consequence of rapidcrystallization under non-equilibrium conditionsŽ .Keller, 1988; London, 1992 .

Ž .2 the core which is mainly characterised byrandomly oriented crystallization via internal nucle-ation processes. It is mostly composed by quartz thatrepresents the only mineral observed in some peg-

Ž .matites cores e.g., Ariakas . Quartz-rich cores canŽcrystallize from melt without a vapor phase London

.et al., 1989 .Regardless the genetic model, the occurrence of

tourmaline-bearing vugs or miarolitic cavities in someŽ .pegmatite cores e.g., Otjimbojo farm reflects that

the pegmatite system became H O-saturated during2

the last stages of crystallization. Miarolitic cavitiesare evidence of vapor saturation prior to the com-

plete crystallization of pegmatite-forming meltŽ . Ž .London, 1986 , and as Jahns 1982 argued suchcavities would indicate the continuity from magmaticto hydrothermal regimes in pegmatite systems. How-ever, the most interesting feature is the availability ofboron in melt to precipitate tourmaline at the late

Ž .stages e.g., Daheim, Berger ; in such cases theincrease of Li in melt composition contributes tostabilize elbaite.

7.3. Boron abundance

Tourmaline represents the main sink for boron inpegmatites from the STB. Hence, an estimation oftourmaline content may provide us an idea of theboron abundance in bulk melt. The stability field oftourmaline hinger on several parameters, some ofwhich are interdependent. Foremost among them arethe activities of B and Al; however, temperature,a , and other compositional parameters as Fe, Mg,H O2

F, and P, are expected to be important because theiractivities control saturation of the melt with regard to

ŽAFM phases and tourmaline Benard et al., 1985;Holtz and Johannes, 1991; Gan and Hess, 1992;

.Wolf and London, 1994, 1997 .In pegmatites from the STB, the tourmaline con-

Žtent varies from accesory quantities e.g., Sandamap. ŽNoord, Etiro to relatively abundant Daheim, Otjim-

.bojo . The boron concentrations of pegmatite melts,however, may be higher than the tourmaline contentsuggests, as a result of the low activities of Fe–Mgcomponents and high Ž4.BrŽ3.B ratio in muscovite-saturated melt and vapor, that limit tourmaline stabil-

Ž .ity Benard et al., 1985 . This may account for theloss of a large fraction of magmatic boron to wall-

Ž .rocks Morgan and London, 1987, 1989 , that makesvery difficult the evaluation of boron abundance.With this aim, it is necessary to examine both thepegmatites and their wallrocks when considering theconcentration of boron in such systems. Unfortu-nately, the wallrock exposures in the STB is nor-mally insufficient to make a reliable evaluation ofthe boron volume lost from the pegmatites. How-ever, the occurrence of large biotite laths at the

Ž .margins of some pegmatites e.g., Etiro suggeststhat the fraction of melt that attained saturation intourmaline was very small, probably due to relativelylow boron contents that limited the crystallization oftourmaline.


Pegmatites from the western part of the STBŽSandamap, Sandamap Noord, Goabeb and Davib-

.Ost include black tourmaline associated with Fe–MnŽ .phosphates, these latter with variable Fer FeqMg

Ž .ratios from 0.62 to 0.75 Keller, 1991 . The forma-tion of black tourmaline can remove the low contentsof Fe–Mg that the pegmatite melts contain, but theoccurrence of Fe–Mn phosphates would mean that:Ž .1 the amount of boron was insufficient in bulk

Ž .melt; 2 much of boron was lost before the crystal-Ž .lization of phosphates; or 3 the presence of P

decrease the stability of tourmaline by lowering theŽ .activity of Al in melt Wolf and London, 1994 , that

favours the formation of Fe–Mn phosphates at theexpense of tourmaline. This is a way of exhaustingferromagnesian components from melts, and if thereis no Li the rest of boron could be lost to the host

Ž .rocks. Nevertheless, although Pichavant 1981 re-ported a Dvrms3 reflecting the volatile nature ofboron in granite systems, on the basis of new values

Ž .of Dvrms1–2, London 1997 concluded thatboron is volatile but only very slightly. The abun-dance of metasomatic tourmaline around pegmatitesresults from the inability of tourmaline-forming reac-tions to buffer the pegmatite melts, and from thewallrocks that represent a reaction medium for thedeposition of tourmaline once melt and host rocks

Ž .put in contact via a fluid phase London et al., 1996 .Evidence of a fluid phase is provided by the

occurrence of vugs and miarolitic cavities, but theseappear mainly in pegmatitic cores and are not anevidence that pegmatite melts were vapor saturatedat the onset of crystallization. The pegmatites arecrystallized prior to lose their fluids, and the ex-change of components between host rocks and peg-

Ž . Ž .matites appears to be sequential London, 1992 : 1infiltration of components from host rocks to peg-

Ž .matite; and 2 subsequent outflow of pegmatite-de-rived fluids at the end-stages of pegmatite crystal-lization. The occurrence of Fe–Mn phosphates inouter zones, associated with subordinate black tour-maline, suggests a relatively low boron content in the

Ž .bulk melt. In some pegmatites e.g., DV 4 thecrystallization of Fe–Mn phosphates in intermediatezones is followed by the formation of Li-bearingtourmaline in cores. As the incorporation of LiqAlin the Y sites of the tourmaline is structurally andenergetically less stable than incorporation of Fe2q

Ž .Foit and Rosenberg, 1979 , the lack of tourmaline inFe–Mn phosphate-bearing assemblages could be the

Žresult of the strong affinity of P with Al Gan and.Hess, 1992; Wolf and London, 1997 that tend to

reduce the stability of tourmaline in P-rich melts.ŽHowever, in Aldehuela de la Boveda Salamanca,´

.Spain we have observed graphic intergrowths ofblack tourmaline and Fe–Mn phosphates that can beexplained by simultaneous crystallization rather thanby replacement. Consequently, the lack of tourma-line in intermediate zones may be due to that theconcentration of boron needed to saturate the meltwith respect to tourmaline was low. The occurrenceof Li-bearing tourmaline in vugs of the core indi-cates that boron became significant at very latestages of the pegmatite crystallization, just whenvapor saturation occurred. With major fractionationB and Li increase promoting the crystallization of

Ž .Li-rich tourmaline in the cores e.g., DV 1 .By comparison with the pegmatites from the

Žwestern part, those from the eastern part Brabant,.Daheim, Berger, and Nestler are characterized by

the absence of Fe–Mn phosphates and presence ofŽF-bearing minerals topaz, fluorite, amblygonite–

.montebrasite, lepidolite . Topaz and fluorite, how-ever, occur in replacement units indicating that theycrystallized from hydrothermal phase, not from melt.Because F dissolves preferentially into aluminosili-

Ž .cate melts rather than in fluids Webster, 1990 ,compared with B, the accumulation of F in meltsreduce the stability field of tourmaline and more Bandror higher Al are needed to saturate melt in

Ž .tourmaline Wolf and London, 1997 . The crystal-Žlization of black tourmaline in outer zones e.g.,

.Berger and Nestler consume most of the Fe inpegmatite melts, and higher concentration of boronand lower temperature are required to attain satura-

Žtion in Li–Al tourmaline Jolliff et al., 1986; Rosen-.berg et al., 1986 . Experimental studies document

that pegmatites with tourmaline as the only AFMsilicate mineral appear to have contained G2 wt.%B O in their bulk melts, probably much more if the2 3

Ž .melts were also rich in F Wolf and London, 1997 .ŽHowever, if melt remains to low temperature F

.6008C it may precipitate tourmaline at lower BŽ . Ž .contents f1 wt.% B O Wolf and London, 1997 .2 3

The existence of melt at lower temperatures is plau-sible because B and F, like H O, depresses solidus2


Žand liquidus temperatures Chorlton and Martin,.1978; Pichavant, 1981, 1987; Manning, 1981 . Con-

sequently, F and temperature would have oppositeeffects on the melt saturation with regard to tourma-line. With increasing F more B is required to saturatemelt in tourmaline but depressing of solidus andliquidus tend to stabilize tourmaline at lower Bcontents. This raises the question if boron abundancein bulk melts can be inferred from the tourmalinecontent in pegmatites enriched in F and other volatileelements.

8. Conclusions

The compositional variations of tourmaline fromŽ .the STB central Namibia may be useful in deci-

phering processes that controlled the evolution ofindividual pegmatites, and in providing evidence of

Ž .the i physical and chemical evolution of the sys-Ž .tems; and ii crystallization processes.

Field relations, distribution, petrographic data, andtourmaline chemistry suggest that two pegmatiticdomains can be delineated in the Southern Tin Belt:Ž .1 a western domain characterized by Fe–Mn phos-phate- and black tourmaline-bearing pegmatites, with

Ž .associated Sn, Nb)Ta, Li, Be mineralization; andŽ .2 an eastern domain that includes more evolvedelbaite-bearing pegmatites, with associated Li)Be,Ž .Sn, Nb)Ta mineralization.

Pegmatites from the eastern domain are character-ized by a fractionation sequence that involved se-quential crystallization from the margins to the coresof the pegmatites. Tourmaline composition changesthrough the pegmatite zones, mostly via the substitu-

Ž 2q .tion Fe ,Mg,Mn LiAl in the Y site. The extenty2

of alkali-defect and proton-loss substitutions becomegreater with fractionation and decreasing tempera-ture. Such trends are also supported by crystal zon-ing patterns.

Pegmatites from the western domain may supportŽ .the model of Uebel 1977 , i.e., simultaneous forma-

tion of two units: the outer unit or ‘shell’ and core.Tourmaline is confined mainly to the pegmatite shell,where the crystal zoning trends are inconsistent withfractionation processes. Precipitation of tourmalinefrom the pegmatite melt through undercooling at themargins andror influx of ferromagnesian compo-

nents from host rocks into the B-bearing melt, maybe a plausible explanation for the development oftourmaline at the margins of these pegmatites.

Although black and green tourmaline are found inbroad association with Sn mineralization in the STB,the boron appears not to have directly controlled thesolubility of Sn and Nb–Ta. Negligible oxidationduring crystallization of the tourmaline indicates fO2

conditions that are inconsistent with the coeval pre-cipitation of cassiterite, thus explaining why theseminerals occur in different pegmatite zones.

The occurrence of numerous granitic intrusions inthe Damara orogen, and the fact that geologicalrelations are insufficiently known, make it difficultto relate the pegmatites of the STB to one or more

Ž .specific parent granite s . Likewise, the source ofboron for the pegmatite melts is a matter of debate.

Acknowledgements

We thank Dr. Ph. de Parseval for assistance inobtaining electron microprobe analyses at the Uni-versite Paul Sabatier. Dr. Fernando Plazaola is also´thanked for assistance with Mossbauer analyses at¨the Universidad del Paıs VascorEHU. We also ex-´press our gratitude to reviewers J.F. Slack and D.London for their constructive comments on themanuscript. This study was carried out with thefinancial support of the Spanish-German Coopera-

Ž . [ ]tion Project HA1997-98 . NA

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