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609
Thz Canadi an M inc ralo gist
vol.
36,
pp.
609-63s
(1998)
OXIDE MINERALS
OF THE SEPARATION RAPIDS RARE-ELEMENT GRANITIC
PEGMATITE
GROUB NORTHWESTERN ONTARIO
ANDREWG.TINDLE'
Departrnent of Earth Sciences, The Open University, Milton Keynes, Buckinghnmshire MK7 6M, U.K.
FRED W. BREAKS
Ontario Geological Survey, Mines and Mineral Research Centre, 933 Ramsey lake Road., Sudbury, Ontario P3E 685
Arsrnacr
The newly discovered Separation Rapids pegmatite group, situated in mafic metavolcanic host-rocks that represent
the eastern extremiry of the Bird River metavolcanic - metasedimentary belt, contains Ontario's first occurrences of
wodginite-group minglals (mainly wodginite and ferrowodginite), the pyrochlore-group minerals stibiomicrolite, stibiobetafite
and ynropyrochlore, fenotapiolite, and probably the first occurrence in North America of nigerite from a granitic pegmatite.
This example of the rare-element class of granitic pegmatites hosts both beryl- and petalite-subtype pegmatites.
Columbite-tantalite and cassiterite are the predominant oxide species. On the basis of columbite-tantalite compositions, the
pegmatites have been divided into an Fe-suite and a Mn-suite. Both beryl and petalite pegmatites occur in each suite. On
the basis of ferrocolumbite compositions, the associated Separation Rapids pluton is considered to be the parent of at least the
Fe-suite ofpegmatites. The Fe suite includes beryl pegmatites within and adjacent to the pluton, in which ferrocolumbite
coexists with ferrowodginite, and, with increasing evolution, petalite-bearing pegmatites that contain ferrotantalite and
wodginite. In individual pegmatites, columbite-tantalite variation is mainly in Tal(Ta + Nb). Minor microlite, antimonian
microlite and stibiomicrolite are found replacing earlier phases.
Cassiterite is the final M-Ta-bearing oxide to crystallize.
Pegmatites belonging to the Mn-suite follow a similar pattern of crystallization, with early manganocolunbite followed by
manganotantalite, the latter coexisting with wodginite. Manganocolumbite within individual samples varies appreciably in
Mn/(Mn + Fe), whereas the variation in manganotantalite is mainly in Tal(Ta + Nb). In pods rich in "cleavelandite" and
Limica within one of the beryl pegmatites, extreme Mn-enrichment has produced near-end-member manganotantalite and
W-bearing wodginite. Microlite is an important. late phase, which is either primary or forms as a replacement, mainly of
wodginite. The presence of microlite, lithian mica and topaz in Mn-suite pegmatites (and aplites) indicates that they were
derived from a more F-rich melt than that which produced the Fe-suite of pegmatites.
Albifization also is more apparent in the
Mn-suite of pegmatites.
The wall zone of Marko's pegmatite,
the largest
body in the eastern subgroup and part of the Mn-suite,
is unique in hosting titanowodginite, "ferrotitanowodginite", stibiobetafite and strtiverite. These Fe-, Ti- and Sb-phases are
considered to have developed as a result of interaction of the pegmatite-forming melt with banded ironstones and Fe-Ti-rich
metavolcanic host-rocks.
Keywords: columbite, tantalite, wodginite, ferrowodginite, titanowodginite, microlite, stibiomicrolite, stibiobetafite, nigerite,
granitic pegmatite, tantalum, tin, Separation Rapids, Ontario.
Solanaetne
Le groupe de massifs de pegmatites granitiques dit de Separation Rapids, situ6 dans des roches h6tes mafiques
m6tavolcaniques qui repr6senteraient
I'aboutissement vers I'est de la ceinture m6tavolcanique et m6tas6dimentaire de Bird
River, contient lds premibres indications de la pr6sence
en Ontario des mindraux du groupe de la wodginite (surtout wodginite
et ferrowodginite), du groupe du pyrochlore (stibiomicrolite, stibiob6tafite et yttropyrochlore), de 1a ferrotapiolite, et tout
probablement du premier exemple en Am6rique du Nord de 1a nig6rite dans un contexte pegmatitique. Ces exemples de
pegmatites granitiques enrichies en 6l6ments
rares comprennent des cortdges I b6ryl et d'autres i p6talite. Columbite-tantalite
et cassit6rite en constituent les oxydes principaux. A la lumibre des compositions de columbite-tantalite, les pegmatites
ddfinissent.
deux suites, une riche en Fe et I'autre, en Mn. On Eouve des pegmatites i b6ryl et des pegmatites d p6talite dans
chaque suite. Selon les compositions de la ferrocolumbite, le pluton de Separation Rapids, avoisinant, serait la source d'au
moins la suite de pegmatites
riche en Fe, dont les pegmatites
d b6ryl i I'intErieur ou prds
du pluton contiennent la fenocolumbite
qui coexiste avec la ferrowodginite. l-es facies plus 6volu6s, d pdtalite, contiennent ferrotantalite et wodginite. Dans les venues
pegmatitiques individuelles, la variation en composition de Ia columbite-tantalite implique surtout Ta./(Ta + Nb). Des traces
de
microlite, microlite stibid et stibiomicrolite remplacent en g6n6ral
les min€raux accessoires
pr6coces.
l,a cassit6rite est le dernier
I E-mail address: a.g.tindle@open.ac.uk
610 THE CANADIAN MINERALOGIST
oxyde porteur de Nb-Ta d cristalliser. Les pegmatites faisant partie de la suite enrichie en Mn suivent un sch6ma de
cristallisation semblable, impliquant manganocolumbite suivie de manganotantalite, cette dernidre en coexistence avec 1a
wodginite. Dans un dchantillon donn6, Ia manganocolumbite varie de fagon importante en Md(Mn + Fe), tandis que la varia.tion
dans la manganotantalite implique surtout Ta.i(Ta
+ Nb). Dans des lentilles riches en "cleavelandite" et en mica lithinifOre au
sein d'une pegmatite d bdryl, un enrichissement extreme en Mn a produit de la manganotantalite dont la composition est proche
du p6le et de la wodginite riche en firngstdne. Le microlite est une phase
tardive importante, soit d'origine primaire ou bien en
remplacement, surtout de la wodginite. La pr6sence
de microlite, mica lithinifdre et topaze dans les pegmatites (et aplites) de
la suite riche en Mn laisse entrevoir une cristallisation d partir d'un magma relativement enrichi en fluor compar6 I celui qui
a produit le cortbge de pegmatites plut6t enrichies en Fe. L albitisation est aussi plus 6vidente dans les pegmatites riches en
Mn. La zone de paroi de la pegmatite de Marko, la plus volumineuse du groupe de I'est et faisant partie de ce groupe riche
en Mn, semble la seule I contenir titanowodginite, "ferrotitanowodginite", stibiob6tafite et striiverite. Ces phases
riches en Fe,
Ti et Sb seraient attribuables l une interaction entre le magma responsable de la pegmatite et les roches encaissantes, des
formations de fer ruban6es et des roches m6tavolcaniques riches en Fe-Ti. (Traduit par la Rddaction)
Mots-cl|s;columbite, tantalite. wodginite, ferrowodginite, titanowodginite, microlite, stibiomicrolite, stibiobetafite, nigerite,
pegmatite granitique, tantale, 6tain, Separation Rapids, Ontario.
IN'TRoDUCTToN
Zoned rare-element (e.9., Be, Li, Nb, Ta, Sn)
pegmatites represent the extremes of fractional
crystalliza_tion of more primitive granitic magmas
(Goad & Cernf 1981,
Shearer et al. 1992,
Breaks &
Moore 1992) and can give rise to major economic
deposits
of these elements. For instance,
the Greenbushes
pegmatite of Western Australia contains half the
world's Ta resource (Partington et al. 1995), whereas
other pegmatites such as Tanco, Manitoba (Crouse er
al. 1979) and Bikita, Zimbabwe (Cooper 1964) are
major Li producers.
Howevero
there are few examples
where all components of the granite - pegmatite
sequence
are exposed,
and consequently
the processes
of differentiation are poorly documented (with a few
notable exceptions;
e.g., Greer Lake, Manitoba: iernf
et al. 1986; Harney Peak, South Dakota: Sheam et al.
1992; Preissac
- Lacorne, Quebec: Mulja et al. 1,995).
Specific facton responsible for the concentration of the
rare metals still remain largely unkn6'q4, although this
is as much a problem of experimental geochemistry as
it is of field exposure.
The Separation Rapids Pegmatite Group offers the
unusual opportunity to examine one of the chemically
and mineralogically most evolved rare-element
pegmatite
groups
andits potential
source,
the Separation
Rapids pluton. In this paper,
we document an electron-
microprobe investigation of the oxide mineralogy,
in particular the M-Ta oxides, as these have been
shown to be especially responsive
to fractionation and
replacement
effects
(Cernf et al. 1985,1986, Abella
et al. 1995, Cernf & NEmec 1995). The geology,
geochemistry and K-feldspar mineral chemistry of the
Treelined Lake granitic complex (the likely source of
the Separation Rapids pluton) and the Separation
Rapids Pegmatite Group will be described by Breaks et
al. (in prep.), and details of the wodginite-group
mineral chemistry are covered in a companion paper
(Tindle et al. 1998).
TABLE
1. StrAMNON RA?IDS
MINERALw
Fesluffib
FetubIte
Mn<dMbiE
h'bndrc
FewodghiF
Wodgnh
F€]i wodghjF
Ti wdgfiE
M@UE
Sdblmi@Iits
Ulmi@ltrc
Sdblob&ff8
Yropyrdlore
SdvedE
CaslFdE
Ilmdre
kIE
ffite
Nigdre
IyrF
L6[ingiE
CHcopydE
FIuonpa6re
AIluaudiE
Ziron
Epldote
Fluodh
Iope
eF/l
Touffie
Gmet
Cordidb
Mphlbole
Blodts
hwaldlts
ilMe
Mlldrc
Albie
K feldsptr
a'je
spodMme
PebUre
Poludh
(.)
(t
.?
(.)
(.)
G)
(.) (.)
.?
.?
1 = Treelhed lrte tdft ompl4 a poEnttal sure of th€ Sep@donkPlds Plub
' 6b conbtu orthopyrcxoe, mdaLde m@ and ruHle;
2 = gpamdd Raptds pluh; 3 - bryl pe8etib (bdE);
4 - P€hIE FcmdB (biliE); 5 - Beryl pegtuEb (MdE);
6 . Pebltre Fgrod6 (Mn-db),
7 = WaI me of Mdko's p%frdE - a PebIF petmdh (Mn4dre).
(.) hplr€ only fomd h me mple; ? quedmble
OXIDE MINERAI.S. SEPARAIION RAPIDS
Pegmatite
Zones Granite Types
6ll
Big Whopper
Pegmatite
(Peg
96-29)
+
o Granite
f aeryr
pegmatite
ffi Intemat Beryt ffi Biotite
granite
and
granodiorite
Peraluminous biotite-muscovite
pegmatitic
granite
E Extemat
Beryt ffi Gamet-biotite
granite
and
pegmatite
$ e"t"tit"
pegmatit€
E Petatite
FIc. 1. General geology and location of rare-element mineral samples of the Separation Rapids Pegmatite Group.
Tne SppanerroN
RApTDS
Punox
The 4 km'zSeparation
Rapids
pluton
(2643
+ 2Ma,
unpubl. U/Pb
monazite
age; Y. t-arbi & R.K. Stevenson,
GEOTOP
Laboratory
Universit€ du
Qudbec
d Montrdal)
is a fertile,
peraluminous
S-type
granite
dominated
by
distinct
pegmatitic granite
units
that largely envelop
a
core of coarse-grained,
K-feldspar porphyritic,
garnet
-
muscovite
- biotite
granite.
It is comparable
in size and
constituent
granitic
units to the fertile, peraluminous
Greer Lake
pegmatitic
granite pluton,
situated
55 km
west-northwest
in the Winnipeg River pegmatite
district
of southeast
Manitoba
(eem! et al. 1981
1.
Similarities
include the
presence
of cordierite,
beryl,
cassiterite
and ferrocolumbite,
and the
primary
layering
and mineral
assemblages of the associated
pegmatites.
Notable petrographic
features of the Separation
Rapids
pluton
include:
(1) widespread
layering
of
pegmafidc
leucogranite, sodic
aplite,
potassic pegmatite
and
coarse-grained granitic
units,
(2) beryl
- garnet
-
muscovite
- biotite pseudomorphs
after cordierite
\/, Fe-surle
I Mn-suite
megacrysts,
(3) metasomatic reaction with widespread
amphibolite enclaveso (4) sporadic ferrocolumbite
and beryl in potassic pegmatite units, and gahnite and
Cd-rich sphalerite in a foliated biotite - muscovite -
garnet granite unit, (5) green muscovite - "cleavelandite"
masses
(pods) probably of replacement origin, hosting
manganocolumbite, manganotantalite and alluaudite,
NaCaFe2*(Mn,Fe,Mg)2@Oa)3,
within potassic pegmatite
units. On the basis of K-feldspar compositions and
other geochemical evidence (Fig. I, Table I, columns
1 and 2), Breaks et al. (in prep.) suggest that the
Separation Rapids pluton was derived from
the Treelined Lake granitic complex, which outcrops
immediately to the north.
RecroNar PecMArrrE ZoNATToN
The Separation Rapids Pegmatite Group (Fig. 1)
comprises two distinct pegmatite clusters (eastern
subgroup, 7.5 km2, and southwestern subgroup,
2.5 kmz) that are spatially related to the Separation
612 THE CANADIAN MINERALOGIST
Rapids
pluton
and hosted
by supracrustal rocks of the
Separation
Lake
Metavolcanic Belt (Blackburn
et al.
l992,Blackburn & Young 1994).The belt correlates
with the 2.74 Ga Bird River Greenstone
Belt of
Manitoba (Timmins et al. 1985),
the host of many
rare-element
pegmatites,
including the
Tanco
pegmatite
(eem! et al. l98l). On the basis of a striking similarity
in geological
setting,
mineral assemblages and
em-
placement
age
with the
various
rare-element
pegmatite
groups
commencing 40 km west in Manitoba,
we
consider that the Separation Rapids Pegmatite Group
constitutes the eastern limits of the Cat Lake
- Winnipeg
River Pegmatite Field.
The southeastern margin of the
Separation Rapids
pluton
and the
proximal
eastern subgroup
pegmatites
(Fig.
l; Peg. l, Lou's, Marko's and Audrey's
pegmatites)
lie within the core and along the eastern flanks
of the
west-plunging Separation
Narrows antiform
(Blackbum
& Young 1994). Pegmatites in the northeastern
part
of
this subgroup
(pegmatites
5-10 and James'pegmatite)
are more
deformed
and hosted
within a 100-m-wide
high-strain zone in which the primary structural
conhols on
pegmatite
emplacement
are uncertain.
The
southwestern
pegmatite
subgroup is a more recent
discovery
which comprises five relatively large
petalite
lenses and swarms of much thinner
pegmatites,
mostly
1 to 10
m in tickness. tlat occur en 4chelon
to these
lenses.
Three regional pegmatite zones have been
recognized
(Breaks
1993, Breaks &Tindle 1996). The
flrst is a beryl (-columbite)
zone within the
southern
half of the Separation Rapids
pluton,
beyond
which
there are two zoned
pegmatite
swarms
that respectively
extend east
to northeast and
southwest of the
pluton.
The external
pegmatites
are
defined
by an
outer beryl
(- columbite
- cassiterite) zone that
partially
envelops
a central
petalite
(- beryl
- cassiterite) zone in the case
of the
eastern subgroup, whereas
in the southwestern
subgroup there is a systematic change from a beryl
(- columbite
- cassiterite)
zone
into a narrow
petalite
zone with increasing
distance
from the
parent granite
(Fie.
1).
The internal
beryl
zone
gosrains
sodic pegmatites
in
which green
beryl, ferrocolumbite and cassiterite are
important
accessory
phases.
The external beryl zone
comprises muscovite and biotite-muscovite-bearing
sodic
pegmatites
and related aplites. They have
essen-
tially the
same
mineralogy as the interior
pegmafites,
and the two have therefore been
grouped
together in
Table
I (column
3).
Exceptional pegmatites
within the beryl zone
include
a topaz-rich
pegmatite
@eg.
304)
and a layered,
8-m-thick
fluorapatite
- garnet
- biotite
- muscovite
sodic
pegmatite
(Peg.
265)
containing
Li-mica-bearing
"cleavelandite'o-rich
pods
up to I by 10
m with minor
white beryl, dark green tourmaline,
purple lithian
muscovite, cassiterite,
microlite and
wodginite.
These
pegmatites
(and
Peg. 263) contain no ferrocolumbite,
but instead
contain manganocolumbite
or mangano-
tantalite (or both). Table l, column 5 summarizes the
mineral assemblage
in pegmatite Peg.265.
The petalite zone comprises 19 petalite- and
ferrowodginite-bearing internally zoned pegmatites
approximately I meter in thickness
or greater
(11 in the
eastern subgroup, 7 in the southwestern subgroup),
hosting accessory beryl, cassiterite, microlite,
ferrocolumbite, ferrotantaliteo with rare nigerite,
stibiomicrolite, uranmicrolite and yttropyrochlore
occurring in some samples. In two of these
pegmatites
(Pegs.5 and 7), albite-rich replacement patches
within or cross-cutting large petalite crystals contain
tungsteniferous wodginite (WO: in the range
2.4-17.3 wt Vo). T\e full mineral assemblage
from
these
pegmatites is listed in Table 1, column 4.
One of the petalite-bearing pegmatites from
the eastern subgroup (Marko's pegmatite) differs
from the others in containing manganocolumbite,
manganotantalite and wodginite rather than the
'Terro] varieties
(Table
I, column 6). It is also the largest
of the eastern
subgroup
pegmatites,
at 8 by 130 m,
and contains a well-developed internal zonation of
five primary pegmatite units dominated by a blocky
K-feldspar - petalite core mantled by a wall zone
of beryl + muscovite + albite + quartz. Oxide
minerals include cassiterite, wodginite, microlite
and manganocolumbite - manganotantalite, which
occur in all units, especially where secondary beryl -
muscovite - albite units replace parts of the central
petalite-rich core.
The wall zone of Marko's pegmatite
hosts a unique
mineral assemblage
(Table 1, column 7) headed
by
stibiobetafite, stibiomicrolite and a wide range of
wodginite species. Rarest of these
is a Fe-Ti-rich mineral
described by Ercit et al. (1992) as hypothetical
"ferrotitanowodginiteo'
on the basis of fwo samples too
small to be adequately
characterized.
Titanowodginite,
ferrowodginite and wodginite are also found in the
wall zone of Marko's pegmatite. The Ti-rich wodginite
species have been described previously only from the
Tanco
pegmatite,
Manitoba
@rcitl9S6,Ercit et al. 1992).
Manganocolumbite, manganotantalite, microlite and
striiverite (tantalian rutile) are the other M-Ta-bearing
phases present.
Several large pegmatites
comprise the petalite zone
of the southwestern subgroup and vary in surface
dimensions
from I to 10 meters
to the 60 by 450 meter
Big Whopper Pegmatite, the latter being the largest
complex-type pegmatite yet discovered in Ontario
(Breaks & Tindle 1996,1997).
Disseminated throughout the Separation Rapids
Group pluton and pegmatites is a very minor sulfide
assemblage,
of which ldllingite, arsenopyrite and
Cd-rich sphalerite are perhaps
the most notable.
FeTa206 Fe-Suite
OKDE MINERAI.S, SEPAR.IiTION RAPIDS
MnTa205 FeTarOu Mn-Suite
FeNb2O5 Mn/(Mn+Fe) MnNb2O5
o Cleavelandlte
pods In Separatlon
Raplds Pluton
Beryl Pegmatltes Petallte
Pegmatlte
a Psg.26tl
I Peg.265
* Peg.3oa
Marko'g
O Grey Granl€ & Layersd PegmatlteApllto
- Petallte Core
Zone md quaru-
u mumvltmlbite rspla@mont
unlts
Separation Rapids
it is perhaps
the most
useful monitor
of fractionation
effects. It is widely distributed,
occurs
in all rock
rypes,
and
only the
limi1afi66
of the sampling
method
(rwo polished
thin sections or mineral separates
were examined from each sample) are thought
to have
prevented
it from being observed in all samples. Its
modal abundance
is highest in the distal petalite
pegmatites,
in particular
in the early-crystallized
part
of
Marko's
pegmatite. It usually
crystallizes earlier and
occurs in lesser
amounts than
cassiterite.
In the Separation
Rapids
pluton,
columbite-tantalite
(Fig.
2a, Table 2, compositions I and 2) with primitive
ferrocolumbite
composition
prevail
[0.1
< Mn/(Md +
Fe) < 0.2, 0.1
< Tal(Ta+M)
< 0.3l.They
form small,
subhedral, inclusion-free,
lath-shaped
or tabular crystals
(0.24.6 mm in length)
included
within pale
brown
Li-rich mica
(zimwaldite?)
or occasionally
in zircon
or
quartz.
Crystal size occasionally
reaches
10
mm
across.
In one sample
(96-260)
from the southwestern
margin
of the pluton, muscovite-"cleavelandite"
pods,
containing alluaudite and considered to be of
replacement
origin, host manganocolumbite
and
manganotanralite
(Fig.
2b).
613
MnTa206
fa) lEnotaplolttef
ta-
\_
\*U-trsetrlslt"l
\ ffi' ao
i*L), .qb*
tranganotantaiEsl
ll
z
+
6
E
F
It
z
+
E,
P
* Wallzono
Frc. 2. Covariation of Mn/(Mn + Fe) versus Tal(Ta + Nb) (at.) for (a) ferrocolumbite, ferrotantalite and ferrotapiolite from
Fe-suite granitic pegmatites and the Separation Rapids pluton, and (b) manganocolumbite and manganotantalite from
Mn-suite pegmatites. Solid venical and horizontal lines indicate variation within individual 5amples. Dashed lines join
coexisting ferrotapiolite - ferrotantalite pairs.
FeNb2O5 Mn/(Mn+Fe) MnNb2O6
@ Separatlon Raplds Pluton
BerylPegmatltes PetalltePegmatltes
a PeS.2 + Peg,262 O Lou's A Peg.7
J Peg.15 A Audrsys E Jams' + Peg.g
* Peg.24 * PBg.ge'g * PaS.1 + Psg.lo
X Psg.9&53 + Peg.9e81
O>me Mnvenarocy
Experimental methods
Mineral analyses
were sltained at the Open University
using a Cambridge Instruments Microscan 9 electron
microprobe equipped with rwo wavelength-dispersion
spectrometers. An operating voltage of 20 kV and a
probe current of 30 nA (measured
on a Faraday cage)
were used. Further details of the methods can be found
in Tindle et al. (1998). In addition, back-scattered
electron images and further quantitative data (obtained
under essentially identical conditions) were collected
on a Cameca SX100 microprobe, also at the Open
University. As most of this work was completed before
the discovery of the southwestern pegmatite group,
the majority of the data can be assumed
to be from the
eastern
pegmatite
subgroup unless otherwise specified.
C o
lumbite-tant alite g
roup
Columbite-tantalite is the "classico'
Nb, Ta-bearing
rare-element species in rare-element pegmatites, and at
lTandnoianGiGl
1,4 .
ftr'
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THE CANADIAN MINERALOGIST
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614
The Separation Rapids pegmatites have columbite
and tantalite compositions that define two discrete
groups,
an Fe-sui[e and a Mn-suite. This grouping does
not equate with beryl and petalite pegmatites, as both
types occur in both groups. Indeed, there is no spatial
discrimination befween the groups, and members
from
each suile are mixed together
in the field. Compositions
of ferrocolumbite and ferrotantalite from selected bervl
and petalite pegmatites
of the eastern subgroup
aie
listed in Table 2.
The Fe-suite contains the Separation Rapids pluton,
all the internal beryl pegmatites, all but three of the
eastern subgroup beryl pegmatites and all but one of
the eastern subgroup petalite pegmatites. Apart from
three beryl pegmatites discussed later in the ferrotapiolite
section, columbite - tantalite from the southwestern
subgroup pegmatites are not considered here, but
preliminary results indicate little difference between
the two subgroups, with Fe- and Mn-suites being repre-
sented
in both regions. Tabular or elongate
laths are
most common, with grain size usually in the range
0.054.8 mm, but larger subhedral
to anhedral crystals
(up to 1.5 cm) are also found in both beryl and petalite
pegmatites.
Although most crystals are subhedral,
it is
unusual to find terminated crystals, as most have
rounded ends, perhaps indicating minor resorption
prior to final crystallization. Further evidence for
resorption and crystallization is provided in Figure 3a,
where the complex zonation observed is most likely to
have developed
as a result of these
processes.
Such
pafterns
of zonation can, howeve! be generated
if only
a shallow subsurface of a crystal is revealed durine
sample
preparation
@.
dernf, p'".r. "o.-oo.). Inclusioni
are uncommon, and there is liffle evidence of alteration
other than at the thin rim of some crystals.
Ferrocolumbite - ferrotantalite may cluster with
monazile or cassiterite,
or occur as inclusions in garnet
or cassiterite.
More commonly, it occurs as inclusions
in or around mica (both muscovite and zinnwaldite?),
but its main occurrence
is in the fine-grained albite-rich
parts of the pegmatites, particularly within petalite-
bearing pegmatites.
There is a clear relationship between ferrocolumbite
from the Separation Rapids pluton and pegmatites
from this suite. The internal beryl pegmatites contain
ferrocolumbile (Fig.2a), with compositions
[Mn/(Mn -r
Fe) in the range 0.2-0.31, intermediate between those
of the Separation Rapids pluton and the external
beryl pegmatites [Mn/(Mn + Fe) in the range 0.3-0.4].
The ratio Ta/(Ta + Nb) is more variable; columbite
from the external beryl pegmatites spans the range
0.08-0.48. The petalite pegmatites predominantly
contain ferrotantalite [Mn/(Mn + Fe) in the range
0.15-0.4, Tal(Ta + Nb) in the range 0.5-0.651, but
ferrocolumbite with Tal(Ta + M) values as low as
0.2 also occur in the petalite pegmatite closest to the
Separation Rapids pluton. Individual pegmatites have
limited variation in Mn/(Mn + Fe). but variation in
615
Tal(Ta + Nb) can be more pronounced
and
is responsible
for the vertical evolution trends
on Figure 2a.Hoizontal
variation within samples
on Figure 2a is mainly from
two bodies of beryl pegmatite with low Tal(Ta + Nb)
values (Pegs.
24, and part of 262).
In pegmatite Peg.1, there appear to be two
generations of ferrotantalite, an early generation
occuring as inclusions in cassiterite
(possibly ixiolite
because of unusually high Sc and Ti contents: Table 2,
composition l1)" and a later, euhedral,
partially altered
generation that forms elongate terminated prismatic
crystals
(Fig. 3b, Table 2, composition l0). Another
unusual phase,
found as inclusions in a single large
grain of cassiterite
within Lou's pegmatite
(Fig. 3c,r,
may also be ixiolite on the basis of its high content
of divalent and tetravalent elements (mainly Ti and
Sc; Table 2, composition l2). It plots in the
manganocolumbite field on Figure 2a.
The Mn-suite is defined on the basis of
manganocolumbite - manganotantalite data from
four eastern subgroup pegmatites, an internal beryl
pegmatite (Pe9.263), two external beryl pegmatites
(one proximal, Peg.265, and one distal, Peg. 304, from
the Separation Rapids pluton), one petalite pegmatite
(Marko's pegmatite) and the "cleavelandite" pods at the
margin of the Separation Rapids granite (93-260).
Crystal form is not dissimilar to the Fe-suite, but
there is perhaps a greater
tendency to form euhedral,
terminated crystals, which are both larger (0.2-3 mm)
and modally more abundant.
This is particularly so in
parts of Markoos pegmatite. There is also a tendency
for the manganocolumbite
to be associated
with the
fi ner-grained, quartz-albite 0ocally "cleavelandite")
portions of the pegmatites, although in one sample
from Marko's pegmatite;:manganocolumbite occurs
mainly as inclusions
in spessartine.
In Marko's pegmatite, manganocolumbite is found
in the outeq earlier crystallized parts of the pegmatite,
such as the layered pegmatite-aplite unit, and
compositionally spans a wide range of Mn/(Mn + Fe),
from 0.6 to 0.85, but with a restricted Tal(Ta + Nb)
of 0.2-0.3 (decreasing
to 0.1 in other pegmatites of
the group). Table 2 lists representative data for
manganocolumbite and manganotantalite from this
suite. These samples do not contain wodginite, and
garnet is generally uncommon, although locally
abundant. Pegmatite
304 is unusual
in containing topaz
in addition to manganocolumbite and spessartine.
The
flat trend in Figure 2b generated by these early units
[as a result of varying Mn/(Mn + Fe) at constant
Tal(Ta + M)l changes direction sharply upward in
the later-crystallized core regions of the pegmatites,
comparable to ffends reported from lepidolite
pegmatites
(eernf & Ercit 1985, Spilde & Shearer
D92, eeng & NEmec 1995). Manganotantalite
with extreme compositions occurs here, approaching
end-member manganotantalite
[Mn/(Mn + Fe) = 0.98,
Tal(Ta + Nb) = 0.971, either in "cleavelandite" - Li
Columbite Group
glnlte
Group
Stibiomicrolite
6t6 THE CANADIAN MINERALOGIST
Flc. 3 False-color back-scattered electron images of Nb-Ta oxides from Separation
Rapids. (a) Complex zonation in
ferrocolumbite - ferrotantalite from Lou's (petalite) pegmatite. (b) Partially altered, late-crystallizing ferrotantalite from
petalite pegmatite (Peg. 7). (c) Complexly zoned Sc-rich manganocolumbite (ixiolite?) in cassiterite. From Lou's (petalite)
pegmatite.
(d) Wodginite core patchily zoned
to a "ferrotitanowodginite" composition in the rim. The dark mineral at bottom
right is stibiomicrolite. Sample from the wall zone of Marko's pegmatite.
(e) Oscillatorily zoned
inclusion of titanowodginite
in cassiterite from the wall zone of Marko's pegmatite. Color key applies to Figures 3, 6 and.7.
mica-rich pods within beryl pegmatite or as inclusions
in microlite (also associated with Li-bearing mica)
within petalite pegmatite. Wodginite is found alongside
the manganotantalite in the evolved parts of these
pegmatites,
but spessartine is absent. These samples
have also clearly undergone
postmagmatic
albitization,
and this has
produced
albite-rich veins cross-cutting
petalite, small masses and veins of apatite
- tourmaline
- quartz - albite that replace petalite, and
"cleavelandite" replacing
margins of blocky K-feldspar.
At Marko's pegmatite, more extensive
replacement
occurs
in two stages:
(i) development
of beryl - Li mica
- albite
patches,
and (ii) development
of saccharoidal
albite. The implication of this replacement origin
is important, as the Ta-rich (and Sn-rich) oxide
minerals are strongly concentrated within many
of the albite-replacement
pods and therefore must
also be postmagmatic. A replacement origin for
O)SDE M$IERALS, SEPARIffION RAPIDS
MnTlTa2O6 F6TITa2O6
617
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rcnow6d-si;tte1
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FeSnTa2O6 MnA/Total A slte MnSnTa2O6 FeSnTa2Os MnA/Totat
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B€ry| Pegmathe Petallte Pegmadte
I Peg.
ms Marko'a
^ Petallle CoreZona
andquatE-
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replacament
unlts
tr Wall zone
- Incluslons
In
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- larye
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ffi&d&;
lffiowods-6G-l
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:;ww;-ff,
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(b) lE rowode-fiEl
lT-od:'nFtl
f-
k ***
-#ff&
Frc. 4. Covariation of Mnhotal A siteversusTiltotzl B site for wodginite-group
minerals
from the Separation
Rapids
pluton and
pegmatites.
(a)
Fe-suite and
(b)
Mn-suite. A-site
elements include Mne,
Fez* and
Li; B-site
elements include
Mns, Fe*, Ti,
Sn, Sc, Sb, Bi, Th, U and
Tar.
BerylPogmatltes PetalltePegmatltes
a AudroYs O LorJs + Peg.6
I P€. 15 El Jamas' A Pq.7
* Peg.s + Pes.e
the "cleavelandite
' pods
occurring at the margin of the
Separation
Rapids
pluton
is also
proposed.
Wo dg inite
-
g
ro
up mine ral s
Wodginite-group minerals from the eastern
subgroup
pegmatites
are the subject of a companion
paper
(Tindle
et al. 1998),
and so are not discussed at
length here.
However, important
points
relevant to the
forthcoming
discussion are repeated here for clarity:
(i) A classification diagram for wodginite-group
minerals
(Mn^/total
A site
versus
Ti/total B site) has
been devised akin to the Mn/(Mn + Fe) versus
Tal(Ta + M) classification
diagram used widely for
columbite-tantalite minerals.
The diagram is suitable
for most wodginite-group
minerals
with the
exception
of lithiowodginite.
Data for wodginite-group
mingl4lg
from Separation Rapids are
plotted
on it (Figs.
4a, b)
and indicate that for the Fe-suite of pegmatites,
fenowodginite is the dominant
species. There is also an
increase in Mn ttrat
parallels
that observed in coexisting
ferrocolumbite. For two evolved samples from
pegmatites
5 and7,
this Mn-enrichment
extends
into
the wodginite
field and tentatively offers the possibility
that
the Fe- and
Mn-suites
may
be
part
of a continuum
of compositions.
(ii) Fenowodginite
from individual samples has
a tendency
to vary more in terms
of Til(total B site)
than in terms
of Mno/(total
A site). A similar pattern
was noted for ferrocolumbite and ferrotantalite
from individual sampleso
where appreciably
more
variation
in Tal(Ta
+ M) was measured
compared
to
Mn/(Mn + Fe).
(iii) The Mn-suite pegmatites
show the greatest
variation in compositiono
and
Marko's
pegmatite
alone
contains four wodginite-group
species
(Fig. 4b).
Wodginite
is by far the most
common
species found
in
this suite and reaches
near-end-member
compositions
in pegmatite Peg.265.
A large crystal
(2 mm across)
with complex internal zonation and replacement
varies in composition
from a wodginite core to a
fenowodginite
and
"fenotitanowodginite"
rim @g. 3d).
Other
data for'Terrotitanowodginite"
and titanowodginite
(Fig.3e) were obtained
from euhedral
to anhedral
zoned inclusions
in cassiterite
crystals.
The increased
Fe and
Ti required
to produce
these
latter
wodginite-
group species is not consistent
with processes
of
magmatic
fractionation,
as
these
elements would be
U+Pb
618
ai
=
o
6
o
F
lt
an
o.2 0.3
Tl /Total Bslte
Ftc. 5. Variation in composition ofpyrochlore-group minerals from Separation Rapids. (a) Classification diagram for the
pyrochlore group (Ti-M-Ta) illustrating the tbree main subgroups. (b) Sb-Na-{a variation diagram. (c) U + Pb-Na-Ca
variation diagram. (d) Covariation of Ti,/total B site with Sb/total A site (at.). Open square:
microlite from Mn-suite beryl
pegmatite (Peg. 265). Open triangle: microlite from Fe-suite
petalite pegmatite (Peg. 7). Open circle: antimonian microlite
and stibiomicrolite from Fe-suite
petalite pegmatite (Lou's). Open five-pointed star: antimonian microlite and stibionicrolite
from the wall zone of Mn-suite pegmatite (Marko's). Shaded five-pointed star: microlite - stibiomicrolite - stibiobetafite
from single crystal in the wall zone of Mn-suite pegmatite (Marko's) at the host-rock contact; solid arrows point to later
compositions. Shaded circle: uranmicrolite from Fe-suite
peralite pegmatite (Peg. 5). Shaded square: bismutomicrolite from
Mn-suite beryl pegmatite (Peg. 265). Shaded
cross: yttropyrochlore from Fe-suite petalite pegmatite (Peg. 96-29). Open
cross: yttrian pyrochlore from Fe-suite petalite pegmatite (Peg. 96-29). Dashed arrow shows replacement trend. Open
four-pointed star: stibiomicrolite from the type locality at Varutrask, Sweden (Groat et al. 1987). Solid four-pointed star:
stibiobetafite from the t]?e locality at VEZae,
Czechoslovakia ((en! et aI.1979). For clarity, not all data for microlite are
plotted.
Ta
expected to be depleted early in the pegmatite's
evolution. An influx of these elements from adiacenr
host-rocks
is proposed.
(iv) High but variable levels ofW are recorded in
many ferrowodginite and wodginite crystals from
Separation Rapids. In the Fe-suite, there is an
indication that W increases
with Mno/(total A site),
especially
in the evolved parts
ofpegmatites 5 and 7
(e.9., in Peg. 5, WO3 reaches 17.36 wt.7o).
Marko's
pegmatite is unusual in having low W contents
(ranging
from below detection
to 1.O2wt.Vo WOr), but
very high levels are recorded in the Mn-suite pegmatite
Peg.265, where WO, reaches 34.63 wt.Vo.
High Bi (up
to 2.35 wt.Vo
Bi2O) is also found in wodginite from
this pegmatite. Such "wolframowodginite" contains
sufficient W to be classified
as a new species, but as
with "ferrotitanowodginite" (Ercit et al. 1992), it must
remain hypothetical until sufficient material is found to
charucteize it fully.
Py ro c
hlo re
-
g roup mine ral s
The pyrochlore group consists of three subgroups,
pyrochlore, betafite and microlite (Fig. 5a). Members
of all three subgroups are represented at Separation
Rapids; these include four members of the microlite
subgroup,
microlite (Ca,Na)2TarO6(O,OH,F),
stibiomi-
crolite (Sb,Ca,Na)r(Ta,M)rO6(O,OH,F), uranmicrolite
(d)
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620 T}IE CANADIAN MINERALOGIST
(U,Ca)ITa,M)rO6(O,OH,F) and, bismutomicrolite
(Bi,Ca)2(Ta,Nb)rO6(OH). Antimonian microlite
intermediate in composition between microlite and
srtbiomicrolite also is found. In addition, stibiobetafite
(Sb,Ca)r(Ti,M,Ta)2O6(OH) and yttopyrochlore
(Y,Na,Ca,U)r-r(M,Ta,Ti)2O6(OH) also are found.
These minerals do not occur in the Separation
Rapids
pluton or those pegmatite bodies with ferrocolumbite or
ferrotantalite whose Mn/(Mn + Fe) values are less than
0.3, and their occurrence does not correlate with the
presence
of either beryl or petalite. Of all the microlite
subgroup specieso
microlite is the one found most
commonly,
such
as in a Fe-suite
petalite pegmatite
(Peg.
7)
and two bodies of Mn-suite
pegmatite
(especially
Marko's,
but also beryl pegmatite Peg.265). Stibiornicrolite is a
very rzue
mineral found in a Fe-suite
petalite pegmatite
(Lou's) and in the wall zone of Marko's pegmatite.
Bismutomicrolite is only found in a Mn-suite beryl
pegmatite Peg.265, whereas uranmicrolite and
yttropyrochlore rue restricted to Fe-suite petalite
pegmatite Peg. 5 and the southwestern
subgroup "Big
Whopper" pegmatite (Peg. 96-29), respectively.
Stibiobetafite has the most restricted occurrence of all,
being found in the wall zone of Marko's pegmatite at
the acfual contact with its host rock, a metasomatized
garnet - biotite - plagioclase
metavolc4nic rock. Table
3 lists representative
compositions of pyrochlore-group
minerals from Separation Rapids, with structural
formulae calculated on the basis
of the B site summins
to 2.00 atoms per formula unit.
Microlite is a strongly associated with Li-bearing
mica (zinnwaldite?), with mostmicrolite crystals being
included within, in contact with, or adjacent to such
mica. Microlite typically forms mottled, irregularly
shaped, inclusion-rich crystals, which partially or
totally replace earlier ferrowodginite, or wodginite
(rarely manganocolumbite).
In Marko's pegmatite, the
presence of microlite after wodginite (Fig. 6a) and
small grains of inclusion-free cassiteritq
growing from
within the microlite outward beyond the original
margin of the wodginite suggests
a reaction involving
the following exchange:
wodginite + Ca,Na,F-rich fluid -+ cassiterite
MnSnTa2Os SnO,
+ microlite
(Ca,Na)2TarO6F
Evidence that the Ca, Na and F components of this
fluid became sufficiently concentrated to react late in
the crystallization sequence is provided in Figure 6b,
where part of a euhedral crystal of wodginite, jutting
out fiom the edge of a large grain of Li-bearing mica
(zinnwaldite?), has been totally replaced,by microlite
prior to being partially included within albite. One
sample from a muscovite-albite replacement unit within
the petalite-rich core zone of Marko's pegmatite hosts
abundantmicrolite with a particularly complex history.
Large anhedral crystals of microlite up to 2.5 mm in
length are found containing elongate inclusions of
manganotantalite (the most manganiferous from
this pegmatite) rimmed by a ferrocolumbite alteration
rim (Fig. 6c). Such microlite crystals often contain
numerous oriented inclusions of ferrocolumbite that
fonn fan-shaped patterns or a symplectitic texture.
Certain zones of these crystals are either porous
and rich in inclusions, or non-porous, with larger,
more discrete inclusions. Most inclusions show some
oscillatory zoning.
Whereas microlite is closely associated with a
Li-rich mica, stibiomicrolite and antimonian microlite
tend to occur associated
with cassiterite and wodginite.
Stibiomicrolite is pale yellow and usually found
as inclusions in, or in contact with, cassiterite,
Exceptions include a stibiomicrolite at the margin of
a "ferrotitanowodginiteo' crystal and a stibiomicrolite
partially replacing a wodginite crystal included in
quartz. A stibiomicrolite inclusion in cassiterite from
Ftc. 6. False-color back-scattered electron images of pyrochlore-group minerals from Separation Rapids. (a) Microlite
(yellow) after wodginite (green) from the albite replacernent unit of Marko's pegmatite. Note the small crystals of cassiterite
(red) growing out from within the microlite and the patchily zoned ferrocolumbite (blue) inclusions in the microlite. (b)
Subhedral wodginite jutting out from the edge of a large grain of lithium-rich mica (zinnwaldite?), in part replaced by
microlile prior to being partially included within untwinned plagioclase.
Sample taken from a muscovite-albite replacement
unit within the petalite core-zone of Marko's pegmatite. (c) Large grain of microlite (yellow) containing an elongate
inclusion of manganotantalite (pale blue) rimmed by and including numerous oriented inclusions of ferrocolumbite
(ixiolite?) (blue). A euhedral crystal of cassiterite (red) grows from its margin. Sample taken from a muscovite-albite
replacement unit within the petalite core-zone of Marko's pegmatite. (d) "Ferrotitanowodginite" inclusion (center) in
cassiterite partially replaced by stibiomicrolite (purple and zoned in parts), which itself is partially replaced by microlite
(yellow region, top right). From the wall zone of Marko's pegmatite. (e) Zoned antirnonian microlite growing along margin
of large grain of cassiterite. The cassiterite hosts many inclusions of titanowodginite (two at bottom right) and
"ferrotitanowodginite". From the wall zone of Marko's pegmatite. (f) Ferrocolumbite - manganocolumbite replaced in
a three-step process starting with microlite, then stibiomicrolite and, finally, stibiobetafite (pink). From the wall zone of
Marko's pegmatite. (g) Euhedral inclusion of uranmicrolite in large grain of fenowodginite from petalite pegmatite
(Peg. 5). (h) Replacement
of tungsteniferous wodginite by bismutomicrolite in beryl pegmatite Peg. 265. For color key, see
FiE. 3.
OXIDE
MINERALS SEPARATION
RAPIDS 621
622 TI{E CANADIAN MINERALOGIST
the wall zone of Marko's pegmatite
clearly replaces
'Terrotitanowodginite" (Frg. 6d), but in another sample
from the same unit, a large (0.5 mm), oscillatorily
zoned
antimonian microlite crystal has grown against
an earlier cassiterite
containing titanowodginite and
"ferrotitanowodginite" inclusions, and thus is most
likely primary fig. 6e). This crystal has an Sb-rich
core (9.99 wt.Vo
Sb2Or),
and the Sb content decreases
outward to antimonian microlite compositions with
3.6wt.Vo
SbrO3
(Table
3, compositions
i and 8). Other
crystals
belonging to the microlile subgroup
have much
higher Sb contents (e.g., Marko's pegmatite, with
20.91 wt.Vo
Sb2O3;
Table 3, anal. 10), which exceeds
the necessary
2O at.Vo Sb in the A site required to name
the mineral as lirue stibiomicrolite Qlogurh 1977
, Groat
et al. 1987).In the Fe-suire petalite pegmatite (Lou's),
oscillatorily zoned inclusions of stibiomicrolite in
cassiterite
are nearly euhedral,
with a form reminiscent
of wodginite (see
Dunn et al. 1978,
Fig. 5). These too
are most likely primary occurrences.
Microlite-subgroup minerals show liftle systematic
variation within an individual pegmatite, but there are
differences among pegmatites (Table 3); however, the
wall zone of Marko's pegmatite
hosts
a suite with com-
positions
ranging ftom microlite (0.2 - 4.6 wt.Vo
Sb2O3)
to antirnonian
microlite (3.8 - 10.5 wt.7o
Sb2O3)
and
stibiomicrolite (9.9 - L2.7 wt.Va
Sb2O).It is apparent
from these
data and,
to a lesser
extent,
those from Louos
pegmatite, that there is a continuum of compositions
from microlite to stibiomicrolite (Figs. 5b, d) and that
where the Ti content is high, so too is the Sb content.
This systematic behavior is more complex than first
appears, as petrographic observations indicate that
stibiomicrolite can crystallize earlier (Fig. 6e) or later
(Fig. 6f) than microlite, and further suggest thar
the initial concentration of Ti and Sb is more related
to the local availability ofthese elements rather than
simply to magmatic fractionation processes.
- (Jranmicrolitehas been observed
in two samples
from a petalite pegmatite (Peg. 5), where it occurs as
small (approximately 50 pm), weakly zoned
iaclusions
in ferrowodginite (Fig.69). The pegmatires are
assumed
to belong to the Fe-suite,
although columbite-
tantalite was not found in either sample.
Uranmicrolite
has a euhedral form, is yellowish brown in color, and
appears
to be an early-crystallizing primary phase.
The
low analytical total (Table 3, composition 12; see also
Fig. 69) is considered to indicate a high H2O content.
Petrographic evidence in the form of radial fractures
typical of radiation damage due to volume expansion
indicates that the crystal is metamict or nearly so. An
alpha-decay
dose of approximately 1.1 x 10rE
o/mg
(based on equation I of Lumpkin et al. 7994) is
consistent
with this interpretation.
Because
of their
rarity, little can be said about how uranmicrolite (or
bismutomicrolite) are related to other microlite-subgroup
minerals
(Fig. 5c).
Bismutomicrolite is ararc mineral that occurs
in the
most evolvedo
Mn-suite beryl pegmatite @eg. 265) in
association
with large pods enriched in "cleavelandite"
and Li-rich mica. It forms as a product of the late-stage
alteration of tungsteniferous wodginite, where it
selectively replaces zone boundaries
and individual
zones of the wodginite. The mineral has a mottled
appearance and is dissected
by many cracks @g. 6h).
Although the mineral contains much less U than
uranmicrolite (Table 3; composition 13), it appears
to
be affected by metamictization. Yeto calculations of
alpha-decay
(approdmately 1.8 x lOtT almg based
on
equation 1 of Lumpkin et al. 1994) indicate only
moderate damage. The very low analytical total is
considered
to reflect a high H2O content. Because
the
major A-site cations, Bi and Fe are not plotted on
Figure 5c, the bismutotnicrolite data plot at anoma-
Iously high U + Pb levels on this diagram. Table 3,
composition l3 shows that the elements
U, Pb, Na and
Ca are all in low abundance.
Stibiobetafite has the highest Sb concentrations of
all the pyrochlore-group minerals found at Separation
Rapids (20.9-25.8 wt.Vo Sb2O3),
but is only found in
the wall zone of Marko's pegmatite.
It occurs as a
diffuse inclusion in a large crystal of wodginite that
exhibits complex internal zonation or replacement
and whose composition varies from a wodginite core
to a ferrowodginite and "ferrotitanowodginite" rim.
Figure 6f illusrabs a second occurrence
of srtbiobetafite
in which crystallization of the pyrochlore-group
mineral appears to have been a three-step process.
Manganocolumbite has been replaced by microlite,
which in turn is replacedby stibiomicrolite.The flnal
step of the process
is the crystallization of oscillatorily
zoned stibiobetafite on the stibiornicrolite. Perhaps
because of a change in volume as manganocolumbite
was replaced by microlite or because some
stibiomicrolite
was dissolved
away frst, much of the sri&iobetafite
was
able to grow in open spaces
to form subhedral crystals.
The change in composition of the pyrochlore-group
minerals in this crystal are shown by an arrow in
Figures 5a, b and d.
As with the transformation of wodginite to microlite,
the sequence of reactions leading to crystallization of
stibiobetafite requires
an initial influx of Ca, Na and F,
which is followed later by an influx or exchange with a
source
rich in Ti and Sb:
manganocolumbite + Ca,Na,F-rich fluid
(Mn,Fe)(M,Ta)rOu -+ microlite
(Ca,Na)lTa,Nb)rO6(OH,D
+ (Fe+Mn) (2)
microlite + Ti, Sb source
-+ stibiomicrolite
(Ca,Na)(Ta,Nb)rOu(F) (Sb,Ca)r(Ta,M,T02O6(OH,D
+ (Ca,Na,F) (3)
OXIDE MINERAI-S, SEPARAI'ION RAPIDS 623
stibiomicrolite + Ti, Sb source
(Sb,Ca)zGa,Nb,Ti)206(OH,F)
-+ srtbiobetafite
(Sb,Ca)(Ti,M,Ta)106(OFT;
+ (Ca,Na,F) (4)
A (Ca,Na,F)-bearing
fluid is a likely constituent
contributing to albitization, which probably developed
as the pegmatite-forming melt evolved and conditions
changed
from magmatic to hydrothennal. However, as
the stibiobetafite (only?) occurs
at the actual
margin of
the wall zone of Marko's pegmatite,
it is reasonable
to
assume the Ti and Sb came from the host rocks.
Ynropyrochlnre
has
been
found in only one sample,
a petalite pegmatite (96-29) from the southwestern
subgroup.
A l50-pm-long relict core of yttropyrochlore
composition (20.92 wt.Vo
Y2O3) is mantled by an
alteration rim with a much lower yttrium content
(5.15 wt.% YrO,) and high Si content (4.67 ut.Vo SiO2)
(Table 3, compositions 15 and 16, and Fig. 7a).The
source of the Y is unknown, but possibilities include its
concentration during rnagmatic fractionation to form
primary yttropyrochlore or crystallization of xenotime
from the pegmatite-fonning melt, later to be dissolved
by albitizing fluids, with the mobilizedY then available
to form yttropyrochlore. Further discoveries are
required before this can be unraveled.
Fenotapiolite
Ferrotapiolite, Fe(T4M)2Ou, is an uncommon oxide
that is spatially restricted to a few Fe-suite beryl
pegmatites of the southwestern subgroup. Influx of
fluids from these
pegmatites
into a small area
(200 m'z)
led to metasomatic alteration of metavolcanic host-rocks,
resulting in elevated Li contents, up to 245 ppm,
compared
to "background" levels of 16
ppm @reaks &
Tindle 1997). Ferrotapiolite was first found in a sodic
pegmatite dyke (Peg. 96-53) as a 30-pm inclusion
associateid
with ferrotantalite (Fig. 7b), both within a
larger grain of cassiterite.
Ferrotapiolite from a second
sodic pegmatite dyke (Peg. 96-9) appears to replace
earlier-formed patchily zoned ferrotantalite (Fig. 7c),
and in a third occurrence from a nrurow, deformed
garnet-albite pegmatite (Peg. 96-8 1
), ferrotapiolite
forms large clusters of grains (600 pm across) intimately
associated with, but later than, ferrotantalite. Table 4
lists analytical data from three coexisting pairs from
the above samples.
These coexisting pairs are similar
' in composition to those from a pegmatite of the
beryl--columbite subtype Ilom Spittal a.d. Drau, Austria
(Cem! et al. 1989), but crossing tie-lines and compo-
sitions within the two-phase field (Fig. 2a) indicate
localized disequilibrilm crystallization and possibly
metastable conditions
(Cern! et al. 1992).
Ferrotapiolite
from Separation Rapids can also contain higher
Ievels of Sc (up to 0.72 wt.Va
Sc2O3) and W (up to
0.61 wt.Vo WOr) than the compiled worldwide dataset
reported
by Wise &Cem! (1996).
Ferrotapiolite has not been found in the eastern
subgroup pegmatites. However, a ferrotapiolite-like
phase occurs in one part of the wall zone of Marko's
pegmatite.
Its unusual composition is attributable to
partial replacement of ferrotapiolite along cleavage
planes by microlite (Fig. 7d, Table 4, compositions
7 and 8). Only Pough (1945) and Eid & von Knorring
(1976) appear to have reported
this process
previously.
As with replacement of wodginite and mangano-
columbite by microlite described aboveo
replacement of
ferrotapiolite by microlite requires interaction with a
(Ca,Na,F)-rich fluid.
Striiverite
This rare mineralo containing 23.8542.49 wt.Vo
TarO5
(Iable 4, compositions 3-5), has
been found only
in the wall zone of Marko's pegmatite. It occurs as a
0.3-mm anhedral inclusion in coarse-grained albite,
where it appears to be replacing manganotantalite.
In a second
occunence, a titanowodginite inclusion
in a large grain of cassiterite that also hosts
"ferrotitanowodginite" inclusions has been
partially
replaced by patchily zoned striiverite. The striiverite
has continued growing out from the margin of the
cassiterite and has
developed both a euhedral habit and
well-defined oscillatory zonation @ig. 7e).
Cassiterite
Cassiterite is common at Separation Rapids,
occurring in all types of granitic pegmatite. Although
its modal abundance is quite variable, it is in many
cases tlte most common oxide present.
In much of the
Separation Rapids pluton, it is either absent or rare,
although crystals up to 3 mm across are found in the
more evolved part of the pluton. The beryl-bearing
pegmatites contain relatively little or no cassiterite,
the exception
being
pegmatitePeg.265, which shows
other mineralogical differences with most of the other
beryl pegmatites (Table 1, column 5). Cassiterite in
pegmatites from the southwestern subgroup is not
discussed here.
It is in the petalite-bearing pegmatites that cassiterite
occurs in the greatest abundance. Pegmatite 7 is
particularly enriched, with visual estimates of modal
abundance
reaching 3-5Vo in zones rich in fluorapatite
and zircon. Noteable enrichment also occurs in both
Lou's and Marko's pegmatites.
However, no cassiterite
has been found in pegmatites I or 10, and abundances
are quite low in pegmatites 6, 8, 9 and I 1.
Cassiterite commonly forms euhedral, diamond-
shaped crystals up to 1.2cm in length (but more
commonly
0.2-0.4 mm in pegmatites
6, 8, 9, and 11),
and exhibits a wide variety of colors in thin section,
from dark brown (almost black) through deep reddish
624 THE
CANADIAN MINERALOGIST
OXIDE MINERALS. SEPARATION RAPIDS
TABLE 4 FERROTAPIOLITE.FERROTANTALI]T
PAIPS AND RELATED MINERAT.s FROM SEPARATION
RAPIDS
Fe hnhlite
Ferrctapiolite
96^53
12
0.03 0.00
13.13 14.38
3.60 0.09
0.74 0.69
27.49 822
54.71 74.61
0.53 0.99
0.14 022
0 08 0.00
0.00 0.00
0.00 0 00
0.00 0.00
0.00 0.00
0.00 0.M
100.44 99.23
No. of oxygens 6 6
Ca 0.002 0.000
Fe 0.784 0.E73
Mn 0.2i8 0.005
Ti 0.040 0.M2
Nb 0.888 0.301
Ta 1.053 1.642
Sn 0 015 0.032
w 0.003 0 005
Pb 0.002 0.000
Th 0000 0000
u 0.000 0 000
sb 0.000 0 000
Bi 0.000 0.000
Sc 0 000 0.003
Tohl 3014 3AO2
Commenb: both are inclusions
in ca$iFriG
F 0.000 0.700
Tobl 3.946 4.739
Fomulas baFd on sum B site = 2.00
625
CaO
MnO
Ti02
Nb20s
Sn02
w03
Pbo
Th02
uo2
sb203
8i203
Sc203
Total
Fe bnhlibe
Ferotapiolite
96-9
34
0.00 0.03
'13.87 14.48
1.01 0.02
1.71 3.40
25.32 9
60
ss.21 70.83
1.1s 0.54
0
49 0.36
0.15 0.00
0
00 0.00
0.00 0.02
0 00 0,00
000 006
o25 0.08
99.'t7 99.41
0.000 0.002
0.840 a.942
4.062 0.001
0.093 0.199
0.829 0338
1.087 t.499
0.033 0.017
0.009 0.oo7
0.003 0.000
0.000 0.000
0.000 0.000
0.000 0.000
0.000 0.001
0.015 0.005
2.973 3.012
fomed formed
early later
Fe tantalire
Ferrolapiolite
9641
56
0 00 0.00
12.96 13.57
1.69 0 24
0.11 0.00
18.97 472
6471 79.02
0.56 1.12
0.60 0.09
o.2a 0.62
0.00 0.00
0.00 0.00
0.20 0.20
0.44 0.00
0.00 0.06
100.52 99.65
0.000 0 000
0822 0947
0.109 1av
0.006 0.000
0.450 0.178
1.334 1.792
0.417 0.037
0012 0 002
0.006 0.014
0.000 0-000
0.000 0.000
0.006 0.007
0 009 0 000
0.000 0.004
2.970 2.998
formed formed
eariy lahr
FerotapioliF-like
Microlitelike
Marko s
78
Na2O 0.47 3 6t)
CaO 155 1067
412c)3 0.00 0.00
FeO 15.39 1.10
MnO 0.03 0.05
T'O2 0.00 0.00
Nb205 0.90 085
Ta2O5 70.60 74.11
SnO2 I 04 1.02
wo3 0.36 0.27
Pbo 0.06 0.12
Tho2 0.05 002
uo2 0 99 1.02
sb203 0E0 061
Bi203 0.o7 0 04
Sc2O3 0.87 0.47
Cs2O O.07 0.08
BaO 432 0.05
5io2 2.60 1.41
F 0.00 2.33
subrotal 96.18 97.41
o=F 0.00 0.9E
Toral 96.18 96.43
Ti 0 000 0.000
Nb 0.M1 0.036
Ta 1 909 1.918
w 0.009 a.007
Sn 0.041 0.039
Al 0.000 0.000
.Fe 1.280 0.084
Mn 0.002 0 004
Ca 0.165 1.088
Na 0.091 0.66
Ba 0.012 0 002
Pb 0002 0003
Th 0.001 0.000
u 0.022 0.0D
sb 0.033 a.024
Bi 0.002 0.001
Cs 0 003 0.003
5c 0.075 0.006
si 0.258 0.135
Ftc. 7. False-color back-scattered electron images of oxide minerals from Separation Rapids. (a) Yttropyrochlore (orange)
replaced by yttrian pyrochlore (brownish orange). From petalite pegmatite
Peg.96-29. (b) Ferrotapiolite (cyan) and
ferrotantalite
(blue) inclusions in cassiterite. From beryl pegmatite
96-53. (c) Ferrotapiolite along margins
ofearlier-
crystallized ferrotantalite. From beryl pegmatite Peg. 9G9. (d) "Fenotapiolite" replaced along cleavages and fractues by
"microlite". From Marko's pegmatite. (e) Patchy zoned striiverite replacing an inclusion of titanowodginite in cassiterite in
the wall zone of Marko's pegmatite. Continued crystallization has produced oscil'latorily zoned strtiverite that grows out
from the cassiterite margin. The dark region on the right is quartz. (f) Euhedral grain of cassiterite containing
ferrocolumbite- and pore-rich zones alternating with inclusion-free zones criss-crossed with needle-shaped
products of
exsolution (rutile?). From beryl pegmatite Peg. 15. (g) Cuspate
gahnite (mid-green) partially replaced
by zinnwaldite? and
cut by later veinlets of (red) secondary Fe-oxides (Separation
Rapids pluton, 94-29). (h) Elongate tabular crystals of nigerite
(pale brown) showing minor zonation, inclusions of cassiterite, and evidence of later deformation that has broken up and
bent some of the crystals. The isolated blue crystal is ferrotantalite. From petalite pegmatite Peg. 10. For other colors, see
key to Fig. 3.
626 THE CANADIAN MINERALOGIST
FeO
MnO
Tio2
Nb2o.5
Sn02
wo3
Pbo
Th02
uo2
Sc2O3
Total
No. of
oxygens
brown, medium brown, pale brown, to a straw-yellow
brown. In some samples,
all these
colors occur in a
single crystal in a series of variably spaced
concentric
zones.
Cassiterite
from the Separation
Rapids
pluton
and Fe-suite beryl pegmatites is relatively free of
inclusions, but in the Fe-suite petalite-bearing
pegmatites,
it is not unusual
for the darker core-regions
to be relatively inclusion-free and for a later, lighter
brown zone to contain trains of small inclusions of
ferrocolumbite near the boundary between the two
zones.
Where multiple zones
occur (such as in beryl
pegmatite Peg. 15), there is alternation between
ferrocolumbite- and pore-rich zones
and other zones
lacking inclusions but full of dark, needle-shaped
products of exsolution (possibly rutile?). Figure 7f
illustrates such an example, in which larger inclusions
stand out from the edges of the crystal, having possibly
been preferentially left behind after an episode of
dissolution.
Cassiterite in the petalite-bearing pegmatites
of the Fe-suite hosts inclusions of ferrocolumbite,
ferrowodginite, Sc-rich manganocolumbite
(or possibly
ixiolite?: Fig. 3c), stibiomicrolite, rare ltillingite and
uraninite; usually it is one of the last, if not the last
oxide mineral, to crystallize. In pegmatite 7, microlite
and a second generation of ferrotantalite (slightly
more Mn-rich than the first) seem to be later. In
the wall zone of Marko's pegmatite (Mn-suite),
cassiterite
hosts inclusions of titanowodeinite and
1
7.A5
0.07
58.28
7.V
1.40
0.v2
0.03
0.02
0.09
0.u
q9.42
44
0.218 0.20'l
0.002 0.035
1.455 1.A1
0.117 0.070
0.215 0.283
0.019 0.020
0.000 0.000
0.000 0.000
0.000 0.000
0.001 0.000
0.001
2.027 2.030
Fe
Mn
Ti
M
Ta
Sn
Pb
Th
U
Total
TABLE 5, STRUVERITE FROM SEPARATION RAPIDS
Mn€uib
WaIl Zone of Mdko's Pegmatibe
"ferroti tanowodgi nite". Elsewhere in Marko's pegmatite,
cassiterite is inclusion-free and forms euhedral crystals
commonly associated
with manganocolumbite and, less
commonly, microlite.
Overall, cassiterite has a limited range of
compositions, and tlere is considerable overlap in FeO,
MnO, MrO5 and Ta2O5 contents among rock types
(Table 5). Of these elements, Ta concentrations are
highest and range from 0.6 to 1.6 wt.Vo Ta2O5 in the
pluton, 0.5 to 2.'7
wt.clo
in the bery1
pegmatites,
and 0.3
to 4.4 wt.Vo
in most of the petalite pegmatites.
Marko's
pegmatite
has a slightly wider range,
0.2-6.2 wt.Vo,
whereas
pegmatite
Peg. 6 has Ta values
up to 8.6 wt.Vo
Ta2O5.
Chemical composition seems related to the
color of the cassiterite;
in the petalite pegmatites,
the
darker zones typically have Nb2O5 + Ta2O5 contents of
4.5 wt.%, whereas
in the lighter zones,
they are nearer
7.5 wt.Vo. There is no systematic
variation in cassiterite
composition among rock types, but all the data fall along
a linear array,
with M + Ta and Fe + Mn predominantly
varying in the ratio of 2:
I corresponding to the coupled
substitution: 3 Sna* <+ 2(Nb,Ta)s*
+ (Fe,Mn)2*.
This
relationship is common to cassiterite from many
rare-element granites and pegmatites worldwide,
and distinctly different to those from epithermal and
hydrothermal vein deposits (Fig. 8). In addition,
cassiterite. compositions
approaching the I :
1 substitution
line suggest the presence
of a Fe3t(M,Ta)Oo
component
in some of the cassiterite, consistent with the substitu-
tion: 2Sne = Fek + Tas*
(eernf & Ercit 1989).
Ilmenite
The occurrence of ilmenite is restricted to the
non-pegmatitic
part of the Separation Rapids
pluton,
where it is rarely found in foliated muscovite - biotite
- (garnet) granodiorite, associated with chloritized
biotite. Columbite-tantalite-group minerals are,
in fact,
much more common throughout the Separation Rapids
pegmatite field.
Scheelite
Scheelite is found only in one petalite pegmatite
(Peg.
7), where it forms very rare 0.1 mm anhedral
crystals
associated
with cassiterite. A high tungsten-
carrying capacity in the pegmatite-forming melt or
fluid of this and two other petalite pegmatites
(Pegs.
5
and 6) is also indicated by the presence
of associated
tungsteniferous columbite or wodginite (or both).
Uraninite
Geiger counter tests indicate that pegmatite Peg. 7,
and
in particular
its cassiterite-rich zones, is significantly
more radioactive than the other pegmatites. Only in this
pegmatite and Peg. 265 has uraninite been positively
2
7.07
1.2'I
55.51
453
1.47
0.01
o.o2
0.01
0.02
100.40
5.23
u.39
8.80
42.49
0.00
0.t2
0.00
0.00
99.a\
0.172
0.173
1.011
0.452
0.67
0.000
0.000
0.000
0.000
2-021
Cassiterlte
from
Separation Rapids
pluton and pogmatites
,/
1:1
OXIDE MINERALS. SEPARAIION RAPIDS 627
0.'16
o.14
identified. In pegmatite Peg.265, it occurs as
euhedral
inclusions
in manganocolumbite,
whereas in Peg. 7, it
occurs as inclusions in cassiterite,
where it has caused
major radiation-induced damage
resulting in radial
fracnring and
the development
of a uraniferous
dift:sion
front. Uraninite may have crystallized in other
pegmatites,
for relict U-Th-Pb (+ M-Ta)-rich phases
are observed, but these are so affected by
metamictization that conclusive identification is
impossible.
fi o.rz
x
o 0.10
$
o o.o8
! u.ub
c
E o.oa
o
L o.oz
| 0.04 0.06 0.08 0.10 0.12
Nb + Ta
( based on
4 orygens)
Frc. 8. Covariation of Nb + Ta with Fe + Mn in cassiterite
from the Separation Rapids pluton and associated bodies
of granitic pegmatite. The rare-element
pegmatite field is
drawn using data from Beauvoir, France ftVang el a/.
1988), Jihalva, Czech
Republic (demf & Nbmec 1995),
Pyrenees, Spain (Abella et al. 7995), Ririwai, Nigeria
(lxer er al. 1987), Sinceni, Swaziland (Trumbull 1995,,
Rondonia, Brazil (Oen et al.1982), Greer Lake, Manitoba
((em! et aI. 1
986), Tanco,Jvlanitoba
(dernf et al. 7981,,
Peerless, South Dakota (Cern! et al. 1985), and Bob
Ingersoll, South Dakota (Spilde & Shearer 1992).The
epithermal and hydrothermal field uses data from Cirotan,
West Java, Indonesia (Marcoux et al. 1993), Rooiberg,
South Africa (Ollila 1986) and Cornwall, England (Moore
& Howie 1979).
- 0.4
-
0.1
0.0 0.1 0.2 0.3 0.4
(Fe+Mg)
/ Al
Frc. 9. Covariation of (Fe + Mg)/Al with (Zn + Mn)/Al
(atomic ratios) for gahnite from Separation Rapids ploned
with igneous and metamorphic gahnite from worldwide
localities. Worldwide data from Ontario (Spry 1982),
Namaqualand (Moore & Reid 1989), Australia (Parr
1992),Glen Muick, Scotland
(Goodman 1993), Poland
(Cook & Dudek 1994), in addition to the compilation of
Batchelor & Kinnaird (1984).
TABLE 6. CASSITERITE FROM SEPARATION RAPIDS
FeO
MnO
Nb205
Te2O5
Sn02
Total
Sepmtion FFSuite B€ryI
RapidsPluton Pegmatiies
91260 Peg. 15 Audrey's
1,23
0.52 0.19 0.57
0.05 0.04 0.97
0.75 0.03 0.21
0.98 0s7 1,.48
97.10 99.20 97.97
99.n 100.03 100.30
0.008 0.024
0.002 0.003
0.001 0.005
0.008 0.020
1.985 1.955
2.0m 2.007
FeSuite
Petalite Pegmtites
Lou's Jmes' Peg. 6
456
023 0.51 "t.75
0.03 0.08 0.07
0.09 0.16 1,.19
1.00 2.33 8.6E
98.64 96.52 88.07
99.99 99.60 99.74
444
0.009 0.022 0.073
0.001 0.003 0.003
0.002 0.004 0.027
0.014 0.0u 0.120
1.975 1.943 1.778
0.026 0.061 0223
Mn-suite Beryl
Pegmatites
Peg.265
78
0.11 0.71
0.10 0.M
0.06 0.96
1.18 2.85
9E.23 95.08
99.68 99.64
44
0.005 0.030
0.004 0.002
0.001 0.022
0.016 0.039
1.974 1.908
0.026 0.@2
Mn-suite Petalite
Pegmatites
Mako's
--9 10
0.13 0.60
0.03 0.20
Q.07 0.12
0.8s 4.39
98.76 94-24
99.M 99.55
44
0.005 0.025
0.001 0.009
0.002 0.003
0.0^12 0.060
1.980 l.9M
0.020 0.097
Wall Zone
N4I\91Pegisle
' Marko's
1-! 12
0.06 0.21
0.04 0.11
0.00 0.40
o.37 1.37
9.67 97.96
100.14 r00.05
44
0.003 0.009
0.002 0.005
0.000 0.009
0.005 0.019
1.992 1.959
0.009 0.oll
No. of orygem 4 4 4
Fe
Mn
Nb
Sn
Total
0.o22
0.002
0.017
0.013
1.950
0.055
628
Gahnite
THE CANADIAN MINERALOGIST
Gahnite, ZnAl2Oo, is a widespread accessory
mineral in rare-element pegmatites
including Greer
Lake, Manitoba (Cernf et al. 1986), central Nigeria
(Batchelor
& Kinnaird 1984), Greenbushes,
Western
Australia (Palache
et al. 1944), Preissac
- Lacorne,
Quebec (M,ulja er al. 1995), northern Moravia, Czech
Republic (Cem! et al. 1992) and the Pyrenees, norrhern
Spain (Abella er al. 1995). It is also reported from a
peraluminous
ganite in New Zealand
(Tulloch 19817.
Batchelor & Kinnaird (1984) noted differences
in com-
position
between
such examples
ofgahnite and those
with a metamorphic origin. The mineral may therefore
be an imponant indicator of hidden rare-element
pegmatites
in areas of glaciated (or other) terranes
where exposure
is limited.
At Separation
Rapids,
gahnite (Table
6, compositions
1 and 2) is found in the
Treelined
Lake granitic complex
and the Separation
Rapids
pluton, where it occurs as
small (0.5-1.5 mm), bright green,
subrounded
crysrals,
commonly associated with mica (muscovite or
zinnwaldite? or both) and in some cases
accompanied
by Cd-rich sphalerite
(3 wt.Vo Cd). A few crystals show
replacement
textures, being totally surrounded and
partially replaced by muscovite, or having a cuspate
margin (Fig. 7g). There is some evidence that gahnite
originally formed larger grains (4-5 mm) and that
TABLE 7, GAHNTIE AND MGERITE FROM SEPARATION RAPIDS
Zn+Mn
FIG. 10. Triangular diagram showing the composition
of nigerite from Separation Rapids,
plotted in terms of
Zn+Mn - Mg - Fe with igneous and metamorphic nigerite
from worldwide localities. Worldwide data from Nigeria
(Bannister & Stadler 1947), Siberia (Ginsburg er a/.
196l), Portugal (van Tassel
1965),Drazil (Kloosterman
1974),
Finland (Burke et al. 1977),Czechoslovakia (dech
et al. 1978), Australia (Grey & Gatehouse 1979), Ontario
(Spry 1982,
Petersen
1986),
Sweden
(Schumacher
el a/.
1987) and Tsomtsaub, Omaruru River, Namibia (Tindle,
unpubl. data).
deformation resulted ih break-up of the crystals with
growrh of a lithiumTticf mica (zinnwaldite?) in the
resulting pressure:slldows. Perhaps surprisingly,
gahnite
has not beegdbserved in any of the pegmatites.
Gahnite from the Separation
Rapids pluton (Fig. 9) is
Fe-rich and falls within the isneous field of Batchelor
& Kinnaird (1984).
Nigerite
The rare mineral nigerite (Zn,Fe2*)(Sn,Zn)2
(Al,Feh),rO22(OH), (611
polymorph) was tust described
by Jacobson & Webb (1947) trom quartz-sillimanite
rocks associated
with tin-bearing pegmatites
of the
Kabba province, central Nigeria, where it occurs with
andalusite, muscovite, gahnite, garnet (probably
spessarti ne-almandine), cassi terite, columbite-tantalite,
chrysoberyl and apatite.
At Lixa, in the Douro Litoral
province of Portugal, nigerite is closely associated with
muscovite in a cassiterite-bearing
pegmatite
that also
hosts lithiophilite, apatite and vivianite (van Tassel
1965). Ginsburg
et al. (1961) desoibed it from a granitic
pegmatite
vein in eastem Siberia, Kloosterman (1974),
from pegmatitic tin-tantalum veins from central
Amap6, Brazil, and Burke er al. (1977), from granitic
pegmatite and associated aplite veins from Kemiri
FeMg
Separafion
Rapids Pluton
Gahnite
94-29
12
11.56 6.01
0 31 0.14
0.2ri 0.14
0.02 0.02
0.t2 0.02
0.02 0.08
0.05 0.05
0-01 0.00
32.14 37.N
100.46 y)45
32 32
1s.846 15.894
2.327 L.227
0.075 0.029
0.'100 0.051
0.00.1 0.004
0.002 0.002
0.00i 0.005
0.005 0.005
0.001 0.000
5.702 6.816
24.062.
24033
Mn€uite Pehlih
AU03
FeO
Mno
Mgo
Tio2
Nb205
Ta2O5
Sn02
w03
z^O
Total
No. of oxygm
AI
Fe
Mn
Mg
Ti
Nb
Sn
z\
Tohl
34
54.14 54.32
11.13 11.98
0.61 0.40
0.00 0.12
0.09 LM
0.32 0.37
038 0.82
28.@ 24.71
0.44 021
329 3.73
99.00 99.12
24 24
11..613 11..478
1.694 1..796
o.c94 0.061
0.000 0.032
0.012 0.32
0.s26 0.030
0.019 0.040
2.075 1.766
0.921 0.010
0.442 0.494
15.97 16.0U
Peg. l Peg.10
9.47 7'1.32
0.46 056
0.08 0.08
0.07 0.42
0.14 0.
021 0.88
21.& 20.00
0.19 0.21
1Z& 70.69
100.51 n32
15.70s rs.589
1.890 2.285
0.093 0.114
0.02 0.029
0.013 0.076
0.015 0.039
0.014 0.058
2.036 1.925
0.01.2 0.013
2.227 1.905
22.032 22.033
Stnrctural fomula calculaed muhg malys3,4 dd5,6 reprst
r6p<tively, the 5H and 24R polltyp6 of nigerite (Grey & Gatehoue 1979)
N
Metamorphic
Sepsauon
Rapids
Pluon
lntemal
86ryl
P€gmalits
Exlemal
Beryl
P€gmattes Petaltle
Pegma$tes
Fe-Sulte
Fereolumbite
Fenotianlalite
Ferowodglnh€
Fenotaplollte
Mlcrollte
Sdblomlcrolite
U@nlcrollte
Yttropyr@hlore
Ca$llorlto
-1r::l
E
::5
tr
r:::l
[a--aT
I;!d-;l m
El
|::::
;t;lr
t;:al
EI
m
3
G::t
Mn-Suite
Mmgao@lumblt€
Msgilotiltaltle
Wodglnlte
Mlcrolite
Blsmutomicrollle
Cffitledte
;-:?
E::i!l
r,:I::::I
E
tr
F:-
:;7t1
f6;tti
r::t=l
[!ir
Marko's Wall Zo
Mmgano@lumbite I
l\rmganohntaltl6
Wodglnite
Tltanowodglnlte
"Ferotlt@wodglntle"
l::H:"
Str0verite
Ca$hedt6
ne (alscItlln-er rll e) ;-rl
K.=l
F::n
lt=rl
@
E
E
tT=
Ftc. 11.
Paragenesis
ofNb-Ta oxides from Separation Rapids.
Island, southwestern Finland. Commonly associated
accessory minerals include cassiterite, columbite-
tantalite, tourmaline, chrysoberyl, and occasionally
andalusite, sillimanite, corundum and garnet. At
Amap6, nigerite and cassiterite commonly occur
together, but not in the presence
of gahnite, whereas
at
Kemio Island, nigerite forms oriented overgrowths on
gahnite,
or in spinel-ftee samples, the nigerite represents
a stage of complete replacement. A lack of gahnite in
the Separation Rapids pegmatites
may indicate total
replacement by nigerite; however, most nigerite fonns
euhedral or subhedral crystals that seem
primary.
Separation Rapids probably represents the first
occulrence in North America of nigerite from a granitic
pegmatite. It has been found in four of the Fe-suite
OXIDE MINERALS, SEPARATION RAPIDS
1.00
629
o
tJ.
+ u.ou
c
E o.5o
c
E 0.40
0.90
Garnei Columblte
/Tantallte
Group
wodginlte
Group
Flc. 12.
Covariation
of Mn/(Mn + Fe) in garnet, columbite
and
wodginite-group minerals
from Separation
Rapids.
Shaded line and open
squares: Separation Rapids
pluton;
thin
solid
line
and filled
diamonds: Fe-suite
pegmatites;
thick
solid line and
open circles: Mn-suite
pegmatites.
petalite pegmatites,
where it has a distinctive golden
brown color and forms small elongate tabular crystals
with rounded margins
or, less
commonly, hexagonal
platy crystals (Fig. 7h). In Peg. 1, crystals reach over
4 mm in length, but in other pegmatites,
grain sizes
are smaller (less than I mm). Muscovite or "squi"
(spodumene
- qvartz intergrowth, alteration products
after petalite) are commonly associated phases.
More rarely, nigerite occurs amongst clusters of
ferrocolumbite-ferrotantalite crystals (Peg. I0). In
some cases, it is accompanied by cassiterite or
spessartine
garnet (or both).
Two distinct compositions
of nigerite are
recognized,
a low-Zn variety with 3.2-6.2 wt.Vo ZnO (Table 6,
compositions 3 and 4), and a high-Zn variery, with
10.7-13.0 wt.VoZnO
(Table
6, compositions
5 and 6).
We tentatively equate these with the 6H and 24R
polytypes (Grey & Gatehouse
1979). Our analytical
data indicate that both varieties occur in individual
samples.
As with gahnite, nigerite from rare-element
pegmatites is distinct ftom that occurring in metamorphic
environments
(Fig. 10), such as at the Geco Cu-Zn
volcanogenic massive sulfide deposit, Manitouwadge
district, Ontario (Petersen
1986).
Drscussrox
An examination of the oxide minerals in the
Separation Rapids pluton and pegmatites, and in
particular columbite-tantalite, has revealed
two major
groupings, an Fe-suite and a Mn-suite. Both beryl- and
petalite-bearing granitic pegmatites are found in each
0.30
0.20
0.10
630
sui0e.
The wall zone of one of the Mn-suite pegmatites
(Marko's pegmatite) has a sufficiently different
assemblage
of minerals that it is described
separately.
Fe-suite
The Fe-suite includes the Separation
Rapids pluton
and the majority of beryl- and petalite-bearing granitic
pegmatites.
T\eZn oxides gahnite and nigerite are also
restricted to this suite. Zn2*
predominantly substitutes
for Fe2*, and usually both elements decrease in
abundance with progressing igneous fractionation.
Nonetheless,
gahnite and nigerite still constitute minor
accessory
minerals,
probably in response
to the low
concentration of ferromagnesian minerals and to the
availability of excess
Al3* in the Separation
Rapids
pluton and pegmatites.
eernf & Hawthorne (1982)
noted that as the fugacity of 52* regulates the form of
Zn precipitation, gahnite and sphalerite are mutually
exclusive
in most of their pegmatite
occurrences.
In the
Separation
Rapids pluton, however, they may occur
together. The irregular shape of gahnite crystals
(Fig. 7g) indicates local disequilibrium and could point
to a xenocrystic origin, the gahnite being derived from
partial melting of the Treelined Lake granitic complex,
where
it also is found. However,
another
possibility is
that the shape of the gahnite may have been produced
by replacement
of mica, which is almost a rule in
"pegmatite-born" gahnite.
The intergranular
fluid is
necessarily alkali-bearing in feldspar-dominated
pegmatites,
and these
alkalit vigorously react with any
peraluminous minerals (P.
Cernf, pers. comm.).
Although a distinct succession
of Nb-Ta minerals
can be recognized within each
pegmatite,
there is a
general sequence from ferrocolumbite to ferrotantalite
throughout the suite (Fig. 1 l). Ferrocolumbire
from
the Separation
Rapids
pluton has the lowest Mn and
Ta content,
and is considered
the most primitive. As
with the internal beryl pegmatites (which have
ferrocolumbite with slightly higher Mn conrenrs),
the
Separation
Rapids pluton does not contain other Nb-Ta
oxide species
than rninor cassiterite.
In most external beryl pegmatites, ferrocolumbite
has higher Mn and Ta contents than in the Separation
Rapids pluton, and it coexists with ferrowodginite in a
few cases.
Ferrowodginite becomes
a characteristic
mineral in the petalite pegmatites,
where it coexists
with either ferrocolumbite or ferrotantalite, and
wodginite is also present
in the evolved parts of two of
these pegmatites.
There is some petrographic evidence that microlite
is a primary crystallizing phase,
but the mineral also
occurs as a replacement product after ferrowodginite or
manganocolumbite.
The other minerals of the microlite
sub
group, uranmi c ro I i
t
e, antimo ni an mi c ro
lit e (w ith
one exception: Fig. 6f) and stibiomicrolite only occur
as inclusions (the first in ferrowodginite, the Sb-bearing
species
in cassiterite)
and are therefore diffrcult to place
in a paragenetic
sequence.
The uranmicrolite and some
antimonian microlite display primary textural features,
such as euhedral habit and oscillatory zoning.
If the oxides had been significantly disturbed by
fluids, it might be expected
that they would exhibit a
variable composition. It was therefore decided to
analyze
garnet (chosen as a relatively stable, primary
mineral that most likely crystallized directly from a
pegmatite-forrning melt) and compare Mn/(Mn + Fe)
values for garnet-,
columbite-tantalite- and wodginite-
group minerals (Fig. l2). With little deviation, the
Nb-Ta oxides and the garnet change composition
systematically; there can be little doubt that these
minerals crystallized in local equilibrium under
identical
(magmatic) conditions, and that they were not seriously
affected by later fluids. Even albite-rich units in the
Separation Rapids pluton and pegmatites
from this
suite show little evidence of replacement features,
and
all members of the Fe-suite are therefore interpreted to
consist of primary magmatic assemblages.
Mn-suite
Two large bodies of pegmatite from the eastern
subgroup essentially define this suite, one an external
beryl-bearing pegmatite (Pee.265), the other a
petalite-bearing pegmatite
(Marko's).
Tlvo other external
beryl pegmatites
are included in the Mn-suite, but neither
the intemal beryl pegmatites,
nor the Separation Rapids
pluton (apart from tIe "cleavelandite"-rich pods) are
represented.
The suite is characterized by early-formed
(petalite-ftee)
units of individual pegmatites
containing
manganocolumbite with appreciable variation in
Mn/(Mn + Fe) within individual samples, followed
by manganocolumbite and manganotantalite with
appreciable
variation in Tal(Ta + Nb) in more evolved
(petalite-rich in Marko's pegmatite)
units. [n these
later
units, wodginite coexists
with manganocolumbite and
manganotantalite, but spessartine
(which can be locally
abundant
in the early crystallized units) is absent in the
later units.
Microlite is restricted to Marko's pegmatite, where
it locally occurs in abundance. As with the Fe-suite,
most microlite is secondary
after wodginite, but where
no evidence exists of a possible precursor phase, it is
likely that this microlite is primary. Bismutomicrolite in
pegmatite Peg.265 is, however, secondary, clearly
replacing earlier (tungsteniferous)
wodginite. Cassiterite
is the final oxide to crystallize. The positive correlation
of Na with F noted at Separation Rapids,
particularly in
the Mn-suite pegmatites
and also reported in microlite
from the Harding pegmatite, New Mexico, suggests
systematic
variation due to magmatic fractionation or a
degree of primary alteration of the microlite itself
during the late magmatic to hydrothermal stages of
pegmatite emplacement
(Lumpkin & Ewtng 1992).
OXIDE MINERALS, SEPARATION RAPIDS 631
A comparison of garnet
with Nb-Ta oxides does
not
reveal the same systematic
variation as that observed in
the Fe-suite
(Fig. l2), even though many samples do
define broadly similar trends. This may be related to
albitization, which is particularly noticeable in the
manganotantalite-bearing samples and discussed
further in Tindle et al. (1998). Although there is
petrographic evidence to suggest that albitization is a
metasomatic phenomenon capable of producing
subsolidus replacement-induced
textures, such as
albite-rich pods replacing earlier petalite crystals, we
agree with London et al. (1989) and Cern! & Lenton
(1995) that most late albite-rich and micaceous units
probably represent the very last residual melt and
should be considered
part ofthe fractionation sequence
(i.e., are
magmatic). However, it is not possible to
consistently distinguish assemblages
formed as late
magmatic
replacements from those
that may be entirely
subsolidus, as a spectrum
of textural features
is evident
in the field.
The rapid nucleation and resulting fine grain-size
of
these
late albite-rich
units can be triggered
by nucleation
of B, R Li and F-bearing minerals (London 1990),
which drastically reduces the solubility of H2O in the
residual melt and leads to exsolution of a supercritical
fluid. Topaz-rich aplitic dykes associated with
pegmatite 304 are a clear example of where this is
likely to have occurred.
We note that although primary albite-bearing
assemblages
are common to both Fe- and Mn-suite
pegmatites,
it is only in the Mn-suite pegmatites that
albite-rich replacement
units are well developed.
We
speculate that this was a consequence
of differences
in
F contents
of the parental pegmatite-forming melts.
Pegmatite-forming melts can, however, evolve toward
an alkaline,
silica-depleted
composition rich in Na, F
and R and become vapor-saturated,
with resulting
increase in solidus temperatures and the formation of
fine-grained, aplitic albite-mica bodies (London 1990).
The Mn-suite pegmatites are most likely to have
achieved
vapor saturation,
but there is less
evidence
for
it in the Fe-suite pegmatites.
This could, therefore, be
another factor contributing to the differences between
the two suites.
On the basis of all our observations,
the parent
of the Fe-suite pegmatites is considered to be the
Separation Rapids pluton. Those
pegmatites containing
ferrocolumbite-ferrotantalite with low Mn/(Mn + Fe)
values (such as
pegmatites
9 and l0) are considered to
be early. As the Separation Rapids magma fractionated,
Mn/(Mn + Fe) increased
in both pluton and later
pegmatites. Columbite-tantalite variation within
individual pegmatites
mainly involves Tal(Ta + Nb),
and this is ascribed
to in situ fractionation.
The Mn-suite pegmatites are more difficult to
explain, as there
is no obvious parent.
Spatially, the
suite is intimately associated with the Fe-suite
pegmatites. The diverging columbite-tantalite trends
and the lack of a continuum of K-feldpar compositions
(Breaks et aI. in prep.) preclude a simple fractionation
model linking the two suites.
There is, however,
the
possibility that the Mn-suite pegmatites were derived
from a thermogravitationally separated F-rich layer
within the Separation
Rapids magma chamber, as
K-feldpar
compositions
(Breaks
et al.,in prep.) indicate
that the Separation Rapids granite is compositionally
layered,
with extreme
Cs (> 6000 ppm) and high Rb
(>150 ppm) values
along
the southwestern
margin.
In
this situation, columbite compositions
from pegmatites
24 and 262 (Fig.2a) could be interpreted as being
derived from a boundary layer between the sources
of
the relatively F-poor Fe-suite and the relatively F-rich
Mn suite.
At this time, there is insufficient evidence to
discriminate between a thermogravitationally layered
magma chamber and one where batches of more
evolved, F-rich, sodic residual melt ponded in the
apical
part of the Separation
Rapids
pluton. It is also
not possible to discount the possibility that the
Mn-suite pegmatites were derived from a totally
independent
F-rich source,
although
there
is no field
evidence
to support the existence
of such a second
pluton.
The wall zone of Marko's pegmatite, a petalite-
absent
pollucite - beryl - muscovite - albite - quartz
unit, is considered here separately from the main
Mn-suite, because of its unique assemblage
of minerals.
It also differs from other Mn-suite pegmatites and the
remainder of Marko's pegmatite in not containing any
albite-replacement
pods or other macro-scale
replacement
assemblages.
Manganocolumbite
and manganotantalite
are both present, but show little systematic variation.
However, wodginite in the wall zone
shows
evidence
of
alteration and replacement, with one Iarge crystal
displaying a texturally complex titanowodginite and
"ferrotitanowodginite" rim (Fig. 3d) comparable
to that
reported in secondary columbite from Finland (Lahti
1987), and secondary
columbite-tantalite from the
Czech Republic (eernf et aL. 7992). One inclusion
of titanowodginite at the edge of a large grain of
cassiterite has been partially replaced by sriiverite
(tantatian
rutile). These
features cannot
be explained
by
normal processes of fractionation (although
extreme
fractionation is required
to crystallize
pollucite). We
thus propose
that there also was interaction with the
host rocks (banded
iron-formation and Fe-Ti tholeiitic
metavolcanic
rocks), which led to an influx of Fe and Ti
into the pegmatite-fonning melt. We speculate
that this
influx occurred whilst the pegmatite was crystallizing,
as other titanowodginite and "ferrotitanowodginite"
crystals found in wallrock samples occur as
inclusions
in what would appear
to be magmatic cassiterite. Host
rocks may also have contributed to the Sb and Ti
enrichment
ofpyrochlore-group minerals
on a localized
scale.
Our conclusions
are supported
by those
ofAbella
et al. (1995), who considered the restricted occurrence
632 THE CANADIAN MINERALOGIST
of niobian and tantalian rutile in the border and wall
zones
of pegmatites from the Cap de Creus pegmatite
field, eastern
Pyrenees,
as being due to host-rock
assimilation.
The evidence presented
above suggests
that at
Separation
Rapids, physicochemical variations due
to halogen and alkali enrichment at the end of
magmatic differentiation, coupled with interaction
of magmatic
and extraneous
fluids, led to conditions
of non-equilibrium crystallization (Ohnensteuer
&
Piantone 1992) in the Mn-suite pegmatites.
Locally
there also was interaction with host rocks.
CoNcr_usroNs
Oxide minerals
have been
characterized
from the
Separation
Rapids
pluton and its associated pegmatites.
Compositions of the Nb-Ta minerals, associated
minerals (petalite, beryl, gahnite, nigerite, zircon,
albite, Li-rich mica, garnet) and other geological
and petrological data (Breaks et aI. in prep., Tindle er
al. 1998) confirm these pegmatites belong to the
rare-element
class,
complex type, petalite subtype.
The following oxide minerals were identified:
ferrocolumbite, ferrotantalite, manganocolumbite,
manganotantalite,
wodginite, ferrowodginite, titano-
wodginite,
"fenotitanowodginite", microlite, antimonian
microlite,
stibiomicrolite,
uranmicrolite,
bismutomicrolite,
stibiobetafi
te, yttropyrochlore, fenotapiolite, striiverite,
cassiterite,
ilmenite, scheelite,
uraninite, gahnite
and
nigerite. Ixiolite also was tentatively identified.
On the basis of compositions
of columbite-group
minerals,
the pegmatites
can be divided into an Fe-suite
and a Mn-suite, both containing beryl and petalite
pegmatites.
The suites
are not spatially separable in the
field.
The Separation Rapids pluton has primitive
compositions
of ferrocolumbite consistent
with it being
the parent
to at least
the Fe-suite
of pegmatites.
The
ferrocolumbite-ferrotantalite trends, as well as the
restricted occurrence of microlite-group minerals
and other F-rich minerals, indicate a rather F-poor
environment.
Late-stage
alteration
of this suite is considered
to be
minimal, as coexisting garnet, columbite-tantalite and
wodginite-group minerals
show systematic variation
inconsistent with major influx of late fluids.
The Mn-suite has quite different characteristics
indicative of a more F-rich environment.
These
include
a greater
abundance
of microlite, the presence
of topaz
in metasomatic
selvedges
(up to 8.5 vtt.Vo
fl, associated
topaz-rich
aplite dykes,
concentrations
ofli-F micas in
evolved pegmatites,
and field and textural evidence
for
replacement
and albitization.
The mineral assemblage
found in the wall zone
of Marko's pegmatite provides evidence for local
interaction of the pegmatite-forming magma with
Fe-, Ti- and probably Sb-rich host rocks.
ln summary, the Nb-Ta oxides are shown to be
excellent indicators
of the chemical evolution of the
leucogranitic and pegmatite-forming melts. Unfortu-
nately, experimental data essential for a more detailed
interpretation
of the conditions of crystallization are
lacking, but they are very desirable.
AcnqowLepcnveurs
Kay Chambers, Brian Ellis and John Watson are
thanked for producing innumerable polished thin
sections
and
blocks for electron-microprobe
analysis.
Thanks are also due to Mrs. Saga von Knorring for
providing nigerite-bearing samples
from the collection
of her late husband (Dr. Oleg von Knorring). Petr
dernf is thanked for his indirect influence on this line
of research. He, Greg Lumpkin and Peter Webb are
thanked for very thorough reviews ofearlier versions
of
this manuscript.
AGT thanks
the Mineralogical Society
for a SeniorTravel Bursary,
and the Ontario Geological
Survey for support in the field.
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