American Mineralogist, Volume 58, pages 831-834, 1973
FerroanGahnitefrom Quartz-Biotite-Almandine
Schist,
Wind River Mountains,Wyoming
B. RoNero Fnosr
De'partmentol GeologicalSciences,Uniuersityof Washington,
Seattle,Washington98I 05
Abstuact
Ferroan gahnite, with compositions ranging from gahnitee-hercynite--Mg spinel,o to
hercyniten,-gahniteo-Mg spinel,3,have been found in quartz-biotite-almandine metasedimentary
rocks from the Double Lake area of the Wind River mountains of Wyoming. It is believed that
the large amount of zinc in these spinels, wbich seemsto have been provided by desulfurization
of nearby ore minerals, stabilized them in the presence of quartz.
fntroduction
Ferroan gahnite, occurring with quartz, biotite,
and almandine, has been found in several rocks from
the Double Lake area in the northeastern Wind River
Mountains, Fremont County, Wyoming. The locality
is 24 kilometers by trail southwest of the nearest
trailhead, Trail Lake Ranch, which itself is 19 kilometers by dirt road from the nearesttown, Dubois.
The area is underlain by a Precambrian gneiss
complex, which has undergone middle-to-upper amphibolite facies metamorphism. The complex shows
two periods of igneous intrusion and migmatization1'
one event is synmetamorphic, the other, post-metamorphic. The pre-migmatitic rocks can be divided
into two typos, a hornblende-calcic andesine amphibolite, and a metasedimentary schist (Frost,
1971). The amphibolite body, the western margin
of which is cut by a post-metamorphic batholith, is
exposedover an area of about five square kilometers.
The eastern contact of the body is locally discordant
to the foliation of the metasedimentaryschist and is
commonly marked by a three-meter-wide zone of
red-weathering metasedimentary schist. Many parts
of this red-weathering zone contain as much as 15
percent opaque minerals. The major opaque mineral
is magnetite. Pyrite is fairly common, usually rimmed
by hematite. Chalcopyrite and sphalerite are rare.
Four of the seven samples found to contain ferroan gahnite were collected from such a red-weathering zone over a distance of a kilometer. The other
three samples did no show the distinctive red
weathering, but also came from within a hundred
meters of the amphibolite-metasediment contact.
Three samplescontaining the spinel were selected
for microprobeanalysis.Two, ZS-1 and ZS-2, came
from the red-weatheringmetasedimentaryschist.
They both contain about 15 percent garnet, in
crystalsup to 1 cm in diameter.They also contain
20 percentbiotite and 60 percentquartz.In sample
ZS-1 the spineloccursthroughoutthe sample,but in
sample ZS-2 it is restricted to a centimeter-wide
quartz-rich layer. Sample ZS-3 does not show the
distinctivered weathering.It displaysa compositional
layeringwith one layer enrichedin quartz and containing green biotite, garnet, and rare spinel, and
with the other layer enrichedin brown biotite and
containingq\attz, spinel, and minor garnet. Such
compositionallayering may exist in 291 and Z,S-2
also, as spineloccurringwith biotite differs in compositionfrom that occurringwith garnetin all three
samples(Table 1). All three samplescontainminor
plagioclase,zircon, apatite, and opaque minerals.
The major opaquemineralis ilmenite;pyrite is rare.
Hematite is a common secondarymineral, causing
the red stains.Massesof finely intergrownchlorite
occur locally in somesamplesand may have formed
from retrogressivebreakdown of cordierite. However,no definitecordieritewas found in any rock in
the area.
In all occurrences,
the spinelforms small,rotrnded
to subhedral grains which are dark g"reenin thin
section.They show smooth,generallyconvexinterfaces, with textures suggestingequilibrium with all
phasesin the rock, including quartz(Fig. 1). It was
the apparenttextural equilibrium with quartz that
prompteda closerinspectionof the spinel.
831
FROST
B. RONALD
832
EMX-sM unit. Acceleration voltages of 15 kV for
the major elements, and 20 kV for the minor elements, were used. Ten to twenty points were measured on each grain analyzed. No variations were
noted either within grains or between grains of the
same group. Full corrections were applied for X-ray
absorption, characteristic fluorescence,atomic number effect, dead time, drift, and background. Standards used include an analyzed suite of garnets and
biotites and, for the spinels, analyzed franklinite and
synthetic MgAl2Oa. In garnet and spinel FeO and
Fe2O, were computed from total iron assuming perfect stoichiometry.
Results
The ferroan gahnite is a mixture of gahnite,
hercynite, and Mg-spinel with the R*tt site largely
occupied by Al. Assuming that the minor Fe*** is
distributed evenly among the end members, compositions range from gahnite6a-hercynite3e-Mg Spinells
to hercyniteae-gahnitqe-Mg Spinel13. These spinels
contain considerably more of the hercynite molecule
than gahnites reported by Winchell (1941) and the
ferroan gahnites reported by Gandhi (1971). They
are similar to spinel reported by Hutton (1957)
from river gravel in New Zealand.
The analyses show that zinc is preferentially in-
Ftc, 1. Photomicrograph of sample Z$1 showing stable
association of green spinel and quartz. B = biotite; Q :
quartz; S = spinel (PPL).
Methods of Andysis
Microprobe analysesof spinel, biotite, and garnet
(Table 1 ) were conducted on carbon-coatedpolished
thin sections. Five elements were measured simultaneously using an Applied Research Laboratories
Taerr 1. Analysesof Spinel, Biotite, and Garnet from the Double Lake Area, Wind River Mountains, Fremont County, Wyoming
SPINEL
zs-l
zs-r
with
with
Biotite
Garnd
lU
"2O3'
137
1 76
150
147
236
02
TOTAL
02
263
240
9953
99,34
zs-2
wirh
giotite
277
IO
292
Garner
242
164
296
04
327
02
zs-3
zs.3
frth
wth
Biotite
Garnd
242
2U
121
225
.22
180
213
26.9
10047
100'tt
9929
fITE
Tio2
154
I 16
125
191
F"2O3
233
FeO
21 0
19,6
19I
FeO
19
MgO
10 O
10.5
to4
MsO
322
426
348
MnO
g
05
16
MnO
I 40
1 39
620
ZnO
m
52
2A
C€O
1 O7
u
KzO
I 70
836
948
Z^O
m2
10016
45
55
19
94-43
93 84
94 26
-
24
'
r p+2
/ Fe+Mg
= Fe2O3
15m6
15638
i
6030
6036
Af fv
2676
2727
2697
At
3764
3 736
At vt
654
7U
743
F"*3
216
.24o
180
135
145
F"+2
4814
4621
Mg
.825
1 011
2966
3AA2
3194
2421
662
833
1 024
1 136
800
818
Ti
m4
004
,m8
004
w
o38
F"O,o,
2726
292
2470
25U
1,24
PROPORTIONS
5 303
2,721
174
30 3
99 86
5 273
424
.362
34.7
5 324
485
4g
35.5
200
si
.313
409
4 613
4 197
3,086
3 666
4 735
4 58
MS
2314
2427
23€7
lvln
180
.187
,841
8.000
8.000
8.000
8.000
8.0@
7.999
Mn
0O5
.007
021
C€
199
143
213
8O4
781
.791
.734
751
760
Zn
023
060
032
Zn
O0O
OOO
Total Y ions 5.902
5,935
5 848
T o t a rR + 2
K
r.6s
1,870
Fe / Fe+Mg
All have lss than
'
15576
1 77
TOTAL
CATION
15515
too
191
Na2O
15687
roo
la 2
TOTAL
15760
19 7
Arzog
Al2o3
1 3I
calculat€d asuming
02 % Ct,O
3,1aO 2
pe.fect fochrometry
Na
ToBlXions
Fe / Fe+Ms
1.723
135
165
057
1858
I 819
1 927
5t2
.507
g1
'
6ot8
854
= b6lowdetstion
5962
A2O
FERROAN GAHNITE
FROM WIND
cluded in the spinel and almost completely excluded
from the garnet. The ZnO content of the garnet is
below a detection limit of 0.03 percent. This is in
agreement with Knorring 09a6) who found that a
garnet coexisting with sphalerite contained less than
0.01 percentZnO.
The biotite coexisting with the spinel contains up
to 0.5 percent ZnO. lt is apparent that micas can
accept fairly large amounts of zinc, e.g. see analyses
of zinc micas from Franklin Furnace, New Jersey
(Frondel and Ito, 1966). Sample ZS-3 contains rwo
types of biotite. The brown biotite from the spinelbearing layers contains more TiO, than the green
biotite from the garnet-bearing layers. The biotites
from the other samples are more nearly homogeneous, although those coexisting with the spinel are
slightly richer in iron and poorer in magnesium than
those coexisting with the garnet.
The Fe'z-/(Fe'?.* Mg) ratios (Tables 2 and 3)
indicate that the order of preference for Fe'. over Mg
is garnet ) spinel ) biotite. The preference of iron
for spinel over coexisting biotite can also be seen
in analysesby Stewart (1942). The distribution of
iron and magnesium between garnet and spinel
(Table 2), however, differs from the results of
Stewart (1942), Knorring and Dearnley (1960),
and Abraham and Schreyer(1971).
Discussion
Green spinel in pelitic metamorphic rocks is
ordinarily considered to be indicative of silica undersaturation, e.g., Friedman (1954). For Mg-spinelhercynite solid solutions at metamorphic tempera'frsrs
2. Fe*'/(Fe*' *
Coexisting Spinel and
Y":.r".f"r
833
RIVER MOUNTAINS
Tesrr
3. Fe*'/(Fe*' *
Mg) for Coexisting Spinel and
Biotite
Stewart,1942
,895
.574
from Hornfels
Abrahamand
Schreyer,197l
.895
.621
from ironstonehorn{els
&4
,791
.751
.541
.512
.507
ferroangahnitepair
almandine
{rom schist
Thispaper:
ZS-1
ZS-2
ZS-3
tures,the presenceof quartzstabilizesthe right hand
side of the following equilibria (cl Richardson,
1 9 6 8F
, i g .6 ) :
3 FeAl,Oa+ 5 SiO, : Fe3Al2
Sisor,+ 2 Al, SiOs
hercyni
te
al mand
2 IV.[gAl,On +
5 SiO, :
sillimanite
i ne
Mg2Aln Si5O1s
quartz
cordieri
te
One would expect, therefore, that spinel in quartzose
metasediments would not be greatly enriched in
either iron or magnesium.Ilowever, as shown above,
zinc is preferentially included in spinel and almost
totally excluded from garnet. This indicates that in
zinc-rich environments spinel and quartz would be
stabilized relative to almandine and sillimanite, or to
cordierite.
The red-weathering zone, near which these spinels
occur, represents a minor concentration of ore
minerals at the margins of what may have been a
mafic intrusion (now represented by amphibolite).
It is postulated that the regional metamorphism
caused desulfurization of these rocks, allowing the
zinc to be incorporated into the oxide phases. A
similar mode of formation for gahnite at the margin
of a metamorphosed sulfide deposit has been des c r i b e d b y J u v e( 1 9 6 7 ) .
Spinel
Stewart,1942
,895
,826
Hercynite
from Hornfels
Knorring,1946
Pehrman,1948
.950
.999
Gahnite-Spessartine
from
pegmatite
Knorringand
Dearnley(1960)
,975
.945
Gahnite.Spessartine
from
pegmatite
Abrahamand
Schreyer,1971
.89
.87 .
from ironstonehornfels
References
Thispaper;
ZS-1
ZS-2
ZS-3
Acknowledgments
This work is an outgrowthof a Mastersthesissupervised
by Dr. Peter Misch. The author would like to thank Dr.
Bernard Evans for his generous help with the probe
analysesand for critically reviewingthis manuscript.Microprobe time was paid for by NSF grant GA-16601.
,781
:738
.760
.854
.820
.830
ferroangahnite.
pair
almandine
from schist
Annlnlu, K., lNo W. Scnneyen(1971) Mineralanalysen
aus einem eisenreichenHornfels des Brockenkontaktes
(Harz). Fortschrift.Mineral. 49, l-4.
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A studyof incompatible
mineralphases.
l. GeoI.
62. 366-374.
834
B. RONALD FROST
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Manuscript receioed, March 22, 19'73; accepted
for publication, MaY 22' 1973.