Nickel-rich chromian muscovite from the Indus suture ophiolite, NW Pakistan:Implications for emerald genesis and exploration
MOHAMMAD ARIF1* and CHARLIE J. MOON2
1Department of Geology, University of Peshawar, Peshawar 25120, Pakistan2Department of Geology, University of Leicester, Leicester LE1 7RH, U.K.
(Received August 11, 2006; Accepted September 15, 2007)
Ubiquitous veins and stockworks of quartz traverse the ophiolitic emerald-hosting, carbonate-altered ultramafic rocksin the Swat Valley. Some of the emerald-bearing quartz veins contain chromian muscovite and tourmaline. In addition,veins and clusters consisting of chromian muscovite and/or tourmaline occur in zones of carbonate-altered rocks wherethe quartz veins are most abundant. The chromian muscovite is characterized by high Mg/Fe ratios (4–9) and containsvariable and in some cases anomalously high concentration of Ni (ranging up to 9 wt% NiO). A detailed investigationreveals that the Ni and Mg entered the chromian muscovite structure as a part of a complex coupled substitution: (FeVI,MnVI, MgVI, NiVI)2+ + [SiIV]4+ ↔ (AlVI, CrVI)3+ + [AlIV]3+. The stable coexistence of quartz, chromian muscovite, tourma-line and emerald suggests that all these phases are cogenetic and precipitated from Si-rich, Al-, Be-, B- and K-bearingfluids related to a single episode of hydrothermal activity. The Mg, Cr and Ni contents in chromian muscovite were mostprobably extracted by the percolating hydrothermal solutions from the host carbonate-altered ultramafic rocks throughwall rock reaction. The observed high variability in the Mg, Cr and Ni contents of chromian muscovite probably reflectslow mobility of these elements during the hydrothermal process or a result of local equilibrium under relatively low Tconditions.
Examples of the occurrence and compositional char-acteristics of chromian muscovite have been describedfrom other areas. These include the corundum-fuchsite(chromian muscovite) rocks in the greenstone belts ofsouthern Africa (Schreyer et al., 1981; Schreyer, 1988;Kerrich et al., 1987, 1988), the chromiferous quartzitesof South India (Raase et al., 1983), the breunnerite(ferroan magnesite)-quartz-chromian muscovite assem-blages in Newfoundland (Chao et al., 1986), the blackschists and quartzites (containing chromian muscovite ±other Cr-bearing silicates) of Outokumpu, Finnish Karelia(Treloar et al., 1987a, b), the green mica schists in theHemlo area of Ontario, Canada (Pan and Fleet, 1991),and a variety of lithologies (quartzites, biotite schist,metsomatized ultramafic rock and vein) from NorthwestNelson, New Zealand (Challis et al., 1995). The compo-sition of chromian muscovite from the present study dif-fers from the previously reported examples in that it con-tains high concentrations of Mg (with Mg/Fe mostly fall-ing between 4 and 9) and Ni (ranging up to 9 wt% NiO).In contrast, the maximum concentration of NiO in previ-ously reported chromian muscovite analyses is 1.11 wt%which occurs in the example from Newfoundland (Chaoet al., 1986).
INTRODUCTION
Emerald, which is commercially exploited at a numberof localities, occurs as nuggets and disseminated grainsin the carbonate-altered ultramafic rocks of the Swat Val-ley, north-western Pakistan (Fig. 1). Crystals of emerald,with or without chromian muscovite and tourmaline (Cr-rich dravite carrying up to 13 wt% Cr2O3), also occurwithin the quartz veins that cut through the highly fissilealtered ultramafic rocks. Detailed studies suggest that allthe three Cr-bearing silicates (chromian muscovite, tour-maline and emerald) are contemporaneous and formed asa result of a single episode of hydrothermal activity re-lated to the injection of quartz veins (Arif et al., 1996).
The present study presents a detailed report on themode of occurrence and chemistry of chromian musco-vite with the aim to investigate its compositional peculi-arities. Such an investigation would also help in under-standing the emerald-forming processes and facilitateplanning for further exploration.
476 M. Arif and C. J. Moon
GENERAL GEOLOGY
A suture which marks the collision of the Indo–Pakistan plate with the Kohistan arc sequence in north-ern Pakistan is known as the Main Mantle Thrust (MMT)and is believed to be the westward extension of theIndus–Tsangbo Suture Zone (ITS) (Treloar et al., 1989).In the Swat Valley, it assumes a broad wedge-shaped com-plex zone with diverse rock assemblages separated byfaults of different ages (Fig. 1). These assemblages arecollectively termed as the Indus suture melange group byKazmi et al. (1984) and here called the MMT MelangeGroup (MMTMG). They are a composite of remnants ofthe lithosphere of the Neotethys that existed between theIndo–Pakistan plate and the Kohistan island arc(Lawrence et al., 1989). A number of melanges (i.e., theShangla blueschist melange, the Charbagh greenschistmelange, and the Mingora ophiolitic melange) have beenmapped within this complex zone (see figure 2 in Kazmiet al., 1984, 1986).
Ultramafic rocks, constituting the dominantlithological type of the ophiolitic member of theMMTMG, occur as small to large lensoidal bodies scat-tered throughout the study area along with frontal rocks(granite gneisses and metasediments) of the Indo–Pakistan plate (Fig. 1). The principal occurrences of theultramafic rocks are in the Barkotkai area, Gujar Kili vil-lage, Spin Obo area and near Mingora town. At places(e.g., near Barkotkai village), they contain small bodiesof chromitites and disseminations of nickeliferous opaquephases (Arif and Jan, 1993; Arif and Moon, 1994). Theyare altered to varying degrees. Although hydration lead-ing to serpentinization is the principal alteration processof the ultramafic rocks, carbonation also has taken placeand produced carbonate-rich mineral assemblages. In anumber of examples, veins of carbonate cut across theserpentine matrix suggesting that carbonation took placeafter the main episode of serpentinization. In the areanorth of Mingora town and at Gujar Kili village, thecarbonate-rich assemblages host emerald mineralization
Fig. 1. Geological map of the Lilaunai–Mingora area, Swat, north-western Pakistan (after Kazmi et al., 1984, 1986). The insetmap shows general location of the study area.
Nickel-rich chromian muscovite from emerald deposits in Pakistan 477
(Fig. 1).The carbonate-rich lithologies, although also occur-
ring as small patches within the serpentinized rocks, aremostly distributed along the contact between theserpentinites and metasediments. These rocks are cut byabundant quartz veins and/or stockworks, and consist ofmagnesite and accessory to trace amounts of spinel(mostly Cr-magnetite-ferritchromite and, in some cases,Cr-rich chromite) accompanied by one or more of thephases including talc, quartz, dolomite, chromian mus-covite and tourmaline. Locally, these rocks also containtrace to accessory amounts of sulphides (pyrrhotite,pentlandite, pyrite, violarite and mackinawite) andsulpharsenides (gersdorffite and cobaltite) as disseminatedgrains and veinlets (Arif, 2003). In addition, chromitite(containing >50 modal % Cr-rich chromite) occurs assmall nodular masses dispersed in these rocks.
SAMPLES AND ANALYTICAL TECHNIQUES
Samples of carbonate-altered ultramafic rocks werecollected from the emerald mines area near the Mingoratown in the Swat Valley (Fig. 1). All the samples werestudied petrographically and selected ones were made intopolished thin sections for scanning electron microscopic(SEM) studies and electron probe micro-analyses(EPMA). Besides, a sample consisting of mechanicallyseparated chromian muscovite flakes was run throughPhilips X-ray diffraction system comprising PW1730generator, PW1710 diffraction control, PW1050/25 de-tector, Philips Long Life Focus Cu X-ray tube and Ni fil-ter, under the following operating conditions: 40 kV, 30mA current, 4–64° 2θ, 0.02° step size at a rate of 1°/min.A portion of the sample was doped using α-corundumand run under the same set of conditions. The peak posi-tions of the main sample were corrected relative to thepeak positions of the doped sample after correcting thelatter using the standard corundum peaks (JCPDS card10-173).
A JEOL Superprobe model JXA-8600, equipped withan on-line computer for ZAF corrections, was used forcarrying out analyses. Quantitative analyses were con-ducted using wavelength dispersive system and naturaland synthetic standards. The analytical conditions wereas follows: 15 kV accelerating voltage; 30 nA probe cur-rent; 20 seconds peak, 10 seconds negative backgroundand 10 seconds positive background counting times. Theaccuracy of the ZAF correction is generally better than2%.
OCCURRENCE OF CHROMIAN MUSCOVITE
Flakes of chromian muscovite with bright green col-our under transmitted light occur in veins in the emerald-
bearing carbonate-altered ultramafic rocks (Fig. 2a). Insome of the rocks, chromian muscovite coexists with tour-maline forming veins and clusters (see figure 6.2 inHammarstrom, 1989). These two minerals also occur asfine-grained disseminations within the host rocks and inthe quartz veins that traverse them. The chromian mus-covite also forms borders around crystals of Cr-richchromite in both the magnesite-rich rocks and chromite-dominated small nodules (Fig. 2b). In one of the sam-ples, chromian muscovite occurs as aggregates, clustersor clots of tiny crystals within a magnesite matrix. Thesemay represent a pseudomorphic transformation from thedisseminated chrome spinel grains that were present inthe parent ultramafic rock. The chromian muscovite rarelyshows alteration to chlorite along the margins of grains.
Fig. 2. Back-scattered SEM images showing the occurrence ofchromian muscovite as a (a) vein in the magnesite (the mainhigh relief matrix)-rich rock and (b) zone around a grain ofCr-rich chromite. Brighter rims of ferroan magnesite surroundthe grains of magnesite.
478 M. Arif and C. J. Moon
S. N
o.1
23
45
67
89
1011
1213
1415
1617
SiO
245
.57
46.4
648
.18
45.8
646
.45
45.7
942
.76
45.0
645
.01
45.4
746
.75
47.3
847
.06
46.0
448
.81
48.1
945
.84
TiO
20.
120.
100.
270.
120.
090.
080.
060.
090.
100.
180.
050.
100.
080.
140.
200.
360.
17A
l 2O
324
.10
24.5
724
.59
22.7
123
.26
22.7
819
.06
21.8
922
.52
23.2
623
.94
22.3
623
.93
20.9
016
.50
21.0
720
.71
Cr 2
O3
6.30
7.61
6.02
5.98
6.51
5.84
4.60
4.54
5.44
5.68
4.91
7.57
7.32
7.20
9.45
10.8
713
.47
FeO
1.29
0.79
0.91
1.06
0.96
1.13
1.35
0.95
0.97
0.87
1.20
1.04
0.78
1.09
0.69
0.78
0.91
MnO
0.01
0.01
0.01
0.02
0.03
0.03
0.03
0.03
0.02
0.03
0.03
0.00
0.03
0.03
0.05
0.04
0.05
MgO
3.24
2.96
3.36
3.58
3.76
3.69
6.12
3.73
3.67
3.11
3.68
4.31
3.84
5.39
9.15
4.70
3.35
NiO
0.17
0.95
1.84
3.32
3.42
4.99
8.75
5.26
2.76
2.22
0.21
0.38
1.06
1.99
0.12
0.06
0.07
CaO
0.01
0.01
0.00
0.02
0.01
0.00
0.09
0.05
0.03
0.01
0.02
0.01
0.05
0.02
0.01
0.00
0.01
Na 2
O0.
360.
380.
380.
390.
350.
330.
270.
360.
350.
360.
250.
310.
330.
290.
290.
340.
32K
2O9.
449.
449.
318.
789.
178.
326.
957.
458.
558.
589.
539.
539.
618.
307.
379.
589.
39T
otal
90.6
193
.28
94.8
791
.84
94.0
192
.98
90.0
489
.41
89.4
289
.77
90.5
792
.99
94.0
991
.39
92.6
495
.99
94.2
9
Str
uctu
ral
form
ulae
on
the
basi
s of
22
oxyg
ens
Si6.
517
6.47
86.
586
6.54
96.
498
6.49
96.
405
6.60
46.
567
6.57
86.
646
6.62
26.
512
6.57
96.
811
6.57
56.
436
AlIV
1.48
31.
522
1.41
41.
451
1.50
21.
501
1.59
51.
396
1.43
31.
422
1.35
41.
378
1.48
81.
421
1.18
91.
425
1.56
4∑
Z8.
000
8.00
08.
000
8.00
08.
000
8.00
08.
000
8.00
08.
000
8.00
08.
000
8.00
08.
000
8.00
08.
000
8.00
08.
000
AlV
I2.
579
2.51
62.
548
2.37
22.
333
2.31
01.
770
2.38
62.
440
2.54
42.
658
2.30
52.
415
2.09
91.
525
1.96
31.
863
Ti
0.01
30.
010
0.02
80.
013
0.00
90.
009
0.00
70.
010
0.01
10.
020
0.00
50.
011
0.00
80.
015
0.02
10.
037
0.01
8C
r0.
712
0.83
90.
651
0.67
50.
720
0.65
50.
545
0.52
60.
628
0.65
00.
552
0.83
70.
801
0.81
41.
043
1.17
31.
495
Fe2+
0.15
40.
092
0.10
40.
127
0.11
20.
134
0.16
90.
116
0.11
80.
105
0.14
30.
122
0.09
00.
130
0.08
10.
089
0.10
7M
n0.
001
0.00
10.
001
0.00
20.
004
0.00
40.
004
0.00
40.
002
0.00
40.
004
0.00
00.
004
0.00
40.
006
0.00
50.
006
Mg
0.69
10.
615
0.68
50.
762
0.78
40.
781
1.36
70.
815
0.79
80.
671
0.78
00.
898
0.79
21.
148
1.90
30.
956
0.70
1N
i0.
020
0.10
70.
202
0.38
10.
385
0.57
01.
054
0.62
00.
324
0.25
80.
024
0.04
30.
118
0.22
90.
013
0.00
70.
008
∑Y
4.17
04.
180
4.21
94.
332
4.34
74.
463
4.91
64.
477
4.32
14.
252
4.16
64.
216
4.22
84.
439
4.59
24.
230
4.19
8
Ca
0.00
20.
001
0.00
00.
003
0.00
10.
000
0.01
40.
008
0.00
50.
002
0.00
30.
001
0.00
70.
003
0.00
10.
000
0.00
2N
a0.
100
0.10
30.
101
0.10
80.
095
0.09
10.
078
0.10
20.
099
0.10
10.
069
0.08
40.
089
0.08
00.
078
0.09
00.
087
K1.
722
1.67
91.
624
1.60
01.
637
1.50
61.
328
1.39
31.
592
1.58
41.
728
1.69
91.
697
1.51
31.
312
1.66
81.
682
∑X
1.82
41.
783
1.72
51.
711
1.73
31.
597
1.42
01.
503
1.69
61.
687
1.80
01.
784
1.79
31.
596
1.39
11.
758
1.77
1
Tot
al13
.994
13.9
6313
.944
14.0
4314
.080
14.0
6014
.336
13.9
8014
.017
13.9
3913
.966
14.0
0014
.021
14.0
3513
.983
13.9
8813
.969
Mg/
Fe2+
4.49
6.68
6.59
6.00
7.00
5.83
8.09
7.03
6.76
6.39
5.45
7.36
8.80
8.83
23.4
910
.74
6.55
Tabl
e 1.
R
epre
sent
ativ
e an
alys
es o
f ch
r om
ian
mus
covi
te
Ana
l yse
s w
i th
l ow
Ni
cont
ent
(<0.
5 w
t % N
i O)
repr
esen
t f u
chsi
t e b
orde
ring
, or
fi l
l ing
cra
cks
i n,
chro
mi t
e gr
ains
.1–
7.
Chl
ori t
e-,
t our
mal
i ne-
, ch
rom
i an
mus
covi
t e-,
chr
ome
spi n
el-b
eari
ng q
uart
z-m
agne
sit e
sch
i st .
8–11
. C
hlor
i te-
, ch
rom
i an
mus
covi
t e-b
eari
ng q
uart
z-m
agne
sit e
sch
i st .
12–1
4.
Chr
omi a
n m
usco
vit e
-, t
ourm
ali n
e-be
ari n
g qu
art z
-mag
nesi
t e s
chi s
t .15
–17.
C
hrom
i an
mus
covi
t e-t
alc-
mag
nesi
t e c
hrom
i ti t
e.
Nickel-rich chromian muscovite from emerald deposits in Pakistan 479
CHEMISTRY OF CHROMIAN MUSCOVITE
Flakes of chromian muscovite were analysed in fivesamples including a chromite-rich rock. All of the spotswere determined for all the major oxides plus Cr2O3 andNiO. In addition, chromian muscovite flakes in some ofthe samples were also analysed for V2O3, BaO, F and Cl.The wt% oxide concentrations of these four componentsare 0.04–0.08, 0.08–0.28, <0.08 and <0.02, respectively.The totals are mostly low compared to typical micas (Ta-ble 1). However, a detailed SEM investigation and a se-ries of EDS spectra of some of the samples show thatthese chromian muscovites do not contain any other ox-ides in addition to the ones for which they have been ana-lysed, at least in amounts large enough to bring the totalsat a par with that of the “ordinary” Cr-free muscovites(~95) (see Deer et al., 1992). Hence the low totals mostprobably reflect the presence of a relatively large amountof hydroxyl water in these chromian muscovites. In otherwords, the studied chromian muscovites are Cr-bearinghydromuscovite (muscovite with high H2O and low K2Ocontent) where K+ ions are partly replaced by (H3O)+ ions(see Deer et al., 1992). The following observationsstrongly support this interpretation regarding the low to-tals of the chromian muscovite analyses under discussion:(i) their K2O content is low and hence the “X” site iscation-deficient relative to Cr-free muscovites (Table 1)and (ii) there is somewhat weak but positive correlationbetween wt% K2O and the total wt% oxide of the analy-ses (r2 = 0.358; plot not included).
Whatever might be the cause of their low totals, theanalyses seem to be reliable as the calculated formulasclosely approximate the ideal muscovite compositionexcept that they contain high amounts of Cr, Mg and, inmost cases, also Ni. More importantly, the X-ray diffrac-tion pattern of a sample of the studied chromian musco-vite closely matches the corresponding data on the “ordi-nary” 2M1 muscovite listed in the JCPDS card 6-263 andchromian muscovite 2M1 reported by Martin-Ramos andRodriguez-Gallego (1982). The only notable differenceis that the XRD trace of the present sample also containssome of the major peaks of the clinochlore 1MIIb varietyof chlorite (JCPDS card 6-12-242; e.g., the diagnosticreflections at 2θ = 12.5°, 18.8°, and 25.2°) (Fig. 3). Thisis to be expected because, as mentioned earlier, thechromian muscovite shows alteration to chlorite.
The compositions are extremely variable in terms ofalmost all the major components even within individualsamples. Except for one analysis, the wt% MgO in thechromian muscovite ranges from 2.39 to 6.18. The outlieranalysis containing abnormally high MgO (9.15 wt%) isfrom the chromite-rich rock representing the Mingoraemerald mines area. The wide range in Al2O3 is the directresult of variation in Cr2O3 suggesting an inverse rela-tionship between these two components. Although the Si–
Ni correlation is not significant, the observed diversityin the concentration of SiO2 seems to be related to theamount of NiO; analyses with abnormally high amountsof NiO have relatively low concentration of SiO2. Simi-larly, the variation and, in many cases, the abnormallylow content of K2O (relative to ordinary muscovite) ap-pear to be mostly in the Ni-rich analyses. In other words,the extreme variation in the composition of the studiedchromian muscovites is largely due to the large ranges inCr2O3 and NiO.
The amount of Cr2O3 in chromian muscovite varieswith the nature of associated phase(s) apparently becauseof local equilibrium. Hence flakes of chromian muscovitescoexisting with chromite crystals in the chromite-richsample have the highest content of Cr2O3 (9.45–13.47wt%) (Table 1). This becomes clearer when the Cr2O3content of chromian muscovite bordering a grain ofchrome spinel is compared with that occurring within the
Fig. 3. The XRD pattern of a sample consisting of mechani-cally separated chromian muscovite from the present study. Thepeak positions are corrected and compared with those of mus-covite 2M1, fuchsite (chromian muscovite) 2M1 and clinochlore1MIIb (see text for details).
480 M. Arif and C. J. Moon
carbonate matrix in one of the quartz-magnesite rocks.The amount of NiO in chromian muscovites is even morevariable and ranges from around detection limit (0.05wt%) to ~9 wt%. It varies from one sample to anotherand flake-to-flake within a given sample. Some of theanalyses performed on chromian muscovite in a givensample are virtually free of NiO while others within thesame sample contain variable amounts of this transitionmetal. The chromian muscovite lying adjacent to chromitegrains is always free of any significant amounts of NiO(Table 1). The maximum concentration of NiO is foundin that chromian muscovite which occurs as veins or in-dividual flakes in the carbonate-talc matrix. However, allthe analyses performed on chromian muscovite that isdistributed as flakes within the magnesite-talc matrix ofone of the samples are consistently devoid of any NiO.The colour of the chromian muscovite changes from brightto dark green with increasing Cr and/or Ni contents.
DISCUSSION
A detailed investigation of a possible relationship ofCr, Ni and Mg with other elements in the chromianmuscovites examined demonstrates that all three elementsshow fairly good negative correlation with octahedral Al(Figs. 4a, b and c). This suggests that at least part of theNi and Mg in the chromian muscovite analyses under dis-cussion substitutes for Al rather than exclusively enter-ing vacant octahedral sites, in a similar way to Li inlepidolites (Deer et al., 1992). It is noteworthy that thecorrelation between Cr and Al is strong but becomesweaker when the Ni-rich analyses are included. Con-versely, the Ni–AlVI correlation is strong but becomes in-significant when the Ni-poor analyses are included. Thatis why Ni-rich compositions are not included in the Crvs. AlVI plot (Fig. 4a) and Ni-poor analyses are excludedfrom the Ni vs. AlVI and Mg vs. AlVI plots (Figs. 4b andc). The relatively poor correlations in these plots couldbe partly due to the Fe3+/Fe2+ portioning, although theamount of total FeO is too low to produce any significanteffect. The slope of the regression line (–1.0133) for Fe +Mn + Ni + Mg versus Si plot (not included) also supportsthis prediction. Alternatively, the coupled nature of ionicsubstitutions, discussed below, and the possible entranceof small amounts of Ni and Mg into vacant octahedralsites probably account for some of the observed scatterof the data points in the Cr–Al, Ni–Al and Mg–Al plots.The results of statistical analysis of the data, included inthe relevant plots in Fig. 4, suggest all the correlationsare extremely significant at the 95% confidence level.
The stronger correlation in plot 5d than that in 4b and4c suggests that the Ni ↔ Al and Mg ↔ Al substitutionsare interrelated. Similarly, the highly significant correla-tion in Fig. 4e indicates that Ni, Mg, Fe and Mn compete
Fig. 4. Compositional variation and inter-element (atomic pro-portion) relationships in chromian muscovite: Only Ni-richanalyses (with NiO > 0.9 wt%) are included in all the plotsexcept A. The data points in A represent only the Ni-poor analy-ses (with NiO < 0.9 wt%). Each of the plots also contains aregression line with its slope value (m) and a value of r2, i.e.,the coefficient of determination, as well as the results of stu-dent’s t-test. The values of r2 enclosed in parentheses representco-efficient of determination where the sum of only Ni and Mg(i.e., excluding Fe and Mn) are plotted as the only divalentcations along the y-axis (see text for detail).
successfully with Cr in substituting for Al in the octahe-dral “Y” site.
The negative Ni+2–VIAl+3 and Mg+2–VIAl+3 correla-tions strongly suggest that, not only Cr, but also Ni andMg go into the “normally” Al-dominated octahedral sites.
Nickel-rich chromian muscovite from emerald deposits in Pakistan 481
However, unlike the Cr+3–VIAl+3 pair, these correlationscannot be explained in terms of a simple substitution be-cause of the involvement of charge difference. That is, itis a coupled heterovalent substitution rather than a sim-ple homovalent substitution. It seems that a greater Al+3
by Si+4 replacement in the tetrahedral “Z” site compen-sates for the charge deficiency resulting from the (Ni,Mg)+2 for Al+3 substitution.
A Si–Al plot is not included in Fig. 4 because of theinsignificantly low, albeit negative, correlation betweenthese two components. However, the very strong correla-tion in Fig. 4f suggests that the Si ↔ Al substitution alsodepends on the (Ni, Mg, Fe, Mn) ↔ Al substitution,thereby indicating “tschermakitic” substitution, i.e., (Mg,Ni, Fe, Mn)2+ + [Si4+] ↔ 2Al3+. The relatively highercorrelation in Fig. 4g than 4f suggests that the (Mg, Ni,Fe, Mn)2+ + [Si4+] ↔ 2Al3+ substitution is also coupledto Al ↔ Cr. The possible relationship of Ni, Mg, Fe andMn concentration with phengitic substitution is demon-strated by a fairly good positive correlation of these com-ponents with the Si/Al ratios (Fig. 4h). To sum up, thefollowing equation demonstrates the overall scheme ofionic substitution on both the tetrahedral and octahedralsites: (FeVI, MnVI, MgVI, NiVI)2+ + [SiIV]4+ ↔ (AlVI,CrVI)3+ + [AlIV]3+.
Field relations outlined in relevant published reportssuggest that the Cr-rich mica formation is closely relatedto hydrothermal alteration of pre-existing rocks. Thisimplies that chromian muscovite forms due to the perco-lation of K-bearing hydrothermal solutions through suit-able host rocks, with Cr being extracted from the latterby wall rock alteration. The character of host rock andmode of occurrence both indicate that the chromian mus-covite under investigation probably also formed by sucha process. However, the present investigation leads us tosuggest that not only Cr but also Ni and Mg were redis-tributed from the host ultramafic rocks. Prior to the for-mation of chromian muscovite, the Ni was probably con-tained in talc, which tends to fractionate this metal withrespect to most other sulphur-free phases including evenolivine (Trommsdorff and Evans, 1974). The greater vari-ability (both inter- and intra-sample) in the concentrations
of Mg, Ni and Cr in the chromian muscovite analysesreported here is most probably due to the extremely lowmobility of these three components or, alternatively, aresult of local equilibrium under relatively low T condi-tions.
The apparent dependence of the concentration of Niin chromian muscovite on the Ni content of the host rocksuggests that Ni in the chromian muscovite in this studyis derived from the local ultramafic rocks (Table 2). Al-though previously reported chromian muscovite analy-ses do not include any data on Ni (with the exception ofthose from Newfoundland; Chao et al., 1986), their to-tals (mostly ~95%) preclude the possibility of the pres-ence of any significant amount of this transition metal.One of the reasons why chromian muscovites from su-perficially similar deposits (e.g., Newfoundland) do notshow enrichment in Ni could be that the Ni content oftheir host rocks is markedly lower or less readily avail-able than the deposits under consideration. The same fac-tor is probably also responsible for the virtual absence ofNi from chromian muscovite in one of the samples fromthe present study. Similarly, the very low abundance ofbulk rock Ni in all the four corundum-chromian musco-vite assemblages (whose chromian muscovite has beenanalysed for elements excluding Ni) from southern Af-rica accounts for the paucity of Ni in their chromian mus-covite (see tables 2, 3 and 5 in Schreyer et al., 1981). Onthe other hand, the high levels of Ni (ranging up to 2220ppm) in the chromian muscovite-bearing rocks, especiallythe quartzites, from Outokumpu suggest that theirchromian muscovite should be richer in Ni. However, thepublished data on chromian muscovites from theOutokumpu quartzites do not include Ni because of itsconcentration below limits of detection (tables 4 and 8 inTreloar, 1987b and pers. comm., 1999). This implies thatNi must be present in some other phases, which occur inthese rocks and show greater affinity for Ni (most prob-ably sulfides, e.g., pentlandite). A further possibility isthat the Ni distribution in the Outokumpu quartzites ismost probably non-uniform and hence the publishedchromian muscovite data represent only the samples withlow bulk rock Ni.
CONCLUSIONS
1. Chromian muscovite (and tourmaline as well as em-erald) most probably originated due to the quartz vein-forming hydrothermal activity.2. The Si-rich hydrothermal fluids also contained Al, Be,B and K, and they invaded the host rocks (ultramaficprotolith) after their carbonate-alteration.3. The Cr, Ni and Mg components in chromian musco-vite were extracted by the hydrothermal solutions fromthe host altered ultramafic rocks through wall rock al-
Number in parentheses shows the number of analyses performed in thegiven sample.
Table 2. Relationship between fuchsite and hostrock nickel
teration. The highly variable amounts of these three met-als in the chromian muscovite analyses probably reflecttheir low mobility in hydrothermal processes.4. The Ni and Mg entered into the structure of the micaas a result of a heterovalent Tschermaks-type coupled sub-stitution.
Acknowledgments—The Association of Commonwealth Uni-versities in the UK financed the studies. Mr. Colin Cunningham,Mr. R. N. Wilson and K. A. Sharkey of the Department of Ge-ology, Leicester University (UK), helped in preparing polishedthin sections, performing probe analyses and carrying out XRDwork, respectively.
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