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Cqnadian MineralogistYol.24, pp. 147-163 (1986)
ZINCIAN SPINEL AND STAUROLITE AS GUIDES TO ORE IN THE APPALACHIANS ANDSCANDINAVIAN CALEDONIDES
PAUL c. SPRY'I AND STEVEN D. SCOTTDepartment of Geologlt, University of Toronto, Toronto, Ontario MSS IAI
ABSTRACT
Zincian spinel or gahnite (Zn,Fe, Mg)Al2Oa occurs inmetamorphosed massive-sulfide deposits, aluminousmetasediments, pegmatites, quartz veins, and metamor-phosed oxide-silicate deposits in at least forty localitieswithin the Appalachians and Scandinavian Caledonides.Most occurrences are associated with metamorphosedmassive-sulfide deposits, in which gahnite is considered toform predominantly by desulfurization reactions involv-ing a member of the system Fe-S-O and either sphalerireand garnet or sphalerite and alurninosilicate. Spinel inquartz veins and pegmatites is thought to be a product ofmetamorphic-hydrothermal solutions and magmaticprocesses, respectively. Spinel in aluminous metasedimentswas probably derived from the metamorphism of metal-liferous shales, in which rocks Zn may originally have beenIinked to organic material. Although gahnite in some sul-fide deposits coexists with zincian staurolite, textural ev!dence suggests that staurolite did not act as a precursor tospinel. The high Zn content in staurolite is likely the resultof desulfurization reactions. Staurolite from the Bleikvasslideposit (Norway) contains up to 8.77 wt.Vo ZnO and isthought to be the most Zn-rich yet recorded. Both gahniteand staurolite are most Zn-rich where associated with sul-fides and may constitute an exploration guide for massive-sulfide deposits in metamorphosed terranes.
Keywords: zincian spinel, gahnite, zincian staurolite,Appalachians, Scandinavian Caledonides. electron-microprobe data, exploration guide.
SoMMAIRE
Le spinelle zincifbre ou gahnite se presente dans les gitesde sulfure massif m6tamorphiques, m6tasddiments alumi-neux, pegmatites, filons de quartz et gites oxyde-silicatemdtamorphiques. On I'a trouv6 dans au moins quaranteendroits dans les Appalaches et les Cal6donides scandina-ves. La plupart des gites sont associds i des sulfures mas-sifs m6tamorphiques, dans lesquels on admet que la gah-nite s'est form6e surtout par des r€actions ded6sulfurisation, impliquant un p6le du systOme Fe-S-O etde la sphaldrite (blende) ainsi que du grenat ou un aluminosilicate. Le spinelle des filons de quartz est consid€rdcomme produit de solutions hydrothermales-mdtamorphiques, celui des pegmatites, comme resultant de
-P..*"t "dd*ss:
Department of Earth Sciences, 253Science Hall I, Iowa State University, Ames, Iowa 5001 l,u.s.A.
processus magmatiques, Le spinelle des metasffiments der-ive probablement du m6tamorphisme de schistes (sedimen-taires), roches dans lesquelles il se peut que le Zn ait 6tdcombin6 i des compos€s organiques. Quoique la gahniteet la staurotide zincifbre coexistent dans certains gites desulfures, les observations texturales font douter que lastaurotide soit l'avant-coureur du spinelle. La haute teneuren Zn de la staurotide r€sulte vraisemblablement des r6ac-tions de d6ulfurisation. La staurotide du gite de Bleikvassli(Norvdge), dont la teneur en ZnO atteint jusqu'i 8.7790(en poids), passe pour la plus riche en Zn connue i ce jour,Gahnite et staurotide sont toutes deux les plus riches enZn li oir elles sont assocides aux sulfures; elles peuvent doncguider la prospection pour gltes de sulfures massifs en ter-rain m6tamorphique.
(traduit par la Rffaction)
Mots-clds: spinelle zincifdre, gahnite, staurotide zincifCre,Appalaches, Calddonides scandinaves, donndes demicrosonde dlectronique, guide d la prospection.
INTRODUCTIoN
Zircian spinel, or gahnite (Zn,Fe,Mg)Al2Oa, iswidely distributed in the Appalachian-Caledonideorogen; based on experience here (Sandhaus 1981,Sundblad 1982) and elsewhere (Sheridan & RaymondlW7, 1984, Spry & Scott 1982, Spry 1984), this phasehas potential as a guide in the exploration formetfirorphosed massive-sulfide deposits. Among thevarious associations of zincian spinel observed, thatwith metamorphosed sulfide deposits is the mostcommon. This has prompted the suggestion (e.9.,Sangster & Scott 1976) that such zincian spinel is aproduct of desulfurization of sphalerite duringmetamorphism. Spinel in and around metamor-phosed massive-sulfide deposits forms a solid solu-tion between ZnN2O4 and FeAl2O4 predominantlyand is commonly spatially associated with sphaleritein addition to pyrite, pyrrhotite or magnetite (or var-ious combinations of these minerals). Calculation of
"f(Or-l(S, relations in the system Zn-Fe-Al-Si-S-O by Wall & England (1979), Spry & Scott (1983a)and Spry (1984), and experiments by Spry (1984) ona simplified SiO2-free system, demonstrate that gah-nite can indeed form by the breakdown of sphaleritecoexisting with either an aluminosficate or garnet.However, considering the assortment of environ-
147
148 THE CANADIAN MINERALOGIST
ments in which gahnite is found, other theories havealso been proposed to account for the formation ofgahnite. These include: (l) reaction of Zn-bearingsilicates [e.9., biotite and staurolite to form spinelduring metamorphism (Stoddard 1979, Dietvorst1980)l; (2) precipitation of spinel from ametamorphic-hydrothermal solution (Wall 1977);and (3) formation from a primary Zn-oxide phaseduring metamorphism (Seenit 1961).
The zincian-spinel-bearing rocks of theAppalachian-Caledonide orogen also contain stau-tolite that is amongst the most Zn-rich yet recorded(e.9., Sandhaus 1981, Spry & Scott 1983b). Zincianstaurolite spatially associated with zincian spinel isthought to be the precursor of spinel in some meta-morphic rocks (Atkin 1978, Stoddard 1979, Spry1982).
This paper documents some of the spinel-bearinglocalities in the Appalachians and ScandinavianCaledonides. We discuss the origin of the spinel inthe light of calculations of stabilify, experiments, tex-tural evidence and field associations, its potential asa guide to sulfide mineralization and the reason forthe anomalous content of Znrn staurolite with whichit is spatially associated.
ElBcrnoN-MICRoPRoBE DATA
The chemical composition of the spinel and ofcoexisting phases was determined at the Universityof Toronto @tec Autoprobe and ARL-EMX elec-tron microprobe equipped with Kevex and Ortecenergy-dispersion silicon detectors, respectively), andat the University of Adelaide (JEOL wavelength-dispersion Superprobe, model 733).
Operating conditions of the Etec Autoprobe andARL-EMX electron microprobe included accelerat-ing voltages of 20 kV and beam currents of 100 nA.Natural and synthetic spinels were used as standardsfor Zn, Fe, Mg and Al, garnet for Mg, Mn, Ca, Fe,Si and Al, rutile for Ti, synthetic pyrrhotite for Feand S, synthetic sphalerite for Zn, Fe and S, syn-thetic chalcopyrite for Cu, synthetic alabandite forMn, and synthetic cadmium sulfide for Cd. Theseelectron microprobes are connected to a PDP/IIcomputer system providing on-line reduction ofenergy-dispersion data by a modified version of thePESTRIPS progxzrm. The techniques used in the pro-gram were described by Statham (1975).
Operating conditions of the JEOL Superprobeincluded an accelerating voltage of 15 kV and a beam
TABLE 1. ZINCIAN SPII{EL LOCAI.ITIES IN TEE A?PAI,ACIUANS
LocaL3ty ceo log lca l c@Iogte l
1 A, Quebec2 Aubun, t{e3 lop6hd, lte4 DavL6, !{a6a
5 gharl"@dt, l{assIordre 8111, I'Iaoa
6 !{lddleto!, coN7 Frankun, NJ6 Sterll.g U111, NJ9 S.E. Ptusylvdl.a
10 sprlDgfield, ud11 UltreraL E111, UdL2 P6tap€co, l'1d13 Valzldco, VaL4 Sulphui, va15 Cofer, Va16 AmiDl.c, vaL7 Julla, va18 Jobnao!, VaL9 Ardel6onvLll-e
(#L8), va20 Stfatfold, Nq2L ole Krob, Nc22 Elk (oob, NC23 Daake, NC24 Spnce Pin€, NC25 Cull@bee, Nc26 sava@h, Nc27 W6yhutta, NC28 orto, NC29 st@dard, ca
30 I'tagede! , ca
31 Caqtoo, Ga32 tlttle Bob, Ga
33 Villa Rica, Ga
@dpeg@tIte?pegmtlte|@d
eulfLdes l t r a.s.?peg@tltepeg@tite@odsod4l@dwdl@d
rud
l@d
m6d
1@d
r@d, qv
@Bd, qv
a€ B.R.|@d B.R.@ d B , R .e a 3 . R .peg@ttte B.R.]@d B.R.@d B.R.l@d B.R.@ d B . R .auLftde vein6 ttr B.R.4aulflde veLda La B.R.
chev6 et a1. (1983)rhla studyrh16 atudyDaaa (1885), Bsertaofa (1972),
rteld & Saggercy (1984)
Flldt (1908)qact locatlou 6kromthis studyFlondel 6 Klelo (1965)cawalho & Selar (1979)Wagoe! & crasfold (1975)sha4trotr (!.923)Shannon (1923)Shatuoo (1923)Ro6s (1935)cralg (1980), saodhaus (1981)
l.{111er (1978)r Saadhau (1981)
cox (1979), Sadhaue (1981)cratg (1980), Saqdha@ (1981)
thls 6tudyth16 study
Sreock (1971)Ross (1935)Roee (1935)cedth (1891)thls atudyRoEe (1935)Roes (1935)Roes (1935)Rose (1935)Llddgletr (1906)
cofer (L953)
ceBth (L862), Roee (1935)
cook (1970) ' Ab!@ &Mccomell (1984)cook (1970), Neathery drbXllste! (1984)
@
l@d
@d
]@d
N , E .N . E .N . E .N . E .
B . R .B . R .
B . R .
a. Depoalt sbm 1o Fig. l " ; N.E. Ns Englatrd; @d seta-@rphosed @6slve gulfide ileposit; a€ al@iao@ Detasedfueqti qe qsrtz vetni @od meta-turphoeed zLnc-oaLde depoalt.
ZINCIAN SPINEL AND STAUROLITE AS GUIDES TO ORE
TABLE 2. ZINCIAN SPINEL L@AIITIES IN IgE SCANDD{AVIAN CALEDONIDES
149
!ocaL-ItyNo.' NaEe
Cs]-ogtcal TectoticSetthq Unlt Reference
1
2
4
56
7
BLelkva8rLl, Nor.
CrskevardoRLpudilal, Swe.ltofjelL, Nor.
Themoe, Nor.
NoBEfJeLLet, Nor.Sklmsdalo, Nor.
VtlLdalsfJ e1t,Nlitrge!, Nor.
R6drngEfJgUetNappef,6dlugsfJal1stNappeRddlogsfJALI-etNappeRiitiogsfJAt"LetNsppeBelam NappeR6alttrgsfJilletNappeBeiam Nappe
Vokes (1.962)
Sudblaat (1982)
sudblad (1982)
Sundblaat (l-982)
Ssdblaal (1982)Jwe (1967)
vokes (L962)
@d
wd
l@d
@d
@d@d
a. Depoalt ahm Ln FIg. 2; Nor' Nomay; Sue. Sv6alen; @d Detmrphosed Maslvesulflde deposlt.
crurent of 2m nA. Natural spinels, simple oxides andmetals were used as standards. The JEOL Super-probe is connected to a PDP/ll computer, whichreduced data by a FORTRAN IV program using atype of correction proposed by Duncumb & Reed(1e68).
ZINCIAN SPINEL.IN THE APPALACHIAN -
SceNonevnn CalBooNtos OnocrN
Zincian spinel is known from at least forty locali-ties in metamorphosed volcanic and sedimentaryrocks in the Appalachians and ScandinavianCaledonides (Tables l, 2) but, in general, is poorlydescribed and treated as a mineralogical curiosity.Occurrences in the U.S. Appalachians are found inmetamorphosed massive-sulfide deposits, aluminousmetasediments, pegmatites, quartz veins and Zn-richoxide bodies within the Blue Ridge and Piedmontgeological provinces and in the probable extensionof the Piedmont province in New England @ig. l).The only reported locality of gahnite in the Cana-dian Appalachians is that by Chev6 et al. (1983), whonoted its occurrence in the small stratiform massiveA sulfide deposit, Eastern Townships, Quebec.
According to Sunblad (1982), all known gahnitelocalities of the Scandinavian Caledonides areassociated with metamorphosed massive-sulfidedeposits contained within the Rddingsfjiillet andBeiarn Nappes of Norway and Sweden (Table 2).These nappes predominantly consist of gneisses andmarbles that were metamorphosed to the amphibo-lite grade. Gahnite-bearing massive-sulfide depositsin the Rcidingsfjiillet Nappe include the Bleikvassli,Mofjell, Thermos and Skbrndesdalen deposits ofNorway and the Graskevardo and Ripudden depositsof Sweden (Fig. 2). The-Villdalsfjell and Niingendeposits are located in the Beiarn Nappe.
Field and textural relations
Of the various geological settings in the
FIc. l. Location of zincian spinel occurrences in theAppalachians. Names of deposits are in Table l.
Appalachian-Caledonide orogen in which gahnite isfound, the association between gahnite andmetamorphosed massive-sulfides is the mo$t com-mon. Gahnite is most often contained within sulfidezones; however, it is also found in quartz veins cross-cutting sulfide zones and in aluminous rocks sur-rounding sulfides. Because of the variable nature of
tffit/"{:;ffi
;d
#r,[€
THE CANADIAN MINERALOGIST
Flc.2. Location of zincian spinel occurrences in the Scan-dinavian Caledonides (modified after Sundblad 1982).Names of deposits are in Table 2.
lM 3. m ssml$s oF zNcN srM-N ZNCN 8TAIBOIIN-BNIXG Sffi
reg5 l t B le tbass l l q t ,w ' 6p , ph , s t ,cp ,aa . * , "p td
,pd 'pod, . td 'rc955(1)h B le lkvss l t ry ,ph ,s t , de ,sp ,6p1d, rud ,z ropcgss(z ) ! B le lbd6 l t qz ,sp ,w,p ,qp ,sp td
mgss( : )b B1€tkes6 l t qz ,s t ,m,sprq ,p f ,Fo ,b tq
res-56( r ) ! . Bre lkvsE l t py ,qz ,sp1,s ,sp ,s t ,b t ,cpd 'Rd 'p lo
wsa (z )b r161hs6 l1 qz ' sp ' s l ,D , py ,po , q t , s ,sPro ,cpd, f 10
rcF56(3)b Bre t :ms l t s t ,b t ,Eu,F ,sp1,Fq,cDr ,p1apcy65b Bre lka6s l l qz ,ep ,py ,b t ,F ,cpa,c lo€pcgoz! Bktbvassu st'qz,sp,ph,po,pyre$681 Bl€lhss1l p,sp,qz,ph,spl'Py's
Pos69i Btsibassll sp,qzrc#70b B leks1 l q3 ,py ,b1 ,sp ,s ,P ,aP1,cp 'm
ms-974 bGf l€116! b 's 'qz ,Bp,py ,8a ,gd ,sp l ,cd , ruo '11ore9984 f rg@s qz '6p 'py 'hb ,8a ,M'cP ' ru"
re$96a stlasdsler qr,ga,@,sP,sP1'bl.s,p1
!l4il36 Arbun pl,@,ga,ap
n9r603 T4sh qa ,p l ,sp
reF24: A ld€rsoNl l l€ q t , py ,b t , sP,hb ' , sPI ,36r ' ' eP ' , p l '
Ee72: Aidst$d116 gzre!
Pc$85D &d€tsoad11€ qz,6P
PC6-844 JollM qu,a,ll
x43573b slerlhg Btll hd'c6'sp'Ba'pF145056b Stsrlhg su1 @,Bp,rh,sp1,c1G47a96c. hd iD rh ,6p ,ca ,b ,qU253I_D DNls @'qz ,4 ,PY ' I I .ga 'cP '
!O1317D . hvts Pt'qt'6P'cPes-104: kvlB 6p,cp'py,qz,cac8O-469 Dd6 py ,sp t ,aa , ry ,P ,qz ,bJ ,cP
c8G4tD Dsts py 'qz '3p ,sp1,8a ,F ,c ] " ,cP
GBG5D- Ms qz ,py ,d ,p t ,sP,sPre
G8o-26: Dads qzrpr ,d ,8a ,sP '@ -r80-45D h tu qz ,u ,Py ,apr ,4 'cP '
1285303b charl@t (rrsp
393064 hrd is 8111 p1 ,q t ,d ,c r ' sP
i l9554 Hrera l K l I sp ,p I ,u ,b l '€P 'qz ,cp 'm 'c le ,sP1 'ch
94677a. spthgfhld s9,q2,@,c11bi8/77t2o utcbell &. sp81957D s1sL6 Hdga sp,fl,qz
120967-53c sttatfoid sp,q2,s!'tE'F1952/c ktoo qe,cp'py,st,q,ql,gr,cho€
r filil-G;ppffidaaof abdec6; a Mlysa of 8tulte 116td
h Tabl6 4i b e1y6€s of Sahlte llstd 10 3pry (1984) i c @lyst6 of
shuollt€ 1let€d tu Tabl€ 5; d tuetal preaeut la tlac€ am6tai
o tural ls secoodaryi e aathophylllto; bl blotlte; ca calclt6;
cal cordtolltei ch chalcocitei cl chlorltai cp chrlcoPFlt€; 6 cryout€i
e epldotel ft ftbroltta; ft franLltllte; 8a ge@ti 8d g€dttte;- ga.galeui
hb lDnbl@de; la bedeabergito; l@ llMtlt6; 11 thtt6; lh hobellte;
B uga6tlt€; fl Ncovltsi Pi Pblo84lt€t P1 plaalocls€i F pynhotite;
W pyrlt6: pF pFoph@tte; qe qurtzl rh rhodoalte; n ntllei
s;r sc1cllei a ,lfth sPtn€1i Bpl sPhal€tlla; st atauroltle; zt ,ltcor'
the geological settings, it is hardly surprising thatgahnite is associated with a variety of minerals (Iable3). Representative compositions of spinel fromseveral deposits, determined by electron microprobe,are presented in Table 4. Additional compositionsare lisred in Spry (1984).
The most detailed study to date on gahnite in theAppalachians is by Sandhaus (1981), who determinedits distribution, chemistry and possible origin in theArminius, Cofer, Julia and Sulphur deposits in theMineral District, Virginia. These deposits are locatedin a sequence of metapelitic sedimentary and maficto felsic volcanic rock$ (Southwick er aL l97l) withinthe Piedmont geological province. Although gahniteis distributed throughout the sulfide deposits, Sand-haus (1981) noted that it is most abundant near ore-wallrock contacts. According to Sandhaus, gahnitein ore zones is predominantly associated with quartz,biotite, garnet, pyrite, sphalerite, chalcopyrite andgalena. Magnetite, pyrrhotite, staurolite, kyanite,amphibole, chlorite and muscovite are associatedwith gahnite in lesser amounts. Sandhaus (1981)reported that gahnite coexists with sphalerite;however, an antipathetic relationship between theseminerals is suggested by the distribution within thedeposits. Despite this relationship, gahnite is poi-kiloblastic and uncorroded where in contact withother phases, including sphalerite.
The Davis mine, Massachusetts, is one of severalmines and pits that lie along a vaguely definedmineral belt within volcanic and sedimentary rocksmetamorphosed to the lower or middle amphibolitegrade (Bwerinofa L972). Gahnite is associated withpyrite in massive sulfides and magnetite in sulfide-bearine chlorite-mica schists surrounding the deposit@werinofa 1972), as well as with pyrite quartzitesand quartz-albite metatuffs (Slack e/ a/. 1983). AtBleikvassli, Norway, spinel occurrences are confinedto massive sulfides, quartz veins and quartz-richschists in the ore zone (Vokes 1962).In massive sul-fides, gahnite coexists with sphalerite, pyrite, pyr-rhotite, quartz and garnet, whereas in quartz-richmuscovite schists it coexists with quartz, phlogopite,muscovite, zincian staurolite, garnet' sphalerite,pyrite and pyrrhotite. There is a particularly closeassociation between gahnite, garnet, quartz and Fesulfides (pyrite or pyrrhotite, or both) and sphaleritein samples from the Davis and Bleikvassli deposits.Sillimanite, although rare, may also coexist with ga}-nite at the Bleikvassli deposit. Gahnite and garnet
exhibit euhedral grain-shapes and noncorroded con-tacts where in contact with the other members of thesystem Zn-Fe-Al-SiS-O (Figs. 3a' b).
Juve (1967) reported on gahnite in lead-zincdeposits in the Hafje[ syncline, Norway. This gah-nite occurs in the ore zone and its immediate wall-rocks, with the grain size and frequency of spineldecreasing away from the ore zone. As with the
ZINCIAN SPINEL AND STAUROLITEAS CUIDES TO ORE 1 5 1
1SLE 4. @reSMATIW COreITIMs OF SPIW ROU ru NP&CTHS N 8(NIMVH CNfMNDSA
slo2r 0.00 O.O0 O.OO O.m 0,05 0.OO 0.00 0.05 0.05 0.00 o.0o o,I5 0.07 o.o5 0.04 0.olt lo2 0.00 0.00 0.00 0.00 0.00 0.m 0.00 0.@ 0.00 0.00 0.00 0.00 0.02 0.02 0.02 0.04^Lt93 36.42 56.06 57.09 47.47 53,71 56.47 s6.17 56.83 51.24 J8.08 55.78 51.34 57.69 56.25 5?.37 t7.80F a & 9 . 6 1 1 . 4 6 6 . 4 1 1 0 . 9 1 5 . 2 9 5 . 3 9 5 , t 7 S . 2 O 1 . 2 9 ? . 6 0 4 . i 9 5 , 1 9 6 . 0 6 5 . 8 9 S . A 5 5 . L 2ho 0,36 0.35 0.29 0.61 0.12 0.m 0.00 0.03 0.12 0.00 0.00 0.00 0.14 0.02 0.07 0.07!s0 0.m 0.16 0.83 0.00 t . t4 1.54 l .16 1.66 t- .76 1.93 t .O9 1.12 1.90 0.59 r.92 t .7t& o n d n d a d d o d 0 , 2 3 r . 3 4 d n d ! d i l d n d d s d dho 32.94 37.00 )5.29 40.49 40.89 37.65 36.88 35.76 33.42 32.95 40.46 35.43 36.81 35.36 33.64 35.56Total 99.33 101.03 99.91 99.€ 10t.20 101.28 101.32 99.53 99.89 100.50 10t.72 100.83 101,69 99.16 100.51 1m.63
stT1A1
Vg&Zn
Atodc proF.tlotu (ory8e! bst€ 24)
0,000 0.m0 0.m0 0.000 0.m8 0.000 0,000 0.009 0.010 0.000 0.000 0.026 0.0u 0.009 0.006 0.m60.0m 0.000 0.000 0.000 0.000 0,000 0.@0 0.000 0.000 0.000 0.000 0.m 0.003 0.003 0.003 0.002
11.818 U.638 12.060 10.703 11.538 11.857 11.824 11.857 t2.OlO 11.926 X1.785 11.965 1t.946 12.031 l l .99l L2.O631.428 1.098 0.961 1.741 0.810 0.80{ 0.a62 0.769 1.085 1,107 0.659 0.768 0.S9 0.894 0.509 0.7970.054 0.052 0.044 0,099 0,018 0.000 0.002 0.005 0.018 0.000 0.000 0.m0 0.020 0.004 0.011 0.0090.000 0.040 0.221 0.000 0.310 0.408 0.357 0.437 0.466 0.492 0.289 0.d54 0.495 0.16t 0.506 0.474
- 0.032 0.165 nd4.323 4,809 4.668 5.721 5.507 4.959 4.864 4.676 4.393 4.236 5.356 4.162 4.&a 4.912 4.661 4.652
Erfti z uvlbs lbpa@r &hsi I lrl06ri Lold's UlU' &daachu€ett6; 4 4789H rfankllu, Nar J€rB€y; 5 14505F Starltry UlI, Nc Jdrdot;6 94677+ sPrlaafleld, hryland; 7 Rl95F &€!al trtU, hryled; I reS-24 hd6r6onv11l€, vtr8tnla: 9 PGs-84 JohrBon, vtlatnta; l0 120967-S srrdtfold,North hrollEi ll U957* slBloa UdBe, Notth &roliui 12 79522+ hron, &oraiai 13 PCS-54 Elsibas€ll, lomayi 14 PGs-t6 sk8nssdaloo, &turi 15 res-9,nostldllet' lonay; 16 PS98 frolms, Nodayi + byal Ootsrlo tus€e Gtaloguo no.i + Sdthso!&n Insrlruro &ialo$€ do,i *a turtcan hse@ of natulalUlBtory &talogue no.i r Total !e a6 F6O: !d lor d6t6d!.di a ntd€lsl assedtaa€s llstsd ln Tabl6 t.
aforementioned Mineral District, Davis and Bleik-vassli deposits, gahnite and garnet from the ffiqeUsyncline form porphyroblasts that appear to be intextural equilibrium with quartz, Fe sulfides andsphalerite. According to Ross (1935), gahnite wascommonly observed in sulfides associated with theOre Knob, Wayhutta 4ad lalzinss deposits in thesouthern Appalachians. Samples were not inves-tigated in this study; therefore, the textural relationsbetween sulfides and gahnite are unknown.
In the Thermos deposit, gahnite occurs withinpyroxene-amphibole-garnet-rich layers Nik 1977,Sundblad 1982) and at Nonsfjellet, gahnite appearsin garnet-mica schists (Sundblad 1982). Coarseeuhedral spinel at Thermos formed in rocks contain-ing pyrite, quarlz, garnet and hornblende. Thespinel-amphibole association is also present atNonsfjellet, where porphyroblastic zincian spinelcoexists with gedrite, anthophyllite and hornblenqe(Figs. 3d, 4a) and in Swedish deposits, where Sund-blad (1982) noted that gahnite coexists withanthophyllite and cummingtonite.
Zn spinel has been found in aluminous metasedi-ments adjacent to metamorphosed sulfide depositsin Bleikvassli, Davis and the Mineral District, withdisseminated sulfides at Charlemont @lint l90S) andStratford, in sulfide-free metasedimentary rocks insoutheastern Pennsylvania (Wagner & Crawford1975) and the Deake mica mine (Genth l89l). cah-nite at Stratford is associated with quartz, stauro-lite, hematite and pyrrhotite, whereas lsselding toFlinr (1908), gahnite at Charlemont coexists withtremolite, chloritoid, feldspar and quartz.
Although gahnite at Ore Knob, Wayhutta and Val-zinse is commonly spatially associated with sulfides,it is also associated with quartz veins or as a replace-ment of quartz-plagioclase segregations. Similar.sulfide-free associations occur at Andersonville,Johnson, Magruder and Standard mines. Gahnite iscoarse grained and euhedral where in contact withquartz and plagioclase. Pegmatites at Auburn
(Maine), Topsham Maine) and Spruce Pine (NorthCarolina) contain gahnite coexisting with quartz,muscovite, plagioclase and other accessory mins1al5.The granoblastic intergrowth among all of theseminerals suggests that gahnite is a primary phaserather than a second€fy minslal.
An unusual and rare blue Co-bearing spinel isreported from the Mineral t{ill, Springfield andPatapsco Qs minss, Maryland, by Shannon (1923).One of his analyses of spinel from the Mineral Hilldeposit gave 1.48 wt.9o CoO. Spinel analyzed by usfrom the Mineral Hill and Springfield mines contains1.34 and 0.23 wt,.$/o CoO, respectively (Table 4).Magnetite in the latter deposit contains 0.83 wt.9oCoO.
Phase relations
Since occurrences ofgahnite in the Appalachian-Scandinavian Caledonide orogen are most oftenassociated with metamorphosed massive sulfides, itis pertinent to look at phase relations in the systemZn-Fe-Al-Si-S0 to determine whether gahnite couldconceivably have been formed by a desulfurizationmechanism in some localities. This simplified sys-tem does not take into account all phases that maybe involved in desulfurization mechanisms, but itdoes incorporate co[lmon phases such as gahnite,alrnandine, aluminosilicate, quartz, sphalerite, pyr-rhotite, pyrite and magnetite.
A Schreinemakers analysis (Schreinemakers 1965)for the system Zn-Fe-Al-Si-S-O (Fie. 5) cau be com-pared with calculated phase-relations using twonatural examples, Mineral District and Bleikvassli,where gahnite is intimately associated with sulfides.For the Mineral District, calculated phase-relationsfor the system Zn-Fe-Al-Si-S-O are projected ontothe /(O)-/(S) plane of Fe-S-O (Frg. O for peakmetamorphic conditions of 465oC, 4.6 kbar,XF$ilsiro,z: 0.55 lbased upon the average ofSandhaus's (1981) microprobe data on garnet co-
ZINCIAN SPINEL AND STAUROLITE AS CUIDES TO ORE 153
existing with gahnitel and taking into account theazns, eFes and the thermal expansion and compres-sibility of sulfide phases. Peak metamorphic condi-tions of 465 oC and 4.6 kbar are based upon garnet-biotite geothermometry and sphalerite geobarome-try (Craig 1980). For the Bleikvassli deposit, phaserelations (Frg. 7) assume peak metamorphic condi-tions of 505oC and 3.8 kbar and Xf$iilsirorz = 0.56[based upon the average of Spry's (1984) microprobedata on garnet coexisting with gahnitel. The presenceof both kyanite and sillimanite at Bleikvassli requiresa minimum temperature and pressure of 505'C and3.76 kbar (Holdaway l97l). This temperature agreeswith a mean value of 487"C (431'C) obtained bySen & Mukherj ee (1972) from sulfur isotope geother-mometry. Sphalerite coexisting with pyrrhotite orpyrite + pyrrhotite from a number of samples hasa composition ranging from 9 to 12 mole Vo FeS.Such compositions yield pressures in excess of 7.6kbar and appe.u to be the result of re-equilibrationupon cooling. Calculations for Figures 6 and 7 weremade using standard free-energy values obtainedfrom Robie et al, (1978) for kyanite, corundum,qtJartz, magnetite, hematite, iron and sphalerite,from Jacob (1970 for gahnite (from the oxides) andzinc oxide, from Richardson & Jeffes (1952) for troi-lite, and the standard Gibbs free-energy change forthe reaction:
FeS + 0.552: FeSz (l)
from Froese & Gunter (1976), The standard free-energy for almandine was derived using the techniqueof Froese (197 3). The free-energy change in the oxi-dation of almandine (data from Froese) is based onthe magnetite-wiistite buffer of Eugster & Wones(1962) and on the quartz-fayalite-magnetite bufferof Wones & Gilbert (1969). So as to remain inter-nally consistent, values of free energy of ahnandinefrom Froese (1973) were rederived using free-energyvalues of magnetite from Robie e/ al. (l%8). An idealionic-solution model was assumed for the almandine-and spessartine-rich garnet based on the results ofGanguly & Kennedy (1974).
Four studies (Pillay el al. 1960, Rezukhina e/ a/.1963, Mclean & Ward 1966, Chhn et al. 1973\havederived values of the free energy of formation of her-cynite. The close agreement in the values of standard-state entropy at298K of Rezukhina et al. (1963) andChan et ol. (1973) usine electrochemical techniques
with that derived from calorimetric measurementsby King (1956) would appear to indicate the superi-ority of these thermodynamic data to those of Pil-lay et al. (1960) and Mclean & Ward (1966), wherethe agreement is not as good.Prllay et al. determinedfree-energy values for hercynite by equilibration ofliquid Fe, corundum and hercynite with H2-H2Ogas mixtures, whereas Mclean & Ward derivedvalues by measuring the oxygen concentration ofliquid Fe in equilibrium with hercynite. Despite apreference for data derived using electrochemicaltechniques, there is excellent agreement between cal-culated chemical compositions of spinel on theZnAlrOo-FeAl2Oa join using Jacob's (1970 free-energy data for gahnite with data for hercynite fromPrllay et al. and Mclean & Ward, assuming idealityfor the gahnite-hercynite solid solution, and theexperimentally derived spinel compositions of Spry(1984). The ideal nature of the gahnite-hercynitesolution is proposed because extremely low valuesof excess enthalpy are obtained (Spry 1984, Spry &Scott, in prep.) utilizing the procedure of Jacob &Alcock (1977), This procedure estimates cation drs-tributions using octahedral site-preference energies.The apparent absence of gahnite-hercynite exsolu-tions in natural assemblages, although not diagnos-tic, supports the view that the excess enthalpy-of-mixing term is small, and the gahnite-hercynite solidsolution is close to being ideal. Spry (1984) showedthat computed spinel compositions using the data ofPrllay et al. (19@) and Mclean & Ward (1966) arealmost identical. The data of Mclean & Ward havebeen used in this study.
In constructing Figures 6 ard 7, the alm-sph-spboundary in the pyrrhotite field is calculated by solv-ing two equilibria:
Fe3A l2S i3O,2+ZnS+52 :Zn&zo|+ 3FeS + 3sio2 + 02 A)
Fe3Al2Si3O12 + 52:FeAl2Oa + 2FeS + 3SiO2 + 02 (3)
To obtain the dm-al boundary, the followingequilibrium is used:
Fe3Al2Si3O12 * 1.5S2 =3FeS + Al2SiOj + 2SiO2 + 1.502 @)
Ftc.3. a. Gahnite (g) and garnet (ga) porphyroblasts in a pyrite (p), sphalerite (s) and quartz matrix, Chlorite (c) formedas a retrograde product; Davis mine, Massachusetts. b. Gahnite-garnet layers in quartz-rich sulfide schists, The opaquemineral is pyrite; Davis mine, Massachusetts. c. Garnet appears to have replaced gahnite in a rock rich in quartz,phlogopite (ph) and sulfide. The sulfide (opaque) consists of pyrite, pyrrhotite and sphalerite; Bleikvassli, Norway.d. Subhedral porplyroblasts ofgahnite intergrown with gedrite (ge), anthophyllite (a), hornblende (h) and quartz.The opaque mineral is pyrite; Nonsfjellet, Norway.
t54 THE CANADIAN MINERALOGIST
ZINCIAN SPINEL AND STAUROLITE AS GUIDES TO ORE 155
whereas for the sp-al-sph boundary the followingtwo equilibria are solved:
ZnAJ'O + SiO2 + 0.552 =ZnS + AlrSiOs + 0.502 (5)
FeAIrOo + SiO2 + 0.552 =FeS + Al2SiO5 + 0.502 (6)
The association of magnetite-pyrrhotite-pyrite inthe Arminius and Sulphur deposits, Mineral District,provided buffering of /(SJ and /(O) ar about10-5'2 and 10-2.6 bars, respectively (Frg. 6). Aslightly lower value of/(OJ and a similar value of/(S) are likely in the Julia deposit, where pyrite andpyrrhotite, but not magnetite, coexist. Magnetitedoes occur in the deposit but is corroded, suggest-ing that /(OJ and /(S2) condirions may initiallyhave been close to the pyrite-pyrrhotite-magnetite(PPM) triple point but subsequently declined (Sand-haus 1981). A common assemblage described bySandhaus is that involving gahnite, garnet, sphaleriteand quartz. According to Figure 6, this assemblagewould be expected at relatively high /(O) condi-tions. The coexistence of gahnite, garnet, quartz,sphalerite and pyrrhotite (Sandhaus l98l) suggesrs
"f(O, -
"f(S, conditions on the abn-sph-sp bound-ary. Despite this, there is no textural evidence to sug-gest that garnet and sphalerite are breaking down.One assemblage described by Sandhaus (1981) thatinvolves gahnite, kyanite, pyrite, sphalerite andquartz implies that/(O, -/(S) condirions were insome places on the equilibrium boundary defined bythe following:
2Al2SiOs + ZnS + FeS, + O, =Z\N2O4+ FeAl2Oa + 2SiO2 + 1.552 e)
Pyrite and pyrrhotite are commonly associatedwith gahnite in the Bleikvassli deposit, whereasmagnetite is an uncommon, but locally abundant,mineral. The presence of pyrite, pyrrhotite andmagnetite requires that f(O) - /(S) conditions wereclose to those expected at the PPM triple point (Fig.7). The presence of the assemblages pyrite, sphalerite,gahnite, pyrrhotite and garnet (Fig. 3c) and pyrite,
sphalerite, pyrrhotite, gahnite and sillimanite sup-ports this prediction.
A Schreinemakers analysis (Schreinemakers 1965)for the system Zn-Fe-Al-Si-S-O in
"f(O, -
"f(S,space shows that five univariant reac'tions have slopesof one-to-one and intersect at an invariant point (Fig.5). Usine Sandhaus's (1981) microprobe data forgahnite from the Mineral District u1i1 lsgpalizingthe gahnite and herclmite contents to 10090 yield aratio of 83 mole 7o gahnite to 17 mole 9o hercynite,which disagrees with the ratio of 98 mole go gahniteto 2 mole Vo hercynite predicted at the pyrite-pyrrhotite boundary (Fig. 6). Our calculations alsopredict a value of 90 mole 9o gahnite (10 mole 9ohercynite) for spinel from Bleikvassli along thepyrite-pyrrhotite join (Fig. 7), whereas analyzedcompositions normalized to l00Vo contain approxi-mately 80 mole 9o gahnite (20 mole 9o hercynite).In addition to the lack of agreement between calcu-lated and natural compositions, reactions involvinggahnite, garnet, aluminosilicate, pyrrhotite andquartz do not intersect at an invariant point (Figs.6, 7) as predicted in Figure 5, These disagreementsare probably due to uncertainties in the free-energydeterminations of gahnite, hercynite and almandinewhen extrapolated to the lower- to middle-amphibolite grades. Spry (1984) has documented theexcellent agreement between calculated and naturalcompositions of zincian spinel at upper amphiboliteto granulite grades. Furthermore, the sequence ofreactions in Figure 7 is inconsistent with that shownin Figure 5. This inconsistency results from the uncer-tainty in free energies of formation of gahnite, her-cynite and almandine and the fact that the alm-aland alm-sph-sp boundaries will shift in /(OJ -
/(Sr) space depending on the choice of the activityof almandine in garnet (ffi), whereas the sp-al-sphboundary will remain fixed regardless of aSfr,.
Genetic specalations
Speculations concerning precursors to gahnitedepend on the preservation ofunmetamorphosed orslightly metamorphosed Zn-bearing rocks at gradesbelow the stability of gahnite. Despite the apparentabsence of obvious precursors to gahnite, the associ-ation of sphalerite and pyrrhotite or pyrite makesthese sulfides likely. It is also likely that aluminosili-cate or garnet breaks down to provide Al and, in thecase of garnet, Fe and Mg, for the formation ofgahnite.
Ftc.4. a. Subhedral gahnite (g) and garnet (ga) porphyroblasts intergrown with quartz (q), gedrite (ge) and hornblende(h). Chlorite (c) has replaced garnet; Nonsfjellet, Norway. b. Intergrowth of subhedral gahnite and stauolite (s)with phlogopite (p) and pyrite (opaque); Bleikvassli, Norway. c. Corroded staurolite in a layer rich in phlogopite,muscovite and quartz adjacent to a gahnite-quartz-pyrite layer. The opaque mineral is pyrite; Bleikvassli, Norway.d. Corroded staurolite and gahnite intergrown with sulfide (opaque) and quartz. Sulfide consists of pyrite, chal-copyrite and sphalerite; Canton mine, Georgia.
156 THE CANADIAN MINERALOGIST
al- alumlnoslllcatealm-almandlnep-pyrrhotlte
sph-sphalerltesp-splnel
rog fsz
Ftc. 5. Schreinemakers analysis of the system Zn-Fe-Al-Si-S-O in/(O) -/(S) space
al
,/|\ + quartz
t,"ln\po sph
al/f\
p9 spnt€fq
In proposing a mechanism for the formation ofgahnite at Broken Hill, Australia, Segnit (1961) sug-gested a reaction between kaolinite and adsorbed Znoxide. A slightly modified form of Segnit's reactionis:
Al2si2o5(oH)a* zno:Z\AI2O4 + 2SiO2 + 2H2O (8)
Althouelr this reaction could conceivably account forthe formation of gahnite in aluminous meta-sediments, there is no documentation of ZnO beingadsorbed onto clays. The available literature onmetalliferous shales suggests tJrat Zn may have origi-nally been linked to organic material (e.9., Vine &Tourtelot 1970, Coveney 1979, Coveney & Martin1983) or phosphate material (Coveney 1979). Thesematerials may subsequently have been removed andthe remaining Zn rcacted with sulfur to formsphalerite. Although no examples have beendocumented in the literature, sphalerite may subse-quently react with aluminous clays (e.9., kaolinite)to form gahnite. In this context, it is significant tonote that slates in the Venn-Stavelot Massif (Kramm
1977) contain gahnite that is intimately associatedwith viridine. In an earlier study, Kramm (1973)showed on the basis of textural evidence that viri-dine and andalusite formed directly from kaolinite.Metamorphism has obliterated precursor phases togahnite in aluminous metasediments in theAppalachian-swedish Caledonide orogen; however,it is conceivablethatZn originally linked to organicor phosphatic matter formed sphalerite and subse-quently reacted with aluminous clays (e.g., kaolinite)to form gahnite by the following reaction:
AI2Si2O,(OH)4+ ZnS + 0.502 :znAl2o4+ 2sio2 + 2H2O + 0.552 (9)
There is a strong possibility that gahnite in peg-matite at Auburn, Topsham and Spruce Pine crys-tallized directly from peraluminous granitic meltsbecause there is no textural evidence to suggest thatgahnite formed as a secondary product. Similarspinel has been reported in acid volcanic rocks else-where (e.g., Phillips et ol. 1981, Tulloch l98l).
Gahnite in quartz yeins that cut across massive sul-fide zones are not spatially related to acid igneous
ZINCIAN SPINEL AND STAUROLITE AS CUIDES TO ORE 157
Frc.6. Calculated phase-relations for the system Zn-Fe-Al-Si-S-O projected onto the Fe-S-O plane as a function of
/(S) and/(O) for peak metamorphic conditions (465'C, 4.6 kbar) affecting the Mineral District' Virginia. See textforlources'of data used in the calculations. Mole Vo ZI!'J2O4 for the gahnite-hercynite solid solution is indicatedon the broad dashed line.
rocks and are thought to have precipitated from aSi-rich hydrothermal solution during metamorphism.
ZINCIAN STAURoLITE IN THE AppeIecTlm _SCANDINAVIAN CALEDoNIDE OnocsN
According to synthesis experiments on Zn- andZn-Fe-staurolite by Griffen (1982), there is no limi-tation to the amount of. Znthat can be incorporatedin staurolite. Despite this, in natural specimens Zncontent has been previously reported 1e lange onlyfrom 0 to 7.7 wt.Vo ZnO. The incorporation of Zns1alilizes staurolite to the upper amphibolite gradein sulfide-free rocks (e.g., Ashworth 1975). However,the most Zn-rich staurolite appears in sulfide-bearingassemblages that have been metamorphosed to thelower or middle amphibolite grade. AnomalouslyZt-rich staurolite is found in a number of depositsin the Appalachian-Scandinavian Caledonide oro-gen. For example, in the Mineral District, staurolitecoexisting with zincian spinel and sphalerite contains
up to 6.9 wt.clo ZnO (Sandhaus l98l). Representa-tive compositions of staurolite that coexist vdth spineland sulfide from three localities in the Appalachian-Scandinavian Caledonide orogen are listed in Table5. Additional compositions are grven by Spry (1984).Included in Table 5 for comparison are data forstaurolite from the Kiipu deposit, Finland, whichoccurs in Precambrian volcanic and sedimentaryrocks metamorphosed to the lower amphibolitegrade.
Field and textural observations
Mineral assemblages of Zn-staurolite-bearingrocks studied are included in Table 3. Zincian stauro-lite from the Bleikvassli deposit contains up to 8.77wt.olo ZnO and is believed to be the most Zn-tichyet recorded. Such staurolite commonly occurs incontact with gahnite and pyrite at Bleikvassli; theredoes not appear to be any reaction between the Zn-bearing phases @ig. 4b). In some specimens,moreover, corroded staurolite in layers rich in mus-
quartzFe3O4
4.6ruARF"ZO3
FeS2
158 THE CANADIAN MINERALOGIST
FIc.7. Calculated phase-relations for the system Zn-Fe-Al-Si-S-O projected onto the Fe-S-O plane as a function of/(S) and/(O) for peak metamorphic conditions (500'C, 3.8 kbar) affecting the Bleikvassli deposit, Norway. Spinelcompositions are given in mole 0/o ZnAl2Oa for gahnite-hercynite solid solutions. See Figure 6 for the explanationof symbols.
covite and phlogopite is isolated from subhedralgrains of spinel that are confined to quartz-sulfideIayers @ig. 4c). Although staurolite and gahniteappear to be corroded in the Canton deposit @ig.4d), coexisting subhedral staurolite, gahnite, quartzand pyrite appear to be a stable assemblage at Strat-ford and at Kiipu.
Genetic speculations
Gahnite - zincian staurolite reactions have beenproposed elsewhere (e.g., Atkin 1978, Stoddard1979, Spry 1982); however, there does not appearto be any reaction bstween these phases at Bleik-vasst, Canton, Stratford and Kiipu. Rather thanstaurolite breaking down to gahnite at Bleikvassli,the presence of corroded staurolite and muscovite,euhedral garnet and rare sillimanite in the biotite-muscovite layers suggests the following reaction
(Thompson & Norton 1968):
staurolite + muscovite + quartz :sillimanite + almandine + biotite + H2O (10)
Pafi of the Zn released from the breakdown ofstaurolite is incorporated into biotite; however,where the remainder of Zn goes is unknown.
There is no textural evidence to support the for-mation of zincian staurolite by desulfurization ofsphalerite and pyrite; however, by analogy, it is pos-sible that staurolite, like gahnite, formed by desul-furization reactions, for example:
[2ZnS] in sphalerite + [2FeS] in pyrrhotite +9Al2Si5 + H2O * 2O2= [Zt2AleSi4O8(OH)]in staurolite + [Fe2AleSiaOz:(OH)] in staurolite
Fe203500 'c, |(BAR
FeS2
-8togfs2
+ SiO2 + 2S2 ( l l )
ZINCIAN SPINEL AND STAUROLITE AS GUIDES TO ORE
TABLE 5. cEE!,fIcAL coMPosITIoNs oF ZINcIAN STAURoLITE Fe-StaUfOlitg
Atomtc proportlms (oxygen basle 69)
sl L1.642 11.589Tt o.L27 0.L24A1 2s.938 26.226Fe 2.L94 2.100!4u 0.098 0.043MC L.297 1".049zt 2.703 2.447
LL.267 LZ.O200.132 0.106
26.L9L 25.7343.194 2.3490.000 0.152L .544 1 .3892.L7L 2.226
1 Pcs-56(1) 3lelkvaes].1, Norray (average of 5 polnts)i2 79522+ Caator, Georgia (average of 4 poltrts);3 f20967-5+ stratforal, Nolth carol-lna (average of3 points) i 4 PcS-61 Klipu, Ilnland (average of 10poitrts); asseoblage contal[s ataurollte, qurtztpyrlte, blotl-te, gahnlte, pFrhotltex, chlotLtex,galem, chalcopjELte mscovLtes; + SmLthsoalatrIEtLtute cataLogue Drnber; x secondaryi s tlaceamut.
Significantly, the most Zn-rich staurolite, that atBleikvassli, coexists with sphalerite. This is to beexpected in view of the phase relations shown inFigure 8, Increasing/(S) at constant/(O) wouldshift the tie lines so that Zn-rich staurolite coexistswith Zn-rich sphalerite.
DrscussroN
Calculations of spinel compositions that shouldbe in equilibrium with associated sulfides in theMineral District and Bleikvassli deposit disagree byas much as 15 mole 0/o ZnN2Onwith those actuallydetermined from natural assemblages (Figs. 6, 7).Part of the explanation for this difference lies in theextremely high sensitivity of calculated spinel com-positions, at given/(O, -/(S) conditions, to thefree energy of hercynite. At 600"C and 2 kbar, forex€rmple, a difference in free energy of formation ofhercynite of 31.4 kJ or only 1.990 of the value ofMclean & Ward (1966) produces a huge (71 moleVo) shift in the computed composition of spinel atthe pyrrhotite-pyrite boundary. The 31.4 kJ is thedifference between the free-energy data for hercyniteformation of Mclean & Ward (196Q and those ofPillay et al. (1960). We have chosen to use the valueof Mclean & Ward because it gives compositionsof spinel closer to our experimental results and natur-ally observed compositions (Spry & Scott 1983a, inprep., Spry 1984), but clearly the free energy of for-mation of hercynite is not well established.
159
Zn-staurolite
Pyrrhotite Sphalerite
Frc.8. The composition of Fe- and Zn-staurolite coexist-ing with sphalerite and pyrrhotite (schematic).
Sundblad (1982) claimed that the critical factorscontrolling the occurence of gahnite in the Scandina-vian Caledonides are metamorphic grade and com-positi&qr of the local rock. The rocks at all gahnitelocalitie"s have been metamorphosed to the amphibo-lite grade. Massive-sulfide deposits in the same ter-rane that were metamorphosed to the greenschistgrade are apparently devoid of gahnite. At firstglance, this would appear to rule out gahnite as anexploration guide for massive sulfides in lower-grademetamorphic rocks. However, zincian spinel hasbeen reported from three greenschist terranes (Frank-lin et al. 1975, Kramm 1977, Hicks et al. 1985).Kramm (1977) recogazed spinel grains that are only8 to l0 pm in leneth in slates, which raises the possi-bility that gahnite is more widespread in low-gradeterranes than commonly believed but may not havebeen recognized because of its grain size. Spinelassociated with metamorphosed massive-sulfidedeposits tend to be Zn-rich (e.9., Vokes 1962, Juve1967, Sandhaus 1981, Sundblad 1982, Spry 1984) andcan, in general, be distinguished on the basis of com-position from spinel associated with aluminousmetasedimentary rocks, which are Fe-rich (Spry &Scott 1983a, in prep., Spry 1984). However, Kramm(1977) and Hicks ef a/. (1985) have shown thataluminous metasedimentary rocks in Belgium andSouth Africa, respectively, contain Zn-rich spinelsimply because of the stabilizing eff€cts of Zn onspinel at low metamorphic grades. Because of sucheffects, caution must be utilized if spinel composi-tion is to be used as an exploration guide for massive-sulfide deposits in low-grade metamorphic terranes.The presence of Zn-enriched spinel does not neces-sarily indicate Zn-rich rocks under such conditionsbecause, for example, Kramm (1977) demonstratedthat some of the most Zn-enriched spinels recordedto date occur in rocks with only 50 ppm Zn.
s10,2 28.23 27.07rtoi o.4o 0.38&2o3 52.77 51.e1FeO* 6.3L 7.52!{nO 0.28 0. l-l-uco 2.08 L.64Z \O 8 .77 7 .73ToraL** 98.84 96.33
26.24 28.550 .41 0 .34
51 .73 51 .868 .89 6 .670 .00 0 .432 . 4 t 2 . 2 .6 .84 7 .27
96.52 97.32
160 THE CANADIAN MINERALOGIST
6o'DtrEco
od2
! abkvaesl lL.,TXn Krnrffisr"ntra fficantor[--'l atunrlnors motasedlment (lttBratu]s)
El metamorptroeeA sufflb(ltioratre)
IJIJIJJIIJI etanome asaoct ted wtthsutttdabut hsvlng rornod by non-deanlffiboraac{lons (Geco, Brol(dr Hill (Aurbana),
Carmbarg)
Frc.9. The Zn content of staurolite in aluminous metasedi-ments and metamorphosed sulfides.
In Figure 9, the Zn content of staurolite from theBleikvassli, Kiipu, Stratford and Canton deposits isplotted together with published compositional datafor staurolite in aluminous metasedimentary rocksand metamorphosed sulfide deposits. Although thereis some overlap in the Zn content of staurolite fromthese two settings, the most Zn-rich are frommetamorphosed massive-sulfide deposits. It isunclear whether all Zn-rich staurolite from massive-sulfide zones has formed by desulfurization of Feand Zn sulfides; however, staurolite that did norform by desulfurization mechanisms (e.g., BrokenHill, Australia; Gamsberg, South Africa and Geco,Ontario), even though adjacent to sulfide zones, con-tains less than 6 wt.9o ZnO. Textural evidence sug-gests that staurolite in retrograde shear-zones thatcut across mineralized zones at Broken Hill formedby the breakdown of alnandine, sillimanite and bio-tite (Laing 1977). Staurolite occurs in cordierite-anthophyllite rocks (Spry 1982) along the margin ofthe Geco deposit. Although the identity of precur-sors to this staurolite is unknown, the widespreadoccurrence of staurolite in gneisses throughout thearea suggests that sphalerite is an unlikely precur-sor. At Gamsberg, staurolite is present in sphalerite-free magrretite - hercynite - garnet - sillimanite rocksthat overlie the economic C-unit of the Gams ironformation (Spry 1984). Textural evidence suggeststhat staurolite may have formed by the breakdownof garnet and sillimanite.
Staurolite in aluminous metasediments contains upto 6 fi.Vo ZnO. That in rocks metamorphosed to
the upper amphibolite grade, in general, containsmore Zn (e.9., Ashworth 1975) than staurolite inrocks from lower-grade metamorphic terranes,regardless of the Zn content of the host rock.However, staurolite that probably formed by desul-furization mechanisms generally contains more than6 w,r/o ZnO (Fig. 9). Therefore,lhe Zn content ofstaurolite may be a useful guide in exploration formetamorphosed sulfide ores, in analogous fashionto zincian spinel.
ACKNOwLEDGEMENTS
The authors are grateful to Drs. G.M. Anderson,L.T. Bryndzia, J.R. Craig, E. Froese and J.J.Fawcett for having read an earlier draft of themanuscript, and particularly to Dr. J.R. Craig andMr. D.J. Sandhaus for generous access to unpub-lished data on the Mineral District. The authors areindebted to Drs. E. Froese, R.F. Martin and R.I.Thorpe for their critical reviews, which led to con-siderable improvement of the manuscript. The fol-lowing provided natural spinel-bearing samples forstudy: R. Bedell, T. Bottrill, J.R. Craig, P. Dunn,J. Gair, W. Griffin, G. Harlow, G. Juve, H. Koark,U. Latvalahti, D. Sangster, J. Slack, 8.. Vik, F.Vokes, F. Wicks and W. Zobel. The authors are alsograteful to Drs. C. Cermignani and M.P. Gorton ofthe University of Toronto and to Drs. R.A. Both andB. Griffen of the University of Adelaide for as-sistance with the electron-microprobe studies. Fund-ing was provided by a Connaught Scholarship, aH.V. Ellsworth Graduate Scholarship, an OntarioGraduate Scholarship, a University of Toronto Bur-sary to P.G. Spry, and a Natural Sciences andEngineering Research of Canada Grant (No. 47069)to S.D. Scott.
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Received October 26, 1984, revised manuscript acceptedSeptember 18, 1985.
