and staurolite are most Zn-rich where associated with sul- fides and may .... metasediments, pegmatites, quartz veins and Zn-rich ...... Open File Rep.71407. & -.
Cqnadian Mineralogist Yol.24, pp. 147-163(1986)
ZINCIANSPINELAND STAUROLITE AS GUIDESTO OREIN THE APPALACHIANSAND SCANDINAVIANCALEDONIDES PAUL c. SPRY'I AND STEVEN D. SCOTT Departmentof Geologlt,Universityof Toronto, Toronto, OntarioMSSIAI
ABSTRACT
processus dermagmatiques, Le spinelledesmetasffiments (sedimendu m6tamorphisme deschistes iveprobablement il sepeutquele Zn ait 6td taires),rochesdanslesquelles organiques. combin6i descompos€s Quoiquela gahnite et la staurotidezincifbrecoexistentdanscertainsgitesde sulfures,les observationstexturalesfont douter que la du spinelle.La hauteteneur staurotidesoitl'avant-coureur desr6acenZn dela staurotider€sultevraisemblablement tionsded6ulfurisation.La staurotidedu gitedeBleikvassli (Norvdge), dontla teneuren ZnOatteintjusqu'i 8.7790 pourla plusricheenZn connuei cejour, (enpoids),passe Gahniteet staurotidesonttoutesdeuxlesplusrichesen Zn li oir ellessontassocides auxsulfures;ellespeuventdonc guiderla prospection pourgltesdesulfuresmassifs enterrain m6tamorphique.
Zincian spinel or gahnite (Zn,Fe, Mg)Al2Oaoccursin metamorphosed massive-sulfide deposits, aluminous metasediments,pegmatites,quartz veins, and metamorphosedoxide-silicatedepositsin at least forty localities within the Appalachians and Scandinavian Caledonides. Most occurrencesare associatedwith metamorphosed massive-sulfide deposits,in which gahniteis consideredto form predominantlyby desulfurizationreactionsinvolving a memberof the systemFe-S-O and either sphalerire and garnet or sphaleriteand alurninosilicate.Spinel in quartz veinsand pegmatitesis thought to be a product of metamorphic-hydrothermal solutions and magmatic processes,respectively.Spinelin aluminous metasediments was probably derived from the metamorphism of metalliferous shales,in which rocks Zn may originally havebeen (traduit par la Rffaction) Iinked to organicmaterial.Although gahnitein somesulfide depositscoexistswith zincian staurolite, textural ev! Mots-clds:spinellezincifdre,gahnite,staurotidezincifCre, dencesuggeststhat staurolite did not act as a precursor to Appalaches,Calddonidesscandinaves,donndesde spinel. The high Zn content in staurolite is likely the result guided la prospection. microsonde dlectronique, of desulfurization reactions.Staurolite from the Bleikvassli deposit (Norway) containsup to 8.77 wt.Vo ZnO and is thought to be the most Zn-rich yet recorded.Both gahnite INTRODUCTIoN and stauroliteare most Zn-rich whereassociatedwith sulfides and may constitute an exploration guide for massiveZircian spinel, or gahnite (Zn,Fe,Mg)Al2Oa,is sulfide depositsin metamorphosedterranes. Keywords: zincian spinel, gahnite, zincian staurolite, Appalachians, ScandinavianCaledonides.electronmicroprobe data, exploration guide.
SoMMAIRE Le spinellezincifbre ou gahnite sepresentedansles gites de sulfure massif m6tamorphiques, m6tasddimentsalumineux, pegmatites, filons de quartz et gites oxyde-silicate mdtamorphiques.On I'a trouv6 dans au moins quarante endroits dans les Appalacheset les Cal6donidesscandinaves. La plupart desgites sont associdsi des sulfures massifs m6tamorphiques,danslesquelson admet que la gahnite s'est form6e surtout par des r€actions de d6sulfurisation, impliquant un p6le du systOmeFe-S-O et de la sphaldrite (blende) ainsi que du grenat ou un alumi nosilicate. Le spinelle des filons de quartz est consid€rd comme produit de solutions hydrothermalesmdtamorphiques,celui despegmatites,comme resultant de -P..*"t Department of Earth Sciences,253 "dd*ss: ScienceHall I, Iowa StateUniversity,Ames, Iowa 5001l,
u.s.A.
widely distributed in the Appalachian-Caledonide orogen; basedon experiencehere (Sandhaus1981, Sundblad 1982)and elsewhere(Sheridan& Raymond lW7, 1984,Spry& Scott 1982,Spry 1984),this phase has potential as a guide in the exploration for metfirorphosed massive-sulfidedeposits.Among the variousassociationsof zincianspinelobserved,that with metamorphosedsulfide depositsis the most common. This has prompted the suggestion(e.9., Sangster& Scott 1976)that suchzincian spinelis a product of desulfurization of sphalerite during metamorphism. Spinel in and around metamorphosedmassive-sulfidedepositsforms a solid solution between ZnN2O4 and FeAl2O4predominantly and is commonly spatially associatedwith sphalerite in addition to pyrite, pyrrhotite or magnetite(or various combinationsof theseminerals). Calculation of relations in the systemZn-Fe-Al-Si-S"f(Or-l(S, O by Wall & England (1979),Spry & Scott (1983a) and Spry (1984),and experimentsby Spry (1984)on a simplified SiO2-freesystem,demonstratethat gahnite can indeedform by the breakdown of sphalerite coexisting with either an aluminosficate or garnet. However, consideringthe assortmentof environ-
147
148
THE CANADIAN MINERALOGIST
DATA ElBcrnoN-MICRoPRoBE
mentsin which gahnite is found, other theorieshave also beenproposedto accountfor the formation of gahnite. Theseinclude: (l) reaction of Zn-bearing silicates[e.9., biotite and staurolite to form spinel during metamorphism (Stoddard 1979, Dietvorst 1980)l; (2) precipitation of spinel from a metamorphic-hydrothermalsolution (Wall 1977); and (3) formation from a primary Zn-oxide phase during metamorphism(Seenit 1961). The zincian-spinel-bearing rocks of the Appalachian-Caledonideorogenalso contain stautolite that is amongstthe most Zn-rich yet recorded (e.9., Sandhaus1981,Spry & Scott 1983b).Zincian staurolitespatially associatedwith zincian spinelis thought to be the precursorof spinelin somemetamorphic rocks (Atkin 1978, Stoddard 1979, Spry 1982). This paperdocumentssomeof the spinel-bearing localities in the Appalachians and Scandinavian Caledonides.We discussthe origin of the spinelin the light of calculationsof stabilify, experiments,textural evidenceand field associations,its potentialas a guideto sulfide mineralizationand the reasonfor the anomalouscontent of Znrn staurolitewith which it is spatially associated.
TABLE 1.
ZINCIAN SPII{EL LOCAI.ITIES
LocaL3ty A, Quebec Aubun, t{e lop6hd, lte DavL6, !{a6a
@d peg@tIte ?pegmtlte |@d
5
eulfLdes ltr a.s. ?peg@tlte peg@tite @od sod 4 l@d wd
20 2L 22 23 24 25 26 27 28 29
gharl"@dt, l{ass Iordre 8111, I'Iaoa !{lddleto!, coN Frankun, NJ Sterll.g U111, NJ S.E. Ptusylvdl.a sprlDgfield, ud UltreraL E111, Ud P6tap€co, l'1d Valzldco, Va Sulphui, va Cofer, Va AmiDl.c, va Julla, va Jobnao!, Va Ardel6onvLll-e (#L8), va Stfatfold, Nq ole Krob, Nc Elk (oob, NC Daake, NC Spnce Pin€, NC Nc Cull@bee, sava@h, Nc W6yhutta, NC orto, NC st@dard, ca
30
I'tagede!
, ca
31 32
Caqtoo, Ga Bob, Ga tlttle
33
Villa
Rica,
Ga
IN TEE A?PAI,ACIUANS
c@Iogtel
ceologlcal
1 2 3 4
6 7 6 9 10 11 L2 13 L4 15 16 L7 18 L9
The chemicalcomposition of the spinel and of coexisting phaseswas determined at the University of Toronto @tec Autoprobe and ARL-EMX electron microprobe equipped with Kevex and Ortec silicon detectors,respectively),and energy-dispersion at the University of Adelaide (JEOL wavelengthdispersionSuperprobe,model 733). Operatingconditionsof the Etec Autoprobe and ARL-EMX electron microprobe included accelerating voltagesof 20 kV and beam currents of 100nA. Natural and synthetic spinelswere usedas standards for Zn, Fe, Mg and Al, garnetfor Mg, Mn, Ca, Fe, Si and Al, rutile for Ti, synthetic pyrrhotite for Fe and S, synthetic sphalerite for Zn, Fe and S, synthetic chalcopyrite for Cu, synthetic alabandite for Mn, and syntheticcadmium sulfide for Cd. These electron microprobesare connectedto a PDP/II computer system providing on-line reduction of data by a modified versionof the energy-dispersion PESTRIPSprogxzrm.The techniquesusedin the program were describedby Statham (1975). Operating conditions of the JEOL Superprobe included an acceleratingvoltageof 15kV and a beam
N ,E . N.E. N .E . N.E.
l@d
rud l@d m6d 1@d r@d,
qv
@Bd,
qv
et a1. (1983) study atudy (1885), Bsertaofa (1972), & Saggercy (1984) (1908) locatlou 6krom this study (1965) 6 Klelo Flondel cawalho & Selar (1979) (1975) Wagoe! & crasfold sha4trotr (!.923) (1923) Shannon Shatuoo (1923) Ro6s (1935) (1980), saodhaus (1981) cralg l.{111er (1978)r Saadhau (1981) cox (1979), Sadhaue (1981) cratg (1980), Saqdha@ (1981) chev6 rhla rh16 Daaa rteld Flldt qact
thls th16
6tudy study
B.R. B.R. B,R. 3.R. B.R. B.R. B.R. B.R. B.R. B.R.
(1971) Sreock Ross (1935) Roee (1935) cedth (1891) thls atudy RoEe (1935) Roes (1935) Roes (1935) Rose (1935) (1906) Llddgletr
B.R.
cofer
@d
B.R. B.R.
]@d
B.R.
ceBth (L862), Roee (1935) & cook (1970)' Ab!@ (1984) Mccomell cook (1970), Neathery d (1984) rbXllste!
a€ |@d @d ea peg@ttte ]@d @d l@d @d auLftde vein6 ttr 4 aulflde veLda La @ l@d
1o Fig. l"; N.E. Ns Englatrd; a. Depoalt sbm gulfide @rphosed @6slve ileposit; a€ al@iao@ turphoeed zLnc-oaLde depoalt.
Detasedfueqti
(L953)
qe qsrtz
vetni
@d seta@od meta-
149
ZINCIAN SPINEL AND STAUROLITE AS GUIDES TO ORE TABLE2. !ocaL-Ity No.'
ZINCIANSPINELL@AIITIES IN IgE SCANDD{AVIAN CALEDONIDES Cs]-ogtcal Setthq
NaEe
1
BLelkva8rLl,
2
Crskevardo RLpudilal, Swe. ltofjelL, Nor.
l@d
4
Themoe,
@d
5 6
NoBEfJeLLet, Nor. Sklmsdalo, Nor.
7
VtlLdalsfJ e1t, Nlitrge!, Nor.
a. Depoalt ahm sulflde deposlt.
Nor.
@d
2;
Nor'
@d @d
Nomay;
crurent of 2m nA. Natural spinels,simpleoxidesand metals were used as standards.The JEOL Superprobe is connectedto a PDP/ll computer, which reduceddata by a FORTRAN IV program using a type of correction proposedby Duncumb & Reed (1e68). ZINCIAN SPINEL.IN THE APPALACHIAN
-
SceNonevnn CalBooNtos OnocrN Zincian spinelis known from at leastforty localities in metamorphosedvolcanic and sedimentary rocks in the Appalachians and Scandinavian Caledonides(Tablesl, 2) but, in general,is poorly describedand treated as a mineralogicalcuriosity. Occurrencesin the U.S. Appalachiansare found in metamorphosedmassive-sulfidedeposits,aluminous metasediments,pegmatites,quartz veinsand Zn-rich oxide bodies within the Blue Ridge and Piedmont geologicalprovincesand in the probable extension of the Piedmont provincein New England @ig. l). The only reported locality of gahnite in the Canadian Appalachiansis that by Chev6et al. (1983),who noted its occurrencein the small stratiform massive A sulfide deposit, EasternTownships, Quebec. According to Sunblad(1982),all known gahnite localities of the Scandinavian Caledonides are associated with metamorphosed massive-sulfide deposits contained within the Rddingsfjiillet and Beiarn Nappesof Norway and Sweden(Table 2). Thesenappespredominantlyconsistof gneisses and marblesthat weremetamorphosedto the amphibolite grade.Gahnite-bearingmassive-sulfidedeposits in the RcidingsfjiilletNappe includethe Bleikvassli, Mofjell, Thermos and Skbrndesdalendepositsof Norway and the Graskevardoand Ripuddendeposits of Sweden(Fig. 2). The-Villdalsfjell and Niingen depositsare located in the Beiarn Nappe. Field and textural relations Of the various geological settings in the
Reference
R6drngEfJgUet Nappe f,6dlugsfJal1st Nappe RddlogsfJALI-et Nappe RiitiogsfJAt"Let Nsppe Belam Nappe R6alttrgsfJillet Nappe Beiam Nappe
wd
Nor.
Ln FIg.
Tectotic Unlt
Sue.
Sv6alen;
@d
Vokes (1.962) Sudblaat
(1982)
sudblad
(1982)
Sundblaat (l-982) Ssdblaal (1982) Jwe (1967) vokes (L962)
Detmrphosed
Maslve
;d tffit /"{:;ffi
FIc. l. Locationof zincianspineloccurrences in the Appalachians. Namesof depositsarein Tablel. Appalachian-Caledonide orogen in which gahnite is found, the association between gahnite and metamorphosedmassive-sulfides is the mo$t common. Gahnite is most often containedwithin sulfide zones;however,it is also found in quartz veinscrosscutting sulfide zonesand in aluminous rocks surrounding sulfides.Becauseof the variablenature of
THE CANADIAN MINERALOGIST
the geological settings, it is hardly surprising that gahniteis associatedwith a variety of minerals(Iable 3). Representativecompositions of spinel from severaldeposits,determinedby electronmicroprobe, are presentedin Table 4. Additional compositions are lisred in Spry (1984). The most detailed study to date on gahnite in the Appalachiansis by Sandhaus(1981),who determined its distribution, chemistry and possible origin in the Arminius, Cofer, Julia and Sulphur depositsin the Mineral District, Virginia. Thesedepositsare located in a sequenceof metapelitic sedimentaryand mafic to felsic volcanic rock$ (Southwicker aL l97l) within the Piedmont geologicalprovince. Although gahnite is distributed throughout the sulfide deposits,Sandhaus(1981)noted that it is most abundantnearorewallrock contacts.According to Sandhaus,gahnite in ore zonesis predominantly associatedwith quartz, biotite, garnet, pyrite, sphalerite,chalcopyriteand galena. Magnetite, pyrrhotite, staurolite, kyanite, amphibole, chlorite and muscovite are associated with gahnite in lesseramounts. Sandhaus(1981) reported that gahnite coexists with sphalerite; however, an antipathetic relationship betweenthese minerals is suggestedby the distribution within the Flc.2. Location of zincian spinel occurrencesin the Scan- deposits.Despitethis relationship, gahnite is poidinavian Caledonides(modified after Sundblad 1982). kiloblastic and uncorroded where in contact with Names of depositsare in Table 2. other phases,including sphalerite. is one of several The Davis mine, Massachusetts, srMoF zNcN m ssml$s lM 3. mines and pits that lie along a vaguely defined Sffi 8TAIBOIIN-BNIXG N ZNCN mineral belt within volcanic and sedimentaryrocks qt,w ' 6p, ph, st,cp,aa.*, Bletbassll reg5lt ,pd 'pod,.td' " p t d metamorphosedto the lower or middle amphibolite de,sp,6p1d,rud,zro Blelkvsslt rc955(1)h ry,ph,st, pcgss(z)! qz,sp,w,p,qp,sptd Blelbd6lt grade (Bwerinofa L972).Gahnite is associatedwith qz,st,m,sprq,pf,Fo,btq B1€tkes6lt mgss(:)b py,qz,sp1,s,sp,st,bt,cpd'Rd'plo BrelkvsElt res-56(r)!. pyrite in massivesulfides and magnetite in sulfidef 10 (z)b qz 'sp ' sl,D, py,po, qt, s,sPro,cpd, r161hs6l1 wsa st,bt,Eu,F,sp1,Fq,cDr,p1a Bret:mslt rcF56(3)b bearinechlorite-mica schistssurroundingthe deposit pcy65b q z , e p , p y , b t , F , c p a , c l o € Brelka6sll pcgoz! st'qz,sp,ph,po,py Bktbvassu @werinofa 1972),as well as with pyrite quartzites p,sp,qz,ph,spl'Py's Bl€lhss1l re$681 sp,qz Pos69i Btsibassll and quartz-albite metatuffs (Slacke/ a/. 1983).At q3,py,b1,sp,s,P,aP1,cp'm Bleks1l rc#70b Bleikvassli,Norway, spinel occurrencesare confined b's'qz,Bp,py,8a,gd,spl,cd,ruo'11o bGfl€116! ms-974 qz'6p'py'hb,8a,M'cP'ru" frg@s re9984 to massive sulfides, quartz veins and quartz-rich qr,ga,@,sP,sP1'bl.s,p1 stlasdsler re$96a pl,@,ga,ap Arbun !l4il36 schistsin the ore zone(Vokes 1962).In massivesulqa,pl,sp T4sh n9r603 qt, py,bt, sP,hb', sPI,36r'' eP', pl' reF24: Ald€rsoNlll€ fides, gahnite coexists with sphalerite, pyrite, pyrgzre! Aidst$d116 Ee72: qz,6P Pc$85D &d€tsoad11€ rhotite, quartz and garnet, whereasin quartz-rich qu,a,ll JollM PC6-844 Btll hd'c6'sp'Ba'pF slerlhg x43573b muscoviteschistsit coexistswith quartz, phlogopite, @,Bp,rh,sp1,c1G su1 145056b Stsrlhg rh,6p,ca,b,q 47a96c. hdiD muscovite, zincian staurolite, garnet' sphalerite, @'qz,4,PY'II.ga'cP' DNls U253I_D hvts pyrite and pyrrhotite. There is a particularly close !O1317D . Pt'qt'6P'cP 6p,cp'py,qz,ca kvlB es-104: py,spt,aa,ry,P,qz,bJ,cP Dd6 c8O-469 associationbetweengahnite, garnet, quartz and Fe py'qz'3p,sp1,8a,F,c]",cP Dsts c8G4tD q z , p y , d , p t , s P , s P r e sulfides(pyrite or pyrrhotite, or both) and sphalerite Ms GBG5Dqzrpr,d,8a,sP'@ Dads G8o-26: in samplesfrom the Davis and Bleikvasslideposits. qz,u,Py,apr,4'cP' htu r80-45D (rrsp charl@t 1285303b Sillimanite, although rare, may also coexistwith ga}p1,qt,d,cr'sP hrdis 8111 393064 sp,pI,u,bl'€P'qz,cp'm'cle,sP1'ch KlI Hreral il9554 nite at the Bleikvassli deposit. Gahnite and garnet s9,q2,@,c11bi spthgfhld 94677a. &. sp utcbell 8/77t2o exhibit euhedralgrain-shapesand noncorroded consp,fl,qz Hdga s1sL6 81957D sp,q2,s!'tE'F sttatfoid 120967-53c tacts wherein contact with the other membersof the qe,cp'py,st,q,ql,gr,cho€ ktoo 1952/c systemZn-Fe-Al-SiS-O (Figs. 3a' b). 116td of 8tulte r filil-G;ppffidaaof a Mlysa abdec6; of llstd 10 3pry (1984) i c @lyst6 of Sahlte h Tabl6 4i b e1y6€s Juve (1967) reported on gahnite in lead-zinc preaeut am6tai la tlac€ tu Tabl€ 5; d tuetal 1let€d shuollt€ ca calclt6; bl blotlte; e aathophylllto; ls secoodaryi o tural in the Hafje[ syncline,Norway. This gahdeposits 6 cryout€i cp chrlcoPFlt€; cl chlorltai ch chalcocitei cal cordtolltei ga.galeui ft franLltllte; ft ftbroltta; 8d g€dttte;8a ge@ti e epldotel in the ore zone and its immediate walloccurs nite hobellte; thtt6; lh 11 l@ llMtlt6; la bedeabergito; hb lDnbl@de; fl Ncovltsi F pynhotite; B uga6tlt€; P1 plaalocls€i Pi Pblo84lt€t with the grain size and frequency of spinel rocks, n ntllei qe qurtzl rh rhodoalte; pF pFoph@tte; W pyrlt6: zt ,ltcor' st atauroltle; Bpl sPhal€tlla; sPtn€1i s;r sc1cllei decreasingaway from the ore zone. As with the a ,lfth
#r,[€
151
ZINCIAN SPINEL AND STAUROLITEAS CUIDES TO ORE 1SLE 4.
@reSMATIW
slo2r tlo2 ^Lt93 Fa& ho !s0 &ond ho Total
0.00 O.O0 O.OO O.m 0,05 0.00 0.00 0.00 0.00 0.00 36.42 56.06 57.09 47.47 53,71 1.46 9.61 6.41 10.91 5.29 0,36 0.35 0.29 0.61 0.12 0.m 0.16 0.83 0.00 t.t4 ndaddod0,23r.34dnd!dildnddsdd 32.94 37.00 )5.29 40.49 40.89 99.33 101.03 99.91 99.€ 10t.20
st T1
0,000 0.0m 11.818 1.428 0.054 0.000
0.m0 0.000 U.638 1.098 0.052 0.040
0.m0 0.000 12.060 0.961 0.044 0.221
0.000 0.000 10.703 1.741 0,099 0.000
4.323
4,809
4.668
5.721
COreITIMs
A1 Vg & Zn
ROU ru
NP&CTHS
N
8(NIMVH
CNfMNDSA
0.OO 0.m 56.47 5.39 0.m 1.54
0.00 0.00 s6.17 5,t7 0.00 l.16
0.05 0.@ 56.83 S.2O 0.03 1.66
0.05 0.00 51.24 1.29 0.12 t-.76
0.00 0.00 J8.08 ?.60 0.00 1.93
o.0o 0.00 55.78 4.i9 0.00 t.O9
o,I5 0.00 51.34 5,19 0.00 1.12
0.07 0.02 57.69 6.06 0.14 1.90
o.o5 0.02 56.25 5.89 0.02 0.59
0.04 0.02 5?.37 S.A5 0.07 r.92
0.ol 0.04 t7.80 5.L2 0.07 t.7t
37.65 101.28
36.88 101.32
35.76 99.53
33.42 99.89
32.95 100.50
40.46 10t.72
35.43 100.83
36.81 101,69
35.36 99.16
33.64 100.51
35.56 1m.63
Atodc 0.m8 0.000 11.538 0.810 0,018 0.310 5.507
OF SPIW
0.000 0,000 11.857 0.80{ 0.000 0.408 0.032 4.959
(ory8e! bst€
24)
0.009 0.000 11.857 0.769 0.005 0.437
0.010 0.000 t2.OlO 1.085 0.018 0.466
0.000 0.000 11.926 1,107 0.000 0.492
0.000 0.000 X1.785 0.659 0.000 0.289
0.026 0.m 11.965 0.768 0.m0 0.d54
0.0u 0.003 1t.946 0.S9 0.020 0.495
0.009 0.003 12.031 0.894 0.004 0.16t
0.006 0.003 ll.99l 0.509 0.011 0.506
4.676
4.393
4.236
5.356
4.162
4.&a
4.912
4.661
proF.tlotu
0,000 0.@0 11.824 0.a62 0.002 0.357 0.165 4.864
0.m6 0.002 L2.O63 0.797 0.009 0.474 nd 4.652
Erfti z uvlbs lbpa@r &hsi I lrl06ri Lold's UlU' &daachu€ett6; 4 4789H rfankllu, Nar J€rB€y; 5 14505F UlI, Nc Jdrdot; Starltry 6 94677+ sPrlaafleld, hryland; 7 Rl95F &€!al trtU, hryled; I reS-24 hd6r6onv11l€, vtr8tnla: srrdtfold, 9 PGs-84 JohrBon, vtlatnta; l0 120967-S North hrollEi ll U957* slBloa UdBe, Notth &roliui 12 79522+ hron, &oraiai &turi 15 res-9, 13 PCS-54 Elsibas€ll, lomayi 14 PGs-t6 sk8nssdaloo, nostldllet' lonay; 16 PS98 *a turtcan frolms, Nodayi + byal Ootsrlo tus€e Gtaloguo of natulal no.i + Sdthso!&n Insrlruro &ialo$€ do,i hse@ UlBtory &talogue no.i r Total !e a6 F6O: !d lor d6t6d!.di a ntd€lsl assedtaa€s llstsd ln Tabl6 t.
aforementionedMineral District, Davis and Bleikvasslideposits,gahniteand garnetfrom the ffiqeU syncline form porphyroblasts that appear to be in textural equilibrium with quartz, Fe sulfides and sphalerite.According to Ross (1935),gahnite was commonly observedin sulfidesassociatedwith the Ore Knob, Wayhutta 4ad lalzinss depositsin the southern Appalachians. Sampleswere not investigated in this study; therefore, the textural relations between sulfides and gahnite are unknown. In the Thermos deposit, gahnite occurs within pyroxene-amphibole-garnet-richlayers Nik 1977, Sundblad1982)and at Nonsfjellet, gahniteappears in garnet-mica schists (Sundblad 1982). Coarse euhedralspinelat Thermos formed in rocks containing pyrite, quarlz, garnet and hornblende. The spinel-amphibole association is also present at Nonsfjellet, where porphyroblastic zincian spinel coexistswith gedrite, anthophyllite and hornblenqe (Figs. 3d, 4a) and in Swedishdeposits,whereSundblad (1982) noted that gahnite coexists with anthophyllite and cummingtonite. Zn spinel has been found in aluminous metasediments adjacentto metamorphosedsulfide deposits in Bleikvassli, Davis and the Mineral District, with disseminatedsulfidesat Charlemont @lint l90S) and Stratford, in sulfide-free metasedimentaryrocks in southeastern Pennsylvania (Wagner & Crawford 1975)and the Deakemica mine (Genth l89l). cahnite at Stratford is associatedwith quartz, staurolite, hematite and pyrrhotite, whereaslsselding to Flinr (1908), gahnite at Charlemont coexistswith tremolite, chloritoid, feldspar and quartz. Although gahniteat Ore Knob, Wayhutta and Valzinse is commonly spatially associatedwith sulfides, it is also associatedwith quartz veins or asa replacement of quartz-plagioclasesegregations.Similar. sulfide-free associations occur at Andersonville, Johnson, Magruder and Standardmines. Gahnite is coarsegrained and euhedral where in contact with quartz and plagioclase. Pegmatites at Auburn
(Maine), Topsham Maine) and SprucePine (North Carolina) contain gahnite coexisting with quartz, muscovite,plagioclaseand other accessorymins1al5. The granoblastic intergrowth among all of these minerals suggeststhat gahnite is a primary phase rather than a second€fyminslal. An unusual and rare blue Co-bearing spinel is reported from the Mineral t{ill, Springfield and PatapscoQs minss, Maryland, by Shannon(1923). One of his analysesof spinel from the Mineral Hill depositgave1.48wt.9o CoO. Spinelanalyzedby us from the Mineral Hill and Springfield minescontains 1.34 and 0.23 wt,.$/oCoO, respectively(Table 4). Magnetitein the latter depositcontains0.83 wt.9o CoO. Phase relations Sinceoccurrencesofgahnite in the AppalachianScandinavian Caledonide orogen are most often associatedwith metamorphosedmassivesulfides, it is pertinent to look at phaserelations in the system Zn-Fe-Al-Si-S0 to determinewhethergahnitecould conceivably have been formed by a desulfurization mechanism in some localities. This simplified system does not take into account all phasesthat may be involved in desulfurization mechanisms,but it doesincorporate co[lmon phasessuch as gahnite, alrnandine, aluminosilicate, quartz, sphalerite, pyrrhotite, pyrite and magnetite. A Schreinemakersanalysis(Schreinemakers1965) for the systemZn-Fe-Al-Si-S-O (Fie. 5) cau be compared with calculated phase-relations using two natural examples,Mineral District and Bleikvassli, wheregahnite is intimately associatedwith sulfides. For the Mineral District, calculated phase-relations for the systemZn-Fe-Al-Si-S-O are projected onto the /(O)-/(S) plane of Fe-S-O (Frg. O for peak metamorphic conditions of 465oC, 4.6 kbar, XF$ilsiro,z: 0.55 lbased upon the average of Sandhaus's(1981)microprobe data on garnet co-
ZINCIAN SPINEL AND STAUROLITE AS CUIDES TO ORE
existing with gahnitel and taking into account the azns,eFesand the thermal expansionand compressibility of sulfide phases.Peak metamorphicconditions of 465oC and 4.6 kbar arebasedupon garnetbiotite geothermometryand sphaleritegeobarometry (Craig 1980).For the Bleikvasslideposit,phase relations (Frg. 7) assumepeak metamorphicconditions of 505oCand 3.8 kbar and Xf$iilsirorz = 0.56 [basedupon the averageof Spry's(1984)microprobe data on garnet coexistingwith gahnitel. The presence of both kyanite and sillimanite at Bleikvasslirequires a minimum temperatureand pressureof 505'C and 3.76kbar (Holdawayl97l). This temperatureagrees with a meanvalueof 487"C (431'C) obtainedby Sen& Mukherj ee(1972)from sulfur isotopegeothermometry. Sphalerite coexisting with pyrrhotite or pyrite + pyrrhotite from a number of sampleshas a compositionranging from 9 to 12 mole Vo FeS. Such compositionsyield pressuresin excessof 7.6 kbar and appe.uto be the result of re-equilibration upon cooling. Calculationsfor Figures6 and 7 were made using standard free-energy values obtained from Robie et al, (1978) for kyanite, corundum, qtJartz, magnetite, hematite, iron and sphalerite, from Jacob(1970 for gahnite(from the oxides)and zinc oxide, from Richardson& Jeffes (1952)for troilite, and the standardGibbs free-energychangefor the reaction: FeS+ 0.552: FeSz
(l)
from Froese& Gunter (1976), The standard freeenergyfor almandinewasderivedusingthe technique of Froese(1973). The free-energychangein the oxidation of almandine(data from Froese)is basedon the magnetite-wiistitebuffer of Eugster& Wones (1962)and on the quartz-fayalite-magnetitebuffer of Wones & Gilbert (1969).So as to remain internally consistent,values of free energy of ahnandine from Froese(1973)wererederivedusingfree-energy valuesof magnetitefrom Robiee/ al. (l%8). An ideal ionic-solution model wasassumedfor the almandineand spessartine-richgarnet basedon the resultsof Ganguly & Kennedy (1974). Four studies (Pillay el al. 1960,Rezukhina e/ a/. 1963,Mclean & Ward 1966,Chhn et al. 1973\have derivedvaluesof the free energyof formation of hercynite. The closeagreementin the valuesof standardstateentropy at298K of Rezukhinaet al. (1963)and Chan et ol. (1973)usine electrochemicaltechniques
153
with that derived from calorimetric measurements by King (1956)would appearto indicatethe superiority of thesethermodynamicdata to those of Pillay et al. (1960)and Mclean & Ward (1966),where the agreementis not asgood.Prllay et al. determined free-energyvalues for hercynite by equilibration of liquid Fe, corundum and hercynite with H2-H2O gas mixtures, whereas Mclean & Ward derived values by measuring the oxygen concentration of liquid Fe in equilibrium with hercynite. Despite a preferencefor data derived using electrochemical techniques,there is excellentagreementbetweencalculated chemical compositions of spinel on the ZnAlrOo-FeAl2Oajoin using Jacob's (1970 freeenergydata for gahnite with data for hercynite from Prllay et al. and Mclean & Ward, assumingideality for the gahnite-hercynitesolid solution, and the experimentallyderivedspinelcompositionsof Spry (1984). The ideal nature of the gahnite-hercynite solution is proposedbecauseextremelylow values of excessenthalpyare obtained(Spry 1984,Spry & Scott, in prep.) utilizing the procedureof Jacob & Alcock (1977),This procedureestimatescation drsenergies. tributions usingoctahedralsite-preference The apparent absenceof gahnite-hercynite exsolualthough not diagnostions in natural assemblages, tic, supports the view that the excessenthalpy-ofmixing term is small, and the gahnite-hercynitesolid solution is closeto being ideal. Spry (1984)showed that computed spinel compositionsusing the data of Prllay et al. (19@) and Mclean & Ward (1966)are almostidentical.The data of Mclean & Ward have beenused in this study. In constructingFigures6 ard 7, the alm-sph-sp boundary in the pyrrhotite field is calculatedby solving two equilibria: Fe3Al2Si3O,2+ZnS+52: Zn&zo|+ 3FeS+ 3sio2 + 02
A)
Fe3Al2Si3O12 + 52: FeAl2Oa+ 2FeS+ 3SiO2+ 02
(3)
To obtain the dm-al boundary, the following equilibrium is used: * 1.5S2= Fe3Al2Si3O12 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) formed as a retrogradeproduct; Davis mine, Massachusetts.b. Gahnite-garnetlayersin quartz-rich sulfide schists,The opaque mineral is pyrite; Davis mine, Massachusetts.c. Garnet appearsto have replaced gahnite in a rock rich in quartz, phlogopite (ph) and sulfide. The sulfide (opaque) consistsof pyrite, pyrrhotite and sphalerite; Bleikvassli, Norway. d. Subhedralporplyroblasts ofgahnite intergrown with gedrite(ge), anthophyllite(a), hornblende(h) and quartz. The opaquemineral is pyrite; Nonsfjellet, Norway.
t54
THE CANADIAN MINERALOGIST
ZINCIAN SPINEL AND STAUROLITE AS GUIDES TO ORE
155
whereasfor the sp-al-sph boundary the following two equilibria are solved:
sphalerite, pyrrhotite, gahnite and sillimanite supports this prediction. A Schreinemakers analysis(Schreinemakers 1965) for the system Zn-Fe-Al-Si-S-O in "f(O, "f(S, ZnAJ'O + SiO2+ 0.552= spaceshowsthat five univariant reac'tionshaveslopes ZnS + AlrSiOs + 0.502 (5) of one-to-oneand intersectat an invariant point (Fig. 5). Usine Sandhaus's(1981)microprobe data for gahnite from the Mineral District u1i1lsgpalizing FeAIrOo+ SiO2+ 0.552= the gahniteand herclmitecontentsto 10090yield a FeS+ Al2SiO5+ 0.502 (6) ratio of 83 mole 7ogahniteto 17mole 9o hercynite, which disagreeswith the ratio of 98 mole gogahnite to 2 mole Vo hercynite predicted at the pyriteThe associationof magnetite-pyrrhotite-pyrite in pyrrhotite boundary (Fig. 6). Our calculationsalso the Arminius and Sulphur deposits,Mineral District, predict a value of 90 mole 9o gahnite (10 mole 9o provided buffering of /(SJ and /(O) ar about hercynite) for spinel from Bleikvassli along the 10-5'2and 10-2.6 bars, respectively (Frg. 6). A pyrite-pyrrhotite join (Fig. 7), whereasanalyzed slightly lower value of/(OJ and a similar value of compositionsnormalizedto l00Vocontain approxi/(S) are likely in the Julia deposit, where pyrite and mately 80 mole 9o gahnite (20 mole 9o hercynite). pyrrhotite, but not magnetite, coexist. Magnetite In addition to the lack of agreementbetweencalcudoesoccur in the depositbut is corroded, suggest- lated and natural compositions,reactionsinvolving ing that /(OJ and /(S2) condirions may initially gahnite, garnet, aluminosilicate, pyrrhotite and have beencloseto the pyrite-pyrrhotite-magnetite quartz do not intersectat an invariant point (Figs. (PPM) triple point but subsequently declined(Sand- 6, 7) as predictedin Figure 5, Thesedisagreements haus 1981). A common assemblagedescribedby are probably due to uncertainties in the free-energy Sandhausis that involving gahnite, garnet, sphalerite determinationsof gahnite, hercynite and almandine and quartz. According to Figure 6, this assemblage when extrapolated to the lower- to middlewould be expectedat relatively high /(O) condi- amphibolitegrades.Spry (1984)hasdocumentedthe tions. The coexistenceof gahnite, garnet, quartz, excellentagreementbetweencalculated and natural sphaleriteand pyrrhotite (Sandhausl98l) suggesrs compositions of zincian spinel at upper amphibolite conditions on the abn-sph-sp bound- to granulite grades.Furthermore, the sequenceof "f(O, "f(S, ary. Despitethis, there is no textural evidenceto sug- reactionsin Figure7 is inconsistentwith that shown gestthat garnet and sphaleriteare breaking down. in Figure 5. This inconsistencyresultsfrom the uncerOne assemblage describedby Sandhaus(1981)that tainty in free energiesof formation of gahnite,herinvolves gahnite, kyanite, pyrite, sphalerite and cynite and almandine and the fact that the alm-al quartz implies that/(O, -/(S) condirionswere in and alm-sph-sp boundaries will shift in /(OJ someplaceson the equilibrium boundary defined by /(Sr) spacedependingon the choiceof the activity the following: of almandinein garnet (ffi), whereasthe sp-al-sph boundary will remain fixed regardlessof aSfr,. 2Al2SiOs+ ZnS + FeS,+ O, = Z\N2O4+ FeAl2Oa+ 2SiO2+ 1.552
Genetic specalations e)
Pyrite and pyrrhotite are commonly associated with gahnite in the Bleikvassli deposit, whereas magnetiteis an uncommon, but locally abundant, mineral. The presenceof pyrite, pyrrhotite and magnetiterequires that f(O) - /(S) conditions were closeto thoseexpectedat the PPM triple point (Fig. 7). The presenceof the assemblages pyrite, sphalerite, gahnite, pyrrhotite and garnet (Fig. 3c) and pyrite,
Speculationsconcerning precursors to gahnite dependon the preservationofunmetamorphosedor slightly metamorphosedZn-bearingrocks at grades belowthe stability of gahnite.Despitethe apparent absenceof obvious precursorsto gahnite, the association of sphaleriteand pyrrhotite or pyrite makes thesesulfides likely. It is also likely that aluminosilicateor garnetbreaksdown to provideAl and, in the caseof garnet, Fe and Mg, for the formation of gahnite.
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, chalcopyrite and sphalerite; Canton mine, Georgia.
156
THE CANADIAN MINERALOGIST al
+ quartz
,/|\
al-alumlnoslllcate alm-almandlne p-pyrrhotlte sph-sphalerlte sp-splnel
t,"ln\ po
sph
al
/f\ p9
t€fq
spn
rog fsz analysisof the systemZn-Fe-Al-Si-S-O in/(O) Ftc. 5. Schreinemakers
In proposing a mechanismfor the formation of gahniteat Broken Hill, Australia, Segnit(1961)suggesteda reaction betweenkaolinite and adsorbedZn oxide.A slightly modified form of Segnit'sreaction is: Al2si2o5(oH)a* zno: Z\AI2O4 + 2SiO2+ 2H2O
(8)
Althouelr this reaction could conceivablyaccountfor the formation of gahnite in aluminous metasediments,thereis no documentationof ZnO being adsorbed onto clays. The available literature on metalliferous shalessuggeststJratZn may haveoriginally beenlinked to organic material (e.9., Vine & Tourtelot 1970,Coveney1979,Coveney& Martin 1983)or phosphatematerial (Coveney1979).These materials may subsequentlyhave beenremoved and the remaining Zn rcacted with sulfur to form sphalerite. Although no examples have been documentedin the literature, sphalerite may subsequently react with aluminous clays (e.9., kaolinite) to form gahnite. In this context, it is significant to note that slatesin the Venn-Stavelot Massif (Kramm
-/(S)
space
1977)contain gahnite that is intimately associated with viridine. In an earlier study, Kramm (1973) showedon the basisof textural evidencethat viridine and andalusite formed directly from kaolinite. Metamorphismhas obliteratedprecursorphasesto gahnite in aluminous metasediments in the Appalachian-swedish Caledonideorogen; however, it is conceivablethatZn originally linked to organic or phosphaticmatter formed sphaleriteand subsequently reactedwith aluminousclays(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 gahnitein pegmatite at Auburn, Topshamand SprucePine crystallized directly from peraluminousgranitic melts becausethere is no textural evidenceto suggestthat gahnite formed as a secondaryproduct. Similar spinelhasbeenreportedin acid volcanicrocks elsewhere (e.g., Phillips et ol. 1981,Tulloch l98l). Gahnitein quartz yeinsthat cut acrossmassivesulfide zonesare not spatially relatedto acid igneous
157
ZINCIAN SPINEL AND STAUROLITE AS CUIDES TO ORE
F"ZO3
4.6ruAR
quartz
Fe3O4
FeS2
for the systemZn-Fe-Al-Si-S-O projectedonto the Fe-S-O plane as a function of Frc.6. Calculatedphase-relations /(S) and/(O) for peak metamorphic conditions (465'C, 4.6 kbar) affecting the Mineral District' Virginia. Seetext forlources'of data usedin the calculations.Mole Vo ZI!'J2O4 for the gahnite-hercynitesolid solution is indicated on the broad dashedline.
rocks and are thought to have precipitatedfrom a up to 6.9 wt.cloZnO (Sandhausl98l). RepresentaSi-rich hydrothermal solution during metamorphism. tive compositionsof staurolitethat coexistvdth spinel and sulfide from three localities in the AppalachianScandinavianCaledonideorogenare listed in Table 5. Additional compositionsaregrvenby Spry (1984). ZINCIANSTAURoLITEIN THEAppeIecTlm _ Included in Table 5 for comparison are data for SCANDINAVIAN CALEDoNIDE OnocsN staurolite from the Kiipu deposit, Finland, which According to synthesisexperimentson Zn- and occurs in Precambrian volcanic and sedimentary Zn-Fe-stauroliteby Griffen (1982),thereis no limi- rocks metamorphosedto the lower amphibolite tation to the amount of.Znthat can be incorporated grade. in staurolite. Despitethis, in natural specimensZn contenthas beenpreviouslyreported1e lange only Field and textural observations from 0 to 7.7 wt.VoZnO. The incorporation of Zn Mineral assemblagesof Zn-staurolite-bearing s1alilizesstauroliteto the upper amphibolite grade in sulfide-freerocks (e.g., Ashworth 1975).However, rocks studiedare includedin Table 3. Zincian staurothe most Zn-rich staurolite appearsin sulfide-bearing lite from the Bleikvasslidepositcontainsup to 8.77 that have beenmetamorphosedto the wt.olo ZnO and is believed to be the most Zn-tich assemblages lower or middle amphibolite grade. Anomalously yet recorded. Such staurolite commonly occursin Zt-rich staurolite is found in a number of deposits contact with gahnite and pyrite at Bleikvassli; there in the Appalachian-ScandinavianCaledonideoro- doesnot appearto be any reactionbetweenthe Zngen.For example,in the Mineral District, staurolite bearing phases @ig. 4b). In some specimens, coexistingwith zincian spineland sphaleritecontains moreover,corrodedstaurolitein layersrich in mus-
158
THE CANADIAN MINERALOGIST
Fe203
'c, 500
|(BAR
FeS2
-8togfs2 FIc.7. Calculatedphase-relationsfor the systemZn-Fe-Al-Si-S-O projectedonto the Fe-S-O plane as a function of /(S) and/(O) for peakmetamorphicconditions(500'C, 3.8 kbar) affectingthe Bleikvasslideposit,Norway. Spinel compositionsare given in mole 0/oZnAl2Oafor gahnite-hercynitesolid solutions.SeeFigure 6 for the explanation of symbols.
covite and phlogopite is isolated from subhedral grains of spinelthat are confined to quartz-sulfide Iayers @ig. 4c). Although staurolite and gahnite appearto be corrodedin the Canton deposit @ig. 4d), coexistingsubhedralstaurolite,gahnite,quartz and pyrite appearto be a stableassemblage at Stratford and at Kiipu. Genetic speculations Gahnite - zincian staurolite reactionshave been proposed elsewhere(e.g., Atkin 1978, Stoddard 1979,Spry 1982);however,there doesnot appear to be any reaction bstweenthesephasesat Bleikvasst, Canton, Stratford and Kiipu. Rather than staurolite breaking down to gahnite at Bleikvassli, the presenceof corrodedstaurolite and muscovite, euhedral garnet and rare sillimanite in the biotitemuscovite layers suggeststhe following reaction
(Thompson & Norton 1968): staurolite+ muscovite+ quartz : sillimanite + almandine+ biotite + H2O
(10)
Pafi of the Zn releasedfrom the breakdown of staurolite is incorporated into biotite; however, where the remainder of Zn goesis unknown. There is no textural evidenceto support the formation of zincian staurolite by desulfurization of sphaleriteand pyrite; however, by analogy, it is possible that staurolite, like gahnite, formed by desulfurization reactions,for example: [2ZnS] in sphalerite + [2FeS]in pyrrhotite + 9Al2Si5+ H2O* 2O2= [Zt2AleSi4O8(OH)] in staurolite + [Fe2AleSiaOz:(OH)]in staurolite (l l) + SiO2+ 2S2
ZINCIAN SPINEL AND STAUROLITE AS GUIDES TO ORE TABLE5. s10,2 rtoi &2o3 FeO* !{nO uco Z\O ToraL**
cEE!,fIcALcoMPosITIoNs oF ZINcIAN STAURoLITE 28.23 o.4o 52.77 6.3L 0.28 2.08 8.77 98.84
27.07 0.38 51.e1 7.52 0. l-lL.64 7.73 96.33
Atomtc proportlms
sl Tt A1 Fe !4u MC zt
L1.642 o.L27 2s.938 2.L94 0.098 L.297 2.703
11.589 0.L24 26.226 2.100 0.043 1".049 2.447
LL.267 0.132 26.L9L 3.194 0.000 L.544 2.L7L
Zn-staurolite
28.55 0.34 51.86 6.67 0.43 2.2. 7.27 97.32
26.24 0.41 51.73 8.89 0.00 2.4t 6.84 96.52
(oxygen basle
Fe-StaUfOlitg
159
69)
LZ.O20 0.106 25.734 2.349 0.152 1.389 2.226
Norray (average of 5 polnts)i 1 Pcs-56(1) 3lelkvaes].1, 2 79522+ Caator, Georgia (average of 4 poltrts); 3 f20967-5+ stratforal, Nolth carol-lna (average of (average of 10 3 points) i 4 PcS-61 Klipu, Ilnland qurtzt poitrts); asseoblage contal[s ataurollte, pyrlte, gahnlte, pFrhotltex, blotl-te, chlotLtex, galem, + SmLthsoalatr chalcopjELte mscovLtes; IEtLtute cataLogue Drnber; x secondaryi s tlace amut.
Pyrrhotite
Sphalerite
coexistof Fe-andZn-staurolite Frc.8.Thecomposition ing with sphalerite and pyrrhotite(schematic).
Sundblad(1982)claimedthat the critical factors controlling the occurence of gahnitein the Scandinavian Caledonidesare metamorphicgradeand compositi&qrof the local rock. The rocks at all gahnite localitie"shavebeenmetamorphosedto the amphibolite grade.Massive-sulfidedepositsin the sameterSignificantly, the most Zn-rich staurolite, that at rane that were metamorphosedto the greenschist Bleikvassli, coexists with sphalerite. This is to be grade are apparently devoid of gahnite. At first expected in view of the phase relations shown in glance,this would appearto rule out gahniteas an Figure 8, Increasing/(S) at constant/(O) would exploration guide for massivesulfidesin lower-grade shift the tie lines so that Zn-rich staurolite coexists metamorphic rocks. However, zincian spinel has with Zn-rich sphalerite. beenreported from threegreenschistterranes(Franklin et al. 1975, Kramm 1977, Hicks et al. 1985). Kramm (1977)recogazed spinelgrains that are only DrscussroN 8 to l0 pm in lenethin slates,which raisesthe possibility that gahniteis more widespreadin low-grade Calculationsof spinel compositionsthat should terranesthan commonly believedbut may not have be in equilibrium with associated sulfides in the been recognizedbecauseof its grain size. Spinel Mineral District and Bleikvassli deposit disagreeby associated with metamorphosed massive-sulfide as much as 15 mole 0/oZnN2Onwith those actually depositstend to be Zn-rich (e.9., Vokes 1962,Juve (Figs. 6, 7). 1967,Sandhaus1981,Sundblad1982,Spry 1984)and determinedfrom natural assemblages Part of the explanation for this difference lies in the can, in general,be distinguishedon the basisof comextremely high sensitivity of calculated spinel com- position from spinel associatedwith aluminous positions, at given/(O, -/(S) conditions, to the metasedimentaryrocks, which are Fe-rich (Spry & free energyof hercynite.At 600"C and 2 kbar, for Scott 1983a,in prep., Spry 1984).However,Kramm (1977) and Hicks ef a/. (1985) have shown that ex€rmple,a difference in free energyof formation of hercyniteof 31.4 kJ or only 1.990of the value of aluminousmetasedimentaryrocks in Belgium and Mclean & Ward (1966)producesa huge (71 mole South Africa, respectively,contain Zn-rich spinel Vo)shift in the computedcompositionof spinel at simply becauseof the stabilizing eff€cts of Zn on the pyrrhotite-pyrite boundary. The 31.4 kJ is the spinelat low metamorphicgrades.Becauseof such differencebetweenthe free-energydata for hercynite effects, caution must be utilized if spinel composiformation of Mclean & Ward (196Q and those of tion is to be usedas an exploration guide for massivePillay et al. (1960).We havechosento usethe value sulfide depositsin low-grade metamorphic terranes. of Mclean & Ward becauseit gives compositions The presenceof Zn-enrichedspineldoesnot necesof spinelcloserto our experimentalresultsand natur- sarily indicate Zn-rich rocks under such conditions ally observedcompositions(Spry & Scott 1983a,in because,for example,Kramm (1977)demonstrated prep., Spry 1984),but clearlythe free energyof forthat someof the most Zn-enrichedspinelsrecorded mation of hercynite is not well established. to date occur in rocks with only 50 ppm Zn.
160
THE CANADIAN MINERALOGIST
abkvaesl ! ffisr"ntra
6
o
Krnr lL.,TXn fficantor
[--'l
atunrlnors motasedlment (lttBratu]s)
El
metamorptroeeAsufflb(ltioratre)
etanome asaoct ted wtthsutttda IJIJIJJIIJI but hsvlng rornod by non-deanlffibo raac{lons (Geco, Brol(dr Hill (Aurbana), Carmbarg)
the upper amphibolite grade, in general, contains more Zn (e.9., Ashworth 1975)than staurolite in rocks from lower-grade metamorphic terranes, regardlessof the Zn content of the host rock. However,staurolitethat probably formed by desulfurization mechanismsgenerallycontains more than 6 w,r/o ZnO (Fig. 9). Therefore,lhe Zn contentof staurolite may be a useful guide in exploration for metamorphosedsulfide ores, in analogousfashion to zincian spinel.
'D tr
E c
ACKNOwLEDGEMENTS
o
The authorsare 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 the manuscript,and particularly to Dr. J.R. Craig and Mr. D.J. Sandhausfor generousaccessto unpublisheddata on the Mineral District. The authorsare indebtedto Drs. E. Froese,R.F. Martin and R.I. Thorpe for their critical reviews, which led to conln staurollio siderableimprovementof the manuscript.The folFrc.9. TheZn contentof staurolitein aluminousmetasedi- lowing provided natural spinel-bearingsamplesfor study: R. Bedell, T. Bottrill, J.R. Craig, P. Dunn, mentsandmetamorphosed sulfides. 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 authorsarealso In Figure9, the Zn contentof staurolitefrom the grateful to Drs. C. Cermignaniand M.P. Gorton of Bleikvassli,Kiipu, Stratford and Canton depositsis of Toronto and to Drs. R.A. Both and plotted togetherwith publishedcompositionaldata the University B. Griffen of the University of Adelaide for asfor staurolitein aluminousmetasedimentaryrocks sistancewith the electron-microprobestudies.Fundand metamorphosedsulfide deposits.Although there ing was provided by a Connaught Scholarship,a is someoverlapin the Zn contentof staurolitefrom H.V. Ellsworth GraduateScholarship,an Ontario these two settings, the most Zn-rich are from Graduate Scholarship,a University of Toronto Burmetamorphosed massive-sulfide deposits. It is sary to P.G. Spry, and a Natural Sciencesand unclear whether all Zn-rich staurolite from massiveEngineeringResearchof CanadaGrant (No. 47069) sulfide zoneshas formed by desulfurizationof Fe to S.D. Scott. and Zn sulfides; however, staurolite that did nor form by desulfurizationmechanisms(e.g., Broken REFERENCES Hill, Australia; Gamsberg,SouthAfrica and Geco, Ontario), eventhough adjacentto sulfide zones,con- AsRAMs,C.E. & McColrmei.l,K.I. (1984):Geologic setting of volcanogenicbaseand preciousmetal tains lessthan 6 wt.9o ZnO. Textural evidencesugdepositsof the westGeorgiaPiedmont:a multiply geststhat staurolite in retrograde shear-zonesthat deformedmetavolcanicterrain. Econ. Geol. 79, cut acrossmineralizedzonesat Broken Hill formed t52t-t539. by the breakdown of alnandine, sillimanite and biotite (Laing 1977). Staurolite occurs in cordierite- Asnwonrn,J.R. (1975):Stauroliteat anomalously high anthophylliterocks (Spry 1982)alongthe margin of grade.Contr, Mineral. Petrology53,281-291, the Geco deposit.Although the identity of precursors to this staurolite is unknown, the widespread Arrurl,B.P. (1978):Hercyniteasa breakdownproduct occurrenceof staurolitein gneissesthroughout the of staurolitefrom within the aureoleof the Ardara Pluton, Co. Donegal, Eire. Mineral. Mag, 42, area suggeststhat sphalerite is an unlikely precur237-239. sor. At Gamsberg,staurolite is presentin sphaleritefree magrretite- hercynite- garnet - sillimanite rocks groups that overlie the economicC-unit of the Gamsiron Bner.wocr,K.C. (1971):Mineralsof the spinel from AlleghanyCounty, North Carolina. Mineral. (Spry formation 1984).Textural evidencesuggests Record2. 4344. that staurolitemay have formed by the breakdown of garnet and sillimanite. Bwnpuxore,O.K. (1972):Geologyand Mineralization Staurolitein aluminousmetasedimentscontainsup M.S. in theDavisMine Area, Rowe,Massachusetts. to 6 fi.Vo ZnO. That in rocks metamorphosedto Amherst,Mass. thesis,Univ. Massachusetts, o d 2
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ReceivedOctober26, 1984,revisedmanuscriptaccepted September18, 1985.
