Canodian MinerologistVol. 28, pp. 37-50 (1990)

VANADIFEROUS ZINCIAN_CHROMIANBASALT.HOSTED ALTERATION

HERCYNITE IN A METAMORPHOSEDZONE. ATIK LAKE, MANITOBA

LOUIS R. BERNIERDepartment of Geological Sciences, McGill University, j450 [Jniversity Street, Montrcful' Quebec H3A 2A7

ABSTRACT

Unusual Y, Zn and Cr concentrations in hercynite(1.30-5.72 wt.9o V2O3, 3.32-17.07 wt.tlo ZnO and5,05-25.71 wt.9o Cr2O3) are reported in a metamor-phosed, tholeiitic basalt-hosted, cordierite-gedrite-bearingalteration zone associated with Fe-As-Zn-Cu-Au-Agmineralization in the Atik Lake area, Manitoba. The grainsof hercynite are very small (<25 pm) and generally sub-hedral. In transmitted light, they have a translucent honey-brown color and are isotropic; in reflected light, they showa low-intensity, pale grey reflectance. Single grains showrelatively uniform'compositions. Cr and V concentrationsin the altered basalts were increased by up to 30 9o as aresult of mass loss due to leaching of mobile cations dur-ing hydrothermal alteration; these elements thus were rela-tively immobile, along with Al, Zr, Ti ar.d Nb. Thevanadiferous zincian-chromian hercynite formed during anamphibolite-facies regional metamorphism (550'C, 2.5kbars) under low lO) and high /(S) conditions.

Keywords: hercynite, vanadiferous, zincian, chromian,hydrothermal alteration, metamorphism, cordierite-ge&ite, Atik Lake, Manitoba, electron-microprobedata, gold mineralization, arsenopyrite, killingite, coul-sonite.

SoMMAIRE

Des concentralions anomales en V, Zn, et Cr(1.30-5 .72v/o par poids de V2O3, 3.32-17 .07 tlo de ZnO et5.05-25.71t/o de Cr2O3) caractdrisent la hercynite desroches i cordi€rite-g6drite d'une zone d'alteration m6ta-morphisde dans un basalte thol6iitique, associ6e a une mine-ralisation de Fe-As-Zn-Cu-Au-Ag dans la rdgion du lacAtik, au Manitoba. Les grains d'hercynite sont trCs petits(<25 pm) et g€n€ralement sub-idiomorphes. En lumibretransmise, ils on1 une couleur brun-miel et sont isotropes,tandis qu'en lumidre r6fl6chie, ils ont une rdflectance grispile. Chaque grain possbde une composition relativementuniforme. Les concentrations de V et Cr dans les basaltesalt€rds ont augmente jusqu'e environ 3090, par la perte demasse due au lessivage des cations mobiles, V et Cr 6tantparticulibrement immobiles durant le processus d'alt6rationhydrothermale, tout comme N, Zr,Ti et Nb. La forma-tion de I'hercynite vanadifbre zincifOre et chromifdre est li6eau m6tamorphisme r6gional dans le facies amphibolite(550'C, 2.5 kbars) sous des conditions de faiblefiO) etde l(S) relativement 6lev6e.

MoB<lls; hercynite, vanadifdre, zincifdre, chromifbre, alt6-ration hydrothermale, m€tamorphisme, cordi€rite-g6drite, lac Atik, Manitoba, donndes de microsondedlectronique, mindralisation aurifbre, arsenopyrite, lcil-lingite, coulsonite.

INTRODUCTION

Vanadiferous zincian-chromian hercynite has onlyrarely been reported as an accessory phase inmetamorphosed hydrothermally mafic and ultra-mafic altered rocks, sulfide ores and associatedmetasediments, as well as in some unaltered ultra-mafic rocks (Table 1). Zincian-chromian spinel hasmore cornmonly been reported without data on thevanadium content (Table 1). This paper reports anew occurrence ofsuch spinel in which the hercynitehas the highest vanadiuirl content ever reported.Small grains (<25 pm) of vanadiferous zincian-chromian hercynite (up to 5.72 wt.9o V2O3 and upto 17 wt.9o ZnO) were found in cordierite-gedriterocks in a metamorphosed alteration pipe in the foot-wall basalt of an auriferous chert and associatedquartz-grunerite-magnetite banded iron-formation(BIF) at Atik Lake, Manitoba'

The origin of the reported range of spinel com-position is assessed through a study of bulk rockchemical changes related to premetamorphichydrothermal alteration, and an estimate of theP-f-fQ)-f(S) conditions of a regional episode oflower amphibolite metamorPhism.

PREVIOUS WORK

From previous work, ocqurences of vanadiferouszincian-chomian spinel and zincian-chromian spinelin mafic and ultramafic rocks can be grouped intotwo broad categories: those in metamorphosedaltered rocks associated with sulfide mineralizatiot'and adjacent metasediments, and those in unminer-alized ultramafic rocks (Table l).

Minerslized areas

Vanadiferous zincian-aluminian chromite incordierite-bearing niccolite ores at Mdlaga, Spain,is considered to have crystallized from a segregatedoxide liquid having unusually high V and Zn con-centrations (Oen 19?3). Groves et al, (1977) envi'sioned direct crystallization of zincian-aluminianchromite from a sulfide-oxide liquid enriched in Al'Ti, Cr, Zn and Mn, to explain the presence of suchspinels in Fe-Ni massive sulfide ores, Western Aus-tralia.

Zoned spinel grains in mineralized ultramafic

37

38 THE CANADIAN MINERALOGIST

TIBLE l.oCCUniEtCES OF (V I|DTFERqTS) ZtICtAX-ClRo||r^t SptlEr t[ ltmEU|LrzED AnEAS

ANEA 16T RC(S

$ 6vltt€ Dlotrlct otr-it chcolcatilarytard. eedl@tt.(ctose to ore: l'l6tas@tlz€dF6, ctr, zn) pefldotlte.

Fa-Il daposlts, Hlneratlzed U'llf€stern AJotralla. Attsred perl-

dotite.

Klorksdorp oold- Detrltal chrc.fletd, Uiteaterersrd, nlte in cgtt.South Afrlca.

Cr-t{l-(Cu-As) nlno- Crd Rocksralization, [6taga, ard a8socistedSp€ln. Alplna t[|.

zxo v?o:t(rt.Z) (rt.f)

0.88-19.09

(rF20)

0.26-5.6?

(rFeo)

o.50-4.71

(rF8)

0.6('-1.00 1.80-2.90

(n:? )

0.36-0.54 0.00-0.52 tntengoc:At-Znr!ilt,

0.54-0.90 0.01-0.30 c:Zn-richr:llt-

0,56-0.71 0.e0-0.41 m ronlrlg LC

cr/R+5 Atln+l zill[G FACIES ORtGm AUnoRS

Or-Co-Zn deposit,qjtokLtrpu, fintafd.

Ni-F6 Thopsorldep6lt, llanltoba.

te-48-Au-Ag-(Zn-Cu)prospects, Atlkl6ke, lhnltoba.

Sel,vage betrc€rl0ro srd lrl'l hostrocks.

Alterod thotel-Itlc bas€tt

llE fha)€r et al.rfEl (19&)

96lear (1967)lrotoar (1987)fretoarot at . (1981)

tlE ? Rlnsslto &IET? Lacharce

(1981 )

llE Thls study.ilEt

lrytla et al.(1987'

Grovea et al.(19V'

utter (19it8)

oen (1973)

Sesl.inger &h;ke (1969)

l.loore (192)

t agnsr &Velde (1985)

onyeagocha(1974)

f,GtoLA

sutflds orc ard 0.95-14.35mtasdlmnt8. 0.50-12.00

(tFt4)Crd.-Ath. rocks 0.$-26.94ln stockeork zd|o (rF24)

0.18-1.76 0. f l -0.98 0.02-0.581.@-3.(D

0.52-1,06 0.05-0.6a 0.3i:l-0.95

c:Cf-For:Zn-AtI nt€lnge

ilA

NE

ilA?

l,lE

UGLA

I,,A

to

GR

5.qr-15.00

(rFz)

0.?0-0.50 0.45-0.85

2.72-17.20 1.03-5-?" 0.6-0.35 0.57-0.92 geak LAr:Zn,A[>c

(crd-ath rock) (n'66)

(ECURnEICES of (VAIAD!FERq,'S) ZiltCtA{-CfiRfitAX SprrEr tt uwrxEMlrzm AREAS

llelgatard aroa, Uttpanaflc rocks 0.13-a.69IoruaY (rF8)

Plan drAtbord, A#vedsonlto- 0.06-8.24Italy. b€arlrlg ulnetts. (rF3)

Tein Slsters Dunlte, Alplne ut 0.03-O.ZzUa8hlngtqr. (rF9)

rA

0.11-0.e1 0.57-0.67 0.20-0.49 c:forrlt- tC ilEchrmlta

0.06

l{ote: t[l: Uttramaflc, cglt: congldnrate, crd.-Ath.! cordlorlte-anthophyttlte, n: rnder of anatpeo, c: core, r: rin,tc: upPer greenshclst' Lt: lqer o{Thlbotlte, A: aryhibotite, lrA: Lgper arftrtbottte, cR: graruilte, fE! ret6s@tlor0,lUl: magnatlom, llEl: mtamrphlsn. -.---: data mt avaltebte.

rocks, Western Australia, have a zincian-aluminianchromite core and a magnetite rim, and have lowerXru than chromite from layered ultramafic com-plexes. These zoned grains of spinel did not crystal-lize directly from a highly magnesian komatiiticparent magma, but have been affected by meta-morphism (Groves et al, 1977), Complex zoning isalso present in zincian chromite in metamorphosedmineralized ultramafic rocks from Sykesville,

Maryland (Wylie el al. 1987), and in vanadiferouszincian-chromian spinel in a stockwork zone andadjacent metasediments, serpentinites and Cu-Co-Zn ores, Outokumpu, Finland (Treloar et al. 1981,Treloar 1987). According to Groves et al. (1977),thecomplexity of the zoning increases with metamorphicgrade. It appears that in these two areas, the origi-nal chromite chemistry was modified by meta-somatism either during sea-floor hydrothermal alter-

39VANADIFEROUS HERCYNITE, ATIK LAKE

ation or during an overprinted regional metamorphicevent. Treloar (1987) interpreted the high levels ofCr, V and Ni in the stockworks to result from theirredistribution by hydrothermal ore-forming fluids.

Rimsaite & Lachance (1971) reported an occurenceof zincian-chromian hercynite in the chloritized sel-vage zone between ore and ultramafic rocks at theThbmpson Ni-Fe deposit, Manitoba, but did not dis-cuss its origin.

The zincian-aluminian chromite reported in theMB5 zone below the Vaal Reef of the KlerksdorpGoldfield, Witwatersrand, was interpreted to bedetrital and derived from the ultramafic rocks in theArchean greenstones (Utter 1978).

Unmineralized areas

Vanadiferous zincian-chromian spinel grains inunmineralized ultramafic rocks (Table l), such as inthe Helgeland area' Norway (Moore 1977), WesternAustralia (Groves et al. 1977), and Twin Sistersdunite complex (Onyeagocha 1974), usually have lessthan 1 wt.9o ZnO andY2O3. Zrnc generally is con-sidered to be inherited from the magmatic stage' asit tends to concentrate in the silicate liquid and par-

tition into the oxide phase (Groves et al. L977)'Anomalous zincian-chromian spinel (up to 8.74wt.9o ZnO) has been reported in minette at PlanD'Albard, Italy; it occurs in altered zones possiblyderived from olivine, and also as inclusions inphlogopite grains (Wagner & Velde 1985). The ori-gin of that zincian-chromian spinel is not known;ihe primary magmatic spinel could have been modi-fied during a metasomatic process indicated by thepresence of altered olivine.

ANALYTICAL Tncrnttque

Electron-microprobe data were obtained with aCameca Microbeam MB-l instrument usingwavelength dispersion. The analyses were conductedat a voltage of 15 kV, a current beam of 8 nA andcounting times on standards and samples of 25seconds. For ilmenite, V and Ti were determinedusing a PET (pentaerythritol) crystal and VKF andTiKcv peaks, respectively, to avoid an overlap oftheV.l(c and TiKp peaks. This problem was not encoun-tered for the spinel grains, as the Ti concentrationsare negligible. The following standards were used:kyanite (Al, Si), magnetite @e), willemite (Zn), diop-side (Mg), Cr2O3 (Cr), spessartine (Mn), rutile (Ti),and vanadinite (V).

X-ray element mapping was done using a JEOL100 CX TEMSCAN scanning-transmission micros-cope in conjunction with an energy-dispersion sys-tem @GT System 4). An accelerating voltage of 25kV and a current of 8 nA were used during X-raymapping.

Gsolocrcar SsrrrNc

The Atik Lake greenstone belt is located in theSachigo Subprovince in the Superior Province (Fig'

l) and extends for 35 km along an east-west trendwith a maximum width of 10 km. A south-facing'subvertical, homoclinal sequence of volcanosedimen-tary units characterizes the belt. Breaks in mafic vol-canism were marked by deposition of clastic andchemical sediments (BIF and auriferous chert)'Periods of quiescence allowed development ofhydrothermal systems (Bernier & Maclean 1989)'

Small-scale alteration pipes and associated strati-form alteration developed in a tholeiitic basalt unit,footwall to a horizon of auriferous chert with relatedgraphitic argillite and silicate-oxide BIF. Cordierite-

E"&it. rocfs formed during the metamorphism. of

iri*-v alteration assemblages in the alterationpipes. il.te of those pipes is well exposed; a detailedmap of it (Fig. 2A) also shows the occurrences of

thJvanadiferous zincian-chromian hercynite'

MtNgnarocY or UNaLreREnAND ALTERED BASALTS

The least-altered basalt is characterized by a meta-morphic assemblage of hornblende and plagioclase(an.^) with a minor amount of quartz and titanite'b.tfrir on the mineralogy of the metamorphosedaltered basalts and their chemistry are presented inBernier & Maclean (1989). The metamorphism ofthe altered tholeiitic basalts in the pipe (Fie' 2A) led

Frc. 1. Location map of the Atik Lake area' Manitoba'Symbols: FF: Flin Flon, LL:, Lynn Lake, LR: Leaf

Rapids, T: ThomPson.

T---I

f r 'i^

f]4..{)4-ll

! aIAI

t :a r r

M A N I T O B A

40

i - ' . - - . .

THE CANADIAN MINERALOGIST

[.''Elursffig14 <,::)

v-uts kFlute Str

1.j,itcVtfrry

7

I l U t l H u a0 5 m

A

@i

# *\\

B

CT (PPM) DISTRIBUTION tN ALTERATION PIPE

et 340-360360-400 ffi

EA 300 -320320-3t 0 fl 280-300

8S.280FIo' 2' A' outcrop map of pipe l. symbols: Utl: least alrered_glomeroporphyritic basalt, Ut2: bleached zone, ut3:cordierite-gedrite-garnet zone, ut4: andalusite-rictr

"f]re, Ut5: pipe condiiti riii.o with cheft and disseminated sul-fides, ut6: silicate-oxide BIF, showing a.poritt"" "r

glr into i6 .;il;;;;; Tl,'o a, fracture patterns, 9: fimit ofoutcrop' Black dots: magnesian tourmaline. B. contour map showing chromiurn concentrations in the alterationpipe' symbols: triangles: location or ru.pi"r "ooiuining

vanadiier"ri a".i--"tromian hercynite.

to the development of variable proportions of cor- is a vanadiferous ilmenite (up to 1.76 wt.Vo V2O3:dierite, gedrite, garnet, chlorite, biotite, andalusite, Table 2) with minor vanadiferous zincian_chromianstaurolite' quartz and plagioclase (Anr-Aner). The hercynite (Tabre 3), rare rutile and extremely scarcemain oxide observed in the cordierite-gedriiirocks magnetite rich in- the coulsonite (FeVrOr) end_

VANADIFEROUS HERCYNITE. ATIK LAKE 4I

TABLE 2. CTEUICIL @}IPGITIOTSI S IITEIITE ll{D I'IACTETI1E II{ TSEALTBED BASALT. AIIT r.AtE

ffiIE

Tff 3. (mru MITTOST OF ffitE Iil ATIT rc []:m MT

(d.z) _qgA -9198 ulq! --!94 --!,19 --w4 --89 -!1?4 -912p glUS

stoz o.03 o.G 0.6 0.m o.@ o,13 o.@ o.07 0.8 0.06

T l O ? 0 . m 0 . @ 0 . 0 2 o . @ 0 . 1 o 0 . 1 o 0 . G

A l o 4 6 , 9 3 l . s 4 3 . @ M . S i l . g 4 1 . @ @ . € s . 7 4 5 . S 3 . S

ft ;o3 13.62 a.g M.1O 13.8 5.08 la. la 18,72 14.13 x.71 2.4

v o l . s 4 . i l l . P ! . $ 1 . 7 S 1 . S 5 . n 2 . 6 2 . @ 2 0 2

r 3 , 3 : o . @ z . @ l . o 7 o . m o . @ 1 . 4 z . @ 4 . 1 4 4 . s 3 . s

F O n . 1 5 8 . m n . 1 a 2 4 , 8 1 9 . S t . @ 4 . 6 1 3 . 4 1 ? . 1 4 3 l ' O 1l t s o 1 . S O . 7 4 1 . 8 2 1 . 5 2 . 6 1 . 9 O . E O . S O . 5 1 o . NM O.1o -- O. l0 -- 0.o1 --m 9.a a.15 a.s e.g \1. fr 10.02 7.15 3.n 3.9 5.S

lotal S,51 s.31 S.e @.9 16.& S.C S.& S'6 S.41 S.E

olt. ta )1102

A1f"

""%.F f "

lrSot'boato

Tl-;!,

USh

ta^ u2c ulzt t 12F u17A t 1?c u17E tJtzl Uln

::1 :i' i:l' 1l' 1l ll-1 i:I l:I lI0 . t 4 0 . l s o . 1 3 0 . 1 1 0 . 1 6 0 . 1 3 0 . @ 8 . 9 4 7 . 0 1

1.16 1.76 0.43 LA 1.@ 1.4i t O.?3 18.64 t7.12

0.@ 1.27 0.@ 1.45 1.85 1.25 0.50 33.41 34.62

€.36 SS.75 €S.s' SS.14 S.11 1@.23 @.23 €A.3A g7.At

44.65 45.51 45.@ 45.34 45.20 45.72 45.950 . 3 0 0 . 1 2 0 . 3 2 0 . 0 3 0 . 1 7 0 . 0 a 0 . 1 1O.32, O.24 0.4*l o.2a 0.34 0.44 O./X'

1 . € 9 1 . 0 4 1 . S a 1 . S 4 1 . S t . S 4 t3o . @0.uto.021.84o . @0.03

- Alollc hPrlloE (oryaon ktB 32)s r " ' o . o l o . o z 0 . o l 0 . G 0 . 0 1 o . 6 o . 0 2 o . 0 2r r l l o . @ o . o 1 o . @ o . @ o . o zA l - 1 3 . 1 ? a . ? { 1 2 , s 1 3 . 0 5 1 5 . @ 1 2 . 6 1 1 . o 1 1 , se l z . B 4 . $ 2 . a z - a t o . s 3 . l s 3 , s 3 . 4

v - o . m o . s o . s o . 3 l o . u o . a 1 . 1 8 o , sFee o.@ o.41 o.a o,@ o.m 0.2{ o.s o.?aFe 8.S 8.41 6.?8 5. t4 3.$ 6.S 8.36 7.05! € 0 . s 0 . 4 0 . s o . € o . ? 1 o . s o . a 0 . ch 0.m --- 0.c --- o.m ---h t . n r . 1 9 l . s 2 . S 2 , S 1 . 8 5 1 . S 0 . 8 1

M-E&n hpofllocU l v o . O . @ O . m O . @ O . 8 O . 1 2 0 . @ 0 . 4 O . 5Sptn. 4.21 3.m 7.41 A.73 A.n A.a7 3.4 4.01& l a . O . S O . @ O . 5 O . @ 0 . @ O . m o . @ O . @tu. 1A,g 31.31 17.€A 16.rc 5.?1 20.@ 21.75 m.AGaln.21.S 13.4: i 1S.Ca X.A 8.n A.A f l '@ 7.Alkrc. S.$ A,gl Q.m €.41 47.16 47.4 €.13 &.O1han. 0.6 0.s o.8 0.@ o.m l .s 2.44 4.74& u l , l . S 8 . O 2 1 . 8 4 1 . S 2 . G 1 . & ? . S 3 , eF 6 . o . m 1 . G 0 . t o . @ O . @ . 0 . @ o . @ 0 . 1 3

i l l ' i l io.o2 0.0{0 . 1 3 0 . 1 0

0 . @0 . 0 50.031 . 9 10 . @o . M

Total 4.@

0 . 0 0 0 . @0.0? 0.050 . 0 3 0 . 0 5

0 . @ o . 0 00 . 0 1 0 . 0 1

o . @ 0 . @0 . 0 4 0 . @0.07 0.05

0 . @ 0 . @0.02 0.02

o.o2 0.o20 . 1 1 0 . 1 0o , 2 1 0 . 2 70 . 6 1 0 . 5 21 . t 3 1 . 1 11 . O 2 1 . O 20 . o o 0 . @0 . @ 0 . @

3 . 0 0 3 . @

0 . @0.070.05

0.000 . 0 1

4 . @

o.01 0 .@o,e o .o29.8 10.45 . ? 4 . S0 . 5 a 0 . €0 . s o . u? . G 4 . 7 0o . m o . a

0 . 7 6 1 . 64 . 0 0 3 . s a 4 . @ 4 . @ 4 . @

Ca1@latlo6 of atoldlo@lrt @d€ oD ttE bsals of 4 (@gFilt€) u I(i]@ilte) ato6 ol o:{ygotr @p€ctlvgty. ----:mt dei@t€d. . Fef3

@tculatod frco Fe& d€t€ml.Fd by stoichloetry. + e dBlemlred byolcropebg Mlla6.

member (Table 2). Traces of V and Cr (< I wt.9o)also are present in garnet and staurolite. Thecordierite-gedrite rocks contain less than 5 vol.9o sul-fides (pyrrhotite, arsenopyrite, l6llingite, chal-copyrite, sphalerite).

l o t a l A . g 3 . S a . S A . g A . B A . S a . S 4 . a n . 8 4 . 8

0.e5 0,42,60 2.80 . @ o . @

3 . 6 A . As . & 1 3 . 1 4

6.S 47.f f i5 . 3 1 4 . 7 8

0 . G o . 6

nB l@tio of th brcFlto-btE @pl6 h tb plts 13 sbm tn Fta@

b by ib ftF! th dlAlb of tb mql6 !l&r. I Fe;03 alculaid frcE

!b Fo3' coDt€lt &iemtrd by (Af*hV) &fict€rc16 on tlE ctffil

stt6. i e doiemld by €l€trcd-olcrcprcb @lFlB. -_ ei delctd.

Ulvo.: ulvcplFl, Spln.! splel, &la.: addtto, tu.: clrcolto, (b.t

gtult6, krc.. brcFlte, hSn. | -ftlt6, &ul.: @ulsDtte, F6.:fdlint!e.

Frc. 3. Bulk-rock compositions plotted onto an ACF diagram. A: Al2O3 + FqO3- Na2o - K2o, C: cao - 3.3 P2o5, F: Feo + Mgo + Mno. Filled circle: freshbasalt, open circle: altered basalt, And: andalusite, St: staurolite, Crd: cordierite,Alrn: almandine, Ged: gedrite, Ath: anthophyllite, Dio: diopside, Hbl: homblende,Pl: plagioclase. Inset: Trend A: lossof Ca during premetamorphic hydrothermalalteration; trend B: losses of Fe+Mg during hydrothermal alteration.

42 THE CANADIAN MINERALOGIST

(\l

o '1 . t ,tr

1.01.1

TiOz

TiOz ZrFIc. 4. Plots of element concentrations in rock from pipe I (major elements plotted as oxides, expressed in wt.go A.

$r9r: TiOl !, TiO2 - Zr, C. Cr - Tio2 (the numbers indicate the hercynite-bearing samplei of altered basalt),D-Y-Zr. Solid lines through data points are derived by linear regression, and the curves reflect a 95go confidenceinterval. Filled circle: fresh basalt, open circle: altered basalt.

1.8

(vl

3ro

120100801.81.11.0

CHEMISTRy op UNar.rsREpAND ALTERED BASEITS

At Atik Lake, unaltered basalt has a tholeiiticaffinity, with trace-element concentrations typical ofocean-floor basalt (Bernier & Maclean 1989). Theabsence of titanite in the altered rocks can beexplained by a loss of Ca during hydrothermal alter-ation at vent sites. This loss of Ca is well depictedon an ACF diagram (Fig. 3), assuming isochemical

TAM,E 4. PEIIFSO!{ @RNE.ATIo!I I.IATRIX Ft'NIIIIFBITE E.EGNTS tN lfiE PIPE

(F15) Al O2 3

A1"% 1.0o

Ttoa 0.s6

?t 0.79v o . 7 1C. O.gt

metamorphism of the greenschist alteration-assemblages (apart from losses in volatiles). Leach-ing of Ca at relatively constant A-F values duringalteration appears to be the main trend (A in Fig.3). Andalusite-rich patches in the pipe @ig. 2A.) canbe explained by extreme losses of Fe + Mg, as shownby trend B (Fig. 3), which terminates near theplagioclase-andalusite line. Silicification is respon-sible for a mass increase of up to 1590 in theseandalusite patches, thus inducing a dilution of theimmobile elements, including Cr and V (Fig. 4). Theloss of the more mobile cations, mainly Si, Ca, Na,Mg and Fe, is largely responsible for a total loss ofup to 3090 of the original mass @ernier & Maclean1989); this leaching led to enrichment in immobilecomponents such as Al, and thus favored the appear-ance of assemblages of aluminous minerals duringmetamorphism.

Figures 4,A' to D show the effect of alteration ont}te behavior of the less mobile elements in the alteredbasalts at Atik Lake. Paired immobile elements havea high correlation-coefficient and plot along an alter-

ttoa b V Cr

1. q)

0.84 r..@o.T7 o.qt0.9:l 0.78

t. oo0 . d / 1 . @

CorelatloD o@fftclgltB @ €lculatedfor 15 rc& eqEpl@ ftlo tbs El,toEttoDpue. ABI!@ are pr*oDted t! Table 3of Bemler & lhcl€e (tn ptw).

VANADIFEROUS HERCYNITE, ATIK LAKE 43

ation trend passing through the composition of theprecursor and origin, which is defined by an increase(mass loss) or a decrease (mass gain) of the immo-bile elements (Maclean & Kranidiotis 1987).Vanadium and chromium behaved as relativelyimmobile elements during the alteration of the AtikLake basalt (Table 4, Figs. 4C, D). Additional evi-dence for the immobility of V and Cr comes froma study by Seyfried & Mottl (1982), who showed thatV and Cr are the most immobile elements among thetransition metals during experimental seawater-basalt interaction at 300"C. Furthermore, Howard& Fisk (1988) showed that an alteration crust con-taining aluminum-rich clays enriched in Ti, V andCr formed by residual weathering during hydrother-mal alteration of basalt on the Gorda Ridge. I havecalculated a total mass-loss'bf up to 7 5t/o in the alter-ation crust from Gorda Ridge, using AlrOr as theimmobile component. This mass loss induced anincrease in Cr concentration from 4ll ppm in thebasaltic glass 1o up to 1060 ppm in the alterationcrusl. At Atik Lake, background values of 270 ppmCr and 315 ppm V in the least-altered tholeiiteincreased by a factor of up to 3090 in the cordierite-gedrite rocks. The presence of vanadiferous zincian-chromian hercynite in the altered rocks is the miner-alogical expression ofthese high-Cr and high-V zonesin the alteration pipe (Figs. 2A, B). Such spinel grainshave been observed only in the metamorphosedaltered rocks, and no "primary" igneous chromiteis found in the least-altered tholeiitic basalt. Thespinel is also absent in the auriferous chert andgrunerite-magnetite BIF overlying the alterationzones. The low concentrations of Cr (l-208 ppm)and V (l-128 ppm) in these chemical sediments indi-cate that Cr and V were not added extensively ashydrothermal components, which suggests that theywere relatively immobile during hydrothermal alter-ation.

Cnvsrer-CneMrsrRy CoNsrpBnerroNs

Spinels of the TM2O4type have one tetrahedrally(Z) and two octahedrally (M) coordinated sites; thesesites can accomodate a large number of cations ofdifferent valences (2 +, 3 +, 4 + ), allowing complexsolid-solutions to occur in nature (O'Neill &Navrotsky 1984). Some elements, such as Fe, V andMn, may have more than one oxidation state, con-tributing to possible electron-exchange reactions. Acrystal-chemistry study of natural Fe-Mg-rich chro-mian spinel (0.15 < Cr < 1.07 atoms per formulauni! by Della Gil.:a,laet al. (1980 shows that: Cr sub-stitutes on both octahedral (M) sites, along with Aland some Mg; on the basis of strong site-preferenceenergies, Cr only occupies octahedrat (M) sites. DellaGiusta el a/. assigned Sia* exclusively to the 7 site,and Ni2+and Tia+ only ro the M sites. The higher

the Cr content, the greater the ordering of Mg in theZ site and Al in the M sites. Zinc has a strong site-preference energy for tetrahedral coordination.Jacob (1970 and Bruckmann-Benke et ol. (1988)found that the tetrahedrally coordinated site of abinary solid-solution of the type Zn(Al'Cr)rOo isalmost entirely occupied by Zn, with very smallamounts of Al. Vanadium has a strong octahedralsite-preference energy and is assigned (both V3+ andV4*) to the M sites (O'Neill & Navrotsky 1984).

For the Atik Lake spinel, I have assumed that allMn is divalent and that all Y is trivalent. Fe3* hasbeen calculated from atomic proportions based on32 oxygen atoms (24 cations) and was obtained asthe difference between the sum of the octahedrallycoordinated cations (Al + Cr+ V3+) and 16.00. End-member proportions were computed with analgorithm for which all Ti, Cr, V, Mn and Mg wereused to form, respectively, ulv6spinel, chromite,coulsonite, galaxite, and spinel. Zn was combinedwith excess Fd* after the formation of magnetiteto form franklinite. Any remaining Znwas combinedwith Al to form gahnite, and the leftover Fe and Alwere attributed to hercynite (Table 3).

Elements having a strong negative correlation(Table 5) are inferred to substitute for each other ina given site (e.9., Al-Cr, Mg-Cr on the M sites;F&*-Zn, Fd*-Mg on the Zsite). Those having astrong positive correlation involve I- and M'typecations (e,g,,Zn-N, Fd*-Cr, Fd*-Fd*), a reflec-tion of specific end-members. Treloar (1987) foundnegative correlations between Zn and Cr, andbetween Fe and Al for spinel at Outokumpu, indicat-ing an inverse relationship between theF eCr 20 a-ZnNtO4 components.

CnenacrspJsrlcs oF THE HERcYNITE

Optical properties

The vanadiferous zincian-chromian hercyniteoccurs as small subhedral grains (<25 pm) havinga honey-brown to dark olive color, strong relief, andlow (R9o = 12) reflectance Grey). The hercynile com-monly is found close to garnet porphyroblasts, rarelyas inclusions, and usually in a matrix of quartz,

rAg.E 5. PEAFSON @BnglTIoN !'tATnIX FOR CoI{CENTSAIIOIIOF CATIONS IN ITERCYNITE FROI'I ATIK I.AKE

(n'€S) Al c| v Fee Fe& t{8A l 1 ,@c| -o.97 t.@v -o . {s 0 .41 1 . (x )Fe$ -o.zs o.ss o.1g l.d)Fe& -o .g l o .?8 o .42 0 .81 1 .@!A o.sl -o'85 -o.rts -0.m -0.87 1.@6 o.&t -0.76 -0.41 -o.m -o.ga o.al l . d )

CoreletloD @fficleils @lculalod bV lt@ Fg@sion

o! 8a bercyatto SntE h @rdls.ito-8e&ite rcka lD tbe pipo'

tho orplolo dat;€ot td aBilablo qpo! .6gwt to tbe-autho''

nopmentatlve hercynlte coalsltloE 9r ahom lD Table 3'

44 THE CANADIAN MINERALOGIST

Flc. 5. SEM image of a grain of vanadiferous zincian-chromian hercynite. Others: X-ray element maps; Al, Cr, Fe[white spot corresponds to an inclusion of pyrrhotite (Po)], Zn, Mn, V, Ca (Cal: calcite inclusion beside the sulfideinclusion), Si (Qtz : large quartz inclusion in the hercynite). Bar scale: l0 pm.

plagioclase or cordierite. The hercynite grains mayhave micrometer-size inclusions of quartz, sulfideand carbonate (Fig. 5).

Chemistry

Electron-microprobe data representativeof the compositional range observed arepresented in Table 3. The following structuralformula expresses the results of 66 microprobe ana-lyses : (F e26!oe -s.esZ!0. oz_0. r zMgo. oz_o.osMn. o.oos)(A1,.,3_,.17Cre.,rr.of e3.io-o., rV3.irr.,,)Oo.

X-ray element mapping shows that single grainsof spinel are relatively homogeneous (Fig. 5), butslight increases in Al and Zn from core to rim weredetected in some samples. A wide range of spinelcompositions is observed within the alteration pipe(Frg. 2) and also at the thin-section scale @igs. 6,7). Compositional variations occur among differentsamples, but also in different grains in the samesample.

When compared to most other occurrences inmineralized areas, the range of spinel compositionsat Atik Lake (Table I, Fig. 6) fills a gap in previ-

Cr

7"e?) o r

$ooo

. O

2 Phases

VANADIFEROUS HERCYNITE. ATIK LAKE 45

.fig{/

"5*Ftc. 6. Al-Cr-Fd+ diagram showing compositional range of (vanadiferous) zincian

spinels. Optical characteristics presented on the side of the diagram are compiledfrom the literature; underlined is the color in reflected light; along Al-Cr andCr-Fd+ sides are shown color and internal reflection in tansmitted light, Dashedline: empirical miscibility gap of Loferski & Lipin (1983). Open circles: Fe-Nideposit of Western Australia, filled triangles: Fe-Zn-Cu-Co deposits, Sykesville,Maryland, open squares: Thompson mine, Manitoba, filled squares: KlerksdorpGoldfield, Witwatersrand, filled circles: Cu-Co-Zn Outokumpu deposits, Fin-land. Compositions from Atik Lake: hatched field, represents 36 compositionsfrom Ul0A, U4, U8, small circle with cross-line: Ul0B, small qpen circle witharrow: U17, black dots: Ul2. Details on location, compositions and referencesare given in Table l.

ously reported data for zincian-chromian spinel. Thespinel has higher AllR3+ (R3+=Al+Cr+V3+-rFe3*; and lower CrlR3* values than spinel inmetamorphosed altered ultramafic rocks. This state-ment is also valid for compositions of spinel in ultra-mafic rocks in unmineralized areas (Onyeagocha1974, Moore 1977,Wagner & Velde 1985; Table l).A spinel from the selvage zone between Ni-ore andits ultramafic host at the Thompson mine, Manitoba(Rimsaite & Lachance 1971), falls within the com-positional range observed at Atik Lake. Core com-positions of spinel in the cordierite-anthophylliterocks from the Outokumpu deposit have muchhigher Cr,/R3+ values than their rim, which is morealuminous and falls within the Atik Lake compos!tional field (Fie. 6; Treloar et al.1981, Treloar 1987).Another striking characteristic of the spinel at AtikLake is its general lack of intense compositional zon-ing, as was reported elsewhere for grains of zincian-chromian spinel that reach up to 700 pm across

(Weiser 1967, Groves et al. 1977, Treloar 1987, Wylieet al. 1987).

A continuum of compositions in natural chro-mium-rich spinel exists between chromian magnetiteand "ferritchromit" through Fe3*-poor, low-aluminum chromite through chromian hercynite topure hercynite, as discussed by Evans & Frost (1975).The zincian-chromian hercynite compositions com-piled from the literature also show this continuum(Fig. 6) and plot next to an empirically determinedmiscibility gap, outlined by Loferski & Lipin (1983).

Interestingly, a euhedral crystal of magnetite con-taining -2.5 wt, 9o A12O3, -70/o CrrO3 and - l7tloVrO3 was found to coexist with the vanadiferouszincian-chromian hercynite in sample Ul2 (Tables2, 3). The slightly low analytical totals for themagnetite (Table 4) could result from the presenceof V4*, as all the vanadium was taken to be V3+ inthe calculations. If this is so, the presence of Va"would lead to overestimation of Fe3* as a conse-

o o oob

6 0od,

-?.9

"fp"r.tgq

46 THE CANADIAN MINERALOCIST

cN

cN

123 Utg L55 1.71AI

t+

+

?+

Ftc. 7. Binary diagrams showing compositional langes of hercynite from Atik Lake.Symbols: half-filled circle: Ul0A, small circle with cross-line: Ul0B, open circle:U4, filled circle: U8, small dots: Ul2, open circle with arrow: Ul7. Correlationcoefficients as in Table 5. Concentrations in number of?ations.

quence of an electron-exchange reaction such asFd* + V:* - Fd* a ya+ gVakiharaetal. l[t).

a o

-"f.

o f l .' j

' t o o f c .

o o0

" f ;o

oo

o €o 8

co

,:"::;+i'"ic

"&

:::.'lg ' ,

&%*

.3.. . t !

t \

o o on n o o

a_'i Et r \ - O

. ;

l ' .t .l ot - t to f

t

y ' o "q o ac

o oo ^ :i"'-.-

tt4 e

I

. t

The hercynite grains in sample Ul2 have high cal-culated concentrations of Fe3+ (Table 3) and plot

VANADIFEROUS HERCYNITE, ATIK LAKE

ocrfo

J

{6

c,20

-21.

-28

-u, 12 -10 -8 -6 -t,r_og (rSz)

Ftc. 8. Log/(O) - logflS) diagram. Shaded area shows the stability field of the hercynite at Atik Lake. Symbols:Hem: hematite, Mt: magnetite, Rt: rutile, Ilm: ilmenite, Po: pyrhotite, Py: pyrite, Lri: lcillingite, Apy: arsenopyrite.Nf"i values shown along the logfs) axis calculated uling equations of Froese & Gunter (1976). The error barsfot the Apy and Ld boundaries are calculated using the A@ uncertainties of Barton & Skinner (1979). Other sourcesof thermodynamic data as indicated in the text.

2

close to one limb of the proposed miscibility gap,whereas the magnetite crystals plot near the oppo-site limb (Fig. 6). For the vanadiferous spinel, afourth component has to be considered; t}te composi-tional fields and tie line are projected from thatfourth component onto the Fe3+-Al-Cr plane. Theeffect on the solvus of adding V to the system is notknown. No evidence of exsolution was found in theAtik Lake spinel. Slightly higherfiO) conditions inthis part of the alteration pipe (Fig. 2A) led to theformation of the coexisting spinel in local areas ofappropriate bulk-compositions. A rare case ofunmixing in primary igneous chromite (Loferski &Lipin 1983) was attributed to metamorphic re-equilibration at temperatures around 600oC. As noprimary igneous spinel has been identified in theunaltered basalt at Atik Lake, a metamorphic ori-gin for these coexisting spinels is indicated.

CoNDITIoNS oF METAMoRpHISM

Peak conditions of metamorphism at Atik Lakeare estimated to have been Z: 550'C and P < 3.0kbars @ernier & Maclean 1989). The pressure is notas well constrained as the temperature. The garnet-hornblende geothermometer of Graham & Powell(1984) gives core and rim temperatures of 557"C and

531oC for sample U2C within the pipe. The garnet-hornblende geobarometer of Kohn & Spear (1989)provides core and rim pressures of 3.3 and 2.8 kbarsfor sample U2C using the activities for the Mg end-members. The core compositions of coexistinggarnet-cordierite pairs in the alteration pipe indicatesmaximum pressrues ranging from 2.5 to 3 kbars atthe condition P = P(HzO) using Figure l0 ofAranovich & Podlesskii (1983). The presence of theassemblage cordierite-chlorite-biotite-muscovite insample U4 indicates a minimum pressure betweenI kbar if X(H2O) = I and 2.5 kbar if X(H2O) =0 at T = 550oC using the mineral compositions andthe thermodynamic data of Berman (1988). Withthese pressure considerations and in the absence ofa precise estimate of the mole fraction of HtO, apeak pressure of 2.5 kbars is chosen. The stabilityfield of the vanadiferous zincian-chromian hercynitecompositions can be located in flSj-flO) spaceusing buffering assemblages and the composition ofpyrrhotite in spinel-bearing samples (Table 6). Thedifferent phase-boundaries plotted in a log /(S)-loel(O) diagram (Fig. 8) were calculated using thethermodynamic data of Clark (1966), Froese & Gun-ter (197O, Barton & Skinner (1979), Berman &Brown (1985), Sharp et al. (1985) and Berman (1988).Different /(S)-lO) buffer assemblages observed in

T=550 C P= 2.5 kbar

48 THE CANADIAN MINERALOGIST

TM A. AVEIACE @GITIOI€ OF EEM1IITE FFO}I AltT rc Mf (o.)-t(s2) FJFmm EsuBtacEs

S&pre (n) F- F^,

u ( 1 6 ) O . 1 2 0 . Su10A (1.3) O.1S 0.7€u10B (12) 0.A O.7ou ( 8) 0.21 0.74w2 ( 8) 0.20 0.?1!17 (12) 0.31 0.a1

g$l$- bs r(s2)apy-lrF-11rr!6PY- lo-p-1 Ia6llFlepo-tlB -4.2

llrpo-rlIlrpo-bi -3.9

l l rpo -3 .5

(B) - nober d borslto amtE @lFd. -: @l@tatd at T - &iOoC dP - 2.8 klor tua prboilto co&Dosttto6 qk O.S{21, o.ffi, 0.s62 for

siplg lr10B, U2 aDd U17 rgspectlwly clng 6q@ttoE of Ft@ & C6i€r(rm). --- bt aElyd. aW: al.ffiplntts, lo: lolllqtte, p:pyFbo i l te , t lo r l l@dto , . t : ru i t lo , E t ! @t t t€ . X-_*z t r t iev* re31,

XilEAI/(Al+hv+Fee). Xary(F6+Ha+ha).

the Atik Lake cordierite-gedrite rocks includearsenopyrite- l6llingite-pymhotite-ilmenite + rutile,pyrrhotite-ilmenite-rutile and pyrrhotite-ilmenite-magnetite (Table 6). The magnetite-bearing sample(Ul2) has a high average X.,[Crl(Cr+Al+V3* +Fe3*)] and Fe3+ content, andthe lowest Xto lZt/(Fe +Mg + Mn+ Zn)1. Calcu-lated log/(S, values based on compositions of thepyrrhotite (method of Froese & Gunter 1976) areshown in Table 6. Sample U4 has the highest Xrnand lowest X., values and appears to have beenbuffered at the lowest values of /(SJ and /(O),represented in Figure 8 by the intersection of thearsenopyrite-lOllingite-pyrrhotite and ilmenite-pyrrhotite-rutile boundaries. Samples UIOA and Bwere buffered at a similar value of log /(S) (:-6.2) as sample U4, but at a higher flO), alongthe arsenopyrite-lOllingite-pyrrhotite boundarywithin the pyrrhotite-ilmenite field, as indicated bythe absence of rutile. A calculated value of )$3, =0.9?47 was obtained by solving reactions (1) (Froese& Gunter 1976) and (2) (Barton & Skinner 1979)byiterations, until the two equations are satisfied bya common value of Jf,$:

(l) nS(in pyrrhotite) : {S, (in vapor)(2) 2FeAs., + 2FeS + Si = 4FeAsS

For reaction (2), the composition of arsenopyrite wasused (37.1 As atomVo) to calculate an activity aFeAsS: 0.8859, and aFeAsr was taken to be unity. Thecalculated value of )S$ contrasts with the measuredvalue Jfi"i = 0.8905 in sample U10B, which leadsto a value of logfiS) = -4.12, which is 2 log unitshigher than that expected from the position ofthe arsenopyrite-ldllingite-pyrrhotite boundary.Although all three minerals are observed within thesame thin section, only ldllingite-arsenopyrite andpyrrhotite-arsenopyrite pairs are observed in con-tact. If the pyrrhotite-arsenopyrite boundary is con-sidered instead, a pyrrhotite composition of l€& :0.8986 is calculated (3).

(3) 2FeAsS * 52 = 2FeS + 2(As,S) liquid(4) 2FeAs2 * 52 = 2FeAsS + 2As

This value of ,4& (0.8986) is much closer to themeasured one (0.8905) in sample Ul0B. Theldllingite-arsenopyrite boundary (4) is calculated tooccur at loglS)= -5.30. If the maximum uncer-tainty of 5 kcal on AG7 (Barton & Skinner 1979) isapplied, the boundary may occur annvhere between-6.4 and -3.9 log.f(S), a range that overlapsboth reaction (2) and (3) boundaries. This suggeststhat the rare pyrrhotite grains (15 pm) observed incontact with arsenopyrite in sample U10B were inlocal equilibrium and that reactions 3 and 4 areindicative of the/(St condition rather than reaction2. The estimated range of /O)-/(SJ values underwhich the vanadiferous zincian-chromian hercyniteformed in the cordierite-gedrite rocks in the AtikLake alteration zone is indicated by the shaded areain Figure 8. In summary, the assemblagearsenopytite-l6llingite-pyrrhotite-ilrnenite fixed t]le/(SJ during metamorphism, but allowed variationsof up to 2.5 log units in /(Or) within thepyrrhotite-ilmenite field. The assemblage ilmenite-pyrrhotite-rutile in sample U8 does not allow a pre-cise estimate oflS)-/(O), but in view of the spinelcompositions, it probably has values within the fieldoutlined in Figure 8, The spinel samples having thehighest contents of chromite and magnetite compo-nents and lowest level of the gahnite componentappear to bi: those buffered at the highest -f(St-/(O) values. Other factors that can influence spinelcompositions are: modal amount of spinel formedin the rock and bulk composition of the rock (Fig.4C).

The f(O)-f(S) conditions prevailing duringmetamorphism in the alteration pipe contrast withthose in the adjacent BIF (magnetite field; Fig. 8),auriferous chert (arsenopyrite-pyrite-pyrrhotite-iknenite; Fig. 8) and associated graphitic argillite(pyrite-pyrrhotite-graphite-rutile; Fie. 8). None ofthese sedimentary horizons contain vanadiferouszincian-chromian hercynite.

DISCUSSION AND CoNCLUSIONS

Genesis of the Atik Lake hercynite

Vanadiferous zincian-chromian hercynite occursin metamorphosed altered basalt (cordierite-gedriterocks) in an alteration pipe. The alteration zone isoverlain by auriferous chert and associated graphiticargillite, and by grunerite-magnetite-bearing BIF(Bernier & Maclean 1989). Primary igneous spinelhas not been observed in the fresh tholeiitic basalt.

Primary igneous chromian spinel compositionsfrom deep-sea basalts, as reported by Haggerty(1981), are generally aluminous (up to 49 wt.qoNzOr), Mg-rich (up to 19 wt.9o MgO) and may beFe3*-rich (up to 11 wt.9o FqO). The reportedcompositions do not show even tracas ofvanadium

F " - v F

o 0.04 0.9o 0.04 o.zo o . G 0 . 1 ?

0.02 0.6 0.210.@ 0.@ o.f l0 . 1 0 0 . 6 0 . 1 1

VANADIFEROUS HERCYNITE, ATIK LAKE 49

and zinc. Spinel in deep-sea basalt occurs as inciu-sions in plagioclase, pyroxene, olivine and glass andmay show complex zoning as a result of interactionbetween early-formed crystals and liquid owing toa temperature decrease (Haggerty 1981). TheCr/R3+ values found in basalt-hosted spinel aresimilar to those in Atik Lake metamorphosed alteredbasalt, and this can readily be attributed to a bulk-composition effect.

At Atik Lake, a premetamorphic stage ofhydrothermal alteration enhanced Cr-V concentra-tions in altered basalts as a result of mass loss (Ber-nier & Maclean 1989). Cr and V behaved as rela-tively immobile elements. Cr3* and V3* cansubstitute for Fe3* and Al3* because of their simi-lar ionic radius; they may have been concentratedin aluminous clays (Howard & Fisk 1988) and chlo-rite when these minerals formed during thehydrothermal aheration of the basalt. Concentrationof Cr in sediment via the clay fraction is common(Burns & Burns 1975, Treloar 1987). Regionalamphibolite-facies metamorphism induced break-down of the greenschist-facies assemblages toproduce cordierite-gedrite rocks. The grains ofvanadiferous zincian-chromian hercynite nucleatedunder relatively low/(O) and high/(S) conditions,at sites of available Cr-V-Al-Zn released by break-down ofprecursor phases. Apart from the hercynite,which contains a major concentration of Cr, tracesof Cr also were found in ilmenite, garnet and stauro-lite, and it is an important component in magnetite,which is scarce. Vanadium was detected in ilmenite,but is a major component (coulsonite) in magnetiteClable 4). The presence of V and Cr in other mineralsof the Atik Lake alteration zone reflects the bulkcomposition of the rocks, acquired during hydrother-mal alteration. The absence of pronounced composi-tional zoning in these spinel grains could be due totheir very small size or may indicate that efficientdiffusion occurred at the peak conditions ofmetamorphism.

Spinel having such unusual compositions inmetamorphosed mafic and ultramafic rocks can pro-vide a useful guide for potential mineralization ofthe Outokumpu type (Cu-Co-Zn), Thompson type(Fe-Ni) or Atik Lake type (Fe-As-Zn-Cu-Au-Ag).Furthermore, the spinel grains are resistant toweathering and can be incorporated into thesedimentary cycle and thus provide useful regionalindicators of mineralization.

ACKNowLEDGEMENTS

This paper represents part of the author's doctoralresearch at McGill University. I thank WestminResources Limited, Barringer-Magenta Limited andPolestar Exploration Inc. for access to their propertyand for research support. Drs. W.H. Maclean, S.

Wood, A.M. Abdel-Rahman and R.F. Martinreviewed a preliminary draft of this manuscript. J.E.Mungall's help on the scanning electron microscopewas greatly appreciated. I acknowledge the finan-cial assistance provided by an FCAR scholarship (LaFormation des Chercheurs et I'Aide d la Recherche).Research costs were partly covered by NaturalSciences and Engineering Research Council grant477L9 to Dr. W.H. Maclean. The manuscript hasalso benefitted from the review of two referees.

RsrgREl.tcps

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50 THE CANADIAN MINERALOGIST

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Received August 29, 1989, revised manwcrtpt acceptedOctober 26, 1989,