METAMORPHISM AND SUPERGENE ALTERATION OF Cu-Ni

Canadian MineralogistYol.22, pp. 13-21 (1984)

ABSTRACT

Four principal types of sulfide ore are recognized at theThiegry'mine, located in the Uchi volcanic belt (SuperiorProvince), northwestern Ontario. These are: disseminatedCu-Ni-Fe sulfides, breccia ore, mylonite ore and borniteore. The sulfides are interpreted to be of magmatic origin,but modified by strong shearing to produce mylonite andbreccia ore. The bornite ore represents enrichment in Cuof blocks of amphibolite schist in the mylonite. Supergenealteration removed iron from the iron-nickel sulfides to pro-duce iron oxides and sulfides, especially violarite enrichedin cobalt and nickel. Violarite (Ni-Fe-rich) appears to co-exist with pentlandite where it formed from pentlandite.Violarite (Ni-Co-rich) generated as the result of metamor-phism does not coedst with pentlandite. This strongly sug-gests that cobalt plays a major role in defining the low-temperature stability fields and explains the two conflic-ting interpretations of assemblages involving violarite inthe system Fe-Ni-S. The modification, during deforma-tion and metamorphism, of primary magmatic sulfides ofsubeconomic grade to an ore deposit at Thierry and otherareas implies that an ulfiamafic-gabbroic body in an in-tensely metamorphosed and deformed terrane might offera better target for er<ploration than ore in lower-grade rocks.Furthermore, the enrichment of platinum-group elements@GE) in mylonite and bornite ores at Thierry indicates thatmobilization into shear zones could produce PGE deposits.

Keywords: copper-nickel sulfi des, metamorphism, defor-mation, platinum-group elements, supergene alteration,Thierry mine, Ontario.

SoMMAIRE

On reconnalt quatre types principaux de sulfures d lamine Thierry, situee dans la ceinture volcanique d'Uchi(province du Supdrieur), dans le Nord-Ouest de I'Ontario.Ce sont: les sulfures diss6min6s de Cu-Ni-Fe. le mineraibr6chifi6, le minerai mylonitis6 et le minerai de bornite. Lessulfures sont considdrds comme 6tant d'origine magmati-que; ils ont 6td modifids par un cisaillement intense qui aproduit le minerai des types mylonitise et br€chifi6. Le mine-rai de bornite repr6sente un enrichissement en cuivre desblocs de schiste amphibolitique inclus dans la mylonite.L'alt6ration supergbne a ddplac6 le fer des sulfures de fer-nickel pour former des oxydes et des sulfures de fer, en par-

lPresent address: Ontario Ministry of Natural Resources,435 James Street, Thunder Bay, Ontario P?E 6E3.

*Publication 09-83 of the Ottawa-Carleton Centre forGeoscience Studies.

METAMORPHISM AND SUPERGENE ALTERATION OF Cu-Ni SULFIDES,THIERRY MINE, NORTHWESTERN ONTARIO*

GEORGE C. PATTERSONIANO DAVID H. WATKINSON

Department of Geology, Carleton University, Ottawa, Ontario KIS 586

ticulier la violarite enrichie en cobalt et en nickel' Une vio-larite riche en Ni-Fe semble coexister avec la pentlanditeld oi elle s'est formde d ses d6pens. Par contre, la violariteriche en Ni-Co due au m6tamorphisme ne coexiste pas avecla penllandite. Le cobalt jouerait donc un r6le cl6 dans laddfinition des champs de stabilit€ ir basse temp6rature etexpliquerait les deux interprdtations contradictoires desassemblages impliquant la violarite dans le systbme Fe-Ni-S, La transformation de mindralisations sub-6conomi-ques en gltes dconomiques, i Thierry et ailleurs, reli6e dla ddformation et au m6tamorphisme des sulfures magma-tiques, implique qu'un massif ultramafique - gabbroilquepourrait offrir de meilleures chances de succds d I'explora-tion dans un terrain inlensdment m€tamorphisd et d6form6que dans un autre moins m€tamorphis6. De plus, I'enri-chissement des 6l6ments du groupe du platine @GP) dansle minerai mylonitisd et celui de bornite i la mine Thierrymontre que la mobilisation dans des zones de cisaillementpourrait donner naissance i des depdts d'EGP.

(Traduit par la R6daction)

Mots-cl6s: sulfures de Cu-Ni, metamorphisme, d6forma-tion, 6l6ments du groupe du platine, alt6ration super-gbne, mine Thierry, Ontario.

INTRODUCTION

The purpose of this paper is to document and ex-plain the various sulfide assemblages at the Thierrymine. Interpretation of these, in conjunction withthe petrographic study ofthe host rocks, illustratesthat copper-nickel deposits associated with mafic toultramafic rocks are not static after their formationby magmatic processes (Patterson 1980, Patterson& Watkinson 1984). Rather, dynamic regionalmetamorphism and supergene alteration can play amajor role in the development of ore bodiesassociated with these rocks.

The Thierry mine, owned by Umex Inc.' is locatedin the Uchi volcanic belt, Superior structural prov-

ince, 450 km northwest of Thunder Bay, Ontario,near the town of Pickle Lake (Patterson & Watkin-son 1984, Fig. l). The geological setting of the mineand surrounding area is described in detail in thepreceding paper (Patterson & Watkinson 1984; seeespecially Fie. 3).

TYPEs oF Cu-Ni Sulpnn Onr

At the Thierry mine, there are four principal types

t3

t4 THE CANADIAN MINERALOGIST

of sulfide: disseminated sulfides, breccia ore,mylonite ore and bornite ore, making up (lVo,40V0, 58v/0, and < lVo, respectively, of the sulfidesmineral assemblages. The disseminated sulfides oc-cur in thick sections of mafic metagabbro and talc-

carbonate schist. Breccia ore occurs as irregular zonesin faults within the mafic metagabbro and mylonite(chlorite-biotite schist). Breccia ore grades intomylonite ore, which occurs in the main northwest-trending shear zone. The bornite ore occurs in

I

i6.a);ra a:::) a; aai.6a # 3":la:u" + -

FIc. l. a, Disseminated sulfides; chalcoplrite, pyrrhotite (light grey) and magnetite (medium grey) intercumulus withrespect to olivine (partly altered to serpentine); specimen K9-208. Field of view 5.6 mm. b. Breccia ore; metigabbrofragments with sulfides penetrating along fractures; specimen TH-14, west pit, 2nd bench. Field ol view 8.5 cm.c. Breccia ore; a fragment of metagabbro rimmed by pyrrhotite (dark gre$ in a matrix of chalcopyrite (white), 600crosscut. Field of view 14 cm. d, Breccia ore; magnetite crystal with a spiral ilrangement of sulfide and silicateinclusions. +032-153. Field of view 1.2 cm. e. Altered breccia ore; dendritic pyrite (white) after pyrrhotite withspongy violarite (gey). 60O-IE-HOR-65. Field of view 2.1 mm. f. Mylonite ore; highly folded and deformed fragmentsof wallrock. Pyrrhotite and chalcopyrite occur as fine disseminations stretched parallel to the foliation. West pit,2nd bench. Field of view 8.5 cm.

METAMORPHISM AND ALTERATION OF Cu-Ni SULFIDES, THIERRY MINE, ONTARIO

TABLE J.. AWRAGE COMPOSITION OF SIJI,FIDES FR'OU THX TSIERFy UINE3 DISSE}{INATED ORE AND

BORNITE ORN

1 5

DISSEMINATEDORE'.

bonite

cubaolte

py!rhotl.te

p€dtLaodl.te

BORNITE ORE

DL11e!lte

carrollr.te

bolnl.te

wlttlchenlte

edplectlte

NO. OFGRAINS S Bt

3 26.9

5 5 ) - C

WEISFT PERCENIFe Co Nl

1 4 . 8 T !

Cu

58.1

24.8

9 .13 29 .8 1 .5s

5 .40 58 .3

3 3 . 6 l l . 3

0 .d . n .d .

IOTAT FOR}'ULA

4 0 . I

3 36 .6

3 32,6

99.8 cu4.35Fe l .26s4

100.3 Cu1.96Fe1.9553

99,2 M7.A4s8

100.6 ug .ZSg

101.5 ML.OSS

95.3 M2.95$q

100.1 Cu4.66Fe1.2754

100.8 Cu2.9B lS2.9

99.6 cuBts2

4

3

I

1

34.7

38.5

25.2

L9.2

: 1 . 4 4

- 0 . 9 9

- 14 .1

43.2 n .d .

61 .0 n .d .

r . 69

r0 .9

60 .8

38 .4

19.4

T r t r a c e , D . d . n o t d e t e c t e d , n . a . n o t M 1 y z e d

metavolcanic blocks included in the mylonite (Pat-terson & Watkinson 1984, Fig. 3). Representativechemical compositions of sulfides and precrous-metalminerals ile presented in Tables 1-5; textural rela-tionships are illustrated in Figures I and 2.

Disseminated sulfides

Disseminated sulfides consist of two types, thoseintercumulus to magmatic silicates (Fig. la) andthose that have been modified during the formationof metamorphic silicates, and are now disseminatedalong cleavages in amphibole and chlorite. Bothtypes consist of chalcopyrite, pyrrhotite, pentlandite,bornite and cubanite intergrown with magnetite; thesulfides are compositionally similar in the two groups(Table l).

Cobalt-rich pentlandile occurs as exsolution flamesand pods in pyrrhotite. Rare emulsion-textures con-sist of pentlandite and chalcopyrite. The pentlanditeshows minor alteration to mackinawite. Cubanite oc-curs as distinct lamellae in chalcopyrite.

Breccio ore

Breccia ore is composed of 20 to 5090 sulfides with50 to 8090 rounded to angular fragments of wallrock.The fragments range from 0.5 cm to 2 m across. Thefragments commonly are shattered, with sulfidespenetrating the fractures (Fig. lb). The fragmentsof wallrock ile predominantly mafic metagabbrowith minor mafic volcanic rocks and mylonite. Thegradation of mylonite to breccia ore, the occurrenceof breccia ore in faults and the presence of mylonitefragments in the breccia ore suggest that the brecciaformed as a result of the shearing process that form-ed the mylonite.

The sulfides consist of chalcopyrite, pyrrhotite,

pentlandite, irregular violarite after pentlandite,lamellar violarite after pyrrhotite, smythite, pyrite,cobalt-rich pyrite, nickel-rich pyrite, sphalerite,galena, vaesite and mackinawite (Table 2). Inunaltered specimens, the breccia ore consists ofchalcopyrite, some with pyrrhotite rims as thick as0.5 cm on fragments (Fig. 1c). Pentlandite occursas flames and pods on the margins of polygonallyrecrystallized pyrrhotite (Fig. 2a). Most of the brec-cia ore shows some degree of aheration (Fig. le). Thepyrrhotite is altered to smythite and lamellar iron-rich violarite along fractures in the pyrrhotite.Pentlandite is altered to a mixture of nickel- andcobalt-rich violarite and nickel-rich pyrite. Ver-miform mackinawite occurs in chalcopyrite. Por-phyroblasts of cobalt-rich pyrite are common inchalcopyrite and pyrrhotite.

Magnetite poikiloblastically encloses all sulfidesin unaltered specimens. Some of the magnetitecrystals contain a spiral trail of inclusions similar tothat in snowball garnet (Fig. 1d), suggesting growthand roiation during metamorphism.

The precious-metal minerals in the breccia ore(Table 3) are merenskyite, moncheite, kotulskite,stuetzite and an unknown phase with the composi-tion Ag3BiTe2. These are found as small (less than0.1 mm in diameter) isolated grains withchalcopyrite, pyrrhotite and pentlandite. The mon-cheite occurs in chalcopyrite, pyrrhotite and pentland-ite with small pods of stuetzite. Merenskyite occurswith lamellae of kotulskite and stuetzite. Onespecimen contains stuetzite intergrown withAg3BiTer.

Mylonite ore

The sulfide mineralogy of the mylonite ore (Table4) is very similar to that of the breccia ore into which

16 THE CANADIAN MINERALOGIST

TABLE 2. AVERACE COMPOSITTON OF SIJLFIDES FROM TEE :SIERRY MINE' BRSCCIA ORX

NO. OF L]EICBT PERCENTBRxccIA oRE cRAtNs s Fe co Nt Cu Zn ToTAt FoR!{ULA

pyrhotlte

PyrLte(cmtch)

pyrlte(N1-r lch)

sEythlte

pentlandl.te

vlolarLte

vLoLarlte(tn po)

sphalerl.te(tn po)

sphal"e!1te(wtth pyrpo)

27 4O.5 57 .6 0 .18 0 .96 o .d . n .d . 99 .2 l tO.gSSO

4 53.7 44 . I 2 .62 Tr o .d . n .d . 100.4 I IO.SSSZ

11 52.4 43 .8 O.25 2 ,67 o .d . n .d . 99 . I u l .O2s2

4 40 .7 56 .6 0 .16 L .66 n .d . n .d . gg . ] g .Ots r t

6 33.6 26.4 3.2O 36.4 0.17 Tr 99.S Mg.AsS

7 4L.7 25.9 2.39 27 .7 97 .7 M3.Otsa

9 40 .8 28 .0 1 .55 27 .3

5 32 .2 6 .22 0 .12 0 .15 1 .5

I 3 2 . 6 7 . 7 L

- 97 .7 M3.1Zs4

59.9 1"00.1 u1.05s

59.9 100.2 u t .O3s


}{BLX 3. CO}IPOSI:rION OF lTE ?RECIOAS.}TETAL UII{ENALS FROU THf, IEIERRY MIN!

t7

Eoocheit€u1.08(Te,Bl.)2

@renakyltelu1.12(Te,Bt)2

oe reoekylte2!11.92(Te,Bt)2

kotuLsklte! t0.99(T€,11)

stuetzite3u5.291e3

Ag3BlTe2

acatrthiteAact.93s

natlve al.lger169, Au)

NO. OFANAIYSES S

I 1 D . d .

, ".:.t o'1'

t " ' : '

t o '1 '

, o . : .

0.42

1 n . d .

tre TotAL

0 , 2 2 9 8 . 30.002

n.d. 100.0

0 . 0 1 8

2.3L tr .d.o . o L 2

0 . 3 8 n . d .0 . 0 0 2

- 0 . 0 1

" . : . 0 . 4 .

Te

o.43

Nl

0.930.016

IEIGrI PERCENT IND ATOMIC RIIIIOBl AS Au Pd Pt

7 , 3 8 o . d . a . d . U . 60.035 0.109

22.80.117

3.800.019

6 0 . 1 8 . 0 1 2 . I 2 a . d . 2 6 . 00 . 4 7 0 . 0 3 9 0 . 0 2 - 0 . 2 4 9

6 1 . 1 6 . 3 8 9 . 5 3 a . d . 1 6 . 90.48 0.031 0.088 - 0.155

5 3 . 5 1 . 8 0 n . d . 0 . d . 4 4 . 30 . 4 2 0 . 0 0 9 0 . 4 1

3 9 . 7 T r 5 6 . 8 L . 5 4 O . 7 50 . 3 1 - 0 . 5 3 0 . 0 0 8 0 . 0 0 7

33.O U,7 40.1 tr .a. o.d.0 . 2 6 0 . 1 3 0 . 3 7

d . d .

o . d .

0 .d .

t r . a .

101.9

99.4

100. 7

99.9

n . d . T r 8 7 . 00 . 8 1

n . d . n . d . 9 1 . 60 . 9 1 0 . 1 1

l. exaolved wltb Lotulsklte. 2. exsolveal wl.rh 6r!etz1re. 3. bigb std. dev. 4. 0.302 Cu,

it grades, except that it has many deformation tex-tures not present in the breccia ore. The sulfides oc-cur molded Erround silicate fragments, as stringerspenetrating rock and crystal fragments, in pressureshadows and in stringers elongated paralel to thefoliation (Fie. 1f). The sulfides tend to occur in thebiotite-rich layers of mylonite. Ductile sulfides suchas chalcopyrite and pyrrhotite tend to occur in thinveinlets and in pressure shadows of silicate fragmentsand cobalt-rich pyrite crystals. Magnetite occurs aseuhedral poikiloblasts in sulfides and as broken andshattered fragments scattered through the matrix ofthe mylonite.

Chalcopyrite shows deformation twins; pyrrhotiteis polygonally lesryslallized, and deformation twinsare cornmon. Pentlandite characterized by alterationto violarite occurs as isolated grains and exsolutionblebs along grain boundaries in pyrrhotite. The pyr-rhotite is altered to smythite and lamellar violarite.

Bornite ore

Within the mylonite, there are blocks as large as20 m in diameter of mafic metavolcanic rocks (am-phibolite schist). The bornite ore occurs as dissemina-tions and in carbonate veins within these blocks. Thesulfides make up 2 to 3Eo of the rock.

The ore minerals are intergrown chalcopyrite andbornite with copper-bismuth sulfosalts (wittichenireand emplectite). Native silver (with acanthite andstuetzite) and rare merenskyite occur as smalldisseminated grains within bornite or chalcopyrite(Table 3).

METAMORPHISM oF THE THIERRY ORES

Metomorphic textures ond structures

Both regional and dynamic metamorphism af-

fected the Cu-Ni sulfides in the Thierry deposit. Theeffect of regional metamorphism is illustrated by thedisseminated ores, in which primary intercumulussulfides were modified to disseminations alongcleavages in amphibole and chlorite. Pyrrhotite com-monly forms polygonal crystals showing exsolutionof pentlandite along grain boundaries, suggestingthat monosulfide solid-solution was formed duringmetamorphism. This is consistent with results ofgeothermometry and geobarometry, which suggestthat regional metamorphism attained temperaturesin the range of 600 to 650oC and pressures from 6.5to 7 kbar (Patterson 1980).

Dynamic metamorphism produced many purelymechanical effects; sulfides were forced into frac-tures and pressure shadows, deformation twins weredeveloped, and magnetite rotated. The occurrenceof mylonitic fragments in the breccia ore and thegradation of breccia ore into mylonitic oredemonstrate that the breccia ore was formed as aresult of dynamic metamorphism.

Chemicul effects of metomorphism

The second major effect of dynamic met€rmor-phism is reflected in the bulk composition of thesulfides. The copper-to-nickel ratio in sulfidesassociated with gabbroic intrusive complexes is nor-mally less than 3 (Naldrett & Cabri 1976). This isthe same value as found in the disseminated sulfidesof the Thierry mine and in the sulfides associatedwith other less deformed mafic intrusive bodies inthe area (Patterson 1980). However, in the brecciaore and mylonite ore, the copper-to-nickel ratiovaries from 5 to 50, averaging 10.5. Furthermore,the bulk mineralogy of the mylonite and breccia oreis unusual, approximately 5090 chalcopyrite and50Vo pyrrhotite + pyrite, compared to 1090chalcopyrite and 9090 pyrrhotite + pyrite in the

1 8 THE CANADIAN MINERALOGIST

AWRAGE COMPOSITION OF SULI'IDES FROM l]TE TTIERRY MINE: MYLONI1E ORETABI"E 4.

NO. OF WEICET PERCSIITGMINS I F" Co Nl Cu Zt TOTAI TORMUIAMTIONI,TE ORE

pyrlhotlte

pyrlte(Nl-rich)

pyr l te(Certch)

peDtlanallte

vLolarlte( ln pn)

sphalerlte(wtth porpy)

1 5 4 0 . 4 5 8 . 7 0 . 1 2 0 . 7 5

3 5 3 . 5 4 5 . 5 0 . 1 4 L . 9 7

7 53.7 44.5 2.7L Tr

3 36.9 26.6 0.44 34.3

- l-00.0 tlO.ZOsa

- L01.1 Mt.01s2

- 100.9 u1.02s2

- 98.2 lte.asS

5 39.9 21.2 I.44 27.2 95.7 M3'12s4

2 31 .6 7 .56 Tr 60 .4 99 '6 ! l t 'O5s

sulfides of other gabbroic complexes.There are several.methods by which sulfides may

become enriched in chalcopyrire relative to pyrrhotiteand penllandite. craig & Kullerud (1969) studied thesystem Fe-Ni-Cu-S at 1000oC and showed that acopper-rich liquid coexists with monosulfide solid-solution (n4ss). This might be a reasonable explana-tion for the Thierry ore if the sulfide could segregatefrom Mss and the silicate magma. However, in afilter-press model, the early precipitation of olivine,pyroxene and plagioclase would probably lock anyimmiscible sulfide droplets between these silicates,making it difficult for such a copper-rich liquid toseparate from the solid sulfides and silicates. Ifthesulfide magma formed a ma$sive unit, it is unlikelythat a copper-rich liquid could migrate because thedensity contrast between the liquid and the.t'l4rs wouldbe small. A similar argument could be made forgeneration of Cu-rich liquid in a system containingoxygen, in which iron oxide and iron-nickel sulfideswould solidify, leaving a copper-rich liquid.

Bowdidge (1970) suggested that the copper-richsulfides at the Thierry mine were generated from thesolidification of a magma that was sulfur-deficient.The nickel and iron would have been concentratedin olivine and pyroxene structures, enriching the re-maining magma in copper. When the magma becamesaturated in sulfur, sulfide liquid enriched in cop-per would precipitate from a magma depleted innickel and iron. However, the ratio of barren maficintrusive rock to ore-bearing rocks at the Thierrymine is very low; virtually all the mafic intrusiverocks contain sulfides.

Verbeek et al. (1972) mentioned that dynamicmetamorphism played an important role inredistributing the sulfides at Thierry. The main con-trast between the Thierry mine ultramafic rocks con-taining ore and the other Kapkichi ultramafic rockscontaining subeconomic €rmounts of sulfide is theamount of shearing and mylonitization. Barrett etsl, (1977) suggested that most economic concentra-tions of iron-nickel sulfides were due to dynamic

metamorphism. They stated that: "although Fe-Nisulfides occur in all metamorphic domains, par-ticularly in the dunitic intrusions, only disseminateddeposits have been discovered in greenschistmetamorphic domains. The matrix and massivesulfides that now represent economic or subeconomicconcentrations of nickel in both volcanic-type anddykelike hosts are confined to amphibolite facies do-mains. Most deposits occur in mid- to high-amphibolite facies domains of dominantly dynamicstyle."

Evidence for the mechanical separation of pyr-rhotite from pentlandite can be seen on a thin-sectionscale in the mylonite ore at the Thierry mine.Pentlandite occurs as isolated grains in silicates,rather than as exsolution lamella in pyrrhotite, asmight be expected as a result of a magmatic origin.The highly variable Cu/Ni and Ni./Fe ratios (Pat-terson 1980) are consistent with this. Removal of acopper-rich liquid from Mss would leave a nickel-rich residuum, which has not been found at theThierry mine. This implies that during dynamicmetamorphism, copper was selectively mobilizedrelative to nickel and iron. The bornite ore mayrepresent further mobilization of copper into blocksof amphibolite caught up in the mylonitic zone.

SupBncrrr AlrBnertoN

The sulfide ores were intensely faulted and frac-tured during dynamic metamorphism. Three distinctassemblages of sulfide were developed in thesesheared rocks. The average composition of thesulfides in altered ore is provided in Table 5.

The first consists of deep violet porous violaritewith octahedral shrinkage cracks after pentlandite(Fig. 2b). Pyrite is partly altered to hematite. Allgradations between the violarite-pyrite assemblageand the fresh pyrrhotite-pentlandite assemblage arepresent (Craie & Higgins 1975). This is similar toalteration at Kambalda (Nickel et al. 1974) and Mt.Windarra, Australia (Watmuff 1974). Evidence for


TABLE 5. AVERACE COI.IPOSITION OF SIILFIDES FROM TEE TUIERRY MINEI A],IERED ORE

l 9

NO. OFALTERED OTX CRAINS

IIEICUT PERCENTCo Nl Cu

0 . 6 9 6 0 . 6

L2.8 34.4

El.llerite

poLydjml te(c(ntch)

polydyDite(Cu-rlch)

v@ELte

py!lte(Cc!1ch)

bonlre

chalcopy!Ite

96.8 M1.04s

97.2 ttZ,g194

97.2 M3.z8s4

100.3 Mt .OSs2

99.9 W.g7s2

97.4 Cu5Fe5.6S4

100.0 Cut.06FeS2

s F e

35,O O.24

Za

! . d .

TOTAI,

4r .8 4 ,24

o .24

Tr

38.6 7 .34 0 .9L 40 .0 10 .3 n .d .

5 1 . 2 0 . 1 6 0 . 5 2

s4.o 44 .72 1 .20

26.2 6 .4O n .d .

34 .3 29 .66 n .d .

4 8 . 4 o . d .

n . d . n . d .

n . d . 6 4 . 8

u . d . 3 6 . 0

tr . d.

a . d .

n . d .

reactions involving loss of iron to oxides (Table 6)can be seen in the partly altered breccia ore. Thenickel released from the violarization of pentlandite(Iable 6, reaction l) combined with smythite (formedby the breakdown of pyrrhotite, reaction 2) to pro-duce iron-rich violarite (reaction 3). In the partlyaltered ores, pentlandite contains flecks and lamellaeof violarite; these follow the octahedral parting inpentlandite (Fig. 2b). Smythite formed flames in pyr-rhotite, and lamellar violarite (Nickel et al. 1974) ap-peared along fractures (Fig. 2c). Continuation ofthisprocess would have produced Ni-rich pyrite (Fig. 2d)and violarite rich in cobalt and nickel (reaction 4).At Kambalda, further alteration produced oxides andhydroxides (Nickel et al, 1974), but at the Thierrymine, an assemblage containing predominantlyvaesite, polydymite, pgite, bornite and chalcopyritewas generated. Vaesite occurs as irregular masseswith cobalt-rich pyrite. Polydymite shows octahedralshrinkage-fractures, implying that it was formed asa result of pentlandite alteration. Bornite developedas rims on chalcopyrite and along shrinkage fracturesin polydymite. This suggests that further loss of ironand sulfur produced vaesite (reaction 4). A continua-tion of this loss produced an assemblage of millerite,polydymite and chalcopyrite (reaction 6). Thepolydymite is very cobalt-rich (12.890 Co). Milleriteoccurs as fine laths in chalcopyrite. Associated withthis assemblage is malachite, native copper,chalcocite and marcasite along joints and fractures.Hematite and magnetite are common in allassemblages.

Vror-amzertou auoLow-TST4pTRATURE RELATIONSHIPS

IN THE SYSTEM Ni:Fe-S

From the preceding description, it is clear that thesystem was not closed during the process ofviolariza-tion. The alteration caused an increase in the Ni/Feand Co,/Fe ratios by removing iron from the sulfide

assemblages in the form of oxides. This enrichmentmay explain some of the unusual nickel-rich deposits,such as the Marbridge mine (Graterol & Naldrettl97l), where a premetamorphic supergene processcould have changed the bulk composition of thesulfides to a nickel-rich composition.

In examining natural sulfides in the system Ni-Fe-S, there appear to be two possible low-temperature configurations (Craig & Scott 1974), onewith millerite-pyrite and the other with pentlandite-violarite. This apparent contradiction hinges on thestability of violarite. If formed from pentlandite,violarite is more Ni-rich than that formed from pyr-rhotite, and equilibrium is thus not attained. Thisis commonly readjusted during metamorphism. Theequilibrium assemblage violarite - millerite - pyrite- magnetite reported at Black Swan (Groves el a/.lW4) and Marbridge (Graterol & Naldrett l97l) maybe explained by the metamorphism of supergene-altered Ni-Fe sulfides.

Kullerud (1969) showed that in general, athiospinel (violarite) breaks down on metamorphismto NiAs-type and FeS2-type structures at varyingtemperatures, depending on compositions:

FerSn = 2 Fer-rS + FeS, at 100"CNi3S4 = 2 NiS + NiS2 at 340oCCorS. : 2 CoS + CoS, at 600'C

According to Craig (1971), FeNirS. has an upper-temperature limit of 461'C in the system Ni-Fe-S.

Consider the metamorphism of a violarite at amiddle amphibolite grade:

(Fe,Ni,Co)rSo = FeSz + 2NiS + (Ni,Co,Fe)3So

The resulting violarite would be nickel-cobalt-richand texturally distinct from that generated by analteration process. The Co-rich violarite reportedfrom Black Swan is unpitted, has a high reflectanceand takes a good polish (Groves et al. 1974). The

20 THE CANADIAN MINERALOGIST

TA.BI,E 6. SCEE},ATIC REACTIONS ]I{VO',VTNG llE GBIEIATION OT'VIOI,ARITE of oxides. Work by Eckstrand (1975) on the Dumontserpentinite implies that sulfur also may be remov-ed, generating alloys of Ni-Fe..Equilibrium cannotbe assumed when considering stability of phases suchas violarite, which is produced during these low-temperature proces$es. The effect of Co on phaserelationships among Ni-Fe-S minerals may also playa major role in stabilizing violarite; this may explainthe two configurations of the low-temperature phaserelationships shown by Craig & Scott (1974).

The unusual CulNi and chalcopyrite-to-pyrrhotiteratios at Thierry are not unique; for example, othercases with unusual Cu/Ni ratios in deformed rocksare the Cu-rich footwall deposits at Sudbury (Abelet al. 1979) and the Shebandowan deposit nearThunder Bay, Ontario (Morton 1982). These mayhave resulted from dynamic processes similar tothose active al the Thierry mine, rather than by thefilter-press method or other strictly magmaticprocesses.

AcrNowt socsr4sNts

We thank T. Verbeek and R. Dehenne of UmexInc. for financial support and for access to thegeological data at the Thierry mine; we also thankJ. Lefebvre (l.Jmex research geologist) for providingprecious-metal and X-ray data, K. Kennedy and B.Zuckerkandl (mine staff) for help in mapping andproducing the mine sections, and C. Bowdidge forproviding the base map in the Kapkichi Lake area.O.R. Eckstrand, of the Geological Survey ofCanada, provided whole-rock compositions and acritical review. Some of the electron-microprobeanalyses of the sulfides were carried out by P. Mor-ton. Typing by E. Lambton is also very much ap-preciated.

REFERENCES

AneL, M.K., BucnaN, R., Coers, C.J'A. & PeNsroNE,M.E. (1979): Copper mineralization in the footwallcomplex, Strathcona mine' Sudbury, Ontaio. Con,Mineral. 17' 275-285.

Bannrrr, F.M., Blxr.rs, R.A., Gnovrs, D.I., Mansror,R.J. & McQuseN, K.G. (1977): Structural historyand metamorphic modification of Archean volcanic-type nickel deposits, Yilgarn Block, WesternAustralia. Econ. Geol. 72, 1195-1223,

BowprncE, C. (1970): Petrography of the KapkichiLake copper-nickel deposits and associated rocks,Pickle Lake area. northwest Ontario. UMEX Inter-nal Rep, TH 196.

Cnerc, J.R. (1971): Violarite stability relalions. Amer.Mineral. 56, 1303-13ll.

l. tr'eqlll.5sg - !'e2NlqSg + N12+ + 2re2+ + 5e?eotlaadLte VlolallteI

2 . ( F e , N t ) 7 S 8 - ( F e , N i ) g S 1 1 + F e 2 +?yrrbotlte Suythlte

3. (Fe'N1)6S11 + N12+ - Fe3N13S6Suyttllte t@lLai Vlolarlte

4. ( Ie,Nl,Co)3sq - Fe2t + 2(I 'e,Nt)52 + (N1'Fe'Co)3s!Violarite I l{l-Pytlte ViolarLte III

5. (Nl, te,co)gsq - Fe2+ + (Nl,re)s2 + (re'co)s2 + Oi 'co,re)3sqVlolarite 11.I Vaealte Co-Pyllte Vl.olatite Iv

6. (Fe,Nl,Cu):Sq - I 'e2+ + Nis + ( [1,cu,re)35qVlolarite II Ul11elLte Vlolallte

violarite generated as a result ofthe violarization ofpentlandite is typically pitted, shows octahedralshrinkage-fractures and takes a poor polish. Violariteforms solid solutions from nickel to cobalt (Tarr1935) and from nickel to iron (Misra & Fleet 1974)end-members. There does not appear to be completesolid-solution between the cobalt and iron end-members. This casts doubt on the practice, in plotsof violarite composition, of evenly distributing cobaltbetween iron and nickel end-members (Misra & Fleetr974).

In summary, violarite (Ni-Fe-rich) appears tocoexist with pentlandite when it results from thealteration of pentlandite. This may not be anequilibrium assemblage. The Ni-Co-rich violaritegenerated during metamorphism may be refractoryand not readily altered to pentlandite. This stronglysuggests that cobalt may play a major role in chang-ing the kinetics of the reactions involved and explainsthe two conflicting configurations in part of thesystem Fe-Ni-S.

CONCLUSIONS

The ores at the Thierry mine have undergone in-tense modification after their initial deposition asmagmatic sulfides. During regional metamorphism,the primary disseminated sulfides were modified bythe recrystallization of the surrounding silicates.Strong dynamic metamorphism mobilized thesulfides into fractures and pressure shadows, andproduced deformation twins and rotation ofmagnetite poikiloblasts in mylonitic and breccia ore.Dynamic metamorphism also significantly changedthe copper-to-nickel ratio of the mylonite and brec-cia compared to disseminated sulfides. The occur-rence of fragments of mylonite in the breccia ore sug-gests that the breccia ore also was formed duringdynamic metamorphism. The process of shearingand faulting significantly increased the penetrationof groundwater into the ore, allowing its supergenealteration. Supergene alteration proceeded by a pro-ces$ that removed iron from the sulfide, producingnickel-cobalt-rich sulfides and iron oxides. Ni-Fesulfides are open to the addition of oxygen, whicheffectively removes Fe from the system in the form


& HrccrNs, J.B. (1975): Cobalt- and iron-rich

2l

violarites from Virginia. Amer. Mineral. 60, 35-38.

- & KurI-snuo, G. (1969): Phase relations in theCu-Fe-Ni-S system and their application tomagmatic ore deposits. .Iz Magmatic Ore Deposits(H.D.B. Wilson, d.). Econ. Geol. Mou4,34+358.

& Scorr, S.D. (1974): Sulfide phase equilibria.Iz Sulfide Mineralogy (P.H. Ribbe, ed.). Mineral.Soc, Amer., Short Course Notes 7, CSI-110.

Ecrsrnaro, O.R. (1975): The Dumont serpentinite: amodel for control of nickeliferous opaque mineralassemblages by alteration reactions in ultramaficrocks. Econ. Geol. 70, 183-201.

Cnerenor., M. & Naronrm, A.J. (1971): Mineralogy ofthe Marbridge no. 3 and no. 4 nickel-iron sulfidedeposits, with some comments on low temperatureequilibration in the Fe-Ni-S system. Econ. Geol. 6,886-900.

Gnovrs, D.I., Huosor.l, D.R. & Hecr, T.B.C. (1974):Modification of iron-nickel sulfides during serpen-tinization and talc-carbonate alteration at BlackSwan, Western Australia. Econ. Geol.69.t26s-t28r.

Kullsnuo, G. (1969): Cubic - hexagonal inversions insome MrS4-type sulfides. Carnegie Inst. Wash.Yearbook 67, 179-182.

Mrsne, K.C. & Fr.ns"r, M.E. (1974): Chemical composi-tion and stability of violarite. Econ. Geol. 69,39t-403.

Monror, P. (1982): Archean Volcanic Stratigraphy,and Petrology and Chernistry of MaJic andUltramaJic Rocks, Chromite, and the Shebandowon

Ni-Cu Mine, Shebandowan, Northwestern Ontorio.Ph.D. thesis, Carleton Univ., Ottawa, Ont.

Nnr-onnrr, A.J. & CesRr, L.J. (1976): Ultramafic andrelated mafic rocks: their classification and genesiswith special reference to the concentration of nickelsulfides and platinum-group elements. Econ, Geol.71 , l13 l -1158.

Nrcxsl, E.H., Ross, J.R. & TnorrvsBn, M.R. (1974):The supergene alteration of pyrrhotite-pentiandiieore at Kambalda, Western Australia. Econ. Geol.69, 93-107.

PnrrBnsoN, G.C. (1980): The Geologt of the KapkichiLake Ultramaftc-Mafic Bodies and Related Cu-NiMineralization, Pickle Lake, Ontario, Ph.D. thesis,Carleton Univ., Ottawa, Ont.

& WerrrNsoN, D.H. (1983): The geology of theThierry Cu-Ni mine, northwestern Ontaio, Can.Mineral. 22.3-ll.

Tann, W.A. (1935): The linnaeite group of cobalt-nickel-iron-copper sulfides. Amer. Mineral. 20.68-80.

Vsngrsr, T., DEHnNvr, R. & Boworocr, C, (1972):Geophysical case-history: the Thierry copper-nickeldeposit in northwestern Ontario, Canada. Int, Geol.Congr. 24th (Montreal) 9, 135-151.

Walvrurr, I.G. (1974): Supergene alteration of the Mt.Windarra nickel sulphide ore deposit, WesternAustralia. Mineral. Deposita 9, 199-221.

Received January 11, 1983, revised manuscript acceptedAugust 29, 1983.

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METAMORPHISM AND SUPERGENE ALTERATION OF Cu-Ni