THE VOISEY’S BAY DEPOSIT, LABRADOR, CANADA

Naldrett,A.J., and Li, C., 2007, The Voisey’s Bay deposit, Labrador, Canada, in Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of MajorDeposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral DepositsDivision, Special Publication No. 5, p. 387-407.

THE VOISEY’S BAY DEPOSIT, LABRADOR, CANADA

ANTHONY J. NALDRETT1 AND CHUSI LI2

1. Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B12. Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405

Corresponding author’s email: [email protected]

Abstract

The Voisey’s Bay Ni-Cu deposit currently comprises 78x106 tonnes of proven, probable, indicated, and inferredmineralization grading 2.20 wt.% Ni, 1.70 wt.% Cu, and 0.134.% Co. It was discovered in October 1994, and stimu-lated an intense program of exploration in 1995, 1996, and 1997 for Ni deposits in neighbouring parts of Labrador. Thedeposit is associated with two 1.334 Ga troctolite intrusive chambers, the upper Eastern Deeps and lower Reid Brookchambers, connected by a 10 to 100 m wide dyke. The igneous rocks are part of the Nain Plutonic Suite that consistsof anorthositic, granitic, troctolitic, and ferrodioritic intrusions emplaced between 2.334 and 1.29 Ga. The mineraliza-tion is composed of massive, semimassive, and disseminated pyrrhotite, pentlandite, and chalcopyrite located withinthe dyke (particularly where it widens) in gneiss adjacent to the dyke and along the line of entry of the dyke to the upperof the two chambers. The sulphides are associated with a magmatic breccia (referred to as feeder breccia or basal brec-cia) and with troctolite that is commonly close to this breccia. The trace element and Pb and Nd isotope geochemistryof the troctolites indicate that they have interacted with crustal rocks, both at depth and close to the level of the intru-sions. Sulphur and oxygen isotopes suggest some interaction with the local gneisses; the radiogenic nature of the Osdemands such interaction. The Ni and Fo content of the olivine indicates that early injections of troctolite experiencedsignificant chalcophile depletion, but that later influxes of magma experienced less depletion, and upgraded early sul-phides in chalcophile metals. The existence and richness of the Voisey’s Bay deposit is attributed to its occurrencewithin a plumbing system that has served to focus repeated pulses of magma. Other prospects discovered during theexploration rush in Labrador are thought to be of lesser size or grade for one or more of the following reasons: (i) themagma with which they interacted was more fractionated than the Voisey’s Bay troctolites, and thus contained less Ni;(ii) there was a lack of adjacent sulphur-rich country rocks with which the magma could interact; and/or (iii) the magmainvolved was not focussed within a well defined conduit.

Résumé

Le gisement de Ni-Cu de Voisey’s Bay renferme actuellement 78 x 106 tonnes de concentrations prouvées, proba-bles, indiquées, et impliquées de minerai présentant des teneurs en Ni de 2,20 % en poids, en Cu de 1,70 % en poids,et Co de 0,134% en poids. Sa découverte en octobre 1994 a suscité un program d’exploration intense en 1995, 1996 et1997 à la recherche de gîtes de Ni dans les parties avoisinantes du Labrador. Le gisement est associé à deux chambresmagmatiques de troctolite de 1,334 Ga, la chambre supérieure Eastern Deeps et la chambre inférieure Reid Brook,reliées par un dyke d’une largeur de 10 à 100 m. Les roches ignées font partie de la suite plutonique de Nain qui secompose d’intrusions anorthositiques, granitiques, troctolitiques et ferrodioritiques mises en place entre 2,334 et 1,29Ga. La minéralisation prend la forme de pyrrhotine, de pentlandite et de chalcopyrite massives, semi-massives et dis-séminées dans le dyke (en particulier aux endroits où il s’élargit), dans les gneiss adjacents au dyke et le long de la lignede pénétration du dyke dans la partie supérieure des deux chambres magmatiques. Les sulfures sont associés à unebrèche magmatique (appelée brèche nourricière ou brèche basale) et à la troctolite couramment présente à proximité decette brèche. La géochimie des éléments en traces et des isotopes de Pb et de Nd des troctolites indique qu’elles ontinteragi avec les roches crustales aussi bien en profondeur qu’à proximité de leur niveau de mise en place dans la croûte.Les isotopes de S et de O laissent croire qu’il y a eu une certaine interaction avec les gneiss locaux; la nature radi-ogénique de Os impose une telle interaction. La teneur en Ni et le contenu en Fo de l’olivine révèle qu’il y a eu unimportant appauvrissement en éléments chalcophiles des injections précoces de troctolite, mais un appauvrissementmoindre des apports ultérieurs de magma, qui ont permis d’enrichir les sulfures préexistants en métaux chalcophiles.L’existence et la richesse du gisement de Voisey’s Bay sont attribuées à son occurrence dans un réseau de canalisationsmagmatiques qui ont servi à concentrer des poussées répétées de magma. Les autres gîtes découverts au moment de laruée vers l’exploration au Labrador seraient de moindres dimensions ou à plus faible teneur pour l’une ou l’autre desraisons suivantes: i) le magma avec lequel il y a eu interaction était davantage fractionné que celui des troctolites deVoisey’s Bay et renfermait ainsi moins de Ni; ii) l’absence de roches encaissantes riches en soufre dans les environsimmédiats avec lesquelles le magma aurait pu interagir; iii) le magma en cause n’était pas concentré dans un conduitbien défini.

History

In the summer of 1993, two prospectors, Albert Chislettand Chris Verbiski, who had been sent to Labrador byDiamond Fields Resources Ltd. to conduct stream samplingfor diamond indicator minerals, discovered a gossan with theblue staining characteristic of copper on a hilltop, 45 kmsouthwest of the village of Nain. An initial 5 hole drill pro-gram was conducted to investigate the occurrence in October1994 and one of the holes intersected a 41.2 m zone of semi-

massive to massive sulphide grading 2.96% Ni, 1.89% Cu,and 0.16% Co within a wider 71.0 m zone that graded 2.23%Ni, 1.47% Cu, and 0.12% Co. A major drilling program wasstarted by Diamond Fields Resources Ltd. in January 1995and by July 1995, 31.7 million tonnes of ore grading 2.83%Ni, 1.68% Cu, and 0.12% Co had been outlined at surface inthe area known as the “Ovoid” and “Mini-Ovoid”. InOctober 1995, a second major discovery, known as the“Eastern Deeps”, was made during a stratigraphic drilling

program. The deposit was acquired by INCO Ltd. in August1996. Reserves and resources as of 2005 (Voisey’s BayNickel Co. website, 27th October 2006) were 32x106 tonnesof proven and probable reserve grading 2.75 wt.% Ni, 1.59wt.% Cu, and 0.14 wt.% Co; 40x106 tonnes of indicatedresource grading 1.89 wt.% Ni, 1.90 wt.% Cu, and 0.12 wt.%Co; and 6x106 tonnes of inferred resource grading 1.9 wt.%Ni, 1.0 wt.% Cu, and 0.2 wt.% Co. The success at Voisey’sBay led to a huge surge in exploration in this part ofLabrador in 1995, 1996, and to a lesser extent in 1997. Manynew Ni-bearing occurrences were located and explored, butthus far none has proved to be of economic interest.

Geology

Regional GeologyThe geology of the Nain area has been described by Ryan

(1990; 1996; 2000), Ryan et al. (1995), and Kerr and Ryan(2000). The deposit has been described by Naldrett et al.

(1996; 1997), Li and Naldrett (1999), Lightfoot and Naldrett(1999), and Evans-Lamswood et al. (2000); aspects of thegeochemistry have been addressed by Lambert et al. (1999;2000), Ripley et al. (1999; 2000), Amelin et al. (2000). Liand Naldrett (2000), Li et al. (2000), and Naldrett et al.(2000a); Naldrett et al. (2000b) described the sulphide min-eralogy.

The deposit is associated with the 1.334 Ga (Amelin et al.,1999) Voisey’s Bay intrusion that belongs to the NainPlutonic Suite and transects the east-dipping 1.85 Ga colli-sional boundary between the Proterozoic Churchill Provinceto the west and the Archean Nain Province to the east (Ryanet al., 1995) (Fig.1). The Churchill Province comprisesreworked Archean rocks and interbanded sulphide- andgraphite-bearing garnet-sillimanite and quartzofeldspathicparagneisses collectively known as “Tasiuyak gneiss”, andminor massive to lineated enderbitic gneiss. The NainProvince comprises interbanded granitic, and intermediateand mafic orthogneisses that exhibit retrogressed granulite-

A.J. Naldrett and C. Li

388

QUEBEC

Nain

LabradorSea

Anaktalik Bay

Voisey's Baydeposit

200 km

GrenvilleProvince

MakkovikProvince

Nain

Province

LABRADOR

ChurchillProvince

62°00'W

56°2

0'N

56°2

5'N

62°10'W

Á‡ÎË‚¬ÓÈÒËÒ ¡ýÈ

Lake

Lake

Lake

Lake

Lake

Reid Brook R.

Reid Brook R.

Reid Brook Zone

Discovery HillZone

Ovoid

Eastern Deeps

Voisey's Bay granite,syenite and monzoniteMakhavinek Lake granite

Anorthosite

Gabbro

Olivine gabbro

Feeder-dike rocks

Massive sulphides

Tasiuyak gneiss

Nain orthogneiss

Quartzite

Abloviak shear zone

Other faults

Enderbitic gneiss

Graphitic gneiss

Troctolite

Mys

huau

intru

sion

Voi

seyí

Bay

intr

usio

n

Normal troctolite

Varied-textured troctolite

Melatroctolite

Early ProterozoicNain Plutonic Suite

Early Proterozoic rocks ofChurchill Province

Archean rocks of Nain Province

Faults

0 1 2 km

Voisey's Bay

Goose Bay

QUEBEC

FIGURE 1. Geology of the Voisey’s Bay area. Drafted using data from Ryan (1990) and Evans-Lamswood et al. (2000).

The Voisey’s Bay Deposit, Labrador, Canada

38

and amphibolite-facies mineral assemblages. The latestmovement along the collisional zone is marked by theAbloviak shear zone, for which the last movement has beendated at 1.73 to 1.75 Ga (Van Kranendonk, 1996).

Two troctolitic intrusions are present in the Voisey’s Bayarea (Fig. 1), the Voisey’s Bay intrusion and the Mushuauintrusion. Both are members of the Nain Plutonic Suite,which comprises a suite of anorthosite, granite, ferrodiorite(absent in the area shown in Fig. 1) and troctolite bodies thatwere emplaced in the interval 1.350 to 1.290 Ga (Ryan,2000). The Voisey’s Bay and Mushuau intrusions were orig-inally regarded as a single complex (Ryan et al., 1995) butAmelin et al. (1999) dated the Voisey’s Bay intrusion at1.334 Ga and Li et al. (2000) reported the age of theMushuau Intrusion as 1.317 to 1.313 Ga. The two intrusionsalso have distinctly different trace element and isotopic sig-natures (see below).

Geology of the Voisey’s Bay IntrusionAt the present level of understanding (Fig. 2), the Voisey’s

Bay intrusion comprises five components: (1) an upperchamber (the “Eastern Deeps”), which comprises an olivinegabbro (OG) sequence, a normal troctolite (NT) sequenceand a varied-textured troctolite (VT) sequence; (2) this is fedfrom below by a gabbroic/troctolitic feeder sheet, which isexposed at surface farther west (the Discovery Hill zone); (3)the “Ovoid”, which is situated between the Eastern Deeps

and the Discovery Hill zone, appears to be the base of theEastern Deeps exposed at surface; (4) the “Mini-Ovoid” liesdirectly west of the Ovoid but is separate from it; (5) thefeeder sheet has been traced 2.5 km west of the Ovoid, andin its western part, the Reid Brook zone, it dips steeply southand appears to widen out into a lower chamber. These aredescribed in turn below.

The Eastern Deeps

The Eastern Deeps chamber (Fig. 2) is exposed at surfaceas a troctolite-olivine gabbro body intruded by syenite andmonzonite (dated at 1.305 Ga, Amelin et al., 1999). Thebasal contact of the troctolite dips uniformly at 25° southeastfrom the vicinity of the Ovoid (Figs. 2B, 3). In this area, thestratigraphic succession comprises a lower sheet of Basal (=Magmatic) Breccia Sequence (BBS) (see description of rocktypes below), overlain by Varied-textured Troctolite or VT,that is in turn overlain by Normal Troctolite (NT). Theuppermost unit in the Eastern Deeps chamber is the OlivineGabbro (OG), which is likely intruded by the underlyingtroctolites (see discussion of olivine compositions below). Afeeder sheet enters the Eastern Deeps from the north andappears horizontal in the north-south section shown inFigure 4. The entry line of the sheet rakes southeast, follow-ing the plunge of the base of the Eastern Deeps, whichaccounts for the apparent dip of the sheet in Figure 3. Severalhundred metres north of the entry line, the sheet assumes a

MBLT

Granite, syeniteand monzonite

Feeder olivinegabbro ( )FOG

Normal troctolite

Varied-texturedtroctolite

Leopard troctolite ( )LT

Legend

Magmatic breccia ( )MB

No dataMassive sulphide

Olivine gabbro

Entry line of the feeder to the chamber

MB

FOG

FOG

FOG

FOG

MB

LT FOG

MB

MINIOVOID

OVOID

OVOID

LTMB

DISCOVERYHILL ZONE

0 1 km

0 1 km

EASTERN DEEPSTasiuyak paragneiss

Nain orthogneiss

Enderbitic gneiss

REID BROOK ZONE

REID BROOK CHAMBER(Lower chamber)

MAGMA CONDUIT (FEEDER DIKE)

(Upper chamber))EASTERN DEEPS CHAMBER

Section-3 lineSection-4 line

Section-1 line

Bh. 166/166ABh. 206

Bh. 7

Bh. 162Bh. 106Bh. 282

Bh. 202

Bh. 231

Section-2line

1

2

3

DISCOVERYHILL ZONEREID BROOK ZONE

Section-5 line

Section-7line Section-6

line

W E

B

A

FIGURE 2 (A) Geological map of the Voisey’s Bay intrusion (modified after Li and Naldrett, 1999). (B) Generalized longitudinal cross-section - the igneousrocks have been projected onto a vertical plane (modified after Lightfoot and Naldrett, 1999). Legend for cross-section: 1 = gabbro-troctolite bodies (mag-matic chambers); 2 = rocks comprising the magma-conduit assemblage; 3 = sulphide mineralization.

steep dip, similar to that of thefeeder to the west. Large blocks ofultramafic rock appear to choke thefeeder sheet in a few places. Thedistribution of the varied-texturedtroctolite shows a strong relation-ship to the north wall of the EasternDeeps chamber (thought to be agrowth fault that has controlledemplacement of the chamber) andto the entry line of the feeder sheet(Fig. 4). It thins out and disappearsto the south.

Sulphide, which is present inminor amounts throughout most ofthe varied-textured troctolite,increases to 10 to 15 modal % (inplaces up to 25%) close to themouth of the feeder as shown inFigure 4. In this vicinity, the basalbreccia sequence contains both dis-seminated and blotchy sulphide andalso lenses of massive sulphide.

Naldrett et al. (1996) haveshown that massive sulphide occursat the base of the Eastern Deeps,both within the feeder sheet (i.e.north of the stippled line represent-ing its line of entry into the cham-ber in Fig. 2A) and up to 100 msouth into the chamber. The basalbreccia sequence is absent or thinnear the line of entry of the feeder,and thickens away from this in asouthwesterly direction, beforethinning out again farther south-west. The basal breccia sequenceseems to form a “bar” just off theentry line of the sheet, with the bestdevelopment of massive sulphidelying between this bar and the entryline.

The stratigraphy of the feedersheet extending north of the cham-ber is a close match to thatobserved in the Discovery Hillzone (see below), although it tendsto be less thick where it has beenstudied so far. The upper part of thesheet consists of unmineralizedOlivine Gabbro (Feeder OlivineGabbro or FOG), either in chilledcontact with the overlying gneiss,or as a mixed zone of troctolite andgneissic fragments. Sulphidesincrease downward and much ofthe disseminated sulphide shows“leopard texture” (LeopardTroctolite or LT) which consists of a troctolite containinginterstitial sulphides and oikocrysts of augite (see descrip-tion below). Sulphide content may increase to about 50%,

below which there is an abrupt change in some holes to mas-sive sulphide, which is underlain by mineralized basal brec-cia sequence. The basal breccia sequence is separated from


390

166230 194

201219 231 263 244

28

189

Boreholes and their numbers

Normal troctolite


Leopard troctolite

Massive sulphide

Country orthogneiss

Magmatic breccia Massive sulfide mineralizatiomdeveloped close to the cross-section line

but not appearing on it

0 200 400 600 800 1000 m

EW


Olivine gabbro

FIGURE 3. Cross-section through Eastern Deeps (modified after Li and Naldrett, 1999). The line of the cross-section is shown in Figure 2A.

166230 194

201219 231 263 244

28

189


Normal troctolite


Leopard troctolite

Massive sulphide

Country orthogneiss

Magmatic breccia Massive sulfide mineralizatiomdeveloped close to the cross-section line

but not appearing on it

0 200 400 600 800 1000 m

EW


Olivine gabbro

FIGURE 4. North-south cross-section across the northern part of the Eastern Deeps chamber. Note the verti-cal north wall of the chamber, the horizontal entry of the feeder sheet, the steepening in the dip of the sheetto the north, and the relationship between the entry line of the feeder and the distribution of varied-texturedtroctolite. This unpublished figure was very kindly provided by P.C. Lightfoot (pers. comm., 2006).


39

the gneiss in most places by a thin zone of sulphide-poor,fine-grained ferrogabbro.

The Ovoid and Mini-Ovoid

The Ovoid consists of up to 110 m of massive sulphideoverlying a variable thickness of basal breccia sequence andminor leopard troctolite, all located within a bowl-shapedstructure and overlain by 10 to 20 m of gravel and clay (Fig.5). A thin (10-20 m thick) feeder sheet joins it from below.The westward continuation of the Ovoid (referred to as theMini-Ovoid) is essentially a thickened variant of theDiscovery Hill zone (see below). It dips 40°N and displaysthe stratigraphic succession typical of this zone, plus a thickzone of massive sulphide between the leopard troctolite andbasal breccia sequence.

The Discovery Hill Zone

In this zone, the thickness of the feeder sheet varies from10 to 100 m; the dip changes from 40°N close to surface andclose to the Mini-Ovoid to a steep southerly orientation atdepth. Where the dip is less than 70°, the stratigraphic suc-cession consists of an upper contact zone of feeder olivinegabbro, which overlies a zone of leopard troctolite that inturn overlies basal breccia sequence (Fig. 6). Mineralizationis restricted to the wider parts of the feeder and occurs as aseries of elongate lenses, within the plane of the sheet, allraking to the east (Fig. 2B).

The Reid Brook Zone

In the Reid Brook zone, the dip of the feeder sheet is inexcess of 70° to the south. The subvertical stratigraphic suc-cession that characterizes the Discovery Hill zone is not welldeveloped here. The breccia sequence that occurs at the basalcontacts of both the Eastern Deeps and Discovery Hill zone(i.e. basal breccia sequence) is referred to as feeder brecciain the Reid Brook zone because the unit is not related to thebase. The feeder breccia is not restricted to the footwall ofthe steeply dipping sheet, but can occur anywhere within it.The feeder olivine gabbro and leopard troctolite are concen-tric with massive sulphides within the feeder in some places.

Evidence of the former presence of the feeder sheet and itsmineralization has been traced well west of Reid Brook (see

Fig. 1). Large masses (xenoliths) of mineralized troctoliteand inclusion-bearing magmatic breccia occur at a prospectknown locally as Ashley, within younger Makovenik Lakegranite, 7 km west-southwest of the Ovoid.

In February 2005, a figure appeared on the INCO Ltd.website showing a major series of intersections of semimas-sive to massive sulphide associated with a change in dip ofthe feeder sheet (Fig. 7). The sulphides occur both within thefeeder and as lenses of massive sulphide that apparently fol-low planes of weakness in the adjacent gneisses (Evans-Lamswood, 2005). No data have been released at the time ofwriting (October 2005) that tie this discovery to publishedaspects of the Voisey’s Bay structure as a whole, althoughthe available information indicates that a major resourceexists in the Reid Brook area at a depth of 200 to 300 m. This

100 m

Massive sulphide

S N

Enderbitic orthogneiss Ultramafic inclusions

Leopard troctolite Troctolite

FIGURE 5. Cross-section through the Ovoid (after Lightfoot and Naldrett,1999).

0 50 100 m

35-65

20-35

10-20

5-10

<5

Sulphidecontents (%)

Feeder olivinegabbro Magmatic breccia

Leopard troctolite Enderbitic gneiss

S N

9389 1 98 102 106 108


FIGURE 6. North-south cross-section through the Discovery Hill zone alongline 5. The location of the section line is shown in Figure 2A. Modifiedafter Li and Naldrett (1999).

is likely to feature prominently inplans for future development atVoisey’s Bay.

Principal Rock Types of theVoisey’s Bay IntrusionOlivine Gabbro andLeucotroctolite

Olivine gabbro (Fig. 8C) is acommon rock-type that occurs,together with several thin (10-30m) layers of gabbro, in the olivinegabbro sequence of the EasternDeeps. It is an olivine-plagioclasecumulate with 50 to 70 modal %plagioclase and 15 to 30 modal %olivine. Augite varies from 10 to 20modal % and occurs mainly aslarge oikocrysts enclosing grains ofolivine and plagioclase. Reactionrims of orthopyroxene surroundingolivine are not common in theolivine gabbro.

The Leucotroctolite (LUT) inthe Reid Brook lower chamber hasa ratio of olivine to plagioclasesimilar to that in the olivine gabbroof the Eastern Deeps. Oikocrystaugite however is less abundant in the leucotroctolite (usu-ally less than 5 modal %) than in the olivine gabbro.Reaction rims of orthopyroxene on olivine are more com-mon in the leucotroctolite than in the olivine gabbro.

Feeder Olivine Gabbro

Feeder Olivine Gabbro (FOG) is a plagioclase+/-olivinecumulate containing a much higher proportion of intercum-ulus minerals (between 30-50 modal %) than either the leu-cotroctolite or olivine gabbro (Fig. 8D). Euhedral tosubeuhedral plagioclase crystals (2-10 mm in length) consti-tute 40 to 50 modal % and are randomly oriented to form aframework for intercumulus minerals. Small amounts ofolivine and augite (less than 10 modal % each) often occuras oikocrysts enclosing grains of plagioclase. Intercumulusminerals include orthopyroxene (5-10 modal %), hornblende(8-15 modal %), biotite (up to 15 modal %), ilmenite (around5 modal %), and small (less than 1 mm) subhedral plagio-clase laths (up to 15 modal %).

Normal Troctolite

Normal Troctolite (NT) is a medium-grained troctolite ofuniform texture (Fig. 8B). It consists of 20 to 40 modal %cumulus olivine and 40 to 65 modal % cumulus plagioclase5 to 10 mm in length. The plagioclase laths are randomly ori-ented and are often partially enclosed by olivine, in someplaces to the extent that olivine oikocrysts surround plagio-clase grains. Oikocryst augite is rare in the normal troctolite,instead small amounts of orthopyroxene (less than 5 modal%) occur as reaction rims on some olivine grains. Raretitanomagnetite occurs as an intercumulus mineral associ-ated with ilmenite. Several bands of ilmenite, 0.5 to 1.5 m

thick, occur in the upper part of the normal troctolite. Smallamounts of augite are present in a myrmekitic intergrowthwith the ilmenite in these bands.

Varied-Textured Troctolite

Varied-textured Troctolite (VT) differs from the normaltroctolite in that it contains up to 25 vol.% gneiss inclusionsand some blotchy sulphide. Plagioclase varies in size fromseveral millimetres to several centimetres in length. Largerplagioclase crystals occasionally enclose grains of euhedralolivine. Pegmatitic plagioclase laths, more than 15 mm inlength, are often observed projecting into patches of sul-phide. Mantling of olivine by orthopyroxene is more com-mon and interstitial hornblende, biotite, and apatite are moreabundant in the varied-textured troctolite than in the normaltroctolite. On the other hand, augite oikocrysts are less abun-dant and have more often been altered to tremolite, particu-larly in areas where they are in direct contact with sulphide.Interstitial ilmenite is less abundant in the varied-texturedtroctolite than in the normal troctolite. Magnetite, which israre in the normal troctolite, is more common in the varied-textured troctolite where it is often associated with gneissicinclusions and patches of sulphide.

Leopard Troctolite

Leopard Troctolite (LT) contains up to 50 vol.% intersti-tial sulphide and about 10 modal % oikocrysts of augite. Sofar leopard troctolite has only been observed within thefeeder. Cumulus plagioclase ranges from 40 to 60 and cumu-lus olivine from 20 to 30 modal %. The augite oikocrystsappear to have grown in situ and, in growing, have pushedinterstitial sulphide aside so that they now appear as black


392

Surface

2.7% Ni, 1.1% Cu, 0.19% Co88.9 m

2.2% Ni, 1.0% Cu, 0.15% Co,51.0 m

Weakly mineralized troctolite

2.0% Ni, 0.7% Cu, 0.13% Co31.8 m

0 50 m

Massive sulphideAssay intersection

S N

FIGURE 7. Cross-section showing recently discovered mineralization in the Reid Brook area of the Voisey’sBay deposit (INCO Ltd. website, March 2005).


39

spots in a matrix with a high sulphide content, thus givingrise to the term ‘leopard troctolite’. The oikocrysts them-selves contain a high proportion of olivine chadocrysts. Withincreasing amounts of sulphide, oikocrysts disappear fromleopard troctolite and the mineralization grades into intersti-tial sulphide ore.

Magmatic Breccias

As described above, Basal Breccia Sequence (BBS) andFeeder Breccia (FB) are similar in mineral composition butrecognized by different names because of their generally dif-ferent inclusion content and different distribution within thefeeder. Both contain more than 25 vol.% of inclusions andvariable amounts of sulphide. Three types of inclusions arepresent: (i) fragments of gneisses (i.e. Tasiuyak gneiss); (ii)melatroctolite/ultramafic rocks in a few places (Fig. 8A);

FIGURE 8. Photomicrographs of principal Voisey’s Bay rock types. (A) Melatroctolite with olivine (Ol) grains enclosed by plagioclase(Pl) oikocrysts. (B) Normal troctolite in which plagioclase (Pl) encloses several olivine (Ol) grains poikilitically. (C) Olivine gabbro from the Eastern Deeps with olivine ina matrix of plagioclase laths. (D) Feeder olivine gabbro with olivine (Ol) grains and plagioclase (Pl) laths poikilitically enclosed in clinopyroxene (Cpx). (E) Xenolith of Tasiuyak gneiss from magmatic breccia showing needles of corundum (Crn) in plagioclase (Pl) and hercynite (Hcn) pseudomorphous aftergarnet with associated magnetite (Mt). (F) Xenolith of Tasiuyak gneiss from Magmatic Breccia with corundum (Crn) in plagioclase (Pl) with hercynite (Hcn)pseudomorphing some of the corundum needles.

and (iii) sulphide-poor troctolite(rare). The troctolite fragments areirregular, several centimetres indiameter, and often have sharpmargins; they have experiencedonly minor alteration including ser-pentinization of olivine and sericit-ization of plagioclase. The ultra-mafic fragments, mainly melatroc-tolite, wehrlite, and dunite, areirregular in shape with diameters ofup to tens of centimetres. Olivine inthe ultramafic rock inclusions areoften highly serpentinized. Theinclusions of gneiss are smaller,ranging from a few millimetres toseveral centimetres in diameter.They have reacted with the enclos-ing troctolite; details of this reac-tion are discussed below.

Sulphides occur in three princi-pal forms in basal brecciasequence/feeder breccia: (i) aszones or stringers of interstitial sul-phide enclosing inclusions andcrosscutting sulphide-poor brecciaore, (ii) as irregular blotches, and(iii) as stringers of massive sul-phide that crosscut other variants ofthis environment.

Ferrogabbro

This is a fine-grained, noncumu-late rock comprising granularpyroxene (less than 10 modal %,mainly clinopyroxene), greenishbrown hornblende (25-40 modal%), small plagioclase laths (30-40modal %), biotite (10-15 modal %),and ilmenite (5-10 modal %) (Fig.8D). Ferrogabbro is generally mas-sive but occasionally exhibits flowbanding particularly in areas wheresulphide is present.

Nickel and Forsterite Contents ofOlivine in Different RockSequences

Plots of nickel (Ni) versusforsterite (Fo) content in olivine are shown in Figures 9A and 9B. Olivine in the melatrocto-lite inclusions observed in the feeder show a steep trend ofdecreasing Ni content with decreasing Fo, in which Nidecreases from 3100 ppm at about 80 mole % Fo to 1500ppm at 72 mole % Fo (Fig. 9A). Olivine in the leucotrocto-lite of the Reid Brook chamber, some troctolites and olivinegabbros of the Eastern Deeps chamber, and Feeder olivinegabbro show a more gentle trend with Ni decreasing from1500 ppm at 75 mole % Fo to as low as 200 ppm at 41 mole% Fo (Fig. 9A). The model curves shown in the figure havebeen derived using Ghiorso and Sack’s (1995) program

MELTS for the fractional crystallization of mafic magmas. Adiscussion of the method and its application to olivine in themineralized intrusions at Noril’sk can be found in Li et al.(2003).

The olivine of the melatroctolite inclusions follows themodel curve for fractional crystallization of the assumed ini-tial magma. Olivine from sulphide-free troctolites from theEastern Deeps chamber and leucotroctolites of the ReidBrook chamber, along with olivine gabbros from the EasternDeeps chamber and the feeder, do not follow the same modelfractionation curve but fall below it. Their behaviour is


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085 80 75 70 65 60 55 50 45 40

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ine

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1110°C33% crystalliaation

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Metatroctolite inclusions in feeder

Troctolite of Easter Deeps Chamber

Leucotroctolite of Reid Brook Chamber

Olivine gabbro of Eastern Deeps

Olivine gabbro of feeder

Normal (sulphide-bearing) troctolite of Eastern Deeps Chamber

Varied-texture troctolite of Eastern Deeps Chamber

Model fractionation curve, original magma - no sulphide removal

Model fractionation curve, sulphide removal, 1 sulphide to 133 olivine

Model fractionation curve, depleted magma, no sulphide removal

FIGURE 9. Plot of nickel (Ni) content of olivine versus forsterite (Fo) content for (A) rocks related to the ini-tial introduction of magma; and for (B) rocks related to the later introduction(s). Model curves developed withthe use of Ghiorso and Sack’s (1995) program MELTS as discussed in the text.


39

explicable if the magma underwent a period of fractionationinvolving the removal of sulphide and silicate in a ratio of1:133 (red line in Fig. 9A), followed by further sulphide-freefractionation of the Ni-depleted magma (purple line in Fig.9A). Li and Naldrett (1999) proposed that interactionbetween the troctolite magma and the surrounding, sulphide-bearing Tasiuyak gneiss had caused this sulphide saturation.

Olivine in the normal troctolite and varied-textured troc-tolite (Fig. 9B), both of which rock types contain minor tosignificant amounts of sulphide, show no systematic trendbut instead form a “cloud” with Fo varying between 73 and52 mole % and Ni between 2000 and 700 ppm. The cloudextends from the model curve for depleted magma up to andabove the curve for undepleted magma. Li and Naldrett(1999) showed that olivine in contact with sulphide reactwith the sulphide, exchanging Ni and Fe. Figure 10 illus-trates the difference in the Ni-Fo covariation in olivine fromthe Normal and varied-textured troctolites at Voisey’s Baybetween those in contact with sulphide and those that do notshow such contact. Olivines in contact with sulphide are dis-tinctly poorer in Ni than those that occur as inclusions in pla-gioclase and are insulated from sulphide. This is evidencethat at Voisey’s Bay reaction between sulphide and olivinehas depleted the olivine in Ni, and therefore that the Ni con-tents shown for the olivine from the Normal and varied-tex-tured troctolites at Voisey’s Bay are minimum values andthat the original contents were likely higher. Geologic rela-tionships indicate that the Normal and varied-textured troc-tolites postdate the other intrusive phases, thus providingevidence for the passage of a later, undepleted magmathough the system.

Geochemistry of Voisey’s Bay Rocks

Major ElementsLi et al. (2000) have shown that major element composi-

tions of the intrusive rocks are controlled essentially by theproportion of cumulus plagioclase, olivine, and trapped sili-cate liquid. Most samples of normal troctolite and varied-textured troctolite in the Eastern Deeps subchamber and theleucotroctolite of the Reid Brook subchamber fall within orclose to the olivine-plagioclase composition array. A fewvaried-textured troctolite samples show abnormally highAl2O3 contents, which is attributed to the presence of high-Al residual gneiss inclusions in the samples. The olivine gab-bro in the Eastern Deeps and the feeder olivine gabbro in theconduit are displaced toward samples represented by thechilled margin rocks of the conduit, reflecting their highercontent of trapped liquid. The basal breccia sequence sam-ples contain abundant Al2O3-rich residual gneiss inclusionsand plot, as would be expected, on the Al2O3-rich side ofolivine-plagioclase array.

Trace ElementsSamples from the conduit and marginal zones of the

Eastern Deeps, Ovoid and Mini-Ovoid, Discovery Hill zone,and Reid Brook zone are shown in Figure 11A (Li et al.,2000) as primitive mantle-normalized spidergram plots (Sunand McDonough, 1989). The samples show strong enrich-ment in the LREE, enrichment in some of the large ionlithophile elements (Rb, Ba, and K), low Th and U contents,

and a slightly negative Ta+Nb anomaly. The elevated Sr andEu concentrations reflect the modal plagioclase content;likewise, Ti is present in oxide minerals (ilmenite and titano-magnetite), and P is present in apatite; both the oxide andapatite contents of the conduit and marginal rocks tend to behigh but variable. Apart from these elements, the spidergramprofiles are very similar, and show no relationship to thenature of the immediate host rocks. The data presented inFigure 11A indicate that the marginal ferrogabbro andolivine gabbro are enriched in trapped liquid relative to thetroctolites, and that the compositions of the finer grained(chilled) marginal rocks are not influenced by chemical con-tributions from the immediate country rocks. The presencethroughout the deposit of inclusions that have originatedfrom paragneiss indicates that equilibration of the melt withcountry rock occurred at depth, and not in situ within theconduit. Indeed, there is every indication that the Voisey’sBay intrusion magma did not react extensively with the wallrocks, although projections of orthogneiss within the EasternDeeps do show extensive local evidence of assimilation asevidenced by the presence of troctolite-gneiss mélange alongthe contact. White feldspathic clots within the varied-tex-tured troctolite, 0.5 to 5 cm in diameter, may also be theresult of such assimilation.

Average primitive mantle-normalized spidergram patternsof troctolitic rocks and olivine gabbro of the Eastern Deepsand Reid Brook chambers are shown in Figure 11B. With theexception of Pb and Ta, the normalized patterns of the var-ied-textured troctolite and normal troctolite are essentiallyindistinguishable. The absolute abundances of trace ele-ments are controlled by the amount of trapped liquid, whichis higher in the olivine gabbro than the varied-textured troc-tolite and normal troctolite. The profiles have a similarshape to the conduit assemblage rocks, with moderate LREEenrichment, and steep negative Th+U and lesser Ta+Nbanomalies.

Figure 11C shows the spidergram patterns of differentgroups of ultramafic inclusions. The mineralized melatrocto-lite inclusions have similar spidergram patterns to the varied-

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66.567.067.568.068.569.069.570.0olivine Fo content (mole%)

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e

sulphideinclusionLinear (sul)Linear (incl)

FIGURE 10. The contrasting nickel (Ni) contents of olivine grains in contactwith sulphide (in leopard troctolite) and those that occur in inclusions inplagioclase and are therefore out of contact with sulphide.

textured troctolite, as would be expected if they are ultra-mafic cumulates that were formed at an early stage with thesulphide saturated magma, and were subsequently broken upand incorporated as cognate xenoliths. A second group ofunmineralized melatroctolite inclusions have spidergrampatterns slightly flatter than the first group. These rocks mayhave crystallized from a less fractionated, less contaminatedand sulphide-undersaturated magma. The third group ofultramafic inclusions is essentially highly altered rocks thathave compositions between dunite and wehrlite, and arecharacterized by no disseminated sulphide and very differentspidergram patterns when compared to the Voisey’s BayIntrusion and other ultramafic inclusions. They are regardedas exotic inclusions picked up by the magma during ascentthough the crust. The gabbroic fragments in the brecciasequence are geochemically like the fine-grained ferrogab-broic rocks developed at the walls of the conduit (both haveCe/Yb = 27 to 33 and MgO = 5 to 8 wt.%).

Different phases of the Voisey’s Bay Intrusion, the nearbyMushuau Intrusion and the Tasiuyak paragneiss, are shownon a plot of La/Sm versus Th/Nb in Figure 12. Most varied-textured troctolite and conduit samples from the Voisey’sBay Intrusion have La/Sm ratios between 3.0 and 4.0 andTh/Nb ratios of less than 0.1. The normal troctolite samples,which are not shown in the plot, have the ratios of La/Sm andTh/Nb similar to the varied-textured troctolite samples. TheTasiuyak paragneiss is characterized by La/Sm between 4.0and 7.0 and Th/Nb between 0.4 and 1.0. The basal brecciasequence samples, which contain high proportions of inclu-sions of what are interpreted to be reacted Tasiuyak parag-

neiss, extend in array from the field of the varied-texturedtroctolite of the Voisey’s Bay Intrusion and conduit rockstowards that of the Tasiuyak paragneiss, lending support tothe interpretation that the inclusions are indeed of Tasiuyakparagneiss. Some conduit and varied-textured troctolite sam-ples also extend toward the field of the Tasiuyak paragneiss,in part due to their content of inclusions. Li et al. (2000) con-cluded that all rocks of the Voisey’s Bay Intrusion are con-sistent with derivation from mantle-derived picritic magmathat underwent an early phase of contamination in which theLa/Sm ratio was increased, but the Th/Nb ratio was unaf-fected, before rising to the present stratigraphic level of theVoisey’s Bay Intrusion. Support for their supposition that theinitial magma at Voisey’s Bay had derived from a primitivemantle comes from the trend of the Voisey’s Bay Intrusionunmineralized melatroctolite inclusions, which provide alink between primitive mantle and that of the Voisey’s BayIntrusion conduit rocks.

Cu/Zr Ratios

Both Cu and Zr are incompatible elements during theearly stages in the fractional crystallization of sulphide-unsaturated mafic/ultramafic magmas, and thus bothincrease in concentration as fractionation proceeds, with theCu/Zr ratio remaining constant. Since in cumulate rocksincompatible elements are contributed by the magma thathas become trapped interstitially between cumulus grains,the Cu/Zr ratio of any sample of cumulate will reflect theratio in the magma that gave rise to the cumulate. Once sul-phide immiscibility develops in magma, Cu is no longer


396

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Reid Brook ZoneDiscovery Hill ZoneMiniovoidEastern Deeps (western part)Eastern Deeps (eastern part)

Leucotroctolite (n=15)Olivine gabbro (Normal troctoliteVaried-textured troctolite

n=9)(n=10)

(Ce<50; n=48)

Ultramafic inclusions:Melatroctolite (min*)MelatroctoliteDunite and wehrlite

(unmin*)

min*=mineralized; unmin*=unmineralized

Varied-textured troctolite (Ce<50; n=48)

Tasiuyak paragneiss (n=8)Enderbitic orthogneiss(high-K group; n=20)Nain orthogneiss(intermediate-mafic; n=10)

A Average conduit rocksC Average ultramafic inclusions

B Average major rock types D Average country rocks

FIGURE 11. Primitive mantle-normalized (Sun and McDonough, 1989) spidergrams for rocks of the Voisey’s Bay intrusion (after Li et al., 2000).


39

incompatible, and the Cu/Zr ratiowill decrease as sulphides areremoved. In the same way, if themagma or cumulates acquire addi-tional sulphides from elsewhere, theCu/Zr ratio will be higher than thatof the initial, sulphide-unsaturatedmagma. Lightfoot et al. (1994)pointed out that Noril’sk basalts thatwere known not to have reactedwith sulphide (Tuklonsky, UpperMorongovsky, Mokuaelevsky) hadCu/Zr ratios between 1 and 3, whilethose that were known to havereacted with sulphide had ratios of<1. Li and Naldrett (1999) showedthat there is a correlation betweenNi depletion shown by olivine andthe Cu/Zr ratios of the rocks hostingthe olivine. Their data are repro-duced in Figure 13 in which it isseen that those rocks with Cu/Zrratios <1 contain less S and alsocontain olivine with generally lowNi contents. Thus, at Voisey’s Bay,Cu/Zr ratios are a useful indicationof rocks that have formed frommagma that has interacted with sul-phide liquid and has thus becomechalcophile depleted.

Reaction between TroctoliticMagma and Inclusions of TasiuyakGneiss

Li and Naldrett (2000) haveshown that inclusions of gneiss inthe basal breccia sequence andfeeder breccia have reacted exten-sively with the enclosing magma(Fig. 15E). In general, the degree ofreaction of the inclusions increasesfrom the Reid Brook zone throughthe Discovery Hill zone to theEastern Deeps. In the course of thisreaction, garnet in the gneiss hasbeen oxidized to form hercyniteand magnetite (Fig. 8E) with therelease of SiO2 to the magma;cordierite has been dehydrated toform hercynite with the release ofSiO2; orthopyroxene and K-feldspar have reacted together toproduce hercynite with the releaseof SiO2 and K2O; plagioclase hasbroken down to produce corundum(Fig. 8E, F), losing SiO2 and Na2Oto the magma and itself becomingenriched in anorthite; and thecorundum has subsequently reactedwith FeO and MgO from theenclosing magma to form hercynite

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Enderbiticorthogneiss

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Trajectory of contaminated tholeites,West Greenland

Primitive mantle (Sun and McDonough, 1989)Tasiuyak paragneiss

Conduit rocks Breccia sequenceVaried-textured troctoliteMelatroctolite inclusion/unmineralizedAverage melatroctolite inclusion/mineralized

Voisey's Bay intrusion

Trajectory ofcontaminated tholeites,Norilísk

Tasiuyakparagneiss

(n=20)

Trend of Voiseiy's Bay troctolitic rocks

Trend of breccia sequence

Melatroctolite and troctoliteMushua intrusion

FIGURE 12. Plot of La/Sm versus Th/Nb for rocks of the Voisey’s Bay and Mushuau intrusions, and countrygneisses (after Li et al., 2000). CMR = chilled margin of the Voisey’s Bay intrusion (n=8); UCPB = uncont-aminated picritic basalts from Noril’sk and West Greenland (after Lightfoot and Hawkesworth, 1997).

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Leucotroctolite

Olivine gabbroNormal troctoliteVaried-textured troctolite

FIGURE 13. Plot of olivine Ni content versus whole rock Cu/Zr ratio (after Li and Naldrett, 1999). Smallarrows indicate that Cu contents in these rocks are lower than the detection limit of 20 ppm that is used incalculating the Cu/Zr ratios for these samples.

(Fig. 8F). The colour of the her-cynite changes progressively frombeige in the Reid Brook zone,through green in the Discovery Hillzone to black in the Eastern Deeps;this colour change reflects anincrease in FeAl2O4 at the expenseof the MgAl2O4 in the hercynite.The inclusions of the EasternDeeps characteristically show aseries of reaction rims rangingfrom labra-dorite through biotite toorthopyroxene, which is thought tobe the consequence of diffusion-controlled activity gradients of theprincipal oxide components. It isseen that interaction with the gneissresults in the addition of SiO2,K2O, and Na2O to the magma. Thisfelsification of magma would haveresulted in a decrease in its abilityto dissolve sulphide and thereforewould have promoted sulphideimmiscibility.

Isotope GeochemistryU-Pb Geochronology

Amelin et al. (1999) have stud-ied the age of the rocks of theVoisey’s Bay area using U-Pbmethods on baddeleyite, zircon,and apatite. Baddeleyite from troc-tolites and gabbros of the Voisey’sBay intrusion gave an age of 1.332 ± 0.001 Ga. Zircons from thenormal troctolite and some fromthe olivine gabbro gave an age coeval with the baddeleyite, but those from varied-texturedtroctolite and Feeder olivine gab-bro gave an age of 1.3050 ± 0.0008 Ga, which is the same asthat shown by granite and syenite that cut the Voisey’s Bayintrusion, and which Amelin et al. (1999) ascribe to a linkbetween the development of the zircons and metamor-phism/metasomatism associated with intrusion of the felsicrocks. Li et al. (2000) obtained an age of 1.3128 ± 0.0013 Gafrom baddeleyite and zircon from the Mushuau intrusion,demonstrating that, despite the similarity in rock types, it is20 Ma younger than the Voisey’s Bay intrusion.

Nd-Pb-Sr Isotopic Data

Amelin et al. (2000) have studied the Nb-Pb-Sr isotopicsystematics of the Voisey’s Bay and nearby Mushuau (1.317-1.313 Ga) intrusions. They found that the Voisey’s Bay intru-sion has the most mantle-like, least contaminated initial iso-topic compositions among the mafic intrusions of the NainPlutonic Suite: εNd = -1 to -2; 87Sr/86Sr(1.320Ga) = 0.7034 to0.7038; 206Pb/204Pb = 15.34 to 15.54; 207Pb/204Pb = 15.10to 15.18; and 208Pb/204Pb = 35.24 to 35.56. These ratioswere found to be uniform in troctolites and gabbros from

throughout the intrusion. Isotopic ratios for the Mushuauintrusion are quite distinct from the Voisey’s Bay intrusionand are typical for the rest of the Nain Plutonic Suite: εNd =-3 to -10, 87Sr/86Sr(1.320Ga) = 0.7034 to 0.7052; 206Pb/204Pb= 14.21 to 14.55; 207Pb/204Pb = 14.63 to 14.77; and208Pb/204Pb = 34.36 to 34.65. Amelin et al. (2000) con-cluded that the primary magmas of the Voisey’s Bay intru-sion were either derived from an enriched continental man-tle, or contaminated by a small amount of crustal materialduring ascent through lower-middle crust, and then by a vol-umetrically minor (probably 8-13%) amount of Tasiuyakgneiss. They attribute the observed variations in theMushuau intrusion, and the reported variations in othermafic intrusions of the Nain Plutonic Suite to assimilation of15 to 35% of a U-depleted Archean crust. They suggest thatthe larger amount of contamination experienced by intru-sions other than the Voisey’s Bay body is due to the magmasgiving rise to these having passed through the crust 15 ormore million years later than the Voisey’s Bay magma, bywhich time the crust had become much hotter due to the pas-sage of preceding magma.


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Tasiuyak gneisssulphidesOs = 8.6Os = +1900

ppbγ

Tasiuyakgneisssulphides

Voisey's Bay normal andvaried-textured troctolites

Basaltic magmaOs = 0.20Os = +70

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Os = 0.4Os = +5200

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Voisey's Baymagmachambers

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FIGURE 14. Plot of initial γOs vs. Re/Os, showing data for Cu-Ni-Co mineralization from the Voisey’s Baydeposit (after Lambert et al., 2000), J-M reef of Stillwater complex (after Lambert et al., 1994), Duluth com-plex (after Ripley et al., 1999), Noril’sk-Talnakh area (after Walker et al., 1994), and Kambalda camp (afterFoster et al., 1996). Modified after Lambert et al. (2000). The inset shows a model in which the picriticmagma (PM), derived from a mantle plume, passes through subcontinental lithospheric mantle (SCLM). Inthe lower crust (LC) it fractionates and forms basaltic magma (BM). Owing to interaction with surroundingrocks, its γOs increases to +70. The basaltic magma (BM) then ascends into the upper crust (UC), where itdevelops a magmatic chamber in the Tasiuyak gneiss (Reid Brook chamber) and assimilates sulphides fromthe gneiss (line 1 in inset and main plot). Tasiuyak sulphides contain 8.6 ppb Os with γOs = +1900. Theassimilated sulphides saturate the basaltic magma, immiscible Fe-sulphide liquid develops and absorbs Ni,Cu, Co, and PGE causing the magma to become strongly depleted in these elements. Fresh, undepletedmagma subsequently enters the magmatic system and forces the previously depleted magma up a verticaldyke to form the Eastern Deeps magmatic chamber. The fresh magma interacts with the early formed sul-phides enriching them in chalcophile metals, and decreasing their Re/Os ratio and γOs value (line 2 in theplot). Sulphide compositions of the Voisey’s Bay deposit correspond to an R-factor of 50 to 500.


39

Re-Os Isotopic Data

Re-Os data are important in the study of an orebodybecause the decay product, 188Os, is concentrated in sul-phide, and thus provides a direct measurement on the sul-phides themselves. Lambert et al. (2000) found that massivesulphide samples from the Voisey’s Bay ores have high Reconcentrations (148-288 ppb) for their Os concentrations(4.8-28 ppb), yielding high Re/Os ratios (2.9-38) that aresimilar to those for massive sulphides from Sudbury and theDuluth Complex. Whole rock Re-Os isotopic data exhibit alarge spread in 187Re/188Os (14-157) but do not define a pre-cise isochron. They attribute this to different “packages” ofsulphide having reacted with magma at different proportionsof magma to sulphide (i.e. different values of R where R =magma/sulphide ratio). However, large whole rock sulphidesamples from the Ovoid yield an imprecise 1320 Maisochron age that is consistent within error with baddeleyiteU-Pb ages from the magmatic system (Amelin et al., 1999).

Lambert et al. (2000) argued that the high initial γOs val-ues (+200 to +1100 = percent deviation in calculated initial187Os/188Os from mantle of the same age) for sulphide-richsamples from the Voisey’s Bay intrusion document signifi-cant magma interactions with older Nain-Churchill Provincecrust. They considered the hypothesis that their Re-Os iso-topic data is the result of contamination of basaltic magmasimilar to fine-grained feeder zone olivine gabbros and troc-tolites with sulphidic/graphitic Proterozoic Tasiuyak parag-neiss (γOs = +1900), followed by an R-factor process (300to >5,000) that improved the tenor (metal concentration in100% sulphide) of the sulphide liquid during transport in theactive Voisey’s Bay magma conduit. However, they pointedout that R-factors of this magnitude are inconsistent withNaldrett et al.’s (2000a) Cu, Ni, and PGE data for the miner-alization. Their preferred model (see Fig. 14 and associated

legend) is that the initial immiscible sulphide liquid inter-acted with a second, chalcophile-element undepleted (>150ppt Os) magma. This model gives R-factors (50-500) that areconsistent with those of Naldrett et al. (2000a), and conformsto that proposed by Li et al. (2000) on the basis of their geo-chemical data.

Oxygen Isotope Data

Ripley et al. (2000) have shown that the δ18O values ofTasiuyak gneiss range from 8.3 to 16.1‰, and the enderbiticand mafic orthogneiss from 5.7 to 8.5 with most values lessthan 7.5 per mil. Reacted inclusions of Tasiuyak gneiss inbasal breccia sequence are depleted in 18O relative toTasiuyak gneiss outside the intrusion, ranging from 4.7 to10.6 per mil. Elevated δ18O values (up to 9.3 per mil) arefound in the troctolitic and noritic matrix to the breccias, butare restricted to less than 2 cm from the edges of inclusions.These isotopic exchange profiles have been produced duringsubsolidus cooling, and do not record the major 18O deple-tion that has been experienced by the gneiss inclusions. It isthought that major contamination has been diluted by largevolumes of later magma that have passed through the sys-tem, disrupting and moving breccia, sulphides, and earlymagma up to a higher level.

Sulphur Isotopes

Ripley et al. (1999; 2002) have shown that δ34S varies forthe different mineralized environments within the Voisey’sBay intrusion: disseminated ore from Eastern Deeps, -2.0 to2.7‰; massive ore from the Ovoid and Mini-Ovoid, -2.1 to0‰; and disseminated mineralization from the Reid Brookzone, -4.1 to 1.4‰. Pyrrhotite from the Tasiuyak gneissranges from -17 to +18.3‰, while that from the Nain gneissranges from 0.2 to 3.3‰. If, as believed, the Tasiuyak gneissis indeed the source of the sulphur in the Voisey’s Bay ores,

Mineralization type

Deposit Zone

Eastern Deeps 5 37.66 3.19 0.85 0.18 145 51 19 34 7.4 16 22 2.82 Ovoid 52 33.76 4.12 2.53 0.18 226 88 7 13 1.8 2 82 2.57 Mini-Ovoid 11 35.65 3.72 2.16 0.18 208 166 8 16 2.3 4 109 1.25 Discovery Hill 1 36.6 2.5 1.02 73 25 9 1.9 36 2.98 Reid Brook 15 37.01 2.84 1.11 97 28 10 11 2.8 4 40 3.42

Eastern Deeps 8 15.9 1.45 0.75 89 50 7 13 2.6 6 35 1.79 Mini-Ovoid 7 13.64 1.42 0.9 0.07 78 70 4 5 1 2 73 1.11 Discovery Hill 18 13.09 1.16 1.28 0.06 77 76 3 9 0.7 1 77 1.01 Reid Brook 9 12.32 1.04 0.69 41 45 3 7 0.8 1 55 0.92

Eastern Deeps 42 8.86 1 0.47 0.06 69 38 4 10 1.3 3 41 1.81 Ovoid 3 7.43 0.83 0.72 0.04 96 41 5 13 2.2 3 46 2.37 Mini-Ovoid 1 15.3 1.24 1.12 0.06 25 112 4 4 1.1 2 31 0.22 Discovery Hill 34 7.16 0.75 0.39 0.06 63 40 2 6 0.6 1 47 1.56 Reid Brook 23 5.86 0.59 0.3 40 25 2 6 0.5 1 29 1.58

Eastern Deeps (>10% sulphide) 21 10.64 1.21 0.75 73 71 6 11 1.9 3 68 1.02 n = number of samples. Contents: S, Ni, Cu, Co – wt.%; Pd, Pt, Rh, Ru, Os, Ir, Au – mg/t (ppb)

Pd Pt Rhn S Ni Cu

Mineralization in Varied textured troctolite

Pd/Pt

Massive Mineralization

Leopard Textured Mineralization

Mineralization in Magmatic Breccia

Ru Ir Os AuCo

TABLE 1. Average composition of mineralization from different environments at Voisey’s Bay.

considerable homogenization must have taken place whilethe sulphur was being incorporated into the magma.

Mineralization within the Voisey’s Bay Intrusion

Types of MineralizationThere are four principal types of mineralization at

Voisey’s Bay; representatives of each are found in most of the mineralized environment: (i) massive sulphides, (ii) leopard textured sulphides, (iii) sulphides in basal brec-cia sequence, and (iv) disseminated sulphides in varied-tex-tured troctolite. In addition, chalcopyrite- and bornite-bear-ing Cu-rich veins within the adjacent gneiss constitute whatis yet an insignificant mineralization type – time will showwhether substantial reserves of this material will be discov-ered, as has been the case at Sudbury and Noril’sk.

The mineralogy of the different types of mineralizationwas described in detail by Naldrett et al. (2000b). Briefdescriptions of the sulphide morphologies are given below.Average chemical compositions of the ores are presented inTable 1.

Massive Sulphide

This consists of massive (>85% sulphides) accumulationsof pyrrhotite (varying proportions of troilite and hexagonalpyrrhotite), pentlandite, chalcopyrite, cubanite, and mag-

netite. Pyrrhotite occurs as coarse crystals, exceeding 10 cmin diameter in some samples. Naldrett et al. (2000b) pointedout that troilite is missing from the Eastern Deeps and mostabundant within the Ovoid. Where present, troilite occurs asfine exsolution lamellae in hexagonal pyrrhotite. Cubaniteoccurs as discrete grains and exsolution lamellae in chal-copyrite and shows a similar distribution to troilite, beingpresent where troilite is present and absent where it is absent.Massive sulphides of the Ovoid, Mini-Ovoid, and ReidBrook zone are characterized by very coarse grains (1-2 cm)of pentlandite, with little or no pentlandite occurring as rimsto pyrrhotite or as lamellae within pyrrhotite (Fig. 15A).Coarse pentlandite grains are rare in the massive sulphide ofthe Eastern Deeps and most pentlandite forms rims aroundcrystals of pyrrhotite or occurs as lamellae within pyrrhotite(Fig. 15B). Magnetite is present in amounts of about 5% byvolume in sulphides at the periphery of the Ovoid, 1% at thecentre; it is rare in the massive sulphide of the Eastern Deeps.

Leopard-Textured Sulphide

As discussed above, the term ‘leopard texture’ is derivedfrom the principal characteristic of this sulphide, which isthat of black spots (augite and olivine oikocrysts ~0.5 cmdiameter) in a yellow matrix (Fig. 15C). The matrix consistsof sulphides (primarily pyrrhotite, pentlandite, and chal-copyrite) that occur interstitial to the principal cumulus sili-


400

FIGURE 15. (A) Typical magmatic sulphide from the Ovoid. Note the large size (fingernail-size) of the pentlandite grains and the lack of pentlandite eitherencircling pyrrhotite crystals as rims or occurring within pyrrhotite as flames. The small black dots within the sulphides are magnetite. (B) Typical magmaticsulphide from the Eastern Deeps. Note that the pentlandite occurs as smaller masses than in photo A and is present both as rims to pyrrhotite grains and withinthe pyrrhotite as flames oriented parallel to the pyrrhotite 0001 basal parting. (C) Typical ‘leopard’ texture. The sulphides occur interstitial to olivine and pla-gioclase but have been displaced by oikocrysts of augite, which also occur interstitial to olivine and plagioclase and now form the large (0.5 cm diameter)black areas. The rock is referred to as leopard troctolite. (D) Blotchy sulphide within varied-textured troctolite. The crystals growing into the sulphide areplagioclase, which is much coarser when hosted by sulphide than in the silicate-dominant areas. (E) Basal (or Magmatic/Feeder) Breccia. The white areasare the remnants of inclusions of Tasiuyak gneiss that have reacted with the enclosing troctolite magma as discussed in the text. Close examination showsthat the white areas, composed largely of anorthite, are rimmed by darker areas largely composed of hercynite. (F) Typical disseminated ore in which sul-phides occur interstitial to olivine and plagioclase. This sample, which contains about 50 vol.% sulphide, is an example of the richest disseminated ore. If thesulphide content exceeds 50%, the sulphides tend to segregate as irregular areas/veins of massive sulphide.


40

cates (plagioclase and olivine). Theproportion of sulphide varies from20 to 50%. In samples with morethan 50% sulphide, the sulphideshave separated from the silicatehost to form small schlieren ofmassive material (Fig. 15F). As hasbeen remarked previously, theblack spots are due to the develop-ment of oikocrysts of augite andolivine, which appear to havegrown interstitial to the other sili-cates, when the sulphides were stillliquid, and to have pushed the latteraside. The mineralogy of the leop-ard textured sulphide is similar tothat of adjacent massive sulphide(Naldrett et al., 2000a). The rockshowing this texture is referred toas ‘leopard troctolite’.

Sulphides in Magmatic Breccias

Mineralization in the basal brec-cia sequence is much more variablethan that of the two preceding min-eralization types. In part it consistsof small lenses of massive sulphide,and in part of veins of leopard troc-tolite. Much of the mineralizationcomprises ‘blotches’ of sulphideinterstitial to the inclusions ofgneiss, troctolite, mela-troctolite,and ultramafic rocks, which arecommon in this rock unit (Li andNaldrett, 1999; Li et al., 2000).Again, the mineralogy tends toreflect that of adjacent massive sul-phide.

Disseminated Sulphides in Varied-Textured Troctolite

Sulphides are abundant (approx.25 vol.%) in the lower part of thevaried-textured troctolite, anddecrease in amount upward. Theyoccur in two forms; the most com-mon is as irregular “blotches”, 10 to 30 cm in diameter, of sulphideintergrown with coarse silicates(Fig. 15D). The silicate minerals inthese blotches comprise plagioclaseand olivine that are very muchcoarser than those in the enclosingrock. In addition, coarse rods ofplagioclase penetrate the sulphideblotches, growing away from pla-gioclase in the enclosing varied-textured troctolite. The sec-ond form of sulphide comprises irregular patches of dissem-inated sulphide, generally less than 20% in amount, whichare present within varied-textured troctolite that displays anormal, medium-grained, igneous fabric.

Nickel, Copper and PGE Tenors in Sulphides fromDifferent Mineralized Environments

Note that all Ni and Cu concentrations quoted in this sec-tion refer to metal content in 100% sulphide and have been

V aried-textured troctolite

Enderbitic orthogneiss

650 0

sulphide content s ( wt.% ) i n r ock

Dep

th (m

)

0

20

2

40

4

60

6

80

8

100

10

670

660

690

680

700

710

720

740

730

750

Ni and Cu content s ( wt.% ) i n 100% sulphide

sulphide contents 10

0

Q

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.

20

30

40

50

60

70

80

90

100

11 0

120

130

Dep

t h( m

)

0 2 4 6 5 3 1 87 Ni and Cu contents (wt.% ) i n 100% sulphide

Co contents (wt.% ) i n 100% sulphide

Cobalt Copper Nickel

170

180

190

200

210

220

230

240

250

260

270

Dep

t h( m

)

0

0

sulphide content s ( wt.% ) i n r oc k 20

2

40

4

60

6

80

8

100

10

sulphide content s

Ni and Cu content s ( wt.% ) i n 100% sulphide

210

220

230

240

250

260

270

280

290

Dep

t h( m

)

0 2 4 6 8 10Ni and Cu content s ( wt.% ) i n 100% sulphide

0 sulphide content s ( wt.% ) i n r ock 20 40 60 80 10

sulphide content s

Magmatic brecci a

Massive sulphide

T asiuyak paragneiss

Normal troctolite

Ni contents after correctio

Leopard troctolite Cu content s

A B

C

Nai n gneiss

D

FIGURE 16. Profiles along holes representative of different mineralized environments at Voisey’s Bay show-ing the variation in Ni and Cu content in 100% sulphides, along with the sulphide content of the rock, and, inthe case of (B) the Co content of the sulphides. (A) Hole 202 through mineralization in the Eastern Deeps.(B) Hole 7 through the Ovoid. (C) Hole 162 through the Discovery Hill zone. (D) Hole 282 through miner-alization in the Reid Brook area. Data from Naldrett et al. (2000b).

taken from Naldrett et al. (2000a)1. Variation in the Ni andCu content of sulphides across typical profiles through theVoisey’s Bay deposit are illustrated in Figure 16. Figure 16Ashows variations along hole 202, which is typical of miner-alization in the Eastern Deeps. It was drilled at the mouth ofthe feeder in a location where a 40 m thick len of massivesulphide occurs in a slight depression in the base of the intru-sion adjoining the feeder mouth. Ni tenor in the massive sul-phide is relatively constant, averaging 3 to 3.5 wt.%. Cutenor is scattered and low, averaging about 1 wt.%. Both Niand Cu rise in sulphides in the basal breccia sequenceSequence that overlies the massive zone, Ni to between 4.5and 5 wt.%, Cu to between 1 and 2 wt.%. In other holesthrough the Eastern Deeps, the varied-textured troctoliteabove the massive and basal breccia-hosted mineralizationcontains variable amounts of sulphides with tenors that areupward of 5 wt.% Ni and 2 to 3 wt.% Cu.

Hole 7 (Fig. 16B) penetrated the massive sulphide occu-pying the centre of the Ovoid. Ni tenor increases upwardfrom an average of 3.90 wt.% over the lowermost 20 m ofmassive sulphide to an average of 4.66 wt.% over the upper-most 20 m. Cu increases even more markedly from an aver-age of 2.06 wt.% over the lowermost 20 m to 4.66 wt.% overthe uppermost. In contrast, Co shows a slight decrease froman average of 0.16 wt.% over the lowermost 20 m to 0.15wt.% over the uppermost. These variations are attributed tofractional crystallization as is discussed below.

Hole 162 (see Fig. 16C) is typical of the mineralization ofthe Discovery Hill zone. It intersected 55 m (true thickness)of leopard troctolite containing 15 m of basal brecciasequence. The sulphide distribution is variable, generallybetween 20 and 40 wt.%, rising to 50 to 60 wt.% in manysamples near the base of the zone. The Ni tenor is very con-sistent, averaging 4.04 wt.%. Cu is more variable but showsno consistent variation with either rock type or position andaverages 3.15 wt.% over the mineralized intersection.

Hole 282 (see Fig. 16D) is typical of those cutting a min-eralized zone in the Reid Brook area that plunges east a shortdistance above the Reid Brook subchamber. The hole cut a25 m thick (true thickness) zone of feeder breccia containinga weak dissemination of sulphide interspersed with stringersof massive sulphide, and then a thin stringer of massive sul-phide in the Tasiuyak gneiss 20 m below the zone of feederbreccia. The Ni and Cu tenors are more scattered than else-where, but average respectively 3.13 wt.% and 1.91 wt.%

over the zone of feeder breccia and 3.20 wt.% and 0.86 wt.%for the massive stringer in the gneiss.

In summary, average Ni tenors within different parts ofthe deposit vary from a low of 3 to 3.5 wt.% in much of themineralization from the Eastern Deeps and Reid Brook zone,through values of 4 to 4.5 wt.% in the Ovoid and DiscoveryHill zone, to 5 to 7 wt.% in sulphides in the varied-texturedtroctolite. Copper tenors are even more variable. In general,massive sulphide is characterized by low Cu and thus highNi/Cu ratios in excess of 2.0 (the Ovoid is a marked excep-tion to this generalization), and disseminated sulphides havevalues less than 2, typically about 1.6.

The results of the analysis of Ni, Cu, and PGE in a total of315 samples from the Voisey’s Bay deposit recalculated tometal content in 100% sulphide (Naldrett et al., 2000a) aresummarized in Table 2. The PGE concentrations are dis-tinctly low in comparison with data from many other Ni-Cumagmatic sulphide deposits. Space does not permit adetailed analysis of the variations here, and readers arereferred to the original paper.

Naldrett et al. (2000a) demonstrated that there is a markedcovariation in Cu, Au, Pd, and Pt within the Ovoid mineral-ization, which they argue is consistent with that to beexpected as a result of the fractional crystallization of mono-sulphide solid solution from a sulphide melt (see variation inNi, Cu, and Co with height in hole 7 in Fig. 16B). They con-cluded that this body of sulphide has cooled upward andinward from the sides, with the crystallization of monosul-phide solid solution first occurring adjacent to the margins.The residual liquid became concentrated toward the top andcentre of the body, enriched in those elements that areincompatible in monosulphide solid solution, i.e. Cu, Au, Pd,and Pt.

Geological Model

The overall plunge of all components of the Voisey’s Baydeposit is about 25° east, with the result that different levelsof the system are exposed at different localities (see Fig. 2).The deepest level is exposed to the west, so that the feederoccurs at surface and the exit of the feeder from the lowerchamber has been intersected in the drilling. The DiscoveryHill zone represents an intermediate level comprising thefeeder between the lower and upper chamber, while theupper chamber is preserved as the Eastern Deeps. It isthought (Naldrett et al., 1996) that the Ovoid represents the


402

1. In calculating the metal content in 100% sulphide, the whole-rock Ni content is taken, on the assumption that it is all in sulphide, and then Ni is calculatedas though it were all in pentlandite containing 36 wt.% Ni (a typical value for Sudbury pentlandite). The sulphur required for the pentlandite is then sub-tracted from the whole rock sulphur content. The same calculation is made for Cu, assuming that it is all present as chalcopyrite. The remaining sulphur isthen calculated as pyrrhotite, so that one can obtain the total sulphide content of the rock by summing the amounts of pentlandite, chalcopyrite, and pyrrhotite.Because the amounts of sulphide in many of the samples in hole 166 and some samples from other holes are relatively low, Ni contained in olivine consti-tutes a significant proportion of the total Ni content of the rock, and has to be corrected for. This has been done by performing the calculation at first with noallowance made for Ni in olivine, and obtaining the composition of the resulting sulphide. The Ni content of the olivine is then calculated using this valueand assuming (1) that the Ni/Fe(olivine) = (Ni/Fe(sulphide))/25, where 25 is the observed exchange partition coefficient between co-existing sulphide andolivine at Voisey’s Bay (see Li and Naldrett, 1999) and (2) that Ni/Fe(sulphide) excludes Fe tied up as chalcopyrite. In making this calculation, it is assumedthat the composition of the olivine is 65 mole% Fo (an average value for olivine of the varied-textured troctolite). Again, assuming that the non-sulphide por-tion of the sample contains 25 wt.% olivine (the observed average content for the varied-textured troctolite), this amount of Ni is then subtracted from thewhole rock Ni content, and the calculation repeated for Ni in 100% sulphide. The whole calculation is iterated until the calculated Ni content in sulphidesvaries by no more than 0.05 wt.% between the last and penultimate iteration. In the case of many samples with less than about 2% sulphide, the calculationnever converges, and these samples have been omitted. Several assumptions are required in making this correction (e.g. the Fo content of the olivine and theamount of olivine in the rock) that are not constant in all of the samples. However, one is using thousands of analyses performed in the course of routineexploration analysis, and it is not practical to measure the olivine composition and modal proportion of olivine for every sample. The reader can assure him-self/herself that the error introduced in this way is relatively small, by repeating the correction procedure assuming, for example, that the Fo content is 60mole% instead of 65 mole%, or that the modal % olivine is 30 instead of 25.


40

base of the upper chamber exposed at surface, although oth-ers (cf. Lightfoot, 1997) believe it to be a swelling in thefeeder conduit.

As discussed above, the systematic relationship betweenthe Ni and Fo content of olivine from the ultramafic rockinclusions, feeder olivine gabbro at all localities, olivine gab-bro of the Eastern Deeps chamber, and leucotroctolite of theReid Brook chamber that is illustrated in Figure 9 suggeststhat these rocks are related to each other by fractionation ofa single magma. The rapid decline in Ni with decreasing Focontent in the olivine, taken in conjunction with low wholerock Cu/Zr (see Fig. 13) ratios for the same samples, sug-gests that the magma from which they crystallized was sul-phide saturated, and became depleted in Ni due to the sepa-ration of sulphide. The appreciably higher Ni content ofolivine in the varied-textured troctolite and normal troctoliteof the Eastern Deeps, coupled with the high whole-rockCu/Zr ratios, indicates that their parental magma was lessfractionated and less chalcophile-depleted than othersequences. Li and Naldrett (1999) concluded, therefore, thatat least two waves of magma were involved at Voisey’s Bay.The first crystallized in a lower chamber to form the ultra-mafic/mafic layers from which the inclusions were derived(Fig. 17A), before fractionating further as it progressed upthe feeder to form the feeder olivine gabbro lining the wallsof the feeder and ultimately the olivine gabbro of the EasternDeeps (Fig. 17B). While in the lower chamber, this magmareacted with gneiss fragments derived from the walls androof. The reaction resulted in sulphide immiscibility devel-oping as fractionation was proceeding, either as a result offelsification of the magma (cf. Li and Naldrett, 1993) and/ordue to the addition of sulphur from the gneiss.

The trace element data of Li et al. (2000) indicate that anearly magma, most probably of picritic composition, frac-tionated and interacted with mid-crustal rocks and acquiredan elevated, non-mantle, La/Sm ratio. The magma thenascended into rocks with a high Th/Nb ratio (Tasiuyakgneiss) where further reaction and incorporation of gneissicinclusions resulted in this signature also being imposed onthe magma. Amelin et al.’s (2000) Nd, Pb, and Sr data sup-port this concept of a two-stage interaction at differentcrustal levels. The Re-Os data of Lambert et al. (2000) alsoprovide strong support for reaction between an early picriticpredecessor of the troctolitic magma and enclosing Tasiuyakgneiss. Ripley et al.’s (1999) S-isotope data are consistentwith the derivation of at least part of the sulphur from thegneiss; the oxygen isotope data (Ripley et al., 2000) againsupport interaction between troctolite and wall rocks.

Li and Naldrett (1999) proposed that the sulphides thatsegregated from the first wave of magma remained withinthe lower chamber but were then picked up by the secondpulse of magma and carried up along the overlying conduit.Calculations based on recently determined partition coeffi-cients for the distribution of Ni between olivine and sulphidemelt (Brenan and Caciagli, 2000) indicate that sulphide liq-uid in equilibrium with magma giving rise to the olivinefound in rocks that formed from the first magma pulse wouldhave contained less than 3.0 wt.% Ni. As Naldrett et al.(2000a) have shown, the Ni content of the sulphide liquidresponsible for the ores was between 3.5 and 4.5 wt.%,which argues strongly that this liquid re-equilibrated with amagma of higher Ni content than that first passing throughthe feeder into the upper chamber. Varying degrees of re-equilibration can account for the variable metal tenors dis-

Ni Cu Pt Pd Rh Ru Ir Os Au

wt.% wt.% ppb ppb ppb ppb ppb ppb ppb

Varied textural troctolite

Eastern Deeps>10% sulphide (21)

Eastern Deeps<10% sulphide (25)

Leopard troctolite

Eastern Deeps (9) 3.41 (0.23) 1.87 (0.50) 107 (75) 154 (126) 16 (3) 23 (20) 6 (1.4) 13 (4) 93 (49)Miniovoid (7) 3.94 (0.24) 2.77 (0.94) 145 (71) 228 (60) 12 (6) 25 (16) 2.9 (0.91) 7 (2) 224 (80)Discovery Hill Zone (12) 3.79 (0.27) 2.23 (0.47) 377 (197) 345 (129) 9 (1) 35 (24) 2.0 (0.8) 3 (1) 337 (73)Reid Brook Zone (20) 3.29 (0.58) 1.98 (1.32) 162 (119) 143 (76) 10 (1) 30 (12) 2.5 (0.5) 4 (2) 189 (119)

Magmatic breccia

Eastern Deeps (44) 4.40 (0.88) 2.47 (1.09) 242 (220) 264 (180) 13 (6) 37 (26) 4.6 (2.5) 9 (6) 245 (176)Ovoid (4) 4.14 (1.10) 3.44 (1.31) 430 (159) 521 (136) 23 (5) 70 (16) 9.1 (1.9) 13 (4) 469 (178)Miniovoid (6) 4.11 (0.85) 3.83 (1.68) 489 (255) 307 (181) 10 (2) 56 (43) 3.6 (2.3) 11 (8) 360 (452)Discovery Hill Zone (18) 4.06 (0.41) 2.20 (0.78) 266 (154) 346 (163) 11 (4) 31 (17) 2.7 (1.7) 5 (3) 278 (168)Reid Brook Zone (26) 3.75 (0.72) 1.69 (0.72) 203 (136) 258 (171) 14 (10) 82 (60) 4.1 (4.2) 7 (5) 200 (143)

Massive sulphide

Eastern Deeps (12) 3.32 (0.99) 1.15 (1.55) 34 (30) 144 (106) 18 (6) 26 (17) 6.6 (2.4) 15 (6) 18 (17)Ovoid (52) 4.61 (1.08) 2.84 (1.40) 123 (111) 252 (115) 8 (3) 17 (10) 2.0 (1.1) 4 (1.5) 93 (130)Miniovoid (11) 3.96 (1.23) 2.30 (2.41) 178 (85) 221 (127) 9 (3) 17 (5) 2.5 (0.6) 5 (2) 116 (48)Reid Brook Zone (16) 2.95 (0.43) 1.17 (0.86) 42 (33) 102 (36) 10 (1) 19 (6) 2.8 (0.8) 4 (3) 60 (67)

Mineralization Type, Location,

and Number of Samples ()

4.72 (1.03) 2.91 (1.38) 360 (309)

4 (7)

354 (264) 19 (9) 31 (34) 6.4 (3.8)

546 (387)

10 (8) 355 (349)

6.08 (1.04) 3.22 (0.93) 447 (251) 354 (347) 19 (10) 60 (88) 5.1 (2.7)

TABLE 2. Average metal contents in 100% sulphide of ores of the Voisey’s Bay deposit (standard deviations in parenthesis) (data fromNaldrett et al. (2000)).

cussed above in the different min-eralized environments of thedeposit. As the second wave ofmagma advanced up the conduitinto the upper chamber (the EasternDeeps), sulphides became concen-trated in hydrodynamic trapswithin the conduit (Evans-Lamswood et al., 2000) and at thepoint where the conduit broadenedout (Fig. 17C) as it entered theupper chamber (Naldrett et al.,1996).

Conclusions as to the Genesis ofthe Voisey’s Bay Deposit

At this stage in the studies of theVoisey’s Bay deposit, certain keyaspects of its genesis haveemerged:

1. The deposit is related to twomagma chambers, one 1.5 kmstratigraphically above the other,connected by a feeder dyke (Fig. 2).

2. Magma, most likely of picriticcomposition, rose into the crustwhere it started to crystallizeand interact with mid-crustalgneisses, acquiring a crustaltrace element and isotopic signa-ture.

3. The magma continued its ascentand swelled out to form thelower (Reid Brook) chamber.Here it reacted with the enclos-ing Tasiuyak gneiss and eventu-ally became saturated in sul-phide. Various degrees of thisreaction are preserved in theinclusions of the basal brecciasequence, feeder breccia, andvaried-textured troctolite.

4. The initial magma lost much ofits Ni as a consequence ofolivine fractionation and separa-tion of immiscible sulphide liq-uid; this liquid also removedother chalcophile elements,including Cu, causing a decreasein the Cu/Zr ratio.

5. The initial magma was forcedout of the lower chamber, alongwith some of the sulphide thathad segregated from it, and roseup the feeder sheet to develop an upper chamber about1 km vertically above the lower chamber. Sulphidesbecame lodged in and near the feeder as the magma roseup it.

6. Fresh, chalcophile element-undepleted magma entered

the lower chamber and was probably responsible for theearly magma being forced upward. The new magmacontinued through the system, disrupting early cumu-lates, picking up sulphides and partially-reacted inclu-sions, and transporting all of these up the feeder system


404

A. Step I

C. Step III

Ovoid

Mini-ovoid

Rei

dB

rook

Zone

Eas

tern

Dee

ps

B. Step II

Uppermagmaticchamber

Feeder

Enderbiticgneiss

Tasiuyakparagneiss

Nainorthogneiss

MSMS

OG

NT

VT

GSM

LUT

FOG

LT

MB

MB

GSM

VT

Lowermagmaticchamber

FIGURE 17. Genetic model for the Voisey’s Bay deposit. (A) Step I: Magma rose to a lower chamber withinTasiuyak gneiss, olivine crystallization occurred with the formation of ultramafic cumulates. The magmareacted with gneiss fragments from collapsed roof, and became sulphide saturated. (B) Step II: New, Ni-unde-pleted magma entered the lower chamber, disrupting early cumulates, and pushing up the early magma to anupper chamber in Nain Gneiss. During transportation some sulphide became lodged in and near the feeder asthe magma rose. (C) Step III: The later magma continued to flow through the system, picking up sulphidesand partially reacted gneiss inclusions and transporting them up the feeder system to spread out over the lowerpart of the upper chamber. The fresh, undepleted magma enriched the sulphides in chalcophile metals. Theinsets show the types of sulphide accumulation that developed. Modified after Li and Naldrett (1999). FOG– Feeder olivine gabbro; GSM – Granite, syenite, and monzonite; LT – Leopard troctolite; LUT –Leucotroctolite; MB – Magmatic breccia; MS – Massive sulphide;NT – Normal troctolite; OG – Olivine gab-bro; VT – Varied-textured troctolite.


40

to spread out in the lower part of the upper chamber.

7. As it progressed through the system, the new magmainteracted with sulphides that were lodged there,upgrading them in chalcophile elements.

What Voisey’s Bay Has to Tell Us about Ni Sulphide Exploration

The Voisey’s Bay deposit reflects many of the major fac-tors that have been recognized as important with respect tothe formation of Ni sulphide deposits (cf. Naldrett, 1997,1999): These include

1. Areas that have been intruded by hot mafic/ultramaficmagma (e.g. rocks with Fo-rich olivine).

2. Locations where these rocks are in close proximity to, orhave cut through, sulphide-bearing country rocks (e.g.the Tasiuyak gneiss at Voisey’s Bay).

3. Field evidence of substantial interaction between thecountry rocks and the intruding magma, such as brecciascomposed of country rocks in an igneous matrix (e.g.the basal breccia/feeder breccia at Voisey’s Bay).

4. Chemical evidence of contamination (e.g. LREE enrich-ment and high Th/Nb ratios at Voisey’s Bay).

5. Chalcophile depletion in the intrusive rocks (e.g. Ni-depleted olivine in the early intrusive phase and lowCu/Zr ratios in the same rocks at Voisey’s Bay).

6. Evidence of an input of fresh, less depleted magma thatcan have enriched the early-forming sulphides (e.g. thenormal troctolite and varied-textured troctolite atVoisey’s Bay).

7. Structure that has channelized the flow of sulphide-bearing magma and caused the sulphides to concentratein hydrodynamic traps (e.g. the connecting dyke atVoisey’s Bay).

All of these factors have been responsible for the forma-tion of the rich mineralization at Voisey’s Bay. Other areas,in which only some of the factors were present, have notproved so far to be of economic interest, as is discussedbelow.

The intense exploration undertaken on intrusions thatform the Nain Plutonic Suite in the aftermath of the Voisey’sBay discovery, coupled with careful documentation of theexploration results by members of the NewfoundlandGeological Survey (Ryan et al., 1995; Kerr, 1999, 2003; Kerrand Ryan, 2000), mean that we have an almost uniqueoverview of the different types of deposits, their genesis, andrelative importance associated with a single igneousprovince.

Kerr and Ryan (2000) have subdivided the deposits andprospects into several categories: A) those related to trocto-lite/olivine gabbro intrusions; B) those related to hybridpyroxenites at the margins of troctolite/olivine gabbro intru-sions; C) those related to anorthosite complexes either a) innorites-gabbros within or close to the anorthosites, or b)within the anorthosites themselves; and D) those related toferrodiorite.

With respect to (B), it has become clear that sulphide min-eralization in hybrid pyroxenites at the margins of largeintrusions has not had access to sufficient quantities of

magma to concentrate Ni and Cu to any great degree, withthe result that the metal tenors of the sulphides are low (Kerrand Ryan, 2000). In addition, the pyroxenitic magmas them-selves have not experienced the dynamic flow regimes thatare necessary to concentrate the sulphides that they carried.

With respect to (C), sulphides within the anorthosites arethe end products of the cooling of small batches of maficmagma that originated during the crystallization of theanorthosites, so that Ni has generally been incorporated intoearly forming silicates and the Ni tenors of the sulphides arelow. Again, these magmas have not experienced the dynamicenvironment necessary to concentrate sulphides or, indeed,to expose the sulphides to large volumes of magma.

With respect to (D), the ferrodiorites are extreme differ-entiates within which sulphides developed at a late stagewhen most of the Ni present in the original magma had beenremoved in mafic silicates.

With respect to (A), the majority of the troctolite/gabbrobodies lie east of the Nain-Churchill suture and they do notseem to have achieved sulphide saturation at an early stage.If one looks at Figure 1, it is clear that the onlytroctolites/gabbros that have developed significant amountsof sulphide are those that have intruded the sulphide-bearingTasiuyak gneiss, namely those at Voisey’s Bay and PantsLake, 80 km south of Voisey’s Bay (also know as theVoisey’s Bay South occurrence). These magmas have hadtheir sulphur content increased through reaction with theTasiuyak gneiss. However, at Pants Lake, so much sulphideappears to have developed in the magmas responsible for theintrusions, that these became very chalcophile depleted. Thisaccounts for the fact that any sulphides that have been foundassociated with them are low in tenor (Kerr and Ryan, 2000;Kerr, 2003). There is no evidence of an input of fresh magmathat could have enriched early forming sulphides.Additionally at Pants Lake, the sulphides have also not beenin the right dynamic environment to have been concentratedto more than 10 to 20 wt.%.

Acknowledgements

This paper is based to a large extent on the research con-ducted under the NSERC Collaborative Research andDevelopment grant held by one of us (AJN) from January1996 to December 1998, the results of which were publishedin Economic Geology in June/July 2000. I am grateful toDiamond Fields Resources Inc. who supported the initialapplication and to INCO Ltd. who continued support. Anumber of distinguished scientists joined our research group,including Ed Ripley (Indiana University), David Lambert(then with Monash University), Yuri Amelin (then with theRoyal Ontario Museum), Peter Lightfoot (INCO Ltd.), andBruce Ryan and Andrew Kerr (Newfoundland GeologicalSurvey). Dr Chusi Li (then at the University of Toronto)undertook the role of leading researcher and project man-ager, and Mohammed Asif, Sasha Krstic, and JagmohanSingh assisted greatly while at the University. Our researchteam met twice a year with geologists working on thedeposit, who included Dan Lee, Dawn Evans-Lamswood,Robert Wheeler, and Colin MacKenzie. Ideas were discussedfreely and it is as a result of these discussions and theresearch that they stimulated that we reached the under-

standing of the deposit that is put forward in EconomicGeology Volume 95, Number 4, and that we have attemptedto summarize here. We are grateful to all of those namedabove for one of the most stimulating and enjoyable researchcollaborations that we have had the good fortune to experi-ence. We would also like to acknowledge, with many thanks,a figure that Peter Lightfoot had used in a presentation in2001, and that he kindly allowed us to reproduce here asFigure 4.

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THE VOISEY’S BAY DEPOSIT, LABRADOR, CANADA