Summary:
The Voisey’s Bay Intrusion (“VBI”) is mafic in composition and is dominantly composed of olivine gabbro and troctolite with variable amounts of leucotroctolite, melatroctolite, olivine norite, gabbronorite and ferrodiorite. The Voisey’s Bay deposits are hosted by the VBI, which has been estimated to be at 1338 Ma old.
The VBI occurs in three intrusions. The first comprises the large, easterly plunging, troctolite chamber overlying the Eastern Deeps deposit. The second is the troctolite dyke, commonly referred to as the “feeder dyke” or “conduit.” It extends north of the Eastern Deeps chamber as a thin, flat-lying, body and then westward, with progressively steeper northerly dips eventually overturning to steep southerly dips. The Ovoid, Mini-Ovoid and Discovery Hill deposits occur in the steep, north-dipping segment while the Reid Brook deposit occurs in the south-dipping portion. The “conduit” ultimately approaches the top of the third body, the Western Deeps troctolite chamber below the Reid Brook deposit at a depth of about 1000 metres. The feeder dyke joins the two chambers.
There are four principal types of sulphide mineralization at Voisey's Bay: massive, leopard-textured, basal breccia, and disseminated in variable troctolite. The last three types are interfingered and cannot be correlated as distinct units.
The deposits making up the Voisey’s Bay Project include Discovery Hill, Eastern Deeps, Mini-Ovoid, Ovoid, Reid Brook and Southeast Extension. The deposits that occur within the feeder include the Reid Brook, Discovery Hill and the Ovoid/Mini-Ovoid. The Eastern Deeps deposit occurs at the junction of a feeder dyke with the Eastern Deeps Chamber. The Ovoid and Mini-Ovoid deposits and a portion of the Southeast Extension deposit constitute the mineralization contained within the proposed open pit for Phase I of the Voisey’s Bay Project.
Sulphide mineralization in all parts of the Voisey’s Bay system is closely similar, consisting mostly of pyrrhotite, pentlandite, chalcopyrite (± cubanite) and variable amounts of magnetite (Naldrett et al., 2000). Mineralized rocks range from coarsegrained, crystalline massive sulphides to disseminated, interstitial sulphides in homogeneous troctolites, to coarse-grained breccia-like sulphides containing troctolitic and reacted country-rock fragments. As in many magmatic sulphide deposits, the effective grade of mineralization at Voisey’s Bay is essentially a function of the amount of sulphides, which usually contain between 3 and 5% Ni, and 1.5 and 2.5% Cu.
The Ovoid and Mini-Ovoid deposits form an elongate, tadpole -shaped feature with a length of 800m along a west-northwest axis and a maximum width of 350m, narrowing to less than 50m at its northwestern end. The maximum vertical depth of the Ovoid deposit approaches 120m. A thin, initially north-dipping feeder dyke extends from the base of the Ovoid, but this dip reverses and eventually dips to the south.
The feeder dyke forms an incomplete marginal skin, 0-20m thick, of variably net-to leopard-textured mineralized troctolite and basal breccia with disseminated sulphides. This marginal skin surrounds a core of massive sulphide mineralization approximately 350m in diameter and up to 120m thick.
Toward the western extension of the Ovoid, the deposit becomes more elongate or trough-like in transition to the more dyke-like form of the Discovery Hill deposit. This area is known as the Mini-Ovoid deposit. The sequence of troctolite intrusion and sulphide deposit is more complete than in the Ovoid deposit. The upper or northern margin of the intrusion in this area consists of unmineralized gabbroic to troctolitic material, either in chilled contact with the overlying gneiss or as a breccia of intrusive and gneiss fragments. Sulphide content increases abruptly downwards into a zone of net- and leopard-textured troctolite with some massive sulphide veins and segregations. Sulphides within this unit are dominantly finer-grained, net-textured and have leopard textures. A large lens of massive sulphide, spatially distinct from the massive sulphide in the Ovoid deposit, occupies the central portion of the Mini-Ovoid deposit and overlies weakly mineralized basal breccia along the lower or southern contact.
The base of the Mini-Ovoid deposit merges into the feeder dyke, which dips about 70º to the north at this location.
In the southeast corner of the Ovoid deposit, mineralized troctolite and basal breccia extend over a buried ridge of footwall gneiss that separates the Ovoid deposit from the Eastern Deeps chamber. Immediately southeast of the Ovoid deposit, the troctolite broadens into the main troctolite intrusion called the Eastern Deeps chamber, host to the Eastern Deeps deposit. The area known as the Southeast Extension is the variably mineralized zone between the Ovoid deposit and the Eastern Deeps deposit. The Southeast Extension mineralization averages between 50m and 100m thick and subcrop at the southeast end of the Ovoid deposit, plunging eastward to a depth of 450m.
The Ovoid/Mini-Ovoid and Southeast Extension deposits are completely covered by 15-20m of unconsolidated glacial and marine sediments.
Nickel distribution in massive sulphide is remarkably consistent throughout the Ovoid and Mini-Ovoid deposits.
Copper grades are considerably more variable than those of nickel. Copper grades in massive sulphide are highest along the southwest side and central portions and lowest along the west, north and east sides of the Ovoid deposit.
Nickel distribution in massive sulphide is remarkably consistent throughout the Ovoid and Mini-Ovoid deposits.
The massive sulphide is composed of approximately 75 per cent pyrrhotite/troilite, 12 per cent pentlandite, 8 per cent chalcopyrite/cubanite and contains 5 per cent magnetite. The pyrrhotite is hexagonal and on average contains about 0.29 per cent nickel. The pyrrhotite to troilite ratio is about 70:30. The troilite contains only trace amounts of nickel.
The pentlandite is coarse grained and often forms coarse “loop textures” rimming large crystals of pyrrhotite. The pentlandite is frequently associated with magnetite, which is dispersed throughout the sulphide minerals. Magnetite can occur in local concentrations, ranging from less than 5 per cent to as high as 30 per cent.
The cubanite occurs as intergrowths with chalcopyrite as exsolution lamellae and rarely as granular aggregates within the chalcopyrite. The chalcopyrite to cubanite ratio averages about 90:10 but high concentrations of cubanite are locally present.