While prospecting in eastern Labrador in 1993, two Newfoundlanders, Albert Chislett and Chris Verbiski of Archean Resources, a small St. John's based company, chipped samples from an iron stained rock outcrop. Within fifteen minutes of standing on the outcrop they realised that they had made a potentially significant mineral discovery. Results came quickly. In November 1994, Diamond Fields, the company that funded the prospectors, announced the discovery of a major nickel-copper-cobalt sulphide orebody, and by July 1995, drilling had defined 31.7 million tons of ore with a grade of 2.83% nickel, 1.68% copper and 0.12% cobalt. The province of Newfoundland was on its way to becoming an important producer of nickel, copper and cobalt.
The geology of Eastern Labrador
Until the Voisey's Bay discovery, it is probably true to say that the igneous rocks and high grade metamorphic rocks of eastern Labrador were not generally considered to be promising areas for base metal prospecting. Indeed, the same iron stained rocks had been sampled at least once before, in 1985, by geologists of the Newfoundland Department of Mines and Energy, in the course of a helicopter-supported survey of the area, but they were not encouraged by the low metal content of the weathered rocks exposed at surface.
Figure 1: (Left) Regional geology of northern Labrador and the location of the Voisey's Bay deposit. (Right) The emplacement of the igneous plutonic rocks associated with the Voisey's Bay deposit following the collision of the Churchill "continent" in the west with the Nain "continent" in the east, and the formation of the intervening Torngat orogen.
The sequence of geological events leading up to the formation of the ore deposit are illustrated in Figure 1. The plutonic igneous rocks that host the orebody were emplaced as magma deep in the crust, close to a linear zone where two ancient continents collided as the ocean basin between them closed. The collision of the continents occurred 1840 to 1860 million years ago, and as the continental crust to the west was thrust below the continent to the east, the sediments of the ancient ocean were pushed deep into the crust to be metamorphosed and folded. That zone of metamorphosed and folded rocks is now exposed at surface, by uplift and erosion, where it forms a belt of gneisses and plutonic igneous rocks known as the Torngat orogen.
Much later, between 1290 and 1340 million years ago, the crust at the Torngat orogen became active once more. This time the crust was under tension and started to pull apart and extend with the formation of steep faults, probably in response to a rising plume in the mantle underlying the crust (Figure 2). It was during this time that the magmas of the igneous rocks of the Nain Plutonic Suite were generated and emplaced into the crust; and it was during this time that the Voisey's Bay orebody was formed.
The two ancient continents, now welded together by the Torngat orogen, are distinguished by their different geological structures, reflecting separate structural histories. The eastern part of the ancient continent to the west is now known as the Churchill Structural Province, but only a sliver of the eastern continent remains, represented by the Nain Structural Province. The rest of the continent has since split off to form Greenland.
The formation of the orebody
The major copper-nickel deposits of the world are associated with igneous rocks that are formed by partial melting of the mantle (Figure 2). The most familiar, mantle-derived igneous rocks are the basalt volcanic flows of the ocean crust. When the same basalt magma is emplaced within the crust, rather than reaching the surface, it cools slowly, to form gabbro, the coarse grained equivalent of basalt. The host rock to the copper-nickel deposit at Voisey's Bay is a type of gabbro.
Compared to other igneous rocks, such as granite, gabbros are dark coloured because they are high in iron and magnesium and low in silica (SiO2), and they also contain comparatively high nickel and copper, although these elements occur only in trace amounts (i.e measured in parts per million rather than percent). When the magma crystallizes, these small amounts of copper and nickel do not form discrete minerals, but are dispersed through the rock, hidden away in silicate minerals where they substitute for the more abundant elements such as magnesium and iron.
Mantle-derived magmas, however, have the potential to form a separate, immiscible magma or liquid of iron sulphide composition into which the small amounts of copper and nickel in the magma may be concentrated and eventually crystallize as discrete sulphide minerals. The iron-rich magmas are capable of dissolving significant amounts of sulphur. Sulphur dissolves in an igneous magma by displacing oxygen atoms from sites where the oxygen is comparatively weakly bonded to a cation. Oxygen bonded with Fe2+ is such a site. Thus only magmas with abundant ferrous iron are capable of taking sulphur into solution in significant amounts.
FeO(in solution) + 1/2S2 = FeS(in solution) + 1/2O2
One of the ways that the magma can pick up sulphur is by passing through sulphur-rich sediments in the crust (Figure 3). At Voisey's Bay the necessary sulphur is found in the metamorphosed sedimentary rocks of the Torngat orogen. The amount of sulphur dissolved in the magma (only a few percentage by weight) depends on several factors, but one of the most important is the temperature of the magma. Thus as it cools, droplets of immiscible iron sulphide liquid appear in the gabbroic magma. These droplets are much more dense than the gabbroic magma, and they fall through the magma to accumulate at the bottom of the magma chamber as a pool of iron sulphide liquid. But as the sulphide droplets pass through the magma, they pick up or scavenge the trace metals that partition preferentially into sulphide liquid rather than the silicate magma; trace elements such as copper and nickel, but others such as cobalt, platinum and palladium, if present will also be concentrated in the sulphide liquid. The sulphide liquid will eventually cool and form sulphide minerals of the ore. At Voisey's Bay the ore consists of, approximately : 75% pyrrhotite, FeS; 12% pent-landite, Fe,Ni9S8; 8% chalcopyrite, CuFeS2; and 5% magnetite Fe3). The last droplets of sulphide liquid that separated will be trapped by the crystallizing gabbroic magma as disseminated sulphide ore overlying the massive sulphides at the bottom of the magma chamber (Figures 3 and 4).
Figure 4: Model for the Voisey's Bay deposit (after Naldrett et al., 1996) The massive sulphides overlie a feeder dyke which is approximately horizontal where it enters the intrusion. It is probable that the immiscible sulphide liquid separated from the gabbroic magma as it was moving along a feeder dyke to its final location.
The discovery gossan
The red iron staining on the weathered rocks, which first attracted the prospectors, is referred to as a gossan by geologists. It is formed by the weathering and oxidation of iron sulphides which invariably occur in base metal ore deposits. In the course of weathering, the iron sulphides (pyrrhotite, FeS, at the Voisey's Bay deposit) are oxidised to insoluble ferric oxy-hydroxides (approximated to Fe(OH)3 in the equation below), and it is this "rust" that gives the gossan its characteristic red colour.
FeS + 2O2 = Fe2+ = SO42-
Fe2+ + 1/4O2 + 5/2H2O = Fe(OH)3 + 2H+
(at low pH this reaction is speeded up by bacteria)
It takes only a comparatively small amount of iron sulphides in a rock to produce a strong red colour and consequently it is reasonably easy for a prospector to spot a gossan. But gossans must be examined very carefully. This is because, in the reaction shown above, hydrogen ions are produced, resulting in acidic waters. Such water will dissolve or leach the base metals in the deposit, leaving only a small amount of metal in a gossan that may have formed by the weathering of a metal-rich sulphide deposit. This was particularly important at Voisey's Bay because at the discovery outcrop, gossan formation and metal leaching had penetrated to greater depths than would normally be expected under the cool climatic conditions of eastern Labrador.
The geologists working for the Geological Survey of Newfoundland and Labrador were discouraged by the low metal content of the gossan. The prospectors were more fortunate, maybe even a bit lucky, because they found fresh, unweathered rock comparatively close to surface, and, as was reported in an article in the Northern Miner, (May 15th 1995), they "could see stringers of chalcopyrite (CuFeS2) shooting through the gabbro." They extrapolated what they saw in their samples to the dimensions of the gossan, approximately 500 m long and between 40 m and 80 m wide, and they knew that they were onto an important discovery.
It has been estimated that the Voisey's Bay Ni-Cu-Co deposit may contain 150 million tons of ore grade material (Financial Post, April 12, 1996). It is one of the most economically significant, geological discoveries in Canada in the last thirty years.
Further reading
Naldrett, A.J., Keats, H., Sparkes, K. and Moore, R. 1996. Geology of the Voisey's Bay Ni-Cu-Co deposit, Labrador, Canada; Exploration and Mining Geology, Vol. 5, No. 2, pp. 169-179.
Ryan, B., Wardle, R.J., Gower, C.F. and Nunn, G.A.G. 1995. Nickel-copper sulphide mineralisation in Labrador: The Voisey's Bay discovery and its exploration implications; Current Research, Report 95-1, Geological Survey, Department of Natural Resources, Government of Newfoundland and Labrador, pp. 109-129.
Ryan, Bruce. 1995. Geology of the Voisey Bay area, Labrador; The Gangue, G.A.C. Mineral Deposits Newsletter, Issue 48, pp. 12-14.