THE 1997-HOWARD-STREET-ROBINSON-LECTURE - THE MESOPROTEROZOIC NAIN PLUTONIC SUITE IN EASTERN CANADA, AND THE SETTING OF THE VOISEYS BAY NI-CU-CO SULFIDE DEPOSIT

Northern Labrador is home to one of the world's classic anorogenic mag matic terranes, the Mesoproterozoic Nain Plutonic Suite (NPS), compris ing a broad spectrum of coalesced basic and silicic intrusions emplace d ca. 1.35-1.29 Ga astride a Paleoproterozoic continental suture zone. Tectonic models for the development of the NPS relate it to processes associated with a mantle plume impinging on the base of the crust. Th e NPS is largely a bimodal igneous terrane, of which the predominant r ocks belong to the anorthositic and granitic components. Troctolitic a nd iron-rich dioritic rocks are subordinate. The Held Brook intrusion, the oldest-recognized troctolitic pluton within the NPS, hosts a majo r ore deposit of magmatic Ni-Cu-Co sulphide minerals that was discover ed north of Voisey's Bay in 1993. The ore is concentrated in three mai n settings: in a troctolitic dyke (Discovery Hill and the Western Exte nsion), in a bowl-shaped zone (the Ovoid, largely massive sulphide) re sting on gneiss, and at the lower levels of a massive troctolite body (the Eastern Deeps). One model for the geometry and distribution of th e Voisey's Bay deposit can be constructed by application of physical a nd chemical processes that would accompany the ascent and deposition o f a sulphide-saturated, mantle-derived silicate magma into a mid- to u pper-crustal dilational chamber. The silicate magma may have liberated a separate immiscible sulphide magma because of sulphide saturation p romoted by interaction with Paleoproterozoic sulphur-bearing paragneis ses at a deeper level, or the sulphide liquid may be totally of juveni le mantle extraction. The origin of the sulphide ore, the complete dis tribution of the ore, and the three dimensional shape of the hosting t roctolitic intrusion are subjects of active study, and no doubt there will be revisions to the present working model.There is also no doubt - when all private sector, government, and aboriginal concerns about t he Voisey's Bay mine development are addressed - that the exploitation of this major world-class nickel-copper-cobait resource will accrue e conomic benefits to the province of Newfoundland and Labrador, as well as the rest of Canada, for decades to come.