Minéralisations en Ni-Cu-(EGP) dans la Province de Grenville : État des connaissances et nouvelles perspectives

Abstract

High-grade metamorphic terrains, such as the Grenville Province, are not traditionally known for their strong potential for Ni-Cu-(PGE) deposits, hence the muted interest over the years for this type of exploration target in the Grenville. However, prospective mafic and ultramafic units for this type of mineralization are abundant in this geological province and a number of them host Ni-Cu-(PGE) showings. Two deposits were mined in the 1970s: the Lac Édouard mine (69 kt at 1.5% Ni, 0.5% Cu: 1973- 1974) and the Lac Renzy mine (717 kt at 0.7% Ni, 0.7% Cu: 1969-1972). In the Grenville, mafic and ultramafic rocks generally form: 1) mafic intrusions (mafic >>> ultramafic; e.g., Lac Volant), mafic to ultramafic intrusions (mafic ~ ultramafic; e.g., Réservoir) and, more rarely, ultramafic intrusions (mafic <<< ultramafic; e.g., Lac Édouard and Lac Renzy); and 2) intrusions associated with the border facies around anorthosite-mangerite-charnockite-granite (AMCG) complexes. The majority of Ni-Cu-(PGE) mineralization is associated with the first type of intrusions (e.g., Lac Édouard, Lac Renzy, and River Valley). The second type, associated with AMCG massifs, have lower potential for Ni-Cu-(PGE) mineralization (e.g., McNickel) but strong potential for Fe-Ti-V deposits. In both cases, most occurrences of Ni-Cu-(PGE) mineralization are in the form of disseminated sulphides and, locally, semi-massive to massive sulphides (e.g., Lac Édouard, Lac Renzy, and Lac Volant). Type 1 intrusions were emplaced during the Mesoproterozoic as plutons, sills or dykes in volcano- sedimentary and volcano-plutonic sequences related to arc-back-arc systems (e.g., Portneuf-Mauricie domain, Lac Renzy, Raglan Hills, and Hart-Jaune terrane) or extensional tectonic environments (e.g., Lac Volant). More rarely, these intrusions emplaced during the Paleoproterozoic and are associated with episodes of rifting (e.g., River Valley). Type 1 intrusions are generally layered, zoned or devoid of internal structure, but all these types seem to have crystallized from fairly primitive mafic parental magmas. Type 2 intrusions were emplaced during the Mesoproterozoic, probably during extensional episodes in an overall convergent tectonic regime, and are associated with mafic parental magmas. They also occur as plutons, sills or dykes with no internal structure or some magmatic layering. The Grenville Province also contains several epigenetic Cu-Ni showings (e.g., 2EZ) that formed as a result of the deformation and metamorphism accompanying the various orogenic phases that affected the province. Nevertheless, these occurrences are generally minor and of limited economic interest. The degree of deformation and metamorphism associated with high-grade metamorphic terrains like the Grenville certainly poses a challenge for Ni-Cu-(PGE) exploration, but the abundance of units that could potentially host such mineralization and the existence of known occurrences suggest that nickel, copper, and PGE resources remain undiscovered in the Grenville.