The Nain-Churchill Boundary and the Nain Plutonic Suite: A Regional Perspective on the Geologic Setting of the Voisey’s Bay Ni-Cu-Co Deposit

Abstract

Northern Labrador is the site of two major lithotectonic elements of eastern Canadian Shield geology. First, it encompasses a Paleoproterozoic continental suture zone: Archean rocks of the Nain province tectonically juxtaposed against Paleoproterozoic and reworked Archean rocks of the Churchill province. The Nain-Churchill suture evolved between 1860 and 1740 Ma, impressing upon the Archean and Paleoproterozoic rocks a regional metamorphic and tectonic signature referred to as the Torngat orogen. An important component of the Torngat orogen is the Tasiuyak gneiss, a Paleoproterozoic metasedimentary unit that occupies most of the eastern margin of the Churchill province. Second, northern Labrador is the site of a world-renowned terrane of Mesoproterozoic anorogenic igneous rocks, the Nain Plutonic Suite, that sits astride the suture and acts as a stitching batholith between the Nain and Churchill provinces. The Nain Plutonic Suite, comprising amalgamated granitic, anorthositic, dioritic, and troctolitic intrusions, largely obscures the suture for nearly half of its total length. The Suite evolved between 1350 and 1290 Ma and is interpreted as a product of magmatism associated with limited intracontinental rifting of the conjoined Nain-Churchill crust above a mantle plume. The Voisey’s Bay Ni-Cu-Co deposit, serendipitously discovered in 1993, resides within one of the oldest troctolitic intrusions of the Nain Plutonic Suite, a 1334 Ma intrusion that transgresses a contact between Archean and Paleoproterozoic gneisses. Genetic controls on the setting of mineralization have been built upon the Nain-Churchill boundary as a zone of weakness that was exploited by mantle magmas and have implicated the Tasiuyak gneiss as a crustal sulfur source. Although the Archean-Paleoproterozoic junction at Voisey’s Bay is presently interpreted as representing the Nain-Churchill boundary, in reality the provincial affinity of the Archean gneisses is equivocal. This uncertainty dictates caution in assigning the Nain-Churchill boundary as the convenient transcrustal conduit that allowed the Voisey’s Bay mafic intrusions rapid transit from their mantle staging area. Modeling the sulfide deposit as being solely a result of sulfur saturation of mafic magma by proximal Paleoproterozoic Tasiuyak paragneisses is also questionable because other genetic processes may have been responsible; for example, desilication of gneiss is strongly indicated by aluminous refractory minerals in vestiges of (fused?) Tasiuyak gneiss. The spatial distribution of the mineralized rock at Voisey’s Bay, apparently within the subchamber plumbing system feeding a larger intrusion and within remnants of the larger intrusion itself, may serve as a paradigm for similar deposits in the Nain area, but the preerosional comparative morphology of that larger intrusion relative to the better preserved ones may never be known. A thorough understanding of the setting and genesis of the Voisey’s Bay deposit will better facilitate establishing exploration targets in other parts of the Nain region, in Labrador as a whole, and in other geologically similar regions of the world.