| Summary: | The Voisey's Bay massive Ni-Cu sulfide deposit is contained within troctolitic (plagioclase + olivine) intrusions of the 1.34 to 1.29 Ga Nain Plutonic Suite, coastal Labrador, Canada, and represents a new setting for magmatic sulfide deposits. Proterozoic anorthosite plutonic suites consist of vast quantities (as much as 20,000 km 2) of cumulate anorthosite-leucotroctolite-leuconorite, intrusions of troctolite and Fe-enriched dioritic rocks, and large granitic batholiths. Crystallization ages span nearly 1,200 m.y. within the Middle Proterozoic (2.12-0.92 Ga). Many Proterozoic anorthosite plutonic suites were emplaced into relatively young Proterozoic crust and some were intruded along Archean-Proterozoic boundaries (e.g. Nain, Labrador; Laramie, Wyoming; Korosten, Ukraine). Nearly all of the largest plutonic suites contain significant areas (thousands of km 2) of plagioclase-rich cumulate rocks that are olivine bearing (olivine anorthosite-leucotroctolite). Troctolitic rocks (20-30% modal olivine) occur in Proterozoic anorthosite plutonic suites, principally as cumulates in layered or massive intrusions (e.g. Kiglapait, Hettasch, Jonathon-Nain) intruded into anorthositic rocks. With the notable exception of the Voisey's Bay deposit, nearly all troctolitic rocks crystallized late in the evolution of individual plutonic suites. Many of the troctolitic intrusions have fine-grained olivine-rich marginal rocks that closely represent parental magma compositions. Numerous fine-grained olivine-normative dikes with high Al basaltic compositions also occur within many plutonic suites (Nain, Harp Lake, Laramie, Adirondack) and share compositional characteristics with the troctolitic magmas. These olivine-normative high Al basaltic magmas record the least evolved compositions in Proterozoic anorthosite plutonic suites (Mg = 0.43-0.64; An = 45-60), have high Al 2O 3 (15-19 wt %) and FeO(total) (9-15 wt %) contents, and typically show the least contaminated Sr and Nd isotope compositions in a given plutonic suite. Many dikes ...
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