The evolution of troctolitic and high Al basaltic magmas in Proterozoic anorthosite plutonic suites and implications for the Voisey's Bay massive Ni-Cu sulfide deposit

The Voisey's Bay massive Ni-Cu sulfide deposit is contained within troctoli tic (plagioclase + olivine) intrusions of the 1.34 to 1.29 Ca Nain Plutonic Suite, coastal Labrador, Canada, and represents a new setting for magmatic sulfide deposits. Proterozoic anorthosite plutonic suites consist of vast quantities las 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. with in the Middle Proterozoic (2.12-0.92 Ga). Many Proterozoic anorthosite plut onic suites were emplaced into relatively young Proterozoic crust and some were intruded along Archean-Proterozoic boundaries (e.g., Nain, Labrador; L aramie, Wyoming; Korosten, Ukraine). Nearly all of the largest plutonic sui tes contain significant areas (thousands of km(2)) of plagioclase-rich cumu late rocks that are olivine bearing (olivine anorthosite-leucotroctolite). Troctolitic rocks (20-30% modal olivine) occur in Proterozoic anorthosite p lutonic 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 troctol itic rocks crystallized late in the evolution of individual plutonic suites . Many of the troctolitic intrusions have fine-grained olivine-rich margina l rocks that closely represent parental magma compositions. Numerous fine-g rained olivine-normative dikes with high Al basaltic compositions also occu r within many plutonic suites (Nain, Harp Lake, Laramie, Adirondack) and sh are compositional characteristics with the troctolitic magmas. These olivin e-normative high Al basaltic mag mas record the least evolved compositions in Proterozoic anorthosite plutonic suites (Mg# = 0.43-0.64; An = 45-60), h ave high Al2O3 (15-19 wt %) and FeOtotal (9-15 wt %) contents, and typicall y show the least contaminated Sr and Nd isotope compositions in a given plu tonic suite. Many dikes and marginal rocks have strong positive Eu anomalie s, a feature of the melt composition that is not related to plagioclase acc umulation. Except at Laramie, the least evolved troctolitic magmas appear t o have about 8 wt percent MgO and corresponding Ni and S concentrations bet ween 100 to 150 ppm and 0.02 to 0.05 wt percent, respectively. Based on clo sed system fractionation in the Kiglapait intrusion, sulfide saturation occ urs late in the crystallization history of troctolitic magmas, after Ni con centrations have been severely depleted by protracted olivine fractionation . The optimal conditions for formation of a Ni-Cu sulfide deposit associate d with a Proterozoic anorthosite plutonic suite require a relatively high M gO parental magma that has not crystallized much prior olivine and an exter nal source of S to promote early sulfide saturation, probably through desul furization reactions as hot troctolitic magmas ascend through the crust. An economically important massive Ni-Cu sulfide deposit on the scale of Voise y's Bay clearly requires large volumes of magma, probably involving continu ous replenishment of relatively Ni rich, unfractionated parental magma in s taging chambers or mixing between magmas in conduits during ascent. A model for the formation of troctolite-associated massive Ni-Cu sulfide deposits is proposed. Depending on the relative degrees of sulfide saturation and fr actionation of the mixed magmas, mixing between troctolitic magmas in a con duit or magma chamber may either poison the system, stopping sulfide segreg ation, ol enhance sulfide segregation.