Re–Os isotopic systematics of the Voisey's Bay Ni–Cu–Co magmatic ore system, Labrador, Canada

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Re and Os concentrations and Os isotopic compositions have been obtained for massive, matrix, and disseminated sulphide ores from three environments within the Voisey's Bay intrusion (the `Ovoid', Eastern Deeps, and Discovery Hill Zone) in order to assess the role of crustal contamination in the genesis of this large Cu–Ni–Co deposit. These samples have high Re concentrations (148 to 288 ppb, in 100% sulphide) for their common Os concentrations (4.8 to 24 ppb, in 100% sulphide), yielding high Re/Os ratios (12 to 33). These data confirm that the magma parental to the Voisey's Bay ore system was broadly basaltic in major element chemistry rather than picritic, consistent with the low Ni/Cu ratio of the ores (∼1.5). Re–Os isotopic data exhibit a limited spread in 187 Re/ 188 Os (57 to 157) and define an imprecise 1323±135 Ma `model 3' isochron, likely the result of small R-factor variations within the ore system. The Re–Os isochron age is within error of 1334 Ma U–Pb ages obtained for baddeleyite from the ore-bearing troctolites, demonstrating that whole rock Re–Os isotopic systematics have remained closed since crystallisation. The initial Os isotopic composition of the isochron ( γ Os=1040±200) implies significant magma interactions with radiogenic Os that most likely resides in the Nain-Churchill Province crust. These data are, therefore, consistent with the parental magma achieving sulphide saturation as a result of contamination by radiogenic crustal components, with further addition of base and precious metals as a function of R-factor. Analyses of sulphide separates from the Proterozoic Tasiuyak (Churchill) and Archaean Nain gneisses confirm that both units contained significant Os (8.6 ppb and 0.38 ppb, respectively) that was very radiogenic at 1334 Ma ( γ Os=1908 and 5202, respectively), yielding crustal residence T CHUR model ages of 2200 to 2400 Ma. However, these model ages may have been affected by Re and/or Os mobility during the 1.85 Ga Torngat and 1.1 Ga Grenville orogeny. Re–Os isotopic modelling suggests that a reasonable fit to the ore data may be obtained via bulk crustal contamination of magmas parental to the Nain Plutonic Suite (NPS) with either Tasiuyak (2% contamination) or Nain (16% contamination) gneiss, followed by an R-factor process (200 to 1000) which improved the tenor of the sulphide liquid during transport in active magma conduits or after deposition in an active/replenished Voisey's Bay intrusion magma chamber. However, the dominance of plagioclase-rich (high Al 2O 3) magmas in the mafic members of the NPS (troctolites and anorthosites) may indicate that other (deeper) forms of lithospheric interaction, potentially involving mafic lower crust, may be crucial to understanding this style of magmatic sulphide ore system.