Geology and petrology of the Voisey's Bay intrusion: reaction of olivine with sulfide and silicate liquids

Abstract

The Voisey's Bay Ni–Cu–Co sulfide deposit is located in Eastern Labrador, and transects the 1.85 Ga collisional boundary between interbanded garnet–sillimanite and quartzo-feldspathic paragneisses (Tasuiyak gneisses) of the Proterozoic Churchill Province to the west and granitic, intermediate and mafic orthogneisses of the Archean Nain Province to the east. The deposit is associated with an upper troctolitic chamber (the Eastern Deeps chamber) which is connected to a lower troctolitic chamber (the Reid Brook chamber) by a gabbroic/troctolitic feeder sheet approximately 1 km in vertical extent. Rock sequences that have been defined to date include Leuco-troctolite (LUT) in the Reid Brook chamber, Feeder Breccia (FB) in the Reid Brook Zone, Feeder Olivine Gabbro (FOG), Leopard Troctolite (LT) and Basal Breccia Sequence (BBS) in the feeder sheet, and Olivine Gabbro (OG), Normal Troctolite (NT) and Varied-textured Troctolite (VT) in the Eastern Deeps chamber. Mineralisation has been found in the feeder sheet, as massive stringers in gneisses flanking the feeder, in the `Ovoid', a 600×300×110 m 3 basin of massive sulfide at surface, and as a zone of massive sulfide that has developed close to the entry of the feeder to the Eastern Deeps chamber. The mineralisation comprises pyrrhotite (both hexagonal and troilite in varying proportions), pentlandite, chalcopyrite, magnetite and minor cubanite. An important component of all mineralized zones is a breccia of gneissic fragments in a troctolitic to noritic matrix referred to as BBS or FB. Considerable reaction has occurred between the troctolite magma and the gneiss inclusions. Olivines in the LUT, OG and FOG have low Ni contents relative to their forsterite (Fo) contents. Olivines in the NT and VT have higher Ni contents but show a great scatter in both their Ni and Fo contents. The Ni content of olivine in the LT increases systematically with decreasing Fo content. All of these variations are explained in terms of fractional crystallisation of magma, sulfide segregation, trapped liquid shift and re-equilibration between sulfide and olivine. Our current understanding of the development of the deposit is that an initial pulse of magma rose to a lower chamber, fractionated to form mafic-ultramafic cumulates and at the same time became sulfide-saturated as a result of interacting with the enclosing, sulfide- and graphite-bearing Tasiuyak gneisses. The magma lost much of its Ni and Cu as it fractionated olivine and segregated sulfides. A new wave of fresh magma entered the lower chamber, forcing the chalcophile-depleted magma, along with some of its contained sulfide, up the feeder to spread out in the upper chamber. Remnants of this chalcophile-depleted magma are preserved as olivine gabbro which forms upper layers in the upper chamber, and which in places has frozen to the walls of the feeder. Sulfides that were swept up in this event were forced out into fractures in the feeder walls, are preserved within thickened zones within the feeder, and were deposited as the magma flow decreased at the entry of the feeder to the upper chamber. Continued influx of the later, undepleted magma resulted in sulfides being picked up, transported farther up the system, upgraded during transportation and then redeposited.