Physical and geometric controls on the distribution of magmatic and sulphide bearing phases within the Voisey's Bay nickel-copper-cobalt deposit, Voisey's Bay, Labrador

Evans-Lamswood, Dawn (1999) Physical and geometric controls on the distribution of magmatic and sulphide bearing phases within the Voisey's Bay nickel-copper-cobalt deposit, Voisey's Bay, Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Voisey's Bay Ni-Cu-Co deposit occurs within fragment-bearing troctolites and olivine gabbros of the 1.34 Ga. Reid Brook Intrusive Complex, an early member of the predominantly anorthositic Nain Plutonic Suite (NPS). The NPS straddles the 1.85 Ga. Cratonic suture between Archean orthogneiss and the Nain Province to the east and Paleoproterozoic paragneiss of the Churchill Province to the west. -- The Reid Brook, Discovery Hill, Mini-Ovoid and Ovoid are mineralized zones within a sub-vertical conduit system, the Ovoid dyke. The Reid Brook zone is the most western mineralized zone of the Voisey's Bay deposit, it contains disseminated and massive sulphide hosted within the dyke and veins in the adjacent country rocks. Mineralization of the Discovery Hill zone, unlike the Reid Brook zone to the west, is confined to the dyke and does not penetrate the country rocks. The Mini-Ovoid is west, but is geologically continuous with the Ovoid. Mineralization consists of massive to semi massive and disseminated sulphides. The Ovoid is a cauldron-shaped body of massive to semi massive sulphides with less extensive parcels of disseminated sulphides. -- Striking east-west, the Ovoid dyke appears to post-date, but physically link two large troctolite intrusions, the Eastern Deeps chamber and the Western Deeps chamber. In the west, the conduit extends from the top of the Western Deeps chamber to the north margin of the Eastern Deeps chamber. Rather than being within the sub-vertical Ovoid dyke, mineralization of the Eastern Deeps zone is located within a sub-horizontal splay from the main conduit where it is connected to the base of the Eastern Deeps chamber. -- All the mineralized zones in the Voisey's Bay deposit comprise magmatic-textured sulphides within fragment-bearing troctolites and olivine gabbros. Sulphides within the system are preferentially concentrated in physical traps where topographic irregularities and variations in conduit morphology favour the capture, containment and precipitation of sulphides through physiochemical changes in the magma. For example, the Eastern Deeps sulphide zone is associated with the line of entry for a feeder conduit close to a structural low in the base of the Eastern Deeps chamber; the Ovoid mineralized appears to fill a bulge in the conduit, and the Reid Brook zone is located close to an axis of inflection along the strike of the conduit. Sulphide deposition is therefore, is ultimately related to a complex interplay of dyke geometry (i.e. changes in dyke trajectory and thickness) and the fluid dynamics of the fragment-bearing magma in a constantly changing and dynamic magma conduit system. -- Dyke geometry is a direct reflection of structural controls. Primary emplacement of the Ovoid dyke and gabbro-troctolite chambers was controlled by east-west lineaments which were established during the Torngat orogeny in response to the Nain/Churchill collision. Syn-magmatic crustal uplift exploited these pre-existing structures, producing local extension in a graben-like system. The extensional faults that resulted provided a media for which magma could easily intrude and be constrained. With the tectonomagmatic evolution of the system, a sub-horizontal fracture system developed, providing an alternative passage for magmatic transgression, producing the Eastern Deeps feeder. Post enlacement deformation invoked oblique stresses on the pre-existing east-west lineaments and extensional faults, resulting in sinistral translation and the development of Reidel brittle shears. -- The Voisey's Bay deposit does not conform to traditional models, where sulphide accumulations are controlled largely by gravitational settling within a magmatic chamber. In contrast, sulphide distribution is controlled by magma emplacement through multiple braids in a dynamic channel-like conduit system.

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