Physical controls associated with the distribution of sulfides in the Voisey's Bay Ni-Cu-Co deposit, Labrador

The Voisey's Bay Ni-Cu-Co sulfide deposits occur within troctolites and oli vine gabbros of the 1.34 Ca Voisey's Bay intrusion, formerly known as the R eid Brook Intrusive Complex. The Voisey's Bay intrusion is the oldest known member of the predominantly anorthositic Nain Plutonic Suite, which stradd les the ca. 1.85 Ca suture between Archean gneisses of the Nain province to the east and Paleoproterozoic rocks of the Churchill province to the west. The Reid Brook, Discovery Hill, Mini-Ovoid, and Ovoid zones are mineralized domains within a subvertical conduit dike system that appears to span betw een two large troctolite intrusions, the Eastern Deeps chamber and the lowe r Western Deeps chamber. The conduit trends east-west and extends from near the top of the Western Deeps chamber, at a depth of 700 m, toward the nort h margin of the Eastern Deeps chamber close to surface. All the mineralized zones in the Voisey's Bay deposits occur as magmatic-te xtured sulfides associated with fragment-bearing troctolites and olivine ga bbros related to the conduit dikes, rather than simply as basal accumulatio ns at the floor of the chambers. Sulfides within the system are preferentia lly concentrated in traps where physical irregularities and changes in cond uit morphology favor the precipitation, capture, and preservation of sulfid es as a result of changes in the velocity and viscosity of a magma. There i s a consistent relationship between different sulfide textures and the phys ical environment existing in the conduits where each of the mineralized zon es occurs. This suggests that sulfides were deposited in their present site s by consistent magmatic processes within the conduits but were locally inf luenced by the physical parameters existing at these sites. Later remobiliz ation or mass movement of sulfides would not generate the consistent textur al relationships observed. Therefore, sulfide deposition is ultimately rela ted to a complex interplay of dike geometry (i.e., changes in dike orientat ion and thickness) and the fluid dynamics in the fragment-bearing magma wit hin a conduit system. i.e., the Ovoid mineralization appears to fill a bulg e in the conduit, and the Reid Brook zone is located close to the axis of i nflection along its strike. The Eastern Deeps zone, although it does occur at the base of the Eastern Deeps chamber, is more specifically associated w ith the line of entry for the feeder conduit close to a structural low at t he base of the Eastern Deeps chamber. The Voisey's Bay deposit does not conform to the traditional model for magm atic sulfide generation such as Sudbury in which sulfides accumulate large ly by gravitational settling within a magmatic chamber, following attainmen t of sulfur saturation as a result of local assimilation. In fact current e vidence suggests, in the Eastern Deeps mineralized zone, that little gravit ational settling took place because a large proportion of sulfides occur as high Ni tenor minerals trapped interstitially in sulfides in coarse phases of the variable-textured troctolite. Furthermore, sulfide saturation and g eneration of magmas with varying sulfide contents clearly appear to have oc curred in chambers at a depth of several kilometers, rather than at the lev els currently exposed. In contrast to gravitational settling, sulfide distr ibution within the Voisey's Bay deposit is controlled by magma emplacement through multiple braids of a dynamic, channel-like conduit dike system.