Magmatic processes associated with the Voisey's Bay Ni-Cu-Co sulphide deposit, Labrador, Canada

Abstract

The Voisey's Bay deposit is considered anomalous within the terms of World-Class magmatic Ni sulphide deposits as it is hosted by low-olivine, low-forsterite troctolites within an anorthosite complex - a combination not previously considered a viable prospect for economic Ni-sulphide mineralisation. The accepted model for generation of a sulphide magma at Voisey's Bay has been contamination of the troctolite parental magma at mid-crustal levels by the sulphidic and graphitic Tasiuyak gneiss. Trace element and mineralogical data presented here suggest that sulphide immiscibility is the result of contamination of the Voisey's Bay magmas by Nain gneiss, probably at the base of the crust. Olivine geochemistry suggests that the source of the Ni at Voisey's Bay is a now departed high-Mg# melt with forsterite >F080. Trace element evidence implies that this melt was plume-like and derived from 7-8 percent partial melting at gamet/spinel facies. The melting temperature was ~1530°C, the pressure was ~3.25 GPa, the ƒ02 ~QFM. Pb isotope, trace element, olivine and sulphide geochemical data imply that there was a sulphide liquid present before the parental magmas encountered the Tasiuyak gneiss. Although the Tasiuyak gneiss has contaminated the Voisey's Bay silicate magmas, the extent of contamination is small and coincidental. The Voisey's Bay intrusion sulphides and silicates exhibit different Pb isotope ratios, this disequilibrium is explained in terms of immiscibility, density contrasts and contamination models. Data presented in this thesis allow the relative timing of the main events in the forming of the Voisey's Bay deposit to be constrained and for a new model of deposit genesis to be proposed. This new model provides new criteria for exploring within anorthosite complexes for another Voisey's Bay- type magmatic Ni-sulphide deposit.