Critical factors for the formation of a nickel–copper deposit in an evolved magma system: lessons from a comparison of the Pants Lake and Voisey's Bay sulfide occurrences in Labrador, Canada

The Voisey's Bay nickel–copper deposit and the Pants Lake sulfide occurrences are the most important mineral systems discovered to date within the Nain Plutonic Suite in northern Labrador. There are many intriguing similarities at both locations. Both are hosted by relatively small troctolite/gabbro bodies that intrude the sulfide-bearing paragneiss of the Churchill Province, and these intrusions contain inclusions of the paragneiss. Similar chemical reactions of the gneissic inclusions with the host magmas are observed at both locations. The reactions resulted in the addition of SiO₂, K₂O, Na₂O and sulfur to the magmas, and are responsible for sulfide-saturation and resultant segregation of immiscible sulfide liquids from the magmas. The initial sulfide liquids in both cases were relatively poor in metals, containing <2.5 wt% Ni and 2 wt% Cu. The sulfides at Pants Lake remained poor in metals because of a lack of subsequent interaction with new, chalcophile-undepleted magma. At Voisey's Bay, the initial sulfides segregated in a dynamic conduit, and were subsequently upgraded in metals to ∼6 wt% Ni and 3 wt% Cu by a new surge of undepleted magma using the same conduit. These sulfides were then concentrated to form large sulfide bodies in the wider parts of the conduit and its entry to an upper chamber in response to a sudden change of liquid velocity in these environments. This study confirms three of the most important factors for the formation of magmatic sulfide deposits in an evolved magma system: (1) contamination of magma with sulfide-bearing country rock to achieve sulfide saturation; (2) a dynamic magmatic system such as a magma conduit to transport large volume of sulfide liquid and to concentrate them in limited localities, and (3) upgrading of metals in the sulfide by new, chalcophile-undepleted magma. Received: 20 February 2000 / Accepted: 14 September 2000