WHY ARE THERE NO MAJOR Ni-Cu SULFIDE DEPOSITS IN LARGE LAYERED MAFIC-ULTRAMAFIC INTRUSIONS

Large, layered intrusions throughout the world are characterized by a remarkable paucity in economically important magmatic Ni-Cu sulfide deposits. The Sudbury intrusion in Canada is a notable exception, but sulfide ores there were formed by a unique combination of processes involving crustal melting in response to a meteorite impact. Economically important magmatic Ni-Cu sulfide deposits tend to occur in magma conduit systems. Such an environment has been documented at Noril'sk, Jinchuan, and the recently discovered Uitkomst deposit of South Africa and the Voisey's Bay intrusion of Labrador, Canada, which are associated with the Bushveld Complex and the Nain Plutonic Suite, respectively. The enhanced ore potential of conduit systems is attributed to their specific flow environment and to the exploitation of the conduits by multiple flows of magma. Sulfides that were entrained and transported by ascending silicate magma were concentrated in the widened parts of the: conduits and, at Voisey's Bay, near the exit of the conduit into a larger chamber, owing to a decrease in the flow velocity of the magma. Further, the precipitated sulfides were upgraded by means of reaction with continued surges of undepleted magma using the same conduit. Thus, exploration for magmatic Ni-Cu sulfide ores should be redirected from layered intrusions to their associated conduit systems.