Nickeliferous gabbroic intrusions of the Pants Lake area, Labrador, Canada: Implications for the development of magmatic sulfides in mafic systems

Abstract

The Pants Lake intrusions are located some 80 kilometers south of the world-class Voisey's Bay Ni-Cu-Co sulfide deposit in Labrador, Canada. They are dominated by olivine gabbro, with subordinate troctolite, melagabbro, peridotite and leucogabbro. They form several sheet-like bodies that were emplaced into metasedimentary gneisses that locally contain sulfides. The largest of these, termed the North and South intrusions, have been dated by other workers at 1322 ′ 2 Ma and 1337 ′ 2 Ma respectively, indicating that they formed in two discrete events. The North intrusion is more varied petrologically, and its three units show ambiguous contact relationships implying that they partly coexisted as liquids. Disseminated sulfide mineralization is widespread near the basal contacts of both intrusions, but massive sulfide mineralization is rare. In the older South intrusion, sulfides are hosted by melagabbro and peridotite of cumulate origin, and probably represent a gravitational accumulation. In the younger North intrusion, they are hosted by a complex mineralized sequence, interpreted to represent two or more influxes of magma charged with sulfides and reacted country-rock fragments. The older South intrusion is geochemically distinct from the younger North intrusion, and more closely resembles the Voisey's Bay Intrusion. Mineralized rocks within both Pants Lake intrusions are geochemically similar to the associated unmineralized rocks, and must be closer related to them. The mafic rocks almost all have low Ni contents and low Cu/Zr ratios that imply extraction of metals by sulfide liquids. This pervasive depletion signature, coupled with consistent Ni/Cu and Ni/Co, and clustering of sulfide metal contents, suggests that sulfide liquids were developed on an intrusion-wide scale, probably at depth, rather than by local processes. Sulfides at Pants Lake typically contain less than 2 percent Ni and 2 percent Cu. These values agree with predictions from the low Ni contents of silicate rocks, and imply a low mass ratio (R factor) of silicate magma to sulfide liquid (R 1000). The pervasive Ni and Cu depletion contrasts with the more localized metal depletion seen at Voisey's Bay. This may indicate that the Pants Lake sulfide liquids were not as effectively upgraded by later batches of undepleted magma. From the perspective of metallogenesis, data from the Pants Lake intrusions support several key concepts proposed m models for the formation of the Voisey's Bay deposit, suggesting that these may apply more widely as controls on the formation of magmatic sulfide deposits in broadly gabbroic magma systems.