Petrogenetic relationship of the variably Ni−Cu−platinum-group element (PGE)-mineralized intrusions of the Thunder Bay North Intrusive Complex, Midcontinent Rift, Canada: Implications for mineral exploration in mafic−ultramafic complexes

The Thunder Bay North Intrusive Complex (TBNIC) of Canada comprises six variably Ni−Cu−platinum-group element (PGE)-mineralized mafic−ultramafic intrusions that intruded the Archean Quetico Subprovince. These are the Current, Escape, Lone Island, Greenwich, and 025 intrusions, with the former three aligned along an east−west-trending intrusion termed the East−West Corridor. These intrusions have a consistent igneous stratigraphy comprising, from bottom to top, peridotite, gabbros, and quartz gabbro. Only the Current and Escape intrusions are known to host sulfide mineralization in the peridotite. Despite the economic significance of the TBNIC, little work has been done to characterize its petrogenetic evolution and the causes of the apparent variable fertility. Our new ages for Escape (1107.6 ± 0.9 Ma) and Greenwich (1105.7 ± 0.9 Ma), combined with previously determined ages for Current (1106.6 ± 1.6 Ma) and Lone Island (1106.3 ± 2.1 Ma), demonstrate that the TBNIC was emplaced as part of the ca. 1.1 Ga Midcontinent Rift event. The similarities in their rare earth element geochemistry and consistently low Th/La−Sr_i values indicate that the intrusions crystallized from genetically related magmas that did not assimilate significant amounts of crustal material. Accordingly, their systematically negative εNd_t values and Nb anomalies likely reflect that their parent magmas were largely sourced from the subdcontinental lithospheric mantle and not the Keweenaw Plume. Given their genetic relationship, the variable fertility of the intrusions cannot be related to processes that occurred in their mantle sources. Since the gabbros have systematically depleted Cu/Zr−Cu/Pd ratios and the peridotites have enriched ratios, the apparent variable fertility is likely related to the timing and location of sulfide segregation and accumulation, with the barren gabbros preserving magmas that lost sulfide liquid during transport to shallow crustal levels, and the mineralized peridotites recording accumulation of sulfide liquid. This interpretation has important implications for exploration as it implies that sulfide mineralization in the apparently barren intrusions may be located at depth in undiscovered ultramafic cumulates.