Magmatic and hydrothermal evolution of the Mesoproterozoic Current PGE-Cu-Ni deposit within the Thunder Bay North Intrusive Complex: Insights from trace element geochemistry and Nd, Sr, O, and H isotopes

Abstract

The Mesoproterozoic Current Intrusion, part of the Thunder Bay North Intrusive Complex, hosts PGE-Cu-Ni mineralization (indicated minerals resource of 8.2 million tonnes grading 0.33% Cu, 0.22% Ni, and 2.7g/t Pt+Pd and an inferred mineral resource of 1.6 million tonnes grading 0.32% Cu, 0.20% Ni, and 1.7g/t Pt+Pd). The intrusion consists of a northwest-trending conduit-type body, consisting of wehrlite, lherzolite, olivine gabbronorite ± troctolite, associated with the earliest stages of the Midcontinent Rift System (MRS) and the Keweenaw plume that intruded Archean rocks. The intrusion is associated with multiple splays related to the Quetico fault that cross the boundaries between the Quetico basin and the Wabigoon terrane of the Superior Province. The intrusion hosts four mineralized zones: the Current and Bridge Zone in the northwest, the Beaver-Cloud Zone in the middle, and the 437-Southeast Anomaly Zone in the southeast. Geochemical analysis reveals that the intrusion displays a well-defined primitive mantle-normalized pattern resembling Oceanic Island Basalt and a lack of continental crust contamination given the slightly positive Nb, La, and Ce anomalies relative to Th. The La/Smn values for the intrusion (from 1.8 to 2.6) indicate a mafic magma was derived from an enriched mantle plume; this is consistent with other intrusions associated with the MRS, including the Nipigon Sills and the Lone Island, the Escape, and the Seagull intrusions. The Sri (from 0.7021 to 0.7043) and εNd (from -1.18 to -4.02) values of the Current Intrusion are slightly lower than those typical values of the mantle source at 1100 Ma. Given the absence of geochemical anomalies that indicate assimilation of the Archean crust, an enriched Subcontitnetal Lithospheric Mantle (SCLM) is suggested to have interacted with the parental magma to generate the slightly negative radiogenic isotope values. Stable isotope analysis suggests that the rocks of the intrusion underwent interactions with mantle-derived fluids (δ2H from −40 to −80‰, δ18O from 5.5 to 7.0‰), meteoric fluids (δ2H <-80‰, δ18O <5.5‰), and crustal-derived fluids (δ18O >7‰). Analysis of the hydrothermal phases reveals three distinct mineral assemblages (domains) that altered the intrusive rocks. i) Domain A, influenced by the interaction of mantle-derived and meteoric fluids, formed antigorite, magnetite ± actinolite-tremolite after olivine, actinolite-tremolite and clinochlore after pyroxene, epidote-sericite and clinochlore after plagioclase, pyrite and magnetite after pyrrhotite, magnetite after pentlandite, secondary pyrrhotite after pyrite, chamosite and pyrite after magnetite. ii) Domain B consists of lizardite-chrysotile and an increase in mineral modal abundances of clinochlore, chamosite, epidote, sericite, pyrite, and secondary magnetite relative to Domain A. Stable isotopes in this domain indicate interaction with mixed mantle-derived and meteoric fluids. iii) Domain C, characterized by talc and carbonate minerals that replace domains A and B, with stable isotopes, indicating crustal-derived fluids. In summary, this study indicates the Current Intrusion was derived from magmas that originated from an enriched mantle plume that interacted with an enriched SCLM as it ascended towards the crust. The documented alteration types resulted from fluid:rock interaction between the host intrusion and fluids derived from mantle-derived, crustal, and meteoric sources. These fluids also generated the mineral volume reduction of primary sulfides.