The petrology, mineralization and regional context of the Thunder mafic to ultramafic intrusion, Midcontinent Rift, Thunder Bay, Ontario

Abstract

The 1108 Ma Thunder mafic to ultramafic intrusion is a small, 800 x 100 x 500 m, Cu- PGE (platinum group element) mineralized body, located on the outskirts of Thunder Bay, Ontario, and associated with the early magmatic stages of the Midcontinent Rift (MCR) based on geochemical similarities to mafic and ultramafic rocks of the Nipigon Embayment and an unpublished 207Pb/206Pb zircon age of 1108.0 ± 1.0 Ma. The Thunder intrusion is similar to the other known mineralized early-rift MCR intrusions; however, it is the only known occurrence hosted by the metavolcanic and metasedimentary assemblages of the Archean Shebandowan greenstone belt. Major textural and geochemical differences can be used to subdivide the intrusion into a lower mafic to ultramafic basal unit and an upper gabbroic unit; the similar trace and rare earth element ratios of the two units suggest a single magmatic pulse that has undergone subsequent fractional crystallization and related cumulate phase layering. The estimated parental composition of the Thunder intrusion has a mg# (MgO/(MgO + 0.9*FeOT)) of 57 which represents a more evolved magma than other early-rift mafic to ultramafic intrusions. This may indicate the involvement of multiple staging chambers during the accent of the parent magma. Trace and rare earth element patterns are consistent with an ocean island basalt source but with indications of contamination such as high Th concentrations and the presence of a negative Nb anomaly. The ԐNdT values from the intrusion range between - 0.7 and + 1.0, with no trends indicative of wall rock participation, whereas the 87Sr/86Sri ratios range from 0.70288 to 0.70611 and trend towards wall rock values of 0.70712 and 0.70873. The weak correlation between ԐNdT and 87Sr/86Sri is also a feature of the Nipigon Sills where it has been interpreted to be due to shallow-level crustal contamination whereas plots of MgO and SiO2 versus ԐNdT indicate contamination at depth by an older crustal source. Ni-Cu-PGE sulphide mineralization (20 m of 0.22 wt. % Cu, 0.06 wt. % Ni, 0.25 ppm Pt and 0.29 ppm Pd) is hosted by feldspathic peridotite in the lower mafic to ultramafic unit adjacent to the footwall of the Thunder intrusion. Sulphides typically occur from 1 - 5 modal %, rarely up to 30 modal %, with textures ranging from medium- to fine-grained disseminated, globular and rarely net-textured. Pyrrhotite, chalcopyrite and rare pentlandite with common secondary marcasite-pyrite replacement occur along with trace michenerite, kotulskite, merenskyite, sperrylite, hessite, electrum and argentian pentlandite. Whole rock geochemical data displays fractionated Ni-Cu-PGE patterns with depletion of iridium-platinum group elements (IPGE) relative to platinum-platinum group elements (PPGE). Sulphide δ34S values from the Thunder intrusion range from -2.0 to +3.8 ‰ and are similar to values for the metavolcanic host rock that range from -3.1 to +2.3 ‰. Two samples from the basal mineralization zone sulphides yield Δ33S values of 0.066 and 0.122 ‰ and one sample from the metavolcanic wall rock yields 0.149 ‰. The δ34S and Δ33S values for the Thunder intrusion fall within range of typical upper mantle. The sulphur source appears to be of mantle origin, however, the assimilation of crustal sulphur is a possibility but hard to distinguish as the wall rock S isotope and S/SeT signature is similar to that of upper mantle.