The petrology, mineralization, and regional context of the Thunder mafic to ultramafic intrusion, Midcontinent Rift, Thunder Bay, Ontario
Authors: Trevisan B E , Hollings P , Ames D E , Rayner N M 
Published: June 2015

The 1108 Ma Thunder mafic to ultramafic intrusion is a small, 800 x 100 x 500 m, Cu-PGE mineralized body, located on the outskirts of Thunder Bay, Ontario. It is associated with the early magmatic stages of the Midcontinent Rift based on geochemical similarities
to mafic and ultramafic rocks of the Nipigon Embayment and a 207Pb/206Pb zircon age of 1108.0 ± 1.0 Ma. The Thunder intrusion is similar to other known mineralized early-rift Midcontinent Rift intrusions; however, it is the only known occurrence of the Midcontinent Rift hosted in an Archean
greenstone belt (Shebandowan). Major textural and geochemical differences can be used to subdivide the intrusion into a lower mafic to ultramafic 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+FeOTot), mole%) of 57, which represents a more evolved magma than other early-rift mafic to ultramafic intrusions and may indicate multiple
staging chambers during the ascent of the parent magma.
Trace and rare earth element patterns are consistent with a mantle plume ocean island basalt-like source but with high-Th concentrations and a negative Nb anomaly. The ?Ndt values of the intrusion range between -0.7 and +1.0, with no trends indicative of progressive wall-rock contamination, whereas
the 87Sr/86Sri ratios range from 0.70288 to 0.70611 and trend towards wall-rock values of between 0.70712 and 0.70873. The radiogenic Sm-Nd and Rb-Sr isotope signature is similar to the contamination trends of the Nipigon sills, which is interpreted to represent contamination by
shallow-basin-filling sedimentary rocks. 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 rock of the Thunder intrusion. Sulphides typically
comprise 1 to 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 together with trace michenerite,
kotulskite, merenskyite, sperrylite, hessite, electrum, and argentian pentlandite. Wholerock geochemical data display fractionated Ni-Cu-PGE patterns with depletion of iridium subgroup relative to the platinum subgroup.
Sulphide d34S values from the Thunder intrusion range from -2.0 to +3.8 per mil and are similar to values for the metavolcanic host rock, which range from -3.1 to +2.3 per mil. Two samples of the basal mineralization zone sulphides yield d33S values of 0.066 and 0.122 per mil and one sample of the
metavolcanic wall rock yields 0.149 per mil. The d34S and d33S values for the Thunder intrusion fall within range for rocks of typical upper mantle composition. The sulphur source is difficult to resolve. It appears to be of mantle origin as the wallrock S isotope values and S/SeTot signature are
similar to that of upper mantle; however, assimilation of crustal sulphur is also a possibility.