Connor Caglioti MSc thesis abstract

The Escape intrusion is a tabular to bladed, mafic–ultramafic chonolith that hosts economic concentrations of PGE–Cu–Ni magmatic sulfide mineralization. The Mesoproterozoic intrusion is located about 50 km northeast of Thunder Bay, Ontario, and with the Current intrusion makes up the Thunder Bay North Intrusive Complex (TBNIC). The intrusive rocks of the TBNIC are part of the 1.1 Ga Midcontinent Rift System (MRS) of North America and were emplaced into the Quetico Basins during early stages of rift development. The fractionated HREE (Gd/Ybcn = 3.18–4.96) signature of the Escape rocks suggests magma derivation from a deep mantle source. Primitive mantle-normalized trace element patterns of the Escape intrusive rocks are similar to ocean-island basalt, as well as multiple MRS-related mafic–ultramafic intrusions (e.g., Hele, Disraeli, Kitto), which is consistent with the mantle-plume hypothesis of MRS formation.

The high-grade zone occurs within the (mostly wehrlitic) peridotite unit of the Escape intrusion, which lies below the weakly mineralized gabbro and hybrid units. The high-grade zone within the Escape intrusion is characterized by intercumulus sulfide mineralization that is net-textured at the core and disseminated at the margins. The primary sulfide mineralization within the net-textured ore is characterized by an assemblage which consists predominantly of pyrrhotite + chalcopyrite + pentlandite + platinum-group minerals (PGMs). The disseminated sulfides of the high-grade zone are composed of a variable assemblage that includes the primary sulfides as well as some or all of the following: native Cu, mackinawite, cubanite, native Ag/electrum, sugakiite, pyrite, and valleriite. In addition to interstitial sulfide mineralization at Escape, numerous centimetre-sized sulfide (± carbonate) veinlets crosscut the groundmass. The sulfide veinlets exhibit complex intergrowths often with mottled textures and variable composition. Phases commonly identified within the sulfide veinlets include cubanite, pyrrhotite, pyrite, pentlandite, chalcopyrite, and mackinawite.

Sulfide trace element compositions obtained via in situ laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) indicate that Pd is concentrated in pentlandite (around 100 ppm) and IPGE (Ir, Os, Ru) are concentrated in pyrrhotite within the net-textured ores. Sulfide minerals from the interstitial–primary assemblage exhibit S/Se ratios within the mantle domain (2850–4350). The high-grade PGE tenors of the deposit were likely the result of moderate magmatic enrichment of the Escape segregated sulfide liquid at R factors of ~7,500, based on numerical models derived from whole-rock, major and trace element geochemistry. Sulfide minerals within the disseminated style are depleted in PGE and S/Se (as low as 668) relative to the net-textured sulfides. These trace element signatures and the distinct sulfide assemblage identified in the disseminated ores are attributed to desulfurization and remobilization of metals during hydrothermal alteration. Sulfide minerals measured within veinlets from the crosscutting assemblage are depleted in PGE and strongly enriched in As (e.g., up to ~900 ppm in pentlandite) relative to the primary–interstitial assemblage, as well as elevated in S/Se (ranging up to 27,896), considerably outside of the mantle range. Various As-bearing PGM were identified within the sulfide veinlets but were not found within the net-textured ores. These features suggest that remobilized metals from the interstitial ores were transported as As-rich bisulfide complexes and emplaced as veinlets within small fractures between cumulus olivine during the later stages of hydrothermal activity.

In situ S-isotopes of Escape sulfides obtained via secondary ion mass spectrometry (SIMS) show that δ34S values range from -3.07 to -0.97‰, and all values for Δ33S and Δ36S fall within a range produced by mass-dependent fractionation (MDF). S-isotopes were also measured in pyrite from Quetico metasedimentary country rocks. There is overlap between Escape and Quetico S-isotopes both in and outside the mantle range. Results from numerical modelling of equilibration between the Escape sulfide liquid and pulses of fresh, uncontaminated melt suggest that the mass-independent fractionation (IDF) signal was erased due to isotopic exchange. It is inconclusive what drove the Escape parental magma to S-saturation, however, the origin of sulfur is likely to be a mixture of mantle source and country rock contamination.