Josh McQuade HBSc thesis abstract

The Eastern Gabbro of the Coldwell Complex is a composite pluton that wraps around the eastern and northern margins of the complex and hosts a number orthomagmatic Ni–Cu–platinum group element (PGE) sulfide deposits (Good et al., 2015; 2019). The most well-characterized deposit is the Marathon conduit-type Cu–PGE deposit situated in the southern portion of the Eastern Gabbro. It, along with the other zones of mineralization, are largely hosted by the Two Duck Lake (TDL) gabbro, a coarse to pegmatitic gabbro that exhibits a subophitic texture. Three zones of mineralization occur in the Marathon deposit: the lower Footwall Zone, the Main Zone, and the upper W Horizon. The Footwall Zone is characterized by the highest Cu/Pd ratios that are within the range of mantle values, whereas the W Horizon is characterized by Cu/Pd values lower than mantle (Brzozowski et al., 2020). Base-metal sulfides (BMS) range from semi-massive to disseminated, and largely comprise variable amounts of pyrrhotite, chalcopyrite, and lesser pentlandite; the W Horizon is unique in that it contains significant bornite, limited pyrrhotite, and low abundance of base metal-bearing sulphides. Several zones of mineralization occur north of the Marathon deposit; from south to north these are the Four Dams and Boyer prospect, the Sally deposit, and the Redstone prospect (Good et al., 2017). Mineralization here is characterized by elevated Cu/Pd ratios and comprises a BMS assemblage similar to the Footwall and Main zones at Marathon, but with notably higher abundances of cubanite (Brzozowski et al., 2020).

 

In the Marathon deposit, the sulfide liquid was carried upwards to the site of emplacement from a sulfide pool located at depth, which formed by the addition of externally derived S from Archean sedimentary rocks. This Archean contamination is evident in the multiple S isotope compositions of BMS in the Footwall and Main zones, namely a negative correlation between δ33S –δ36S and non-zero Δ33S, which is characteristic of Archean S reservoirs (Shahabi Far et al., 2018). Similarly, the S/Se ratio of BMS in the Footwall Zone are elevated compared to mantle values, indicative of the addition of external S to the mineralizing system (Brzozowski et al., 2020).

 

In contrast to the Marathon deposit, BMS in the northern zones of mineralization are characterized by S/Se ratios that are largely lower than mantle ranges, indicating either i) contamination of these mineralized zones by a low S/Se source or ii) S loss. To attempt to address this ambiguity, the S isotope (δ34S) composition of BMS at Four Dams, Boyer, Sally, and Redstone were characterized. In all of these zones of mineralization, BMS are characterized by δ34S values within the range of mantle values (Four Dams = 0.2 to 2.3, Boyer = 0.3 to 2.6, Sally = -0.7 to 2.5, Redstone = -0.1 to 2.6). Considering the propensity of Ni–Cu–PGE systems to record evidence of external S addition, and the general inefficiency of other saturation mechanisms (e.g., decreasing fO2, magma mixing; Ripley and Li, 2013) to generate economic sulfide deposits, the lack of δ34S signature could be the result of either i) dilution/destruction of the isotopic signature by interaction of the contaminated sulfide liquid with uncontaminated silicate melt (i.e., R factor), or ii) contamination by an Archean S source characterized by δ34S ~ 0‰. To assess the effect of R factor on the S isotope composition of BMS, δ34S was compared to bulk-rock Cu/Pd (i.e., a proxy for R factor). Although the Cu/Pd values of these mineralized zones varies over more than two orders of magnitude (525–909,000), no systematic change is evident in the δ34S composition of BMS, suggesting that dilution of the contaminant signature may not be the cause of the mantle-like values, but rather that the contaminant contained S that was processed through the Archean atmosphere. Given the current dataset, it is not possible to robustly assess the role of Archean contamination in the S saturation history of the northern mineralized zones in the Eastern Gabbro. This preliminary study, therefore, serves as the foundation for a future study characterizing the multiple S isotope composition of these mineralized zones and development of a more detailed mineral deposit model.