The Marathon conduit-type Cu–platinum-group element (PGE) deposit occurs within the Proterozoic Coldwell alkaline complex, Ontario, Canada, and comprises three zones of mineralization that occur at varying distances from the contact with Archean country rocks. Multiple sulfur isotope studies (δ34S, Δ33S, Δ36S) of sulfides from the Marathon deposit show that Δ33S and Δ36S measurements, as well as the relationship between them, are critical in demonstrating crustal S contamination. This is particularly significant for magmatic Cu-PGE deposits in contact with Archean country rocks where the δ34S of sulfides are within the range of typical mantle values (0‰ ± 2‰), as is the case at the Marathon deposit. Although the δ34S values of sulfides from the Marathon deposit did not reflect any evidence of crustal S assimilation, the Δ33S range (−0.91‰ to 0.00‰), and a negative correlation between Δ33S and Δ36S, indicate that S was derived from both the mantle and Archean supracrustal rocks. Given that the country rocks contain insignificant amounts of sulfide, crustal S assimilation must have occurred at depth, prior to magma emplacement. The magnitude of the Δ33S anomaly decreases with increasing distance from the basal contact, suggesting either that S contamination was greatest in the mineralized zone that occurred in close proximity to the contact or that crustal S signatures have been diluted in the mineralized zones further from the contact.