Identifying externally derived sulfur in conduit-type Cu–platinum-group element deposits: The importance of multiple sulfur isotope studies

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.