Kelsie Ojaste, MSc

The offset dykes of the Sudbury Igneous Complex (Ontario, Canada) host significant magmatic Ni-Cu-PGE deposits rich in precious metals (e.g., platinum-group elements, gold), making them superlative sites for understanding metal solubility in fluids and magmatic sulfide differentiation. Despite extensive study, the origin(s) and character of volatile components and their impact on precious metal distribution in the Sudbury sulfide ores remain controversial. Here we aim to evaluate the evidence for volatile activity in differentiated sulfide ores from the distal Podolsky North deposit in the Whistle offset dyke, at the northeast margin of the Sudbury Igneous Complex. We have examined a ~30 cm long channel-cut section, including massive sulfide, chloritic alteration/reaction rim and host gneiss, across the margin of a ~2 m thick sulfide vein. Our integrated analytical approach includes utilizing optical petrography, mineral chemistry (by scanning electron microscopy and microprobe) and stable sulfur isotope measurements. 

Petrographic observations revealed a diverse mineralogy of aggregated platinum-group minerals (PGM), and accessory Ag±Bi tellurides within the chalcopyrite-dominated sulfide vein material. Fractured surfaces from the interior of the vein expose decrepitated fluid inclusions, observed as salt evaporate haloes (evaporated ‘liquid’ phase), together with solid halides and pyrosmalite [(Fe²⁺,Mn)₈Si₆O₁₅(OH,Cl)₁₀] daughter phases. It is noteworthy that there is trace Pd present in a few of the halides, and chlorargyrite [AgCl] is commonly observed associated with the fluid inclusions. Epidote occurs in separate parallel bands (i.e., two close to the margin within the sulfide vein and one in the alteration halo), and electron microprobe data reveal this mineral has a relatively uniform composition across the studied section. Chlorite exhibits a minor decrease in Ni concentration away from the vein. Sulfur isotope measurements (on chalcopyrite) indicate variable δ³⁴S compositions within the vein but relatively homogeneous δ³⁴S in the chloritic rim and host rock.

Our findings suggest that fluid interactions played a role in the localization and enrichment of precious metals within the highly differentiated sulfides in the Whistle offset dyke, highlighting the complex interplay between volatile activity and metal distribution during solidification of these rocks. By providing new insights into the relationship between volatile components and metal solubility, this study contributes to broader models of magmatic-hydrothermal systems, with potential applications for guiding exploration for precious metal-enriched deposits.