Sulphur, oxygen, and carbon isotope geochemistry of the Windy Craggy mafic-siliciclastic (Besshi-type) volcanogenic massive sulphide deposit, British Columbia

The Windy Craggy deposit in northwestern British Columbia is the world’s largest mafic siliciclastic (Besshi-type) volcanogenic massive sulfide (VMS) deposit, containing ~300 Mt grading 1.38 wt.% Cu, 0.069 wt.% Co, and 0.2 g/t Au. To evaluate the sources of sulfur and carbon, and assess magmatic contributions to ore formation, we analysed sulfur, carbon, and oxygen isotopes in massive sulphide, carbonate gangue minerals, and host rocks. Whole-rock sulfur isotope compositions range from δ34S = −10.1 to +4.8‰ and are consistent with mixing between sulfur leached from footwall volcanic rocks and magmatic sulfur modified by SO2 disproportionation, with only a minor possible sedimentary contribution. Carbonate δ13C values range from −11.8 to +29.6‰ (mostly −5.9 to +7.7‰), whereas δ18O values range from +7.0 to +26.6‰. Most samples define a mixing trend between mantle-derived carbon and marine carbonate, indicating variable inputs of magmatic CO2 and seawater-derived carbon. Trace elements commonly enriched in magmatic fluids (As, Ag, Mo, Sb, and Tl) are strongly correlated and increase with decreasing δ34S. Principal component analysis similarly links these elements with δ13C and total carbon, providing compelling evidence that magmatic volatiles supplied sulfur, carbon, and metals to the Windy Craggy hydrothermal system.