SULFUR ISOTOPE GEOCHEMISTRY AND GEOLOGY OF THE KEMESS NORTH PORPHYRY AU-CU DEPOSIT, TOODOGGONE DISTRICT, BRITISH COLUMBIA, CANADA

The Kemess North porphyry Au-Cu deposit (407 Mt resource @ 0.41 g/t Au and 0.22% Cu) is associated with a suite of Early Jurassic, high-K, calc-alkaline granitoids that has been emplaced into coeval, arc-related, volcano-sedimentary rocks, which fill a northwesterly elongated intra-arc basin. The different stages of vein formation and related alteration events were characterized in order to understand hydrothermal fluid evolution and evaluate the potential of the porphyry system to generate a late stage of epithermal Au-Ag mineralization. Sulfide δ³⁴S values from Early-stage magnetite stringer veins and slightly younger quartz-magnetite-pyrite ± chalcopyrite ± molybdenite veins average 0.9 ± 0.5‰. These veins have chlorite alteration halos and host the bulk of the Au-Cu mineralization in the 202.7 ± 1.9 Ma monzonite porphyry. The Early-stage veins are cut by quartz-pyrite-chalcopyrite ± molybdenite and pyrite-chalcopyrite veins, which have strong sericite-chlorite (phyllic) alteration halos and average sulfide δ³⁴S values of 0.2 ± 1.4‰. Late-stage anhydrite ± pyrite veins have average pyrite δ³⁴S values of 0.3 ± 0.3‰ and anhydrite δ³⁴S values of 11.7 ± 0.5‰. The average formation temperature of the Late-stage veins using equilibrium sulfur isotope fractionations between anhydrite-pyrite pairs is 474 ± 26 ºC. Intersection of the lines of best fit for anhydrite and pyrite δ³⁴S data (n = 4 pairs) on a δ versus δΔ diagram yields an estimate of 4.2 ‰ for the unfractionated sulfur source. This value is consistent with a magmatic source of sulfur and the anhydrite and pyrite δ³⁴S values are typical of those measured in calc-alkaline porphyry Cu-Mo-Au deposits worldwide. The acute vein angles-to-core-axes in unoriented drillcore indicate that all vein-types at Kemess North dip 30º to 40° to the horizontal. This indicates that a preferred dip angle for veins exists. Structurally, the deposit has undergone significant post-mineralization dismemberment. An E-striking, steeply S-dipping, fault truncates the northern extremity of the orebody and structurally juxtaposes mineralized monzonite and Takla Group andesite against poorly mineralized Toodoggone volcaniclastic rocks to the north. Subsequent NW- to NE-striking normal faults have created a horst-and-graben style block shuffling of the orebody.