Lithogeochemical Investigation of the Big Bulk Cu-Au System, Northwestern British Columbia: Implications for Exploration in the Stewart–Iskut Mineral District, Canadian Cordillera

The Big Bulk Cu–Au porphyry system is located at the southern end of the Stewart–Iskut mineral district in the Stikine volcanic arc terrane, which hosts numerous Late Triassic to Early Jurassic porphyry Cu–Au and epithermal Au deposits, including the Kerr–Sulphurets–Mitchell (KSM) system. Although Big Bulk and KSM have historically been variably classified as alkaline or calc-alkaline systems within a syn-subduction setting, recent studies have proposed post-subduction, back-arc, or stalled-subduction emplacement associated with localized pull-apart basin development. Here, new lithogeochemical data from the Big Bulk stock are integrated with published data from the Sulphurets intrusions at KSM to constrain petrogenesis and assess commonly used magmatic-fertility criteria. Emplaced during the early stages of Hazelton arc volcanism, the Big Bulk stock is gabbroic to dioritic, subalkaline, and calc-alkaline to transitional in affinity, with enrichment in large-ion lithophile elements (LILE), pronounced negative Nb-Ta-Ti anomalies, and relatively low Nb/Yb and Ta/Yb ratios. Integrated with regional paleogeographic reconstructions, geochronological constraints, and comparison with contemporaneous Hazelton arc volcanic rocks, these characteristics favor emplacement in a proximal rear-arc to incipient back-arc setting that remained strongly influenced by active subduction and are less consistent with a mature back-arc or strongly enriched post-subduction setting. The Sulphurets intrusions are generally more evolved, predominantly dioritic and high-K calc-alkaline, and show greater enrichment in LILE and high-field-strength elements. Both mineralized suites show contrasting distributions on commonly cited porphyry Cu ± Au fertility diagrams (e.g., unprospective using La/Yb- and V/Sc-based criteria), but exhibit elevated Al2O3/TiO2 values comparable to those of globally Cu ± Au-productive intrusions, although the Big Bulk stock extends to lower SiO2 contents. These discrepancies may partly reflect differences in magma evolution within island-arc systems and highlight the limitations of applying fertility indicators developed largely from more evolved continental-arc suites to contrasting tectonomagmatic settings. Accordingly, the lithogeochemical characteristics shared by the mineralized Big Bulk and Sulphurets intrusions, including their elevated Al2O3/TiO2 values, may provide a useful guide for exploration of similar systems within the Stikine terrane.