ARE THERE ELEPHANTS HIDING IN THE JURASSIC OF YUKON? A TECTONOMAGMATIC PERSPECTIVE ON PORPHYRY PROSPECTIVITY

Within British Columbia, Triassic-Jurassic felsic plutonic and intrusive rocks of the Stikinia and Quesnellia terranes are very much considered “elephant country”, and are spatially and genetically associated with some of the most significant porphyry Cu-Mo deposits in North America. However, where these terranes meet and merge in central Yukon, little significant mineralization has been discovered in similarly aged felsic plutonic rocks and they host only one operating Cu mine, despite outcrop covering hundreds of square kilometers. The reasons for this discrepancy are not clear. In order to provide assessment of the tectonomagmatic setting and porphyry prospectivity of Jurassic plutonic rocks within Yukon, 65 unmineralized samples from across the district have been analyzed for their whole rock major and trace element geochemistry, and their zircon Ce⁴⁺/Ce³⁺ ratios were determined by laser ablation ICP-MS. Most samples plot within calc-alkaline to high K calc-alkaline fields within a plot of SiO₂ vs. K₂O, although data display significant scatter and lack any strong trend, suggestive of later disturbance of the K content. Rocks from all sampling locations show HFSE depletions and negative Nb anomalies characteristic of suprasubduction zone environments, and commonly depleted HREE patterns are suggestive of melt generated below the depth of garnet stability. Plots of Sr/Y vs. Y and La/Yb_N vs. Yb_N both indicate significant adakite-like characteristics in a subset of the samples analyzed. Taken together, the suprasubduction zone character and presence of adakite-like magmatism have been suggested as strong indicators of high porphyry potential in other parts of the world. Zircon Ce⁴⁺/Ce³⁺ values were highly variable and ranged up to more than 2100, an order of magnitude greater than previous studies have suggested as a porphyry prospectivity threshold. However, aluminum-in-hornblende data and Cu and Mo contents well below crustal abundances suggest that Jurassic plutonic rocks may represent the mid-crustal, volatile-depleted residue of porphyry-related magmatism that has been eroded away. Future exploration for Cu-Mo-Au mineralization within Yukon will depend on understanding of tectonic uplift across the region, and better constraint on the genesis of known mineralization.