U–Pb–Hf and morphological evolution of zircon from granites associated with world-class tungsten skarn deposits in the northern Canadian Cordillera

The northern Canadian Cordillera is the most significant tungsten district in North America. Here, high-grade tungsten skarn deposits are associated with small, reduced, high-K calc-alkaline, S-type biotite granite plutons belonging to the 102–96 Ma Tungsten plutonic suite (TPS). A detailed U–Pb–Hf and morphological study of magmatic zircon from plutons in the southern half of the TPS belt was undertaken to better understand magmatic processes leading to the generation of the associated tungsten deposits. Antecrystic zircon from the TPS plutons began crystallizing during a transpressional regime ca. 117 Ma, suggesting the TPS magmas were active for up to 21 Myr prior to their upper crustal emplacement and final crystallization. This prolonged magmatic activity necessitates a magma origin in long-lived, deep crustal magma chambers. Hafnium isotopic compositions in zircon for the southern TPS as a whole form a non-radiogenic, univariate, and relatively wide ranging population (ε_Hfi = −17.6 ± 4.5), but U–Pb–Hf trends become apparent when the data are sub-divided into sample groups with similar age, zircon morphology, and geographic location. These evolutionary trends in magmatic zircon are most simply explained by interactions between the parent melt and dissolving inherited zircon grains. This, along with changing zircon morphology, is consistent with gradual cooling and crystallization pathways exhibited by S-type magmas. Differing evolutionary trends in the U–Pb–Hf isotopic data between sample groups, however, suggest there were multiple magma batches that evolved independently, possibly in separate pockets within large, deep magma chambers. Zircon morphologies also suggest some grains in all sample groups were equilibrated with hotter and more alkaline magmas, although there is no textural or compositional evidence in the zircon for mixing of magmas with widely different compositions. An unconstrained inversion of local aeromagnetic data indicates reduced batholiths could be present 4–6⁺ km below the surface and that the plutons are apophysies to (or, higher level injections from) these deeper bodies. Although these batholiths can only be short-term holding chambers for magmas ascending from deep crustal levels, they may have been important for the segregation of mineralizing fluids. Since no single magmatic evolutionary pattern in the unaltered TPS plutons can be definitively linked to tungsten mineralization, pulses of mineralizing fluid may have been derived instead from the underlying batholiths. The 20⁺ Myr duration of deep magmatic activity exhibited by the TPS is similar to timeframes suggested for magmas associated with tungsten deposits in southern China, and may have allowed extended fractionation of a large volume of crustally derived magma to concentrate tungsten into late-stage melts. The emplacement of upper crustal batholiths and plutons in both regions during or following a transition to an extensional tectonic regime suggests the relaxed geodynamic regime may have been important in the ascension of metal-rich magmas and (or) fluids, ultimately resulting globally important tungsten deposits.