Metallogeny of an Early to Middle Jurassic arc, Iskut River area, northwestern British Columbia

Abstract

The Iskut River area of northwestern British Columbia is a relatively small (approx 100 x 60 km) but metallogenetically significant portion of the Canadian Cordillera. In the Iskut River area, Mesozoic volcanic-arc rocks and related intrusions of the Stikine terrane are host to a wide variety of mineral deposits, most of which formed in Early to Middle Jurassic time. Porphyry-style mineralization includes the Kerr Cu-Au porphyry deposit, the Mitchell-Sulphurets porphyry deposits, and the Red Bluff prospect, all related to calc-alkaline to subalkaline Early Jurassic intrusions. Precious metal-rich veins at the Johnny Mountain and Snip mines are spatially and genetically associated with the Red Bluff intrusion. These deposits show a progression from shear-hosted laminated mesothermal veins at deeper levels (Snip mine) to thick, tabular dilational veins at higher levels (Johnny Mountain mine). In the Sulphurets camp, vein-hosted mineralization of the west zone exhibits a variety of vein textures suggesting a relatively shallow level of formation. The timing and stratigraphic setting of advanced argillic alteration at the Treaty Glacier prospect indicates near-surface formation. Finally, the Eskay Creek mine is presently producing from high-grade, precious and base metal volcanic massive sulfide-sulfosalt mineralization that formed on the sea floor in the Middle Jurassic. In addition, several other styles of mineralization are recognized in the Iskut area including the Triassic Granduc volcanogenic massive sulfide deposit, gold-rich skarns such as McLymont Creek of probable Early Jurassic age, and gabbro-hosted magmatic nickel sulfide mineralization at Nickel Mountain of uncertain age.Integrated regional mapping, deposit-specific studies, geochronology, lithogeochemistry, and biostratigraphy have demonstrated that Early to Middle Jurassic mineralization developed within the evolving Hazelton Group volcanic arc. Volcanism during early stages of arc evolution ( approximately 200-180 Ma) is characterized by calc-alkaline andesites to dacites. Most Jurassic intrusions in the region are coeval with this episode of volcanism and are dominated by a potassium feldspar megacrystic suite of granitic to granodioritic plutons. Mineralization which is temporally and spatially associated with this period of magmatism includes porphyry-style deposits and precious metal-rich vein systems, which often show strong structural localization. A younger period of volcanism ( approximately 180-170 Ma) is bimodal (rhyolite-dacite and basalt-basaltic andesite) and contains both calc-alkaline and tholeiitic signatures. Mineralization during this later stage of the arc includes high-level alteration systems and submarine exhalative mineralization. Regional contractional deformation in mid-Cretaceous time resulted in significant structural modification of individual deposits, obscuring many of the primary field relationships.