THE KERR-SULPHURETS-MITCHELL DEPOSIT, BRITISH COLUMBIA: A SUITE OF DISMEMBERED CU-AU-AG-MO PORPHYRY ORE BODIES

The Kerr-Sulphurets-Mitchell (KSM) Project, owned by Seabridge Gold Ltd., hosts one of the world’s largest undeveloped reserves of copper and gold. Located in northwest British Columbia, Canada, the KSM district covers an area of roughly two by ten kilometers and preserves large segments of a calc-alkalic magmatic-hydrothermal complex made up of four separate porphyry ore bodies. Combined, these ore zones hold an immense total proven and probable reserve of 38 million ounces of gold, 9.9 billion pounds of copper, 191 million ounces of silver and 213 million pounds of molybdenum. These resources are contained in 2.16 billion tonnes of ore with an average grade of 0.55 g/t Au, 0.21% Cu, 2.74 g/t Ag and 44.7 ppm Mo.

Cretaceous accretionary processes have resulted in significant deformation and dissection of the KSM ore bodies: imbricate thrust faults have dismembered large segments of the deposits; zones with strong phyllic alteration and resulting low competency have been preferentially deformed; and folding due to tectonic compressional stresses has resulted in considerable structural and geometrical complexities within the entire district. The mineralization at KSM is stockwork vein-hosted to disseminated, and typically occurs as chalcopyrite, pyrite, molybdenite and lesser sulfosalts, with gold and silver correlating strongly with sulphide minerals. Mineralization within each of the four main KSM ore bodies is predominantly hypogene and associated primarily with hydrothermal activity centered on intermediate composition, porphyritic, Jurassic hypabyssal intrusions. Each of the four intrusive centers features multiple generations of texturally and mineralogically distinct, diorite to monzodiorite dykes and stocks belonging to the Sulphurets suite, which are hosted in arc-related, volcanic and sedimentary assemblages of the Stikinia terrane. In addition to coherent, hypabyssal intrusions, the deposits also feature hydrothermal breccias and intrusion breccias, which occasionally host significant mineralization. An ongoing study of the petrogenesis of the KSM intrusive suite, through an analysis of petrographic and textural properties, new U-Pb zircon ages, whole rock geochemistry and zircon trace element geochemistry has shed new light on the formation and evolution of the KSM system.