The Geology, Petrology and Geochronology of the Archean Côté Gold Large-Tonnage, Low-Grade Intrusion-Related Au(-Cu) Deposit, Swayze Greenstone Belt, Ontario, Canada

Abstract

The Archean Côté Gold Au(-Cu) deposit is the first large gold deposit discovered in the Swayze greenstone belt of the Abitibi Subprovince. The deposit is a low-grade, large-tonnage type with a combined indicated and inferred resource of 8.65 M oz Au. The deposit is hosted by the Chester intrusive complex (CIC), a multi-phase, subvolcanic intrusion comprised of low-Al tonalite, diorite and quartz diorite, plus magmatic and hydrothermal breccia bodies. The age of the tonalite and dioritic phases is constrained at 2741 to 2739 ± 1 Ma using high-precision ID-TIMS U-Pb zircon geochronology. Although these phases are co-temporal and co-spatial, they appear to be petrogenetically unrelated. The CIC was emplaced into mafic metavolcanic rocks of the Arbutus Formation whose geochemistry reflects a back-arc environment. The tonalite of the CIC is coeval and co-genetic with the felsic to intermediate metavolcanic rocks of the Yeo Formation. Emplacement of the CIC into a shallow crustal level is inferred based on the incorporation of screens and inclusions of the Yeo Formation and is supported by the presence of textures in tonalite and dioritic rocks (e.g., granophyres, miarolitic cavities and pegmatites), as well as Al-in-hornblende geobarometry results of ≤1.3 ± 0.6 kbars. The CIC is petrologically similar to other subvolcanic, low-Al tonalite-trondhjemite-diorite intrusions that underlie VMS-type deposits and which themselves may contain syn-intrusion mineralization. Several geochemically unrelated dykes and deformation events crosscut and postdate the CIC.