Abstract:

The recently discovered Côté Gold deposit, located in the southeast limb of the Swayze greenstone belt, Abitibi Subprovince, is an Archean low-grade, high-tonnage Au(-Cu) deposit. The deposit is hosted by the 2741 Ma Chester intrusive complex (CIC), a high-level, multi-phase, laccolithic-shaped synvolcanic intrusion composed of several tonalite and diorite phases. Although a close temporal and spatial relationship exists between the phases of the CIC, whole-rock geochemistry on least-altered samples suggests they are petrogenetically unrelated. Notably, the tonalite phases are characterized chemically as low-Al type, which is atypical of the tonalite-trondhjemite-granodiorite suites in the Archean Superior Province and elsewhere, but typical of primitive arc submarine environments. The mineralized system at the Côté Gold deposit is cospatial with a multi-phase, magmatic-hydrothermal breccia body that contains multiple brecciation events and matrices and is overprinted by several alteration types (biotite, sericite, silica-sodic). Whole-rock geochemistry indicates that the deposit is Au(-Cu) only and is relatively depleted in other elements (e.g. As, F, Bi, Te). The overlap of Re-Os dates on syn-gold-deposition molybdenite (2739 ± 9 Ma) with the 2741 Ma CIC is consistent with field observations that suggest an overlap of magmatic and hydrothermal events. The results of this study have defined a new significant early stage gold metallogenic event in the Abitibi Subprovince at 2740 Ma and the deposit provides a guide for future exploration in other composite, subvolcanic, low-Al intrusions in the Archean.