The Archean low-grade, large-tonnage Côté Gold Au(-Cu) deposit is the first large gold deposit discovered in the Swayze greenstone belt, Ontario, Canada. The deposit is hosted by the Chester Intrusive Complex, a low-Al composite, subvolcanic intrusion composed of tonalite, quartz diorite, and diorite that was previously constrained to ca. 2741 to 2739 Ma (U-Pb zircon). Presented here is the first detailed study of the mineralization and related alteration, along with the relative and absolute age (U-Pb, Re-Os) constraints on gold mineralization.

The earliest hydrothermal stage is represented by rare Au-bearing amphibole-rich veins and breccias. The main ore stage consists of biotite-rich alteration centered on an Au(-Cu)–bearing magmatic-hydrothermal biotite breccia body with spatially related disseminated biotite and veins of both sheeted and stockwork type. Extensive fracture-controlled and replacement-style Au ± Cu-bearing muscovite alteration overprints biotite-altered rocks in the core of the deposit. Barren fracture-controlled and disseminated epidote alteration is localized to the north of the deposit and above the magmatic-hydrothermal biotite breccia. Late, texturally destructive albite alteration overprints the mineralized hydrothermal alteration in the deposit core.

U-Pb isotope dilution-thermal ionization mass spectrometry and laser ablation-inductively coupled plasmamass spectrometry ages for hydrothermal titanite from amphibole (2745 ± 3 Ma) and albite (2737.5 +2.2/–1.8, 2745 ± 9, and 2736 ± 7 Ma) alteration assemblages constrain hydrothermal activity to ca. 2740 Ma. The timing of gold and sulfide mineralization is also constrained by two Re-Os molybdenite ages of 2736.1 ± 11.4 (biotite alteration) and 2746.8 ± 11.4 Ma (muscovite alteration). These new ages overlap with the ca. 2741 to 2739 Ma magmatism for the Chester Intrusive Complex, thereby suggesting a synintrusion, magmatic-hydrothermal origin for the gold mineralization and related alteration. This is significant, as it represents a new gold metallogenic event in the Abitibi subprovince, for which regional importance remains to be defined.

Considering the spatial association of the deposit with a dioritic intrusion and the temporal overlap of igneous activity with alteration (i.e., amphibole, biotite, sericite) and mineralization (i.e., breccias, veins, disseminations), the deposit is interpreted as an Archean magmatic-hydrothermal ore system sharing analogies with Phanerozoic Au-Cu porphyry deposits. It suggests that Archean porphyry-type deposits can form in low-Al composite intrusions, which are known to host Cu-Mo-Au breccia, vein, and disseminated mineralization and underlie temporally and genetically related felsic to intermediate volcanic rocks that host volcanogenic massive sulfide deposits.

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