Geology of the Archean Côté Gold Au(-Cu) intrusion-related deposit, Swayze Greenstone Belt, Ontario

The Côté Gold Au(-Cu) deposit is a low-grade, large-tonnage type deposit with an
\nindicated resource of 269 Mt averaging 0.88 g/t Au (7.61 M oz) and an inferred resource of 44
\nMt averaging 0.74 g/t Au (1.04 M oz) at a cut-off grade of 0.3 g/t Au. The deposit is located in
\nthe Swayze greenstone belt (SGB), part of the larger and gold-rich Abitibi Subprovince and is
\nthe first large gold deposit to be discovered in the SGB.
\nThe deposit is hosted by multi-phase tonalite, diorite and quartz diorite intrusions of the
\nChester intrusive complex (CIC), a multi-phased, laccolith-shaped synvolcanic intrusion.
\nCrosscutting relationships suggest coeval emplacement of the host rocks and high-precision UPb
\nID-TIMS zircon geochronology results constrain the CIC to ca. 2741 to 2739 Ma. Although a
\nclose temporal relationship exists between the tonalite and dioritic phases, they appear to be
\npetrogenetically unrelated. The CIC is demonstrated to be petrologically, texturally and
\ngeochemically similar to low-Al tonalite-trondhjemite-diorite composite intrusions known to
\nunderlie VMS-type mineralization. The CIC was emplaced into tholeiitic mafic metavolcanic
\nrocks of the Arbutus Formation that are interpreted to have formed in a back-arc environment.
\nThe CIC is coeval and cogenetic with the felsic to intermediate rocks of the Yeo Formation.
\nEmplacement of the CIC into a shallow crustal level is inferred based on the incorporation of
\nscreens and inclusions of the Yeo Formation, consistent with the presence of textures, such as
\ngranophyres, miarolitic cavities and pegmatites, and Al-in hornblende geobarometry results of
\n≤1.3 ± 0.6 kbars. The intrusive complex is intruded by several younger and petrogenetically
\nunrelated dike phases, as well as overprinted by regional deformation events.
\niv
\nThe deposit has a large ore envelope (1200 m by 300 m) within which several styles of
\nmineralization (breccias, disseminations and veins) that are co-spatial and cogenetic to
\nhydrothermal alteration are recognized. The hydrothermal stages consist of: (1) a rare series of
\ngold-bearing amphibole-rich veins and breccias spatially restricted to the south and central parts
\nof the deposit; (2) an extensive, main-stage biotite alteration that is centred on Au- and Cubearing
\nhydrothermal biotite breccia bodies. Outwards from the breccia are veins (stockworks
\nand sheeted arrays) and disseminations; (3) an extensive, Au ± Cu-bearing fracture-controlled
\nand replacement-style muscovite alteration in the centre of the deposit; (4) an unmineralized
\nfracture-controlled and disseminated epidote alteration that is localized to the north of the
\ndeposit; (5) a texturally destructive albite alteration occurs in the central part of the deposit; and
\n(6) chlorite alteration that overprints biotite alteration. Drill core observations suggest that all the
\nhydrothermal events, except chloritization, broadly overlap with magmatism and are crudely
\nzoned on the deposit scale; chloritization is considered to be metamorphic in origin.
\nTo constrain the timing of hydrothermal events U-Pb ID-TIMS, SHRIMP and LA ICPMS
\ndating of four hydrothermal titanites from amphibole and albite alteration assemblages and
\nRe-Os dating of two molybdenite samples were analyzed. The hydrothermal titanite ages are
\ncentred on a ca. 2740 Ma and the molybdenite ages are 2736.1 ± 11.4 Ma and 2746.8 ± 11.4 Ma.
\nThese ages overlap with magmatism and strongly support a syn-intrusion timing for alteration
\nand gold mineralization. Importantly, this age represents a new metallogenic gold event in the
\nAbitibi Subprovince. In addition, two rutile ages at ca. 2667 Ma were obtained and indicate the
\ndeposit was affected by subsequent thermal events, such as regional metamorphism.
\nSeveral of the alteration types (i.e., amphibole, biotite, muscovite and epidote) are
\nconsidered to be genetically related to the dioritic phases of the CIC based on relative timing,
\nv
\ndistribution, mineralogy and mineral chemistry and whole-rock geochemistry. The progressive
\ncrystallization of the dioritic magma chamber is considered to have resulted in the generation of
\nmineralized magmatic-hydrothermal fluids that are thought to be responsible for generating the
\nCôté Gold deposit. The sequence of alteration types, their zonation in the deposit and their
\nchemistry (i.e., gains and losses, REE patterns, metal associations and isotopes) is consistent
\nwith the early hydrothermal stages (i.e., amphibole and biotite) having formed from high
\ntemperature magmatic-derived dioritic fluids and the later alteration types (i.e., muscovite and
\nepidote) having formed from cooling magmatic fluids mixed with seawater.
\nThe development of albite alteration is thought to be the result of volatile exsolution of
\ntonalite based on its syn-intrusion timing and the Na-rich nature of the fluids. Although albite
\nalteration may be gold-bearing, the erratic distribution of the mineralization and the leaching
\ncapacity of fluids suggest earlier gold was remobilized during this event.
\nConsidering the characteristics of the host rocks, the demonstrated overlap of magmatic
\nand hydrothermal events at ca. 2740 Ma, the nature and distribution of alteration, the style of the
\nassociated mineralization and the geochemical characteristics of the hydrothermal fluids the Côté
\nGold deposit is considered to be intrusion-related in origin and formed in a manner similar to
\nPhanerozoic porphyry-type deposits.

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