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Title: Geology of the Archean Côté Gold Au(-Cu) intrusion-related deposit, Swayze Greenstone Belt, Ontario
Authors: Katz, Laura Rachel
Keywords: Côté Gold deposit;Chester intrusive complex;low-Al composite intrusion;Swayze greenstone belt
Issue Date: 5-Apr-2016
Abstract: The Côté Gold Au(-Cu) deposit is a low-grade, large-tonnage type deposit with an indicated resource of 269 Mt averaging 0.88 g/t Au (7.61 M oz) and an inferred resource of 44 Mt 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 the Swayze greenstone belt (SGB), part of the larger and gold-rich Abitibi Subprovince and is the first large gold deposit to be discovered in the SGB. The deposit is hosted by multi-phase tonalite, diorite and quartz diorite intrusions of the Chester intrusive complex (CIC), a multi-phased, laccolith-shaped synvolcanic intrusion. Crosscutting relationships suggest coeval emplacement of the host rocks and high-precision UPb ID-TIMS zircon geochronology results constrain the CIC to ca. 2741 to 2739 Ma. Although a close temporal relationship exists between the tonalite and dioritic phases, they appear to be petrogenetically unrelated. The CIC is demonstrated to be petrologically, texturally and geochemically similar to low-Al tonalite-trondhjemite-diorite composite intrusions known to underlie VMS-type mineralization. The CIC was emplaced into tholeiitic mafic metavolcanic rocks of the Arbutus Formation that are interpreted to have formed in a back-arc environment. The CIC is coeval and cogenetic with the felsic to intermediate 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, consistent with the presence of textures, such as granophyres, miarolitic cavities and pegmatites, and Al-in hornblende geobarometry results of ≤1.3 ± 0.6 kbars. The intrusive complex is intruded by several younger and petrogenetically unrelated dike phases, as well as overprinted by regional deformation events. iv The deposit has a large ore envelope (1200 m by 300 m) within which several styles of mineralization (breccias, disseminations and veins) that are co-spatial and cogenetic to hydrothermal alteration are recognized. The hydrothermal stages consist of: (1) a rare series of gold-bearing amphibole-rich veins and breccias spatially restricted to the south and central parts of the deposit; (2) an extensive, main-stage biotite alteration that is centred on Au- and Cubearing hydrothermal biotite breccia bodies. Outwards from the breccia are veins (stockworks and sheeted arrays) and disseminations; (3) an extensive, Au ± Cu-bearing fracture-controlled and replacement-style muscovite alteration in the centre of the deposit; (4) an unmineralized fracture-controlled and disseminated epidote alteration that is localized to the north of the deposit; (5) a texturally destructive albite alteration occurs in the central part of the deposit; and (6) chlorite alteration that overprints biotite alteration. Drill core observations suggest that all the hydrothermal events, except chloritization, broadly overlap with magmatism and are crudely zoned on the deposit scale; chloritization is considered to be metamorphic in origin. To constrain the timing of hydrothermal events U-Pb ID-TIMS, SHRIMP and LA ICPMS dating of four hydrothermal titanites from amphibole and albite alteration assemblages and Re-Os dating of two molybdenite samples were analyzed. The hydrothermal titanite ages are centred on a ca. 2740 Ma and the molybdenite ages are 2736.1 ± 11.4 Ma and 2746.8 ± 11.4 Ma. These ages overlap with magmatism and strongly support a syn-intrusion timing for alteration and gold mineralization. Importantly, this age represents a new metallogenic gold event in the Abitibi Subprovince. In addition, two rutile ages at ca. 2667 Ma were obtained and indicate the deposit was affected by subsequent thermal events, such as regional metamorphism. Several of the alteration types (i.e., amphibole, biotite, muscovite and epidote) are considered to be genetically related to the dioritic phases of the CIC based on relative timing, v distribution, mineralogy and mineral chemistry and whole-rock geochemistry. The progressive crystallization of the dioritic magma chamber is considered to have resulted in the generation of mineralized magmatic-hydrothermal fluids that are thought to be responsible for generating the Côté Gold deposit. The sequence of alteration types, their zonation in the deposit and their chemistry (i.e., gains and losses, REE patterns, metal associations and isotopes) is consistent with the early hydrothermal stages (i.e., amphibole and biotite) having formed from high temperature magmatic-derived dioritic fluids and the later alteration types (i.e., muscovite and epidote) having formed from cooling magmatic fluids mixed with seawater. The development of albite alteration is thought to be the result of volatile exsolution of tonalite based on its syn-intrusion timing and the Na-rich nature of the fluids. Although albite alteration may be gold-bearing, the erratic distribution of the mineralization and the leaching capacity of fluids suggest earlier gold was remobilized during this event. Considering the characteristics of the host rocks, the demonstrated overlap of magmatic and hydrothermal events at ca. 2740 Ma, the nature and distribution of alteration, the style of the associated mineralization and the geochemical characteristics of the hydrothermal fluids the Côté Gold deposit is considered to be intrusion-related in origin and formed in a manner similar to Phanerozoic porphyry-type deposits.
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Doctoral theses

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