Abstract

The structure and stratigraphy of the Chapais-Chibougamau mining camp in the Archean Abitibi greenstone belt of the Superior Craton, Quebec, Canada, is reevaluated from new aeromagnetic data, geochronology, and field studies to provide a framework for syngenetic and epigenetic mineralization. Mineralization in the Chapais-Chibougamau area is synvolcanic (volcanogenic massive sulfide (VMS)), synmagmatic (Fe-Ti-V, Ni-Cu, Au-Ag-Cu-Zn-Pb veins, Chibougamau-type Cu-Au veins), and shear zone hosted (including Opemiska Cu-Au veins subtype). VMS deposits occur within felsic volcanic rocks of tholeiitic affinity and mafic to felsic volcanic rocks of transitional to calc-alkaline affinity at the top of three volcanic cycles of the Roy Group. VMS deposits and early polymetallic veins (Au-Ag-Cu-Zn-Pb) are associated with NNW- to NNE-striking synvolcanic faults. Fe-Ti-V deposits occur within the Layered zone of the Lac Doré Complex, especially where it thickens in areas of interpreted synmagmatic faults. Subeconomic magmatic Ni-Cu deposits occur at the contacts of mafic-ultramafic or tonalitic intrusions. “Chibougamau-type” Cu-Au veins formed through magmatic-hydrothermal processes. They are cut by dikes that predate regional D2 deformation but are located within, and are deformed by, NW- and NE-trending D2 shear zones. Shear zone-hosted Au deposits are developed preferentially within regional E-W–trending deformation corridors and along NNE-striking sinistral shear zones while “Opemiska-type” Cu-Au veins occur within regional overturned anticlines in mafic sills of the Cummings Complex.

U-Pb dating of detrital zircons provides maximum depositional ages for metasedimentary rocks of the Blondeau Formation of the Roy Group and the Stella Formation of the Opémisca Group (which unconformably overlies the Roy Group). The Blondeau Formation is <2721 ± 3 Ma, which is coeval to the age of the Windfall Member in the Urban-Barry Belt. Both show similar exploration potential as they host early polymetallic (Au-Ag-Cu-Zn-Pb) veins. The Stella Formation is <2704 ± 2 Ma and is unaffected by first generation folds, thus providing a time constraint for the earliest folding event (D1). This new age also indicates that sedimentation in the Chapais-Chibougamau area occurred earlier than for the Timiskaming-type assemblages in the southern Abitibi greenstone belt, but the structural setting and presence of alkaline intrusions suggests similar potential for syenite-associated Au deposits in the Stella Formation.

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