Structural Controls and Deformation History of the Orogenic Island Gold Deposit, Michipicoten Greenstone Belt, Ontario

Abstract

Island Gold is a currently producing mine on the high-grade orogenic Island Gold deposit in northern Ontario. It is located within the southern domain of the regional Goudreau Lake Deformation Zone (GLDZ), which trends east-west through the Michipicoten greenstone belt of the Wawa-Abitibi terrane. The study area encompasses the Island Gold deposit and is located along the northern limb of the Goudreau Anticline, a regional-scale fold attributed to D1 deformation. D2 consists of regional greenschist-facies metamorphism, camp-scale F2 folds, associated steep axial-planar foliation S2, moderately to steeply east-plunging stretching lineation L2a, and sub-horizontal slickenside striations L2b. D3 structures are camp- and outcrop-scale F3 folding, which deforms S2 foliation into shallowly-plunging Z-folds, weakly developed axial-planar cleavage S3, and brittle reverse faults. The Island Gold deposit forms a mineralized corridor south of the trondhjemitic Webb Lake Stock intrusion. The main Lochalsh, Island, Island Deep, and Extension 1 and 2 Zones consist of steeply dipping, subparallel ore zones of laminated V1 quartz veins and V2 veinlets within a silicic-sericitic alteration package. V3 conjugate quartz-carbonate extensional veins cross-cut V1 and V2 ore veins. Offset to the north of the main zones is the Goudreau Zone, which contains both sub-vertical and sub-horizontal ore zones with VGD ore veins. All pre-existing vein sets and structures were overprinted by V4 tourmaline veins. U-Pb zircon geochronology analyses from this study place the age of the mineralized Webb Lake Stock at 2724.1±4.3 Ma and the age of the post-mineralization I2M intrusion at 2672.2±3.5 Ma, which constrain the upper and lower absolute limits on timing of mineralization. Youngest detrital zircon ages from the overlying Doré metasedimentary rocks, which show D2 greenschist-facies metamorphism, further constrain this timing to between 2680±3 Ma and 2672.2±3.5 Ma. The GLDZ formed during D2 deformation along a major lithologic contact. The Island Gold deposit V1 and V2 ore veins were emplaced sub-parallel to S2 foliation along a strain shadow created by the Webb Lake Stock during D2 north-side-up, sinistral transpression. Subsequent D3 deformation folded and sheared the ore zones and V3 veins were emplaced in areas of high competency contrast.