Geology, structural evolution, and hydrothermal alteration of the Island gold deposit, Michipicoten greenstone belt, Ontario

Abstract

The Archean Island Gold deposit is situated in the Michipicoten greenstone belt within the Abitibi-Wawa subprovince. Gold mineralization is hosted in dacitic volcanic and volcaniclastic rocks, gabbro, and a tonalite-trondhjemite body (Webb Lake stock) within the southern domain of the east-west-trending Goudreau Lake deformation zone. The area is highly prospective, however, the controls on high-grade mineralization and the timing of events in the Michipicoten belt remain to be firmly established and compared with major gold systems of the southern Abitibi belt. Several phases of deformation have affected the host rocks. The first deformation (D1) event produced regional-scale folds. The second deformation (D2) event resulted in camp-scale folds (F2), a steep axial planar foliation (S2), regional greenschist-grade metamorphism, and the formation of the Goudreau Lake deformation zone. The third deformation (D3) event is expressed by brittle reverse faults, camp- to outcrop-scale folds (F3), and a weakly developed axial planar (S3) cleavage. Gold mineralization in the main Island Gold ore zones was emplaced along the Goudreau Lake deformation zone during the D2 deformation event. The Goudreau Lake deformation zone is inferred to be the major conduit that focused the gold-bearing fluids. The main ore zones dip steeply towards the south and consist of shear-hosted laminated quartz veins (V1) and associated extensional veinlets (V2). This V1-V2 veining is sub-parallel to the S2 foliation and concentrated in the strain shadow of the pre-mineralization Webb Lake stock during D2 north-side-up, sinistral transpression. Shallowly dipping, extensional quartz veins (VGD) also host economic gold mineralization in the Goudreau Zone, which is located north of the main zones. The VGD and V1-V2 veins are cut by extensional quartz veins (V3) that formed during the third deformation (D3) event in areas of high competency contrast. Post-mineralization tourmaline-quartz veins (V4) locally overprint the V3 veins and pre-existing structures. Alteration envelopes associated with auriferous veining at the Island Gold deposit range from the centimetre to metre scale and are enriched in Au, K2O, Rb, S, and Te, and depleted in Na2O. Alteration minerals associated with ore zones generally include biotite, Ca-Mg-Fe carbonates, chlorite, plagioclase, quartz, sulphides (pyrite±pyrrhotite±chalcopyrite), and white mica (muscovite±phengite). Weak alteration associated with the late V3 non-auriferous veins is variable and local. The upper and lower absolute limits on the timing of gold mineralization are constrained by the U-Pb zircon ages of the mineralized Webb Lake stock (2724.1 ± 4.3 Ma) and the post-mineralization silica-poor diorite-monzodiorite (I2M) dykes (2672.2 ± 3.5 Ma). The youngest detrital zircon ages from overlying Doré sedimentary rocks, which were affected by syn-D2 greenschist-facies metamorphism, further constrain the maximum age limit for the timing of gold mineralization to 2680 ± 3 Ma. This timing (2680 ± 3 Ma to 2672.2 ± 3.5 Ma) falls within the typical age range for gold deposits in greenstone belts of the Superior Province.