The Magino Gold Deposit, Ontario, Canada: An Overprinted Archean Intrusion-Related Gold Deposit

Abstract

Abstract The Magino gold deposit, located within the Michipicoten greenstone belt of the Wawa subprovince (Ontario, Canada), is a past-producing underground mine which has recently begun production as an open-pit gold mine with 4.5 Moz of Au in resources. Gold is primarily hosted within the ca. 2724 Ma Webb Lake stock, a steeply dipping, tabular, multiphase tonalitic intrusion oriented parallel to the penetrative regional S2 foliation along the Goudreau Lake deformation zone. The Magino deposit underwent two gold mineralization events (Au1, Au2) and three ductile deformation events (D2, D3, D4). The main gold event (Au1) is expressed by pervasive biotitic/phengitic alteration of the stock and the emplacement of auriferous sugary quartz veins. The veins have a molybdenite Re-Os age of 2731 ± 6.9 Ma, which is indistinguishable from that of the host Webb Lake stock. Quartz-feldspar porphyry dikes cogenetic with the Webb Lake stock cut across the veins and are transposed, stretched, and folded parallel to the S2 foliation within high-strain corridors in the stock, supporting a pre-D2 timing for the Au1 event. These D2 high-strain corridors formed during localization of deformation along Au1 hydrothermally altered zones. The Au2 event occurred during the D2 event and is associated with the emplacement of N-trending quartz-tourmaline-carbonate veins with albitic alteration selvages, which were later deformed during D3 dextral reactivation of the high-strain corridors. Later D4 vertical loading produced a subhorizontal crenulation cleavage, which is overprinted by chloritoid porphyroblasts that grew during late upper greenschist facies peak metamorphism. The Magino deposit is an example of an Archean intrusion-related gold system, which was structurally modified and overprinted by a syndeformation epigenetic gold mineralization event and late metamorphism.