The Role of Transverse Faults in Late Gold Mineralization: A Structural Study of the Maskinonge Lake Fault, Michipicoten Greenstone Belt, Ontario

Abstract

The Maskinonge Lake fault is a late NW-striking fault that occurs at a high angle to the structural features of the gold-producing Goudreau Lake Deformation Zone (GLDZ) in the Michipicoten greenstone belt. The fault is populated with zones of high-grade gold mineralization along its strike and represents an uncommon setting for orogenic gold deposits in the Superior Province. Three phases of fault movement (D4–D6) are identified that postdated the main structural features of the GLDZ (D1–D3). D4 dextral shearing resulted in the initiation of the Maskinonge Lake fault and the development of fault-parallel foliation. The fault was reactivated during D5 sinistral progressive brittle-ductile deformation and included the emplacement of steeply dipping fault-parallel quartz vein arrays and extensional en echelon veins. Structural analysis of these veins shows that structures associated with D5 deformation are subvertical and occurred during transpression. D6 sinistral faulting is responsible for the brecciation of earlier features and ~1 km of strike-slip stratigraphic displacement with little to no vertical movement. The Pine-Breccia gold occurrence is found along the Maskinonge Lake fault and consists of rocks that have undergone pervasive silica alteration within the fault zone. The centre of the alteration halo contains a gold- and copper-bearing fault-filling “main vein”. Both the fault and main vein are located along the contacts of a NW-trending Matachewan dyke, while gold concentrations in the main vein are enhanced at the intersection between the Maskinonge Lake fault and ENE-striking shear zones. These structural controls provide additional constraints for favourable sites of mineralization along late high-angle transverse faults and support these structures as viable gold exploration targets.