The structural evolution of the missanabie-renabie gold district: Pre-orogenic veins in an orogenic gold setting and their influence on the formation of hybrid deposits

The Missanabie-Renabie gold district in the Wawa gold camp of the Archean Superior craton, Ontario, Canada, is characterized by laminated saccharoidal quartz veins within shear zones. Over 50 years, from 1941 to 1991, 1.1 million ounces of gold were produced from a number of these veins hosted by shear zones that overprint 2720.8 ± 1.4 Ma biotite tonalite of the Missinaibi Lake batholith. Because of their laminated nature and association with shear zones, the veins were interpreted as syn-orogenic fault-fill veins that formed during the migration of hydrothermal fluids along actively deforming shear zones. A structural study of well-exposed, mechanically stripped outcrops indicates that the veins were emplaced in brittle structures prior to the onset of regional ductile deformation and that they subsequently localized the formation of the shear zones in which they are hosted. The veins define isoclinal F1 folds and are overprinted by a regional S2 cleavage, L2 stretching lineation, and F2 folds. The shear zones hosting the veins formed as D3 sinistral-reverse shearing became localized in the mechanically weak alteration envelopes surrounding the veins. Syn-orogenic hydrothermal fluids locally infiltrated the shear zones during D3 and formed gold-barren veins of quartz ± epidote ± chlorite ± K-feldspar ± hematite associated with hematite-bearing alteration zones. Dextral reactivation of the shear zones during D4 produced Z-shaped drag folds, dextral shear bands, and NW-striking quartz tension gashes. Massive pyrite veins with an orientation similar to the quartz tension gashes were emplaced either during or subsequent to D4 and represent a second gold mineralization event. Other granitoid-hosted mines in the Wawa gold camp, such as the Magino deposit and those associated with the Jubilee stock, also comprise pre-orogenic auriferous quartz veins, suggesting that this style of mineralization defines a number of economic deposits in the camp. The Missanabie-Renabie gold district exemplifies how pre-orogenic veins acted as planar anisotropies that localized the development of shear zones, the channeling of successive pulses of syn- to post-orogenic hydrothermal fluids, and resulted in the formation of hybrid deposits comprising both pre- and syn-orogenic veins. Although the main gold mineralization event occurred prior to regional deformation, the hybrid deposits display characteristics that are typical of orogenic gold deposits. This study emphasizes that hybrid deposits are likely an underrecognized style of mineralization in Archean gold camps and contributes to an understanding of why multiple styles and ages of mineralization overlap spatially in the Superior craton and globally.