Damaged Goods in the Marmion Tonalites, Superior Province, Ontario: Deformation Fabrics in a Gold-Bearing Damage Zone.

Nils Backeberg, McGill University
Christie Rowe, McGill University

The Archean Hammond Reef gold deposit in NW Ontario, Canada, is hosted in the Mesoarchean Marmion tonalite gneisses of the south-central Wabigoon subprovince. The deposit lies on the western margin of the Marmion tonalites within a fault damage zone 2 km east of the Marmion Shear Zone, which bounds the tonalite terrane to the Finlayson Lake greenstone belt. The mineralization at Hammond Reef is structurally controlled but not vein-hosted. It is characterized by low-grade disseminations of gold-pyrite in a pervasive altered- to partially altered zone.

A kinematic study of the adjacent Finlayson Lake greenstone belt, where metamorphic conditions and structural fabrics are better preserved than in the felsic gneiss, reveals the regional deformation history. We have identified a prolonged structural, magmatic and fluid flow history of the Finlayson Lake greenstone belt and its eastern margin from peak ductile deformation at depths below 20km to shallow reactivation. We aim to relate the deformation history to the damage zone formation in the Marmion terrane. The Marmion Shear Zone is intruded by a late felsic granodiorite, the Diversion Stock, which varies in width from 1 to 2 km along the full length of the structure. This coarser grained, younger intrusion acts as a rigid block within the shear zone. Shallow reactivation of the structure, related to the later stages of the regional deformation history, promoted the development of a brittle damage zone across the contact of the late intrusion and the Marmion tonalites. These new findings explain the 1 to 2 km distance separation of the gold deposit from the original terrane boundary and Marmion Shear Zone. An anastomosing network of brittle fractures and sericite-altered feldspars defines the foliation in the damage zone. The foliation is defined by predominantly sericite and in some cases chlorite, depending on the host rock assemblages and alteration intensity. Feldspar was preferentially fractured and altered to form the sericite matrix, which promoted further localization of the deformation and minor shearing during the development of the damage zone. This style of deformation and complete breakdown of primary feldspar in the damage zone is a possible cause for the pervasive permeability network to be developed and may have facilitated disseminated gold mineralization.