The Island Belt Silver Region lies within a northeast-trending fracture zone in Proterozoic sedimentary and igneous rocks of the Southern Province of the Canadian Shield. It comprises a number of vein occurrences on offshore islands in northwestern Lake Superior and along its shoreline south and east of the City of Thunder Bay, Ontario. The belt-like spatial association of these occurrences and their similar vein attitudes is due to structural localization in linear fracture zones related to regional rifting and subsidence of the Lake Superior Basin in the late Proterozoic time. Not all of these veins are argentiferous, and some contain only trace amounts of silver. Ore and gangue mineralogies vary widely between sites from simple quartz-calcite ± barite veins with galena, pyrite and sphalerite to complex "five-element" (Ag-Bi-Ni-Co-As) assemblages such as that at Silver Islet; the latter is unique in its complexity. Vein systems may be simple, composite or fill brecciated shear zones. The predominant primary textures are those related to open-space filling. These include rhythmic crustification, euhedral and multi-stage crystal growth, zonation and hydrothermal brecciation. Secondary textures, resulting from weathering, oxidation-reduction and supergene processes, are related to near surface alteration while deformation textures are probably due to ongoing fault movement and post-crystallization tectonism. The more complexly mineralized veins crosscut igneous intrusive bodies at high angles approaching 90° and are usually associated with gabbroic rocks, while veins associated with diabase are relatively mineralogically simple.
The Mainland Belt Silver Region is found in a fracture zone that parallels that of the Island Belt and was formed during the same tectonic event. The respective mineralogies of the veins of the two Belts are superficially similar. The major differences are related to the homogeneity of the Mainland Belt deposits and the relative heterogeneity of those in the Island Belt, whose occurrences are associated with later diabasic and gabbroic intrusives. The mafic rocks of both Belts are petrogenetically related.
The Island Belt occurrences may be related, but variances arising from an episodic vein-filling history imply a complex regional metallogeny. The source of the vein metals is perhaps a combination of the distillation of metals from Rove sedimentary rocks and input of metals such as Ni, Co and Ag from an associated mafic intrusion, which is the heat source for both metal mobilization and hydrothermal circulation.