The origin, structure, and history of the Brunswick iron-formation and massive sulfides of the Brunswick 6 and 12 mines are reviewed. New evidence concerning structure, stratigraphy, and alteration of the orebodies is presented.Five generations of folds are described, of which the earliest three have an important influence on the large-scale structure of the mine areas. The earliest folds, F 1 , are large and overturned to the north and are related to coeval thrusting. These folds have not been recognized previously. The main orebody folds are upright, isoclinal, locally downward-facing structures of the F 2 generation.The mineralization is interpreted as syngenetic, but evidence is presented to show that previously interpreted feeder pipes do not exist in a presently recognizable form and that previously interpreted syngenetic alteration is in fact a later diagenetic or postdiagenetic feature. Remobilization of the sulfide mineralization continued until after the F 3 deformation and concentration of the sulfides may be a relatively late feature, but the possibility of the main concentration being syngenetic cannot be ruled out. The evidence is obscured by later deformation and alteration.The present form of the orebodies is largely a function of deformation and the sulfide layers are thickened locally by folding. The orebodies are located in tight structural basins formed by interference of F 1 and F 2 .