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•, 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 downwardfacing structures of the F2 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 Fa 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• andThe area between, and surrounding, the two mines was remapped in detail (Fig. 2) and all relevant drill logs, accumulated since 1953, were reevaluated, and in some cases logged by the first author, and incorporated in this study. Form surface mapping (Hobbs et al., 1976, p. 366) was used extensively because of a lack of good marker horizons. There is considerable variation in magnetic deviation close to the iron-formation; during mapping, measurements were checked, and where necessary, made by means of a solar compass that is accurate to ca. 3 ø (compass as designed by Ten Haaf and Wensink, 1962).All available geologic data including that collected by mine geologists was incorporated in our synthesis of the large-scale structure. Age Rocks in the area of the Brunswick mines are ascribed to the Tetagouche Group (Skinner, 1974) and are generally considered to be Ordovician in age. The dating is based on correlation with Early to Middle Ordovician quartzites from central New Brunswick (Poole, 1967) and sparce fossil occurrences in the upper parts of the Tetagouche sequence (Skinner, 1974;Kennedy et al., 1979). Psammites at the base of the Tetagouche Group are probably older than Llanvirn and it has been suggested that deposition began in Cambrian times (Fyffe, 1982).