Stratigraphy, structure and metallization Piskahegan-Rolling Dam area (northern Appalachians, New Brunswick, Canada)

The main structure in the map area is a northeasterly trending antiform: the “St David Dome”. The core of this structure consists of Ordovician pelitic metasediments, which are flanked by Silurian metagreywackes and quartz wackes. Three distinct phases of folding have deformed the metasediments in the area. The early or main phase is represented by close to sub-isoclinal cleavage folds, which trend roughly parallel with the St David Dome. The second phase folds trend approximately parallel with the first phase folds, but they are all overturned in a southerly direction and they produce typical crenulations in the first phase slaty cleavage planes. A conjugate set of chevron-type folds, which trend obliquely to the earlier folds, was produced during the third phase of deformation. Most of the rocks in the map-area have been metamorphosed to some extent. The higher grades of metamorphism postdate the main phase folding and at least continued into the second phase of deformation. The earliest intrusions in the area are Ordovician gabbroic rocks in the western part of the map area. Other gabbroic rocks, in the southern part of the map area, are possibly of Devonian age. The southeastern part of the map area is occupied by a large Devonian granite mass. A number of adamellite stocks intrude the metasediments along the northern contact of this large igneous body. These small intrusions probably represent a late phase of the granitic intrusions. Moreover it is shown that they are contemporaneous with or postdate the F3 folding phase. Northwesterly trending wrench faults are very prominent in the map area and the latest movement postdates the granitic intrusions. The volcanism in the eastern part of the map area is related to a narrow block which is delimited by two wrench faults. Stretching of this block by a north-northwesterly tensional stress probably permitted volcanic material to reach the surface and in the final stages of volcanic activity, it provided access for mineralizing fluids. Other tin and some base metal occurrences are related to the adamellite intrusions. In the Rolling-Dam area, arsenopyrite-gold mineralization occurs in fractures which are related to F3 folds. These fractures occur mostly near the core, but also along the flanks of the St David Dome, which appears to be mostly intruded by adamellite. Nickeliferous pyrrhotite and some pentlandite are associated with some of the gabbroic rocks in the western part of the map area.