Structural evolution of the Chugach-Prince William terrane at the hinge of the orocline in Prince William Sound, and implications for ore deposits

The Chugach-Prince William terrane is a Mesozoic through Tertiary accretionary complex that lies along coastal southern and southeastern Alaska. In Prince William Sound, the regional structural fabric bends about 90°, forming an orocline. Rocks at the hinge of the orocline consist of turbidites, conglomerate, and minor volcanic rocks and limestone. The structural geology in the hinge region defines a number of domains (each >15 km²) consisting of kilometer-scale tight folds. Adjacent domains may have up to a 90° difference in the strike of bedding and trend of fold axes. Four granite to tonalitic or gabbro plutons are dated or inferred to be about 35 Main age, and all were emplaced after regional folding. Base-metal sulfide mineral occurrences, barren quartz veins, and strikeslip late faults locally cut the plutons and generally strike north-south. The mineral occurrences often have a dendritic network of quartz veinlets adjacent to the mineralized zone and brecciated wall rock within the zone. Oroclinal bending, in the style of bending a bar, is consistent with the origin of the complicated domainal geometry of the structures. The Contact fault, thought by some workers to juxtapose two parts of the accretionary prism, truncates one of these 35-Ma plutons with strike-slip offset, but previous reverse motion cannot be ruled out. A magmatic source for the ore-forming fluids is consistent with the structural and mineralogical data. The north-south orientation of mineralized zones suggests that east-west extension occurred possibly during release of fluids from the plutons, which locally hydrofractured the wall rocks and allowed migration of ore-forming fluids along preexisting fractures.